Weather Notes
Origin of the Atmosphere:
- The original atmosphere was much different than it is today
- A process called Outgassing created earth’s early atmosphere:
- Outgassing- volcanoes vented (gave off) mostly water vapor,
carbon dioxide, nitrogen and hydrogen.
- As Earth cooled, the water vapor turned to liquid water and
formed the early oceans.
- Over time as plants evolved, they used CO2 to do photosynthesis
and gave off oxygen.
- Our current atmosphere is 78% Nitrogen, 21% oxygen, 1% other
gases
I. Atmosphere Conditions
 Weather- the state of the atmosphere at a given
location over a short period of time
A. Temperature
- Measured in Celsius, Fahrenheit, Kelvin
- Convert using ESRT
- Shown on maps using isolines
B. Heat
- The atmosphere is heated many ways:
o Direct Insolation (energy from the sun)
o Conduction from surface
Weather Notes
o Re-radiation from surface
o Condensation from formation of fog, dew, clouds, frost
o Friction between surface and atmosphere due to Coriolis
effect.
- Heat is transferred throughout the atmosphere by convection
 When a gas expands, temp. decreases
o As air rises/expands, temp. goes down.
 When a gas contracts, temp. increases
o As air descends/contracts, temp goes up
C. Air Pressure & Density
- Are directly related
- Air Pressure- the pressure due to weight of overlying
atmosphere pushing down on any given area
- Barometer measures A.P.
- Standard A.P.=
o 14.7 lbs/sq.in.
o 29.92 in. of Hg (inches of Mercury)
o 1013.2 mb
- Shown using isobars on weather maps
Weather Notes
D. Temperature & Air Pressure
- As temp. increases, Density & Air pressure decrease
E. Water vapor & Air Pressure (A.P)
- As amount of Water vapor increases, Density & A.P decrease
*Why? Because dry air is HEAVIER than moist air!
Nitrogen’s molecular weight is 28 amu while a water vapor molecule is only 18!!
- High pressure systems are often associated with clear skies &
low/no precip.
- Low pressure systems are often associated with cloudy skies &
precip.
F. Altitude & Air Pressure
- As altitude increases, pressure and density decrease
Weather Notes
II. Winds
- Caused by differences in Air pressure
o Differences in A.P are caused by changing temp. and
water vapor content.
A. Air Pressure Gradient
- The difference in Air pressure over a specific distance.
- Shown by isobars
o The closer together the isobars, the steeper (greater) the
gradient
- A large gradient (change in air pressure) means: HIGH WINDS!!
**Look for isobars close together and you’ll find an area with high winds!
B. Land & Sea Breezes (AKA. Local Winds)
First, understand SPECIFIC HEATSpecific heat- the amount of energy
needed to raise a material 1 C. Water
has a very high specific heat so it takes
longer to heat and cool off. Land surfaces
(like sand on a beach) have a low specific
heat so it heats and cools very quickly.
Sea Breeze-
Weather Notes
- Land heats faster than water during the day
- Air above the land heats faster and becomes less dense (lower
air pressure) causing it to rise.
- Cooler, more dense air from above the sea flows into land
replacing risen air
- Land Breeze- Warmer air forms over water during the night
- The warmer air over the water becomes less dense (lower air
pressure) and rises
- Cooler air from land flows out to sea replacing the risen air
C. Direction of Winds
- Air (winds) moves from areas of high to low pressure
- Higher pressure gradient creates higher wind speeds!
- High Pressure areas produce winds that blow outward from the
center and turn clockwise (Anti-cyclones)
- Low Pressure areas produce winds that blow inward towards
the center and turn counter clockwise (cyclones)
- A wind is named for the direction it comes from
o Ex. Wind blowing towards the North is a South Win
Weather Notes
III.
Circulation of Air in the Troposphere
A. Convection Cells
- Unequal insolation on earth = unequal heating of surface =
differences in air pressure
- Results in series of convection cells at various latitudes
- See ESRT, pg.14
- Areas of convergence- where air from convection currents
come together (low pressure)
- Areas of divergence- where air from convection currents come
apart (high pressure)
B. Jet Streams
- Fast moving winds, 200mph or more
- Move at top of troposphere, flowing easterly
- Drive formation of weather & movement of weather
C. Planetary Wind Belts
- AKA. Prevailing Winds
- Shift seasonally due to changing angle of insolation which
results in changing pressure gradients
- Cause seasonal weather changes (Ex. Monsoons)
Weather Notes
- US Prevailing winds = Southwesterly
o Move SW to NE
D. Ocean Currents
- Caused by prevailing winds
- NRG transferred to water
- Can also shift position N or S reflecting seasonal changes in
prevailing winds
Weather Notes
IV.
Atmospheric Moisture
- Moisture enters atmosphere by
o Transpiration- water vapor released by plants
o Evaporation
A. How to increase the rate of Evaporation
1. Higher temp
2. More Surface area
3. Less moisture in air
4. More air movement
B. Humidity
- Absolute Humidity- amount of water vapor in the air
- Relative humidity- ratio of how much water vapor air has and
how much it can hold
- RH & Temp
o If temp increases, RH decreases
- Dew Point- temperature at which air is saturated with water
vapor
o Below this temp. water vapor will condense
Weather Notes
C. Cloud Formation
- When temperature in atmosphere drops below dew point.
- A cloud is a collection of water (can be liquid or crystal form)
- Cloud Cover- fraction/percent of total sky covered by clouds
- Low Clouds/On surface= FOG
- Needs condensation surface- a place for water to condense
o Dew- condenses on grass/leaves
o Clouds/Fog- condense on aerosols in air (volcanic ash,
dust, pollutants)
- Both air temp & Dew point DECREASE with increase in
ALTITUDE- The altitude where temp & dew point are the same
is the level where clouds form by condensation.
o CLOUD BASE ALTITUDE
D. Precipitation
- Ice crystals or water droplets must come together
in clouds to become big enough so they will fall
due to gravity
Formation of:
1. Rain- Temp. in cloud above 32° F, liquid droplets (.2mm diameter)
Weather Notes
2. Drizzle- Temp. in cloud above 32 °F, liquid droplets (.2-.5 mm
diameter)
3. Snow- Temp. in cloud below 32 °F, ice crystals
4. Sleet- Temp. in cloud above 32° F, liquid droplets freezes because
air closer to ground is below 32° F
5. Freezing Rain- Cloud of liquid droplets above 32° F, freezes in
contact with feature of earth’s surface.
6. Hail- forms in thunderstorm clouds, layers of ice/snow/liquid
water
E. Atmospheric Transparency & Precipitation
- Effected by the amount of pollution
o More pollution = less transparency / Less pollution = more
transparency
- PRECIPTATION will INCREASE atmospheric transparency! (i.e it
makes it cleaner and clearer!)