What is the major factor increasing the temperature of death valley when prevailing winds are relatively dry?

The very high temperatures in the Death Valley desert are influenced by few factors:
1. It's shape, long and narrow.
2. It's 68 meters below see level.
3. It is surrounded by high mountains, which traps the hot air.
4. The vegetation is very sparse, and the air very clear and dry, which allows the sunlight to heat the desert's surface.

Exposed great deposits of coal and iron ore