Meteorology · Level R3 · 5 parts · about 15 min

Pressure, air masses, depressions and fronts

5 short parts: Pressure · Air masses · Depressions · Front passage · Highs. Each part explains the idea, then gives the points to remember, the common mistakes, a worked case and a practice question; the Academy unit adds all 31 check questions.

1 · Pressure

Air pressure, the barometer and wind

Differences in air pressure make the wind. The barometer's trend, not its number, is your best early warning.

What pressure is

Air pressure is the weight of air pressing on each unit area. Forecasts give it in hectopascals (hPa); one hPa equals one millibar (mb). The standard sea-level value is 1013.25 hPa. Real sea-level pressure usually lies between about 950 and 1050 hPa. On a chart, an isobar is a line joining places of equal pressure.

The barometer

On board you use an aneroid barometer, a sealed metal capsule that flexes as pressure changes, or an electronic one. Set it against a nearby official sea-level reading. Then log it regularly, at least every three hours at sea, and more often when it is falling.

Tendency matters

The words on an old dial, "rain" or "fair", are unreliable. What matters is the tendency, the change over the last three hours. Met Office terms: slowly 0.1–1.5 hPa, plain rising or falling 1.6–3.5 hPa, quickly 3.6–6 hPa, very rapidly over 6 hPa. A quick fall warns of strengthening wind; check the forecast at once.

Why the wind blows

A pressure gradient is a change in pressure over distance. A steeper gradient generally drives stronger wind. Air is pushed from high towards low pressure. Earth's rotation, the Coriolis effect, turns moving air right in the Northern Hemisphere and left in the Southern. Northern Hemisphere winds circle lows anticlockwise and highs clockwise; both reverse in the south.

Buys Ballot's law

In the Northern Hemisphere, stand with your back to the wind: low pressure is on your left. At the surface, friction turns the wind a little towards the low, so the low is slightly ahead of your left side. In the Southern Hemisphere, low pressure is on your right. The rule fails near the equator and in local winds.

Key figures

Standard sea-level pressure: 1013.25 hPa; 1 hPa = 1 mb
Falling slowly: 0.1–1.5 hPa in 3 hours
Falling quickly: 3.6–6 hPa in 3 hours
Falling very rapidly: More than 6 hPa in 3 hours
Northern Hemisphere circulation: Anticlockwise round a low, clockwise round a high
Buys Ballot (north): Back to the wind: low pressure on your left

Remember

Pressure is given in hPa (= mb); standard is 1013.25 hPa.
Log the barometer; the 3-hour tendency is what counts.
A quick fall, over 3.5 hPa in 3 hours, warns of stronger wind.
North: wind goes anticlockwise round lows, clockwise round highs.
Back to the wind in the north: the low is on your left.

Common mistakes

Wrong: A reading of 1000 hPa always means bad weather.
Right: The absolute value means little on its own. Watch the trend: a steady 1000 hPa can be quiet, a quick fall from 1020 hPa can bring a gale.
Wrong: Wind blows straight from high to low pressure.
Right: The Earth's rotation turns it, so it blows round the systems, crossing the isobars only slightly at the surface.
Wrong: Buys Ballot's law is the same in both hemispheres.
Right: In the south it is reversed: back to the wind, the low is on your right.

Worked case

At 0900 your barometer reads 1012 hPa, at 1200 it reads 1007 hPa. You are in the Mediterranean with your back to a south-easterly wind. What do you learn?

Show the answer

A fall of 5 hPa in 3 hours is "falling quickly": the wind is likely to strengthen soon. With your back to a south-easterly, you face north-west. The low is roughly south-west of you, slightly ahead of your left side. Check the latest forecast and warnings now, and head for shelter if they confirm stronger wind.

Practice question

What is the standard atmospheric pressure at sea level?

1000.00 hPa
1013.25 hPa
1030.50 hPa
980.00 hPa

Show the answer

Answer: B. Standard sea-level pressure is 1013.25 hPa. Actual pressure varies, mostly between about 950 and 1050 hPa.

2 · Air masses

Air masses: where the air comes from

An air mass carries the weather of its source. Know where the air has come from and you can expect its cloud, visibility and showers.

What an air mass is

An air mass is a huge body of air, often over a thousand kilometres across, with broadly similar temperature and humidity across it at any given height. It gets them by staying over one source region, such as the Arctic, the subtropical ocean or a desert.

