STANDARD GRADE DEPRESSIONS John Smith Invergordon Academy

LOW PRESSURE SYSTEMS How depressions form What happens at the warm front? What happens at the cold front? What happens in the warm sector? What happens at the occluded front?

Cold air will be shown in darker blue. LOW PRESSURE SYSTEMS Imagine an area out in the North Atlantic Ocean, to the West of the UK where the air is fairly cool. This cool air is represented by the light blue background on the following slides. Cold air will be shown in darker blue. Warm air will be shown in red.

THE AIRMASSES OVER THE ATLANTIC Imagine you are looking down on a large area of the North Atlantic Ocean. The air is quite cool (light blue). In this area, different types of air masses meet. We will look at two of these. 1. Cold Arctic air blowing from the North East 2. Warm moist Tropical air blowing from the South West

THE AIRMASSES MEET…… Cold Arctic air blows from the North East Warm, moist Tropical air blows from the South West

……BUT THEY DO NOT MIX The two sets of air do not mix together As they blow past each other, friction causes them to swirl round

WARM AND COLD SECTORS Cold Arctic air This means that we now have a cool area (shown by the light blue background)…. …with a wedge of very cold Arctic air (shown by the darker blue triangle)…. There is also a wedge of warm moist Tropical air. (red triangle)... …this is called the COLD SECTOR. Warm, moist Tropical air …this is called the WARM SECTOR.

These systems usually move either East, or North East DIRECTION OF MOVEMENT These systems usually move either East, or North East Warm, moist Tropical air Cold Arctic air Warm, moist Tropical air Cold Arctic air Warm, moist Tropical air Cold Arctic air Warm, moist Tropical air Cold Arctic air Warm, moist Tropical air Cold Arctic air Warm, moist Tropical air Cold Arctic air

WARM AND COLD FRONTS The line between the warm sector and the cold sector is called the COLD FRONT. Cold Arctic air Cool air Warm, moist Tropical air The line between the cooler air and the warm sector is called the WARM FRONT.

A SIMPLE CROSS SECTION THROUGH A DEPRESSION THE WARM SECTOR (warm, moist air) C O L D F R O N T W A R M F R O N T COLD SECTOR (cold, dense air) Dense cool air

CONDITIONS AT STATION X (1) Warm, moist Tropical air Cold Arctic air X What conditions would be felt at X? Cool Cold Warm

CONDITIONS AT STATION X (2) Warm, moist Tropical air Cold Arctic air Warm, moist Tropical air Cold Arctic air Warm, moist Tropical air Cold Arctic air Warm, moist Tropical air Cold Arctic air What has passed over X? Warm Front Western Front Cold Front What conditions would be felt at X? Cool Cold Warm X From which way are the winds blowing at X? South East North West South West

CONDITIONS AT STATION X (3) Warm, moist Tropical air Cold Arctic air Warm, moist Tropical air Cold Arctic air Warm, moist Tropical air Cold Arctic air Warm, moist Tropical air Cold Arctic air What has passed over X? Warm Front Western Front Cold Front X What conditions would be felt at X? Cool Cold Warm From which way are the winds blowing at X? North East North West South West

WHAT HAVE YOU LEARNED SO FAR? Low pressure systems are also called depressions Cold Arctic winds blow from the North East Warm moist Tropical winds blow from the South West The wedge of cold air is called the cold sector The wedge of warm air is called the warm sector The leading edge of the cold sector is the cold front The leading edge of the warm sector is the warm front

WHAT HAPPENS AT THE WARM FRONT? Warm air begins to rise over the cooler air As this air rises, it begins to cool Cool air can hold less water vapour than warm air Water vapour begins to condense into water droplets Water droplets begin to form clouds The first – and highest – type of cloud to form along the warm front is called Cirrus

AIR RISING ALONG THE WARM FRONT (1) These high-level wispy clouds are called CIRRUS Water vapour condenses and forms clouds As the warm air rises, it cools. Warm air is forced to rise over denser, cool air. Dense cool air

CIRRUS CLOUDS

PRECIPITATION AT THE WARM FRONT Clouds form lower down, and give prolonged rain CIRRUS These are CUMULUS More moist air rises and cools Dense cool air

CUMULUS CLOUDS

THE WARM FRONT - SUMMARY CIRRUS are very high and wispy. They are usually the first clouds we see as the warm front approaches CUMULUS clouds bring prolonged rain at the warm front More moist air rises over the cooler air. As it does so it cools. Dense cool air Warm Front

AN APPROACHING WARM FRONT CIRRUS CUMULUS

WHAT HAPPENS AT THE COLD FRONT? The cold Arctic air moves faster than the warm air. The cold Arctic air is denser than the warm air. The cold Arctic air undercuts the warm air, forcing it up. Water vapour begins to condense into water droplets Water droplets begin to form very tall clouds The clouds along the cold front are called Cumulonimbus.

THE COLD FRONT The cold front rapidly undercuts the air in the warm sector, making it rise very quickly. The Warm Sector (warm, moist air)

PRECIPITATION AT THE COLD FRONT Very tall clouds are formed by the rapidly rising air. These are called CUMULONIMBUS. CUMULONIMBUS give very heavy showers, sometimes with thunder and lightning. The Warm Sector (warm, moist air)

CUMULONIMBUS CLOUDS

IN THE WARM SECTOR Warm, moist winds blow from the South West Air is forced to rise over cooler air Condensation occurs, and forms Stratus clouds The sky is very overcast Showers are common

THE WARM SECTOR Warm, moist air in the warm sector is rising. Warm front Dense cool air

CLOUD & PRECIPITATION IN THE WARM SECTOR Warm, moist air in the warm sector is rising. The clouds formed here are mostly Stratus Stratus are flat layer clouds. They give showers. STRATUS W A R M F R O N T Dense cool air

CLOUD & PRECIPITATION – SUMMARY 1 CUMULONIMBUS CIRRUS CUMULUS STRATUS W A R M F R O N T Dense cool air HEAVY SHOWERS LIGHT SHOWERS PERSISTENT RAIN

A SATELLITE IMAGE OF A DEPRESSION

CROSS SECTION THROUGH A DEPRESSION CLOUD PRECIPITATION TEMPERATURE WIND DIRECTION AIR PRESSURE Cumulonimbus Stratus Cumulus Cirrus Heavy showers Showers Prolonged rain Cool Cold Warm N.W. or N S.W. E or N.E. Rising Falling Low