Seasons & Weather
Squalls Behave Differently Over Water Than Over Land
Showers crossing the sea are less disrupted by terrain, arrive with sharper wind shifts and can be tracked visually for a long way before they reach you.
A shower that would be unremarkable inland can arrive at a coast as a violent few minutes of wind and rain. The difference is in what the sea does not do to it.
Water offers no friction and no obstruction
Air moving over land is slowed and broken up by hills, trees and buildings, which disrupts the organised structure of a shower cell.
Over water, surface friction is low and there is nothing to break the flow, so cells stay organised and move faster.
They also arrive with their downdraught intact, which is the part that produces the sudden wind rather than the rain.
The wind shift is the dangerous part
A squall's outflow spreads ahead of the rain as a gust front, arriving before the shower and from a different direction than the prevailing wind.
For anything under sail or at anchor, that direction change matters more than the increase in speed, because gear is set for the previous wind.
The shift can be substantial and it can reverse within a few minutes as the cell passes, which is why squalls catch out people who saw them coming.
A warm sea feeds them
Showers over water draw heat and moisture from the surface, so a sea that is still warm from summer supports stronger cells than a cold one does.
This is why autumn and early winter produce the most vigorous coastal showers in many regions, with the sea at its warmest relative to the air above it.
Cold air arriving over that warm water is the classic setup, and it can generate heavy showers with lightning from an otherwise ordinary weather pattern.
You can see them coming a long way off
The uninterrupted horizon at sea allows a shower to be watched approaching for a considerable time, with a visible rain shaft and a dark base.
Judging whether it will pass to one side or come over you is a matter of watching its bearing, which is the same technique used to assess collision risk.
If the bearing stays constant while the cell grows, it is coming to you, and that is the point at which to act rather than to keep watching.
How they change a coastal day
Squally conditions produce brilliant clear intervals between showers, which is why such days often provide the best light and the best visibility.
They also produce short, sharp interruptions rather than a general washout, so a plan with shelter available at intervals still works.
The forecast term for this is usually a mention of showers with a note about their character, and the words about gusts are the ones that matter.
Questions readers ask
Why is the coast windier than the forecast says?
Land forecasts are produced for typical inland exposure, and models smooth out coastal terrain. An open shoreline, a headland or a cliff top routinely experiences considerably more wind than the grid square value.
Which forecast should I use for a beach day?
The inshore waters forecast for the relevant stretch, together with a wave model and a local tide table. The general land forecast tells you about temperature and rain, and very little about what the sea will be doing.
