Rain and snow
Read the monthly rain chart, the wet and dry months, and snow on the ground, and know what these numbers can't size.
Rain, month by month #
The first card starts with a sentence: about how much it rains in a year, the month the wet season peaks, and the driest month. Below it, a bar for each month shows liquid precipitation in inches.
The line under the chart says where the numbers come from. Rainfall uses the best the station has:
- A multi-year average, when the station's record has one. Thin whiskers on each bar run from that month's driest year to its wettest.
- A typical-year composite, one sample year for each month, when it doesn't.
If a station recorded no precipitation, the card says so. Pick a nearby airport station instead. See When a station has no data.
Snow #
The Snow card shows the average depth of snow on the ground each month, in inches, and the months it sits there. When the record has it, the sentence also gives the deepest cover in the typical year.
Snow on the ground isn't snowfall, and it isn't a roof snow load. Ground snow load comes from ASCE 7 (the ASCE Hazard Tool) and your local building department.
If the station recorded effectively no snow cover, or no snow data at all, the card says that instead.
The numbers #
Under the snow chart:
- Annual precipitation, in inches.
- Wettest month and Driest month, with their totals.
- Avg relative humidity across the year, when the station has it.
What these numbers are for #
Monthly totals show the wet season, for planning when to schedule site work and where water will stand. They don't size gutters, scuppers or roof drains: those use the design rainfall intensity (the 100-year, 1-hour rate in IPC §1106 and NOAA Atlas 14), set by your plumbing engineer or the local code.
Where snow shows up, keep roof valleys and entries out of the paths where snow slides off.
The charts follow the Colors setting above the study. See Chart colors.