- Joined
- Saturday January 15, 2011
- Location
- Norfolk, England
N.B. I wrote the majority of the following posts during late 2016 and early 2017.
The Jet Stream (at least the one which affects the UK and countries on a similar latitude) is a band of fast-moving air located roughly 30-40,000 feet above the surface, around the tropopause (the divide between the troposphere and stratosphere, respectively the lowest and second-lowest layers of the atmosphere). Globally, there are actually four jets - each hemisphere (northern and southern) has a polar jet (the one that concerns us) located in the upper-mid latitudes, and a weaker subtropical jet much closer to the equator. For the avoidance of doubt, in this thread, any mention of the jet is a reference to the northern hemisphere polar jet. It flows from west to east, and its velocity and path varies depending on global factors that I'll mention at a later date.
Jets are caused by circulations of air rising from near the equator (warm air rises above colder air as its density is lower). This air drifts northwards at altitude, and, as it cools, it sinks towards the surface, then flows southwards again. What converts this into a westerly flow is the Coriolis Effect, which is an apparent force caused by the fact that the circumference of the earth is largest at the equator and drops to almost nothing at the poles. Quoting from the follwing site:
http://www.theweatherprediction.com/habyhints/27/
The Jet Stream (at least the one which affects the UK and countries on a similar latitude) is a band of fast-moving air located roughly 30-40,000 feet above the surface, around the tropopause (the divide between the troposphere and stratosphere, respectively the lowest and second-lowest layers of the atmosphere). Globally, there are actually four jets - each hemisphere (northern and southern) has a polar jet (the one that concerns us) located in the upper-mid latitudes, and a weaker subtropical jet much closer to the equator. For the avoidance of doubt, in this thread, any mention of the jet is a reference to the northern hemisphere polar jet. It flows from west to east, and its velocity and path varies depending on global factors that I'll mention at a later date.
Jets are caused by circulations of air rising from near the equator (warm air rises above colder air as its density is lower). This air drifts northwards at altitude, and, as it cools, it sinks towards the surface, then flows southwards again. What converts this into a westerly flow is the Coriolis Effect, which is an apparent force caused by the fact that the circumference of the earth is largest at the equator and drops to almost nothing at the poles. Quoting from the follwing site:
http://www.theweatherprediction.com/habyhints/27/
What is happening is that, from a viewpoint in space, air is rising near the equator with the same high angular velocity as the surface below thanks to the large circumference of the earth at that point. As it flows north, however, the surface below is, realtive to the parcel of air, moving more slowly - it has lower angular velocity. From the space observer's point of view, the air is flowing due north; it's just that the surface below is rotating more slowly as the circumference reduces. However, from the frame of reference of an observer on the ground (say in London), the flow appears to be a westerly, as that person is unable to compare their angular velocity with that of the surface in the area the air originated from. It's a difficult concept to explain, and requires some thought to grasp, but, once you 'get it', the reason for the westerly jet flow is suddenly obvious. To quote from the above site (which I think explains it as eloquently as anywhere else):The Coriolis force is an apparent force. From an earth observer, it is an apparent curving of a wind flow. The earth spins counterclockwise when viewed from the North Pole and clockwise when viewed from the South Pole. Therefore, the Coriolis deflection is the opposite in the Northern Hemisphere as compared to the Southern Hemisphere. If you stand on the North Pole, your body will make a complete counterclockwise rotation in 24 hours. However, if you are on the equator, your body will not rotate but will rather face forward as you move with the earth. The Coriolis force is a maximum at the pole (perfect spin) and a minimum at the equator (no earth generated spin). The earth's linear velocity (distance per unit time) is a maximum at the equator and a minimum at the pole.
An important concept to grasp is that, in the northern hemisphere, this rightwards deflection applies to air moving parallel with the surface in any compass direction in the absence of other effects. This idea is fundamental to understanding the development and maturation of areas of high and low pressure. For now, the important concept is that, all else being equal, weather in the northern hemisphere tends to come from the west or south-west, hence why that is our prevailing wind direction.An air parcel in the Northern Hemisphere moving from the equator toward the pole will carry its higher angular velocity as it moves north. This will cause the air parcel to deflect to the right of its path of motion. If an air parcel moves north to south in the Northern Hemisphere, it will carry its lower angular velocity with it. Since it is moving into a region of higher angular velocity, the earth will spin underneath the air parcel, causing again, an apparent deflection to the right. The word deflection is used because it is relative to an earth observer. Someone watching the air parcel from space would not see the parcel deflect but would rather see the parcel moving straight and the earth rotating out from under the air parcel.
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