Wilton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wilton 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	17% 7% 2 6% 4 4 4 4 6% 7% 38%	130475
	Night	17% 8% 2 6% 4 4 4 4 5% 6% 40%	130358
Transparency	Day	29% 38% 7% 14% 10% 2	133995
	Night	28% 37% 8% 14% 11% 2	134925
Seeing	Day	25% 18% 31% 18% 7% 2	16500
	Night	23% 17% 33% 21% 6%	25537

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	10% 6% 3 10% 6% 6% 6% 5% 6% 6% 36%	4661
	11th to 20th	Day	13% 8% 3 7% 4 4 4 4 5% 6% 42%	4650
	21st to end	Day	12% 10% 3 6% 3 3 3 3 5% 6% 45%	5124
February	1st to 10th	Day	15% 8% 2 7% 5% 4 5% 5% 6% 6% 35%	1923
	11th to 20th	Day	12% 7% 3 8% 5% 4 4 4 6% 9% 39%	1998
	21st to end	Day	17% 10% 3 7% 5% 4 4 4 5% 4 37%	1650
March	1st to 10th	Day	19% 11% 3 7% 4 4 5% 3 4 5% 35%	4476
	11th to 20th	Day	15% 9% 2 5% 4 4 4 4 6% 7% 40%	4736
	21st to end	Day	23% 10% 2 6% 3 3 4 3 5% 7% 33%	5832
April	1st to 10th	Day	18% 10% 2 6% 5% 4 4 4 4 5% 37%	2400
	11th to 20th	Day	17% 6% 2 6% 4 4 4 4 6% 7% 38%	2377
	21st to end	Day	21% 7% 2 7% 5% 4 4 4 5% 6% 35%	2600
May	1st to 10th	Day	31% 9% 2 7% 4 4 4 3 5% 6% 24%	5886
	11th to 20th	Day	20% 8% 3 8% 5% 5% 5% 4 5% 6% 30%	6087
	21st to end	Day	20% 7% 3 6% 4 4 5% 4 6% 8% 35%	6949
June	1st to 10th	Day	19% 6% 2 6% 5% 5% 5% 4 6% 8% 34%	2800
	11th to 20th	Day	17% 5% 2 6% 5% 4 5% 5% 6% 8% 37%	2800
	21st to end	Day	18% 7% 2 6% 6% 4 5% 4 5% 8% 35%	2800
July	1st to 10th	Day	15% 6% 2 7% 5% 5% 6% 5% 7% 9% 34%	6360
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 11% 38%	6360
	21st to end	Day	15% 6% 2 6% 4 4 4 3 6% 8% 42%	6926
August	1st to 10th	Day	15% 6% 2 6% 6% 5% 5% 4 7% 8% 36%	4349
	11th to 20th	Day	14% 7% 3 7% 5% 5% 5% 4 6% 8% 37%	4089
	21st to end	Day	9% 4 2 6% 4 5% 6% 4 7% 10% 43%	4312
September	1st to 10th	Day	13% 5% 1 6% 4 4 4 4 6% 7% 46%	2400
	11th to 20th	Day	14% 6% 1 5% 4 3 3 3 4 5% 52%	2215
	21st to end	Day	17% 5% 3 5% 3 5% 3 3 5% 7% 44%	2200
October	1st to 10th	Day	23% 5% 2 5% 5% 4 4 3 4 5% 41%	2200
	11th to 20th	Day	16% 5% 1 5% 4 4 4 3 5% 8% 45%	1932
	21st to end	Day	21% 7% 2 6% 4 4 5% 3 4 6% 39%	1980
November	1st to 10th	Day	14% 5% 3 6% 4 4 3 3 5% 5% 48%	1851
	11th to 20th	Day	17% 6% 2 5% 3 4 3 4 5% 5% 47%	1890
	21st to end	Day	18% 7% 2 5% 4 5% 4 3 5% 5% 42%	1868
December	1st to 10th	Day	15% 8% 2 6% 4 3 4 4 6% 7% 42%	2680
	11th to 20th	Day	20% 6% 2 5% 3 3 3 4 6% 5% 43%	3066
	21st to end	Day	18% 11% 3 7% 4 4 4 3 4 6% 37%	4048
January	1st to 10th	Night	12% 7% 2 7% 4 4 4 4 5% 7% 43%	8234
	11th to 20th	Night	13% 10% 3 9% 5% 5% 5% 4 5% 6% 36%	8235
	21st to end	Night	13% 9% 3 6% 4 4 4 4 5% 6% 41%	9057
February	1st to 10th	Night	14% 10% 3 6% 3 4 4 4 5% 7% 39%	2693
	11th to 20th	Night	12% 8% 2 6% 5% 5% 5% 4 6% 6% 40%	2399
	21st to end	Night	14% 9% 3 7% 5% 3 4 4 5% 6% 40%	1980
