How to Read Historical Weather Data: A Beginner's Guide
Understanding climate averages, percentiles, and historical records can be confusing. This guide explains what the numbers mean and how to use them for planning.
Averages vs. Normals
When you see "average high temperature" for a month, that's typically a 30-year average - the mean of all daily high temperatures recorded during that month over three decades. The National Weather Service updates these "Climate Normals" every 10 years (currently using 1991-2020). This site uses Open-Meteo data covering 1994-2023.
What Does "Average High" Actually Mean?
The average high temperature for July in Phoenix might be 106°F. This doesn't mean every July day reaches exactly 106°F - it's the mean of all July highs over 30 years. In practice, some days will be 102°F, others 112°F. About half of all July days will be hotter than 106°F and half cooler.
Understanding Record Highs and Lows
Record highs and lows represent the single most extreme reading ever observed at a location. A record high of 118°F in Phoenix doesn't mean you'll experience that on your trip - but it tells you the upper bound of what's physically possible at that location.
Rain Days vs. Total Rainfall
"Average rain days" counts days with measurable precipitation (typically 0.01 inches or more). A location might have 10 rain days in July but only 1.5 inches total - meaning many light showers. Another location might have 3 rain days but 4 inches total - meaning infrequent but heavy storms.
How to Use This Data for Planning
For outdoor events, look at both rain days and average high temperatures for the month. A location with 8 rain days in May but average highs of 72°F is probably more pleasant than one with 3 rain days but average highs of 95°F. The ideal is low rain days, comfortable temperatures (65-80°F for most people), and low humidity.
Sunshine Hours Explained
Sunshine duration is measured in hours per day when direct solar radiation exceeds a threshold (120 W/m²). A "sunny day" in meteorological terms means at least 6-8 hours of direct sunshine. Cloudy days reduce this significantly even without rain.