1. Descending Air: At the horse latitudes (around 30° North and South), air from the upper atmosphere descends. As this air descends, it compresses and warms up. This warming process prevents the formation of clouds and precipitation, leading to a dry environment.
2. Wind Patterns: The trade winds blow from the subtropical high-pressure belts towards the equator. In the northern hemisphere, they blow northeast to southwest, and in the southern hemisphere, they blow southeast to northwest. In both cases, these winds move away from the landmasses, making them offshore winds that don't bring moisture. The westerlies, which are on-shore and can bring rain, blow towards the poles, outside the desert regions.
3. Low Relative Humidity: The constant high pressure and descending air create conditions where the air can hold a lot of moisture, but there is very little moisture available. This results in very low relative humidity, which means the air is extremely dry. With low humidity, condensation is not possible, so clouds and rain cannot form.