5.2 Weather Systems, Fronts & Forecasting

Air Masses

An air mass is a large body of air with fairly uniform temperature and moisture, taking on the characteristics of the source region where it forms. The Regents names air masses with a two-letter code: a lowercase letter for moisture and an uppercase letter for temperature.

Most summer humidity in New York arrives in mT air from the Gulf of Mexico and Atlantic, while winter cold snaps bring cP air from central Canada. Where two different air masses meet, the boundary is a front. The Regents frequently gives the source region and asks for the code, or gives a code and asks which weather it brings - so anchor each letter to a meaning: the lowercase letter (m or c) always describes moisture based on whether the source was ocean or land, and the uppercase letter (T or P) always describes temperature based on whether the source was near the equator or the poles.

An air mass gradually loses its source-region traits as it travels, but it still strongly influences the weather wherever it moves.

Fronts

A front is the boundary between two air masses of different density (temperature/moisture). The four types each produce signature weather:

• Cold front - a colder, denser air mass advances and wedges under warmer air, forcing it up steeply. This causes rapid lifting, tall cumulonimbus clouds, and brief but intense thunderstorms, followed by cooler, clearer air.
• Warm front - a warmer, less-dense air mass advances and rides over retreating cold air on a gentle slope. This produces widespread, steady, light-to-moderate precipitation over a long period.
• Stationary front - two air masses meet but neither advances; clouds and precipitation can linger for days.
• Occluded front - a fast cold front overtakes a warm front and lifts the warm air completely off the ground, often near the center of a mature low-pressure system.

On a weather map, fronts use standard symbols: a cold front is a blue line with triangles pointing in the direction it moves, a warm front is a red line with semicircles, a stationary front alternates triangles and semicircles on opposite sides, and an occluded front combines both symbols in purple. After a cold front passes, expect temperature to drop, pressure to rise, and skies to clear; after a warm front passes, expect temperature to rise and skies to gradually clear. Tracking these changes across a sequence of station models is a classic constructed-response task.

High vs. Low Pressure and Rotation

Pressure systems are the engines of weather. The single most tested fact here is rotation direction in the Northern Hemisphere, set by the Coriolis effect:

• Low pressure (cyclone) - air rises at the center, drawing surface winds inward and counterclockwise. Rising air cools adiabatically, so lows bring clouds, precipitation, and stormy weather.
• High pressure (anticyclone) - air sinks at the center and spreads outward and clockwise. Sinking air warms and dries, so highs bring fair, clear weather.

Memorize: L = Low = Lousy weather = counterclockwise inward; H = High = Happy weather = clockwise outward (Northern Hemisphere). Pressure is measured in millibars (mb); standard sea-level pressure is about 1013.2 mb.

Reading the Station Model

A station model is a compact symbol that records the weather at one location. The Regents reference tables include the Station Model key. Core elements:

• Air temperature (degrees F) at upper left; dewpoint at lower left.
• Barometric pressure at upper right, written in a shorthand: append a decimal and read "3-9-9" as 1039.9 mb and "2-4-7" as 1024.7 mb (add 9 or 10 in front so the value lands near 1000).
• Wind shown by a staff pointing from the direction the wind blows, with barbs indicating speed.
• Cloud cover shown by how much of the center circle is shaded; present weather symbols sit to the left.

Isobars and Pressure Gradient

Isobars are lines connecting points of equal air pressure on a weather map. Their spacing reveals wind strength: closely spaced isobars indicate a steep pressure gradient and strong winds, while widely spaced isobars indicate gentle winds. Wind generally blows across isobars slightly, from high toward low pressure, curving because of the Coriolis effect.

Storm Tracks and Severe Weather

In the United States, weather systems generally move west to east, steered by the prevailing westerlies, though coastal lows (nor'easters) can track up the East Coast. Two severe-weather systems are commonly tested:

• Hurricanes - large tropical low-pressure systems that form over warm ocean water (above ~27 degrees C), drawing energy from latent heat of condensation; they weaken over land.
• Tornadoes - small, intense, rapidly rotating columns often spawned along strong cold fronts and supercell thunderstorms, with the highest U.S. frequency in "Tornado Alley."

Both are low-pressure systems, so winds spiral counterclockwise and inward in the Northern Hemisphere. A useful forecasting idea the Regents tests is that because systems move generally eastward, the weather to your west is often your weather a day from now; students may be asked to predict conditions for a city downwind of an approaching low or front.