ATMO 201 Exam 2 (Ch 5,6, & 7)

*ice-crystal (Bergeron) process*

*A process that produces precipitation. The process involves tiny ice crystals in a supercooled cloud growing larger at the expense of the surrounding liquid droplets.*

*Cloud droplets grow by collision and stick to other cloud droplets by coalescence. If this happens enough, the droplets grow large enough to overcome cloud updraft.*

*Describe how the process of collision and coalescence produces precipitation.*

*In an environment of ice crystals and super-cooled cloud drops, the ice grows at the expense of the super-cooled drops. This occurs because the saturation vapor pressure required over ice is less than that required over water at the same temperature.*

*How does the ice-crystal (Bergeron) process produce precipitation? What is the main premise behind this process?*

*size - cloud droplets are 100 times smaller than raindrops*

*What is the primary difference between a cloud droplet and a raindrop*

*(B) a warm towering cumulus congestus cloud because rising air currents slow the rate at which droplets fall, allowing them to grow in size*

*Would the collision-and-coalescence process work better at producing rain in* *(a) warm, thick nimbostratus cloud or* *(b) a warm, towering cumulus congestus cloud?*

*collision-coalescence process*

*the process of producing precipitation by liquid particles (cloud droplets and rain drops) colliding and joining (coalescing)*

adiabatic process

A process that takes place without a transfer of heat between the system (such as an air parcel) and its surroundings. In an adiabatic process, compression always results in warming, and expansion results in cooling.

subsidence inversion

A temperature inversion produced by compressional warming—the adiabatic warming of a layer of sinking air.

rotor clouds

A turbulent cummuliform type of cloud that appears to slowly spin about a horizontal axis behind the leading edge of a thunderstorm's gust front.

neutral stability

An atmospheric condition that exists in dry air when the environmental lapse rate equals the dry adiabatic rate. In saturated air the environmental lapse rate equals the moist adiabatic rate.

absolutely unstable atmosphere

An atmospheric condition that exists when the environmental lapse rate is greater than the dry adiabatic rate. This results in a lifted parcel of air being warmer than the air around it.

conditionally unstable atmosphere

An atmospheric condition that exists when the environmental lapse rate is less than the dry adiabatic rate but greater than the moist adiabatic rate.

absolutely stable atmosphere

An atmospheric condition that exists when the environmental lapse rate is less than the moist adiabatic rate. This results in a lifted parcel of air being colder than the air around it.

parcel of air

An imaginary small body of air a few meters wide that is used to explain the behavior of air.

Water droplet = high pressure Ice crystal = low pressure Ice crystal grows at the expense of the water droplet.

Bergeron Process

Hail occurs when the air is very unstable, i.e. the difference of temperature between the ground and the top of the troposphere is great. When that happens, the rising air of a convection never manages to cool down enough to stop rising and it does so all the way up. When that happens, the friction of the ice crystals in the cloud may even form static electricity and a thunderstorm. It is only in those conditions that raindrops keep falling "upward" until they grow very big and freeze into pellets of ice, when reaching the tropopause and eventually come down as hail.

Describe how hail might form in a cumulonimbus cloud.

"A major hazard to aviation, icing reduces aircraft efficiency by increasing weight. Icinghas other adverse eff ects, depending on where it forms. On a wing or fuselage, ice can disruptthe air flow and decrease the plane's flying capability. When ice forms in the air intake of theengine, it robs the engine of air, causing a reduction in power. Icing may also aff ect th

Describe how icing can be hazardous to aircaft.

insulator

Describe some of the positive consequence of a snowfall.

Clouds in a stable atmosphere tend to spread out horizontally, resulting in cirrostratus, altostratus, nimbostratus or stratus forming in the stable air. Clouds are more likely to grow vertically in an unstable/conditionally unstable atmosphere, as the warm air parcel from the ground tends to keep rising upwards.

Describe the general characteristics of clouds associated with stable and unstable atmospheres.

Snowflakes fall through moist air that forms a thin film of water around the snowflakes that acts like glue when it comes in contact with other flakes.

Explain how snowflakes in a cloud grow large and heavy enough to fall toward the ground.

The environmental lapse rate is the actual decrease of temperature with altitude, while the dry adiabatic lapse rate is the rate at which air temperature would drop with altitude due to expansion.

Explain the difference between environmental lapse rate and dry adiabatic rate.

Injecting clouds with small particles to act as nuclei so the cloud particles will grow enough to fall to the surface as precipitation.

Explain the main principle behind cloud seeding.

With an inversion, warm air overlies cold air; rising air is becoming colder while the air around it is getting warmer. This causes the rising air to sink, creating stability.

Explain why an inversion represents an absolutely stable atmosphere.

Over a cool water surface there is nothing to keep the air moving upwards, and so cumulus clouds are absent there.

Explain why cumulus clouds are conspicuously absent over a cool water surface.

