lec 12 Flashcards
how much of the atmos is water
0-4%. its highly variable due to its low retention time
this percent encompasses both vapour, clouds and free molecules
what are the different categories of humidity?
vapor pressure
abs humidity
mixing ratio
relative humidity
what is total atoms pressure?
the weight of all gas molecules pushing down (from all layers)
partial pressure
what fraction of total presure is imparted by a specific compound. we can calc but not measure directly
what is humidity and how is it measured
the measurement of water vapor in the air. measured via a hygrometer. is uses a substance with a known ‘reaction’ to the amount of vapor in an air parcel (it will expand or contract)
what happens when water vapour is added to a system
generally will result in the expansion of the substance in the hygrometer
what is saturation
the condition where an air parcel obtains the max amount of water at a certain temp
evaporation dynamics at temps
at warmer temps = more water will evap
at cooler temps more water will condense
how does saturation vapor pressure change with temp (over water)
warmer air has the capacity to hold more water than cold water
what is super cooling
at very low temps condensation can occur without water solidifying. this is why we can observe saturation at temps below 0.
how does saturation vapor pressure change with temp (over ice)
it will follow the same trend as over water (higher capacity at warmer temps), though overall saturation will be LOWER as fewer water molecules are able to effectively escape via sublimation.
sublimation
the transition from a solid to a gas without passing through a liquid medium
what controls net evap rate
vapor pressure deficit dictated by region (is there space for vapor)
the availability of water governed by transpiration
turbulence (mixing of air via convection or mechanical mixing)
total energy available to promote phase change
what is the equaition for total energy?
Q*= QH + QE + QG
QH= sensible heat
QE= latents heat
QG= ground conduction
total energy= (Kincoming-Koutgoing)+(Lincoming-Loutgoing)
what is evap
water moles turning into water vapour at the surface of a water body (any temp)
what is boiling
evaporation occuring through an entire volume of valter at a specific temp (100)
what is the boiling point temp
when the saturation vapor pressure is equal to the atmospheric pressure
at lower pressure the boiling point decreases
what is absolute humidity?
it is that mass of water vapor contained in a volume of air
measure in kg/m^3
vapour pressure
the partial pressure exerted by vapor in the atmos
humidity mixing ratio
the mass of vapour to the mass of dry air
g/kg
max humidity
the maximum mount of water vapour an air parcel can hold at a given temp
specific humidity
the mass of vapor in an air parcel at a given time, commonly less than the capacity.
g of vapor/kg of dry air
relative humidity
the ratio of specific over the max hum, measuring how close an air parcel is to saturation.
how does relative humidity change
1) when the vapor pressure changes
2) when the temperature of the air parcel changes
at lower temps the max capacity is lower (particles become closer together and less room as available for vapor) and vice versa. this way the specific hum is commonly higher at night, even with the same amount fo water present
more vapor in the air at the same temp means more of the open space in the parcel is occupied. this will increase the ratio, moving it closer to sat
dew temp
the temperature where a parcel of air will become saturated. this involves the specific hum or measured vapor pressure.
what is meant by a high dew point
it means there is a greater amount of moisture in the air, TF the air will need to cool substantially for saturation to occur. with the high amount of vapour, heat is retained better (water is a GHG).
vapor pressure deficit
the diff between the saturation vapor pressure and the measure vapor pressure
dew point depression
the difference between the air temp and the dew point temp
how is realtive humidity measured?
psychometry.
what is psychometry
a device that uses a wet and dry bulb to measure relative humidity. In unsaturated air, after spinning it in the air the wet bulb will have become dry. in saturated air the wet bulb will remain wet. Dew to the evap of the moisture there will be a difference in temp between the wet and dry blub. this difference quantifies the RH
how does humidity impact temp
vapor is a GHG. areas with a high RH will retain outgoing radiation much more efficiently than areas with a low humidity.
that’s is why a humid region of 30 deg feels warmer than a dry region of 30 deg. in humid situations, the body is unable to remove sweat via evap.
how does low humidity impact dew point?
low humidity regions often experience warmer dew points in respect to their environment. This often leads to low cloud formation
