|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
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
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
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