Depressions Notes[1]

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Depressions
List of facts
1. It is an area of Low Pressure
2. Air pressure is shown by isobars drawn on the map
3. Pressure readings are shown in millibars on the isobar lines.
4. These pressure readings are lowest in the centre of the depression and increase in fours as you
move out from the centre of the depression.
5. Where the isobars are closest together that is where you will find the strongest winds
6. A depression contains two fronts – the warm front and the cold front
7. You always get rainfall at Fronts (Frontal Rainfall)
 Warm air meets cold air at a front - the warm air is lighter and rises over the cold air
- as the warm air rises it becomes colder, the moisture in the air condenses and forms
clouds and rain falls
 Study Figure 11.9 and paragraph 2 on page 191 of New Wider World
 Long period of continual rain at warm front
 Rain takes the form of showers, often heavy and thundery, at the cold front
8. In between the two fronts is an area of warm air called the Warm Sector. Because the air is
warmer, the air can hold more moisture so it is usually drier in the warm sector and if there is
any precipitation it is usually in the form of light rain or drizzle
 Study Figure 11.11 on page 192 of New Wider World and read notes on this page
 Study Figure 11.13 of a depression on a synoptic chart. Can you identify the occluded
front, warm front, cold front, warm sector.
9. Winds in a depression blow in an anti-clockwise direction moving in towards the centre. This
means that the winds are usually from a S.E. direction as the warm front approaches and then
backs to S, then S.W or W in the warm sector and finally NW at the cold front
10. Occluded Front occurs when the cold front has caught up with the warm front and the two join
together, producing lighter rain
Weather Through A Depression
Weather Elements
Cloud Cover
Precipitation
Temperature
Wind Direction
Wind Speed
Warm Front
Completely overcast
Stratus cloud 8 oktas
Long period of
continuous rain
Warm Sector
Broken cloud
Cumulus 6 oktas
Mainly dry
Maybe some Drizzle or
light rain
Cold Front
Broken cloud
Cumulus 5 oktas
Showers
Heavy, often thundery
showers. Hail
Cool air before the
front
Winds from S.E.
Strong winds
Warm as this is warm
air
Winds from S.W. or W
Winds slightly more
gentle
Cold air follows the
cold front
Winds from N.W.
Very strong winds,
especially with the
showers
Exam Type Questions
1. Write a description of the weather at the warm front
2. Write a description of the weather in the warm sector
3. Write a description of the weather at the cold front
4. Write a description of the weather at Point B as a depression passes
overhead
Sample Answer
The weather at Belfast is being influenced by the warm front. Warm air is rising over cold air so
clouds have formed and the sky is overcast and there is a long period of rain(frontal rain).
Temperatures are cool and the wind direction is from the SE and is changing towards a Southerly
direction. Winds are strong because isobars are close together
The weather will change as the warm sector will approach and this area of warm air will
cause temperatures to rise. The warmer air is able to hold more moisture so the weather will be drier
for a period with perhaps some drizzle or light rain. Wind speeds will lessen slightly and will be
coming from a SW or W direction. There will be some clear spells so cloud cover will be about 6
oktas.
This will be followed by the cold front where the cold air will cause temperatures to drop.
This is a front where cold air is pushing under warm air causing it to rise so clouds are forming to give
heavy showers. There will be clear spells between the showers so cloud cover will be around 5 oktas.
Winds will be strong, especially accompanying the showers and winds will be blowing from the NW.
The wind pattern throughout the depression shows that they blow in an anti-clockwise direction,
moving in towards the centre of the depression.
5. Describe how two elements of the weather change as a depression
passes over
6. Explain why two elements of the weather change as a depression
passes over
Precipitation
As the warm front passes – there is a long period of steady rain (3/4 hours)
This is because warm air is rising over cold air at the warm front and is becoming colder,
condensation occurs, clouds form and rain falls
As warm sector passes – some drizzle or light rain; may even be a dry spell
Air is warmer and is not rising so it can hold more moisture and there is less rainfall
As cold front passes – heavy showers, often with thunder
Because cold air is pushing under warm air at the cold front forming high cumulus clouds
which bring very heavy showers
Cloud Cover
As the warm front passes – 8/8 cloud cover, layers of low stratus cloud
This is because warm air is rising over cold air at the warm front and is becoming colder,
condensation occurs and layers of low stratus clouds form covering the sky
As warm sector passes – less cloud cover, 4/8, broken cloud.
Air is warmer and is not rising so it can hold more moisture and fewer clouds are formed
and there are sunny periods
As cold front passes – 5/8 cloud cover, heavy showers, sunny spells
Because cold air is pushing under warm air at the cold front forming groups of high
cumulus clouds separated with breaks of clear sky
Temperature
As the warm front passes – cold air at the warm front causes a drop in temperature
As warm sector passes – rise in temperature as this is warm tropical air
As cold front passes – coldest temperatures as cold air moves down from the NW
Wind Direction
As winds in a depression blow in an anticlockwise direction in towards the centre, you
will find the following pattern of wind direction in a depression
As the warm front passes – winds blow from the SE
As warm sector passes – winds blow from the SW
As cold front passes – winds blow from the NW
Wind Speed
As the warm front approaches – winds become stronger because isobars in the depression
are closer together.
As warm sector passes – the winds ease in strength (become lighter) as the isobars are
further apart
As cold front passes – these are the strongest as the isobars are usually closest together at
this part of the depression
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