---
title: "Air Masses, Fronts and Weather Maps"
book: "Primary & Middle School Earth and Space Sciences"
subject: earth
language: en
chapter: 25
exercises: 12
source: https://one-course.com/books/earth/1/en/chapter/25-air-masses-fronts-and-weather-maps
license: CC-BY-NC-SA-4.0
credit: "One Earth and Space Sciences Book, One Course (one-course.com)"
---

# Chapter 25 — Air Masses, Fronts and Weather Maps

The morning is bright and still. Around noon, thin white streaks appear very high in the sky, and slowly spread into a pale veil that dims the Sun. In the afternoon the [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud) thicken and come lower; by evening a steady [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) is falling. During the night it turns warm and damp, then a dark line of [cloud](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud) brings a short burst of heavy [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) and a gust of wind. The next morning is clear, fresh and windy. In two days a *[depression](#def-g8-fronts-and-weather-maps-depression)* has passed over the town, with its two [fronts](#def-g8-fronts-and-weather-maps-front).

![A curtain of rain arrives over a town under a dark cloud base, while the sky is still clear behind the church (AI-generated illustration of a generic scene).](https://one-course.com/images/onecourse/chapters/earth-1/g8-fronts-and-weather-maps/img-d95a652048f6.jpg)

*A curtain of [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) arrives over a town under a dark [cloud](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud) base, while the sky is still clear behind the church (AI-generated illustration of a generic scene).*

[Weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) forecasters saw it coming days before. This chapter shows how: the air of our skies comes in large masses, the masses meet along [fronts](#def-g8-fronts-and-weather-maps-front), and [maps](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) of the pressure show where they are going.

**You already know.**

Air presses on everything it touches; a barometer measures this pressure in hectopascals (the physics books), and at [sea level](https://one-course.com/books/earth/1/en/chapter/9-the-physical-map-of-the-world#def-g4-world-physical-map-altitude) it is about $1013\,\mathrm{hPa}$ ([Chapter 23](https://one-course.com/books/earth/1/en/chapter/23-the-atmosphere#ch-g8-the-atmosphere)). [Clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud) form when damp air rises and cools ([Chapter 7](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#ch-g4-clouds-rain-snow)). At the ground, the wind blows from high pressure towards low pressure, and the Earth’s rotation turns it aside: to the right in the [northern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator). Between about $30{}^{\circ}$ and $60{}^{\circ}$, the [westerlies](https://one-course.com/books/earth/1/en/chapter/24-winds-and-the-global-circulation#def-g8-winds-global-circulation-global-winds) bring one [storm](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-storm) after another ([Chapter 24](https://one-course.com/books/earth/1/en/chapter/24-winds-and-the-global-circulation#ch-g8-winds-global-circulation)).

## 25.1 Air masses

Air that stays for days over the same place takes on the character of the surface beneath it. Over a warm [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) it becomes warm and damp; over a frozen land, cold and dry. When it moves away, it carries this character with it, sometimes for thousands of kilometres.

**Definition 25.1 (Air mass).**

An *air mass* is a very large body of air in which the temperature and the humidity are nearly the same everywhere at a given height. It takes them from the region where it formed, its *source region*.

Forecasters name an [air mass](#def-g8-fronts-and-weather-maps-air-mass) with two letters. The first says where it formed: *c* (continental) over land, so it is dry; *m* (maritime) over an [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), so it is moist. The second says how cold it is: *A* (arctic) over the polar regions, *P* (polar) over the high [latitudes](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude), *T* (tropical) over the low [latitudes](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude). Air from a hot [desert](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-desert) is cT, hot and dry; air from the Arctic is cA, very cold and dry.

![Typical source regions of air masses around the North Atlantic (schematic): over the Arctic, the cold continents, the northern ocean, the subtropical ocean and the Sahara. Blue: cold air masses; red: warm ones.](https://one-course.com/images/onecourse/chapters/earth-1/g8-fronts-and-weather-maps/fig-9f967e93405e.svg)

*Typical source regions of air masses around the North Atlantic (schematic): over the Arctic, the cold [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent), the northern [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), the subtropical [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) and the Sahara. Blue: cold air masses; red: warm ones.*

## 25.2 Fronts

When an [air mass](#def-g8-fronts-and-weather-maps-air-mass) moves, it meets another one. The two do not mix easily: they stay apart, with a narrow boundary between them where the [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) changes.

