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 2024-03-28.

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	9% 7% 3 5% 4 2 4 4 6% 9% 48%	78931
	Night	11% 8% 3 6% 5% 3 4 4 5% 7% 46%	81091
Transparency	Day	32% 5% 9% 19% 22% 13%	78653
	Night	38% 4 9% 19% 18% 11%	80892
Seeing	Day	11% 11% 28% 32% 14% 3	14560
	Night	13% 7% 16% 32% 26% 6%	24013

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	15% 7% 3 5% 5% 3 4 5% 6% 8% 39%	2743
	11th to 20th	Day	18% 9% 4 8% 6% 3 5% 4 5% 6% 33%	2860
	21st to end	Day	20% 10% 4 7% 6% 3 4 5% 5% 6% 30%	3146
February	1st to 10th	Day	16% 6% 2 5% 4 3 4 4 4 6% 45%	1710
	11th to 20th	Day	13% 7% 3 5% 4 3 4 4 6% 9% 41%	1728
	21st to end	Day	12% 7% 3 6% 4 2 3 4 5% 7% 48%	1570
March	1st to 10th	Day	14% 8% 4 7% 5% 3 5% 4 5% 8% 37%	2075
	11th to 20th	Day	15% 7% 3 6% 6% 3 5% 4 6% 8% 39%	2090
	21st to end	Day	11% 5% 2 4 5% 3 4 4 6% 9% 47%	2272
April	1st to 10th	Day	13% 6% 3 5% 5% 3 4 5% 5% 8% 44%	2160
	11th to 20th	Day	7% 6% 3 5% 3 2 3 4 6% 8% 54%	2129
	21st to end	Day	5% 4 2 4 3 1 3 4 6% 11% 56%	2160
May	1st to 10th	Day	6% 6% 2 4 3 1 3 3 5% 9% 56%	2255
	11th to 20th	Day	6% 6% 3 4 3 2 3 3 5% 10% 54%	2321
	21st to end	Day	5% 10% 3 5% 3 2 3 4 6% 10% 49%	2723
June	1st to 10th	Day	7% 6% 2 4 4 2 3 4 5% 10% 54%	2520
	11th to 20th	Day	2 4 2 3 3 1 3 3 6% 10% 64%	2520
	21st to end	Day	3 7% 3 4 3 2 3 4 7% 11% 54%	2520
July	1st to 10th	Day	2 9% 2 4 3 2 2 4 7% 14% 51%	2502
	11th to 20th	Day	2 7% 3 5% 4 2 4 5% 8% 16% 44%	2340
	21st to end	Day	1 7% 2 4 4 2 4 4 7% 17% 47%	2546
August	1st to 10th	Day	3 9% 3 5% 5% 2 4 5% 9% 12% 42%	2328
	11th to 20th	Day	2 6% 4 5% 3 2 3 4 8% 14% 49%	2206
	21st to end	Day	2 9% 3 5% 4 2 4 4 7% 12% 47%	2376
September	1st to 10th	Day	2 4 2 3 3 2 3 4 4 12% 60%	2160
	11th to 20th	Day	4 9% 2 5% 3 2 3 4 6% 11% 52%	2160
	21st to end	Day	4 3 1 3 3 2 2 3 4 10% 66%	1896
October	1st to 10th	Day	5% 4 2 3 3 2 3 2 4 7% 64%	1800
	11th to 20th	Day	8% 5% 3 5% 4 2 3 3 4 8% 56%	1800
	21st to end	Day	9% 5% 2 4 4 2 3 3 4 7% 57%	1980
November	1st to 10th	Day	8% 5% 2 4 4 3 3 4 5% 8% 54%	1850
	11th to 20th	Day	11% 7% 3 6% 5% 3 4 4 4 6% 48%	1710
	21st to end	Day	11% 5% 2 5% 4 2 3 4 4 7% 54%	1533
December	1st to 10th	Day	15% 7% 4 5% 4 2 4 4 4 7% 45%	1520
	11th to 20th	Day	25% 10% 4 5% 5% 3 3 4 5% 6% 31%	1928
	21st to end	Day	21% 8% 3 6% 5% 3 4 4 6% 8% 34%	2794
January	1st to 10th	Night	14% 8% 3 5% 5% 3 4 4 5% 7% 42%	4193
	11th to 20th	Night	19% 9% 3 7% 6% 4 5% 4 5% 5% 32%	4110
	21st to end	Night	21% 11% 4 7% 6% 4 4 4 4 6% 29%	4521
February	1st to 10th	Night	16% 6% 2 4 4 2 3 4 4 6% 48%	2470
	11th to 20th	Night	11% 7% 3 5% 5% 3 3 5% 6% 8% 44%	2336
	21st to end	Night	12% 6% 2 4 4 2 4 4 5% 7% 50%	1884
March	1st to 10th	Night	16% 8% 4 6% 4 3 5% 5% 5% 6% 39%	2103
	11th to 20th	Night	16% 7% 3 4 4 3 4 4 6% 7% 42%	2090