Lifting or lowering a layer of air changes causes the air to stretch out or compact as the air density aloft increases or decreases. When the air stretches out, it cools more quickly. The air at the top of the layer will be cooler than the air at the bottom, which steepens the environmental lapse rate and increases instability.

How and why does lifting or lowering a layer of air change its stability?

When cirroform clouds lie directly above a lower cloud deck, ice crystals may descend from the higher cloud and seed the cloud below it.

How are clouds naturally seeded?

Sleet forms when there is a layer of above freezing temperatures below cloud base but the temperatures at the surface are below freezing. Hail is produced inside a cumulonimbus cloud by accretion of super-cooled liquid water onto a large frozen embryo caught in a strong updraft.

How do the atmospheric conditions that produce sleet differ from those that produce hail?

During the summer months, bodies of water, including the great lakes, absorb huge amounts of energy from the sun and from the warm air that passes over them. They become huge reservoirs of heat. The surrounding land, in contrast, cannot store heat nearly as effectively. Consequently during the autumn and winter, the temperature of the land drops quickly, whereas water bodies lose their heat more gradually and cool slowly.

How does atmospheric stability influence the formation of lake-effect snowstorms?

Light rain or drizzle falls on a surface that is at a temperature below freezing (0C/32F). The ice is thin and thus is transparent, blending in with the often black pavement underneath and making it dangerous for drivers or pedestrians.

How does black ice form?

Drizzle droplets are smaller than rain droplets.

How does rain differ from drizzle?

a radiosonde

How would one normally obtain the environmental lapse rate?

When the environmental lapse rate moves above the dry adiabatic rate

If the atmosphere is conditionally unstable, what condition is necessary to bring on instability?

(B) 15F (-9C)

Inside a cloud, at which of the following temperatures would you expect to see the most supercooled cloud droplet? (A) 35F (2C) (B) 15F (-9C) (C) -40F (-40C)

Unstable

Is strong convection more likely in a stable or unstable atmosphere? Explain your reasoning.

Any process that creates a steeper lapse rate will cause the air to become less stable. This could be warming near the surface or cooling aloft or both.

List and explain several processes by which a stable atmosphere can be made unstable.

1. Lower a layer of air 2. The environmental lapse rate decreases 3. Cooling of surface air 4. Cold advection

List and explain several processes by which an unstable atmosphere can be made stable.

If the atmosphere is stable, they will remain small. If the atmosphere is unstable they will grow into cumulonimbus.

On a warm, sunny day the sky is dotted with small developing cumulus clouds. How does the stability of the atmosphere determine whether these clouds will remain small or grow into cumulonimbus clouds (thunderstorms)?

freezing rain

Rain or drizzle that falls in liquid form and then freezes upon striking a cold object or ground. Can produce a coating of ice on objects which is called glaze.

Takes into account the average temperature and precipitation values to define drought severity.

The Palmer Drought Severity Index uses what weather elements to assess drought severity?

condensation level

The level above the surface marking the base of a cummuli-form cloud

orographic uplift

The lifting of air over a topographic barrier. Clouds that form in this lifting process are called orographic clouds.

entertainment

The mixing of environmental air into a pre-existing air current or cloud so that the environmental air becomes part of the current or cloud.

moist adiabatic rate

The rate of change of temperature in a rising or descending saturated air parcel. The rate of cooling or warming varies but a common value of 6C per 1000.

dry adiabatic rate

The rate of change of temperature in a rising or descending unsaturated air parcel. The rate of adiabatic cooling or warming is about 10C per 1000m.

rain shadow

The region on the leeside of a mountain where the precipitation is noticeably less than on the windward side.

convective instability

The state of an unsaturated layer of air where the upper part is dry and the lower part is humid and the entire layer is lifted to a point where it becomes saturated and unstable.

When the rising air is very cold; not much moisture in the air (dry)

Under what conditions would the moist adiabatic rate of cooling be almost equal to the dry adiabatic rate?

The atmosphere is stable when the environmental lapse rate is small. Consequently, the atmosphere tends to become more stable as the air aloft warms or the surface air cools.

What is a stable atmosphere and how can it form?

The addition/subtraction of heat from air without external heating/cooling.

What is an adiabatic process?

Sleet is water drops that refreeze on their way to the surface. Freezing rain remains rain but freezes on contact with surface objects.

What is the difference between freezing rain and sleet?

freezing rain

What type of precipitation does certain things to object that it may potentially freeze on?

wettest - Marwsynram, India (crest of the southern slopes of the Khasi Hills) driest - Arica, Chile (lowest annual rainfall)

Where are the wettest and driest regions of the world located?

snow pellets

White, opaque, approximately round ice particles between 2 and 5 mm in diameter that form in a cloud either from the sticking together of ice crystals or from the process of accretion. Also called graupel.

During the afternoon, the ground is warmed by the mid-day sun. This in turn heats the air at the ground, which forms a warm air bubble that rises and eventually becomes a cumulus cloud. This doesn't happen often at night because the ground is cooler at night.

