Chilly weather could make April coldest since 1975

``March comes in like a lion and goes out like a lamb'' -- or is it the other way around? To look at the temperatures for March and April, one would think the calendar pages had switched.

Last month in San Jose, the average maximum temperature was 69 degrees, 3.7 degrees above the normal temperature of 65.3 degrees. There were also 14 days when the high reached 70 degrees or greater, and on four days it was at least 80 degrees.

So far in April, the average maximum temperature in San Jose is 64 degrees, or 6 degrees below normal, and only 4 days have reached 70 degrees. If San Jose has only normal temperatures for the rest of the month, this will go into the record books as the chilliest April in San Jose since 1975.

The cool weather has been the result of a series of unseasonable low-pressure systems that have dipped south from the Gulf of Alaska and then persisted over the Golden State. Typically, there might be a couple of these systems, bringing a day or two of mild weather until they were replaced by a high-pressure system and warmer weather. So far this April, however, there have been at least a half-dozen low-pressure systems that have swept into the state and lingered.

Forecast charts do indicate that temporary relief is in sight, with high-pressure and somewhat warmer conditions beginning this week. But this may break down, bringing cooler weather next weekend and the beginning of next week.

Q  I don't know if you've been able to notice this phenomenon down there where the air's not so clear, but for the past two days, April 11 and 12, here in Sea Ranch (on the coast about 85 miles north of San Francisco), there has been a significant dimming of the sun by a fairly high layer of what I assume is dust. It gives a fairly uniform, grayish cast to the sky, and though shadows are present they are not as distinct as usual. Do you have any leads on this?  Ken Holmes - Sea Ranch

A  It was indeed dust that you were seeing, the result of a major Asian dust storm. On April 6-8, there were strong winds across the Gobi Desert in northern China and Mongolia that lifted dust into the upper atmosphere. This dust cloud was dense enough that it was visible on weather satellite images, as well as from the ground, when it reached North America nearly a week later, and even as it moved out over the North Atlantic on April 14.

According to the Navy Research Laboratory in Monterey, this is the largest such event since April 1998. The dust is transported by winds in the lower atmosphere at an altitude between 2,000 and 23,000 feet. NRL monitors the origin of this type of storm, and their five-day forecast of the dust cloud reaching the West Coast was accurate within about 12 hours.

I have put together a small Web page about this event with some links to additional resources at http://ggweather.com/dust.htm.

Q  Has the overnight temperature anywhere ever stayed above 100 degrees? What's the record? Thanks.  Jim Richey - Newark

A  This is not a commonly used statistic, and it took some digging to get even a partial answer. To answer your first question, there have been numerous hot desert locations around the world where the highest minimum temperature has remained above 100 degrees.

I could not locate an absolute world record, but in exploring Death Valley's climate data I think I came close, as well as finding out some other interesting information about this unique location. On five separate summertime occasions since 1911, the lowest temperature of the day in Death Valley was 110 degrees. (But it's a dry heat!)

The combination of Death Valley's elevation, about 200 feet below sea level, and its location in the lee of the Sierra Nevada make it the hottest and driest spot in North America. A reading of 134 degrees on July 10, 1913, was the world record until Azizia, Libya, took the honor with 136 degrees in 1922.

In the summer of 1974, there were 134 consecutive days in Death Valley when the maximum reading was more than 100 degrees. The average high temperature for the months of July through September ranges from 106 to 115 degrees. The hottest month is July, with an average high of 115 degrees, an average low of 88 degrees, and the resulting average for the month at 102 degrees.

Like many other places, the El Niņo season of 1997-98 brought record rainfall to Death Valley, with a seasonal total of 6.09 inches, about 3 times the annual average. In 1929 and 1953, there was zero rainfall at Death Valley, and for a 40-month period in 1931-1934, there was only 0.64 inches.