Never underestimate the power of the wind to cool the body, especially when wet, at sea. Wind chill explained by Weather expert Chris Tibbs.

Meteorologically spring is an interesting time of year as the weather can be quite volatile, particularly the sunshine and showers we associate with April and the squalls they bring (although statistically March can be just as showery).

The temperature can be remarkably variable on land but stays much colder at sea.

Although we generally focus on the wind forecast, we should also take into account the temperature of both the air and the water, particularly in the spring.

The movement of air cools the human body and we call this wind chill; on BBC forecasts it is termed a ‘feels-like temperature’. Wind chill is a common part of the forecast in many countries and recognised by mountaineers for years.

Initial experiments were made in Antarctica timing how long it took bottles of water to freeze in a given wind strength by hanging them near the anemometer.

Although there is no universally accepted formula for determining wind chill, the basic principles are the same whatever system we use, giving a good idea just how cold it will feel.

Wind chill

We lose heat from our bodies by convection, conduction and radiation.

When the wind blows it not only removes a boundary layer of warmth generated from convection, but conduction also comes into play from the wind blowing on our face continuing to reduce temperature.

If we are wet then the cooling effect of evaporation will cause our temperature to fall further. As a lot of the effect is felt on bare skin, good clothing will help in preventing heat loss.

A hat will also help, although the amount of heat lost through the head is nothing like as great as sometimes quoted: I have seen figures of between 40 and 80 per cent, but the consensus is much lower (around 10 per cent).

Although there are various ways of estimating wind chill a typical example is that in 10°C air temperature a wind speed of 20 knots will give an equivalent temperature of 2.7°C, which would drop to around -1.5°C in 40 knots.

Water temperature

When looking at wind chill we need to take into account the water temperature – sea water temperature lags behind the seasons by a couple of months, so the coldest water around Northern European coasts is near the end of February, slowly warming by Easter.

As the temperature of the air over the sea is largely dictated by the sea’s temperature, our temperature at sea will generally be colder than over the land. Add the wind chill factor from a strong wind and we will feel considerably colder out on the water than on land.

Water temperature affects the stability of the lower layers of the atmosphere. Cold water will tend to make the air more stable by cooling the lower levels of the boundary layer and this has the effect of increasing drag.

Given the same isobar spacing/pressure gradient, wind over cold water will tend to be slowed and backed with the opposite effect over warm water.

This is not just a seasonal effect as we find changes in wind when crossing warm ocean currents, for example the Gulf Stream. Not only will we often find the wind veering over the warm water, but the cloud structure may also be different.

Weather expert Chris Tibbs on how to predict and cope with fog at sea

Cold water currents will have the opposite effect and it can be quite interesting near the boundary between warm and cold water.

On one passage from Brazil to South Africa where the warm Brazil current meets the cold Falklands current, we found bands of fog over the cold water clearing with an increase in wind over the warmer waters.

The cold and warm currents run parallel to each other, meandering like a river, so at times we were passing from warm to cold and back again.

This also seemed a popular area for whales, which was great to see but became quite scary after a midnight collision (no damage to the boat and hopefully none to the whale).