Ireland's Gloomy New Year's Weather Explained

The past two days have been a very dark and murky in many parts, with fog and low stratus giving a very gloomy feel to things, writes iWeather Online's synoptics forecaster Fergal Tierney. You might ask why are conditions like this when we have an anticyclone pretty much right over us. Don’t anticyclones bring dry sunny weather? Well, yes they do, but not always.

UKMO fax chart via
Anticyclones are areas of high atmospheric pressure, caused by the subsidence of air from the upper atmosphere. As the air subsides it warms adiabatically at a rate of 9.8°C/km. This warming reduces relative humidity, which dissipates cloudiness and generally gives us our fine sunny weather.

The subsiding air causes an inversion (warm air above cold) at the top of the Planetary Boundary Layer (the lowest layer of the atmosphere where daily solar heating causes turbulent mixing). In this inversion, temperature actually rises with height, which acts as a “cap”, trapping moisture and pollutants below. This is what is referred to as “Anticyclonic Gloom”, and occurs in winter or late Spring, when relatively warm airmasses pass over colder seas.
* Dewpoint is the temperature to which air will need to cool for saturation (100% relative humidity) to occur.
Let’s have a look at the midnight sounding from Castor Bay above, which shows the temperature and dewpoint* profile of the atmosphere, as measured by the instruments attached to their weather balloon. The left axis represents height, as measured in pressure levels (with the actual heights shown in metres), and the horizontal axis represents temperature, in °C. Where the temperature and dewpoint curves are together shows where the atmosphere is saturated, i.e. cloudy, and where they’re far apart is where the air is dry, i.e. low relative humidity. So we can see that the surface lies at around 1025hPa, and the two curves are together at around 4°C, meaning it is foggy. They separate further up, then meet again from around 920hPa (around 900m) to 890hPa (around 1,100m, meaning there is a layer of cloud between these two heights. Above this layer we can see the subsidence inversion, where the temperature rises with height and the dewpoint decreases, meaning very low relative humidity and hence no cloud. This warm layer above colder is very stable, therefore preventing any thermals from rising through it. This causes all moisture and pollutants to remain in the PBL, decreasing visibility and giving us that “gloom”.

With the current airmass originating near the Azores a few days ago and passing north and then eastwards over increasingly cold seas, this caused it to cool to near its dewpoint and cause the layer of cloudiness below the inversion. The combination of saturated ground and calm winds is what is leading to fog at the surface. With a cold front moving down from the north overnight tonight, the inversion will be eroded and we will see conditions clear as convection is allowed to build throughout the full depth of the troposphere. Of course, this cold front marks the arrival of colder air, which we all know can bring its own set of problems far greater than just a murky grey day!

For more on the arriving cold spell stay tuned to Peter’s Daily Forecasts and other updates on IWO.