To find the mean of the given data set, add all the temperatures and divide by the number of data points.

The temperatures are: 89, 80, 83, 89, 91, 87, 84.

1. Add the temperatures:
   $89 + 80 + 83 + 89 + 91 + 87 + 84 = 603$

2. Divide by the number of data points (7):

   $\frac{603}{7} \approx 86.1$

So, the mean temperature is approximately 86.1.