Standeford and Andreas Observatories
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Standeford and Andreas Observatories clear sky chart from 2005-11-06 to 2026-08-15.

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	12% 8% 3 8% 5% 4 5% 4 6% 7% 39%	144367
	Night	12% 9% 3 8% 5% 4 4 4 5% 6% 39%	137850
Transparency	Day	43% 6% 13% 25% 12% 1	147963
	Night	44% 5% 13% 24% 13% 1	142333
Seeing	Day	17% 11% 31% 26% 12% 3	18476
	Night	16% 10% 33% 31% 10% 1	28278

By Month

Percentage Cloud Cover SE (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	7% 15% 6% 20% 9% 7% 5% 4 4 6% 16%	4691
	11th to 20th	Day	5% 12% 6% 12% 7% 7% 9% 7% 9% 7% 18%	5010
	21st to end	Day	5% 12% 6% 13% 6% 6% 8% 4 5% 6% 29%	5812
February	1st to 10th	Day	8% 12% 5% 12% 8% 7% 6% 5% 6% 6% 25%	1890
	11th to 20th	Day	9% 11% 4 10% 6% 5% 6% 5% 6% 7% 30%	1844
	21st to end	Day	13% 9% 3 10% 6% 5% 5% 3 6% 7% 32%	1601
March	1st to 10th	Day	14% 8% 3 6% 4 3 4 3 5% 7% 43%	5046
	11th to 20th	Day	11% 9% 3 5% 4 3 4 3 5% 8% 44%	5090
	21st to end	Day	24% 11% 4 10% 6% 4 4 3 5% 6% 24%	5583
April	1st to 10th	Day	18% 11% 3 7% 4 4 4 4 6% 7% 32%	2505
	11th to 20th	Day	19% 12% 3 7% 4 4 4 3 5% 6% 32%	2531
	21st to end	Day	24% 9% 3 6% 5% 4 4 4 6% 6% 30%	2730
May	1st to 10th	Day	11% 6% 2 5% 4 4 5% 4 6% 7% 48%	7712
	11th to 20th	Day	16% 7% 2 5% 3 3 3 3 5% 7% 46%	8222
	21st to end	Day	17% 7% 3 7% 5% 5% 5% 4 5% 7% 37%	9047
June	1st to 10th	Day	11% 5% 2 6% 3 4 4 4 5% 9% 46%	2940
	11th to 20th	Day	14% 6% 2 6% 5% 5% 5% 4 7% 7% 40%	2940
	21st to end	Day	11% 5% 2 6% 5% 4 4 4 6% 9% 43%	2940
July	1st to 10th	Day	10% 6% 3 7% 5% 4 5% 4 7% 10% 40%	8280
	11th to 20th	Day	6% 3 2 4 3 3 3 3 5% 8% 60%	8280
	21st to end	Day	7% 4 2 6% 4 5% 5% 5% 7% 9% 47%	8581
August	1st to 10th	Day	12% 6% 3 7% 5% 3 4 3 6% 7% 45%	4364
	11th to 20th	Day	12% 6% 3 7% 4 4 5% 4 7% 9% 41%	4224
	21st to end	Day	8% 5% 2 6% 4 4 4 4 6% 7% 50%	4493
September	1st to 10th	Day	13% 5% 2 5% 3 3 4 3 6% 7% 49%	2400
	11th to 20th	Day	11% 5% 1 5% 4 4 4 3 4 7% 50%	2316
	21st to end	Day	13% 6% 2 6% 3 3 4 3 5% 7% 48%	2135
October	1st to 10th	Day	15% 6% 2 5% 4 4 4 4 5% 5% 46%	2000
	11th to 20th	Day	16% 8% 3 5% 3 4 4 3 4 6% 44%	2000
	21st to end	Day	19% 9% 4 5% 3 3 3 3 5% 6% 39%	2200
November	1st to 10th	Day	11% 10% 3 7% 4 3 4 3 6% 7% 41%	1752
	11th to 20th	Day	10% 14% 5% 11% 5% 4 4 3 5% 6% 33%	1680
