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 2024-11-04.

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% 7% 2 5% 3 3 3 3 6% 9% 50%	105015
	Night	9% 9% 3 7% 4 3 3 3 5% 6% 48%	102750
Transparency	Day	16% 46% 4 11% 13% 10%	104777
	Night	21% 46% 5% 10% 10% 8%	102579
Seeing	Day	11% 11% 28% 33% 15% 3	15444
	Night	13% 7% 16% 32% 26% 6%	24745

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% 8% 2 6% 5% 4 4 4 6% 8% 39%	2743
	11th to 20th	Day	18% 9% 4 9% 5% 4 4 3 5% 6% 33%	2860
	21st to end	Day	20% 10% 3 8% 5% 5% 4 4 5% 6% 30%	3146
February	1st to 10th	Day	16% 7% 2 5% 4 4 4 3 4 6% 45%	1710
	11th to 20th	Day	13% 7% 3 6% 4 4 3 3 7% 9% 41%	1728
	21st to end	Day	11% 7% 2 6% 3 3 3 3 5% 7% 48%	1570
March	1st to 10th	Day	13% 10% 4 10% 6% 6% 5% 4 5% 7% 30%	3427
	11th to 20th	Day	10% 5% 2 5% 4 3 3 3 5% 8% 52%	3490
	21st to end	Day	13% 7% 3 6% 5% 5% 4 4 7% 9% 37%	3853
April	1st to 10th	Day	12% 7% 3 5% 4 3 3 4 5% 8% 44%	2280
	11th to 20th	Day	7% 6% 2 5% 3 3 3 4 6% 8% 52%	2249
	21st to end	Day	5% 4 2 5% 3 2 3 3 6% 11% 56%	2280
May	1st to 10th	Day	6% 4 1 3 2 1 2 2 3 6% 70%	3973
	11th to 20th	Day	3 5% 2 4 2 2 3 3 7% 10% 58%	4117
	21st to end	Day	3 8% 2 3 2 2 2 3 4 8% 63%	4813
June	1st to 10th	Day	6% 6% 2 4 3 2 3 3 6% 10% 54%	2660
	11th to 20th	Day	2 4 1 3 3 2 3 2 6% 10% 64%	2660
	21st to end	Day	3 7% 2 4 2 3 3 3 7% 11% 54%	2660
July	1st to 10th	Day	1 5% 1 3 1 2 2 2 5% 10% 68%	4389
	11th to 20th	Day	1 5% 2 5% 3 3 4 3 8% 15% 51%	4110
	21st to end	Day	1 11% 3 4 3 4 4 3 7% 16% 44%	4455
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% 6% 2 4 4 4 3 3 5% 8% 54%	1881
	11th to 20th	Day	11% 8% 3 7% 4 4 4 3 4 6% 47%	1710
	21st to end	Day	10% 5% 2 5% 3 3 3 3 4 7% 54%	1533
December	1st to 10th	Day	15% 7% 3 6% 3 3 4 4 4 7% 44%	1520
	11th to 20th	Day	25% 10% 3 6% 4 3 3 3 5% 6% 31%	1928
	21st to end	Day	21% 8% 3 6% 4 3 4 3 6% 7% 34%	2794
January	1st to 10th	Night	14% 8% 3 6% 4 4 4 3 5% 7% 42%	4193
	11th to 20th	Night	19% 9% 3 8% 5% 5% 5% 3 5% 5% 32%	4110
	21st to end	Night	21% 11% 3 9% 5% 4 4 3 4 6% 29%	4521
February	1st to 10th	Night	16% 7% 2 5% 3 3 3 3 4 6% 48%	2470
	11th to 20th	Night	11% 8% 2 6% 4 4 4 4 6% 8% 43%	2336
	21st to end	Night	11% 6% 2 5% 4 3 4 3 5% 6% 50%	1884
March	1st to 10th	Night	13% 11% 4 8% 5% 4 5% 4 6% 6% 36%	3445
	11th to 20th	Night	12% 6% 3 4 3 3 3 3 4 6% 53%	3490
	21st to end	Night	13% 11% 3 7% 5% 4 4 3 4 7% 38%	3799