March	1st to 10th	Night	22% 9% 2 5% 4 4 4 3 5% 6% 37%	5364
	11th to 20th	Night	14% 9% 4 8% 4 4 5% 4 5% 6% 38%	5221
	21st to end	Night	25% 8% 2 5% 3 3 4 3 6% 6% 35%	5082
April	1st to 10th	Night	18% 9% 3 6% 5% 5% 4 3 5% 6% 36%	2000
	11th to 20th	Night	20% 7% 2 5% 4 4 4 4 5% 6% 40%	1971
	21st to end	Night	21% 6% 3 7% 3 4 4 3 3 5% 41%	1800
May	1st to 10th	Night	33% 10% 2 5% 4 3 3 3 5% 7% 24%	4054
	11th to 20th	Night	16% 9% 3 9% 6% 5% 5% 4 6% 7% 30%	3837
	21st to end	Night	22% 8% 2 7% 5% 4 4 3 5% 6% 34%	3926
June	1st to 10th	Night	18% 8% 3 6% 5% 3 4 3 4 7% 39%	1600
	11th to 20th	Night	19% 6% 1 6% 4 4 4 4 6% 8% 38%	1600
	21st to end	Night	16% 7% 3 5% 4 4 4 4 6% 7% 40%	1600
July	1st to 10th	Night	13% 5% 2 6% 5% 4 4 4 6% 8% 41%	3600
	11th to 20th	Night	11% 6% 2 6% 4 4 5% 4 6% 9% 41%	3600
	21st to end	Night	12% 5% 2 5% 4 4 3 3 5% 6% 50%	3926
August	1st to 10th	Night	20% 6% 2 6% 5% 5% 5% 4 5% 6% 36%	2767
	11th to 20th	Night	12% 8% 3 7% 5% 5% 5% 3 6% 7% 38%	2954
	21st to end	Night	9% 4 1 4 3 3 3 3 5% 8% 58%	3586
September	1st to 10th	Night	14% 4 2 5% 3 3 4 3 5% 7% 51%	2000
	11th to 20th	Night	13% 5% 1 4 3 3 2 3 5% 5% 54%	2183
	21st to end	Night	16% 5% 2 6% 3 4 4 3 5% 6% 47%	2200
October	1st to 10th	Night	20% 6% 2 5% 4 3 4 3 4 6% 43%	2200
	11th to 20th	Night	16% 5% 2 4 4 4 5% 4 5% 7% 46%	2464
	21st to end	Night	22% 7% 2 5% 3 4 4 4 5% 6% 38%	2860
November	1st to 10th	Night	15% 6% 2 5% 4 4 5% 3 4 6% 47%	2667
	11th to 20th	Night	16% 6% 2 5% 4 4 4 3 4 6% 46%	2730
	21st to end	Night	15% 7% 2 5% 4 4 4 3 5% 7% 44%	2750
December	1st to 10th	Night	16% 11% 3 6% 4 4 3 3 6% 6% 39%	4720
	11th to 20th	Night	21% 8% 3 6% 3 3 3 3 4 5% 41%	5348
	21st to end	Night	19% 9% 3 6% 3 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	37% 22% 9% 19% 12% 1	4662
	11th to 20th	Day	32% 22% 8% 18% 17% 4	4680
	21st to end	Day	32% 23% 6% 18% 18% 2	5124
February	1st to 10th	Day	18% 59% 7% 9% 6% 1	1923
	11th to 20th	Day	18% 59% 3 8% 9% 2	2007
	21st to end	Day	16% 62% 5% 9% 7% 1	1650
March	1st to 10th	Day	40% 30% 5% 8% 8% 9%	4479
	11th to 20th	Day	30% 32% 11% 18% 9% 1	4736
	21st to end	Day	39% 29% 5% 11% 12% 5%	5832
April	1st to 10th	Day	22% 62% 2 7% 5% 2	2400
	11th to 20th	Day	16% 63% 4 8% 8% 1	2377
	21st to end	Day	19% 62% 3 7% 8% 1	2600
May	1st to 10th	Day	46% 32% 9% 8% 5%	5897
	11th to 20th	Day	40% 32% 11% 11% 5%	6091
	21st to end	Day	35% 31% 8% 15% 9%	6949
June	1st to 10th	Day	16% 63% 5% 10% 7%	2800
	11th to 20th	Day	16% 62% 5% 9% 7% 1	2800
	21st to end	Day	16% 62% 5% 9% 7% 1	2800
July	1st to 10th	Day	35% 28% 10% 15% 12%	6290
	11th to 20th	Day	28% 29% 13% 19% 10% 1	6285
	21st to end	Day	30% 26% 8% 21% 14% 1	6728
August	1st to 10th	Day	30% 39% 10% 15% 6%	4335
	11th to 20th	Day	28% 39% 10% 17% 6%	4128
	21st to end	Day	19% 38% 9% 20% 12% 1	4312