	21st to end	Day	14% 10% 4 9% 5% 3 5% 4 5% 8% 34%	1680
December	1st to 10th	Day	7% 10% 3 13% 10% 7% 6% 4 5% 7% 28%	2690
	11th to 20th	Day	11% 15% 6% 12% 8% 7% 5% 4 5% 6% 22%	3088
	21st to end	Day	19% 18% 7% 20% 5% 4 3 2 3 4 15%	4070
January	1st to 10th	Night	8% 12% 5% 17% 8% 8% 6% 4 5% 5% 23%	8232
	11th to 20th	Night	5% 11% 4 13% 8% 7% 10% 7% 7% 6% 21%	7964
	21st to end	Night	6% 12% 6% 10% 6% 5% 6% 4 6% 6% 34%	8402
February	1st to 10th	Night	8% 13% 4 11% 7% 5% 4 4 4 6% 33%	2730
	11th to 20th	Night	8% 10% 4 8% 6% 6% 5% 3 5% 7% 38%	2663
	21st to end	Night	11% 8% 3 9% 7% 5% 5% 4 5% 7% 36%	2209
March	1st to 10th	Night	11% 8% 2 6% 4 3 3 3 4 6% 50%	5041
	11th to 20th	Night	11% 7% 2 6% 4 4 4 4 6% 9% 43%	5090
	21st to end	Night	24% 11% 4 8% 4 3 3 3 4 6% 30%	5588
April	1st to 10th	Night	14% 12% 3 6% 5% 4 4 3 5% 6% 38%	2113
	11th to 20th	Night	19% 9% 4 7% 4 3 4 3 5% 6% 37%	2035
	21st to end	Night	19% 11% 3 7% 4 3 4 3 4 7% 36%	1890
May	1st to 10th	Night	10% 5% 2 5% 4 3 4 3 5% 6% 53%	5157
	11th to 20th	Night	15% 6% 2 4 3 3 3 3 4 6% 50%	4670
	21st to end	Night	19% 7% 3 7% 4 4 4 3 5% 6% 38%	5139
June	1st to 10th	Night	10% 5% 2 4 4 4 4 4 5% 8% 52%	1680
	11th to 20th	Night	11% 5% 3 5% 4 4 5% 4 7% 7% 44%	1680
	21st to end	Night	9% 5% 2 5% 3 3 3 3 6% 8% 54%	1680
July	1st to 10th	Night	9% 4 2 6% 4 4 5% 5% 8% 10% 43%	4708
	11th to 20th	Night	5% 4 1 4 3 3 3 3 5% 8% 62%	4707
	21st to end	Night	9% 4 2 6% 4 4 4 4 6% 8% 52%	5509
August	1st to 10th	Night	11% 6% 2 5% 4 4 4 3 5% 7% 49%	3016
	11th to 20th	Night	13% 5% 1 4 3 3 3 3 5% 9% 50%	2924
	21st to end	Night	9% 5% 2 6% 4 3 3 3 4 7% 53%	3408
September	1st to 10th	Night	10% 4 2 5% 3 3 3 2 4 6% 58%	2000
	11th to 20th	Night	11% 4 1 5% 3 3 3 3 6% 8% 51%	2080
	21st to end	Night	10% 5% 2 5% 4 3 3 3 5% 6% 54%	2244
October	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 5% 52%	2400
	11th to 20th	Night	16% 9% 2 6% 2 3 4 3 5% 6% 44%	2400
	21st to end	Night	18% 10% 3 6% 3 3 3 2 5% 6% 40%	2640
November	1st to 10th	Night	12% 8% 3 6% 4 4 4 3 5% 6% 45%	2762
	11th to 20th	Night	12% 12% 4 8% 5% 4 4 3 5% 5% 38%	2940
	21st to end	Night	13% 11% 3 9% 5% 4 4 3 4 7% 38%	2940
December	1st to 10th	Night	6% 9% 4 13% 7% 7% 5% 4 5% 7% 33%	4720
	11th to 20th	Night	11% 14% 6% 13% 8% 5% 5% 3 5% 5% 27%	5339
	21st to end	Night	21% 14% 8% 17% 5% 4 4 3 4 4 17%	7150

