Winds

Wind is the horizontal velocity of air at a point. Winds are always known by the direction from which they come, thus a wind blowing from the west is a westerly.

Winds

Three of the factors that govern the strength and direction of the wind are:

• the pressure gradient (the difference in pressure between two points)
• the Earth's rotation
• friction

These all act at the same time but we can consider them separately.

• Pressure Gradients arise from differences in temperature. When a column of air is heated it expands and becomes less dense than its surroundings. This sets up a slight pressure gradient and cooler, denser air moves in, lifting the warm air away from the surface. A convection circulation is then set up. Small pressure gradients at certain altitudes produce strong horizontal winds.
• Air cannot flow directly from high to low pressure areas due to the Earth's rotation which is faster at the equator than at the poles. Air moving from the equator to the poles moves eastward faster than the underlying surface and thus curves to the right in the northern hemisphere. Air moving from the poles to the equator moves slower than the underlying surface and thus also curves to the right. This is known as the Coriolis effect and produces the global wind patterns. Air flowing out of the subtropical highs towards the equator forms the trade winds. On the polar side of the subtropical highs, the winds are deflected to give rise to the prevailing weather. When there is a change in wind direction we use two terms:
  • veer (wind direction swings clockwise)
  • back (wind direction swings anticlockwise)
• Friction slows the surface wind and weakens the Coriolis effect, causing the air to curve towards low pressure. The amount of change depends upon the surface.

Wind Speed

In meteorological observations the mean wind is usually an average taken over ten minutes preceding the time of observation. A gust is a fluctuation above the mean typically lasting from a few seconds to a few minutes. Windflow is always turbulent due to the roughness of the Earth's surface. Gustiness is much increased by the presence of buildings, trees and rocky country, but at the same time the mean wind is decreased by friction. For this reason winds over the sea are usually less gusty then over land. Wind speed, by international agreement, is measured in knots (though some nations favour metres per second). Wind direction is measured in degrees from true north.

A knot is 1.152 mph (0.514 m/s). The knot unit has its origins in naval history. A knot is one nautical mile per hour, where a nautical mile is one minute (one sixtieth of a degree) of latitude at the Earth's surface. 8 kn. are roughly equal to 10 mph (4 m/s).

Wind speeds are often given on the Beaufort scale which was originally devised for ships at sea. In forecasts for ships or aircraft, wind speed is usually quoted in knots. Meteorologist use m/s but km/h is easier for most people to visualize.

Beaufort Scale

In 1805 Captain Beaufort devised a simple numerical scale of wind forces in terms of the resulting state of the sea. The Beaufort wind scale is still widely used in area forecasts for shipping and has been extended for use on land..

Wind Chill

The feeling of temperature outdoors depends greatly on wind speed. Even slight winds give greater cooling effects than still air on any object. Weather forecasts in cold weather often give the wind chill equivalent temperature which takes into account extra cooling of the human body due to the wind. There is no accurate method of measuring this (as it depends on the amount of your body exposed, clothing worn, humidity, sunshine and shelter).

This wind chill poses a severe hazard when low temperatures and moderate wind speeds are experienced together.

At low temperatures a rough guide is to take the actual air temperature and subtract 1 °C and also the wind speed in m/ s

Example: An air temperature of 0 °C and a wind speed of 5 m/s is approximately equal to -6 °C in calm conditions.