Ask A Scientist© General Science Archive

 

> >  name       Lawrence
> >  status     other
> >  age        60s
>
> >  Question - Does water evaporate faster(inchs per day) from a smaller
> > surface area
> >ie. a spa vs a larger surface area ie. a swimming pool? My spa (7 ft dia)
> >loses 1 1/2 in. per day while my swimming pool (22' x 42') loses 1/2 in.
> >per day. My pool builder says that is normal!
>
>No, water doesn't evaporate from a smaller surface area than a larger one.
>There may be a couple different reasons why the spa loses more than the pool
>though!
>
>1. If the spa is heated it will evaporate faster.
>2. If the bubble jets are on (if the spa is equipped with them, I am
>guessing it is like a hot tub?)
>3. If the spa is smaller than the pool and is painted with a dark color,
>the water will warm faster in the spa than the pool water would warm, this
>would just be the sunlight heating two different volumes of water.(this
>would occur regardless of color but would be more apparent if the paint was
>dark) The only thing I could suggest is leave a thermometer in both and
>compare the temperatures throughout the day and see if they vary by much.
>
>Hope this helps
Dr. Baldwin
=========================================================
>Isn't your spa warmer than your pool? If so, this is the reason its water
>evaporates faster.
>
>Tim Mooney
=========================================================
>Lawrence,
>
>I'm going to guess that in general you keep your spa warmer than your
>pool. Water evaporation is related to temperature, so that would explain a
>lot of it.
>
>If they're being used by the same number of people each day, the spa will
>also lose water faster, as the water absorbing in the suits and hair and
>skin of each user will be the same, but since the spa is smaller, each bit
>lost represents a larger percentage of the total volume.
>
>
>Donald Yee Ph.D.               San Francisco Estuary Institute
>180 Richmond Field Station,     1325 South 46th St. Richmond, CA 94804
========================================================
>Dear Lawrence-
>
>   There are several factors affecting the rate of evaporation of a liquid.
>Probably the most important two factors are the temperature of the liquid
>that is evaporating, and the humidity of the air into which the water is
>evaporating.
>
>If the air is "dry," the water will evaporate faster than if the air is very
>humid. And warmer water evaporates faster than cold water. Probably the
>temperature of the spa water is warmer than that of the swimming pool. And
>possibly the air around the spa is drier than that over the swimming pool.
>
>   Other factors being equal, the size of the container of the water should
>not matter.
>
>Wendell Bechtold, meteorologist
>Forecaster, National Weather Service
>Weather Forecast Office, St. Louis, MO
=========================================================
Hello,
>
>There are many factor the can affect the rate of evaporation from a
>pool: volume, surface area, wind, location, chemistry of water, air
>temperature and moisture, color of the pool walls (different colors
>absorb at different rates), etc.
>
>Water is heated up by the sun and by the warm walls of the pool.
>Infrared radiation is absorbed by water, mostly in the top 0.1 mm or so,
>while the visible light goes deeper in water. Warm water -having lower
>density- moves to the surface when it is further heated and evaporated.
>The mechanism is complex, however, it is not difficult to understand why
>the SPA loses more water than a pool - the question you posed.
>
>SPA has a smaller volume of water than the pool. As such, for the same
>amount of sunlight impinging on the surfaces of the pool and the SPA,
>the water in the SPA is warmer, leading to more evaporation. Warm
>ground may make additional heat contribution. In fact, you can measure
>the water temperature at regular interval in the two reservoirs and
>correlate the results with evaporation rates.
>
>AK

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