San Francisco State University Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Francisco State University Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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 6% 9% 51%	124050
	Night	9% 8% 3 6% 4 3 3 3 5% 6% 49%	123965
Transparency	Day	18% 39% 5% 13% 15% 10%	123813
	Night	22% 39% 5% 12% 13% 8%	123791
Seeing	Day	11% 11% 28% 33% 14% 3	16337
	Night	13% 7% 17% 32% 26% 6%	25650

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	12% 6% 2 5% 4 4 4 4 6% 9% 44%	3941
	11th to 20th	Day	12% 7% 3 6% 4 3 4 3 4 5% 48%	4100
	21st to end	Day	16% 9% 3 7% 4 4 4 3 5% 6% 39%	4510
February	1st to 10th	Day	17% 7% 2 5% 4 4 4 3 4 6% 45%	1800
	11th to 20th	Day	13% 7% 3 7% 4 4 3 4 7% 9% 40%	1822
	21st to end	Day	11% 8% 2 6% 3 3 3 3 5% 7% 47%	1650
March	1st to 10th	Day	12% 9% 3 8% 6% 6% 5% 4 6% 8% 33%	4924
	11th to 20th	Day	11% 6% 2 5% 4 3 4 3 6% 8% 48%	5000
	21st to end	Day	11% 7% 3 6% 5% 5% 5% 4 7% 10% 37%	5527
April	1st to 10th	Day	12% 7% 3 5% 5% 3 3 4 5% 8% 45%	2400
	11th to 20th	Day	7% 6% 2 5% 3 3 4 4 6% 8% 52%	2369
	21st to end	Day	5% 5% 2 5% 3 2 3 3 6% 11% 55%	2400
May	1st to 10th	Day	4 3 1 3 2 1 2 2 4 7% 70%	5691
	11th to 20th	Day	3 3 2 3 2 2 3 3 6% 9% 65%	5900
	21st to end	Day	2 6% 2 3 2 2 2 3 5% 9% 65%	6902
June	1st to 10th	Day	6% 6% 2 5% 3 3 4 3 6% 10% 53%	2800
	11th to 20th	Day	2 4 1 3 3 2 3 3 6% 10% 64%	2800
	21st to end	Day	3 7% 2 4 2 3 3 3 7% 11% 55%	2800
July	1st to 10th	Day	1 5% 2 3 2 2 2 2 5% 10% 67%	4528
	11th to 20th	Day	1 5% 2 6% 3 3 4 3 8% 15% 49%	4240
	21st to end	Day	1 11% 3 4 3 4 4 3 7% 16% 44%	4456
August	1st to 10th	Day	2 8% 4 8% 5% 4 6% 5% 12% 16% 29%	4091
	11th to 20th	Day	1 5% 3 6% 3 3 4 3 6% 10% 57%	3856
	21st to end	Day	2 7% 2 6% 3 3 4 4 7% 14% 48%	4180
September	1st to 10th	Day	2 4 2 4 3 2 3 3 5% 12% 59%	2280
	11th to 20th	Day	4 9% 2 6% 3 3 3 3 6% 11% 51%	2280
	21st to end	Day	4 3 1 4 3 3 2 2 4 9% 65%	2000
October	1st to 10th	Day	3 3 1 3 2 3 2 2 5% 7% 68%	3180
	11th to 20th	Day	5% 8% 3 6% 3 3 3 2 4 7% 56%	3161
	21st to end	Day	6% 5% 2 5% 3 3 2 3 6% 8% 57%	3448
November	1st to 10th	Day	8% 5% 1 4 3 4 3 3 5% 8% 55%	1949
	11th to 20th	Day	11% 8% 3 7% 4 3 4 3 4 6% 47%	1800
	21st to end	Day	11% 6% 2 5% 3 3 3 3 4 7% 53%	1613
December	1st to 10th	Day	9% 5% 2 4 3 3 3 3 6% 10% 52%	2620
	11th to 20th	Day	21% 9% 3 8% 5% 5% 5% 4 6% 7% 27%	3028
	21st to end	Day	25% 10% 3 6% 4 4 5% 3 6% 7% 28%	4004
January	1st to 10th	Night	10% 6% 3 5% 4 4 4 3 7% 9% 45%	6002
	11th to 20th	Night	13% 7% 3 7% 5% 4 4 2 5% 4 45%	5880
	21st to end	Night	15% 9% 3 7% 4 4 3 3 4 6% 42%	6468
February	1st to 10th	Night	17% 7% 2 5% 3 3 3 3 4 6% 47%	2600
	11th to 20th	Night	11% 8% 2 6% 4 4 4 4 6% 8% 43%	2462
	21st to end	Night	11% 6% 2 5% 4 3 4 3 5% 7% 51%	1980
March	1st to 10th	Night	12% 10% 4 8% 5% 4 5% 4 6% 7% 37%	4955
