Climate and average weather in Altadena
In Altadena, the summers are hot, dry, and clear and the winters are mild and mostly clear.
Over the course of the year, the temperature in Altadena typically varies from 43°F to 89°F, and is rarely below 35°F or above 97°F. August is the hottest month of the year, while December is the coldest.
Based on the travel score, the best times of year to visit Altadena for warm-weather activities are from late March to early July and mid September to late November.
- Hottest
- Aug
- 89°
- Coldest
- Dec
- 43°
- Wettest
- Feb
- 3.7 in
- Driest
- Aug
- 0.0 in
- Rain / yr
- 15.3 in
- 37 wet days
- Daylight
- 9.7–14.3 h
- shortest–longest
- Elevation
- 1,358 ft
- 34.2°N
- Clearest
- Aug
- 11% cloud
Altadena is census-designated place in Los Angeles County, California, United States.
- Density
- 1,892/km²
- people per km²
- Founded
- 1887
Air quality: WHO Ambient Air Quality Database. Density & description: Wikidata (CC0), GeoNames.
Altadena weather by month, at a glance
The whole year in one picture — sky, rain, humidity, temperature and travel score. Click any band to jump to its full chart.
Each stripe is one measure across the whole year, stacked so a single day reads top to bottom. The sky band runs overcast grey on top down to clear blue at the base — more blue means sunnier — followed by the rainfall curve, the humidity band, a temperature strip that tints each stretch of the year by how warm its average high is, and the travel-score line at the foot. Scan straight down any date to read that day across every measure at once; the score climbs highest around late March to early July.
Temperature through the year
Daily average highs and lows in Altadena, from 1991–2020 of climate data. The shaded bands show the range most days fall in — 10th to 90th percentile.
The hot season lasts 3.5 months, from June 18 to October 2, with daily high temperatures above 82°F. The hottest month of the year in Altadena is August, with an average high of 89°F and low of 65°F.
The cool season lasts 3.6 months, from November 25 to March 12, with daily high temperatures below 66°F. The coldest month of the year in Altadena is December, with an average low of 43°F and high of 62°F.
Days typically run about 21°F warmer than nights year-round.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Avg high (°F) | 62 | 63 | 66 | 71 | 75 | 81 | 87 | 89 | 85 | 78 | 69 | 62 |
| Avg low (°F) | 44 | 44 | 46 | 49 | 54 | 59 | 64 | 65 | 62 | 56 | 48 | 43 |
| Avg high (°F) | Avg low (°F) | |
|---|---|---|
| Jan | 62 | 44 |
| Feb | 63 | 44 |
| Mar | 66 | 46 |
| Apr | 71 | 49 |
| May | 75 | 54 |
| Jun | 81 | 59 |
| Jul | 87 | 64 |
| Aug | 89 | 65 |
| Sep | 85 | 62 |
| Oct | 78 | 56 |
| Nov | 69 | 48 |
| Dec | 62 | 43 |
Average hourly temperature in Altadena
A compact picture of the whole year, hour by hour: each column is a day, each row an hour, and the color is the modeled average temperature. Darker overlays mark night, lighter ones the civil-twilight hours around sunrise and sunset.
Read left to right for the day of the year and top to bottom for the hour of the day; the color is the modeled temperature, cool blues through to hot reds on the same scale as the other charts. A vertical slice is a single day's swing — coolest just before dawn, warmest in the mid-afternoon — while a horizontal slice follows one hour across the seasons. The darker overlay marks night and the lighter one the twilight around sunrise and sunset. Colors run warmest on August afternoons, near 89°F, and coldest before dawn in December, near 43°F.
