Studies

Eyewall replacement: why a typhoon suddenly growsThe peak wind dips a little, and the area affected gets wider · 2 min read
Hong Kong's typhoon signals, explainedWhat each of the five levels means in wind speed, and why the No. 8 comes in four directions · 2 min read
Why a storm to the south brings easterly windsCyclones spin anticlockwise north of the equator, so the wind where you stand depends on which side of the storm you are on · 3 min read
What an ensemble forecast is, and what it is notA spray of tracks is a statement about uncertainty, not a menu of outcomes · 2 min read
How to read a METARThe line of apparent gibberish airports publish, taken apart piece by piece · 2 min read
Knots, km/h and the Beaufort scaleTwo agencies can quote the same storm 20 per cent apart. They are not disagreeing; they are measuring different things · 3 min read
Storm surge: why low-lying areas flood before the wind peaksThe flooding is not rain. It is sea water pushed ashore — and its worst moment is not the windiest one · 2 min read
How the No. 8 signal chance is computedForty perturbed tracks, and a count of how many bring gale-force wind to Hong Kong's offshore stations · 4 min read
Wind radii: how big a typhoon really isThe gale radius on a storm page is the agency's figure; its threshold sits below a No. 8, and the circle is not really a circle · 3 min read
Where the rain nowcast comes fromThe four half-hour cells on the Today page are the Observatory's radar-based gridded rainfall, and why there is no minute-by-minute bar · 3 min read