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 2024-01-15.

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% 6% 3 5% 5% 3 4 5% 5% 7% 39%	74816
	Night	17% 7% 3 5% 5% 3 4 4 5% 7% 41%	74949
Transparency	Day	43% 6% 11% 21% 15% 2	74521
	Night	43% 6% 12% 22% 15% 2	74778
Seeing	Day	25% 17% 31% 18% 7% 2	13850
	Night	23% 17% 33% 21% 6% 1	22887

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% 8% 3 7% 6% 3 4 5% 6% 7% 40%	1517
	11th to 20th	Day	16% 8% 3 6% 6% 3 5% 5% 5% 8% 36%	1472
	21st to end	Day	12% 9% 3 5% 5% 3 4 4 5% 8% 42%	1584
February	1st to 10th	Day	17% 8% 3 7% 6% 4 5% 7% 5% 6% 33%	1648
	11th to 20th	Day	13% 7% 3 7% 6% 4 4 4 6% 8% 38%	1748
	21st to end	Day	17% 10% 4 6% 5% 4 4 4 5% 5% 35%	1480
March	1st to 10th	Day	15% 8% 3 6% 5% 4 5% 4 5% 7% 38%	1797
	11th to 20th	Day	16% 8% 3 6% 5% 3 5% 5% 5% 7% 39%	1876
	21st to end	Day	17% 8% 3 7% 5% 3 4 4 4 6% 41%	2322
April	1st to 10th	Day	17% 8% 3 6% 6% 3 5% 5% 4 6% 38%	2160
	11th to 20th	Day	16% 6% 2 6% 5% 3 4 4 6% 7% 39%	2136
	21st to end	Day	22% 7% 2 6% 5% 3 5% 5% 5% 6% 33%	2340
May	1st to 10th	Day	23% 9% 3 6% 5% 3 4 4 4 6% 34%	2340
	11th to 20th	Day	18% 6% 3 5% 5% 3 5% 5% 4 6% 40%	2387
	21st to end	Day	15% 4 3 6% 5% 3 5% 4 5% 8% 42%	2725
June	1st to 10th	Day	19% 6% 2 6% 6% 4 5% 5% 6% 8% 34%	2520
	11th to 20th	Day	18% 5% 3 5% 6% 3 5% 5% 6% 8% 37%	2520
	21st to end	Day	18% 6% 3 6% 6% 3 5% 5% 6% 8% 34%	2520
July	1st to 10th	Day	15% 5% 2 5% 6% 3 5% 5% 5% 8% 41%	2520
	11th to 20th	Day	12% 5% 2 5% 7% 3 6% 5% 6% 9% 38%	2520
	21st to end	Day	13% 5% 3 5% 6% 4 5% 6% 7% 9% 38%	2744
August	1st to 10th	Day	11% 6% 2 6% 6% 4 5% 6% 7% 9% 38%	2395
	11th to 20th	Day	12% 6% 2 5% 6% 4 5% 6% 7% 8% 38%	2263
	21st to end	Day	13% 4 2 5% 5% 3 5% 5% 7% 8% 43%	2376
September	1st to 10th	Day	14% 4 2 6% 5% 3 4 4 6% 7% 45%	2160
	11th to 20th	Day	15% 6% 2 4 5% 2 3 3 4 5% 51%	1994
	21st to end	Day	16% 4 3 4 4 3 4 4 5% 7% 46%	1980
October	1st to 10th	Day	24% 6% 2 5% 6% 3 4 4 4 5% 38%	1980
	11th to 20th	Day	15% 5% 2 4 4 3 4 4 5% 9% 44%	1740
	21st to end	Day	23% 7% 3 5% 4 2 5% 4 4 6% 37%	1782
November	1st to 10th	Day	15% 5% 3 5% 4 3 4 3 4 6% 48%	1671
	11th to 20th	Day	19% 5% 2 4 4 3 3 4 5% 5% 46%	1710
	21st to end	Day	18% 6% 2 5% 4 3 5% 4 5% 5% 42%	1691
December	1st to 10th	Day	18% 8% 3 6% 6% 3 4 5% 5% 6% 36%	1520
	11th to 20th	Day	22% 6% 2 4 4 3 3 5% 5% 5% 42%	1906
	21st to end	Day	21% 12% 3 5% 5% 3 3 4 4 7% 33%	2772
January	1st to 10th	Night	14% 9% 3 5% 4 3 4 4 5% 6% 44%	2649
	11th to 20th	Night	17% 9% 2 5% 6% 3 5% 4 6% 7% 36%	2577
	21st to end	Night	12% 8% 4 6% 5% 3 4 5% 5% 6% 41%	2772
February	1st to 10th	Night	16% 10% 3 6% 5% 3 5% 5% 5% 7% 37%	2308
	11th to 20th	Night	12% 8% 3 5% 5% 3 6% 5% 5% 6% 40%	2098