September	1st to 10th	Day	15% 58% 6% 13% 8% 1	2400
	11th to 20th	Day	17% 57% 4 9% 12% 1	2215
	21st to end	Day	18% 57% 4 11% 8% 1	2200
October	1st to 10th	Day	27% 38% 5% 14% 14% 2	3590
	11th to 20th	Day	19% 37% 5% 19% 19% 3	3122
	21st to end	Day	19% 36% 5% 24% 14% 2	3180
November	1st to 10th	Day	20% 53% 7% 11% 8% 1	1851
	11th to 20th	Day	18% 54% 4 9% 14% 2	1890
	21st to end	Day	19% 54% 6% 11% 8% 1	1868
December	1st to 10th	Day	25% 36% 9% 22% 7%	2680
	11th to 20th	Day	29% 31% 8% 13% 15% 4	3066
	21st to end	Day	36% 28% 5% 15% 11% 5%	4048
January	1st to 10th	Night	32% 22% 11% 22% 11% 2	8234
	11th to 20th	Night	37% 22% 9% 14% 12% 4	8280
	21st to end	Night	36% 23% 7% 15% 16% 4	9057
February	1st to 10th	Night	18% 59% 5% 9% 7% 1	2693
	11th to 20th	Night	17% 59% 5% 7% 9% 2	2401
	21st to end	Night	17% 62% 3 9% 6% 3	1980
March	1st to 10th	Night	36% 30% 7% 9% 12% 6%	5374
	11th to 20th	Night	32% 32% 12% 13% 9% 1	5222
	21st to end	Night	38% 29% 4 8% 16% 5%	5082
April	1st to 10th	Night	22% 62% 3 6% 6% 2	2000
	11th to 20th	Night	18% 63% 4 6% 8% 1	1971
	21st to end	Night	18% 62% 4 7% 9% 1	1800
May	1st to 10th	Night	46% 31% 10% 8% 5%	4060
	11th to 20th	Night	38% 34% 11% 11% 6%	3838
	21st to end	Night	39% 31% 10% 12% 7% 1	3927
June	1st to 10th	Night	17% 62% 3 11% 8%	1600
	11th to 20th	Night	15% 64% 6% 10% 6%	1600
	21st to end	Night	15% 61% 4 12% 6% 1	1600
July	1st to 10th	Night	29% 28% 11% 19% 12%	3560
	11th to 20th	Night	30% 29% 14% 13% 13% 1	3552
	21st to end	Night	26% 27% 8% 25% 13%	3809
August	1st to 10th	Night	34% 38% 8% 12% 7% 1	2757
	11th to 20th	Night	29% 39% 10% 16% 6%	2981
	21st to end	Night	13% 37% 12% 22% 15% 1	3586
September	1st to 10th	Night	14% 57% 5% 13% 10%	2000
	11th to 20th	Night	15% 57% 4 11% 13% 1	2183
	21st to end	Night	19% 58% 4 13% 7% 1	2200
October	1st to 10th	Night	23% 37% 11% 17% 11% 1	3590
	11th to 20th	Night	14% 36% 7% 18% 22% 3	4013
	21st to end	Night	20% 36% 7% 25% 10% 1	4610
November	1st to 10th	Night	19% 54% 4 13% 9% 2	2667
	11th to 20th	Night	17% 54% 5% 10% 13% 2	2730
	21st to end	Night	17% 55% 7% 13% 8% 1	2750
December	1st to 10th	Night	30% 36% 7% 19% 8%	4720
	11th to 20th	Night	33% 31% 8% 9% 17% 2	5348
	21st to end	Night	35% 28% 6% 16% 13% 3	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	33% 24% 29% 14% 1	220
	11th to 20th	Day	36% 23% 30% 8% 2 1	220
	21st to end	Day	30% 33% 28% 7% 2	242
February	1st to 10th	Day	32% 28% 30% 9% 1	398
	11th to 20th	Day	36% 27% 27% 9%	516
	21st to end	Day	31% 25% 29% 12% 2	414
March	1st to 10th	Day	32% 23% 32% 10% 3	500
	11th to 20th	Day	32% 28% 28% 8% 3 1	500
	21st to end	Day	34% 21% 31% 8% 4 1	550
April	1st to 10th	Day	36% 22% 27% 10% 5%	500
	11th to 20th	Day	34% 22% 30% 9% 4 1	492
	21st to end	Day	29% 22% 29% 13% 5% 1	500
