Weatherwatch with Irv Lee

Coastal Meteorology

Airfields near the coast can enjoy (suffer?) different weather from inland airfields - and it can change during the day. Circuit flyers beware!

Living and flying near the UK coast or islands can bring advantages and disadvantages to the UK pilot. Let's examine the differences found by proximity to the sea.

Winds
On sunny days with light winds inland, wind vectors at coastal fields can change dramatically while you are en route. This can surprise a less vigilant pilot on arrival when the runway or crosswind is completely different to plans made before departure.

I have taken off on runway 06 at inland Old Sarum at lunchtime with northerly light winds then arrived at Shoreham, on the coast, to find 20 knot winds and runway 23 in use.

Unlike the sea, land temperatures go through large changes on a sunny day. Land heating, even by a few degrees, causes the air in contact with the surface to rise. The air on the sea surface is drawn horizontally onto land, an effect known as an 'onshore breeze'.

In stable conditions, a complete circulation sets up before lunch, with air coming in from the sea, rising over land, heading back out to sea at height, and descending back over the sea. In late afternoon, land cools again and the reverse process can set up, giving first a lull in the wind, then a complete reversal in direction on the ground as a lighter 'offshore' breeze establishes itself.

Cloud
The onshore breeze process involves air rising. In stable conditions, small cumulus clouds will form in the rising air, while in unstable conditions this can be the trigger for towering cumulus and even thunderstorms, especially with the higher moisture content in the air near coastal areas.

In less sunny conditions, or with a stronger airflow, with sea breezes suppressed, coasts facing the wind flow can be relatively free of cloud compared to conditions just a few miles inland.

In a stable, non frontal flow, the air, charged with moisture from a sea crossing, is forced to rise by the first ranges of hills it meets, usually a little way from the coast. Rising causes moisture release, giving cloud and possibly rain inland.

In practical terms, in a south-westerly airflow, my airfield, Popham, in north Hampshire, could be suffering low cloud all day, yet pilots are having fun at Bournemouth and Sandown.

Fog
Islands in particular can suffer greatly from addiction fog, usually formed by moist warmer air being cooled from contact with cold seas.

Water vapour is released as 'ground level cloud', or fog to you or me. Banks of fog then slowly roll in over islands or coasts to envelop nearby airfields.

In my area, the Isle of Wight airfields and the Channel Islands can occasionally be closed all day while mainland Britain enjoys good flying weather. On the other hand, coastal or island airfields can be clear when radiation fog is widespread inland (Weather Watch 1), if the drop in land temperature overnight has been suppressed sufficiently by the nearby, more temperature-stable, sea.