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 2025-05-26.

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	5% 4 2 5% 3 3 4 3 5% 7% 59%	118075
	Night	6% 5% 2 5% 4 4 4 3 5% 7% 55%	120085
Transparency	Day	23% 6% 10% 19% 24% 17%	117835
	Night	27% 7% 11% 19% 21% 15%	119915
Seeing	Day	7% 9% 24% 32% 21% 7%	16121
	Night	8% 5% 15% 35% 30% 6%	25567

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	9% 6% 3 7% 4 4 4 4 6% 9% 45%	3932
	11th to 20th	Day	8% 6% 3 8% 5% 5% 4 3 4 5% 51%	4100
	21st to end	Day	12% 8% 3 8% 5% 5% 4 3 5% 7% 40%	4510
February	1st to 10th	Day	10% 7% 3 7% 5% 4 5% 4 5% 6% 45%	1800
	11th to 20th	Day	10% 7% 3 7% 5% 3 4 4 7% 8% 42%	1849
	21st to end	Day	10% 6% 3 6% 5% 5% 4 3 6% 6% 46%	1650
March	1st to 10th	Day	8% 7% 3 9% 6% 6% 7% 5% 8% 8% 33%	4881
	11th to 20th	Day	6% 4 2 6% 4 5% 6% 5% 6% 9% 47%	5000
	21st to end	Day	7% 5% 2 6% 6% 6% 6% 5% 8% 10% 38%	5565
April	1st to 10th	Day	9% 7% 2 6% 4 3 4 4 6% 8% 48%	2400
	11th to 20th	Day	8% 5% 2 4 3 3 4 3 5% 7% 56%	2369
	21st to end	Day	5% 4 2 3 4 3 3 4 5% 8% 59%	2400
May	1st to 10th	Day	4 3 1 2 2 2 2 2 4 5% 73%	4033
	11th to 20th	Day	2 2 1 2 2 1 2 2 3 5% 78%	4263
	21st to end	Day	2 5% 2 3 2 2 2 2 4 6% 69%	4886
June	1st to 10th	Day	4 4 1 3 2 3 3 3 5% 7% 63%	2660
	11th to 20th	Day	2 2 1 2 1 2 2 2 4 6% 77%	2660
	21st to end	Day	2 2 1 2 2 2 2 1 3 6% 77%	2660
July	1st to 10th	Day	1 1 1 1 2 2 3 4 84%	4402
	11th to 20th	Day	1 2 1 2 2 2 3 3 4 7% 75%	4110
	21st to end	Day	1 1 2 1 1 2 2 4 7% 78%	4455
August	1st to 10th	Day	1 2 1 3 2 3 4 4 7% 8% 67%	4091
	11th to 20th	Day	1 2 1 2 2 2 4 6% 80%	3819
	21st to end	Day	1 1 2 2 2 4 3 4 7% 73%	4180
September	1st to 10th	Day	1 2 1 2 2 2 2 2 3 5% 78%	2280
	11th to 20th	Day	2 4 1 4 3 3 3 3 5% 7% 66%	2280
	21st to end	Day	2 2 1 3 2 2 2 2 4 5% 76%	2014
October	1st to 10th	Day	2 2 1 3 3 2 3 2 4 6% 73%	3180
	11th to 20th	Day	3 3 1 4 3 3 3 3 5% 7% 65%	3161
	21st to end	Day	3 3 1 4 3 4 3 3 6% 9% 62%	3448
November	1st to 10th	Day	5% 5% 2 4 4 4 4 4 5% 8% 55%	1934
	11th to 20th	Day	6% 7% 2 6% 5% 4 5% 4 5% 7% 50%	1800
	21st to end	Day	9% 5% 2 5% 4 3 3 4 5% 8% 52%	1651
December	1st to 10th	Day	5% 4 2 6% 5% 3 4 3 6% 10% 53%	2620
	11th to 20th	Day	10% 8% 4 9% 6% 7% 5% 5% 8% 8% 30%	3028
	21st to end	Day	11% 10% 4 10% 6% 6% 7% 5% 6% 8% 28%	4004
January	1st to 10th	Night	8% 6% 3 6% 3 4 5% 4 6% 9% 45%	6024
	11th to 20th	Night	9% 7% 4 8% 5% 5% 4 3 4 6% 46%	5880
	21st to end	Night	11% 9% 3 9% 5% 5% 4 4 4 6% 41%	6468
February	1st to 10th	Night	10% 7% 2 6% 5% 4 4 3 4 6% 48%	2600
	11th to 20th	Night	9% 6% 3 6% 5% 5% 5% 5% 6% 8% 43%	2435
	21st to end	Night	8% 5% 2 6% 5% 4 4 4 5% 7% 51%	1980
March	1st to 10th	Night	9% 8% 3 9% 6% 5% 5% 5% 7% 7% 37%	4998
	11th to 20th	Night	9% 6% 3 6% 4 4 3 3 5% 6% 49%	5000