**Definition 25.2 (Front).**

A *front* is the boundary at the ground between two air masses of different temperature and humidity. At a *cold front*, cold air pushes into warmer air and takes its place; at a *warm front*, warm air pushes into colder air and takes its place.

Cold air is denser than warm air. At a [cold front](#def-g8-fronts-and-weather-maps-front), the cold air slides under the warm air like a wedge and lifts it up sharply. The warm, damp air rises fast and builds towering [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud), [cumulus](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud-families) and [cumulonimbus](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud-families): the [front](#def-g8-fronts-and-weather-maps-front) brings heavy showers, sometimes [hail](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) and thunder, gusts of wind and a sudden drop in temperature. Once it has passed, the [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) stops and the sky clears. A [cold front](#def-g8-fronts-and-weather-maps-front) moves fast, up to twice as fast as a [warm front](#def-g8-fronts-and-weather-maps-front).

![A cold front seen from the side (heights much exaggerated). The cold air pushes under the warm air like a wedge; the warm air is lifted steeply and builds towering clouds.](https://one-course.com/images/onecourse/chapters/earth-1/g8-fronts-and-weather-maps/fig-850516211692.svg)

*A [cold front](#def-g8-fronts-and-weather-maps-front) seen from the side (heights much exaggerated). The cold air pushes under the warm air like a wedge; the warm air is lifted steeply and builds towering [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud).*

At a [warm front](#def-g8-fronts-and-weather-maps-front), the light warm air cannot push the cold air away from the ground: it climbs slowly over it, on a long and gentle slope. Its [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud) therefore appear far ahead of the [front](#def-g8-fronts-and-weather-maps-front), high in the sky: first thin streaks of [cirrus](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud-families), then a veil of cirrostratus, then grey altostratus. As the [front](#def-g8-fronts-and-weather-maps-front) comes nearer, the [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud) get lower and thicker and a long, steady [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) begins. After it has passed, the air is warmer and damper.

![A warm front seen from the side (heights much exaggerated). The warm air climbs slowly over the cold air; its clouds come lower and thicker as the front approaches, and the rain is long and steady.](https://one-course.com/images/onecourse/chapters/earth-1/g8-fronts-and-weather-maps/fig-e06f6d5bcede.svg)

*A [warm front](#def-g8-fronts-and-weather-maps-front) seen from the side (heights much exaggerated). The warm air climbs slowly over the cold air; its [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud) come lower and thicker as the [front](#def-g8-fronts-and-weather-maps-front) approaches, and the [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) is long and steady.*

On a [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map), a [cold front](#def-g8-fronts-and-weather-maps-front) is drawn as a blue line with triangles, a [warm front](#def-g8-fronts-and-weather-maps-front) as a red line with half circles. The symbols sit on the side towards which the [front](#def-g8-fronts-and-weather-maps-front) is moving. When a fast [cold front](#def-g8-fronts-and-weather-maps-front) catches up with a [warm front](#def-g8-fronts-and-weather-maps-front), the two join into an *occluded [front](#def-g8-fronts-and-weather-maps-front)*, drawn in purple with both symbols.

## 25.3 Depressions and anticyclones

[Fronts](#def-g8-fronts-and-weather-maps-front) are born in the great whirls of air that cross the middle [latitudes](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude). Where air rises, the pressure at the ground is lower than all around; where it sinks, the pressure is higher.

**Definition 25.3 (Depression and anticyclone).**

A *depression*, or low, is a region where the pressure at [sea level](https://one-course.com/books/earth/1/en/chapter/9-the-physical-map-of-the-world#def-g4-world-physical-map-altitude) is lower than all around it. An *anticyclone*, or high, is a region where it is higher than all around it.