	11th to 20th	Night	14% 7% 3 5% 3 3 3 3 4 7% 49%	5000
	21st to end	Night	13% 9% 3 6% 4 4 4 3 4 8% 41%	5447
April	1st to 10th	Night	9% 8% 3 6% 4 4 4 3 5% 7% 48%	2000
	11th to 20th	Night	9% 8% 2 6% 2 4 3 2 4 6% 54%	1979
	21st to end	Night	4 5% 2 5% 4 3 3 3 5% 8% 59%	2000
May	1st to 10th	Night	4 5% 2 4 2 2 2 2 3 4 70%	4294
	11th to 20th	Night	2 8% 4 7% 4 3 3 2 4 6% 59%	3992
	21st to end	Night	2 7% 2 4 2 2 2 2 4 5% 69%	3972
June	1st to 10th	Night	5% 8% 4 7% 4 4 4 3 5% 9% 48%	1600
	11th to 20th	Night	2 5% 3 6% 3 3 2 2 3 8% 64%	1600
	21st to end	Night	3 9% 4 6% 3 3 2 2 3 7% 58%	1600
July	1st to 10th	Night	7% 3 7% 3 2 2 2 2 6% 66%	2647
	11th to 20th	Night	2 10% 4 11% 5% 3 2 3 5% 7% 48%	2950
	21st to end	Night	1 22% 6% 10% 4 3 3 2 5% 6% 37%	3109
August	1st to 10th	Night	3 19% 8% 14% 5% 4 3 3 5% 7% 30%	2848
	11th to 20th	Night	1 10% 4 8% 4 3 2 2 5% 6% 54%	2987
	21st to end	Night	2 10% 4 9% 4 3 4 3 6% 7% 49%	3498
September	1st to 10th	Night	1 6% 3 7% 4 2 4 3 5% 7% 58%	1900
	11th to 20th	Night	4 11% 3 8% 3 2 3 3 5% 8% 50%	1900
	21st to end	Night	3 5% 2 7% 3 2 3 3 3 7% 61%	2176
October	1st to 10th	Night	3 3 2 4 2 3 3 2 5% 6% 67%	3800
	11th to 20th	Night	5% 9% 3 7% 4 3 3 3 6% 6% 52%	3777
	21st to end	Night	7% 5% 2 5% 3 3 3 3 5% 5% 59%	4121
November	1st to 10th	Night	7% 6% 2 7% 3 3 3 3 4 7% 55%	2348
	11th to 20th	Night	9% 8% 2 5% 4 4 4 3 5% 8% 49%	2600
	21st to end	Night	10% 5% 2 4 3 3 4 3 5% 7% 54%	2785
December	1st to 10th	Night	9% 4 2 4 3 3 4 3 5% 6% 56%	4560
	11th to 20th	Night	22% 9% 3 7% 4 5% 5% 4 7% 8% 28%	5176
	21st to end	Night	25% 8% 3 7% 4 4 5% 4 5% 6% 30%	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	26% 28% 14% 16% 12% 3	3941
	11th to 20th	Day	33% 26% 6% 9% 14% 12%	4100
	21st to end	Day	36% 28% 4 9% 8% 15%	4510
February	1st to 10th	Day	20% 62% 3 7% 7% 2	1800
	11th to 20th	Day	19% 61% 3 7% 7% 2	1831
	21st to end	Day	12% 62% 4 12% 7% 3	1650
March	1st to 10th	Day	37% 30% 5% 14% 13% 1	4924
	11th to 20th	Day	22% 28% 13% 22% 7% 7%	5000
	21st to end	Day	30% 31% 14% 19% 6%	5527
April	1st to 10th	Day	15% 62% 4 11% 7% 1	2400
	11th to 20th	Day	12% 62% 6% 11% 7% 2	2372
	21st to end	Day	10% 62% 7% 11% 9% 1	2400
May	1st to 10th	Day	8% 27% 6% 23% 22% 13%	5691
	11th to 20th	Day	8% 30% 4 25% 29% 4	5903
	21st to end	Day	9% 27% 5% 25% 32% 2	6904
June	1st to 10th	Day	9% 61% 3 12% 10% 5%	2800
	11th to 20th	Day	5% 62% 2 11% 16% 5%	2800
	21st to end	Day	8% 61% 1 4 13% 12%	2800
July	1st to 10th	Day	7% 35% 1 4 7% 47%	4460
	11th to 20th	Day	11% 35% 2 4 13% 35%	4171
	21st to end	Day	14% 34% 1 6% 22% 23%	4316
August	1st to 10th	Day	22% 37% 1 8% 19% 13%	4076
	11th to 20th	Day	12% 37% 1 7% 31% 12%	3895
	21st to end	Day	13% 37% 3 10% 23% 14%	4179
September	1st to 10th	Day	9% 60% 1 6% 13% 11%	2280
