Jet Stream and Polar Front
Jet streams are fast flowing, narrow air currents found in the atmospheres of some planets, including Earth. The main jet streams are located near the tropopause. The major jet streams on Earth are westerly winds (flowing west to east). Their paths typically have a meandering shape; jet streams may start, stop, split into two or more parts, combine into one stream, or flow in various directions including the opposite direction of most of the jet. The strongest jet streams are the polar jets, at around 7–12 km (23,000–39,000 ft) above sea level, and the higher and somewhat weaker subtropical jets at around 10–16 km (33,000–52,000 ft). The northern hemisphere and the southern hemisphere each have both a polar jet and a subtropical jet. The northern hemisphere polar jet flows over the middle to northern latitudes of North America, Europe, and Asia and their intervening oceans, while the southern hemisphere polar jet mostly circles Antarctica all year round.
Jet streams are caused by a combination of a planet’s rotation on its axis and atmospheric heating (by solar radiation and, on some planets other than Earth, internal heat). The Coriolis effect describes how a planet’s surface and atmosphere rotate fastest at the planet’s equator while virtually not rotating at all at the poles. Jet streams form near boundaries of adjacent air masses with significant differences in temperature, such as the polar region and the warmer air towards the equator.
Flight time can be dramatically affected by either flying with the flow or against the flow of a jet stream. Clear-air turbulence, a potential hazard to aircraft passenger safety, often is found in a jet stream’s vicinity.
Polar jet streams are typically located near the 250 hPa pressure level, or 7 kilometres (4.3 mi) to 12 kilometres (7.5 mi) above sea level, while the weaker subtropical jet streams are much higher, between 10 kilometres (6.2 mi) and 16 kilometres (9.9 mi) above sea level. In each hemisphere, both upper-level jet streams form near breaks in the tropopause, which is at a higher altitude near the equator than it is over the poles, with large changes in its height occurring near the location of the jet stream. The northern hemisphere polar jet stream is most commonly found between latitudes 30°N and 60°N, while the northern subtropical jet stream located close to latitude 30°N. The upper level jet stream is said to “follow the sun” as it moves northward during the warm season, or late spring and summer, and southward during the cold season, or autumn and winter.
The width of a jet stream is typically a few hundred miles and its vertical thickness often less than three miles.
The wind speeds vary according to the temperature gradient, exceeding 92 kilometres per hour (50 kn), although speeds of over 398 kilometres per hour (215 kn) have been measured. Meteorologists now understand that the path of jet streams steers cyclonic storm systems at lower levels in the atmosphere, and so knowledge of their course has become an important part of weather forecasting. For example, in 2007, Britain experienced severe flooding as a result of the polar jet staying south for the summer.
The polar and subtropical jets merge at some locations and times, while at other times they are well separated.
Associated with jet streams is a phenomenon known as clear air turbulence (CAT), caused by vertical and horizontal windshear connected to the jet streams.
The CAT is strongest on the cold air side of the jet, next to and just underneath the axis of the jet.
There are two factors which create CAT – a rapid change of wind speed and a rapid change of wind direction. A curved jet steam provides both factors. Furthermore, an associated ‘deep trough’ implies that there is a strong pressure gradient which also increases wind speed.
Remember, high air mass temp – high pressure aloft, low air mass temp – low presure aloft.
So the sharp differences in the temps of the whole air masses between sub-tropical and polar generate big pressure differences up near the tropopause. This means there is a big horizontal pressure difference across the front and therefore a big jet.
Actually in the jet, across the jet core, there is very little horizontal temp difference. There is, however, a big horizontal pressure difference.
The polar front jet core is in the warm air but above the cold air tropopause at a point where the temperature in the warm air troposphere is the same as the cold air stratosphere. If the cold tropopause is -40?C then above that point in the cold sector the temperature is constant. In the warm air at the same level as the cold tropopause the temperature will be warmer, say -34?C, but will decrease with altitude. At the point where the temperature in the warm air is the same as the -40?C in the cold air stratosphere you will find the strongest wind speed.
During the Summer the North Atlantic polar front runs from “Newfoundland to Norway” but in the Winter it runs from “Florida to Folkston” so it moves South. The cold polar air is colder in the winter than in the summer. The warm sub-tropical air over the ocean changes temperature very little between winter and summer but, because it is being sourced from a lower latitude in the winter, it tends to be slightly warmer. This gives a greater temperature contrast between the cold air and the warm air in the winter and, therefore, a stronger thermal wind component.
