Danbury
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Danbury clear sky chart from 2005-11-06 to 2026-10-29.

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	17% 7% 2 6% 4 4 4 4 5% 7% 39%	149467
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 41%	142590
Transparency	Day	43% 6% 12% 22% 15% 2	152984
	Night	43% 5% 12% 22% 16% 3	147162
Seeing	Day	24% 17% 30% 19% 8% 2	17545
	Night	22% 15% 30% 24% 8% 1	26560

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	12% 7% 3 7% 4 4 5% 3 5% 5% 43%	4661
	11th to 20th	Day	15% 9% 3 6% 4 4 4 3 4 5% 42%	4650
	21st to end	Day	15% 8% 3 6% 4 4 4 4 5% 7% 41%	5400
February	1st to 10th	Day	16% 7% 2 7% 5% 5% 5% 4 5% 6% 38%	1924
	11th to 20th	Day	15% 8% 3 6% 5% 4 4 4 5% 7% 39%	2048
	21st to end	Day	19% 9% 3 6% 4 4 4 3 4 6% 38%	1730
March	1st to 10th	Day	23% 8% 2 6% 4 4 3 3 3 5% 40%	4576
	11th to 20th	Day	14% 6% 3 6% 4 4 4 4 6% 7% 42%	4796
	21st to end	Day	19% 8% 2 6% 4 4 4 4 5% 6% 38%	6102
April	1st to 10th	Day	19% 9% 2 7% 4 3 4 3 6% 7% 36%	2520
	11th to 20th	Day	17% 7% 2 6% 3 4 4 3 5% 7% 40%	2494
	21st to end	Day	20% 6% 2 5% 4 4 4 4 5% 6% 40%	2599
May	1st to 10th	Day	26% 8% 2 5% 3 4 4 3 5% 8% 31%	7639
	11th to 20th	Day	19% 7% 3 7% 4 4 4 3 5% 6% 38%	8100
	21st to end	Day	23% 7% 2 6% 4 4 4 4 6% 7% 32%	9060
June	1st to 10th	Day	17% 5% 2 5% 4 5% 4 5% 6% 9% 37%	2940
	11th to 20th	Day	16% 7% 2 6% 5% 4 5% 5% 6% 9% 36%	2940
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 4 7% 10% 37%	2940
July	1st to 10th	Day	15% 6% 2 6% 5% 5% 5% 4 6% 10% 36%	8280
	11th to 20th	Day	10% 6% 2 6% 4 5% 5% 4 7% 11% 38%	8280
	21st to end	Day	15% 7% 2 6% 5% 4 4 4 5% 8% 40%	8993
August	1st to 10th	Day	16% 7% 3 7% 5% 6% 6% 5% 7% 9% 29%	6238
	11th to 20th	Day	11% 6% 2 7% 4 4 4 4 6% 9% 43%	5488
	21st to end	Day	11% 6% 3 6% 4 4 5% 4 6% 10% 42%	6080
September	1st to 10th	Day	13% 7% 2 6% 5% 4 5% 4 6% 6% 43%	2520
	11th to 20th	Day	13% 6% 2 5% 3 3 4 3 5% 7% 50%	2401
	21st to end	Day	18% 7% 3 7% 4 5% 4 3 5% 6% 38%	2310
October	1st to 10th	Day	20% 5% 2 6% 4 4 3 3 4 5% 44%	2234
	11th to 20th	Day	14% 6% 2 6% 3 4 3 3 5% 8% 46%	2100
	21st to end	Day	21% 5% 2 5% 4 3 4 4 4 6% 42%	2068
November	1st to 10th	Day	12% 6% 2 6% 3 3 3 3 5% 6% 50%	1851
	11th to 20th	Day	18% 5% 2 4 3 4 4 3 5% 7% 46%	1890
	21st to end	Day	17% 7% 3 5% 4 4 3 3 6% 6% 42%	1821
December	1st to 10th	Day	16% 7% 3 6% 4 4 3 3 4 7% 44%	2680
	11th to 20th	Day	21% 6% 3 5% 3 3 3 3 4 6% 44%	3066
	21st to end	Day	18% 10% 3 7% 4 4 4 3 4 6% 37%	4048
January	1st to 10th	Night	14% 8% 2 6% 4 4 4 3 5% 6% 43%	8234
	11th to 20th	Night	15% 9% 3 7% 4 3 4 3 4 6% 41%	8235
	21st to end	Night	15% 9% 2 6% 4 4 4 3 4 6% 43%	8778
February	1st to 10th	Night	16% 9% 3 6% 5% 4 4 3 5% 6% 40%	2693
	11th to 20th	Night	16% 9% 3 5% 5% 4 4 4 5% 7% 39%	2458
	21st to end	Night	18% 8% 3 7% 4 3 4 3 5% 5% 41%	2076
