>Sorry this is a little long winded!!

Now that we’re steadily creeping our way into September, this is usually a time when I ask myself, when’s our first frost going to be? Back on September 18th, 2007 I photographed frost that had formed during the overnight hours on cars, house roofs and grassy surfaces. Steam came off warm bodies of water such as the local canal in Kirkintilloch (roughly 10 miles northeast of Glasgow) however as I’ve come to know, throughout the clear, calm nights of summer, we see temps drop, and they meet the dew point, anywhere that lies either in a dip or over relatively warm water, fog forms close to the surface, on the chillier of those mornings, dew lies on the car.

However, we certainly here in low lying central Scotland don’t get frost usually beyond late April though I have seen frost in June. September both last year and 2007 saw frost by mid-month. To get frost to form, it’s usually as nights become longer and skies are perfectly clear and not a breath of wind. Surfaces that cool faster such as cars, blades of grass, signage, roofing tend to 1) collect moisture in our somewhat moist, damp air, and of course helped along by cooling air and condensation formation after sunset. Then as the temperature perhaps drops into the 3-4C or 36 to 40F range, the surface may be around freezing as low lying areas can be colder at your feet than headheight.

As nights become even longer through October, November temperatures can get colder and frosts thicker. Soon the heat in road and pavement cool suffiently enough that even these surfaces start to glaze over with frost and perhsp icy patches and winter will have arrived as sunrise is late and sunset early. Our best change of early frosts is when we have a touch of Indian summer. High pressure sits overhead providing daytime sunshine and temperatures have have been known to warm into the 60s to even 70s well into September, but those warm days give way to chilly nights as those clear skies soon radiatiate the daytime heat back to space by night, often you could waken to having white grass and cars.. I don’t believe we shall recieve the early frost this year, it may be as late as October the way we’re going right now. Too much west to east flow aloft is bringing depression after depression to Scotland. Sure, it’s feeling cold with the day after day rainfall, but until we get a more settled pattern, we won’t have sufficient nights of night-long clear skies and calm winds to allow heat to escape. These early fall nights need longer periods of clear skies, light winds, whereas a few hours in mid-winter and ice can form. I have known some nights to have heavy rain falling at midnight or 1am, only get get up and out for work at 4am and have a solid sheet of frozen rain drops, that are a nightmare to scrape off your windshield, roads, pavements become like ice rinks and for me are likely the most hazardous form of ice/frost.
SCOTLAND’S HEAVY, THICK FROSTS
Because we are a small island in the northeast Atlantic and Arctic or Siberian high’s do settle from time to time or even a breif break in between Atlantic depressions which can provide clear, calm nights in between periods of unsettled and stormy weather, cold nights and frost take place, but we not only “feel” much colder than we actually are, but we appear to be frozen in time some mornings when literally a half ice of frost forms on every surface during the heart of winter and when every ourdoor object or surface is cold. We could be at FREEZING point and look colder than Churchill, Manitoba where it could be 40 below zero.. Our air mass is always “wet”, loaded with moisture because every air mass that comes to Scotland, must travel across water before it arrives here, the Atlantic and North Sea pulls in moisture and that moisture freezes into the form of frost as temperature drop below freezing. When we have a cold Arctic air mass in place and skies remain sunny throughout the day, frost sometimes doesn’t leave the ground and every night gets colder, I wonder whether thick frost covered ground could act in a similar way to snow covered ground, keeping the air above colder? I’ve seen frost thick and expand as air masses haveremained in place for perhaps a week or more, freezing any water body making nights colder and colder. When snow is covering the ground, recent years have not had the appropriate cold air in place to support bitter cold nights… 2002-2003 was the last time we saw truely bitter nights because frost on the ground has stayed unfrozen throughout the cloudless days and crystal clear, still calm nights have allowed stronger radiational cooling. Frozen pipes are more of a risk with little or no snow cover as the cold can freeze deeper with no thermal protection from the snow. When snow in covering the ground, this to a point protects under ground pipes from freezing since snow reflects the the cold, dense air from penetrating deep into the ground and simply radiates it ack to space, that is the reason for the most bitter nights when snow is present on the ground and the air simply grows colder and colder.

The rains are battering off my attic windows are usual, and frost is likely nowhere on the horizon, certainly not until the pattern changes and we get clear nights and sunny days. Perhaps we can still get a few warm, summer-like days if only high pressure could settle across Scotland and I could get in another couple of BBQ’s before the cold of fall and winter comes in. After all it’s only September..

DIFFERENCE BETWEEN SCOTTISH AND NORTH AMERICAN AIR MASSES

Winter cold is remarkably different across much of North America than in the United Kingdom. Because of the direct discharge of highly-concentrated Arctic air that’s bundled in Polar High’s that originate in northern Canada’s Arctic region and drop south as far as Texas, air can and does be much colder than the UK ever gets. But also, the air is extremely dry, simply because these Arctic high’s not only form many miles inland and away from moisture sources, they also drop south and still remain untouched by a large body of water, so therefore even on bitter nights, frost can be absent from outdoor surfaces.

During both visits to Chicago and New Jersey during Arctic outbreaks and temperatures as low as 4 degrees, there was not a even a touch of frost, My friend Derek in New Jersey drove us around and one or two of those very cold mornings when it was down in the low teens and snow was covering the ground, he did not need to clear anything off the windshield. Ice from melted snow was indeed solid on the edge of the windshield but other than that it was completely void of frost.. As already stated, here it can get to freezing just as it’s always a touch time scraping the windshield from frost or ice.. That’s the striking difference between polar maritime air and polar continental. Id prefer the polar continental as I love cold but don’t enjoy so much the scraping.. Their drier air gets drier as the core of Arctic cold settles in. Whilst in Jersey, we saw a 4-8 inch snowstorm, leaving areas very wet and sloppy, then came the Arctic high from Canada, and all of a sudden the next morning after a cleared out sky during the overnight hours and plummeting temps down to 12 degrees, brisk winds which made for wind chills down around zero, that snow was dried out completely, ringing out any moisture that was within the snow after it had fallen. All the moisture that was Atlantic originated was soon changed from a maritime composition to continental. The heavy, wet snow, became, dry and powdery..

Thanks for reading.
-Mark