Altadena weather by month
Every month at a glance. Click a month for its detailed page.
| Month | High / Low (°F) | Rainfall (inches) | Wet days | Daylight (hours) | Travel score |
|---|---|---|---|---|---|
| Jan | 62° / 44° | 3.4 | 5.8 | 10 | 6.1 |
| Feb | 63° / 44° | 3.7 | 6.7 | 10.7 | 6.1 |
| Mar | 66° / 46° | 2.5 | 6.1 | 11.7 | 6.9 |
| Apr | 71° / 49° | 1.0 | 3.6 | 12.8 | 7.9 |
| May | 75° / 54° | 0.5 | 2.3 | 13.8 | 8.8 |
| Jun | 81° / 59° | 0.1 | 0.5 | 14.2 | 8.4 |
| Jul | 87° / 64° | 0.1 | 0.6 | 14 | 6.4 |
| Aug | 89° / 65° | 0.0 | 0.2 | 13.3 | 5.9 |
| Sep | 85° / 62° | 0.2 | 0.9 | 12.2 | 7.0 |
| Oct | 78° / 56° | 0.7 | 2.3 | 11.1 | 8.7 |
| Nov | 69° / 48° | 0.9 | 3 | 10.2 | 7.9 |
| Dec | 62° / 43° | 2.3 | 5.2 | 9.8 | 6.1 |
Chance of rain
How likely it is to rain on a typical day, and how that shifts across the wetter and drier seasons.
Altadena gets about 15.3 in of precipitation a year. On a typical day the chance of rain is mostly dry, but that rises and falls sharply with the seasons.
The wetter season lasts 5.0 months, from around Nov 22 to Apr 21, peaking in February at about 27% chance on a given day. The drier stretch runs the rest of the year, bottoming out around Aug 14.
February is the wettest month by volume, with 3.7 in over about 7 wet days; August is the driest, with 0.0 in.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Rain on a day (%) | 19 | 24 | 20 | 12 | 7 | 2 | 2 | 1 | 3 | 7 | 10 | 17 |
| Rain days | 6 | 7 | 6 | 4 | 2 | 1 | 1 | 0 | 1 | 2 | 3 | 5 |
| Rain on a day (%) | Rain days | |
|---|---|---|
| Jan | 19 | 6 |
| Feb | 24 | 7 |
| Mar | 20 | 6 |
| Apr | 12 | 4 |
| May | 7 | 2 |
| Jun | 2 | 1 |
| Jul | 2 | 1 |
| Aug | 1 | 0 |
| Sep | 3 | 1 |
| Oct | 7 | 2 |
| Nov | 10 | 3 |
| Dec | 17 | 5 |
Monthly rainfall
Over a full year, Altadena collects about 15.3 in of rain across 37 wet days — roughly 1.3 in in an average month.
It doesn’t fall evenly. February is the wettest month with 3.7 in, while August is the driest at just 0.0 in. The three wettest months alone bring about 63% of the year’s total.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Rainfall (in) | 3.4 | 3.7 | 2.5 | 1.0 | 0.5 | 0.1 | 0.1 | 0.0 | 0.2 | 0.7 | 0.9 | 2.3 |
| Rain days | 6 | 7 | 6 | 4 | 2 | 1 | 1 | 0 | 1 | 2 | 3 | 5 |
| Rainfall (in) | Rain days | |
|---|---|---|
| Jan | 3.4 | 6 |
| Feb | 3.7 | 7 |
| Mar | 2.5 | 6 |
| Apr | 1.0 | 4 |
| May | 0.5 | 2 |
| Jun | 0.1 | 1 |
| Jul | 0.1 | 1 |
| Aug | 0.0 | 0 |
| Sep | 0.2 | 1 |
| Oct | 0.7 | 2 |
| Nov | 0.9 | 3 |
| Dec | 2.3 | 5 |
Clouds and sunshine
How often the daytime sky is clear, partly cloudy or overcast.
In Altadena, the average share of the sky covered by clouds shows significant seasonal variation over the course of the year.
The clearer part of the year begins around May 16 and lasts for 5.8 months, ending around November 9. The clearest month is August, when the sky is clear, mostly clear, or partly cloudy 95% of the time on average.
