Convert in real time between the most common units in meteorology and hydrology. Edit either of the two fields and the other updates instantly.
Almost every equivalence on this page is exact by definition, not approximate, and knowing that settles arguments. The knot is one nautical mile per hour, and the nautical mile has been exactly 1,852 metres since 1929, so 1 knot is exactly 1.852 km/h and 0.5144 m/s. The hectopascal and the millibar are the same quantity under two names: 1 hPa = 100 Pa = 1 mbar exactly, so an old chart in millibars and a current one in hectopascals read with the same number. And a millimetre of rain is exactly one litre per square metre: a millimetre of water spread over a square metre occupies a litre. That is why forecasts use the two terms interchangeably.
There are three places where care is needed. First, wind: the «mean» speed in a forecast is averaged over ten minutes, while the maximum gust is an instantaneous three-second value, and on a stormy day the gust can be 50 % above the mean with nothing wrong. Second, rain: a millimetre is a depth of water, not a volume, which is why it does not depend on the size of the gauge. Third, aviation pressure, given in inches of mercury (1 inHg = 33.8639 hPa) and, in European QNH, in hectopascals.
A marine forecast does not talk in kilometres per hour but in Beaufort force, because the scale describes the wind and the sea state it produces at the same time. This is the official conversion used by the World Meteorological Organization.
| Force | Description | Knots | km/h | Sea state |
|---|---|---|---|---|
| 0 | Calm | < 1 | < 1 | Mirror |
| 1 | Light air | 1-3 | 1-5 | Ripples, no foam |
| 2 | Light breeze | 4-6 | 6-11 | Short waves, glassy crests |
| 3 | Gentle breeze | 7-10 | 12-19 | Scattered whitecaps |
| 4 | Moderate breeze | 11-16 | 20-28 | 1 m waves, frequent whitecaps |
| 5 | Fresh breeze | 17-21 | 29-38 | 2 m waves, white foam |
| 6 | Strong breeze | 22-27 | 39-49 | 3 m waves, breaking crests |
| 7 | Near gale | 28-33 | 50-61 | Foam in streaks |
| 8 | Gale | 34-40 | 62-74 | 5.5 m waves, long breakers |
| 9 | Severe gale | 41-47 | 75-88 | 7 m waves, reduced visibility |
| 10 | Storm | 48-55 | 89-102 | 9 m waves, sea white |
| 11 | Violent storm | 56-63 | 103-117 | 11.5 m waves |
| 12 | Hurricane | ≥ 64 | ≥ 118 | Sea covered in foam |
Spanish reservoir data from the Ministry for the Ecological Transition is published in cubic hectometres, and it is worth having the scale in mind: 1 hm³ = 1,000,000 m³ = 1,000 million litres. For a sense of proportion, domestic consumption in a city of a hundred thousand people is around 5 hm³ a year, and Spanish reservoirs total some 56,000 hm³ of capacity. A flow of 1 m³/s sustained for a whole day delivers 0.0864 hm³.
