drought yet despite records for low rainfall
Special to the Mercury
It didn't rain much in the past year, but are parts of California
really in a drought?
The 2001-2002 rainfall season just ended, and most of Southern
California had less than a third of normal rainfall. A number of
cities went through their driest season on record.
Between July 1, 2001, and June 30 this year, Los Angeles had 4.42
inches of rain -- only 29 percent of normal and the driest season
since records began in 1877. San Diego got just 3.02 inches or 34
percent of normal, the driest since the city's records began in
1850. Long Beach was even drier with 1.89 inches, a meager 15
percent of normal. Palm Springs was bone-dry with 0.41 inches for
the entire season. That's just 8 percent of normal.
Even with such minimal rainfall, there is no widespread drought
in Southern California. How come? This is the perfect case to show
that it takes more than lack of rainfall to make a drought.
The American Meteorological Society's ``Glossary of Meteorology''
defines drought as a ``period of abnormally dry weather sufficiently
long enough to cause a serious hydrological imbalance.''
Consequently, if there is little impact on the water supply to the
general populace, industry or agriculture, low rainfall doesn't
necessarily equate to drought.
This is certainly the case in Southern California, because much
of the water used in the region's generally arid climate is
imported; it does not fall from the sky. Typically 60 to 80 percent
of Southern California's water is imported from the Colorado River
and from Northern California. The rest comes from groundwater below
much of the coastal plain. Some mountain areas of Southern
California are ``fractured rock,'' which doesn't hold groundwater,
so all water for those regions has to be imported.
However, while most of the region isn't affected by what is
defined as a drought, there are some people who are severely
affected by the lack of water. Among them are grassland ranchers who
rely entirely on rainfall, not irrigation, and firefighters who are
facing tinder-dry wildlands just waiting for a spark.
Q When a long-range
forecast predicts above-average precipitation, does it mean an
above-average number of storms with varying amounts of
precipitation, or above-average amounts of total rainfall?
Dan Carlentine -
A Would you believe,
none of the above. The long-range 30- and 90-day forecasts from the
National Weather Service show the probability of above- or
below-average temperatures or rain. They begin with the assumption
that there are equal chances -- 33 percent -- of normal,
above-normal and below-normal conditions.
For example, if the 90-day temperature forecast map shows an
``A'' surrounded by a contour line marked ``20,'' the chance of
above-normal temperatures is 20 percent greater than 33 percent, or
a 53 percent chance. This methodology does not address the amount
above normal, or in the case of rain, the number of rainy days. For
30- and 90-day forecasts, see
Q The recent forecast
was for high pressure with hot, dry weather. Can you explain the
association between high pressure and hot, dry weather? Hot, dry air
is lighter, so it seems this weather should accompany low pressure
readings. Curtis Panasuk -San
A There seems to be a
great deal of misunderstanding about the relationship between high
or low pressure and the resulting weather. The biggest source of
this confusion is television weathercasters who don't tell you
whether they are referring to high pressure near the surface of
Earth or high pressure in the upper atmosphere.
The forecast you are referring to was made when there was a large
area of high pressure in the upper atmosphere covering much of the
western United States. High pressure aloft is associated with
sinking air that warms by compression as it gets closer to the
ground. This sinking air also results in mostly clear skies.
Conversely, if there is low pressure in the upper atmosphere,
there is generally rising air that can lead to clouds and sometimes
High pressure at the surface is usually associated with cooler
air, while a nearby area with warm rising air results in lower
barometric pressure. This causes our usual sea breeze. The waters of
the Pacific cool the air mass directly above, and the resulting
heavier air means higher pressures. As the land warms up during our
long summer days, the air directly above warms and rises, creating
lower pressure. Since nature abhors anything resembling a vacuum,
the cool air from the high pressure offshore rushes inland toward
the lower pressures, and we have a sea breeze.
Q What causes the gaps
in ``stacked lenticular'' clouds? I understand how lenticular clouds
form at the top of a standing wave. But what causes the clear zones
between them? Why not just one big lenticular cloud? I guess this
may be a bit esoteric for the Weather Corner, but I've wondered for
years and never found anyone who knows.
Art Botterell -
A Nothing is too
esoteric for the Weather Corner. First, the basics: Lenticular
clouds form over or in the lee of a mountain as the wind lifts moist
air high enough for condensation to occur. The cloud that forms is
shaped like a lens with a fairly flat bottom and a smooth rounded
top that follows the contour of the wind as it flows over the
mountain in the form of a wave.
There is sometimes stratified moisture in the air that can cause
several lenticular clouds to form, one on top of another in a stack.
There is very little if any gap between the layers, and the result
is what some describe as a pagoda. Check out
Q How many gallons of
water falls from 0.01 inches of rain in a 1-square-kilometer area?
Ted Schlaepfer -
A There are about 1.55
billion square inches in a square kilometer. A layer of water
one-hundredth of an inch deep over that area would equal
approximately 15.5 million cubic inches. Because one cubic inch is
about 0.004 gallons, the total for one-hundredth of an inch of water
over a square kilometer would be just slightly more than 67,000