Term
| What does the ‘relative humidity’ represent? |
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Definition
| The relative humidity (RH) is the ratio of the amount of water vapor actual in the air to the maximum amount of water vapor required for saturation at that particular temperature (and pressure). Or in other words, it is the ratio of the air’s water vapor current content to its possible full capacity. |
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Term
| An air temperature of 10˚F with an actual vapor pressure of 1.9 mb and saturated vapor pressure of 2.4 mb would have a RH of? |
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Definition
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Term
| An air temperature of 50˚F with an actual vapor pressure of 4.6 mb and saturated vapor pressure of 12.3 mb would have a RH of? |
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Definition
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Term
| Explain why the air on a hot, humid day is less dense than on a hot, dry day. |
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Definition
| At the same temperature and at the same atmospheric level (pressure), humid air weighs less than dry air. This is due to the fact that water vapor molecules weigh less than the dry air molecules that they are replacing. (The molecular weight of water vapor = 18, while the molecular weight of dry air = 29). |
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Term
| What is the ‘dew-point temperature’? |
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Definition
| This is the temperature to which air would have to be cooled (with no change in air pressure or moisture content) for saturation to occur. The dew-point is a good indicator of the air’s actual water vapor content. High dew-points indicate high water vapor content; low dew-points, low water vapor content. Addition of water vapor to the air increases the dew-point temperature; removing water vapor lowers the dew-point temperature. |
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Term
| Calculate the relative humidity (RH) for an air parcel with a dew-point temperature of 45˚F and an air temperature of 70˚F. |
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Definition
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Term
| An air parcel has a dew-point temp of 30˚F and an air temp of 80˚F. The actual vapor pressure is 5.6 mb and the saturated vapor pressure is 35.0 mb. What is the relative humidity? |
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Definition
RH = actual vapor pressure (e)
saturated vapor pressure (es)
RH = 5.6 mb / 35.0 mb x 100%
RH = 16% |
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Term
Air Dew Point Actual Saturated
City Temp Temp Vapor Press Vapor Press
Miami 85°F 75°F 29.6mb 41.0mb
Fargo 45°F 45°F 10.2mb 10.2mb
Chicago 65°F 50°F 12.3mb 21.0mb
Dallas 95°F 65°F 21.0mb 56.2mb
1. Which city has the 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? |
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Definition
1. Fargo (Temp and Dew Pt Temp are the same value therefore 100% RH)
2. Fargo (Has the lowest Dew Pt Temp and/or Actual Vapor Pressure of all 4 cities)
3. Miami (Has the highest Dew Pt Temp and/or Actual Vapor Pressure of all 4 cities)
4. Dallas (Has the highest Temp and Saturated Vapor Pressure of all 4 cities, therefore the highest capacity to hold the most water vapor if all 4 locations were saturated at there current temperatures) |
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Term
| Why do relative humidities (RH) seldom reach 100% in polluted air? |
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Definition
| In polluted air, condensation begins on hygroscopic particles when the RH is less than 100% (≈75%). Condensation removes water vapor from the air which lowers the dew-point temperature. This increases the spread or difference between air temperature and dew-point temperature, therefore keeping the RH values slightly less than 100%. |
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Term
| What must the relative humidity be for wet haze to occur? |
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Definition
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Term
| What are the four methods of Fog formation? |
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Definition
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 |
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Term
| What are the five types of Fog? |
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Definition
1. Radiation
2. Advection
3. Upslope
4. Evaporation
- Steam
- Frontal
5. Ice |
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Term
| Provide a brief explanation on the process involved in the formation of ‘radiation fog’ and a description of the characteristics of ‘radiation fog’. |
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Definition
| The method of formation for ‘radiation fog’ is lowering the air temperature to the dew-point temperature. This will occur 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 light. ‘Radiation fog’ is shallow, sometimes referred to as ‘ground fog’. It is located in low lying areas and is most commonly seen around sunrise. |
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Term
| What conditions are necessary for the formation of ‘Advection Fog’ ? |
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Definition
| The method for the formation for ‘advection fog’ is lowering the air temperature to the dew-point temperature. This will occur when warm, moist air is advected over a colder surface. The colder surface could consist of an area of cold water or cold land. As the wind moves the warmer air over the colder surface, the air temperature will be drawn down to the dew-point temperature causing saturation to occur and the development of the fog. Advection fog can be dense, widespread, and long lasting making it more dangerous than radiation fog. |
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Term
| A January snowfall covers central Arkansas with 5 inches of snow. The following day a south wind brings thick dense fog to this region. Explain what has apparently happened and name the type of fog that formed. |
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Definition
| 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’. |
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Term
| What conditions are necessary for the formation of ‘Upslope Fog’? |
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Definition
| The method of formation is to lower the air temperature to the dew-point temperature. ‘Upslope Fog’ 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. The Front Range of the Rocky Mountains is a prime area for ‘Upslope Fog’ development when a moist air mass is moved into the region by an easterly wind component. |
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Term
| What is the difference between ‘Steam Fog’ and ‘Advection Fog’? |
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Definition
‘Steam Fog’ forms when cool air moves over a body of warmer water. Due to the thermal properties of water, water remains relatively warm during the autumn months, therefore ‘steam fog’ is more common over water during cool Fall mornings.
