Grubbs Road Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grubbs Road Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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% 7% 2 6% 4 4 4 4 6% 8% 43%	126804
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 45%	124424
Transparency	Day	39% 4 11% 25% 18% 3	130437
	Night	38% 3 10% 25% 19% 5%	128868
Seeing	Day	15% 8% 23% 25% 20% 9%	16929
	Night	16% 7% 25% 32% 18% 2	26720

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	16% 9% 3 9% 4 3 3 3 5% 6% 38%	3617
	11th to 20th	Day	14% 8% 3 7% 4 3 3 3 4 5% 48%	3872
	21st to end	Day	20% 9% 3 6% 4 3 3 3 4 6% 40%	4488
February	1st to 10th	Day	20% 10% 3 6% 4 3 4 3 5% 5% 37%	1965
	11th to 20th	Day	19% 10% 3 7% 4 4 3 3 4 5% 38%	1948
	21st to end	Day	19% 10% 2 6% 4 4 3 3 4 6% 37%	1650
March	1st to 10th	Day	16% 12% 3 7% 4 4 3 3 4 5% 38%	4480
	11th to 20th	Day	13% 7% 2 6% 3 3 3 3 4 6% 49%	4791
	21st to end	Day	12% 7% 3 7% 5% 5% 4 3 5% 6% 43%	5921
April	1st to 10th	Day	14% 6% 2 5% 4 3 4 4 5% 8% 45%	2400
	11th to 20th	Day	14% 8% 3 7% 5% 4 4 3 5% 6% 41%	2374
	21st to end	Day	16% 8% 2 5% 5% 4 4 3 5% 8% 39%	2400
May	1st to 10th	Day	16% 8% 3 8% 5% 5% 6% 5% 7% 9% 27%	5630
	11th to 20th	Day	15% 8% 3 8% 5% 5% 5% 4 7% 8% 31%	5861
	21st to end	Day	15% 7% 3 6% 5% 4 5% 4 6% 7% 38%	6752
June	1st to 10th	Day	9% 5% 1 5% 4 5% 5% 5% 7% 10% 44%	2800
	11th to 20th	Day	5% 4 2 5% 4 5% 6% 5% 8% 10% 47%	2800
	21st to end	Day	5% 5% 2 5% 5% 5% 6% 4 8% 10% 46%	2800
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 9% 43%	6360
	11th to 20th	Day	5% 4 2 6% 5% 5% 5% 5% 8% 12% 43%	6360
	21st to end	Day	6% 7% 3 7% 5% 6% 5% 5% 7% 11% 37%	6657
August	1st to 10th	Day	4 3 1 4 3 3 4 4 6% 10% 57%	3911
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 6% 9% 45%	3849
	21st to end	Day	5% 3 1 5% 3 4 4 3 6% 8% 57%	4312
September	1st to 10th	Day	10% 5% 2 6% 3 4 4 5% 6% 7% 48%	2400
	11th to 20th	Day	11% 5% 2 5% 4 3 4 3 5% 8% 48%	2292
	21st to end	Day	13% 5% 2 5% 4 3 4 3 4 6% 51%	2200
October	1st to 10th	Day	9% 4 1 5% 3 3 3 3 5% 5% 60%	2147
	11th to 20th	Day	13% 6% 2 5% 3 3 3 2 4 5% 56%	2000
	21st to end	Day	19% 7% 1 4 3 3 4 2 4 5% 48%	2130
November	1st to 10th	Day	11% 5% 2 6% 4 3 4 4 3 6% 53%	1852
	11th to 20th	Day	15% 7% 2 5% 3 3 3 3 4 5% 49%	1890
	21st to end	Day	18% 8% 3 5% 4 4 4 3 4 5% 42%	1890
December	1st to 10th	Day	14% 9% 2 5% 4 4 4 2 4 5% 47%	2959
	11th to 20th	Day	16% 9% 2 6% 3 4 4 3 5% 6% 41%	3064
	21st to end	Day	21% 12% 3 8% 5% 4 4 3 4 5% 29%	3982
January	1st to 10th	Night	17% 10% 3 9% 3 4 4 3 5% 6% 37%	6348
	11th to 20th	Night	14% 8% 3 7% 4 4 4 3 4 4 46%	6105
	21st to end	Night	21% 8% 3 5% 3 3 4 2 4 5% 42%	6490
February	1st to 10th	Night	20% 10% 3 7% 4 4 4 3 5% 6% 35%	2434
	11th to 20th	Night	19% 8% 3 7% 4 3 3 3 4 5% 41%	2338
	21st to end	Night	18% 8% 3 8% 4 3 5% 3 4 5% 39%	1980
