Hot temperatures and a lack of rainfall and conditions for vegetative growth result in a dry, barren environment as you would expect for a southern desert.
The climate in most deserts is quite hot and dry during much of the summer. However, during the monsoon season in the southwest United States from about June 15 - September 30 the humidity will frequently be higher than normal.
California
Yes, there are a number of dry lakes, especially in the desert regions of southern California.
No, some deserts are cold all year long. Other deserts have a pronounced winter season when it can get quite cold. All deserts experience a period of cooler weather during the winter.
The air mass that typically produces warm, dry conditions in the summer is the continental tropical (cT) air mass. This air mass originates over hot, dry land areas, such as deserts, and is characterized by high temperatures and low humidity. When it moves into regions during the summer, it can lead to hot and dry weather, often resulting in heat waves and arid conditions.
That depends upon the specific desert and season of the year. In deserts of the American southwest, the humidity may drop to less than 10 percent in the dry winter but may be 50-60 percent (or more) during the summer rainy season.
Highlands are warm and dry during the summer, but, cool and wet, during the winter.
Only in California
All deserts are dry. That's why they are called deserts.
All deserts are, by definition, relatively dry.
The mountain ranges in the Central Highlands (Pleiku/Kontum areas) can get pretty dry. Just like the forests in California...nice and wet during winter...dry and a fire danger in the summer.
Southern California is home to part of the Sonoran and Mojave Deserts. Even areas that are not "desert' are relatively dry in the southern half of California.