Meteorology Test Chapters 4,5,6 & 7
Provide a brief explanation on the process involved in the formation of 'Radiation Fog' and a description of the characteristics of 'Radiation Fog'
'Radiation Fog' is lowering the air temperature to the dew-point temperature occurs during the process of radiational cooling during the overnight hours. the best conditions for the formation of 'radiation fog' is when the overnight skies are clear and the wind is light 'radiation fog is shallow, aka ground fog located in low lying areas and is most commonly seen around sunrise
clear
0 clear below 24,000' AGL (ASOS) 0 clear skies (human observation) 0 to 5%
What are the four methods of fog formation?
1) Cool the air to saturation - Decrease the air temp to the dew-point temp 2) Add moisture to the air - Increase the dew-point temp to the air temp 3) Add moisture to colder air and mix to saturation - Lower air temp while increasing dew-point temp 4) Mix two unsaturated air parcels to create a saturated air parcel
1. on a typical summer day, how does atmospheric stability change from the daytime to nighttime? 2. What kind of stability is needed is needed for the development of a thunderstorm. in other words, what kind of atmospheric stability will allow the thunderstorm to grow in the vertical?
1. During the daytime, incoming solar radiation will cause the earth's surface and the lowest region of the troposphere to warm causing the atmosphere to become more unstable overnight hours, radiational cooling will cause the earth's surface and lowest region of the troposphere to cool causing the atmosphere to become more stable 2. for a thunderstorm to grow vertically, the temp within the thunderstorm mass must be warmer than the surrounding environmental air and now it's able to build in the vertical because the atmosphere in the region of thunderstorm development is unstable
Refer to slide 33 chapter 4 1. which city has highest relative humidity? 2. which city has the least amount of water vapor content? 3. which city has the greatest amount of water vapor content? 4. which city has the capacity to hold the greatest amount of water vapor?
1. Fargo because Temp and Dew point are the same = 100% RH 2. Fargo has the lowest Dew point temp and/ or actual vapor pressure 3. Miami has the highest dew point temp and/or actual vapor pressure 4. Dallas has the highest temp and saturated vapor pressure = highest capacity to hold the most water vapor
What are the five types of fog?
1. Radiation 2. Advection 3. Upslope 4. Evaporation • Steam • Frontal 5. Ice
Explain two ways that relative humidity can be changed
1. changing the air's water vapor content while holding the temperature constant: - when water vapor is added to air, with no changes in air temp, the RH increases - when water vapor is removed from the air, with no change in air temp, the RH decreases 2. changing the air temperature with no change in water vapor content: - an increase in air temp while holding water vapor contain constant will lower the RH - a decrease in air temp while holding water vapor contain constant will raise the RH
what mechanisms can cause the atmosphere to become more unstable?
1. cooling aloft a) cold air advection above the surface b) radiational cooling off of top of clouds 2. warming at the surface a) daytime solar heating b) warm air advection on the surface c) cool air moving over a warmer surface
list 3 lapse rates, and their numerical values that are used for a rising or descending parcel of air
1. dry lapse rate (10C/km) 2. dew-point lapse rate (2C/km) 3. Moist lapse rate (6C/km)
name the four mechanisms that are responsible for the development of the majority of the clouds we observe
1. surface heating and free convection 2. topography (orographic lift) forced ascent 3. widespread ascent (convergence of surface air) 4. uplift along weather fronts, air masses of different density interacting with one another
what processes in the atmosphere can cause the atmosphere to become more stable?
1. warm air advection above the surface 2. downward movement of air above (subsidence) that will cause the air temp above to warm due to compression 3. cold air advection on and/or near the surface 4. overnight radiational cooling on the surface 5. air moving over a colder surface which will cause the surface air temp to decrease
What are the necessary conditions needed to determine the correct use of the various lapse rates for a parcel of air?
