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These are fast blowing winds of a steady velocity moving in a narrow zone of the upper layer of the atmosphere.
Their speed varies from about 110 km/h in summer to about 184 km/h in winter. Jet streams are located approximately over 27°-30° north latitude, therefore, they are known as subtropical westerly jet  streams. A number of separate jet streams have been identified. The most constant are the mid-latitude and the sub tropical jet stream. The western cyclonic disturbances experienced in the north and north-western parts of the country are brought in by this westerly flow.
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The jet stream is a strong flowing ribbon of air that flows around our planet high up in the atmosphere, at around the level of the tropopause. Situated between the troposphere and the stratosphere, the Jet Stream is approximately 11 kilometres above the surface of the Earth at the poles and around 17 kilometres above the surface of the Earth at the equator. The jet stream flows at around 160kmph (100mph). We often hear that the jet stream is responsible for influencing the weather in the UK, so it is natural to wonder what causes the jet stream and why it has such an influence on the weather we experience on the ground.