Alto sits near the bottom of U.S. towns for July heat
Nights in January average 44°. Winter and summer sit 28° apart. By January nights alone, it runs milder than 93% of U.S. towns. For walks and days out, the stretch from April to November brings highs of 62–72°.
A year in Alto at a glance
Each month’s typical high and low, and what it brings.
What stands out about Alto’s climate
Compared with towns across the U.S., with Alto’s place in California beside it.
What a year in Alto feels like
Typical highs and lows by season and by month, from NOAA’s 30-year normals.
Frost is rare
Frost and day counts come from NOAA’s station at San Francisco Dwtn, CA, 11 miles from the town’s center, at about the same height.
The 2023 USDA map puts Alto’s center in zone 10a: the coldest night of a typical winter, averaged over 1991–2020, reaches about 31°. Plant labels use the zone; frost dates tell you when to plant.
90° days by month
Average days at 90° or more
About 30 inches of rain a year
December is the wettest month and July the driest. Rain falls on about 96 days a year. Snow is rare: about 0.0" in a normal year at Muir Woods, CA, so most winters see none.
Rain + melted snow by month
Inches of water
Heating and cooling a home in Alto
96% of the work is heating, so the furnace matters far more than the air conditioner. Months averaging under 55° run from December through March.
Degree days add up how far each day’s average falls below (heating) or rises above (cooling) 65° over a year. NOAA normals at San Francisco Dwtn, CA.
Living with Alto’s climate
What the numbers mean for a garden, a free afternoon and a bad-weather day.
Zone 10a. Frost almost never comes, so tender plants can often stay out through winter.
Months with highs of 62–84° and lows of 40° or more: April, May, June, July, August, September, October and November.
Parks & trails →Of the yearly building losses FEMA expects in Alto from natural hazards, river flooding is the largest share (48%); heat waves, cold waves, winter storms and ice storms together are 1%. Normals are averages; the hazards page covers the extremes.
Natural hazards →Alto and its neighbors
Mill Valley the hottest July afternoons (72°, 1° warmer). Differences under a degree are within what the 5 km grid can tell apart. Tap a town to see its climate.
Climates most like Alto’s, farther away
TownRatings’ own comparison of NOAA 1991–2020 normals, not a NOAA product: places of 5,000+ at least 150 miles away, one per state, closest on January low, July high, precipitation and snow.
| Town | Jan. low | July high | Precip. | Snow |
|---|---|---|---|---|
| Alto, CA | 44° | 70° | 30" | 0.0" |
| Port Angeles, WA | 37° | 69° | 28" | 0.4" |
| Makawao, HI | 53° | 75° | 23" | 0.0" |
| Scappoose, OR | 34° | 81° | 41" | 0.8" |
| Nantucket, MA | 26° | 76° | 44" | 25" |
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 Alto averaged 1.4° warmer than the 1900s. The biggest change is in January (+2.2°).
A bigger shift than in 56% 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 Alto compares
Against 12,954 U.S. towns of 2,500 or more (one per 5 km climate cell), with Alto’s place among California’s.
Alto weather by month
Highs, lows and precipitation from NOAA’s grid at Alto’s center; snow, wet days and snow on the ground from Muir Woods, CA. 1991–2020 normals.
| Month | High | Low | Rain + melt | Snow | Wet days | Snow on ground | 30-yr range |
|---|---|---|---|---|---|---|---|
| January | 56° | 44° | 5.8" | 0.0" | 14 | 0 | 47° – 55° |
| February | 59° | 46° | 5.8" | 0.0" | 13 | 0 | 48° – 58° |
| March | 61° | 47° | 4.2" | 0.0" | 12 | 0 | 49° – 59° |
| April | 63° | 48° | 2.0" | 0.0" | 9 | 0 | 52° – 59° |
| May | 66° | 50° | 0.9" | 0.0" | 5 | 0 | 54° – 63° |
| June | 70° | 52° | 0.2" | 0.0" | 4 | 0 | 58° – 63° |
| July | 70° | 54° | 0.0" | 0.0" | 2 | 0 | 59° – 65° |
| August | 72° | 55° | 0.1" | 0.0" | 3 | 0 | 61° – 67° |
| September | 72° | 55° | 0.1" | 0.0" | 3 | 0 | 60° – 68° |
| October | 69° | 52° | 1.4" | 0.0" | 6 | 0 | 58° – 66° |
| November | 62° | 48° | 3.3" | 0.0" | 10 | 0 | 49° – 58° |
| December | 56° | 44° | 6.5" | 0.0" | 15 | 0 | 47° – 55° |
| Year | 65° | 50° | 30.3" | 0.0" | 96 | 0 |
How much a month can swing
A normal is an average. March swings the most: from 1991 to 2020 its average ranged from 49° to 59°. June is the most reliable, staying within 5°.
Where Alto’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 Alto’s center.
- Snow: Muir Woods, CA.
- Frost, freezing nights, 90° and 100° days, heating and cooling: San Francisco Dwtn, CA, 11 miles from the town’s center, at about the same height; the nearest station that passes our checks for distance, height and agreement with the grid.
- 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 Alto’s climate
What is the weather like in Alto?
A typical July afternoon reaches 70°, a January night drops to 44°, and snow is almost unheard of. Milder January nights than 93% of U.S. towns; cooler July afternoons than all but 83 U.S. towns.
How cold does it get in Alto in winter?
January nights average 44°. A night at 32° or below comes in only some winters (0.0 a year on average). In the coldest January of the last 30 years the month averaged 47°.
How much snow does Alto get?
About 0.0 inches in a normal winter at Muir Woods, CA, so most winters see none.
How hot does it get in Alto?
July, with a typical high of 70° and a low of 54°. About 3 days a year reach 90° (San Francisco Dwtn, CA).
When is the first and last frost in Alto?
Most years have no frost at all: a 32° night comes in about 0% of winters at San Francisco Dwtn, CA, usually in December or January.
What growing zone is Alto in?
Zone 10a on the 2023 USDA Plant Hardiness Zone Map: the coldest night of a typical winter reaches about 31°.
What is the best time of year to visit Alto?
By one simple rule (highs of 62–84° and lows of 40° or more), April, May, June, July, August, September, October and November. The rule is ours, not NOAA’s; the month table has the numbers to judge by.
Is Alto getting warmer?
Yes: 1991–2020 averaged 1.4° warmer than 1901–2000 on NOAA’s grid. A bigger shift than 56% of U.S. towns.
Neighbors talk weather
Gardens, snow, and the best days of the year in Alto.
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.