Windsor
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Windsor clear sky chart from 2005-11-06 to 2025-09-04.

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	14% 9% 3 8% 5% 5% 5% 4 6% 7% 34%	125553
	Night	14% 10% 4 8% 5% 4 4 3 5% 6% 35%	123246
Transparency	Day	46% 6% 13% 21% 12% 1	129137
	Night	48% 5% 12% 21% 13% 1	127731
Seeing	Day	16% 9% 28% 27% 15% 6%	16440
	Night	18% 8% 28% 30% 13% 2	26079

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% 15% 7% 16% 5% 5% 4 3 4 5% 24%	3617
	11th to 20th	Day	14% 15% 5% 16% 8% 7% 7% 4 5% 6% 12%	3725
	21st to end	Day	22% 15% 6% 13% 6% 5% 5% 3 5% 5% 15%	4466
February	1st to 10th	Day	15% 15% 4 11% 8% 6% 5% 4 5% 6% 22%	1800
	11th to 20th	Day	13% 13% 5% 12% 7% 6% 5% 4 6% 6% 23%	1871
	21st to end	Day	17% 12% 4 11% 5% 5% 4 3 6% 7% 27%	1650
March	1st to 10th	Day	16% 12% 5% 11% 5% 4 4 3 4 5% 30%	4652
	11th to 20th	Day	12% 12% 3 8% 5% 5% 5% 3 6% 7% 35%	4980
	21st to end	Day	17% 14% 4 9% 6% 4 4 3 5% 6% 28%	5465
April	1st to 10th	Day	16% 12% 3 6% 5% 5% 5% 4 6% 7% 32%	2290
	11th to 20th	Day	17% 9% 2 8% 4 5% 5% 4 6% 6% 35%	2556
	21st to end	Day	18% 9% 3 7% 5% 4 4 4 5% 7% 35%	2600
May	1st to 10th	Day	19% 10% 3 8% 5% 4 4 4 4 6% 34%	5862
	11th to 20th	Day	18% 7% 2 6% 5% 5% 5% 4 6% 8% 34%	5861
	21st to end	Day	15% 6% 2 6% 4 4 4 4 6% 7% 42%	6433
June	1st to 10th	Day	10% 6% 3 6% 5% 5% 5% 5% 7% 8% 41%	2831
	11th to 20th	Day	10% 6% 2 7% 5% 5% 5% 5% 6% 8% 41%	3000
	21st to end	Day	9% 5% 3 5% 5% 5% 5% 5% 8% 10% 39%	2874
July	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 4 7% 9% 43%	6360
	11th to 20th	Day	7% 4 2 6% 5% 5% 6% 5% 7% 10% 45%	6320
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 7% 9% 45%	6424
August	1st to 10th	Day	13% 4 2 5% 4 5% 5% 4 6% 9% 43%	4234
	11th to 20th	Day	10% 5% 2 6% 4 6% 6% 6% 8% 11% 35%	4174
	21st to end	Day	9% 5% 2 7% 5% 6% 5% 5% 7% 9% 41%	4675
September	1st to 10th	Day	9% 6% 2 7% 4 5% 5% 4 6% 7% 45%	2161
	11th to 20th	Day	9% 4 2 6% 4 4 5% 5% 6% 9% 48%	2090
	21st to end	Day	16% 6% 3 6% 5% 3 5% 4 5% 7% 40%	2090
October	1st to 10th	Day	12% 5% 3 6% 5% 4 5% 4 6% 8% 43%	2019
	11th to 20th	Day	16% 9% 2 7% 5% 5% 4 3 5% 8% 36%	1900
	21st to end	Day	18% 10% 3 7% 5% 5% 5% 4 5% 6% 31%	1971
November	1st to 10th	Day	11% 8% 3 7% 5% 4 5% 4 6% 7% 40%	1762
