Thickness and Thermal Wind AOS 101 Section 301 April 13th, 2009

Pressure vs. Height Pressure decreases with height, but can also decrease as you move north/south/east/west

Pressure vs. Height 300 mb Z Z 500 mb 1000 mb X X We can imagine pressure surfaces stacked on top of one another

Pressure vs. Height Z X 1000 mb 300 mb 500 mb 500 mb everywhere Pressure Y X At a given height, there is no pressure gradient if the pressure surface is flat, just like the height surface you are looking at

Pressure vs. Height Pressure 300 mb 500 mb Z Y 300 mb 1000 mb 400 mb 500 mb X X A pressure gradient only appears when pressure surfaces develop hills and valleys

Pressure vs. Height Height above ground 300 mb 500 mb Z Y 1 km 1000 mb 2 km 3 km X X When we stay on a pressure surface, we observe that the height of that surface above the ground is low in a region of low pressure, and is high in a region of high pressure.

Pressure vs. Height Low Pressure = Low Height Height above ground 300 mb 500 mb Z Y 1 km 1000 mb 2 km 3 km X X Low Pressure = Low Height High Pressure = High Height

Thickness What can cause pressure surfaces to develop hills and valleys? Consider the how much space warm air occupies compared to cold air…

Thickness Here is a column of air at room temperature

Thickness Here is a column of air at room temperature If I were to cool this column of air …

Thickness Here is a column of air at room temperature If I were to cool this column of air … It would shrink in size. This is because cold air is more dense than warm air (recall the lecture on buoyancy) The same amount of air is still there (i.e. it has the same weight, and causes the same amount of pressure beneath it), the column is just shorter when the air is cooled.

Thickness Here is a column of air at room temperature If I were to cool this column of air … It would shrink in size. This is because cold air is more dense than warm air (recall the lecture on buoyancy) The same amount of air is still there (i.e. it has the same weight, and causes the same amount of pressure beneath it), the column is just shorter when the air is cooled. We refer to this phenomenon as the “thickness” of an air column.

Thickness Thickness = the vertical distance between two pressure surfaces This is entirely dependent upon the temperature of the air between those pressure surfaces: Cold Air = Low Thickness Warm Air = High Thickness

Thickness COOL WARM

Thickness Imagine the atmosphere is a ‘block’ of fluid that pushes down with 1000 mb of pressure at the bottom 1000 mb 800 mb 600 mb 400 mb 200 mb 0 mb

Thickness Imagine the atmosphere is a ‘block’ of fluid that pushes down with 1000 mb of pressure at the bottom The block starts out at a uniform temperature – the thickness of the atmosphere is the same everywhere. 1000 mb 800 mb 600 mb 400 mb 200 mb 0 mb

Thickness Height is the same everywhere Y X 0 mb 200 mb 400 mb 600 mb

Thickness Height is the same everywhere Y X 0 mb 200 mb 400 mb 600 mb

Thickness Height is the same everywhere Y X 0 mb 200 mb 400 mb 600 mb

Thickness Imagine the atmosphere is a ‘block’ of fluid that pushes down with 1000 mb of pressure at the bottom The block starts out at a uniform temperature – the thickness of the atmosphere is the same everywhere. Now we make the block cold on the west side and warm on the east side… 1000 mb 800 mb 600 mb 400 mb 200 mb 0 mb COLD WARM

Thickness 0 mb Imagine the atmosphere is a ‘block’ of fluid that pushes down with 1000 mb of pressure at the bottom The block starts out at a uniform temperature – the thickness of the atmosphere is the same everywhere. Now we make the block cold on the west side and warm on the east side… The pressure surfaces develop a steep slope, with the low end on the cold side and the high end on the high side 200 mb 400 mb 600 mb 800 mb 1000 mb

Thickness 0 mb The 1000 mb pressure surface is still flat – there is the same amount of fluid above the surface whether you are on the cold side or the warm side 200 mb 400 mb Height is the same everywhere 600 mb 800 mb 1000 mb Y X

Thickness 0 mb The 1000 mb pressure surface is still flat – there is the same amount of fluid above the surface whether you are on the cold side or the warm side But at upper levels, a pressure gradient appears … 200 mb 400 mb 600 mb 800 mb L H 1000 mb Y X

Thickness 0 mb The 1000 mb pressure surface is still flat – there is the same amount of fluid above the surface whether you are on the cold side or the warm side But at upper levels, a pressure gradient appears … Which gets stronger as you go up. 200 mb 400 mb 600 mb 800 mb L H 1000 mb Y X

Thickness 0 mb The 1000 mb pressure surface is still flat – there is the same amount of fluid above the surface whether you are on the cold side or the warm side But at upper levels, a pressure gradient appears … Which gets stronger as you go up. 200 mb 400 mb 600 mb 800 mb L H 1000 mb Y X

Thickness 0 mb The 1000 mb pressure surface is still flat – there is the same amount of fluid above the surface whether you are on the cold side or the warm side But at upper levels, a pressure gradient appears … Which gets stronger as you go up. 200 mb 400 mb 600 mb 800 mb L H 1000 mb Y X

Thickness and Thermal Wind A temperature gradient forces the geostrophic wind to blow differently at different levels of the atmospher As you go up in the atmosphere, the geostrophic wind blows more and more strongly keeping cold air to the left We call this the thermal wind

Thickness and Thermal Wind More technically, the thermal wind is defined as “the difference in the geostrophic wind between two pressure levels” This vertical change in the geostrophic wind is entirely dependent on thickness: the “thermal wind” points parallel to lines of constant thickness, with low thicknesses to the left

500 mb 850 mb

500 mb Thermal wind between 850 mb and 500 mb 850 mb

WARM 500 mb Thermal wind between 850 mb and 500 mb 850 mb COLD

Thickness and Thermal Wind North Pole The northern hemisphere is typified by cold air to the north and warm air to the south Equator

Thickness and Thermal Wind The northern hemisphere is typified by cold air to the north and warm air to the south The height contours at upper levels are typified by low heights to the north, and high heights to the south, consistent with thickness h1 h2 h3 H

Thickness and Thermal Wind This is why anomalously low pressure/height is referred to as a “trough”, and anomalously high pressure/height is referred to as a “ridge” h1 h2 H L h3

Thickness and Thermal Wind Because mid-latitude cyclones mix warm and cold air along their fronts, they create ridges and troughs at upper levels by changing the temperature field:

Cyclone pushes warm air in front of it to the east, and pulls down cold air behind it to the west

Cyclone at upper levels sits above cold air, appearing slightly west of surface cyclone.

Cyclone at upper levels sits above cold air, appearing slightly west of surface cyclone. As it turns out, this is a good place for the upper level low – it helps the lower level low get stronger, which in turn helps the upper level low get stronger – CYCLOGENESIS But, that is a story for another time… L L