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 2024-11-30.

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% 5% 4 4 4 6% 7% 38%	101493
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 40%	98093
Transparency	Day	24% 47% 7% 12% 9% 1	105031
	Night	24% 47% 7% 12% 9% 1	102644
Seeing	Day	25% 17% 31% 18% 7% 2	15043
	Night	23% 17% 33% 21% 6% 1	23969

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	13% 7% 2 7% 5% 4 4 3 6% 6% 43%	2517
	11th to 20th	Day	16% 7% 4 9% 5% 4 4 4 5% 5% 38%	2520
	21st to end	Day	18% 13% 3 7% 4 4 4 3 5% 6% 33%	2772
February	1st to 10th	Day	16% 9% 2 7% 5% 4 5% 5% 5% 6% 35%	1739
	11th to 20th	Day	13% 7% 3 8% 5% 4 4 4 6% 8% 38%	1848
	21st to end	Day	17% 10% 3 6% 5% 5% 4 4 5% 4 36%	1570
March	1st to 10th	Day	22% 14% 3 7% 4 4 5% 3 4 5% 30%	3116
	11th to 20th	Day	13% 8% 2 5% 4 4 5% 4 6% 8% 41%	3306
	21st to end	Day	20% 10% 2 7% 4 3 4 3 5% 7% 35%	4070
April	1st to 10th	Day	17% 10% 2 6% 5% 4 5% 4 4 5% 37%	2280
	11th to 20th	Day	17% 6% 2 7% 4 4 4 4 6% 7% 38%	2257
	21st to end	Day	21% 7% 2 7% 5% 4 4 4 5% 6% 34%	2470
May	1st to 10th	Day	22% 11% 3 8% 5% 4 4 4 5% 6% 28%	4106
	11th to 20th	Day	23% 7% 3 7% 5% 5% 5% 3 5% 6% 32%	4237
	21st to end	Day	14% 5% 2 5% 4 4 4 3 6% 9% 44%	4837
June	1st to 10th	Day	18% 6% 2 7% 5% 5% 5% 4 6% 8% 34%	2660
	11th to 20th	Day	17% 5% 2 6% 5% 4 5% 5% 6% 9% 37%	2660
	21st to end	Day	18% 7% 2 6% 6% 4 5% 4 6% 8% 34%	2660
July	1st to 10th	Day	13% 6% 2 7% 5% 5% 6% 5% 6% 10% 37%	4440
	11th to 20th	Day	11% 5% 2 6% 5% 5% 6% 5% 7% 11% 38%	4440
	21st to end	Day	17% 8% 3 7% 5% 4 5% 4 7% 7% 35%	4814
August	1st to 10th	Day	15% 6% 2 7% 6% 5% 5% 4 7% 8% 35%	4215
	11th to 20th	Day	14% 7% 3 7% 5% 5% 5% 4 6% 7% 36%	3962
	21st to end	Day	9% 4 2 6% 4 5% 6% 4 8% 10% 42%	4180
September	1st to 10th	Day	13% 5% 1 6% 4 4 4 3 6% 7% 46%	2280
	11th to 20th	Day	15% 6% 1 5% 4 3 3 3 4 5% 51%	2104
	21st to end	Day	17% 5% 3 5% 3 4 3 3 5% 7% 44%	2090
October	1st to 10th	Day	23% 6% 2 6% 5% 4 4 3 5% 5% 38%	2090
	11th to 20th	Day	14% 6% 1 5% 4 4 4 3 5% 8% 45%	1836
	21st to end	Day	21% 7% 2 6% 4 3 5% 3 4 6% 39%	1881
November	1st to 10th	Day	14% 5% 3 6% 4 4 3 3 5% 5% 49%	1761
	11th to 20th	Day	18% 5% 2 5% 3 3 3 4 5% 5% 48%	1800
	21st to end	Day	18% 7% 2 5% 4 5% 4 3 5% 5% 42%	1777
December	1st to 10th	Day	17% 8% 3 7% 5% 4 4 4 5% 6% 36%	1520
	11th to 20th	Day	22% 6% 2 4 4 4 3 4 5% 5% 42%	1906
	21st to end	Day	21% 12% 3 7% 4 3 3 3 5% 6% 33%	2772
January	1st to 10th	Night	16% 8% 2 5% 3 3 4 3 5% 5% 46%	4429
	11th to 20th	Night	13% 8% 3 9% 5% 4 5% 3 6% 6% 38%	4440
	21st to end	Night	20% 13% 5% 8% 5% 4 4 3 4 5% 31%	4884
February	1st to 10th	Night	15% 10% 3 6% 4 4 4 5% 6% 7% 38%	2437
	11th to 20th	Night	12% 8% 2 6% 5% 5% 5% 4 6% 6% 41%	2218
	21st to end	Night	15% 9% 3 6% 5% 3 3 4 5% 6% 41%	1884
