Hildene Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hildene Observatory clear sky chart from 2005-11-06 to 2026-07-31.

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% 12% 4 10% 6% 5% 4 3 5% 6% 29%	138123
	Night	16% 13% 5% 11% 6% 5% 4 3 4 5% 28%	134910
Transparency	Day	53% 6% 11% 19% 10% 1	141619
	Night	55% 6% 10% 17% 10% 1	139503
Seeing	Day	25% 20% 29% 17% 7% 2	17019
	Night	23% 21% 27% 21% 8% 1	25960

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	9% 17% 8% 25% 10% 4 4 3 3 3 16%	4661
	11th to 20th	Day	8% 15% 8% 20% 11% 6% 4 2 3 4 19%	4650
	21st to end	Day	11% 15% 7% 12% 8% 7% 5% 3 4 4 25%	5139
February	1st to 10th	Day	13% 15% 5% 14% 9% 5% 4 3 5% 5% 22%	1894
	11th to 20th	Day	10% 15% 7% 18% 7% 5% 4 3 4 4 24%	2048
	21st to end	Day	15% 17% 6% 11% 7% 3 4 3 3 5% 26%	1730
March	1st to 10th	Day	15% 18% 7% 13% 5% 4 3 3 3 4 24%	4576
	11th to 20th	Day	13% 16% 7% 9% 4 4 4 3 5% 5% 31%	4771
	21st to end	Day	20% 11% 5% 10% 5% 4 3 3 3 5% 30%	6102
April	1st to 10th	Day	18% 17% 5% 8% 4 3 3 3 5% 4 30%	2520
	11th to 20th	Day	15% 14% 4 8% 5% 5% 4 4 5% 5% 32%	2494
	21st to end	Day	19% 14% 4 8% 4 4 4 3 5% 5% 30%	2667
May	1st to 10th	Day	26% 13% 4 9% 5% 4 5% 3 5% 5% 21%	7691
	11th to 20th	Day	18% 12% 3 7% 5% 4 5% 3 6% 6% 29%	8221
	21st to end	Day	22% 11% 3 8% 5% 4 5% 4 5% 7% 27%	9060
June	1st to 10th	Day	19% 9% 3 8% 5% 5% 5% 4 7% 7% 28%	2940
	11th to 20th	Day	19% 7% 3 7% 5% 5% 5% 4 6% 8% 30%	2940
	21st to end	Day	20% 9% 3 7% 6% 5% 5% 4 6% 7% 29%	2940
July	1st to 10th	Day	13% 8% 3 8% 5% 5% 5% 4 6% 10% 31%	6500
	11th to 20th	Day	11% 7% 2 8% 6% 6% 6% 5% 7% 9% 33%	6500
	21st to end	Day	16% 7% 3 7% 4 4 5% 4 5% 7% 38%	7066
August	1st to 10th	Day	17% 8% 3 7% 5% 5% 5% 4 6% 7% 31%	4508
	11th to 20th	Day	15% 8% 3 7% 5% 5% 6% 4 6% 8% 31%	3949
	21st to end	Day	13% 7% 3 6% 4 5% 5% 5% 7% 9% 37%	4312
September	1st to 10th	Day	15% 7% 3 5% 4 3 5% 5% 6% 6% 41%	2400
	11th to 20th	Day	15% 7% 2 5% 4 4 4 3 6% 7% 45%	2278
	21st to end	Day	17% 8% 2 5% 4 5% 5% 4 6% 7% 38%	2200
October	1st to 10th	Day	21% 11% 3 7% 4 4 4 3 4 4 35%	2133
	11th to 20th	Day	16% 12% 3 7% 4 4 5% 3 5% 6% 35%	1965
	21st to end	Day	22% 16% 4 7% 5% 3 4 3 5% 5% 27%	1980
November	1st to 10th	Day	13% 16% 3 6% 4 3 4 3 5% 6% 37%	1851
	11th to 20th	Day	15% 16% 5% 8% 5% 4 3 3 4 4 32%	1890
	21st to end	Day	19% 15% 5% 10% 6% 4 4 3 4 5% 26%	1753
December	1st to 10th	Day	13% 20% 6% 17% 6% 5% 5% 3 3 4 19%	2680
	11th to 20th	Day	16% 18% 9% 12% 6% 4 3 2 2 4 24%	3066
	21st to end	Day	14% 18% 5% 14% 5% 4 3 3 4 5% 25%	4048
January	1st to 10th	Night	11% 15% 8% 24% 7% 4 3 2 3 4 19%	8234
	11th to 20th	Night	9% 16% 11% 22% 11% 6% 4 3 3 3 13%	8235
	21st to end	Night	11% 11% 4 13% 9% 9% 6% 4 4 5% 22%	9039
February	1st to 10th	Night	12% 16% 6% 14% 8% 6% 4 3 4 5% 22%	2725
	11th to 20th	Night	10% 13% 6% 13% 10% 7% 6% 4 5% 4 22%	2458