	21st to end	Night	13% 7% 4 4 4 2 4 3 3 6% 49%	2238
April	1st to 10th	Night	10% 6% 4 6% 4 2 5% 4 5% 7% 47%	1800
	11th to 20th	Night	10% 7% 2 4 3 2 3 3 4 6% 56%	1779
	21st to end	Night	4 4 1 4 4 2 3 3 5% 9% 60%	1800
May	1st to 10th	Night	4 8% 3 5% 5% 3 4 3 4 6% 56%	1702
	11th to 20th	Night	3 8% 4 7% 5% 2 4 4 4 8% 51%	1568
	21st to end	Night	6% 14% 2 6% 3 2 4 3 3 6% 51%	1552
June	1st to 10th	Night	5% 7% 3 7% 4 3 4 4 4 10% 49%	1440
	11th to 20th	Night	2 5% 3 6% 4 2 3 2 3 9% 62%	1440
	21st to end	Night	3 9% 4 6% 3 3 3 2 3 7% 57%	1440
July	1st to 10th	Night	1 10% 4 8% 4 2 3 4 3 9% 51%	1452
	11th to 20th	Night	2 11% 4 7% 6% 3 3 4 6% 12% 41%	1620
	21st to end	Night	2 17% 8% 11% 6% 3 4 4 5% 7% 34%	1773
August	1st to 10th	Night	4 18% 4 9% 5% 3 3 4 4 8% 39%	1611
	11th to 20th	Night	2 13% 4 8% 6% 4 4 5% 6% 9% 38%	1693
	21st to end	Night	2 12% 4 8% 5% 2 4 4 6% 8% 44%	1980
September	1st to 10th	Night	1 6% 3 6% 4 2 4 3 5% 8% 59%	1800
	11th to 20th	Night	4 11% 3 6% 4 1 3 4 5% 8% 51%	1800
	21st to end	Night	3 5% 2 7% 4 2 3 3 3 7% 62%	2060
October	1st to 10th	Night	6% 5% 3 5% 3 2 3 3 3 6% 62%	2160
	11th to 20th	Night	6% 7% 2 5% 5% 2 3 4 4 7% 55%	2160
	21st to end	Night	7% 6% 2 5% 4 2 3 4 4 6% 56%	2376
November	1st to 10th	Night	7% 6% 2 6% 4 2 3 3 4 7% 54%	2227
	11th to 20th	Night	9% 8% 3 4 4 3 4 4 5% 8% 49%	2470
	21st to end	Night	9% 4 2 3 4 3 4 4 4 7% 56%	2645
December	1st to 10th	Night	16% 7% 3 5% 4 2 4 5% 4 6% 46%	2660
	11th to 20th	Night	28% 9% 3 5% 4 2 4 4 5% 6% 32%	3276
	21st to end	Night	18% 7% 3 6% 6% 3 5% 5% 6% 6% 35%	4862

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	43% 5% 14% 28% 8% 3	2743
	11th to 20th	Day	54% 4 12% 13% 10% 7%	2860
	21st to end	Day	54% 5% 9% 16% 11% 4	3146
February	1st to 10th	Day	41% 3 8% 19% 21% 8%	1710
	11th to 20th	Day	40% 5% 9% 20% 21% 6%	1737
	21st to end	Day	36% 4 10% 26% 18% 6%	1570
March	1st to 10th	Day	45% 5% 11% 21% 15% 3	2075
	11th to 20th	Day	43% 5% 10% 22% 15% 5%	2090
	21st to end	Day	35% 6% 16% 24% 16% 4	2272
April	1st to 10th	Day	39% 5% 14% 19% 20% 4	2160
	11th to 20th	Day	29% 4 16% 25% 20% 6%	2132
	21st to end	Day	23% 4 15% 30% 23% 4	2160
May	1st to 10th	Day	26% 4 9% 28% 28% 6%	2255
	11th to 20th	Day	26% 5% 9% 30% 22% 8%	2324
	21st to end	Day	30% 6% 13% 25% 22% 3	2724
June	1st to 10th	Day	27% 4 12% 26% 23% 8%	2520
	11th to 20th	Day	17% 5% 9% 21% 35% 14%	2520
	21st to end	Day	21% 7% 3 13% 31% 24%	2520
July	1st to 10th	Day	21% 6% 2 10% 24% 36%	2434
	11th to 20th	Day	24% 9% 5% 9% 22% 32%	2271
	21st to end	Day	22% 8% 2 10% 26% 33%	2405
August	1st to 10th	Day	28% 9% 2 12% 27% 22%	2312
	11th to 20th	Day	23% 6% 1 11% 33% 27%	2206
	21st to end	Day	25% 9% 2 12% 28% 24%	2376
September	1st to 10th	Day	19% 5% 1 9% 30% 36%	2160
	11th to 20th	Day	27% 4 4 23% 28% 14%	2160
