Chapter 10, Wind 10-7
Figure 10-9. Upper Air Wind Flow
10.5 Surface Wind
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.
Figure 10-10. Surface Wind Forces
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 a nd 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.
Chapter 10, Wind 10-8
Figure 10-11. Surface Wind Flow
10.6 Local Winds
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).
Figure 10-12. Local Wind Circulation
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
Chapter 10, Wind 10-9
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.
10.6.1 Sea 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.
Figure 10-13. Sea Breeze
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.
10.6.1.1 Sea Breeze Front
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.
Chapter 10, Wind 10-10
Figure 10-14. Sea Breeze Front
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.
10.6.1.2 Effects of Coastline Shape
Locally, the shape of the coastline plays an important role in the development of convection along sea
breezes (see Figure 10-15). A narrow peninsula or island is generally an area of strong convective
development during the late morning or early afternoon. This is because the sea breezes that formed along
opposing shores merge near the center of the peninsula or island.
Chapter 10, Wind 10-11
Figure 10-15. Effects of Coastline Shape on a Sea Breeze
In Figure 10-15, convergence occurs where sea breezes merge from opposite directions. Stronger lift may
be sufficient to initiate showers and thunderstorms if the air mass is sufficiently moist and unstable.
10.6.2 Land Breeze
A land breeze (see Figure 10-16) is a coastal breeze blowing from land to sea caused by the temperature
difference when the sea surface is warmer than the adjacent land. Land breezes usually occur at night and
during early morning.
Chapter 10, Wind 10-12
Figure 10-16. Land Breeze
Air above the land becomes cooler (denser) than air above the water due to conduction. This is because
land cools faster than water. Low-level pressure gradients develop with higher pressure over the cooler land
and lower pressure over the warmer water.
Low-level winds develop in the direction of the PGF. Thus, the wind blows from the land to the water. The
land breeze is usually weaker than the sea breeze. The air rises over water and sinks over land. Clouds and
precipitation may develop in the rising air over the water.
10.6.3 Lake Breeze
A lake breeze (see Figure 10-17) is a local wind that blows from the surface of a large lake onto the shores
during the afternoon and is caused by the temperature difference when the lake surface is colder than the
adjacent land. The lake breeze is similar in origin to the sea breeze and is common in the Great Lakes. Both
occur during the warm season, primarily spring and summer. Both are easiest to detect in light synoptic
wind conditions.