‘Advection Fog’ forms when warm, moist air moves over a colder surface. This is more likely to occur in Winter and Spring, therefore ‘advection fog’ is more common during the late Winter and early Spring months. |
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Term
| Name the type of fog that most often forms when warm rain drops fall into an unsaturated layer of colder air near the surface. |
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Definition
| ‘Frontal Fog’ This fog usually develops in the shallow layer of cold air just ahead of an approaching warm front or behind a passing cold front. ‘Frontal fog’ falls into the category of ‘Evaporation Fog’ since the method of formation is a combination of lowering the air temperature while adding moisture to the atmosphere. |
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Term
Which clouds are associated with each of the following characteristics.
1. Low cloud, puffy elements but shallow, may produce a brief snow shower during the winter season.
2. Light to moderate steady rain or snow, no lightning.
3. Nickname is ‘Mackerel Sky".
4. High, thin, layered, ice crystals, can produce a halo.
5. Mid-level, ice crystals and water droplets, rounded individual puffy elements.
6. Cotton ball looking, water droplets, often seen on warm summer afternoons. |
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Definition
1. Stratocumulus
2. Nimbostratus
3. Cirrocumulus
4. Cirrostratus
5. Altocumulus
6. Cumulus |
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Term
| Define stable and unstable when relating the terms to the atmosphere. |
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Definition
Stability in 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 will not be able to rise because it is more dense or heavier than the surrounding air. If the parcel is forced to rise, it will return to its place of origin once the lifting mechanism dissipates because the temperature of the parcel is still colder than the surrounding air. (Stable)
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. (Unstable) |
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Term
| Define what an adiabatic process is? |
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Definition
| 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. |
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Term
| Why do we use 10˚C/km for the dry adiabatic lapse rate and only 6˚C/km for the moist adiabatic lapse rate? |
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Definition
| Once the air parcel becomes saturated (100% RH), the latent heat of condensation (600 cal/gr) will add sensible heat to the parcel which in turn will offset the cooling due to expansion and slow the rate of cooling within the parcel. Hence the cooling rate will decrease from 10°C/km (dry) to 6°C/km (moist). |
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Term
| 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? |
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Definition
The dry lapse rate applies to the cooling or warming of a parcel of air that is rising or descending through the atmosphere. The dry lapse rate has a constant value.
The environmental lapse rate indicates the overall vertical temperature profile of the atmosphere. These temperatures are measured twice daily by the release of radiosonde balloons at the various National Weather Service Offices.
The 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. |
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Term
| List the various lapse rates that we use for a rising or descending parcel of air. |
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Definition
1. Dry Lapse Rate (10°C/km)
2. Dew-Point Lapse Rate (2°C/km)
3. Moist Lapse Rate (6°C/km) |
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Term
| What are the necessary conditions to determine the correct use of the various lapse rates. |
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Definition
1. The dry lapse rate is used for a parcel’s temperature when the parcel of air is unsaturated (RH < 100%). -10°C/km in a rising parcel and +10°C/km in a sinking parcel.
