Now that we have examined Earth’s large-scale circulation, let us turn briefly to winds that influence much smaller areas. Remember that all winds are produced for the same reason: pressure differences that arise because of temperature differences caused by unequal heating of Earth’s surface. Local winds are small-scale winds produced by a locally generated pressure gradient. Those described here are caused either by topographic effects or by variations in surface composition in the immediate area.

Land and Sea Breezes

In coastal areas during the warm summer months, the land is heated more intensely during the daylight hours than is the adjacent body of water. As a result, the air above the land surface heats, expands, and rises, creating an area of lower pressure. A sea breeze then develops because cooler air over the water (higher pressure) moves toward the warmer land (lower pressure) (▼A). The sea breeze begins to develop shortly before noon and generally reaches its greatest intensity during the mid- to late afternoon. These relatively cool winds can be a significant moderating influence on afternoon temperatures in coastal areas. Small-scale sea breezes can also develop along the shores of large lakes. People who live in a city near the Great Lakes, such as Chicago, recognize this lake effect, especially in the summer. They are reminded daily by weather reports of the cooler temperatures near the lake as compared to warmer outlying areas. At night, the reverse may take place: The land cools more rapidly than the sea, and the land breeze develops (▼B).

Mountain and Valley Breezes

A daily wind similar to land and sea breezes occurs in many mountainous regions. During daylight hours, the air along the slopes of the mountains is heated more intensely than the air at the same elevation over the valley floor. Because this warmer air is less dense, it glides up along the slope and generates a valley breeze (▼A). The occurrence of these daytime upslope breezes can often be identified by the cumulus clouds that develop on adjacent mountain peaks.

After sunset, the pattern may reverse. Rapid radiation cooling along the mountain slopes produces a layer of cooler air next to the ground. Because cool air is denser than warm air, it drains downslope into the valley. This movement of air is called a mountain breeze (▼B). The same type of cool air drainage can occur in places that have very modest slopes. The result is that the coldest pockets of air are usually in the lowest spots. Like many other winds, mountain and valley breezes have seasonal tendencies. Although valley breezes are most common during the warm season, when solar heating is most intense, mountain breezes tend to be more dominant in the cold season.

Chinook and Santa Ana Winds

Warm, dry winds sometimes move down the eastern slopes of the Rockies, where they are called chinooks. Such winds are often created when a strong pressure gradient develops in a mountainous region. As the air descends the leeward slopes of the mountains, it is heated adiabatically (by compression). Because condensation may have occurred as the air ascended the windward side, releasing latent heat, the air descending the leeward slope will be warmer and drier than it was at a similar elevation on the windward side. Although the temperature of these winds is generally less than 10°C (50°F), which is not particularly warm, the winds occur mostly in the winter and spring, when the affected areas may be experiencing below-freezing temperatures. Thus, by comparison, these dry, warm winds often bring a drastic change. When the ground has a snow cover, these winds are known to melt it in short order. A Chinook-like wind that occurs in southern California is the Santa Ana. This hot, desiccating wind greatly increases the threat of fire in this already dry area (▼). The warm, dry downslope Chinook-like winds are not unique to the United States; Foehn winds blowing over the Alps warm areas of Austria, southern Germany, and Switzerland, and in Argentina, zonda winds occur over the eastern slopes of the Andes Mountains.

• Local winds are small-scale winds produced by a locally generated pressure gradient. Sea and land breezes form along coasts and are brought about by temperature contrasts between land and water. Valley and mountain breezes occur in mountainous areas where the air along slopes heats differently than does the air at the same elevation over the valley floor. Chinook and Santa Ana winds are warm, dry winds created when air descends the leeward side of a mountain and warms by compression.

• chinooks: Local winds blowing down the leeward side of a mountain due to strong pressure gradients, and that are adiabatically warmed (by compression).

• land breeze: A local wind blowing from land toward the water during the night in coastal areas.

• local winds: Small-scale winds produced by a locally generated pressure gradient. Examples include land and sea breezes and mountain and valley breezes.

• mountain breeze: The nightly downslope winds commonly encountered in mountain valleys.

• Santa Ana: The local name given to the strong chinook winds in southern California.

• sea breeze: A local wind blowing from the sea towards the land during the afternoon in coastal areas.

• valleybreeze: The daily upslope winds commonly encountered in a mountain valley.