The weather map in Figure 1 shows a contour map of the temperature function for
the states of California and Nevada at 3:00 PM on a day in October. The level curves, or
isothermals, join locations with the same temperature. The partial derivative at a location
such as Reno is the rate of change of temperature with respect to distance if we travel east
from Reno; is the rate of change of temperature if we travel north. But what if we want
to know the rate of change of temperature when we travel southeast (toward Las Vegas), or
in some other direction? In this section we introduce a type of derivative, called a directional
derivative, that enables us to find the rate of change of a function of two or more
variables in any direction.