San Francisco
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Francisco clear sky chart from 2005-11-06 to 2025-09-04.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	7% 6% 2 5% 3 3 3 3 5% 9% 55%	134606
	Night	9% 8% 3 6% 4 3 3 3 5% 6% 49%	131555
Transparency	Day	29% 4 8% 20% 24% 15%	129519
	Night	36% 3 8% 19% 21% 13%	127633
Seeing	Day	10% 12% 29% 32% 14% 3	16502
	Night	12% 7% 17% 33% 26% 5%	25786

By Month

Percentage Cloud Cover SW (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	13% 6% 2 5% 4 4 4 4 6% 9% 45%	3933
	11th to 20th	Day	13% 7% 3 6% 4 4 3 2 3 5% 49%	4100
	21st to end	Day	16% 10% 3 7% 4 4 4 3 4 6% 40%	4510
February	1st to 10th	Day	17% 7% 2 5% 3 4 4 3 4 6% 45%	1800
	11th to 20th	Day	13% 8% 2 6% 4 4 4 4 6% 9% 41%	1825
	21st to end	Day	11% 7% 2 5% 4 3 3 4 5% 7% 48%	1650
March	1st to 10th	Day	12% 9% 3 8% 5% 5% 5% 4 6% 8% 35%	4908
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 5% 8% 49%	5000
	21st to end	Day	11% 7% 2 6% 5% 5% 5% 5% 7% 10% 37%	5529
April	1st to 10th	Day	11% 7% 3 6% 3 4 4 4 5% 8% 47%	2400
	11th to 20th	Day	8% 5% 2 6% 3 4 4 4 5% 8% 53%	2369
	21st to end	Day	4 5% 2 4 3 3 3 3 6% 10% 57%	2400
May	1st to 10th	Day	4 3 1 3 2 1 2 2 3 6% 75%	5676
	11th to 20th	Day	3 3 1 3 2 2 2 2 4 9% 69%	5922
	21st to end	Day	2 5% 1 3 2 2 2 2 4 8% 70%	6902
June	1st to 10th	Day	5% 6% 2 4 3 3 3 3 5% 10% 56%	2800
	11th to 20th	Day	1 5% 1 3 2 2 2 2 6% 8% 67%	2800
	21st to end	Day	3 6% 2 4 2 2 3 2 4 9% 62%	2800
July	1st to 10th	Day	4 2 4 2 2 2 2 3 8% 71%	6287
	11th to 20th	Day	1 6% 2 5% 2 3 3 2 4 10% 60%	5880
	21st to end	Day	1 15% 3 6% 3 3 3 2 5% 12% 46%	6402
August	1st to 10th	Day	1 6% 2 5% 3 3 4 3 6% 11% 57%	5777
	11th to 20th	Day	1 6% 2 5% 2 2 2 2 5% 10% 62%	5560
	21st to end	Day	1 5% 2 5% 3 3 3 3 6% 12% 57%	5981
September	1st to 10th	Day	2 4 2 4 3 2 2 2 4 9% 66%	2317
	11th to 20th	Day	4 7% 3 5% 4 3 3 2 5% 9% 55%	2280
	21st to end	Day	3 3 2 4 3 2 2 1 3 7% 69%	2002
October	1st to 10th	Day	3 3 1 3 2 2 3 2 5% 7% 69%	3180
	11th to 20th	Day	6% 6% 2 5% 3 2 3 2 4 6% 60%	3161
	21st to end	Day	6% 5% 2 5% 3 3 3 3 6% 7% 58%	3448
November	1st to 10th	Day	7% 6% 2 4 3 3 3 3 5% 8% 56%	1940
	11th to 20th	Day	11% 9% 2 6% 5% 3 3 3 4 8% 47%	1800
	21st to end	Day	10% 7% 1 4 4 4 3 2 5% 7% 53%	1615
December	1st to 10th	Day	8% 4 2 4 3 3 4 3 6% 10% 52%	2620
	11th to 20th	Day	21% 8% 2 8% 5% 4 4 4 7% 8% 29%	3028
	21st to end	Day	25% 10% 3 6% 3 4 4 4 5% 7% 29%	4004
January	1st to 10th	Night	10% 6% 3 5% 4 3 4 3 5% 9% 48%	6023
	11th to 20th	Night	13% 7% 2 7% 4 4 4 3 5% 5% 46%	5880
	21st to end	Night	15% 9% 3 6% 4 4 4 3 4 6% 42%	6468
February	1st to 10th	Night	17% 7% 2 4 4 3 3 3 4 6% 47%	2600
	11th to 20th	Night	11% 8% 3 6% 3 4 4 4 6% 8% 43%	2460
	21st to end	Night	11% 6% 1 4 4 3 4 3 5% 6% 51%	1980
March	1st to 10th	Night	13% 10% 3 8% 4 4 4 4 5% 6% 38%	4984
