Frost Alerts

I have a wireless soil/air sensor so can see when the air temp just above soil drops.
Jimmy

I've learnt living here in the nethers of Lincpolnshire that if the weather says 6C I always drop it by 4 as the position I live in does not reflect the temps that might occur a mile down the road.

Our potager is about 2weeks behind our neighbour's who are just across the road from us
Always good to have a rough idea though!

Someone has been on the Brandy.. personal antifreeze

The trouble with any sort of app is that it will use the raw data from one of the forecasting models, often the GFS. While this will give you a general idea, it has no human interpretation at all, and can be seriously misleading as very local factors can make a large difference.

The best way to go about deciding whether a frost is likely is to take note of the general weather forecast for your area, then look outside at the state of the sky and the wind. A clear sky, forecast to stay clear, plus a light wind that is not forecast to pick up during the night is a recipe for possible frost. The longer the hours with no daylight the colder it will get, and it is almost always coldest just after dawn.

Last night was a superb example of what I mean. Our forecast here was for a minimum temperature of 6C, and these are often reasonably accurate for my location. However, by 5pm the sky was completely clear and the wind had dropped to nothing. Although the temperature was around 15 degrees, I went to the allotment and rigged up a net cover over the tomatoes, hoping for a little protection as the net might trap some warm air and stop the cold air sinking onto the plants.

This is what happened:



(sorry about the size of the screenshot - the latest windows update has broken my usual screenshot software).

The red line is the air temperature, and you can see that every time there is a breath of wind it rises as the wind mixes the layers of air. But when the wind drops the cold air sinks again and it gets colder, reaching its minimum of 2.2C at about 8am. After that, on a sunny morning, the temperature shoots up, aided by a light breeze. (The green line is the dewpoint - the temperature can't drop below this, but the dewpoint can of course drop during the night).

Of course it isn't foolproof to forecast this way. It can be cloudy in the evening and clear up during the night, allowing the temperature to drop suddenly, particularly if the cloud was associated with a cold front. But in general, clear skies + no wind + long nights = frost, particularly if the air mass is cold.

Sighs. I know you're all right, I was just hoping for a failsafe that didn't require me to check the forecast and local conditions daily, which I just can't guarantee I can do.

It's moot now, anyway, as the pumpkins are happily on the windowsill (looking gorgeous as it happens) and there's nothing else on the plot that will be harmed by frost at this stage.

what is the definition of frost?

One thing that I find fascinating where I live is it's locality
if it's frosty out the front, it may well not be frosty in the back. Even weirder, one of the cars out of the front seems to get frost even when nowhere else is. I assume these are all down to local climactic features. (or the car is used for ice-cream smuggling...)

Good article here explaining the different types of frost. Whether you see white or not will depend on how wet the surface is. Car windscreens tend to mist up before the temperature gets cold enough to freeze, so you get ice on them, whereas sometimes if a surface is dry it can appear not to be frosty when it is below zero.

It always used to bug me that the windscreen frosted up when the back window didn't. Having learned a lot more about why frost forms I now understand that it is because the windscreen slopes at a more acute angle, so it catches the cold air as it sinks, whereas the more vertical back window doesn't.

Nice link Pen