Chapters 6, 7 and 8. Review the three forces that control wind, high- and low-pressure circulation, convergence, divergence, and how wind is measured.
Chapter 6 Videos
Chapter 6 — Air Pressure and Winds
Chapter 6 — Wind Flow, Part 2
Understanding Air Pressure
Air pressure is the force exerted by the weight of air above a surface and by collisions of gas molecules. Standard sea-level pressure is approximately 1013.25 millibars (hPa), 29.92 inches of mercury, or 760 mm Hg.
Pressure Units
1 millibar (mb) = 100 pascals (Pa) = 100 newtons per square meter (N/m²).
Pressure Instruments
Mercury barometer, aneroid barometer (without liquid), and barograph (records pressure changes).
Pressure decreases with increasing altitude, most rapidly near Earth's surface. The standard atmosphere describes an idealized vertical pressure profile. Temperature and moisture influence air density and pressure patterns, but actual surface pressure also depends on the amount and distribution of air in the atmospheric column.
The Three Forces That Control Wind
Wind results from horizontal differences in pressure. Without Earth's rotation and friction, air would flow directly from higher to lower pressure.
1. Pressure-Gradient Force
Starts air moving from higher toward lower pressure. Closely spaced isobars mean a stronger pressure gradient and generally stronger winds; widely spaced isobars indicate a weaker gradient.
2. Coriolis Effect
Deflects moving air because Earth rotates. To the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
3. Friction
Slows wind near Earth's surface, reducing Coriolis deflection and causing surface winds to cross isobars toward lower pressure.
Isobars
Lines on weather maps connecting places with equal atmospheric pressure. Their spacing helps reveal the pressure-gradient strength.
Coriolis, Friction and Geostrophic Wind
The Coriolis effect increases with wind speed and latitude. It is strongest at the poles and zero at the equator. Friction is important near the surface and becomes much less significant higher in the atmosphere.
Geostrophic wind occurs when the pressure-gradient force and Coriolis force balance. It flows approximately parallel to straight isobars above the friction layer. Gradient wind follows curved isobars.
Cyclonic and Anticyclonic Flow
Low Pressure — Cyclone
Counterclockwise circulation in the Northern Hemisphere. Surface winds spiral inward, causing convergence, with rising air often favoring clouds and precipitation.
High Pressure — Anticyclone
Clockwise circulation in the Northern Hemisphere. Surface winds spiral outward, causing divergence, with sinking air often favoring fair weather.
Circulation reverses in the Southern Hemisphere. Surface friction causes winds to cross isobars, rather than follow them exactly.
Pressure Systems, Troughs and Ridges
Trough
An elongated region of relatively low pressure, often associated with cooler, unsettled or stormy conditions.
Ridge
An elongated region of relatively high pressure, often associated with fairer, warmer conditions.
A surface low can strengthen when divergence aloft helps remove air from the atmospheric column, supporting rising motion and surface convergence. Inadequate divergence aloft can weaken the system. The barometric tendency—whether pressure is rising or falling over several hours—helps with short-range forecasting.
Measuring Wind
Weather observers measure wind direction and wind speed. Wind direction tells where the wind is coming from: a north wind blows from north toward south.
Wind Vane
Measures wind direction.
Cup Anemometer
Measures wind speed.
Aerovane
Combines wind direction and speed sensing.
Prevailing Wind
The direction from which wind most frequently blows at a location.
What to Know for Test 3
- The three forces that control wind: pressure-gradient, Coriolis and friction
- Why closely spaced isobars often mean stronger winds
- How winds circulate around highs and lows in both hemispheres
- Cyclonic versus anticyclonic flow
- Where air converges, diverges, rises and sinks
- Surface wind versus upper-level wind; why winds are often faster aloft
- Geostrophic and gradient winds
- Wind direction is the direction from which the wind blows
- How barometers, wind vanes and anemometers work