Self-defeating Sunshine
By Sean Downey on April 10, 2012, 1:11pm
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You wake up to bright sunny skies, nary a cloud in the sky, and you think it’s going to be a gorgeous day. The day wears on, and you see some fair weather clouds form. “No big deal” you say and continue on with your day. Next thing you know you’re outside walking around and it starts to rain on you (without your umbrella) because it was so nice out.
This happens a lot this time of year – cold lows spinning nearby combine with sunshine to create instability showers. Take a look at the visible shot above - you can see a huge gyre of clouds situated over Quebec and northern New England. This is an upper level low; its circulation is allowing cooler air to infiltrate south and eastward. Look across Pennsylvania – see all those puffy cumulus clouds forming? Those are indicative of cold air advection – cold air moving into a particular region. So why do the clouds and showers form? Well, what happens is that the sun heats the Earth’s surface. The air is heated and becomes buoyant to the point that it begins to rise (becomes unstable). As it rises, it begins to cool and condense into water vapor (forming a cloud).
Because there’s so much cold air aloft, clouds form, and eventually showers will form. Typically these showers are widely scattered in nature, as is the case today, but as is often the case with these lows, troughs of low pressure will swing through and act as a focal point for more widespread showers – as will be the case tomorrow and Thursday. This is why the forecast for the next few days is calling for the chance of showers. Hit or miss showers. Self defeating sunshine.
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