A thunderstorm just rolled through here in Mississauga, Ontario. Before the rain hit, it got very very dark, almost alarmingly so. Then it lightened up and the torrent began.
My question: why is it so much darker before the rain starts? Are the clouds thicker there on the edge of the storm?
Rigardu, kaj vi ekvidos.
I think cumulo-nimbus t-storm type clouds can reach 30,000 feet or more. That would seem to block out a lot of sun.
Just look out if they look kinda greenish. That’s one sign of a potential tornado.
As a meteorologist, even I can’t think of a definite answer off the top of my head. Like Mjollnir said, cumulonimbus clouds are very tall, reaching 60,000 feet in some instances, and they do block out the sun. Also, sunlight makes clouds appear darker.
Another guess I have is that you can see the dark clouds from a relatively far distance. When it rains heavily, it is difficult to see many cloud features.
Last comment, referring to the “green skies” comment. Green skies in and of themselves don’t necessarily mean a tornado is imminent. Greenish skies aren’t that rare. Rather, they are due to hail in the cloud. However, large hail is usually only produced in the most severe thunderstorms, which also may produce tornadoes, so they are related.
Two WAGs:[list=1][li]The squall cloud precedes the storm. It is dark gray, sometimes blueish, and associated with violent wind and turbulence but rarely if ever with rain. It is not very thick, though, so I doubt it has much ability to block sunlight by itself.[/li][li]The back edge of the storm (most often the west edge in the middle lattitudes of the N. Hemisphere) is sloped backwards. The cloud looks something like this[/list=1][/li]
(sun)
*************
|******** -----> movement toward east
|******
||**********
||*************
|||||
||||
Here the asterisks represent cloud, and vertical lines represent rain. (Some of the rain evaporates before reaching the ground.) As you can see, in the afternoon when the sun is in the west, sunlight can reach the ground pretty easily even where it’s raining, but to reach the ground near the leading edge of the storm, it has to pass through much more cloud. Again, not much more than a WAG
WAG
Thick clouds arrive.
It gets dark.
Your pupils adjust to lower levels of light.
It doesn’t seem so dark.
I thought of the ‘pupils adjusting’ thing too, but when the sky was at its darkest, I could look to the west, and see lighter sky. When this arrives, the rain began. So I’m not convinced that simple eye adjustment explains it.
We were supposed to get thunderstorms here this weekend (Asheville, NC) so I was going to test a few theories. However, I was subjected to an entire weekend of sunny weather with highs in the high seventies. Damn it all.
One thing I wanted to check was if it got darker when the trailing edge of the storm passed overhead. If anyone has a chance to observe this, please do so and report your findings here.