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	69% 5% 11% 11% 5%	4692
	11th to 20th	Day	61% 4 10% 16% 9%	5010
	21st to end	Day	55% 5% 14% 20% 7%	5812
February	1st to 10th	Day	57% 7% 13% 15% 8% 1	1890
	11th to 20th	Day	51% 7% 13% 21% 6% 1	1853
	21st to end	Day	50% 7% 14% 19% 10%	1601
March	1st to 10th	Day	40% 5% 16% 28% 9% 2	5046
	11th to 20th	Day	38% 5% 14% 29% 12% 2	5090
	21st to end	Day	61% 6% 11% 17% 5%	5585
April	1st to 10th	Day	50% 6% 13% 20% 9% 1	2505
	11th to 20th	Day	52% 6% 11% 19% 10% 1	2531
	21st to end	Day	53% 7% 11% 20% 8% 1	2730
May	1st to 10th	Day	33% 6% 11% 31% 20%	7750
	11th to 20th	Day	38% 5% 8% 26% 18% 5%	8226
	21st to end	Day	46% 6% 10% 27% 10% 1	9061
June	1st to 10th	Day	34% 8% 14% 28% 14% 1	2940
	11th to 20th	Day	40% 8% 13% 22% 16% 1	2940
	21st to end	Day	36% 8% 15% 26% 12% 1	2940
July	1st to 10th	Day	38% 7% 18% 29% 7%	8211
	11th to 20th	Day	23% 5% 14% 37% 20% 1	8205
	21st to end	Day	31% 7% 16% 33% 12% 1	8394
August	1st to 10th	Day	38% 6% 12% 30% 12% 2	4350
	11th to 20th	Day	38% 7% 17% 29% 8%	4263
	21st to end	Day	33% 7% 16% 29% 14% 1	4492
September	1st to 10th	Day	35% 6% 13% 31% 15%	2400
	11th to 20th	Day	33% 7% 17% 30% 12% 1	2316
	21st to end	Day	36% 6% 12% 30% 15% 1	2135
October	1st to 10th	Day	29% 4 7% 34% 25% 1	3260
	11th to 20th	Day	38% 5% 11% 24% 21% 1	3240
	21st to end	Day	41% 4 13% 30% 11%	3538
November	1st to 10th	Day	42% 7% 14% 20% 15% 3	1752
	11th to 20th	Day	52% 5% 13% 20% 9% 1	1680
	21st to end	Day	48% 6% 18% 20% 7% 1	1680
December	1st to 10th	Day	55% 7% 15% 17% 6% 1	2690
	11th to 20th	Day	64% 5% 15% 12% 4	3085
	21st to end	Day	74% 4 9% 9% 3	4070
January	1st to 10th	Night	62% 5% 13% 15% 4	8233
	11th to 20th	Night	60% 5% 11% 15% 9% 1	7964
	21st to end	Night	50% 4 12% 23% 10%	8402
February	1st to 10th	Night	52% 5% 13% 20% 8% 1	2730
	11th to 20th	Night	46% 6% 13% 25% 9% 2	2664
	21st to end	Night	46% 6% 13% 21% 12% 2	2209
March	1st to 10th	Night	36% 5% 11% 32% 14% 2	5041
	11th to 20th	Night	37% 5% 12% 30% 15% 1	5090
	21st to end	Night	57% 5% 11% 17% 9% 1	5588
April	1st to 10th	Night	46% 6% 13% 23% 12% 1	2113
	11th to 20th	Night	47% 6% 9% 18% 16% 3	2035
	21st to end	Night	48% 6% 9% 21% 14% 2	1890
May	1st to 10th	Night	32% 5% 9% 31% 22% 1	5184
	11th to 20th	Night	35% 5% 8% 20% 23% 7%	4671
	21st to end	Night	46% 6% 9% 26% 12% 2	5149
June	1st to 10th	Night	31% 6% 15% 30% 16% 2	1680
	11th to 20th	Night	36% 7% 14% 21% 19% 3	1680
	21st to end	Night	29% 6% 15% 30% 17% 2	1680
July	1st to 10th	Night	34% 7% 20% 28% 11% 1	4670
	11th to 20th	Night	22% 5% 12% 37% 23% 1	4662
	21st to end	Night	30% 6% 16% 32% 15% 1	5385
August	1st to 10th	Night	35% 6% 12% 28% 16% 3	3006
	11th to 20th	Night	32% 6% 17% 31% 14% 1	2951
	21st to end	Night	31% 6% 18% 27% 17% 2	3408
September	1st to 10th	Night	28% 4 13% 32% 20% 1	2000
	11th to 20th	Night	30% 7% 17% 30% 16% 1	2080
	21st to end	Night	31% 6% 11% 29% 20% 2	2244
October	1st to 10th	Night	23% 3 7% 32% 31% 4	3920
	11th to 20th	Night	30% 4 13% 26% 23% 4	3899
	21st to end	Night	42% 4 11% 29% 14% 1	4252
November	1st to 10th	Night	40% 5% 15% 23% 15% 2	2762
	11th to 20th	Night	48% 7% 12% 20% 12% 1	2940
	21st to end	Night	47% 6% 16% 23% 8% 1	2940
December	1st to 10th	Night	50% 5% 18% 20% 6%	4720
	11th to 20th	Night	59% 6% 13% 18% 4	5341
	21st to end	Night	71% 5% 9% 12% 3	7150

