Published Tuesday, June 18,  2002 in the San Jose Mercury News


Forecast for Wet Weather Can Be Confusing

by Jan Null
Special to the Mercury News

There appears to be about a 24 percent chance that some people are confused when a forecast says there is a 30 percent chance of rain, according to a recent reader survey I conducted. Many more are baffled by a forecast for a 30 percent chance of showers -- including a National Weather Service forecaster who sent in a response to the Weather Corner User Survey.

The idea to use probabilities for whether it was going to rain began with the National Weather Service in 1965. The original concept was to provide a risk-benefit assessment for people to whom the occurrence of rain was critical. For example, a contractor might decide to pour concrete if the chance of rain is only 30 percent, but he might decide not to pour if it's 60 percent.

In response to survey questions about a forecast for a 30 percent chance of measurable rain, 76 percent of the survey's respondents knew that rain was not expected -- but about a quarter did not. Seventy-two percent correctly stated that there was a three in 10 chance of measurable rain occurring.

Based on the NWS Operations Manual, a 30 percent chance of rain means that given similar meteorological conditions, the forecaster thinks rain will occur three out of 10 times for the area in question. Or, to state the converse, rain is NOT expected seven out of 10 times. Additionally, the probability has nothing to do with the amount of rain, how long it will rain or the percentage of the area that will get rain.

However, when the forecast says there is a ``30 percent chance of showers,'' the confusion increases -- only 13 percent of survey respondents correctly answered that 30 percent of the area would get measurable precipitation. According to the NWS, when showers are mentioned in a forecast, there is a high likelihood of them occurring somewhere in the area, and thus the probability refers to the amount of the area in the forecast that will get wet.


Q. I recently saw a forecast that described an ``upper-level trough'' positioned over the Pacific Northwest. Can you please explain the terms ``upper-level troughs'' and ``upper-level high pressure,'' using simple terms, and how they affect our weather? Does this mean the air pressure is monitored at different altitudes like layers of a cake? Curtis Panasuk - San Carlos

A. Upper-level troughs and ridges are descriptive terms for regions of low and high pressure in the upper atmosphere. On a weather map, a ``trough'' shows up as a valley while a ``ridge'' looks a hill. When you see references to upper-level troughs and ridges, they commonly are at about 18,000 feet, which is the level that has half the atmospheric pressure of the surface and is frequently used by meteorologists to follow storm tracks.

Generally, troughs are associated with rising air, which leads to the formation of clouds and, under the right circumstances, to precipitation. Conversely, there is sinking air associated with ridges, with clear skies and typically warmer weather being the result. However, an important consideration in the final weather outcome is whether there is a corresponding area of high or low pressure at the surface, which tends to enhance the effects of the ridge or trough.

Q.  How can you tell, when the wind picks up, if it's because an area of high pressure is being replaced by a low or vice versa, excluding the use of instruments such as a barometer? Tom Duke - San Francisco

A Winds are produced due to differences in barometric pressure, with air moving from an area of high pressure toward an area of low pressure, and it is moot as to which replaces the other. The wind speed is a function of how much the pressure varies over a given difference in what is referred to as the ``pressure gradient.''

Barometers remain the main method of measuring atmospheric pressure over the land, at sea and in the upper atmosphere, using a barometer attached to an aircraft or weather balloon. Without a barometer, the temperature of surface air masses can be used to generalize where pressure would be higher or lower. High pressure at the surface is generally associated with cooler air, while low pressure usually coincides with warmer air.

Q.  In the past 100 years, how many days in July have had measurable rain in the Bay Area? I am 44 years old, and it has rained on my birthday -- July 17 -- only once. Andrea Puck - Santa Clara

A.  The longest consecutive rainfall record in California is for downtown San Francisco, with 153 years of records. During that time, it has rained on 67 occasions -- or about 1 percent of the time -- for any day in July. Twice in that period, it has rained on July 17, the last time being in 1958 when 0.02 inches fell. Interestingly enough, the 17th is bracketed by the historically wettest and driest days of July. During the past 153 years, it rained five times on July 16, and it has never rained on July 18. The only other days of the year that have never had measurable rain were July 7 and 14.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to him c/o WeatherCorner, 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 Please indicate in your e-mail what city you live in.