2. The dew-point lapse rate is used for a parcel’s dew-point temperature when the parcel of air is unsaturated (RH < 100%). -2°C/km in a rising parcel and +2°C/km in a sinking parcel.
3. The moist lapse rate is used for the parcel’s temperature and dew-point temperature when the parcel of air reaches saturation (RH = 100%) -6°C/km for a rising parcel. Note: The moist lapse rate is not used in sinking air because the parcel will warm and RH values will fall to values below 100% |
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Term
| What is an absolutely stable atmosphere? |
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Definition
| When a parcel of air remains colder than the surrounding air while being lifted, we say the air is stable. In other words, when the environmental lapse rate is less than the moist adiabatic lapse rate (6°C/km) the atmosphere is absolutely stable. |
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Term
| What conditions cause a stable atmosphere? |
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Definition
| The environmental lapse rate is small when stable conditions prevail. In other words the difference in the air temperature from the surface to those aloft is relatively small. Therefore we need warm air aloft or cool air at the surface or a combination of both to achieve this relatively small temperature decrease with height. |
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Term
| What processes in the atmosphere can cause the atmosphere to become more stable? |
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Definition
1) Warm air advection aloft.
2) Downward movement of air aloft (subsidence) that will cause the air temperatures aloft to warm due to compression.
3) Cold air advection in the lower levels of the atmosphere.
4) Nighttime radiational cooling at the surface.
5) Air moving over a cold surface which will cause the surface air temperature to decrease . |
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Term
| What kind of clouds are associated with a stable atmosphere? |
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Definition
| When stable air is forced to rise it will have a tendency to spread out. The clouds that form under this condition will also spread out forming layers. Therefore clouds with this characteristic will be either cirrostratus, altostratus, stratus or nimbostratus. |
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Term
| What is an unstable atmosphere? |
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Definition
| 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. |
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Term
| What conditions cause an unstable atmosphere? |
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Definition
| 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. |
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Term
| What mechanisms can cause the atmosphere to become more unstable? |
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Definition
1. Cooling aloft
a. Cold air advection
b. Radiational cooling aloft
2. Warming at the surface
a. Daytime solar heating
b. Warm air advection
c. Cool air moving over a warmer surface |
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Term
Use your newfound knowledge on the topic of stability to answer these two questions.
1. On a typical summer day, how does atmospheric stability change from the daytime to nighttime?
2. What kind of stability 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? |
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Definition
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. During the 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. In order for a thunderstorm to grow vertically, the temperature within the thunderstorm mass must be warmer than the surrounding environmental air. Under this condition the thunderstorm is able to build in the vertical because the atmosphere in the region of thunderstorm development is unstable. |
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Term
| Define “conditionally unstable”? |
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Definition
| This occurs when the environmental lapse rate falls between the moist and dry adiabatic lapse rate. Conditionally unstable means that, if unsaturated air could be lifted to a level where it becomes saturated, instability would result. |
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Term
| Define “convective instability”? |
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Definition
| The potential instability brought about by the lifting of a stable layer whose surface is “humid” and whose top is “dry” is called convective instability. This type of instability is associated with severe thunderstorms. |
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Term
| Name the four mechanisms that are responsible for the development of the majority of the clouds we observe. |
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Definition
1. Surface heating and free convection
2. Topography (orographic lift)
3. Widespread ascent (convergence of surface air) Example: Low pressure system
4. Uplift along weather fronts |
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Term
| What type of cloud formations are typically associated a warm front and cold front? |
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Definition
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. |
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Term
| The temperature at the surface is 25˚C and the dew-point temperature is 15˚C. Cumulus clouds form in the afternoon. What will be the approximate base of these cumulus clouds? |
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Definition
| Cloud base = [(Temp - dewpoint) / 2.5 ] 1000 feet. |
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Term
| Explain why rain shadows form on the leeward side of mountains. |
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Definition
| Precipitation falls on the windward side of the mountain. |
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Term
| Question: The surface temp of a parcel of air is 24°C and the dew-point temp is 16°C. Using the adiabatic process, the parcel of air is lifted 2000 m (2 km). What is the temp and dew-point temp of the air parcel at the 2000 m (2 km) height? |
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Definition
First we need to know the various lapse rates.
