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 2025-07-21.

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% 10% 3 8% 5% 5% 5% 4 5% 6% 31%	120396
	Night	17% 12% 4 9% 5% 5% 4 3 5% 6% 30%	118700
Transparency	Day	53% 4 12% 21% 9% 1	123906
	Night	55% 4 11% 19% 10% 1	123351
Seeing	Day	22% 14% 30% 22% 10% 3	15377
	Night	23% 10% 29% 28% 9% 1	24438

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	16% 18% 6% 14% 6% 6% 4 3 4 4 19%	3597
	11th to 20th	Day	12% 16% 7% 14% 6% 5% 5% 4 5% 5% 21%	3570
	21st to end	Day	13% 17% 7% 13% 7% 6% 5% 4 5% 6% 19%	4112
February	1st to 10th	Day	16% 15% 5% 11% 7% 5% 5% 3 5% 5% 24%	1800
	11th to 20th	Day	14% 15% 5% 11% 7% 6% 3 3 4 6% 25%	1786
	21st to end	Day	19% 15% 4 10% 6% 4 5% 4 4 5% 24%	1650
March	1st to 10th	Day	18% 12% 4 11% 6% 4 5% 4 5% 5% 25%	4476
	11th to 20th	Day	11% 13% 4 9% 5% 5% 6% 4 5% 7% 30%	4925
	21st to end	Day	18% 10% 3 8% 6% 4 4 3 4 5% 37%	5821
April	1st to 10th	Day	21% 13% 3 7% 4 3 3 3 5% 6% 31%	2400
	11th to 20th	Day	22% 10% 3 8% 4 4 3 3 3 6% 35%	2374
	21st to end	Day	23% 9% 3 7% 4 4 4 3 5% 5% 31%	2518
May	1st to 10th	Day	24% 11% 4 9% 5% 5% 4 4 5% 6% 24%	6346
	11th to 20th	Day	17% 9% 3 10% 5% 5% 5% 4 5% 7% 30%	6318
	21st to end	Day	21% 8% 3 7% 4 4 4 4 6% 7% 33%	6949
June	1st to 10th	Day	22% 8% 2 7% 5% 5% 5% 4 6% 6% 30%	2800
	11th to 20th	Day	15% 6% 2 6% 5% 5% 5% 4 6% 8% 39%	2800
	21st to end	Day	20% 7% 3 7% 5% 4 5% 4 6% 7% 32%	2800
July	1st to 10th	Day	12% 6% 2 8% 5% 5% 6% 5% 7% 9% 36%	4580
	11th to 20th	Day	13% 6% 2 6% 5% 5% 6% 5% 7% 9% 38%	4580
	21st to end	Day	11% 6% 2 6% 4 5% 5% 4 7% 9% 43%	4814
August	1st to 10th	Day	15% 8% 3 7% 5% 4 5% 4 6% 8% 35%	4378
	11th to 20th	Day	17% 6% 2 6% 4 4 5% 4 6% 8% 39%	3951
	21st to end	Day	17% 5% 2 5% 4 5% 5% 4 6% 8% 39%	4180
September	1st to 10th	Day	13% 5% 2 7% 4 4 5% 4 6% 7% 43%	2280
	11th to 20th	Day	14% 5% 2 5% 4 5% 3 3 5% 7% 47%	2262
	21st to end	Day	19% 5% 2 6% 3 5% 4 4 6% 7% 39%	2057
October	1st to 10th	Day	21% 6% 3 8% 4 4 5% 4 4 6% 35%	1900
	11th to 20th	Day	20% 9% 3 9% 6% 5% 5% 3 5% 6% 29%	1900
	21st to end	Day	26% 11% 3 7% 5% 4 4 3 5% 6% 27%	1920
November	1st to 10th	Day	16% 10% 3 7% 5% 5% 4 5% 6% 8% 31%	1761
	11th to 20th	Day	17% 13% 3 8% 6% 4 5% 3 5% 5% 30%	1645
	21st to end	Day	21% 15% 4 10% 6% 5% 4 4 4 6% 21%	1600
December	1st to 10th	Day	21% 19% 5% 11% 6% 5% 5% 4 5% 5% 14%	2600
	11th to 20th	Day	19% 15% 5% 11% 6% 5% 5% 4 5% 4 23%	2986
	21st to end	Day	21% 16% 4 12% 5% 6% 6% 3 4 3 20%	3960
January	1st to 10th	Night	15% 20% 8% 14% 5% 4 3 2 4 4 20%	6349
	11th to 20th	Night	11% 16% 8% 15% 8% 6% 6% 4 5% 4 17%	6315
	21st to end	Night	16% 17% 6% 12% 6% 5% 4 3 5% 5% 21%	6840
February	1st to 10th	Night	15% 17% 5% 11% 7% 5% 5% 4 5% 5% 22%	2600
	11th to 20th	Night	14% 14% 4 12% 7% 5% 4 3 5% 5% 27%	2499
