Goodwin Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Goodwin Mountain Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	2 4 2 5% 4 4 5% 5% 8% 10% 52%	140320
	Night	1 3 1 4 3 3 4 4 7% 10% 59%	138284
Transparency	Day	21% 13% 17% 24% 18% 7%	135082
	Night	17% 13% 19% 26% 19% 6%	134529
Seeing	Day	12% 25% 35% 20% 7% 1	17458
	Night	8% 20% 35% 26% 9% 1	27618

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	2 1 5% 4 4 4 4 9% 16% 52%	4720
	11th to 20th	Day	1 2 3 5% 6% 5% 8% 10% 61%	5280
	21st to end	Day	1 1 2 2 3 5% 4 9% 11% 63%	5844
February	1st to 10th	Day	2 1 5% 5% 7% 7% 7% 11% 12% 43%	1890
	11th to 20th	Day	3 2 6% 6% 6% 8% 6% 10% 11% 43%	1845
	21st to end	Day	1 1 5% 5% 6% 8% 9% 10% 12% 43%	1611
March	1st to 10th	Day	6% 3 6% 4 4 5% 5% 9% 10% 47%	4977
	11th to 20th	Day	2 2 6% 4 4 4 4 7% 12% 54%	5000
	21st to end	Day	1 7% 3 7% 5% 5% 7% 7% 13% 13% 34%	5486
April	1st to 10th	Day	1 7% 2 8% 5% 5% 7% 7% 9% 11% 38%	2351
	11th to 20th	Day	1 7% 3 7% 6% 7% 7% 5% 9% 10% 37%	2438
	21st to end	Day	3 11% 4 8% 6% 6% 7% 5% 7% 9% 34%	2600
May	1st to 10th	Day	3 8% 3 8% 6% 7% 7% 5% 8% 9% 36%	5880
	11th to 20th	Day	3 7% 2 6% 5% 5% 6% 5% 9% 10% 42%	6202
	21st to end	Day	3 5% 2 5% 5% 6% 6% 6% 9% 11% 42%	6925
June	1st to 10th	Day	5% 4 2 6% 5% 5% 6% 6% 7% 9% 44%	2800
	11th to 20th	Day	4 4 1 4 4 4 4 5% 8% 9% 52%	2800
	21st to end	Day	2 2 1 4 3 4 4 4 7% 8% 61%	2800
July	1st to 10th	Day	1 1 1 3 4 4 5% 5% 7% 9% 59%	6339
	11th to 20th	Day	1 1 1 3 4 5% 6% 6% 8% 8% 57%	6306
	21st to end	Day	2 2 1 4 4 5% 6% 5% 8% 10% 53%	6401
August	1st to 10th	Day	2 2 1 4 4 5% 6% 5% 8% 9% 54%	5779
	11th to 20th	Day	2 2 1 3 3 3 5% 4 8% 9% 61%	5798
	21st to end	Day	3 3 1 4 3 3 4 4 5% 8% 61%	6279
September	1st to 10th	Day	3 4 1 4 4 3 4 3 6% 8% 61%	2400
	11th to 20th	Day	5% 3 1 3 3 3 4 4 5% 8% 61%	2262
	21st to end	Day	3 5% 2 5% 3 3 3 3 5% 7% 61%	2188
October	1st to 10th	Day	3 6% 1 4 2 3 3 4 5% 5% 66%	3270
	11th to 20th	Day	1 3 1 4 3 3 3 3 6% 9% 64%	3250
	21st to end	Day	4 3 4 3 3 3 3 4 9% 63%	3548
November	1st to 10th	Day	1 3 1 4 4 4 4 5% 9% 11% 54%	1793
	11th to 20th	Day	2 1 4 3 4 4 5% 10% 11% 55%	1680
	21st to end	Day	1 2 1 5% 4 4 4 4 6% 11% 59%	1680
December	1st to 10th	Day	1 1 3 3 3 4 3 7% 10% 66%	2700
	11th to 20th	Day	1 1 2 2 2 4 5% 8% 12% 61%	3106
	21st to end	Day	2 1 4 4 4 6% 6% 10% 15% 48%	4092
January	1st to 10th	Night	1 1 3 3 4 5% 6% 10% 14% 53%	8201
	11th to 20th	Night	3 5% 5% 5% 5% 8% 13% 57%	7756
	21st to end	Night	1 1 3 3 3 4 4 8% 11% 60%	8370
February	1st to 10th	Night	2 2 5% 3 4 5% 6% 10% 12% 51%	2730
	11th to 20th	Night	3 2 5% 4 4 5% 5% 9% 12% 52%	2662
	21st to end	Night	2 2 6% 4 4 5% 5% 10% 12% 49%	2192
March	1st to 10th	Night	8% 3 5% 4 3 4 4 9% 11% 49%	4962
	11th to 20th	Night	3 2 6% 3 4 4 3 6% 11% 59%	5000