Dry Lapse Rate = 10°C/km
Dew-Point Lapse Rate = 2°C/km
Moist Lapse Rate = 6°C/km
Next we need to know when to use them.
If parcel is unsaturated use the Dry LR for parcel temp and Dew-Point LR for parcel dew-point temp.
If parcel is saturated use the Moist LR for both the parcel temp and dew-point temp. Initial Temp = 24°C and Dew-Point Temp = 16°C The air parcel is unsaturated due to the spread in the Temp and Dew-Point Temp of 8°C). To start, let’s just move the parcel up 1 km initially. The new Temp will be 14°C (24 – 10 = 14) and the new Dew-Pt Temp will be 14°C (16 – 2 = 14) at the 1 km level.
At the 1km height we are now saturated since the Temp and Dew-Pt Temp are equal at 14°C. So we must use the Moist LR for both the Temp and Dew-Pt Temp for the last 1 km rise.
The new Temp at the 2 km level will be 8°C (14 – 6 = 8) and the new Dew-Pt Temp will be 8°C (14 – 6 = 8). Therefore the correct answer is Temp 8°C and Dew-Pt Temp 8°C at a height of 2 km. |
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Term
| Describe how the process of collision-and-coalescence produces precipitation. |
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Definition
This process is known as the ‘Warm Rain Process’ since the temperatures within the cloud are above freezing
1) Cloud droplets are of varying sizes within the cloud mass because of the varying strengths of the updrafts within the cloud.
2) Coalescences occurs between the cloud droplets when the larger cloud droplets fall and collide with smaller droplets.
3) The drops will eventually grow to a size that overcomes the upward vertical velocity within the cloud enabling the drops to fall to the surface. |
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Term
| When a cloud consists of water droplets and ice crystals, is the saturation vapor pressure greater over the water droplets or over the ice crystals? |
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Definition
| The saturation vapor pressure above a water surface is greater than the saturation vapor pressure above an ice surface. This is true because it is easier for water molecules to escape from a water droplet than an ice surface. |
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Term
| Explain how the Bergeron process ‘Ice Crystal Process’ increases the size of ice crystals? |
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Definition
| Bergeron Process → Ice crystals grow larger at the expense of the surrounding water droplets since the saturated vapor pressure over ice is less than that over water at the same subfreezing temperature. The water vapor molecules will diffuse towards the ice crystal causing the ice crystal to grow while the super-cooled liquid water droplet will decrease in size due to evaporation since the water droplet must replace the water vapor molecules that it lost to the ice crystal. |
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Term
| Can the collision-coalescence (Warm Rain) and Bergeron (Ice Crystal) process for precipitation production exist in the same cloud formation? |
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Definition
| Yes, If the cloud has a vertical temperature structure that allows for regions of ice crystals and liquid droplets. This holds true for a cumulonimbus cloud. Temperatures in the mid and upper portion of the cumulonimbus cloud are cold enough to support ice crystals and supercooled liquid water droplets while the lower portion of the cumulonimbus cloud is warm enough to support liquid droplets only. |
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Term
| A cloud has air temperatures in a range around -10˚C (14˚F). Why are there many more cloud droplets than ice crystals in this cloud even though the air temperatures are well below the freezing temperature of 0˚C? |
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Definition
| The number of ice-forming nuclei available in the atmosphere is small for temperatures between 0°C (32°F) and -15˚C (5˚F). Therefore a lot of super-cooled liquid water can exist in clouds with temperatures in this range. However, as the temperatures decrease, more particles become active and promote freezing. Particles of clay become active around -15˚C (5˚F). |
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Term
| List and describe three ways in which ice crystals can form in a cloud. |
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Definition
1. Homogeneous Freezing → The freezing of pure water without the benefit of some nucleus. At temperatures around -40˚C, enough molecules within the water droplet join together in a rigid pattern to form an ice embryo.
2. Deposition → deposition nuclei will allow water vapor to deposit as ice directly onto their surfaces in cold, saturated air, bypassing the liquid phase.
