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 2024-01-12.

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	8% 4 2 3 3 2 3 3 4 6% 63%	74728
	Night	9% 4 1 3 3 2 3 3 4 6% 61%	75011
Transparency	Day	24% 4 9% 19% 25% 20%	74443
	Night	26% 4 9% 20% 24% 18%	74816
Seeing	Day	8% 4 14% 27% 31% 17%	14093
	Night	10% 2 12% 39% 32% 4	23536

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	22% 8% 3 6% 5% 3 4 5% 5% 7% 32%	1519
	11th to 20th	Day	15% 6% 3 5% 5% 3 4 4 4 8% 43%	1620
	21st to end	Day	15% 7% 3 5% 5% 3 5% 4 5% 7% 39%	1782
February	1st to 10th	Day	16% 6% 4 5% 5% 3 4 3 4 6% 46%	1620
	11th to 20th	Day	14% 5% 3 4 4 3 4 4 4 8% 48%	1692
	21st to end	Day	15% 5% 3 4 4 2 3 3 5% 7% 49%	1480
March	1st to 10th	Day	16% 7% 3 5% 6% 2 5% 6% 6% 7% 37%	1869
	11th to 20th	Day	18% 6% 2 4 4 2 5% 4 6% 7% 41%	1980
	21st to end	Day	13% 5% 2 5% 4 3 4 5% 5% 7% 46%	2254
April	1st to 10th	Day	12% 6% 2 5% 3 3 3 4 5% 7% 48%	2160
	11th to 20th	Day	10% 4 2 4 3 2 3 3 4 6% 58%	2129
	21st to end	Day	7% 4 1 2 3 1 2 4 4 8% 64%	2160
May	1st to 10th	Day	4 3 2 3 3 2 3 3 4 6% 68%	2186
	11th to 20th	Day	4 3 2 3 3 2 3 3 4 7% 68%	2459
	21st to end	Day	4 3 1 3 2 2 3 3 4 7% 66%	2723
June	1st to 10th	Day	4 3 1 2 3 1 3 3 4 7% 69%	2520
	11th to 20th	Day	1 1 1 2 1 2 3 3 4 82%	2520
	21st to end	Day	2 1 1 1 1 2 2 2 4 83%	2520
July	1st to 10th	Day	1 1 1 1 1 1 2 2 4 87%	2520
	11th to 20th	Day	1 1 1 1 2 1 2 4 86%	2495
	21st to end	Day	1 1 1 1 1 1 2 3 3 5% 83%	2546
August	1st to 10th	Day	1 2 1 1 2 1 2 2 2 4 83%	2328
	11th to 20th	Day	1 1 1 1 1 1 2 3 5% 85%	2152
	21st to end	Day	1 1 1 1 1 1 2 2 3 87%	2376
September	1st to 10th	Day	1 1 1 1 1 1 1 2 4 87%	2160
	11th to 20th	Day	2 2 1 2 2 1 2 3 3 5% 76%	2160
	21st to end	Day	3 2 1 1 1 1 1 2 3 4 79%	1941
October	1st to 10th	Day	2 2 1 2 3 2 3 3 4 5% 73%	1800
	11th to 20th	Day	6% 2 1 3 3 2 2 3 5% 5% 70%	1800
	21st to end	Day	7% 3 1 3 3 2 2 3 4 6% 65%	1980
November	1st to 10th	Day	8% 4 2 3 4 3 4 4 5% 7% 55%	1733
	11th to 20th	Day	11% 5% 2 4 5% 3 3 4 5% 7% 50%	1710
	21st to end	Day	11% 6% 2 5% 3 2 3 4 5% 8% 51%	1592
December	1st to 10th	Day	14% 7% 4 6% 5% 3 4 3 4 8% 43%	1520
	11th to 20th	Day	26% 10% 4 8% 5% 2 4 3 4 6% 27%	1928
	21st to end	Day	14% 7% 3 7% 9% 4 7% 6% 7% 9% 27%	2794
January	1st to 10th	Night	18% 6% 3 5% 5% 2 4 4 6% 8% 41%	2646
	11th to 20th	Night	12% 6% 2 5% 5% 3 5% 5% 5% 6% 47%	2376
	21st to end	Night	16% 5% 2 5% 4 2 4 4 5% 8% 46%	2574
February	1st to 10th	Night	16% 5% 2 4 3 2 3 3 4 5% 53%	2340
	11th to 20th	Night	13% 3 2 4 5% 2 4 4 6% 8% 51%	2155
	21st to end	Night	15% 4 2 4 3 2 3 3 4 6% 53%	1776
March	1st to 10th	Night	16% 8% 3 5% 5% 3 3 4 5% 6% 44%	2088
	11th to 20th	Night	17% 4 2 4 4 2 4 4 5% 7% 47%	1980
	21st to end	Night	14% 5% 1 4 3 2 4 4 4 7% 50%	2102
