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 2025-11-18.

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	7% 4 1 4 3 2 3 2 4 5% 66%	135469
	Night	9% 4 1 4 3 3 3 2 4 5% 63%	132321
Transparency	Day	22% 3 7% 19% 25% 23%	130406
	Night	25% 4 9% 20% 23% 20%	128367
Seeing	Day	8% 4 14% 27% 31% 17%	16597
	Night	10% 2 13% 40% 32% 4	25907

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	19% 7% 2 7% 5% 5% 4 4 6% 7% 35%	3639
	11th to 20th	Day	14% 7% 3 7% 4 4 3 2 3 5% 48%	4074
	21st to end	Day	17% 7% 3 7% 5% 4 4 3 4 6% 40%	4510
February	1st to 10th	Day	17% 7% 4 6% 4 4 3 3 4 5% 44%	1800
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 4 8% 44%	1885
	21st to end	Day	14% 5% 3 5% 4 3 3 3 5% 7% 48%	1650
March	1st to 10th	Day	12% 8% 3 8% 5% 5% 5% 5% 5% 7% 36%	4691
	11th to 20th	Day	14% 5% 2 5% 4 3 4 3 5% 6% 48%	5000
	21st to end	Day	12% 6% 2 6% 4 4 5% 4 7% 8% 41%	5678
April	1st to 10th	Day	12% 7% 2 5% 3 4 3 3 5% 7% 49%	2400
	11th to 20th	Day	9% 5% 1 4 3 3 3 3 4 6% 58%	2369
	21st to end	Day	6% 4 1 2 2 2 3 3 5% 8% 63%	2400
May	1st to 10th	Day	4 2 1 2 1 2 2 1 2 4 79%	5518
	11th to 20th	Day	2 1 1 2 1 1 2 2 3 5% 81%	6233
	21st to end	Day	2 1 1 2 1 1 2 1 3 5% 82%	6902
June	1st to 10th	Day	4 3 1 2 2 2 3 3 4 6% 71%	2800
	11th to 20th	Day	1 1 1 1 2 2 2 3 4 83%	2800
	21st to end	Day	2 1 1 1 1 1 1 2 4 84%	2800
July	1st to 10th	Day	1 1 1 1 3 93%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 88%	6243
	21st to end	Day	2 1 2 2 1 2 2 3 4 81%	6402
August	1st to 10th	Day	1 1 1 1 1 1 1 3 4 85%	5777
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 88%	5432
	21st to end	Day	1 1 1 1 1 2 1 2 4 84%	5984
September	1st to 10th	Day	1 1 1 1 1 1 1 1 2 4 87%	2400
	11th to 20th	Day	2 2 1 2 2 2 2 2 4 5% 76%	2400
	21st to end	Day	3 2 1 2 1 2 2 2 3 4 79%	2155
October	1st to 10th	Day	2 1 1 3 2 2 2 2 4 6% 76%	3280
	11th to 20th	Day	4 2 1 4 2 2 2 2 3 5% 71%	3261
	21st to end	Day	4 4 1 4 3 3 3 3 4 6% 64%	3558
November	1st to 10th	Day	8% 4 1 4 3 4 3 3 5% 7% 57%	1919
	11th to 20th	Day	12% 6% 2 5% 4 4 4 4 6% 7% 48%	1863
	21st to end	Day	12% 7% 1 5% 3 2 4 3 5% 8% 50%	1675
December	1st to 10th	Day	8% 4 3 5% 4 4 4 3 6% 10% 50%	2620
	11th to 20th	Day	24% 10% 4 10% 6% 4 5% 3 5% 7% 22%	3028
	21st to end	Day	22% 9% 3 9% 7% 5% 6% 4 7% 7% 21%	4004
January	1st to 10th	Night	10% 5% 2 7% 4 4 4 4 7% 9% 44%	6306
	11th to 20th	Night	15% 5% 2 5% 3 4 4 3 4 4 51%	5902
	21st to end	Night	15% 7% 2 6% 4 4 4 3 5% 6% 45%	6468
February	1st to 10th	Night	18% 6% 2 5% 3 3 3 2 4 5% 50%	2600
	11th to 20th	Night	13% 4 2 5% 4 3 4 3 6% 7% 49%	2402
	21st to end	Night	14% 5% 2 4 2 3 4 3 4 7% 53%	1980
March	1st to 10th	Night	14% 8% 3 6% 4 4 4 3 5% 7% 41%	5200
	11th to 20th	Night	16% 5% 2 4 3 3 3 2 4 5% 51%	5000
