When a mass of cold air overtakes a mass of warm air, the cold air being denser, stays on the surface and undercuts the warm air violently. Surface friction tends to slowdown the surface air while a sharp fall in temperature, a rise in pressure and rapid clearing usually occur with the passage of the cold front.

When the progress of time as a depression advances, the cold front gradually overtakes the warm front and lifts the warm sector entirely from the ground. It is simply a case of the cold air catching up with itself as it flows around the depression. Thus only one front remains, which is called an occluded front or occlusion. An occluded depression soon commences to fill up and die away.

A developing cyclone typically has a preceding warm front (the leading edge of a warm moist air mass) and a faster moving cold front (the leading edge of a colder drier air mass wrapping around the storm). North of the warm front is a mass of cooler air that was in place before the storm even entered the region.

As the storm intensifies, the cold front rotates around the storm and catches the warm front. This forms an occluded front, which is the boundary that separates the new cold air mass (to the west) from the older cool air mass already in place north of the warm front. Symbolically, an occluded front is represented by a solid line with alternating triangles and circles pointing the direction the front is moving. On coloured weather maps, an occluded front is drawn with a solid purple line.

Changes in temperature, dew point temperature, and wind direction can occur with the passage of an occluded front. In the map below, temperatures ahead (east of) the front were reported in the low 40's while temperatures behind (west of) the front were in the 20's and 30's. The lower dew point temperatures behind the front indicate the presence of drier air.

A noticeable wind shift also occurred across the occluded front. East of the front, winds were reported from the east-southeast while behind the front, winds were from the west-southwest. Common characteristics associated with occluded fronts have been listed in the table below.

The cold air, in the distance it has travelled, may have undergone considerable change. Therefore it may not be as cold as the air it is overtaking. In this case (cool air advancing on colder air), the front is known as an occluded warm front or a warm occlusion and has the characteristics of a warm front, with low cloud and continuous rain and drizzle. It the warm air is unstable, heavy cumulus or cumulonimbus cloud may be embedded in the stratiform cloud bank.

It the cold air is colder than the air it is overtaking (cold air advancing on cool air), the front is known as an occluded cold front or a cold occlusion. A cold occlusion has much the same characteristics as a warm front, with low cloud and continuous rain. If the warm air is unstable, cumulonimbus and thunderstorms are likely to occur, with the violent turbulence, lightning and icing conditions associated with these clouds.

In Canada, the term upper front refers to a non-occlusion situation.  Sometimes, cold air advancing across the country may encounter a shallow layer of colder air resting on the surface or trapped in a topographical depression. The advancing cold air rides up over the colder, heavier air. The cold front which is the leading edge of the advancing cold air, therefore, leaves the ground and moves along the top of the colder air. It is then known as an upper cold front.

Sometimes, the structure of the advancing cold front is such that the cold air forms a shallow layer for some distance along the ground in advance of the main body of cold air. The frontal surface If the main mass of cold air, in this situation, will usually be very steep. The line along which the frontal surface steepens is also known as an upper cold front.

On occasion, an advancing warm front rides up over a pool or layer of cold air trapped on the ground. A station on the ground does not experience a change of air mass because the front passes overhead. This is known as an upper warm front.

Sometimes, the surface of the cold air that is retreating ahead of an advancing warm front is almost flat for some distance ahead of the surface front and then steepens abruptly. The line along which the surface of the retreating cold air steepens sharply is also called an upper warm front.

frontal weather

The theory of the polar front, which for the sake of simplicity has been described in the form of its original conception, might leave the impression that depressions form only along some well defined line Iying somewhere midway between the poles and the equator. Air masses are in a constant state of formation over all the land and water areas of the world. Once formed, they tend to move away from the source regions over which they form. The same frontal processes and phenomena occur whenever a mass of warm air and a mass of cold air come in contact.

There is a widespread impression among pilots that fronts always bring bad weather and that all bad weather is frontal. Actually some fronts have little or no weather associated with them. A slight change of temperature and a windshift may be the only evidence that the front has gone through. And, of course, bad weather can develop without the passage of a front. Fog, for example, generally occurs when no fronts are present and severe thunderstorms may develop in an air mass, which has no frontal characteristics.

Another common misconception is that the front is a thin wall of weather. This false idea is perhaps occasioned by the line that indicates a front on a weather map. The line on the map only shows the surface location at which the pressure change, windshift and temperature change occur. The actual weather associated with the front may extend over an area many miles in width, both well ahead and also for many miles behind the actual line on the weather map.

Frontolysis means a front, which is decreasing in intensity.

If you examine the diagrams showing fronts on a weather map, you will notice that all fronts lie in regions of lower pressure. The isobars are bent sharply at a front. These two factors are characteristic of all fronts.

weather at the cold front

Cold fronts are not all the same. The weather associated with a cold front may vary from a minor windshift to severe thunderstorms, low ceilings, restricted visibility and violent gusty winds. The severity of the weather is determined by the moisture content and stability of the warm air mass that the cold air mass is undercutting and the speed of the advancing cold front.

