Midwinter lows in Rocky Boy's Agency sink below those of 93% of U.S. towns
Midsummer highs settle at about 80°. The seasons are far apart: from a winter night to a summer afternoon the average spans 70°. Its July afternoons stay fresher than in 91% of U.S. towns. The frost-free window for a garden runs roughly May 17 to September 25.
A year in Rocky Boy's Agency at a glance
Each month’s typical high and low, and what it brings.
What stands out about Rocky Boy's Agency’s climate
Compared with towns across the U.S., with Rocky Boy's Agency’s place in Montana beside it.
What a year in Rocky Boy's Agency feels like
Typical highs and lows by season and by month, from NOAA’s 30-year normals.
About 128 frost-free days a year
In half of years the last 32° night comes by May 17 and the first by September 25. Frost dates and degree days from Big Sandy, MT, 16 miles away.
The 2023 USDA map puts Rocky Boy's Agency’s center in zone 4a: the coldest night of a typical winter, averaged over 1991–2020, reaches about −25°. Plant labels use the zone; frost dates tell you when to plant.
Freezing nights by month
Average nights at or below 32°
90° days by month
Average days at 90° or more
About 18 inches of rain a year
June is the wettest month and December the driest.
Rain + melted snow by month
Inches of water
Heating and cooling a home in Rocky Boy's Agency
94% of the work is heating, so the furnace matters far more than the air conditioner. Months averaging under 55° run from September through May.
Degree days add up how far each day’s average falls below (heating) or rises above (cooling) 65° over a year. NOAA normals at Big Sandy, MT.
Living with Rocky Boy's Agency’s climate
What the numbers mean for a garden, a free afternoon and a bad-weather day.
128 frost-free days in zone 4a. The last frost comes by May 17 in half of years, so many gardeners wait a week or two past that to set out tomatoes. The first fall frost comes by Sep 25.
Months with highs of 62–84° and lows of 40° or more: June, July, August and September.
Parks & trails →Cold waves are 71% of the yearly building losses FEMA expects in Rocky Boy's Agency from natural hazards, the largest share. Normals are averages; the hazards page covers the extremes.
Natural hazards →Rocky Boy's Agency and its neighbors
Every town next door is within a degree of Rocky Boy's Agency in January and July, closer than the 5 km grid can tell apart. Tap a town to see its climate.
Temperatures and rain come from NOAA’s 5 km grid, so each town gets its own value. Snow and frost-free days come from the nearest weather station, so neighbors often share them.
Warmer than it used to be
The last 30 years in Rocky Boy's Agency averaged 1.7° warmer than the 1900s. The biggest change is in January (+5.1°).
A bigger shift than in 72% of U.S. towns.
Change by month
The two periods overlap (1991–2000 is in both), which makes the gap a little smaller than it would be between separate periods.
How Rocky Boy's Agency compares
Against 12,954 U.S. towns of 2,500 or more (one per 5 km climate cell), with Rocky Boy's Agency’s place among Montana’s.
Rocky Boy's Agency weather by month
Highs, lows and precipitation from NOAA’s grid at Rocky Boy's Agency’s center. 1991–2020 normals.
| Month | High | Low | Rain + melt | 30-yr range |
|---|---|---|---|---|
| January | 32° | 11° | 0.7" | 8° – 35° |
| February | 34° | 12° | 0.7" | −2° – 38° |
| March | 43° | 20° | 0.7" | 14° – 40° |
| April | 53° | 29° | 1.5" | 35° – 45° |
| May | 62° | 38° | 2.8" | 46° – 56° |
| June | 71° | 45° | 3.5" | 55° – 63° |
| July | 80° | 50° | 2.1" | 56° – 72° |
| August | 80° | 49° | 1.4" | 59° – 70° |
| September | 69° | 41° | 1.5" | 50° – 61° |
| October | 54° | 31° | 1.5" | 35° – 49° |
| November | 42° | 21° | 0.9" | 19° – 41° |
| December | 33° | 13° | 0.6" | 9° – 33° |
| Year | 54° | 30° | 17.7" |
How much a month can swing
A normal is an average. February swings the most: from 1991 to 2020 its average ranged from -2° to 38°. June is the most reliable, staying within 8°.
Where Rocky Boy's Agency’s numbers come from
Normals are 30-year averages, not a forecast. How TownRatings reads climate data →
- Temperatures and precipitation: NOAA’s gridded 1991–2020 normals for the 5 km cell at Rocky Boy's Agency’s center.
- Frost dates, heating and cooling: Big Sandy, MT, 16 miles away.
- Growing zone: USDA Plant Hardiness Zone Map, 2023. USDA Agricultural Research Service and PRISM Group, Oregon State University, prism.oregonstate.edu/phzm, accessed 8 Oct 2026.
- Then and now: NOAA’s gridded normals for 1901–2000 at the same cell.
- Storms and extremes: FEMA National Risk Index (expected annual losses to buildings).
Common questions about Rocky Boy's Agency’s climate
What is the weather like in Rocky Boy's Agency?
A typical July afternoon reaches 80°, a January night drops to 11°. Colder January nights than 93% of U.S. towns; cooler July afternoons than 91% of U.S. towns.
How cold does it get in Rocky Boy's Agency in winter?
January nights average 11°. About 180 nights a year reach 32° or below and 29 reach 0° or below. In the coldest January of the last 30 years the month averaged 8°.
How hot does it get in Rocky Boy's Agency?
July, with a typical high of 80° and a low of 50°. About 28 days a year reach 90° (Big Sandy, MT).
When is the first and last frost in Rocky Boy's Agency?
In half of years the last spring frost (32°) comes by May 17 and the first fall frost by September 25, about 128 frost-free days, at Big Sandy, MT.
What growing zone is Rocky Boy's Agency in?
Zone 4a on the 2023 USDA Plant Hardiness Zone Map: the coldest night of a typical winter reaches about −25°.
What is the best time of year to visit Rocky Boy's Agency?
By one simple rule (highs of 62–84° and lows of 40° or more), June, July, August and September. The rule is ours, not NOAA’s; the month table has the numbers to judge by.
Is Rocky Boy's Agency getting warmer?
Yes: 1991–2020 averaged 1.7° warmer than 1901–2000 on NOAA’s grid. A bigger shift than 72% of U.S. towns.
Neighbors talk weather
Gardens, snow, and the best days of the year in Rocky Boy's Agency.
Source: NOAA NCEI U.S. Climate Normals, 1991–2020 and 1901–2000; USDA/PRISM 2023 hardiness zones; FEMA National Risk Index. Today’s forecast: National Weather Service.