WEATHER CORNER

BY JAN NULL
Special to the Mercury News

Average high this July was 1.8 degrees below normal

THOUGHT July was cooler than normal? Or hotter? I've had people tell me both within the past week. Actually, it was a pretty normal July if we look at the average daily high temperature, and it's a great example of how a seemingly atypical month ends up being just about average statistically.

The average daily high temperature for July over the past 50 years is 82 degrees. This year, July ended up with an average high of 80.2 -- with 10 days being above normal and 21 below. Bottom line: ``normal'' is just the average of the extremes.

Q.  I have read in a couple of garden magazines that there is a new plant heat-zone map. Being an avid gardener, I generally pay attention to the USDA plant-hardiness map, to ensure that all my new selections can withstand the low cold temperatures in my zone. But apparently, this new heat tolerance map shows another factor that gardeners must be aware of -- how a plant can withstand high temperatures -- almost as important. Can you tell me how they calculated the heat zones and which zone the Fremont-San Jose area will be a part of? Kathy Nunes - Fremont

A.  The new plant heat-zone map was developed by the American Horticultural Society as a measure of heat stress on plants. This complements the USDA plant-hardiness maps that determine winter hardiness based on average annual minimum temperatures. Heat zones are
figured from the average number of days per year above 86 degrees. This temperature was chosen because it's the point above which cellular proteins in plants are damaged.

Because of wide variation in temperatures in the Fremont-San Jose region, areas near San Francisco Bay fall into plant heat Zone 4 with 14 to 30 days above, while warmer inland areas are in Zone 5 with 31 to 45 days above 86 degrees.

The map is available on the society's Web site -- but only for members. More information is available at www.ahs.org on the Web.

Q. How is the ultraviolet (UV) index determined? How sensitive is the UV index to the accuracy of the weather forecast? What about local influences, such as altitude, reflections from water or snow and things like that? Ed Taft  - Mountain View

A.  The UV index is a next-day forecast of the amount of UV radiation expected to reach Earth's surface at the time when the sun is highest in the sky (solar noon). The amount of UV radiation reaching the surface is computed using only the forecast ozone data, forecast cloud amounts and
the elevation of the location.

The UV index can range from 0 (at night) to 15 or 16 (in the tropics at high elevations under clear skies). The higher the UV index, the smaller the time it takes before skin damage occurs.

Q.  I will be traveling to Thailand for two weeks and will very likely land in Bangkok on Nov. 23, a time of celebration. Will there be a full moon? Don Streepey - San Jose 

A.  Yes, the full moon is Nov. 23, the same day it's full here. The moon appears full when it is on the opposite side of Earth from the sun. This happens every 29 days all over Earth.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to Weather Corner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone and fax them at (510) 657-2246 or e-mail them weathercorner@ggweather.com.