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Treebeard's Stumper Answer
14 May 1999

Pumping Up for a Ride

Here's a simple experiment to try while tuning up your bike for the Friday afternoon Refugio Ridge Ride over the mountains and to the beach. Pump up your bike tire with a hand pump and feel the bottom of the pump. It will be hot. Now wait a moment and release the pump (and a puff of air) from the tire and quickly feel the valve stem on the tire. It should feel cool. Can you explain these different results? Isn't it the same air in each case? Hint: It's not friction that makes it so hard to pump up a tire with a hand pump!


A bicycle pump gets hot because you are compressing the air within. The tire valve feels cool because the escaping air is expanding. Friction is a minor factor. These are adiabatic processes, temperature shifts due to pressure changes rather than any transfer of heat from outside. There is an ancient fire-making tool called a fire piston. We see the same heating effect (and potential for fire) when summer winds are compressed while moving down the canyons causing hot and dangerous Santa Ana and Sundowner winds.

Notes:

There are many examples of adiabatic heating and cooling. Diesel engines and glo-plug model airplane engines don't need a spark plug. They use the heat generated as the returning piston compresses the fuel and air in the cylinder to ignite the mixture. Dip your finger in a fast-evaporating solvent like alcohol or acetone and you'll feel how cold it feels when the solvent expands into a vapor. Refrigerators and air conditioners use the same principle, compressing a coolant like freon or ammonia (which generates heat) and letting it evaporate again to cool things down.

Adiabatic processes also make weather. This is much on my mind here in the mountains behind Santa Barbara at the start of a long hot summer following a dry La Niña winter. Our mountains are a natural fire piston, and the results can be devastating. I'm especially nervous after our unusual 10 inch snow storm last March brought down so many trees. The woods are full of kindling.

Both sides of the stumper can be seen in our local weather patterns. Everything warms at sunrise, and warm air rises. This pulls in cool marine air - usually visible as a fog layer - with an on-shore flow. In time, the air warms up above the dew point and the fog "burns off" becoming invisible water vapor rather than visible droplets. As the warm air continues to rise, it enters an area of lower pressure and undergoes adiabatic cooling, usually becoming visible as billowing cumulous clouds and thunderheads, though it rarely rains here in summer. At night everything cools down and the fog returns.

Santa Ana winds are something else. They develop when there is high pressure over the interior deserts and low pressure on the coast. This sets up an off-shore airflow from the desert to the coast. But the mountains are in the way. The already warm desert air is funneled down through the mountain passes and heated by compression, gaining as much as 5.5 degrees per 1000 feet of elevation change. These are the famous devil winds that effect Los Angeles. The Doors sang about them, and Raymond Chandler and Ross MacDonald wrote classic mysteries with the winds as a major player. They effect us.

Santa Barbara is different. We are isloated from the interior deserts, but we have our own local Santa Ana-like winds formed by our unique local geography. The Santa Ynez Valley runs east-west for 70 miles, but it is only about 10 miles from the ocean, separated by the steep 4,000 foot high Santa Ynez Mountains. (See map.) The Santa Ynez Valley is much warmer than the coast in the summer. When conditions are right, the hot valley air rises to the level where it can spill over the mountain passes towards the coast. It then warms by adiabatic compression. This typically happens late in the day, hence the local name sundowner winds. This is strictly a local effect

I live on top of the mountains, where it's usually much warmer than the coast. But during a sundowner, the situation is reversed. The compressing air can heat to 100 degrees and more as it descends the mountain wall. For many years, Goleta (next to Santa Barbara on the coast) held the U.S. temperature record of 133 degrees from a sundowner heat burst event on June 17, 1859. Ray Ford describes this incident on his Santa Barbara Outdoors site.

I found this event hard to believe when I first read about it in a book by local historian Walker Tompkins. The wind was called a simoon, an Arabic word, and the whole story seemed impossible. Then on June 27, 1990, it happened again. Sundowner winds brought the temperature in Goleta to 108 degrees and the arson-caused Painted Cave Fire took off and threatened the city of Santa Barbara itself. We were at the beach and couldn't get home. We spent the night with friends in town watching the fire on the hills and listening to exploding propane tanks. We did get home the next day, and stayed there. The winds returned to normal and turned the fire back into the mountains towards us. The weather and the Hot Shot fire crews and the bombers managed to keep the fire away from most mountain homes, including ours.

The fire destroyed 500 homes, and killed local resident Andrea Gurka. Ray Ford tells the story of this fire. A simple bike pump shows the principle, and summer is coming...

Here are some Web links for further research:

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Copyright © 1999 by Marc Kummel / mkummel@rain.org