At the surface of the Earth, all three forces come into play. As frictional force slows the wind speed, Coriolis force decreases. However, friction does not affect PGF. PGF and Coriolis force are no longer in balance. The stronger PGF turns the wind at an angle across the isobars toward lower pressure until the three forces balance, as shown in Figure 10-10.

The angle of surface wind to isobars is about 10° over water, increasing to as high as 45° over rugged terrain. The end result is, in the Northern Hemisphere, the surface wind spirals clockwise and outward from high pressure and counterclockwise and inward into low pressure (see Figure 10-11). In mountainous regions, one often has difficulty relating surface wind to pressure gradient because of immense friction and because of local terrain effects on pressure.

Local winds are small-scale wind field systems driven by diurnal heating or cooling of the ground. Air temperature differences develop over adjacent surfaces. Air in contact with the ground heats during the day and cools at night. Low-level pressure gradients develop with higher pressure over the cooler, denser air, and lower pressure over the warmer, less dense air (see Figure 10-12).

Low-level winds develop in the direction of the PGF. Coriolis force is insignificant, because the circulation’s dimension (less than 100 mi) and lifespan (less than 12 hours) are too short for significant Coriolis deflection. Thus, the wind generally blows from a high-pressure cool surface to a low-pressure warm surface. Air rises over the warmer surface and sinks over the cooler surface. A local wind circulation is easiest to identify when synoptic-scale wind patterns are weak.

Local winds include sea breeze, land breeze, lake breeze, lake effect, valley breeze, mountain-plains wind circulation, and mountain breeze.

A sea breeze (see Figure 10-13) is a coastal local wind that blows from sea to land and is caused by temperature differences when the sea surface is colder than the adjacent land. Sea breezes usually blow on relatively calm, sunny, summer days.

Air above the land becomes warmer (less dense) than air above the water. This is because land heats up faster than water. Low-level pressure gradients develop with lower pressure over the warmer land and higher pressure over the cooler water.

Low-level winds develop in the direction of the PGF. Thus, the wind blows from the water to the land. The air rises over land and sinks over water. Clouds (and precipitation) may develop in the rising air over land with cloud dissipation over the sinking air offshore.

A sea breeze front (see Figure 10-14) is the horizontal discontinuity in temperature and humidity that marks the leading edge of the intrusion of cooler, moister marine air associated with a sea breeze. It often produces a wind shift and enhanced cumulus clouds along its leading edge. Cumuliform clouds may be absent if the air mass being lifted over land is dry or stable.

A sea breeze front’s position and movement are influenced by coastline shape, low-level wind direction and speed, and temperature difference between land and sea surface. This temperature difference can be affected by the presence of cloud cover over land and the diurnal cycle. The depth of convection is usually too shallow for precipitation to develop. However, sea breeze fronts can be a lifting mechanism for shower and thunderstorm development.