By Dr. Robert M MacKay Clark College Physics & Meteorology Thermal Circulation By Dr. Robert M MacKay Clark College Physics & Meteorology

A tale of two identical cities City 1 and City 2 both have identical air masses above them. Here we look at a layer of air from 1000 mb to 900 mb which is the bottom 10% of the atmosphere.

The air above City 1 cools and that above City 2 warms The air above City 1 cools and that above City 2 warms. This causes the layer above City 1 to contract and that above city 2 to expand. At this point the pressure at the bottom and top of each layer is still 1000 mb and 900 mb respectively.

Air pressure aloft is High above City 2 The air pressure at the top of city 1 is still 900 mb, but at the same altitude the air pressure above city 2 is somewhere between 900 mb and 1000 mb. Let’s say that the 900 mb level above city 1 is 1 km. Then from the figure above we might estimate the pressure at an altitude of 1 km above city 2 to be about 940 mb. Regardless it is clear that there is a horizontal pressure gradient established at an altitude of 1 km. At this point the surface pressures above city 1 and city 2 are still 1000 mb.

Winds aloft blow from city 2 to city 1. The pressure gradient at 1 km cause air to flow from above city 2 to a region above city 1.

The divergence of air from above city 2 causes the surface air pressure at city 2 to drop below 1000 mb and the convergence of air into the region above city 1 causes the surface air pressure at city 1 to increase above 1000 mb. This creates a surface high pressure above the cold city 1 and a surface low pressure above warm city 2.

The horizontal surface pressure gradient causes air to flow from the region above city 1 back to the region above city 2.

The rise warm air above city 2 and the sinking cold air above city 1 complete the thermal circulation loop.

Clouds often form with rising moist air and hence for a thermal circulation clouds may form in the rising warm air. Sinking air is typically associated with clear skies.

Examples of Thermal Circulations

Sea Breeze during the day

Land Breeze at night

Very dry South East Asian Monsoon

Very wet South East Asian Monsoon

Mountain or Valley breeze??

Upward motion of Valley breeze made visible by clouds

Sea breeze convergence zone in Florida giving rise to very frequent thunderstorms. Florida has more thunderstorms each year than anywhere else in the US.

FIGURE 9.25 Typically, during the summer over Florida, converging sea breezes in the afternoon produce uplift that enhances thunderstorm development and rainfall. However, when westerly surface winds dominate and a ridge of high pressure forms over the area, thunderstorm activity diminishes, and dry conditions prevail.

Figure 7.6: Typically, during the summer over Florida, converging sea breezes in the afternoon produce uplift that enhances thunderstorm development and rainfall. However, when westerly surface winds dominate and a ridge of high pressure forms over the area, thunderstorm activity diminishes, and dry conditions prevail. Stepped Art Fig. 9-25, p. 241

FIGURE 9.26 Surface heating and lifting of air along a converging sea breeze combine to form thunderstorms almost daily during the summer in southern Florida.

FIGURE 9.27 The convergence of two lake breezes and their influence on the maximum temperature during July in upper Michigan.

Local Winds Santa Anna Winds Desert winds Warm dry that blows from east or northeast don canyons into S. California Very fast, desiccates vegetation, providing fuel for fires Desert winds Dust storms, sand storms, dust devil, haboob

FIGURE 9.39 Surface weather map showing Santa Ana conditions in January. Maximum temperatures for this particular day are given in oF. Observe that the downslope winds blowing into southern California raised temperatures into the upper 80s, while elsewhere temperature readings were much lower.

FIGURE 9.40 Strong northeasterly Santa Ana winds on October 23, 2007, blew the smoke from massive wild fires (red dots) across southern California out over the Pacific Ocean.