Pool owners often wonder why they have to refill their pools more frequently during the summer months. One of the primary reasons is evaporation, especially on hot, sunny days like when the temperature reaches 90°F. In this article, we’ll explore how much water evaporates from a pool on a 90 degree day, what factors influence evaporation rates, and what you can do to minimize water loss and maintain your pool efficiently.
Understanding Pool Evaporation Basics
Evaporation is the process by which water changes from a liquid to a vapor and escapes into the atmosphere. It is a natural phenomenon that occurs in all bodies of water, including swimming pools, and is most noticeable during hot and dry conditions.
What Causes Evaporation to Increase on a 90°F Day?
When the air temperature reaches 90°F, several environmental factors can contribute to increased evaporation from a pool:
- Higher Temperatures: Warmer air can hold more moisture, which encourages faster evaporation.
- Increased Sun Exposure: Solar radiation heats the surface of the pool water, increasing the energy available for evaporation.
- Lower Humidity: Low humidity means the air is “thirsty” for moisture, promoting faster evaporation.
- Wind Speed: Moving air across the pool surface removes water vapor, allowing more evaporation to occur.
These factors work together to determine the actual rate of evaporation from a pool on any given day.
Measuring Evaporation: The Numbers Behind the Heat
To understand how much water evaporates on a 90 degree day, let’s break it down into actionable units.
Average Evaporation Rate on 90°F Days
Pool evaporation rates can vary, but under typical summer conditions with high temperatures and moderate wind, you can expect:
| Pool Size (in square feet) | Average Daily Evaporation (in gallons per day) |
|---|---|
| 300 sq ft | 15-25 |
| 500 sq ft | 25-35 |
| 1,000 sq ft | 50-75 |
| 1,500 sq ft | 75-100 |
This chart assumes a typical 90°F day with moderate humidity and wind speeds around 5-10 mph. These numbers represent a conservative estimate and can be much higher under extreme weather conditions.
How Evaporation is Measured
Evaporation can be measured using several methods, including:
- Evaporation Pans: Small containers of water placed near the pool to mimic evaporation conditions.
- Water Level Indicators: Marking the pool water level and measuring the drop over a 24-hour period.
- Smart Sensors: Devices that use ultrasonic technology or float systems to monitor water levels automatically.
Factors That Increase or Decrease Pool Evaporation
While temperature is a major factor, it’s not the only one that affects how much water evaporates from your pool. Let’s dive into each key variable.
Temperature
As mentioned, a 90°F day is synonymous with high evaporation potential. However, the water’s surface temperature itself can be cooler or warmer than air temperature, depending on the time of day, cloud cover, and wind.
Water Temperature vs. Air Temperature
The rate of evaporation depends more on the temperature difference between the water and the air. The greater the difference, the faster the evaporation. For instance:
- If the water is at 85°F and the air is at 90°F, evaporation proceeds moderately.
- If the pool water is heated to 90°F and the air remains the same, evaporation increases significantly.
Humidity Levels
Relative humidity (RH) is a measure of how much moisture the air currently holds compared to its maximum capacity at a given temperature. On a 90°F day:
- High Humidity (80-100% RH): Evaporation slows down.
- Moderate Humidity (40-60% RH): Standard evaporation conditions.
- Low Humidity (under 40% RH): Pool water evaporates rapidly.
Understanding your local humidity is essential to predict how much water your pool may lose in a day.
Wind Speed
Wind plays a critical role in the evaporation process by:
- Removing the moist boundary layer that forms over the pool’s surface.
- Introducing drier air that can absorb more water vapor.
Wind speeds over 10 mph can significantly increase evaporation rates. Even a light breeze can boost daily water loss.
Precipitation and Cloud Cover
On a 90°F day, evaporation is generally highest during the afternoon when the sun is most intense. Cloud cover, however, reflects some of the sun’s energy, reducing surface temperature and evaporation.
Rainfall adds water to the pool, so after a storm or a humid morning, evaporation for the day might effectively cancel out.
Calculating Evaporation: From Theory to Practice
To precisely determine how much water will evaporate from a pool on a 90 degree day, some mathematical models and formulas can be used.
The U.S. Bureau of Reclamation Formula
One of the common methods to estimate pool water evaporation is the U.S. Bureau of Reclamation formula, which uses surface area, wind speed, and vapor pressure difference between the pool surface and the surrounding air.
The simplified version for backyard pools:
E = k ⋅ A ⋅ (e_s – e_a)
Where:
– E = evaporation rate (in gallons/day)
– k = evaporation constant (varies with wind speed)
– A = surface area of the pool (in square feet)
– e_s = vapor pressure at the surface (based on pool water temp)
– e_a = actual vapor pressure in the air (based on air temp and humidity)
Example Calculation for a Residential Pool
Let’s take a 500 sq ft pool on a 90°F day with:
– Air temperature = 90°F
– Water temperature = 85°F
– Relative humidity = 40%
– Wind speed = 10 mph
Estimated evaporation per day: 25–35 gallons
Breaking it down:
– A 1,000 sq ft commercial pool could lose up to 100 gallons in a single day under similar conditions.