The cloudier part of the year begins around November 10 and lasts for 6.1 months, ending around May 15. The cloudiest month is February, when it is overcast or mostly cloudy 42% of the time.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Clearer (%) | 60 | 58 | 62 | 71 | 78 | 89 | 92 | 95 | 93 | 85 | 75 | 64 |
| Cloudier (%) | 40 | 42 | 38 | 29 | 22 | 11 | 8 | 5 | 7 | 15 | 25 | 36 |
| Clearer (%) | Cloudier (%) | |
|---|---|---|
| Jan | 60 | 40 |
| Feb | 58 | 42 |
| Mar | 62 | 38 |
| Apr | 71 | 29 |
| May | 78 | 22 |
| Jun | 89 | 11 |
| Jul | 92 | 8 |
| Aug | 95 | 5 |
| Sep | 93 | 7 |
| Oct | 85 | 15 |
| Nov | 75 | 25 |
| Dec | 64 | 36 |
Daylight, sunrise and sunset
Hours of daylight through the year, with sunrise and sunset times.
The gold band is daylight — sunrise runs along its lower edge, sunset along the upper — so it widens toward summer and pinches toward winter, with night the dark area above and below. The longest day, around June 14, brings about 14 h 18 min of light; the shortest, around December 13, only 9 h 42 min — a swing of roughly 4 h 36 min.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Daylight (h) | 10.0 | 10.7 | 11.7 | 12.8 | 13.8 | 14.2 | 14.0 | 13.3 | 12.2 | 11.1 | 10.2 | 9.8 |
| Sunrise (AM) | 7:03 | 6:45 | 6:10 | 5:27 | 4:56 | 4:45 | 4:56 | 5:18 | 5:41 | 6:04 | 6:32 | 6:57 |
| Sunset (PM) | 5:00 | 5:29 | 5:54 | 6:18 | 6:42 | 7:01 | 7:00 | 6:35 | 5:53 | 5:11 | 4:43 | 4:41 |
| Daylight (h) | Sunrise (AM) | Sunset (PM) | |
|---|---|---|---|
| Jan | 10.0 | 7:03 | 5:00 |
| Feb | 10.7 | 6:45 | 5:29 |
| Mar | 11.7 | 6:10 | 5:54 |
| Apr | 12.8 | 5:27 | 6:18 |
| May | 13.8 | 4:56 | 6:42 |
| Jun | 14.2 | 4:45 | 7:01 |
| Jul | 14.0 | 4:56 | 7:00 |
| Aug | 13.3 | 5:18 | 6:35 |
| Sep | 12.2 | 5:41 | 5:53 |
| Oct | 11.1 | 6:04 | 5:11 |
| Nov | 10.2 | 6:32 | 4:43 |
| Dec | 9.8 | 6:57 | 4:41 |
Sunny days
How many genuinely sunny days each month brings — days the sky stays clear or mostly clear — banded into plain-language zones.
Altadena averages about 20 sunny days a month — counting the days the sky stays clear or mostly clear — with a clear seasonal swing between 12 and 28 days.
The sunnier stretch runs 5.5 months, from around May 23 to Nov 6. The sunniest month is August, with roughly 28 sunny days (brilliant).
The greyest month is February, down near 13 sunny days — about 52% fewer than the August peak.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Sunny days | 15 | 12 | 14 | 17 | 20 | 25 | 27 | 28 | 26 | 24 | 19 | 16 |
| Sky | Mixed | Mixed | Mixed | Bright | Bright | Brilliant | Brilliant | Brilliant | Brilliant | Brilliant | Bright | Bright |
| Sunny days | Sky | |
|---|---|---|
| Jan | 15 | Mixed |
| Feb | 12 | Mixed |
| Mar | 14 | Mixed |
| Apr | 17 | Bright |
| May | 20 | Bright |
| Jun | 25 | Brilliant |
| Jul | 27 | Brilliant |
| Aug | 28 | Brilliant |
| Sep | 26 | Brilliant |
| Oct | 24 | Brilliant |
| Nov | 19 | Bright |
| Dec | 16 | Bright |
Humidity comfort
Share of days at each humidity comfort level, based on the dew point — the measure that tracks how muggy it actually feels.