	21st to end	Night	12% 16% 6% 14% 6% 4 3 4 5% 5% 26%	2076
March	1st to 10th	Night	18% 11% 5% 13% 7% 5% 4 3 4 6% 24%	5484
	11th to 20th	Night	13% 15% 7% 10% 5% 4 4 2 4 6% 30%	5391
	21st to end	Night	22% 16% 5% 12% 4 3 3 2 3 4 26%	5072
April	1st to 10th	Night	20% 13% 5% 8% 5% 4 4 3 5% 6% 28%	2100
	11th to 20th	Night	19% 15% 3 7% 4 3 3 4 5% 5% 32%	2075
	21st to end	Night	19% 13% 4 7% 4 4 4 3 4 6% 33%	1953
May	1st to 10th	Night	27% 14% 3 8% 5% 4 4 3 5% 5% 23%	5259
	11th to 20th	Night	17% 12% 4 9% 5% 5% 4 4 5% 7% 27%	4642
	21st to end	Night	24% 11% 3 7% 4 4 5% 3 5% 6% 27%	5114
June	1st to 10th	Night	19% 10% 2 6% 4 4 4 3 6% 6% 35%	1680
	11th to 20th	Night	20% 8% 2 6% 4 4 4 3 5% 8% 36%	1680
	21st to end	Night	17% 7% 3 6% 4 5% 5% 4 5% 7% 37%	1680
July	1st to 10th	Night	13% 9% 3 7% 5% 4 5% 4 6% 8% 36%	3680
	11th to 20th	Night	19% 9% 4 7% 5% 5% 5% 4 5% 6% 33%	3680
	21st to end	Night	12% 8% 3 7% 4 5% 4 3 5% 6% 43%	4006
August	1st to 10th	Night	26% 8% 3 5% 3 3 3 3 5% 7% 34%	2609
	11th to 20th	Night	16% 9% 3 7% 4 5% 5% 4 5% 7% 36%	3094
	21st to end	Night	11% 9% 2 5% 4 3 4 4 5% 9% 44%	3586
September	1st to 10th	Night	16% 8% 2 6% 4 3 3 3 4 6% 45%	2000
	11th to 20th	Night	16% 5% 2 5% 3 2 3 3 5% 6% 49%	2121
	21st to end	Night	17% 7% 2 6% 5% 4 4 3 4 5% 41%	2200
October	1st to 10th	Night	21% 10% 2 7% 4 4 4 3 4 6% 35%	2265
	11th to 20th	Night	16% 12% 3 8% 4 4 5% 4 4 6% 36%	2435
	21st to end	Night	21% 14% 3 8% 6% 4 4 4 4 5% 27%	2860
November	1st to 10th	Night	14% 14% 4 9% 4 4 3 3 4 5% 36%	2667
	11th to 20th	Night	15% 15% 6% 7% 5% 4 4 3 4 5% 32%	2730
	21st to end	Night	13% 16% 6% 10% 6% 5% 4 3 5% 6% 27%	2862
December	1st to 10th	Night	15% 20% 9% 19% 5% 4 3 3 3 4 15%	4720
	11th to 20th	Night	19% 18% 6% 15% 6% 3 3 2 3 3 22%	5348
	21st to end	Night	15% 18% 7% 13% 5% 4 4 3 4 4 23%	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	70% 5% 7% 14% 4	4662
	11th to 20th	Day	67% 5% 5% 11% 10% 2	4680
	21st to end	Day	60% 4 7% 19% 8% 2	5139
February	1st to 10th	Day	61% 6% 9% 15% 8% 1	1894
	11th to 20th	Day	61% 6% 7% 15% 9% 1	2057
	21st to end	Day	59% 6% 8% 15% 11% 1	1730
March	1st to 10th	Day	62% 4 9% 11% 9% 5%	4579
	11th to 20th	Day	53% 5% 15% 16% 8% 3	4771
	21st to end	Day	54% 6% 6% 13% 18% 3	6102
April	1st to 10th	Day	57% 5% 8% 18% 10% 2	2520
	11th to 20th	Day	52% 6% 10% 18% 13% 1	2494
	21st to end	Day	55% 6% 8% 17% 12% 2	2667
May	1st to 10th	Day	62% 6% 10% 14% 8%	7703
	11th to 20th	Day	53% 6% 14% 18% 8% 1	8225
	21st to end	Day	54% 6% 10% 23% 6%	9061
June	1st to 10th	Day	51% 7% 14% 18% 9%	2940
	11th to 20th	Day	50% 6% 12% 19% 12% 1	2940
	21st to end	Day	53% 7% 13% 20% 8%	2940
July	1st to 10th	Day	47% 6% 13% 23% 10%	6430
	11th to 20th	Day	42% 7% 16% 23% 12% 1	6425
	21st to end	Day	44% 5% 13% 28% 10%	6868
August	1st to 10th	Day	49% 7% 14% 20% 10%	4494
	11th to 20th	Day	47% 7% 16% 24% 5%	3988
