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 2025-07-06.

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% 38%	119741
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 41%	119037
Transparency	Day	43% 6% 12% 21% 15% 3	123301
	Night	42% 5% 12% 22% 16% 3	123638
Seeing	Day	24% 17% 30% 19% 8% 2	15803
	Night	22% 15% 30% 24% 8% 1	24899

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% 6% 3 7% 4 4 5% 3 5% 5% 45%	3597
	11th to 20th	Day	14% 9% 3 7% 5% 5% 5% 3 4 5% 40%	3570
	21st to end	Day	17% 9% 3 7% 4 5% 4 3 4 5% 39%	4167
February	1st to 10th	Day	17% 8% 2 7% 5% 5% 5% 4 5% 6% 37%	1832
	11th to 20th	Day	14% 8% 3 6% 5% 4 4 4 5% 7% 40%	1948
	21st to end	Day	19% 9% 3 6% 4 4 4 3 4 6% 38%	1650
March	1st to 10th	Day	23% 8% 2 6% 4 4 3 3 4 5% 40%	4476
	11th to 20th	Day	13% 6% 3 6% 4 4 4 4 6% 7% 42%	4693
	21st to end	Day	19% 8% 2 6% 4 4 4 4 5% 6% 38%	5970
April	1st to 10th	Day	19% 9% 2 7% 4 4 4 3 6% 7% 35%	2400
	11th to 20th	Day	17% 7% 2 6% 4 4 4 3 5% 7% 40%	2374
	21st to end	Day	20% 6% 2 5% 4 4 4 4 5% 6% 40%	2475
May	1st to 10th	Day	30% 8% 2 6% 3 4 4 3 5% 8% 26%	5886
	11th to 20th	Day	21% 8% 3 7% 4 4 4 3 5% 6% 35%	6224
	21st to end	Day	21% 7% 2 6% 4 4 4 4 6% 8% 35%	6949
June	1st to 10th	Day	18% 6% 2 5% 4 5% 4 5% 6% 9% 36%	2800
	11th to 20th	Day	16% 7% 2 6% 5% 4 5% 5% 6% 9% 35%	2800
	21st to end	Day	13% 6% 3 6% 5% 5% 5% 4 7% 10% 37%	2800
July	1st to 10th	Day	11% 6% 2 7% 6% 6% 6% 5% 7% 10% 35%	4510
	11th to 20th	Day	13% 6% 2 6% 4 5% 5% 4 7% 9% 38%	4440
	21st to end	Day	15% 8% 3 7% 5% 5% 5% 5% 6% 7% 33%	4814
August	1st to 10th	Day	20% 7% 2 6% 5% 5% 5% 4 6% 8% 31%	4254
	11th to 20th	Day	11% 7% 2 7% 4 4 4 4 6% 9% 41%	3730
	21st to end	Day	10% 6% 3 5% 4 4 5% 4 6% 9% 45%	4180
September	1st to 10th	Day	13% 7% 2 6% 5% 4 5% 4 6% 5% 43%	2280
	11th to 20th	Day	13% 5% 2 5% 3 3 4 3 4 7% 49%	2173
	21st to end	Day	17% 7% 3 6% 4 4 4 3 5% 6% 40%	2090
October	1st to 10th	Day	22% 6% 2 6% 4 4 4 3 4 5% 41%	2022
	11th to 20th	Day	14% 6% 2 6% 4 4 3 3 6% 7% 46%	1900
	21st to end	Day	22% 5% 2 5% 4 3 5% 4 4 6% 40%	1895
November	1st to 10th	Day	13% 6% 2 6% 3 3 3 3 5% 6% 50%	1761
	11th to 20th	Day	19% 5% 2 5% 3 3 3 2 5% 7% 46%	1800
	21st to end	Day	17% 6% 2 5% 4 4 3 3 6% 7% 43%	1735
December	1st to 10th	Day	16% 7% 3 5% 4 4 3 3 4 7% 44%	2600
	11th to 20th	Day	21% 6% 3 5% 3 3 3 3 4 6% 44%	2986
	21st to end	Day	18% 10% 4 7% 4 4 4 3 4 6% 37%	3960
January	1st to 10th	Night	14% 6% 2 5% 3 4 4 4 6% 7% 46%	6349
	11th to 20th	Night	14% 8% 3 7% 4 3 4 3 5% 6% 44%	6315
	21st to end	Night	17% 9% 3 7% 4 4 4 3 4 6% 39%	6786
February	1st to 10th	Night	16% 9% 3 6% 5% 4 4 3 5% 6% 40%	2565
	11th to 20th	Night	16% 9% 3 5% 4 4 3 4 5% 7% 40%	2338
	21st to end	Night	18% 8% 3 6% 4 3 4 3 5% 5% 41%	1980
March	1st to 10th	Night	20% 10% 2 5% 3 3 3 3 5% 6% 41%	5364
