Download Local Winds

Local Winds
Sea Breezes and Land Breezes
The wind felt when large areas of land are next to large bodies of water are called sea and
land breezes. These winds are created by convections currents that develop due to differences
in heating and cooling characteristics of land and water. Because land and water heat and
cool at different rates the density of air above these substances are affected. When the surface
is cool, the air molecules move slower and the air becomes more dense. When the surface is
warm, the air molecules move faster and the air becomes less dense. Air of low density next
to air of high density is called an air pressure gradient. When an air pressure gradient
exists, the more dense air flows toward the less dense air seeking equilibrium. This air
movement is the main cause of wind on our planet.
A sea breeze (figure 1) refers to wind that comes from the sea. Sea breezes occur when the
air above the sea is cooler than the air above land. Recall from our experimentation that water
changes temperature slower than all other substance we compared it to. During the day, the
water would resist a rise in temperature and potentially cause an air pressure gradient. The
cooler air molecules above the water have less energy, move slower, and therefore are more
dense. The land heats at a faster rate than water causing the air molecules to heat up, move
faster, take up more room and therefore become less dense. The dense cool air above water
flows toward the air above land because the crowded particles in the high pressure cool air
move toward the less crowded particles in the low pressure warm air. A sea breeze usually
begins in midmorning and reaches its maximum strength in the later afternoon when the
greatest temperature and pressure differences exist. It dies down at sunset when air
temperature and pressure once again become similar across the two surfaces.
Colder Air
Higher Density
Higher Pressure
Sea Breeze!
Warmer Air
Lower Density
Lower Pressure
Figure 1: Daytime development of sea breeze.
When the Earth spins so that the sun is no longer shining on the surface (at sunset), the land
and water stop receiving the Suns’ energy and begin to cool (Figure 2). Recall the water will
cool at a slower rate than land. So during the night the land will eventually become colder
than water. The land surface now being cooler than the water creates a high air pressure area
above it (cooler air contracts, becomes denser, and creates high pressure air). Conversely, the
water, which cools off much more slowly than the land, creates a relatively low air pressure
area above it. Wind flow reverses and now moves from the land to the open ocean. This type
of localized air flow is called a land breeze.
Cooler Air
Higher Density
Higher Pressure
Warmer Air
Lower Density
Lower Pressure
Land Breeze!
Figure 2: Nighttime development of land breeze.
Valley and Mountain Breezes
Valley and Mountain breezes are common in regions with large changes in elevation. A
valley breeze develops during the day as the sun heats the land surface and air at the valley
bottom and sides (Figure 3). As the air heats it becomes less dense and more buoyant and
begins to flow gently up the valley sides. Upward movement of the air rising along the sides
of the mountain is usually limited by the presence of a temperature inversion layer (due to
colder air at higher temperatures). When the rising air currents meet the inversion they are
forced to move horizontally (sideways) and then back down to the valley floor. This creates a
self-contained circulation system. If conditions are right, the rising air can condense and form
into cumuliform (puffy white) clouds.
During the night, the air along the mountain slopes begins to cool quickly (Figure 4). As the
air cools, it becomes more dense and begins to flow downslope causing a mountain breeze.
Convergence of the draining air occurs at the valley floor and forces the air to push vertically
upward. The upward movement is usually limited by the presence of a temperature
conversion which forces the air to begin moving horizontally. This horizontal movement
completes the convection cell. In narrowing terrain, mountain breezes can attain speeds as
high has 150 kilometers per hour.
Figure 3: Daytime development of a valley breeze as the air along the
sides of the mountains is warmed and rises.
Figure 4: Nighttime development of a mountain breeze as air along the
mountain sides cools quickly and sinks.
Local Winds
Answer the following questions in your journal. You
may draw pictures with written explanations or write
in complete sentences. Although you should feel free
to work with your partners, do not use your partners’
answers. Your answers should be your own creative
work.
1. What is an “air pressure gradient”? Remember to explain in your own
words or create a labeled diagram.
2. Why does air rise when heated? Explain thoroughly from a particle level
(in other words, discuss the movement of the air molecules as it relates to
temperature) (The answer is not in the reading. Think about what we’ve
learned this year.)
3. During a sea breeze, why does air move toward the land?
4. If you were relaxing at Bellingham’s Boulevard Park (this is a park next
to Bellingham Bay) on a warm summer day, what type of wind would you
expect to feel and why?
5. Why do sea breezes stop and land breezes begin after the sun goes down?
Explain thoroughly.
6. In a valley breeze, which direction does the wind blow and why? If you
draw a picture, make sure to include descriptions that explain and the time of
day!
7. In a mountain breeze, which direction does the wind blow and why? If
you draw a picture, make sure to include descriptions that explain and the
time of day!