High Pressure and Radiative Cooling

Fergal Tierney writes:

Below is a high resolution satellite image of a crisp, sunny Ireland today, though the patches of dense fog that lingered in some midland places are clearly visible. Ireland is currently under the influence of a strong anticyclone, or area of high pressure, which is centred over the eastern part of the country.

MODIS Satellite picture, taken at 1235UTC on Thursday, 20th January, 2011. Click image for high resolution version (250m/pixel)

As explained in the article on Anticyclonic Gloom, anticyclones are areas of descending, or subsiding, air. As the air subsides it warms (at a rate of 9.8°C/km) and the dewpoint decreases, contributing to extremely low relative humidities above around 1000m (the top of the atmospheric boundary layer). This can be seen in the upper air sounding from Valentia this morning, which shows a strong subsidence inversion at the 900hPa level (~1,100 metres). In the space of just ~100 metres, the temperature curve shoots from -3°C to +2°C, and the dewpoint drops from -4°C to -29°C (and to -40°C just above that). This resulted in a drop in relative humidity of from 94% to just 8% (and 3% just above). No wonder there were no clouds!!

So why was there so much fog in places? It is due to this very absence of cloud. Nighttime lasts around 16 hours at this time of the year, so there is a net negative energy balance, i.e. the ground loses more heat (longwave) energy during the night than can be recovered during the day. On a cloudy night the process of longwave radiation loss to space is limited, as the water in the clouds is an absorber of infrared radiation and thus acts as a blanket. Take away this blanket, and the longwave radiation is free to radiate out into the emptiness of space, lost forever. Temperatures plummet during the night, eventually reaching the dewpoint, resulting in 100% relative humidity. Any further cooling leads to condensation in the form of mist and fog. In some places, especially in the midlands, this fog can be hard to shift, due to a variety of reasons:

  • light winds, which prevent enough turbulent mixing to occur to mix in drier air from above the fog layer,
  • weak solar energy, which is not enough to reach through to heat the ground and reduce the relative humidity,
  • topographical factors, such as low-lying ground, where cold air gathers,
  • saturated ground. Some soil-types are more prone to saturation than others, therefore will contribute more to the humidity of the overlying airmass,
  • frost/snow cover. Frost or lying snow increases the albedo (reflectivity) of the ground, therefore more of the suns energy is wasted due to reflection.
  • nearby lakes or rivers, such as the Shannon and its lakes!

As the high slowly shifts westwards over the next few days it will allow cloudier air to spill over from the north. This will lead to less radiative cooling and milder nights, though it will not feel quite as pleasant during the day. I suppose it's a matter of what your own preference is - cold and sunny, or "less-cold" and gloomy!