Geography · Ch 15 — Atmospheric Circulation and Weather Systems
World Distribution of Sea Level Pressure
World Distribution of Sea Level Pressure
The map is a global view of sea-level pressure for January, drawn with isobars spaced at 5-millibar intervals from 995 mb up to 1030 mb. These isobars snake across both oceans and continents, and wherever they close into loops they mark centres of action — labelled H for high pressure and L for low pressure. The bold line running near the equator is the ITCZ, the Inter Tropical Convergence Zone, with arrows on either side pointing toward it, showing surface winds converging into the equatorial low.
What the January pattern teaches is that the textbook's neat belts — equatorial low, subtropical highs, subpolar lows, polar highs — are an idealised average, not a fixed reality. In January the continents are cold, so the landmasses of Asia and North America develop strong thermal highs, while the oceans, being relatively warmer, host low-pressure cells. The isobars over the Northern Hemisphere bulge and bend sharply around these centres, and the subtropical high belt is broken into discrete cells over the oceans rather than forming a continuous band. The Southern Hemisphere, by contrast, shows a more regular, zonal arrangement of isobars because its landmasses are smaller and less disruptive. …
The map is a world view of sea-level pressure for July, drawn with isobars that run from 995 mb to 1020 mb. These isobars are the thin lines that trace out the pressure field, and on top of them sit the lettered centres: H for high pressure and L for low pressure. The H and L marks are not scattered randomly — they sit over specific oceans and continents, showing where the pressure systems actually anchor themselves in the northern summer.
The most striking feature is the ITCZ, drawn as a bold line that has shifted north of the equator. This is the intertropical convergence zone, and in July it follows the thermal equator — the belt of maximum heating — which has moved into the Northern Hemisphere because the sun now stands overhead there. Wind-direction arrows converge onto this line from both sides, showing that air is being drawn into the low-pressure trough it represents. Where those arrows meet, air rises, clouds form, and rainfall is heavy.
What the diagram teaches is that the pressure belts are not fixed bands wrapped around the globe. In January the ITCZ sits near or south of the equator, and the subtropical highs are strong, continuous belts over the oceans. By July the whole pattern has shifted northward, and the continents tell a different story. Over the heated landmasses of Asia and North America, the pressure falls and thermal lows appear — these are the L centres marked on the map. Over the cooler oceans, the subtropical highs strengthen and show up as the H centres. So the map is really a picture of how land and sea respond differently to the same seasonal heating: land heats fast and develops low pressure, while the ocean stays relatively cool and holds high pressure. …
The pressure we feel at the surface is not uniform across the globe. If you could strip away the effect of altitude and look at pressure at sea level everywhere, you would find a pattern of alternating low and high belts running roughly parallel to the equator. That pattern is the backbone of global wind systems, and it shifts with the seasons.
The book presents this distribution for two months: January and July. These two maps (Figures 9.2 and 9.3 in the chapter) show how the belts move north and south with the Sun's apparent path, but the basic structure remains recognisable in both.
Starting at the equator, the sea level pressure is low. This belt is called the equatorial low. The warmth there causes air to rise, which lowers surface pressure.
Moving poleward, along roughly 30° N and 30° S, you find belts of high pressure. These are the subtropical highs. Air that rose at the equator sinks here, compressing and raising surface pressure.
Further poleward still, along about 60° N and 60° S, the pressure is low again. These belts are the subpolar lows. Here, warmer subtropical air meets colder polar air, and the rising motion creates low pressure at the surface.
Finally, near the poles themselves, the pressure is high. These are the polar highs, where intensely cold, dense air sits heavily on the surface.
A crucial point the book stresses: these pressure belts are not permanent. They are average conditions, and they shift position with the seasons — moving north in July and south in January. They also break apart over continents, especially in the Northern Hemisphere, where land heats and cools faster than oceans. …
The world distribution of sea level pressure in January and July is shown in Figures 9.2 and 9.3. Near the equator the sea level pressure is low, and the area is known as the equatorial low. Along 30 degrees N and 30 degrees S are found the high-pressure areas known as the subtropical highs. Further pole-wards along 60 degrees N and 60 degrees S, the low-pressure belts are termed the sub polar lows. Near the poles the pressure is high, known as the polar high. These pressure belts are not permanent in nature -- they oscillate with the apparent movem …