	21st to end	Night	16% 15% 4 9% 6% 4 4 4 4 4 28%	1980
March	1st to 10th	Night	21% 14% 4 8% 5% 4 4 2 4 4 29%	5364
	11th to 20th	Night	14% 13% 3 8% 4 4 5% 3 6% 8% 31%	5030
	21st to end	Night	19% 11% 3 7% 5% 5% 4 3 5% 5% 34%	5110
April	1st to 10th	Night	21% 10% 4 8% 4 5% 4 3 4 5% 32%	2000
	11th to 20th	Night	20% 11% 3 7% 5% 4 4 4 5% 5% 35%	1975
	21st to end	Night	22% 9% 3 7% 5% 4 3 3 4 6% 35%	1900
May	1st to 10th	Night	24% 10% 3 9% 5% 5% 5% 4 5% 7% 23%	3594
	11th to 20th	Night	16% 9% 3 8% 5% 4 4 3 6% 7% 36%	3575
	21st to end	Night	16% 10% 3 8% 5% 4 5% 4 5% 7% 32%	3926
June	1st to 10th	Night	18% 7% 3 7% 5% 6% 6% 4 6% 9% 30%	1600
	11th to 20th	Night	16% 6% 3 6% 4 4 4 3 6% 9% 40%	1600
	21st to end	Night	19% 7% 3 6% 4 5% 4 4 5% 8% 35%	1600
July	1st to 10th	Night	12% 7% 3 8% 6% 5% 5% 4 7% 9% 35%	2600
	11th to 20th	Night	13% 6% 2 6% 4 5% 4 4 6% 9% 41%	2600
	21st to end	Night	11% 6% 2 6% 4 5% 5% 5% 6% 8% 41%	2739
August	1st to 10th	Night	14% 7% 3 7% 4 4 4 4 5% 8% 39%	2535
	11th to 20th	Night	13% 8% 3 7% 4 5% 4 4 6% 8% 39%	2844
	21st to end	Night	11% 7% 2 6% 4 5% 6% 4 6% 8% 41%	3476
September	1st to 10th	Night	13% 6% 2 6% 4 4 5% 3 6% 6% 45%	1900
	11th to 20th	Night	14% 5% 2 4 4 3 3 3 5% 6% 50%	1917
	21st to end	Night	16% 6% 3 6% 4 4 4 4 5% 6% 43%	2121
October	1st to 10th	Night	18% 7% 3 6% 4 4 5% 4 5% 7% 37%	2280
	11th to 20th	Night	20% 8% 3 8% 5% 4 4 3 5% 6% 34%	2280
	21st to end	Night	25% 9% 3 8% 4 4 4 3 5% 6% 28%	2676
November	1st to 10th	Night	19% 10% 3 7% 5% 4 4 3 4 7% 35%	2537
	11th to 20th	Night	18% 12% 3 8% 5% 5% 4 3 4 6% 32%	2754
	21st to end	Night	21% 14% 5% 9% 5% 4 4 3 5% 6% 24%	2800
December	1st to 10th	Night	20% 16% 6% 12% 6% 5% 5% 3 5% 5% 17%	4580
	11th to 20th	Night	23% 15% 4 9% 5% 4 4 3 3 4 25%	5208
	21st to end	Night	26% 16% 4 11% 4 4 4 3 4 4 20%	6996

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	70% 4 10% 12% 4 1	3598
	11th to 20th	Day	65% 4 11% 12% 8%	3600
	21st to end	Day	67% 4 10% 12% 7%	4112
February	1st to 10th	Day	63% 4 8% 20% 4 1	1800
	11th to 20th	Day	60% 4 10% 17% 8% 1	1795
	21st to end	Day	62% 5% 10% 13% 9% 1	1650
March	1st to 10th	Day	62% 2 9% 15% 10% 3	4479
	11th to 20th	Day	53% 4 17% 16% 9% 1	4925
	21st to end	Day	51% 3 6% 21% 17% 2	5821
April	1st to 10th	Day	55% 4 6% 20% 13% 2	2400
	11th to 20th	Day	52% 4 7% 21% 15% 2	2374
	21st to end	Day	54% 4 8% 17% 14% 3	2518
May	1st to 10th	Day	60% 5% 8% 19% 8% 1	6357
	11th to 20th	Day	54% 4 12% 19% 9% 1	6322
	21st to end	Day	51% 4 12% 22% 11% 1	6949
June	1st to 10th	Day	52% 5% 11% 19% 12% 1	2800
	11th to 20th	Day	43% 5% 12% 26% 12% 2	2800
	21st to end	Day	50% 6% 12% 21% 11% 1	2800
July	1st to 10th	Day	43% 4 13% 27% 12%	4510
	11th to 20th	Day	41% 4 15% 31% 9%	4505
	21st to end	Day	38% 6% 13% 33% 10%	4661
August	1st to 10th	Day	47% 5% 15% 24% 9%	4364