Why are cumulus clouds more frequently observed during the afternoon than at night?

The weight of the ice is heavy which brings down or breaks power lines.

Why are ice storms often responsible for the loss of power to many thousands, and sometimes millions, of people?

Moist adiabatic rates are usually slower because condensation in the rising air adds latent heat.

Why are moist and dry adiabatic rates of cooling different?

Because the tropopause is very stable, and thus resists the cloud from continuing to grow upwards, and thus it must spread out at the top.

Why do cumulonimbus clouds (thunderstorms) often have flat tops?

Cumuliform clouds are mostly present in unstable air and cold fronts. When you have precipitation in those conditions, it is often showery in nature. The unstable air also makes it visible and puffier clouds. In the contrast, stratiform clouds are present in stable air and warm fronts. The stable air possibly is suggestive of steady rain (not showers), calm winds, lower visibility, and flatter clouds (153).

Why do heavy showers usually fall form cumuliform clouds? Why does steady precipitation normally fall from stratiform clouds?

hail starts out as rain that is caught in an updraft and frozen. it then falls back through the wet zone and collects more water and is uplifted again and frozen. This process is repeated until the hail is too heavy for the updraft. For the process to start it has to warm enough for rain. In the winter it is often too cold so you just get snow and the strong updrafts needed to for hail are much less common

Why is hail more common in summer than in winter?

The air always contains water vapor.

Why is it NEVER too cold to snow?

mid-latitude cyclonic storm

a cyclonic storm that most often forms along a front in middle and high latitudes. also called an extratropical cyclone, a depression, and a low. It is not a tropical cyclone or hurricane.

flash flood

a flood that rises and falls quite rapidly with little or no advance warning, usually as the result of intense rainfall over a relatively small area

contour lines (on isobaric charts)

a line that connects points of equal elevation above a reference level, most often sea level

surface map

a map that shows the distribution of sea-level pressure with isobars and weather phenomena. also called a surface chart

avalanche

a mass of snow moving rapidly downhill along a steep mountain slope

drought

a period of abnormally dry weather sufficiently long enough to cause serious effects on agriculture and other activities in the affected area

blizzard

a severe weather condition characterized by low temperatures and strong winds (greater than 35 mi/hr) bearing a great amount of snow either falling or blowing. When these conditions continue after the falling snow has ended, it is termed a ground blizzard

training

a situation where a series of individual thunderstorms keep moving over the same location

snow

a solid form of precipitation composed of ice crystals in complex hexagonal form

black ice

a thin layer of ice appearing dark in color

sleet

a type of precipitation consisting of transparent pellets of ice 5 mm or less in diameter. same as ice pellets.

rime

a white or milky granular deposit of ice formed by the rapid freezing of supercooled water drops as they come in contact with an object in below-freezing air.

ice storm

a winter storm characterized by a substantial amount of precipitation in the form of freezing rain, freezing drizzle, or sleet.

anticyclone

an area of high atmospheric pressure around which the wind blows clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere

Palmer Drought Severity Index

an index that determines moisture deficiencies by comparing the precipitation received in an area during a given period of time with the average amount expected during the same period

snow squall

an intermittent heavy shower of snow that greatly reduces visibility

precipitation

any form of water particles -- liquid or solid -- that falls form the atmosphere and reaches the ground

a warm process (can only happen if temp. is above freezing) cumulonimbus

collision-coalescence

ridges

elongated areas of high atmospheric pressure

troughs

elongated areas of low atmospheric pressure

shower (rain)

intermittent precipitation from a cumuliform cloud, usually of short duration but often heavy

flurries (of snow)

light showers of snow that fall intermittently

isobars

lines connecting points of equal pressure

supercooled (water droplets)

liquid droplets at temperatures below 0C (32F)

rain

precipitation in the form of liquid water drops that have diameters greater than that of drizzle

snow grains

precipitation in the form of very small, opaque grains of ice. The solid equivalent of drizzle.

virga

precipitation that falls from a cloud but evaporates before reaching the ground

drizzle

small water drops between 0.2 and 0.5 mm in diameter that fall slowly and reduce visibility more than light rain

hailstreak

the accumulation of hail at the earth's surface along a relatively long (10km), narrow (2km) band

sea-level pressure

the atmospheric pressure at mean sea level

ice jam flooding

the break-up of river ice that gets caught in a narrow channel and produces local flooding, most often in spring

cloud seeding

the introduction of artificial substances (usually silver iodide or dry ice) into a cloud for the purpose of either modifying its development or increasing its precipitation

coalescence

the merging of cloud droplets into a single larger droplet

environmental lapse rate

the rate of decrease of air temperature with elevation. It is most often measured with a radiosonde.

hailstone

transparent or partially opaque particles of ice that range in size from that of a pea to that of golf balls