March	1st to 10th	Night	18% 10% 3 7% 4 4 4 3 4 5% 38%	5373
	11th to 20th	Night	12% 7% 2 5% 4 4 4 3 5% 6% 49%	5210
	21st to end	Night	13% 7% 2 7% 5% 5% 4 3 5% 6% 42%	5008
April	1st to 10th	Night	14% 5% 2 4 3 4 4 4 5% 7% 48%	2000
	11th to 20th	Night	14% 6% 2 5% 3 3 4 3 5% 7% 48%	1975
	21st to end	Night	15% 7% 2 6% 4 4 4 3 6% 7% 41%	2000
May	1st to 10th	Night	18% 8% 2 8% 5% 3 4 4 6% 8% 34%	4262
	11th to 20th	Night	15% 8% 3 9% 5% 4 4 4 5% 6% 36%	4032
	21st to end	Night	14% 6% 2 6% 4 5% 6% 4 6% 8% 39%	4100
June	1st to 10th	Night	9% 7% 2 6% 3 4 4 4 7% 7% 47%	1600
	11th to 20th	Night	5% 3 1 5% 3 3 5% 5% 6% 8% 56%	1600
	21st to end	Night	3 3 1 3 4 5% 5% 4 7% 12% 53%	1600
July	1st to 10th	Night	8% 5% 2 6% 4 4 5% 4 6% 9% 47%	3618
	11th to 20th	Night	4 4 2 5% 4 5% 5% 4 8% 11% 48%	3617
	21st to end	Night	5% 6% 2 7% 5% 4 5% 4 7% 12% 43%	4210
August	1st to 10th	Night	3 2 1 4 3 4 4 3 6% 8% 62%	3234
	11th to 20th	Night	5% 3 1 5% 4 5% 5% 4 6% 10% 53%	3200
	21st to end	Night	4 4 1 3 3 3 3 3 5% 8% 63%	3586
September	1st to 10th	Night	9% 5% 2 5% 3 3 3 3 4 7% 57%	2000
	11th to 20th	Night	9% 5% 2 4 3 4 4 3 6% 7% 54%	2086
	21st to end	Night	11% 4 2 5% 3 3 3 4 4 7% 55%	2200
October	1st to 10th	Night	8% 3 1 4 2 3 3 2 4 6% 63%	2250
	11th to 20th	Night	12% 5% 1 5% 4 3 3 3 4 5% 57%	2400
	21st to end	Night	18% 7% 2 4 3 3 3 3 5% 5% 48%	2710
November	1st to 10th	Night	12% 4 2 4 3 3 3 3 4 6% 54%	2666
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 4 6% 53%	2730
	21st to end	Night	17% 7% 2 4 4 3 3 2 4 6% 47%	2730
December	1st to 10th	Night	18% 8% 2 5% 3 3 4 2 4 4 47%	4438
	11th to 20th	Night	18% 8% 3 5% 4 3 3 2 4 6% 44%	5298
	21st to end	Night	21% 10% 3 8% 4 4 4 4 4 5% 33%	6996

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	48% 2 11% 25% 13% 2	3618
	11th to 20th	Day	40% 3 6% 29% 21% 2	3872
	21st to end	Day	46% 2 9% 19% 17% 7%	4488
February	1st to 10th	Day	48% 3 10% 24% 13% 2	1965
	11th to 20th	Day	49% 4 9% 21% 13% 5%	1958
	21st to end	Day	50% 3 8% 20% 14% 5%	1650
March	1st to 10th	Day	50% 2 5% 21% 16% 6%	4480
	11th to 20th	Day	37% 3 7% 25% 22% 6%	4791
	21st to end	Day	42% 3 10% 27% 18%	5923
April	1st to 10th	Day	38% 3 11% 26% 19% 2	2400
	11th to 20th	Day	43% 3 9% 23% 18% 4	2374
	21st to end	Day	44% 3 10% 27% 15% 1	2400
May	1st to 10th	Day	52% 5% 14% 18% 10% 1	5665
	11th to 20th	Day	49% 6% 13% 21% 10% 1	5865
	21st to end	Day	44% 4 9% 23% 16% 3	6766
June	1st to 10th	Day	34% 5% 13% 26% 21% 1	2800
	11th to 20th	Day	29% 6% 12% 30% 21% 3	2800
	21st to end	Day	31% 6% 16% 24% 18% 5%	2800
July	1st to 10th	Day	36% 7% 11% 26% 18% 2	6291
	11th to 20th	Day	31% 8% 17% 33% 9% 2	6285
	21st to end	Day	39% 7% 20% 25% 8% 1	6468
August	1st to 10th	Day	22% 5% 15% 32% 24% 1	3898
	11th to 20th	Day	34% 5% 13% 28% 18% 2	3885
	21st to end	Day	24% 5% 11% 36% 21% 3	4311