The wind blows from the high towards the low, but the Earth’s rotation turns it aside, so it ends up blowing almost around the centres. In the [northern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) it turns anticlockwise around a [depression](#def-g8-fronts-and-weather-maps-depression) and clockwise around an [anticyclone](#def-g8-fronts-and-weather-maps-depression); in the [southern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), the other way round. In a [depression](#def-g8-fronts-and-weather-maps-depression), the air flows in near the ground, rises, cools and forms [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud): [depressions](#def-g8-fronts-and-weather-maps-depression) bring [cloud](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud), [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) and wind, and their [fronts](#def-g8-fronts-and-weather-maps-front). In an [anticyclone](#def-g8-fronts-and-weather-maps-depression), the air sinks and warms as it comes down: [anticyclones](#def-g8-fronts-and-weather-maps-depression) bring dry, settled [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) with light winds. A typical [anticyclone](#def-g8-fronts-and-weather-maps-depression) has a pressure of about $1020\,\mathrm{hPa}$ or more at its centre; a deep [depression](#def-g8-fronts-and-weather-maps-depression) falls below $980\,\mathrm{hPa}$.

![A depression south-west of Iceland (the island at the top right) seen by NASA’s Aqua satellite on 4 September 2003. Its clouds spiral in towards the centre, turning anticlockwise as in every depression of the northern hemisphere. NASA/GSFC, MODIS Rapid Response Team, Jacques Descloitres (public domain, via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g8-fronts-and-weather-maps/img-b7a0a7ecffb0.jpg)

*A [depression](#def-g8-fronts-and-weather-maps-depression) south-west of Iceland (the island at the top right) seen by NASA’s Aqua satellite on 4 September 2003. Its [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud) spiral in towards the centre, turning anticlockwise as in every [depression](#def-g8-fronts-and-weather-maps-depression) of the [northern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator). NASA/GSFC, MODIS Rapid Response Team, Jacques Descloitres (public domain, via Wikimedia Commons).*

## 25.4 Isobars and weather maps

To see the [depressions](#def-g8-fronts-and-weather-maps-depression) and [anticyclones](#def-g8-fronts-and-weather-maps-depression), forecasters draw [maps](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) of the pressure. Every station measures its pressure and reduces it to [sea level](https://one-course.com/books/earth/1/en/chapter/9-the-physical-map-of-the-world#def-g4-world-physical-map-altitude), so that a station on a [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) can be compared with one on the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). Then lines are drawn through all the places where the pressure is the same.

**Definition 25.4 (Isobar).**

An *isobar* is a line on a [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) joining places where the pressure at [sea level](https://one-course.com/books/earth/1/en/chapter/9-the-physical-map-of-the-world#def-g4-world-physical-map-altitude) is the same. Isobars are labelled in hectopascals and are often drawn every $4\,\mathrm{hPa}$.

Around a [depression](#def-g8-fronts-and-weather-maps-depression) the [isobars](#def-g8-fronts-and-weather-maps-isobar) form closed loops, with the lowest pressure at the centre, marked L; around an [anticyclone](#def-g8-fronts-and-weather-maps-depression) they form loops with the highest pressure at the centre, marked H. Where the [isobars](#def-g8-fronts-and-weather-maps-isobar) are close together, the pressure changes quickly from one place to the next, and the wind is strong; where they are far apart, the wind is light.

![A weather map of an imaginary day in the northern hemisphere, computed from a model of the pressure. Isobars every 4\, hPa; the depression L (980\, hPa) carries a warm front (red) and a cold front (blue); the anticyclone H (1034\, hPa) lies to the east. Green arrows: the wind, longer where it is stronger. T: the town of the problem.](https://one-course.com/images/onecourse/chapters/earth-1/g8-fronts-and-weather-maps/fig-c6392c0bec10.svg)

*A [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) of an imaginary day in the [northern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), computed from a model of the pressure. [Isobars](#def-g8-fronts-and-weather-maps-isobar) every $4\,\mathrm{hPa}$; the [depression](#def-g8-fronts-and-weather-maps-depression) L ($980\,\mathrm{hPa}$) carries a [warm front](#def-g8-fronts-and-weather-maps-front) (red) and a [cold front](#def-g8-fronts-and-weather-maps-front) (blue); the [anticyclone](#def-g8-fronts-and-weather-maps-depression) H ($1034\,\mathrm{hPa}$) lies to the east. Green arrows: the wind, longer where it is stronger. T: the town of the problem.*

**Method 25.5 (Reading a weather map).**

1. Find the centres: L for the [depressions](#def-g8-fronts-and-weather-maps-depression) , H for the [anticyclones](#def-g8-fronts-and-weather-maps-depression) , with their pressures.
2. Read the [isobars](#def-g8-fronts-and-weather-maps-isobar) : their values rise towards H and fall towards L. Close [isobars](#def-g8-fronts-and-weather-maps-isobar) mean a strong wind.
3. Find the wind’s direction: in the [northern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) it blows nearly along the [isobars](#def-g8-fronts-and-weather-maps-isobar) , with the low pressure on its left, turned a little towards the low.
4. Find the [fronts](#def-g8-fronts-and-weather-maps-front) and the way they move (the side of their symbols), then the air on each side: what is coming next?

