The tropopause

Why is the tropopause twice as high over the equator?

Higher at the equator

A slice of the air from the pole (left) to the equator (right).

1. The troposphere is not the same depth everywhere It reaches up 16-18 km over the equator but only 8-10 km over the poles.
2. Why: rising heat Over the hot equator, strong convection carries warm air very high before it stops cooling. Over the cold poles the air does not rise so far. Rule: the warmer the surface, the higher the tropopause.
3. Higher means colder Because the air keeps cooling all the way up, the high tropical tropopause is very cold (−70 to −75 °C); the low polar one is warmer (−40 to −45 °C).
4. What sets its height Five things: surface temperature, latitude, season, the spread of land and sea, and the weather situation of the day (synoptic situation). It is lower in winter than in summer. Tropopause height: equator 16-18 km, poles 8-10 km, India 16-16.5 km.

Memory hook: Hot below, high above, cold on top.

Takeaway: The warmer the surface, the higher the tropopause: 16-18 km over the equator, 8-10 km over the poles, lower in winter. The high tropical tropopause is also the coldest.

Can the North Pole be warmer than the equator?

Reversal above 8 km

Two temperature soundings, one over each.

1. Over the equator Warm at the ground, and it keeps cooling all the way to a tropopause near 17 km, where it is about −75 °C.
2. Over the pole Cold at the ground, but the tropopause comes early, near 9 km. Above it the temperature stops falling and stays near −45 °C.
3. The lines cross Above about 8 km the polar column is warmer than the equatorial one at the same height: the temperature (and density) pattern reverses above 8 km. Above 8 km the lowest temperatures are over the equator.

Memory hook: Low lid at the pole = warmer up high.

Takeaway: Above about 8 km the polar air has already stopped cooling (its tropopause is low), while equatorial air keeps cooling up to 16-18 km. So high up the poles are warmer, and denser air is reversed too.

Is the tropopause one smooth surface?

Three tropopauses

A slice from the North Pole (left) to the equator (right).

1. The polar tropopause From the pole to about 45-60° latitude it lies near the 300 hPa level, about 8-9 km. In winter, when polar air pushes south, it is sometimes noticed over Srinagar.
2. The middle tropopause Between the two breaks it lies near 200 hPa. Over India it is about 11.5 km with about −45 °C; in winter it can reach as far south as 23° N.
3. The tropical tropopause From the equator to about 35-40° it lies near 100 hPa. Over India it is about 16-16.5 km.
4. The breaks Where one step meets the next the tropopause changes height suddenly. These breaks (or folds) are near 60° and 40° latitude; the one near 40° is the more marked.
5. Jet streams live at the breaks The polar front jet stream sits at the southern end of the polar tropopause; the subtropical jet stream at the southern end of the middle tropopause.
6. Over India India (8-37° N) is under the tropical tropopause, at 16-16.5 km, with the subtropical jet overhead in the north in winter. Tropopause breaks: about 40° and 60°. The 40° one is more prominent.

Memory hook: 300, 200, 100 hPa from pole to equator. Jets at the breaks.

Takeaway: The tropopause comes in three steps: polar (~300 hPa), middle (~200 hPa) and tropical (~100 hPa), with breaks near 60° and 40° where the jet streams blow. Over India the tropical one is at 16-16.5 km.

Where does the weather stop?

The lid on the weather

A thunderstorm over Africa, photographed from the space station. Tap the pins.

• Flat top
• The anvil. The storm's rising air reached the tropopause and could rise no further, so it spread sideways. Clouds rarely go higher than the tropopause.
• Overshooting top
• The strongest updraught punches a little way into the stratosphere before falling back: a sign of a very violent storm.
• Spreading downwind
• Strong winds just below the tropopause stretch the anvil out downwind.
• Small cumulus
• Ordinary fair-weather cumulus, only 1-2 km tall, deep in the troposphere.

Memory hook: Anvil top = tropopause.

Takeaway: The tropopause acts as a lid: rising storm air hits the stable stratosphere and spreads out sideways into a flat anvil.