1. when the dry lapse rate used for a parcel's temperature when the parcel of air is unsaturated (RH < 100%) -10C/km in a rising parcel and +10C/km in a sinking parcel 2. the dew-point lapse rate for a parcel's temp when the parcel is unsaturated (RH <100%) -2C/km in a rising parcel and +2C/km in a sinking parcel 3. the moist lapse rate is used for the parcel's temp and dew-point temp when the parcel of air reaches saturation (RH = 100%) -6C/km for a rising parcel the moist lapse rate isn't used in sinking rate because the parcel will warm and RH values will fall to values below 100%
Few
1/8-2/8 few clouds >5 to <- 25%
Scattered
3/8-4/8 Partly Cloudy >25 to <-50%
Broken
5/8-7/8 Mostly cloudy >50 to <-87%
Overcast
8/8 Cloudy >87 to 100%
cumulus congestus
A towering cloud that has not fully developed into a thunderstorm
cumulonimbus
A type of cumulus cloud that is tall and gray, and associated with thunderstorms.
In a volume of air, how does the 'actual vapor pressure (e)' differ from the 'saturation vapor pressure (es)'?
Actual Vapor Pressure: indicates the air's total or current water vapor content Saturation Vapor pressure describes how much water vapor is necessary to make the air saturated at any given temperature, or it describes the max amount of water vapor the air could hold at a given temperature the air has to be saturated or have a relative humidity of 100% for them to be the same
altostratus
Clouds at mid-level that form a light sheet that lets the sun or moon show as a bright spot.
nimbostratus
Clouds that are low-level, uniform layer, usually very dark, that bring strong precipitation
stratus
Clouds that form in flat layers and often cover much of the sky.
What is the difference between dry and wet haze?
Dry haze: particles (aerosols) in the atmosphere that selectively scatter some rays of sunlight which will give the atmosphere a bluish, brownish, or yellowish color causing a decrease in visibility Wet haze: particles (aerosols) in the atmosphere in which the condensation process has started to occur when relative humidity values reach 75%. Their particles are slightly larger than dry haze particles and will scatter rays of sunlight more effectively than dry haze giving the atmosphere a dull gray or white appearance causing greater reduction in visibility
Cumulus
Fluffy, white clouds, usually with flat bottoms, that look like rounded piles of cotton.
cirrostratus
High elevation clouds, layered across the sky
cirrocumulus
Look like cotton balls, indicate storm on the way
stratocumulus
Low elevation layered clouds with tops and bottoms rounded
altocumulus
Medium level cumulus cloud that is higher than regular cumulus clouds. Lead to precipitation.
an are parcel has a dew-point temp of 30F and an air temp of 80F. The actual vapor pressure is 5.6mb and the saturated vapor pressure is 35.0mb. what is the relative humidity?
RH = [actual vapor pressure (e) / saturated vapor pressure (es)] x 100% RH = 5.6mb / 35.0mb x 100% RH = 16%
During a summer day, explain why the relative humidity values will change significantly while the amount of water vapor remains fairly constant throughout the day
RH doesn't tell us how much water vapor is actually in the air, it tells us how close the air is to being saturated at a given temperature - the air's total water vapor content varies only slightly during the entire day, so it's changing air temperature that primarily regulates the daily variation in RH - this causes the highest RH values to occur during the coldest part of the day, early morning, and the lowest RH values to occur during the warmest part of the day, mid to late afternoon
then the 'relative humidity' is given, why is it also important to know the air temperature?
RH values indicate the amount of water vapor in an air mass relative to its temperature warmer air has the capacity to hold more water vapor a high RH value in a cold air mass could actually contain less actual water vapor that a low RH value in a warm air mass examples: an air temp of 10F with an actual vapor pressure of 1.9 mb and saturated vapor pressure of 2.4 mb would have a RH of 79%
A January snowfall covers central Arkansas with 5in of snow. hThe following day a south wind brings thick dense fog to this region. explain what has apparently happened and name the type of fog that is formed.
The cold snowy surface cooled the relatively warm, moist air mass from the south to its dew-point temperature. Since the warm, moist air mass was advected over a colder region of snow by the south wind, the type of fog created was 'advection fog'
What conditions cause an unstable atmosphere?
The environmental lapse rate is large (steep) when unstable conditions prevail. In other words the difference in the air temperature from the surface to those aloft is relatively large. Therefore we need cold air aloft and warm air at the surface to steepen the environmental lapse rate.
