ATSC SNOW

what does diurnal slope flow refer to

- a process that happens daily on mountains that is caused by thermal/temperature changes over the course of a day due to heating from the sun - causes upslope flows/anabatic winds in the day as well as sometimes return flows - causes katabatic winds and eventually cold pools in night

arctic air

- aka arctic airmass - presents two major hazards to skiers: bitterly cold air and outflow inds

describe low pressure systems

- aka cyclones - associated with bad weather including heavy precipitation, strong winds, and low visibility - hazardous and most difficult to ski in

faceted crystals

- aka facets - form when a strong vertical temperature gradient exists and the water vapour is moving quickly, thus growth of facets happens quickly - lots of space between snow crystals - weak bonds and make the snowpack unstable

rounded crystals

- aka rounds - produced when temperature gradients are weak and water vapour moves slowly - occurs when there is a weak vertical temp gradient within the snowpack (less than 1C per 10cm depth or 10C per m) - tightly packed crystals - strongly bonded and make the snowpack stronger and more stable

what are hazards associated with anabatic winds

- anabatic winds occur in the day due to the diurnal slope flow process - in the summer, it can cause showers and thunderstorm but this is not a hazard for skiers except in the late spring - the wind speeds are not a hazard (10-20km/h) but you should be aware of the warm temperatures it can bring to mountain slopes on top of regular daytime heating

wind sheltered areas

- areas sheltered from wind like mountain valleys and treed areas - valleys are wind sheltered because they are sheltered from surrounding ridges and peaks, which cause friction and turbulence that dissipate wind

why do gap winds matter to skiers

- brings faster winds and cold temperatures - modifies wind direction and which slopes are wind affected and wind sheltered

what are hazards associated with katabatic winds

- can may first and last skis of the day very cold - valley fog and cloud formation caused by cold air pooling - cold temperatures

what are the limitations and positives of IR imagery

- can only infer where cloud tops are so can't distinguish between different cloud types like thunderstorms - but you can infer storm and frontal locations by shape and organization

what are secondary indicators of a frontal zone

- change in wind direction from ahead of the front to behind it called wind shift - pressure trough - often higher relative humidity

what is needed for surface hoar to form

- clear skies, usually at night, causing the snow surface temperature to cool greatly because of radiative cooling - calm or near calm winds, less than 15km/h but a small amount of wind to deposits the vapour as ice (can be provided by downslope flows) - strong temperature inversion above the snow surface (air above the snow is relatively warmer than the snow but still quite cold)

IR imagery

- detect radiation in the thermal part of the spectrum; how hot or cold objects are - temps often shown in kelvins; to convert to celsius subtract 273 - warm objects come from lower in the atmosphere like low clouds, the ground, and the ocean (dark greys) - cold temps come from objects higher like higher cloud tops - impossible to distinguish between low clouds

snow pits

- dug by backcountry skiers and avalanche professionals to find information about the snowpack - 1-2 m deep - analysis includes layer identification, snow crystal type, hardness tests, and stability tests

how does wind speed affect surface cooling or heating

- during the day, the wind can reduce the heat to some extent - turbulence can cause mechanical mixing which makes the mountain slope air temperature closer to its free air temperature - at night, turbulence mixes warmer air from above down close to the surface to reduce cooling which again makes mountain slope air closer to its free ait temperature

in the spring, all surfaces undergo daily melt/freeze cycles and skiers like to hit these slopes when the sun first softens them but when they're not too slushy. which areas would they go to?

- easterly will soften first, then southerly - westerly will go to slush too quickly in the springtime

why do skiers care about cold air pooling

- effects last beyond sunrise as the drainage lasts all night; when the sun comes out, it can take several hours for the cold pool to dissipate; you may dress warmer than necessary - it can persist for days - a lot of people ski at night - backcountry trips may start or end in the dark and often involve camping; it is best to not camp in the valley where it will be very cold

what hazards are associated with warm fronts on mountains

- flat light - precipitation that may change characteristics (snow to rain or snow becoming more dense and wet) - reduced visibility from blowing snow (more associated with cold fronts but still possible) - thicker clouds and fog (warm air holds more moisture than cooler so the clouds are thicker and denser, sometimes described as soupy)

when does avalanche control occur

- following heavy snowfall or multiple small snowfalls so not every day -

in what situations do you not need to interpolate a temperature

- for a frontal passage - for inversions because it can vary widely

describe coastal snow climate

- frequent snowfall - high density newly fallen snow - warm temps - coastal location - deep snowpack - most avalanches occur during or soon after storms (48 hours) - infrequent persistent weak layers - lower avalanche danger - infrequent cold arctic air

what are hazards associated with cold temperatures

- frostbite - hypothermia - cold temperatures can lead to a formation of a weak snow layer at or near the surface which can cause an avalanche

surface hoar/hoarfrost/frost

- frozen version of dew - caused by deposition onto the snow surface when the air temperature falls below the frostpoint temperature - ice crystals on top of snow

what are the different types of snow surfaces

- hard packed, soft packed, fresh powder, dry, moist, melting, slush

what is it like to ski in upside down snow and who fares better

- hard to ski in because the ski tips want to dive under the snow so it is hard to stay above surface - snowboarders have an advantage because they can float easier - skeirs with fat skies fare better - telemark skiers suffer most and can faceplant

what snow surface conditions occur after an arctic air outbreak

- hardpacked but not fun like curdoroy because it is like concrete, super fast, and a bit dangerous if you fall

what are hazards associated with warm temperatures

- heat stroke - hyperthermia - avalanche danger

how do cold fronts affect visibility on mountains

- heavy snowfall - blowing snow - clouds and fog - blizzard conditions

upside down snow and when it occurs

- high density snow on top of low density snow - occurs at warm fronts - temperatures increase following a warm front leading to higher density snow falling as the warm front passes - can also occur when it gets windy towards the end of a storm

where is the best place in winter to find preserved powder

- high elevation slopes that are in the shade all or most of the day, so steep east or north facing slopes - these are also the areas where faceting and surface hoar exists

how can you recognize arctic air on MSLP maps (mean sea level pressure)

