Wednesday April 29th
Tuesday gave us such a gloomy, grey morning with brief light drizzle in the air, that combined with the strong northeasterly wind meant a cool day. The sun did break through in the afternoon that allowed the temperature to rise to a maximum of 14.2C at 16.30, however, this was 0.1C below average and made it the coolest day since the 21st. The minimum of 5.3C was logged at 05.03 early Wednesday being 1.4C above the late April average. The cool northeasterly breeze kept brisk throughout the night gusting into double figures.
Wednesday began quite the opposite from Tuesday, thankfully, with sunshine shortly after sunrise, although the persistent northeasterly, veering into the east later, is still blowing quite strong and will do so throughout the day, thus making it feel cooler outside than that indicated on a radiation shielded thermometer.
There are signs that the recent dominant high pressure is edging northeastwards towards eastern Europe and an Atlantic depression edging closer. The barometric pressure has been edging lower for the past two days, with a current barometric pressure of 1023.2mb at 08.00, which is still high, that will give us two more fine days with plenty of sunshine before it departs however, we will still be dominated by the persistent wind that has veered a few degrees to come from the east for a couple of days. This is the result of the squeeze between the two pressure systems as the air rushes from the high to the low pressure.
The high pressure is also keeping weather fronts at bay for a couple of days, although the forecast surface pressure charts indicate that the depression over Iberia will edge closer that will throw a weather front over the western approaches today, that is likely to cross the country late on Friday into Saturday. Sadly this will bring unsettled conditions for the bank holiday with lower temperatures by day and cloudier conditions.
From time to time there are discussions around condensation trails left by aircraft under certain conditions and if they might affect the climate.
I attach the first part of an interesting article discussing this subject that has just been released, that might be of interest.
Contrails, short for condensation trails, are the white streaks often seen in the sky behind aircraft. The international cloud atlas, which classifies clouds, has a category just for them: cirrus homogenitus, an example of man-made clouds.
Contrails contribute to climate change, adding to the warming by the carbon dioxide emitted by aviation. Although the exact amount of warming caused by these wispy-looking clouds is uncertain, what is understood now suggests that reducing the number of contrails has the potential to reduce the climate impact of flights.
Contrails are made of ice crystals. These reflect sunlight, causing the Earth’s surface to receive less energy, but at the same time trapping some of Earth’s outgoing infrared radiation. Depending on the balance between those two opposite effects, a net loss of energy or a gain of energy, individual contrails can be either warming or cooling over their lifetime, but warming dominates once averaged over the global, annual contrail population.
How are they made?
Contrails form behind aircraft at around an altitude of 10-1lkm. They only form in sufficiently cold and humid regions of the atmosphere, where water vapour condenses on to the soot particles emitted by aircraft engines to form liquid droplets, which freeze into ice crystals. The regions with the most contrails are over Europe, the North Atlantic, and eastern North America. They are rarer in Asia.
Soot particles are needed to form contrails, yet even engines that emit very few soot particles still generate contrails. Other particles, often formed in the engine plume, take over and lead to contrail formation. But some combinations of fuel and engine technology may yet provide a way to form fewer contrails, or at least contrails with a smaller climate impact.
The characteristics of a contrail depend initially on the size, shape and engine position of the aircraft that created it, but atmospheric conditions are ultimately more important.
In a dry atmosphere, contrails only last a few minutes and cover a tiny surface area: their climate impact is negligible. But if the atmosphere remains cold and moist enough, many contrails form, grow, and come together to form fields of ice clouds, called contrail cirrus.
Contrail cirrus impact the climate because they last for several hours and can cover large areas, sometimes spanning entire countries, as has been observed over the UK and France, for example.