Fronts and Depressions
Weather & Meteorology Fundamentals
Course 1 · Chapter 5 · Fronts and Depressions
Most of the changeable weather in the middle latitudes comes from one pattern: a mass of warm air meeting a mass of cold air along a boundary, with a spinning low-pressure system forming on that boundary. Learn to recognise the air masses, the fronts and the life cycle of the low, and the symbols on a weather chart start to tell a story. This chapter builds on the pressure and wind ideas of Weather & Meteorology Fundamentals 4. Dates and figures are given from general knowledge and flagged where they should be checked.
Air Masses
An air mass is a very large body of air with fairly uniform temperature and moisture, which takes on the character of the region where it forms, called its source region. Air that sits for days over a cold ocean becomes cool and moist; air that sits over a hot desert becomes hot and dry. Air masses are named by their source: polar or tropical for the temperature, and maritime or continental for the moisture.
| Air mass | Source | Typical feel in the British Isles |
|---|---|---|
| Polar maritime | Cold seas to the north-west | Cool and showery, with good visibility between showers |
| Tropical maritime | Warm seas to the south-west | Mild, damp and cloudy, with drizzle and hill fog |
| Polar continental | Northern Europe and Siberia | Very cold and dry in winter, brought by easterly winds |
| Tropical continental | North Africa and southern Europe | Hot and dry in summer, often hazy |
| Arctic maritime | Arctic Ocean | Very cold, with snow showers in winter |
Fronts
A front is the boundary between two air masses of different temperature. The term was borrowed from the military language of the First World War by the Norwegian meteorologists who developed the idea, led by Vilhelm Bjerknes and his colleagues in Bergen (check the names, the date and the military link). Fronts are drawn on charts as lines with symbols along them.
Warm front
Warm air advances and rides up over cooler air along a gentle slope. Cloud thickens and lowers over many hours, from high wispy cloud to thick grey layers, with steady rain, then the rain stops and it turns milder. Drawn as a red line with semicircles.
Cold front
Cold air advances and undercuts the warmer air along a steeper slope, pushing it up quickly. The belt of cloud is narrow, with heavy rain or showers, sometimes thunder, then the sky clears and it turns colder and brighter. Drawn as a blue line with triangles.
Occluded front
A cold front moves faster than the warm front ahead of it and catches it up, lifting the warm air off the ground. The weather is a mixture of both fronts. Drawn as a purple line with both symbols.
Stationary front
A boundary that is barely moving, with cloud and rain that can linger for a day or more in the same area.
The Life of a Depression
Depression is the traditional British name for a low-pressure system in the mid-latitudes; the word cyclone is also used. Such a low develops along the polar front, the boundary between cold polar air and warm tropical air. The Norwegian model describes a life cycle of several stages (check the details, which have been refined by later research).
- A wave forms. A kink appears on the front, with warm air pushing into the cold on one side and cold air pushing into the warm on the other.
- The low deepens. Pressure at the centre falls, the winds strengthen and circulate anticlockwise, and a warm front and a cold front become distinct, with a warm sector between them.
- The cold front catches up. The cold front moves faster than the warm front and closes the warm sector, forming an occlusion.
- The low fills. With the warm air lifted off the ground, the system loses its source of energy, the centre fills, and the depression decays over a few days.
The jet stream plays a key part. Where it provides divergence aloft, drawing air away from the top of a developing low, the surface pressure falls faster. Rapid deepening of this kind has been called explosive cyclogenesis or, informally, a weather bomb (check the term and the threshold used for it).
Highs, Ridges and Troughs
Between the depressions come calmer spells. A ridge of high pressure, or an anticyclone that settles over the country, gives light winds and clear skies, with fog or frost in winter and heat in summer. In winter, a blocking high that sits in place for days or weeks can divert the Atlantic depressions away and bring prolonged cold or dry spells (check the term and examples). A trough is an elongated area of low pressure, often with a line of showers or a weak front along it.
Exercises
A cool, showery north-westerly airstream is replaced by a mild, damp, cloudy south-westerly. Name the air mass in each case and say where each comes from.
Describe the sequence of weather as a depression passes over you, from the first sign of the warm front to the clearance after the cold front, saying what the pressure does at each stage.
Explain how an occluded front forms, and why the depression weakens afterwards.
Quick Reference
- Air mass: a large body of air with uniform temperature and moisture, named by source
- Front: the boundary between two air masses
- Warm front: gentle slope, thickening cloud, steady rain, then milder
- Cold front: steep slope, narrow belt of heavy rain, then colder and brighter
- Occlusion: the cold front catches the warm front and lifts the warm air
- Depression: a mid-latitude low that forms on the polar front and passes through a life cycle
- Anticyclone: a high with settled weather; a blocking high can divert the depressions
Next chapter: Storms and Extremes, which covers thunderstorms, tornadoes, hurricanes, snow and ice, and the Great Storm of 1987.