What makes the wind blow? A difference in air pressure is what causes the wind to blow. Wind is simply air moving from a place of high air pressure to a place of low air pressure. The greater the difference in air pressure, the stronger the winds.
What Causes Differences in Air Pressure?A) Differential Heating Due to the curvature and tilt of the planet. Cold temperatures at the poles and hot conditions near the equator. Temperature extremes produce their own air pressure patterns. For instance, extreme cold produces high air pressure. Why? – Think about if you were to open a window in your bedroom on a cold and calm winters night and you stood away from the window in the middle of the room. On what part of your body would you first feel the cold? Near your feet. This is because cold air is dense and the air molecules are sinking. On a larger scale, a cold air mass pushing down on the earths surface creates an area of high air pressure.
What Causes Differences in Air Pressure?A) Differential Heating On the other hand, extreme heat produces low air pressure. – Think about the air rising above a camp fire or the heat radiating above an asphalt road on a hot summers day. The air molecules as they are heated, begin to expand and leave the earths surface putting less pressure on it. On a larger scale, this hot air creates an area of low air pressure.
What Causes Differences in Air Pressure?B) Processes of Precipitation Generate LowPressure In convectional precipitation, the heat of the sun warms the ground and the ground then heats the air above it. This causes the air molecules to rise leaving low pressure on the earths surface. Air from surrounding areas moves from higher pressure to low pressure (= wind)
What Causes Differences in Air Pressure? B) Processes of Precipitation Generate LowPressure With orographic precipitation it is not extreme heat that causes the air to rise. – Rather, the presence of mountains on the windward side forces the air to rise and create lower air pressure at the surface. – In contrast, on the rainshadow or leeward side of the mountains the air is descending and pushing down the earths surface producing higher air pressure in comparison to the windward side.
What Causes Differences in Air Pressure? B) Processes of Precipitation Generate LowPressure With frontal precipitation, it is the meeting of two different air masses that causes the air to rise. The warmer air mass is forced above the colder air mass leaving lower air pressure at the surface. Air from surrounding areas moves from higher pressure to low pressure (= wind)
What Causes Differences in Air Pressure?c) Convection Currents in the Atmosphere The deserts of the world are associated with high air pressure whether they are hot such as the Sahara or cold such as the Canadian Tundra. Recall that high air pressure is consistently associated with dry conditions because the air is descending. Air that is descending is not rising, cooling, condensing, and forming clouds readily. A convection current of air rising at the equator where the average annual temperature is even hotter than the Sahara desert. – Deserts, such as the Sahara, can get quite cool at night due to the loss of heat with clear skies. – The air that rises over the equator, cools, condenses, and forms clouds and considerable precipitation via convective processes. – After releasing its moisture, this air proceeds northward and descends over the Sahara desert generating high air pressure and dry conditions.
Global Pattern of Prevailing Surface Winds On a global scale, the prevailing winds at the surface follow the same principles described above namely that – wind is simply air moving from a place of high air pressure to a place of low air pressure – extreme temperatures produce their own pressure patterns – rising air is associated with low air pressure, and – subsiding air is associated with high air pressure
Global Pattern of Prevailing Surface Winds Coriolis Effect – Since the Earth is rotating on its axis: – Winds moving anywhere in the Northern hemisphere will turn to the right. – Winds moving anywhere in the Southern hemisphere will turn to the left.
Global Pattern of Prevailing Surface Winds A wind is always named for the direction that it is coming from. The winds near the equator are often relatively light, hence the area has been named the doldrums by sailors. Similarly, light winds under stable high air pressure near thirty degrees north and south have resulted in the regions being nicknamed the horse latitudes. – Some sailors, frustrated by their lack of progress under such conditions, would resort to throwing their horses overboard to lighten the load and presumably increase their speed; hence the nickname.
Low Pressure Areas Low pressure areas are also known as depressions or mid-latitude cyclones. In the northern hemisphere, low pressure areas rotate counterclockwise versus clockwise in the southern hemisphere. – This is similar for the direction water goes down a drain. – This is all due to the rotation of the earth about its axis. – On satellite photo, the low pressure areas are labeled with an (L). Low pressure areas are associated with clouds and precipitation which is evidence of unstable conditions.
High Pressure Areas On a satellite photo, the high pressure areas (H) are associated with clearer conditions. High means dry These areas are also known as ridges or anticyclones. – In the northern hemisphere, they rotate clockwise and the reverse in the southern hemisphere. They produce stable conditions in the atmosphere.
Local Scale Winds - Sea Breeze Land heats up faster than water. Consequently, as the land heats up on a clear, hot summers day, localized low air pressure develops as the air molecules expand and rise. In contrast, over the ocean the temperature is cooler and consequently the air pressure is relatively higher. Since wind is simply air moving from a place of higher air pressure to a place of lower air pressure, a local sea breeze develops often in the afternoon.
Local Scale Winds - Land Breeze Land cools down faster than water. Consequently, as the land cools down on a clear, summers night, localized high air pressure develops as the air molecules descend towards the earths surface. In contrast, over the ocean the temperature remains warmer and consequently the air pressure is relatively lower. The net result is a local land breeze as the air moves from the land to the sea.
Larger Scale Winds - Monsoons Same principles as land and seas breezes but on a large scale – a subcontinental scale. A monsoon is typically associated with India and its surrounding countries On the Indian subcontinent, there are two types of monsoons – a Southwest Summer Monsoon and – a Northeast Winter Monsoon. Over India during the Summer, it becomes extremely hot (i.e. 40 degrees Celsius). – This extreme heat generates low air pressure. In contrast, the Indian Ocean is relatively cooler and therefore has relatively higher air pressure. The air starts to move from the ocean to the land as this pressure differential is established. This is the onset of the monsoon season which brings torrential precipitation as moisture laden air moves onshore.
Larger Scale Winds - Monsoons On the other hand, for much of the rest of the year it is quite dry. This is due to an offshore wind blowing from the land to the sea. Northeast Winter Monsoon: extreme cold over the Himalayas and the Tibetan Plateau helps to form high air pressure. Over the Indian Ocean it is considerably warmer resulting in relatively lower air pressure. Again, air moves from a place of high air pressure to low air pressure.
Upper Air Westerlies and the Jet Stream Another key factor that influences our weather is upper level winds. Notice in the satellite images, the path of the jet stream as shown by the white line. In the northern hemisphere, the jet stream flows in a general west to east progression moving frontal systems along its path.
Chinook Winds Foehn winds Warm rainshadow winds that blow down the leeward side of mountain ranges – East side of Rockies Calgary, AB Air warms as it compresses back towards lower elevations