This morning, the temperature in SE Scotland had been down to -0.6C in my garden, which gets rounded to -1C in the met. world. For statistical reasons, always round to the odd, avoids a warm or cold bias.
So outside, it was a cold sparkly white world, with frost on grass, leaves and pavements. I was trying to explain to my youngest child how you get a frost at zero because that is the temperature that water freezes at. She told me that 0 (zero the hero) is nothing and that is the number you start at, so didn’t get into a discussion about -1C or -5C as seen in the Highlands. In the scientific world, -273C is nothing, where you truly start at, 0 degrees Kelvin, no energy, we’ll do that next week.
However, there was a frost on Monday morning and temperatures and only dipped to +2.7C (can you tell I’ve got a new thermometer outside?)
There is an Air Frost (a true frost) when the air temperature falls to zero Celsius, and a ground frost, normally below 3C. You might get a touch of frost on the grass below 5C – grass frost. This is due to different types of surface and their ability to hold heat. Sensors for recording temperatures are placed on tarmac/concrete and at different depths. This helps forecast ice on roads. So with some autumn sunshine, a tarmac road may hold onto that heat longer during a clear night, than the grass. A dark soil field will absorb more heat from the sun than a pale AstroTurf pitch. The UK Met Office record twice daily NCMs (National Climatological Message) which include precipitation types, sunshine hours (you see these in the press reports) and different depth and surface type temperatures, grass/concrete, D30cm/ D100cm.
For an air frost, the UK would be away from summer months, have lighter winds, clear skies and time for the temperatures to fall before morning sunshine warms everything up again. Obviously northern Scotland gets a frost before Cornwall due to its latitude. Top of Nevis range gets a frost before Skegness beach due to altitude. If the air is still, the effect of radiative cooling (heat being lost from the earth out into the atmosphere at night) works on the same parcel of air, and so continue to cool. If a breeze moves the parcels of air on, one cools, blown away, next one comes along, cools a bit, moves on and the frost doesn’t form so readily.
Also if cloud cover increases in the night, it acts as a blanket and stops the flow of heat out and away from the lower atmosphere, cooling is reduced. Temperatures don’t fall so much.
Forecasting for the roads, so gritters can do their work to stop ice on the highways, generally has 3 stages of alert. Green – all fine, Red – Ice risk – would be a night for gritting, and Amber, urgh the plague of road ice forecasting. Incoming frontal cloud, might just save the temperatures above freezing, showers will wash all the grit off the roads and then freeze, Also, is there enough moisture in the air, using dew point temperatures. When the air is very dry, there isn’t the moisture to form ice.
Might be simpler to just blame Jack Frost, he’s been busy.
