An evaporative cooler — also known in some parts of the country as a swamp cooler — is an energy efficient way to cool your home without breaking the bank with your summertime electricity bill. These machines work using a basic scientific principle known for centuries: namely, that as water evaporates and changes from liquid to gas, it reduces the temperatures of anything around it.
Evaporative cooling is the reason why sweat works. When your skin is wet, that excess moisture evaporates into the air and leaves behind cooler skin, which helps your body stay comfortable in the heat. However, if you’ve ever suffered through a hot, muggy day, you know that there are limits to your body’s cooling capacity. When the air if very humid, there’s just not much room for the air to absorb additional moisture, so your sweat doesn’t evaporate much — if at all. This is why a hot, humid day can be so much more miserable than the much-touted “dry heat” of Arizona and the American Southwest.
How an Evaporative Cooler Works
Think of an evaporative cooler as a machine that helps your home “sweat” to cool off. The machine works by speeding up the evaporation process with a simple fan to draw in hot, dry air and spit out more humid, cold air. An evaporative cooler has a specially designed pad that absorbs water from a reservoir. The pad has many layers to increase its surface area so that water can evaporate more quickly. A fan brings a supply of air directly across the pad, where contact with warm air stimulates water molecules to evaporate and turn to gas. This process results in cool air that is then blown out of the unit to make your home more comfortable. For people in dry climates, the more humid air that comes out of an evaporative cooler can be a bonus for comfort as well, as skin and nasal passages generally do better with some degree of humidity for good health. ( See What Are Evaporative Coolers? Here’s Everything You Need to Know)
Knowing that an evaporative cooler works similarly to the way your body naturally cools itself with sweat, you may find yourself wondering: Should I buy an evaporative cooler where I live?
That’s a great question!
To answer it, you’ll need to understand some specifics about the atmosphere in your area, plus a general knowledge of some key weather terms.
The Ultimate Evaporative Cooler Humidity Chart
Wondering if an evaporative cooler is a good purchase for your home? The right environment for an evaporative cooler depends on both air temperature and relative humidity where you plan to use the cooler.
On any given day, Mother Nature can deliver a wide range of air temperatures and humidity. Air changes in either category will influence how well your evaporative cooler works. There’s an optimal range for coolers to have a big impact on dropping temperatures in your home, as you can see highlighted in the chart above.To understand what you’re looking at in the grid, you’ll first need to know some basic weather terms.
Air temperature is probably exactly what you think it is: how hot or cold the air feels based on a standard thermometer reading. To be more specific, air temperature here refers to what scientists and engineers call “dry bulb temperature,” which means temperature measured by a thermometer without any other factors taken into account.
Knowing the air temperature alone won’t help you know if your evaporative cooler will work efficiently, though, because such a measurement doesn’t take into account the humidity — a crucial factor when it comes to quick evaporation, as explained above. To factor in humidity, engineers sometimes take a “wet bulb temperature,” which involves wrapping a thermometer in wet fabric. As water evaporates from that fabric into the air, it cools the temperature around the thermometer down a bit, and you get a reading that takes into account how humidity affects cooling. If the wet bulb temperature is significantly colder than the dry bulb temperature, it means the humidity is low and you have good evaporation going on — great conditions for using an evaporative cooler.
Unless you want to do a science experiment, though, wet bulb temperature isn’t usually helpful information for the average person. That’s why this chart ignores it in favor of standard air temperature, and instead deals with …
Relative humidity is measured as a percentage that expresses how saturated the air is with water. Hot air can hold more water than cold air, so the same amount of humidity — also known as absolute humidity — on a cold day will feel more humid than it would on a hot day.
To account for the ways temperatures affect humidity, meteorologists tend to talk in terms of relative humidity instead. This percentage takes the amount of moisture in the air and compares it to the absolute maximum amount of water that air at that temperatur e can hold. If you see a reading of 100 percent relative humidity, it means the air is maxed out on moisture and evaporation will be impossible. That’s why summertime humidity readings in the upper 90s are so dreaded on humid climates like the East Coast and in the Deep South — water can’t evaporate well, and you’re likely to feel miserable once you break a sweat— even if it’s not that hot out.
How to Read the Chart
If you take a measurement of the air temperature and relative humidity in your town, you can use this chart to determine if you have optimal conditions to run an evaporative cooler. Just find the intersection of temperature (on the right) and relative humidity (across the top) to the number on the grid where both measurements meet. This is the temperature you can expect to achieve with your evaporative cooler.
For example, if it’s 85 degrees and 40 percent relative humidity where you are, you’ll see that you can bring your temperature down to 72 degrees with an evaporative cooler. That’s ideal for comfort! You’ll also notice that this temperature is highlighted, which means these conditions are in the optimal zone for getting the most out of evaporative cooling. You may be able to use an evaporative cooler in more humid or hotter conditions, but the cool-off you experience won’t be as strong. It’s up to you to look at the temperature results on the grid to decide if an evaporative cooler can provide enough chill to keep you happy during the summer months, or if you’d prefer to have a traditional compressor air conditioner instead.
Should You Invest in an Evaporative Cooler?
Taking a temperature and humidity measurement on any given will let you know how well your evaporative air cooler will work on that day, but how can you tell if you live in a place where it makes sense to rely on this type of cooling for most of the year?
It’s helpful to consider the average temperatures and humidity in your area during different seasons. You can try a Google search for information on your town, or go to Weather Underground to look up the weather station that’s closest to your house. Once you find it, you can enter any date in history to get high and low temperatures, humidity readings, dew point and more. This should give you a clear idea about your averages over time and whether you should get one for yourself.
If all that sounds too time consuming, take a look at this handy map instead:
As a general rule of thumb, the farther west you live, the better luck you’ll have with evaporative cooling. Here humidity levels and ambient temperatures generally fall in the right range for evaporative air conditioning systems to do their thing.
Using Portable Evaporative Coolers Outside
In the east, the moisture content of the air may generally be too high to use an evaporative cooler indoors — they do, after all, cause a humidity increase inside as you use them. However, you may enjoy using an evaporative cooler outside on days when the outdoor air has a low percent relative humidity. Portable evaporative coolers can make patio parties, picnics and days by the pool a bit more comfortable as you can harness the evaporative cooling process to blow colder air on you while outside. Because you don’t have to worry about things getting clammy outdoors, this type of air conditioning makes sense for an outdoor room to keep things comfortable without overworking a compressor air conditioner.
How to Get the Best Airflow From Your Evaporative Cooler
Whether indoors or out, evaporative coolers work best in areas with low humidity. To see the evaporation rate up, make sure you provide a constant flow of fresh air to your cooler. This means that, unlike a standard air conditioner, you should keep a window open to allow air movement to bring in dry air and sweep out extra humidity from the cooler. You’ll have to experiment to find just the right balance of dry, fresh air and more humid conditioned air, but it can be done. If you live in an arid climate, cooler air can be yours for less money when you invest in an evaporative cooler.