- high pressure is associated with an arctic airmass

why is right side up snow safer

- higher density snow under low density snow is a stable configuration - stable snow is less prone to avalanches

when does a weak vertical temperature gradient occur within a snowpack

- if the bottom of the snowpack is near 0C, and the top surface is also near 0C because of cloudy/windy nighttime conditions that keep the surface warmer, it stops temperature inversions - deeper snowpacks can also have this since temperature differences are more spread out

how do the wind and temperature interact to create upside down or right side up snow

- if there is a warm front passing, but towards the end of the storm there are strong winds, you can have a snow layer that is right side up in the lower part due to decreasing temps but then becomes upside down towards the top due to winds but this would still be unstable because you just need one unstable layer for the whole thing to be unstable

in winter, inversions can persist for weeks because

- if there is high pressure - if there is snow on the ground, which causes the air to cool more - if there is clear and calm skies - if the incoming solar radiation comes at an angle, causing the warming to be reduced * most of these do happen

why are facets and surface hoars hazardous

- if they are present anywhere in the snowpack, it is an unstable snowpack - they also persist for long periods of time

when does faceting occur more

- in colder, continental climates with drier, clearer weather and shallower snowpacks

how does cloud cover relate to surface heating and cooling

- in sunny days, the unshaded ground receives the most insolation so this means the daytime heating is maximized - if there is cloud cover, the insolation is blocked - under partly cloudy skies, half of the sun's energy makes the surface - at night, if there is no cloud cover, the ground loses the heat to space - at night, if there is cloud cover, the clouds absorb the heat and send it back to the earth so the surface does not cool as much as it would under clear skies

what kind of conditions bring wind storms to the BC south coast

- in the BC south coast, two types of low pressure systems bring large-scale high winds which cause power outages and tree damage; often occur at fronts

when are wet sluffs more common

- in the springtime and in maritime snow cimates

in what situations do steep north facing slopes not get any insolation

- in the winter when they are in shadow all day and thus only get diffused insolation

what is a hazard associated with high pressure system

- insolation (incoming solar radiation) - when you are at high elevation, insolation is worse because there is less atmosphere above you - snow reflects 90% of light - insolation can be so bad that people get sun burn on the roof of their mouth - snow blindness = temporary blindness

how does an arctic air mass originate

- it forms in the arctic in cold, clear conditions found under a high pressure system - the air is blown southward by northerly or NE winds

where does the arctic air travel

- it is very dense so does not move up mountains like the rockies since stable air doesn't like to move vertically upwards - but once it piles up deep on the east side of the rockies, it moves south and west towards the BC coast through valleys and lower mountain passes and then into the northern plains of the US

how are large scale winds modified

- large scale winds are modified by terrain features - the wind accelerates when it passes ridge tops and can get channeled through valleys and gaps in terrain

wet slab

- less common type of slab -

wet slab

- less common type of slab - springtime phenomenon - results from meltwater or rain going through a snowpack and a pre-existing weak layer becomes wet enough that its bonds lose strength and fail - occur in springtime when the snowpack experiences for the first time all season several nights of above freezing temps in a row

preparation for off ski pistes

- less frequent - might keep moguls (bumps) or clear them if too big - has extreme terrain like in the backcountry with bowls, couloirs, and cliffs - ski resorts still try to eliminate avalanches here -

sluff

- loose snow avalanche - made up of surface and/or near surface snow that is not well-bonded - recognized by their V shape - does not usually do major structural damage or bury people though possible

describe continental snow climates

- low snowfall - low density snow - cold temperatures - in land location far from the coast - shallow snowpack - persistent weak layers - higher avalanche danger

why is upside down snow dangerous

- makes for bad avalanche conditions - unstable snow configuration

what precautions should be taken at temperature -15

- might be time to call off the trip unless you are thoroughly prepared

surface cooling in cloudy nights vs clear skies nights

- minimum, like a few degrees - as large as 10 degrees

describe transitional snow climates

- mix of maritime and continental - frequent snowfall with moderate amounts of snow each storm - moderate density snow - snowpack of moderate depth - some storm snow avalanches - infrequent persistent weak layers - lower avalanche danger due to windy conditions to some extent

describe the findings of Dr. Wolf Read on BC South coast wind storms

- most frequent in first half of winter (Nov-Jan) - most approach from SW, producing strong SE winds, some approach from W producing strong W winds - lows deepen as they approach the coast with a minimum of 99.4 kPa or lower - lower min pressures = stronger gradient = stronger winds

describe strongly bonded snow layers

- mostly comprised of old stellar dendrites that are now rounded crystals aka rounds - more dense and hard - four finger to one finger hardness - newly fallen high density snow (snow that falls during warmer and windier conditions) - wind-slab (one finger to pencil hardness) - less likely to collapse and fail

wind exposed location

- mountain peaks and ridgetops where there is less landmass or ground to slow down wind by friction - wind is worse higher up in the troposphere on mountains and ridges than on the ground because of friction

does high pressure guarantee nice weather

- no, occasionally there can be strong winds during a period of high pressure, sometimes because of a low approaching or other local effects - sometimes high pressure is associated with very cold temps especially in valleys and can also cause valley clouds and fog

