Pressure, Wind, and Air Masses

Air has weight, and air pressure is that weight pressing down on everything below. You measure it with a barometer. Two scales are in common use, millibars and inches of mercury. One standard atmosphere is 1013.2 millibars, or 29.92 inches. The 2026 Reference Tables no longer print a conversion between the two, though page 18 does show how pressure is coded on a station model.

Two things lower the pressure of a column of air. Warming it makes it expand and thin out. Adding water vapor also lowers it, because a water molecule is lighter than the nitrogen and oxygen molecules it pushes out of the way. That surprises most people, but humid air really is lighter than dry air. So warm, moist air means low pressure, and cold, dry air means high pressure. That one rule explains most of a weather map. A falling barometer means warm moist air is arriving and storms are likely. A rising barometer means cool dry air and clearing skies.

Pressure systems are the main features on every weather map. A HIGH, or anticyclone, is a broad area of cool, dry, sinking air. Sinking air warms up and dries out, so highs bring fair skies and calm weather. A LOW, or cyclone, is an area of warm, moist, rising air. Rising air is how clouds form, as you saw in the last chapter, so lows bring clouds, precipitation, and storms.

In the Northern Hemisphere, surface winds spiral clockwise and outward from a high. They spiral counterclockwise and inward toward a low, where the converging air has nowhere to go but up. Learn to see those two spirals and a weather map starts to move in your head.

Wind is air flowing from high pressure toward low pressure. The bigger the pressure difference over a given distance, the stronger the wind. This is the same gradient idea from Unit 2, applied to pressure. Earth's rotation then bends everything that moves to the right in the Northern Hemisphere. That is the Coriolis effect, and it is why air spirals around highs and lows instead of flowing in straight lines. The effect only shows up over large distances and long times, in things like hurricanes and jet streams. It does not affect sinks and toilets. The direction water swirls down a drain is set by the shape of the basin, not by which hemisphere you are in. Winds are always named for the direction they come from. A northwest wind blows from the northwest.

The sea breeze is a small version of the whole system. On a summer afternoon the land heats faster than the water. Air rises over the warm land, pressure there drops, and cool air flows in off the sea. At night the flow reverses. One beach day contains the entire logic of this chapter.

Scale that logic up to the whole planet and you get the wind and pressure belts charted on page 19 of your Reference Tables. Intense heating at the equator makes a belt of rising air and low pressure, wet and stormy. That air sinks back down near 30 degrees latitude, making belts of high pressure, clear skies, and the world's great deserts. Between the belts blow the planetary winds, bent by the Coriolis effect. The tropics get the trade winds. The middle latitudes, where New York sits, get the prevailing southwesterlies.

Those southwest winds matter for the whole unit. They steer air masses, fronts, and entire storm systems across the United States from west to east. That is why tomorrow's weather in Pleasantville is usually today's weather in Ohio.

An air mass is a huge body of air, often a thousand kilometers across. It has sat over one region long enough to take on that region's temperature and moisture. The two-letter codes on weather maps tell you where it formed. The lowercase letter gives moisture. Use m for maritime, which formed over water and is humid, or c for continental, which formed over land and is dry. The capital letter gives temperature. Use T for tropical warmth, P for polar cold, and A for even colder arctic air.

Two air masses run New York's weather. cP is the cold dry air that pours down from central Canada. mT is the warm humid air that streams up from the Gulf of Mexico. cT and mP play smaller parts. Most of the state's storms are cP and mT colliding, and the boundaries where they meet, fronts, are the subject of the next chapter.

Build an air mass. Choose the surface it forms over and the latitude it forms at. The lab writes its two-letter code, describes its character, and tells you what it brings when it reaches New York.

Air pressure is the weight of the air. Warmth and moisture both lower it, so warm humid air builds lows and cold dry air builds highs. Highs have sinking air, spiral clockwise and outward, and bring fair weather. Lows have rising air, spiral counterclockwise and inward, and bring storms. Wind runs down the pressure gradient from high to low, bent to the right by the Coriolis effect. The planet-scale version of all this is the wind belt chart on your Reference Tables, including the prevailing southwesterlies that carry our weather from west to east. Air masses carry the temperature and moisture of the region they formed in, recorded in a two-letter code. When cP air meets mT air over New York, the next chapter begins.

Ten Regents-style questions, one at a time in a focused view, each with an instant explanation. The set reshuffles when you reach the end, so you can keep practicing as long as you like.