This illustrates how critical large-scale weather conditions are to water usage and maintenance costs—especially for public swimming pools and water parks.
Signs Your Pool Is Losing Water to Evaporation
Not all pool water loss is due to evaporation. It’s essential to distinguish between normal evaporation and leaks.
Bucket Test for Leak Detection
The bucket test is a simple, effective method to determine whether your water loss is due to evaporation or a leak:
- Place a bucket filled with water on a pool step so that the water levels inside the bucket and the pool are the same.
- Mark both water levels.
- Let the setup sit for 24 hours without using the pool or running the pump.
- Compare the water levels after 24 hours.
- If both levels drop by the same amount: Evaporation is the cause.
- If the pool water drops more than the bucket water: There may be a leak.
This test helps pool owners avoid unnecessary repairs and focus on real issues like structural degradation or equipment failure.
Automated Evaporation Monitors
Smartpool devices now enable real-time monitoring of evaporation and water levels. These sensors can alert homeowners to abnormal loss patterns and even integrate with pool automation systems.
Cost Implications of Pool Evaporation
Pool evaporation isn’t just a nuisance—it also has direct financial consequences.
Water Costs
In many regions, municipal water costs are rising due to scarcity and environmental regulations. Refilling your pool can cost anywhere from $0.005 to $0.02 per gallon, depending on local water rates.
For example, a 90°F day with 75 gallons of loss results in:
– At $0.005/gallon = ~$0.375/day
– At $0.02/gallon = ~$1.50/day
These costs add up quickly over a summer season, especially in dry climates.
Energy and Chemical Costs
When water evaporates, it carries with it essential pool chemicals like chlorine and stabilizers. Homeowners typically need to:
- Re-add chemicals to maintain balance
- Run additional filtration cycles to maintain water clarity
- Heat replacement water to desired temperatures, increasing energy use
This can increase your annual pool maintenance costs by hundreds of dollars.
Strategies to Reduce Pool Evaporation
Minimizing evaporation doesn’t just save you money—it also contributes to water conservation and environmental sustainability.
Use a Pool Cover
One of the most effective ways to reduce evaporation is by covering your pool when it’s not in use. Pool covers:
– Reduce evaporation by 30–90%
– Help retain heat, lowering overall energy costs
– Decrease chemical loss
There are several types available:
– Solar blankets: Absorb sunlight and insulate
– Liquid solar blankets: Evaporate slowly on the water surface
– Solid safety covers: Double as barriers for evaporation and safety
Install a Windbreak
Simple landscaping features like fences, lattice walls, or hedges can significantly reduce wind speeds across the surface of your pool. Even a modest barrier can reduce evaporation by up to 30%.
Optimize Pool Water Temperature
Keeping your pool water a few degrees cooler than the air temperature can reduce the evaporation potential. You may also consider heating the pool later in the day or turning off heating when it’s not being used.
Minimize Splash and Fountain Use
Excessive splashing—whether from pool entertainment features like waterfalls, fountains, or playful children—exposes more water to air and wind, increasing evaporation.
Use Evaporation Reducing Chemicals
“Liquid solar blankets” like Evaporation Suppression Technologies (EST) float on the surface and form a thin, invisible layer that reduces water loss without affecting water chemistry.
Regional Variations Across the United States
Pool evaporation varies depending on a region’s climate.
Southwest U.S. (Arizona, Nevada, New Mexico)
These areas experience extreme heat, low humidity, and high evaporation:
- Average summer evaporation rate: 0.25+ inches/day
- A 1,000 sq ft pool can lose over 150 gallons/day
West Coast (California, Oregon)
While California often gets hot, coastal regions have high humidity and sea breezes. This produces moderate evaporation rates:
- Central Valley and inland cities: 80–100 gallons/day
- Coastal areas (e.g., San Diego): 30–50 gallons/day
Midwest & Northeast (Ohio, New York, Pennsylvania)
Humidity is higher in summer, which slightly reduces evaporation. However, sudden temperature spikes (like a 90°F day) can still lead to significant loss:
- Typical loss: 40–70 gallons/day
Southern U.S. (Florida, Georgia, Louisiana)
Though hot, southern states have high humidity during the summer:
- Daytime evaporation lower than in arid areas
- Night evaporation significantly reduced due to moisture retention
Conclusion: Understanding and Managing Evaporation
On a 90 degree day, your swimming pool can lose anywhere from 15 to 100+ gallons of water through evaporation alone—depending on environmental conditions, size, and the pool’s location. By understanding the driving factors and using tools like covers, windbreaks, and monitoring technology, you can dramatically reduce water loss, lower your utility bills, and contribute to sustainable water use.