	21st to end	Night	16% 7% 2 5% 4 4 3 3 5% 8% 45%	5284
April	1st to 10th	Night	10% 5% 2 5% 3 4 3 3 4 6% 55%	2000
	11th to 20th	Night	8% 4 1 4 3 3 3 2 6% 6% 60%	1979
	21st to end	Night	6% 3 1 3 2 2 3 3 4 7% 66%	2000
May	1st to 10th	Night	2 2 1 2 2 2 2 2 3 5% 79%	4475
	11th to 20th	Night	2 2 1 2 1 2 2 2 3 5% 80%	3647
	21st to end	Night	1 1 1 1 1 2 1 2 4 85%	3972
June	1st to 10th	Night	4 3 1 3 3 2 2 3 5% 5% 71%	1600
	11th to 20th	Night	1 1 1 1 2 1 1 3 4 86%	1600
	21st to end	Night	1 2 1 1 1 1 1 1 2 3 85%	1600
July	1st to 10th	Night	1 1 1 1 1 1 1 93%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 1 2 88%	3703
	21st to end	Night	1 1 2 1 2 2 2 3 5% 81%	4472
August	1st to 10th	Night	1 1 1 2 1 1 1 2 2 4 84%	4035
	11th to 20th	Night	1 1 1 1 1 2 3 91%	4432
	21st to end	Night	2 1 1 1 1 1 2 1 2 3 84%	5015
September	1st to 10th	Night	1 1 1 1 1 1 2 4 88%	2000
	11th to 20th	Night	3 2 1 2 2 2 2 2 2 4 78%	2000
	21st to end	Night	2 1 1 1 2 2 2 2 3 5% 82%	2240
October	1st to 10th	Night	3 2 1 2 2 2 2 2 3 4 78%	3920
	11th to 20th	Night	3 2 1 3 1 2 2 2 3 5% 76%	3897
	21st to end	Night	5% 3 1 3 2 3 4 3 4 5% 68%	4253
November	1st to 10th	Night	9% 4 1 3 2 3 3 3 6% 8% 59%	2598
	11th to 20th	Night	9% 4 2 5% 3 2 3 4 5% 7% 56%	2691
	21st to end	Night	12% 4 1 4 3 3 3 2 4 6% 58%	2722
December	1st to 10th	Night	9% 3 1 3 3 3 4 3 5% 6% 60%	4560
	11th to 20th	Night	21% 8% 3 7% 4 5% 5% 5% 7% 8% 27%	5176
	21st to end	Night	22% 7% 3 8% 6% 6% 6% 5% 6% 6% 25%	6952

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	47% 7% 13% 20% 9% 3	3639
	11th to 20th	Day	41% 4 7% 18% 18% 12%	4074
	21st to end	Day	45% 5% 8% 14% 25% 4	4510
February	1st to 10th	Day	44% 4 8% 22% 16% 7%	1800
	11th to 20th	Day	39% 5% 12% 21% 17% 6%	1894
	21st to end	Day	36% 4 10% 24% 20% 6%	1650
March	1st to 10th	Day	44% 7% 9% 21% 17% 2	4691
	11th to 20th	Day	35% 5% 13% 27% 12% 8%	5000
	21st to end	Day	38% 7% 14% 27% 12% 2	5678
April	1st to 10th	Day	34% 5% 11% 25% 18% 8%	2400
	11th to 20th	Day	28% 5% 11% 26% 22% 7%	2372
	21st to end	Day	20% 5% 13% 29% 27% 6%	2400
May	1st to 10th	Day	13% 2 6% 28% 31% 21%	5518
	11th to 20th	Day	9% 2 6% 30% 40% 13%	6236
	21st to end	Day	8% 2 8% 29% 41% 12%	6904
June	1st to 10th	Day	16% 3 10% 28% 29% 14%	2800
	11th to 20th	Day	7% 2 7% 21% 36% 27%	2800
	21st to end	Day	8% 1 4 11% 36% 40%	2800
July	1st to 10th	Day	2 1 7% 19% 70%	6251
	11th to 20th	Day	5% 1 2 5% 14% 73%	6168
	21st to end	Day	8% 2 3 14% 29% 45%	6217
August	1st to 10th	Day	9% 1 2 7% 26% 54%	4206
	11th to 20th	Day	4 1 2 9% 40% 45%	3924
	21st to end	Day	6% 1 3 10% 38% 42%	4311
September	1st to 10th	Day	6% 1 2 11% 30% 50%	2400
	11th to 20th	Day	12% 2 6% 21% 38% 20%	2400
	21st to end	Day	12% 2 4 19% 35% 28%	2155
