Published Tuesday, May 6, 2003 in the San Jose Mercury News

Weather Corner

How cool was April? Very, and wet

Special to the Mercury News

There had better be a lot of May flowers after the overabundance of April showers, that
along with much cooler than normal temperatures have frankly made a lot of people pretty

A succession of cold troughs of low pressure from the Gulf of Alaska made April 2003 the
coolest since 1975 over most of northern and central California, while it was the wettest
April since 1983.

In San Jose there was 3.05 inches of rain for April, the wettest since 1983 when 3.90
inches was recorded. The unsettled weather pattern also made it the coldest April since 1975
with an average temperature of 55.9 degrees; 4.3 degrees below normal. San Francisco picked up
3.60 inches of rain for their 14th wettest April since records began in 1850, and it as the
wettest since 1978 when 3.78 inches fell. The record for April in San Francisco is 10.06
inches in 1880.

The average temperature of 54.1 degrees was the coolest April in San Francisco since 1975
when it was 51.9 degrees.

Statewide, the showery weather pushed seasonal rainfall and snow totals to near or above
seasonal normals in most areas and added smiles to water managers' faces. The 8-Station
Precipitation Index that monitors the rainfall in the northern Sierra Nevada and Shasta
regions that feed into the Sacramento River system is at 109 percent of the seasonal total and
Lake Shasta is full and having to release water so that flooding does not occur.

Likewise, the Sierra Nevada snowpack is at 120% of normal which is up from just 70 percent
of normal on April 1st. Both are key to California's water health as the Sacramento Basin
provides almost 50 percent of the state's useable water supply.

Locally the near normal rainfall has not completely filled the reservoirs but the region's
groundwater supplies are in excellent shape and groundwater is the source of nearly half of
the water supply from the Santa Clara County Water District.

What makes the season especially interesting is how it makes a counterpoint to the
mythology that just because it was an El Nino winter that it should have rained a lot. Instead
El Nino is just one piece in the intricate, and intriguing, global climate puzzle, and this
year it turned out not to be that important a factor for California.

Q.  What are moonbows & sun dogs? A book I had for children asks if you have ever seen a
double rainbow. Which we have but it also asks if you have ever seen a moonbow or a sun dog. I
have to assume that they are weather related. Janice McCourt - Parsons, KS

A.  A moonbow is another name for a lunar rainbow and results when the light source is
the moon instead of the sun. Because the moon's light is much weaker than the sun's, a moonbow
is very difficult to see. Sundogs, technically called parhelion, are another optical phenomena
and result from sunlight passing through slowly falling hexagonal plate-like ice crystals.
They are seen as bright colored spots at the same height and appear at 22 degrees on either
side of the sun.

Q.  Why is it that particular localized areas of Northern California historically receive much more rainfall than others? (i.e. Santa Cruz mountains, Cazadero, Kentfield) Christy Somers - Santa Rosa

A.  The biggest factor for the distribution of rainfall in California is the topography,
especially the mountain ranges that extend north and south in the state. Because these
mountains are generally perpendicular to the upper level winds that bring our storms from the
Pacific, they are very efficient at producing lift for the storm clouds. This rising air
causes more precipitation to occur in the mountains while on the lee side of the mountains
there is considerably less rain. There is also a decrease in the rain the farther from the
ocean that a person travels.

If you were to look at the rises and falls in the topography along a line through Santa
Cruz, Ben Lomond in the Santa Cruz Mountains, San Jose, Mount Hamilton, across the Central
Valley, through Yosemite and into Nevada it would very closely resemble a graph of the
rainfall at points along the way. To illustrate this, Santa Cruz gets 31 inches of rain a
year, Ben Lomond 48 inches, San Jose 15 inches, Mount Hamilton 23 inches and Los Banos 8

Q.  What was the climate like during the Jurassic period? Robina Walton - Tiburon

A.  The Jurassic period, between about 150 and 200 million years ago, was a warm and
humid period. Temperatures were on the order of 20 degrees warmer than today and the polar
regions were ice-free which meant that the oceans were considerably higher than they are
today. The warmer temperature also meant that the range of mild weather plants and animals was
much greater than it is today.

Q.  Are there monsoons in Arizona and are they similar to the monsoons in India? Sharon Yuki - Redwood City

A.  The term monsoon comes from the Arabic mausim, ""a season,'' and is a name for
seasonal winds. This is contrary to the common usage which implies it is a rain event. This
comes the fact that these seasonal winds often do bring rain to different parts of the world.
This is the case in both India and Arizona, but the magnitude of the monsoon-induced rain in
India and other part of southern Asia is significantly greater than in the Southwest United

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to him c/o Weather Corner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone questions at (510) 657-2246, fax them to (510) 315-3015 or e-mail them to or fill out a form online at