How to calculate weather days

Counting weather days means going through a month one day at a time and asking whether the weather met the rules in your contract. The steps are simple. The details, such as what to do when a reading sits exactly on the threshold, are where counts differ. This guide is general information, not legal advice.

Write down the rules exactly

Copy the wording from your contract instead of working from memory. For each type of weather, note the number and the words around it:

• Rain. “At least 0.10 inch” and “more than 0.10 inch” are different rules.
• Cold and heat. “Below 32°F” and “at or below 32°F” are different rules.
• Wind. Note the speed, and whether the contract means sustained wind or a gust. The National Weather Service explains sustained wind in its glossary.

If your contract gives a number of weather days per month that the schedule already allows, write that down too.

Choose the weather station

Weather is measured at stations, not at your site. Pick the station closest to the job that has a complete record for the month, and note its name and distance. If two people choose different stations, they will get different counts. NOAA keeps these daily station records in a database called GHCN-Daily.

Go through the month one day at a time

For each day, compare the station’s daily reading with each rule. In WeatheredOut, a day potentially qualifies if any rule you turned on is met. Your contract may combine rules differently, for example by requiring two conditions together, so check before you rely on a count.

Watch for the edge cases

• Exactly on the threshold. The words decide. “At least” includes the number and “more than” does not. WeatheredOut counts wind only when it is strictly above your number.
• Trace rain. A trace is an amount too small to measure. WeatheredOut counts trace precipitation as 0.00 inch and flags the day.
• Missing readings. If a station reported no value for a day, the day cannot be counted either way. WeatheredOut lists it as insufficient data.
• Multi-day totals. Sometimes a station reports one total that covers several days. WeatheredOut marks those days as needing review instead of counting them.

A worked example

These numbers are made up to show the method. They are not real observations.

Suppose the rule is that a day counts when precipitation is at least 0.10 inch. A station reports this for seven days:

That is 3 potential qualifying weather days out of 7. If the rule said “more than 0.10 inch”, Day 3 would not count and the total would be 2.

Compare the count with your baseline

If your contract lists a number of weather days per month that the schedule already allows, subtract it from your count. WeatheredOut calls that number the baseline and shows the result as potential days over the baseline, never below zero. With 3 potential qualifying days and a baseline of 5, the result is 0. With 8 days and a baseline of 5, it is 3.

The baseline can be one number you enter, a table with a number for each month, or the average of the same month over the previous 5 or 10 years at the same weather station. If an average is not a whole number, you choose whether a half rounds up, rounds down, or the number is left as it is, because contracts differ.

Whether those days change your schedule or your price is decided by your contract, not by the count. The guide on anticipated adverse weather explains how contracts tend to word the allowance.

Common questions