Our fine and settled spell of weather continues this week as an intense area of high pressure refuses to shift. Hour after hour of long-awaited sunshine, tempered only by some wisps of high cirrus clouds and contrails. We could have done with this back during our real summer, but better late than never. But why does high pressure mean sunny weather?
Firstly, high pressure (anticyclones) usually means sunny weather, but not always. The reason for this will come later, but first we’ll look at why this one is giving us our sunniest spell since June.
Today’s midday sounding from Valentia illustrates things nicely. This is a plot of temperature (red) and dewpoint (green) measured by a radiosonde attached to a weather balloon as it rose through the atmosphere.
In an anticyclone the air is subsiding (sinking), unlike in low pressure, where it rises and cools to form clouds and rain. Subsiding air compresses and warms, just like the air in your bicycle pump heats up as it compresses. The rate of warming is 9.8 °C per kilometre, and a layer of warm air (subsidence inversion) forms a few thousand feet above the surface (red curve). As the air is coming from upper layers of the troposphere it is very dry, causing a very sharp drop in dewpoint within the inversion (green curve). This very warm and dry air means clouds are absent, subsidence being the nemesis to cloudiness.
We can see that the surface temperature and dewpoint are around 16 and 10 °C, respectively. The temperature falls quickly with height to 10 °C in the first 700 hundred metres but then rises sharply again to about 13 °C and remains at that temperature up to around 850 hPa (1640 m). This warm layer is the inversion. After that it falls steadily to -69 °C at the tropopause (~13,500 m), where it then starts rising again slowly through the stratosphere.
Dewpoint shows the opposite effect, taking a huge dive from 8 °C at 600 m to -20 °C at 1640 m. This translates to a drop in relative humidity from 80 to 10 %, falling further to just 3% as we continue further up. No cloud is going to fom in air that dry!
We did have some cirrus, however, due to much higher relative humidity above 400 hPa (7570 m). Many of these cirrus were caused by persistent contrails, as relative humidities relative to ice were above 100% at some levels (frost point, not dewpoint, is more important at very low temperatures, and lies between the green and red curves). As the sun is now lower in the sky (the same height as on St. Patrick’s Day) more of its energy is reflected off this cirrus, meaning temperatures are struggling to reach the 20 °C mark.
So why do we sometimes get layers of cloud even in high pressure? This occurs when the lowest layer is moist and saturated near the inversion, with cloud forming but being forced to spread out by the warm air above it. The current airmass is very dry in its lowest layers, meaning it never reaches saturation, except when it forms fog at the surface during the nighttime cooling. This process is described in more detail in this previous article.
The chart for tomorrow shows the current high pressure area extending back over Ireland, ensuring a few more sunny days before a probable return to Atlantic business as usual from next week.