May	1st to 10th	Day	35% 19% 28% 13% 4 1	503
	11th to 20th	Day	23% 19% 36% 15% 5% 2	527
	21st to end	Day	19% 15% 36% 22% 6% 1	573
June	1st to 10th	Day	21% 15% 37% 18% 7% 2	530
	11th to 20th	Day	19% 16% 35% 22% 7% 1	530
	21st to end	Day	20% 14% 36% 20% 7% 3	530
July	1st to 10th	Day	19% 9% 30% 28% 11% 2	525
	11th to 20th	Day	15% 8% 35% 28% 11% 3	524
	21st to end	Day	14% 11% 36% 30% 7% 2	576
August	1st to 10th	Day	13% 8% 34% 28% 13% 4	526
	11th to 20th	Day	17% 9% 29% 27% 13% 4	519
	21st to end	Day	13% 11% 34% 27% 11% 4	583
September	1st to 10th	Day	19% 10% 30% 21% 15% 4	530
	11th to 20th	Day	18% 10% 29% 18% 16% 9%	530
	21st to end	Day	20% 13% 29% 24% 10% 5%	530
October	1st to 10th	Day	28% 14% 23% 20% 11% 4	530
	11th to 20th	Day	24% 16% 30% 22% 6% 2	475
	21st to end	Day	30% 16% 32% 17% 5% 1	462
November	1st to 10th	Day	25% 21% 27% 18% 7% 1	425
	11th to 20th	Day	31% 22% 27% 17% 3 1	430
	21st to end	Day	33% 21% 29% 13% 4	408
December	1st to 10th	Day	30% 30% 25% 12% 4	220
	11th to 20th	Day	41% 23% 28% 6% 1	220
	21st to end	Day	35% 22% 28% 11% 4	242
January	1st to 10th	Night	24% 26% 35% 13% 2	840
	11th to 20th	Night	31% 21% 32% 14% 2	840
	21st to end	Night	22% 23% 37% 15% 2	924
February	1st to 10th	Night	23% 23% 37% 15% 2	840
	11th to 20th	Night	24% 26% 37% 12% 1	804
	21st to end	Night	27% 25% 33% 13% 2	660
March	1st to 10th	Night	28% 22% 32% 16% 2	800
	11th to 20th	Night	27% 22% 32% 16% 3	800
	21st to end	Night	27% 20% 34% 15% 4	880
April	1st to 10th	Night	29% 20% 32% 17% 2	800
	11th to 20th	Night	26% 22% 32% 16% 4	787
	21st to end	Night	26% 20% 33% 18% 2	600
May	1st to 10th	Night	29% 13% 34% 20% 4	600
	11th to 20th	Night	22% 14% 37% 22% 5%	513
	21st to end	Night	21% 8% 38% 27% 5%	433
June	1st to 10th	Night	19% 13% 36% 25% 7%	400
	11th to 20th	Night	21% 9% 30% 32% 8% 1	400
	21st to end	Night	17% 6% 38% 33% 7%	400
July	1st to 10th	Night	16% 7% 33% 36% 7% 1	390
	11th to 20th	Night	16% 4 29% 39% 11% 1	388
	21st to end	Night	13% 5% 33% 39% 10%	438
August	1st to 10th	Night	15% 5% 32% 36% 12%	501
	11th to 20th	Night	15% 8% 29% 34% 13% 1	635
	21st to end	Night	16% 5% 28% 35% 15% 1	880
September	1st to 10th	Night	16% 8% 31% 33% 11% 1	800
	11th to 20th	Night	16% 6% 29% 29% 17% 2	800
	21st to end	Night	19% 10% 28% 29% 12% 2	800
October	1st to 10th	Night	26% 10% 27% 25% 11% 2	800
	11th to 20th	Night	21% 15% 32% 23% 8% 1	800
	21st to end	Night	27% 17% 30% 21% 5%	880
November	1st to 10th	Night	20% 19% 33% 20% 7% 1	820
	11th to 20th	Night	24% 22% 31% 17% 6% 1	840
	21st to end	Night	26% 20% 33% 17% 4	840
December	1st to 10th	Night	26% 22% 36% 13% 3	840
	11th to 20th	Night	29% 22% 32% 16% 3	840
	21st to end	Night	29% 21% 32% 16% 2	924

Page generated at 2026-02-13 01:42:16. Version ac07

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