Atmospheric Air Pressure measured in millibars by meterorologists; 1013.2 millibars at sea level.
Mercury Barometer - measures air pressure by allowing the atmosphere to force mecury up an evacuated sealed tube. Atmospheric pressure is indicated by how high the mercury rises. Atmosphereic pressure is measured in inches or millimeters of mercury. Sea level pressure is 29.92 inches or 760mm of mercury.
Aneroid barometer - doesn't use any liquid, uses a spring mechanism.
Barograph - an instrument that continually records pressure changes.
Air flows from arieas of high pressure to areas of low pressure creating wind. Unequal heating of the Earth creates pressure differences. Winds are driven by solar energy.
Factors include:
The greater the differences in air pressure the greater the wind speed. Weather maps convey pressure data using isobars - lines connecting points of equal pressure. Closely spaced isobars indicate a steep pressure gradient and high winds, whereas widely spaced isobars indicate a weak pressure gradient and light winds.
All free moving objects or fluids, including the wind, are deflected to the right of their path of motion in the Northern Hemisphere and to the left in the Southern Hemisphere. The deflection is apparent.
The deflection:
Frictional effects are only important within the first few kilometers of the atmosphere.
Geostrophic winds - upper air winds influenced only by pressure gradients and the Coriolis effect and not by friction.
Jet Streams - fast moving rivers of air (75 to 150 mph) direction is west to east across the US.
Effect of friction is to slow speed and reduce the Coriolis effect.
Pressure Centers: cyclones=low pressure center; anticyclone=high pressure center
Cyclonic and Anticyclonic Winds: In the Northern hemisphere winds blow inward and counterclockwise around a low. In the Southern hemisphere winds blow clockwise around a low. Winds travel in the opposite direction around a high.
A net inflow of air at the surface (cyclone) is called surface convergence and is accompanied by divergences aloft.
A net outflow of air at the surface (anticyclone) is called surface divergence and is accompanied by convergence aloft.
Rising air is associated with cloud formation. Subsidence produces clear skies. Cyclones and Anticyclones pair up and cause air to circulate.
Barometric tendency or pressure indicates, in a general sense, what to expect from the weather.
Low pressure zones move west to east across the United States.
Global circulation on a rotating Earth consists of three cells in each hemisphere.
Equatorial Low at the equator warm air rises
Polar High at the poles cold air sinks
Polar Front westerlies of mid-latitudes meet polar easterlies
Subtropical High 30o Latitude both north and south - location of the World's great deserts
Trade winds- blow from subtropical high to equator, deflected to the west.
Westerlies- blow from subtropical high to ploar front, deflected to the east.
Polar Easterlies- blow from the poles to the polar front, deflected to the west.
Land disrupts pressure zones and complicates weather patterns. There is more land in the Northern Hemisphere.
Monsoons - seasonal changes in wind directin due to unequal heating and cooling of land and sea.
Circulatin in the Mid-Latitudes is best described as complicated and chaotic.
Sea and Land Breezes: A sea breeze develops because cooler air over the water moves towards warmer land, a land breeze is the reverse.
Valley and Mountain Breezes: Uneven heating of air on slopes and air over valleys at the same altitude cause these breezes.
Chinook Winds: Warm dry winds on the eastern slopes of the Rockies. Descending air warms by comprossion. Word means "snow eater:.
Santa Ana Winds: Hot, desiccating wind in Southern California.
Direction: Wind Vane; a device that always points into the wind.
Speed: Cup Anemometer
Winds are given a direction based on their origins. Prevailing wind is a term used to describe a predominate direction of wind flow.