Published Tuesday, Apr 20, 2004 in the San Jose Mercury News

Weather Corner

April Showers (and March Warmth) Bring Allergy Weather

Special to the Mercury News

Coupled with a record warm March, April's showers will bring May flowers, but the May flowers will bring on lots of allergies around the Bay Area.

The combination of early warmth and now some moisture means a run on tissues and antihistamines at local pharmacies. Just look at that fine coating of yellow pollen on your car.

Trees typically pollinate first, from late in February through May. Then come the allergens from grasses, which are most prevalent in May, June and July. Finally in late summer and fall, people experience allergies associated with weeds. Molds peak in late spring and then again in late fall.

Each type of plant or spore has a fairly consistent pollination period from year to year, but the weather can exacerbate the effects, increasing or decreasing the number of allergens and how much is in the air at any given time. The three weather elements most responsible for hay fever are the temperature, wind and rain.

The air temperature and even the soil temperature affect the germination and rate of growth of plants. Consequently, a warmer-than-normal March and first week of April got this process off to a fast start.

One of the primary methods of pollination is via the wind, which can carry pollen, mold spores and dust many miles. In recent weeks as the inland areas of California have warmed, the afternoon sea breeze has kicked in significantly. That and the recent unsettled weather pattern has kept conditions right for flying kites and spreading pollen.

The relationship between rain and allergies is a bit more complicated. On one hand, precipitation aids in the springtime growth of flora, and this increases the number of potential allergens. However, on days when it is actually raining, airborne particles are scrubbed from the air for some short-term relief. With the rainfall season rapidly coming to a close, the abundance of airborne allergens will rise again. The worst days for allergy sufferers are when the weather is warm, dry and windy.

In severe cases, it is necessary for hay fever sufferers to move to areas where the mix of flora and weather minimizes the amount of allergic pollution. Locally, that can be near the coast where the westerly winds from the Pacific keep the air relatively ``clean'' of allergens. Others may relocate to the desert where there is less vegetation -- but sometimes find that the new habitat has allergens to which they have not built up a tolerance.

Q I was disappointed that your April 6 article did not explain why a certain phenomenon occurs some days during baseball games, causing sportscasters to make statements about the weight of the air. I have pondered this and could only explain this by suggesting that maybe the viscosity of the air changes. Norman J. Martin - Saratoga

A In all fluids, liquid and gaseous, viscosity is a factor as it indicates how much a substance resists flowing. Another way to describe viscosity is in terms of a substance's ``stickiness.'' Fluids like honey and thick motor oil have high viscosities, while water and gas are low-viscosity fluids. Even though gases, including the air we breathe, behave like a liquid in many ways, air's viscosity is about 100 times less. Consequently, even though the air's viscosity increases with higher humidities, the effects on the movement of a baseball are negligible.

Q I recently saw a newspaper account stating that crop circles in Mexico were thought to be done either by an extraterrestrial landing or by wind currents. Is it possible that a circular wind path could cause crop circles to appear? Anne Foster - Sebastopol

A I'm glad you didn't ask me about the extraterrestrials. While there are some circumstances where wind patterns on the ground are somewhat circular, they do not come close to the symmetry in crop circles.

The downdrafts from a thunderstorm sometimes strike the Earth from above and then spread out in a somewhat circular pattern. The same is true with the path of a tornado. While the damage and debris patterns show the circular nature of these storms, they are too irregular to be mistaken for a crop circle.

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