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 6% 50%	5000
	21st to end	Night	14% 9% 2 6% 4 3 4 3 4 7% 42%	5432
April	1st to 10th	Night	9% 7% 3 7% 4 3 3 3 5% 7% 48%	2000
	11th to 20th	Night	9% 8% 2 5% 3 3 3 2 4 7% 54%	1979
	21st to end	Night	4 5% 2 5% 3 3 3 3 5% 8% 59%	2000
May	1st to 10th	Night	4 5% 2 4 3 2 2 2 3 4 71%	4295
	11th to 20th	Night	2 6% 3 8% 4 3 3 3 4 6% 58%	3982
	21st to end	Night	2 5% 1 3 2 2 2 3 3 5% 70%	3972
June	1st to 10th	Night	4 7% 3 7% 4 4 4 3 5% 9% 48%	1600
	11th to 20th	Night	2 5% 2 5% 4 3 2 2 4 7% 63%	1600
	21st to end	Night	3 8% 3 7% 3 3 3 2 4 7% 58%	1600
July	1st to 10th	Night	7% 4 9% 4 3 2 3 3 6% 60%	3655
	11th to 20th	Night	1 14% 7% 12% 5% 4 3 2 4 6% 42%	4100
	21st to end	Night	2 31% 5% 8% 4 3 3 3 4 5% 33%	4472
August	1st to 10th	Night	2 14% 5% 9% 4 4 4 5% 5% 6% 43%	4035
	11th to 20th	Night	1 13% 3 8% 3 3 3 2 4 5% 54%	4300
	21st to end	Night	1 8% 4 9% 5% 5% 5% 4 6% 6% 48%	5015
September	1st to 10th	Night	1 6% 3 5% 3 3 3 3 5% 8% 61%	1930
	11th to 20th	Night	4 11% 3 7% 3 3 4 3 5% 8% 49%	1900
	21st to end	Night	3 6% 2 6% 3 2 3 2 4 8% 61%	2171
October	1st to 10th	Night	3 3 2 4 2 3 2 3 4 6% 67%	3800
	11th to 20th	Night	5% 10% 3 6% 3 3 3 4 4 6% 55%	3777
	21st to end	Night	7% 5% 2 5% 3 3 3 3 5% 6% 58%	4121
November	1st to 10th	Night	7% 7% 2 6% 4 3 3 3 4 8% 55%	2355
	11th to 20th	Night	9% 8% 3 4 3 3 4 3 5% 8% 51%	2600
	21st to end	Night	10% 5% 2 5% 4 3 3 3 5% 7% 54%	2781
December	1st to 10th	Night	9% 4 2 5% 3 3 4 3 5% 7% 56%	4560
	11th to 20th	Night	22% 9% 3 7% 4 4 5% 4 6% 8% 29%	5176
	21st to end	Night	24% 8% 3 7% 4 4 5% 3 5% 5% 32%	6952

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	37% 5% 18% 21% 15% 4	3933
	11th to 20th	Day	40% 3 9% 14% 19% 15%	4100
	21st to end	Day	46% 4 7% 12% 14% 17%	4510
February	1st to 10th	Day	42% 3 7% 19% 21% 8%	1800
	11th to 20th	Day	40% 5% 10% 20% 20% 6%	1834
	21st to end	Day	36% 4 9% 26% 18% 6%	1650
March	1st to 10th	Day	47% 5% 9% 20% 18% 2	4908
	11th to 20th	Day	34% 3 17% 28% 10% 8%	5000
	21st to end	Day	40% 6% 18% 25% 10% 1	5529
April	1st to 10th	Day	37% 4 14% 22% 20% 3	2400
	11th to 20th	Day	30% 4 15% 25% 19% 6%	2372
	21st to end	Day	23% 4 15% 31% 23% 4	2400
May	1st to 10th	Day	15% 2 8% 30% 30% 16%	5676
	11th to 20th	Day	13% 5% 6% 34% 35% 6%	5925
	21st to end	Day	16% 3 8% 32% 38% 2	6904
June	1st to 10th	Day	26% 4 11% 27% 24% 7%	2800
	11th to 20th	Day	16% 4 8% 21% 36% 14%	2800
	21st to end	Day	20% 7% 3 13% 31% 26%	2800
July	1st to 10th	Day	14% 3 1 8% 23% 50%	6219
	11th to 20th	Day	21% 4 2 4 28% 41%	5811
	21st to end	Day	34% 3 2 13% 24% 24%	6217
August	1st to 10th	Day	30% 5% 2 12% 30% 21%	4206
	11th to 20th	Day	19% 3 1 10% 43% 24%	4019
	21st to end	Day	22% 6% 3 13% 34% 24%	4311
September	1st to 10th	Day	18% 5% 1 9% 31% 36%	2317
	11th to 20th	Day	28% 4 4 23% 28% 13%	2280