	11th to 20th	Day	14% 11% 3 7% 5% 6% 5% 4 6% 6% 33%	1800
	21st to end	Day	17% 13% 4 8% 6% 5% 5% 3 6% 6% 27%	1660
December	1st to 10th	Day	13% 13% 6% 13% 5% 5% 5% 4 6% 6% 23%	2600
	11th to 20th	Day	21% 18% 6% 7% 5% 4 5% 4 5% 6% 20%	2922
	21st to end	Day	27% 18% 7% 12% 5% 5% 3 2 3 5% 13%	3858
January	1st to 10th	Night	13% 15% 6% 11% 5% 4 5% 3 5% 6% 27%	6348
	11th to 20th	Night	11% 16% 8% 13% 7% 6% 5% 4 6% 6% 18%	6247
	21st to end	Night	19% 16% 7% 10% 5% 4 4 3 3 4 25%	6490
February	1st to 10th	Night	16% 16% 5% 10% 7% 5% 5% 3 5% 5% 24%	2600
	11th to 20th	Night	15% 13% 4 11% 6% 5% 5% 4 5% 6% 27%	2411
	21st to end	Night	18% 12% 4 9% 5% 4 5% 3 4 5% 30%	1980
March	1st to 10th	Night	16% 11% 4 7% 4 4 4 3 5% 6% 35%	5200
	11th to 20th	Night	9% 8% 3 6% 5% 4 5% 4 6% 8% 41%	4980
	21st to end	Night	15% 9% 3 8% 4 5% 5% 3 4 6% 37%	5467
April	1st to 10th	Night	17% 10% 3 6% 5% 5% 4 3 5% 7% 36%	2128
	11th to 20th	Night	17% 8% 2 6% 4 4 4 3 6% 8% 37%	1792
	21st to end	Night	16% 8% 3 6% 4 4 4 4 6% 6% 39%	1800
May	1st to 10th	Night	21% 9% 3 8% 5% 5% 4 3 5% 6% 32%	4033
	11th to 20th	Night	16% 7% 3 7% 5% 4 5% 3 5% 7% 38%	4032
	21st to end	Night	14% 6% 2 5% 4 4 4 3 5% 7% 44%	4421
June	1st to 10th	Night	12% 7% 2 5% 4 3 4 3 6% 8% 47%	1553
	11th to 20th	Night	10% 5% 3 6% 5% 5% 5% 4 7% 9% 43%	1400
	21st to end	Night	10% 4 2 6% 4 3 5% 3 6% 8% 48%	1515
July	1st to 10th	Night	8% 4 2 5% 4 4 5% 4 6% 10% 49%	3618
	11th to 20th	Night	10% 4 2 5% 4 4 5% 4 7% 9% 47%	3651
	21st to end	Night	8% 4 2 6% 4 4 5% 3 6% 8% 50%	4427
August	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 5% 7% 47%	2915
	11th to 20th	Night	9% 5% 2 6% 4 4 5% 4 5% 7% 49%	2875
	21st to end	Night	8% 4 2 7% 5% 4 6% 4 6% 8% 48%	3223
September	1st to 10th	Night	10% 5% 2 6% 5% 4 4 4 5% 6% 49%	2060
	11th to 20th	Night	10% 4 1 5% 3 3 3 4 5% 6% 55%	2090
	21st to end	Night	14% 8% 3 7% 4 4 3 4 5% 6% 42%	2090
October	1st to 10th	Night	12% 6% 2 6% 5% 4 3 3 4 7% 47%	2160
	11th to 20th	Night	17% 8% 2 8% 5% 4 4 3 6% 6% 38%	2280
	21st to end	Night	19% 9% 3 7% 5% 5% 4 4 4 6% 34%	2626
November	1st to 10th	Night	12% 7% 3 7% 4 4 4 4 5% 6% 44%	2536
	11th to 20th	Night	16% 10% 3 8% 5% 5% 4 5% 6% 6% 32%	2600
	21st to end	Night	19% 13% 4 8% 4 4 4 3 5% 7% 28%	2736
December	1st to 10th	Night	13% 13% 6% 11% 7% 5% 5% 4 5% 6% 25%	4580
