Signs of an approaching storm and finding its centre
1The lesson in plain words
Swell, pressure and cloud changes can give early warning of a cyclone. Read the clues together and check the latest advisory.
Swell first
Swell is waves that have left the wind that made them. A new long swell from an unusual direction can warn of a distant cyclone before its winds arrive. Check the advisory. See Onboard forecasting for swell timing.
The barometer
Tropical pressure normally follows a regular twice-daily wave of up to about 3 hPa. Loss of that rhythm is a warning. Reeds gives a traditional warning clue: sea-level pressure about 3 hPa below the local seasonal normal after daily-wave allowance. Pilot charts give normal pressure. Check the advisory and other signs. This does not prove a cyclone or fix its distance or track. An accelerating fall can mean approach, deepening, or both.
Sky and air
Streaks of cirrus, high ice cloud, may fan out from the storm's direction hundreds of miles ahead. Sunsets can be unusually coloured, and visibility unusually clear. Closer in, a dark wall of cloud, the bar, rises on the horizon. Squalls come more often and harder. The centre may still be 100–200 miles away when the bar arrives.
Finding the centre
Buys Ballot's law links wind and pressure. In the northern hemisphere, stand with your back to the true wind: low pressure is on your left. Because the wind spirals inwards, the centre is a little forward of your left beam. Facing the wind, it lies roughly 90°–135° to your right. In the southern hemisphere it is the mirror: back to the wind, the centre is on your right.
Confirm, don't guess
These signs are clues, not a position. Get the latest advisory, plot the centre and compare it with what you observe. If your observations and the forecast disagree, act on the worse of the two and report your weather to the forecasters if you can.
2Key data
- Early warning
- Long swell from an unusual direction can be an early warning.
- Daily pressure wave
- Up to about 3 hPa; its loss is a warning sign
- Pressure warning
- Reeds clue: corrected pressure ~3 hPa below seasonal normal
- The bar
- Dark cloud wall; centre may be 100–200 miles away
- Buys Ballot (north)
- Back to the wind, low on the left
- Buys Ballot (south)
- Back to the wind, low on the right
3Key points
- 1
A new long swell from an unusual direction is often the first sign of a distant storm.
- 2
Reeds clue: after sea-level and daily-wave corrections, ~3 hPa below the local seasonal normal warrants suspicion.
- 3
Cirrus, a vivid sky and then the dark bar mark the approach of the storm's cloud shield.
- 4
North: back to the wind, the centre is on your left, slightly forward of the beam. South: on your right.
- 5
Signs are clues: confirm the storm's position with the latest official advisory.
4Frequent mistakes
Wrong: The barometer barely moves in the tropics, so it is no use for storm warning.
Right: Its small, regular daily wave makes any change stand out. Compare with the seasonal normal at the same local time.
Wrong: With your back to the wind, the centre of a northern hemisphere storm is directly behind you.
Right: It is on your left, a little forward of the beam, because the wind spirals in towards the centre.
Wrong: Growing swell proves the storm is heading for you.
Right: Swell shows where waves were made, not where the storm is going. Check its track in the advisory.
5Practice case & exam keys
At 15° N in the Atlantic in late August, the barograph's twice-daily wave has flattened and pressure reads 1009 hPa; the pilot chart gives about 1013 hPa as normal. A long swell is arriving from the east-south-east and cirrus is spreading from the same side. The true wind is north-east. What do you conclude and do?
Show the answer
The pressure and cloud signs justify suspecting a cyclone. Facing the north-east true wind, the northern rule suggests its centre between south-east and south. Swell can indicate an earlier storm position, so the clues do not fix the centre. Get the latest NHC advisory, compare it with your observations and act on the worse case.
Exam keys
- First signs: long swell, cirrus, loss of daily wave; Reeds clue: ~3 hPa below local seasonal normal after sea-level/daily-wave corrections.
- The bar: a dark cloud wall; the centre may still be 100–200 miles away.
- Buys Ballot, north: back to the wind, low on the left. South: back to the wind, low on the right.
- Facing the wind in the north, the centre lies roughly 90°–135° to the right.
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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