Snow Forecast



Precip snow level


Forecasting kind of precipitation in winter
via Ivens method
Theta W max (°C)
Theta W surface (°C)
Thickness of warm layer (hPa)
Thickness of stable layer (hPa)
Forecasted amount of precipitation (mm)
Duration precipitation (hour)
Is warm layer discontinuous Yes No

Critical amount of precipitation for snow mm
Critical amount of precipitation for wet snow mm
Melting layer parameter mm


Used formulas:

rc = 0.03 * (Tw * dwl + Twg * dsl)
i = precipitation / duration precipitation
rca = rc - 2.497 * i0.68 - 1.524 * i0.34 - 0.235;
sp = rc - rca

Tw: maximum wet bulb temperature
Twg: wet bulb temperature at surface
dwl: thickness warm layer
dsl: thickness stable layer
rc: critical amount of precipitation for snow
rca: critical amount of precipitation for wet snow
sp: melting layer parameter

Ivens, A.A.M., Forecasting the kind of precipitation in winter.
Proc. Symp. Mesoscale Analysis & Forecasting, Vancouver, Canada, 17-19 August 1987, ESA SP-282 (August 1987).




From the greek letter 'theta' and subscript 'w', used to denote wet bulb potential temperature (q.v.) - one of a group of pseudo-conservative (q.v.) properties of air masses.

 wet bulb potential (often abbreviated to WBPT, or 'theta-W') A relatively conservative property within any one air mass that is derived from the temperature and humidity values of a particular air sample for a particular level: usually 850 or 500 hPa. Very warm/very humid southerlies for example (in NW Europe) would have typical 850hPa WBPT values well in excess of 16 degC, and perhaps as high as 20 degC or more; polar maritime air streams would have values typically 5 to 10 degC, but these values would be much lower in the depths of winter.

From the wet-bulb temperature, follow the saturation adiabat to the 1000 hPa isobar.The isotherm value at this intersection equals the wet-bulb potential temperature at the given pressure.

In this example, air at 700 hPa with T = -5°C and Td = -13°C has a wet-bulb temperature of -7°C and a wet-bulb potential temperature of 10°C.

Thickness and its uses

Thickness' is a measure of how warm or cold a layer of the atmosphere is, usually a layer in the lowest 5 km of the troposphere; high values mean warm air, and low values mean cold air.

It would be perfectly feasible to define the average temperature of a layer in the atmosphere by calculating its mean value in degrees C (or Kelvin) between two vertical points, but an easier, practical way to measure this same mean temperature between two levels can be gained by subtracting the lower height value of the appropriate isobaric surface from the upper.
Thus one measure of thickness commonly quoted is = height (500 hPa surface) - height (1000 hPa surface)

Advection is simply the meteorologists word for movement of air in bulk. When we talk about warm advection, we mean that warm air replaces colder air, and vice-versa. These 'bulk' movements of air of differing temperatures can be seen very well on thickness charts, and differential advection, important in studies of stabilisation / de-stabilisation, can also be inferred by considering advection of partial thicknesses.

Total Thickness (500-1000 hPa) isopleths (when shown in combination with other fields) are conventionally drawn as long-dash lines, with the values either thus [540] or white numerals on a black/solid rectangle. Certain isopleths are considered 'standard', mainly for historical reasons: They are listed hereunder, with the colour code convention used by the UK Met.Office on internal charts.
474 - red 492 - purple 510 - brown 528 - blue 546 - green 564 - red 582 - purple

Rain and snow are equally likely when the 500-1000 hPa thickness is about 5225 gpm (or 522 dam).
Rain is rare when the 500-1000 hPa thickness is less than 5190 gpm.
Snow is extremely rare when the 500-1000 hPa thickness is greater than 5395 gpm

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