March	1st to 10th	Night	25% 11% 2 5% 4 4 4 3 4 5% 33%	3724
	11th to 20th	Night	11% 9% 3 7% 4 5% 6% 5% 5% 6% 39%	3651
	21st to end	Night	23% 8% 2 5% 3 3 4 3 6% 6% 36%	3544
April	1st to 10th	Night	18% 8% 3 6% 5% 5% 4 3 5% 6% 37%	1900
	11th to 20th	Night	20% 7% 2 5% 4 4 4 4 5% 6% 40%	1871
	21st to end	Night	21% 7% 3 7% 3 4 4 3 4 5% 40%	1710
May	1st to 10th	Night	25% 12% 3 7% 5% 4 4 3 6% 8% 26%	2834
	11th to 20th	Night	19% 8% 3 9% 5% 5% 5% 4 6% 6% 28%	2673
	21st to end	Night	16% 6% 2 6% 4 3 4 3 6% 7% 43%	2738
June	1st to 10th	Night	18% 7% 3 6% 5% 3 4 3 4 7% 38%	1520
	11th to 20th	Night	20% 6% 1 6% 4 4 4 4 6% 7% 38%	1520
	21st to end	Night	17% 6% 3 5% 4 4 4 4 6% 7% 40%	1520
July	1st to 10th	Night	11% 5% 2 6% 6% 5% 5% 5% 7% 8% 40%	2520
	11th to 20th	Night	12% 6% 2 7% 4 3 5% 4 5% 9% 42%	2520
	21st to end	Night	17% 7% 3 5% 5% 4 4 3 5% 6% 42%	2739
August	1st to 10th	Night	20% 6% 2 7% 5% 5% 5% 4 5% 6% 36%	2682
	11th to 20th	Night	13% 8% 3 7% 5% 5% 5% 3 6% 6% 38%	2862
	21st to end	Night	9% 4 1 4 3 3 3 3 5% 8% 58%	3476
September	1st to 10th	Night	14% 4 2 5% 3 3 3 3 5% 6% 50%	1900
	11th to 20th	Night	14% 5% 1 4 3 2 2 3 5% 5% 53%	2074
	21st to end	Night	15% 5% 2 6% 2 4 4 3 5% 6% 47%	2090
October	1st to 10th	Night	21% 6% 2 5% 4 3 4 3 5% 6% 41%	2090
	11th to 20th	Night	16% 5% 2 4 4 4 5% 4 5% 7% 46%	2340
	21st to end	Night	22% 7% 2 5% 3 4 4 3 5% 7% 38%	2717
November	1st to 10th	Night	15% 5% 2 5% 4 4 4 3 4 6% 47%	2537
	11th to 20th	Night	17% 6% 2 5% 4 4 4 3 4 6% 46%	2600
	21st to end	Night	16% 7% 2 4 4 4 4 3 5% 7% 44%	2617
December	1st to 10th	Night	17% 9% 3 6% 4 4 4 3 6% 6% 37%	2660
	11th to 20th	Night	19% 7% 3 5% 4 4 3 4 5% 6% 41%	3288
	21st to end	Night	21% 10% 4 7% 4 4 4 3 5% 6% 33%	4884

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	25% 38% 7% 23% 6% 1	2518
	11th to 20th	Day	28% 39% 6% 19% 7% 1	2520
	21st to end	Day	37% 42% 4 11% 5% 1	2772
February	1st to 10th	Day	16% 65% 6% 8% 4 1	1739
	11th to 20th	Day	17% 64% 3 6% 9% 1	1857
	21st to end	Day	15% 65% 4 8% 6% 1	1570
March	1st to 10th	Day	40% 42% 4 6% 7% 1	3119
	11th to 20th	Day	19% 45% 11% 13% 11% 2	3306
	21st to end	Day	32% 41% 4 9% 10% 5%	4070
April	1st to 10th	Day	20% 65% 2 7% 5% 2	2280
	11th to 20th	Day	14% 66% 4 8% 7% 1	2257
	21st to end	Day	19% 65% 3 6% 7% 1	2470
May	1st to 10th	Day	36% 44% 8% 8% 4	4120
	11th to 20th	Day	34% 43% 10% 9% 3	4241
	21st to end	Day	21% 43% 8% 16% 12% 1	4837
June	1st to 10th	Day	14% 66% 5% 9% 6%	2660
	11th to 20th	Day	14% 65% 4 9% 7% 1	2660
	21st to end	Day	16% 65% 4 9% 6%	2660
July	1st to 10th	Day	26% 38% 9% 14% 12% 1	4370
	11th to 20th	Day	24% 38% 12% 17% 8% 1	4365
	21st to end	Day	33% 36% 7% 13% 10% 1	4661
August	1st to 10th	Day	30% 39% 10% 15% 6%	4201
	11th to 20th	Day	28% 40% 10% 17% 5%	4001
	21st to end	Day	20% 39% 9% 20% 11% 1	4180