does high pressure always mean light winds

- no, sometimes when a high pressure system is approaching and a low is leaving, there is a pressure gradient where the winds can be strong - outflow winds also cause strong winds in high pressures

what are low pressure troughs and how are they identified on maps

- not completely surrounded by higher pressure

wind chill

- not the actual temperature but the apparent temperature, or how cold it feels due to combined temperature and wind - wind chill below 0 risks discomfort and over a long period, hypothermia - wind chill below -10 risks frostbite and hypothermia after a short amount of time

water vapour satellite imagery

- not useful to look at water vapour - shows water vapour in the upper half of the troposphere - useful in the absence of visible/IR imagery - helps to see large scale flow and features not covered in this course

katabatic winds

- occurs at night during the diurnal slope flow process - katabatic winds develop at night in a high pressure system when there is radiative cooling (ground loses heat and the air next to ground cools) - the colder, denser air drains down the mountainside and into the valley due to gravity - 10-20km/h wind speeds and 10-100 m depth - leads to cold pools of air

anabatic wind/upslope flow

- occurs because of the diurnal slope flow - when radiative heating causes air to warm near the surface of the ground and that warm air rises up along the mountainside - 10-20km/h and can rise hundreds of metres

return flow

- occurs in the daytime diurnal slope flow process, and specifically with anabatic wind/upslope flow - when the rising air goes up, air must descend to replace the leaving air - basically a cycle of recyling

what should you always consider before skiing

- recent snow history and the current season

what qualities do people look for in ski pistes

- safe - smooth/even - durable - interesting terrain - visually attractive - good grip

radiative cooling of the ground

- second way inversions occur - when on clear calm nights, the ground cools - in the winter, when there is snow on the ground, an inversion is strengthened - also in the winter, the incoming solar energy comes at an angle so the ground heating is reduced; ground does not get very warm - in winter, inversions in mountain valleys can persist for weeks under high pressure

describe moist snow surfaces and why it occurs

- self explanatory - occurs because of location (coast mountains) - warm temps (warm frontal passage) - sometimes people call snow cement which is snow with very high moisture content, super dense and you get stuck in it

visible satellite imagery

- shows the black and white version of what you would see if you were in space looking down on earth - sunlight illuminated the cloud tops so you cannot use it at nighttime

how does a strong vertical gradient occur

- since the bottom of the snowpack is fixed near 0C, you get a large gradient when the snow surface is very cold - you don't need extremely cold air to get a cold snow surface - like in temperature inversions - a strong temperature gradient also occurs when there is a shallow snowpack; you can get a stronger temperature gradient in a shallow snowpack and a weak gradient in a deep snowpack

what are examples of transitional snow climates

- slkirks, monashees, purcells, and caribou ranges in BC - wasatch in utah and tetons in wyoming - much of the european alps - the northern japanese alps

describe hard packed snow surface and how it occurs

- snow becomes hardpacked when it hasn't snowed in a few days and has become compressed by skiers and/or grooming machines

describe weakly bonded snow layers

- snow that has gone faceting - low density and fewer bonds between crystals - more likely to collapse or fail - surface hoar when it forms at the surface and is buried by more snow - newly fallen low density snow which is shortlived as once it falls, it will start settling

storm snow

- snow that is currently falling or newly-fallen within the last 48 hours - changes in snow density due to a storm can lead to unstable snow setups but these are shortlived, lasting 12-48 hours as the new snow rapidly settles

blowing snow

- snow that is falling and blowing or snow that's being lifted; it only takes 14km/h snow to pick snow off the ground and start moving it - hazard associated with cold fronts

describe melting snow surface and why it occurs

- snow that is melting and can cause suction like feeling - happens in springtime in sunny areas when snow warms up to 0C

what causes wet sluff

- snowmelt from radiation, or by significant rainfaill

what hazards are associated with temperatures near 0 degrees

- snowmelt occurs which can cause avalanches - when there is a prolonged period of above freezing weather, wet slab avalanches become more likely - if precipitation falls above 0 degrees, it can fall as rain which also has consequences for avalanches

avalanche control

- sometimes ski patrol use an explosive early in the morning to trigger an avalanche; if an avalanche happens, it's done, if it doesn't, it guarantees its stable - also includes snowpack assessment

where is wet/heavy snow most often found and why

- southerly and westerly aspects because the largest amount of insolation is on the southerly, and westerly gets insolation at a time of day when the air temp is warmest esp in the spring

subsidence

- subsidence is sinking motion that is associated with high pressure systems - when air descends, it warms and becomes drier because of the dry adiabatic lapse rate - when the high pressure system is strong and sustained over time, the descending layer may become warmer than the layers below it (like those in a mountain valley) which may cause a warm layer over a cold layer, or an inversion - one of the ways inversion happens

what factors affect heating and cooling of the ground surface

- sun angle - cloud cover - wind speed

how does the sun angle affect surface heating and cooling

- sun angle = the angle at which the sun's rays strike the earth's surface which can vary by hour as well as daily and by seasons - when the sun angle is low, the heating from the sun is much weaker

what destroys surface hoar

- sunlight or warm air causing sublimation (ice to vapour) - strong winds either by sublimation or mechanically, if it blows the crystals away - melting/refreezing again like in daytime - rain or freezing rain

describe powder snow surface conditions and how it occurs

- super sought after - follow substantial snowfall between 10-50cm and soemtimes more - fresh powder is when it is untouched - can also occur through wind loading = loose snow is blown

what are hazards associated with surface hoar

- surface hoar can lead to avalanches - it is feathery and delicate so when it is on the surface of snow, there is no danger but when it is a weak layer within the snowpack, it can collapse since there are few bonds between the ice crystals and lots of empty air space

