Below is the code for the weather example.
Lines 36 read the input from the worksheet.
Lines 8  10 initialize the variables.
The "for" loop beginning on line 16 runs the main iterations of the model.
The "if" block beginning on line 17 sets the value of rain, snow, or clear
according to the value of the Rnd() function.
The "if" block beginning on line 26 is triggered each time the refresh interval
is reached. It uses the Mod function (modulo division). Modulo
division returns the remainder after dividing one number by another. So,
for instance, if we wanted to refresh the screen every 50 iterations, we would
wait for the total number of iterations to be evenly divisible by 50. As
the program iterates, the function "CurrentIteration Mod Interval" would return
the following values:
Iteration 98: 
98 Mod 50 = 48 
Iteration 99:

99 Mod 50 = 49 
Iteration 100: 
100 Mod 50 = 0 
So when Iteration reaches 100, the formula evaluated by the If statement is
TRUE, and the refresh interval code is executed. The same occurs again at
Iterations 150, 200, and so on.
Lines 27  30 simply update the output section of the worksheet using methods
we've described before. What's new here is lines 32  37. These
cause the program to pause execution for 1/10th of one second and explicitly
allow other processes on the computer to execute. This 1/10th of a second
is enough time to allow the worksheet to refresh the bar chart, but is short
enough that it only adds a total of 1 second to the overall execution time
assuming refresh intervals equal to 1/10 the total number of iterations.
We'll explain that code section in a little more detail below.
Here's the explanation of lines 32  37:

Line 32 simply declares the two
variables used, and is not really an essential line.

Line 33 establishes the length of
time, in seconds, that the program execution will be paused, and stores that
value in the PauseTime variable.

Line 34 reads the value of VBA's
internal timer and stores it in the variable named Start.

The Do Loop in lines 35  37 keeps
repeating as long as the time lapsed since reading the Timer is less than
the established PauseTime.

Line 36, "DoEvents", tells the
computer it can perform other tasks while the current function is paused.

Once the PauseTime has lapsed, the
program exits the Loop on line 37, and on line 40 returns to process the
next simulation iteration.
