Clouds do not precipitate, or release rain or snow, in deserts primarily because of low humidity.

Low humidity means that the air in the desert contains very little moisture. When there is not enough moisture in the air, clouds cannot form. Clouds are formed when warm, moist air rises and cools, causing the moisture to condense into tiny water droplets or ice crystals. In deserts, where the air is dry and lacks moisture, this process does not occur.

It is important to understand that the other factors mentioned in the options - low pressure, high wind velocity, and high temperature - can influence the formation and behavior of clouds but are not the primary reason why clouds do not precipitate in deserts.

Low pressure systems can contribute to the formation of clouds, but their absence alone does not explain the lack of precipitation in deserts. Similarly, high wind velocity can affect cloud formation and development, but it is not the main reason why clouds do not produce rain in deserts. High temperature can increase evaporation and reduce the relative humidity, which can contribute to the lack of precipitation in deserts, but it is not the sole factor responsible.

In conclusion, low humidity is the primary factor that prevents clouds from precipitating in deserts.