Union, Missouri climate: Normal weather by month, frost dates and records
Union's normal year runs from 23°F January nights to 91°F July afternoons, with 45.0 inches of precipitation and 14 inches of snow a year. (NOAA 1991-2020 normals)
In metric: a yearly mean of 13.8°C, January lows of −5°C, July highs of 33°C, 1,142 mm of precipitation a year.
- NOAA stationUnion
- Station IDUSC00238515
- Location38.44°N 91.00°W
- Elevation540 ft (165 m)
- From the centerunder 1 mile
- Population13,198
On October 23, the normal high in Union is 68°F and the normal low 43°F, and the chance of measurable rain or snow is 28%.
Normals describe a typical year at the station, not a forecast. The National Weather Service forecast for Union.
- Jan 43° 23°
- Feb 48° 26°
- Mar 59° 35°
- Apr 70° 45°
- May 78° 55°
- Jun 86° 63°
- Jul 91° 67°
- Aug 89° 66°
- Sep 82° 57°
- Oct 70° 46°
- Nov 57° 35°
- Dec 46° 27°
Each month: normal high (large) and low, °F. Fill: the month's normal mean, colder blue to hotter red.
What is the weather like in Union each month?
July is the warmest month, with normal highs of 91°F; January the coldest, with normal lows of 23°F. The most precipitation falls in April (4.99 in), the least in February (2.45 in).
| Month | High | Low | Mean | Precip. | Snow | Wet days | 90°F days | Freezing nights | Daylight |
|---|---|---|---|---|---|---|---|---|---|
| January | 43°F | 23°F | 33.0°F | 2.72 | 4.7 | 9 | <1 | 25 | 9 h 50 min |
| February | 48°F | 26°F | 37.2°F | 2.45 | 4.1 | 7 | <1 | 20 | 10 h 40 min |
| March | 59°F | 35°F | 47.2°F | 3.87 | 1.8 | 10 | <1 | 14 | 11 h 50 min |
| April | 70°F | 45°F | 57.7°F | 4.99 | 0.2 | 11 | <1 | 4 | 13 h 10 min |
| May | 78°F | 55°F | 66.5°F | 4.85 | 0.0 | 12 | 1 | <1 | 14 h 10 min |
| June | 86°F | 63°F | 74.7°F | 4.24 | 0.0 | 10 | 9 | <1 | 14 h 50 min |
| July | 91°F | 67°F | 79.0°F | 4.74 | 0.0 | 8 | 17 | <1 | 14 h 40 min |
| August | 89°F | 66°F | 77.6°F | 3.84 | 0.0 | 8 | 16 | <1 | 13 h 40 min |
| September | 82°F | 57°F | 69.3°F | 3.63 | 0.0 | 8 | 5 | <1 | 12 h 30 min |
| October | 70°F | 46°F | 57.9°F | 3.31 | 0.0 | 8 | 1 | 3 | 11 h 20 min |
| November | 57°F | 35°F | 46.1°F | 3.66 | 0.7 | 9 | <1 | 13 | 10 h 10 min |
| December | 46°F | 27°F | 36.7°F | 2.66 | 2.9 | 9 | <1 | 22 | 9 h 30 min |
| Year (annual) | 68°F | 45°F | 56.9°F | 44.96 | 14.4 | 108 | 50 | 101 |
The same normals in °C and millimetres
| Month | High | Low | Mean | Precip. | Snow |
|---|---|---|---|---|---|
| January | 6°C | −5°C | 0.6°C | 69 mm | 12 cm |
| February | 9°C | −3°C | 2.9°C | 62 mm | 10 cm |
| March | 15°C | 2°C | 8.4°C | 98 mm | 5 cm |
| April | 21°C | 7°C | 14.3°C | 127 mm | 1 cm |
| May | 26°C | 13°C | 19.2°C | 123 mm | 0 cm |
| June | 30°C | 17°C | 23.7°C | 108 mm | 0 cm |
| July | 33°C | 20°C | 26.1°C | 120 mm | 0 cm |
| August | 32°C | 19°C | 25.3°C | 98 mm | 0 cm |
| September | 28°C | 14°C | 20.7°C | 92 mm | 0 cm |
| October | 21°C | 8°C | 14.4°C | 84 mm | 0 cm |
| November | 14°C | 2°C | 7.8°C | 93 mm | 2 cm |
| December | 8°C | −3°C | 2.6°C | 68 mm | 7 cm |
Source: NOAA NCEI 1991-2020 monthly normals, station USC00238515; annual normals. Wet days: 0.01 inch or more; 90°F days and freezing nights: NOAA's day counts. Daylight: sunrise to sunset on the 15th, computed from the latitude.
