why does a desert cooler cool better on a hot dry day ?

A desert cooler, also known as an evaporative cooler, has the ability to cool better on a hot dry day because of the principle of evaporative cooling.

Here's how it works:

1. Water is stored in the cooler's reservoir or tank.
2. A pump circulates this water onto a cooling pad or medium made of porous material.
3. A fan draws in the hot, dry air from outside and passes it through the wet cooling pad.
4. As the air moves through the pad, it comes in contact with the water present on the pad's surface.
5. The hot air absorbs moisture from the pad through evaporation.
6. The evaporation process requires energy, which is taken from the surrounding air, resulting in a drop in temperature.
7. The cooled air is then blown out into the room or area.

Now, let's see why a desert cooler performs better on a hot dry day:

1. Dry air has a higher capacity to absorb moisture compared to humid air. When the hot, dry air enters the cooler and passes through the wet cooling pad, it can absorb more water molecules, increasing the evaporation process.
2. The faster rate of evaporation leads to a greater reduction in temperature, resulting in cooler air being blown into the room.
3. On hot dry days, when the humidity is low, the evaporative cooling effect is more efficient since the dry air can hold more water vapor, and the cooling pad can easily evaporate the moisture.
4. In contrast, on humid days, when the air already contains a significant amount of moisture, the cooling pad has less capacity to evaporate water. This reduces the efficiency of the evaporative cooling process and results in less effective cooling.

So, the drier the air, the better the desert cooler performs due to a more efficient evaporation process and greater temperature reduction.

I assume you are referring to those bags hooked on the front of 18 wheelers. Water evaporates better on hot dry days and evaporation is the secret of cooling. It works with the human body also. We sweat, it evaporates, we get cool.