By David Braine
Sea breezes can be a welcome relief during summer heat. BBC Broadcast Meteorologist, David Braine, explains what they are.
Since Victorian times we have sought the clean air of the seaside, not least for the health giving power of maritime air but also the affects of the sea temperature to give us some relief from the heat.
Inland temperatures during the summer can soar into the high 20's or even 30's, however nearer the coast it can be much more comfortable with temperatures reaching no more than 20-24 degrees as the sea breezes set in and bring relief. So how does the sea breeze form and what are the effects?
Traditionally sailors know only too well of the effects of this sea breeze, often relying on it for summer sailing on the coast when the otherwise light winds would be insufficient for racing. As with all winds the generating force is temperature, specifically temperature differences between two areas, in this case the sea and the land.
During summer, the temperature difference between the land and the sea is at its greatest, in fact because of length of time it takes to heat the ocean, often the maximum sea temperature is reached in September or early October in contrast to the land where daytime maximum temperatures normally peak in July or August.
This illustrates the difference in heat capacity between the land and the sea. On the land the air temperature quickly warms to 30 degrees yet the sea surface in the same day will barely change in temperature, as the heated surface waters are quickly mixed with cooler water below.
The result is that sea temperatures rarely get above 15 or 16 degrees in UK waters, whilst the land often reaches 20 degrees and above, and with a temperature difference comes a pressure difference. As the land warms through the day the warm air rises and forms an area of low pressure at the surface. This then sucks in air off the sea and a circulation slowly develops with a returning current of air (often at 2000ft) moving out to sea and then descending several miles offshore. The effect is a circulation with low pressure over the land (and resultant cloud build up) and high pressure out at sea (and resultant clear skies).
The sea breeze once it develops can reach a force 5 or 6 on the Beaufort scale, and penetrate quite a long way inland, sometimes as far as 20 or 30 miles by mid-afternoon. It is also acted on by the effects of the earth's rotation (coriolis force) such that it starts blowing perpendicular to the coast and by the end of the day as it starts to ease off it will blow parallel to the coast (having been turned to the right in the northern hemisphere).
On a grand scale the monsoon rains of India are nothing but a huge sea breeze effect. The land over India gradually heating up over the early summer months to create an area of low pressure. This massive column of rising air drags in from the Indian Ocean the moisture laden sea breeze that deluges much of the continent with the monsoon rains but also brings relief to the pre-monsoon excessive heat.
Several factors influence the sea breezes around the waters of the UK. The temperature contrast needs to be sufficient, the air needs to be reasonably unstable (have the potential to form cloud), and the winds within the general circulation need to be sufficiently light to allow the sea breeze circulation to set up.
If it does happen, the sea breeze is a gift to sailors who would otherwise be becalmed, but more especially it brings cleaner, fresher air into coastal regions. More importantly it brings relief from the heat, pollution, and pollen that can make life uncomfortable for those that suffer inland, perhaps one good reason to head for the beach in the summer months!
Other features in the Weather Basics series:
- Forecasters Biography: David Braine