Names

Two letters describe it. Maritime (m) air formed over the sea and is moist; continental (c) air formed over land and is dry. Polar (P) air is cold; tropical (T) air is warm; Arctic (A) air is colder still. So mP is maritime polar air: cool and moist.

Changed by the journey

Air changes as it travels. Cold air moving over a warmer sea is heated from below and becomes unstable: heap clouds and showers. Warm air moving over a colder sea is cooled from below and becomes stable: low cloud, drizzle and sea fog. The Stability lesson in the Clouds unit explains why.

Air masses in Europe

mP from the north-west: cool, showery, very good visibility. mT from the south-west: mild, moist, low cloud, drizzle, sometimes fog. cP from the east in winter: very cold and dry, with snow showers where it crosses warmer sea. cT from North Africa: hot, dry, hazy and sometimes dusty. Arctic air from the north: very cold, with wintry showers.

Where they meet

Where two air masses meet, they do not mix easily. The boundary between them is a front. The polar front between cold polar air and warm tropical air is where most Atlantic depressions are born.

Key figures

mP (maritime polar): Cool, moist; showers, gusts, very good visibility
mT (maritime tropical): Mild, moist; stratus, drizzle, poor visibility, sea fog
cP (continental polar): Very cold, dry in winter; snow showers over warm sea
cT (continental tropical): Hot, dry; haze and dust, poor visibility
Front: Boundary between two air masses

Remember

An air mass takes the temperature and humidity of its source.
m = maritime (moist), c = continental (dry), P = polar, T = tropical.
Cold air over warmer sea: unstable, showers.
Warm air over colder sea: stable, low cloud and fog.
The boundary between air masses is a front.

Common mistakes

Wrong: Polar air always brings poor visibility.
Right: Maritime polar air over a warmer sea is unstable: visibility is usually very good between showers.
Wrong: Tropical air always means fine sunny weather.
Right: Maritime tropical air over a cooler sea often brings low cloud, drizzle and fog.

Worked case

In spring off north-west Spain, the forecast says a mild, moist south-westerly flow will replace a cool north-westerly. The sea is still cool. Would you keep a landfall plan that depends on seeing shore marks?

Show the answer

Cool, showery mP air with good visibility gives way to mild, moist mT air. Over the cool sea, the new air cools from below and becomes stable. Expect low cloud, drizzle and possible sea fog. Do not rely on seeing shore marks. Replan for poor visibility; postpone if you cannot make the landfall safely.

Practice question

Warm air forms over a tropical ocean. Which air-mass label fits?

Show the answer

Answer: A. The ocean source makes it maritime (m); the warm tropical source gives T.

3 · Depressions

Depressions: birth, life and movement

A depression grows on the polar front as a wave with a warm and a cold front. It deepens, occludes, then slowly fills.

What a depression is

A depression, or low, is an area of low pressure with closed isobars. In the Northern Hemisphere the wind circles it anticlockwise. Mid-latitude depressions bring most of the wind and rain to north-west Europe.

Birth on the polar front

It usually starts as a wave on the polar front. Warm air pushes towards the pole and cold air pushes towards the equator. Pressure falls at the tip of the wave. A warm front forms ahead of the warm air and a cold front behind it. Between them lies the warm sector.

Deepening and occlusion

A low is deepening when its central pressure falls. Winds often strengthen if the pressure gradient steepens. The cold front moves faster than the warm front and catches it. Warm air is lifted off the surface and an occluded front forms. The low is usually deepest near this stage. It later fills: central pressure rises. It often slows and winds ease, but check the local gradient and forecast.

Movement

A young low usually moves in the direction of the isobars in its warm sector, in the North Atlantic often east or north-east at 20–30 knots. Old, occluded lows slow down and may stall. New secondary lows can form on a trailing cold front and develop fast.

Where the wind is

The wind is strongest where the isobars are closest together. Explosive deepening means a central-pressure fall of at least about 24 hPa in 24 hours at 60° latitude, with a lower threshold farther south. Such rapid development can turn a moderate forecast into a storm. Always check the latest warnings.

