arduino weather shield and wind speed
When debugging a problem like this one, print out values of the variables that the calculation depends on.

In this case, you may find that both windClicks and deltaTime are 0 or 0.0.  The IEEE 754 floating-point-operations specification dictates a result of NaN (not-a-number) for various zero over zero calculations.  It dictates a result of +Inf or -Inf for calculations like 7/0.0 or -13/0.0.  [See, for example, the stackoverflow question, Why does division by zero with floating point (or double precision) numbers not throw an exception.]

If arithmetic generates a NaN or +Inf result, subsequent arithmetic using that result will also produce NaN or +Inf results.

To deal with the problem, you could use deltaTime &gt; 0 as a required pre-condition for calculation of windSpeed.  That is, have an if-statement protecting the calculation block.