Stanford
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stanford clear sky chart from 2005-11-06 to 2025-11-03.

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 2 4 3 3 4 3 5% 7% 61%	135191
	Night	5% 5% 2 5% 4 4 4 3 5% 7% 56%	132317
Transparency	Day	22% 6% 10% 20% 25% 17%	130124
	Night	26% 7% 11% 20% 22% 15%	128370
Seeing	Day	7% 9% 24% 32% 21% 8%	16766
	Night	8% 5% 15% 35% 31% 6%	26092

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%	3933
	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 8% 6% 6% 7% 5% 8% 8% 33%	4883
	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%	5563
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	3 2 1 2 1 2 2 1 3 5% 76%	5603
	11th to 20th	Day	2 2 1 2 2 1 2 2 3 5% 79%	5925
	21st to end	Day	2 3 1 2 2 2 2 2 4 7% 73%	6902
June	1st to 10th	Day	4 4 1 3 2 3 3 3 5% 7% 64%	2800
	11th to 20th	Day	2 2 1 1 1 1 2 2 4 6% 78%	2800
	21st to end	Day	2 2 1 2 2 2 2 1 3 6% 77%	2800
July	1st to 10th	Day	1 1 1 1 2 2 3 4 83%	6286
	11th to 20th	Day	1 2 2 2 4 3 4 7% 76%	5880
	21st to end	Day	1 1 3 2 2 3 3 5% 8% 72%	6402
August	1st to 10th	Day	1 1 1 2 2 2 3 3 5% 7% 74%	5777
	11th to 20th	Day	1 3 2 2 3 2 4 6% 77%	5500
	21st to end	Day	1 1 2 2 2 5% 3 5% 8% 72%	5984
September	1st to 10th	Day	1 2 1 3 2 2 3 2 3 5% 77%	2400
	11th to 20th	Day	2 4 2 5% 3 3 3 3 5% 7% 65%	2400
	21st to end	Day	2 2 1 3 2 2 2 2 4 5% 75%	2119
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%	1955
	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%	1650
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%	6021
	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%	2438
	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%	4995
	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%	5410
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 4 1 3 2 2 3 2 4 4 73%	4386
	11th to 20th	Night	1 2 1 2 3 2 4 3 4 8% 69%	3982
	21st to end	Night	1 3 1 3 2 2 2 2 4 5% 74%	3972
June	1st to 10th	Night	3 4 1 4 3 3 3 3 6% 9% 62%	1600
	11th to 20th	Night	1 1 2 2 2 2 2 4 5% 79%	1600
	21st to end	Night	1 3 1 3 2 2 2 2 3 5% 77%	1600
July	1st to 10th	Night	1 1 1 1 1 1 3 4 85%	3657
	11th to 20th	Night	1 1 2 2 3 4 5% 5% 6% 72%	4100
	21st to end	Night	1 1 3 3 3 4 5% 11% 11% 58%	4472
August	1st to 10th	Night	1 2 1 3 2 2 3 3 5% 7% 71%	4035
	11th to 20th	Night	2 1 2 2 3 3 3 6% 6% 72%	4363
	21st to end	Night	2 1 3 2 3 6% 5% 5% 7% 68%	5015
September	1st to 10th	Night	1 2 1 2 2 3 3 3 4 7% 71%	2000
	11th to 20th	Night	3 5% 2 4 3 3 3 3 5% 7% 63%	2000
	21st to end	Night	2 2 1 3 2 2 3 3 4 6% 73%	2276
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 4 3 5% 6% 63%	4253
November	1st to 10th	Night	6% 5% 2 4 3 3 3 4 5% 7% 57%	2385
	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	3933
	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	4883
	11th to 20th	Day	29% 9% 16% 28% 11% 7%	5000
	21st to end	Day	35% 13% 17% 23% 10% 1	5563
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	13% 4 9% 29% 32% 14%	5603
	11th to 20th	Day	10% 3 10% 32% 36% 9%	5928
	21st to end	Day	13% 6% 11% 31% 36% 3	6904
June	1st to 10th	Day	20% 7% 14% 26% 25% 8%	2800
	11th to 20th	Day	9% 4 11% 24% 36% 16%	2800
	21st to end	Day	10% 3 7% 16% 34% 30%	2800
July	1st to 10th	Day	4 2 4 12% 23% 54%	6218
	11th to 20th	Day	6% 3 6% 10% 20% 54%	5811
	21st to end	Day	8% 4 6% 20% 33% 29%	6217
