Windsor Castle Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Windsor Castle Park clear sky chart from 2005-11-06 to 2025-12-03.

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% 6% 2 6% 4 4 5% 4 6% 8% 42%	126619
	Night	13% 6% 2 6% 4 4 4 4 5% 7% 44%	124080
Transparency	Day	39% 4 12% 23% 17% 4	130212
	Night	40% 4 10% 22% 19% 5%	128556
Seeing	Day	17% 11% 27% 25% 15% 5%	16656
	Night	17% 9% 28% 31% 13% 2	26266

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	14% 4 1 4 3 3 3 3 3 4 56%	3873
	11th to 20th	Day	11% 7% 2 5% 3 3 3 3 4 5% 54%	4060
	21st to end	Day	11% 8% 3 7% 4 4 5% 3 5% 5% 45%	4466
February	1st to 10th	Day	12% 9% 3 6% 4 5% 5% 4 6% 7% 40%	1800
	11th to 20th	Day	20% 6% 2 5% 4 3 3 3 4 6% 44%	1890
	21st to end	Day	15% 8% 2 6% 4 4 5% 4 6% 6% 40%	1650
March	1st to 10th	Day	19% 8% 3 6% 4 4 4 3 4 6% 40%	4817
	11th to 20th	Day	10% 7% 2 5% 4 3 4 2 4 6% 52%	4980
	21st to end	Day	13% 6% 2 5% 4 4 4 3 5% 7% 46%	5597
April	1st to 10th	Day	12% 6% 2 5% 4 3 4 3 5% 7% 49%	2400
	11th to 20th	Day	9% 6% 1 5% 4 4 5% 4 5% 7% 48%	2374
	21st to end	Day	13% 7% 2 6% 5% 4 4 4 6% 6% 44%	2400
May	1st to 10th	Day	11% 8% 3 8% 5% 5% 5% 4 7% 9% 34%	5444
	11th to 20th	Day	17% 8% 3 7% 5% 5% 5% 4 6% 9% 32%	6031
	21st to end	Day	13% 6% 2 6% 4 4 4 4 6% 8% 43%	6934
June	1st to 10th	Day	10% 5% 2 5% 4 5% 5% 4 8% 10% 42%	2800
	11th to 20th	Day	10% 6% 2 6% 4 5% 4 5% 8% 11% 40%	2800
	21st to end	Day	6% 5% 3 5% 6% 5% 6% 5% 8% 12% 40%	2800
July	1st to 10th	Day	7% 5% 2 7% 5% 5% 6% 6% 8% 12% 38%	6360
	11th to 20th	Day	9% 7% 3 9% 6% 6% 6% 5% 8% 11% 29%	5952
	21st to end	Day	5% 5% 3 7% 6% 6% 7% 6% 9% 12% 34%	6424
August	1st to 10th	Day	14% 9% 4 9% 6% 6% 6% 5% 8% 10% 24%	4234
	11th to 20th	Day	9% 6% 3 8% 6% 6% 6% 5% 7% 9% 34%	3850
	21st to end	Day	7% 5% 2 6% 4 5% 5% 5% 8% 11% 43%	4312
September	1st to 10th	Day	14% 5% 2 6% 4 4 4 4 7% 11% 39%	2400
	11th to 20th	Day	15% 7% 2 7% 5% 4 5% 4 5% 6% 41%	2400
	21st to end	Day	21% 8% 2 6% 4 5% 4 3 5% 6% 36%	2129
October	1st to 10th	Day	16% 6% 2 6% 4 3 4 3 5% 6% 46%	2000
	11th to 20th	Day	15% 6% 2 5% 4 3 3 3 5% 5% 51%	2000
	21st to end	Day	12% 5% 2 5% 4 4 4 3 4 5% 52%	2200
November	1st to 10th	Day	14% 5% 2 5% 3 3 4 3 4 6% 51%	1982
	11th to 20th	Day	13% 7% 2 5% 3 3 2 2 4 6% 52%	1890
	21st to end	Day	14% 4 2 6% 4 3 4 3 3 6% 52%	1804
December	1st to 10th	Day	11% 5% 2 6% 4 3 3 3 5% 5% 54%	2616
	11th to 20th	Day	14% 8% 3 6% 4 3 3 3 5% 5% 46%	2990
	21st to end	Day	12% 6% 2 5% 4 3 3 3 5% 7% 49%	3960
January	1st to 10th	Night	12% 6% 3 6% 3 3 3 3 4 5% 52%	6055
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 4 5% 53%	5900
	21st to end	Night	14% 7% 2 6% 3 3 3 3 4 6% 49%	6490
February	1st to 10th	Night	16% 7% 3 5% 4 4 3 3 5% 7% 43%	2600
	11th to 20th	Night	21% 8% 2 5% 4 3 4 4 5% 6% 40%	2393
	21st to end	Night	15% 8% 2 5% 4 3 3 3 4 5% 47%	1980
