How does 850 hPa temperature relate to the snow level?
T850 describes temperature at one atmospheric level. A simple linear scenario uses T(z) = T850 + Γ × (Z850 − z) / 1,000, with heights in metres and Γ in °C/km. At Γ = 6, moving 100 m higher corresponds to 0.6 °C lower temperature within that scenario. The 4, 6 and 8 °C/km columns test sensitivity; they are not a calibrated forecast or fixed atmospheric constants.
Inversions, warm layers and changing humidity with height can invalidate this simple extrapolation. The dry-bulb freezing level differs from the wet-bulb zero level and from the actual snowfall limit. Source: NOAA/NWS, vertical profiles and winter precipitation .
NW or NE wind: what changes for snow?
Drier air can cool more through evaporation of precipitation, but it can also evaporate much of it before it reaches the ground. This mechanism may matter in a northwesterly flow; direction alone does not guarantee low humidity or snow. Check dew point, wet-bulb temperature and the actual air flow. NWS case study: dry air, wet-bulb temperature and precipitation-type changes .
Aegean sea-effect snow: can a northeasterly wind enhance snowfall?
Yes. Cold air crossing warmer water can acquire heat and moisture, helping clouds and precipitation develop. The air track, wind changes with height, depth of the cold layer and uplift over terrain influence where precipitation is concentrated. Sea effect is the marine counterpart of lake effect. Study of marine enhancement of southern European snowfall in January 2017 .
The study of storm Elpis in January 2022 linked Attica’s snowfall to the large-scale circulation and terrain. Simulations with warmer sea temperatures increased snow mainly at higher elevations, while a cooler sea favoured longer-lasting snow cover lower down. Northeasterly winds and marine moisture therefore do not have one fixed effect everywhere. Patlakas et al., University of Athens / HNMS study of Elpis .
Can it snow above 0 °C?
Yes. Near-surface temperature is only part of the picture. Flakes pass through layers with different temperatures and humidity. Evaporation and melting cool them, while a deep warm layer can turn them into rain. A single thermometer reading cannot determine whether it will snow. Source: NOAA/NWS, winter precipitation types .
Humidity, dew point and wet-bulb temperature
Dew point describes how far air must cool to reach saturation. Wet-bulb temperature estimates the temperature it can approach through evaporative cooling. In saturated air both equal air temperature. Lower humidity favours more evaporative cooling, but very dry air can also evaporate precipitation before it reaches the surface. Source: FAO, atmospheric humidity .
How much snow can 10 mm of precipitation produce?
If all precipitation falls as snow at a 10:1 ratio, 10 mm of water corresponds to 10 cm of fresh snow. At 5:1 it gives 5 cm; at 15:1 it gives 15 cm. The ratio varies with crystal structure, in-cloud temperature and wind, and cannot be reliably inferred from ground temperature alone. Source: NOAA/NWS, snow-to-water ratios .
Snowfall and settling in Agios Stefanos
Snowfall is the fresh snow falling from the sky. Settled depth is what remains after melting, compaction and wind redistribution. In Agios Stefanos, Attica, at 380 m, the actual location elevation and full vertical profile matter. Local corrections to air-temperature forecasts do not validate snow-depth estimates.
14-day snow and weather forecast for Agios Stefanos · Greece weather forecast with map · Satellites and radar · Historical station measurements
Calculation method and limitations
Wet-bulb temperature is solved numerically from the air–water energy balance and station pressure. Saturation vapour pressure and dew point are referenced to liquid water (Magnus, 17.62 / 243.12). The liquid-water thermodynamic wet-bulb equation is used; below freezing it approximates supercooled water, not an ice bulb. Pressure is estimated from the standard atmosphere when no observation is supplied. Energy-balance equation: ASHRAE / PsychroLib
Centimetres remain conditional: precipitation in mm × snow fraction / 100 × ratio / 10. A 10:1 ratio is a reference, not a locally calibrated default. The 5:1 and 15:1 examples show sensitivity, not a statistical confidence interval. Precipitation type, the ground and the event’s evolution determine what remains.
Snowfall calculator for Greece: compare 850 hPa temperature and geopotential height with elevation, surface wet-bulb temperature and dew point. Explore Aegean sea-effect conditions and conditional snowfall depth. Linear lapse-rate scenarios are not a forecast of precipitation type or snow settling.
Why is one number at 850 hPa insufficient?
The calculator examines surface wet-bulb temperature separately from levels where temperature, humidity and height are all supplied. It estimates zero crossings and the total depth of positive wet-bulb temperature by linear interpolation between these points. Unsampled warm layers can be missed; the result is not a verified snowfall elevation. Sims & Liu (2015) .
The 0.5 and 1 °C wet-bulb thresholds are two published benchmarks. When they disagree, the result remains transitional. A warm layer triggers a melting or mixed-precipitation indication. A 2025 study found that even more complex surface-based methods struggle near freezing. Jennings et al. (2025) .
The GFS button imports the station’s next available hourly forecast, including the 925/850 hPa levels. Hourly precipitation covers the interval ending at the displayed time. The import uses one model, without the blended forecast’s calibration. It does not supply ground or sea temperature. Source: GFS through Open-Meteo .