Highlighting Greater Than Average Dry Durations

Heather has a worksheet that, in column A, has a series of dates. These represent the dates during the past 25 years on which a weather station noted at least 0.01 inches of precipitation. Thus, the duration between two consecutive dates is a "dry duration." Heather would like to highlight any dates in the list where the dry duration is greater than the average dry duration for all dates in the list.

The easiest way to do this is to add another column to your data. This one will be used to store the dry durations. Assuming your dates are sorted and in column A (beginning with cell A2), then place the following formula in cell B3:

=A3-A2

Copy this formula down for as many rows as necessary, and you now have all the dry durations. Now you can create a conditional formatting rule that will do the evaluation. Follow these steps:

1. Select all your dates, beginning with cell A3. (I'll assume that you are selecting A3:A4750.)
2. Make sure the Home tab of the ribbon is displayed.
3. Click the Conditional Formatting tool, within the Styles group. Excel displays a palette of options related to conditional formatting.
4. Click New Rule. Excel displays the New Formatting Rule dialog box.
5. In the Select a Rule Type area at the top of the dialog box, choose Use a Formula to Determine Which Cells to Format.
6. In the Format Values Where This Formula Is True box, enter the following: =B3>AVERAGE(B3:B4750). This formula will return True if the cell next to the date contains a value that is larger than the average dry duration.
7. Click Format to display the Format Cells dialog box.
8. Make any changes to the font and/or cells to highlight True values, as desired.
9. Click OK to close the Format Cells dialog box.
10. Click OK to close the New Formatting Rule dialog box. Excel applies the conditional format to the selected cells in column A.

If you don't want to add the dry duration column to your data, then you will benefit by using a macro to do the formatting, rather than using a conditional format. The following macro removes any explicit formatting from the cells in the range A2 through whatever is the last cell in the column. It then adds a helper column (B) and calculates the dry durations, placing them in that column. It then calculates the average of those dry periods and, if the dry period is greater than the average, it formats the date associated with that dry period as bold and red. Finally, the macro deletes the helper column it previously added.

Sub DryPeriod()
    Dim RowNum As Long
    Dim MaxRow As Long
    Dim AvDuration As Long

    Application.ScreenUpdating = False
    MaxRow = ActiveSheet.UsedRange.Rows.Count
    With Range("A2:A" & MaxRow).Font
        .ColorIndex = xlAutomatic
        .Bold = False
    End With

    Range("B1").EntireColumn.Insert
    For RowNum = 3 To MaxRow
        If Cells(RowNum, 1) > "" Then
            Cells(RowNum, 2) = Cells(RowNum, 1) - Cells(RowNum - 1, 1)
        End If
    Next RowNum

    AvDuration = Application.WorksheetFunction.Average(Range("B3:B" & MaxRow))
    For RowNum = 3 To MaxRow
        If Cells(RowNum, 2) > AvDuration Then
            Cells(RowNum, 1).Font.Color = vbRed
            Cells(RowNum, 1).Font.Bold = True
        End If
    Next RowNum

    Range("B1").EntireColumn.Delete
    Application.ScreenUpdating = True
End Sub

You can run the macro as many times as you would like, which is why it first sets the formatting in column A to the default each time it is run.

ExcelTips is your source for cost-effective Microsoft Excel training. This tip (8923) applies to Microsoft Excel 2007, 2010, 2013, 2016, 2019, 2021, and Excel in Microsoft 365.