Types of Jet Streams
The Polar Jet Streams: These Jet Streams are located 50°-60° North/South of the equator and at 35,000 Feet (About 6.6 Miles), is a powerful, current of wind that acts as a boundary line, separating the extremely cold polar air (North) from the warm sub-tropical air (South). The Speed of the Polar Jet Stream varies depending on the time of the year. When it is winter in the Northern hemisphere it can reach up to 300 miles an hour but it has been measured at a speed of 400 miles an hour in southwest Scotland.
The Subtropical Jet Stream: These strong jet streams are located at about 30° North/South of the equator at a height of about 41,000 feet (About 8.1 Miles) and along with the Polar Jet Stream is responsible for many types of weather patterns, depending on the time of the year.
The Equatorial Jet Stream: The equatorial Jet Stream is located 7° to 10° North of the Equator at an altitude of 50,000 Feet (About 9.5 Miles). This stream only travels from Asia to Africa and is the strongest during the summer months of July and August. This Stream travels in an easterly direction and is formed when there is a great temperature change with the coldest air nearest the equator.
|Intense summer heating of Indian sub-continent and coast of West Africa transfers heat to upper air where a marked temperature difference develops between upper air over land and sea. Hot air (over land) is north
of cold air (over sea) so result is an easterly jet.
The African Easterly Jet Stream: The African Easterly Jet Stream forms at an altitude of 12,000 to 15,000 Feet and is sometimes present during the summer months of July and August. This jet stream is located right under the Equatorial Jet Stream at (About 2.2 to 2.9 Miles) high.
The Polar-Night Jet Stream: This jet stream is only active during the winter months of the Northern Hemisphere and is located at 60° North only, at a height of about 80,000 feet (About 15.2 Miles).
Arctic Jet Stream: In winters, long polar night cools the polar air forming an arctic air mass between (roughly) 70 deg latitude and poles. The temperature difference between polar and arctic air causes arctic jet stream. The core is at 20,000 feet and max speed around 80 knots.
Polar Jet Stream typically found at 30,000 feet with speed of around 200 knots whereas Sub-Tropical Jet found at 40,000 feet with speeds around 100 knots.
Jet Streaming Along the Polar Front
The polar front and its associated jet stream have a major influence on the weather conditions surrounding it. Many storms form along the polar front in the vicinity of the jet stream’s maximum winds. Because of the association of the jet stream with the polar front, media weather maps often include the jet stream’s position as a rough indicator of continental divisions between warm and cold air masses. Here is why.
From a climatological viewpoint, the position of the polar front forms a more-or-less even band around the globe at mid-latitudes in both hemispheres. The average polar-front position slips north and south with the seasons in response to the annual hemispheric heating cycle. Like my waistline’s annual cycle, the polar frontal belt shrinks poleward in the summer months and then expands toward the subtropics during the winter.
From a meteorological viewpoint, however, the polar front is not an evenly encircling latitude belt but an undulating zone around the globe that is ever-changing as masses of warm and cold air push away from their regions of origin. Of particular interest to meteorologists and weather forecasters is the ever-changing pattern of long-waves that form around the polar front. These long-waves, very visible on polar projection maps, undulate around the hemisphere with three to six wave cycles. (Long waves are also known as Rossby waves in honour of Swedish meteorologist C.G. Rossby who gave us many early insights into the impacts of upper atmosphere features on weather.)
At times, that global belt fits tight, having three or four, small-amplitude undulations (little north-south latitude variation) around the hemisphere. At other times, it has as many as six large-amplitude loops (great north-south variation). How those wave loops sit over the hemisphere, or portion thereof, determines what temperature regimes are experienced on the surface below.
When a long section of the polar front, as seen on continental weather maps, is smooth like the surface of a calm sea, the upper-level winds, including associated jet streams, run generally parallel to the latitude lines, a condition called zonal flow. Under zonal-flow, north-south undulations of the frontal boundary are small, and the surface temperatures across the continent, as seen in the isotherm (lines of equal temperature) pattern on the weather map, layer in zonal (east-west) bands with warm air to the south and cold air to the north.
But like occupants of a small boat riding that calm surface, we may be lulled during persistent zonal flow by a stretch of nearly indistinguishable day-to-day temperature changes, often remaining near the climatological mean for that time of year. Despite the unfounded desires of those living in the mid-latitudes to believe there is a truly “normal” state of weather from which departures are abnormal, such stable zonal patterns are but a temporary state. We live not in a meteorological Camelot, where it never rains til after sundown — or the Tropics where it always rains in late afternoon — but in a region subject to very changeable-weather regimes.