	21st to end	Day	39% 7% 15% 28% 10% 1	4312
September	1st to 10th	Day	40% 7% 11% 29% 13% 1	2400
	11th to 20th	Day	39% 6% 10% 26% 19% 1	2278
	21st to end	Day	44% 7% 11% 24% 13% 2	2200
October	1st to 10th	Day	50% 6% 7% 20% 16% 1	3471
	11th to 20th	Day	45% 4 12% 22% 12% 4	3181
	21st to end	Day	43% 6% 11% 27% 11% 2	3180
November	1st to 10th	Day	46% 7% 13% 21% 13% 1	1851
	11th to 20th	Day	53% 6% 9% 16% 15% 2	1890
	21st to end	Day	59% 6% 13% 16% 6% 1	1753
December	1st to 10th	Day	66% 6% 8% 15% 5% 1	2680
	11th to 20th	Day	63% 4 8% 14% 10% 1	3066
	21st to end	Day	60% 5% 7% 12% 12% 4	4048
January	1st to 10th	Night	67% 5% 7% 14% 6% 1	8234
	11th to 20th	Night	72% 7% 6% 8% 6% 1	8280
	21st to end	Night	58% 7% 8% 13% 11% 3	9039
February	1st to 10th	Night	61% 6% 10% 15% 7% 1	2725
	11th to 20th	Night	58% 7% 9% 15% 9% 1	2460
	21st to end	Night	57% 6% 10% 18% 8% 2	2076
March	1st to 10th	Night	58% 7% 9% 14% 10% 2	5494
	11th to 20th	Night	52% 5% 16% 16% 8% 2	5392
	21st to end	Night	59% 6% 4 11% 17% 2	5072
April	1st to 10th	Night	55% 6% 10% 18% 11% 1	2100
	11th to 20th	Night	53% 6% 9% 19% 13% 1	2075
	21st to end	Night	52% 6% 8% 17% 13% 3	1953
May	1st to 10th	Night	60% 6% 10% 14% 9% 1	5265
	11th to 20th	Night	54% 7% 13% 18% 7% 1	4643
	21st to end	Night	56% 6% 9% 20% 8% 1	5115
June	1st to 10th	Night	48% 6% 13% 21% 11%	1680
	11th to 20th	Night	46% 7% 16% 20% 11%	1680
	21st to end	Night	45% 7% 14% 23% 10% 1	1680
July	1st to 10th	Night	43% 6% 16% 23% 11%	3640
	11th to 20th	Night	49% 7% 12% 20% 12% 1	3632
	21st to end	Night	40% 6% 13% 30% 10% 1	3889
August	1st to 10th	Night	50% 6% 15% 20% 8% 1	2599
	11th to 20th	Night	45% 7% 18% 22% 8%	3121
	21st to end	Night	34% 6% 18% 31% 10% 1	3586
September	1st to 10th	Night	40% 5% 13% 28% 14% 1	2000
	11th to 20th	Night	36% 5% 10% 29% 18% 1	2121
	21st to end	Night	44% 5% 10% 25% 14% 2	2200
October	1st to 10th	Night	50% 6% 11% 21% 10% 1	3707
	11th to 20th	Night	42% 6% 12% 21% 15% 4	3958
	21st to end	Night	46% 5% 11% 27% 9% 1	4610
November	1st to 10th	Night	48% 6% 11% 19% 13% 2	2667
	11th to 20th	Night	52% 7% 9% 17% 12% 3	2730
	21st to end	Night	53% 8% 14% 17% 7% 1	2862
December	1st to 10th	Night	68% 6% 8% 13% 4	4720
	11th to 20th	Night	65% 6% 8% 9% 12% 1	5348
	21st to end	Night	63% 6% 7% 9% 11% 4	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	31% 28% 27% 11% 4	220
	11th to 20th	Day	33% 30% 26% 8% 1 1	220
	21st to end	Day	31% 31% 27% 8% 3	244
February	1st to 10th	Day	34% 37% 22% 6% 1	334
	11th to 20th	Day	30% 32% 27% 9% 1	536
	21st to end	Day	32% 29% 26% 11% 2	446
March	1st to 10th	Day	32% 27% 28% 12% 1	540
	11th to 20th	Day	33% 28% 26% 9% 3 1	540
	21st to end	Day	35% 26% 24% 9% 4 2	594
April	1st to 10th	Day	38% 22% 24% 11% 4 1	540
	11th to 20th	Day	35% 24% 23% 13% 5% 1	532
	21st to end	Day	30% 21% 30% 13% 4 2	540
