Take the weather with you

The Beaufort scale

We have been experiencing some very high winds recently. If you look at the Nodehill Weather station, wind speeds are given in knots. The table below helps you to convert this into mph or the Force of the wind according to the Beaufort scale and gives you physical effects of these winds to help you identify them without an anemometer (wind gauge).

 

Weather in low pressure systems

This image shows a classic low pressure system

(c) Crown copyright [2006], data supplied by the Met Office

Because low pressure systems are unstable and controlled by fronts, sequences of weather are more complicated. They usually travel across the country from west to east and there is a sequence to the weather as described below.

Warm front (red semicircles on chart)

6-12 hours before the warm front arrives, winds will come from the south west and high cirrus clouds become blocked by lower level stratus clouds. These clouds bring rain which becomes heavier as the warm front approaches.

As the warm front passes over, rain eases to drizzle. The wind swings to a westerly direction and the air is warmer. Weather is generally mild and cloudy occasional drizzle in spring, autumn and winter but clear skies in summer.

Cold front (blue triangles on chart)

This is more violent with thick, black nimbostratus clouds which bring heavy rain or snow and stronger gusting winds.

The cold front passes through quickly and the wind veers to the north-west bringing brighter, clearer weather and the clouds lift to leave cirrus again.

Occluded front (purple semicircles and triangles on chart)

Sometimes the cold front catches up with the warm front and the mixture of the two is called an occluded front.
This behaves like a cold front with a narrow band of rain and after it passes winds will come from the North West and the weather will clear.

What are low pressure systems?

What are low pressure systems (depressions)

Low pressure systems are formed when cold and warm air battle away in the atmosphere. Warm air wins at the front of the system creating a warm front and pushes the cold air back forming a cold front behind it.

Winds in the upper atmosphere push depressions in an easterly direction. Depressions which affect the UK usually come across the Atlantic from America. When a depression passes over you get weather associated with the warm front first followed by the cold front.

Air travels around a depression in an anti-clockwise direction parallel to isobars. These are lines which join points of equal pressure which is measured in millibars (mb). The closer the isobars are together, the stronger the winds.