Humidity comfort is graded by the dew point — the temperature at which the air would be fully saturated — because it tracks how muggy it actually feels far better than relative humidity does. A low dew point feels dry and crisp; a high one feels sticky and oppressive no matter the reading on a humidity gauge.
In Altadena, the air stays dry or comfortable essentially all year. Even in July, the muggiest month, it feels humid or worse only about 14% of the time — humidity is not something you need to plan around here.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Dry (%) | 100 | 100 | 100 | 100 | 92 | 73 | 43 | 50 | 71 | 91 | 99 | 100 |
| Comfortable (%) | 0 | 0 | 0 | 0 | 8 | 26 | 44 | 39 | 24 | 8 | 1 | 0 |
| Humid (%) | 0 | 0 | 0 | 0 | 0 | 1 | 13 | 11 | 5 | 1 | 0 | 0 |
| Muggy (%) | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 |
| Dry (%) | Comfortable (%) | Humid (%) | Muggy (%) | |
|---|---|---|---|---|
| Jan | 100 | 0 | 0 | 0 |
| Feb | 100 | 0 | 0 | 0 |
| Mar | 100 | 0 | 0 | 0 |
| Apr | 100 | 0 | 0 | 0 |
| May | 92 | 8 | 0 | 0 |
| Jun | 73 | 26 | 1 | 0 |
| Jul | 43 | 44 | 13 | 1 |
| Aug | 50 | 39 | 11 | 0 |
| Sep | 71 | 24 | 5 | 0 |
| Oct | 91 | 8 | 1 | 0 |
| Nov | 99 | 1 | 0 | 0 |
| Dec | 100 | 0 | 0 | 0 |
Wind
Average wind speed through the year, banded into plain-language speed zones so a number reads as 'breezy' or 'calm' without knowing the scale.
This is the average sustained wind speed at the standard 10 metres above open ground — the steady flow a weather station reports, not the sharper gusts that come and go, which can run half again as fast. It's the number that says whether a place is generally breezy or calm.
Wind in Altadena is steady company: average speeds sit near 4 mph — calm — in every month of the year, with no meaningful windy or calm season.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Wind speed (mph) | 5 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | 5 |
| Feels | calm | calm | calm | calm | calm | calm | calm | calm | calm | calm | calm | calm |
| Wind speed (mph) | Feels | |
|---|---|---|
| Jan | 5 | calm |
| Feb | 4 | calm |
| Mar | 4 | calm |
| Apr | 4 | calm |
| May | 4 | calm |
| Jun | 4 | calm |
| Jul | 4 | calm |
| Aug | 4 | calm |
| Sep | 4 | calm |
| Oct | 4 | calm |
| Nov | 4 | calm |
| Dec | 5 | calm |
Best time to visit Altadena
To characterize how pleasant the weather is in Altadena throughout the year, we compute two travel scores — one for general sightseeing, one for beach and pool weather.
Around early April, Altadena is still near the start of its best-weather window — great conditions with the peak-season crowds not yet at full swing.
Inferred from the travel-score curve; month-by-month visitor numbers are a planned addition.
The travel score favors clear, rainless days with daytime highs between 64°F and 82°F. Based on this score, the best times of year to visit Altadena for general outdoor tourist activities run from late March to early July and again from mid September to late November, with a peak score in the last week of May.
The travel score (filled area), and its constituents: the temperature score (red line), the cloud cover score (blue line), and the precipitation score (green line).