within the troposphere, the initial source of heating and cooling is

- the ground surface - the ground heats the air next to the surface and other processes transfer that air up - at night, especially in clear skies, the surface cools and the largest amount of cooling is often at the ground surface making the air near the ground the coolest

what does diurnal slope flow cause in the day

- the sun heats the ground surface (radiative heating) and the ground warms the air above the surface - the air rises and travels up the mountainside, as opposed to rising vertically - this thermally inducded upward motion is called upslope flow or anabatic wind

what is a limitation of wind speeds on wind maps

- the wind speeds are estimates in the 'free atmosphere' of the elevation and do not account friction from the surface which slows down the speed

how can you recognize an arctic airmass on maps in general

- tight contours representing the cold front; arctic front and temperatures of -15 and colder behind it - N and NE winds - high pressure

melt/freeze crust

- type of crust layer - when above freezing temperatures occur, melting the snow, then refreezes

sun crust

- type of crust layer - when radiative heating leads to above freezing temperatures and this melt/freeze crust is then referred to a sun crust

rain crust

- type of crust layer - when rain on snow occurs and is followed by below freezing temperatures a rain crust forms

curduroy snow

- type of snow report; means hardpacked

off-piste

- ungroomed areas where terrain tends to be more extreme; for advanced skiers - includes traditional-looking runs left ungroomed, glades, and ungroomed alpines

what steps are taken after the first initial one to made a downhill ski racing piste hard

- using grooming machines, skis, or feet in hard boots to compress snow (though not uniform) - man made snow - water injection (best overnight when clear skies and cold temps) - chemicals (mixing salt or fertilizer, but bad for the environment and effective for short periods of time so needs to be done right before a race); banned for environmental and health reasons in some places

what happens after a cold front passes

- usually there is a drying trend which leads to breaks in the cumulus clouds and the precipitations - convective showers are possible (short lived and heavy showers) because the air mass is unstable due to cold air moving aloft

what are the hazards associated with cold air pooling

- very cold air in valleys and valley bottoms - cold air temps in the down slope of drainage flows - valley cloud/fog limiting visibility - freezing fog when temps within the fog are below freezing

describe crust layers

- very thin (few mm to few cm) - quite dense and hard; pencil or knife - crusts don't fail but can lead to failure

what is the disadvantage of putting mountain ranges in climate categories

- weather always changes so they don't always work

when does right side up snow occur

- when a cold front happens - temperatures decrease after a cold front, so the snow decreases in density as the front passes leading to right side up snow (high density on low density) - it is also windier at a front, and less windy following so both temps and wind work to make right side up snow

slab avalanches

- when a layer beneath the surface fails and a cohesive layer abov it fractures into a block or blocks which slide down - typically larger than sluff avalanches - more destructive - two types: hard and soft and less common wet slab

lee shadowing

- when air descends on the other side of the mountain after oreographic uplift, it is drier since the moisture was removed in the form of precipitation, and as it moves down, it warms adiabatically by 10 degrees per km (warms faster than it had cooled on the windward side) - final result is warmer and direr air after crosses the mountain

oreographic uplift and adiabatic cooling

- when air lifts up and over mountains; as it ascends it cools adiabatically meaning for each km it rises, it cools by 10 degrees

airmass transformation

- when air, because of lee shadowing, is warmer and drier - air cools adiabatically during oreographic lift and loses its moisture as precipitation, as the air descends, it warms adiabatically as it descends on the leeward side

what is the venturi/bernoulli effect

- when air/wind flow over a mountain goes through convergence and accelerates/speeds up

valley cloud and fog

- when in mountain valleys, during an inversion, moisture gets trapped within the valley; sometimes pollution also gets trapped (inversion is stable since cold air is below warm air, which can create fog/cloud that lasts for days)

what is spring wax

- when skiers put this type of wax on in springtime to reduce the suction feel of melting snow

wet sluff

- when snow has a high liquid water content and is more likely to be swept along because it has more mass

when does right side up snow occur

- when temperatures and/or wind speed decrease during the course of a storm or storm cycle

describe sea level pressure (SLP)/mean sea level pressure (MSLP)

- when the pressure on maps is calculated not at the surface pressure, but what the pressure would be if reduced to the sea level to show us a nice looking map of highs and lows which we can use to predict weather

describe slush surface and why it occurs

- whole top layer melts - occurs in springtime in the afternoon - whole snowpack is near 0C - fun to ski in since it moves around like powder but dense

gap winds

- wind blowing through a gap in the mountain -occurs when there are strong winds perpendicular to a large mountain range - occurs a lot on BC coast mountains - most common when arctic air approaches the north mountains from NE; most of the air cannot go through since it is dense - but since the arctic air dammed behind the mountains has high pressure, the pressure gradient moves the air to the region of low pressure to the west and southwest over the pacific ocean (over the mountain)

mature trees aged 50+ are advantageous to skiers because

- wind sheltered (but beware of tree bombs) - generally safe from avalanches

why are lows and troughs associated with bad weather

- winds converge towards the centre of the low and blow from high pressure to low pressure - in the northern hemisphere, the coriolis effect turns these winds to the right so the air rotates counterclockwise around a cyclone - the upward motion of air in a moist atmosphere cases clouds and precipitation and other bad weather - * the cyclone looks like arrows going up counterclockwise

why does high pressure bring good weather

- winds diverge away from the centre flowing from high to low pressure, and turning right in a clockwise circulation - wind tends to be lighter because the pressure difference is weak under high pressure

winds speeds as shown on pressure level maps

- winds shown on slopes and valleys are only loosely related to speeds on pressure level maps because they are sheltered by surrounding ridgelines - BUT they will be moderate and strong still in alpine or treeless areas

describe dry snow surface and why it occurs

- you say this when there is no liquid water but in reality there is a small amount - occurs because of location (BC interior famous for champagne snow) and very low temps due to cold fronts for example

what is a comfortable skiing range

-8 to 3, but each person is different

what are examples of coastal mountains

1. coast mountains in bc 2. cascades and sierra nevada in USA 3. mountains in western norway

what are the different snow climates

1. coastal/maritime 2. transitional (aka intermountain in USA) 3. continental

how do you forecast temperature in the mountains using pressure level maps?

