Graystar Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Graystar Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	16% 7% 2 6% 4 4 4 4 5% 7% 41%	130675
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 42%	130139
Transparency	Day	40% 6% 13% 23% 16% 2	134202
	Night	42% 5% 11% 22% 16% 3	134699
Seeing	Day	20% 14% 31% 24% 10% 2	16616
	Night	20% 12% 30% 27% 10% 1	25846

By Month

Percentage Cloud Cover NE (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	13% 6% 2 5% 4 4 3 3 4 6% 50%	4661
	11th to 20th	Day	15% 8% 2 6% 3 4 4 3 4 5% 47%	4650
	21st to end	Day	15% 8% 3 7% 4 4 4 3 5% 6% 41%	5496
February	1st to 10th	Day	16% 8% 2 6% 4 4 4 4 4 7% 40%	1946
	11th to 20th	Day	15% 6% 2 7% 5% 3 3 3 5% 7% 45%	1998
	21st to end	Day	20% 7% 3 6% 4 3 4 4 5% 6% 38%	1650
March	1st to 10th	Day	22% 7% 2 5% 3 3 3 3 4 5% 43%	4476
	11th to 20th	Day	11% 10% 3 5% 4 3 4 4 6% 8% 43%	4694
	21st to end	Day	18% 7% 2 6% 3 3 4 3 5% 6% 42%	5970
April	1st to 10th	Day	19% 7% 2 6% 4 3 4 3 5% 7% 38%	2400
	11th to 20th	Day	18% 6% 2 6% 4 4 4 3 5% 7% 41%	2374
	21st to end	Day	18% 7% 2 5% 4 4 4 4 6% 7% 40%	2453
May	1st to 10th	Day	28% 9% 2 5% 3 3 4 3 5% 8% 29%	5886
	11th to 20th	Day	24% 9% 2 6% 4 4 3 3 4 6% 36%	6151
	21st to end	Day	21% 7% 2 5% 4 4 4 4 6% 8% 35%	6949
June	1st to 10th	Day	18% 5% 2 5% 4 4 4 5% 6% 8% 39%	2800
	11th to 20th	Day	15% 7% 3 5% 5% 4 5% 4 6% 7% 40%	2800
	21st to end	Day	11% 6% 2 6% 5% 4 4 4 6% 9% 44%	2800
July	1st to 10th	Day	12% 7% 3 7% 5% 5% 5% 4 6% 10% 37%	6360
	11th to 20th	Day	11% 7% 2 6% 5% 5% 6% 5% 8% 11% 34%	6360
	21st to end	Day	13% 6% 2 6% 4 4 5% 4 6% 9% 41%	6926
August	1st to 10th	Day	17% 6% 3 7% 4 4 4 4 6% 10% 35%	4289
	11th to 20th	Day	13% 6% 2 6% 4 4 5% 4 7% 9% 41%	3850
	21st to end	Day	8% 5% 2 5% 4 4 6% 4 6% 9% 48%	4312
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 3 5% 7% 47%	2400
	11th to 20th	Day	11% 6% 2 5% 3 3 3 3 4 8% 51%	2285
	21st to end	Day	19% 7% 3 6% 4 4 4 3 6% 6% 39%	2200
October	1st to 10th	Day	19% 5% 2 5% 4 4 4 3 4 7% 43%	2135
	11th to 20th	Day	16% 4 2 4 4 3 4 3 5% 7% 47%	2000
	21st to end	Day	18% 6% 1 5% 3 4 3 3 5% 7% 44%	2011
November	1st to 10th	Day	14% 5% 2 5% 4 3 3 3 4 5% 52%	1851
	11th to 20th	Day	15% 6% 2 5% 3 3 4 3 4 6% 49%	1890
	21st to end	Day	16% 7% 2 6% 3 4 3 3 4 8% 45%	1858
December	1st to 10th	Day	15% 6% 2 6% 4 4 3 4 5% 7% 45%	2680
	11th to 20th	Day	22% 7% 2 3 3 2 2 3 4 5% 46%	3066
	21st to end	Day	18% 8% 3 6% 4 4 4 3 5% 6% 39%	4048
January	1st to 10th	Night	13% 8% 3 6% 3 3 3 2 4 6% 49%	8234
	11th to 20th	Night	16% 8% 2 6% 3 3 3 3 4 5% 47%	8235
	21st to end	Night	16% 9% 2 6% 4 4 5% 3 4 5% 43%	8681
February	1st to 10th	Night	16% 9% 3 5% 4 3 4 3 5% 6% 41%	2672
	11th to 20th	Night	19% 7% 3 5% 4 3 4 3 4 6% 42%	2399
	21st to end	Night	19% 7% 2 6% 4 5% 3 3 4 6% 42%	1980
