Wednesday, April 20, 2011

Question : In Brasilia during the raining season it often starts sunny and hot in the morning, but begins changing at around mid-day. The wind picks up. Clouds appear as if from no-where. The weather turns cold. And suddenly it's raining heavily.

Right now, in London, we're seeing the same thing. Gloriously sunny mornings ... that suddenly turn. Cool breeze, clouds out of nowhere ... maybe rain. At mid-day.

My question : Brasilia is almost 1000km from the sea. And further from the Amazon basin where a lot of the water comes from. London is only thirty or so miles from the sea. Presumably water that evaporates from the sea (or Amazon) takes a very different amount of time to form into clouds and find its way to London or Brasilia.

So why the similarity of this pattern? Is it coincidence? Is it that the clouds are not "pre-formed" water carriers, but actually spontaneously form over Brasilia and London for the same reasons in response to the same conditions at around mid-day?


John Powers said...

"So why the similarity of this pattern?"in re afternoon rains.

I hope somebody who knows answers your question; I'm not such a somebody. But I'll offer that your question seems apparently simple, that is a question that seems as though it ought to have a simple answer.

Something I've noticed as I've tried to understand the science around the general subject of global climate change is that things aren't often simple, yet there's a tendency to think they are.

Most of us have knowledge about water and changes of state as a result of temperature. We have some knowledge of humidity and some sense about vapor pressure. Because we have such everyday understandings we feel pretty confident that rainy weather ought to be pretty easy to understand. In reality the relationships between a few simple facts quickly become rather complicated.

One of the similarities in your London weather and weather you're describing in Brasilia is unstable air being forced aloft. Stability is determined by the temperature of the atmosphere at various heights. There are mechanisms whereby stable air can be forced aloft too.

In general a gray overcast day with light drizzle is a day where stable air is forced aloft and days where clouds build to big cauliflower shapes are days when ascending air is unstable.

The sorts of geographic differences between London and Brasilia you point to have more consequence to situations where stable air is forced aloft than to the sort of afternoon showers you're asking about.

John Powers said...

Oh my comment shows my ignorance. Something I know is wrong is "unstable air being forced aloft." Unstable air ascends freely because of its own buoyancy.

One of the difficulties in understanding greenhouse gases in the atmosphere is the physics involved in the movement of the gases upward. It's definitely not the case I understand this movement well. I point out my ignorance because the sorts of layman explanations of what happens tend to make it harder to understand what actually happens.

Cloud formation is I think much better understood, but not so obvious as we tend to imagine. We're all familiar with condensation. Near the earth's surface heat is readily exchanged between the ground and the air above. But air is a poor conductor of heat, so above a few thousand meters exchange of heat like the ground warming the air is hardly a factor. Compressed air is the major heat source. So air pressure plays an interesting role in cloud formation.

John Mashey's blog Deep Climate keeps a eye on climate change deniers. Not to your post, just going to my point that the expectation that the topic is simple isn't really true. There's some really good scientific explanations in posts.

Also I want to link to one of many articles Mashey has done in re Edward Wegman a statistician at George Mason University.

The Wegman case is a little deep in the weeds of US domestic policy. However it might be interesting to you because part of Wegman's work involves social network analysis of peer review of scientific papers.

Keep in mind that the Koch's are major funders of George Mason.

Scribe said...

The same-ish pattern emerged when we were in the Swiss Alps a few summers ago - often huge storms around 3-4pm. Some of the water must have come from the large lakes, but walking around, you could also actually see mist/steam rising from the trees.

I'd suspect there's a lot more water content around than just lakes and rivers - and more susceptible to heat as it's not flowing? Anything that stores water, including vegetation, marshes, etc, would probably be a major storm battery.