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Meteorology

 


Contents:



 

What is meteorology?

Meteorology is quite simply the study of the weather.

When we talk about weather we are concerned with several different elements:

Understanding these and how they are generated allow us to predict the weather.

 

What makes the weather?

 

Wind

Wind is the movement of air across the earth's surface. Winds blow from areas of high pressure toward areas of low pressure, down what is called the pressure gradient.

The direction from which a wind is blowing is very significant in weather prediction. In the Northern Hemisphere....

North Westerly winds = colder, drier weather
Easterly winds = cloudiness and precipitation
Southerly winds = warm, humid weather

 

Pressure

Atmospheric pressure by itself has little significance in weather forecasting. It is the change in pressure that is important.

Falling pressure = a storm is approaching
Rising pressure = approach or continuation of fair weather

 

Temperature

Temperature changes also are significant in weather forecasting. In the Northern Hemisphere....

Rising temperatures are associated with southerly winds
Falling temperatures are associated with northerly winds

Night time temperatures may be 5-6ºC lower in rural areas than in urban areas because of concentrations of man-made sources of heat in the cities.

 

Humidity

Water vapour in the air is what clouds, fog, rain, and snow are made from. The amount of moisture in the air is known as humidity.

Warm air can hold more moisture than cold air can.

Relative humidity is the proportion of water vapour actually in the air at a given temperature. It may vary from almost none over deserts to as much as 100 percent in thick fog or rain.

 

Clouds

Clouds often signal an imminent weather change.

Rising cloud levels indicate clearing weather
Thickening and lowering clouds signify precipitation

The main cloud types are:

The terms alto, meaning "high", and nimbus, meaning "rain" further describe clouds. (e.g. cumulus nimbus describe large billowing rain clouds)

 

Fog

Fog is a cloud whose base is on the ground. Like clouds, it forms when moist air cools below its dew point.

There are 4 types of fog:

 

Radiation fog

Forms on still, clear nights where the heat of the day radiates into the air and cools. It generally occurs in high pressure areas with the land cooling rapidly.

 

Advection fog

Forms when warm, moist air lies over cold water. When advection fog forms at sea, it is known as 'sea fog'.

This usually happens in spring and early summer whilst the sea is still cold. A complete change in weather pattern is often required to disperse the persistent sea fog.

 

Frontal fog

Forms where a warm, moist front encounters a colder polar front. This usually forms as low level cloud which can fall to sea level. When this happens, land masses and lighthouses may be obscured.

 

Artic sea smoke

Occurs where cold air flows over warmer water. This is because the air is very cold and the sea is actually warmer (relatively speaking).

This type of fog is rarely encountered!

 

Methods of weather forecasting

Before setting out, you should always check the weather for the period you expect to be at sea. Although it may be clear and favourable when you set off, conditions may deteriorate rapidly.

The first signs of weather changes can be spotted by using the above observations in conjunction with the information portrayed in the official forecast.

For the mariner, weather forecasts can be obtained from:

NB: The shipping forecast tends to be quite general so it is best to gain more detailed weather information for your area as well.

 

Understanding the shipping forecast

The shipping forecast is split into 4 main areas:

  1. General Synopsis
  2. Shipping areas
  3. Inshore waters
  4. Costal stations

1. The general synopsis indicates the main weather pattern for the UK in terms of fronts and pressure systems.

2. The actual shipping forecast covers the main shipping areas around the UK waters. Weather details covered are:

Shipping areas with the same forecast are bunched together

(e.g. Dover, Wight, Portland, Plymouth )

The shipping areas around the UK are:

Bailey Lundy
Biscay Malin
Cromarty North Utsire
Dogger Plymouth
Dover Portland
Faeroes Rockall
Fair Isle Shannon
Fastnet Sole
Fisher South East Iceland
FitzRoy (formerly Finisterre) South Utsire
Forties Thames
Fourth Trafalgar
German Bight Tyne
Hebrides Viking
Humber Wight
Irish Sea  

In Birkenhead we are generally interested in the area Irish Sea

Example:

IRISH SEA
SOUTHEASTERLY 5 TO 7, VEERING WESTERLY 3. RAIN AT FIRST. MODERATE
WITH FOG PATCHES, BECOMING GOOD

 

3. The inshore waters section gives the forecast for the costal regions of the UK. Weather details reported here are:

Again, here in Birkenhead, we are interested in the area Colwyn Bay to the Mull of Galloway including the Isle of Man.

Example:

Outlook for the following 24 hours:
Wind: southwest 3 backing south or southeast 5 or 6 locally 7 for a time.
Weather: rain at times.
Visibility: moderate or good locally poor at times.
Sea State: slight or moderate locally rough later.

 

Beaufort wind and sea scales

 

4. The last part of the forecast is from the costal stations. They report such details as:

The nearest costal station to us is Liverpool Crosby.

The map below shows the costal waters and costal stations for the UK.

 

 

Understanding the terminology

The above marine forecasts seem pretty simple, but hidden within them is a whole host of important information. Notice that words such as 'soon' and 'rising slowly'. These, and many words like them, have particular meaning in a shipping forecast.

 

Times:

Imminent = any time within the next 6 hours

Soon = 6 to 12 hours from time of issue

Later = beyond 12 hours from time of issue

 

Wind:

Veering = a clockwise wind shift

Backing = an anti-clockwise wind shift

Cyclonic = an anti-clockwise rotating wind movement

(e.g. easterly veering southerly)

 

Visibility:

Good = more than 5 miles

Moderate = 2 to 5 miles

Poor = up to 2 miles

Fog = less than half a mile

Haze = a dry atmosphere

Mist = atmosphere is relatively wet

 

Pressure:


Steady no appreciable change.
Rising or falling slowly not enough change to appreciably affect the wind speed.
Rising expect some decrease in wind
Falling expect some increase in wind
Rising quickly usually leads to a rapid drop in wind speed
Falling quickly expect a marked increase in wind strength, but not usually to gale force.
Rising very rapidly usually follows the passage of a low centre and may indicate stronger wind from a new direction.
Falling very rapidly expect a gale to develop very soon if it has not already done so.
Now falling implies that the pressure has only just started to drop, so we don't yet know how quickly it is falling. All we can say is that the trend is possibly towards poorer weather.
Now rising likewise, the pressure was actually falling until very recently - but maybe there is now a move towards better weather.

glossary of all other metrological terms

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