Winchester
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Winchester clear sky chart from 2005-11-06 to 2026-07-16.

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	12% 8% 3 7% 5% 4 4 4 6% 8% 40%	138208
	Night	13% 9% 3 7% 4 4 4 3 5% 7% 41%	134765
Transparency	Day	41% 7% 12% 23% 15% 2	141759
	Night	41% 6% 11% 23% 16% 2	139294
Seeing	Day	14% 10% 27% 27% 17% 5%	18018
	Night	16% 9% 30% 31% 13% 1	27818

By Month

Percentage Cloud Cover SE (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	12% 10% 6% 14% 5% 4 4 3 5% 6% 30%	4691
	11th to 20th	Day	8% 10% 4 9% 7% 7% 7% 4 6% 5% 34%	4950
	21st to end	Day	13% 16% 7% 9% 5% 3 3 2 4 4 33%	5812
February	1st to 10th	Day	15% 11% 5% 9% 5% 5% 5% 3 5% 7% 32%	1890
	11th to 20th	Day	13% 11% 3 7% 6% 5% 5% 4 5% 7% 33%	1998
	21st to end	Day	14% 10% 4 8% 5% 4 4 2 5% 5% 38%	1730
March	1st to 10th	Day	16% 11% 3 5% 3 3 3 2 3 5% 46%	4627
	11th to 20th	Day	8% 7% 2 5% 3 4 4 4 6% 8% 49%	5090
	21st to end	Day	15% 10% 3 8% 5% 5% 5% 4 6% 8% 33%	5900
April	1st to 10th	Day	15% 8% 2 7% 5% 4 4 4 6% 7% 38%	2520
	11th to 20th	Day	15% 9% 3 6% 5% 4 4 4 5% 7% 38%	2494
	21st to end	Day	18% 6% 2 8% 5% 4 5% 4 6% 6% 36%	2520
May	1st to 10th	Day	15% 8% 3 8% 5% 4 5% 4 6% 9% 34%	7367
	11th to 20th	Day	14% 7% 3 7% 4 4 4 3 5% 7% 41%	8222
	21st to end	Day	14% 8% 3 7% 5% 5% 5% 4 6% 8% 35%	9047
June	1st to 10th	Day	8% 5% 2 5% 4 4 5% 5% 7% 9% 45%	2940
	11th to 20th	Day	9% 4 2 5% 5% 4 5% 5% 7% 9% 45%	2940
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 4 7% 10% 42%	2940
July	1st to 10th	Day	10% 5% 2 5% 4 4 4 4 7% 10% 44%	6500
	11th to 20th	Day	7% 7% 3 8% 5% 5% 6% 5% 7% 10% 38%	6431
	21st to end	Day	8% 6% 2 7% 5% 5% 5% 5% 8% 11% 37%	6706
August	1st to 10th	Day	5% 4 2 6% 4 4 4 4 7% 10% 49%	4234
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 7% 10% 42%	3883
	21st to end	Day	5% 4 2 5% 4 4 4 5% 7% 11% 50%	4312
September	1st to 10th	Day	12% 4 2 5% 3 4 4 3 6% 6% 52%	2400
	11th to 20th	Day	9% 5% 2 5% 4 3 3 5% 6% 7% 51%	2375
	21st to end	Day	9% 5% 2 5% 4 3 5% 3 6% 7% 51%	2187
October	1st to 10th	Day	11% 4 2 5% 3 4 3 4 5% 6% 53%	2000
	11th to 20th	Day	14% 8% 2 6% 3 3 3 3 4 7% 47%	2000
	21st to end	Day	18% 9% 3 6% 3 3 3 3 3 6% 43%	2153
November	1st to 10th	Day	11% 6% 2 5% 4 4 3 4 5% 6% 50%	1852
	11th to 20th	Day	14% 8% 3 7% 3 4 3 3 4 6% 45%	1890
	21st to end	Day	15% 11% 5% 8% 4 3 3 3 5% 4 38%	1759
December	1st to 10th	Day	8% 7% 3 7% 4 3 4 3 4 6% 50%	2690
	11th to 20th	Day	13% 10% 6% 9% 6% 4 5% 3 5% 6% 33%	3088
	21st to end	Day	23% 19% 6% 11% 5% 2 3 3 4 4 20%	4070
January	1st to 10th	Night	12% 12% 5% 10% 5% 4 4 3 4 5% 35%	8232
	11th to 20th	Night	9% 11% 4 11% 5% 4 6% 4 5% 5% 35%	8022
	21st to end	Night	14% 14% 5% 10% 5% 3 3 2 4 5% 34%	8402
February	1st to 10th	Night	15% 11% 6% 8% 4 4 5% 4 5% 6% 33%	2730
	11th to 20th	Night	14% 9% 4 7% 4 4 4 4 5% 6% 38%	2506
