By: Anna Mae CababarosMS EST- 1

By: Anna Mae CababarosMS EST- 1

- refers to the weight of the Earth's atmosphere pressing down on everything at the surface.

• Another way to define air pressure is that it is the force exerted against a surface by the continuous collision of gas molecules.

• Average air pressure at sea level is about 1 kilogram per square centimeter, or 14.7 pounds per square inch.

MEASURING AIR PRESSURE

Simple Mercury Barometer

Simple Mercury Barometer

• If air pressure decreases, the column of mercury falls; if the pressure increases, the column rises.

• Standard atmospheric pressure at sea level equals 29.92 inches of mercury.

Aneroid Barometer

this instrument has a partially evacuated chamber that changes shape, compressing as atmospheric pressure increases, and expanding as pressure decreases.

Barograph

•Wind is the result of horizontal differences in air pressure. •If Earth did not rotate and there were no friction, air would flow directly from areas of higher pressure to areas of lower pressure. •However, because both factors exist, wind is controlled by a combination of:–(1) the pressure-gradient force,–(2) the Coriolis force, and–(3) friction.

Pressure-Gradient Force• The pressure-gradient force is the primary

driving force of wind that results from pressure changes that occur over a given distance, as depicted by the spacing of isobars, lines drawn on maps that connect places of equal air pressure.

•The spacing of isobars indicates the amount of pressure change occurring over a given distance, expressed as the pressure gradient.

Coriolis Effect

• The Coriolis force produces a deviation in the path of wind due to Earth's rotation (to the right in the Northern Hemisphere and to the left in the Southern Hemisphere).

• The amount of Coriolis deflection also increases with wind speed.

Is always directed at right angles to the direction of airflow.

Affects only wind direction, not wind speed.

Is affected by wind speed.

Is strongest at the poles and weakens equatorward, becoming nonexistent at the equator.

Friction with Earth’s Surface

• It acts to slow the movement of air.• Friction, which significantly influences airflow near Earth's

surface, is negligible above a height of a few kilometers. • Winds in which the Coriolis force is exactly equal and

opposite to the pressure gradient force are called geostrophic winds.

• Geostrophic winds flow in a straight path, parallel to the isobars, with velocities proportional to the pressure-gradient force.

• Winds that blow at a constant speed parallel to curved isobars are termed gradient winds.

Highs and Lows

Lows (Cyclone) – centers of low pressure

Highs (Anticyclone) – high-pressure centers

Winds move from higher pressure to lower pressure and are deflected to the right or left by Earth’s rotation.

In a Northern Hemisphere surface anticyclone, winds blow outward and clockwise.

Regardless of the hemisphere, friction causes a net inflow (convergence) around a cyclone and a net outflow (divergence) around an anticyclone.

General Circulation of the Atmosphere

The underlying cause of wind is unequal heating of earth’s surface.

The atmosphere acts as a giant heat-transfer system.

Ocean currents also contribute to the global heat transfer,

Series of Prevailing Winds:

Polar Regions- Polar easterliesNear the Equator- Easterly Trade winds

Middle Latitude- Westerlies

The flow is divided into six cells, three in each regions:

a)Equatorial Low (Doldrums)- the air that flows up from the equator forms an equatorial zone of low pressure, a region of calm.b) Subtropical High (Horse latitudes)- the region of light winds.c) Westerlies- the north-ward flowing air current turns right.d) Northeast trade winds-the southward air also deflected to the right.e) Polar easterlies- the air aloft gradually moves , continues to cool and descends at the polar region. There it cools further at the surface, and flows southward.

This force causes the winds in the Northern hemisphere to move from the East towards the West below the subtropical high, and from the West towards the East above the subtropical high.

Above the subtropical highs in the Northern Hemisphere, and below the subtropical highs in the Southern Hemisphere, winds blow from the West, towards the East.

The subpolar lows generally cause the winds above 60 ° latitude to move from the East towards the West.

LOCAL

WINDS

• small-scale winds produced by a locally generated pressure gradient.

• it is caused either by topographic effects or variations in surface composition in the immediate area.

All winds are produced for the same reason: Pressure differences that arise because of temperature differences that are caused by unequal heating of Earth’s surface.

Land and Sea Breeze

Mountain and Valley Breeze

Wind Vane

Wind Sock

Cup Anemometer

Average annual precipitation in millimeters

THE END…