1. find the free air temperature (the temp at a particular level of the atmosphere assuming no heating or cooling from the surface) 2. make an adjustment based on the heating or cooling that occurs at the ground surface

what do you do if you are given the pressure level map at an elevation different to where you want to ski?

1. find the temperature from the pressure level map you have been given 2. use the dry/wet air tool (if over 80% saturation, use wet, if under use dry; for dry, has to be under 80% RH AND a. winds are 40km/h aka strong or greater, b. in the daytime during spring or c. in the afternoon on sunny winter days) and plot your temp on the right section, then follow the green line to the elevation of concern 3. at that point, go straight down to read the temp

what makes an avalanche

1. pre-existing weak layer and 2. a trigger like new snowfall, new wind deposited snow, rapid temperature change, or human weight

how are snow layers measured for avalanche danger and stability

1. snow pits 2. hand hardness test

how do inversions occur

1. subsidence due to high pressure 2. radiative cooling of the ground

what are the two exceptions to alpines being generally wind sheltered areas

1. thermally driven flows (diurnal slope flow and katabatic winds) 2. terrain-channelled flows (fix this later)

under dry adiabatic conditions, temperatures decrease with height at a rate of...

10 degrees per 1000 metres (9.8C/km)

pressure at sea level is typically around

100 kPa, which is 0m elevation

how much wind is needed to pick up dry powdery snow off the ground

14 km/h

how do you find the rain snow line

1st method: use the vertical interpolation tool to find the elevation of the freezing level then subtract 300m; use moist air tool 2nd method: use a time height plot, find the 0 isotherm, convert pressure level to elevation, and subtract 300m 3rd method: meteogram 4th method: use weather station observations from different resorts and interpolate

the rain snow line is roughly ___ below the freezing level

300m

under wet adiabatic conditions, temperatures decreases with height at a rate of...

6 degrees per 1000m (6C/km) *SLOWER than the dry adiabatic rate

it is rare for water content in snowpacks to exceed ___%

8

what is a good pressure level map for grouse?

85 kPa because it is close to the resort elevation (85 is 1500m, and grouse's peak elevation is 1250 m.

snow should be groomed after each ___ of snowfall

`20cm

high pressure ridges are always connected to

a high pressure centre; they are an area that increases in size as it gets to the centre; ridge is the extended area increasing in pressure as it gets to the circle, the high is the enclosed circle itself

what is an avalanche

a mass of snow that moves quickly down a mountain with two main types: loose-snow (sluff) and slab

ensemble forecast

a meteogram (plot graph) with many models run at once to produce an average of the forecasts

a strong counter clockwise wind field on a pressure level map indicates

a strong low pressure system

what is a front

a transition zone defined by strong horizontal temperature gradient

what do skiers hope for after a long period of high pressure which creates surface hoars

a warm storm to either cause winds to eliminate the surface hoar or rain

overnight in particular, thermally-driven downslope flows aka katabatic winds, can bring moderate winds to

alpine valleys

if RH is 90% below the elevation of Grouse on a mountain, what can you expect?

an unbroken layer of cloud below you called an undercast

pressure level maps are often ___ loops that show ___

animated loops that show future forecasted weather

frost

another word for surface hoar

what temperature is "unusually warm" or "unseasonably warm" in skiing

anything above 5 degrees

the coldest temperatures occur when

arctic air is present; real hazard for frostbite, hypothermia, and wind chill will occur

what air temp is found at warm fronts

around 0 degrees Celsius

if an oreographic uplift occurs with humid air, then...

as the air cools, it reaches its dew point and condenses to form clouds and often precipitation too

why do facets and surface hoars develop in continental climates

because of the cold, clear nights

why are surface maps problematic for recognizing fronts

because temperature decreases rapidly with height which makes it almost impossible to see a front in complex terrain

the snow after a cold front passes if often the ___ for skiing

best

when using a pressure level map for a location in mind, it is best to use

best to use a pressure level map that is at or above the elevation of concern

as humidity increases, the snow crystals tend to get

bigger and more branches or dendrites on them

anabatic cumulus cloud

caused by diurnal slope flows and specifically anabatic wind/upslope flow; if there is enough moisture present, this cloud forms

if RH is less than 50% for an elevation of concern, what can you expect?

cloud free conditions in that level

whistler is in which mountain range

coast mountains

the strongest winds occur during ___ fronts

cold

ahead of a warm front is

colder air

blizzard

combination of heavy snow, blowing snow, clouds and fog; associated with cold fronts

which snow climate often sees arctic air

continental snow climates

how do crusts lead to failure in a snowpack

crusts can block the flow of water vapour, causing a weak layer of facets to form right above them in the snowpack

how often does grooming occur in ski resorts

daily

wind at 200km/h results in

dangerous conditions where the wind can knock you off your feet or downhill; tents will be destroyed and in cold, frostbite is almost instant

when does minimum sun angle occur

december 21st, in the winter solstice

cold air is ___ than warm air

denser; that is why it sinks

frostbite

dewpoint temperature if the air is below 0C; instead of dew, frost forms

easterly aspects mean

east facing slopes

rain-snow line

elevation where precipitation transitions from rain to snow and is lower than the freezing level; means vertical sense

depth hoar

facets deep down in the snowpack that become large and angular

the weaker the pressure gradient on a pressure level map, the ___ isobars

farther

why is right side up snow nice

feels bouncier and floatier to ski in; if it is deep enough, the low density snow sprays and billows up around you

at what time do east and south-east facing slopes get the most insolation

first thing in the morning when temps are colder

as temperature decreases below 0C, snow crystals form changes type three times from

flat plates, to long columns or needles, back to plates, and then columns and plates

surface hoar forms best on...

