Stanford Faculty Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stanford Faculty Observatory clear sky chart from 2005-11-06 to 2026-09-29.

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	4 4 1 4 3 3 4 3 5% 7% 62%	157440
	Night	5% 5% 2 5% 4 4 4 4 5% 7% 56%	151511
Transparency	Day	21% 5% 9% 20% 26% 18%	152374
	Night	25% 6% 11% 19% 23% 16%	147566
Seeing	Day	6% 9% 24% 32% 21% 8%	17943
	Night	8% 5% 15% 35% 30% 6%	27252

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	10% 7% 3 7% 4 4 5% 4 6% 9% 43%	5099
	11th to 20th	Day	6% 4 2 6% 4 4 3 3 4 6% 57%	5340
	21st to end	Day	10% 8% 3 8% 5% 5% 5% 3 6% 8% 40%	5843
February	1st to 10th	Day	10% 7% 3 6% 5% 4 5% 4 5% 6% 46%	1890
	11th to 20th	Day	11% 7% 3 7% 5% 4 5% 4 7% 8% 40%	1944
	21st to end	Day	9% 6% 3 6% 5% 5% 4 3 6% 7% 45%	1730
March	1st to 10th	Day	8% 7% 3 8% 6% 6% 7% 5% 8% 8% 34%	4989
	11th to 20th	Day	6% 4 2 6% 4 5% 5% 5% 6% 9% 48%	5110
	21st to end	Day	7% 5% 2 6% 6% 6% 6% 5% 8% 10% 38%	5688
April	1st to 10th	Day	9% 7% 2 6% 4 3 4 4 6% 8% 48%	2520
	11th to 20th	Day	7% 5% 2 4 3 3 4 4 5% 7% 55%	2489
	21st to end	Day	5% 4 2 3 4 3 3 3 6% 8% 58%	2520
May	1st to 10th	Day	3 3 1 4 2 3 3 2 4 6% 68%	7245
	11th to 20th	Day	2 1 1 2 1 1 2 1 2 6% 81%	7707
	21st to end	Day	2 3 1 2 2 2 3 3 5% 7% 70%	8992
June	1st to 10th	Day	4 4 1 3 2 3 3 3 5% 7% 65%	2940
	11th to 20th	Day	1 2 1 1 1 2 2 2 4 7% 78%	2940
	21st to end	Day	1 2 1 2 2 2 2 2 3 6% 77%	2940
July	1st to 10th	Day	1 1 1 2 3 2 3 5% 81%	8188
	11th to 20th	Day	2 2 1 3 2 3 4 3 4 6% 71%	7650
	21st to end	Day	1 1 2 2 2 3 3 5% 7% 76%	8304
August	1st to 10th	Day	1 1 2 1 2 3 3 5% 6% 77%	7547
	11th to 20th	Day	1 1 3 3 3 4 3 5% 7% 69%	7174
	21st to end	Day	1 1 1 3 2 3 5% 3 5% 9% 67%	7750
September	1st to 10th	Day	1 2 1 3 2 2 3 2 4 5% 76%	2520
	11th to 20th	Day	2 4 1 5% 3 3 3 3 5% 7% 65%	2520
	21st to end	Day	2 2 1 3 2 2 2 2 4 6% 75%	2207
October	1st to 10th	Day	2 2 1 3 3 2 3 2 4 6% 72%	3280
	11th to 20th	Day	3 3 1 3 3 3 3 3 5% 7% 65%	3261
	21st to end	Day	3 3 1 4 3 4 3 3 6% 9% 62%	3558
November	1st to 10th	Day	5% 5% 2 4 4 4 4 4 5% 8% 55%	2032
	11th to 20th	Day	7% 7% 2 6% 5% 4 5% 4 5% 7% 48%	1890
	21st to end	Day	9% 5% 2 5% 4 4 3 4 5% 8% 53%	1733
December	1st to 10th	Day	4 3 2 6% 5% 3 4 3 6% 10% 53%	2700
	11th to 20th	Day	10% 7% 4 9% 6% 7% 5% 5% 8% 8% 31%	3108
	21st to end	Day	11% 10% 4 10% 5% 6% 7% 5% 6% 8% 28%	4092
January	1st to 10th	Night	10% 7% 3 7% 4 5% 5% 4 6% 8% 42%	7799
	11th to 20th	Night	7% 6% 3 6% 4 4 3 3 5% 7% 52%	7650
	21st to end	Night	9% 9% 3 9% 5% 5% 5% 4 5% 7% 39%	8371
February	1st to 10th	Night	10% 7% 2 6% 5% 4 4 3 4 6% 48%	2730
	11th to 20th	Night	9% 6% 3 6% 5% 5% 5% 4 6% 8% 42%	2560
	21st to end	Night	8% 6% 2 6% 5% 4 4 4 5% 7% 50%	2076
March	1st to 10th	Night	9% 8% 3 8% 6% 5% 5% 5% 6% 7% 38%	5110