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Travel score (0–10) | 6.1 | 6.1 | 6.8 | 7.9 | 8.8 | 8.4 | 6.4 | 5.9 | 7.0 | 8.6 | 7.9 | 6.1 |
| Rating | good | good | good | ideal | ideal | ideal | good | good | good | ideal | ideal | good |
| Travel score (0–10) | Rating | |
|---|---|---|
| Jan | 6.1 | good |
| Feb | 6.1 | good |
| Mar | 6.8 | good |
| Apr | 7.9 | ideal |
| May | 8.8 | ideal |
| Jun | 8.4 | ideal |
| Jul | 6.4 | good |
| Aug | 5.9 | good |
| Sep | 7.0 | good |
| Oct | 8.6 | ideal |
| Nov | 7.9 | ideal |
| Dec | 6.1 | good |
The beach/pool score favors clear, rainless days with daytime highs between 75°F and 93°F. Based on this score, the best time of year to visit Altadena for hot-weather activities is from late May to late October, with a peak score in the last week of June.
The beach/pool score (filled area), and its constituents: the temperature score (red line), the cloud cover score (blue line), and the precipitation score (green line).
What to pack for Altadena, by month
Month by month, what's worth putting in the bag — from the temperature range, rain chance, sun and wind. Tap a month for a quick packing note.
For a warm-season trip around August (highs near 89°F), the essentials are light clothes and sun protection. In the cold season around December (highs near 62°F), pack a warm layer.
In August, Altadena sees daytime highs around 89°F and nights near 65°F. Pack light clothes and sun protection.
Sea temperature near Altadena
Average ocean temperature at the nearest coast, month by month, banded into swim-comfort zones — the other half of the beach question.
The sea at the nearest coast (about 26 mi away) is warmest in August at about 68°F and coldest in February near 59°F.
It's comfortable for a long swim from Aug 4 to Sep 13 — about 1.3 months when the water holds above 68°F.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Sea temperature (°F) | 59 | 59 | 59 | 59 | 62 | 65 | 67 | 68 | 68 | 66 | 63 | 60 |
| Comfort | Bracing | Bracing | Bracing | Cool | Cool | Cool | Cool | Refreshing | Refreshing | Cool | Cool | Cool |
| Sea temperature (°F) | Comfort | |
|---|---|---|
| Jan | 59 | Bracing |
| Feb | 59 | Bracing |
| Mar | 59 | Bracing |
| Apr | 59 | Cool |
| May | 62 | Cool |
| Jun | 65 | Cool |
| Jul | 67 | Cool |
| Aug | 68 | Refreshing |
| Sep | 68 | Refreshing |
| Oct | 66 | Cool |
| Nov | 63 | Cool |
| Dec | 60 | Cool |
UV index (midday) in Altadena by month
Clear-sky midday UV index through the year, shaded by the WHO exposure levels — how much sun protection each month calls for.
Under clear skies, the midday UV index in Altadena peaks around July 11 at about 11 (extreme) — sunscreen, a hat and midday shade are essential. It bottoms out near December 16 at about 3 (moderate).
The index reaches 3 or more — the level where the WHO recommends sun protection — in 11 months of the year.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| UV index (midday) | 4 | 5 | 5 | 8 | 8 | 11 | 11 | 10 | 8 | 6 | 3 | 3 |
| Level | Moderate | Moderate | Moderate | Very high | Very high | Extreme | Extreme | Extreme | Very high | High | Moderate | Moderate |
| UV index (midday) | Level | |
|---|---|---|
| Jan | 4 | Moderate |
| Feb | 5 | Moderate |
| Mar | 5 | Moderate |
| Apr | 8 | Very high |
| May | 8 | Very high |
| Jun | 11 | Extreme |
| Jul | 11 | Extreme |
| Aug | 10 | Extreme |
| Sep | 8 | Very high |
| Oct | 6 | High |
| Nov | 3 | Moderate |
| Dec | 3 | Moderate |
Average hourly UV index in Altadena
Modeled clear-sky UV for every hour of every day — when in the day sun protection actually matters.
The whole year hour by hour: each column is a day, each row an hour, and the color is the modeled clear-sky UV index — the midday peak scaled down as the sun sits lower in the sky. Darker overlays mark night, lighter ones the civil-twilight hours around sunrise and sunset.
