Weather: Pressure, Winds, and Precipitation
AIR PRESSURE
1. Measure of the force of air pressing down on the Earth’s surface
2. Equal to the weight of the air directly above any point on Earth
3. Depends on the density of the air, denser air has more mass so it exerts
more air pressure on the Earth
4. Air pressure depends on the density of the air in a particular place
Density is affected by:
Temperature: Density decreases when heated, so places with high
temperature usually have low air pressure (causes air to separate and rain to
move in). As temperature increases density decreases
Water Vapor: Moist air is less dense because a water molecule has less mass
than a molecule of N or O, so…. Air with a large amount of water vapor in it has
low air pressure. As water vapor increases density decreases
Elevation: As elevation increases air becomes thinner and less dense. As
elevation increases density decreases
A barometer measures the pressure of the air
High Pressure “H” Formation: When air mass cools over land/water. The
cooling air causes air to shrink inward. Air in upper troposphere is pulled
downward not allowing moist air into upper areas where it can condense.
This process leaves MORE air weight at the surface = HIGH
Result: Sunny, nice day
Low Pressure “L” Formation: When air mass warms over land/water. The
warming causes air to expand upward. Air in upper troposphere is pushed away
allowing moist air into upper areas where it can condense resulting in
precipitation.
This process leaves LESS air weight at the surface = LOW
Result: Clouds, possibly storms
Winds
Both local winds and global winds are caused by unequal heating of the
atmosphere
Winds travel from areas of high pressure to areas of low pressure
Winds curve to the right in the Northern hemisphere and to the left in the
Southern hemisphere due to the Coriolis effect, which is caused by the Earth’s
rotation.
Local Winds blow all directions, short distance
Global Winds blow specific direction, long distances
1. Doldrums are located at the equator where the winds are very calm, air is
warm and rises producing a low pressure belt
2. Trade winds are from the equator to 30 degrees N/S where the equatorial
winds cool and sink, winds are warm and steady
3. Horse latitudes are located at about 30 degrees N/S, winds are calm with
little precipitation
4. Prevailing westerlies are located between 40 and 60 degrees latitude,
caused by cool sinking air moving toward the poles, winds are usually
strong
5. Polar easterlies are cold, but weak winds caused by the rising and sinking
of the westerlies, the cause of many weather changes in the U.S.
6. The jet stream is a narrow belt of strong, high speed, high pressure air
that flows across the Earth in a west to east direction.
Wind is measured by an anemometer for speed and a weather vane for
direction
Winds are named for the direction in which they come (Nor'easters blow from
the North East, Prevailing Westerlies blow from the West
Moisture in the Air
Water vapor or moisture in the air is called humidity
Water condenses at a point called the dew point
Relative humidity: percent of moisture the air holds relative to the amount it
could hold at a particular temperature
Precipitation
1. When water vapor in the clouds becomes too heavy, it can fall as rain, sleet,
snow, or hail
2. Cloud droplets must first increase in size until gravity pulls them down
3. If they pass through extremely cold layers of air, they can freeze and produce
sleet
4. Snow is formed when water vapor changes directly into a solid
5. Hail is formed when water droplets hit ice pellets
6. A rain gauge is used to measure precipitation
Precipitation Video
Weather Patterns:
Air Masses
Changes in the weather are caused by very large areas of air with specific
temperature and humidity called air masses
There are four main air masses that affect the weather in the United States,
named for the regions they originate in
Maritime Tropical, Maritime Polar, Continental Tropical, Continental Polar
Maritime masses
Maritime tropical: originates near the equator, it brings warm moist air in
summer. In the winter it brings rain or snow
Maritime polar: forms over Pacific year round and in N. Atlantic in the summer.
It brings cool temps. to east and fog to west in summer, in winter heavy snow
and cold
Continental masses
The continental tropical forms over land in the Southwestern US it brings dry
hot air
The continental polar mass forms over the land in Canada and brings very cold
dry air into the US
Fronts
Fronts form when large air masses meet that have different temperatures and
humidity
The weather at a front can be very unstable and stormy
Cold Front
A cold front forms when a mass of cold air pushes under a mass of warm air
Violent storms, even tornadoes can be associated with these fronts
Fair, cooler, and drier air usually follows
Warm Front
A warm front forms when a mass of warm air overrides a mass of cooler air
Rain showers can accompany a warm front
Hot and humid weather follows
Occluded Front
When a very fast moving cold air mass overtakes a slower moving warmer air
mass, an occluded front forms
It produces less severe weather (rain) than a warm or cold front
Stationary Front
A stationary front is formed when a cold air mass and a warm air mass meet
and do not move
This produces several days of rain
1. Weather Instruments
Barometer: measures air pressure, standard air pressure is 0 at sea level,
two types: Mercury and Aneroid
2. Psychrometer: measures relative humidity
3. Hygrometer: measures humidity
4. Anemometer: measures wind speed
5. Rain gauge and Thermometer
Predicting the Weather
Meteorologists interpret weather info. from local weather observers, balloons,
satellites, super computers and weather stations around the world
Map Lines
Iso means to have the same as
Isotherms: areas sharing the same temperature
Isobars: areas sharing the same barometric pressure
Controlling the weather
Cloud Seeding: involves sprinkling of dry ice or silver iodide crystals into super
cooled layers of stratus clouds. This is to decrease the severity of hail storms or
hurricanes
Sometimes dry ice is sprinkled into fog to decrease its thickness, most fog is
unaffected by this
What are the advantages and disadvantages of “controlling” the weather?
Weather Careers
Broadcast meteorologist
Meteorology Professor
Meteorology Consulting (air quality, air pollution)
Commercial Meteorology (working with airlines, electric companies)
Research Meteorologist
Atmospheric Scientist
Specialty areas: climatology, micrometeorology, cloud physics, cloud
electrification, synoptic meteorology, radar, satellite, hydrology /
hydrometeorology, aviation