Shady Grove Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Shady Grove 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	11% 6% 2 5% 4 4 4 3 5% 7% 50%	131330
	Night	11% 5% 2 5% 3 3 3 3 5% 6% 54%	129820
Transparency	Day	34% 5% 10% 25% 21% 6%	134982
	Night	31% 4 9% 24% 24% 8%	134232
Seeing	Day	13% 7% 23% 28% 22% 7%	17034
	Night	13% 8% 27% 34% 17% 2	27016

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	11% 8% 3 6% 3 4 3 3 4 6% 49%	5289
	11th to 20th	Day	6% 5% 2 5% 3 4 3 2 4 5% 62%	5310
	21st to end	Day	19% 8% 2 5% 2 2 2 2 4 5% 47%	5812
February	1st to 10th	Day	17% 9% 3 6% 4 3 3 2 4 5% 44%	1890
	11th to 20th	Day	15% 9% 3 5% 4 4 3 3 4 5% 44%	1957
	21st to end	Day	17% 9% 3 5% 3 3 3 3 4 6% 45%	1717
March	1st to 10th	Day	16% 7% 2 5% 3 3 3 3 3 4 51%	4936
	11th to 20th	Day	9% 5% 2 5% 4 3 4 3 4 6% 54%	4980
	21st to end	Day	14% 9% 2 7% 4 4 4 4 5% 7% 40%	5462
April	1st to 10th	Day	12% 6% 2 5% 4 4 4 3 5% 7% 48%	2205
	11th to 20th	Day	16% 6% 2 5% 4 4 3 3 5% 5% 47%	2374
	21st to end	Day	17% 7% 3 7% 4 4 3 3 4 6% 41%	2508
May	1st to 10th	Day	17% 8% 3 6% 4 4 4 3 5% 6% 41%	5862
	11th to 20th	Day	12% 6% 2 7% 5% 4 5% 3 5% 7% 44%	5862
	21st to end	Day	15% 7% 3 7% 5% 5% 5% 4 5% 7% 37%	6434
June	1st to 10th	Day	8% 5% 2 6% 4 3 5% 4 7% 9% 48%	2600
	11th to 20th	Day	7% 5% 2 5% 4 4 4 4 7% 9% 49%	2600
	21st to end	Day	4 3 1 3 4 3 4 3 5% 8% 59%	2600
July	1st to 10th	Day	6% 5% 2 6% 5% 5% 6% 5% 7% 10% 42%	5900
	11th to 20th	Day	4 4 2 5% 4 3 4 3 5% 9% 57%	5900
	21st to end	Day	4 3 1 4 3 3 5% 4 6% 9% 58%	6422
August	1st to 10th	Day	3 3 1 5% 3 4 4 3 6% 8% 60%	4234
	11th to 20th	Day	8% 3 2 5% 3 4 4 4 6% 8% 54%	4171
	21st to end	Day	5% 3 2 4 4 4 5% 4 6% 8% 55%	4384
September	1st to 10th	Day	11% 5% 1 4 3 4 4 4 5% 6% 53%	2246
	11th to 20th	Day	10% 3 2 6% 4 4 5% 4 6% 8% 50%	2200
	21st to end	Day	8% 3 1 5% 4 4 5% 3 5% 6% 57%	2200
October	1st to 10th	Day	6% 4 1 5% 4 4 3 3 5% 5% 60%	2065
	11th to 20th	Day	11% 4 1 4 2 3 3 1 3 5% 63%	2000
	21st to end	Day	16% 6% 2 5% 3 3 3 2 4 4 53%	2200
November	1st to 10th	Day	10% 6% 2 4 3 2 3 4 5% 5% 57%	2058
	11th to 20th	Day	10% 5% 2 4 2 3 3 2 4 5% 61%	1964
	21st to end	Day	15% 7% 2 3 2 2 3 3 4 7% 53%	1890
December	1st to 10th	Day	14% 6% 1 5% 3 2 3 3 4 6% 52%	3030
	11th to 20th	Day	15% 7% 3 5% 3 3 3 2 4 6% 48%	3481
	21st to end	Day	20% 10% 2 5% 4 3 4 2 4 5% 41%	4587
January	1st to 10th	Night	13% 6% 2 8% 3 3 3 3 5% 6% 46%	7634
	11th to 20th	Night	5% 4 2 4 2 3 3 2 3 6% 67%	7680
	21st to end	Night	22% 7% 2 5% 4 3 3 3 4 5% 43%	8402
February	1st to 10th	Night	15% 7% 2 6% 4 3 2 2 4 6% 49%	2730
	11th to 20th	Night	16% 9% 3 5% 3 3 2 2 4 6% 46%	2456
	21st to end	Night	14% 9% 2 6% 3 3 3 2 4 5% 50%	1911
March	1st to 10th	Night	18% 7% 3 6% 3 2 3 2 4 5% 48%	4919
	11th to 20th	Night	9% 4 2 4 3 3 3 2 4 6% 60%	4980
