Winds Notes

What is Wind? Wind is the sideways movement of air. The air always moves from an area of HIGH pressure to an area of LOW pressure. Differences in air pressure are caused by differences in how places are heated by the sun. Warm air has less pressure than cooler air.

PGF is Pressure Gradient Force. In other words, which direction is the pressure going? Remember that cities typically have higher temperatures, and therefore lower pressure than surrounding areas.

What is Wind? The name of a wind tells you where the wind is coming from. Land breeze, sea breeze, westerlies, polar easterlies, etc. Wind speed is measured with an anemometer. Blowing winds can reduce body heat, this is known as a wind-chill factor. Higher winds equal lower wind-chills. You’ve probably seen a weather forecast stating that it’s 42°F, with a wind-chill of 36°F.

Anemometers work just like the speedometer in your car. The faster the winds are, the faster the cups will move. Remember that speed = distance/time.

Local Winds Local winds are winds that blow over a short distance. A cool breeze blowing in from the water over a beach is an example of a local wind. Unequal heating often happens near lakes and ocean coasts, as well as in densely packed cities.

Local Winds A sea breeze is a local wind that blows from an ocean. A lake breeze is a local wind that blows from a lake. Sea breezes occur during the day due to the land heating up much faster than the water (water has a high specific heat) and creating rising air a low pressure.

Local Winds A land breeze is a local wind that blows from the land over onto a lake or ocean. Land breezes occur at night as the land cools off very rapidly, creating sinking air and high pressure, whereas the water will be releasing the energy stored from the daylight, creating rising air and low pressure.

Global Winds A global wind is a wind that blows steadily over very large distances. A global wind blows from the same direction every time. The sun heats Earth unequally due to Earth being a sphere. Sunlight strikes most directly near 0° latitude, and strikes at less direct angles as the latitude becomes greater. Due to this, air has a tendency to rise at the equator (Low Pressure) and sink at the poles (High Pressure) just like ocean currents, but this is occurring in the troposphere.

Global Winds Of course, we can’t forget that Earth also rotates towards the East. This is going to curve winds just like it does ocean currents. This is the Coriolis effect. Northern hemisphere winds generally curve to the right. Southern hemisphere winds generally curve to the left.

Global Wind Belts There are 6 “belts” of wind that circle the entire planet. Three belts in each the northern and southern hemispheres. Between each of these belts, there are areas of very calm, very little winds.

Global Wind Belts From the equator (0°) to 30° in each hemisphere, there are the trade winds. From 0°-30°N, it’s the NE trade winds. From 0°-30°S, it’s the SE trade winds. Remember that the names of winds tell you where they come from. Where these two trade winds meet, winds are rapidly warmed by the sun and rise, creating the doldrums. Doldrums is an Old English word for “foolish,” for those who got stuck in this area of no winds.

Global Wind Belts From 30°N - 60°N, as well as 30°S - 60°S, winds always blow from the west. These two prevailing wind belts are known as westerlies. Where the westerlies meet the trade winds at 30°N and S, the area of calm winds form known as the horse latitudes. This is because as sailors got stuck in this windless latitude, they would either have to eat the horses, or throw them overboard.

Global Wind Belts From 60°N to 90°N, as well as from 60°S to 90°S, a prevailing wind will always blow directly from the poles outwards. Remember that air drops quickly over the poles due to the temperature/density/pressure change. As the winds blow away from the poles, they will curve in response to the Coriolis effect. They will always curve towards the west. These winds are the polar easterlies.

Global Wind Belts About 10 km above Earth’s surface (250mb), there are bands of high-speed winds that travel without friction to slow them down. These are jet streams. These winds are hundreds of kilometers wide, blow from west to east, reach speeds of km/hr, and control the paths of storms in the United States.