Good Morning everyone who is a regular on zoomradar.com . I know I haven’t posted in a while but workload has increased here. Well anyway. Tonight I am going to write on Global wind circulations, regional winds and local winds that occur on our planet Earth. There are 3 important ways that determine the strength and direction of the wind: A – The difference between high and low pressure creates pressure gradient, B- The rotation of the Earth, and C – The friction at the surface of the Earth. Wind is defined as the direction the air is blowing, i.e. westerlies blow from west to east.
1.) GLOBAL CIRCULATION
1. The global circulation is dominated by the imbalance between the heating in the tropical regions and the cold arctic and Antarctic air at the Poles. Because of Earth’s rotation, the circulation does not form one cell or feature in each hemisphere. Instead three distinct feature exists. These features are as follows:
A. Hadley Cell and Tropics – Warm air rises in the tropics consistently throughout the year, yielding great big thunderstorms adjacent the equator, as these thunderstorms grow the rising air yields to descending air near 30 degrees North and South latitude. This zone here is responsible for all your tropical weather Hurricanes and Typhoons, Tropical Storms and Depressions. At this location easterly winds blow. How? here’s my explanation – Remember air descending at 30 degrees North right . This is where the Subtropical High belt is (anticyclone). From here, air at the surface flows both back toward the tropics – creating trade winds throughout our entire planet. The trade winds create an area of convergence and precipitation called the ITCZ or Intertropical Convergence Zones. The Birthplace of Tropical Cyclones.
B. Subtropical High Belt – The Subtropical High Pressure systems are created from descending Air from the tropics and the Hadley cells. These High Pressure zones are located or centered at 30 degrees North and South Latitude around our planet. They have a part in creating the trade winds when air at the surface back towards the tropics, and they have a part in creating the westerlies when air is blowing towards the Poles. remember from previous blogs Air Flow around High pressure is clockwise, so air is rotating in all directions. Great deserts are located here: The Sahara, and the Arabian deserts come to mind.
C. Westerlies – Belts of the westerlies winds lie on the Poleward side of the Subtropical Highs North in the Northern Hemisphere and South in the Southern Hemisphere. The latitudes they occupy is between 35 – 65 degrees North and South(particularly in the Northern Hemisphere). Westerlies vary in strength and position depending on ocean decadal oscillations that are present. For example in a positive AMO phase westerlies are strong blowing in a zonal flow west to east. In a negative AMO phase there is a more meridional flow north to south in the Eastern U.S. so westerlies are weaker in this phase. Westerlies however are all important for weather in these temperate regions. Like I said the prevailing winds are from the west and southwest in the Northern Hemisphere and the west and northwest in the Southern Hemisphere. These westerlies steer weather systems Eastward around the globe. Winds do rotate around High and Low pressure though, High pressure winds rotate clockwise and Low Pressure winds rotate counterclockwise. The opposite is true in the Southern Hemisphere. At this zone cloud cover and precipitation varies because weather is dominated by Low Pressure and High Pressure Systems.
D. Polar Easterlies – The cold Polar easterlies spread down to about 65 degrees Latitude North and South, where warming from the ocean produces Low pressure areas like the Aleutian Low and the Icelandic Low which are nearly stationary. I will say this though, both Lows are much stronger during winter than summer. the Siberian High is also strong and produces bitterly cold air in Siberia, Asian and Eastern Europe during the winter, and summer the Asian monsoon and Low Pressure take over these regions as the Siberian High weakens in summer. The Polar front is front is produced from sharp contrast in temperatures from the Cold Siberian Land mass and the Kuroshio current which is warm from japan and points south.(Aleutian Low). The icelandic Polar Front is created similarly by the North Atlantic drift, which is an extension of the Gulf Stream and the cold areas of Canada and Greenland.
E. Polar Highs – Polar Highs in part create the polar easterlies in part by rotating air clockwise equatorward. This process creates a sharp temperature boundary located at 65 degrees latitude both North and South. These features are responsible for bitterly cold air and Tundra to Ice Cap conditions. Note Antarctica is colder than the Arctic Region because it is the tallest average land region on Earth. The Arctic is surrounded by water which does radiate heat regardless of how cold it gets.
Here is a picture by goes-r.gov depicting what I just wrote about. I hope this clarifies anything you didn’t understand.