**Example 25.6 (The weather at the town T).**

On the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map), T lies south of the [warm front](#def-g8-fronts-and-weather-maps-front) and east of the [cold front](#def-g8-fronts-and-weather-maps-front): in the warm air between the two [fronts](#def-g8-fronts-and-weather-maps-front). The [warm front](#def-g8-fronts-and-weather-maps-front) has already passed it. The [isobars](#def-g8-fronts-and-weather-maps-isobar) run from the south-west to the north-east there, with the low to the north-west: the wind blows from the south-west, warm and damp. The [cold front](#def-g8-fronts-and-weather-maps-front) is coming from the west. Its triangles point east, towards the town: within hours there will be a short spell of heavy showers and gusts, then the wind will turn to the west or north-west, the air will get cooler and the sky will clear.

A barometer gives its own warning, even without a [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map). Suppose the whole [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) above drifted east, without changing, at $40\,\mathrm{km}$ an hour, and a recording barometer stood at a station S, east of the [fronts](#def-g8-fronts-and-weather-maps-front). Over two and a half days it would draw the record below.

![The record of a barometer at the station S while the map above drifts east at 40\, km an hour (an invented situation, computed from the map’s pressure field). Dashed lines: the warm front and the cold front passing the station.](https://one-course.com/images/onecourse/chapters/earth-1/g8-fronts-and-weather-maps/fig-7e29c7dd6b7e.svg)

*The record of a barometer at the station S while the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) above drifts east at $40\,\mathrm{km}$ an hour (an invented situation, computed from the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map)’s pressure field). Dashed lines: the [warm front](#def-g8-fronts-and-weather-maps-front) and the [cold front](#def-g8-fronts-and-weather-maps-front) passing the station.*

**Example 25.7 (Reading a barometer’s record).**

For the first 8 hours the pressure rises to about $1031\,\mathrm{hPa}$: the [anticyclone](#def-g8-fronts-and-weather-maps-depression) passes over S, with settled [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather). Then it falls for 30 hours, by about $45\,\mathrm{hPa}$: the [depression](#def-g8-fronts-and-weather-maps-depression) is coming. It falls fastest a few hours before the [warm front](#def-g8-fronts-and-weather-maps-front) passes, more slowly in the warm air behind it, and reaches its lowest, about $986\,\mathrm{hPa}$, just as the [cold front](#def-g8-fronts-and-weather-maps-front) passes. Then it rises: the [depression](#def-g8-fronts-and-weather-maps-depression) is moving away, and the cooler, clearer air behind the [cold front](#def-g8-fronts-and-weather-maps-front) arrives.

## 25.5 How a forecast is made

**Definition 25.8 (Weather forecast).**

A *weather forecast* is a statement of the [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) expected at a place over the coming hours or days, worked out from observations of the atmosphere and from the laws of physics.

A forecast is made in three steps. First, [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) services around the world make millions of observations every day: temperature, rainfall, pressure, wind and humidity, measured at ground stations, at [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), by [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) balloons, by radar and from satellites. Then these observations are fed into computer models: very powerful computers work out, step by step, how the atmosphere will change from its present state. Finally, forecasters read the results, compare them with what they know of their region, and write the forecast.

**History — The first public forecast.**

In 1854 a naval officer, Robert FitzRoy, was put at the head of a new [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) office in London. Every morning at 9 o’clock, stations around the [coasts](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) measured the [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) and sent their readings to London by the telegraph, the newest invention of the day. In 1859 a terrible [storm](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-storm), the *Royal Charter* gale, sank many ships; FitzRoy then set up a service of [storm](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-storm) warnings for sailors, which began in February 1861. On 1 August 1861 he went further and published the first public [weather forecast](#def-g8-fronts-and-weather-maps-forecast) in a newspaper, *The Times*. His forecasts looked one or two days ahead and were very general; they were the start of the forecasts we read today.

