Thickness and Thermal Wind

Background on Thickness

When you hear the word thickness you usually are talking about a layer. You can also describe our atmosphere's thickness. So how do we do that?

Let look at this presentation: Thickness and Wind

Summary of the Thickness and Wind presentation:

• Cold air is more dense, therefore thinner
• Warm air is less dense, therefore thicker
• Temperature is the only factor that changes the thickness of a layer
• When you have a temperature contrast, you create height variations for a layer
• Height variation create a pressure gradient
• Pressure gradient creates a PGF
• The change in the Geostrophic Wind is directly proportional to the horizontal temperature gradient
  This is the Thermal (temperature) Wind relationship

Thermal Wind

A horizontal thermal gradient creates a PGF at upper levels. As you increase in altitude, the pressure gradient between the warm column and the cool column increase. Last week we saw that wind in geostrophic balance, balances the PGF and Coriolis force. As the PGF increases the magnitude of the wind will increase and so will the Coriolis force. In this figure, the size of the green circles represent the magnitude of the geostrophic wind and the x in the circle represents the tail end of the directional arrow, so we are looking at an arrow pointing into away from us.

The vertical change in geostrophic wind is called the geostrophic vertical shear. Since the geostrophic vertical shear is directly proportional to the horizontal temperature gradient, it is also called the Thermal Wind

So the Thermal wind is not an actual wind, but a the difference between two winds at different levels.

Wind Vectors

The thermal wind describes the vertical geostrophic wind shear. So how do we define it?

V_T = V_{upper level} - V_{lower level} or V_{upper level} = V_T + V_{lower level}


Veering Winds with height (clockwise rotation with height)


Backing winds with height (counterclockwise rotation with height)

Veering and Backing Winds

The direction of the Thermal wind will tell us about the thermal structure of the atmosphere. The thermal wind always is point parallel to lines of constant thickness with lower thicknesses to the left, therefore, the thermal wind will always have the colder air to the left. So what does this tell us about the veering and backing winds?

This is the case of Veering wind we saw before. If colder air is always to the left, then we have Warm Air advection
Veering winds clockwise with height are associated with warm air advection

This is the case of backing wind we saw before. If colder air is always to the left, then we have Cold Air advection
Backing winds counterclockwise with height are associated with cold air advection