What km and NM mean on a model
Grid spacing, why a model sees features about five times larger than its grid, and what that means near a coast.
See it for yourself
Move the slider. The map is the same each time; only the grid changes. Each box becomes land or sea depending on what covers most of it, which is what a model’s land-sea mask does.
- Fjord, 6 km widelost
- Strait, 4 km widelost
- Island, about 12 km acrossvisible
Typical of: global ensembles about 25 km; global models 9 to 13 km; regional models 2 to 7 km; the finest local models 1 to 1.5 km. The map is 100 km (54 NM) wide and made up.
What “km” and “NM” refer to
The number is the grid spacing: the distance between two neighbouring points where the model keeps a value. It is shown in kilometres in most documents. A sailor thinks in nautical miles, and one nautical mile is exactly 1.852 km, so 9 km is about 4.9 NM and 2.5 km is about 1.3 NM. You can switch the unit used across this site in the clock menu at the top, under Distances.
Models given in degrees
GRIB files and many data sets describe the grid in degrees, for example 0.25°. One degree of latitude is about 60 NM everywhere, so 0.25° of latitude is 15 NM. But the distance covered by a degree of longitude shrinks towards the poles, so the boxes get narrower as you go north or south.
| Latitude | 0.25° north-south | 0.25° east-west |
|---|---|---|
| 0° | 28 km · 15 NM | 28 km · 15 NM |
| 30° | 28 km · 15 NM | 24 km · 13 NM |
| 45° | 28 km · 15 NM | 20 km · 11 NM |
| 60° | 28 km · 15 NM | 14 km · 8 NM |
What the model can actually see
A single value in a box cannot describe anything that varies inside the box, and numerical methods smooth the sharpest features too. As a rule of thumb a model shows a feature well only if it spans about five to seven grid spacings. This is the effective resolution, and it is always coarser than the grid.
| Model | Grid spacing | In nautical miles | Smallest features shown well |
|---|---|---|---|
| GFS | 13 km | 7 NM | 65 to 91 km |
| ICON | 13 km | 7 NM | 65 to 91 km |
| NAM | 12 km | 6.5 NM | 60 to 84 km |
| ECMWF IFS | 9 km | 4.9 NM | 45 to 63 km |
| ARPEGE | 7 km | 3.8 NM | 35 to 49 km |
| ICON-EU | 6.5 km | 3.5 NM | 33 to 46 km |
| HRRR | 3 km | 1.6 NM | 15 to 21 km |
| ICON-D2 | 2.2 km | 1.2 NM | 11 to 15 km |
| UKV | 1.5 km | 0.8 NM | 8 to 11 km |
| AROME | 1.3 km | 0.7 NM | 7 to 9 km |
The last column uses the rule of thumb of 5 to 7 grid spacings: see the text above.
- Coastlines and islands. A box is either land or sea (or a mix), so narrow straits, small islands and fjords can disappear, and the wind near the coast is a blend of land and sea.
- Terrain. Mountains are smoothed: peaks come out lower and valleys shallower. Winds funnelled through valleys and gaps are weaker in the model than in reality.
- Sea breezes and squalls. These work on scales of a few to a few tens of kilometres. Models of about 4 km or finer can begin to produce them; coarser models cannot, or place them wrongly.
Does a finer grid always mean a better forecast?
- Not automatically. A model with a fine grid but poor starting data or physics can be sharply wrong. A sharp storm in the wrong place can score worse than a smooth, blurred one.
- It costs range. Because fine grids are expensive, the finest models cover small areas and short periods, typically one to three days.
- It helps where the land matters. The gain is largest near coasts, islands, mountains, and for showers and squalls. Over open water far from land a 9 to 13 km model is often just as good.