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	15% 15% 35% 23% 10% 2	330
	11th to 20th	Day	21% 14% 37% 22% 5% 1	330
	21st to end	Day	13% 13% 40% 25% 7% 1	363
February	1st to 10th	Day	18% 15% 42% 22% 3 1	330
	11th to 20th	Day	14% 17% 39% 27% 4	330
	21st to end	Day	21% 16% 37% 25% 1	273
March	1st to 10th	Day	20% 13% 43% 21% 3	330
	11th to 20th	Day	26% 14% 32% 22% 5% 1	330
	21st to end	Day	30% 13% 30% 20% 6% 1	363
April	1st to 10th	Day	26% 17% 31% 22% 3	525
	11th to 20th	Day	30% 13% 26% 22% 8% 1	569
	21st to end	Day	29% 14% 28% 20% 8% 2	750
May	1st to 10th	Day	20% 12% 33% 23% 9% 2	753
	11th to 20th	Day	22% 11% 29% 24% 11% 3	775
	21st to end	Day	20% 7% 28% 30% 13% 2	844
June	1st to 10th	Day	12% 9% 26% 32% 17% 4	780
	11th to 20th	Day	14% 10% 35% 29% 11% 2	780
	21st to end	Day	14% 9% 33% 29% 12% 4	780
July	1st to 10th	Day	8% 5% 23% 32% 25% 8%	771
	11th to 20th	Day	9% 5% 26% 34% 19% 7%	768
	21st to end	Day	7% 4 26% 38% 20% 5%	849
August	1st to 10th	Day	10% 4 22% 33% 25% 6%	776
	11th to 20th	Day	12% 5% 23% 31% 23% 6%	736
	21st to end	Day	10% 5% 28% 32% 19% 6%	699
September	1st to 10th	Day	16% 9% 26% 29% 17% 4	530
	11th to 20th	Day	11% 6% 30% 33% 17% 4	446
	21st to end	Day	17% 12% 27% 25% 13% 6%	330
October	1st to 10th	Day	19% 11% 29% 25% 12% 4	330
	11th to 20th	Day	21% 18% 36% 20% 5% 1	330
	21st to end	Day	29% 15% 36% 14% 6%	363
November	1st to 10th	Day	22% 18% 34% 20% 6% 2	330
	11th to 20th	Day	18% 21% 41% 15% 5%	330
	21st to end	Day	23% 17% 38% 18% 4	330
December	1st to 10th	Day	16% 10% 45% 23% 6%	330
	11th to 20th	Day	19% 14% 40% 22% 5%	330
	21st to end	Day	27% 16% 35% 19% 3	363
January	1st to 10th	Night	12% 12% 38% 30% 8% 1	1050
	11th to 20th	Night	15% 13% 35% 31% 6% 1	1050
	21st to end	Night	17% 13% 35% 28% 6%	1155
February	1st to 10th	Night	15% 14% 38% 29% 5%	1050
	11th to 20th	Night	14% 12% 41% 26% 6% 1	1029
	21st to end	Night	15% 12% 36% 29% 7% 1	825
March	1st to 10th	Night	17% 13% 36% 25% 8% 1	642
	11th to 20th	Night	20% 10% 38% 25% 6% 1	630
	21st to end	Night	24% 10% 32% 25% 8% 1	693
April	1st to 10th	Night	20% 12% 38% 25% 5%	630
	11th to 20th	Night	24% 12% 30% 27% 6% 1	623
	21st to end	Night	23% 11% 33% 26% 6% 1	630
May	1st to 10th	Night	18% 9% 32% 31% 9% 1	630
	11th to 20th	Night	19% 9% 32% 29% 10% 1	621
	21st to end	Night	20% 6% 28% 33% 13%	683
June	1st to 10th	Night	12% 5% 29% 39% 14%	630
	11th to 20th	Night	15% 5% 32% 39% 9%	630
	21st to end	Night	10% 6% 29% 40% 14% 1	630
July	1st to 10th	Night	6% 3 22% 39% 28% 3	615
	11th to 20th	Night	4 4 24% 45% 21% 2	612
	21st to end	Night	5% 3 23% 48% 20% 1	690
August	1st to 10th	Night	9% 3 18% 47% 22% 1	627
	11th to 20th	Night	11% 2 17% 42% 24% 3	606
	21st to end	Night	8% 2 24% 41% 23% 2	660
September	1st to 10th	Night	13% 4 22% 43% 16% 2	600
	11th to 20th	Night	12% 5% 32% 33% 16% 2	600
	21st to end	Night	12% 9% 30% 34% 15% 1	644
October	1st to 10th	Night	17% 8% 32% 30% 13% 1	800
	11th to 20th	Night	21% 12% 33% 28% 6% 1	800
	21st to end	Night	24% 12% 31% 27% 6%	880
November	1st to 10th	Night	17% 12% 37% 26% 7% 1	958
	11th to 20th	Night	18% 13% 39% 23% 7%	1050
	21st to end	Night	18% 14% 38% 23% 7%	1050
December	1st to 10th	Night	16% 11% 38% 29% 6% 1	1050
	11th to 20th	Night	20% 10% 36% 27% 6% 1	1050
	21st to end	Night	25% 11% 33% 25% 5%	1155

Page generated at 2026-08-12 02:37:18. Version ac07

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