March	1st to 10th	Night	20% 9% 2 5% 3 3 3 3 4 5% 44%	5364
	11th to 20th	Night	15% 10% 3 5% 4 3 4 3 5% 5% 43%	5233
	21st to end	Night	19% 9% 3 6% 4 4 3 3 5% 5% 39%	4962
April	1st to 10th	Night	19% 7% 3 6% 5% 4 5% 3 5% 6% 38%	2000
	11th to 20th	Night	19% 7% 3 5% 4 4 5% 4 5% 6% 39%	1975
	21st to end	Night	18% 8% 2 6% 4 3 4 3 4 6% 42%	1945
May	1st to 10th	Night	25% 10% 4 8% 3 4 4 3 5% 7% 27%	4054
	11th to 20th	Night	23% 10% 3 6% 4 4 4 3 4 5% 34%	3772
	21st to end	Night	23% 9% 2 7% 3 3 4 3 5% 7% 35%	3926
June	1st to 10th	Night	18% 7% 2 4 5% 4 4 3 6% 7% 40%	1600
	11th to 20th	Night	15% 7% 3 6% 3 4 4 5% 6% 10% 38%	1600
	21st to end	Night	11% 5% 3 6% 4 5% 6% 4 7% 9% 39%	1600
July	1st to 10th	Night	13% 10% 3 8% 5% 4 5% 4 6% 8% 34%	3600
	11th to 20th	Night	16% 10% 3 8% 4 5% 4 4 6% 8% 31%	3600
	21st to end	Night	14% 7% 2 5% 4 4 4 3 5% 8% 42%	3926
August	1st to 10th	Night	23% 6% 3 6% 4 5% 4 3 6% 7% 32%	2824
	11th to 20th	Night	14% 6% 2 6% 5% 5% 4 4 6% 7% 40%	3197
	21st to end	Night	9% 5% 2 4 4 4 4 3 5% 8% 52%	3586
September	1st to 10th	Night	14% 5% 2 6% 4 5% 5% 5% 5% 6% 44%	2000
	11th to 20th	Night	12% 5% 2 5% 4 3 3 2 4 6% 52%	2113
	21st to end	Night	18% 6% 2 7% 4 4 4 3 4 7% 41%	2200
October	1st to 10th	Night	19% 8% 3 5% 4 3 4 3 4 6% 43%	2262
	11th to 20th	Night	16% 5% 2 5% 3 3 3 3 5% 6% 49%	2400
	21st to end	Night	20% 6% 2 5% 3 3 3 2 4 6% 44%	2827
November	1st to 10th	Night	12% 6% 2 5% 4 3 4 3 4 6% 52%	2667
	11th to 20th	Night	19% 6% 2 4 3 3 2 3 4 6% 49%	2730
	21st to end	Night	15% 4 2 5% 4 4 4 4 5% 6% 47%	2757
December	1st to 10th	Night	16% 7% 2 5% 3 3 4 2 4 6% 48%	4720
	11th to 20th	Night	25% 7% 2 4 3 3 3 2 4 4 44%	5348
	21st to end	Night	19% 8% 2 6% 3 3 4 3 5% 5% 42%	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	36% 3 11% 31% 18% 2	4662
	11th to 20th	Day	39% 3 12% 23% 18% 5%	4680
	21st to end	Day	42% 4 14% 23% 15% 1	5496
February	1st to 10th	Day	44% 5% 15% 21% 13% 1	1946
	11th to 20th	Day	42% 5% 10% 25% 17% 2	2007
	21st to end	Day	44% 5% 12% 22% 14% 3	1650
March	1st to 10th	Day	40% 5% 12% 21% 15% 8%	4479
	11th to 20th	Day	36% 6% 14% 27% 14% 2	4694
	21st to end	Day	42% 4 10% 15% 24% 5%	5970
April	1st to 10th	Day	42% 5% 11% 21% 17% 4	2400
	11th to 20th	Day	39% 6% 12% 23% 17% 3	2374
	21st to end	Day	40% 5% 10% 22% 18% 4	2453
May	1st to 10th	Day	53% 7% 15% 14% 10% 1	5897
	11th to 20th	Day	49% 5% 12% 20% 14% 1	6155
	21st to end	Day	45% 6% 11% 22% 14% 1	6949
June	1st to 10th	Day	40% 8% 13% 23% 15% 1	2800
	11th to 20th	Day	40% 8% 13% 24% 13% 2	2800
	21st to end	Day	35% 8% 15% 25% 16% 1	2800
July	1st to 10th	Day	42% 10% 11% 20% 15%	6290
	11th to 20th	Day	40% 10% 19% 20% 9% 1	6285
	21st to end	Day	37% 8% 13% 22% 19% 1	6728
August	1st to 10th	Day	42% 8% 16% 24% 9%	4275
	11th to 20th	Day	35% 8% 15% 32% 10% 1	3886
	21st to end	Day	28% 7% 14% 29% 20% 1	4312