April	1st to 10th	Night	11% 5% 2 4 4 3 4 4 4 6% 53%	1800
	11th to 20th	Night	9% 3 1 4 4 2 3 3 6% 6% 61%	1779
	21st to end	Night	6% 2 2 2 2 2 3 4 4 7% 66%	1800
May	1st to 10th	Night	5% 2 1 2 3 2 3 3 4 6% 69%	1771
	11th to 20th	Night	4 2 1 2 2 2 3 3 4 7% 70%	1433
	21st to end	Night	4 2 1 2 2 2 3 3 4 6% 72%	1552
June	1st to 10th	Night	4 3 1 3 3 2 2 3 4 5% 71%	1440
	11th to 20th	Night	1 1 1 1 1 1 2 3 4 84%	1440
	21st to end	Night	1 2 1 1 2 1 1 2 2 4 84%	1440
July	1st to 10th	Night	1 1 1 1 1 2 2 2 89%	1440
	11th to 20th	Night	1 1 1 1 1 1 1 1 3 87%	1461
	21st to end	Night	1 1 2 2 2 2 2 4 5% 80%	1773
August	1st to 10th	Night	2 2 1 2 2 1 2 2 2 4 80%	1611
	11th to 20th	Night	1 1 1 1 1 2 3 3 5% 81%	1747
	21st to end	Night	2 1 1 1 1 1 2 2 3 87%	1980
September	1st to 10th	Night	1 1 1 1 1 1 1 2 4 88%	1800
	11th to 20th	Night	3 2 1 1 2 1 2 2 2 4 80%	1800
	21st to end	Night	1 1 1 1 2 2 2 2 3 5% 81%	2014
October	1st to 10th	Night	5% 2 1 2 3 1 3 2 4 4 74%	2160
	11th to 20th	Night	5% 2 1 2 2 2 2 3 4 6% 71%	2160
	21st to end	Night	5% 3 1 2 2 1 3 3 3 5% 71%	2376
November	1st to 10th	Night	9% 4 2 3 2 2 3 3 6% 8% 58%	2344
	11th to 20th	Night	9% 3 2 4 3 2 3 5% 5% 7% 58%	2470
	21st to end	Night	11% 3 1 4 4 2 3 3 4 6% 59%	2585
December	1st to 10th	Night	16% 5% 2 4 4 3 3 4 4 6% 50%	2660
	11th to 20th	Night	22% 7% 3 5% 5% 3 4 5% 6% 7% 34%	3276
	21st to end	Night	18% 6% 3 7% 7% 4 6% 6% 6% 6% 31%	4862

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	52% 6% 13% 16% 11% 2	1519
	11th to 20th	Day	41% 5% 12% 17% 18% 7%	1620
	21st to end	Day	45% 5% 10% 18% 17% 4	1782
February	1st to 10th	Day	42% 3 8% 24% 16% 7%	1620
	11th to 20th	Day	37% 5% 11% 22% 19% 6%	1701
	21st to end	Day	36% 4 10% 23% 20% 6%	1480
March	1st to 10th	Day	44% 6% 12% 20% 14% 4	1869
	11th to 20th	Day	41% 4 10% 24% 17% 4	1980
	21st to end	Day	36% 8% 14% 20% 18% 4	2254
April	1st to 10th	Day	35% 5% 12% 24% 16% 8%	2160
	11th to 20th	Day	28% 5% 11% 27% 23% 8%	2132
	21st to end	Day	20% 4 13% 29% 28% 7%	2160
May	1st to 10th	Day	19% 3 9% 26% 31% 11%	2186
	11th to 20th	Day	18% 4 8% 30% 29% 12%	2462
	21st to end	Day	18% 5% 14% 28% 26% 9%	2724
June	1st to 10th	Day	17% 3 11% 27% 28% 14%	2520
	11th to 20th	Day	7% 2 8% 21% 35% 27%	2520
	21st to end	Day	8% 1 4 12% 37% 38%	2520
July	1st to 10th	Day	4 1 3 10% 26% 55%	2451
	11th to 20th	Day	6% 1 4 12% 26% 51%	2420
	21st to end	Day	7% 1 3 9% 25% 54%	2405
August	1st to 10th	Day	9% 2 3 11% 32% 44%	2312
	11th to 20th	Day	5% 1 2 12% 34% 45%	2152
	21st to end	Day	6% 2 2 12% 34% 45%	2376
September	1st to 10th	Day	6% 1 2 10% 30% 52%	2160
	11th to 20th	Day	11% 2 6% 21% 38% 22%	2160
	21st to end	Day	11% 2 4 19% 35% 28%	1941
October	1st to 10th	Day	14% 3 8% 18% 30% 28%	1800