March	1st to 10th	Night	20% 10% 2 5% 3 3 3 3 4 6% 41%	5484
	11th to 20th	Night	15% 8% 3 6% 4 4 4 3 5% 6% 43%	5364
	21st to end	Night	19% 10% 3 6% 3 3 4 3 4 5% 39%	5072
April	1st to 10th	Night	20% 6% 3 5% 4 5% 4 4 5% 7% 37%	2100
	11th to 20th	Night	18% 8% 2 6% 5% 4 4 4 4 8% 38%	2075
	21st to end	Night	20% 7% 2 6% 4 3 4 2 5% 5% 41%	2021
May	1st to 10th	Night	26% 9% 4 7% 4 3 3 3 4 7% 30%	5259
	11th to 20th	Night	19% 9% 2 7% 4 4 4 4 5% 8% 35%	4803
	21st to end	Night	22% 9% 3 7% 4 4 4 3 5% 6% 34%	5114
June	1st to 10th	Night	18% 6% 2 5% 5% 4 3 4 5% 7% 42%	1680
	11th to 20th	Night	15% 5% 3 6% 5% 4 4 5% 6% 9% 38%	1680
	21st to end	Night	12% 6% 2 6% 4 4 4 4 7% 9% 42%	1680
July	1st to 10th	Night	16% 6% 2 6% 4 4 5% 4 6% 9% 37%	4680
	11th to 20th	Night	13% 8% 3 7% 4 5% 4 4 6% 9% 37%	4680
	21st to end	Night	15% 6% 2 5% 4 4 4 4 6% 7% 44%	5084
August	1st to 10th	Night	19% 7% 3 8% 5% 4 5% 5% 6% 7% 31%	3847
	11th to 20th	Night	14% 7% 3 6% 5% 5% 5% 4 6% 8% 39%	4557
	21st to end	Night	13% 6% 2 5% 3 3 4 3 5% 7% 49%	5045
September	1st to 10th	Night	16% 7% 2 6% 3 4 4 3 4 6% 46%	2100
	11th to 20th	Night	12% 5% 2 6% 3 3 3 2 4 6% 53%	2217
	21st to end	Night	20% 7% 2 5% 3 4 4 4 4 5% 42%	2310
October	1st to 10th	Night	19% 6% 2 5% 4 3 3 3 4 6% 45%	2383
	11th to 20th	Night	15% 6% 2 6% 3 3 3 3 5% 7% 48%	2520
	21st to end	Night	20% 6% 2 5% 3 4 4 3 5% 6% 43%	2948
November	1st to 10th	Night	12% 6% 2 5% 3 3 4 4 6% 6% 48%	2667
	11th to 20th	Night	18% 7% 2 4 3 3 3 3 5% 6% 46%	2730
	21st to end	Night	15% 6% 2 5% 4 4 4 3 5% 7% 45%	2798
December	1st to 10th	Night	16% 8% 2 5% 3 4 3 3 4 5% 47%	4720
	11th to 20th	Night	24% 8% 2 4 3 3 3 3 4 5% 41%	5348
	21st to end	Night	19% 9% 3 7% 4 4 3 3 5% 5% 39%	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	41% 4 10% 26% 16% 3	4662
	11th to 20th	Day	44% 4 10% 20% 15% 7%	4680
	21st to end	Day	43% 4 10% 25% 17% 1	5400
February	1st to 10th	Day	46% 5% 12% 21% 15% 2	1924
	11th to 20th	Day	43% 5% 11% 22% 17% 2	2057
	21st to end	Day	48% 4 11% 22% 13% 2	1730
March	1st to 10th	Day	46% 4 12% 16% 13% 9%	4579
	11th to 20th	Day	38% 6% 16% 27% 10% 3	4796
	21st to end	Day	46% 4 10% 13% 21% 6%	6102
April	1st to 10th	Day	46% 6% 10% 19% 15% 4	2520
	11th to 20th	Day	42% 6% 12% 20% 17% 2	2494
	21st to end	Day	43% 6% 10% 21% 17% 3	2599
May	1st to 10th	Day	52% 5% 13% 16% 12% 1	7651
	11th to 20th	Day	46% 6% 12% 21% 14% 1	8104
	21st to end	Day	50% 5% 12% 20% 12% 1	9061
June	1st to 10th	Day	42% 8% 14% 21% 14% 1	2940
	11th to 20th	Day	42% 8% 14% 22% 12% 1	2940
	21st to end	Day	41% 8% 14% 22% 13% 1	2940
July	1st to 10th	Day	44% 8% 14% 19% 15% 1	8210
	11th to 20th	Day	38% 7% 16% 27% 12% 1	8205
	21st to end	Day	42% 5% 11% 23% 18% 1	8795
August	1st to 10th	Day	49% 8% 16% 18% 9%	6224
	11th to 20th	Day	37% 6% 18% 32% 8%	5524
	21st to end	Day	38% 6% 13% 27% 14% 2	6080