	21st to end	Night	1 4 2 7% 3 4 5% 6% 9% 10% 49%	5489
April	1st to 10th	Night	1 6% 2 6% 4 4 6% 5% 9% 9% 48%	2048
	11th to 20th	Night	2 5% 3 7% 4 4 5% 6% 8% 11% 46%	1908
	21st to end	Night	3 8% 3 6% 4 5% 4 4 7% 9% 47%	1800
May	1st to 10th	Night	4 10% 3 6% 4 4 4 4 6% 9% 45%	4100
	11th to 20th	Night	3 6% 2 5% 4 3 5% 4 6% 8% 54%	3709
	21st to end	Night	3 4 1 4 3 4 5% 5% 9% 13% 50%	3972
June	1st to 10th	Night	2 3 1 4 3 4 4 5% 8% 11% 54%	1600
	11th to 20th	Night	3 3 1 4 3 2 3 3 5% 9% 64%	1600
	21st to end	Night	1 1 1 2 2 2 3 3 6% 7% 72%	1493
July	1st to 10th	Night	2 2 1 2 3 2 3 3 5% 8% 70%	3640
	11th to 20th	Night	1 2 3 3 4 6% 9% 71%	3657
	21st to end	Night	1 1 2 2 2 3 4 7% 9% 68%	4472
August	1st to 10th	Night	2 3 2 3 2 3 3 3 5% 8% 66%	4033
	11th to 20th	Night	1 1 1 2 2 3 4 4 6% 9% 67%	4051
	21st to end	Night	2 3 2 5% 3 3 3 3 4 7% 65%	4685
September	1st to 10th	Night	2 2 1 3 3 3 3 3 5% 8% 69%	2000
	11th to 20th	Night	3 4 1 3 2 2 3 3 7% 9% 62%	2135
	21st to end	Night	3 4 1 3 2 2 3 2 4 8% 67%	2211
October	1st to 10th	Night	2 4 1 4 2 2 3 2 4 5% 70%	3920
	11th to 20th	Night	2 3 1 3 3 3 5% 4 7% 9% 60%	3898
	21st to end	Night	1 4 2 4 2 2 3 3 7% 10% 62%	4252
November	1st to 10th	Night	1 2 1 4 2 3 4 5% 7% 12% 61%	2699
	11th to 20th	Night	2 1 4 2 3 4 4 8% 11% 60%	2940
	21st to end	Night	3 1 3 3 3 4 4 7% 11% 60%	2940
December	1st to 10th	Night	1 1 2 2 2 4 4 7% 10% 68%	4710
	11th to 20th	Night	2 1 3 2 3 3 3 7% 13% 64%	5321
	21st to end	Night	2 2 4 3 4 4 5% 9% 13% 56%	7128

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	17% 15% 29% 24% 15% 2	4723
	11th to 20th	Day	11% 15% 23% 25% 17% 8%	5280
	21st to end	Day	10% 16% 19% 25% 22% 7%	5844
February	1st to 10th	Day	25% 23% 23% 19% 8% 1	1890
	11th to 20th	Day	27% 19% 23% 22% 7% 3	1854
	21st to end	Day	27% 21% 21% 17% 11% 3	1611
March	1st to 10th	Day	24% 15% 17% 25% 12% 7%	4977
	11th to 20th	Day	18% 17% 18% 28% 18% 1	5000
	21st to end	Day	28% 21% 24% 20% 6%	5486
April	1st to 10th	Day	33% 19% 19% 20% 8% 1	2351
	11th to 20th	Day	34% 16% 22% 20% 7%	2441
	21st to end	Day	41% 16% 18% 20% 5%	2600
May	1st to 10th	Day	31% 13% 21% 27% 7% 1	5880
	11th to 20th	Day	26% 13% 18% 25% 15% 2	6206
	21st to end	Day	26% 15% 20% 27% 12%	6926
June	1st to 10th	Day	30% 16% 19% 22% 11% 1	2800
	11th to 20th	Day	24% 12% 14% 23% 20% 7%	2800
	21st to end	Day	16% 9% 12% 27% 27% 10%	2800
July	1st to 10th	Day	15% 8% 10% 17% 27% 22%	6271
	11th to 20th	Day	15% 9% 10% 19% 25% 22%	6231
	21st to end	Day	19% 9% 11% 18% 33% 11%	6217
August	1st to 10th	Day	21% 10% 11% 18% 25% 14%	4208
	11th to 20th	Day	17% 8% 10% 22% 31% 12%	4197
	21st to end	Day	16% 6% 9% 18% 35% 15%	4522
September	1st to 10th	Day	20% 9% 10% 18% 32% 11%	2400
	11th to 20th	Day	20% 9% 14% 22% 26% 8%	2262
	21st to end	Day	23% 7% 12% 29% 23% 5%	2188