Keeping track of these losses allows for better planning and conservation. Whether you’re managing a residential oasis or a commercial facility, controlling evaporation is a smart, responsible choice.
So next time you’re enjoying the cool water of your pool on a hot 90°F day, remember that nature is quietly pulling moisture from the surface into the sky. Monitoring that invisible loss is just as important as keeping your pool clean and ready for enjoyment.
How much water typically evaporates from a pool on a 90-degree day?
On a typical 90-degree day, a standard residential pool can lose anywhere from 1/4 inch to 1/2 inch of water per day due to evaporation. The exact amount depends on various factors including humidity, wind speed, and the surface area of the pool. In dry, arid climates with strong winds, evaporation can be on the higher end of that range, while more humid conditions can reduce the daily loss.
Water evaporation occurs when the air surrounding the pool surface becomes saturated with less moisture than it can hold, pulling water molecules into the atmosphere. While 90 degrees is generally a strong indicator of increased evaporation, the combination of temperature with other environmental factors will ultimately determine the rate at which water evaporates from any given pool.
What factors influence the evaporation rate from a swimming pool?
Evaporation rate is influenced by a number of interrelated environmental and physical factors. The primary ones include temperature, humidity, wind speed, and the surface area of the pool. Higher temperatures increase the energy in water molecules, making them more likely to escape into the air. Lower humidity allows air to absorb more water, increasing evaporation, while wind can carry away moist air, also accelerating the process.
Additionally, whether the pool is in use or covered can greatly impact the amount of water loss. Pools that are frequently used or left uncovered will lose more water than those with covers or are sheltered by structures or landscaping. Water features such as fountains or waterfalls also contribute to higher evaporation rates by increasing surface agitation and exposure to air.
How can I calculate the evaporation from my pool on a 90-degree day?
A simple method to estimate evaporation is to use the “bucket test.” Place a bucket filled with water on the second step of the pool and mark both the bucket water level and the pool water level. After 24 hours, compare the water loss. If the pool water evaporates faster than the bucket, which is not affected by splashing or backwashing, you’ll know evaporation is a significant cause of water loss.
For a more scientific approach, evaporation calculation formulas like the U.S. Environmental Protection Agency (EPA) equation or the Stiver and Mackay method can be used. These models factor in the surface area of your pool, air temperature, wind speed, and the vapor pressure of the water. Plugging in these values on a calculator or spreadsheet can give you a more accurate estimate of daily evaporation levels.
Is evaporation the only reason my pool loses water?
No, evaporation is not the only cause of water loss from swimming pools. Other common reasons include splash-out during use, backwashing during filter cleaning, and even leaks. A pool that drops more than a half-inch of water per day may have a leak, especially if evaporation isn’t expected to account for such a significant decrease under the current weather conditions.
To distinguish between evaporation and a leak, perform the bucket test described earlier. If the water in the pool drops significantly more than the water in the bucket, you may have a leak that requires further inspection. It’s important to repair leaks promptly to avoid structural damage and higher water bills.
How does wind affect the evaporation from a swimming pool?
Wind plays a major role in increasing evaporation from pool surfaces. When air moves over the surface of the pool at higher speeds, it reduces the humidity directly above the water, allowing more water molecules to escape into the atmosphere. This is why unsheltered pools tend to lose more water than those surrounded by fencing, trees, or windbreaks.
Pool owners in windy areas can greatly reduce evaporation by using wind barriers to minimize exposure. Although it may not be possible to eliminate natural wind currents, even basic interventions like installing a fence or planting tall shrubs can significantly lower the daily rate of water loss on hot, windy days such as when temperatures reach 90 degrees.
Does a pool cover help reduce evaporation on hot days?
Yes, using a pool cover can significantly reduce evaporation—especially on days with high temperatures and low humidity. Pool covers act as a barrier between the pool water and the air, limiting the ability of water molecules to escape into the atmosphere. Covers also reduce heat loss, which helps maintain water temperature, particularly at night or in cooler environments.
There are different types of pool covers, ranging from solar blankets to automatic safety covers, all of which can help reduce evaporation. Choosing the right cover depends on the pool size, climate, and usage. Many pool owners find that using a cover is a cost-effective way to conserve water, reduce chemical use, and maintain pool cleanliness—particularly during hot summer months when evaporation rates are at their peak.
What impact does humidity have on pool water evaporation at 90 degrees?
Humidity, or the amount of moisture in the air, has a significant impact on the rate at which pool water evaporates. On a 90-degree day with high humidity, the air is already close to saturation, meaning it cannot absorb large amounts of water from the pool surface. This results in lower evaporation compared to when the same temperature occurs in a dry or arid climate.
In contrast, low humidity increases the evaporation rate dramatically, even if the temperature remains the same. Areas with desert-like conditions will see much more water loss than coastal regions. Knowing the local weather conditions, particularly the dew point and relative humidity, can help pool owners better estimate and manage their pool’s water usage on hot days.