September	1st to 10th	Day	40% 7% 10% 23% 18% 2	2520
	11th to 20th	Day	34% 5% 10% 23% 24% 2	2401
	21st to end	Day	46% 6% 10% 20% 15% 3	2310
October	1st to 10th	Day	38% 5% 10% 19% 25% 5%	3572
	11th to 20th	Day	29% 4 9% 25% 28% 5%	3340
	21st to end	Day	30% 3 15% 24% 23% 4	3268
November	1st to 10th	Day	36% 5% 12% 25% 20% 3	1851
	11th to 20th	Day	38% 4 11% 23% 21% 4	1890
	21st to end	Day	42% 4 13% 27% 11% 2	1821
December	1st to 10th	Day	41% 4 16% 27% 11% 1	2680
	11th to 20th	Day	43% 3 14% 21% 16% 3	3066
	21st to end	Day	49% 5% 8% 21% 15% 3	4048
January	1st to 10th	Night	40% 4 12% 24% 18% 2	8234
	11th to 20th	Night	44% 4 12% 18% 15% 7%	8280
	21st to end	Night	44% 3 10% 20% 19% 4	8778
February	1st to 10th	Night	46% 5% 11% 23% 13% 2	2693
	11th to 20th	Night	44% 5% 10% 20% 18% 4	2460
	21st to end	Night	45% 4 9% 23% 15% 4	2076
March	1st to 10th	Night	44% 4 12% 18% 15% 7%	5494
	11th to 20th	Night	41% 6% 15% 23% 12% 4	5365
	21st to end	Night	48% 4 7% 13% 23% 5%	5072
April	1st to 10th	Night	46% 6% 10% 19% 15% 3	2100
	11th to 20th	Night	45% 5% 12% 19% 19% 1	2075
	21st to end	Night	44% 5% 9% 21% 17% 4	2021
May	1st to 10th	Night	54% 5% 12% 16% 12% 1	5265
	11th to 20th	Night	47% 7% 12% 20% 12% 2	4804
	21st to end	Night	52% 5% 12% 21% 9% 1	5115
June	1st to 10th	Night	42% 6% 10% 22% 18% 2	1680
	11th to 20th	Night	41% 7% 16% 25% 10% 1	1680
	21st to end	Night	36% 9% 15% 25% 14% 1	1680
July	1st to 10th	Night	44% 7% 15% 20% 13% 1	4640
	11th to 20th	Night	43% 6% 14% 24% 13% 1	4632
	21st to end	Night	38% 6% 14% 24% 18% 1	4967
August	1st to 10th	Night	50% 8% 14% 17% 11% 1	3837
	11th to 20th	Night	42% 7% 16% 29% 6%	4587
	21st to end	Night	35% 5% 13% 32% 15% 1	5045
September	1st to 10th	Night	41% 5% 9% 26% 18% 1	2100
	11th to 20th	Night	33% 5% 11% 24% 25% 2	2217
	21st to end	Night	43% 6% 8% 21% 20% 2	2310
October	1st to 10th	Night	38% 5% 11% 23% 20% 2	3825
	11th to 20th	Night	27% 4 9% 28% 28% 4	4019
	21st to end	Night	33% 4 13% 29% 19% 3	4698
November	1st to 10th	Night	35% 6% 12% 23% 20% 4	2667
	11th to 20th	Night	39% 4 10% 23% 20% 4	2730
	21st to end	Night	39% 5% 13% 30% 12% 1	2798
December	1st to 10th	Night	40% 4 12% 31% 13% 1	4720
	11th to 20th	Night	46% 4 12% 19% 16% 3	5348
	21st to end	Night	46% 5% 10% 18% 16% 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	36% 23% 25% 13% 2	220
	11th to 20th	Day	33% 28% 30% 9% 1	220
	21st to end	Day	25% 33% 30% 8% 4	292
February	1st to 10th	Day	33% 26% 31% 10% 1	364
	11th to 20th	Day	30% 28% 32% 9% 1	536
	21st to end	Day	31% 25% 31% 11% 2	446
March	1st to 10th	Day	31% 20% 33% 13% 3	540
	11th to 20th	Day	29% 26% 31% 11% 2 1	540
	21st to end	Day	32% 26% 29% 9% 3 1	594
April	1st to 10th	Day	34% 25% 24% 10% 5% 1	540
	11th to 20th	Day	29% 25% 27% 12% 5% 2	532
	21st to end	Day	31% 18% 33% 13% 4 1	540