	21st to end	Night	8% 9% 3 8% 4 5% 5% 4 6% 7% 42%	5408
April	1st to 10th	Night	7% 7% 2 6% 5% 4 4 4 6% 8% 48%	2000
	11th to 20th	Night	6% 5% 2 6% 4 3 4 3 5% 8% 54%	1979
	21st to end	Night	4 3 1 4 3 3 4 3 5% 8% 61%	2000
May	1st to 10th	Night	2 5% 1 3 3 3 3 2 4 5% 69%	3158
	11th to 20th	Night	1 3 1 3 3 2 4 3 4 8% 68%	2840
	21st to end	Night	2 3 1 3 2 3 3 3 5% 6% 71%	2805
June	1st to 10th	Night	3 4 1 4 3 3 3 3 6% 9% 61%	1520
	11th to 20th	Night	1 1 2 2 2 2 2 4 6% 78%	1520
	21st to end	Night	1 3 1 3 2 2 2 2 3 5% 76%	1520
July	1st to 10th	Night	2 1 1 1 1 1 2 5% 84%	2554
	11th to 20th	Night	1 1 1 1 2 2 3 4 5% 7% 73%	2860
	21st to end	Night	1 1 3 2 3 4 4 9% 10% 64%	3109
August	1st to 10th	Night	2 3 1 4 3 3 4 4 7% 9% 61%	2848
	11th to 20th	Night	3 1 2 1 2 2 2 4 7% 74%	3025
	21st to end	Night	1 2 1 3 3 3 5% 4 4 7% 69%	3498
September	1st to 10th	Night	1 2 1 2 2 3 3 3 4 8% 72%	1900
	11th to 20th	Night	3 5% 2 4 3 3 3 3 5% 7% 64%	1900
	21st to end	Night	1 2 1 3 2 2 3 2 4 6% 74%	2161
October	1st to 10th	Night	2 2 1 2 2 3 3 2 4 6% 73%	3800
	11th to 20th	Night	4 3 1 4 3 2 3 3 6% 7% 63%	3777
	21st to end	Night	3 4 2 4 3 3 3 3 5% 6% 64%	4121
November	1st to 10th	Night	6% 5% 2 4 3 3 3 4 5% 7% 57%	2364
	11th to 20th	Night	7% 6% 3 5% 4 4 4 3 6% 8% 52%	2600
	21st to end	Night	6% 6% 2 4 4 4 4 3 5% 7% 55%	2745
December	1st to 10th	Night	5% 4 2 5% 3 4 4 3 5% 7% 58%	4560
	11th to 20th	Night	12% 9% 3 9% 6% 5% 6% 5% 7% 11% 28%	5176
	21st to end	Night	12% 9% 3 10% 6% 6% 6% 5% 6% 7% 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	33% 9% 18% 20% 16% 4	3932
	11th to 20th	Day	35% 7% 8% 13% 21% 16%	4100
	21st to end	Day	42% 7% 7% 12% 15% 17%	4510
February	1st to 10th	Day	39% 6% 9% 18% 18% 10%	1800
	11th to 20th	Day	39% 6% 12% 17% 19% 6%	1858
	21st to end	Day	38% 6% 9% 22% 18% 6%	1650
March	1st to 10th	Day	42% 10% 10% 17% 18% 2	4881
	11th to 20th	Day	29% 9% 16% 28% 11% 7%	5000
	21st to end	Day	35% 13% 17% 23% 10% 1	5565
April	1st to 10th	Day	35% 7% 14% 22% 19% 4	2400
	11th to 20th	Day	28% 7% 16% 24% 20% 5%	2372
	21st to end	Day	24% 7% 14% 28% 22% 4	2400
May	1st to 10th	Day	16% 5% 11% 30% 27% 12%	4033
	11th to 20th	Day	11% 4 10% 26% 36% 13%	4266
	21st to end	Day	17% 8% 14% 29% 30% 3	4887
June	1st to 10th	Day	21% 8% 14% 25% 24% 8%	2660
	11th to 20th	Day	10% 4 11% 24% 35% 15%	2660
	21st to end	Day	11% 4 7% 16% 32% 30%	2660
July	1st to 10th	Day	5% 2 5% 10% 15% 62%	4333
	11th to 20th	Day	8% 3 6% 11% 16% 55%	4041
	21st to end	Day	6% 2 5% 11% 36% 40%	4315
August	1st to 10th	Day	12% 4 7% 15% 31% 31%	4076
	11th to 20th	Day	7% 3 5% 12% 44% 28%	3857
	21st to end	Day	9% 4 7% 16% 36% 28%	4179
September	1st to 10th	Day	9% 3 5% 14% 29% 40%	2280
	11th to 20th	Day	18% 5% 10% 24% 27% 16%	2280
	21st to end	Day	11% 4 8% 21% 34% 22%	2014
October	1st to 10th	Day	14% 3 6% 12% 28% 38%	3180
