After the extended period of pleasant warmth that we experienced throughout June, July and the early part of August, this last week certainly came as a bit of a shock as a Polar air mass, which moved down over Ireland last weekend, brought with it a much cooler feel to the air. Many people commented that it felt more like October than August, with many others saying that they had to put the heating on for the first time in months.
So why did it feel so cool this week? Particularly when daytime temperatures still managed to reach into the mid to high teens on most days this week along with good amounts of sunshine? The answer is not so much to do with the actual temperature or sunshine values themselves, but more to do with the humidly value (water content of the air) and its effect on the feel of the air as we actual experience it.
The air mass which lay over the country this past week was sourced over Polar regions, and because of this, it was relatively dry. Colder air masses tend to be drier because cold air cannot hold onto the same levels of moisture that warm air can. On route to Ireland & NW Europe, Polar air masses such as this particular are modified a to an extent as they cross over the colossal expanse of the north Atlantic, picking up both heat and moisture, allowing cumulus clouds to form which can bring showers between the lengthy spells of blue sky – just as we seen throughout this week.
Despite these modifications, Polar air masses retain much of their original characteristics by the time they reach Ireland, and one of these is drier air, thus, lower relative humidity values.
Relatively humidity (abbreviation: “RH”) is the measure of water content in a sample of air at a given temperature. (a more detailed, scientific overview of relative humidity contained within this article by IWOs Fergal Tierney) Basically, the higher the water content in a pocket of air, the more humid the air will feel — and the more humid the air, the slower sweat will evaporate from skin.
When RH values are low, such as they have been throughout this week, then the easier it is for sweat to evaporate from our skin. Evaporation has a cooling effect on any surface where it is taking place, including our skin, and this is one of the reasons why some of us may have felt pretty cool this week, despite the apparently respectable daytime temperatures.
If you find this hard to believe, try this small experiment. Dip your hand, or even just one finger, in a basin of water, then go outside for a minute or two and hold your wetted hand (or finger) up. You will instantly notice that your wet hand will feel much cooler than your dry one. This is the direct result of a phenomenon known as ‘evaporative cooling’, and this will be all the more apparent when relative humidity values are low, and less so when they are high. Just to illustrate this point a little further, a daytime air temperature of 16°c coupled with a 60% RH value will have a stronger cooling effect on your skin than 16°C with RH value of 95%, because the evaporation rate is higher when RH values are low – and vice-versa.