Frontal Changes MET

This is one of the most detailed but important explanations, as the variants of the questions here are numerous. It is helpful if you are able to draw both a cross section and a plan view of a polar front depression (this has a warm front followed by a cold front), as these will greatly aid your understand of the explanations below. The explanation will take each variant and explain what happens at each stage in the passage of the polar front depression.

Pressure (plan view of PFDPFD —Primary Flight Display)

As the warm front approaches, pressure falls as you are crossing isobars towards the centre of the low. As the warm front passes, pressure stops falling. In the warm sector, pressure remains constant as you are generally following the line of an isobar. As the cold front passes, pressure starts increasing. After the cold front passes, pressure increases as you are crossing isobars away from the low.

Temperature (plan view of PFDPFD —Primary Flight Display)

Remember that fronts separate air masses, which themselves have a generally similar temperature.

As the warm front approaches, temperature remains constant or increases slightly. As the warm front passes, there is a marked increase (leading edge of warm air passes you). In the warm sector, temperature remains constant. As the cold front passes, there is a marked decrease (leading edge of cold air passes you). After the cold front passes, temperature remains relatively constant.

Surface wind (plan view of PFDPFD —Primary Flight Display)

At both the passage of the warm and the cold front, the isobar direction changes anticlockwise, meaning the wind backs.

Cloud cover and base (cross section of PFDPFD —Primary Flight Display)

As the warm front approaches, cloud will thicken and lower. As the warm front passes, there will either be an improvement or remain constant depending on the warm sector. In the warm sector in the UK in winter, the cloud will remain low and overcast. In the warm sector in the Europe in summer, the cloud will dissipate and rise (Fair weather cumulus.) As the cold front passes, the cloud will thicken and lower (cumulonimbus). After the cold front passes, the cloud will dissipate and rise (cumulus).

Cloud type (cross section of PFDPFD —Primary Flight Display)

As the warm front approaches, the cloud will go from CICI —Cirrus to a lowering of the stratus types: CSCS —1) Met: Cirrostratus2) Air Law: Contracting State, ASAS —Altostratus, NSNS —Nimbostratus and STST —Stratus in that order. As the warm front passes, there will either be an improvement or remain constant depending on the warm sector. In the warm sector in the UK in winter, the cloud will remain as low STST —Stratus. In the warm sector in the Europe in summer, the cloud will dissipate and rise to fair weather CUCU —Cumulus. As the cold front passes, the cloud will thicken and lower to CBCB —Cumulonimbuss. After the cold front passes, the cloud will dissipate and rise to CUCU —Cumuluss.

Precipitation type (cross section of PFDPFD —Primary Flight Display)

As the warm front approaches, you will get increasing amounts of drizzle and rain (or even freezing rain) from the NSNS —Nimbostratus clouds. As the warm front passes, there will either be an improvement or remain constant depending on the warm sector. In the warm sector in the UK in winter, the low STST —Stratus will give drizzle. In the warm sector in the Europe in summer, the cloud rises fair weather CUCU —Cumulus, given little precipitation. As the cold front passes, the cloud will thicken to CBCB —Cumulonimbuss, giving heavy rain and possible hail. After the cold front passes, the CUCU —Cumuluss will give showers of rain.