Why do clouds disappear during a solar eclipse?
- Published
It might sound odd, but in certain weather situations clouds can disappear from the skies during a solar eclipse.
The disappearance of clouds is just one of the several eerie impacts such as darkening skies and absence of bird song that are likely to be observed during this year's total solar eclipse.
The event will take place on 8 April, with several minutes of totality experienced in a narrow corridor across Mexico, the US and Canada.
An eerie chill
As the Moon passes in front of the Sun, casting a shadow on Earth and turning day to dusk, the most noticeable meteorological affect to many will be the drop in temperature.
In blocking out our main source of energy we may notice the drop in warmth on our skins immediately, but it can take up to 20 minutes for the air temperature to reach its lowest point.
This is because of the way our air is actually warmed. It is not warmed directly by the Sun, but instead by the Sun鈥檚 warmth radiating back out of the ground, and this takes a little longer.
The amount the air temperature drops will vary depending on the overriding weather conditions, but most places under the line of totality can expect a drop of at least a couple of degrees.
During the eclipse across Europe in August 1999, Szczawnica in southern Poland observed a temperature fall of as much as 11 Celsius (20F).
It is that change in temperature that also drives other weather changes.
Calm in the darkness
The wind can fall lighter and change in direction slightly. As the air cools it becomes slightly denser (heavier) and so has greater friction with the ground.
This causes it to slow slightly and take an anti-clockwise shift, so a south-westerly wind would become slightly more southerly.
These changes were highlighted in a 1901 study by American meteorologist Henry Helm Clayton, following an eclipse in May 1900.
Drops in wind strength and temperature can also have an impact on energy demand and renewable energy generation, but the effects of an eclipse are now factored into modern weather computer models to help energy companies in managing these.
Disappearing clouds
Perhaps the most magical weather change during an eclipse is the sight of clouds evaporating.
Now, this will not happen in all situations, and not necessarily with every cloud in the sky.
The type of cloud present ahead of the eclipse is crucial. Cumulus clouds, those fluffy fair weather clouds, are the ones likely to be affected the most.
They form on and are supported by pockets of air rising from the ground and condensing into clouds.
A specific temperature is required for that process to begin and continue. That will vary depending on the atmospheric conditions of the day, but should the temperature fall below that crucial value, as it may do during the process of the eclipse, the skies are liable to clear.
Once the eclipse is over, and if temperatures return to where they were before, the clouds should start to reappear. The breeze will pick up too, and nature should wake up from its brief pause.
- Published22 March