	11th to 20th	Day	10% 61% 2 10% 12% 6%	2280
	21st to end	Day	8% 59% 2 10% 15% 6%	2000
October	1st to 10th	Day	8% 36% 1 6% 19% 31%	3180
	11th to 20th	Day	18% 36% 4 15% 16% 11%	3161
	21st to end	Day	17% 36% 5% 20% 16% 7%	3448
November	1st to 10th	Day	8% 56% 4 16% 12% 5%	1949
	11th to 20th	Day	17% 57% 4 12% 8% 2	1800
	21st to end	Day	17% 57% 4 8% 10% 3	1613
December	1st to 10th	Day	17% 35% 4 23% 18% 3	2620
	11th to 20th	Day	39% 31% 6% 10% 11% 2	3028
	21st to end	Day	42% 26% 9% 15% 6% 1	4004
January	1st to 10th	Night	24% 29% 18% 17% 9% 3	6002
	11th to 20th	Night	35% 27% 7% 11% 13% 7%	5880
	21st to end	Night	34% 28% 4 8% 15% 10%	6468
February	1st to 10th	Night	19% 61% 3 7% 6% 3	2600
	11th to 20th	Night	17% 61% 5% 8% 7% 3	2464
	21st to end	Night	11% 62% 4 15% 6% 3	1980
March	1st to 10th	Night	35% 30% 8% 15% 12% 1	4955
	11th to 20th	Night	25% 28% 10% 19% 12% 7%	5000
	21st to end	Night	32% 30% 11% 19% 7%	5447
April	1st to 10th	Night	13% 62% 4 11% 8% 2	2000
	11th to 20th	Night	14% 62% 3 11% 7% 2	1979
	21st to end	Night	10% 64% 5% 12% 8% 2	2000
May	1st to 10th	Night	13% 28% 5% 20% 24% 10%	4291
	11th to 20th	Night	19% 29% 5% 23% 19% 6%	3993
	21st to end	Night	11% 27% 7% 23% 31% 1	3972
June	1st to 10th	Night	13% 63% 3 8% 10% 4	1600
	11th to 20th	Night	8% 62% 2 9% 15% 4	1600
	21st to end	Night	9% 61% 1 4 12% 13%	1600
July	1st to 10th	Night	12% 35% 1 2 7% 42%	2607
	11th to 20th	Night	21% 34% 2 3 8% 32%	2896
	21st to end	Night	26% 35% 1 3 15% 21%	3011
August	1st to 10th	Night	37% 36% 1 5% 14% 7%	2839
	11th to 20th	Night	16% 37% 1 4 32% 11%	3014
	21st to end	Night	18% 37% 3 9% 20% 13%	3498
September	1st to 10th	Night	11% 60% 2 6% 12% 9%	1900
	11th to 20th	Night	11% 60% 2 10% 11% 6%	1900
	21st to end	Night	10% 60% 2 10% 10% 8%	2176
October	1st to 10th	Night	8% 36% 1 4 17% 34%	3800
	11th to 20th	Night	20% 36% 4 14% 14% 12%	3777
	21st to end	Night	18% 36% 3 15% 17% 11%	4121
November	1st to 10th	Night	11% 55% 3 14% 11% 6%	2348
	11th to 20th	Night	16% 57% 5% 13% 7% 2	2600
	21st to end	Night	16% 57% 4 9% 9% 4	2785
December	1st to 10th	Night	17% 36% 4 21% 16% 5%	4560
	11th to 20th	Night	39% 32% 6% 11% 10% 2	5176
	21st to end	Night	43% 27% 9% 15% 5% 1	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	27% 8% 27% 22% 14% 1	333
	11th to 20th	Day	19% 9% 24% 35% 12% 2	400
	21st to end	Day	14% 8% 28% 34% 15% 2	440
February	1st to 10th	Day	19% 11% 25% 28% 16% 2	400
	11th to 20th	Day	17% 16% 30% 26% 11%	430
	21st to end	Day	13% 14% 30% 28% 12% 2	414
March	1st to 10th	Day	22% 16% 29% 22% 9% 1	500
	11th to 20th	Day	22% 12% 27% 24% 12% 4	500
	21st to end	Day	16% 18% 31% 25% 9% 1	550
April	1st to 10th	Day	15% 18% 32% 22% 10% 2	500
	11th to 20th	Day	12% 20% 32% 24% 10% 3	492
	21st to end	Day	9% 19% 32% 26% 9% 5%	500
May	1st to 10th	Day	11% 16% 34% 26% 10% 2	503