Thus, when a zonal flow pattern cuts off the exchange of heat between the polar region and the tropics, great thermal contrasts develop across the polar front. And this becomes the zonal pattern’s downfall. Eventually, some chink, some small perturbation, develops in the zonal pattern, and a short wave, with initially small north-south extent, arises upon the polar front. If the short wave amplifies (grows in size), it can distort the flow into a new pattern that crosses the latitude lines, a pattern termed meridional. In meridional flow, cold air rushes southward while warm air streams northward.
As the polar-front meridional pattern rises into Rossby waves of deep north-south extent, it severely rocks our “temperature boat.” When the wave, and associated jet stream, plunges southward, we find ourselves in a deep upper-level trough with temperatures quite cold below it. When we rise high on the upper-level ridge, or crest of the passing Rossby wave, the surface experiences warm, even hot, temperatures.
Often, the intense temperature changes brought by the meridional flow are expressed by frontal patterns associated with a surface low-pressure system: warm air moving northward behind the warm front; cold air descending southward behind the cold front. Not only are cyclonic storms spawned in the vicinity of the peak jet stream winds (called jet streaks), but they are often pushed rapidly eastward by the jet stream above.
So when you look at a weather map with the polar jet stream position drawn on it, you can generally tell what type of weather you will be having. If north of the jet, it should be relatively cold, if south, warmer conditions should prevail. And, if the jet stream is flowing overhead, look for stormy weather to dominate.
Exam Question Tips:
An aircraft is flying from south to north, above the polar front jet stream, at FL 400 in the southern hemisphere. What change, if any, in temperature will be experienced?
Answer: It decreases.
If you are flying above the polar front jet at FL400 from the cold pole towards the warm equator, you are in the stratosphere above the jet, so the temperature will remain constant at whatever temperature it reached at the tropopause. As you fly north you are flying into the troposphere but at a high altitude so the temperature will be colder than it was in the low troposphere where the jet was.
Rule of thumb. If you cross below the jet the temperature change is normal. Flying from the cold to warm side the temperature will increase, and vice versa.
If you cross above the jet the temperature change is reversed. Flying from cold to warm the temperature decreases, and vice versa.
If you fly at the height of the core there is no temperature change, regardless of which way you go through it.
A new vesion of the crossing through the core question, is as follows :
You are crossing the polar front jet in the northern hemisphere and flying from south to north at the height of the core. Is there a bigger temperature gradient or a bigger pressure gradient?
If you are in the northern hemisphere and crossing the polar front jet from south to north, you are flying from the warm sub tropic air to the cold polar air. But at the height of the core there is no temperature change. However you will have jet speed winds from your left, and you can only have strong winds if you have a strong pressure gradient.
So the answer is a greater pressure gradient.
A wind sounding in the region of a polar front jet stream gives the following wind profile(northern hemisphere)
900 220/20 kts
800 220/25 kts
700 230/35 kts
500 260/60 kts
400 280/85 kts
300 300/100 kts
250 310/120 kts
200 310/80 kts
In this case, Jet stream is associated with a warm front
Q. Where is the projection of the polar front jet stream on the surface most likely to be found in relation to the cold and warm fronts of a depression?
(a) 50 to 200 NM behind the cold front and 300 to 450 NM ahead of the warm front
(b) Up to 100 NM either side of the cold front and up to 200 NM either side of the warm front
(c) Up to 200 NM either side of the cold front and up to 200 NM either side of the warm front
(d) 300 to 450 NM behind the cold front and 50 to 200 NM ahead of the warm front
Answer is (a): The cold front is steeper than the warm front so the surface position of the cold front jet is closed to the surface position of the cold front than the warm front jet is to the surface position of the warm front.
Considering the North Atlantic region between 30 and 65 deg N, the mean position of the polar front during:
“Summer” – Extends from Newfoundland to N Scotland.
“Winter” – Extends from Florida to SW England.
The length, width and height of a typical mid-latitude jet stream is 1000 nautical miles, 150 nautical miles and 18000 feet respectively. Depth width ration of about 1:100.
Greatest windshear is on the cold side of the jet, but still in warm air, and this is the place for worst turbulence. In ref to windshear, speed may go from 60 to 150 knots in 150 km, or 5000 feet vertically.
The easterly jet is a jet stream that occurs only in the summer of the northern hemisphere at approx. 45000 ft, from south east Asia extending over southern India to central Africa.