Published Tuesday, January 19, 1999, in the San Jose Mercury News


Special to the Mercury News

Q. need some clarification on the definition of a ``hard freeze'' and how it differs from a ``frost.'' 
Mark Merritt - Santa Cruz

Q.  Why can there be frost on lawns, rooftops and cars when both the city's official temperature and my outdoor thermometer read 36 degrees or ever higher? In Fremont two weeks ago, we had frost every morning, even when the official temperature was 38. On some mornings I have even observed a lone shingle on a rooftop with frost, or just a single car on the block.
Dave Herstein - Fremont

A.  We all know that water freezes and becomes ice at 32 degrees Fahrenheit. When the temperature dips to about 28 degrees it's termed a ``hard freeze.'' This is the air temperature when unprotected seasonal vegetation is damaged and the ground surface is frozen.

Frost occurs when water vapor in the air freezes onto a surface. This can happen even when the air temperature is above 32 degrees because many objects cool faster than the air. When their temperature dips below freezing, moisture from the air will accumulate as ice crystals through a process called ``deposition.'' This can occur when the air temperature is as high as 36 or 38 degrees.

We also get ``frozen dew'' in the Bay Area. This happens when water condenses onto an object that is about 32 degrees and then the object cools to below freezing. This leaves a pebble-like surface that's really tough on your windshield wipers.

Q.  My favorite Web weather source publishes a term that I'm not familiar with -- heating degree-days. Could you please define?
Harry Welch - San Jose

Q.  Do you know where I can find degree-day information for San Jose? I am looking for the info for the last year or so.
Frank Thomas - San Jose

A. Heating engineers developed the concept of heating degree-days as a useful index of energy requirements. They found when the daily mean temperature is lower than 65 degrees, most buildings require heat to maintain an inside temperature of 70 degrees. The daily mean temperature is obtained by adding together the maximum and minimum temperatures reported for the day and dividing by two. Each degree of mean temperature below 65 is counted as one
heating degree-day. Thus, if the maximum temperature is 70 degrees and minimum 52 degrees, the average would be 61 degrees and four heating degree-days would be produced. If the daily mean temperature is 65
degrees or higher, the heating degree-day total is zero.

Conversely the cooling degree-day statistic is the summer counterpart. When the mean is greater than 65 degrees the CDDs accumulate. Degree-days, usually with a different base from the 65 degrees for heating and cooling, are also used by growers to estimate plant growth.  Average heating and cooling degree-day information for San Jose (and
about 300 other California sites) can be found at: and

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 telephone and fax questions at (510) 657-2246 or e-mail them to (weathercorner@