Posted Updated

Mike Moss

A question we receive here in the WeatherCenter from time to time pertains to how much evaporation (transformation of liquid water to water vapor) or evapotranspiration (conversion of liquid water to water vapor in the process of supporting plant life) occurs over a given time, and where to find average or observed data reagrding this phenomenon. It has importance of course in terms of agriculture, horticulture and lawn maintenance, along with larger scale water supplies as applied to lake and pond levels.

Over the course of a year, the climate in our region is such that increasing temperatures drive evaporation rates to levels that generally exceed precipitation during the summer months (for a net loss of liquid water), more or less match long term precipitation averages for parts of the spring and fall, and falls below wintertime precipitation rates (for a net gain in stored liquid water). Of course, from year to year there can be quite a bit of variability in this behavior.

In general, those of us around the Triangle area experience monthly evaporation and evapotranspiration rates that run roughly between one and two inches during the December to January time frame, and see peak evaporation rates around six to seven inches for June and July, with a fairly smooth increase through the spring and decrease through the fall connecting those values. These numbers are based largely on Class A Pan evaporation measurements taken in Chapel Hill, and published in the book "North Carolina Weather and Climate," by Dr Peter Robinson of UNC-CH.

For those who might be interested in looking up more specific monthly estimates or observed data, there are some resources on the web that make that possible. In particular, the State Climate Office has each of these types of data available for selected locations, some theoretical (calculated based on measurements of other weather parameters) and others measured using an instrument called an "atmometer" (also known as an "evaporimeter") that approximates evapotranspiration rates froman agricultural crop. Here are addresses for these two kinds of information.

First, you can get estimated average monthly evapotranspiration values for various sites around the southeast, including the Raleigh-Durham airport, for example, at

As for measured values, several of the "Econet" sites (green dots) on the map at

provide atmometer data on an hourly, daily and monthly basis. Note that to select a station from the map, you need to click the "information" button above the map first.

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