March	1st to 10th	Night	18% 7% 3 6% 4 4 3 3 4 5% 43%	5020
	11th to 20th	Night	12% 5% 2 6% 4 3 4 3 4 6% 49%	4980
	21st to end	Night	14% 7% 2 5% 4 4 4 3 5% 6% 47%	5333
April	1st to 10th	Night	12% 7% 2 6% 4 5% 5% 4 6% 7% 43%	2000
	11th to 20th	Night	12% 6% 3 5% 4 3 3 3 5% 7% 50%	1975
	21st to end	Night	13% 6% 2 4 5% 4 4 3 5% 7% 46%	2000
May	1st to 10th	Night	12% 7% 3 9% 5% 6% 6% 4 6% 9% 32%	4490
	11th to 20th	Night	19% 8% 3 7% 5% 4 4 3 5% 7% 33%	3862
	21st to end	Night	14% 8% 3 7% 4 4 4 3 6% 8% 38%	3920
June	1st to 10th	Night	11% 5% 2 6% 4 4 5% 5% 8% 11% 41%	1600
	11th to 20th	Night	9% 6% 2 7% 5% 4 5% 5% 7% 11% 40%	1600
	21st to end	Night	6% 5% 2 6% 6% 5% 5% 5% 8% 9% 43%	1600
July	1st to 10th	Night	7% 5% 2 7% 6% 5% 6% 6% 7% 13% 36%	3598
	11th to 20th	Night	9% 7% 3 8% 5% 6% 6% 6% 9% 11% 30%	4000
	21st to end	Night	9% 7% 3 8% 5% 5% 5% 5% 7% 9% 35%	4427
August	1st to 10th	Night	17% 8% 3 8% 4 5% 5% 5% 7% 10% 28%	2915
	11th to 20th	Night	8% 8% 2 8% 5% 5% 5% 4 6% 9% 39%	3163
	21st to end	Night	8% 5% 2 6% 4 4 4 4 7% 10% 45%	3586
September	1st to 10th	Night	12% 5% 2 6% 4 4 3 4 6% 9% 44%	2000
	11th to 20th	Night	13% 6% 2 8% 4 4 5% 4 6% 6% 43%	2000
	21st to end	Night	20% 7% 2 7% 5% 4 4 3 5% 7% 35%	2265
October	1st to 10th	Night	15% 5% 2 6% 4 4 4 2 5% 6% 48%	2400
	11th to 20th	Night	16% 5% 2 5% 3 3 3 3 4 5% 51%	2400
	21st to end	Night	14% 5% 2 5% 3 3 3 3 5% 5% 52%	2640
November	1st to 10th	Night	14% 7% 2 5% 3 3 3 3 5% 5% 51%	2535
	11th to 20th	Night	15% 6% 2 5% 3 3 3 3 5% 7% 48%	2730
	21st to end	Night	11% 5% 2 5% 3 4 4 3 5% 7% 50%	2813
December	1st to 10th	Night	10% 4 2 6% 3 4 4 3 5% 6% 53%	4609
	11th to 20th	Night	16% 8% 3 7% 5% 5% 5% 3 4 6% 39%	5205
	21st to end	Night	11% 6% 2 6% 4 4 4 4 5% 6% 49%	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	34% 2 3 25% 22% 14%	3874
	11th to 20th	Day	35% 2 4 19% 32% 8%	4060
	21st to end	Day	42% 2 7% 27% 18% 3	4466
February	1st to 10th	Day	43% 3 10% 25% 15% 4	1800
	11th to 20th	Day	44% 3 6% 21% 20% 6%	1899
	21st to end	Day	45% 2 10% 18% 19% 5%	1650
March	1st to 10th	Day	47% 2 7% 19% 18% 8%	4817
	11th to 20th	Day	35% 2 8% 21% 25% 9%	4980
	21st to end	Day	39% 3 14% 19% 20% 5%	5597
April	1st to 10th	Day	36% 3 11% 22% 21% 7%	2400
	11th to 20th	Day	34% 2 12% 26% 20% 6%	2374
	21st to end	Day	41% 4 11% 23% 17% 5%	2400
May	1st to 10th	Day	46% 5% 17% 19% 12% 1	5476
	11th to 20th	Day	50% 4 13% 22% 10% 1	6035
	21st to end	Day	40% 4 14% 25% 16% 2	6949
June	1st to 10th	Day	36% 6% 15% 26% 16% 1	2800
	11th to 20th	Day	37% 5% 16% 25% 16% 2	2800
	21st to end	Day	35% 6% 16% 27% 15% 2	2800
July	1st to 10th	Day	37% 9% 20% 21% 11% 3	6290
	11th to 20th	Day	47% 10% 23% 16% 4 1	5881
	21st to end	Day	38% 9% 15% 22% 14% 1	6240
August	1st to 10th	Day	53% 6% 16% 19% 5%	4221
	11th to 20th	Day	43% 7% 15% 23% 12%	3886
	21st to end	Day	33% 5% 14% 28% 17% 2	4312