	21st to end	Night	16% 8% 3 7% 5% 3 4 3 4 5% 42%	2076
March	1st to 10th	Night	19% 11% 4 6% 3 3 3 2 3 5% 41%	5476
	11th to 20th	Night	12% 8% 2 5% 3 3 3 3 4 5% 51%	5090
	21st to end	Night	16% 10% 3 8% 5% 4 5% 4 5% 7% 33%	5271
April	1st to 10th	Night	14% 7% 3 6% 5% 5% 4 3 5% 7% 42%	2100
	11th to 20th	Night	14% 8% 2 5% 4 3 4 4 5% 8% 44%	2075
	21st to end	Night	18% 7% 3 6% 4 4 4 3 5% 6% 40%	2100
May	1st to 10th	Night	17% 8% 3 7% 5% 5% 4 3 6% 7% 35%	5483
	11th to 20th	Night	12% 5% 2 6% 4 4 5% 4 6% 9% 43%	4670
	21st to end	Night	14% 7% 3 7% 4 5% 5% 4 6% 8% 37%	5139
June	1st to 10th	Night	8% 4 1 5% 4 3 4 4 6% 8% 52%	1680
	11th to 20th	Night	8% 5% 2 5% 4 4 5% 4 6% 10% 48%	1680
	21st to end	Night	6% 4 1 6% 4 4 5% 4 7% 9% 49%	1680
July	1st to 10th	Night	9% 5% 2 6% 4 5% 4 4 6% 9% 45%	3698
	11th to 20th	Night	5% 5% 2 6% 5% 5% 5% 4 7% 9% 47%	3657
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 5% 8% 10% 40%	4162
August	1st to 10th	Night	4 3 1 4 4 3 3 4 6% 9% 58%	2926
	11th to 20th	Night	8% 5% 2 5% 4 5% 4 3 6% 8% 50%	3165
	21st to end	Night	4 3 1 5% 3 4 4 4 7% 9% 55%	3586
September	1st to 10th	Night	10% 3 2 4 3 3 3 3 5% 7% 56%	2000
	11th to 20th	Night	8% 4 2 5% 3 3 5% 3 6% 8% 53%	2003
	21st to end	Night	9% 5% 2 5% 4 3 4 3 5% 6% 56%	2210
October	1st to 10th	Night	9% 4 2 5% 3 3 3 3 5% 6% 57%	2400
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 4 6% 51%	2400
	21st to end	Night	18% 8% 2 6% 3 3 3 3 4 6% 44%	2685
November	1st to 10th	Night	10% 6% 2 5% 4 3 3 3 4 7% 52%	2666
	11th to 20th	Night	12% 7% 3 6% 4 3 4 2 4 7% 48%	2730
	21st to end	Night	16% 10% 3 6% 4 3 4 3 4 5% 41%	2856
December	1st to 10th	Night	11% 9% 3 7% 5% 4 3 3 4 5% 46%	4720
	11th to 20th	Night	15% 10% 3 8% 4 4 4 3 5% 6% 38%	5339
	21st to end	Night	24% 18% 5% 10% 5% 4 4 3 3 4 20%	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	53% 6% 12% 19% 9% 1	4692
	11th to 20th	Day	49% 6% 13% 18% 11% 3	4950
	21st to end	Day	56% 4 9% 20% 9% 2	5812
February	1st to 10th	Day	52% 5% 13% 20% 9% 1	1890
	11th to 20th	Day	49% 6% 11% 20% 12% 2	2008
	21st to end	Day	48% 5% 11% 24% 12% 2	1730
March	1st to 10th	Day	43% 4 7% 23% 22% 2	4627
	11th to 20th	Day	31% 6% 12% 28% 21% 2	5090
	21st to end	Day	48% 7% 17% 21% 6% 1	5902
April	1st to 10th	Day	44% 7% 12% 23% 14% 1	2520
	11th to 20th	Day	45% 6% 10% 24% 13% 2	2494
	21st to end	Day	46% 7% 12% 21% 13% 1	2520
May	1st to 10th	Day	44% 8% 13% 21% 13% 1	7402
	11th to 20th	Day	42% 6% 10% 24% 16% 2	8226
	21st to end	Day	45% 8% 10% 21% 16% 1	9061
June	1st to 10th	Day	32% 9% 15% 26% 16% 3	2940
	11th to 20th	Day	32% 8% 12% 24% 21% 3	2940
	21st to end	Day	35% 9% 14% 22% 17% 3	2940
July	1st to 10th	Day	33% 6% 14% 28% 17% 2	6431
	11th to 20th	Day	38% 9% 14% 23% 15% 1	6356
	21st to end	Day	36% 10% 18% 26% 9% 1	6515
August	1st to 10th	Day	27% 8% 18% 34% 12% 1	4220
