x Changes in temperature, water vapor content and movement of air are responsible
for horizontal variation in atmospheric pressure
x Coriolis Effect ± Effect of the rotation of the Earth on the objects on it.
x Adiabatic Cooling ± As air moves up it expands and cools, as it moves down it
compresses and warms.
o For dry air the cooling rate is 10° C for every 1000m
x Adiabatic Lapse Rates
o The saturated adiabatic lapse rate is less than the dry adiabatic lapse rate
due to the release of energy during condensation
o Temperature to which a parcel of air would need to be cooled to reach
x Stable vs. Unstable Air
o Tendency of parcel of air to remain in place or to change its vertical
position is referred as atmospheric stability
o Air mass is said to be unstable if its parcels rise until they reach air of
similar temperature and density
o Thunderstorms form when an air mass becomes so unstable
Unstable means that the air in the lowest layers is unusually warm
and humid, or that the upper layers are unusually cool, or both.
x Convection ± +HDWLQJRIWKH(DUWK¶VVXUIDFHleads to rapidly rising parcels of air
(convection) with subsequent condensation and cloud formation.
x Condensation and Cloud Formation
Water vapor in the air changes to a liquid and forms dew, fog, or
Water vapor requires a surface to condense on
x Possible condensation surfaces on the ground can be the
grass, a car window, etc.
x Possible condensation surfaces in the atmosphere are tiny
bits of particulate matter called condensation nuclei
o Dust, smoke, etc.
o Ocean salt crystals which serve as hygroscopic
x Cumulonimbus ± The main type of cloud when talking about thunderstorms and
x Cold and Warm Fronts
o Cold Front ± cold air rapidly displaces relatively warm air as it advances.
Heavy precipitation results along a narrow band.
o Warm Front ± advancing warm air rises slowly over the adjacent cooler air
mass. Associated with widespread cloud with moderate, steady
x Stationary and Occluded Fronts
o Stationary Front
Flow of air on both sides of the front is almost parallel to the line
of the front
Surface position of the front does not move