Winds

Did you know about wind chill in the Arctic?    

Air pressure gradient, or the difference between regions of high and low air pressure, impels air in the direction of lowest pressure, creating wind. The larger the air pressure gradient, the greater the wind speed. Several other factors interact to affect wind speed and direction. The most important of these are the Coriolis effect and friction.

Coriolis Effect

The earth's rotation creates an apparent force ("Coriolis force") that deflects moving air to the right of its initial direction in the Northern Hemisphere and to the left of its initial direction in the Southern Hemisphere.

The magnitude of the deflection, or "Coriolis effect," varies significantly with latitude. The Coriolis effect is zero at the equator and increases to a maximum at the poles. The effect is proportional to wind speed; that is, deflection increases as wind strengthens. The resultant balance between the pressure force and the Coriolis force is such that, in the absence of surface friction, air moves parallel to isobars (lines of equal pressure). This is the geostrophic wind.

The Coriolis force explains why winds circulate around high and low pressure systems as opposed to blowing in the direction of the pressure gradient.

The following figure shows how wind is deflected in each hemisphere.

Coriolic Effect

Friction

Air moving over the earth's surface creates friction, which affects the lowest one kilometer of the atmosphere. Friction can interact with other forces to change the wind direction. Above the so-called Atmospheric Boundary Layer where friction is negligible, the pressure gradient force and the Coriolis force are in balance and the wind blows parallel to the isobars. This is called the geostrophic wind. At lower elevations where friction can not be neglected, the wind has a component pointing toward the lower pressure (and away from the higher pressure).

Local Winds

Local winds result from thermal differences that generate a local pressure gradient.

Sea Breezes

Sea breezes form during the day as the sun heats the land. The warm air rises and cool air from the ocean blows in and under the rising air. At night, the land cools faster than the ocean and the wind direction reverses to blow offshore.

Mountain and Valley Winds

Mountain and valley winds are part of the localized air circulation that develops along mountain slopes heated by solar radiation after winter snows have melted. Differences in air density as air is heated by day and cooled by night lead to an up-valley wind by day and a down-valley wind by night.

By day, the air in contact with the bare valley walls facing the sun is heated and, as it rises, neighboring cooler and denser air flows in an adiabatic wind. At night, the mountain slopes cool rapidly and the air in contact with the slopes is chilled. The now cold, dense air flows downslope as a katabatic wind. These winds are generally shallow (100-300 meters in altitude) with speeds of about two to four meters per second.

Large Scale Katabatic Winds

Katabatic winds occur when cooled, dense air flows down slopes. Over extensive snow-covered plateaus or highlands large-scale katabatic drainage winds may develop. This is common over the Greenland ice sheet. In some places katabatic winds are channeled by mountain valleys, and the wind accelerates to potentially destructive speeds. Steep slopes can also accelerate the katabatic flow. Along the edge of the massive Greenland ice sheet, katabatic winds frequently exceed 100 kilometers per hour.

Large Scale Circulation

The following figure shows the major large-scale wind systems of the planet, as well as the polar front boundary, where extratropical cyclones (migrating low pressure systems) tend to form.

global circulation patterns

Unlike westerlies and trade winds, the polar easterlies are not a global wind belt. Instead, easterly winds occur mainly on the poleward sides of the Icelandic and Aleutian lows.

Measuring and Reporting Wind

Meteorologists usually report wind speed in meters per second (m/s). Ship observations may be reported in knots (one knot is one nautical mile per hour or about 1.15 miles per hour, approximately 0.5 meters per second). The Beaufort scale, named after Admiral Sir Francis Beaufort (1774-1857), allows observers on ships to judge wind strength by the ocean's appearance. For example, Beaufort Force 9, also known as a "strong gale," is indicated by dense foam blowing from the tops of breaking waves. Force 9 compares to a wind speed of 21 m/s to 24 m/s.

Wind direction is always reported as the direction the wind is coming from. For example, a wind out of the west is reported as a west wind, or wind direction 270 degrees. Interestingly, this convention is the opposite of that used by oceanographers for ocean currents.

Winds in the Arctic

The Arctic winter is characterized by high winds with snowstorms between calm periods. With little to slow them, Arctic winds scour open areas, and deposit loads of snow in sheltered areas. Nevertheless, in the Arctic, winter surface wind speeds are often lower than in summer due to the frequent occurrence of inversions (when warm air tops a surface cold layer). The inversion layer decouples surface wind from stronger upper layer winds.