September	1st to 10th	Day	34% 5% 10% 26% 21% 4	2400
	11th to 20th	Day	35% 5% 7% 24% 26% 3	2292
	21st to end	Day	34% 4 7% 24% 26% 6%	2200
October	1st to 10th	Day	18% 2 6% 17% 42% 15%	3498
	11th to 20th	Day	23% 2 6% 23% 30% 17%	3240
	21st to end	Day	31% 2 5% 27% 29% 6%	3420
November	1st to 10th	Day	33% 4 8% 27% 23% 5%	1852
	11th to 20th	Day	38% 2 7% 23% 24% 6%	1890
	21st to end	Day	45% 3 6% 25% 18% 3	1890
December	1st to 10th	Day	41% 3 8% 30% 17% 1	2959
	11th to 20th	Day	45% 3 10% 24% 16% 3	3061
	21st to end	Day	59% 2 10% 19% 9% 1	3982
January	1st to 10th	Night	49% 2 9% 27% 13% 1	6348
	11th to 20th	Night	43% 2 6% 26% 22% 2	6105
	21st to end	Night	46% 2 6% 25% 16% 5%	6490
February	1st to 10th	Night	51% 3 9% 21% 15% 1	2434
	11th to 20th	Night	46% 3 8% 22% 18% 4	2339
	21st to end	Night	47% 3 8% 20% 19% 4	1980
March	1st to 10th	Night	49% 3 6% 16% 18% 8%	5373
	11th to 20th	Night	38% 2 8% 23% 17% 12%	5210
	21st to end	Night	43% 3 9% 28% 15% 2	5008
April	1st to 10th	Night	36% 4 10% 27% 19% 4	2000
	11th to 20th	Night	38% 3 9% 23% 21% 6%	1975
	21st to end	Night	42% 4 8% 25% 17% 3	2000
May	1st to 10th	Night	49% 4 14% 22% 9% 1	4292
	11th to 20th	Night	48% 5% 14% 19% 12% 1	4033
	21st to end	Night	43% 4 10% 26% 14% 3	4108
June	1st to 10th	Night	33% 5% 13% 23% 23% 2	1600
	11th to 20th	Night	25% 4 13% 30% 23% 5%	1600
	21st to end	Night	22% 5% 18% 28% 20% 7%	1600
July	1st to 10th	Night	33% 6% 13% 24% 21% 3	3580
	11th to 20th	Night	28% 8% 20% 30% 12% 3	3572
	21st to end	Night	33% 6% 21% 29% 10%	4088
August	1st to 10th	Night	19% 5% 15% 37% 23% 1	3223
	11th to 20th	Night	27% 5% 15% 32% 18% 3	3230
	21st to end	Night	20% 4 10% 37% 26% 4	3586
September	1st to 10th	Night	29% 4 8% 30% 23% 6%	2000
	11th to 20th	Night	29% 6% 9% 28% 25% 4	2086
	21st to end	Night	30% 4 9% 24% 26% 7%	2200
October	1st to 10th	Night	17% 3 5% 16% 42% 18%	3679
	11th to 20th	Night	21% 2 5% 18% 40% 14%	3899
	21st to end	Night	29% 2 5% 25% 29% 10%	4370
November	1st to 10th	Night	32% 3 7% 27% 26% 6%	2666
	11th to 20th	Night	33% 2 6% 25% 24% 9%	2730
	21st to end	Night	40% 2 7% 26% 21% 3	2730
December	1st to 10th	Night	41% 3 7% 27% 20% 2	4438
	11th to 20th	Night	43% 3 8% 24% 21% 2	5300
	21st to end	Night	53% 3 9% 23% 10% 2	6996

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% 14% 38% 17% 3 1	300
	11th to 20th	Day	23% 19% 37% 17% 4	300
	21st to end	Day	20% 16% 43% 16% 5%	330
February	1st to 10th	Day	26% 15% 35% 18% 4 1	300
	11th to 20th	Day	29% 20% 38% 11% 2	300
	21st to end	Day	31% 14% 36% 16% 2	249
March	1st to 10th	Day	23% 13% 39% 18% 6%	300
	11th to 20th	Day	23% 17% 39% 18% 2	300
	21st to end	Day	23% 16% 36% 18% 7% 1	530
April	1st to 10th	Day	21% 11% 32% 26% 10%	500
	11th to 20th	Day	18% 18% 27% 24% 11% 2	492
	21st to end	Day	19% 12% 32% 24% 10% 3	500
