Meteorology Chapter 6

Air Pressure

(atmospheric pressure) air pressure is the force exerted on a surface by the weight of the air above it. The internationally recognized unit for measuring this pressure is the kilopascal.

Standard Atmosphere

A hypothetical vertical distribution of the atmospheric temperature, pressure, and density, which by international agreement is considered to be representative of the atmosphere for pressure-altimeter calibrations and other purposes.

Anitcyclone

A large body of air in which the atmospheric pressure is higher than the pressure in the surrounding air. The winds blow clockwise around an anticyclone in in the Northern Hemisphere.

Isobar

A line of equal barometric pressure on a weather map.

Millibar

A metric unit of atmospheric pressure. 1 mb = 100 Pa (pascal). Normal surface pressure is approximately 1013 millibars.

Aerovane

Aerovanes are commonly used at many weather stations and airports to measure both wind direction and speed. They are similar to wind vanes and cup anemometers except have three-bladed propellers attached to the end of the vane.

Wind

Air in motion relative to the surface of the earth.

Cup Anemometer

An anemometer is a device used for measuring the speed of wind, and is also a common weather station instrument.

Coriolis Force

An apparent force caused by the rotation of the Earth. In the Northern Hemisphere winds are deflected to the right, and in the Southern Hemisphere to the left. In synoptic scale weather systems (hurricanes and large mid-latitude storms), the Coriolis force causes the air to rotate around a low pressure center in a cyclonic direction. The air flowing around a hurricane spins counter-clockwise in the northern hemisphere

Cyclone

An area of low pressure around which winds blow counterclockwise in the Northern Hemisphere. Also the term used for a hurricane in the Indian Ocean and in the Western Pacific Ocean.

Convergence

An atmospheric condition that exists when the winds cause a horizontal net inflow of air into a specified region. Divergence is the opposite, where winds cause a horizontal net outflow of air from a specified region.

Ridge

An elongated area of high pressure in the atmosphere. Opposite of a trough.

Trough

An elongated area of relatively low atmospheric pressure surface or aloft. Usually not associated with a closed circulation, and thus used to distinguish from a closed low. The opposite of ridge.

Aneroid Barometer

An instrument built around a metal structure that bends with changing air pressure. These changes are recorded on a pointer that moves back and forth across a printed scale.

Wind Vane

An instrument that determines the direction from which a wind is blowing.

Mercury Barometer

An instrument that measures barometric pressure by measuring the level of mercury in a column

Barograph

An instrument that provides a continuous record of atmospheric pressure.

Cyclonic Flow

Circulation (or rotation) which is in the same sense as the Earth's rotation, i.e., counterclockwise (in the Northern Hemisphere) as would be seen from above.

Buys Ballot's Law

In the Northern Hemisphere, if a person stands with his back to the wind, the atmospheric pressure is low to the left, high to the right.

Barometric Tendency

The amount and direction of change in barometer readings over a three-hour period.

Pressure Tendency

The character and amount of atmospheric pressure change during a specified period of time, usually the 3-hour period preceding an observation.

Divergence

The expansion or spreading out of a vector field usually said of horizontal winds. It is the opposite of convergence.

Pressure Gradient

The rate of decrease of pressure with distance at a fixed level.

Prevailing Wind

a wind from the direction that is predominant at a particular place or season.

Geo strophic Wind

a wind whose direction and speed are determined by a balance of the pressure-gradient force and the force due to the earth's rotation.

Gradient Wind

a wind with a velocity and direction that are mathematically defined by the balanced relationship of the pressure gradient force to the centrifugal force and the Coriolis force

Anticyclone Flow

describes the movement of air around a high pressure, and rotation about the local vertical opposite the earth's rotation. This is clockwise in the Northern Hemisphere.

Hydrostatic Equilibruim

hydrostatic balance when it is at rest, or when the flow velocity at each point is constant over time

Newton

the SI unit of force.

Friction

the action of one surface or object rubbing against another