	21st to end	Day	18% 3 3 23% 31% 21%	1896
October	1st to 10th	Day	21% 4 6% 18% 31% 21%	1800
	11th to 20th	Day	28% 5% 7% 18% 23% 20%	1800
	21st to end	Day	28% 4 8% 20% 22% 17%	1980
November	1st to 10th	Day	29% 4 11% 27% 18% 11%	1850
	11th to 20th	Day	38% 5% 8% 22% 20% 6%	1710
	21st to end	Day	31% 4 10% 21% 24% 10%	1533
December	1st to 10th	Day	40% 5% 11% 19% 17% 8%	1520
	11th to 20th	Day	55% 5% 9% 14% 15% 4	1928
	21st to end	Day	49% 5% 14% 21% 10% 3	2794
January	1st to 10th	Night	41% 5% 18% 26% 7% 3	4193
	11th to 20th	Night	54% 4 13% 12% 9% 7%	4110
	21st to end	Night	57% 4 9% 14% 11% 5%	4521
February	1st to 10th	Night	38% 3 9% 21% 18% 10%	2470
	11th to 20th	Night	38% 5% 10% 19% 21% 7%	2338
	21st to end	Night	33% 4 11% 30% 17% 4	1884
March	1st to 10th	Night	46% 5% 11% 18% 16% 4	2103
	11th to 20th	Night	41% 5% 8% 23% 18% 6%	2090
	21st to end	Night	38% 4 14% 23% 17% 4	2238
April	1st to 10th	Night	37% 4 14% 21% 19% 5%	1800
	11th to 20th	Night	31% 4 13% 26% 19% 8%	1779
	21st to end	Night	24% 5% 12% 30% 23% 7%	1800
May	1st to 10th	Night	30% 4 7% 24% 24% 10%	1702
	11th to 20th	Night	32% 4 10% 23% 24% 7%	1569
	21st to end	Night	36% 4 13% 23% 21% 4	1552
June	1st to 10th	Night	33% 6% 8% 22% 24% 7%	1440
	11th to 20th	Night	25% 2 7% 18% 33% 15%	1440
	21st to end	Night	29% 3 3 13% 27% 24%	1440
July	1st to 10th	Night	30% 5% 1 8% 24% 32%	1412
	11th to 20th	Night	35% 5% 4 10% 16% 28%	1566
	21st to end	Night	49% 6% 1 5% 15% 25%	1674
August	1st to 10th	Night	44% 4 1 10% 23% 17%	1602
	11th to 20th	Night	39% 6% 2 9% 20% 23%	1693
	21st to end	Night	36% 6% 2 11% 24% 21%	1980
September	1st to 10th	Night	23% 5% 3 10% 26% 34%	1800
	11th to 20th	Night	33% 3 5% 21% 23% 14%	1800
	21st to end	Night	23% 4 3 20% 28% 21%	2060
October	1st to 10th	Night	25% 4 7% 16% 25% 24%	2160
	11th to 20th	Night	29% 4 6% 19% 20% 22%	2160
	21st to end	Night	30% 4 6% 21% 22% 19%	2376
November	1st to 10th	Night	31% 5% 9% 25% 18% 12%	2227
	11th to 20th	Night	33% 5% 10% 27% 18% 8%	2470
	21st to end	Night	29% 5% 10% 22% 24% 11%	2645
December	1st to 10th	Night	40% 4 9% 19% 18% 10%	2660
	11th to 20th	Night	54% 4 9% 15% 15% 3	3276
	21st to end	Night	48% 6% 15% 17% 10% 4	4862

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	29% 8% 28% 20% 14% 1	306
	11th to 20th	Day	20% 9% 22% 35% 12% 2	370
	21st to end	Day	14% 8% 29% 33% 14% 2	407
February	1st to 10th	Day	17% 11% 26% 28% 16% 2	370
	11th to 20th	Day	17% 16% 30% 26% 11% 1	396
	21st to end	Day	13% 14% 31% 27% 13% 2	382
March	1st to 10th	Day	22% 15% 29% 22% 9% 1	460
	11th to 20th	Day	21% 12% 27% 24% 13% 4	460
	21st to end	Day	15% 18% 32% 26% 8% 1	491
April	1st to 10th	Day	16% 18% 32% 22% 10% 2	420
	11th to 20th	Day	11% 22% 34% 22% 8% 3	412
	21st to end	Day	7% 20% 33% 25% 10% 5%	420
May	1st to 10th	Day	12% 17% 32% 26% 9% 2	423