	21st to end	Night	15% 8% 4 5% 6% 2 3 5% 5% 6% 40%	1776
March	1st to 10th	Night	14% 8% 2 6% 5% 4 5% 4 4 6% 42%	2150
	11th to 20th	Night	16% 8% 3 5% 5% 3 6% 6% 6% 5% 38%	2081
	21st to end	Night	17% 7% 3 5% 5% 2 4 3 5% 7% 43%	2030
April	1st to 10th	Night	17% 8% 3 5% 6% 3 4 4 5% 7% 38%	1800
	11th to 20th	Night	19% 5% 3 5% 4 3 4 5% 5% 7% 41%	1772
	21st to end	Night	22% 6% 3 6% 4 3 5% 4 3 6% 39%	1620
May	1st to 10th	Night	22% 8% 3 6% 6% 3 4 4 5% 7% 32%	1620
	11th to 20th	Night	18% 6% 3 6% 5% 3 4 4 5% 6% 39%	1523
	21st to end	Night	16% 6% 3 6% 6% 2 5% 4 5% 7% 40%	1550
June	1st to 10th	Night	18% 7% 3 5% 5% 3 4 4 4 7% 38%	1440
	11th to 20th	Night	21% 6% 2 5% 5% 3 5% 5% 6% 7% 38%	1440
	21st to end	Night	16% 5% 3 4 4 3 4 4 6% 8% 42%	1440
July	1st to 10th	Night	16% 4 2 4 5% 3 5% 4 7% 7% 43%	1440
	11th to 20th	Night	13% 6% 3 5% 5% 3 5% 6% 6% 10% 40%	1440
	21st to end	Night	13% 4 2 4 4 3 4 5% 6% 8% 47%	1574
August	1st to 10th	Night	9% 6% 2 5% 5% 3 5% 4 6% 8% 46%	1524
	11th to 20th	Night	12% 7% 3 5% 6% 4 5% 6% 7% 7% 38%	1634
	21st to end	Night	16% 4 1 4 4 2 3 3 5% 8% 49%	1980
September	1st to 10th	Night	14% 4 2 4 4 2 3 4 5% 7% 50%	1800
	11th to 20th	Night	15% 5% 2 4 4 2 3 4 5% 6% 53%	1964
	21st to end	Night	15% 5% 2 5% 3 2 4 4 5% 6% 49%	1980
October	1st to 10th	Night	22% 6% 2 4 4 3 4 3 4 5% 41%	1980
	11th to 20th	Night	16% 5% 2 3 4 3 5% 5% 5% 7% 44%	2216
	21st to end	Night	23% 7% 2 5% 4 2 4 4 5% 7% 36%	2574
November	1st to 10th	Night	15% 5% 2 5% 4 3 4 4 4 6% 47%	2407
	11th to 20th	Night	17% 6% 2 4 4 3 4 4 4 6% 45%	2470
	21st to end	Night	15% 7% 2 4 4 3 4 4 5% 8% 44%	2488
December	1st to 10th	Night	18% 9% 3 5% 5% 3 5% 4 6% 6% 37%	2660
	11th to 20th	Night	19% 7% 3 5% 4 3 4 4 5% 6% 41%	3288
	21st to end	Night	21% 9% 4 6% 4 3 4 4 5% 7% 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	44% 5% 10% 26% 14% 2	1518
	11th to 20th	Day	46% 8% 13% 19% 12% 2	1472
	21st to end	Day	41% 5% 13% 24% 14% 3	1584
February	1st to 10th	Day	49% 7% 12% 18% 11% 2	1648
	11th to 20th	Day	44% 7% 11% 21% 14% 4	1757
	21st to end	Day	51% 5% 9% 18% 15% 3	1480
March	1st to 10th	Day	46% 4 13% 20% 12% 5%	1800
	11th to 20th	Day	45% 5% 11% 19% 16% 4	1876
	21st to end	Day	47% 5% 8% 18% 17% 6%	2322
April	1st to 10th	Day	47% 5% 8% 19% 17% 4	2160
	11th to 20th	Day	43% 6% 11% 20% 17% 3	2136
	21st to end	Day	51% 6% 7% 15% 17% 4	2340
May	1st to 10th	Day	51% 5% 9% 20% 13% 2	2340
	11th to 20th	Day	44% 6% 10% 19% 18% 3	2391
	21st to end	Day	41% 7% 14% 23% 13% 2	2725
June	1st to 10th	Day	47% 8% 13% 21% 10% 1	2520
	11th to 20th	Day	44% 7% 12% 19% 16% 3	2520
	21st to end	Day	46% 8% 12% 21% 11% 1	2520
July	1st to 10th	Day	41% 7% 12% 22% 17% 2	2450
	11th to 20th	Day	39% 9% 14% 25% 12% 1	2445
	21st to end	Day	39% 7% 15% 23% 14% 3	2591