October	1st to 10th	Day	21% 6% 9% 21% 30% 13%	3270
	11th to 20th	Day	15% 9% 16% 35% 19% 6%	3250
	21st to end	Day	17% 10% 15% 37% 19% 2	3548
November	1st to 10th	Day	20% 17% 20% 25% 16% 2	1793
	11th to 20th	Day	17% 20% 21% 26% 12% 3	1680
	21st to end	Day	21% 16% 21% 24% 15% 3	1680
December	1st to 10th	Day	13% 15% 23% 28% 17% 4	2700
	11th to 20th	Day	13% 21% 30% 27% 8% 1	3104
	21st to end	Day	19% 16% 18% 26% 14% 8%	4092
January	1st to 10th	Night	13% 18% 25% 24% 18% 1	8202
	11th to 20th	Night	15% 13% 25% 27% 14% 5%	7756
	21st to end	Night	13% 15% 20% 25% 21% 6%	8370
February	1st to 10th	Night	19% 22% 26% 22% 8% 2	2730
	11th to 20th	Night	21% 19% 23% 22% 11% 3	2663
	21st to end	Night	23% 20% 21% 20% 11% 4	2192
March	1st to 10th	Night	22% 16% 22% 21% 13% 6%	4962
	11th to 20th	Night	15% 16% 20% 32% 17% 1	5000
	21st to end	Night	21% 24% 24% 24% 6% 1	5489
April	1st to 10th	Night	27% 17% 24% 19% 11% 2	2048
	11th to 20th	Night	26% 19% 23% 23% 8% 1	1908
	21st to end	Night	30% 15% 24% 26% 6%	1800
May	1st to 10th	Night	28% 13% 18% 28% 11% 1	4097
	11th to 20th	Night	23% 10% 18% 33% 13% 3	3710
	21st to end	Night	19% 13% 25% 30% 12% 1	3972
June	1st to 10th	Night	22% 15% 25% 26% 11% 1	1600
	11th to 20th	Night	17% 10% 18% 21% 24% 10%	1600
	21st to end	Night	9% 8% 13% 28% 30% 12%	1493
July	1st to 10th	Night	12% 5% 9% 22% 33% 19%	3600
	11th to 20th	Night	7% 5% 9% 23% 35% 20%	3609
	21st to end	Night	10% 6% 8% 23% 36% 18%	4344
August	1st to 10th	Night	18% 7% 10% 22% 31% 12%	2928
	11th to 20th	Night	16% 8% 9% 27% 27% 12%	2928
	21st to end	Night	11% 5% 8% 24% 36% 16%	3373
September	1st to 10th	Night	14% 7% 12% 23% 34% 10%	2000
	11th to 20th	Night	18% 8% 15% 26% 26% 7%	2135
	21st to end	Night	18% 8% 12% 30% 27% 5%	2211
October	1st to 10th	Night	17% 7% 10% 26% 30% 10%	3920
	11th to 20th	Night	17% 11% 16% 31% 22% 3	3898
	21st to end	Night	17% 10% 16% 34% 18% 5%	4252
November	1st to 10th	Night	15% 18% 19% 30% 15% 3	2699
	11th to 20th	Night	15% 18% 27% 26% 11% 3	2940
	21st to end	Night	16% 16% 23% 27% 13% 5%	2940
December	1st to 10th	Night	10% 16% 27% 27% 18% 2	4710
	11th to 20th	Night	13% 17% 29% 28% 12% 1	5322
	21st to end	Night	17% 14% 20% 26% 14% 9%	7128

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% 27% 27% 18% 7% 1	330
	11th to 20th	Day	16% 35% 28% 12% 7% 2	330
	21st to end	Day	11% 32% 37% 15% 4	363
February	1st to 10th	Day	23% 42% 25% 8% 2	330
	11th to 20th	Day	19% 34% 32% 10% 4	330
	21st to end	Day	27% 29% 29% 11% 3	314
March	1st to 10th	Day	17% 33% 37% 10% 2	509
	11th to 20th	Day	12% 33% 36% 16% 3	500
	21st to end	Day	17% 30% 37% 13% 2	550
April	1st to 10th	Day	18% 32% 36% 12% 3	500
	11th to 20th	Day	17% 36% 33% 12% 2	492
	21st to end	Day	14% 29% 30% 21% 4 1	500
May	1st to 10th	Day	15% 25% 34% 17% 8% 1	503