	11th to 20th	Night	15% 8% 3 6% 4 4 4 3 4 6% 43%	5247
	21st to end	Night	19% 10% 3 6% 4 3 4 3 4 5% 39%	4962
April	1st to 10th	Night	19% 6% 3 5% 5% 5% 4 4 5% 7% 36%	2000
	11th to 20th	Night	18% 8% 2 6% 5% 4 4 4 4 8% 38%	1975
	21st to end	Night	19% 7% 2 6% 4 3 4 2 5% 5% 41%	1925
May	1st to 10th	Night	29% 9% 3 7% 4 3 3 3 4 7% 26%	4054
	11th to 20th	Night	21% 10% 2 6% 3 4 4 4 5% 7% 34%	3699
	21st to end	Night	22% 8% 2 6% 4 4 4 3 5% 6% 36%	3926
June	1st to 10th	Night	18% 6% 2 5% 5% 4 4 4 5% 7% 41%	1600
	11th to 20th	Night	16% 5% 3 6% 5% 4 4 4 7% 9% 39%	1600
	21st to end	Night	12% 6% 2 6% 4 4 4 4 7% 9% 42%	1600
July	1st to 10th	Night	12% 6% 2 6% 5% 5% 6% 4 7% 9% 38%	2560
	11th to 20th	Night	15% 6% 2 7% 4 6% 4 4 6% 9% 37%	2520
	21st to end	Night	18% 8% 2 6% 5% 4 4 4 6% 7% 36%	2739
August	1st to 10th	Night	21% 7% 3 8% 5% 4 5% 4 5% 7% 30%	2641
	11th to 20th	Night	15% 7% 2 7% 5% 4 5% 3 6% 7% 40%	3097
	21st to end	Night	11% 5% 2 4 3 3 4 3 5% 7% 53%	3476
September	1st to 10th	Night	15% 7% 2 6% 4 4 3 3 4 6% 46%	1900
	11th to 20th	Night	13% 5% 2 5% 3 3 3 2 4 6% 54%	2005
	21st to end	Night	19% 7% 2 5% 3 3 4 4 4 5% 44%	2090
October	1st to 10th	Night	20% 6% 2 6% 4 3 3 3 4 6% 42%	2156
	11th to 20th	Night	15% 6% 2 6% 3 3 3 3 4 6% 48%	2280
	21st to end	Night	22% 7% 2 5% 3 4 4 3 5% 6% 41%	2702
November	1st to 10th	Night	12% 6% 2 5% 4 3 4 4 6% 6% 49%	2537
	11th to 20th	Night	19% 7% 2 4 3 3 3 2 5% 6% 46%	2600
	21st to end	Night	15% 6% 2 5% 4 4 3 3 5% 7% 45%	2665
December	1st to 10th	Night	16% 8% 2 5% 3 3 3 3 3 5% 47%	4580
	11th to 20th	Night	24% 8% 2 5% 3 3 3 3 4 5% 41%	5208
	21st to end	Night	19% 9% 3 7% 4 4 3 2 5% 5% 39%	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	39% 5% 8% 28% 17% 3	3598
	11th to 20th	Day	45% 5% 11% 21% 13% 6%	3600
	21st to end	Day	48% 4 8% 23% 15% 1	4167
February	1st to 10th	Day	47% 5% 12% 21% 14% 1	1832
	11th to 20th	Day	42% 5% 11% 22% 17% 3	1957
	21st to end	Day	48% 4 10% 22% 13% 3	1650
March	1st to 10th	Day	46% 4 12% 16% 13% 9%	4479
	11th to 20th	Day	37% 6% 16% 28% 10% 3	4693
	21st to end	Day	45% 4 10% 13% 21% 6%	5970
April	1st to 10th	Day	47% 6% 10% 19% 15% 4	2400
	11th to 20th	Day	43% 6% 11% 21% 17% 2	2374
	21st to end	Day	43% 6% 10% 21% 16% 3	2475
May	1st to 10th	Day	56% 6% 15% 13% 10% 1	5897
	11th to 20th	Day	49% 7% 11% 18% 13% 1	6228
	21st to end	Day	46% 6% 11% 22% 13% 1	6949
June	1st to 10th	Day	43% 8% 14% 21% 13% 1	2800
	11th to 20th	Day	43% 8% 14% 22% 11% 1	2800
	21st to end	Day	41% 8% 14% 23% 13% 1	2800
July	1st to 10th	Day	43% 8% 12% 20% 15% 1	4440
	11th to 20th	Day	40% 7% 16% 22% 13% 1	4365
	21st to end	Day	48% 7% 12% 17% 14% 1	4661
August	1st to 10th	Day	49% 8% 15% 21% 8%	4240
	11th to 20th	Day	38% 7% 16% 29% 10%	3766
	21st to end	Day	35% 6% 12% 29% 17% 1	4180
September	1st to 10th	Day	40% 8% 10% 22% 18% 1	2280