	11th to 20th	Day	36% 7% 15% 28% 14% 1	3919
	21st to end	Day	26% 9% 17% 30% 15% 3	4311
September	1st to 10th	Day	31% 7% 10% 25% 21% 5%	2400
	11th to 20th	Day	29% 6% 10% 28% 23% 4	2375
	21st to end	Day	31% 6% 10% 27% 23% 2	2187
October	1st to 10th	Day	21% 3 8% 17% 46% 5%	3260
	11th to 20th	Day	25% 4 8% 21% 36% 7%	3240
	21st to end	Day	38% 4 8% 25% 23% 2	3455
November	1st to 10th	Day	34% 5% 13% 25% 19% 4	1852
	11th to 20th	Day	42% 4 11% 23% 18% 3	1890
	21st to end	Day	49% 5% 11% 23% 12%	1759
December	1st to 10th	Day	36% 5% 17% 30% 12%	2690
	11th to 20th	Day	51% 6% 11% 21% 10% 2	3085
	21st to end	Day	67% 6% 11% 11% 5%	4070
January	1st to 10th	Night	51% 6% 14% 21% 7%	8233
	11th to 20th	Night	50% 4 8% 20% 15% 2	8022
	21st to end	Night	54% 4 11% 20% 10% 2	8402
February	1st to 10th	Night	51% 6% 11% 20% 10% 2	2730
	11th to 20th	Night	46% 5% 12% 21% 15% 2	2507
	21st to end	Night	44% 5% 12% 23% 14% 3	2076
March	1st to 10th	Night	47% 4 7% 22% 18% 2	5476
	11th to 20th	Night	35% 5% 8% 28% 20% 5%	5090
	21st to end	Night	48% 7% 11% 24% 8% 2	5271
April	1st to 10th	Night	41% 6% 11% 24% 15% 3	2100
	11th to 20th	Night	37% 7% 12% 24% 17% 4	2075
	21st to end	Night	44% 5% 11% 22% 16% 2	2100
May	1st to 10th	Night	45% 8% 12% 23% 10% 1	5513
	11th to 20th	Night	36% 8% 13% 25% 15% 3	4671
	21st to end	Night	43% 7% 12% 19% 17% 2	5149
June	1st to 10th	Night	27% 8% 14% 29% 17% 4	1680
	11th to 20th	Night	29% 8% 11% 22% 26% 4	1680
	21st to end	Night	29% 9% 16% 24% 18% 5%	1680
July	1st to 10th	Night	34% 6% 11% 30% 16% 2	3660
	11th to 20th	Night	29% 9% 15% 29% 16% 1	3612
	21st to end	Night	34% 9% 19% 25% 12% 1	4042
August	1st to 10th	Night	21% 8% 19% 35% 16% 1	2916
	11th to 20th	Night	32% 6% 13% 29% 20% 1	3195
	21st to end	Night	23% 7% 18% 31% 17% 4	3586
September	1st to 10th	Night	29% 5% 11% 24% 25% 7%	2000
	11th to 20th	Night	27% 6% 11% 24% 27% 5%	2003
	21st to end	Night	29% 6% 10% 24% 27% 5%	2210
October	1st to 10th	Night	19% 4 7% 20% 45% 5%	3920
	11th to 20th	Night	24% 4 9% 21% 35% 6%	3899
	21st to end	Night	33% 4 9% 24% 25% 5%	4333
November	1st to 10th	Night	32% 5% 12% 25% 20% 5%	2666
	11th to 20th	Night	37% 5% 9% 28% 19% 2	2730
	21st to end	Night	46% 4 10% 25% 14% 1	2856
December	1st to 10th	Night	41% 4 11% 31% 12% 1	4720
	11th to 20th	Night	46% 5% 13% 23% 12% 1	5341
	21st to end	Night	67% 5% 11% 11% 5% 1	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	20% 18% 34% 22% 6% 1	330
	11th to 20th	Day	22% 20% 39% 16% 3	330
	21st to end	Day	16% 16% 42% 21% 4	363
February	1st to 10th	Day	23% 13% 43% 16% 5%	330
	11th to 20th	Day	22% 18% 39% 17% 5%	330
	21st to end	Day	21% 17% 42% 19% 1	273
March	1st to 10th	Day	20% 19% 43% 16% 2	330
	11th to 20th	Day	22% 20% 34% 18% 5%	330
	21st to end	Day	26% 17% 35% 17% 4 1	499
April	1st to 10th	Day	21% 15% 34% 23% 6% 1	540
	11th to 20th	Day	18% 19% 31% 21% 10% 1	532