May	1st to 10th	Day	34% 19% 29% 12% 4 1	543
	11th to 20th	Day	24% 17% 35% 17% 6% 1	567
	21st to end	Day	24% 12% 31% 22% 9% 2	617
June	1st to 10th	Day	20% 14% 30% 24% 10% 2	570
	11th to 20th	Day	20% 13% 35% 21% 9% 1	570
	21st to end	Day	16% 12% 31% 27% 12% 1	570
July	1st to 10th	Day	16% 8% 30% 28% 15% 3	565
	11th to 20th	Day	12% 7% 32% 31% 16% 2	564
	21st to end	Day	14% 10% 33% 29% 12% 2	620
August	1st to 10th	Day	14% 9% 28% 30% 14% 5%	566
	11th to 20th	Day	14% 7% 29% 31% 14% 5%	559
	21st to end	Day	10% 12% 30% 28% 13% 7%	627
September	1st to 10th	Day	20% 11% 29% 20% 15% 4	570
	11th to 20th	Day	15% 9% 28% 27% 12% 8%	570
	21st to end	Day	22% 14% 31% 19% 10% 4	570
October	1st to 10th	Day	26% 11% 27% 18% 14% 4	570
	11th to 20th	Day	20% 21% 29% 20% 8% 2	570
	21st to end	Day	29% 13% 31% 17% 9% 1	495
November	1st to 10th	Day	20% 19% 29% 22% 8% 2	425
	11th to 20th	Day	26% 25% 27% 17% 5% 1	430
	21st to end	Day	30% 21% 28% 17% 4 1	361
December	1st to 10th	Day	31% 22% 29% 13% 4	220
	11th to 20th	Day	38% 25% 25% 10% 2	220
	21st to end	Day	28% 23% 31% 14% 4 1	242
January	1st to 10th	Night	25% 20% 35% 18% 3	840
	11th to 20th	Night	28% 21% 30% 19% 2	840
	21st to end	Night	23% 23% 35% 17% 2	924
February	1st to 10th	Night	24% 20% 34% 20% 2	840
	11th to 20th	Night	27% 22% 35% 14% 1	823
	21st to end	Night	28% 20% 33% 17% 2	692
March	1st to 10th	Night	26% 21% 32% 18% 3	840
	11th to 20th	Night	24% 24% 32% 16% 3	840
	21st to end	Night	25% 21% 34% 16% 4	924
April	1st to 10th	Night	28% 21% 31% 18% 3	840
	11th to 20th	Night	24% 19% 35% 19% 3	831
	21st to end	Night	25% 19% 33% 18% 5%	761
May	1st to 10th	Night	27% 15% 33% 20% 5%	630
	11th to 20th	Night	22% 17% 34% 22% 5%	476
	21st to end	Night	24% 7% 27% 34% 8% 1	455
June	1st to 10th	Night	18% 9% 29% 33% 11% 1	420
	11th to 20th	Night	18% 8% 30% 32% 12%	420
	21st to end	Night	14% 5% 26% 44% 10%	420
July	1st to 10th	Night	16% 5% 22% 41% 16%	410
	11th to 20th	Night	13% 5% 25% 40% 15% 2	408
	21st to end	Night	13% 5% 25% 40% 16% 1	460
August	1st to 10th	Night	12% 3 25% 42% 17% 1	495
	11th to 20th	Night	16% 5% 22% 36% 19% 2	830
	21st to end	Night	16% 5% 21% 38% 18% 2	924
September	1st to 10th	Night	18% 6% 28% 33% 13% 2	840
	11th to 20th	Night	15% 7% 23% 33% 19% 4	840
	21st to end	Night	24% 8% 25% 29% 14% 1	840
October	1st to 10th	Night	21% 11% 27% 26% 14% 2	840
	11th to 20th	Night	18% 15% 30% 27% 9% 1	840
	21st to end	Night	25% 12% 34% 21% 8%	913
November	1st to 10th	Night	18% 14% 33% 27% 8% 1	820
	11th to 20th	Night	23% 18% 31% 21% 6% 1	840
	21st to end	Night	23% 18% 30% 23% 5% 1	840
December	1st to 10th	Night	26% 19% 31% 19% 4 1	840
	11th to 20th	Night	28% 20% 29% 20% 3	840
	21st to end	Night	27% 18% 34% 19% 2	924

Page generated at 2026-10-26 04:37:52. Version ac07

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