	21st to end	Day	18% 3 3 22% 32% 21%	2002
October	1st to 10th	Day	17% 3 3 12% 30% 36%	3180
	11th to 20th	Day	28% 3 5% 22% 24% 18%	3161
	21st to end	Day	27% 2 7% 27% 24% 13%	3448
November	1st to 10th	Day	28% 4 11% 28% 19% 11%	1940
	11th to 20th	Day	38% 5% 9% 22% 20% 6%	1800
	21st to end	Day	32% 4 10% 21% 23% 10%	1615
December	1st to 10th	Day	29% 3 7% 30% 26% 6%	2620
	11th to 20th	Day	54% 3 10% 16% 15% 2	3028
	21st to end	Day	54% 4 13% 19% 8% 2	4004
January	1st to 10th	Night	35% 5% 21% 23% 12% 4	6023
	11th to 20th	Night	43% 3 9% 16% 17% 11%	5880
	21st to end	Night	45% 3 7% 12% 20% 13%	6468
February	1st to 10th	Night	39% 3 8% 20% 19% 10%	2600
	11th to 20th	Night	39% 4 11% 19% 20% 7%	2462
	21st to end	Night	33% 4 10% 30% 18% 5%	1980
March	1st to 10th	Night	45% 4 11% 21% 16% 3	4984
	11th to 20th	Night	36% 3 12% 26% 16% 8%	5000
	21st to end	Night	43% 5% 15% 24% 11% 2	5432
April	1st to 10th	Night	36% 4 13% 24% 19% 4	2000
	11th to 20th	Night	33% 4 12% 25% 19% 7%	1979
	21st to end	Night	24% 5% 12% 31% 22% 6%	2000
May	1st to 10th	Night	20% 2 6% 27% 32% 13%	4292
	11th to 20th	Night	26% 4 7% 29% 26% 8%	3983
	21st to end	Night	19% 2 9% 30% 38% 2	3972
June	1st to 10th	Night	33% 6% 9% 22% 24% 6%	1600
	11th to 20th	Night	23% 2 6% 18% 36% 16%	1600
	21st to end	Night	27% 4 3 12% 28% 26%	1600
July	1st to 10th	Night	26% 3 1 7% 20% 43%	3615
	11th to 20th	Night	43% 3 2 5% 13% 34%	4046
	21st to end	Night	52% 2 1 7% 19% 19%	4345
August	1st to 10th	Night	50% 3 1 9% 24% 14%	2929
	11th to 20th	Night	29% 3 1 8% 38% 21%	3112
	21st to end	Night	30% 3 3 12% 30% 22%	3608
September	1st to 10th	Night	23% 4 2 10% 26% 34%	1930
	11th to 20th	Night	33% 3 5% 21% 24% 13%	1900
	21st to end	Night	24% 4 3 20% 28% 22%	2171
October	1st to 10th	Night	19% 2 4 10% 25% 40%	3800
	11th to 20th	Night	30% 2 6% 21% 21% 20%	3777
	21st to end	Night	28% 2 5% 23% 26% 16%	4121
November	1st to 10th	Night	30% 4 9% 26% 19% 11%	2355
	11th to 20th	Night	33% 5% 10% 27% 18% 8%	2600
	21st to end	Night	30% 5% 10% 21% 23% 11%	2781
December	1st to 10th	Night	30% 3 6% 28% 24% 9%	4560
	11th to 20th	Night	53% 3 10% 17% 15% 3	5176
	21st to end	Night	55% 4 13% 18% 8% 2	6952

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	25% 11% 29% 22% 13% 1	325
	11th to 20th	Day	18% 11% 24% 34% 12% 2	400
	21st to end	Day	14% 8% 29% 32% 14% 2	440
February	1st to 10th	Day	20% 12% 26% 25% 16% 2	400
	11th to 20th	Day	19% 15% 30% 26% 11%	430
	21st to end	Day	13% 15% 30% 28% 12% 2	414
March	1st to 10th	Day	21% 17% 32% 20% 10% 1	500
	11th to 20th	Day	20% 15% 27% 23% 11% 4	500
	21st to end	Day	15% 20% 32% 24% 8% 1	550
April	1st to 10th	Day	16% 17% 32% 23% 10% 2	500
	11th to 20th	Day	13% 20% 32% 23% 10% 3	492
	21st to end	Day	7% 23% 31% 25% 10% 4	500
May	1st to 10th	Day	10% 17% 35% 25% 10% 2	503
