Local winds result from air moving between small low and high pressure systems. High and low pressure cells are created by a variety of conditions. Some local winds have very important effects on the weather and climate of some regions.
Land and Sea Breezes
Since water has a very high specific heat, it maintains its temperature well. So water heats and cools more slowly than land. If there is a large temperature difference between the surface of the sea (or a large lake) and the land next to it, high and low pressure regions form. This creates local winds.
· Sea breezes blow from the cooler ocean over the warmer land in summer. Where is the high pressure zone and where is the low pressure zone (Figure below)? Sea breezes blow at about 10 to 20 km (6 to 12 miles) per hour and lower air temperature much as 5 to 10oC (9 to 18oF).
· Land breezes blow from the land to the sea in winter. Where is the high pressure zone and where is the low pressure zone? Some warmer air from the ocean rises and then sinks on land, causing the temperature over the land to become warmer.
How do sea and land breezes moderate coastal climates?
Land and sea breezes create the pleasant climate for which Southern California is known. The effect of land and sea breezes are felt only about 50 to 100 km (30 to 60 miles) inland. This same cooling and warming effect occurs to a smaller degree during day and night, because land warms and cools faster than the ocean.
Monsoon winds are larger scale versions of land and sea breezes; they blow from the sea onto the land in summer and from the land onto the sea in winter. Monsoon winds occur where very hot summer lands are next to the sea. Thunderstorms are common during monsoons (Figure below).
In the southwestern United States relatively cool moist air sucked in from the Gulf of Mexico and the Gulf of California meets air that has been heated by scorching desert temperatures.
The most important monsoon in the world occurs each year over the Indian subcontinent. More than two billion residents of India and southeastern Asia depend on monsoon rains for their drinking and irrigation water. Back in the days of sailing ships, seasonal shifts in the monsoon winds carried goods back and forth between India and Africa.
Mountain and Valley Breezes
Temperature differences between mountains and valleys create mountain and valley breezes. During the day, air on mountain slopes is heated more than air at the same elevation over an adjacent valley. As the day progresses, warm air rises and draws the cool air up from the valley, creating a valley breeze. At night the mountain slopes cool more quickly than the nearby valley, which causes a mountain breeze to flow downhill.
Katabatic winds move up and down slopes, but they are stronger mountain and valley breezes. Katabatic winds form over a high land area, like a high plateau. The plateau is usually surrounded on almost all sides by mountains. In winter, the plateau grows cold. The air above the plateau grows cold and sinks down from the plateau through gaps in the mountains. Wind speeds depend on the difference in air pressure over the plateau and over the surroundings. Katabatic winds form over many continental areas. Extremely cold katabatic winds blow over Antarctica and Greenland.
Chinook winds (or Foehn winds) develop when air is forced up over a mountain range. This takes place, for example, when the westerly winds bring air from the Pacific Ocean over the Sierra Nevada Mountains in California. As the relatively warm, moist air rises over the windward side of the mountains, it cools and contracts. If the air is humid, it may form clouds and drop rain or snow. When the air sinks on the leeward side of the mountains, it forms a high pressure zone. The windward side of a mountain range is the side that receives the wind; the leeward side is the side where air sinks.
The descending air warms and creates strong, dry winds. Chinook winds can raise temperatures more than 20oC (36oF) in an hour and they rapidly decrease humidity. Snow on the leeward side of the mountain melts quickly. If precipitation falls as the air rises over the mountains, the air will be dry as it sinks on the leeward size. This dry, sinking air causes a rainshadow effect (Figure below), which creates many of the world’s deserts.
As air rises over a mountain it cools and loses moisture, then warms by compression on the leeward side. The resulting warm and dry winds are Chinook winds. The leeward side of the mountain experiences rainshadow effect.
Santa Ana Winds
Santa Ana winds are created in the late fall and winter when the Great Basin east of the Sierra Nevada cools, creating a high pressure zone. The high pressure forces winds downhill and in a clockwise direction (because of Coriolis). The air pressure rises, so temperature rises and humidity falls. The winds blow across the Southwestern deserts and then race downhill and westward toward the ocean. Air is forced through canyons cutting the San Gabriel and San Bernardino mountains. (Figure below).
The winds are especially fast through Santa Ana Canyon, for which they are named. Santa Ana winds blow dust and smoke westward over the Pacific from Southern California.
The Santa Ana winds often arrive at the end of California’s long summer drought season. The hot, dry winds dry out the landscape even more. If a fire starts, it can spread quickly, causing large-scale devastation (Figure below).
In October 2007, Santa Ana winds fueled many fires that together burned 426,000 acres of wild land and more than 1,500 homes in Southern California.
High summer temperatures on the desert create high winds, which are often associated with monsoon storms. Desert winds pick up dust because there is not as much vegetation to hold down the dirt and sand. (Figure below). A haboob forms in the downdrafts on the front of a thunderstorm.
A haboob in the Phoenix metropolitan area, Arizona.
Dust devils, also called whirlwinds, form as the ground becomes so hot that the air above it heats and rises. Air flows into the low pressure and begins to spin. Dust devils are small and short-lived, but they may cause damage.