	11th to 20th	Night	9% 6% 3 6% 4 4 3 3 5% 6% 50%	5110
	21st to end	Night	8% 9% 3 8% 4 5% 5% 4 6% 7% 42%	5527
April	1st to 10th	Night	7% 7% 2 6% 5% 4 4 4 7% 7% 48%	2100
	11th to 20th	Night	6% 5% 2 6% 4 3 4 3 5% 8% 53%	2079
	21st to end	Night	4 3 1 4 3 3 4 3 6% 8% 60%	2100
May	1st to 10th	Night	2 5% 2 4 3 3 3 3 5% 5% 65%	5705
	11th to 20th	Night	1 2 1 2 2 2 4 3 4 7% 73%	5158
	21st to end	Night	1 3 1 3 2 2 3 3 5% 5% 71%	5181
June	1st to 10th	Night	3 3 1 4 3 3 3 3 6% 9% 62%	1680
	11th to 20th	Night	1 1 2 2 1 2 2 4 5% 79%	1680
	21st to end	Night	1 3 1 3 2 2 2 2 3 5% 77%	1680
July	1st to 10th	Night	1 1 2 2 3 2 3 5% 81%	4754
	11th to 20th	Night	1 2 1 3 3 3 3 4 4 5% 69%	5340
	21st to end	Night	1 1 3 3 3 4 4 9% 10% 63%	5806
August	1st to 10th	Night	1 2 1 2 2 2 3 3 5% 6% 74%	5275
	11th to 20th	Night	2 1 3 3 4 5% 4 7% 8% 63%	5687
	21st to end	Night	1 2 1 3 2 3 5% 5% 5% 7% 66%	6499
September	1st to 10th	Night	1 2 1 2 2 4 3 3 4 7% 70%	2100
	11th to 20th	Night	3 4 2 4 3 3 3 3 5% 7% 64%	2100
	21st to end	Night	1 2 1 3 2 2 3 3 4 6% 73%	2365
October	1st to 10th	Night	2 2 1 2 2 3 3 2 4 6% 73%	3920
	11th to 20th	Night	4 4 1 4 3 2 3 3 6% 7% 63%	3897
	21st to end	Night	3 4 2 4 3 3 3 3 5% 6% 63%	4253
November	1st to 10th	Night	6% 5% 2 4 3 3 4 4 5% 7% 57%	2485
	11th to 20th	Night	7% 6% 3 5% 4 4 4 3 6% 8% 50%	2730
	21st to end	Night	6% 5% 2 4 4 4 4 4 5% 7% 55%	2882
December	1st to 10th	Night	5% 4 2 5% 3 4 4 3 5% 7% 58%	4700
	11th to 20th	Night	12% 9% 3 9% 6% 5% 6% 5% 7% 11% 28%	5316
	21st to end	Night	12% 9% 3 10% 6% 6% 6% 5% 6% 7% 30%	7106

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	35% 9% 17% 22% 14% 3	5100
	11th to 20th	Day	28% 5% 7% 13% 27% 20%	5340
	21st to end	Day	39% 7% 8% 16% 17% 13%	5843
February	1st to 10th	Day	38% 6% 9% 19% 18% 10%	1890
	11th to 20th	Day	40% 7% 12% 17% 18% 6%	1953
	21st to end	Day	38% 6% 11% 22% 17% 6%	1730
March	1st to 10th	Day	42% 10% 10% 18% 18% 2	4989
	11th to 20th	Day	29% 8% 15% 27% 12% 8%	5110
	21st to end	Day	35% 12% 17% 23% 11% 2	5688
April	1st to 10th	Day	35% 7% 14% 21% 19% 4	2520
	11th to 20th	Day	28% 7% 16% 24% 20% 5%	2492
	21st to end	Day	24% 7% 14% 29% 22% 4	2520
May	1st to 10th	Day	17% 4 8% 26% 32% 12%	7245
	11th to 20th	Day	8% 3 8% 29% 39% 13%	7710
	21st to end	Day	12% 6% 11% 31% 36% 4	8994
June	1st to 10th	Day	19% 7% 14% 25% 26% 8%	2940
	11th to 20th	Day	9% 4 11% 23% 37% 17%	2940
	21st to end	Day	10% 4 7% 16% 34% 29%	2940
July	1st to 10th	Day	5% 2 5% 13% 28% 47%	8120
	11th to 20th	Day	12% 3 5% 9% 18% 53%	7581
	21st to end	Day	7% 3 5% 16% 32% 37%	8119
August	1st to 10th	Day	9% 3 6% 12% 37% 33%	5976
	11th to 20th	Day	10% 5% 8% 20% 36% 20%	5656
	21st to end	Day	13% 4 7% 20% 37% 20%	6077
September	1st to 10th	Day	10% 3 6% 15% 29% 36%	2520
	11th to 20th	Day	18% 5% 10% 23% 28% 15%	2520
	21st to end	Day	11% 4 8% 22% 33% 21%	2207