	11th to 20th	Day	43% 4 16% 29% 8%	3990
	21st to end	Day	43% 4 17% 25% 11%	4180
September	1st to 10th	Day	39% 6% 13% 29% 12%	2280
	11th to 20th	Day	37% 5% 12% 33% 14%	2262
	21st to end	Day	43% 5% 14% 27% 10%	2057
October	1st to 10th	Day	47% 4 11% 30% 8%	3160
	11th to 20th	Day	48% 3 16% 26% 7%	3140
	21st to end	Day	49% 3 17% 24% 7%	3145
November	1st to 10th	Day	50% 4 14% 23% 7% 1	1761
	11th to 20th	Day	54% 5% 12% 17% 10% 1	1645
	21st to end	Day	64% 6% 13% 13% 5%	1600
December	1st to 10th	Day	72% 4 12% 10% 1	2600
	11th to 20th	Day	64% 5% 10% 15% 6%	2986
	21st to end	Day	69% 3 8% 16% 4	3960
January	1st to 10th	Night	69% 3 12% 12% 4	6349
	11th to 20th	Night	69% 4 10% 11% 5%	6360
	21st to end	Night	67% 3 8% 11% 10% 1	6840
February	1st to 10th	Night	64% 4 10% 16% 6% 1	2600
	11th to 20th	Night	59% 5% 10% 19% 8%	2501
	21st to end	Night	59% 4 8% 18% 10% 1	1980
March	1st to 10th	Night	60% 3 6% 16% 12% 2	5374
	11th to 20th	Night	51% 4 18% 17% 7% 3	5031
	21st to end	Night	53% 3 5% 24% 13% 2	5110
April	1st to 10th	Night	55% 3 9% 19% 13% 2	2000
	11th to 20th	Night	50% 6% 7% 20% 15% 2	1975
	21st to end	Night	52% 3 9% 17% 16% 3	1900
May	1st to 10th	Night	60% 4 9% 17% 8% 2	3600
	11th to 20th	Night	48% 5% 13% 20% 13% 1	3576
	21st to end	Night	51% 4 13% 18% 13% 1	3927
June	1st to 10th	Night	49% 7% 12% 19% 12% 1	1600
	11th to 20th	Night	41% 5% 13% 22% 17% 2	1600
	21st to end	Night	47% 5% 13% 20% 14% 1	1600
July	1st to 10th	Night	45% 4 15% 21% 14% 1	2560
	11th to 20th	Night	40% 4 14% 33% 9%	2552
	21st to end	Night	40% 6% 15% 24% 15%	2651
August	1st to 10th	Night	44% 4 16% 26% 10%	2525
	11th to 20th	Night	44% 3 16% 29% 8%	2871
	21st to end	Night	41% 4 16% 26% 13% 1	3476
September	1st to 10th	Night	39% 6% 11% 28% 15% 1	1900
	11th to 20th	Night	35% 4 13% 34% 13% 1	1917
	21st to end	Night	41% 5% 12% 29% 13%	2121
October	1st to 10th	Night	43% 4 14% 30% 10%	3800
	11th to 20th	Night	45% 3 15% 28% 9%	3779
	21st to end	Night	50% 3 16% 21% 9%	4401
November	1st to 10th	Night	51% 4 13% 21% 10% 1	2537
	11th to 20th	Night	53% 5% 12% 18% 10% 1	2754
	21st to end	Night	62% 5% 12% 16% 5%	2800
December	1st to 10th	Night	71% 3 12% 12% 3	4580
	11th to 20th	Night	65% 3 8% 15% 9% 1	5208
	21st to end	Night	69% 3 6% 13% 9%	6996

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	23% 16% 36% 20% 7%	200
	11th to 20th	Day	27% 18% 35% 14% 6% 1	200
	21st to end	Day	22% 17% 36% 19% 6%	251
February	1st to 10th	Day	29% 21% 34% 12% 4	300
	11th to 20th	Day	23% 21% 39% 15% 2	333
	21st to end	Day	28% 15% 37% 19% 2	414
March	1st to 10th	Day	24% 14% 35% 20% 7%	500
	11th to 20th	Day	22% 15% 37% 20% 5% 1	500
	21st to end	Day	28% 16% 30% 17% 7% 1	550
April	1st to 10th	Day	27% 18% 32% 17% 6% 1	500
	11th to 20th	Day	28% 20% 25% 17% 6% 3	492
	21st to end	Day	29% 18% 33% 11% 8% 1	500