October	1st to 10th	Day	11% 2 5% 12% 27% 43%	3280
	11th to 20th	Day	16% 3 6% 23% 34% 18%	3261
	21st to end	Day	22% 3 8% 19% 35% 14%	3558
November	1st to 10th	Day	27% 4 11% 26% 24% 9%	1919
	11th to 20th	Day	36% 4 13% 23% 20% 4	1863
	21st to end	Day	34% 5% 11% 23% 21% 8%	1675
December	1st to 10th	Day	30% 5% 12% 30% 17% 5%	2620
	11th to 20th	Day	60% 6% 8% 14% 9% 2	3028
	21st to end	Day	57% 8% 14% 13% 7% 1	4004
January	1st to 10th	Night	35% 7% 18% 26% 9% 4	6306
	11th to 20th	Night	37% 5% 10% 18% 22% 9%	5902
	21st to end	Night	40% 4 10% 14% 23% 9%	6468
February	1st to 10th	Night	38% 4 8% 22% 21% 8%	2600
	11th to 20th	Night	34% 4 10% 23% 22% 6%	2403
	21st to end	Night	33% 4 9% 27% 22% 6%	1980
March	1st to 10th	Night	41% 6% 11% 19% 19% 4	5200
	11th to 20th	Night	35% 5% 14% 23% 14% 9%	5000
	21st to end	Night	38% 6% 15% 24% 14% 3	5284
April	1st to 10th	Night	30% 5% 12% 24% 23% 7%	2000
	11th to 20th	Night	25% 4 12% 28% 20% 11%	1979
	21st to end	Night	19% 4 13% 30% 28% 6%	2000
May	1st to 10th	Night	11% 3 10% 27% 30% 20%	4472
	11th to 20th	Night	10% 2 7% 32% 39% 11%	3647
	21st to end	Night	7% 2 8% 33% 39% 11%	3972
June	1st to 10th	Night	16% 3 11% 27% 30% 12%	1600
	11th to 20th	Night	6% 2 8% 20% 37% 28%	1600
	21st to end	Night	8% 1 3 13% 33% 43%	1600
July	1st to 10th	Night	3 1 7% 22% 66%	3600
	11th to 20th	Night	6% 1 2 5% 13% 74%	3655
	21st to end	Night	7% 2 3 16% 27% 45%	4345
August	1st to 10th	Night	11% 1 2 8% 28% 49%	2929
	11th to 20th	Night	4 1 2 9% 38% 47%	3208
	21st to end	Night	7% 1 2 9% 37% 45%	3608
September	1st to 10th	Night	4 2 3 10% 31% 51%	2000
	11th to 20th	Night	12% 2 5% 21% 37% 23%	2000
	21st to end	Night	9% 2 5% 18% 34% 32%	2240
October	1st to 10th	Night	12% 2 4 11% 21% 49%	3920
	11th to 20th	Night	12% 3 6% 27% 30% 22%	3897
	21st to end	Night	20% 3 5% 20% 31% 20%	4253
November	1st to 10th	Night	24% 4 11% 25% 23% 14%	2598
	11th to 20th	Night	27% 4 11% 26% 24% 8%	2691
	21st to end	Night	29% 3 8% 23% 26% 10%	2722
December	1st to 10th	Night	26% 4 8% 29% 23% 9%	4560
	11th to 20th	Night	50% 8% 10% 18% 11% 3	5176
	21st to end	Night	55% 8% 12% 15% 7% 2	6952

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% 7% 12% 23% 20% 8%	200
	11th to 20th	Day	18% 6% 13% 27% 22% 14%	387
	21st to end	Day	16% 4 15% 25% 25% 16%	440
February	1st to 10th	Day	20% 8% 15% 19% 23% 16%	400
	11th to 20th	Day	14% 7% 23% 22% 21% 13%	462
	21st to end	Day	14% 8% 18% 26% 26% 8%	414
March	1st to 10th	Day	18% 9% 17% 27% 22% 6%	500
	11th to 20th	Day	21% 7% 18% 22% 20% 13%	500
	21st to end	Day	15% 8% 20% 26% 21% 10%	550
April	1st to 10th	Day	14% 7% 16% 30% 19% 13%	500
	11th to 20th	Day	11% 6% 19% 28% 22% 14%	492
	21st to end	Day	6% 5% 15% 31% 27% 17%	500
May	1st to 10th	Day	4 6% 23% 28% 24% 15%	503