UV falls off far faster than brightness as the sun drops, so even in July the burn window is concentrated in the middle of the day: about 8.5 hours a day reach index 3 (protection recommended), and 6 hours reach 6 or more. Mornings and late afternoons stay comparatively gentle year round.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| UV ≥ 3 (h/day) | 2.5 | 4 | 5 | 7 | 7.5 | 8.5 | 8.5 | 8 | 7 | 5 | 2.5 | 0 |
| UV ≥ 6 (h/day) | 0 | 0 | 0 | 3.5 | 4 | 6 | 6 | 5.5 | 4 | 0 | 0 | 0 |
| UV ≥ 3 (h/day) | UV ≥ 6 (h/day) | |
|---|---|---|
| Jan | 2.5 | 0 |
| Feb | 4 | 0 |
| Mar | 5 | 0 |
| Apr | 7 | 3.5 |
| May | 7.5 | 4 |
| Jun | 8.5 | 6 |
| Jul | 8.5 | 6 |
| Aug | 8 | 5.5 |
| Sep | 7 | 4 |
| Oct | 5 | 0 |
| Nov | 2.5 | 0 |
| Dec | 0 | 0 |
Air quality in Altadena by month
Average fine-particle pollution (PM2.5) through the year — so you can see which months the air is worst, not just the yearly average.
Air quality in Altadena is worst in December — around 29 µg/m³ PM2.5 (moderate), about 2.4× the cleanest month (May, 12 µg/m³).
Modeled monthly averages (CAMS reanalysis, 2015–2024).
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| PM2.5 (µg/m³) | 18 | 17 | 17 | 15 | 12 | 14 | 25 | 26 | 24 | 18 | 23 | 29 |
| Level | Moderate | Moderate | Moderate | Moderate | Good | Moderate | Moderate | Moderate | Moderate | Moderate | Moderate | Moderate |
| PM2.5 (µg/m³) | Level | |
|---|---|---|
| Jan | 18 | Moderate |
| Feb | 17 | Moderate |
| Mar | 17 | Moderate |
| Apr | 15 | Moderate |
| May | 12 | Good |
| Jun | 14 | Moderate |
| Jul | 25 | Moderate |
| Aug | 26 | Moderate |
| Sep | 24 | Moderate |
| Oct | 18 | Moderate |
| Nov | 23 | Moderate |
| Dec | 29 | Moderate |
Growing season
Altadena is frost-free year round — even the coldest nights rarely reach freezing, so there is no start or end to the growing season.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Frost-free chance (%) | 99 | 99 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 98 |
| Frost nights | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| Frost-free chance (%) | Frost nights | |
|---|---|---|
| Jan | 99 | 0 |
| Feb | 99 | 0 |
| Mar | 100 | 0 |
| Apr | 100 | 0 |
| May | 100 | 0 |
| Jun | 100 | 0 |
| Jul | 100 | 0 |
| Aug | 100 | 0 |
| Sep | 100 | 0 |
| Oct | 100 | 0 |
| Nov | 100 | 0 |
| Dec | 98 | 0 |
Growing degree days
Growing degree days tally heat above 50°F: each day adds however many degrees its average temperature runs over that threshold, and cold days add nothing. Altadena banks about 4,920 degree days a year, and the running total — which growers use to time plantings and predict maturity — climbs fastest around August.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Growing degree days (°F·days) | 86 | 98 | 193 | 295 | 452 | 602 | 791 | 835 | 712 | 521 | 260 | 74 |
| Growing degree days (°F·days) | |
|---|---|
| Jan | 86 |
| Feb | 98 |
| Mar | 193 |
| Apr | 295 |
| May | 452 |
| Jun | 602 |
| Jul | 791 |
| Aug | 835 |
| Sep | 712 |
| Oct | 521 |
| Nov | 260 |
| Dec | 74 |
Solar energy
Average daily shortwave solar energy reaching the ground, banded into brightness zones so the number means something at a glance.
Solar energy is how much sunlight actually reaches the ground each day, measured in kilowatt-hours per square metre (kWh/m²) — the combined effect of how long the sun is up, how high it climbs, and how much cloud gets in the way. It's the raw fuel a solar panel has to work with: for scale, 1 kWh/m² a day is dim and wintry, while 7+ is desert-bright.