May	1st to 10th	Day	36% 22% 24% 11% 6% 1	543
	11th to 20th	Day	23% 20% 34% 17% 4 1	567
	21st to end	Day	24% 18% 29% 20% 7% 2	617
June	1st to 10th	Day	20% 18% 34% 19% 9% 1	570
	11th to 20th	Day	23% 15% 32% 22% 8% 1	570
	21st to end	Day	21% 17% 35% 18% 7% 2	570
July	1st to 10th	Day	16% 13% 36% 23% 11% 2	565
	11th to 20th	Day	15% 9% 38% 27% 10% 2	564
	21st to end	Day	15% 15% 38% 22% 9% 2	616
August	1st to 10th	Day	13% 11% 30% 29% 13% 5%	526
	11th to 20th	Day	16% 12% 30% 23% 14% 5%	519
	21st to end	Day	14% 14% 28% 27% 11% 5%	583
September	1st to 10th	Day	17% 13% 27% 23% 13% 6%	530
	11th to 20th	Day	13% 15% 26% 21% 16% 10%	530
	21st to end	Day	17% 15% 32% 18% 11% 7%	530
October	1st to 10th	Day	26% 15% 25% 18% 12% 4	530
	11th to 20th	Day	22% 22% 29% 18% 8% 1	511
	21st to end	Day	31% 19% 28% 18% 3 1	462
November	1st to 10th	Day	28% 23% 24% 17% 7% 1	425
	11th to 20th	Day	26% 28% 27% 13% 5% 1	430
	21st to end	Day	34% 26% 29% 8% 2	293
December	1st to 10th	Day	31% 30% 23% 11% 4	220
	11th to 20th	Day	37% 31% 22% 8% 1	220
	21st to end	Day	36% 23% 27% 12% 2	242
January	1st to 10th	Night	22% 32% 31% 14% 2	840
	11th to 20th	Night	27% 27% 27% 17% 2	840
	21st to end	Night	20% 30% 30% 17% 3	924
February	1st to 10th	Night	22% 29% 33% 15% 1	840
	11th to 20th	Night	23% 32% 30% 13% 2	823
	21st to end	Night	25% 31% 30% 13% 2	692
March	1st to 10th	Night	26% 24% 31% 16% 3	840
	11th to 20th	Night	27% 25% 28% 16% 3	840
	21st to end	Night	28% 25% 24% 16% 6% 1	924
April	1st to 10th	Night	28% 23% 27% 18% 3	840
	11th to 20th	Night	26% 23% 27% 17% 7%	831
	21st to end	Night	28% 20% 27% 19% 5%	693
May	1st to 10th	Night	27% 18% 27% 21% 7%	630
	11th to 20th	Night	19% 18% 29% 27% 6%	417
	21st to end	Night	19% 12% 28% 29% 11% 1	455
June	1st to 10th	Night	17% 14% 27% 29% 12% 1	420
	11th to 20th	Night	21% 12% 24% 33% 10%	420
	21st to end	Night	17% 9% 28% 35% 10% 1	420
July	1st to 10th	Night	14% 7% 27% 37% 15% 1	410
	11th to 20th	Night	16% 6% 24% 37% 15% 1	408
	21st to end	Night	12% 7% 30% 35% 14% 1	458
August	1st to 10th	Night	14% 7% 20% 39% 19%	408
	11th to 20th	Night	16% 8% 21% 32% 19% 4	723
	21st to end	Night	16% 8% 24% 30% 19% 2	880
September	1st to 10th	Night	16% 11% 26% 31% 15% 3	800
	11th to 20th	Night	16% 9% 20% 30% 19% 6%	800
	21st to end	Night	20% 12% 25% 26% 14% 3	800
October	1st to 10th	Night	25% 14% 22% 24% 13% 3	800
	11th to 20th	Night	21% 17% 28% 24% 10% 1	800
	21st to end	Night	28% 22% 26% 19% 5% 1	880
November	1st to 10th	Night	24% 22% 28% 20% 6% 1	820
	11th to 20th	Night	23% 26% 27% 15% 7% 2	840
	21st to end	Night	24% 27% 29% 14% 6% 1	840
December	1st to 10th	Night	29% 29% 25% 13% 3	840
	11th to 20th	Night	30% 29% 23% 15% 4	840
	21st to end	Night	28% 31% 24% 14% 2	924

Page generated at 2026-07-28 03:25:19. Version ac07

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