September	1st to 10th	Day	35% 6% 15% 22% 19% 2	2400
	11th to 20th	Day	31% 5% 14% 25% 24% 2	2285
	21st to end	Day	45% 6% 11% 18% 19% 2	2200
October	1st to 10th	Day	32% 6% 13% 22% 22% 4	3473
	11th to 20th	Day	24% 5% 12% 24% 31% 4	3240
	21st to end	Day	28% 5% 16% 26% 22% 3	3223
November	1st to 10th	Day	35% 4 14% 24% 20% 2	1851
	11th to 20th	Day	37% 4 13% 22% 21% 3	1890
	21st to end	Day	39% 4 15% 29% 10% 2	1858
December	1st to 10th	Day	40% 4 16% 27% 12% 1	2680
	11th to 20th	Day	42% 3 14% 24% 15% 3	3066
	21st to end	Day	45% 5% 10% 22% 16% 2	4048
January	1st to 10th	Night	38% 3 12% 25% 20% 2	8234
	11th to 20th	Night	41% 4 12% 21% 17% 6%	8280
	21st to end	Night	44% 4 9% 20% 18% 4	8681
February	1st to 10th	Night	44% 6% 13% 23% 12% 2	2672
	11th to 20th	Night	44% 5% 9% 21% 17% 4	2401
	21st to end	Night	44% 4 9% 22% 17% 4	1980
March	1st to 10th	Night	43% 3 11% 21% 13% 9%	5374
	11th to 20th	Night	43% 5% 12% 24% 12% 4	5234
	21st to end	Night	46% 5% 7% 14% 23% 5%	4962
April	1st to 10th	Night	47% 5% 10% 19% 16% 4	2000
	11th to 20th	Night	44% 6% 9% 18% 21% 2	1975
	21st to end	Night	43% 6% 9% 19% 19% 5%	1945
May	1st to 10th	Night	56% 7% 12% 14% 10% 2	4060
	11th to 20th	Night	52% 5% 12% 15% 14% 2	3773
	21st to end	Night	48% 7% 12% 20% 12% 2	3927
June	1st to 10th	Night	43% 5% 10% 20% 20% 1	1600
	11th to 20th	Night	39% 9% 14% 25% 12% 1	1600
	21st to end	Night	39% 9% 12% 25% 13% 2	1600
July	1st to 10th	Night	46% 9% 11% 21% 12% 1	3560
	11th to 20th	Night	47% 8% 14% 22% 8% 1	3552
	21st to end	Night	39% 8% 14% 23% 16% 1	3809
August	1st to 10th	Night	50% 8% 17% 16% 9% 1	2814
	11th to 20th	Night	38% 9% 15% 28% 10% 1	3227
	21st to end	Night	28% 8% 12% 33% 18% 1	3586
September	1st to 10th	Night	41% 6% 10% 24% 18% 1	2000
	11th to 20th	Night	34% 6% 11% 24% 23% 3	2113
	21st to end	Night	45% 5% 8% 20% 19% 2	2200
October	1st to 10th	Night	38% 5% 11% 23% 20% 3	3704
	11th to 20th	Night	25% 4 10% 26% 30% 4	3899
	21st to end	Night	31% 4 12% 29% 22% 2	4565
November	1st to 10th	Night	36% 4 13% 22% 20% 4	2667
	11th to 20th	Night	40% 4 10% 22% 20% 4	2730
	21st to end	Night	38% 5% 13% 27% 15% 1	2757
December	1st to 10th	Night	39% 5% 13% 28% 13% 3	4720
	11th to 20th	Night	46% 3 11% 24% 14% 3	5348
	21st to end	Night	45% 4 12% 19% 17% 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	26% 25% 30% 15% 4	220
	11th to 20th	Day	23% 30% 31% 14% 2	220
	21st to end	Day	22% 22% 39% 13% 4 1	309
February	1st to 10th	Day	23% 21% 39% 15% 1	386
	11th to 20th	Day	25% 21% 37% 15% 2	516
	21st to end	Day	26% 22% 33% 15% 4	414
March	1st to 10th	Day	24% 22% 33% 19% 3	500
	11th to 20th	Day	18% 22% 39% 17% 3 1	500
	21st to end	Day	25% 20% 33% 16% 4 1	550
April	1st to 10th	Day	23% 23% 33% 15% 5% 1	500
	11th to 20th	Day	24% 16% 34% 18% 7%	492
	21st to end	Day	25% 16% 36% 18% 5% 1	500
