Weather what you need to know

There are two classes of weather:
1. Wide area weather systems; and
2.Local weather events

Wide area weather systems

There is really only one aspect of all weather that really matters to powerboaters and that is wind strength and direction. If we can predict that and avoid getting caught in strong winds, then we are doing well.

The key to understanding wide area weather is to understand low pressure systems, or depressions as they are called. Depressions are areas of low pressure caused by heated air rising. Warm air is less dense than cold air and therefore exerts less pressure on the surface of the earth and us low pressure systems.

How deressions are formed - Fig 1.

Low pressure systems tend to originate where there is heat, both in the form of sun and warm water. For us living where we do, that means that our low pressure systems tend to originate in the warmer parts of the Atlantic such as the Caribbean.

In our position on the globe, the prevailing air stream blows from the southwest, which is why the commonest wind we get in reasonably settled conditions is a south westerly. (Winds are always described by where they blow FROM, and not by where they are blowing TO).

So, in the Caribbean the winds are wafting over the hot islands and warm seas and being heated. As they warm up, they rise casing a low pressure underneath. The higher pressure air around the edges will try to equalise the pressure, and so at the surface level we get winds as the cooler, high pressure air pours into the low pressure area.

The picture above shows that the air is spinning, spiralling into the centre of the low pressure area, just like water running down a plughole. The reason for both these effects is the same. The earth is spinning inside its atmosphere. It makes one complete rotation every 24 hours. The atmosphere, or air, being a gas, doesn´t move exactly with the solid earth. The air nearest the surface is dragged by friction, so any air near the surface that is trying to move in a different direction to the spin of the earth will be affected by the friction and pulled off course, or turned. The net effect is that it spins. The water spins down the plughole for the same reason. This effect is called the Corialis effect in our hemisphere, the winds spiral in an anti-clockwise direction.

The hotter the air gets, the lower the surface pressure and the higher the wind speeds as the colder, high pressure air tries to equalise the pressure or ‘fill’ the depression as the weathermen say.

So, all depression are associated with strong winds. These winds rotate around the depression so as it passes over us we get winds from every direction at some stage.

What makes the depression travel are the higher level prevailing south westerly´s. How fast the whole depression moves is determined by how strongly these winds are blowing. How deep the depression is depends on what the moving depression travels over.

Rain

Warm air can hold more moisture than cold air (frost is caused by water condensing out of the air as it cools at night). If a depression has been formed over the sea, then the warm air that is rising inside will be moist and will carry water high into the sky to form clouds. Since most of our depressions come across the Atlantic, they will have a lot of water in them. For us then, depressions mean windy, wet conditions.

Summary so far

Our prevailing winds blow from the south west. Depressions formed in the warmer climates of the Caribbean are blown in a north easterly direction towards us. Depressions have winds spiralling in an anti-clockwise direction around them and the strength of these winds is determined by how deep the depression is (how low the pressure gets). Because they were formed and travel over the sea, they carry a lot of moisture in them which we get as rain.

The insides of a depression

As the depression moves, it eventually crashes against colder, denser air. The less dense, warm air of the depression is forced to rise over the denser, colder air. As teh hot air climbs, it cools and forms clouds. This impact area is called a warm front as it is warm air crashing into colder air. The first sign in the sky that a warm front is on its way is wispy high clouds, often called mares tails (Cirrus).

The warmer air climbs over the colder air on a gentle slope. As a result, the distance from the very front edge with the first traces of cirrus clouds to the warm air at ground level is of the order of 500-600 nm. The cirrus clouds will persist and thicken, often forming mackerel skies until they give way to nimbo stratus clouds, while the edge of the front is still 200 or so miles away.

Fronts travel at varying speeds of up to 45kn. A front is considered slow moving if it is travelling at 15kn. So when you see the grey cloud mass appearing over the horizon behind the cirrus clouds, the front is only 200nm distant. A front travelling at a moderate speed of 30kn will be overhead in a little over 6 hours. The rain will start much earlier, usually when the front itself is still 100nm or so away.

As the front comes in, visibility will fall and the wind will strengthen and back (the direction it blow from will move anti-clockwise). Typically, rain will be steady and persistent rather than squally. The strength of the wind you experience will depend on where the depression is centred relative to your position and the depth (Pressure) at the centre of the system.

The wind strength is highest at the edges of the system (where the isobars on the surface pressure chart are bunched closest together) at the centre of the system the winds will be calmer, maybe even still.

Anatomy of a depression

Within the depression, there will be a number of fronts where warm air and cold air masses meet. Leading the way will be a warm front, and following behind will be a cold front. The cold front moves faster than the warm front and eventually catches it up, creating a mixed, or occluded, front.

In the area between the two fronts, the skies will be completely overcast, visibility will be low and light rains may persist.

As the cold front approaches, the winds will strengthen and veer (The direction they blow from will move clockwise), bigger, fiercer clouds will appear (Cumulo-Nimbus) giving squally, heavy rain showers and maybe thunder storms.

As the cold front passes, the air will become clear, visibility become very good and the skies dotted with large cumulus clouds, maybe with frequent, heavy showers.

All in all, the approach of a depression in the Atlantic to the west of us is a good reason to stay ashore.

Local weather events

The combination of warm weather, mountains and estuaries can combine to create localised weather events that can pose just as big a threat to mariners as do the wide area systems. Generally, but not always, of short duration these events can create winds that appear very quickly and can easily reach gale force (Force 8) strength.

Localised winds - Sea Breezes

On a hot summers day, the land gets heated by the sun. The sea, however, is a large heat sink and does not get much warmer in the same time period. By the afternoon, clouds are bubbling up over the land and a sea breeze starts as air, warmed by the land rises and colder air from the sea rushes in.

We can immediately tell two things about these winds. They will slack off when the land cools in the evening and they are on-shore winds, i.e. they blow from the sea towards the land. The term breeze can be misleading since these winds can easily blow at force 5 (18-22kn).

Sea breezes can occur anywhere in summer but are not dangerous to boaters although they can make the journey home lumpy.

Adiabatic and Katabatic winds

Adiabatic means ‘heated by compression’ and Adiabatic winds are caused by moist air rising over a mountain where it looses moisture through precipitation (Rain) on the lee side as it descends, streaming down mountain passes under gravity. As the air falls, it heats up. The warm air is funnelled between the mountains and streams out to sea. These winds can easily reach gale force and occur very suddenly.

Katabatic Winds, which include the Bora in the Adriatic and the Mistral in France as typical examples, are also caused by wind rushing down mountain slopes. In this case, however, the cause is a rapid cooling of dry air at the top of the mountain. The cold air rushes down the mountain, warming as it goes. It may, however, still be unseasonably cold by the time it reaches the bottom. In places where it can be funnelled by the shape of the terrain, these winds can become very strong indeed!

The conditions necessary for an adiabatic and a katabatic wind can be forecast, and areas subject to them include them in their local weather forecasting, so if you are thinking of renting a boat while away on holiday, say in Greece or Croatia, it pays to get the local forecast unless you want to find yourself in a sudden force 8.

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