Our weather passes across these islands from south of west to north of east in a series of highs and lows.

Depression This is an area of low pressure which, when newly formed, consists of an area of warm and moist air, roughly triangular in shape, projecting into cooler air. This system travels between 20-30 knots across the North Atlantic. Pressure is lowest at the apex of the warm sector (the peak of the triangle). This is the centre of the depression. At the leading edge of the warm sector, where it rises over the cooler air ahead, is a long line of rain forming the ‘warm front’.

At the following edge - with cooler air pushing under the warm sector – is another line of cloud and rain; the ‘cold front’. The lines drawn on a weather chart through areas of equal pressure are called isobars. These are straight in the warm sector but sweep round the cold part of the depression centre from the warm front to the cold front behind - see diagram.

Plan of a Depression

C.F. Cold front W.S. Warm sector W. Wind W.F. Warm front D Direction of travel C. Centre

Section through Depression

Ci. Cirrus C.st. Cirrostratus A.st. Altostratus N.st. Nimbostratus A.cu. Altocumulus Ni. Nimbus

Depressions are said to travel approximately parallel to the straight isobars in the warm sector. The wind blows anti-clockwise around an area of low pressure, not suite parallel to the isobars but cutting slightly in towards the centre. Thus, if you stand facing the wind of a depression, the centre of the depression lies over your right shoulder.

At the approach of a front of the warm type, the barometer will begin to fall, the cloud ceiling get lower with sheet type cloud and continuous steady light rain will fall. As the front passes by, the wind veers, the barometer becomes steady and the temperature rises. Varying amounts of sheet cloud and drizzle can be expected during the first half of the warm sector but later the cloud will break up and patches of blue sky will be seen until the cold front approaches.

During the passage of the cold front heavy showers of rain may fall, the wind will veer again and the barometer begins to rise as the temperature falls. If they can be predicted, these veering winds can often be put to advantage when cruising - during long passage making ocean-racing navigators often win or lose a race gambling on these wind changes.

When a low-pressure system is dying, the warm sector may be lifted up entirely from the surface, the following cold front catches up the warm front - leaving only one line of rain cloud. This is an occluded front. The occluded front may be of a warm front type with continuous rain or a cold front type with showers, depending if the air behind the occlusion is warm or cold. The wind veers sharply during the passage of these fronts.

Secondary depressions are small low-pressure systems which form within the isobars of a larger low system. They may be just a bulge in the parent isobars or a complete system enclosed by their own isobars with, or without, associated fronts. The secondary systems travel anti-clockwise around their parent. They are often very vigorous and bring gales causing ships to seek shelter and you and I to worry about our tent and our mooring. A sudden drop in pressure and a backing wind may be the only warning -

"When the wind backs and the weather glass falls, Keep an eye lifted for gales and squalls."

Held up on the Beaulieu river one summer by a fierce wind and sudden heavy showers, I peered out of the tent anxiously watching the direction of the cloud. I was due back in Chichester by midday on the morrow. The barometer (ex RAF altimeter), which had fallen quickly, was now steady. I was waiting for the wind to veer - the front to pass by - when the wind would most likely moderate. I awoke in the night, at about 4 a.m., to find 'Beachcomber.' banging against another dinghy. The wind had veered and the barometer was just rising. I brewed some tea and cooked breakfast. It was still blowing fairly hard when I got under way around 6 a.m. but gradually, as the morning passed, the wind moderated and I made Chichester Harbour in time. This front, either because it was small, or fast moving, gave short notice and was soon past.

Of the Barometer - "Long foretold - long last. Short notice - soon past."

Some low-pressure systems lose their shape, the isobars being almost parallel running north-south with a sharp veer at its southernmost point. These are often squeezed out of shape by high-pressure systems around them - hence the "Trough of Low Pressure”.

Anticyclones are high-pressure systems and the opposite of depressions. An area of high pressure is enclosed by isobars, the wind blowing around the High clockwise and crossing the isobars at a slight angle away from the centre. There are no fronts. Anticyclones have little energy - they move slowly - and they may hang around for weeks dissolving all low-pressure systems which attack their flanks. The winds associated with them are light and, in summer, give as blue skies.

Without any sheet cloud, free radiation of heat varies things a bit. Huge Cumulus clouds may be formed during the day and may give rise to thunder storms and showers or “cloud bursts” over the land. As the land heats up quicker than the sea, causing rising convection currents, a light breeze off the sea might flow inshore to replace the rising currents. These usually begin around 10.00 hrs. and reach a maximum about 15.00 hrs. At sundown the reverse happens; the land cools quickly, the sea is relatively warm, convection currents rise up from the sea surface and a breeze begins off the land, usually at about 20.00 hrs. and reaching a maximum at 03.00 hrs.

These land and sea breezes are only noticeable when the anticyclone is producing little or no wind, and may be useful if you are hugging the coast, especially at night when often the true wind will tend to die with the sun. How often do we sail away in the morning with a nice force 3 wind, the harbour entrance tempts us and out we go. Then, during the evening, the wind slowly dies, we miss the last of the flood and can still be heard swearing at 9 p.m. as we sweat and row against the ebb to get back home?

Dew is caused by the cooling surface at night and is prevented if sheet cloud blankets the surface and prevents it cooling. “ Whilst there's dew upon the grass, Rain will never come to pass."

Fog at sea is caused by warm humid air from the south or south-west blowing over a cooler sea. They call it mist if you can see 1000 yards.

As a 'Low' may be squeezed by surrounding 'Highs', so a 'High' may be squeezed up by surrounding 'Lows', giving rise to a 'Ridge of High Pressure’, so watch your barometer.

“Quick rise after low, Foretells a stronger blow."

The stronger blow being caused by the following low. Another jingle:

“If the wind stays N.W. for three days without rain, It will be eight days before it comes south again."

The outer isobars of a large slow moving anticyclone forming off the coasts of Spain or France may give us northerly winds for ten days or more, before they change direction as it drifts over us. A good sign in the summer. In winter, anticyclones cause high fog - a low dark ceiling, an ‘anticyclonic’ gloom and smog over towns.

Clouds Very little has been said over clouds and their significance, but they are a subject of a book in themselves. Not often seen but worth a mention as it is seen on nice days is a long dark cloud stretching right across the sea from one horizon to the other, travelling fast from east to west - a ‘Line Squall’.

Heavy sudden rain, even thunder and lightning, with a violent wind occur as this cloud passes over. The barometer will dive as it approaches, and the wind veer as it goes by. The actual passage of the squall lasts only a few minutes, but it is very violent for all that, stirring up a nasty sea. The sight of a line squall advancing should make any skipper lower all sail, get head to wind with oars or something, and prepare for a battering. Cold air from the west undercutting warm moist air is said to be the cause.

May I recommend two Met. Publications: -

No. M0247 "A short course in Elementary Meteorology” by W H Pick – price about 6/- and No. M0233 "Cloud Forms" - about 3/- with coloured photos and cloud descriptions; both are obtainable from good booksellers.

Here's to many Highs this summer, but don’t forget the lulling of the breeze at night - my wife never believes me when I get home hours late.