The Evaporation Pan, also known as the Class A Evaporation Pan, and Evaporation Substation, is used to determine evaporation rates from day-to-day. It is perhaps the most uncommon, yet most important measuring device an observer can have.
The pan itself is made of monel, a nickel alloy created by metallurgist, David H. Browne. The alloy is primarily composed of nickel, copper, some iron and various trace elements. Monel is highly resistant to corrosion, making it perfect for use as an evaporation pan.
The pan has a diameter of 47.5 inches and a depth of 10 inches, and is filled to 2 inches of the top, but never filled to the top to avoid splash out due to wind. The pan rests flat on a wood-framed base consisting of 2″ x 6″ pressure-treated dimensional lumber.
Evaporation is measured daily as the depth of water (in inches) evaporates from the pan. The measurement day begins with the pan filled to exactly two inches (5 cm) from the pan top. At the end of 24 hours, the amount of water to refill the pan to exactly two inches from its top is measured.
If precipitation occurs in the 24-hour period, it is taken into account in calculating the evaporation. Sometimes precipitation is greater than evaporation, and measured increments of water must be dipped from the pan. Evaporation cannot be measured in a Class A pan when the pan’s water surface is frozen.
The Class A Evaporation Pan is of limited use on days with rainfall events of >1.5 inches unless it is emptied more than once per 24hours. Possible splash out may occur, and if enough rainfall occurs, the pan will simply overflow making measurements near impossible.
Evaporation Pan Water Statistics: (when the pan is filled to 9″ (22.86 cm)):
Pan volume = 7,809.5623″ (19,836.288242 cm) of water = 1.04723″ (2.659964 cm) of water = 7.833 gallons (29.651131 liters) of water = 65.345 lbs. (29.640492 kg) of water.
The above photo shows the complete evaporation pan set up. This photo is from the National Weather Service Office in North Platte, Nebraska. The pan sits on a wood frame in a grassy area; on the right you can see the cup anemometer used for measuring miles of wind that passes over the pan water. The metal box at the base of the anemometer contains a odometer which shows the observer the total number of miles and can be reset, usually, by depressing a small mechanical button. This cup anemometer is often called a Totalizing Anemometer, and in most other countries measures wind in kilometers rather than in miles.
Inside the pan on the right side you can see the still well, and on top of that you can see the hook gauge micrometer. Notice how the micrometer’s 3 legs support it on top of the still well. This is how it measures from the same height every day to ensure accurate measurements. On the left of the pan, you can see a metal hanger-looking device sticking out of the water; that is the handle that connects to the six’s thermometer. Finally, on the right side of the pan, you can see a faucet, which is used to refill the pan each day. The hook gauge is set to a pre-determined height (this same height is used daily), and an observer will fill the pan slowly until the water level reaches the hook gauge and the meniscus of water “pops” past the hook’s tip.
~ Steve Woodruff and Devin Lussier