flat, open surface; if there are trees or rocks it can inhibit formation due to radiative warming from those objects

if RH is 90% on a map for Grouse mountain's elevation (85 kPa) what can you expect?

fog, as 90% RH represents overcast which on a mountain would mean fog with little to no visibility

where does water vapour travel in the snowpack

from warmer to colder, so generally from bottom to top; it travels upwards but the speed at which it goes depends on how strong the temperature difference aka gradient is

what are hazards associated with cold fronts

frostbite and hypothermia

what do barbs mean on a wind barb

half barb = 10km/h or 5kts full barb = 20km/h or 10 kts triangles or flags = 100km/h

why does hardness relate to snow stability

hardness is related to density and cohesiveness of the layer

light winds are associated with what kind of pressure

high pressure; identifying regions of high pressure on a map allows you to recognize regions of light wind

70 kPa winds greather than 60km/h will bring

high wind speeds to wind exposed areas in mountains of southern BC

oreographic lift results in more precipitation at

higher elevations, because there are colder temps

what happens if the weather that makes hoarfrost persists

hoarfrost can reach several cm

what does the size of hoarfrost depend on

how long the weather conditiions of clear skies, calmish winds, and strong temp inversion remains

snowcat

how ski pistes are groomed; has tools that churn the snow and compact the top 10-15cm of it to increase density and hardness

what is the exception to a ridge being a windexposed area

if you are on a ridge surrounded by other ridges, the friction and turbulence might slow down the winds somewhat but still, it will be stronger than winds below

in what conditions can surface hoar occur in a large area

in alpine areas after a long period of high pressure

at what time do west and south west facing slopes get the most insolation

in the afternoon when temperatures are warmest

the sun angle above the horizon ___ into spring

increases

frostbite

injury where the skin freezes

if you have a pressure level map for 85 kPa and 70 kPa, but you are skiing at an elevation in between, what do you do

interpolate using the two maps you do have

what happens when air travels down in a high pressure system

it sinks downward and thus becomes warmer and drier which is why high pressure systems are associated with dry conditions, clearer skies, and lack of precipiation

can hoarfrost form in the day?

it usually forms at night but in the day it can form on north facing slopes because they are in the shade and continue to cool radiatively with little to no radiative heating; if it grows here, it might not melt and grow for days

when does maximum sun angle occur

june 21st, summer solstice

where are transitional climates

just inland from the coast

what has more turbulence, anabatic winds or katabatic winds?

katabatic, because there is no upward motion and they are shallower (?)

tree bombs

large clumps of snow suspended in tree branches that may fall and hit skiers during high winds; while heavily treed areas may not experience strong winds, this can be a concern

wind direction, whether it goes up or down valley, usually depends on

larger scale pressure pattern

in what time frame do you need to consider new snowfall density changes

last 24 hours

if there is a weak pressure gradient, the winds will be

light

how are low pressure centres identified on a pressure map

low pressure centres are areas of low pressure completely surrounded by higher pressure and thus encircled by at least one isobar where the isobars increase in pressure around the one low circle

what is the best map to see wind

low-level wind forecast map at the surface or at 85 kPa pressure level BUT these maps do not account for friction which slows down wind

soft slabs

lower density snow that has more bonding than newly fallen, non wind affected snow; results in smaller avalanches that break up more as they slide down

85 kPa is about ___ at whistler

mid-mountain

at what time do south facing slopes get the most insolation

midday when temps are warmer

alpines are ___ sheltered from strong ridges than ridges and peaks

more sheltered; alpines are high elevation, treeless areas; can occasionally see strong winds

slopes that face east and west receive ___ insolation than north-facing slopes but ___ than south facing slopes

more; less

stellar dendrites

most recognizable type of snowflake

in mature tall forests, strong winds

never make it to the forest floor

graupel

new snow crystals that form from accretion, when a snow crystal collides with water droplets that stick to it; looks like ice cream dots

is the rain snow line the same as the freezing level

no because snow crystals fall from below freezing air to above freezing air and take time to melt depending on temp gradient, humidity, and size of crystals

will there always be precipitation or clouds during oreographic lift

no, if the air approaching the mountain is dry it won't. this happens at inland locations away from the coast

can you groom a piste that has freshly fallen snow

no; it should be done not when the snow has low density so you need to wait for it to become dense by settling

northerly/shady aspects means

north facing slopes

where is most likely to be crust free

northerly and easterly

wet adiabatic lapse rate

once the air is saturated (humid), the air cools at a slower rate (this is about mountains and oreographic uplift)

if the RH is 90% on a map of elevation above grouse mountain's elevation, (more than 85 kPa), what can you expect?

overcast conditions over grouse

in real life, people almost always ___ wind speeds

overestimate

when are stable vertical temperature profiles most common

overnight and into the early morning during the winter, especially when there is snow on the ground; when the sun disappears, the ground loses its heat and starts to cool; optimal cooling occurs when there is clear skies (high pressure); in these conditions, the cold air sinks into valleys because it is so dense

what is another indicator of highs/lows on maps

precipitation and wind; precipiation is associated with lows; winds going counterclockwise are associated with lows; winds going clockwise are associated with highs

as you go up in the atosphere, elevation increases but --- decreases

pressure

what kind of precipitation occurs at cold fronts

range of snow, ice pellets, graupel, and rain

adiabatic lapse rate

rate in which the temperature changes in the atmosphere (?)