	11th to 20th	Day	17% 4 8% 21% 26% 24%	1800
	21st to end	Day	22% 3 8% 20% 29% 19%	1980
November	1st to 10th	Day	28% 4 10% 26% 23% 10%	1733
	11th to 20th	Day	34% 4 13% 24% 21% 4	1710
	21st to end	Day	33% 5% 11% 23% 20% 8%	1592
December	1st to 10th	Day	42% 5% 12% 19% 15% 6%	1520
	11th to 20th	Day	60% 5% 7% 13% 12% 4	1928
	21st to end	Day	50% 10% 16% 13% 8% 2	2794
January	1st to 10th	Night	42% 5% 14% 23% 11% 5%	2646
	11th to 20th	Night	37% 6% 12% 19% 14% 11%	2376
	21st to end	Night	38% 5% 13% 17% 20% 7%	2574
February	1st to 10th	Night	35% 4 8% 24% 21% 8%	2340
	11th to 20th	Night	31% 4 10% 23% 24% 7%	2156
	21st to end	Night	33% 3 10% 26% 22% 6%	1776
March	1st to 10th	Night	42% 4 11% 19% 18% 6%	2088
	11th to 20th	Night	38% 4 9% 22% 21% 7%	1980
	21st to end	Night	34% 6% 11% 22% 20% 8%	2102
April	1st to 10th	Night	32% 5% 12% 22% 22% 8%	1800
	11th to 20th	Night	25% 4 12% 27% 21% 12%	1779
	21st to end	Night	19% 4 13% 29% 28% 7%	1800
May	1st to 10th	Night	17% 5% 10% 24% 31% 13%	1771
	11th to 20th	Night	15% 3 11% 32% 26% 12%	1433
	21st to end	Night	14% 3 15% 34% 25% 8%	1552
June	1st to 10th	Night	17% 3 12% 26% 29% 13%	1440
	11th to 20th	Night	6% 2 9% 21% 34% 27%	1440
	21st to end	Night	9% 1 3 14% 33% 41%	1440
July	1st to 10th	Night	5% 1 2 10% 29% 53%	1400
	11th to 20th	Night	6% 1 3 10% 25% 54%	1413
	21st to end	Night	10% 1 2 9% 27% 51%	1674
August	1st to 10th	Night	11% 1 2 13% 30% 42%	1602
	11th to 20th	Night	7% 2 3 11% 29% 48%	1747
	21st to end	Night	7% 1 2 10% 33% 47%	1980
September	1st to 10th	Night	4 2 3 9% 32% 51%	1800
	11th to 20th	Night	11% 2 4 20% 38% 24%	1800
	21st to end	Night	9% 2 5% 19% 34% 32%	2014
October	1st to 10th	Night	15% 3 6% 18% 28% 29%	2160
	11th to 20th	Night	15% 3 7% 18% 28% 28%	2160
	21st to end	Night	16% 3 7% 22% 29% 24%	2376
November	1st to 10th	Night	25% 4 10% 24% 22% 15%	2344
	11th to 20th	Night	26% 4 11% 26% 26% 9%	2470
	21st to end	Night	28% 3 8% 23% 26% 10%	2585
December	1st to 10th	Night	36% 5% 10% 21% 18% 9%	2660
	11th to 20th	Night	49% 6% 10% 16% 15% 4	3276
	21st to end	Night	50% 9% 11% 17% 9% 3	4862

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	35% 6% 12% 22% 18% 7%	180
	11th to 20th	Day	18% 6% 13% 27% 22% 14%	332
	21st to end	Day	17% 4 15% 24% 24% 16%	374
February	1st to 10th	Day	16% 8% 13% 19% 24% 19%	340
	11th to 20th	Day	14% 7% 24% 20% 21% 15%	389
	21st to end	Day	15% 10% 19% 24% 24% 8%	346
March	1st to 10th	Day	19% 9% 17% 28% 21% 6%	420
	11th to 20th	Day	22% 5% 16% 23% 21% 13%	420
	21st to end	Day	15% 7% 21% 26% 22% 9%	462
April	1st to 10th	Day	16% 7% 17% 31% 17% 12%	420
	11th to 20th	Day	12% 6% 20% 29% 20% 12%	412
	21st to end	Day	6% 5% 16% 31% 25% 16%	420
May	1st to 10th	Day	4 6% 24% 28% 25% 14%	423