What conditions are necessary for the formation of 'upslope fog'?
The method of formation is to lower the air temp to the dew-point temp forms when moist air slowly rises, cools, and condenses over elevated terrain the cooling process results from the air rising and cooling due to expansion front range of the Rocky Mountains is a prime area for this kind of fog development when a moist air mass is moved into the region by an easterly wind component
Why are IR images computer enhanced with color?
To increase the contrast between features and their backgrounds, a color scale was developed to enhance finer features within a cloud mass or surface objects.
what is an unstable atmosphere?
When a parcel of air remains warmer than the surrounding air while being lifted, we say the air is unstable. In other words, when the environmental lapse rate is greater than the dry lapse rate (10°C/km) the atmosphere is absolutely unstable.
cirrus
Wispy, feathery clouds made of ice crystals that form at high levels.
what type of cloud formations are typically associated a warm front and cold front
air that is forced to ride up and over the gentle slope of a warm front will produce layered clouds in the stratus category because the atmosphere is mainly stable air that is forced upward along a more aggressive cold front will tend to build vertically and be in the cumulus category because the atmosphere is mainly unstable
Why do hot, humid summer days usually feel hotter than hot, dry summer days?
because the evaporation rate would be slower
the temp at the surface is 25C and the dew-point temp is 15C cumulus clouds form in the afternoon. what will be the approximate base of these cumulus ?
cloud base = (T-Td)/2.5C x 1,000ft
What does the saturation vapor pressure (es) primarily depend upon?
depends primarily on the air's temperature because warmer air has the potential to hold more water vapor
the surface temp of a parcel of air is 24 C and the dew-point temp is 16 C using the adiabatic process lift a parcel of air to the 2km level. what is the temp and dew-point temp of the air parcel at the 2km height
dry lapse rate = 10 C/km dew-point lapse rate = 2 C/km moist lapse rate = 6 C/km if parcel is unsaturated use the dry LR for parcel dew-point temp if parcel is saturated use the moist LR for both the parcel temp and dew-point temp
list the lapse rate used for the vertical temp profile of the atmosphere
environmental not a constant number because the atmospheric temperatures are always changing so the calculations will vary to calculate the rate, we use the temp data provided by a upper-level sounding that was collected during the ascent of a radiosonde balloon
What are the 3 primary factors that influence the rate of evaporation?
evaporation occurs when more water molecules are leaving a water surface than are returning Factors: 1. water temperature: warmer water = greater rate of evaporation 2. wind speed: stronger winds will enhance evaporation 3. moisture content of air above water surface: drier air = higher potential for evaporation
steam fog
forms when cool/cold air is located over a body of warmer water, due to the thermal properties (specific heat) of water, water temps cool down slower during the autumn months more common over water during cool/cold fall mornings
advection fog
forms when warm, moist air moves over a colder surface more likely to occur in winter and spring and so it's more common during the late winter and early spring months
name the type of fog that most often forms when warm rain drops fall into an unsaturated layer of colder air near the surface
frontal fog it usually develops in the shallow layer of cold air just ahead of an approaching warm front or behind a passing cold front falls into the category of evaporation fog, the method of formation is a combination of lowering the air temp while adding moisture to the atmosphere
how do the three states of water differ?
gas: water vapor molecules move freely, mixing well with other atoms and molecules liquid: water molecules are closer together (greater density than ice) so they constantly bump each other solid: the molecules arrange themselves into an orderly pattern, locked into a rigid position, not able to move about freely
The orbit pattern for a 'geostationary' and 'polar-orbiting' weather satellite
geostationary satellites revolve around the earth at the same rate that the earth rotates so it remains above a fixed spot on the equator and monitors the same area of the earth's surface constantly height of orbit is 22,300 miles polar-orbiting satellites scan north to south which closely align with the meridian (longitude lines) move over the southern and norther polar regions on each pass with one orbit taking approximately 90-100 min height of orbit varies from 225-500 miles
What does the Relative Humidity Value tell us?
