An evaporative cooler, more commonly known as a swamp cooler or desert cooler, can generally be used for outdoor cooling as long as it is set-up correctly and rated for safe use in the kind of location where you will be using it. But some are easier to use for outdoor cooling than others.
Avoid styles that have air vents coming out the bottom of the unit. These are designed to be permanently mounted to the roof of a building with cool air vents leading from the unit down into the building.
Look instead for coolers with air vents coming out the side, or out of the top with air flow adaptes than can direct the air flow where you need it. Also look for features such as wheels that can make them easier to move around and use outdoors.
Swamp coolers are generally classified by how they work or by their size.
Single-stage are the most common and least expensive to purchase. They create more humidity than two-stage units -- as much as 70 - 80% relative humidity. This means their cooling will be most effective in only the driest climates and weather. The chart below shows the range of cooling you can expect from a single-stage unit.
Single-stage units with "direct" technology use the most basic technology: warm air blowing through the pads and the cooled air blowing directly out of the unit. These are the least expensive and easiest to find. Single-stage units with "indirect" technology add a heat exchanger to the unit. This may lower the amount of humidity produced by the unit, but it can also lower the cooling somewhat.
A two-stage swamp cooler (also called "indirect-direct") produces less humidity than a single-stage unit -- around 50 - 70% relative humidity. But because of the way it's made, this does not lower the cooling effect. In fact, the air from these can be up to 6°F / 3.5°C cooler than air from a direct cooler. These are more expensive than similar units that use direct cooling, and also require more maintenance. They are mostly available in larger units, but this technology will gradually make its way down to smaller portable swamp coolers, too.
In the first (indirect) stage the warm air goes through a cooling chamber where it is pre-cooled. In the second (direct) stage this pre-cooled air is blown through the water soaked pads and then into the area being cooled. Because the air going through the pads is already cooled a bit, it needs less moisture to lower it to the desired temperature. Since it picks up less moisture, it picks up less humidity to provide the same degree of cooling as a single-stage unit. Less humidity means it can cool in a wider range of weather conditions than a single-stage unit.
Another fairly new technology uses dessicants (materials such as silica) that remove moisture from the air. Fresh air first passes through this material to dehumidify it before going through the single-stage or two-stage cooling process.