Meteorology 106


Wind is air motion with respect to the Earth's surface.
Wind typically refers to horizontal air movement.
Updrafts and downdrafts are terms typically applied to vertical air movement.
Wind is the result of differences in pressure in different location - pressure gradient
Air moves from high pressure to low pressure.
Air will continue to move until the pressures are equal.
The greater the pressure gradient (i.e. the greater the difference between two location's pressures) the greater the force of wind.

Causes of Unequal Pressure on the Earth's Surface

Differences in pressures over the surface of the Earth are primarily caused by uneven heating of the surface by the Sun.
Coastal areas show this process with the development of sea-breezes.

Daytime Onshore Breezes

Land is heated by the sun, which heats the air above the surface.
This air becomes less dense and begins to rise. A low pressure area is created.
The air over the water stays cooler due to the 4 thermal properties of water
The cooler air forms a high pressure area.
Air moves from high to low pressure, creating a cool breeze which moves onshore.

Offshore Breezes

At night, the land cools more rapidly than the ocean, thus creating a high pressure area over the land.
Water retains its heat longer than land, keeps the air above the water warm, creating a low pressure area.
The breeze which develops moves offshore.

Wind Measurement

Two measurements are needed to characterize air movement.

Coriolis Effect

Apparent force which causes free moving objects to appear to be deflected from its path of motion.
Cause by the rotation of the Earth
Northern Hemisphere:
Southern Hemisphere:

Low Pressure Centers

High Pressure Centers

Geostrophic Winds

At low altitudes - winds move across isobars at an angle (Coriolis effect)
At high altitudes (7-14 km) winds flow parallel to the isobars.
Coriolis effect still influences the high altitude winds

Vertical Air Flow

Diverging air in a high pressure center
Results in subsiding air from aloft
Descending air does what?

Converging air in a low pressure center
Results in rising air aloft
Ascending air does what?