Key figures

Deepening / filling: Central pressure falling / rising
Usual movement: Along the warm-sector isobars; often E or NE at 20–30 kn
Occlusion: Cold front catches warm front; warm air lifted aloft
Explosive deepening: At least about 24 hPa in 24 hours at 60°; lower farther south
Strongest wind: Where the isobars are closest together

Remember

A depression usually starts as a wave on the polar front.
It has a warm front, a warm sector and a cold front.
Deepening: central pressure falls; filling: it rises. Winds often strengthen or ease respectively.
The cold front catches the warm front to form an occlusion.
A young low moves along its warm-sector isobars.

Common mistakes

Wrong: A deepening low means the pressure is rising.
Right: Deepening means central pressure falls; filling means it rises. Winds often strengthen or ease respectively, but local wind depends on the pressure gradient.
Wrong: The strongest wind is always at the centre of a low.
Right: Near the centre the wind can be light. The strongest wind is where the isobars are tightest.
Wrong: Once a low occludes, the danger is over.
Right: A low is often deepest around occlusion, and secondary lows can form. Keep checking the warnings.

Worked case

At 1200 today, the synopsis reads: "Low 1002, 300 miles west of Ireland, deepening rapidly, expected 975 Irish Sea by 1200 tomorrow." What does that tell you about the next day?

Show the answer

The low is forecast to deepen by 27 hPa in 24 hours, meeting explosive-deepening criteria at this latitude. It is heading east towards the Irish Sea. Severe winds may develop, but their location depends on the pressure gradient. Check the wind forecast and warnings, and arrange shelter before conditions deteriorate.

Practice question

What does it mean when a depression is "filling"?

Its central pressure is falling
It is moving faster towards you
Its central pressure is rising
It is forming a new warm sector

Show the answer

Answer: C. Filling means central pressure rises. Winds often ease, but the wind at your position depends on the surrounding pressure gradient and the track of the low.

4 · Front passage

A depression passes: the sequence of a front

If a young Atlantic low passes north of you and both fronts cross your position, expect a warm front, a warm sector, then a cold front.

Ahead of the warm front

Here an eastward-moving Atlantic low passes north of you in the Northern Hemisphere, and both fronts cross your position. Watch for high wisps (cirrus), a thin veil (cirrostratus) sometimes with a halo, a grey sheet (altostratus), then thick rain cloud (nimbostratus). The barometer falls steadily. The wind backs (turns anticlockwise), for example from south-west to south, and strengthens. Rain becomes continuous and visibility falls.

At the warm front

As the warm front passes, the wind veers (turns clockwise), for example from south to south-west. The fall of the barometer slows or stops. The rain eases to drizzle and the temperature rises.

In the warm sector

In the warm sector the wind stays fairly steady from the south-west. Pressure is steady or falls slowly. The air is mild and moist: low sheet cloud (stratus), drizzle and poor visibility, with fog where it crosses a cold sea.

At the cold front

The cold front often passes quickly. The wind veers sharply, for example from south-west to west or north-west, often in a squall (a sudden, strong increase in wind). Heavy rain falls, sometimes with thunder or hail. Then the barometer starts to rise, often quickly, and the temperature drops.

Behind the cold front

Cooler air brings heap clouds, showers, gusts and very good visibility between the showers. An occluded front gives a mix of both fronts: a long band of rain and a wind veer, but no warm sector. If an eastward-moving low passes to your south instead, the wind backs steadily, for example from east to north-east to north, with no warm sector.

Key figures

Before the warm front: Pressure falls; wind backs and freshens; rain sets in
Warm front passing: Wind veers; fall slows; drizzle; temperature rises
Warm sector: Steady SW wind and pressure; stratus, drizzle, poor visibility
Cold front passing: Sharp veer, squalls, heavy rain; pressure rises, colder
Behind the cold front: Showers, gusts, very good visibility
Eastward-moving low passing south (NH): Wind backs steadily; no warm sector

Remember

Ahead of a warm front: falling pressure, backing wind, thickening cloud.
At each front the wind veers (low passing north, Northern Hemisphere).
Warm sector: mild, steady wind, low cloud, poor visibility.
Cold front: sharp veer, squalls, then rising pressure and showers.
Eastward-moving low passing south of you in the north: the wind backs instead.

