Atmospheric Moisture & Precipitation Patterns
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what is humidity?
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amount of water vapor in the atmosphere
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what is vapor pressure?
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the part of the atmosphere's pressure that is exerted by water vapor; water molecules make up only a small part of the total atmosphere, and therefore vapor pressure composes a small part of the total atmospheric pressure
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what is saturation vapor pressure?
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the maximum vapor pressure that can exist at a given temperature
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what is the relationship between saturation vapor pressure and temperature? that is, if temperature changes, what happens to saturation vapor pressure?
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the higher the temperature, the greater the amount of water vapor that could exist, and therefore, a rise in temperature causes a rise in saturation vapor pressure
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suppose that on a given day, the vapor pressure is 12 mb and that it remains the same throughout the day. the temperature is 20 degrees Celsius; at that temperature, the saturation vapor pressure is 24 mb...so, what is the relative humidity?
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RH = 100 x actual vapor/max vapor pressure possible RH = 100 x 12/24 RH = 100 x .5 RH = 50%
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is the air saturated?
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no, at saturation, it would have 24 mb of vapor; it only has 12; it is halfway to saturation
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will dew form under these conditions? why or why not?
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no; dew forms when the air becomes saturated and some of the water vapor begins to condense out of the air and onto grass, windshields, etc.; the air in this example is not saturated
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what is dew point?
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dew point is the temperature to which the air must cool in order to reach saturation
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if dew point one day is 13 mb and on the next day it's 20 mb, what has happened to the vapor pressure of the atmosphere?
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the vapor pressure has risen - on the second day, the vapor is exerting greater pressure because there is more vapor in the atmosphere; the air is more humid, in other words, so it doesn't have to be cooled as far to reach saturation
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what does it mean for the air temperature to change adiabatically?
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-adiabatic means without heat exchange; temperature change without addition or subtraction of energy, change in temperature due to compressing or expanding -diabatic heating: heat is added to the air; heat or cool something by adding or subtracting energy to it; ex. Sun warming earth
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what is the difference between stable and unstable air?
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-if an air parcel is stable, it is cooler than the air around it, and therefore denser; it will tend to remain in pace or to descend to a lower altitude; stable air that remains or seeks low altitudes is not conducive to precipitation -on the other hand, if an air parcel is unstable, it will tend to rise because it is warmer and more buoyant than the surrounding air; unstable air that rises is conducive to precipitation; a good example is our warm, humid air that often occupies Texas during spring; the air is often unstable, and once it begins to rise, it rises strongly and produces powerful thunderstorms
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if there is a rising plume of air in the atmosphere - for example, where wind blows against a mountain range and forces the air upward over the mountain - what happens to the temperature of the air as it rises? why?
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temperature declines adiabatically, that is, by expansion; rising air expands and cools, sinking air is compressed and becomes warmer as a result
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if air rises high enough that it cools to dew point, what happens? (what are the atmospheric features that form?)
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clouds form; we can see the point in the atmosphere where dew point is reached, because that is where the bottom of the clouds form
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when clouds form, what are the consequences in terms of latent heat?
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clouds form when water vapor condenses; that is, when water changes state from gas to liquid; therefore, latent heat is released as sensible heat, warming the atmosphere around the clouds
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what are condensation nuclei?
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small particles onto which water vapor condenses to form cloud droplets
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what is the collision-coalescence model of precipitation formation?
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the process through which tiny cloud droplets collide with each other, coalescing and becoming large enough to fall as rain drops
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what is the Bergeron ice crystals model of precipitation formation?
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-first you need to know that water can exist as a liquid at temperatures below the freezing point; water that is still liquid below freezing point is said to be supercooled -the ice crystals model of precipitation involves supercooled water droplets coexisting with tiny ice crystals in the same cloud; as water evaporates from the water droplets, it accumulates onto the ice crystals, which eventually grow large enough to fall as snowflakes
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name three main cloud types, and describe briefly
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cumulus: puffy clouds that grow thick vertically, sometimes growing to become cumulonimbus (rain-producing) clouds stratus: clouds that form a layer; these are usually featureless and gray, typically covering the entire sky; when they produce precipitation, they are called nimbostratus clouds cirrus: wispy clouds at high altitudes
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what is the difference between clouds and fog?
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they are the same, except fog occurs as a cloud layer on the ground
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what is an air mass?