	11th to 20th	Day	6% 3 22% 27% 24% 17%	448
	21st to end	Day	5% 3 16% 26% 29% 21%	485
June	1st to 10th	Day	4 3 18% 27% 31% 17%	450
	11th to 20th	Day	1 4 16% 30% 37% 12%	450
	21st to end	Day	1 1 11% 30% 38% 19%	450
July	1st to 10th	Day	1 1 7% 30% 45% 16%	446
	11th to 20th	Day	1 7% 29% 45% 19%	444
	21st to end	Day	7% 29% 44% 19%	487
August	1st to 10th	Day	2 11% 33% 37% 17%	447
	11th to 20th	Day	1 10% 30% 38% 20%	437
	21st to end	Day	1 1 7% 28% 44% 19%	495
September	1st to 10th	Day	1 8% 27% 44% 19%	450
	11th to 20th	Day	2 3 12% 29% 34% 20%	450
	21st to end	Day	3 3 9% 27% 36% 22%	450
October	1st to 10th	Day	3 3 16% 26% 29% 23%	450
	11th to 20th	Day	5% 4 10% 19% 32% 29%	450
	21st to end	Day	8% 5% 12% 21% 32% 22%	495
November	1st to 10th	Day	9% 5% 12% 19% 32% 22%	329
	11th to 20th	Day	11% 3 9% 26% 34% 17%	270
	21st to end	Day	14% 2 7% 27% 35% 15%	214
December	1st to 10th	Day	19% 4 11% 28% 21% 16%	180
	11th to 20th	Day	16% 6% 14% 23% 28% 14%	180
	21st to end	Day	19% 6% 19% 24% 25% 8%	198
January	1st to 10th	Night	21% 3 14% 35% 23% 6%	760
	11th to 20th	Night	15% 3 11% 41% 25% 5%	728
	21st to end	Night	18% 3 10% 38% 26% 5%	792
February	1st to 10th	Night	17% 4 13% 36% 26% 3	720
	11th to 20th	Night	14% 4 15% 37% 26% 4	704
	21st to end	Night	16% 4 17% 42% 19% 3	592
March	1st to 10th	Night	19% 4 16% 35% 23% 2	720
	11th to 20th	Night	18% 4 13% 37% 25% 3	720
	21st to end	Night	15% 4 16% 41% 22% 2	792
April	1st to 10th	Night	13% 3 19% 40% 24% 2	720
	11th to 20th	Night	7% 5% 21% 40% 23% 4	711
	21st to end	Night	6% 2 21% 37% 30% 3	720
May	1st to 10th	Night	5% 2 21% 41% 28% 4	691
	11th to 20th	Night	5% 1 12% 44% 35% 2	371
	21st to end	Night	5% 2 17% 40% 35% 2	389
June	1st to 10th	Night	5% 1 15% 43% 34% 1	360
	11th to 20th	Night	1 1 14% 50% 32% 1	360
	21st to end	Night	1 1 10% 49% 37% 2	360
July	1st to 10th	Night	1 4 48% 46% 1	350
	11th to 20th	Night	1 4 46% 47% 1	369
	21st to end	Night	1 3 36% 56% 4	591
August	1st to 10th	Night	3 4 42% 48% 3	537
	11th to 20th	Night	1 8% 44% 42% 4	679
	21st to end	Night	2 5% 40% 49% 4	792
September	1st to 10th	Night	1 1 7% 42% 47% 3	720
	11th to 20th	Night	4 1 10% 40% 38% 6%	720
	21st to end	Night	2 1 9% 39% 41% 7%	720
October	1st to 10th	Night	6% 2 10% 41% 35% 7%	720
	11th to 20th	Night	5% 2 11% 35% 40% 6%	720
	21st to end	Night	5% 3 10% 35% 36% 11%	792
November	1st to 10th	Night	10% 2 11% 38% 32% 7%	740
	11th to 20th	Night	10% 1 9% 43% 31% 6%	760
	21st to end	Night	12% 3 12% 39% 29% 5%	760
December	1st to 10th	Night	18% 2 13% 39% 23% 6%	760
	11th to 20th	Night	24% 4 11% 35% 24% 3	760
	21st to end	Night	22% 4 16% 34% 22% 2	836

Page generated at 2024-01-09 13:24:22. Version ac07

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