October	1st to 10th	Day	14% 3 6% 12% 28% 37%	3280
	11th to 20th	Day	17% 5% 10% 24% 29% 15%	3261
	21st to end	Day	19% 5% 9% 25% 29% 14%	3558
November	1st to 10th	Day	28% 5% 12% 23% 21% 11%	2032
	11th to 20th	Day	34% 7% 10% 22% 20% 7%	1890
	21st to end	Day	30% 5% 11% 18% 27% 9%	1733
December	1st to 10th	Day	26% 5% 8% 25% 30% 7%	2700
	11th to 20th	Day	45% 8% 13% 15% 14% 5%	3108
	21st to end	Day	50% 10% 13% 16% 10% 2	4092
January	1st to 10th	Night	36% 10% 18% 21% 13% 3	7799
	11th to 20th	Night	31% 6% 8% 13% 27% 15%	7650
	21st to end	Night	41% 6% 8% 14% 18% 12%	8371
February	1st to 10th	Night	37% 6% 10% 18% 19% 11%	2730
	11th to 20th	Night	38% 7% 11% 18% 19% 6%	2562
	21st to end	Night	34% 7% 13% 27% 15% 4	2076
March	1st to 10th	Night	42% 10% 12% 18% 16% 2	5110
	11th to 20th	Night	33% 8% 12% 24% 16% 7%	5110
	21st to end	Night	40% 11% 14% 21% 12% 2	5527
April	1st to 10th	Night	32% 8% 17% 22% 17% 4	2100
	11th to 20th	Night	29% 8% 14% 24% 18% 7%	2079
	21st to end	Night	20% 9% 14% 32% 20% 5%	2100
May	1st to 10th	Night	20% 6% 9% 23% 31% 11%	5703
	11th to 20th	Night	9% 4 13% 28% 31% 15%	5159
	21st to end	Night	13% 7% 13% 27% 37% 3	5182
June	1st to 10th	Night	19% 8% 13% 24% 27% 8%	1680
	11th to 20th	Night	9% 6% 9% 22% 34% 20%	1680
	21st to end	Night	11% 4 7% 16% 34% 28%	1680
July	1st to 10th	Night	4 2 5% 13% 27% 49%	4714
	11th to 20th	Night	12% 5% 5% 12% 15% 52%	5286
	21st to end	Night	8% 5% 8% 18% 31% 31%	5679
August	1st to 10th	Night	11% 4 7% 14% 28% 36%	4169
	11th to 20th	Night	12% 4 11% 18% 34% 20%	4474
	21st to end	Night	12% 4 8% 22% 33% 19%	5092
September	1st to 10th	Night	12% 5% 8% 14% 25% 36%	2100
	11th to 20th	Night	18% 6% 11% 23% 29% 14%	2100
	21st to end	Night	13% 5% 8% 21% 29% 24%	2365
October	1st to 10th	Night	13% 3 7% 11% 25% 40%	3920
	11th to 20th	Night	18% 4 10% 25% 25% 18%	3897
	21st to end	Night	21% 4 9% 22% 27% 18%	4253
November	1st to 10th	Night	26% 6% 12% 24% 19% 12%	2485
	11th to 20th	Night	32% 6% 12% 24% 18% 8%	2730
	21st to end	Night	28% 6% 12% 21% 21% 12%	2882
December	1st to 10th	Night	26% 6% 8% 24% 26% 10%	4700
	11th to 20th	Night	46% 7% 12% 15% 15% 4	5316
	21st to end	Night	50% 9% 15% 15% 8% 3	7106

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	19% 14% 25% 24% 13% 4	350
	11th to 20th	Day	12% 8% 22% 32% 21% 5%	430
	21st to end	Day	11% 9% 27% 29% 19% 5%	473
February	1st to 10th	Day	13% 16% 25% 25% 15% 6%	430
	11th to 20th	Day	17% 17% 28% 25% 12% 2	477
	21st to end	Day	9% 15% 28% 34% 13% 2	446
March	1st to 10th	Day	15% 16% 31% 27% 10% 2	540
	11th to 20th	Day	15% 14% 25% 25% 17% 4	540
	21st to end	Day	11% 16% 34% 25% 11% 3	594
April	1st to 10th	Day	11% 15% 32% 26% 13% 4	540
	11th to 20th	Day	10% 17% 29% 25% 14% 5%	532
	21st to end	Day	6% 16% 29% 30% 14% 5%	540
May	1st to 10th	Day	4 13% 34% 29% 15% 6%	543
