Ch 5 Atmospheric Pressure and Wind

Intertropical Convergence Zone

0⁰-23.5⁰ N & S. Produces low pressure 80"-100" of rainfall.

Subtropical High's

23.5⁰- 35⁰ Latitude N & S Hemisphere. Represent cells of high pressure and subsiding air. Air curculation anticyclonic, weather always clear, warm, and calm. Represent deserts and absence of wind. Air circulation clockwise in N and counterclockwise S Hemisphere.

Westerlies

30 °- 60 ° latitude N & S Hemispheres. Wind system of midlatitudes. Winds flow from W-E. Subtropical jet stream. Strong winds. The westerlies originate as a result of pressure differences between the subtropical high-pressure zone and the subpolar low-pressure zone.

Upper Atmosphere Pressure Gradient Force and Coriolis Effect

Aboove 1000m/3300 ft. An approximate balance develops between the pressure gradient force and the Coriolis effect deflection, resulting in a geostrophic wind blowing parallel to the isobars; wind circulates clockwise around high and counter clockwise around a low in the Northern Hemisphere

Density at higher altitudes

Air is less dense and there is a corresponding decrease in pressure.

Pressure Gradient Force only

Air would flow down the pressure gradient away from high pressure and toward low pressure corssing the isobars at an angle of 90⁰

Sea Level Pressure

Average sea level pressure is 1013.25 milibars.

Influence on Air Pressure

Density, Temperature and Movement

Factors influencing atmospheric pressure

Density, Temperature, and vertical movement of air

Direction of wind movement

Determined by the interaction of three factors: pressure gradient, Coriolis effect, and friction

Wind Speed

Determined by the pressure gradient. Steep gradient, air moves swiftly; gentle gradient, movement is slow.

Atmospheric Pressure

Force exerted by gas molecules on some area of the Earth's surface or on any other body

Pressure

Force gas exerts in some specified area of the container walls Pressure always decreases vertically with height. Air pressure is exerted equally in all directions.

Lower Atmosphere Pressure Gradient Force, Coriolis Effect and Friction

Friction slows the wind (which results in less Coriolis effect deflection). Wind diverges clockwise out of a high and converges counterclockwise into a low in the Northern Hemisphere

Density at lower altitudes

Gas molecules of the atmosphere are packed more densely together. Density is higher, more molecular collision, higher pressure at lower altitudes

Santa Ana Winds

High pressure cell. The wind diverges clockwise out of the high, bringing dry, warm northerly or easter winds to the coast. High speed, high temperature, and extreme dryness.

Anticyclone

High pressure circulation patterns. High pressure cell, air descends and diverges.

Wind

Horizontal air movement. Small scale vertical motions are referred to as updrafts and downdrafts. Large scale vertical motions are referred to as ascents and subsidences. More air is involved in horizontal movement than vertical.

Temperature

If air is heated, the molecules become more agitated and the speed increases. Results in higher pressure.

Barometers

Instruments used to measure atmospheric pressure. Average sea level pressure using a barometer is 760 millimeters or 29.92 inches.

Isobar

Isobars are lines of connecting points of equal atmospheric pressure drawn on a meteorological map

Cyclone

Low pressure circulation patterns. Low pressure cell, air converges and rises.

Density

Mass of matter in a unit volume. The denser the gas, the greater the pressure exerts

Millibar

Measure of pressure, consisting of one-thousandth part of a bar. Expression of force per square area. One millibar is defined as 1000 dynes per square centimeter.

Polar High's

Polar regions are high-pressure cells. High elevation very cold. Air movement is typically anticylonic. Air from above sinks down into the high and diverges horizontilly near the surface, clockwise in N Hemisphere and counterclockwise in S Hemisphere. Regions under the polar high also experience very low levels of precipitation, known as "polar deserts".

When referring to air pressure

Prominent on such maps are roughly circular or oval areas characterized as being HIGH or LOW pressure. In similar way a RIDGE is an elongated area of relatively high pressure, while a trough is an elongated area of low pressure.

Pressure Gradient

Representing the steepness of the pressure slope (the abruptness of the pressure change over a distance)

Monsoon

Seasonal reversal of winds; a general onshore movement in summer and a general offshore flow in winter, with a very distictive seasonal precipitation regime

Dynamic High

Strongly descending air is usually associated with high pressure at the surface

Dynamic Low

Strongly rising air is usually associated with low pressure at the surface

Thermal High

Very cold surface conditions are often associated with high pressure at the surface

Thermal Low

Very warm surface conditions are often associated with relatively low pressure at the surface

Geostrophic Wind

Wind that moves parallel to the isobars as a result of the balance between the pressure gradient force and the Coriolis effect.