August	1st to 10th	Day	39% 9% 15% 23% 13% 1	2381
	11th to 20th	Day	40% 9% 14% 23% 12% 2	2263
	21st to end	Day	35% 8% 12% 25% 17% 3	2376
September	1st to 10th	Day	37% 6% 13% 24% 18% 2	2160
	11th to 20th	Day	35% 5% 8% 26% 24% 2	1994
	21st to end	Day	38% 5% 10% 23% 21% 3	1980
October	1st to 10th	Day	47% 6% 8% 21% 14% 4	1980
	11th to 20th	Day	37% 7% 12% 29% 13% 2	1740
	21st to end	Day	48% 6% 10% 20% 13% 3	1782
November	1st to 10th	Day	39% 4 12% 26% 17% 2	1671
	11th to 20th	Day	40% 6% 10% 22% 19% 3	1710
	21st to end	Day	43% 5% 17% 21% 13% 2	1691
December	1st to 10th	Day	48% 6% 14% 16% 15% 1	1520
	11th to 20th	Day	44% 5% 11% 19% 17% 3	1906
	21st to end	Day	51% 7% 12% 18% 11% 1	2772
January	1st to 10th	Night	41% 5% 12% 24% 15% 3	2649
	11th to 20th	Night	48% 7% 14% 20% 8% 3	2577
	21st to end	Night	44% 5% 10% 20% 18% 3	2772
February	1st to 10th	Night	46% 7% 13% 19% 12% 3	2308
	11th to 20th	Night	44% 5% 12% 20% 15% 3	2100
	21st to end	Night	43% 6% 9% 23% 15% 4	1776
March	1st to 10th	Night	44% 5% 10% 23% 14% 4	2160
	11th to 20th	Night	45% 8% 10% 17% 16% 4	2082
	21st to end	Night	43% 5% 9% 18% 19% 5%	2030
April	1st to 10th	Night	46% 5% 11% 19% 16% 3	1800
	11th to 20th	Night	44% 5% 10% 21% 18% 2	1772
	21st to end	Night	46% 6% 12% 18% 14% 4	1620
May	1st to 10th	Night	51% 5% 10% 19% 12% 3	1620
	11th to 20th	Night	44% 7% 13% 17% 15% 3	1524
	21st to end	Night	44% 7% 13% 21% 12% 1	1551
June	1st to 10th	Night	47% 6% 11% 23% 11% 1	1440
	11th to 20th	Night	45% 8% 12% 19% 14% 2	1440
	21st to end	Night	39% 10% 15% 22% 13% 1	1440
July	1st to 10th	Night	40% 8% 13% 24% 14% 1	1400
	11th to 20th	Night	39% 8% 17% 24% 11%	1392
	21st to end	Night	34% 7% 16% 29% 12% 2	1486
August	1st to 10th	Night	35% 8% 15% 24% 16% 1	1514
	11th to 20th	Night	43% 8% 15% 21% 12% 1	1634
	21st to end	Night	34% 7% 12% 27% 19% 1	1980
September	1st to 10th	Night	34% 7% 13% 26% 19% 1	1800
	11th to 20th	Night	33% 5% 11% 25% 25% 2	1964
	21st to end	Night	36% 6% 10% 28% 18% 3	1980
October	1st to 10th	Night	45% 6% 11% 22% 13% 3	1980
	11th to 20th	Night	38% 8% 13% 27% 13% 1	2216
	21st to end	Night	48% 5% 13% 21% 12% 1	2574
November	1st to 10th	Night	39% 6% 11% 22% 19% 3	2407
	11th to 20th	Night	41% 4 12% 22% 19% 3	2470
	21st to end	Night	41% 6% 14% 26% 13% 1	2488
December	1st to 10th	Night	47% 6% 12% 21% 12% 1	2660
	11th to 20th	Night	42% 6% 15% 17% 18% 2	3288
	21st to end	Night	50% 7% 10% 24% 10% 1	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	34% 22% 30% 10% 2 1	168
	21st to end	Day	26% 30% 34% 8% 3	176
February	1st to 10th	Day	35% 28% 28% 7% 2	303
	11th to 20th	Day	35% 27% 28% 10%	416
	21st to end	Day	32% 25% 29% 12% 1	346
March	1st to 10th	Day	30% 23% 34% 10% 3	420
	11th to 20th	Day	31% 28% 29% 8% 2 1	420