	11th to 20th	Day	7% 13% 31% 29% 16% 5%	568
	21st to end	Day	6% 11% 27% 31% 17% 8%	617
June	1st to 10th	Day	4 9% 28% 31% 21% 8%	570
	11th to 20th	Day	1 8% 29% 36% 20% 6%	570
	21st to end	Day	1 6% 22% 37% 25% 9%	570
July	1st to 10th	Day	1 4 17% 43% 27% 8%	566
	11th to 20th	Day	1 1 15% 40% 32% 12%	564
	21st to end	Day	1 1 18% 40% 30% 10%	619
August	1st to 10th	Day	2 2 20% 38% 24% 14%	567
	11th to 20th	Day	1 2 20% 39% 26% 11%	557
	21st to end	Day	1 2 18% 38% 31% 11%	627
September	1st to 10th	Day	1 4 15% 38% 31% 11%	570
	11th to 20th	Day	2 7% 24% 34% 24% 9%	570
	21st to end	Day	3 7% 18% 33% 29% 10%	563
October	1st to 10th	Day	2 7% 22% 32% 24% 13%	530
	11th to 20th	Day	5% 8% 18% 30% 27% 12%	530
	21st to end	Day	6% 5% 25% 30% 25% 10%	583
November	1st to 10th	Day	6% 9% 23% 25% 25% 12%	508
	11th to 20th	Day	6% 10% 19% 30% 23% 11%	330
	21st to end	Day	9% 5% 21% 28% 29% 8%	247
December	1st to 10th	Day	6% 10% 18% 29% 27% 10%	220
	11th to 20th	Day	10% 12% 19% 23% 25% 11%	220
	21st to end	Day	15% 19% 21% 26% 16% 2	242
January	1st to 10th	Night	17% 7% 18% 33% 21% 5%	840
	11th to 20th	Night	12% 6% 15% 36% 25% 6%	840
	21st to end	Night	15% 5% 14% 37% 23% 5%	924
February	1st to 10th	Night	13% 7% 17% 39% 20% 4	840
	11th to 20th	Night	12% 8% 22% 34% 19% 5%	824
	21st to end	Night	11% 8% 20% 41% 18% 3	692
March	1st to 10th	Night	13% 11% 20% 32% 21% 2	840
	11th to 20th	Night	13% 9% 17% 34% 24% 4	840
	21st to end	Night	11% 9% 22% 38% 18% 2	924
April	1st to 10th	Night	9% 8% 21% 38% 21% 3	840
	11th to 20th	Night	8% 11% 22% 35% 21% 4	831
	21st to end	Night	5% 5% 25% 35% 24% 5%	840
May	1st to 10th	Night	5% 4 22% 39% 26% 5%	767
	11th to 20th	Night	3 5% 19% 35% 32% 5%	575
	21st to end	Night	4 6% 18% 37% 29% 5%	455
June	1st to 10th	Night	3 4 16% 43% 31% 3	420
	11th to 20th	Night	1 2 15% 46% 34% 1	420
	21st to end	Night	1 4 10% 38% 45% 3	420
July	1st to 10th	Night	1 2 8% 34% 51% 3	411
	11th to 20th	Night	1 1 3 32% 49% 14%	612
	21st to end	Night	1 1 6% 26% 55% 11%	690
August	1st to 10th	Night	4 4 6% 28% 48% 11%	627
	11th to 20th	Night	1 2 11% 34% 44% 8%	760
	21st to end	Night	1 1 9% 33% 46% 10%	924
September	1st to 10th	Night	2 2 10% 33% 43% 10%	840
	11th to 20th	Night	5% 4 13% 33% 37% 8%	840
	21st to end	Night	2 2 11% 32% 42% 11%	832
October	1st to 10th	Night	5% 3 11% 37% 37% 9%	800
	11th to 20th	Night	7% 3 12% 30% 37% 11%	800
	21st to end	Night	5% 5% 10% 36% 33% 12%	880
November	1st to 10th	Night	7% 4 15% 31% 34% 10%	820
	11th to 20th	Night	10% 4 13% 38% 29% 7%	840
	21st to end	Night	9% 4 14% 41% 27% 6%	840
December	1st to 10th	Night	10% 5% 16% 36% 26% 6%	840
	11th to 20th	Night	16% 6% 17% 36% 21% 4	840
	21st to end	Night	17% 10% 22% 32% 18% 2	924

Page generated at 2026-09-26 01:18:41. Version ac07

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