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a large, distinct body of air that has similar characteristics of temperature and humidity throughout
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cP
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name: continental polar humidity: low temperature: low source area for cP air masses that affect North America: primarily north-central Canada
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mP
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name: maritime polar humidity: fairly high temperature: moderately low, but not as low as cP because they originate over oceans source area for mP air masses that affect North America: northern Pacific Ocean, northern Atlantic Ocean
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cT
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name: continental tropical humidity: low temperature: very high source area for cP air masses that affect North America: small area in northern Mexico, west Texas; but the Old World (northern Africa/Arabian Peninsula) has a much larger source area for cT air masses
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mT
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name: maritime tropical humidity: very humid temperature: high, but not as high as cT source area for cP air masses that affect North America: Gulf of Mexico and tropical/subtropical Atlantic, primarily; also tropical/subtropical areas of eastern Pacific
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what is convergent lifting? where are some parts of the earth that you think would favor this type of lifting?
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lifting associated with air converging at the surface from different directions; favored in areas prone to convergence; especially the ITCZ
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what is convectional lifting? where would favor this type of lifting?
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lifting of air associated with surface hearing, which warms the overlying air and causes it to become more buoyant and rise; common in the tropics (along ITCZ) and in subtropical locations such as southeastern U.S. during summer; also common in areas such as the central U.S. where humid, unstable air releases latent heat as clouds form, therefore becoming more buoyant
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what is orographic lifting? where does this happen?
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lifting as air is forced over a mountain range by wind; common on the windward side of mountains, for example, the western slopes of midlatitude mountains (such as Pacific Northwest of North America) where westerly winds prevail; another example is the eastern slopes of tropical mountains, such as Hawaii, where westerly winds (trade winds) prevail
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what is a rain shadow?
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the dry area downwind of mountains, where less rain falls because much of the moisture has been removed by orographic precipitation over the mountains upwind
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what is frontal lifting? where does this happen?
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lifting of warm air where two air masses interact along a front; the warm air has to rise over the cold air because it is less dense than cold air; this lifting is common in the midlatitudes, which are common conflict zones for cold (polar) and warm (tropical) air masses
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how does latent heat release help fuel a tropical storm?
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as clouds form in the storm, the condensation releases latent heat (a lot of it); the energy released in this way becomes sensible heat, which warms the atmosphere at cloud level to a temperature that exceeds that of the surrounding air; this warmed air becomes even more buoyant than before and continues to rise explosively, fueling the continued development of the storm
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in the northern hemisphere, tropical storms occur most frequently in September. why?
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by late summer, the tropical northern oceans have warmed all summer, but they have not begun to cool because they retain heat for longer than the land; this warm water favors convection, and when a storm system becomes organized and cloud formation begins, latent heat is released, fueling storm development, which continues to cause more air to rise, resulting in additional latent heat release and growth of the storm
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what two basic ingredients are needed for precipitation to occur?
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-moisture in the atmosphere -atmospheric lifting (to cool the air to dew point)
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if the air temperature falls to dew point, what will happen to some of the water vapor in the air?
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it will condense to form cloud droplets (which are liquid)
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describe the changes in wind direction, temperature, humidity, precipitation, and clouds that typically accompany the passage of a cold front here in College Station
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-wind shifts from southerly to northerly OR southwesterly to northwesterly -humidity declines -temperature declines -precipitation may fall because of lifting along the front -clouds often are present along the front, but after the front passes, the sky will clear
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why does so much precipitation occur in the equatorial sections of central Africa?
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-it's warm and therefore the air can contain a lot of moisture -the ITCZ contributes to convergence and lifting; also, strong solar heating contributes to convectional lifting
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why is the climate of central Australia very dry?
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-it's in the subtropical high pressure zone -also, it's in a continental interior, which is isolated to some extent from the oceanic moisture supply
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why does a lot of precipitation fall along the Pacific Northwest region of North America?
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midlatitude cyclones and fronts lift the humid Pacific air; also, the westerly winds encounter mountains, causing orographic lifting
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why is the climate of central asia dry?
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-it's the middle of a large landmass, distant from oceanic moisture sources -it's in the rain shadow of the Himalayans which prevent moist air from the Indian Ocean or elsewhere from entering central Asia
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how can you determine where has the highest wind speeds on a graph?
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where the isobars are closest together
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where has the highest dew point?
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where the warm, humid air is that is transported into the midlatitude cyclone from the south
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when does condensation occur?
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when temperature falls to dew point
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does get warmer or cooler as it expands?
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cooler
