We try to frequently use the prefixes/adjectives listed on this page to describe precipitation and cloud cover. If these descriptors are properly understood by our users, then the forecast will appear much more accurate to you... compared to if the terms are not interpreted correctly.
Precipitation is defined as falling liquid or ice from a cloud. (as such, dew and frost are not precipitation, even though they may sometimes accumulate to a minor measurement in rain gauges). There are two primary types of precipitation. These are stratiform, and cumuliform (referred to as convective/convection). While both types produce rain or snow at the surface, there are major differences in the way they form. Further more, the effects at the surface from stratiform and convective precipitation can often be very different. Since this is not really meant to be an educational section for the two terms, the processes which lead to formation of them will not be defined more extensively on this page.
-- In a typical forecast, when correctly used, the term "rain" or "snow" (i.e., "chance of rain/snow") means a stratiform rain/snow event. In most cases, stratiform rain/snow events are characterized by widespread steady precipitation. These type days would typically feature an overcast day with low (often dark) clouds from horizen to horizen over a large region... often several states. Stratiform precipitation will also usually cover the same large region... with rain or snow usually affecting most or all people for an extended period of time. However, not all stratiform events have such an extensive region of clouds/precipitation. Some will instead have a single period of rain lasting for a couple or few hours, or multiple periods of rain throughout the course of a day. The following terms are the most commonly used to describe stratiform events:
Other prefixes may occasonally be used to describe rain events, due to many events being different in their outcome. Thus, from time to time, other adjectives may be needed to improvise. Also, these adjectives are especially used in the short term (<72 hours), with a more broad wording in the longer term.
-- In a typical forecast, when correctly worded, all showers (and thunderstorms) are convective in nature. In general, convective precipitation is characterized by multiple breaks in the precipitation, being scattered in nature and produced from individual, or clusters of, cumulus clouds of some type. Convective precipitation can sometimes (especially during the warm season) develop very rapidly, and dissipate after not too long. Further more, due to the relatively small size of individual cumulus/convective clouds, convective precipitation may often affect only some areas while other areas are left dry. In some cases one side of a town may be affected while the other is not. However, there are convective precipitation events that feature widespread showers/thunderstorms in which all or most locations see them... occasionally even lasting for hours at any given location. Lastly, it is important to remember that the term shower does NOT in any way imply the intensity of the precipitation falling. The heaviest rainfall typically comes from showers, whether localized or widespread. The following terms are the most commonly used, in our forecasts, to describe stratiform events:
Other prefixes may occasonally be used to describe shower events, due to many events being different in their outcome. Thus, from time to time, other adjectives may be needed to improvise. Also, these adjectives are especially used in the short term (<72 hours), with a more broad wording in the longer term.
We describe cloud cover in our forecasts usually by the amount of sky that is covered by clouds... NOT necessarily by whether the sun is or is not shining. Thus, to get an accurate sky cover condition, the entire sky must be looked at... NOT just whether the sun is or is not shining. For example, as you can see in the descriptions below, it is still possible to briefly have sunshine under a mostly cloudy sky. Likewise, it is possible to briefly not have sunshine under a mostly sunny sky. The only exception to this rule, sometimes, is with the presence of thin, high clouds (cirrus clouds). Since thin high clouds can sometimes cover much of the sky, but also still allow plenty of sunshine to occur due to their thinness, a mostly or partly sunny wording may be used even though a lot of the sky contains high clouds.
Occasionally... a few other terms, some listed below, may be used to customize the forecast for a particular day:
Yet other ways of describing cloud cover may be used... such as Sunshine though high clouds, etc. These are self explanatory, and used at discretion of the forecaster when conditions warrant.