	11th to 20th	Day	9% 21% 31% 28% 9% 2	528
	21st to end	Day	9% 17% 31% 29% 11% 3	573
June	1st to 10th	Day	9% 11% 36% 30% 11% 3	530
	11th to 20th	Day	5% 13% 37% 33% 11% 2	530
	21st to end	Day	3 11% 35% 38% 12% 2	530
July	1st to 10th	Day	5% 6% 32% 43% 13% 1	526
	11th to 20th	Day	2 5% 25% 50% 16% 2	524
	21st to end	Day	6% 3 26% 49% 13% 3	575
August	1st to 10th	Day	6% 6% 31% 44% 10% 3	527
	11th to 20th	Day	5% 3 31% 42% 16% 3	517
	21st to end	Day	5% 4 22% 45% 18% 5%	583
September	1st to 10th	Day	4 5% 27% 39% 21% 5%	503
	11th to 20th	Day	6% 11% 27% 37% 14% 5%	490
	21st to end	Day	4 9% 25% 34% 21% 6%	490
October	1st to 10th	Day	5% 9% 27% 30% 19% 10%	490
	11th to 20th	Day	8% 10% 24% 27% 23% 8%	490
	21st to end	Day	11% 7% 25% 29% 22% 6%	539
November	1st to 10th	Day	10% 10% 23% 27% 24% 6%	476
	11th to 20th	Day	12% 8% 18% 33% 24% 6%	300
	21st to end	Day	13% 7% 27% 34% 17% 2	207
December	1st to 10th	Day	18% 10% 20% 29% 20% 5%	200
	11th to 20th	Day	18% 14% 23% 30% 15% 1	200
	21st to end	Day	23% 12% 26% 29% 10%	220
January	1st to 10th	Night	19% 7% 15% 32% 22% 5%	800
	11th to 20th	Night	16% 8% 13% 34% 25% 5%	800
	21st to end	Night	18% 6% 14% 35% 24% 3	880
February	1st to 10th	Night	20% 6% 14% 33% 24% 3	800
	11th to 20th	Night	15% 11% 17% 34% 18% 5%	784
	21st to end	Night	14% 10% 21% 32% 20% 2	660
March	1st to 10th	Night	20% 12% 19% 28% 19% 3	800
	11th to 20th	Night	19% 9% 17% 30% 22% 4	800
	21st to end	Night	16% 11% 26% 28% 17% 2	880
April	1st to 10th	Night	12% 10% 26% 33% 16% 3	800
	11th to 20th	Night	14% 14% 21% 30% 17% 4	791
	21st to end	Night	6% 10% 28% 30% 22% 4	800
May	1st to 10th	Night	8% 7% 25% 34% 22% 3	691
	11th to 20th	Night	8% 5% 22% 34% 29% 3	552
	21st to end	Night	12% 10% 21% 34% 20% 3	433
June	1st to 10th	Night	7% 6% 24% 37% 24% 2	400
	11th to 20th	Night	3 6% 20% 49% 19% 3	400
	21st to end	Night	6% 4 18% 39% 30% 4	400
July	1st to 10th	Night	4 4 14% 46% 28% 4	392
	11th to 20th	Night	8% 3 7% 40% 34% 8%	582
	21st to end	Night	12% 6% 9% 33% 33% 7%	657
August	1st to 10th	Night	13% 4 13% 34% 29% 6%	597
	11th to 20th	Night	9% 4 16% 36% 29% 7%	699
	21st to end	Night	7% 3 11% 38% 33% 7%	880
September	1st to 10th	Night	3 2 13% 36% 36% 10%	772
	11th to 20th	Night	8% 5% 17% 34% 30% 7%	760
	21st to end	Night	4 4 14% 31% 35% 11%	760
October	1st to 10th	Night	8% 4 13% 33% 33% 9%	760
	11th to 20th	Night	8% 6% 12% 29% 35% 9%	760
	21st to end	Night	10% 4 12% 29% 33% 12%	836
November	1st to 10th	Night	9% 7% 14% 28% 34% 8%	780
	11th to 20th	Night	12% 4 14% 33% 30% 7%	800
	21st to end	Night	12% 6% 12% 38% 27% 6%	800
December	1st to 10th	Night	18% 6% 14% 33% 23% 6%	800
	11th to 20th	Night	25% 7% 13% 30% 21% 4	800
	21st to end	Night	22% 9% 19% 30% 19% 1	880

Page generated at 2025-08-31 22:51:04. Version ac07

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