Common mistakes

Wrong: When the rain stops in the warm sector, the bad weather is over.
Right: The warm sector is often a lull. The cold front, with squalls and a sharp wind shift, is still to come.
Wrong: A rising barometer after a cold front means the wind will drop at once.
Right: A quick rise often comes with strong, gusty wind behind the front. Wait for the wind to ease before increasing sail.
Wrong: Every front gives exactly the textbook sequence.
Right: Real fronts vary from weak to violent. Use the sequence as a guide and confirm with the forecast and barometer.

Worked case

You are at anchor in a bay open to the north-west. The forecast gives a low passing to your north tonight: "Southwest 5 to 7, veering northwest 6 to 8 after the cold front." Is the anchorage safe?

Show the answer

No. The bay is sheltered now from the south-west, but after the cold front the wind will veer to the north-west and blow straight in at up to force 8, often in squalls. The wind would push you towards the shore at night. Move before dark to a harbour or anchorage sheltered from the north-west.

Practice question

A warm front approaches from the west. What does the barometer usually do?

It rises steadily
It stays exactly steady
It rises, then falls
It falls steadily

Show the answer

Answer: D. Pressure falls steadily ahead of a warm front. The fall slows or stops as it passes.

5 · Highs

Highs, ridges, troughs and cols

A high brings sinking air, light wind and settled weather, but also haze and fog. Ridges, troughs and cols are the shapes between systems.

Anticyclones

A high, or anticyclone, is an area of high pressure with closed isobars. In the Northern Hemisphere the wind circles it clockwise, spiralling slightly outwards. Its isobars are usually wide apart, so wind near the centre is light. Highs move slowly and can stay for days, steering lows around them.

Weather in a high

Air sinks and warms, making it stable: vertical mixing is limited. An inversion, a layer where temperature increases with height, can trap haze and moisture below. Fair weather is common, but persistent low cloud can bring anticyclonic gloom. On calm nights, cooling near the surface can form fog. Strong winds can blow at the edge where a high lies beside a low.

Ridges

A ridge is a tongue of high pressure stretching out from a high; its isobars bulge outwards from the centre. It often brings fair weather and lighter winds. Between travelling lows, this may be a short lull. Check the forecast for how long it will last.

Troughs

A trough is a tongue of low pressure stretching out from a low. Many troughs carry a front or a line of showers. As one passes, the wind often veers in the Northern Hemisphere and showers or squalls may follow.

Cols

A col is the area between two highs and two lows, arranged like a cross. Pressure gradients are weak, so wind is light and variable. Cols can bring fog in winter, or thunderstorms in summer. Wind direction can change without warning when you leave one.

Key figures

High (NH): Clockwise, slightly outwards; light wind near centre
Sinking air: Stable air: fair, hazy or cloudy; calm-night cooling can form fog
Ridge: Extension of high pressure; often fair with lighter winds
Trough: Tongue of low pressure: showers, wind shift
Col: Between two highs and two lows: light, variable wind

Remember

Northern Hemisphere: wind circles a high clockwise.
Sinking air in a high is stable: fair, but hazy or foggy.
Strong wind can blow on the edge of a high next to a low.
A ridge often brings fair weather and lighter winds; a trough brings showers and a wind shift.
A col gives light, variable wind, fog or thunderstorms.

Common mistakes

Wrong: A high always means sunshine and perfect visibility.
Right: Sinking air traps haze and can give persistent low cloud or fog, especially in winter.
Wrong: There is never strong wind under high pressure.
Right: Where a high lies beside a low, the isobars can be close together and the wind strong.

Worked case

In October a large high sits over France for several days. The forecast for the Gulf of Lion gives light variable winds, "fog patches" at night and visibility "moderate or poor, occasionally very poor". You plan a night passage to the Balearics. Does the high make this an easy night passage?

Show the answer

No. Light wind does not guarantee good visibility. Stable air can trap haze; fog can drift off the cooled land or form where moist air crosses cooler water. Expect poor visibility and possible motoring. Choose a better departure window if your crew or equipment cannot manage those conditions, and check for stronger wind near the high's edge.

Practice question

In the Northern Hemisphere, how does the wind circulate round a high?

Anticlockwise, spiralling slightly in
Clockwise, spiralling slightly out
Anticlockwise, spiralling slightly out
Clockwise, spiralling slightly in

Show the answer

Answer: B. Wind circles a Northern Hemisphere high clockwise and spirals slightly outwards near the surface.