Reading the sky: fronts, showers and the crossed-winds rule
1The lesson in plain words
The sky shows what the air is doing above you. Use cloud sequences and cloud movement together with the barometer, never on their own.
A front announced
On board, watch whether high cloud spreads from upwind, thickens and lowers while the barometer falls. These signs support an approaching warm front. Often the first cirrus comes half a day to a day before the rain. Scattered cirrus that never thickens often means nothing.
Halo and mackerel sky
A halo is a ring round the sun or moon, usually about 22° from its centre. A mackerel sky is rippled cloud, like fish scales. Both can come ahead of a front, so watch whether the cloud thickens and lowers. Old sayings about them are only a prompt to look again.
Towers in the morning
Cumulus clouds that grow tall and hard-edged by late morning, called towering cumulus, show unstable air: showers or thunderstorms may follow later in the day. Small turrets on a mid-level cloud layer early in the morning, called altocumulus castellanus, are another warning of thunderstorms later. Flat, shallow cumulus that stays small usually means fair weather.
The crossed-winds rule
Watch which way the high or middle clouds move, compared with the surface wind. In the Northern Hemisphere, stand with your back to the surface wind. If the high clouds come from your left, the weather will probably get worse. From your right, it will probably improve. Parallel to the surface wind: little change. In the Southern Hemisphere, reverse left and right.
Using the rule well
Use the general wind over open water, corrected for your boat’s motion. Avoid sea breezes and wind bent by land. Hold a steady heading and allow for yaw, the boat swinging from side to side; do not mistake mast movement for cloud movement. Note the compass direction high or middle clouds come from. Low cloud largely follows surface wind. Treat the rule as a clue and check it against pressure and the latest bulletin.
2Key data
- Reliable front sign
- High cloud from upwind thickens and lowers; barometer falls
- First cirrus to rain
- Often about half a day to a day; varies a lot
- Halo
- A halo prompts cloud and pressure checks; no forecast on its own
- Crossed winds (north)
- Back to surface wind; high cloud from left: worse; from right: better
- Thunder warning
- Towering cumulus by late morning; altocumulus castellanus early
- Spanish terms
- Halo; cielo aborregado (mackerel sky); cúmulos de desarrollo
3Key points
- 1
A front is likely when high cloud thickens and lowers while the barometer falls.
- 2
Halo and mackerel sky mean look again; they are not forecasts on their own.
- 3
Towering cumulus by late morning warns of afternoon showers or thunder.
- 4
North, back to the surface wind: high cloud from your left means worse weather.
- 5
Use high or middle cloud and the open-sea wind for the crossed-winds rule.
4Frequent mistakes
Wrong: Every halo means a gale within 12 hours.
Right: A halo is a prompt to watch. Check whether the cloud thickens and lowers and the barometer falls before drawing conclusions.
Wrong: Use low, fast-moving cloud for the crossed-winds rule.
Right: Low cloud moves almost with the surface wind. Use high or middle cloud, which shows the wind aloft.
Wrong: Apply the crossed-winds rule to a sea breeze near the coast.
Right: A sea breeze is a local wind. Use the general wind over open water, or the rule gives false answers.
5Practice case & exam keys
In the English Channel you have a steady westerly wind. With your back to it, you see cirrus streaks moving from your left, slowly thickening. The barometer has fallen 1.2 hPa in 3 hours. What do you expect, and what do you do?
Show the answer
With your back to a westerly you face east, so cloud from your left comes from the north. By the crossed-winds rule, high cloud from the left in the Northern Hemisphere means the weather will probably get worse. Thickening cirrus and a slow fall agree: a front may arrive within about a day. Get the latest forecast, plan your stops, and keep logging the sky and the barometer.
Exam keys
- Warm-front sign is reliable when cloud thickens and lowers and pressure falls.
- A halo or mackerel sky prompts further checks; neither predicts weather alone.
- Crossed winds (NH): back to surface wind, high cloud from left = deterioration.
- Towering cumulus or altocumulus castellanus in the morning: thunder risk later.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
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