Clouds from Both Sides

Monday, September 10th, 2012

Whenever possible, I try to book a window seat on plane flights and look at clouds. If I remember to take my camera out of the carry-on bag, I like to shoot pictures of the pretty or interesting clouds and share them with other cloud afficianadoes.

Boundary Layer Clouds

You can see the atmospheric boundary layer from a plane by looking for the flatish cloud tops.

Here’s a picture I took on a return flight from Colorado last week just as the sun was going down. When the tops of clouds form a fairly flat layer like this, it can indicate a demarcation in the atmosphere where conditions change from a turbulent air mass below the cloud tops where most of what we experience as weather oocurs to a more stable layer of the atmosphere. (The exception being really strong thunder-head clouds that punch through the others… when you see those anvil-shaped clouds from a plane, the pilot is usually trying to skirt around the often powerful storms below).

That transition point where clouds flatten out signifies the top of what’s technically known as the atmospheric boundary layer or planetary boundary layer. Generally about one or two kilometers thick, the boundary layer is affected by daytime heating and nighttime cooling, surface winds, fog and most clouds… in other words, weather.

Surprisingly the temperature above the boundary layer is generally warmer than the layer below. The way meteorologists traditionally measure the height of the boundary layer is by sending up weather balloons that continously measures temperature as they rise. When the temperature of air take a clear turn from gradual cooling towards warmth that signifies the top of the boundary layer. The National Weather Service for the Bay Area launches a weather balloon from the Oakland Airport twice a day to measure the height of the boundary layers and collect other atmospheric data for their forecasts.

As the Exploratorium prepares to move to the piers, we are making plans to install instruments and sensors that will monitor weather conditions, including an instrument to  measure the height of the boundary layer without having to launch balloons (although we’d love to also launch weather balloons!). Called a radiometer, it detects the temperature inversion through microwave radiation measurments. It’s one of the instruments that will make up our “Wired Pier,”  a set of sensors that will collect data about the Bay waters and atmosphere.

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