The Jet Stream And The Global Circulation

Uneven heating makes the atmosphere move heat from the equator toward the poles. On a rotating planet, that movement organizes into a set of great circulation belts, and wrapped around the world between them are rivers of fast wind near the top of the troposphere: the jet streams. They steer storms, separate warm air from cold, and are among the most important features on any forecast map.

What Are The Hadley, Ferrel And Polar Cells?

If Earth did not rotate, air might rise at the equator, flow toward each pole aloft, sink there and return along the surface in one giant loop per hemisphere. Rotation breaks that loop into three cells:

• **The Hadley Cell (0-30 degrees): Intense heating at the equator drives rising air in a band of thunderstorms called the Intertropical Convergence Zone (ITCZ). Air spreads poleward aloft, is turned eastward by Coriolis, and sinks near 30 degrees latitude, creating the subtropical highs and many of the world's deserts. At the surface, air returns toward the equator as the trade winds**, blowing from the northeast in the Northern Hemisphere.
• **The Ferrel Cell (30-60 degrees): A mid-latitude belt where surface winds blow mainly from the west, the prevailing westerlies**. The Ferrel cell is less a real loop than the average effect of countless migrating storms.
• **The Polar Cell (60-90 degrees): Cold, dense air sinks over the poles and flows outward along the surface as the polar easterlies, meeting milder air near 60 degrees at the polar front**, where rising air closes the loop.

What Causes The Jet Stream?

Jet streams form where temperature changes sharply over a short distance. Recall that pressure falls faster with height in cold, dense air than in warm air. So at 10 km up, pressure on the warm side of a strong temperature contrast is much higher than on the cold side. That creates a strong pressure gradient aloft, and geostrophic balance turns it into fast west-to-east wind. This relationship is called the thermal wind: the stronger the horizontal temperature contrast, the stronger the wind increases with height.

Fast winds near the tropopause that steer storms; troughs and ridges in the jet shape our weather.

How High And How Fast Is The Jet Stream?

• The polar jet sits above the polar front, roughly 9-12 km up (about 250-300 mb), typically between 40 and 60 degrees latitude. It is the one that matters most for daily weather in the United States. It dives south in winter, sometimes reaching the Gulf Coast, and retreats toward Canada in summer.
• The subtropical jet sits at the poleward edge of the Hadley cell near 30 degrees, higher up at 12-14 km (about 200 mb). It is steadier, strongest in winter, and often carries moisture from the tropical Pacific across Mexico and the southern United States.

Core speeds are typically 100-150 mph (90-130 knots), and winter jets over Japan and the western Atlantic occasionally exceed 250 mph. Eastbound airline flights ride the jet to save time and fuel; westbound flights avoid it.

How Does The Jet Stream Affect Weather?

The polar jet does not flow straight around the globe. It meanders north and south in large undulations called Rossby waves, usually four to six around the hemisphere. Where the jet dips south, it forms an upper-level trough carrying cold air; where it bulges north, it forms an upper-level ridge with warm air.

• Ahead of (east of) an upper trough, air aloft diverges, surface pressure falls, air rises and storms develop. This is where surface lows are born and strengthen.
• Under an upper ridge, air sinks, and weather is warm and quiet.

Waves can be progressive, moving steadily east, or become amplified and nearly stationary. A stuck pattern, called a blocking pattern, can lock one region into heat or drought while another sees repeated storms.

Jet Streaks

Along the jet, winds are not uniform. A Jet Streak is a concentrated core of maximum wind within the jet, often a few hundred kilometers long. As air speeds up entering a streak and slows down leaving it, it creates zones of divergence aloft. Imagine the streak divided into four quadrants:

• The right entrance and left exit regions favor upper-level divergence and rising motion.
• The other two quadrants favor convergence aloft and sinking.

Forecasters watch the left-exit region of a jet streak closely for rapid storm development and severe weather.

What Is A 500 mb Map?

Forecasters often study the 500 mb map, which shows the height of the 500 mb pressure surface, near 5,500 m (18,000 ft). This level sits in the middle of the troposphere and shows the overall wave pattern clearly.

• Height contours are drawn in decameters; 576 dam means the 500 mb surface is 5,760 m up.
• Lower heights mean colder air below, so troughs show up as southward dips in the contours.
• Winds blow parallel to the contours, stronger where they are tightly packed.
• Small ripples moving through the pattern, called shortwaves, often trigger clusters of storms.