	11th to 20th	Day	17% 5% 10% 24% 28% 16%	3161
	21st to end	Day	18% 5% 9% 25% 29% 14%	3448
November	1st to 10th	Day	28% 5% 11% 23% 21% 12%	1934
	11th to 20th	Day	32% 7% 10% 23% 21% 7%	1800
	21st to end	Day	31% 5% 11% 17% 27% 9%	1651
December	1st to 10th	Day	26% 5% 8% 24% 30% 7%	2620
	11th to 20th	Day	45% 8% 13% 15% 14% 4	3028
	21st to end	Day	50% 10% 13% 16% 10% 2	4004
January	1st to 10th	Night	32% 10% 20% 21% 14% 3	6024
	11th to 20th	Night	38% 7% 10% 13% 21% 11%	5880
	21st to end	Night	43% 6% 8% 10% 18% 15%	6468
February	1st to 10th	Night	37% 6% 10% 18% 19% 11%	2600
	11th to 20th	Night	38% 7% 11% 18% 20% 6%	2437
	21st to end	Night	33% 7% 12% 28% 15% 5%	1980
March	1st to 10th	Night	42% 10% 12% 18% 16% 3	4998
	11th to 20th	Night	34% 8% 12% 24% 15% 7%	5000
	21st to end	Night	41% 11% 14% 21% 11% 2	5408
April	1st to 10th	Night	33% 8% 16% 23% 16% 4	2000
	11th to 20th	Night	29% 8% 13% 25% 18% 7%	1979
	21st to end	Night	20% 9% 14% 32% 20% 6%	2000
May	1st to 10th	Night	18% 7% 11% 26% 25% 13%	3158
	11th to 20th	Night	13% 6% 16% 24% 23% 17%	2841
	21st to end	Night	14% 10% 15% 28% 28% 4	2805
June	1st to 10th	Night	20% 9% 14% 24% 25% 8%	1520
	11th to 20th	Night	10% 6% 10% 23% 32% 19%	1520
	21st to end	Night	12% 4 7% 16% 32% 28%	1520
July	1st to 10th	Night	5% 3 5% 9% 16% 63%	2514
	11th to 20th	Night	7% 5% 6% 15% 16% 51%	2806
	21st to end	Night	8% 4 8% 13% 33% 34%	3011
August	1st to 10th	Night	15% 5% 8% 17% 27% 28%	2839
	11th to 20th	Night	9% 3 6% 11% 43% 27%	3053
	21st to end	Night	11% 5% 9% 17% 33% 25%	3498
September	1st to 10th	Night	11% 5% 7% 13% 25% 39%	1900
	11th to 20th	Night	19% 6% 11% 23% 27% 14%	1900
	21st to end	Night	12% 5% 8% 21% 29% 25%	2161
October	1st to 10th	Night	13% 3 7% 11% 25% 42%	3800
	11th to 20th	Night	18% 4 11% 24% 25% 18%	3777
	21st to end	Night	21% 4 8% 22% 26% 19%	4121
November	1st to 10th	Night	26% 6% 12% 24% 19% 13%	2364
	11th to 20th	Night	30% 7% 12% 24% 19% 8%	2600
	21st to end	Night	28% 6% 11% 20% 22% 13%	2745
December	1st to 10th	Night	26% 6% 8% 24% 27% 10%	4560
	11th to 20th	Night	46% 7% 12% 15% 16% 3	5176
	21st to end	Night	49% 10% 15% 15% 8% 3	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	20% 12% 25% 26% 14% 4	324
	11th to 20th	Day	13% 8% 24% 32% 18% 5%	400
	21st to end	Day	12% 9% 27% 28% 20% 5%	440
February	1st to 10th	Day	14% 17% 25% 23% 16% 6%	400
	11th to 20th	Day	15% 17% 28% 25% 13% 2	442
	21st to end	Day	10% 14% 28% 32% 14% 2	414
March	1st to 10th	Day	16% 16% 30% 27% 9% 2	500
	11th to 20th	Day	16% 15% 26% 25% 14% 4	500
	21st to end	Day	11% 17% 35% 24% 10% 3	550
April	1st to 10th	Day	12% 15% 33% 25% 12% 4	500
	11th to 20th	Day	10% 16% 29% 25% 13% 6%	492
	21st to end	Day	5% 16% 30% 29% 14% 6%	500
May	1st to 10th	Day	5% 13% 34% 29% 14% 5%	503