	11th to 20th	Day	35% 5% 10% 23% 25% 2	2173
	21st to end	Day	44% 6% 10% 21% 16% 3	2090
October	1st to 10th	Day	39% 5% 10% 18% 24% 5%	3360
	11th to 20th	Day	28% 4 9% 25% 29% 5%	3140
	21st to end	Day	31% 3 15% 24% 23% 3	3095
November	1st to 10th	Day	35% 5% 12% 25% 20% 3	1761
	11th to 20th	Day	38% 4 11% 23% 20% 4	1800
	21st to end	Day	40% 4 14% 28% 12% 2	1735
December	1st to 10th	Day	41% 4 16% 27% 11% 1	2600
	11th to 20th	Day	43% 3 14% 21% 16% 3	2986
	21st to end	Day	48% 5% 8% 20% 15% 3	3960
January	1st to 10th	Night	36% 4 10% 26% 21% 3	6349
	11th to 20th	Night	41% 4 15% 20% 14% 6%	6360
	21st to end	Night	47% 3 11% 18% 17% 5%	6786
February	1st to 10th	Night	47% 5% 12% 22% 12% 2	2565
	11th to 20th	Night	44% 5% 10% 19% 18% 4	2340
	21st to end	Night	45% 4 9% 22% 16% 4	1980
March	1st to 10th	Night	44% 4 12% 18% 14% 8%	5374
	11th to 20th	Night	41% 6% 15% 23% 12% 4	5248
	21st to end	Night	47% 4 7% 14% 23% 5%	4962
April	1st to 10th	Night	46% 7% 10% 20% 14% 3	2000
	11th to 20th	Night	45% 5% 12% 19% 19% 1	1975
	21st to end	Night	44% 5% 8% 22% 16% 4	1925
May	1st to 10th	Night	58% 6% 13% 13% 10% 1	4060
	11th to 20th	Night	49% 8% 12% 15% 13% 2	3700
	21st to end	Night	48% 6% 14% 22% 10% 1	3927
June	1st to 10th	Night	44% 7% 10% 21% 17% 2	1600
	11th to 20th	Night	41% 7% 16% 25% 10% 1	1600
	21st to end	Night	36% 9% 15% 25% 14% 1	1600
July	1st to 10th	Night	41% 9% 16% 18% 14% 1	2520
	11th to 20th	Night	44% 6% 15% 23% 12% 1	2472
	21st to end	Night	46% 7% 15% 14% 17% 2	2651
August	1st to 10th	Night	52% 8% 14% 17% 9% 1	2631
	11th to 20th	Night	43% 7% 14% 28% 8% 1	3127
	21st to end	Night	30% 6% 12% 34% 17% 1	3476
September	1st to 10th	Night	40% 5% 9% 26% 18% 1	1900
	11th to 20th	Night	32% 5% 11% 25% 25% 2	2005
	21st to end	Night	42% 6% 8% 21% 20% 2	2090
October	1st to 10th	Night	40% 5% 11% 22% 20% 2	3598
	11th to 20th	Night	26% 4 9% 28% 28% 4	3779
	21st to end	Night	33% 4 13% 29% 18% 2	4452
November	1st to 10th	Night	35% 6% 12% 23% 21% 4	2537
	11th to 20th	Night	40% 4 10% 23% 20% 4	2600
	21st to end	Night	39% 5% 13% 30% 12% 1	2665
December	1st to 10th	Night	40% 4 12% 31% 12% 1	4580
	11th to 20th	Night	47% 4 12% 19% 16% 3	5208
	21st to end	Night	47% 5% 10% 18% 16% 4	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	38% 22% 26% 13% 2	200
	11th to 20th	Day	31% 28% 31% 10% 1	200
	21st to end	Day	24% 33% 30% 8% 5%	265
February	1st to 10th	Day	34% 27% 29% 9% 1	332
	11th to 20th	Day	31% 29% 30% 9% 1	496
	21st to end	Day	31% 25% 30% 11% 2	414
March	1st to 10th	Day	31% 20% 33% 13% 3	500
	11th to 20th	Day	28% 26% 31% 11% 2 1	500
	21st to end	Day	31% 24% 31% 9% 3 1	550
April	1st to 10th	Day	34% 26% 24% 10% 6% 1	500
	11th to 20th	Day	30% 25% 26% 13% 5% 1	492
	21st to end	Day	30% 18% 33% 14% 4 1	500
May	1st to 10th	Day	35% 18% 29% 12% 5% 1	503