how close an air mass is to saturation (100%) - warm air with a high relative humidity value would contain a lot of moisture but cold air with a high relative humidity would not contain a lot of moisture - relative humidity values aren't a good indicator of how much moisture is actual in the air
what is the difference between hygroscopic nuclei and hydrophobic nuclei
hygroscopic nuclei are 'water seeking' and water vapor condenses upon these surfaces when the RH values are considerably lower than 100% (75%), these nuclei are made up of sulfates hydrophobic nuclei are 'water repelling' therefore relative humidity values must be near or above 100% to promote condensation
Explain why the condensation process becomes more efficient when the air becomes colder
if the air is chilled, the molecules move more slowly and are more apt to stick and condense onto the nuclei
It is raining. How can you tell if the cloud type is nimbostratus or cumulonimbus.
if the precipitation event is a light to moderate steady rain or snow with no lightning, the the could type would be nimbostratus if the precipitation event consists of a moderate to heavy rain shower accompanied by lightning, then the cloud type is cumulonimbus
What conditions are necessary for the formation of 'advection fog'?
lowering the air temperature to the dew-point temperature warm, moist air is advected over a colder surface the colder surface could consist of an area of cold water or cold land the wind moves the warmer air over the colder surfaces the air temp will be drawn down to the dew-point temp causing saturation to occur and the development of fog can be dense, widespread, and long lasting making it MORE dangerous than radiation fog
On what instrument for measuring atmospheric moisture can you find a wet and dry bulb?
on an aspirated or sling psychrometer
Why do we use 10C/km for the dry lapse rate and only 6 C/km for the moist lapse rate?
once air parcel becomes saturated, the latent heat of condensation (600 cal/gr) will add heat to the parcel and offset the cooling due to expansion and slow the rate of cooling within the parcel the cooling rate will decrease from 10 C/km (dry) to 6 C/km (moist)
Explain why rain shadows form on the leeward side of mountains.
precipitation falls on the windward side of the mountain. lets take a closer look at the flow of an air parcel up and over a mountain
calculate the stability of a parcel of air at the 1km and 2km level starting temp for the parcel is 30C and the dew-pt temp is 22C environmental temps are 22C at the 1km level and 12 C at the 2km level answer this question you will need to find parcels temps
slide 41
Stable
the atmosphere relates to a comparison of temperatures between a parcel of air and the air surrounding the parcel when a parcel of air is colder than the surrounding air, it won't be able to rise because it's more dense or heavier than the surrounding air the parcel is forced to rise, it will return to its place of origin once the lifting mechanism dissipates because the temp of the parcel is still colder than the surrounding air
Explain the difference between the 'dry lapse rate' and the 'environmental lapse rate'. how do we obtain the environmental temperatures in order to calculate the 'environmental lapse rates'?
the cooling or warming of a parcel of air that is rising or descending thought the atmosphere constant value the environmental lapse rate indicates the overall vertical temp profile of the atmosphere measured twice daily by the release of radiosonde balloons at the various national weather service offices environmental lapse rates can and do change regularly and can even change from one layer of the atmosphere to the next layer of the atmosphere
what conditions cause a stable atmosphere?
the environmental lapse rate is small when stable conditions prevail the difference in the air temp from the surface to those aloft is relatively small we need warm air aloft or cool air at the surface or a combination of both to achieve this relatively small temp decrease with height
Ceiling
the height of the lowest layer of clouds above the surface that are either broken or overcast but not thin
What is the 'wet-bulb temperature'?
the lowest temperature that can be reached by evaporating water into the air
what does the 'relative humidity' represent?
the ratio of the amount of water vapor actually in the air to the max amount of water vapor required for saturation at a particular temperature and pressure. the ratio of the air's water vapor current content to its potential full capacity
explain why it takes longer to cook vegetables in the mountains than at sea level
the saturation vapor pressure is directly related to the temperature of the water vapor in the bubbles higher water vapor temps = higher vapor pressure values any change in atmospheric pressure will change the temp that the water boils increase in air pressure = increases boiling point and vice versa pressure decreases with hight, the higher you go, the lower the boiling point and the longer it takes to cook veggies
what is the 'wet-bulb depression'?