	21st to end	Day	32% 20% 32% 9% 5% 2	462
April	1st to 10th	Day	33% 22% 28% 10% 6%	420
	11th to 20th	Day	30% 21% 32% 11% 5% 1	412
	21st to end	Day	31% 22% 29% 13% 5% 1	420
May	1st to 10th	Day	35% 19% 28% 12% 4 1	423
	11th to 20th	Day	23% 19% 38% 14% 5% 2	447
	21st to end	Day	18% 16% 36% 22% 7% 1	485
June	1st to 10th	Day	20% 16% 36% 18% 7% 2	450
	11th to 20th	Day	18% 15% 38% 20% 7% 1	450
	21st to end	Day	19% 14% 37% 20% 7% 3	450
July	1st to 10th	Day	20% 8% 28% 29% 13% 3	445
	11th to 20th	Day	15% 8% 35% 29% 11% 3	444
	21st to end	Day	13% 10% 35% 31% 8% 2	488
August	1st to 10th	Day	13% 9% 33% 28% 14% 3	446
	11th to 20th	Day	16% 10% 31% 26% 13% 3	439
	21st to end	Day	14% 11% 31% 29% 10% 5%	495
September	1st to 10th	Day	20% 10% 28% 22% 16% 4	450
	11th to 20th	Day	20% 12% 30% 17% 13% 7%	450
	21st to end	Day	19% 14% 30% 24% 9% 5%	450
October	1st to 10th	Day	31% 14% 23% 19% 10% 3	450
	11th to 20th	Day	21% 15% 33% 23% 7% 1	405
	21st to end	Day	32% 16% 30% 17% 5% 1	396
November	1st to 10th	Day	22% 21% 27% 21% 8% 1	365
	11th to 20th	Day	32% 21% 26% 17% 3 1	370
	21st to end	Day	29% 21% 30% 15% 5% 1	351
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% 31% 15% 2	737
	21st to end	Night	21% 23% 38% 16% 3	792
February	1st to 10th	Night	25% 23% 36% 14% 2	720
	11th to 20th	Night	23% 26% 38% 13% 1	703
	21st to end	Night	28% 24% 33% 13% 1	592
March	1st to 10th	Night	26% 22% 34% 16% 2	720
	11th to 20th	Night	27% 21% 34% 15% 3	720
	21st to end	Night	26% 21% 33% 16% 4	792
April	1st to 10th	Night	28% 19% 32% 19% 3	720
	11th to 20th	Night	24% 22% 32% 18% 5%	708
	21st to end	Night	26% 20% 33% 18% 2	540
May	1st to 10th	Night	29% 13% 35% 20% 4	540
	11th to 20th	Night	23% 14% 37% 22% 4	462
	21st to end	Night	20% 7% 39% 28% 6%	389
June	1st to 10th	Night	20% 12% 36% 24% 8%	360
	11th to 20th	Night	22% 9% 30% 32% 7% 1	360
	21st to end	Night	16% 5% 39% 33% 6%	360
July	1st to 10th	Night	16% 7% 32% 36% 8% 1	350
	11th to 20th	Night	15% 4 30% 39% 11% 1	348
	21st to end	Night	13% 5% 33% 39% 10%	394
August	1st to 10th	Night	14% 5% 33% 36% 12%	450
	11th to 20th	Night	14% 8% 29% 35% 12% 1	570
	21st to end	Night	17% 6% 26% 35% 15% 1	792
September	1st to 10th	Night	16% 8% 31% 32% 12% 1	720
	11th to 20th	Night	17% 7% 32% 26% 16% 2	720
	21st to end	Night	18% 11% 30% 27% 13% 2	720
October	1st to 10th	Night	28% 10% 27% 24% 9% 2	720
	11th to 20th	Night	20% 15% 33% 23% 8% 1	720
	21st to end	Night	28% 18% 30% 20% 5%	792
November	1st to 10th	Night	20% 18% 33% 21% 8% 1	740
	11th to 20th	Night	24% 21% 30% 17% 6% 1	760
	21st to end	Night	26% 20% 33% 17% 4	760
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 2024-01-11 22:33:48. Version ac07

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