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FLOODS
Floods are not only caused by excessive amounts of rain but by other, sometimes more severe phenomena. Floods can be caused by the blockage of water by a dam or the collapse of an existing dam. Beavers are one of natures finest dam builders and have been known to alter established ecosystems by flooding areas up-stream of their dams and drying areas down-stream. A sudden removal of this dam can cause water to rush down the river much as it would if a large man-made dam would collapse.
Another more common type of dam is formed in spring when ice breaks-up and flows downstream until it gets bloked. This will further impede the continuing flow of even more ice blocks. Coupled with water from the winter's melting snow, the flood potential is exacerbated.
When rivers flood, the time of peak flooding, or in otherwords,
when the river crests, can be calculated by examining the surrounding
terrain including the flood plain. The larger the river, the longer
the time between the peak rains and the peak flooding.
FOG
One of the strangest of all weather phenomena occured in the December of 1952, when over 4,000 deaths were attributed to one of the worst cases of fog and smoke to beseige London. The smog from the persistant fog and the smoke from coal-burning stoves asphixiated thousands while hundreds more died as a result of freak accident induced by the visibilty, which at times was less than a couple of feet. Although, killer fog like that of the Great London Fog are rare indeed, fog is an extremely common occurance in almost every part of the globe.
Fog is nothing more than clouds at ground level. As long as the
air at the surface is cool enough to reach saturation, fog will
form. This layer of fog can be anywhere from a few feet to several
thousand feet thick.
- Radiation Fog - Radiation fog will form whan the ground
cools radiationally so that the air coming in contact with the
surface will cool to its dew point temperature. Usually around
100 m thick.
- Advection Fog - Formed when warm air is blown over a
cold surface so that the is cooled to its dew point. Fog off the
coast of California is partially cooled when the air travels across
the cool ocean, in addition to picking up more moisture. As the
air travels inland it is forced to rise up the mountains and adiabatically
cools even more.
- Frontal Fog - As a warm front appears, continuous rain
over a long period of time may saturate the air thus creating
frontal fog.
- Upslope Fog - This type of fog will form when the air
near the ground is cool enough so that it will not rise and moist
enough that when it moves upslope, can cool to its dew point.
This can take place over several hundred kilometers. For instance,
in Kansas, if an east wind is blowing, air starting out at 300m
Above Sea Level (ASL) on the eastern border will rise and cool adiabatically as
it reaches the western border of the state which is 1200m ASL.
- Steam Fog - (also known as Arctic Sea Smoke) This fog, seen commonly in the winter when a person breathes, is usually only a few meters thick. It forms over water or a wet surface when it is warmer than the surrounding air (for instance, over hot pavement in the summer). As the air is warmed and moistened, it then remixes until it reaches saturation above the surface.
A box of air with T=10°C; RH=90%; w=7.0
A second box with T=-20°C; RH=50%; w=.392
Mixing the two boxes gives us a box with an average temperature of -5°C and a mixing ratio of 3.696. @-5°C, ws=2.64.
Therefore, RH=3.696 / 2.64 = 140% ! Even though neither parcel of air was saturated, when they were mixed, fog formed.
(The temperature of the box is actually a little warmer bacause as they were mixed and water condensed, latent heat was released.)