Fast moving cold fronts may travel across the country with a speed of 30 knots or more.  If the warm air that is being undercut by the cold air mass is very moist and unstable, towering cumulus clouds and thunderstorms are likely to develop. Heavy rain or hail may be associated with the front. A slower moving cold front advancing on more stable and drier air in the warm sector will produce less severe weather conditions, stratus or altocumulus clouds with light or no precipitation.

A long line of cumulus clouds on the western horizon is usually an indication of an approaching cold front. Sometimes a deck of altocumulus cloud or decks of stratus and stratocumulus extending ahead of the front will mask the main frontal cloud from the view of the high flying or low flying pilot respectively.

weather changes

Surface Wind: The wind direction will always veer as the front passes. Gustiness may be associated with the windshift.

In flying through a cold front, the windshift may be quite abrupt and occurs at the frontal surface rather than at the front. The windshift is always such that an alteration in course to starboard is required, no matter which way you are flying through the front.

Temperature: On the ground, the temperature may drop sharply as the front passes, but usually it drops gradually. The air immediately behind the front has been warmed in passing over the warm ground. Therefore, it may be several hours before the temperature drops to the true value of the cold air mass. In flying through a cold front, there will be a noticeable temperature change when passing through the frontal surface.

Visibility: Visibility usually improves after passage of a cold front. If the front is moving fairly rapidly, the width of frontal weather generally is less than 50 miles. If the front is moving slowly, however, flight operations may be affected for many hours.

Pressure: The approach of a cold front is accompanied by a decrease in pressure. A marked rise will be noticed when the front has passed.

Turbulence: Turbulence may be associated with the cold front if it is active, although thunderstorms are not always present. Even in cases where there are no clouds, turbulence may be a problem. As a rule, flight through an active cold front can be expected to be rough.

Precipitation: The frontal rain or snow is usually narrow, especially if it is showery in character. Icing in the turbulent cumulus clouds can be severe.

Warm front changes are usually less pronounced than cold front changes. The change is also generally very gradual. However, the weather at a warm front is usually more extensive and may cover thousands of square miles. A wide variety of weather characterizes warm fronts. The weather may even vary along a given front.

The degree of overrunning and the moisture content and stability of the overrunning warm air determine the seventy of the weather. If the warm air is very moist, the cloud deck forming in the overrunning air may extend for hundreds of miles up the slope of the retreating cold air. It the warm air is unstable, thunderstorms may be embedded in the cloud deck.

High cirrus cloud is the first sign of the approach of an active warm front. Cirrostratus soon follows (the high thin cloud which causes a halo around the sun or moon). The cloud gradually thickens and the base lowers until a solid deck of altostratus/altocumulus covers the area. Low nimbostratus moves in, merging with the altostratus. With the result that a solid deck of cloud extending from near the surface to 25,000 feet or more covers the whole area. Precipitation is usually heavy.

weather changes

Windshift: With the passage of a warm front, the wind will veer, but the change will be much more gradual than in the case of a cold front.

When flying through a warm front, the windshift will occur at the frontal surface and will be more noticeable at lower levels. When flying through a warm front, the windshift is such that a course alteration to starboard is necessary.

Temperature: The warm front brings a gradual rise in temperature. A pilot flying through the frontal surface will notice a more abrupt temperature rise.

Visibility: Low ceilings and restricted visibility are associated with warm fronts and, because warm fronts usually move quite slowly, these conditions persist for considerable time.

When rain falls from the overrunning warm air, masses of irregular cloud with very low bases form in the cold air. Fog is frequently a condition 50 nautical miles ahead of an advancing warm front.

Turbulence: Cumulonimbus clouds are frequently embedded in the main cloud deck and these storms are responsible for the most severe turbulence associated with a warm front. However, these storms and the turbulence they occasion are less severe than those associated with cold fronts. The principal problem with these storms is that they cannot be located by sight since they are embedded in the main cloud cover.

Precipitation: The first precipitation begins in the region where the altostratus layer of cloud is from 8000 to 12,000 feet above the ground. As the front approaches, the precipitation becomes heavier. Occasional very heavy precipitation is an indication of the presence of thunderstorms.

winter warm fronts

In winter, when temperatures in the cold air are below freezing and temperatures in the lower levels of the warm air are above freezing, snow and freezing rain can be expected.

Snow falls from that part of the warm air cloud that is high and therefore below freezing in temperature. From the lower cloud, where temperatures are above freezing, rain falls. However, as the rain falls through the cold air (of the cold air mass that the warm air is overrunning), it becomes supercooled and will freeze on contact with any cold object. This is known as freezing rain (ZR).

In the area ahead of the freezing rain, there is a region where the rain falling through the cold air becomes sufficiently supercooled to freeze and falls to the ground as ice pellets (IP). A pilot approaching the frontal surface at higher altitudes may not encounter the ice pellets, but the pilot flying at quite low altitudes can expect to encounter snow, ice pellets and then freezing rain.