September	1st to 10th	Day	14% 61% 6% 11% 8% 1	2280
	11th to 20th	Day	16% 60% 3 9% 11% 1	2104
	21st to end	Day	17% 60% 4 10% 8% 1	2090
October	1st to 10th	Day	27% 39% 6% 13% 13% 2	3480
	11th to 20th	Day	18% 38% 4 18% 20% 3	3026
	21st to end	Day	18% 38% 4 23% 14% 2	3081
November	1st to 10th	Day	18% 56% 7% 11% 8% 1	1761
	11th to 20th	Day	17% 56% 4 7% 14% 2	1800
	21st to end	Day	17% 57% 6% 10% 8% 1	1777
December	1st to 10th	Day	19% 61% 6% 6% 8%	1520
	11th to 20th	Day	24% 48% 5% 10% 10% 3	1906
	21st to end	Day	33% 40% 6% 12% 8% 1	2772
January	1st to 10th	Night	24% 38% 8% 20% 7% 3	4429
	11th to 20th	Night	26% 39% 8% 15% 9% 2	4440
	21st to end	Night	42% 40% 5% 9% 4 1	4884
February	1st to 10th	Night	16% 65% 5% 7% 5% 1	2437
	11th to 20th	Night	15% 64% 4 6% 8% 2	2220
	21st to end	Night	15% 65% 3 8% 6% 3	1884
March	1st to 10th	Night	36% 42% 5% 9% 5% 2	3734
	11th to 20th	Night	24% 44% 8% 12% 10% 1	3652
	21st to end	Night	32% 41% 4 7% 12% 5%	3544
April	1st to 10th	Night	19% 65% 3 5% 6% 1	1900
	11th to 20th	Night	16% 66% 4 6% 7% 1	1871
	21st to end	Night	17% 65% 3 6% 7% 1	1710
May	1st to 10th	Night	36% 44% 8% 9% 3	2840
	11th to 20th	Night	36% 45% 9% 7% 3	2674
	21st to end	Night	24% 43% 10% 13% 9% 1	2739
June	1st to 10th	Night	15% 65% 3 10% 7%	1520
	11th to 20th	Night	14% 66% 5% 9% 6%	1520
	21st to end	Night	14% 64% 4 11% 6% 1	1520
July	1st to 10th	Night	24% 39% 10% 15% 11% 1	2480
	11th to 20th	Night	26% 39% 9% 14% 11%	2472
	21st to end	Night	31% 37% 7% 13% 12%	2651
August	1st to 10th	Night	35% 39% 8% 12% 6%	2672
	11th to 20th	Night	29% 40% 10% 15% 6%	2889
	21st to end	Night	13% 38% 12% 21% 15%	3476
September	1st to 10th	Night	13% 60% 5% 12% 9%	1900
	11th to 20th	Night	15% 60% 4 9% 11% 1	2074
	21st to end	Night	17% 60% 3 12% 6% 1	2090
October	1st to 10th	Night	24% 38% 12% 16% 10% 1	3480
	11th to 20th	Night	13% 37% 7% 17% 23% 3	3889
	21st to end	Night	20% 37% 7% 25% 10% 1	4467
November	1st to 10th	Night	18% 57% 4 12% 9% 2	2537
	11th to 20th	Night	16% 56% 4 8% 13% 2	2600
	21st to end	Night	16% 57% 7% 12% 7% 1	2617
December	1st to 10th	Night	20% 60% 5% 8% 6%	2660
	11th to 20th	Night	23% 50% 6% 8% 12% 1	3288
	21st to end	Night	33% 40% 6% 15% 6%	4884

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	31% 24% 28% 15% 1	180
	11th to 20th	Day	33% 23% 31% 9% 2 1	180
	21st to end	Day	26% 29% 33% 8% 3	198
February	1st to 10th	Day	32% 28% 29% 10% 1	334
	11th to 20th	Day	35% 28% 27% 10%	456
	21st to end	Day	32% 24% 29% 13% 2	382
March	1st to 10th	Day	31% 23% 33% 10% 3	460
	11th to 20th	Day	32% 27% 29% 8% 3 1	460
	21st to end	Day	33% 21% 31% 9% 5% 1	506
April	1st to 10th	Day	33% 22% 28% 10% 5%	460
	11th to 20th	Day	31% 21% 32% 10% 4 1	452
	21st to end	Day	29% 23% 29% 14% 5% 1	460
May	1st to 10th	Day	33% 19% 30% 13% 4 1	463
