Evaporative Cooler Chart (Humidity, Efficiency, Temperature Swamp Coolers)

Will using an evaporative cooler be effective? This is exactly what the evaporative cooler chart (also called the swamp cooler chart) tells you. In this chart, we are going to look at the relationship between temperature and humidity for the use of evaporative coolers; this will tell us how efficient a swamp cooler can be in your area.

Namely, all evaporative coolers use the principle of water evaporation to cool the air. As you might imagine, relative humidity levels play a key role here (we usually refer to the chart below as the evaporative cooler humidity chart because of this). Essentially, this is what every swamp cooler does:

1. Pull in warm and dry air.
2. Run it over a water-saturated medium; water evaporates to vapor in the air. This both decreases the temperature of the air and increases the humidity of the air.
3. Expel colder but more humid air. According to the US Department of Energy, evaporative coolers can lower the air temperature “by 15°F to 40°F”. In short, if used in correct temperature-humidity conditions, these swamp coolers can be very effective.

Now, as we can see, to effectively use an evaporative cooler, we need air with these two properties:

1. Air must be warm. We usually have no problem finding warm air in the summer. In fact, the whole point of using a swamp cooler is to cool the warm air.
2. Air must be dry. Here we encounter a problem because humidity levels in the summer are quite high. At what humidity level do evaporative coolers become ineffective? We can still cool up to 80% relative humidity levels air but with decreasing efficiency. Example: According to the swamp cooler chart, at 10% humidity levels and 100°F, you can lower the temperature by 27°F to 73°F. At 80% humidity levels and 80°F, you can lower the temperature by only 3°F to 77°F.

As you might suspect, there is a clear compromise between the temperature of the air we are cooling and the humidity of that air. How do tell what kind of temperature and humidity levels still make sense?

To answer this, we always use the swamp cooler temperature humidity chart. It will essentially tell us at what temperature and humidity it’s too humid to effectively use an evaporative cooler.

Before we check out the evap cooler chart (and, even more importantly, explain how to use it), let’s have two things in mind:

• Evaporative coolers are most effective in high temperature (85°F or more) and lower humidity (below 50% relative humidity) situations.
• Evaporative coolers are least effective in lower temperature (below 75°F) and high humidity (above 60% relative humidity) situations.

The swamp cooler humidity chart spans from 75°F to 125°F temperature and from 2% to 80% relative humidity levels. It tells you at which temperatures and moisture levels using an evaporative cooler is effective. What is more, it tells you how many degrees does a swamp cooler lower. For HVAC professionals: This is based on the Mollier diagram and uses dry-bulb and wet-bulb temperatures for a direct evaporative cooler process.

Note: This evaporative cooler chart is quite complex to read and understand. We are going to look at it and explain further how to read it. Below that, we are going to look at a chart that tells us how many degrees can an evaporative cooler cool (so-called swamp cooler temperature chart), based on the evap cooler chart. If you don’t get your answer here, you can use the comments below, tell us the temperature and humidity levels you have in mind, and we will try to help you out.

With this in mind, let’s have a look at the evaporative cooler or swamp cooler humidity chart (scrolling right and down will help, it’s a big graph):

Swamp Cooler Humidity Chart

Here is how to read this graph:

You have to look at the temperature and relative humidity in a space where you will use an evaporative cooler. Let’s say you have 90°F heat. Alright, now you can see what temperature drop you will be able to achieve at different humidity levels:

• At 2% relative humidity, the swamp cooler will be able to drop the temperature from 90°F to 64°F. That’s a -26°F drop.
• At 5% relative humidity, the swamp cooler will be able to drop the temperature from 90°F to 65°F. That’s a -25°F drop.
• At 10% relative humidity, the swamp cooler will be able to drop the temperature from 90°F to 67°F. That’s a -23°F drop.
• At 15% relative humidity, the swamp cooler will be able to drop the temperature from 90°F to 69°F. That’s a -21°F drop.
• At 20% relative humidity, the swamp cooler will be able to drop the temperature from 90°F to 70°F. That’s a -20°F drop.
• At 25% relative humidity, the swamp cooler will be able to drop the temperature from 90°F to 72°F. That’s a -18°F drop.
• At 30% relative humidity, the swamp cooler will be able to drop the temperature from 90°F to 74°F. That’s a -16°F drop.
• At 35% relative humidity, the swamp cooler will be able to drop the temperature from 90°F to 76°F. That’s a -14°F drop.
• At 40% relative humidity, the swamp cooler will be able to drop the temperature from 90°F to 77°F. That’s a -13°F drop.
• At 45% relative humidity, the swamp cooler will be able to drop the temperature from 90°F to 78°F. That’s a -12°F drop.
• At 50% relative humidity, the swamp cooler will be able to drop the temperature from 90°F to 79°F. That’s a -11°F drop.
• At 55% relative humidity, the swamp cooler will be able to drop the temperature from 90°F to 81°F. That’s a -9°F drop.
• At 60% relative humidity, the swamp cooler will be able to drop the temperature from 90°F to 82°F. That’s a -8°F drop.
• At 65% relative humidity, the swamp cooler will be able to drop the temperature from 90°F to 84°F. That’s a -6°F drop.
• At 70% relative humidity, the swamp cooler will be able to drop the temperature from 90°F to 85°F. That’s a -5°F drop.
• At 75% relative humidity, the swamp cooler will be able to drop the temperature from 90°F to 87°F. That’s a -3°F drop.
• At 80% relative humidity and 90°F, the evaporative coolers are not effective.