	21st to end	Night	12% 6% 2 6% 4 4 4 3 5% 7% 46%	5467
April	1st to 10th	Night	12% 5% 3 4 4 3 3 3 5% 6% 51%	2194
	11th to 20th	Night	13% 5% 1 5% 4 3 3 2 4 7% 53%	1975
	21st to end	Night	14% 6% 2 6% 4 3 3 2 5% 7% 49%	1888
May	1st to 10th	Night	17% 6% 2 5% 4 3 4 3 5% 7% 45%	4053
	11th to 20th	Night	11% 5% 2 6% 4 4 4 4 6% 8% 48%	4053
	21st to end	Night	12% 5% 2 6% 4 4 4 4 6% 8% 45%	4443
June	1st to 10th	Night	5% 4 1 6% 3 3 3 3 6% 9% 58%	1800
	11th to 20th	Night	6% 3 1 3 3 3 3 3 5% 9% 60%	1800
	21st to end	Night	2 2 1 3 3 3 3 3 5% 8% 69%	1800
July	1st to 10th	Night	6% 5% 2 6% 4 5% 5% 5% 7% 10% 47%	4078
	11th to 20th	Night	3 3 1 3 2 2 3 3 5% 9% 65%	4077
	21st to end	Night	3 2 1 3 2 3 3 3 7% 10% 63%	4449
August	1st to 10th	Night	2 1 1 3 2 4 4 4 5% 8% 67%	2926
	11th to 20th	Night	4 3 1 3 3 4 3 4 6% 10% 61%	2888
	21st to end	Night	3 3 1 4 3 3 3 4 5% 8% 62%	3513
September	1st to 10th	Night	8% 5% 2 5% 3 3 4 3 4 6% 58%	2150
	11th to 20th	Night	9% 4 2 5% 4 3 4 4 6% 6% 54%	2200
	21st to end	Night	7% 3 2 4 3 3 3 3 5% 5% 62%	2200
October	1st to 10th	Night	7% 3 2 4 3 3 4 3 4 6% 63%	2314
	11th to 20th	Night	11% 4 1 4 3 2 3 2 3 4 63%	2400
	21st to end	Night	16% 5% 2 4 2 3 2 2 3 5% 56%	2640
November	1st to 10th	Night	11% 5% 2 3 3 2 2 3 3 5% 61%	2460
	11th to 20th	Night	8% 5% 2 4 3 3 3 2 4 5% 61%	2651
	21st to end	Night	14% 6% 1 4 3 2 3 3 4 5% 55%	2730
December	1st to 10th	Night	10% 8% 2 5% 3 3 3 2 4 6% 54%	4380
	11th to 20th	Night	16% 6% 2 5% 3 3 2 2 3 4 53%	4946
	21st to end	Night	19% 9% 2 5% 3 3 3 2 4 4 47%	6633

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	39% 3 9% 26% 22% 2	5290
	11th to 20th	Day	27% 4 7% 21% 35% 5%	5310
	21st to end	Day	42% 3 5% 20% 25% 6%	5812
February	1st to 10th	Day	45% 4 9% 20% 18% 4	1890
	11th to 20th	Day	44% 4 8% 23% 17% 4	1967
	21st to end	Day	43% 3 6% 17% 22% 9%	1717
March	1st to 10th	Day	38% 3 6% 16% 19% 19%	4936
	11th to 20th	Day	32% 4 8% 18% 20% 17%	4980
	21st to end	Day	43% 5% 10% 28% 13% 1	5464
April	1st to 10th	Day	37% 5% 11% 22% 19% 7%	2205
	11th to 20th	Day	38% 4 11% 22% 19% 5%	2374
	21st to end	Day	44% 5% 10% 20% 19% 2	2508
May	1st to 10th	Day	44% 5% 10% 23% 15% 2	5900
	11th to 20th	Day	39% 5% 12% 22% 18% 3	5866
	21st to end	Day	45% 7% 12% 18% 16% 3	6447
June	1st to 10th	Day	31% 8% 15% 29% 14% 3	2600
	11th to 20th	Day	30% 7% 14% 28% 19% 2	2600
	21st to end	Day	23% 6% 15% 30% 22% 4	2600
July	1st to 10th	Day	34% 6% 17% 29% 12% 2	5836
	11th to 20th	Day	25% 4 14% 35% 21% 2	5831
	21st to end	Day	21% 6% 16% 38% 19% 1	6240
August	1st to 10th	Day	22% 5% 13% 27% 28% 5%	4220
	11th to 20th	Day	28% 5% 17% 33% 15% 2	4210
	21st to end	Day	25% 6% 14% 38% 14% 3	4383
September	1st to 10th	Day	31% 5% 9% 24% 22% 10%	2246
	11th to 20th	Day	31% 7% 11% 24% 20% 7%	2200