why is wax applied to skis and snowboards

reduce friction and go faster

what is used to represent clouds on pressure level maps

relative humidity (RH) which is the ratio of how much moisture is actually in the air to how much the air is able to hold expressed as a percentage

hard slabs

result of high density, well bonded snow which remain hard and cohesive blocks as they slide

when is the best time to groom a piste

right after closing so the piste has maximum time to hard before opening the next day; only exception is when there is snowfall in the overnight forecast

what is better for skiing, right side up snow or upside down snow

right side up snow because the vertical density gradient lifts the skis or snowboard making it easier and it's safer because it is more stable

what is the first step to preparing a ski race course

same steps as for ski piste for recreation

cold and warm fronts look __ on maps

same; the best way to tell is to see an animation and see if the cold or warm air is advancing or neither

what are other words for ski pistes

ski runs, groomed runs, or groomers

surface hoar over large areas can cause what type of avalanche

slab avalanche

diamond dust

smallest snow crystals that often glisten under the sun and found in extreme cold weather

columns and needles

snow crystals that are just columns and needles

what is the best tool for upside down snow

snowboards

snow metamorphism

snowpack evolution

is the snowpack colder at the top or the bottom and why

snowpack is at or close to 0 degrees at the bottom due to some heating from the ground so the top of the snowpack is colder on average than the bottom

what is the most important factor that influences the evolution of the snowpack

snowpack temperature gradient

southerly aspect/sunny/solar aspects means

south facing slopes

in the winter, the sun strikes which facing slopes more directly and which facing slopes less direction

south facing slopes; north facing slopes

where are crusts most often found

southerly and westerly aspects if the snow surface refreezes overnight, which happens if there is no cloud cover

where are sun crusts more common in the northern hemisphere

southerly facing aspects while northerly facing aspects remain crust free

insolation on ___ and ___ slopes increases meltwater in the top layers of snow, increasing the chances of ___ and ___ which can be an issue in the backcountry

southerly; westerly; wet sluffs; wet slabs

high pressure is associated with nice weather and a ___ temperature profile

stable

temperature inversions are statically ___

stable; because the temperature increases with height and so the cold air is under the warm air

what types of clouds are associated with warm fronts

stratiform; it would come in this order (highest to lowest): cirrus, altostratus, stratus, nimbostratus and associated rain

when the sun hits the earth perpendicularly, the heating is...

strong and the most concentrated, aka the warmest

describe the sun angle in the winter

sun is at a lower angle and tends to be in the southern part of the sky as it rises in the southeast and sets in the southwest (in summer, rises in the northeast and sets in the northwest)

why is precipitation associated with cold fronts

the cold air behind the front undercuts the warmer air ahead, causing the warmer air to rise up, cool, condense, and form tall cumulonimbus clouds which can bring a band of precipitation along the frontal zone (10-100km wide); this can bring moderate to heavy snowfall

the plain end of the wind barb indicates

the direction that the wind is going to

katabatic wind

the downslope movement of cold air that occurs when there is no large wind flows, which leads to cold air pooling within mountain valleys. occurs under high pressure because you need clear and calm skies for the air to cool so much that it sinks downwards. if there were clouds, they would absorb the heat from the ground and radiate it back to the ground making the air not as cool.

analysis map

the first map in a pressure level map loop

forecast maps

the pressure level maps in an animated loop that show the evolution of the weather into the future

why do postfrontal clouds dissipate in the evenings in cold fronts

the sun heated ground

free air temperature

the temperature at a particular level of the atmosphere assuming no heating or cooling from the surface

what is one advantage of anabatic winds/upslope flow

they help dissipate valley fog/clouds and cold pools/inversions

it is best to use IR and satellite

together to find threatening clouds and clear air

what snow climate is called the goldilocks

transitional

which snow climate is optimal for skiing

transitional cliamtes since there are cooler temps compared to coastal and maritime and the snow is in medium density

often, snow becomes bonded enough to form soft slab after __ days on the ground

two

a stable layer on top of an unstable layer is

unstable

what type of gradient is in snowpacks

vertical temperature gradient

what are some characteristics of arctic air

very cold, dense, shallow, and stable; since it is so dense, it flows quickly like water

what are the main limitations of visible and IR imagery

visible = can't see at night IR = can't see low clouds - these two forces combine during ski season because often valley clouds form at night so it is impossible to see on both IR and visible imagery in the early morning in winter; best way to overcome is to look at surface observations through webcams or some airports report cloud cover and visibility overnight

what are the three types of satellites used by meteorologists

visible, infared, and water vapour

what is the most extreme example of a wind exposed location

volcanic peaks because they rise so far above surrounding terrain

low pressure systems have both

warm and cold fronts

ahead of a cold front is

warmer air

what types of water exists in the snowpack

water exists in all three phases in the snowpack, even a small amount of liquid water

temperature gradients in a warm front are often ___ than the temperature gradient in a cold front

weaker

in the real world, what happens when a warm front approaches

weather deteriorates

westerly aspects means

west facing slopes

what is a warning sign of wet slab

wet sluffs

when do you use the wet air tool to interpolate temperatures at an elevation of concern when given a pressure level map of a different elevation

when RH is above 80%

when does condensation (aka clouds and fog) occur?