September	1st to 10th	Day	40% 6% 15% 18% 17% 4	2400
	11th to 20th	Day	44% 4 9% 22% 19% 2	2400
	21st to end	Day	50% 4 8% 20% 16% 2	2129
October	1st to 10th	Day	32% 4 10% 24% 26% 4	3260
	11th to 20th	Day	26% 2 5% 20% 38% 9%	3240
	21st to end	Day	24% 2 7% 27% 27% 13%	3538
November	1st to 10th	Day	36% 3 8% 28% 21% 4	1982
	11th to 20th	Day	35% 2 10% 24% 23% 6%	1890
	21st to end	Day	36% 3 7% 27% 22% 5%	1804
December	1st to 10th	Day	34% 2 8% 28% 23% 5%	2616
	11th to 20th	Day	42% 3 12% 21% 17% 5%	2986
	21st to end	Day	36% 2 13% 29% 19% 1	3960
January	1st to 10th	Night	36% 1 5% 21% 27% 10%	6055
	11th to 20th	Night	36% 2 5% 21% 30% 6%	5900
	21st to end	Night	39% 2 6% 27% 21% 5%	6490
February	1st to 10th	Night	43% 2 10% 24% 17% 4	2600
	11th to 20th	Night	47% 2 6% 21% 19% 6%	2395
	21st to end	Night	41% 2 7% 22% 19% 8%	1980
March	1st to 10th	Night	45% 3 6% 19% 19% 8%	5020
	11th to 20th	Night	38% 3 8% 18% 23% 11%	4980
	21st to end	Night	39% 3 9% 25% 17% 7%	5333
April	1st to 10th	Night	40% 4 11% 17% 23% 5%	2000
	11th to 20th	Night	36% 2 10% 25% 21% 6%	1975
	21st to end	Night	38% 3 11% 26% 18% 4	2000
May	1st to 10th	Night	49% 5% 15% 19% 11% 1	4520
	11th to 20th	Night	51% 5% 11% 24% 9% 1	3863
	21st to end	Night	45% 4 14% 24% 12% 1	3927
June	1st to 10th	Night	36% 7% 15% 25% 16% 1	1600
	11th to 20th	Night	39% 6% 18% 22% 15% 1	1600
	21st to end	Night	35% 6% 14% 26% 16% 3	1600
July	1st to 10th	Night	39% 10% 18% 21% 11% 2	3560
	11th to 20th	Night	45% 12% 22% 16% 4 1	3951
	21st to end	Night	43% 8% 13% 21% 15%	4297
August	1st to 10th	Night	51% 6% 19% 20% 5%	2904
	11th to 20th	Night	41% 6% 18% 22% 13%	3193
	21st to end	Night	33% 5% 14% 26% 21% 2	3586
September	1st to 10th	Night	37% 6% 16% 22% 16% 4	2000
	11th to 20th	Night	41% 4 9% 23% 20% 3	2000
	21st to end	Night	50% 4 10% 17% 17% 2	2265
October	1st to 10th	Night	38% 4 9% 23% 24% 3	3920
	11th to 20th	Night	27% 3 5% 20% 33% 12%	3899
	21st to end	Night	23% 2 6% 30% 25% 14%	4252
November	1st to 10th	Night	37% 3 8% 22% 26% 4	2535
	11th to 20th	Night	38% 2 9% 21% 21% 9%	2730
	21st to end	Night	36% 2 8% 26% 23% 5%	2813
December	1st to 10th	Night	33% 3 7% 26% 24% 6%	4609
	11th to 20th	Night	48% 3 10% 17% 16% 6%	5208
	21st to end	Night	37% 3 13% 28% 18% 1	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	25% 23% 37% 10% 5%	260
	11th to 20th	Day	24% 28% 33% 14% 1	300
	21st to end	Day	21% 23% 40% 13% 2	330
February	1st to 10th	Day	21% 24% 40% 11% 4	300
	11th to 20th	Day	29% 24% 31% 13% 2	438
	21st to end	Day	25% 19% 35% 19% 3	414
March	1st to 10th	Day	21% 20% 36% 18% 4 1	500
	11th to 20th	Day	23% 19% 32% 21% 4 1	500
	21st to end	Day	22% 18% 37% 16% 5% 1	550
April	1st to 10th	Day	22% 16% 40% 15% 6% 1	500
	11th to 20th	Day	14% 17% 37% 19% 9% 3	492
	21st to end	Day	17% 14% 34% 25% 9% 2	500
