HELPERG Data · Guide · Weather
How to read an hourly weather forecast
In an hourly weather forecast, temperature and wind describe one instant while precipitation is a total for the hour that follows. How to read each value, its unit and its time zone.
The short answer
Each row of an hourly weather forecast starts at a time. Temperature, wind, humidity, pressure and cloud cover are values for that instant. Precipitation is different: it is the total over the hour that starts at the row’s time, and the weather symbol summarises that same hour. Read “14:00 · 18 °C · 0.6 mm” as “18 °C at 14:00, and 0.6 mm in total between 14:00 and 15:00”.
That split comes from the source. MET Norway’s Locationforecast, which HELPERG reads, separates instantvalues, valid at one point in time, from period values, which are totals or minima and maxima over the next 1, 6 or 12 hours.
What each value in an hourly row means
| Value | Describes | Unit (HELPERG default) | How to read it |
|---|---|---|---|
| Temperature | the instant | °C | Air temperature 2 m above the ground, as the model computes it for the location. |
| Feels like | the instant | °C | The provider’s apparent temperature. A dash means the provider published none for that time. |
| Wind speed | the instant | m/s | A 10-minute mean at 10 m above the ground. Gusts are higher than this figure. |
| Wind direction | the instant | degrees | Where the wind blows from: 0° is north, 90° east, 180° south, 270° west. |
| Precipitation | the hour that starts | mm | Total rain, or snow as melted water, expected over that hour. 1 mm equals 1 litre per square metre. |
| Chance of precipitation | the hour that starts | % | Published by the source for some areas only. Elsewhere it is absent, and a dash is shown. |
| Cloud cover, humidity | the instant | % | The share of the sky covered, and relative humidity. |
| Pressure | the instant | hPa | Air pressure reduced to sea level. 1 hPa equals 1 millibar. |
| Symbol | the hour that starts | — | A summary of the hour’s cloud and precipitation, not a reading at the row’s time. |
A worked example
Illustrative row, not current data. 14:00 · 18 °C · wind 4 m/s from 250° · precipitation 0.6 mm · chance of precipitation 40%.
- At 14:00 the model expects 18 °C. The wind blows from the west-south-west (250°) at about 4 m/s, which is roughly 14 km/h.
- Between 14:00 and 15:00 it expects 0.6 mm of precipitation in total: light rain if it falls. It is not a rate at 14:00.
- 40% is the chance that precipitation occurs in that hour, as the source computes it. It does not say that 40% of the hour will be wet.
Converting the units
| From | To | Rule |
|---|---|---|
| m/s | km/h | multiply by 3.6 (exact) |
| m/s | knots | divide by 0.514444 (1 knot is 1852 m per hour) |
| m/s | mph | multiply by 2.23694 |
| °C | °F | multiply by 9/5, then add 32 |
| mm | inches | divide by 25.4 (exact) |
| hPa | inHg | multiply by 0.02953 |
Times, days and how far ahead
- Time zone. A row’s time is shown in the zone you choose. The instant itself is fixed in UTC. A day boundary depends on the zone, so the same hours can fall on different dates in Prague and in Los Angeles.
- How far. One-hour periods exist only in the short-range part of the source’s forecast. HELPERG’s API returns up to 48 hourly and 10 daily values, as far as the source publishes them, and the weather page asks for that maximum.
- Daily values. HELPERG assembles them from the source’s sub-daily values for the day in your time zone, and labels them as derived by HELPERG. They are not a separate forecast.
Common misreadings
- Reading the precipitation amount as a rate at the row’s time. It is a total over the hour that follows.
- Adding the hourly temperatures, or the chances of precipitation. Only amounts can be summed: 0.6 mm and 0.4 mm make 1.0 mm over two hours.
- Taking the wind speed as the strongest wind. It is a 10-minute mean, and gusts are higher.
- Reading a missing chance of precipitation as “no chance”. Outside the areas where the source publishes it, there is no value at all.
Each value is a model’s expectation for a grid point, not a measurement. The guideweather forecast vs observations explains what that means for the “current conditions” HELPERG shows. Field names and units are in theweather API quickstart.