Saturday, December 19, 2015

In a fog!

Spending my teen age years in the shadow of the Sierras, I thought of the mountains as my back yard... In wintertime, when the snow was too heavy for hiking in the high mountains, there were accessible locations in the lower mountains.  One winter day I had been exploring with my friend Charlie and late in the afternoon we were hiking along a trail high above a deep valley.  We were aware that the temperature was falling - and in the span of a few minutes the valley below up to our level filled with a thick fog... 

I do not have a photo from that day - but this give you the idea
It was amazing to look over this solid looking firmament of clouds, with billows and ripples and see the sun shining bright and clear.  Our concern was to get back to the car before we were caught up in this cloud - and we did - and once away from that valley the fog lifted.  Now I know that the sudden development of the fog was due to normal cooling of the deep valley air...As air cools it is no longer able to hold as much water.  The point where the air is saturated in called its dew point, and if air cools below that point it must form tiny cloud droplets of 'cloud'.  When this happens close to the ground we call it fog.  What we saw that day was a sudden winter time drop of air temperature in that valley and its moisture had no where else to go but into fog.

Flat bottom - think rising air - air forced to rise past its dew point -
Look at a heated swimming pool or a bathtub with hot water in a cold room and the cool air above the heated water can not hold as much moisture as the air above the warmer surface.  In the San Joaquin valley, the same happens with Tule fog - Soil is a little warmer than the air above- just like the heated pool - and moisture from the soil forms fog in the air... With Tule fog the soil isn’t much warmer than the air, but the difference is enough to form fog.

Oh how I hate Tule fog - sometimes it lasts for many days -
Water warmer than the air ( even a few degrees)-  moisture laden air rises into the cool air and fog forms - think steamy bath tub
The sun normally heats the ground, which heats the air in contact with it... Warm air rises... as it does so it cools off...It cools until it reaches its dew point... and as it rises further - a cloud forms in the air...  But suppose it continues to rise and cool - eventually it reaches the freezing point and ice crystals form from the cloud particles.  If there is a system of vertical winds within the cloud, the ice crystals and the liquid droplets stick together and grow bigger and bigger until they are so heavy they fall out of the cloud - Viola! Raindrops.  All of these steps are needed for rain to form... Next time you see it raining - look up and imagine this vertical zone of mixing - and imagine each rain drop falling thousands of feet before hitting on your head!

Mountain "thunder head" - big enough to have a region of freezing and vertical winds to produce rain drops

This raises another issue and that is freezing point on the earth's surface... Easy to imagine how on a cold night, heat is being lost into space, and the temperature drops... if the temperature drops below 32 F (0 C) ... the moisture in the air does something surprising it changes directly into ice crystals without first going through a liquid stage. It is the formation of frost!  Here is the mystery - often this time of year when my thermometer reads several degrees above freezing there is still thick frost formed on roof tops... How can that be?
Aircraft wing would give off heat quickly  - and would allow moisture to form frost...
  The fact is that frost can not form unless that surface is below freezing. My thermometer is a several feet above the level of the soil - and air currents bring in slightly warmer air... Some materials radiate heat more effectively than others and the roof has lost enough heat that the air sitting above it is below freezing. 

At times is there is no vertical mixing the moisture in an air mass can be converted into an ice crystal cloud - We most often see them in the summer time - formed high above our head...They have the thin wispy nature very different in appearance from our more common cumulus clouds.  

Summer cirrus clouds made off high altitude ice crystals