Forecasts have become much better. Over the past decades, forecasters have gained about one extra day of good forecast every ten years: a forecast for four days ahead is now as good as a forecast for one day ahead was thirty years ago. But the further ahead a forecast looks, the less sure it is, because small errors in the starting observations grow as the computer goes forward in time.

**In the field — Watching a front arrive.**

On a day when the forecast announces a [front](#def-g8-fronts-and-weather-maps-front), look at the sky every few hours and note the time, the [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud), the wind and the temperature. Before a [warm front](#def-g8-fronts-and-weather-maps-front), do high streaks of [cirrus](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud-families) come first? When a [cold front](#def-g8-fronts-and-weather-maps-front) passes, does the temperature drop and the wind change? Compare your notes with the [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) of the day.

## 25.6 Exercises

**Exercise 25.1 ★.**

Give the meaning of each pair of letters and say whether the [air mass](#def-g8-fronts-and-weather-maps-air-mass) is warm or cold, damp or dry: (a) mT; (b) cP; (c) cT; (d) mP.

**Solution of Exercise 25.1.**

(a) Maritime tropical: warm and damp. (b) Continental polar: cold and dry. (c) Continental tropical: hot and dry. (d) Maritime polar: cold and damp.

**Exercise 25.2 ★.**

Match each sign of a [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) with its meaning: a blue line with triangles; a red line with half circles; a line joining places with the same pressure; the letter H; the letter L.

**Solution of Exercise 25.2.**

Blue line with triangles: a [cold front](#def-g8-fronts-and-weather-maps-front). Red line with half circles: a [warm front](#def-g8-fronts-and-weather-maps-front). Line joining places with the same pressure: an [isobar](#def-g8-fronts-and-weather-maps-isobar). H: the centre of an [anticyclone](#def-g8-fronts-and-weather-maps-depression) (high pressure). L: the centre of a [depression](#def-g8-fronts-and-weather-maps-depression) (low pressure).

**Exercise 25.3 ★.**

True or false? Correct the false sentences. (a) [Anticyclones](#def-g8-fronts-and-weather-maps-depression) usually bring settled [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather). (b) The [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud) of a [warm front](#def-g8-fronts-and-weather-maps-front) arrive after the [front](#def-g8-fronts-and-weather-maps-front). (c) Close [isobars](#def-g8-fronts-and-weather-maps-isobar) mean a light wind. (d) Cold air is denser than warm air.

**Solution of Exercise 25.3.**

(a) True. (b) False: they arrive before it, far ahead, high in the sky. (c) False: close [isobars](#def-g8-fronts-and-weather-maps-isobar) mean a strong wind. (d) True.

**Exercise 25.4 ★.**

On the [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) of this chapter: (a) what is the pressure at the centre of the [depression](#def-g8-fronts-and-weather-maps-depression)? (b) Of the [anticyclone](#def-g8-fronts-and-weather-maps-depression)? (c) How many hectopascals separate two neighbouring [isobars](#def-g8-fronts-and-weather-maps-isobar)? (d) By how many hectopascals does the pressure differ between the two centres?

**Solution of Exercise 25.4.**

(a) $980\,\mathrm{hPa}$. (b) $1034\,\mathrm{hPa}$. (c) $4\,\mathrm{hPa}$. (d) $1034 -
980 = 54\,\mathrm{hPa}$.

**Exercise 25.5 ★★.**

Put these events in the order in which they happen when a [warm front](#def-g8-fronts-and-weather-maps-front) approaches and passes: steady [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation); thin streaks of [cirrus](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud-families); a grey veil of altostratus; warmer, damper air; the [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud) come lower and thicker.

**Solution of Exercise 25.5.**

Thin streaks of [cirrus](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud-families); a grey veil of altostratus; the [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud) come lower and thicker; steady [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation); warmer, damper air.

**Exercise 25.6 ★★.**

Explain why the [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud) of a [cold front](#def-g8-fronts-and-weather-maps-front) are towering, while those of a [warm front](#def-g8-fronts-and-weather-maps-front) are spread in layers. Use the words *wedge*, *steep*, *gentle slope*.

**Solution of Exercise 25.6.**

At a [cold front](#def-g8-fronts-and-weather-maps-front), the dense cold air pushes under the warm air like a *wedge*: the warm air is lifted on a *steep* slope, rises fast and builds tall [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud). At a [warm front](#def-g8-fronts-and-weather-maps-front), the warm air climbs slowly over the cold air on a *gentle slope*: it rises little by little over a long distance, and its [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud) spread out in layers.