April	1st to 10th	Night	9% 8% 3 6% 4 4 4 3 5% 7% 48%	1900
	11th to 20th	Night	9% 7% 2 5% 2 4 3 2 4 6% 55%	1879
	21st to end	Night	4 4 2 4 4 3 3 3 5% 8% 60%	1900
May	1st to 10th	Night	4 6% 2 4 3 2 2 2 3 4 68%	2998
	11th to 20th	Night	2 10% 5% 9% 4 2 3 2 3 6% 55%	2780
	21st to end	Night	3 9% 1 4 2 3 3 2 2 5% 66%	2762
June	1st to 10th	Night	5% 8% 3 6% 3 4 4 3 5% 9% 49%	1520
	11th to 20th	Night	2 5% 3 6% 4 3 2 2 3 8% 62%	1520
	21st to end	Night	3 9% 4 6% 3 3 2 2 3 7% 58%	1520
July	1st to 10th	Night	7% 3 6% 3 2 2 2 2 6% 66%	2566
	11th to 20th	Night	2 10% 4 10% 5% 3 3 3 5% 7% 49%	2860
	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 5% 7% 54%	2268
	11th to 20th	Night	9% 8% 2 5% 4 4 4 3 5% 7% 49%	2470
	21st to end	Night	9% 5% 2 4 3 4 4 3 5% 6% 56%	2645
December	1st to 10th	Night	15% 7% 3 5% 4 3 4 4 4 5% 46%	2660
	11th to 20th	Night	28% 10% 3 5% 3 3 3 3 5% 6% 31%	3276
	21st to end	Night	18% 8% 3 7% 5% 4 5% 4 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	27% 38% 9% 21% 3 1	2743
	11th to 20th	Day	43% 37% 9% 6% 2 2	2860
	21st to end	Day	39% 40% 6% 11% 3 1	3146
February	1st to 10th	Day	18% 65% 3 6% 6% 1	1710
	11th to 20th	Day	17% 64% 2 7% 8% 2	1737
	21st to end	Day	10% 65% 4 12% 6% 2	1570
March	1st to 10th	Day	38% 42% 6% 7% 6% 1	3427
	11th to 20th	Day	13% 40% 8% 21% 7% 10%	3490
	21st to end	Day	27% 42% 12% 15% 4 1	3853
April	1st to 10th	Day	15% 65% 4 9% 6% 1	2280
	11th to 20th	Day	10% 66% 6% 11% 7% 1	2252
	21st to end	Day	8% 65% 6% 10% 9% 1	2280
May	1st to 10th	Day	9% 39% 7% 21% 13% 11%	3973
	11th to 20th	Day	9% 43% 2 15% 25% 5%	4120
	21st to end	Day	11% 39% 5% 21% 22% 2	4814
June	1st to 10th	Day	8% 65% 3 11% 9% 5%	2660
	11th to 20th	Day	5% 65% 2 11% 14% 4	2660
	21st to end	Day	8% 64% 2 4 12% 11%	2660
July	1st to 10th	Day	7% 36% 1 3 5% 48%	4321
	11th to 20th	Day	11% 36% 2 4 11% 36%	4041
	21st to end	Day	14% 34% 1 6% 22% 23%	4315
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% 58% 4 15% 11% 5%	1881
	11th to 20th	Day	16% 60% 3 11% 7% 2	1710
	21st to end	Day	15% 60% 4 7% 10% 3	1533
December	1st to 10th	Day	20% 61% 5% 7% 4 4	1520
	11th to 20th	Day	32% 48% 3 5% 9% 3	1928
	21st to end	Day	31% 37% 9% 15% 7% 2	2794
January	1st to 10th	Night	26% 38% 13% 18% 3 2	4193
	11th to 20th	Night	43% 38% 9% 5% 3 2	4110
	21st to end	Night	42% 40% 5% 8% 4 2	4521
February	1st to 10th	Night	17% 65% 4 7% 5% 3	2470
	11th to 20th	Night	15% 64% 4 7% 7% 3	2338
	21st to end	Night	10% 65% 4 14% 5% 2	1884