Altadena receives about 5.7 kWh/m² a day of solar energy on average — bright — with a clear seasonal swing between 2.9 and 8.2 kWh/m².
The brighter stretch of the year lasts 6.3 months, from around Mar 22 to Sep 28. The brightest month is June, averaging about 8.1 kWh/m² (intense).
The dimmest month is December, down near 2.9 kWh/m² — roughly 64% less than the June peak.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Solar energy (kWh/m²) | 3.1 | 4.0 | 5.4 | 6.8 | 7.6 | 8.2 | 7.9 | 7.4 | 6.2 | 4.8 | 3.6 | 3.0 |
| Level | Moderate | Moderate | Bright | Intense | Intense | Intense | Intense | Intense | Intense | Bright | Moderate | Moderate |
| Solar energy (kWh/m²) | Level | |
|---|---|---|
| Jan | 3.1 | Moderate |
| Feb | 4.0 | Moderate |
| Mar | 5.4 | Bright |
| Apr | 6.8 | Intense |
| May | 7.6 | Intense |
| Jun | 8.2 | Intense |
| Jul | 7.9 | Intense |
| Aug | 7.4 | Intense |
| Sep | 6.2 | Intense |
| Oct | 4.8 | Bright |
| Nov | 3.6 | Moderate |
| Dec | 3.0 | Moderate |
Where is Altadena?
Altadena sits at 34.2°N, 118.1°W, 1,358 ft above sea level, in United States. The nearest listed city is Pasadena, 3.1 mi away.
Topography around Altadena
How much the land rises and falls, and what covers it, within 2, 10 and 50 miles of Altadena.
Within 2 miles, the terrain around Altadena has large variations in elevation — a maximum elevation change of 2,904 ft around an average of 1,493 ft above sea level; within 10 miles, very significant variations in elevation — a maximum elevation change of 5,932 ft; within 50 miles, extreme variations in elevation — a maximum elevation change of 10,013 ft.
The land around Altadena is covered by trees (43%) and artificial surfaces (40%) within 2 miles, artificial surfaces (35%), trees (31%) and shrubs (29%) within 10 miles, and artificial surfaces (24%) and grassland (21%) within 50 miles.
United States travel facts
- Power plugs
- Type A / B
- 120 V · 60 Hz
- Prices
- ≈ 100% of US
- price level, 2025
- GDP per capita
- $90,027
- 2025
- Happiness
- 6.8 / 10
- World Happiness, 2025
- Safe drinking water
- 98%
- of population, 2024
- Homicide rate
- 5.8
- per 100k, 2023
- Road deaths
- 12.7
- per 100k/yr, 2019
- Income group
- High
- World Bank
- Religion
Sources: World Bank, US State Department, World Happiness Report, WHO/UNICEF, EF EPI, CIA World Factbook.
Data sources
Temperature, precipitation, cloud, humidity, wind and solar figures are a 1991–2020 climatology for Altadena (34.1897°, -118.1312°, elevation 1,358 ft), computed from hourly ERA5 reanalysis — contains modified Copernicus Climate Change Service information. Sun figures are astronomical calculations, not observations. City data © GeoNames, CC BY 4.0. Map outlines from Natural Earth (public domain); interactive map © Google. Sea temperatures are the NOAA OISST v2.1 1991–2020 monthly climatology (public domain), sampled at the nearest ocean point. UV index is derived from CAMS (Copernicus Atmosphere Monitoring Service) clear-sky UV data — contains modified Copernicus Atmosphere Monitoring Service information. Monthly air-quality (PM2.5) seasonality is from the CAMS EAC4 reanalysis (Copernicus Atmosphere Monitoring Service), scaled to the WHO annual mean where available. Topography is sampled from the Copernicus DEM GLO-90 elevation model (© ESA/Airbus) and ESA WorldCover 2021 land cover (CC BY 4.0).