May	1st to 10th	Day	25% 14% 30% 22% 7% 2	503
	11th to 20th	Day	18% 13% 37% 19% 9% 3	527
	21st to end	Day	20% 15% 29% 22% 13% 2	573
June	1st to 10th	Day	25% 12% 29% 24% 8% 2	530
	11th to 20th	Day	15% 13% 32% 26% 10% 3	530
	21st to end	Day	23% 11% 32% 21% 10% 3	530
July	1st to 10th	Day	14% 8% 28% 33% 15% 3	525
	11th to 20th	Day	13% 8% 29% 32% 15% 3	524
	21st to end	Day	14% 7% 29% 33% 14% 2	532
August	1st to 10th	Day	16% 7% 23% 34% 16% 4	486
	11th to 20th	Day	15% 5% 23% 29% 20% 8%	479
	21st to end	Day	15% 10% 27% 26% 14% 8%	539
September	1st to 10th	Day	14% 9% 29% 26% 13% 10%	490
	11th to 20th	Day	12% 7% 23% 23% 21% 14%	490
	21st to end	Day	19% 9% 30% 20% 17% 4	490
October	1st to 10th	Day	22% 12% 28% 19% 13% 6%	490
	11th to 20th	Day	24% 19% 27% 23% 7% 1	490
	21st to end	Day	31% 15% 26% 22% 5%	449
November	1st to 10th	Day	26% 18% 28% 19% 7% 1	395
	11th to 20th	Day	24% 18% 36% 14% 7% 1	245
	21st to end	Day	27% 23% 31% 13% 7% 1	200
December	1st to 10th	Day	32% 14% 30% 21% 5%	200
	11th to 20th	Day	28% 25% 32% 11% 4 1	200
	21st to end	Day	32% 16% 30% 15% 5% 2	220
January	1st to 10th	Night	27% 14% 32% 24% 4	800
	11th to 20th	Night	29% 13% 30% 23% 5% 1	800
	21st to end	Night	21% 10% 34% 29% 6%	880
February	1st to 10th	Night	21% 15% 37% 24% 4	800
	11th to 20th	Night	21% 15% 37% 23% 4	783
	21st to end	Night	24% 15% 32% 26% 3	660
March	1st to 10th	Night	25% 13% 32% 25% 5%	800
	11th to 20th	Night	24% 16% 33% 23% 5%	800
	21st to end	Night	24% 12% 29% 26% 8%	880
April	1st to 10th	Night	27% 12% 29% 28% 4	800
	11th to 20th	Night	25% 14% 31% 22% 8%	791
	21st to end	Night	27% 11% 28% 25% 8% 1	700
May	1st to 10th	Night	27% 8% 28% 27% 9% 2	400
	11th to 20th	Night	19% 7% 34% 31% 8%	394
	21st to end	Night	15% 7% 38% 28% 12%	433
June	1st to 10th	Night	18% 7% 30% 36% 9%	400
	11th to 20th	Night	18% 7% 30% 32% 12% 1	400
	21st to end	Night	19% 5% 31% 31% 13% 1	400
July	1st to 10th	Night	11% 6% 27% 43% 13%	390
	11th to 20th	Night	16% 3 21% 41% 17% 1	388
	21st to end	Night	12% 3 25% 49% 11% 1	416
August	1st to 10th	Night	15% 3 22% 41% 18% 1	394
	11th to 20th	Night	15% 2 19% 38% 25% 2	597
	21st to end	Night	15% 5% 26% 34% 18% 2	836
September	1st to 10th	Night	15% 7% 25% 35% 18% 1	760
	11th to 20th	Night	16% 5% 19% 32% 23% 4	760
	21st to end	Night	17% 7% 26% 28% 21% 2	760
October	1st to 10th	Night	21% 8% 29% 26% 15% 2	760
	11th to 20th	Night	24% 11% 30% 26% 9% 1	760
	21st to end	Night	28% 12% 27% 25% 7%	836
November	1st to 10th	Night	26% 14% 26% 26% 7% 1	780
	11th to 20th	Night	25% 14% 31% 21% 9%	800
	21st to end	Night	28% 15% 28% 22% 7%	800
December	1st to 10th	Night	30% 13% 29% 24% 4	800
	11th to 20th	Night	29% 14% 31% 21% 5% 1	800
	21st to end	Night	33% 12% 29% 23% 4	880

Page generated at 2025-07-18 01:10:05. Version ac07

Copyright © 2025 Attilla Danko.