	11th to 20th	Day	15% 20% 33% 21% 9% 1	681
	21st to end	Day	15% 19% 29% 26% 9% 2	789
June	1st to 10th	Day	8% 22% 33% 22% 13% 2	730
	11th to 20th	Day	10% 28% 37% 18% 5% 1	730
	21st to end	Day	4 21% 36% 27% 10% 2	730
July	1st to 10th	Day	4 15% 36% 28% 15% 2	721
	11th to 20th	Day	3 16% 41% 27% 11% 2	710
	21st to end	Day	6% 14% 41% 27% 10% 1	575
August	1st to 10th	Day	6% 15% 43% 27% 8% 1	527
	11th to 20th	Day	4 14% 43% 27% 10% 3	517
	21st to end	Day	7% 19% 39% 24% 9% 2	583
September	1st to 10th	Day	5% 16% 41% 28% 9% 2	530
	11th to 20th	Day	10% 22% 35% 22% 8% 2	530
	21st to end	Day	8% 22% 38% 22% 8% 1	518
October	1st to 10th	Day	9% 22% 35% 22% 11% 1	330
	11th to 20th	Day	20% 20% 32% 17% 9% 2	330
	21st to end	Day	17% 30% 37% 13% 4	363
November	1st to 10th	Day	16% 33% 33% 12% 4 1	330
	11th to 20th	Day	17% 36% 29% 16% 1 1	330
	21st to end	Day	11% 32% 38% 16% 3	330
December	1st to 10th	Day	23% 29% 28% 15% 4 1	330
	11th to 20th	Day	22% 33% 28% 13% 5%	330
	21st to end	Day	23% 31% 24% 14% 4 3	363
January	1st to 10th	Night	12% 26% 34% 23% 4 1	1050
	11th to 20th	Night	11% 30% 34% 15% 7% 2	881
	21st to end	Night	6% 25% 43% 20% 5% 1	924
February	1st to 10th	Night	10% 33% 34% 18% 5%	840
	11th to 20th	Night	9% 29% 41% 15% 6% 1	824
	21st to end	Night	14% 29% 35% 16% 5% 1	692
March	1st to 10th	Night	7% 25% 40% 22% 5% 1	808
	11th to 20th	Night	7% 25% 37% 25% 7% 1	800
	21st to end	Night	6% 21% 41% 26% 5%	880
April	1st to 10th	Night	10% 22% 43% 19% 6%	648
	11th to 20th	Night	6% 24% 42% 21% 6% 1	593
	21st to end	Night	8% 19% 35% 28% 9% 2	600
May	1st to 10th	Night	14% 15% 31% 27% 12% 2	600
	11th to 20th	Night	12% 12% 31% 29% 14% 2	591
	21st to end	Night	10% 14% 30% 33% 13% 1	650
June	1st to 10th	Night	6% 18% 30% 29% 17% 1	600
	11th to 20th	Night	8% 20% 41% 26% 6%	600
	21st to end	Night	2 8% 31% 40% 18% 1	493
July	1st to 10th	Night	2 6% 28% 41% 23% 1	585
	11th to 20th	Night	2 9% 27% 46% 17% 1	582
	21st to end	Night	2 3 25% 50% 18% 2	657
August	1st to 10th	Night	5% 6% 24% 46% 19% 1	597
	11th to 20th	Night	3 4 26% 47% 20% 2	594
	21st to end	Night	3 7% 28% 40% 22% 1	660
September	1st to 10th	Night	3 5% 30% 42% 18% 3	600
	11th to 20th	Night	5% 10% 29% 40% 12% 4	735
	21st to end	Night	8% 10% 32% 33% 14% 4	800
October	1st to 10th	Night	9% 14% 36% 29% 11% 2	800
	11th to 20th	Night	9% 20% 33% 25% 10% 2	800
	21st to end	Night	8% 21% 42% 20% 7% 2	880
November	1st to 10th	Night	8% 29% 39% 17% 6% 1	899
	11th to 20th	Night	14% 27% 38% 17% 4	1050
	21st to end	Night	6% 28% 39% 20% 5% 1	1050
December	1st to 10th	Night	13% 30% 34% 17% 5%	1050
	11th to 20th	Night	12% 29% 38% 16% 4	1050
	21st to end	Night	12% 28% 36% 15% 6% 2	1155

Page generated at 2026-02-27 23:40:24. Version ac07

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