When is the best weather to visit Union?
Normal highs sit between 65°F and 85°F in April, May, September and October. Of those, September has the fewest days with rain (8).
That is a measurement, not a recommendation: it counts the months whose normal afternoon high sits between 65°F and 85°F, where a light layer is enough by day.
Source: NOAA NCEI 1991-2020 monthly normals, station USC00238515.
When is the last frost in Union?
In Union, the last spring frost (32°F) comes after April 11 in half of years and after April 28 in one year in ten; the first autumn frost comes before October 23 in half of years and before October 9 in one year in ten.
Frost-free in half of yearsFrost in 1 to 9 years out of 10Frost most years
- Last spring frost, half of years after
- April 111 year in 10 after April 28; 9 in 10 after March 26
- First autumn frost, half of years before
- October 231 year in 10 before October 9; 9 in 10 before November 5
NOAA puts the median frost-free season at 194 days, and the season without a hard freeze (28°F) at 217 days.
A hard freeze (28°F) comes after March 31 in half of springs and before November 3 in half of autumns.
The USDA's 2023 Plant Hardiness Zone Map puts Union in zone 6b in ZIP code 63084. By the map's own method, the average of each year's coldest night over 1991-2020, Union's NOAA station is in zone 6b: the coldest night of a typical year reaches −3°F (18 complete years).
Frost dates for every NOAA station in Missouri · Frost dates by ZIP code
Source: NOAA NCEI 1991-2020 annual normals (frost probabilities), station USC00238515; USDA 2023 Plant Hardiness Zone Map, ZIP listings: PRISM Group, Oregon State University, https://prism.oregonstate.edu, accessed October 8, 2026; the station zone: ACIS daily lows, 1991-2020. A 10% date means one year in ten has a frost after it in spring or before it in autumn.
How hot and how cold does it get in Union?
In a normal year in Union, 50 days a year reach 90°F (4 of them 100°F or more), 101 nights fall to 32°F or colder and 108 days bring measurable precipitation.
- Days at 90°F or more
- 50a normal year
- Days at 100°F or more
- 4a normal year
- Nights at 32°F or colder
- 101a normal year
- Days that stay below freezing
- 14high of 32°F or less
- Days with 0.01 inch of precipitation
- 108a normal year
- Days with 1 inch of snow or more
- 5a normal year
The highest temperature in this station's record (since 1937) is 118°F, on July 14, 1954; the lowest is −26°F, on January 17, 1977.
The wettest calendar year on that record is 1993, with 66.85 in; the snowiest season is 1959–60, with 49.0 inches.
Source: NOAA NCEI 1991-2020 annual normals, station USC00238515; records: ACIS (NOAA Regional Climate Centers), station USC00238515. Records use complete months only; a month missing more than three days is left out.
How sunny is Union?
Over a year 75% of the sunlight that would reach the ground under clear skies gets through the clouds in the area around Union, 4.2 kWh per square metre a day. July is the sunniest month (83%) and February the cloudiest (66%).
| Month | Share of clear-sky sunshine | Sunshine, kWh/m² a day |
|---|---|---|
| January | 67% | 2.2 |
| February | 66% | 2.9 |
| March | 67% | 3.8 |
| April | 72% | 5.0 |
| May | 73% | 5.7 |
| June | 81% | 6.5 |
| July | 83% | 6.4 |
| August | 81% | 5.6 |
| September | 81% | 4.8 |
| October | 75% | 3.4 |
| November | 72% | 2.4 |
| December | 67% | 1.9 |
Source: NASA POWER solar climatology, 2001-2020 (CERES SYN1deg). Satellite-based, for the 1-degree cell (about 100 km across) around the station, so it describes the area rather than the airport.
