Jet Streams

At a Glance

• Height About 9 to 14 km, near the tropopause
• Minimum speed Roughly 110 km per hour
• Polar front jet About 40 to 60 degrees, westerly
• Subtropical jet About 25 to 35 degrees, westerly
• Tropical Easterly Jet Summer jet over peninsular India
• Exam link UPSC Prelims, Geography: Storms, Cyclones, Jet Streams and Air Masses

Where Does It Fit?

This is the sixth topic of the category on storms, cyclones, jet streams and air masses. The previous topics covered storms at the surface, and this one moves to the fast winds aloft that steer them. The next topic covers air masses and fronts. Previous topic: Naming, Warning and Effects of Tropical Cyclones. Next topic: Air Masses and Fronts.

What Are Jet Streams and Why Do They Form?

In short: jet streams are rivers of very fast air at the top of the troposphere, found where warm and cold air meet.

A jet stream is a narrow current of strong wind, a few hundred kilometres wide and a few kilometres thick, that flows near the tropopause. It is created by two things. First, a strong horizontal temperature contrast produces a sharp pressure gradient aloft, as in Atmospheric Pressure, Isobars and Vertical Variation. Second, the Coriolis force turns the flow, so that the air moves along the gradient and not across it. The winds blow mainly from the west to the east in the middle latitudes, because the upper air in the three-cell system is westerly, as in General Circulation of the Atmosphere. Jet streams are strongest in winter, when the temperature contrast between the equator and the poles is greatest. Their discovery came during the Second World War, when high-flying bomber pilots found that they were being slowed or speeded by unusually strong winds.

What Are the Main Jet Streams?

The jet streams do not flow in straight lines. They form large waves, called Rossby waves, with ridges and troughs that move slowly and bring cold or warm air into different regions. When the waves are very deep, the weather in one place can stay the same for days, as in an extended cold spell or heat wave.

How Do Jet Streams Affect India?

• Winter: the subtropical westerly jet is split by the Tibetan plateau into two branches. The northern branch blows along the northern edge of the plateau. The southern branch flows south of the Himalaya over northern India, at around 25 degrees north, and carries western disturbances, the mid-latitude cyclones that bring winter rain and snow to northwest India, which benefits the rabi wheat crop, as in Temperate Cyclones.
• Onset of the monsoon: in early June the southern branch of the jet weakens and moves north of the Himalaya, and the Tibetan plateau heats up. The rapid shift is linked with the burst of the southwest monsoon over India, as in Mechanism of the Indian Monsoon.
• Monsoon season: the Tropical Easterly Jet forms over the peninsula and strengthens the upper-level divergence that supports rising air and rain. A strong Tropical Easterly Jet is generally linked to a good monsoon, while a weak jet is often seen in deficient years.
• Cyclones: jet streams help to steer tropical cyclones and decide whether they recurve, as in Path, Intensity Scale and Breeding Grounds of Tropical Cyclones.

Why Do Jet Streams Matter Beyond India?

The polar front jet marks the track of temperate storms of North America and Europe and decides which areas get warm or cold spells. Airlines use the jet stream to save fuel, because an eastbound flight in a tailwind can be shorter by an hour or more than the westbound return. A strong subtropical jet over the Pacific in an El Niño year changes the winter in North America, as in El Niño and Its Effects. Climate scientists are studying whether the rapid warming of the Arctic, which reduces the temperature contrast, makes the jet stream weaker and wavier. The evidence is still debated, so it is better to say that this link is a research question and not a settled fact. As an analytical point, the jet stream shows that the weather of one place cannot be understood without the upper air, which is why Indian forecasting depends on balloon soundings, satellites and numerical models.

Why Is This Topic Important?

• Facts Types and heights asked
• India Western disturbances and monsoon onset
• Climate Wavy jet and extreme weather

What Came Next?

The next topic describes air masses and fronts. Next topic: Air Masses and Fronts.

Key People and Terms

• Jet stream Fast narrow upper air current
• Tropopause Boundary between troposphere and stratosphere
• Rossby waves Large meanders of the jet
• Western disturbance Winter low from the west
• Tropical Easterly Jet Summer easterly jet over India
• Findlater jet Low-level Somali jet

Exam Corner

Mains Angle

Questions link jets to the monsoon. These points add depth.

• Winter Western disturbances
• Summer Shift and easterly jet
• Change Arctic warming debate