	21st to end	Day	27% 6% 8% 23% 29% 8%	2200
October	1st to 10th	Day	17% 3 7% 12% 40% 22%	3364
	11th to 20th	Day	17% 2 4 20% 38% 18%	3240
	21st to end	Day	29% 3 6% 29% 27% 7%	3538
November	1st to 10th	Day	28% 4 8% 25% 28% 7%	2058
	11th to 20th	Day	28% 3 8% 27% 25% 10%	1964
	21st to end	Day	33% 3 9% 23% 26% 6%	1890
December	1st to 10th	Day	35% 3 8% 35% 16% 2	3030
	11th to 20th	Day	39% 3 5% 22% 26% 5%	3479
	21st to end	Day	47% 5% 7% 21% 17% 3	4587
January	1st to 10th	Night	41% 3 8% 25% 21% 2	7635
	11th to 20th	Night	24% 4 7% 17% 39% 10%	7680
	21st to end	Night	46% 4 5% 15% 25% 6%	8402
February	1st to 10th	Night	39% 3 10% 23% 19% 5%	2730
	11th to 20th	Night	42% 4 7% 22% 20% 5%	2456
	21st to end	Night	37% 4 7% 18% 24% 11%	1911
March	1st to 10th	Night	40% 4 5% 15% 22% 14%	4919
	11th to 20th	Night	27% 3 7% 23% 19% 20%	4980
	21st to end	Night	37% 5% 11% 24% 20% 3	5467
April	1st to 10th	Night	33% 6% 11% 20% 19% 11%	2194
	11th to 20th	Night	32% 5% 9% 25% 18% 11%	1975
	21st to end	Night	35% 5% 9% 26% 22% 3	1888
May	1st to 10th	Night	38% 7% 13% 23% 15% 4	4080
	11th to 20th	Night	34% 6% 13% 22% 21% 6%	4054
	21st to end	Night	35% 7% 16% 21% 17% 4	4452
June	1st to 10th	Night	22% 7% 16% 31% 20% 4	1800
	11th to 20th	Night	21% 5% 14% 31% 25% 4	1800
	21st to end	Night	14% 5% 16% 31% 27% 7%	1800
July	1st to 10th	Night	32% 5% 18% 27% 15% 3	4035
	11th to 20th	Night	17% 4 14% 35% 27% 3	4026
	21st to end	Night	16% 5% 17% 38% 22% 2	4320
August	1st to 10th	Night	15% 5% 12% 27% 32% 9%	2916
	11th to 20th	Night	19% 4 17% 34% 22% 3	2915
	21st to end	Night	19% 5% 12% 38% 22% 4	3513
September	1st to 10th	Night	27% 5% 9% 26% 23% 11%	2150
	11th to 20th	Night	29% 6% 10% 23% 23% 9%	2200
	21st to end	Night	24% 4 8% 24% 32% 9%	2200
October	1st to 10th	Night	14% 2 6% 14% 37% 27%	3782
	11th to 20th	Night	19% 2 4 13% 42% 19%	3899
	21st to end	Night	29% 3 4 29% 25% 10%	4252
November	1st to 10th	Night	27% 4 7% 24% 28% 10%	2460
	11th to 20th	Night	27% 3 8% 23% 27% 11%	2651
	21st to end	Night	33% 3 8% 22% 27% 7%	2730
December	1st to 10th	Night	34% 3 7% 32% 20% 4	4380
	11th to 20th	Night	38% 3 5% 20% 24% 10%	4947
	21st to end	Night	41% 4 9% 23% 18% 5%	6633

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	19% 14% 37% 22% 6% 1	330
	11th to 20th	Day	19% 19% 37% 20% 5%	330
	21st to end	Day	18% 14% 40% 21% 7% 1	363
February	1st to 10th	Day	26% 12% 35% 20% 7%	330
	11th to 20th	Day	26% 18% 36% 16% 4	315
	21st to end	Day	24% 16% 39% 15% 5% 1	249
March	1st to 10th	Day	20% 15% 35% 22% 7% 1	300
	11th to 20th	Day	24% 15% 38% 17% 5% 1	300
	21st to end	Day	20% 15% 42% 18% 5% 1	330
April	1st to 10th	Day	19% 16% 37% 18% 7% 2	305
	11th to 20th	Day	22% 13% 35% 25% 5%	492
	21st to end	Day	19% 13% 33% 24% 9% 2	608
May	1st to 10th	Day	15% 8% 35% 31% 11% 1	703
