Chapter 10, Wind 10-13
Figure 10-17. Lake Breeze
As with sea breezes, thunderstorms are favored in the upward motion branch of the lake breeze circulation.
This is especially true where breezes from adjacent lakes collide.
The strength of the lake breeze circulation is affected by a lake’s depth. A shallow lake (e.g., Lake Erie and
Lake St. Clair) warms up rapidly and is less effective as the source of a lake breeze in summer than a deep
lake (e.g., the other Great Lakes).
Figure 10-18. Sea Breeze/Lake Breeze Example [National Aeronautics and Space Administration (NASA)]
Chapter 10, Wind 10-14
In Figure 10-18, the sinking air behind the lake breeze inhibits clouds over Lake Ontario and Lake Erie and
for miles inland.
10.6.4 Valley Breeze
A valley breeze (see Figure 10-19) is a wind that ascends a mountain valley during the day. Air in contact
with the sloping terrain becomes warmer (less dense) than air above the valley. This is because the air in
contact with the sloping terrain heats up faster than air above the valley.
Figure 10-19. Valley Breeze
Pressure gradients develop (along a horizontal reference) with lower pressure over the warmer sloping
terrain and higher pressure over the cooler valley . Winds develop in the direction of the PGF. Thus, the
wind blows from the valley up the mountain slopes. Air rises over sloping terrain and sinks over the valley.
Clouds and precipitation may develop over mountain slopes.
10.6.5 Mountain-Plains Wind System
A mountain-plains wind system (see Figure 10-20) is the diurnal cycle of local winds between a mountain
or mountain range and the adjacent plains. During the daytime, this wind system is the equivalent of
one-half of a valley breeze. Air in contact with the sloping terrain becomes warmer (less dense) than air
above the plains. This is because the air in contact with the sloping terrain heats up faster than the air above
the plains.
Chapter 10, Wind 10-15
Figure 10-20. Mountain-Plains Wind System
Pressure gradients develop (along a horizontal reference) with lower pressure over the warmer sloping
terrain and higher pressure over the cooler plains. Winds develop in the direction of the PGF. Thus, the
wind blows from the plains up the mountain slopes . There is a weak return flow aloft. Clouds and
precipitation may develop in the rising air over the mountain.
10.6.6 Mountain Breeze
A mountain breeze (see Figure 10-21) is the nightly downslope winds commonly encountered in mountain
valleys. Air in contact with the sloping terrain cools faster than air above the valley. Pressure over the
sloping terrain is higher than over the valleys (along a horizontal reference). Coo ler air over the sloping
terrain is denser than warmer air over the valley.
Chapter 10, Wind 10-16
Figure 10-21. Mountain Breeze
Surface wind flows from the mountain down the sloping terrain into the valley. Air rises over the valley
and sinks over the sloping terrain.
10.7 Adverse Winds
10.7.1 Crosswind
A crosswind is a wind that has a component directed perpendicularly to the heading of an aircraft
(see Figure 10-22). The potential of drift produced by crosswind is critical to air navigation and can have
its biggest impact during take off and landing. Airplanes take off and land more efficiently when oriented
into the wind. The aircraft’s groundspeed is minimized, a shorter runway is required to achieve lift-off, and
the pilot has more time to make adjustments necessary for a smooth landing. As the wind turns more
perpendicular to the runway to become a crosswind, the airplane’s directional control is affected. If a pilot
does not correctly compensate for the crosswind, the aircraft may drift off the side of the runway or side
load on landing gear might occur. In extreme cases, the landing gear may collapse.
Chapter 10, Wind 10-17
Figure 10-22. Crosswind Climb Flightpath
10.7.2 Gust
A gust is a fluctuation of wind speed with variations of 10 knots (kt) or more between peaks and lulls.
Even if the airplane is oriented into the wind, gusts during takeoff and landing cause airspeed fluctuations
that can cause problems for pilots. A gust increases airspeed, which increases lift and may cause an aircraft
to briefly balloon up. Once the gust ends, a sudden decrease of airspeed occurs, which decreases lift and
causes the aircraft to sink. Gusty winds at the point of touchdown provide significant challenges to a
safe landing.
10.7.3 Tailwind
A tailwind is a wind with a component of motion from behind the aircraft.
A tailwind can be hazardous during both takeoff and landing. A longer takeoff roll is necessary because a
higher groundspeed is needed to generate sufficient lift, and the aircraft may roll off the end of the runway
before lift-off. Also, a smaller initial climb gradient occurs during takeoff, which may be insufficient to
clear obstacles at the end of the runway. During a landing, a longer landing roll is needed because the
aircraft will touch down at a higher groundspeed. Wind should always be considered in takeoff performance
planning.
Chapter 10, Wind 10-18
10.7.4 Variable Wind/Sudden Wind Shift
A variable wind is a wind that changes direction frequently, while a sudden wind shift is a line or narrow
zone along which there is an abrupt change of wind direction. Both, even at low wind speeds, can make
takeoffs and landings difficult. A headwind can quickly become a crosswind or tailwind.
10.7.5 Wind Shear
See Chapter 19, Turbulence, for information on wind shear.
10.7.6 Adverse Mountain Winds
See Chapter 16, Mountain Weather, for information on adverse mountain winds.
10.7.7 Atmospheric Disturbances in Mountainous Areas
See Chapter 19, Turbulence, for information on mountain-related turbulence.