	11th to 20th	Day	7% 12% 31% 29% 16% 5%	528
	21st to end	Day	6% 12% 27% 31% 16% 7%	550
June	1st to 10th	Day	4 10% 29% 31% 20% 6%	490
	11th to 20th	Day	2 9% 29% 36% 19% 6%	490
	21st to end	Day	1 4 22% 40% 24% 8%	490
July	1st to 10th	Day	2 4 15% 44% 28% 7%	486
	11th to 20th	Day	1 1 14% 42% 32% 10%	484
	21st to end	Day	1 1 16% 40% 32% 10%	531
August	1st to 10th	Day	2 2 21% 40% 24% 10%	487
	11th to 20th	Day	1 2 21% 40% 26% 11%	477
	21st to end	Day	2 18% 39% 30% 10%	539
September	1st to 10th	Day	1 4 13% 40% 31% 11%	490
	11th to 20th	Day	2 7% 24% 34% 24% 9%	490
	21st to end	Day	2 8% 17% 34% 29% 10%	490
October	1st to 10th	Day	2 7% 22% 34% 23% 13%	490
	11th to 20th	Day	5% 8% 18% 30% 27% 12%	490
	21st to end	Day	6% 5% 24% 31% 24% 9%	539
November	1st to 10th	Day	6% 9% 24% 25% 25% 10%	470
	11th to 20th	Day	5% 9% 19% 30% 25% 12%	300
	21st to end	Day	9% 5% 23% 28% 29% 6%	225
December	1st to 10th	Day	7% 12% 19% 29% 25% 10%	200
	11th to 20th	Day	11% 13% 20% 24% 23% 10%	200
	21st to end	Day	14% 16% 23% 27% 16% 3	220
January	1st to 10th	Night	16% 6% 17% 34% 22% 5%	800
	11th to 20th	Night	12% 6% 16% 36% 24% 7%	800
	21st to end	Night	16% 5% 14% 37% 23% 5%	880
February	1st to 10th	Night	13% 8% 17% 37% 21% 4	800
	11th to 20th	Night	12% 7% 21% 35% 20% 5%	784
	21st to end	Night	11% 8% 20% 40% 18% 3	660
March	1st to 10th	Night	14% 11% 19% 31% 22% 3	800
	11th to 20th	Night	14% 9% 18% 33% 23% 4	800
	21st to end	Night	12% 9% 22% 36% 18% 2	880
April	1st to 10th	Night	10% 8% 22% 38% 21% 3	800
	11th to 20th	Night	8% 11% 21% 34% 21% 5%	791
	21st to end	Night	5% 6% 25% 35% 24% 6%	800
May	1st to 10th	Night	5% 4 22% 39% 26% 5%	730
	11th to 20th	Night	4 5% 18% 35% 33% 5%	547
	21st to end	Night	4 6% 18% 37% 29% 5%	422
June	1st to 10th	Night	3 4 16% 43% 31% 3	380
	11th to 20th	Night	1 2 15% 47% 34% 1	380
	21st to end	Night	1 4 9% 39% 44% 3	380
July	1st to 10th	Night	1 2 6% 32% 54% 4	371
	11th to 20th	Night	1 3 31% 50% 14%	552
	21st to end	Night	1 1 6% 25% 56% 11%	624
August	1st to 10th	Night	5% 4 5% 29% 48% 10%	567
	11th to 20th	Night	1 2 11% 34% 45% 8%	687
	21st to end	Night	1 1 9% 32% 46% 11%	836
September	1st to 10th	Night	2 2 10% 32% 44% 10%	760
	11th to 20th	Night	5% 4 13% 33% 38% 8%	760
	21st to end	Night	2 2 11% 31% 43% 11%	760
October	1st to 10th	Night	4 2 11% 36% 37% 10%	760
	11th to 20th	Night	6% 3 11% 30% 39% 10%	760
	21st to end	Night	5% 5% 10% 35% 33% 13%	836
November	1st to 10th	Night	7% 4 15% 31% 33% 9%	780
	11th to 20th	Night	9% 3 13% 38% 31% 7%	800
	21st to end	Night	9% 4 15% 40% 26% 6%	800
December	1st to 10th	Night	11% 5% 17% 36% 25% 6%	800
	11th to 20th	Night	17% 6% 18% 35% 20% 4	800
	21st to end	Night	16% 9% 22% 32% 18% 2	880

Page generated at 2025-05-23 06:21:01. Version ac07

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