American Meteorological Society Certification (AMS) Practice

A high pressure system is typically associated with descending air, which leads to clearer skies and generally fair weather. As the air descends, it warms up and dries out, resulting in lower humidity levels. This phenomenon occurs because the cooling effect of rising air, which can lead to condensation and cloud formation, is absent in high pressure systems. Therefore, over time, a persistent high pressure system will likely lead to reduced humidity in the atmosphere.

In contrast, rainfall and cloud formation are more characteristic of low pressure systems, where air rises and cools, promoting moisture accumulation and precipitation. While increased temperatures can also occur in high pressure systems due to the warming of descending air, this answer specifically highlights the significant and long-term impact of such systems, which is the reduction of humidity.