May	1st to 10th	Day	30% 16% 31% 17% 4 1	503
	11th to 20th	Day	21% 13% 35% 25% 5% 1	527
	21st to end	Day	20% 9% 29% 26% 13% 3	573
June	1st to 10th	Day	20% 8% 32% 27% 12% 2	530
	11th to 20th	Day	16% 10% 33% 28% 11% 2	530
	21st to end	Day	13% 8% 34% 29% 13% 3	530
July	1st to 10th	Day	13% 6% 25% 34% 18% 4	525
	11th to 20th	Day	9% 5% 26% 39% 17% 4	524
	21st to end	Day	13% 7% 30% 33% 15% 3	576
August	1st to 10th	Day	13% 6% 27% 32% 17% 5%	526
	11th to 20th	Day	12% 7% 23% 37% 17% 4	519
	21st to end	Day	11% 7% 29% 34% 13% 6%	583
September	1st to 10th	Day	19% 6% 28% 25% 18% 4	530
	11th to 20th	Day	13% 6% 26% 30% 16% 8%	530
	21st to end	Day	21% 10% 29% 24% 13% 4	530
October	1st to 10th	Day	23% 8% 26% 22% 15% 5%	530
	11th to 20th	Day	17% 15% 31% 23% 12% 2	530
	21st to end	Day	22% 13% 35% 20% 8% 2	478
November	1st to 10th	Day	15% 16% 30% 26% 10% 2	425
	11th to 20th	Day	21% 23% 28% 22% 6% 1	430
	21st to end	Day	20% 20% 31% 22% 6% 1	398
December	1st to 10th	Day	23% 20% 36% 16% 5%	220
	11th to 20th	Day	32% 20% 30% 15% 4	220
	21st to end	Day	26% 18% 34% 14% 7%	242
January	1st to 10th	Night	20% 21% 37% 20% 3	840
	11th to 20th	Night	26% 19% 30% 22% 3	840
	21st to end	Night	21% 19% 38% 21% 2	924
February	1st to 10th	Night	21% 18% 36% 22% 2	840
	11th to 20th	Night	25% 20% 38% 16% 1	804
	21st to end	Night	24% 18% 35% 19% 4	660
March	1st to 10th	Night	23% 17% 36% 19% 5%	800
	11th to 20th	Night	22% 13% 38% 22% 5% 1	800
	21st to end	Night	22% 15% 36% 22% 5%	880
April	1st to 10th	Night	23% 16% 35% 21% 5%	800
	11th to 20th	Night	23% 11% 33% 27% 5%	791
	21st to end	Night	22% 12% 33% 27% 6% 1	745
May	1st to 10th	Night	24% 11% 31% 25% 8% 1	600
	11th to 20th	Night	23% 10% 33% 28% 6%	487
	21st to end	Night	22% 4 21% 33% 17% 3	433
June	1st to 10th	Night	20% 6% 25% 35% 15%	400
	11th to 20th	Night	15% 5% 24% 40% 16% 1	400
	21st to end	Night	12% 7% 23% 42% 16% 1	400
July	1st to 10th	Night	14% 4 19% 45% 17% 1	390
	11th to 20th	Night	14% 4 19% 41% 20% 3	388
	21st to end	Night	12% 4 21% 42% 21%	438
August	1st to 10th	Night	13% 2 22% 42% 20% 2	532
	11th to 20th	Night	15% 3 18% 38% 24% 2	790
	21st to end	Night	15% 3 17% 39% 23% 3	880
September	1st to 10th	Night	16% 5% 24% 36% 17% 3	800
	11th to 20th	Night	15% 4 21% 34% 21% 5%	800
	21st to end	Night	21% 5% 25% 31% 18% 2	800
October	1st to 10th	Night	23% 8% 24% 26% 16% 3	800
	11th to 20th	Night	17% 13% 28% 28% 13% 1	800
	21st to end	Night	24% 11% 33% 24% 8%	880
November	1st to 10th	Night	15% 12% 32% 28% 11% 1	820
	11th to 20th	Night	21% 15% 32% 24% 7%	840
	21st to end	Night	18% 18% 30% 26% 6% 1	840
December	1st to 10th	Night	24% 18% 34% 19% 4 1	840
	11th to 20th	Night	25% 18% 32% 23% 3	840
	21st to end	Night	24% 15% 36% 21% 3	924

Page generated at 2026-02-13 01:42:17. Version ac07

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