	11th to 20th	Day	16% 8% 26% 32% 15% 4	725
	21st to end	Day	13% 6% 28% 28% 19% 6%	789
June	1st to 10th	Day	8% 3 15% 35% 30% 9%	730
	11th to 20th	Day	8% 2 16% 41% 26% 7%	730
	21st to end	Day	2 2 16% 39% 33% 8%	730
July	1st to 10th	Day	8% 1 9% 32% 35% 15%	721
	11th to 20th	Day	4 1 6% 34% 42% 12%	718
	21st to end	Day	5% 1 5% 29% 42% 18%	794
August	1st to 10th	Day	5% 1 5% 38% 41% 9%	726
	11th to 20th	Day	9% 1 5% 33% 41% 11%	715
	21st to end	Day	8% 2 5% 26% 38% 21%	629
September	1st to 10th	Day	11% 2 8% 21% 38% 21%	376
	11th to 20th	Day	3 3 7% 32% 37% 17%	330
	21st to end	Day	10% 3 16% 24% 33% 13%	330
October	1st to 10th	Day	5% 3 22% 33% 25% 12%	330
	11th to 20th	Day	10% 5% 32% 32% 18% 2	330
	21st to end	Day	26% 9% 25% 23% 14% 2	363
November	1st to 10th	Day	16% 5% 38% 26% 12% 3	330
	11th to 20th	Day	15% 9% 34% 29% 11% 2	330
	21st to end	Day	19% 13% 37% 23% 8%	330
December	1st to 10th	Day	16% 16% 31% 27% 9% 1	330
	11th to 20th	Day	25% 9% 40% 20% 6%	330
	21st to end	Day	21% 15% 35% 22% 7%	363
January	1st to 10th	Night	16% 13% 35% 29% 6%	1050
	11th to 20th	Night	15% 16% 37% 27% 4	1050
	21st to end	Night	17% 10% 36% 32% 5%	1155
February	1st to 10th	Night	19% 12% 36% 28% 5%	1050
	11th to 20th	Night	23% 16% 34% 24% 3	861
	21st to end	Night	19% 14% 38% 26% 3	591
March	1st to 10th	Night	19% 11% 33% 33% 4	600
	11th to 20th	Night	19% 11% 37% 29% 4	600
	21st to end	Night	14% 12% 38% 28% 7%	660
April	1st to 10th	Night	15% 11% 39% 29% 6%	600
	11th to 20th	Night	15% 12% 35% 30% 9%	593
	21st to end	Night	16% 8% 36% 30% 10% 1	600
May	1st to 10th	Night	13% 6% 35% 37% 9% 1	600
	11th to 20th	Night	12% 4 27% 39% 16% 2	591
	21st to end	Night	10% 3 26% 37% 21% 3	650
June	1st to 10th	Night	6% 3 15% 39% 35% 3	600
	11th to 20th	Night	6% 1 18% 49% 25% 1	600
	21st to end	Night	3 1 14% 46% 32% 5%	600
July	1st to 10th	Night	4 1 6% 41% 41% 6%	585
	11th to 20th	Night	2 1 8% 40% 44% 6%	582
	21st to end	Night	3 2 4 40% 45% 6%	657
August	1st to 10th	Night	3 1 6% 40% 46% 5%	597
	11th to 20th	Night	6% 1 5% 38% 46% 4	594
	21st to end	Night	5% 1 5% 36% 47% 6%	660
September	1st to 10th	Night	9% 2 7% 37% 42% 4	600
	11th to 20th	Night	11% 1 9% 37% 37% 5%	600
	21st to end	Night	9% 1 16% 37% 33% 5%	600
October	1st to 10th	Night	8% 3 19% 38% 29% 4	714
	11th to 20th	Night	13% 6% 27% 38% 16%	800
	21st to end	Night	19% 8% 27% 32% 14% 1	880
November	1st to 10th	Night	14% 5% 32% 36% 13% 1	820
	11th to 20th	Night	10% 12% 32% 36% 10%	971
	21st to end	Night	18% 10% 34% 29% 8%	1050
December	1st to 10th	Night	16% 12% 33% 32% 6%	1050
	11th to 20th	Night	21% 11% 34% 29% 5%	1050
	21st to end	Night	18% 12% 37% 27% 5%	1155

Page generated at 2026-02-13 00:42:18. Version ac07

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