May	1st to 10th	Day	15% 16% 37% 21% 9% 3	503
	11th to 20th	Day	19% 9% 32% 25% 13% 2	527
	21st to end	Day	18% 6% 24% 25% 20% 6%	573
June	1st to 10th	Day	12% 5% 22% 31% 25% 4	530
	11th to 20th	Day	11% 6% 20% 34% 20% 8%	530
	21st to end	Day	9% 5% 18% 34% 26% 8%	530
July	1st to 10th	Day	9% 4 12% 34% 32% 8%	525
	11th to 20th	Day	7% 2 12% 38% 29% 12%	524
	21st to end	Day	7% 2 18% 37% 26% 11%	576
August	1st to 10th	Day	14% 4 19% 32% 25% 8%	526
	11th to 20th	Day	12% 4 14% 29% 30% 11%	519
	21st to end	Day	10% 3 16% 32% 25% 13%	583
September	1st to 10th	Day	15% 3 19% 29% 20% 14%	530
	11th to 20th	Day	17% 5% 18% 30% 18% 12%	530
	21st to end	Day	28% 7% 20% 25% 12% 7%	530
October	1st to 10th	Day	19% 5% 19% 25% 22% 9%	530
	11th to 20th	Day	16% 12% 27% 25% 17% 2	530
	21st to end	Day	15% 12% 31% 25% 12% 5%	583
November	1st to 10th	Day	17% 9% 32% 23% 14% 3	495
	11th to 20th	Day	17% 17% 31% 25% 9% 1	430
	21st to end	Day	18% 17% 27% 24% 12% 2	344
December	1st to 10th	Day	20% 21% 38% 15% 5% 1	204
	11th to 20th	Day	22% 15% 37% 21% 6%	200
	21st to end	Day	22% 17% 33% 18% 9% 1	220
January	1st to 10th	Night	18% 17% 38% 24% 4	800
	11th to 20th	Night	21% 13% 37% 25% 5%	800
	21st to end	Night	19% 13% 40% 27% 1	880
February	1st to 10th	Night	20% 12% 41% 22% 4	800
	11th to 20th	Night	25% 15% 40% 18% 1	783
	21st to end	Night	20% 17% 38% 23% 3	660
March	1st to 10th	Night	17% 15% 36% 27% 5%	800
	11th to 20th	Night	21% 15% 36% 23% 6%	800
	21st to end	Night	19% 14% 39% 23% 6%	880
April	1st to 10th	Night	18% 14% 41% 22% 4	800
	11th to 20th	Night	15% 11% 38% 25% 9% 1	791
	21st to end	Night	16% 9% 36% 33% 6% 1	800
May	1st to 10th	Night	16% 10% 35% 32% 7% 1	800
	11th to 20th	Night	17% 5% 29% 38% 10% 1	525
	21st to end	Night	14% 3 17% 44% 19% 2	433
June	1st to 10th	Night	12% 5% 19% 45% 18% 2	400
	11th to 20th	Night	10% 3 18% 46% 22% 2	400
	21st to end	Night	7% 4 17% 47% 25% 1	400
July	1st to 10th	Night	9% 2 14% 48% 24% 2	390
	11th to 20th	Night	7% 2 8% 45% 31% 6%	553
	21st to end	Night	8% 4 14% 44% 27% 3	657
August	1st to 10th	Night	12% 2 12% 42% 28% 5%	597
	11th to 20th	Night	12% 3 10% 44% 27% 5%	768
	21st to end	Night	11% 3 12% 37% 31% 6%	880
September	1st to 10th	Night	14% 3 14% 39% 26% 5%	800
	11th to 20th	Night	16% 4 14% 34% 28% 4	800
	21st to end	Night	24% 5% 17% 33% 19% 2	800
October	1st to 10th	Night	17% 5% 18% 31% 24% 5%	800
	11th to 20th	Night	19% 8% 25% 31% 15% 2	800
	21st to end	Night	16% 6% 28% 32% 16% 2	880
November	1st to 10th	Night	18% 6% 28% 30% 16% 2	820
	11th to 20th	Night	18% 10% 31% 30% 9% 1	840
	21st to end	Night	15% 9% 32% 32% 11% 1	840
December	1st to 10th	Night	20% 11% 35% 26% 7% 1	809
	11th to 20th	Night	19% 9% 36% 29% 7%	800
	21st to end	Night	17% 11% 33% 32% 6%	880

Page generated at 2025-11-30 01:00:57. Version ac07

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