A = latest available for your selected chart
B = an earlier forecast for A’s exact valid time
Source
Thickness & 850 hPa temperature guide
A quick UK air-mass guide for the dashed 1000–500 hPa thickness contours. Colours match the drawing pens; 1 dam = 10 metres.
Approximate associations: the 850 hPa ranges below are experience-based forecasting aids, not a validated conversion table or limits. Actual temperatures can fall outside them: thickness describes the whole layer, whereas 850 hPa samples one level.
| Thickness | Approx. 850 hPa temperature | UK weather interpretation |
|---|---|---|
| 564 dam | +10 to +15°C | Warm air mass. Can support hot weather in summer with sunshine and favourable surface conditions; thickness alone does not establish a heatwave. In winter, warmth aloft may be separated from colder surface air. |
| 546 dam | 0 to +4°C | Often relatively mild maritime air, with rain favoured at low levels. Surface temperatures vary with cloud, wind and inversions; elevated terrain or a cold layer near the ground can alter precipitation type. |
| 528 dam | −7 to −5°C | Traditional UK snow-risk indicator. Snow becomes more plausible, including at low levels when the lower atmosphere is cold enough. Maritime exposure, wet-bulb temperature and precipitation intensity affect marginal situations. |
| 510 dam | −15 to −10°C | Very cold air mass with strong potential for low-level snow where precipitation occurs. Heavy snow showers require moisture and instability; settling depends on surface conditions and snowfall rate. |
| 492 dam | Below about −18°C | Exceptionally cold air mass. Severe frost and very cold daytime conditions are possible. Snowfall still needs moisture and lift; wind chill depends on the wind as well as temperature. |
For precipitation type and settling, check the full temperature/wet-bulb profile, melting layers, elevation and ground conditions alongside these indicators.
Background: RMetS thickness guide · UK Weather Forecasts snow criteria · NWS thickness explanation. These references support the general principles, not the exact paired ranges above.
0% = latest (A) · 100% = previous (B)
How to use this page
Choose one of the four Icelandic forecasts (00Z, 06Z, 12Z or 18Z), then T+24 through T+120. Each Icelandic button keeps the latest PPVE89 chart for that cycle; these are T+24 forecasts with their printed run and valid dates shown. Each chart remains until its own replacement arrives.
Previous / Next follows the buttons from Iceland 00Z through T+120.
On mobile: with Pen off, tap Chart A to advance one chart. After the last chart, tapping starts again at the first. Scroll or pinch to zoom as usual.
Chart B and the extra panels compare earlier issue times for Chart A’s exact valid time. They never replace your selected lead in Chart A. Run and valid dates are shown on every chart.
Each source is checked independently every five minutes. The page checks for updates every minute and on return, keeping the selected chart. Drawings save automatically for everyone. Use Pen, Undo and Clear mine as before. Wipe is available for matching image layouts.
Archive details
| Valid UTC | Run UTC | Lead | Source | Downloaded UTC | Image |
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