	11th to 20th	Day	13% 16% 33% 26% 9% 2	448
	21st to end	Day	12% 16% 31% 26% 11% 3	485
June	1st to 10th	Day	8% 14% 36% 30% 10% 3	450
	11th to 20th	Day	4 14% 35% 36% 10% 2	450
	21st to end	Day	6% 8% 35% 38% 11% 2	450
July	1st to 10th	Day	7% 7% 29% 45% 12% 1	446
	11th to 20th	Day	4 3 24% 50% 17% 2	444
	21st to end	Day	5% 3 25% 49% 15% 3	487
August	1st to 10th	Day	8% 6% 31% 44% 9% 2	447
	11th to 20th	Day	6% 3 28% 42% 17% 4	437
	21st to end	Day	6% 4 22% 45% 18% 4	495
September	1st to 10th	Day	4 5% 24% 40% 21% 6%	450
	11th to 20th	Day	7% 8% 26% 39% 15% 6%	450
	21st to end	Day	4 9% 25% 34% 22% 6%	450
October	1st to 10th	Day	6% 8% 27% 30% 21% 9%	450
	11th to 20th	Day	7% 10% 21% 28% 24% 9%	450
	21st to end	Day	12% 7% 22% 28% 24% 7%	495
November	1st to 10th	Day	10% 9% 22% 28% 24% 6%	446
	11th to 20th	Day	12% 6% 15% 32% 27% 7%	270
	21st to end	Day	10% 6% 25% 36% 20% 3	185
December	1st to 10th	Day	22% 9% 18% 28% 18% 4	180
	11th to 20th	Day	16% 14% 24% 32% 13% 1	180
	21st to end	Day	20% 11% 26% 31% 11% 1	198
January	1st to 10th	Night	21% 7% 14% 30% 23% 5%	760
	11th to 20th	Night	17% 8% 13% 32% 25% 6%	760
	21st to end	Night	18% 6% 14% 32% 25% 3	836
February	1st to 10th	Night	19% 7% 16% 31% 24% 4	760
	11th to 20th	Night	15% 11% 18% 31% 20% 5%	744
	21st to end	Night	14% 11% 21% 31% 20% 3	628
March	1st to 10th	Night	21% 11% 19% 27% 19% 3	760
	11th to 20th	Night	19% 8% 17% 30% 22% 3	760
	21st to end	Night	15% 10% 27% 29% 17% 2	820
April	1st to 10th	Night	13% 10% 28% 30% 17% 3	720
	11th to 20th	Night	14% 15% 22% 28% 16% 4	711
	21st to end	Night	6% 11% 28% 28% 22% 4	720
May	1st to 10th	Night	9% 7% 26% 31% 23% 4	622
	11th to 20th	Night	9% 4 22% 34% 28% 4	506
	21st to end	Night	14% 13% 19% 31% 18% 5%	389
June	1st to 10th	Night	8% 8% 23% 33% 26% 3	360
	11th to 20th	Night	5% 6% 20% 45% 21% 3	360
	21st to end	Night	9% 1 19% 36% 31% 4	360
July	1st to 10th	Night	7% 2 13% 42% 29% 6%	362
	11th to 20th	Night	10% 3 7% 38% 32% 9%	522
	21st to end	Night	14% 5% 8% 31% 34% 8%	591
August	1st to 10th	Night	19% 4 12% 30% 30% 6%	537
	11th to 20th	Night	11% 4 13% 34% 30% 8%	625
	21st to end	Night	10% 3 11% 37% 31% 8%	792
September	1st to 10th	Night	4 3 12% 34% 36% 11%	720
	11th to 20th	Night	9% 5% 15% 33% 31% 7%	720
	21st to end	Night	6% 4 14% 30% 34% 12%	720
October	1st to 10th	Night	8% 5% 13% 32% 33% 9%	720
	11th to 20th	Night	8% 6% 12% 28% 35% 10%	720
	21st to end	Night	11% 5% 11% 28% 32% 13%	792
November	1st to 10th	Night	9% 7% 14% 27% 35% 9%	740
	11th to 20th	Night	12% 3 12% 33% 32% 7%	760
	21st to end	Night	11% 6% 13% 38% 28% 6%	760
December	1st to 10th	Night	19% 7% 13% 32% 23% 6%	760
	11th to 20th	Night	25% 7% 14% 29% 20% 4	760
	21st to end	Night	21% 11% 17% 31% 20% 1	836

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