the temperature difference between the wet bulb and dry bulb (air temperature) is called the wet-bulb depression
Calculate the RH for an air parcel with a dew-point temperature of 45F and an air temperature of 70F you'll need to use the appropriate table of page 107 (11th edition) of lecture book
the temperatures ARE NOT ENOUGH information to calculate the exact RH values - knowing the temperatures, go into the chart and find necessary water vapor info to calculate RH - first find the saturation vapor pressure for the dew point temp (numerator) then saturation vapor pressure for the air temp (denominator) and this indicates the capacity of water vapor the air could hold at that temp
What does the Dew- Point temperature tell us?
the value we want to use to identify how much water vapor is available when the dew-point temp is increasing, the amount of water vapor available is increasing
What are the ideal weather conditions for the formation of dew and frost?
they can both form on nights with clear to mostly clear skies and calm winds when objects near the surface become colder than the dew-point temp, dew will form (condensation) on those objects when objects near the surface become colder than the frost-point temp, frost will form (deposition) on those objects
what is happening that enables you to see your breath on a cold morning? does the air temp have to be below freezing for this to occur?
this can happen because warm, moist air from your mouth mixes with cooler air in the atmosphere - super saturated mixture = condensation occurs within the exhaled breath - small water droplet form from the condensation process - air temp DOESN'T have to be below freezing for this to occur
What is the 'dew-point temperature'?
this is the temperature that air would have to be cooled for saturation to occur - good indicator of the air's actual water vapor content - water vapor into the air increases the dew-point temperature - removing water vapor from the air lowers the dew-point temperature
conditionally unstable
this occurs when the environmental lapse rate is greater than the moist adiabatic lapse rate and less than the dry adiabatic lapse rate. If an unsaturated air parcel is somehow lifted to a level where it becomes saturated, instability may result.
explain how visible (vis) and infrared (IR) images can be used to distinguish: a) thick clouds from thin clouds b) high clouds from low clouds
visible measures sunlight reflected from the tops of the clouds (albedo). thick clouds have a higher albedo so they appear brighter than thin clouds with a lower albedo IR images measure the radiation emitted by the tops of clouds or the earth's surface if no clouds are present. Low clouds (warm) radiate more IR than high clouds (cold). an artificial gray scale is assigned to the various temperatures, gray for lower, warm clouds and white for higher, cold clouds
Explain why the air on a hot, humid day is LESS DENSE than the air Lon a hot, dry day
water vapor molecules weigh less than the dry air molecules that they are replacing at the same temperature and at the same atmospheric level (pressure), humid air weighs less than dry air Two air masses: 1. Air temp 80F, Dew Pt Temp 40F 2. Air temp 80F, Dew Pt Temp 60F why is air mass #2 less dense that #1? because we have to look at the water vapor content and #2 has the highest water vapor content therefore making it the least dense (lighter) air mass
unstable
when a parcel of air is warmer than the surrounding air, it will have buoyancy because it is less dense or lighter than the surrounding air and it will be able to rise on its own freely
what is an adiabatic process
when a rising parcel of air expands and cools or when a sinking parcel of air compresses and warms with no exchange of heat with the surrounding air outside of the parcel, this situation is called an adiabatic process
what kind of clouds are associated with a stable atmosphere?
when stable air is forced to rise, it will spread out. the clouds that form under this condition also spread forming layers cirrostratus altostratus stratus nimbostratus (stratus = layered)
what is an absolutely stable atmosphere?
when the environmental lapse rate is less than the moist adiabatic lapse rate (6 C/km) the atmosphere is absolutely stable
how is the difference between dew- point temperature and air temperature (dew-point depression) related to the relative humidity?
when the two are far apart, the RH is low and when the two are close together, the RH is high
It's 7am, the skies are clear, the wind is calm, the air temp is 37F and the dew-point temp is 34F. your car is parked on the street and the windshield is covered from frost. Why?
your car windshield was able to cool down to a temp below freezing due to the infrared radiation loss during the overnight hours. Since the windshield temp was colder than the dew-point temp and at a temp below freezing, frost formed on the windshield instead of dew
