Answer
Weather temperature is increasing.
Weather temperature is decreasing.
Weather temperature is constant.
Work Step by Step
In general, definition of the slope is simply rate of change. It represents rate at which something changes for given period.
If the slope of the line is positive, then the temperature is increasing by certain rate as time goes by.
If the slope of the line is negative, then the temperature is decreasing by certain rate.
And if the slope is zero, it means that the temperature is constant. The function has a form of $y=b$, that is straight, constant line with no change in $y$.