What Are Jet Streams?
Jet streams are narrow bands of fast-moving air in the upper layers of Earth’s atmosphere. These winds flow at high speeds, usually between 110 and 250 kilometers per hour (70 to 155 miles per hour), but can go even faster. They occur around 9 to 16 kilometers (6 to 10 miles) above the Earth’s surface, just below the tropopause (the boundary between the troposphere and the stratosphere). Jet streams are a natural result of Earth’s rotation, temperature differences, and atmospheric pressure gradients.
Why Do Jet Streams Exist?
What are geostrophic Winds?
Jet streams are Geostrophic Winds and they form because of the uneven heating of the Earth’s surface and the planet’s rotation. Here’s how it works:
The velocity and direction of wind result from the combined effects of various wind-generating forces. In the upper atmosphere, approximately 2-3 km above the Earth’s surface, winds are free from the frictional effects of the surface. These winds are primarily controlled by the pressure gradient force (PGF) and the Coriolis force.
When an air parcel initially at rest is subjected to the PGF, it moves from a region of high pressure to low pressure. However, as the air parcel begins to move, it is deflected by the Coriolis force—to the right in the northern hemisphere and to the left in the southern hemisphere. As the wind gains speed, the Coriolis force increases until it balances the pressure gradient force. At this stage, the wind flows parallel to the isobars and perpendicular to the PGF. This phenomenon is referred to as geostrophic wind.
Characteristics of Jet Streams
- Narrow and Concentrated: Jet streams are not very wide—usually 50 to 150 kilometers (30 to 95 miles) across—but they stretch for thousands of kilometers around the globe.
- High Speed: Their high speeds make them a key component of atmospheric circulation.
- Flow Direction: They generally flow from west to east in both hemispheres due to the Earth’s rotation.
- Meandering Path: Jet streams don’t move in a straight line; they meander, forming ridges and troughs. This meandering is influenced by temperature contrasts and Earth’s geography.
Types of Jet Streams
- Polar Jet Stream:
- Found at higher latitudes, near 60° North and South.
- It forms where cold polar air meets warmer air from mid-latitudes, creating a strong temperature gradient.
- The polar jet is stronger in winter when the temperature difference is greater.
- Subtropical Jet Stream:
- Located near 30° North and South.
- Caused by Earth’s rotation and the temperature difference between the tropics and subtropics.
- It is more stable and continuous, particularly in the southern hemisphere.
- Tropical Easterly Jet Stream:
- Found in the tropics, mainly over Asia and Africa during the northern hemisphere summer.
- This jet stream is linked to the monsoon systems and plays a role in seasonal weather patterns.
- Temporary Jet Streams:
- These can form due to unusual atmospheric conditions, such as the Somali Jet or the African Easterly Jet.