	11th to 20th	Day	23% 18% 36% 16% 5% 1	487
	21st to end	Day	18% 16% 36% 23% 6% 1	529
June	1st to 10th	Day	20% 16% 36% 18% 7% 2	490
	11th to 20th	Day	17% 16% 36% 22% 8% 1	490
	21st to end	Day	20% 15% 36% 19% 7% 3	490
July	1st to 10th	Day	19% 8% 29% 29% 12% 3	485
	11th to 20th	Day	14% 8% 36% 28% 11% 3	484
	21st to end	Day	14% 11% 36% 30% 8% 2	532
August	1st to 10th	Day	14% 8% 34% 28% 13% 3	486
	11th to 20th	Day	17% 10% 31% 25% 13% 4	479
	21st to end	Day	13% 11% 32% 29% 11% 5%	539
September	1st to 10th	Day	20% 10% 29% 21% 15% 4	490
	11th to 20th	Day	19% 11% 28% 18% 14% 9%	490
	21st to end	Day	20% 13% 29% 23% 10% 5%	490
October	1st to 10th	Day	29% 14% 23% 20% 10% 3	490
	11th to 20th	Day	23% 16% 31% 23% 6% 2	440
	21st to end	Day	30% 16% 31% 17% 5% 1	429
November	1st to 10th	Day	24% 21% 28% 19% 7% 1	395
	11th to 20th	Day	32% 21% 26% 17% 3 1	400
	21st to end	Day	32% 21% 29% 14% 4 1	379
December	1st to 10th	Day	29% 30% 25% 12% 4	180
	11th to 20th	Day	39% 22% 32% 6% 2	180
	21st to end	Day	33% 24% 29% 10% 3 1	198
January	1st to 10th	Night	23% 25% 36% 14% 2	760
	11th to 20th	Night	30% 22% 32% 14% 2	760
	21st to end	Night	22% 22% 37% 16% 3	836
February	1st to 10th	Night	24% 23% 37% 15% 1	760
	11th to 20th	Night	23% 27% 38% 12% 1	743
	21st to end	Night	27% 24% 32% 14% 2	628
March	1st to 10th	Night	27% 21% 33% 16% 2	760
	11th to 20th	Night	26% 22% 34% 15% 3	760
	21st to end	Night	27% 20% 34% 15% 4	836
April	1st to 10th	Night	28% 19% 33% 18% 3	760
	11th to 20th	Night	25% 22% 32% 17% 4	747
	21st to end	Night	26% 20% 33% 18% 2	570
May	1st to 10th	Night	28% 13% 35% 20% 4	570
	11th to 20th	Night	23% 14% 36% 22% 5%	487
	21st to end	Night	20% 9% 38% 27% 6%	411
June	1st to 10th	Night	19% 13% 35% 25% 8%	380
	11th to 20th	Night	22% 8% 30% 32% 7% 1	380
	21st to end	Night	17% 6% 39% 32% 7%	380
July	1st to 10th	Night	16% 7% 33% 36% 8% 1	370
	11th to 20th	Night	15% 4 30% 39% 11% 1	368
	21st to end	Night	14% 5% 32% 38% 11%	416
August	1st to 10th	Night	15% 5% 33% 35% 12%	476
	11th to 20th	Night	14% 8% 30% 34% 13% 1	603
	21st to end	Night	16% 5% 26% 36% 16% 1	836
September	1st to 10th	Night	16% 8% 31% 32% 12% 1	760
	11th to 20th	Night	17% 7% 30% 27% 18% 3	760
	21st to end	Night	19% 10% 29% 28% 12% 2	760
October	1st to 10th	Night	27% 10% 27% 25% 10% 2	760
	11th to 20th	Night	20% 15% 32% 23% 8% 1	760
	21st to end	Night	27% 18% 30% 20% 5%	836
November	1st to 10th	Night	19% 18% 34% 21% 7% 1	780
	11th to 20th	Night	24% 22% 30% 17% 6% 1	800
	21st to end	Night	27% 20% 33% 17% 4	800
December	1st to 10th	Night	26% 22% 36% 13% 3	760
	11th to 20th	Night	27% 23% 32% 16% 3	760
	21st to end	Night	29% 22% 32% 15% 1	836

Page generated at 2024-11-27 17:30:54. Version ac07

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