Depressions
Weather fronts
A In meteorological terms, a depression is an area of low air pressure. is an area of low pressure which moves from west to east in the northern hemisphere.
Low pressure systems can be identified from a synoptic chart due to:
- A band of cloud formed when warm air meets cold air, and the cold air pushes the warm air upwards.
- A band of cloud formed when warm air meets cold air, and the warm air rises above the cold air.
- possible A band of thick cloud formed when the cold front catches up with the warm front.
- tightly packed isobars
- Lines on a synoptic chart which join places of equal air pressure. showing pressure decreasing towards the centre from about 1004mb
Warm front
In a low pressure system the warm front is the first to pass over. This occurs when warm air meets cold air and the warm air rises above it. Warm fronts bring steady continuous rain.
Cold front
The next front to pass over is the cold front. This occurs when cold air meets warm air. The cold air pushes the warm air upwards. Cold fronts bring heavy rain showers.
Occluded front
When the cold front catches up with the warm front the result is an occluded front. Occluded fronts bring sudden downpours of heavy rain.
All three fronts are associated with rain.
Watch the following clip to help make sense of weather fronts.
Depression characteristics
Where isobars are close together the wind is greatest. This is because of a rapid change in air pressure.
- Wind - winds blow anticlockwise in a depression and wind blows along the isobars. You can work out the wind direction by following the isobars in an anticlockwise direction.
- Wet - where warm air meets cold air, the warm air is pushed upwards where it cools, condenses and precipitates (usually as rain). A front is a band of cloud and clouds bring rain.
- Temperature - in general, the An area of warm air between the warm front and the cold front in a depression. behind the warm front brings warmer temperatures and the An area of colder air that surrounds the wedge of warm air in the warm sector in a depression. behind the cold front brings cooler temperatures.
As a depression passes over the following changes occur.
The table below explains the changes as a depression passes over.
| Factors | 5 (End) | 4 (Cold Front) | 3 | 2 (Warm Front) | 1 (Start) |
| Cloud cover and type | Broken-up cumulus | Tall/heavy cumulonimbus | Sparse stratus | Low and thick | High wispy cirrus |
| Pressure | Increasing | Still low | Lowest | Decreasing | High |
| Temperature | Cool | Warm | Warm | Cool | Cool |
| Rainfall | Showers | Heavy | Mainly dry | Drizzle | Dry |
| Factors | Cloud cover and type |
|---|---|
| 5 (End) | Broken-up cumulus |
| 4 (Cold Front) | Tall/heavy cumulonimbus |
| 3 | Sparse stratus |
| 2 (Warm Front) | Low and thick |
| 1 (Start) | High wispy cirrus |
| Factors | Pressure |
|---|---|
| 5 (End) | Increasing |
| 4 (Cold Front) | Still low |
| 3 | Lowest |
| 2 (Warm Front) | Decreasing |
| 1 (Start) | High |
| Factors | Temperature |
|---|---|
| 5 (End) | Cool |
| 4 (Cold Front) | Warm |
| 3 | Warm |
| 2 (Warm Front) | Cool |
| 1 (Start) | Cool |
| Factors | Rainfall |
|---|---|
| 5 (End) | Showers |
| 4 (Cold Front) | Heavy |
| 3 | Mainly dry |
| 2 (Warm Front) | Drizzle |
| 1 (Start) | Dry |