when RH reaches 100% aka saturation; if there is 100% RH on a pressure level map, there will be clouds/fog BUT in the real world, if the map says RH is `70%, expect to start seeing cloud formation; if RH is greater than 90%, expect overcast cloud cover

statically stable air

when a cool layer of air is beneath a warm layer of air (cool air is more dense) and there is no vertical motion

accretion

when a snow crystal falls through the atmosphere and it collides with tiny water droplets that freeze and stick to it; the new snow crystal that forms are called graupel

deposition

when a substance undergoes a phase change from gas directly to solid without the liquid stage in between; when water changes from vapour to ice, it literally deposits itself

capping inversion

when an inversion occurs anywhere else within the troposphere and it behaves like a lid or a cap, trapping the air below; the air below does not travel any higher than the inversion layer of cold air below warm air

right side up snow

when heavy snow is under lighter snow; less dense snow on top of dense snow

aggregation

when ice crystals stick to other ice crystals forming irregular shapes called aggregates

what is cold air pooling

when in high pressure, which causes calm and clear skies as well as a stable temperature profile, cold air above the ground surface pools or drains into valleys where clouds or fog can form; this can cause hazards for skiing

stationary front

when neither air mass advances

persistent weak layer

when surface hoar layer is buried for weeks and becomes this

temperature inversion

when temperature increases with height

when is the hardening process of pistes best

when temps are close to 0C

what are arctic outflow winds

when the arctic air mass is pushed to the coast (?) and other areas

when do facets occur in a snowpack

when the temperature gradient is larger than 1 degree per 10cm depth, or 10C/m

isothermal

when the whole snowpack is around 0C; happens in slush surfaces

flat light

when there are increasing high clouds because of the approach of a low pressure system in which the clouds diffuse the light, and the snow surface no longer has definition which can be dangerous because you can't tell uphill from downhill

whiteout

when there is a cloud/fog at a mountain elevation in which you are in where you can't see anything; it's white in every direction; hazard associated with cold fronts

along-valley channeling of wind

when wind direction is modified by the valleys and the wind goes along the valleys

white out

when you cannot see in front of you

hypothermia

when your body loses heat faster than it can produce and your body temperature lowers to below 95F (normal body temp is 98.6)

groomed ski pistes

where majority of people skiing and snowboarding spend time recreationally and where downhill racing occurs

what are secondary indicators of fronts

wind direction; wind shifts; moisture (for cold fronts)

describe the winds associated with warm fronts in the northern hemisphere

winds ahead of a warm front come from E, behind it they shift to more S

in the south coast of BC, what is the typical wind shift of cold fronts

winds ahead of cold fronts are SE, and winds behind are SW

oreographic lift is strongest when

winds are perpendicular to mountains

if pressure level winds for your altitude are less than __, you will likely have light wind speeds

winds under 50km/h

in the northern hemisphere, what direction are winds in a cold front

winsd ahead of a cold front come from the SE, S, or SW and behind it, come from the W or NW

how can you recognize arctic air on temperature maps

- 85 kPa temperature maps (which are 1500m above sea level but still shalow enough to show the arctic air) which show a strong temperature gradient (contours packed tight) associated with a cold front, as the cold front ushers in the arctic air (called arctic fronts) - you will see cold temperatures behind the front (ticht contours) of -15 and colder

what precautions should be taken at temperatures of -10

- a lot of layers - face covered - chemical warmers in boots and gloves

describe high pressure systems

- aka anticyclone - area where pressure is higher than in surrounding regions - best for skiing as it makes it more enjoyable and also safer

hand hardness test

- applying light pressure to each snowpack layer - hardest to softest = fist, four fingers, one finger, pencil, and knife

examples of continental locations

- rocky mountains - pamirs (asia) - brooks range (usa)

wind slabs

- slabs avalanches due to high wind speeds

what can you expect when there is an inversion on a mountain

- warmer temps at the middle/top and colder in the valley - surface hoar formation in the valleys due to cold processes - warmer temps at upper elevations can cause melting and wet-type avalanches - valley cloud and fog

describe soft packed snow surface and how it occurs

- when freshly fallen snow has been somewhat compressed by snowcats and skiers - nice to ski in - happens when it hasn't snowed in a few days or just lightly snowed

pressure level maps that are a few hundred metres above most ridgelines give you a ___ indication of wind speeds in wind exposed areas

a better; for most southern bc mountains, 70 kPa is good

how can you identify a high pressure centre or ridge on a map

a high pressure centre will be an enclosed circle of highest pressure surrounding be decreasing pressure and a ridge will be where the pressure is high in most directions and the wind is also clockwise

gradient

change in a property over a distance

what hardness test do weak snow layers pass

fist hardness or four fingers

why does snow have to be hard for downhill ski racing

if it is not hard, the skis will cut into the ski piste making them slow for turns or cause them to slip

are warm fronts associated with no hazards?

no

does a snowpack stop changing once snow stops falling

no

NWP

numerical weather prediction models

only ___ weak layer is needed for a snowpack to fail

one

calm winds allow for the most

surface cooling

what's best to recognize a front

surface pressure level maps because they allow you to see temps at pressure level located above terrain effects

dewpoint

temperature in which water vapour condenses into liquid droplets while being cooled

cold air pooling can strengthen an already surface ___ ___

temperature inversion (where temp increases with height)

wind shift

when wind direction changes across a front

the end of the barbs with the barbs indicates

where it is coming from

how can you recognize a front from a surface map

where the isobars are close together

does avalanche control occur in the backcountry too

yes

when can you use the dry air tool to interpolate the temperature of an elevation of concern when given a pressure level map of a different elevation

- RH is below 80% - AND if the winds are moderate/strong (40km/h), in the daytime during spring or in the afternoon on sunny days