	11th to 20th	Day	13% 15% 32% 29% 10% 2	528
	21st to end	Day	10% 15% 32% 28% 12% 3	573
June	1st to 10th	Day	7% 12% 36% 32% 10% 3	530
	11th to 20th	Day	4 14% 36% 35% 9% 2	530
	21st to end	Day	5% 7% 35% 40% 10% 2	530
July	1st to 10th	Day	6% 6% 29% 46% 13% 1	526
	11th to 20th	Day	5% 3 23% 51% 16% 2	524
	21st to end	Day	5% 3 26% 49% 14% 3	532
August	1st to 10th	Day	8% 5% 31% 46% 9% 1	487
	11th to 20th	Day	6% 4 31% 39% 16% 4	477
	21st to end	Day	6% 4 23% 44% 19% 5%	539
September	1st to 10th	Day	4 5% 26% 39% 20% 6%	490
	11th to 20th	Day	8% 10% 25% 38% 14% 5%	490
	21st to end	Day	5% 9% 24% 35% 22% 6%	490
October	1st to 10th	Day	5% 7% 27% 30% 22% 9%	490
	11th to 20th	Day	8% 11% 22% 28% 24% 8%	490
	21st to end	Day	11% 7% 23% 29% 23% 6%	539
November	1st to 10th	Day	9% 9% 23% 28% 24% 7%	485
	11th to 20th	Day	13% 7% 17% 31% 25% 7%	300
	21st to end	Day	13% 6% 26% 34% 19% 2	205
December	1st to 10th	Day	20% 8% 20% 29% 20% 4	200
	11th to 20th	Day	17% 14% 25% 33% 12% 1	200
	21st to end	Day	23% 11% 25% 31% 10%	220
January	1st to 10th	Night	21% 7% 15% 31% 22% 5%	800
	11th to 20th	Night	16% 7% 14% 33% 25% 5%	800
	21st to end	Night	18% 6% 14% 33% 26% 4	880
February	1st to 10th	Night	20% 7% 16% 31% 23% 4	800
	11th to 20th	Night	16% 11% 18% 31% 19% 5%	784
	21st to end	Night	13% 11% 20% 33% 20% 3	660
March	1st to 10th	Night	21% 11% 19% 27% 19% 3	800
	11th to 20th	Night	20% 9% 17% 30% 22% 3	800
	21st to end	Night	15% 11% 27% 28% 17% 2	880
April	1st to 10th	Night	13% 10% 27% 30% 17% 3	800
	11th to 20th	Night	14% 14% 21% 30% 17% 4	791
	21st to end	Night	7% 11% 28% 29% 22% 4	800
May	1st to 10th	Night	9% 7% 26% 32% 22% 4	690
	11th to 20th	Night	9% 4 22% 34% 27% 4	562
	21st to end	Night	13% 12% 20% 33% 18% 4	433
June	1st to 10th	Night	8% 8% 23% 35% 25% 2	400
	11th to 20th	Night	5% 7% 23% 45% 20% 2	400
	21st to end	Night	9% 2 20% 36% 30% 4	400
July	1st to 10th	Night	6% 2 14% 44% 27% 5%	404
	11th to 20th	Night	10% 3 8% 39% 31% 8%	582
	21st to end	Night	14% 5% 8% 32% 33% 8%	624
August	1st to 10th	Night	18% 3 12% 32% 29% 6%	567
	11th to 20th	Night	11% 4 15% 34% 28% 7%	661
	21st to end	Night	10% 3 12% 38% 30% 8%	836
September	1st to 10th	Night	4 3 13% 34% 36% 10%	760
	11th to 20th	Night	9% 6% 16% 33% 30% 7%	760
	21st to end	Night	6% 4 14% 30% 35% 11%	760
October	1st to 10th	Night	8% 5% 13% 32% 33% 10%	760
	11th to 20th	Night	9% 6% 13% 28% 34% 10%	760
	21st to end	Night	11% 5% 12% 28% 32% 13%	836
November	1st to 10th	Night	9% 7% 13% 28% 34% 8%	780
	11th to 20th	Night	12% 4 13% 33% 31% 7%	800
	21st to end	Night	12% 6% 13% 38% 26% 5%	800
December	1st to 10th	Night	19% 7% 14% 32% 23% 6%	800
	11th to 20th	Night	25% 7% 14% 30% 20% 4	800
	21st to end	Night	22% 11% 17% 31% 19% 1	880

Page generated at 2025-07-17 22:09:59. Version ac07

Copyright © 2025 Attilla Danko.