	21st to end	Day	19% 14% 36% 24% 6% 1	540
May	1st to 10th	Day	16% 11% 34% 26% 9% 2	560
	11th to 20th	Day	17% 11% 29% 27% 14% 3	775
	21st to end	Day	14% 7% 30% 29% 14% 6%	844
June	1st to 10th	Day	8% 5% 24% 33% 24% 7%	780
	11th to 20th	Day	10% 4 23% 34% 22% 6%	780
	21st to end	Day	9% 6% 26% 34% 21% 4	780
July	1st to 10th	Day	9% 3 13% 32% 28% 15%	771
	11th to 20th	Day	8% 3 19% 33% 28% 9%	744
	21st to end	Day	10% 2 15% 33% 34% 7%	794
August	1st to 10th	Day	8% 3 15% 39% 27% 8%	726
	11th to 20th	Day	9% 2 16% 28% 31% 13%	541
	21st to end	Day	7% 2 13% 29% 37% 11%	583
September	1st to 10th	Day	12% 3 15% 30% 28% 11%	530
	11th to 20th	Day	8% 2 12% 30% 32% 15%	530
	21st to end	Day	10% 8% 20% 30% 24% 8%	517
October	1st to 10th	Day	11% 6% 25% 28% 21% 8%	330
	11th to 20th	Day	12% 22% 28% 22% 14% 2	330
	21st to end	Day	27% 16% 32% 18% 6% 1	363
November	1st to 10th	Day	11% 11% 36% 28% 10% 4	330
	11th to 20th	Day	17% 18% 35% 17% 12% 1	330
	21st to end	Day	23% 15% 31% 22% 8% 1	330
December	1st to 10th	Day	12% 15% 42% 21% 6% 3	330
	11th to 20th	Day	21% 16% 35% 21% 6%	330
	21st to end	Day	25% 20% 37% 15% 4	363
January	1st to 10th	Night	16% 14% 40% 25% 4	1050
	11th to 20th	Night	18% 18% 38% 21% 5%	1050
	21st to end	Night	19% 14% 38% 26% 3	1155
February	1st to 10th	Night	21% 13% 38% 25% 4	1050
	11th to 20th	Night	20% 15% 37% 25% 3	871
	21st to end	Night	21% 16% 38% 22% 2	692
March	1st to 10th	Night	21% 14% 39% 22% 4	836
	11th to 20th	Night	22% 12% 34% 27% 5%	630
	21st to end	Night	24% 14% 33% 24% 6%	693
April	1st to 10th	Night	18% 12% 37% 28% 4	630
	11th to 20th	Night	16% 12% 37% 27% 8%	623
	21st to end	Night	20% 10% 36% 27% 7%	630
May	1st to 10th	Night	16% 6% 34% 34% 9% 1	630
	11th to 20th	Night	14% 6% 29% 38% 11% 1	621
	21st to end	Night	12% 4 27% 36% 19% 3	683
June	1st to 10th	Night	9% 5% 22% 43% 21% 1	630
	11th to 20th	Night	8% 3 23% 45% 19% 2	630
	21st to end	Night	7% 3 22% 44% 22% 1	630
July	1st to 10th	Night	10% 2 10% 37% 37% 4	615
	11th to 20th	Night	5% 2 16% 46% 29% 2	597
	21st to end	Night	7% 2 13% 44% 33% 2	657
August	1st to 10th	Night	8% 2 11% 46% 31% 3	597
	11th to 20th	Night	9% 2 11% 41% 32% 5%	594
	21st to end	Night	5% 1 13% 38% 41% 3	660
September	1st to 10th	Night	11% 3 16% 37% 30% 4	600
	11th to 20th	Night	9% 1 13% 39% 32% 5%	603
	21st to end	Night	11% 5% 22% 33% 25% 4	800
October	1st to 10th	Night	10% 4 28% 33% 23% 3	800
	11th to 20th	Night	16% 8% 33% 32% 11% 1	800
	21st to end	Night	22% 12% 32% 27% 7%	880
November	1st to 10th	Night	14% 9% 39% 29% 9%	820
	11th to 20th	Night	16% 12% 38% 28% 6% 1	840
	21st to end	Night	21% 13% 35% 25% 6%	966
December	1st to 10th	Night	20% 11% 38% 25% 6% 1	1050
	11th to 20th	Night	23% 10% 33% 28% 6%	1050
	21st to end	Night	25% 14% 35% 23% 2	1155

Page generated at 2026-07-13 02:14:51. Version ac07

Copyright © 2026 Attilla Danko.