>>Just wondered why on some very cold 0 degree mornings there is no frost and on other 'warmer' (+2) I could have thin ice?
All to do with relative humidity (RH) and the dew point.
There's a certain amount of moisture in the air. RH is a% being the moisture level as a proportion of the maximum moisture the air could hold at that temperature.
When the temp drops, RH goes up - colder air can hold less water. At the dew point, the temp at which the air is saturated with moisture, water condenses. If the temperature goes below freezing below the dew point you will have water and it will be frozen.
The phenomenon of frost on the windscreen when the general temp is above zero is because the layer next to the screen is at a lower temp. That could be because it has been colder, and has not melted yet, or evaporation could be a factor especially if there's wind.
If it's below freezing but there's no frost on the window, the dew point is probably below the current temperature.
Play with this. You can enter a RH and a temp, and it will calculate the dew point. So e.g. if its 5c and 90% RH, you can calculate that dew will form if the temp drops by 1.5C.
www.calculator.net/dew-point-calculator.html
I have some of these. Sometimes interesting to see what's going on.
smile.amazon.co.uk/EEEKit-Temperature-Thermometer-Hygrometer-Greenhouse/dp/B07GPT8HPY/
Last edited by: Manatee on Wed 7 Dec 22 at 11:35
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