	11th to 20th	Day	25% 17% 34% 17% 6% 1	527
	21st to end	Day	22% 11% 31% 24% 9% 2	573
June	1st to 10th	Day	21% 13% 31% 23% 10% 2	530
	11th to 20th	Day	20% 13% 34% 22% 10% 2	530
	21st to end	Day	16% 12% 31% 28% 12% 1	530
July	1st to 10th	Day	15% 8% 31% 26% 16% 3	505
	11th to 20th	Day	13% 6% 31% 32% 16% 2	484
	21st to end	Day	14% 10% 33% 29% 12% 1	532
August	1st to 10th	Day	15% 9% 27% 31% 13% 5%	486
	11th to 20th	Day	15% 7% 28% 31% 14% 5%	479
	21st to end	Day	11% 12% 28% 27% 14% 8%	539
September	1st to 10th	Day	19% 11% 27% 21% 17% 5%	490
	11th to 20th	Day	15% 10% 28% 27% 12% 9%	490
	21st to end	Day	21% 14% 31% 19% 10% 4	490
October	1st to 10th	Day	29% 11% 26% 17% 14% 3	490
	11th to 20th	Day	17% 21% 30% 21% 8% 2	490
	21st to end	Day	30% 13% 29% 18% 8% 1	436
November	1st to 10th	Day	20% 18% 29% 23% 8% 2	395
	11th to 20th	Day	26% 24% 28% 17% 5% 1	400
	21st to end	Day	28% 21% 28% 18% 4 1	335
December	1st to 10th	Day	31% 22% 29% 15% 4 1	200
	11th to 20th	Day	36% 26% 25% 11% 3	200
	21st to end	Day	25% 24% 30% 15% 5% 1	220
January	1st to 10th	Night	25% 21% 35% 18% 3	800
	11th to 20th	Night	27% 21% 31% 20% 2	800
	21st to end	Night	23% 22% 35% 18% 2	880
February	1st to 10th	Night	25% 20% 35% 20% 2	800
	11th to 20th	Night	27% 23% 35% 14% 1	783
	21st to end	Night	28% 20% 33% 17% 3	660
March	1st to 10th	Night	26% 21% 33% 18% 3	800
	11th to 20th	Night	24% 24% 33% 17% 3	800
	21st to end	Night	25% 21% 34% 16% 4	880
April	1st to 10th	Night	27% 21% 32% 18% 3	800
	11th to 20th	Night	24% 19% 34% 19% 4	791
	21st to end	Night	24% 20% 33% 18% 5%	725
May	1st to 10th	Night	28% 14% 33% 20% 5%	600
	11th to 20th	Night	22% 17% 33% 22% 5%	453
	21st to end	Night	23% 7% 26% 35% 9% 1	433
June	1st to 10th	Night	19% 9% 29% 32% 11% 1	400
	11th to 20th	Night	18% 7% 29% 34% 13%	400
	21st to end	Night	14% 5% 27% 44% 10%	400
July	1st to 10th	Night	15% 5% 22% 40% 17% 1	380
	11th to 20th	Night	12% 5% 24% 41% 16% 2	368
	21st to end	Night	14% 5% 25% 40% 16% 1	416
August	1st to 10th	Night	12% 3 27% 41% 16% 1	448
	11th to 20th	Night	16% 5% 22% 35% 20% 2	750
	21st to end	Night	16% 5% 19% 38% 19% 2	836
September	1st to 10th	Night	17% 6% 29% 32% 14% 2	760
	11th to 20th	Night	16% 7% 22% 31% 19% 4	760
	21st to end	Night	22% 8% 25% 29% 14% 2	760
October	1st to 10th	Night	23% 11% 26% 26% 13% 1	760
	11th to 20th	Night	17% 14% 31% 27% 9% 1	760
	21st to end	Night	26% 12% 34% 20% 7%	836
November	1st to 10th	Night	16% 14% 34% 27% 8% 1	780
	11th to 20th	Night	23% 17% 31% 21% 6% 1	800
	21st to end	Night	24% 17% 31% 22% 6% 1	800
December	1st to 10th	Night	26% 19% 31% 19% 4 1	800
	11th to 20th	Night	27% 20% 29% 21% 3	800
	21st to end	Night	27% 17% 34% 19% 2	880

Page generated at 2025-07-03 01:30:12. Version ac07

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