**Exercise 25.7 ★★.**

On the [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map): (a) where is the wind strongest, near L or near H? How can you tell? (b) North of L, from which direction does the wind blow? (c) South of H? (d) Does the wind turn clockwise or anticlockwise around L?

**Solution of Exercise 25.7.**

(a) Near L: the [isobars](#def-g8-fronts-and-weather-maps-isobar) are closer together there, and the arrows are longer. (b) From the east (the arrows point west, a little towards the south-west). (c) From the east (the arrows point west). (d) Anticlockwise.

**Exercise 25.8 ★★.**

A barometer in a town reads $1016\,\mathrm{hPa}$ at 6:00, $1009\,\mathrm{hPa}$ at 12:00 and $1003\,\mathrm{hPa}$ at 18:00. (a) By how much does the pressure fall in 12 hours? (b) What is coming towards the town, a [depression](#def-g8-fronts-and-weather-maps-depression) or an [anticyclone](#def-g8-fronts-and-weather-maps-depression)? (c) What [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) can the town expect?

**Solution of Exercise 25.8.**

(a) $1016 - 1003 = 13\,\mathrm{hPa}$. (b) A [depression](#def-g8-fronts-and-weather-maps-depression): the pressure keeps falling. (c) [Clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud), wind and [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation), probably with a [front](#def-g8-fronts-and-weather-maps-front).

**Exercise 25.9 ★★.**

Air that has stayed for a week over the Sahara moves north over the Mediterranean [Sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). (a) Name this [air mass](#def-g8-fronts-and-weather-maps-air-mass) with its two letters. (b) Is it hot or cold, damp or dry, when it leaves? (c) What could change in its character as it crosses the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean)?

**Solution of Exercise 25.9.**

(a) cT, continental tropical. (b) Hot and dry. (c) Over the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) it takes up water vapour: it becomes damper, and the air near the water is cooled a little by the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean).

**Exercise 25.10 ★★.**

A forecast for four days ahead is now as good as a forecast for one day ahead was thirty years ago. (a) How many days of good forecast have been gained in thirty years? (b) How many is that for each ten years? (c) If the same progress went on, in how many years would a five-day forecast be as good as today’s one-day forecast?

**Solution of Exercise 25.10.**

(a) $4 - 1 = 3$ days. (b) $3 \div 3 = 1$ day for each ten years. (c) It needs $5 - 1 = 4$ more days: about 40 years.

**Exercise 25.11 ★★★.**

On the [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map), use the [scale](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-scale) bar. (a) About how many kilometres separate the centre L from the centre H? (b) Near L, two neighbouring [isobars](#def-g8-fronts-and-weather-maps-isobar) are about $80\,\mathrm{km}$ apart. By how many hectopascals does the pressure change over $100\,\mathrm{km}$ there? (c) Near H, two neighbouring [isobars](#def-g8-fronts-and-weather-maps-isobar) are about $140\,\mathrm{km}$ apart. Same question. (d) Where is the wind stronger? Do the arrows of the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) agree?

**Solution of Exercise 25.11.**

(a) About $1350\,\mathrm{km}$ (four and a half [scale](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-scale) bars). (b) $4\,\mathrm{hPa}$ over $80\,\mathrm{km}$: $4 \div 80 \times 100 = 5\,\mathrm{hPa}$ per $100\,\mathrm{km}$. (c) $4 \div 140 \times 100 \approx 2.9\,\mathrm{hPa}$ per $100\,\mathrm{km}$. (d) Near L, where the pressure changes faster; yes, the arrows are longer near L.

**Exercise 25.12 ★★★.**

A [cold front](#def-g8-fronts-and-weather-maps-front) lies $300\,\mathrm{km}$ west of a town and moves east at $50\,\mathrm{km}$ per hour; a [warm front](#def-g8-fronts-and-weather-maps-front) lies $200\,\mathrm{km}$ west of another town and moves at half that speed. (a) Which [front](#def-g8-fronts-and-weather-maps-front) reaches its town first, and how many hours earlier? (b) Describe the [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) each town will see when its [front](#def-g8-fronts-and-weather-maps-front) passes. (c) Which town will be warmer once its [front](#def-g8-fronts-and-weather-maps-front) has passed?

