Fronts

A front is the boundary zone between two air masses of different properties (T, humidity, density). At fronts, lifting occurs → clouds, precipitation, weather change.

Source: WMO; AMS Glossary; DWD; FAA-H-8083-25B PHAK Ch. 12.

Definition

"Front" refers to a boundary between two air masses with lifting processes and weather phenomena:

At a front the warmer (lighter) air is lifted over the colder (denser) one.
Result: adiabatic cooling → clouds → precipitation.

"Air-mass boundary" vs front

The "boundary of an air mass" is a boundary between two different air masses at which weather phenomena hardly occur:

A pure boundary without active lifting → no weather.
A front is actively associated with weather phenomena.

Front types

Warm front (WF).
Cold front (CF).
Occluded front.
Stationary front.

Warm front

A warm front develops when warm, light air gently slides over denser cold air ahead, causing gradual ascent that leads to layered cloud formation and steady precipitation.

Properties:

Speed: slow (~15-25 km/h).
Slope: very shallow (1
to 1
) — at 1.5 km altitude the front is 150-300 km ahead of the surface position.
A warm front typically moves slower than a cold front with a shallower slope, where at 1.5 km height it lies about 320 km ahead of its surface position in the Northern Hemisphere, producing more uniform and widespread precipitation compared to cold fronts.

Cloud sequence on approach: Ci → Cs → As → Ns → St (see cloud-types lesson).

Weather ahead of the front: A pilot flying through a warm front in accordance with VFR needs to expect low clouds and a reduction of visibility in precipitation. Ahead of an approaching warm front, expect light to heavy precipitation (rain or snow) depending on available moisture and lifting intensity, as warm air ascends over cooler air, leading to widespread cloud formation and steady rain.

Winter phenomena at warm fronts:

Snowfall or freezing rain are possible at a warm front in winter.
Icing due to freezing rain has to be expected in front of warm fronts or warm front occlusion after winterly cold spells (see precipitation lesson).

Cold front

A cold front develops when faster-moving cold air wedges under warmer air ahead, lifting it off the ground and triggering cloud formation and precipitation.

Properties:

Speed: fast (30-60 km/h, sometimes more).
Slope: steep (1
to 1
) → compact weather zone.
Precipitation: heavy, short (showers, thunderstorms).

Summer weather at a cold front: In summer, cold fronts typically feature convective clouds such as cumulus (Cu), towering cumulus (TCu), and cumulonimbus (Cb), along with showers and thunderstorms due to the rapid lifting of warm, moist air by the advancing cold air mass.

Visibility reduction: Visibility greatly reduced at the front due to showers of precipitation, dense cumuliform clouds (Cu, Cb), showers and thunderstorms, descending cloud base, and turbulence characterize an approaching cold front in summer.

Dark wall of clouds: A dark wall of clouds signalling an approaching cold front poses an urgent take-off danger from greatly intensifying and gusty winds due to the rapid onset of unstable air and strong pressure gradients behind the front.

Winter weather: convective clouds (Cu, Cb), snow showers, fewer thunderstorms.

Backside weather

On the backside of a cold front, typical weather phenomena include cumuliform clouds, good thermals, gusty winds, and possible showers due to the unstable air mass that follows.

In meteorology "backside weather" means the typical weather behind a cold front: excellent visibility, increasing atmospheric pressure.

After a cold front passes, visibility becomes excellent due to the influx of drier, clearer air, while atmospheric pressure increases as the front is typically followed by a building high-pressure system.

Warm sector

The term "warm sector" is used for the area between a warm front and a cold front:

Warm air mass (mT) between the two fronts.
Typical weather: low-layer clouds (Sc, St), drizzle, poor visibility in haze.

South-westerly wind, scattered clouds, hardly any precipitation, moderate to good visibility are typical weather for a warm sector in summer.

High fog, drizzle, and poor visibility are typical in a winter warm sector.

Occlusion

An "occlusion process" refers to the merging of a cold front and a warm front in a dynamic low-pressure area, where the warm sector air is lifted off the ground, leading to the formation of an occluded front:

Cold front is faster than warm front → catches up.
Both merge → warm sector lifted off the ground.

Two types depending on which cold air is denser:

Cold occlusion

The air mass behind the occlusion is colder than the air ahead (for the cold-front type) → following denser cold air undercuts the leading cold air. Weather resembles a cold front (abrupt lift, showers from Cb).

Warm occlusion

In a warm-front occlusion, the air mass behind the occlusion is warmer than the air mass in front of the occlusion:

The leading cold air is denser → the following less-dense cold air slides over it.
Weather resembles a warm front (gradual lift, steady precipitation from Ns).

In occluded fronts, a warm occlusion mimics warm-front weather (gradual lifting, steady precipitation from nimbostratus) because the overtaking cold air is less dense and slides over the denser cold air ahead, while a cold occlusion resembles cold-front conditions (abrupt lifting, showers from cumulonimbus) as the denser overtaking cold air undercuts the less dense cold air ahead.

Wind changes in a low-pressure system (NH)

As a low-pressure area with fronts approaches in the Northern Hemisphere, wind directions change as follows: backing from southwest to southeast ahead of the warm front, remaining southwest in the warm sector (behind warm front and ahead of cold front), and veering from west to northwest behind the cold front.

In an ideal cyclone in the Northern Hemisphere, wind ahead of a cold front is typically southwest, becoming gusty and increasing during passage, then veering to northwest behind the front due to the system's counterclockwise circulation and pressure gradient changes.

Pressure-system sequence (W→E)

In a low-pressure system approaching from west to east, the sequence of pressure areas experienced by an observer is the warm front, warm sector, cold front, and backside due to the cyclone's counterclockwise rotation and frontal arrangement in the Northern Hemisphere.

Display on weather charts

In a colour surface weather chart, a cold front is represented as a line coloured blue:

Warm front: red.
Cold front: blue.
Occlusion: purple/magenta.
Stationary front: alternating red/blue.