· Water has high specific heat, so its temperature changes very slowly relative to the temperature of the land. This is the reason for sea and land breezes and monsoon winds.
· The cause of all of these winds is the differential heating of Earth's surface, whether it's due to the difference in water and land, the difference with altitude, or something else.
· Winds blow up and down slope, on and off land and sea, through deserts or over mountain passes.
In order to access referenced diagrams please click on the link below.
Mountain and Valley Breezes
In a similar behavior as land and sea breezes, mountains (hilly) and surrounding valleys also have breezes called Mountain and Valley Breezes.
During the day, the air over the mountain slope heats up more than the air at the foot of the mountain. The warm air over the slope reduces in density. A low pressure is created at the top of the mountain and high pressure from the cool air below forces a cool breeze to move upward. This condition generates a breeze which we call Valley breeze, and it is very common during warmer months when there is a lot of heating from the sun.
In the night, it is a lot cooler as the sun goes to sleep. So the air at the upper slope of the mountain cools off very quickly and becomes dense. A high pressure is created. At this time, the air at the valley floor is a lot warmer (low pressure) and is forced to give way to colder air moving down the slope towards the valley floor. This is called mountain breeze, and it is a lot common in the colder months when there is less warming from the sun.
In order to access referenced diagrams please click on the link below.
Hurricane Formation: How Hurricanes Form in the Sahara Desert
The Birth of Atlantic Hurricanes
Contributed by Associate Writer Sharon Tomlinson
In the United States, the eastern and Gulf coasts are in danger of being slammed by hurricanes during hurricane season from June through November. But why?
Hurricanes that hit the eastern coast of the United States are born many miles away in the Sahara desert. The waters in the North Atlantic Ocean are typically at their warmest while the Sahara is at its hottest from June through November, so the chances of a hurricane are highest during those times.
Before we begin, if you need extra help, or need to see a video on hurricane formation, click this link - Hurricane Formation Videos. Sometimes seeing the images in action can further clarify understanding.
A hurricane is a complex weather system that can be simply explained as a funnel of warm, wet air.
Think of your sink, and the water swirling down the drain; now, put that swirl up in the sky. Oh, and magnify it about a thousand times in size and speed. A hurricane is a non-frontal system whose air has a distinct circular flow. It is created for those of us in the USA, when hot air over the Sahara is released into the North Atlantic.
The Sahara, whose land mass is almost that of the continental United States, is the largest “hot” desert in the world. It is also the second largest desert overall. (Antarctica is classified as a “cold” desert.) In the Sahara, there is an art to living where there is little to no water, and the day-night-day temperatures can swing 30 degrees in a few hours. There are great swirling winds over the Sahara carrying sands over the Mediterranean, bringing storms into England, and dropping sands on the beaches of eastern Florida.
The greatest mass of the Sahara lies in the horse latitudes. Defined by light winds and hot dry weather, the name came from the calm on water during which sailing ships were said to have jettisoned their cargoes. It is thought that sailors would eat horses in transit to survive until winds picked up and moved them to their destinations.
The temperatures of the land mass of western North Africa grows so hot the air over this area rises to create the Africa easterly jet. Here is the womb of the mother Sahara, and here is where the hurricanes are delivered.
A column of hot air swirls upward three miles and spreads as it races to the west coast where it dips toward the ocean. The air picks up moisture from the warm waters and continues its race westward. The flow of the ocean and the spin of the earth combined with the dry winds of the desert and the warm moist air off the Atlantic horse latitudes make this desert child grow. It spins and flies over the water, sometimes it spends its life out over the open ocean, never reaching landfall.
When it does reach land, the winds can have grown and the entire size of the storm can reach massive proportions. The storm receives a name as any child receives a name. It is never just a windstorm. Broken away from the safety of its mother, the desert, and father, the ocean, it will beat itself to death over land, but not before doing great damage and spawning minor storms and tornadoes of its own.
Across the ocean, on the other side of the Atlantic, the Sahara continues to birth these great storms during the fertile season from June through November. As the winds return to just carrying sand across the waters, the United States can breathe a sigh of relief as the desert rests.
Local Winds Quiz
Please answer 30 points worth of questions in paragraph form
1. Please explain the following characteristics of Low Pressure air systems in the Northern Hemisphere. Be sure to include; Temperature, Volume, Density, Change in Altitude, direction of Rotation, ability to hold water vapor, and the altitude where divergence occurs. (5 points)
2. Please explain the following characteristics of High Pressure air systems in the Northern Hemisphere. Be sure to include; Temperature, Volume, Density, Change in Altitude, direction of Rotation, ability to hold water vapor, and the altitude where divergence occurs. (5 points)
3. Please draw and explain how a Land/Sea Breeze phenomenon occurs. Be sure to include cloud /fog development. (10 points)
4. Please draw and explain how a Mountain/Valley wind phenomenon occurs. Be sure to include cloud /fog development. (10 points)
5. Please draw and explain how a how a rain Shadow phenomenon occurs. Be sure to include cloud /fog development. (20 points)
6. Please explain how the Sahara Desert contributes to hurricane formation in the Atlantic Ocean. Be sure to include high and low pressure system development, storm surge, what keeps the hurricane going and what changes lead to its destruction (30 points)