Seasonal Winds (World of Earth Science)
Awind in low-latitude climates that seasonally changes direction between winter and summer is called a monsoon, and is a typical example of seasonal winds. Monsoons usually blow from the land in winter (called the dry phase, because it carries cool, dry air), and to the land in summer (called the wet phase, because it carries warm, moist air), causing a drastic change in the precipitation and temperature patterns of the area.
The word "monsoon" originates from the Arabic mauzim, meaning season. It was first used to depict the winds in the Arabian Sea, but later it was extended for seasonally changing wind systems all over the world. The main reason for monsoons is the difference in the heating of land and water surfaces, which results in land-ocean pressure differences. On a small scale, heat is transferred by land-sea breezes, to maintain the energy balance between land and water. On a larger scale, in winter when the air over the continents is colder than over the oceans, a large, high-pressure area builds up over Siberia, resulting in air motion over the Indian Ocean and South China, causing dry, clear skies for East and South Asia. This is the winter monsoon. The opposite of this happens in summer; the air over the continents is much warmer than over the ocean, leading to moisture-carrying wind moving from the ocean towards the continent. When the humid air unites with relatively drier west airflow and crosses over mountains, it rises, reaches its saturation point, and thunderstorms and heavy showers develop. This is the summer monsoon in Southwest Asiaind blowing from the ocean to the continent with wet, rainy weather patterns.
Although the most pronounced monsoon system is in eastern and southern Asia, monsoons can also be observed in West Africa, Australia, or the Pacific Ocean. Even in the southwestern United States, a smaller scale monsoonal circulation system exists (called North American Monsoon, Mexican monsoon, or Arizona monsoon). The North American Monsoon is a regional-scale circulation over southwest North America between July and September, bringing dramatic increases in rainfall in a normally arid region of Arizona, New Mexico, and northwestern Mexico. It is a monsoonal circulation because of its similarities to the original Southwest Asian monsoonhe west or northwest winds turn more south or southeast, bringing moisture from the Pacific Ocean, Gulf of California and Gulf of Mexico. As the moist air moves in, it is lifted up due to the mountains, which, combined with daytime heating from the Sun, causes thunderstorms.
The monsoon is an important feature of atmospheric circulation, because large areas in the tropics and subtropics are under the influence of monsoons, bringing humid air from over the oceans to produce rain over the land. In highly populated areas (e.g., Asia or India), this precipitation is essential for agriculture and food crop production. Sometimes a strong monsoon circulation can also bring flooding. Or, if the monsoon is late in a certain year, it can cause droughts.
A similar phenomenon to the monsoon also occurs in a smaller spatial and temporal scale, the mountain and valley breezes. The main reason they occur is also the difference in heating of the areas: during the day, the valley and the air around it warm and, because it is less dense, the air rises, and thus, a gentle upslope wind occurs. This wind is called the valley breeze. If the upslope valley winds carry sufficient moisture in the air, showers, even thunderstorms can develop in the early afternoon, during the warmest part of the day. The opposite happens at night, when the slopes cool down quickly, causing the surrounding air also to cool and glide down from the mountain to the valley, forming a mountain breeze (also called gravity winds or drainage winds). Although technically any kind of downslope wind is called a katabatic (or fall) wind, this term is usually used for a significantly stronger wind than a mountain breeze.
For katabatic winds carrying cold air, their ideal circumstances are mountains with steep downhill slopes and an elevated plateau. If winter snow accumulates on the plateau, it makes the surrounding air very cold, which then starts to move down as a cold, moderate breeze and can become a destructive, fast wind if it passes through a narrow canyon or channel. These katabatic winds have different names in different areas of the world. The bora is a northeast cold wind with speeds of sometimes more than 115 miles per hour (100 knots), blowing along the northern coast of the Adriatic Sea, when polar cold air from Russia moves down from a high plateau, reaching the lowlands. The mistral is a similar, although less violent cold wind in France, which moves down from the western mountains into the Rhone Valley then out to the Mediterranean Sea, often causing frost damage to vineyards. Even in Greenland and Antarctica there are occasional cold, strong katabatic winds.
Among the katabatic winds carrying warm air, the chinook wind is a dry warm wind, moving down the eastern slope of the Rocky Mountains in a narrow area between northeast New Mexico and Canada. When strong westerly winds blow over a north-south mountain, it produces low pressure on the eastward side of the mountain, forcing the air downhill, and causing a compressional heating. The chinook causes the temperature to rise over an area sharply, resulting in a sharp drop in the relative humidity. If chinooks move over heavy snow cover, they can even melt and evaporate a foot of snow in less than a day. The chinook is important because it can bring relief from a strong winter, uncovering grass, which can be fed to the livestock. A similar wind in the Alps is called foehn, a dry, warm wind descending the mountain slope then flowing across flat lands below. A warm and dry wind in South California blowing from the east or northeast is called the Santa Ana wind (named from the Santa Ana Canyon). Because this air originates in the desert, it is dry, and becomes even drier as it is heated. Brush fires and dried vegetation can follow the Santa Ana wind.
See also Air masses and fronts; Land and sea breeze