2. GLOBAL WINDS
A. Jet Streams – The Jetstream is found in the part of the atmosphere called the Tropopause. The tropopause is a region between the troposphere, which is the surface to about 35-40,000 feet(give or take 5000 feet) to the stratosphere which is above 45 to 50 thousand feet(give or take 5000 feet). The stratosphere is where find ozone or O3 which protects living things from the Sub’s UV rays. The troposphere and tropopause is highest over the tropics and lowest over the Polar Regions. Jetstreams form great temperature contrasts in both the subtropical regions and polar regions. Hence, Subtropical Jet and Polar Jet streams. The Subtropical Jet Stream is generally found near Subtropical High Belt and the Polar Jet Stream is usually found near the Polar Front. Jetstreams are fast-flowing rivers of air that can reach 200+mph that snake around the world in both hemispheres. Like polar Fronts they are variable in strength and may disappear at times. for example, he Polar Jet Stream is much stronger in the Winter than compared to the Summer. Because the General Circulation of the Earth shifts South during the winter. Severe Weather and Tornadoes as well as Blizzards are associated with the Polar Jet Stream. This depends what season you are in and where the Jetstream is located. If a location is southeast of the Jetstream, severe weather is usually the case, because of convergent air and the surface and divergent air aloft. Cyclogenesis or Low Pressure formation occurs here too. The Subtropical Jet is usually strengthen by an El Nino year. the stronger the El Nino generally the stronger the Subtropical Jetstream for the Southern Tier of the U.S. Snow can occur in the deep south and Appalachians. Severe weather can occur in Texas, Florida and even in Southern California. Finally the Jetstream has a great effect on storm growth, movement, maturity, and decay.
B. Coriolis Force – The Coriolis is an apparent wind force. The winds do not flow North to South and South to North. The winds or objects curve to the right in the Northern Hemisphere and curve to the Left in the Southern Hemisphere. The Coriolis Force is responsible for the spin or ROTATION of storms. For example Hurricanes and Typhoon develop their rotation or counterclockwise or clockwise rotation depending on what hemisphere you are in from the Coriolis Force. That is why you do not see tropical Cyclone development at the equator because the Coriolis is zero at the equator and gets stronger as you move poleward. The Coriolis Force is responsible for Global Wind Patterns. Air flowing from Mid-Latitude High-Pressure Zones(Subtropical Highs) towards the equator creates Trade winds, northeasterly in the Northern Hemisphere and Southeasterly in the Southern Hemisphere. Air flowing toward the Poles from te same Subtropical Highs creates the dominant Westerlies over temperate latitudes (35-65 degrees Latitude North and South). Finally, air flowing from the Poles toward the middle latitudes become Polar Easterlies.
C. Geostrophic Winds and Friction – Geostrophic Winds apply to air the free flows and rapidly flows above 1600-2000 feet. Generally where friction from land has NO effect. This wind freely flows around Low and High Pressure Systems unabated. Geostrophic wind Balance is exactly equal in strength to the Coriolis Force but acting in the opposite direction of the Pressure Gradient Force. Geostrophic Winds flow in Relatively straight paths, parallel to isobars(lines of equal pressure). Friction – occurs when winds are at the surface. Winds at the surface will be different in both strength and direction compared to Geostrophic winds. The lowest layer of Air is SLOWED due to friction and the Coriolis Force is reduced or eliminated. As a result winds no longer flow around High and Low Pressure areas. Instead, air flows out of High Pressure and into the Center of Low Pressure. Friction relative to geostrophic winds, the surface wind is backed in the Northern Hemisphere, veered in the Southern. The amount of change in the wind direction and speed depends upon the nature of the surface. Over the oceans, the change and speed is 10-20 degrees and over land it can reach 30-50 degrees. Topography effects wind as well. A North-South mountain Chain effects wind direction and speed. THe Rockies are a good example. The Rockies cause Low Pressure to form on the Lee side or downwind side. I.E. Lee Side Lows and Panhandle Hooks are two Rocky Mountain Low Pressure Systems.
Ths is an example of the average position of the Polar Jetstream at different seasons. The bottom location is where the Jet is located in the Winter. Notice how much stronger and amplified it is. Jet Stream winds average 125km/hr compared to 60km/hr in the summer. However for Northern Tier states severe weather can occur in the summer due to the proximity of the Polar Jet.
This is where the Geostrophic Winds are balanced. Air flow is Freely flowing around the High Pressure Center, opposite the Pressure Gradient Force but in Harmony with the Coriolis Force.
3. WIND SPEEDS
Meteorologists measure wind speeds on land at a standard height of 33 feet or 10 meters, but use other heights at sea(buoys). Speeds are usually quoted in knots and/or meters per second, the former for aviation purposes. For us civilians I am going to use Miles Per Hour or (MPH). There I a scale to measure wind called the Beaufort Scale that measure wind from calm wind o a gentle breeze all the way to storm conditions. I will provide graphics here to show the wind chart.
Here is the Beaufort Scale depicting various wind speeds and conditions. The scale shows how objects and structures are effected by the wind.
