Changes of State of Water
Heat = energy Adding heat to an object makes
atoms vibrate fasterHeat ≠ temperature!Temperature = average of all
molecular motion
Change of State of Water
Latent heat – energy added to or released by substance w/out changing temperature
Latent = “hidden”Ex: Beaker w/ ice and water
Humidity
Describes amount of water vapor in air
- water vapor sits in spaces between dry air gases
Saturation = spaces filled to capacity w/ water vapor
Dry Air Analysis (consistent worldwide)
Nitrogen = 78%Oxygen (O2) = 21%
Argon = <1%Carbon Dioxide = 0.035%TOTAL 100%
Water Vapor Capacity Table
Temperature dependent
Hotter air = molecules moving farther apart = more space for water vapor
Relative Humidity
- measures how close air is to saturation w/ water vapor
- expressed as a percentage- is temperature dependent!
True or False?
Air with a higher relative humidity has more water vapor in it than air with a lower relative humidity?
Relative Humidity
Question:If temp. increases w/out changing
the mixing ratio, what happens to relative humidity?
Factors that change relative humidity
1) Change in air temperatureAssuming no change in mixing
ratio:Increase temp. = __________ RHDecrease temp. = _________ RH
Factors that change relative humidity
2) Change amount of water vapora) Add water vapor through ____________b) Remove water vapor by ____________
Dew Point
Temperature at which air is saturated without changing the mixing ratio
- air has to ________to reach dew point
Examples of humidity problems
Temp (oC) M.R. (g) R.H. 1. 25 4 20%2. 10 3.5 50%What is dew point of Example 2?
Necessary conditions for condensation
1) Saturated Air (air at dew point)2) Condensation Nuclei
Ex: dust, salt, smoke, ash“hygroscopic particles”
Saturated air
Two methods for cooling air to dew point:
1) Environmental Lapse Rate- air is cooler the further you are
from Earth’s surface- rate changes daily
Cooling to Dew Point: Method #2
2) Adiabatic Temp. Change- caused by changing air’s volume* No heat (energy) is added or
removed from the air!
Adiabatic Temperature Changes
Air expands = temp. decreasesEx: aerosol spray can
Air compresses = temp. increasesEx: bicycle tire pump heating up
Cloud Formation
As air rises, it expands & coolsDry adiabatic rate = cooling of
unsaturated air- constant 10o C per km
Cloud Formation
Wet adiabatic rate = cooling of saturated air
- always < 10o C per kmVaries depending on amount of
condensation in cloud(latent heat warms surrounding
air)
Review cloud forming process:
4. Air reaches dew point- altitude varies based on how
much water vapor is in rising air5. Condensation begins
Mechanisms forcing air to rise:
1) Orographic Lifting - physical barrier to wind
Clouds form on windward side of mountain
As air descends, it compresses & heats up adiabatically
Mechanisms forcing air to rise:
2) (hot air rises) LocaConvective lifting lly, caused
by intense heating on summer afternoons
- forms cumulus clouds
Mechanisms forcing air to rise:
3) ConvergenceSurface air moves from high
pressure to low pressure (wind)
Mechanisms forcing air to rise:
4) Frontal WedgingCool air acts as a barrier forcing
warmer air to riseRegional cause of clouds
Fog
Cloud with its base at or very near the ground
Ex: Radiation fog (ground fog)- fairly clear nights w/ high relative
humidity
Radiation Fog
Air above ground cools rapidly to dew point
- after sunrise, fog evaporates from ground up (“lifts”)
Stable Air
Air resists vertical displacement- air at surface is colder than
temp. of surrounding air- Force air to riseEx: Orographic lifting
Unstable Air
Air that does not resist vertical displacement
- air at surface is warmer than surrounding air
- begins and continues to rise on its own (ex: “convective lifting”)
Weather Conditions
Stable Air:a) Characteristically sheet-like
layered clouds (stratus)Ppt.- producing = nimbostratusb) light to moderate ppt. of long
duration
Weather with stable air:
c) Other phenomena:- poor visibility & haze- fog, sleet, glaze- inversions (smog alerts)
Weather Conditions
Unstable Air:a) Clouds w/ considerable vertical
developmentb) Short episodes of heavy ppt. (cumulonimbus clouds)