Air Masses, Fronts, and Pressure Systems

An air mass is a very large body of air with fairly uniform temperature and humidity, a front is the boundary between two different air masses, and high- and low-pressure systems are regions where the atmospheric pressure is higher or lower than the surroundings and which control the weather. In brief: air masses take their properties from the surface where they form (cold or warm, dry or moist). Where a cold air mass advances on warm air a cold front forms, bringing a narrow band of heavy rain; where warm air advances over cold air a warm front forms, with widespread, steady rain. A low-pressure system (depression or cyclone) has rising air, cloud and rain, and winds blowing counterclockwise inward in the Northern Hemisphere; a high-pressure system (anticyclone) has sinking air, clear skies and clockwise winds outward.

Most day-to-day weather in the middle latitudes comes from the passage of fronts and pressure systems. This guide describes air masses, the types of front, high and low pressure systems, compares them in a table, works through examples of the timing and slope of fronts, outlines the history of the frontal model and corrects common mistakes.

What are air masses?

An air mass is a huge body of air, hundreds to thousands of kilometers across, that has taken on the temperature and moisture of its source region, an extensive, uniform surface where air stays for days. Air masses are classified by latitude of origin (polar or tropical, with arctic and equatorial types) and by surface (continental, dry, or maritime, moist). The main types are: continental polar (cold and dry, from the interior of Canada or Siberia), maritime polar (cool and moist, from the northern oceans), continental tropical (hot and dry, from deserts such as the Sahara) and maritime tropical (warm and moist, from warm oceans such as the Gulf of Mexico). When an air mass moves, it is changed by the surface beneath it: cold air over a warm sea becomes unstable and gives showers.

What are fronts?

Air masses with different densities do not mix easily; the boundary is called a front. Warmer, lighter air rises over the colder, denser air, producing cloud and precipitation.

• Cold front: cold air advances and undercuts warm air, forcing it to rise steeply (a slope of roughly 1 in 50 to 1 in 100). It produces a narrow band of heavy showers or thunderstorms, followed by a drop in temperature, a clearing sky and a shift of wind (in the Northern Hemisphere, from southwest to northwest). Cold fronts move fast (around 30–50 km/h). On weather maps it is a blue line with triangles.
• Warm front: warm air advances and slides up over the retreating cold air with a gentle slope (roughly 1 in 150 to 1 in 200). Cloud thickens gradually (cirrus, then altostratus, then nimbostratus), with steady rain over a wide area before the front passes. After the front, the temperature rises and the rain stops. It moves more slowly (about 15–30 km/h). It is shown as a red line with semicircles.
• Occluded front: a cold front catches up with a warm front, lifting the warm air completely off the ground. It gives a mixture of the weather of both fronts and marks the mature stage of a depression. It is a purple line with triangles and semicircles.
• Stationary front: the boundary hardly moves, and cloud and rain can persist for days.

What are high- and low-pressure systems?

A low-pressure system (depression, cyclone) is an area of lower pressure at its center, with air converging and rising. Rising air cools, condenses and forms cloud and rain, so lows bring unsettled, windy weather. In the Northern Hemisphere, the winds spiral counterclockwise into the center (clockwise in the Southern Hemisphere). Mid-latitude depressions form along the polar front, and typically measure about 1,000–2,000 km across, with central pressures around 980–1,000 hPa. A high-pressure system (anticyclone) has higher pressure at the center, with air sinking and diverging at the surface. Sinking air warms and dries, so highs bring clear, settled weather: hot and sunny in summer, but cold with frost or fog in winter. Winds spiral clockwise outward in the Northern Hemisphere and are generally light. On a map, closely spaced isobars mean strong winds, and widely spaced ones mean light winds.

Fronts and pressure systems compared

Worked example: timing a front

1. Arrival time: a cold front is 240 km west of a town and moves east at 40 km/h. It arrives in 240 ÷ 40 = 6 hours.
2. Duration of the rain band: if the band of heavy rain is 60 km wide, it passes the town in 60 ÷ 40 = 1.5 hours.
3. Warm front: a warm front moving at 25 km/h, with rain falling up to 300 km ahead of the front line, would bring 300 ÷ 25 = 12 hours of rain and thickening cloud before the front passes.
4. Frontal slope: for a cold front slope of 1 in 60, at a point 120 km behind the front line the cold air wedge is 120 ÷ 60 = 2 km deep. For a warm front with a slope of 1 in 150, the warm air lies over the cold air at a height of 2 km at 300 km ahead of the front on the ground.
5. Pressure change: the pressure at a station falls from 1,016 hPa to 998 hPa in 9 hours as a low approaches. The rate is 18 ÷ 9 = 2 hPa per hour, which is rapid and suggests strong winds are likely.
6. Wind direction: in a Northern Hemisphere low centered to the west of a town, the wind blows counterclockwise around it, so at the town, to the east of the center, the wind is from the south.

How did the frontal model develop?

The Norwegian Cyclone Model was developed by Vilhelm Bjerknes and his colleagues in Bergen, Norway, during and soon after the First World War. They introduced the terms “front” (by analogy with the battle front of the war) and described the life cycle of a depression along the polar front, with warm and cold sectors. The ideas of air masses were developed by Tor Bergeron in the 1920s. The model is still the basis of weather maps today, although computer models and satellites have added a more detailed understanding of how fronts and jet streams work.

Which mistakes are common?

Mistake 1: Thinking that a cold front brings warmer weather

A cold front is the advancing edge of colder air. The temperature drops after it passes.

Mistake 2: Mixing up the rain of warm and cold fronts

Warm fronts give long, steady rain, and cold fronts give short, heavy showers.

Mistake 3: Forgetting the wind pattern around lows and highs

Lows have converging, rising air, and in the Northern Hemisphere the surface winds turn counterclockwise around them. Highs have the opposite pattern.

Mistake 4: Assuming that highs always mean warm weather

In winter, anticyclones can bring frost, fog and cold, clear nights.