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 WHAT IS THE THERMAL WIND?

METEOROLOGIST JEFF HABY

You may run across the term thermal wind from time to time in meteorology. When looking up information on this topic you will likely run into an equation and/or a semi-complicated definition. In this hint the thermal wind will be explained in basic terms so that you can understand how this wind develops and what influences the direction and magnitude of the wind.

The first word in the term is thermal. Thermal as you may have guessed deals with temperature. The thermal wind is set up by a change in temperature over a change in distance. When thinking of how the thermal wind sets up think of the polar jet stream. To the north of the polar jet stream the air is cold. Since the air is cold the thickness values (and heights) are lower since cold air is more dense. To the south of the polar jet stream the air is warm. Since air is warm the thickness values are higher since warm air is less dense. A north to south temperature gradient is set up and the height values slope over this distance. When height values slope (think of height contours close together on upper level charts) the pressure gradient force is put into action. It is the Pressure Gradient Force that causes the wind to blow. Whether it is the jet stream, a mid-latitude cyclone or a sea breeze it is the change in temperature over distance that sets the wind in motion. The thermal wind occurs above the boundary layer since friction is not an influence on altering the wind direction aloft.

The wind direction in association with the jet stream generally travels from west to east. This is because the Pressure Gradient Force moves air from higher heights toward lower heights and the Coriolis deflection deflects the air to the right of the path of motion in the Northern Hemisphere. Thus, air moving from south toward north is deflected to the east due to Earth's rotation. The jet stream has a meandering flow pattern to it. Instead of the flow perfectly flowing from west to east the flow moves around troughs and ridges.

When analyzing a sample of air moving within the Northern Hemisphere polar jet stream there will always be colder air to the left of the path of motion of the air and warmer air to the right of the path of motion. This will be the case whether the parcel of air is traveling perfectly west to east or traveling around a trough or ridge.

The thermal wind flow parallel to thickness lines. Remember that thickness is a function of temperature. Thus, when a global or synoptic scale temperature gradient sets up such as that with the jet stream or a mid-latitude cyclone, meteorologists can predict what the upper level wind direction will be in relation to that temperature gradient.

The magnitude of the wind will be a function of how strong the temperature gradient is. When the height contours or thickness values of packed close together then the wind will be strong.

The thermal wind is important for forecasting purposes. Storms will tend to move with the thermal wind. The thermal wind can be thought of as a steering influence for the direction and magnitude that storms move.

In conclusion, the thermal wind is a wind that flow parallel to the temperature gradient in the troposphere. The thermal wind explains the magnitude and direction the wind will take when a temperature change occurs over a horizontal distance.