How much heating and cooling does Union need?
A normal year in Union brings 4,400 heating degree days, 18% fewer than the median U.S. place (5,373), and 1,485 cooling degree days, 64% more than the median (908). Five months need 300 or more heating degree days, a monthly mean of about 55°F or colder. For gardens and crops, NOAA counts 4,333 growing degree days a year above 50°F.
| Month | Heating | Cooling |
|---|---|---|
| January | 993 | 0 |
| February | 779 | 1 |
| March | 558 | 6 |
| April | 254 | 35 |
| May | 72 | 117 |
| June | 5 | 297 |
| July | 0 | 434 |
| August | 1 | 391 |
| September | 42 | 173 |
| October | 249 | 29 |
| November | 568 | 3 |
| December | 879 | 0 |
| Year (annual) | 4,400 | 1,485 |
Source: NOAA NCEI 1991-2020 monthly normals, station USC00238515. A degree day is one degree of a day's mean temperature below 65°F (heating) or above it (cooling); utilities use them to estimate energy use.
How does Union's climate compare?
Union is warmer than 108 of the other 133 places with their own NOAA record in Missouri, and wetter than 76. Across the 7,247 U.S. stations with normals, its annual mean of 56.9°F is warmer than 66%.
In the Köppen classification computed from these normals, Union is Cfa: temperate, no dry season, hot summer. What the classes mean.
Places with the most similar year elsewhere
Measured month by month on the normal high, low and precipitation, among places of 20,000 or more people in other states:
Source: NOAA NCEI 1991-2020 normals for every station PlainClimate covers. Similarity: the root-mean-square difference of the 36 monthly figures, precipitation weighted so an inch counts like 6°F.
Which places near Union share this climate record?
9 places have no NOAA station with 1991-2020 normals of their own, and this station is the nearest full record. A place's own climate can differ from these figures, by several degrees where it sits much higher or lower than the station, nearer the coast, or inside a large city's heat; for the larger places the next nearest records are named with their yearly mean, so the spread around them is visible, and places of 50,000 or more have their own page with NOAA's gridded normals.
- Sullivan, MO6,925 people · 18 mi SW
- St. Clair, MO4,748 people · 7 mi S
- Villa Ridge, MO2,994 people · 7 mi E
- Lake St. Clair, MO670 people · 8 mi S
- Parkway, MO642 people · 8 mi S
- Oak Grove Village, MO405 people · 17 mi SW
- Stanton, MO307 people · 13 mi SW
- Miramiguoa Park, MO123 people · 15 mi S
- Charmwood, MO16 people · 12 mi SW
Source: Census Bureau 2024 Gazetteer (place centers) and Vintage 2024 population estimates. Distance and direction from the station to each place's center.
Questions about Union's climate
What is the coldest month in Union?
January, with a normal high of 43°F and a normal low of 23°F. The warmest is July (91°F by day, 67°F at night).
Does it snow in Union?
Yes: 14 inches in a normal year, falling on 10 days with 0.1 inch or more, the most in January (4.7 inches).
What is the rainiest month in Union?
April, with 4.99 in of precipitation in a normal year; the driest is February with 2.45 in.
What plant hardiness zone is Union in?
The USDA's 2023 map puts Union in zone 6b in ZIP code 63084. By the same method at Union's NOAA station, the coldest night of a typical year reaches −3°F (18 years, 1991-2020): zone 6b.
How late can frost come in Union?
In half of years the last 32°F night comes after April 11; in one year in ten it comes after April 28. Waiting until April 28 leaves about a one-in-ten chance of a later frost.
Nearby and next
- Missouri climateThe statewide normals and every place in the state
- Washington, MO7 miles from this station, 54.8°F a year
- Rosebud, MO20 miles from this station, 53.0°F a year
- St Louis Spirit of St. Louis Airport, MO24 miles from this station, 56.1°F a year
- Weldon Spring, MO25 miles from this station, 55.2°F a year
- Compare two placesMonth by month, side by side
Download these normals (CSV) · JSON · How these figures are made