May	1st to 10th	Day	18% 13% 25% 29% 11% 3	503
	11th to 20th	Day	18% 7% 23% 27% 17% 9%	527
	21st to end	Day	11% 5% 23% 28% 22% 11%	710
June	1st to 10th	Day	11% 5% 17% 26% 27% 15%	730
	11th to 20th	Day	6% 2 14% 35% 31% 12%	730
	21st to end	Day	6% 3 17% 35% 25% 14%	730
July	1st to 10th	Day	7% 2 9% 26% 32% 23%	721
	11th to 20th	Day	6% 2 10% 28% 36% 18%	718
	21st to end	Day	7% 3 11% 26% 35% 19%	794
August	1st to 10th	Day	6% 3 9% 29% 37% 17%	532
	11th to 20th	Day	11% 3 10% 28% 32% 16%	519
	21st to end	Day	9% 2 9% 24% 35% 21%	583
September	1st to 10th	Day	8% 3 11% 26% 34% 17%	530
	11th to 20th	Day	11% 1 8% 26% 35% 20%	530
	21st to end	Day	18% 3 18% 25% 24% 12%	530
October	1st to 10th	Day	9% 2 18% 33% 31% 7%	477
	11th to 20th	Day	14% 8% 31% 28% 14% 7%	330
	21st to end	Day	25% 9% 31% 23% 10% 2	363
November	1st to 10th	Day	16% 10% 30% 27% 13% 3	330
	11th to 20th	Day	11% 14% 36% 27% 12% 1	330
	21st to end	Day	23% 11% 29% 25% 10% 2	330
December	1st to 10th	Day	22% 15% 32% 20% 9% 2	330
	11th to 20th	Day	23% 13% 33% 22% 10%	321
	21st to end	Day	25% 17% 34% 21% 3	330
January	1st to 10th	Night	22% 11% 35% 27% 5%	1000
	11th to 20th	Night	23% 12% 39% 22% 4	1000
	21st to end	Night	22% 9% 39% 27% 2	1100
February	1st to 10th	Night	25% 8% 37% 27% 3	834
	11th to 20th	Night	23% 14% 39% 22% 2	783
	21st to end	Night	22% 14% 39% 23% 2	660
March	1st to 10th	Night	22% 10% 33% 31% 5%	800
	11th to 20th	Night	21% 11% 34% 29% 5%	700
	21st to end	Night	19% 12% 31% 29% 9%	660
April	1st to 10th	Night	16% 10% 37% 30% 7%	600
	11th to 20th	Night	17% 10% 29% 32% 11% 1	593
	21st to end	Night	18% 5% 33% 32% 12% 1	600
May	1st to 10th	Night	18% 7% 27% 34% 13% 1	600
	11th to 20th	Night	14% 5% 21% 37% 20% 2	591
	21st to end	Night	9% 4 17% 39% 27% 3	650
June	1st to 10th	Night	12% 4 12% 39% 30% 4	600
	11th to 20th	Night	6% 2 10% 45% 34% 3	600
	21st to end	Night	4 12% 44% 36% 5%	600
July	1st to 10th	Night	6% 2 8% 28% 46% 10%	585
	11th to 20th	Night	8% 2 8% 29% 45% 9%	582
	21st to end	Night	5% 2 8% 31% 45% 9%	657
August	1st to 10th	Night	5% 2 5% 34% 47% 7%	597
	11th to 20th	Night	7% 1 8% 36% 42% 5%	594
	21st to end	Night	7% 2 5% 31% 46% 8%	660
September	1st to 10th	Night	11% 2 9% 35% 38% 6%	600
	11th to 20th	Night	10% 1 9% 33% 40% 6%	686
	21st to end	Night	11% 3 15% 36% 29% 6%	800
October	1st to 10th	Night	9% 3 17% 38% 29% 4	800
	11th to 20th	Night	14% 4 27% 35% 18% 2	800
	21st to end	Night	21% 7% 27% 34% 10% 1	880
November	1st to 10th	Night	14% 5% 28% 39% 13% 1	820
	11th to 20th	Night	15% 8% 33% 33% 11%	840
	21st to end	Night	20% 8% 29% 31% 10% 1	840
December	1st to 10th	Night	23% 9% 29% 31% 7%	872
	11th to 20th	Night	22% 7% 33% 31% 7%	1036
	21st to end	Night	26% 11% 31% 28% 4	1100

Page generated at 2025-12-15 00:13:56. Version ac07

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