August	1st to 10th	Day	12% 4 7% 15% 32% 31%	4206
	11th to 20th	Day	7% 3 5% 13% 44% 29%	3982
	21st to end	Day	9% 4 7% 16% 37% 27%	4311
September	1st to 10th	Day	10% 3 5% 15% 29% 38%	2400
	11th to 20th	Day	19% 5% 10% 24% 27% 15%	2400
	21st to end	Day	11% 4 8% 22% 33% 22%	2119
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% 11% 23% 21% 12%	1955
	11th to 20th	Day	32% 7% 10% 23% 21% 7%	1800
	21st to end	Day	31% 5% 11% 17% 27% 9%	1650
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	6021
	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%	2440
	21st to end	Night	33% 7% 12% 28% 15% 5%	1980
March	1st to 10th	Night	42% 10% 12% 18% 16% 3	4995
	11th to 20th	Night	34% 8% 12% 24% 15% 7%	5000
	21st to end	Night	41% 11% 14% 21% 11% 2	5410
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	16% 5% 9% 27% 29% 14%	4383
	11th to 20th	Night	12% 5% 16% 31% 25% 12%	3983
	21st to end	Night	13% 8% 11% 28% 37% 3	3972
June	1st to 10th	Night	19% 8% 14% 25% 26% 8%	1600
	11th to 20th	Night	9% 6% 9% 22% 34% 19%	1600
	21st to end	Night	12% 4 7% 16% 33% 28%	1600
July	1st to 10th	Night	4 2 4 11% 23% 56%	3617
	11th to 20th	Night	5% 5% 6% 15% 15% 53%	4046
	21st to end	Night	8% 5% 9% 22% 31% 24%	4345
August	1st to 10th	Night	15% 5% 8% 17% 28% 28%	2929
	11th to 20th	Night	9% 3 6% 12% 43% 27%	3150
	21st to end	Night	10% 5% 9% 16% 34% 25%	3608
September	1st to 10th	Night	12% 5% 7% 14% 25% 37%	2000
	11th to 20th	Night	19% 6% 11% 24% 27% 14%	2000
	21st to end	Night	13% 5% 8% 21% 29% 25%	2276
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% 26% 18%	4253
November	1st to 10th	Night	26% 6% 12% 24% 19% 13%	2385
	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	325
	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% 32% 16% 8%	573
June	1st to 10th	Day	4 9% 27% 31% 21% 8%	530
	11th to 20th	Day	2 9% 30% 36% 19% 5%	530
	21st to end	Day	1 5% 22% 38% 25% 9%	530
July	1st to 10th	Day	2 4 16% 44% 28% 8%	526
	11th to 20th	Day	1 1 13% 41% 33% 12%	524
	21st to end	Day	1 1 15% 41% 32% 10%	575
August	1st to 10th	Day	2 3 22% 38% 24% 12%	527
	11th to 20th	Day	1 2 20% 40% 26% 11%	517
	21st to end	Day	2 17% 38% 31% 11%	583
September	1st to 10th	Day	1 4 15% 39% 31% 10%	530
	11th to 20th	Day	3 6% 23% 34% 25% 10%	530
	21st to end	Day	3 8% 18% 33% 29% 10%	530
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% 24% 10%	583
November	1st to 10th	Day	6% 9% 24% 26% 25% 10%	479
	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%	552
	21st to end	Night	4 6% 18% 37% 30% 4	433
June	1st to 10th	Night	3 4 15% 43% 32% 3	400
	11th to 20th	Night	1 2 16% 47% 34% 1	400
	21st to end	Night	1 4 9% 38% 45% 3	400
July	1st to 10th	Night	1 2 7% 34% 52% 4	394
	11th to 20th	Night	1 3 30% 51% 14%	582
	21st to end	Night	1 1 6% 25% 56% 11%	657
August	1st to 10th	Night	4 4 6% 29% 46% 10%	597
	11th to 20th	Night	1 2 11% 35% 44% 8%	724
	21st to end	Night	1 1 9% 32% 46% 11%	880
September	1st to 10th	Night	2 2 10% 33% 43% 10%	800
	11th to 20th	Night	5% 4 13% 33% 38% 8%	800
	21st to end	Night	2 2 11% 32% 42% 11%	800
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% 35% 33% 12%	880
November	1st to 10th	Night	7% 4 15% 31% 33% 9%	788
	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 2025-10-30 23:02:37. Version ac07

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