3. Contact Freezing → Supercooled liquid water freezes on contact with a contact nuclei. The nuclei can be just about anything. This process is the dominate production of ice crystals in some clouds. |
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Term
| Why is it foolish to perform a cloud seeding operation with silver iodide or any of the other seeding agents when the sky is clear? |
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Definition
Seeding does not generate clouds. The first ingredient in any seeding project is the presence of clouds. For the best results, 1) the cloud must be cold enough in the upper portions to contain super-cooled liquid water, and 2) be lacking in ice crystals and ice nuclei.
Cloud seeding adds the necessary ice nuclei that will enhance ice crystal growth and the number of ice crystals within the cloud (Bergeron process). |
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Term
| Define the precipitation type “Virga”. |
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Definition
| Precipitation that falls from a cloud but evaporates before reaching the ground. Under the right conditions, the evaporation of the rain drops below cloud base can cause a significant low-level wind shear event on the surface. |
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Term
| What atmospheric conditions are necessary for snow to remain snow and not melt into a raindrop when falling into air temperatures below cloud base and on the surface that are slightly above freezing? |
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Definition
| In order for falling snowflakes to survive in air with temperatures much above freezing, the air must be unsaturated and the wet-bulb temperature must be at freezing or below. The snowflake will partially melt in the warmer air but due to the dryness of the air, the evaporation process will cool the air and snowflake. This will allow the snowflake to fall earthward before melting. Eventually the air will cool to the wet-bulb temperature and become saturated providing no warm air advection is taking place. |
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Term
| North Dakota winters can be harsh. You should understand the local terminology. What is the difference between a ‘Ground Blizzard’ and a ‘Blizzard’? |
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Definition
North Dakota is unique because it has a lot of open country.
A Ground Blizzard is classified as an event when: Blizzard conditions are occurring in the open country due to the blowing and drifting of existing snow. Snow does not have to be falling to create these conditions. Visibilities in cities will be much better than those in the open country
A Blizzard is classified with the following definition: Issued for winter storms with sustained or frequent winds of 35 mph or higher with considerable falling and/or blowing snow that frequently reduces visibility to 1/4 of a mile or less. These conditions are expected to prevail for a minimum of 3 hours. |
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Term
| How do the atmospheric conditions that produce sleet differ from those that produce hail? |
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Definition
Sleet forms when there is a layer of above freezing temperatures below cloud base but the temperatures at the surface are below freezing. A snowflake falling out of the cloud will melt in the above freezing layer producing a rain drop. The below freezing surface layer must be thick and cold enough to refreeze the raindrop before reaching the ground.
Hail is produced inside a cumulonimbus cloud by accretion of super-cooled liquid water onto a large frozen embryo caught in a strong updraft. Continuous cycles through areas of super-cooled liquid water will cause the hailstone to increase in size until it can overcome the upward vertical velocity and eventually fall to the earth’s surface. |
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Term
| What is the difference between sleet and freezing rain? |
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Definition
Sleet is usually a transparent or translucent ice ball around .2 inches or less in diameter. When it hits the ground it can bounce. The formation of sleet was discussed in the previous question.
Freezing rain is liquid (super-cooled) that will freeze on contact with objects that are below freezing on the surface. The determining factor between sleet and freezing rain as a precipitation type, is the depth and coldness of the subfreezing layer near the surface. |
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Term
| What conditions are most favorable for the occurrence of aircraft icing? |
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Definition
| Aircraft icing occurs when an aircraft is flying through clouds that contain a significant amount of super-cooled liquid water droplets. The best aircraft icing conditions occur when the cloud temperature ranges from 0 to -15ºC. At these temperatures a lot of super-cooled liquid water can be present within the cloud because of a lack of active ice nuclei at these temperatures. |
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Term
| What is the difference between glaze and rime icing? |
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Definition
Glaze icing forms when large super-cooled liquid drops strike the aircraft and spread out before freezing. The solid sheet of ice that forms is smooth and transparent. Glaze ice can build up quickly, is heavy, and difficult to remove, even with modern de-icers.
Rime icing forms when an aircraft flies through a cloud containing small super-cooled liquid droplets. These droplets will freeze before they have time to spread out, leaving a rough and brittle coating of ice on the wing and other exposed parts of the aircraft. Rime ice usually appears white since air is trapped between the frozen droplets. Thou lighter than glaze ice, rime icing will redistribute the airflow over the wing more than glaze icing will. |
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