	11th to 20th	Day	5% 4 20% 27% 26% 18%	528
	21st to end	Day	4 2 15% 28% 30% 21%	573
June	1st to 10th	Day	4 2 16% 27% 33% 17%	530
	11th to 20th	Day	1 4 16% 30% 38% 11%	530
	21st to end	Day	1 1 12% 30% 38% 19%	530
July	1st to 10th	Day	1 1 8% 31% 42% 17%	526
	11th to 20th	Day	1 6% 30% 43% 19%	524
	21st to end	Day	1 1 7% 29% 43% 20%	575
August	1st to 10th	Day	2 11% 33% 36% 18%	527
	11th to 20th	Day	1 1 10% 30% 38% 20%	517
	21st to end	Day	1 1 7% 28% 41% 22%	583
September	1st to 10th	Day	2 9% 28% 42% 19%	530
	11th to 20th	Day	2 3 12% 27% 35% 20%	530
	21st to end	Day	2 3 9% 28% 35% 23%	530
October	1st to 10th	Day	3 3 15% 26% 29% 25%	530
	11th to 20th	Day	5% 5% 11% 19% 31% 29%	530
	21st to end	Day	8% 4 12% 22% 32% 21%	583
November	1st to 10th	Day	8% 5% 12% 19% 31% 24%	395
	11th to 20th	Day	13% 3 10% 27% 31% 15%	321
	21st to end	Day	15% 3 8% 27% 34% 14%	237
December	1st to 10th	Day	17% 4 11% 27% 24% 18%	200
	11th to 20th	Day	18% 5% 13% 23% 29% 13%	200
	21st to end	Day	20% 7% 18% 25% 23% 7%	220
January	1st to 10th	Night	20% 3 14% 35% 23% 5%	800
	11th to 20th	Night	15% 4 11% 40% 26% 4	800
	21st to end	Night	18% 3 10% 38% 26% 5%	880
February	1st to 10th	Night	19% 5% 13% 36% 24% 3	800
	11th to 20th	Night	15% 4 15% 38% 24% 4	784
	21st to end	Night	15% 3 16% 44% 19% 3	660
March	1st to 10th	Night	18% 4 17% 35% 24% 2	800
	11th to 20th	Night	18% 5% 14% 37% 24% 3	800
	21st to end	Night	15% 4 16% 40% 22% 2	880
April	1st to 10th	Night	12% 3 20% 40% 24% 2	800
	11th to 20th	Night	7% 5% 20% 41% 23% 4	791
	21st to end	Night	6% 2 21% 38% 30% 3	800
May	1st to 10th	Night	4 2 22% 41% 27% 4	767
	11th to 20th	Night	5% 1 13% 45% 35% 2	412
	21st to end	Night	4 2 16% 42% 34% 2	433
June	1st to 10th	Night	4 1 15% 45% 34% 1	400
	11th to 20th	Night	1 1 15% 50% 32% 1	400
	21st to end	Night	1 1 10% 49% 38% 2	400
July	1st to 10th	Night	1 5% 48% 45% 1	390
	11th to 20th	Night	1 4 46% 48% 1	412
	21st to end	Night	1 3 36% 55% 4	657
August	1st to 10th	Night	3 5% 42% 48% 3	597
	11th to 20th	Night	1 9% 45% 41% 4	756
	21st to end	Night	2 6% 40% 48% 4	880
September	1st to 10th	Night	1 1 8% 43% 46% 3	800
	11th to 20th	Night	4 1 10% 41% 37% 6%	800
	21st to end	Night	2 1 9% 40% 41% 7%	800
October	1st to 10th	Night	5% 2 9% 41% 36% 7%	800
	11th to 20th	Night	6% 3 11% 36% 39% 6%	800
	21st to end	Night	5% 3 10% 37% 35% 10%	880
November	1st to 10th	Night	9% 2 12% 37% 32% 7%	820
	11th to 20th	Night	10% 2 11% 43% 29% 5%	828
	21st to end	Night	13% 3 13% 39% 29% 5%	800
December	1st to 10th	Night	17% 2 13% 39% 23% 7%	800
	11th to 20th	Night	24% 4 11% 35% 24% 3	800
	21st to end	Night	22% 4 16% 35% 21% 2	880

Page generated at 2025-11-14 21:56:00. Version ac07

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