You will notice that the drop in temperature evaporative cooler producers are the highest in low humidity levels (at 2% humidity, there is a -26°F drop). At high humidity levels, the drop in temperature becomes less significant (-3°F at 75%). At very high humidity levels, evaporative coolers are not effective; that’s why you have blank spots in the evaporative cooler humidity chart above.

The optimum efficacy of evaporative coolers are temperature-humidity states where the cooler is capable of lowering the temperature anywhere from 70°F to 75°F. In the case of 90°F outdoor temperature, these swamp coolers have optimum efficacy at 20%, 25%, and 30% humidity levels. In the chart for evaporative coolers above, you will see these states are bolded.

Here is the list of temperature-humidity states where the use of a swamp cooler is most effective (optimum efficacy with a drop to 70-75°F temperature):

• At 75°F, the evaporative cooler has the optimum efficiency at 70% – 80% relative humidity.
• At 80°F, the evaporative cooler has the optimum efficiency at 50% – 65% relative humidity.
• At 85°F, the evaporative cooler has the optimum efficiency at 30% – 55% relative humidity.
• At 90°F, the evaporative cooler has the optimum efficiency at 20% – 30% relative humidity.
• At 95°F, the evaporative cooler has the optimum efficiency at 10% – 20%  relative humidity.
• At 100°F, the evaporative cooler has the optimum efficiency at 5% – 10% relative humidity.
• At 105°F, the evaporative cooler has the optimum efficiency at 2% relative humidity.
• At 110°F – 125°F, it is just physically impossible for an evaporative cooler to lower the temperature below 76°F, no matter how dry the air is.

In many cases, it is also useful to see by how many degrees does a swamp cooler lower the temperature. This is usually referred to as evaporative cooler temperature chart. Let’s check this one as well (you can calculate one yourself from the humidity chart as well):

Evaporative Cooler Temperature Chart

This swamp cooler temperature tells us how much can an evaporative cooler lower the temperature at different temperatures and relative humidity levels.

Let’s look at an example of 100°F. If you have the unfortunate of suffering a 100°F heat in the summer, here is how much an evaporative cooler (the effect, as you know, depends heavily on the relative humidity levels):

•  At 2% relative humidity, a swamp cooler can lower the temperature by -31°F.
•  At 5% relative humidity, a swamp cooler can lower the temperature by -29°F.
•  At 10% relative humidity, a swamp cooler can lower the temperature by -27°F.
•  At 15% relative humidity, a swamp cooler can lower the temperature by -24°F.
•  At 20% relative humidity, a swamp cooler can lower the temperature by -22°F.
•  At 25% relative humidity, a swamp cooler can lower the temperature by -20°F.
•  At 30% relative humidity, a swamp cooler can lower the temperature by -18°F.
•  At 35% relative humidity, a swamp cooler can lower the temperature by -17°F.
•  At 40% relative humidity, a swamp cooler can lower the temperature by -15°F.
•  At 45% relative humidity, a swamp cooler can lower the temperature by -13°F.
•  At 50% relative humidity, a swamp cooler can lower the temperature by -12°F.

What about at 60%, 70%, and 80% relative humidity? Unfortunately, if the air is so moist, the evaporative cooler will have little effect in lowering the temperature at 100°F outdoor temperature.

Hopefully, now you understand the relationship between temperature and humidity levels for evaporative coolers. With the evaporative cooler humidity chart and evaporative cooler temperature chart, you can pretty much figure out if it makes sense to use a swamp cooler or not.

If you are still looking for your answer, you can use the comments below; we’ll try to help you out, just give us some insight.

If you need an advice on which evaporative cooler to choose, you can consult this post about the best evaporative coolers on the market.