Unit 4 :Meterology
A. Air Pressure
1. Air pressure – the weight of air pressing down on Earth
2. temperature and altitude determine air pressure
3. the more air particles are present, the more air density or pressure exists
4. the less air particles are present, the less air density or pressure exists
5. High pressure
a. air is more dense
b. associated with cold air
(more dense than warm air)
c. associated with sinking air
6. Low pressure
a. air is less dense
b. associated with warm air
(less dense than cold air)
c. associated with rising air
7. warm air rises, cold air sinks
8. air cools as it rises
9. air pressure is measured using a
tool called a barometer
10. winds are formed when air from a
high pressure (more dense) moves to
areas of low pressure (less dense)
B. Air Mass
1.air mass – large mass of air that
takes on the temperature and moisture
characteristics of the surface below it
2. air moves from an area of high pressure
(more dense) to areas of lower pressure
(less dense)
3. a high pressure system
a. rotating mass of cool, dry air rotating in a downward clockwise motion
b. creates sunny and pleasant weather
4. a low pressure system
a. rotating mass of warm, moist air rotating an upward counterclockwise
motion
b. creates stormy, windy weather
5. air masses are classified by their temperature and humidity (moist or dry)
6. air masses in the US move west to east
C. Fronts
1. there are 4 major air masses that
effect the weather in North America
a. continental polar (cP)
b. continental tropical (cT)
c. maritime polar (mP)
d. maritime tropical (mT)
2. most weather occurs along the border of these air masses at boundaries called
fronts
3. Cold front
a. occurs when a cold air mass
overtakes a warm air mass
b. this pushes the warm air
mass upward
c. creates severe weather lasting
a short period of time
d. represented by blue triangles
on a weather map
4. Warm front
a. occurs when a warm air mass
slides up and over a cold air mass
b. this creates rainy weather that
lasts for several days
c. represented by red semicircles
on a weather map
5. Stationary front
a. occurs when a cold and
warm air mass meet but do
not move in either direction
b. this creates sluggish winds
and precipitation
c. represented by alternating blue
triangles and red semicircles
6. Occluded front
a. occurs when two cold air masses merge and force warmer air between
them to rise
b. this creates high winds and heavy precipitations
c. represented by purple alternating triangles and semicircles
D. Clouds
1. cloud – a visible mass of condensed water in the atmosphere made up of
mixtures of tiny droplets of water or tiny crystals of ice
2. form when water evaporates from Earth’s surface
3. classified according to shape and altitude
4. prefixes:
a. cirro – describes high clouds (above 6000m)
b. alto – describes middle clouds (between 2000m and 6000m)
c. strato – describes low clouds (below 2000m)
E. Types of Clouds
1. cirrus
a. high, white, feathery cloud
b. usually associated with fair weather
2. cumulus
a. thick, puffy masses
b. usually associated with fair weather
3. stratus
a. occurs in layers and often cover the whole sky
b. usually associated with fair weather
4. nimbus
a. dark, grey clouds with ragged edges
b. associated with precipitation
F. Fronts and Clouds
1. cold front – cumulonimbus clouds are large puffy rain clouds associated with
severe thunderstorms along a cold front
2. warm front – cirrus, stratus, and nimbostratus clouds are associated with the
passing of a warm front
G. Water Vapor
1. humidity – the amount of water vapor in the air
2. relative humidity – ratio of water vapor in a volume of air relative to how much
water vapor that volume of air is capable of holding
a. warm air is capable of holding more moisture than cool air
b. if you increased the temperature of a room without adding more
moisture, the relative humidity of the room would decrease
c. relative humidity is expressed as a percentage
d. measured by a hygrometer
3. saturation – point at which the air holds as much water vapor as it possibly can
a. warm saturated air contains more water vapor than cold saturated air
4. dew point – temperature to which air must be cooled at the same pressure to
reach saturation
a. if the same air continued to cool past the dew point, the air’s excess water
vapor would condense to lead to precipitation
5. condensation (gas to a solid) occurs only when air is saturated
a. this leads to precipitation (rain, snow, freezing rain (glaze), sleet, hail)
6. saturation, relative humidity, and dew point are used to measure humidity