March	1st to 10th	Night	33% 42% 10% 7% 7% 1	3445
	11th to 20th	Night	17% 39% 6% 16% 14% 9%	3490
	21st to end	Night	32% 41% 9% 14% 4 1	3799
April	1st to 10th	Night	13% 65% 4 9% 7% 2	1900
	11th to 20th	Night	12% 66% 4 11% 6% 2	1879
	21st to end	Night	8% 67% 5% 10% 7% 2	1900
May	1st to 10th	Night	12% 40% 6% 18% 16% 8%	2998
	11th to 20th	Night	20% 42% 4 11% 15% 8%	2781
	21st to end	Night	12% 39% 8% 20% 20% 1	2762
June	1st to 10th	Night	10% 66% 3 7% 9% 4	1520
	11th to 20th	Night	8% 65% 2 9% 13% 3	1520
	21st to end	Night	9% 64% 1 3 11% 13%	1520
July	1st to 10th	Night	11% 36% 1 2 6% 43%	2526
	11th to 20th	Night	19% 35% 2 3 8% 32%	2806
	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% 57% 3 13% 10% 6%	2268
	11th to 20th	Night	15% 60% 4 12% 6% 3	2470
	21st to end	Night	14% 60% 4 9% 9% 4	2645
December	1st to 10th	Night	19% 61% 5% 7% 5% 4	2660
	11th to 20th	Night	31% 49% 3 6% 8% 3	3276
	21st to end	Night	31% 38% 10% 13% 6% 2	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	506
April	1st to 10th	Day	16% 18% 31% 22% 10% 2	460
	11th to 20th	Day	11% 21% 32% 23% 9% 3	452
	21st to end	Day	7% 20% 33% 25% 10% 5%	460
May	1st to 10th	Day	12% 17% 33% 27% 10% 2	463
	11th to 20th	Day	12% 15% 32% 29% 10% 2	488
	21st to end	Day	11% 15% 31% 27% 13% 3	529
June	1st to 10th	Day	7% 13% 37% 30% 10% 3	490
	11th to 20th	Day	4 13% 36% 36% 10% 2	490
	21st to end	Day	6% 7% 36% 39% 11% 2	490
July	1st to 10th	Day	6% 6% 27% 47% 13% 1	486
	11th to 20th	Day	4 3 24% 50% 17% 2	484
	21st to end	Day	5% 3 26% 49% 14% 3	531
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	10% 10% 23% 27% 24% 6%	459
	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	836
April	1st to 10th	Night	13% 10% 27% 30% 17% 3	760
	11th to 20th	Night	14% 15% 21% 29% 17% 4	751
	21st to end	Night	6% 11% 28% 28% 21% 4	760
May	1st to 10th	Night	9% 7% 26% 31% 23% 4	656
	11th to 20th	Night	9% 4 21% 34% 28% 4	534
	21st to end	Night	13% 12% 19% 32% 19% 5%	411
June	1st to 10th	Night	8% 8% 23% 34% 25% 2	380
	11th to 20th	Night	5% 6% 21% 45% 21% 2	380
	21st to end	Night	9% 1 19% 37% 30% 4	380
July	1st to 10th	Night	7% 2 13% 44% 28% 6%	383
	11th to 20th	Night	9% 3 8% 39% 31% 9%	552
	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% 14% 27% 34% 9%	754
	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

Page generated at 2024-11-01 19:47:29. Version ac07

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