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Treebeard's Stumper Answer
1 February 2002

The Big Chill

It was cold enough for heavy snow this week in central California, but there wasn't enough precipitation for more than a dusting in the mountains. On Monday morning, highway workers had to dump sand and salt on Cold Springs Bridge to melt the ice. Why was the bridge icy when the rest of the highway was clear? The harder stumper is so easy to ask. Heat rises, cold sinks, and the mountains are closer to the sun, so why does snow usually fall in the mountains instead of the valleys? You might also remember that Death Valley is both the lowest and hottest place in the US!

Cold Springs Bridge on San Marcos Pass is a beautiful single
span arch bridge that won engineering awards when it was
built in 1962. Why are icy bridges a special road hazard?
Snow in the backcountry, but the valley is clear. It's a familiar
sight, even here in central California. Heat rises and cold settles,
so why is there usually more snow in the high mountains?
 
Another curious stumper is that on some clear mornings, I can see a frost line in the foothills, with noticeable frost only below the line. I've never managed to capture a photo of this. The Alaska Science Forum also mentions this. See last week's Frosty Leaves stumper.


Bridges, overpasses, and birdbaths freeze before roads because they are exposed on all sides, concrete and steel are better heat conductors, and there is nothing underneath to hold the day's heat. Warm air does rise, but it expands into lower pressure and undergoes adiabatic cooling, just like a spray can feels cold when you release pressure. Dry air cools 5.4 °F for every 1,000 feet it rises. That's more than 20 °F of cooling for a 4,000 foot mountain, enough to bring snow. Cold air warms as it sinks for the same reason, which can create perfect winter days and dangerous summer fire weather with Sundowner winds.

Notes:

I admit, we're mostly novices with snow and ice here in Central California. Now that I think of it, of course bridges, overpasses, and birdbaths freeze before roads. They're more exposed. I'll be careful driving on frosty mornings like we had last week!

Adiabatic processes are temperature shifts due to pressure changes (and gas laws) rather than any heat transfer from outside. A bicycle pump is a familiar example. The pump feels warm when you use it because you are compressing the air within. The tire valve feels cool when you release the pump because the escaping air is expanding. Friction is a minor factor. I discuss this in my Pumping Up for a Ride (14 May 1999) stumper.

There are many examples of adiabatic heating and cooling. Diesel engines and glo-plug model airplane engines don't need a spark plug because they use the heat generated by the returning piston to compress and ignite the fuel and air mix in the cylinder. Dip your finger in a 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 this principle in different directions, compressing a coolant like freon or ammonia which generates heat and letting it evaporate again to cool things down. Heat pumps work in both directions to heat and cool with the same mechanism. I understand all this, but I'm still impressed with the Servel propane-burning refrigerator in our old cabin that kept my drinks cold with a flame. That's another stumper!

Winter storms are not just containers of rain that move across the country. No cloud could hold that much moisture! Storms are moving areas of low pressure that cause rising air to cool and condense moisture along the front, creating new rain. Winter storms are continually re-creating themselves as they move across the country. It is the vertical movement of local air that makes weather happen.

The atmosphere is held to Earth by gravity that diminishes with (the square of) distance, so successive layers of the atmosphere take up more volume and have less pressure from above. It may seem that the Earth is so large that slight differences in elevation wouldn't make much difference. But think of floating in a pool or hot tub. You rise and fall with every breath. Small differences in density and pressure do make a difference. The atmosphere is not static like the layers of an onion. It is a dynamic equilibrium of warm air rising (and cooling) and cold air sinking (and warming). The result is weather!

Last week in Santa Barbara, we had arctic temperatures in the teens at night. This week we had high pressure and sinking air that gave us perfect warm days. But that clear dry air allowed radiatice cooling at night. The result was large temperature swings from the 20s at night to high 70s in daytime. These 50 °F temperature swings are remarkable. I like it!

We had regular rains before Christmas, but it's been mostly clear and dry since then during what should be our rainiest months. I'm worried about our fire weather next summer. Just as snow falls in the mountains because of adiabatic cooling, sometimes the hottest days of the year come in the summer because of adiabatic heating.

Sundowner and Santa Ana winds 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 and dry desert air is funneled down through the mountain passes and heated by compression, gaining 5.4 degrees per 1000 feet of descent. 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. I'm not looking forward to this fire weather.

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Copyright © 2002 by Marc Kummel / mkummel@rain.org