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
|