4. REGIONAL AND LOCAL WINDS OF THE WORLD
Regional and local winds affect and effect areas that range for 10s of miles to 100s of miles. The first I am going to talk about is the:
A. Sea and Land Breezes – Sea and land breezes develop due to the differential heating of the land and water. Sea Breeze – A good example is the Sea Breeze Convergent Boundary in Florida. During the Summer wet season there is a large thermal gradient or temperature difference between the very hot land and the relatively cooler waters(80s)s compared to upper 90s land temperatures inland. So in the Daytime the land heats much faster than the water, so Low Pressure develops on Land and High pressure develops over the relatively cooler waters. Remember above Air and wind flow from High to Low Pressure. Thus we have a onshore flow. In Florida and the Southeast in the Summer Sea Breeze fronts for on this flow and initiate thunderstorms in the late mornings to afternoon time periods. Actually the sea breezes on the West Coast are stronger than the East Coast due to a stronger temperature gradient. The only reason why it does not rain out west due to this phenomenom is because the West coast has a cold ocean current which lends to absolute stability. In the East we have the Gulf stream which produces instability and storms. So the Sea Breeze develops due to a differential in Land and Ocean temperatures. The Air and Wind flow is ONSHORE, and Showers and Thunderstorms develop where unstable air is present(SouthEast Coast of U.S.). Temperature gradient in the West Coast brings in stable marine inversion layer. Fog in summer out West can last to the afternoon hours.
LandBreeze – This is the opposite of Seabreeze. At night the Land cools much faster than water because water has a High Specific Heat Capacity – in such in holds on to heat more effectively than Land. Ok, so High Pressure over land due to rapid cooling and Low Pressure over the Ocean due to warm temperatures there. Winds blow from High Pressure to Low Pressure. So, we have a OFFSHORE wind, these winds blow form Land to Sea. Land breezes are weaker over West Coast than the East Coast, because at night the land cools off much faster than water, water stays relatively warm.
Lake Breezes – Large areas of deep water, such as the Great Lakes in North America, produce their own systems of winds, similar to Land and Sea Breezes due to temperature gradient contrasts like sea breezes. Summer convective thunderstorms can be produced by these winds.
B. Valley Winds and Mountain Winds –
Valley winds – arise when hillsides or mountainsides are heated by the Sun on a direct angle. The warm air rises, and a wind starts to blow either along its length up the slope toward the High Ridge top, or towards the head. If the warm humid air is lifted above the condensation level where Relative Humidity is 100% and clouds form along the ridge rain can precipitation can occur. A similar process sometimes occurs when the valley has been filled with shallow overnight fog. The warmth of the sunlight mixes out the fog and forms patchy low stratus clouds. These then drift up the valley and mountainsides, producing a line of stratus along the slopes of Mountains. actually valley breezes can initiate big thunderstorms in the mountains, Climatology in the Rocky mountains has a summer convective Max due to valley breeze thunderstorms.
Mountain winds – The are cool winds compared to valley winds. Mountain winds are the nighttime counterpart to valley winds. At night, the upper slopes of mountains and valleys cool more quickly than the more sheltered valleys by radiating heat away to space. he air in contace with them is cooled, becomes denser, and heavier and slides or rolls downhill. Such winds begin shortly after sunset and continue after dawn. If several valleys drain into 1 larger one, there may be a considerable flow of air down toward the plains, but wind speeds tends to be les than corresponding valley winds(up to 7 mph).
C. Chinook/Fohn Winds and Fall/Katabatic Winds –
Chinook/Fohn – Chinook winds and Fohn winds are the same dry warm warms. Chinook in North America and Fohn in Europe. Thiese winds are produced when air rises over mountains, produces precipitation on the windward side, but when it starts to descend to the leeward, it loses all of its moisture. Chinook in Native American language means “snow eater”. Just as air cools as it rises, it warms and dries when air descends, but dry air warms faster than saturated air. When air is drawn across a mountain range , the air to leeward to downward side is much warmer and drier than at the same level as the windward side. This is called adiabatic warming. Such winds are known as Fohn winds, term used from the Alps in Europe. The sudden arrival of a Fohn may cause a dramatic rise in temperature and cause a sudden thaw and rapid snow melt. The Chinook which is the same as Fohn is the same descending lee side wind having the same effect.
D. Fall/Katabatic – Fall or katabatic winds are cold winds compared to the Fohn/Chinook counterparts. These arise when cold air has accumulated over high land or very high palteaus, cascades downhill, sometimes with the help of a strong pressure gradient. Despite some warming with its descent, such winds remain cold due to the extreme cold it originated from. These winds maybe extremely violent, especially in Antarctica, where Commonwealth Bay(Home of the Blizzard) has recorded wind speeds of 200+mph. Examples of Katabatic winds are the Mistral Wind in the Rhone Valley in France, and the Bora Winds in the Alps.
This is a perfect example of sea breeze in action. High pressure over the Ocean or water and Low Pressure over the Land.