**Solution of Exercise 25.12.**

(a) The [cold front](#def-g8-fronts-and-weather-maps-front): $300 \div 50 = 6$ hours. The [warm front](#def-g8-fronts-and-weather-maps-front) moves at $25\,\mathrm{km}$ per hour: $200 \div 25 = 8$ hours. The [cold front](#def-g8-fronts-and-weather-maps-front) arrives 2 hours earlier. (b) First town: towering [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud), heavy showers, gusts, then cooler air and a clearing sky. Second town: lowering layered [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud) and a long, steady [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation), then warmer, damper air. (c) The second town, reached by warm air.

## 25.7 Problem: Will the match be rained off?

**Problem 25.1.**

Weekend problem — a football match is planned for 15:00 in the town T of the weather map; the cold front is coming: its speed is measured on two maps and the hour it reaches the town is forecast

At 09:00 a young forecaster looks at the [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) of this chapter, then at the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) below: it shows the [cold front](#def-g8-fronts-and-weather-maps-front) near the town at 06:00 and at 09:00, on a grid of squares of $20\,\mathrm{km}$.

![](https://one-course.com/images/onecourse/chapters/earth-1/g8-fronts-and-weather-maps/fig-69428343f196.svg)

**Part I — Reading the map.**

1. On the [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) of the chapter, is the town in the warm air or in the cold air? Which [front](#def-g8-fronts-and-weather-maps-front) has already passed over it?
2. From which direction does the wind blow at the town?
3. Which way is the [cold front](#def-g8-fronts-and-weather-maps-front) moving? How can you tell from its symbols?
4. What [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) will the [cold front](#def-g8-fronts-and-weather-maps-front) bring when it passes?

**Part II — The front’s speed.**

5. On the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) above, at the height of the town, how far east is the [front](#def-g8-fronts-and-weather-maps-front) at 06:00? At 09:00?
6. How many kilometres has the [front](#def-g8-fronts-and-weather-maps-front) moved in these three hours?
7. What is its speed, in kilometres per hour?
8. At 09:00, how many kilometres separate the [front](#def-g8-fronts-and-weather-maps-front) from the town?

**Part III — The forecast.**

9. If the [front](#def-g8-fronts-and-weather-maps-front) keeps the same speed, how many hours will it need to reach the town?
10. At what time will it reach the town?
11. Suppose the heavy showers last about one hour after the [front](#def-g8-fronts-and-weather-maps-front) passes. Will it still be raining at 15:00?
12. The [front](#def-g8-fronts-and-weather-maps-front) could slow down to $30\,\mathrm{km}$ per hour. When would it reach the town then? Write the forecast for the match in one or two sentences, giving both possible hours.

**Solution of Problem 25.1.**

**1.** In the warm air, between the two [fronts](#def-g8-fronts-and-weather-maps-front). The [warm front](#def-g8-fronts-and-weather-maps-front) has already passed.

**2.** From the south-west.

**3.** Towards the east: the triangles of a [cold front](#def-g8-fronts-and-weather-maps-front) lie on the side towards which it moves, and they point east, towards the town.

**4.** Towering [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud), heavy showers, perhaps thunder and [hail](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation), gusts of wind; then cooler air, a wind from the west or north-west and a clearing sky.

**5.** At $180\,\mathrm{km}$ at 06:00 and at $300\,\mathrm{km}$ at 09:00.

**6.** $300 - 180 = 120\,\mathrm{km}$.

**7.** $120 \div 3 = 40\,\mathrm{km}$ per hour.

**8.** The town is at $480\,\mathrm{km}$: $480 - 300 = 180\,\mathrm{km}$.

**9.** $180 \div 40 = 4.5$ hours, that is 4 hours and 30 minutes.

**10.** 09:00 + 4 h 30 min = **13:30**.

**11.** No: the showers should end at about 14:30, before the kick-off.

**12.** $180 \div 30 = 6$ hours: at 15:00, just at the kick-off. Forecast: “The [cold front](#def-g8-fronts-and-weather-maps-front) should reach the town at about **13:30**, with an hour of heavy showers and gusts, then clearing skies: the match can most likely be played. If the [front](#def-g8-fronts-and-weather-maps-front) slows down, it could arrive at 15:00 and delay the match.”