Moisture, Adiabatic Process, Latent Heat, Humidity Flashcards
what are some characteristics of water in the atmosphere?
- a minor, variable gas (0-4%)
- mostly in lower troposphere and mostly from ocean
- transported high by clouds
- transported horizontally by winds
- large fluctuations in concentration, spatially and temporally
is water an ideal gas? what is an ideal gas?
- yes
- ideal gas: molecules behave elastically, don’t exchange energy
- volume and pressure are proportional to the number of molecules present
partial pressure and what determines it?
the pressure contributed by a single gas in a mixture of gases
- amount of a given gas determines partial pressure
what is atmospheric pressure at sea level and how to calculate partial pressure?
at sea level = 1013.2 mb (let’s say 1000)
nitrogen is 78% so so partial pressure is 780 MB, and water is 0-4 mb
why is the amount of vapor in an air mass important to it’s mass or density?
because mass is directly related to molecule weight of combined gases
which is lighter, moist or dry air?
moist air because water vapor molecules are lighter than other dry air molecules such as nitrogen
phase changes:
freezing, melting, evaporation, condensation, deposition, sublimation
freezing = liquid to solid
melting = solid to liquid
evaporation = liquid to gas
condensation = gas to liquid
deposition = gas to solid
sublimation = solid to gas
latent heat
energy involved in a phase change of water
- to go from less to more active, you need to get molecules moving faster, thus latent heat is used
how much latent heat is released when water vapor condenses? how about during a hurricane?
- 600cal/gram (enough to raise temp of 2.5tsp from freezing to boiling)
- equivalent to 400 20-megaton nuclear bombs, or enough for all USA electrical consumption for 1/2 year
humidity
measure of amount of water vapor in atmosphere
absolute humidity
- mass of water vapor/volume of air
- g/m^3
- affected by changing volume
specific humidity
- mass of water vapor/ total mass of air
- g/kg
- not affected by volume changes
- mixing ratio
mixing ratio
- mass of vapor/mass of dry air
- g/kg
- similar to specific humidity
- both SH and MR are constant unless moisture changes
relative humidity
- vapor content of air mass/vapor content at saturation
- dependent upon vapor content and temperature
- expressed as a percent
- RH = vapor pressure/saturation vapor pressure
saturation vapor pressure
max partial pressure for a given temp
why does warm air have a greater capacity for storing moisture?
because as temp increases, molecules move faster, so water vapor is moving so quickly it doesn’t condense out
as temp increases, relative humidity___.
decreases
how would you increase relative humidity?
add moisture or decrease temp
dew point
temp to which parcel must be cooled for water vapor to condense into water
frost point
when dew point falls below freezing
what does adiabatic mean?
without heat - temp changes without adding or removing heat
expansion causes ____, compression causes ____
cooling, warming
dry adiabatic lapse rate (DAR)
- rate at which dry air changes temp as it rises
- DAR = 10C/1000m
moist adiabatic lapse rate (MAR)
- rate at which air at saturation changes temp as it rises
- MAR = 6C/1000m (technically 4-9.9C)
why is MAR < DAR
because latent heat is released by condensing water
lifting condensation level (LCL)
where air moving up the atmosphere will cool at the DAR until it reaches condesation (aka clouds form), air then moves up at the MAR
environmental lapse rate (ELR)
- actual decrease in temp with increase in altitude at a given location
- ELR ~ 6.4C/1000m
- varies by local conditions
stability
a measure of the tendency for vertical motion to continue in the direction of displacement from a starting point
stable conditions
- ELR<MAR<DAR
- air mass will be cooler and denser than surrounding air
- resists
unstable conditions
- ELR>DAR>MAR
- parcel is warmer and less dense
- does not resist
conditionally unstable
- DAR>ELR>MAR
- resists if unsaturated
absolutely unstable
-ELR>MAR>DAR
- does not resist
what are two ways to promote stability
- cool surface - loss of LW at night or advection of cool air
- warm upper atmosphere - under high pressure, sinking air will warm
what are two ways to promote instability?
- heat surface - daytime heating, advect warm air
- cool air aloft
when is air typically most and least stable?
most: evening and early morning
least: mid-afternoon
