9-1 CHAPTER 9. LOCAL WINDS
9.1 Description. 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 9-1).
Figure 9-1. Local Wind Circulation
Low-level winds develop in the direction of the Pressure Gradient Force (PGF). Coriolis force is insignificant because the circulation’s dimension (less than 100 miles) and life span (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.
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9.2 Hazards. Local winds can produce aviation weather hazards. Turbulence and shifting surface winds are common. Clouds and precipitation (including thunderstorms) can develop in the rising air over the warmer surface given sufficient moisture and lift.
9.3 Sea Breeze. A sea breeze (see Figure 9-2) is a coastal local wind that blows from sea to land, and 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 9-2. 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.
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9.3.1 Sea Breeze Front. A sea breeze front (see Figure 9-3) 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.
Figure 9-3. 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.
9.3.2 Effects of Coastline Shape. Locally, the shape of the coastline plays an important role in the development of convection along sea breezes (see Figure 9-4). 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 formed along opposing shores merge near the peninsula’s or island’s center.
9-4 Figure 9-4. Effects of Coastline Shape on a Sea Breeze
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.
9.4 Land Breeze. A land breeze (see Figure 9-5) 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.
9-5 Figure 9-5. 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.
9.5 Lake Breeze. A lake breeze (see Figure 9-6) 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.
9-6 Figure 9-6. 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).
9-7 Figure 9-7. Sea Breeze/Lake Breeze Example
The sinking air behind the lake breeze inhibits clouds over Lake Ontario and Lake Erie and for miles inland (National Aeronautics and Space Administration (NASA)).
9.6 Valley Breeze. A valley breeze (see Figure 9-8) 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 9-8. Valley Breeze
9-8 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.
9.7 Mountain-Plains Wind System. A mountain-plains wind system (see Figure 9-9) 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.
Figure 9-9. 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.
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9.8 Mountain Breeze. A mountain breeze (see Figure 9-10) 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). Cooler air over the sloping terrain is denser than warmer air over the valley.
Figure 9-10. 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.
10-1 CHAPTER 10. AIR MASSES, FRONTS, AND THE WAVE CYCLONE MODEL
10.1 Air Masses. An air mass is a large body of air with generally uniform temperature and humidity. The area from which an air mass originates is called a source region.
Air mass source regions range from extensive snow-covered polar areas to deserts to tropical oceans. The United States is not a favorable source region because of the relatively frequent passage of weather disturbances that disrupt any opportunity for an air mass to stagnate and take on the properties of the underlying region. The longer the air mass stays over its source region, the more likely it will acquire the properties of the surface below.
10.1.1 Air Mass Classification. Air masses are classified according to the temperature and moisture properties of their source regions (see Figure 10-1).
10.1.1.1 Temperature Properties:
• Arctic (A)—An extremely deep cold air mass which develops mostly in winter over arctic surfaces of ice and snow.
• Polar (P)—A relatively shallow cool to cold air mass which develops over high latitudes.
• Tropical (T)—A warm to hot air mass which develops over low latitudes.
10.1.1.2 Moisture Properties:
• Continental (c)—A dry air mass which develops over land.
• Maritime (m)—A moist air mass which develops over water.
10.1.1.3 Five Air Masses. When this classification scheme is applied, the following five air masses may be identified:
• Continental Arctic (cA)—Cold, dry.
• Continental Polar (cP)—Cold, dry.
• Continental Tropical (cT)—Hot, dry.
• Maritime Polar (mP)—Cool, moist.
• Maritime Tropical (mT)—Warm, moist.
Note: Maritime Arctic (mA) is not listed since it seldom, if ever, forms.
10-2 Figure 10-1. Air Mass Classification
10.1.2 Air Mass Modification. As these air masses move around the Earth, they can begin to acquire different attributes. For example, in winter an arctic air mass (very cold and dry air) can move over the ocean, picking up some warmth and moisture from the warmer ocean and becoming a maritime polar (mP) air mass—one that is still fairly cold but contains moisture. If that same polar air mass moves south from Canada into the southern United States, it will pick up some of the warmth of the ground, but due to lack of moisture it remains very dry. This is called a continental polar (cP) air mass.
The Gulf Coast states and the eastern third of the country commonly experience the tropical air mass in the summer. Continental tropical (cT) air is dry air pumped north, off of the Mexican Plateau. If it becomes stagnant over the Midwest, a drought may result.
Maritime tropical (mT) air is air from the tropics which has moved north over cooler water.
A warm, moist air mass moving over a cold surface (see Figure 10-2) produces stable air associated with stratiform clouds, fog, and drizzle.
Figure 10-2. Air Mass Modification—Warm, Moist Air Mass Moving Over a Cold Surface
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10.1.2.1 Lake Effect. Lake effect is the effect of any lake in modifying the weather near its shore and for some distance downwind. In the United States, the term is applied specifically to the region around the Great Lakes, and sometimes the Great Salt Lake in Utah. A lake effect can sometimes generate spectacular snowfall amounts to the lee of the Great Lakes. This phenomenon is termed lake effect snow.
In fall and winter, cumuliform clouds and showers often develop in bands over, and to the lee of, large, ice-free lakes (see Figure 10-3). As initially cold, dry, stable polar air over land flows over the relatively warm water, the air is heated and moistened, and stability decreases. Shallow cumuliform clouds develop with low tops. The strength of the convection increases with increasing temperature differences between warm water and cold air, increasing wind speeds, and decreasing relative humidity within the cold, dry air.
Figure 10-3. Lake Effect
10.2 Fronts. Air masses can control the weather for a relatively long time period ranging from days to months. Most weather occurs along the periphery of these air masses at boundaries called fronts. A front is a boundary or transition zone between two air masses. Fronts are classified by which type of air mass (cold or warm) is replacing the other (see Figure 10-4).