	11th to 20th	Night	16% 18% 6% 10% 5% 5% 5% 3 4 6% 22%	5269
	21st to end	Night	27% 19% 5% 10% 5% 4 3 3 4 4 16%	7113

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	60% 7% 10% 15% 6% 1	3618
	11th to 20th	Day	68% 5% 11% 8% 7%	3725
	21st to end	Day	69% 5% 10% 10% 6%	4466
February	1st to 10th	Day	60% 6% 10% 15% 7% 1	1800
	11th to 20th	Day	59% 6% 10% 17% 7% 1	1880
	21st to end	Day	56% 5% 12% 18% 7% 1	1650
March	1st to 10th	Day	54% 6% 15% 17% 8% 1	4652
	11th to 20th	Day	46% 6% 16% 22% 9% 1	4980
	21st to end	Day	55% 5% 12% 16% 11% 1	5465
April	1st to 10th	Day	50% 6% 13% 18% 12% 1	2290
	11th to 20th	Day	47% 5% 12% 20% 13% 2	2556
	21st to end	Day	47% 6% 13% 20% 13% 2	2600
May	1st to 10th	Day	50% 5% 10% 19% 12% 3	5897
	11th to 20th	Day	45% 8% 13% 22% 11% 2	5865
	21st to end	Day	40% 6% 11% 21% 17% 5%	6447
June	1st to 10th	Day	37% 7% 13% 27% 14% 2	2831
	11th to 20th	Day	38% 7% 14% 22% 16% 3	3000
	21st to end	Day	35% 9% 14% 23% 17% 2	2874
July	1st to 10th	Day	35% 6% 13% 27% 18% 2	6291
	11th to 20th	Day	30% 7% 15% 29% 17% 1	6245
	21st to end	Day	34% 5% 15% 22% 21% 2	6240
August	1st to 10th	Day	35% 7% 15% 28% 14% 1	4221
	11th to 20th	Day	36% 10% 14% 31% 8% 1	4213
	21st to end	Day	35% 7% 16% 28% 14% 1	4674
September	1st to 10th	Day	36% 6% 16% 25% 15% 2	2161
	11th to 20th	Day	31% 6% 14% 33% 16% 1	2090
	21st to end	Day	43% 5% 14% 25% 13%	2090
October	1st to 10th	Day	29% 5% 14% 31% 20% 1	3357
	11th to 20th	Day	38% 6% 13% 25% 18%	3140
	21st to end	Day	43% 5% 15% 23% 13% 1	3221
November	1st to 10th	Day	43% 6% 12% 25% 12% 1	1762
	11th to 20th	Day	49% 6% 15% 20% 9% 1	1800
	21st to end	Day	57% 6% 12% 17% 7% 1	1660
December	1st to 10th	Day	60% 6% 17% 13% 4	2600
	11th to 20th	Day	64% 6% 11% 13% 6%	2918
	21st to end	Day	75% 5% 7% 8% 5%	3858
January	1st to 10th	Night	57% 6% 10% 19% 7% 1	6348
	11th to 20th	Night	64% 6% 11% 13% 6%	6247
	21st to end	Night	62% 4 11% 13% 10%	6490
February	1st to 10th	Night	62% 6% 11% 15% 6% 1	2600
	11th to 20th	Night	56% 5% 10% 19% 9% 1	2413
	21st to end	Night	55% 5% 12% 18% 8% 1	1980
March	1st to 10th	Night	49% 5% 14% 22% 10% 1	5200
	11th to 20th	Night	38% 6% 14% 25% 15% 1	4980
	21st to end	Night	48% 5% 10% 19% 16% 1	5467
April	1st to 10th	Night	49% 4 10% 21% 13% 2	2128
	11th to 20th	Night	45% 5% 12% 19% 17% 2	1792
	21st to end	Night	44% 5% 10% 20% 17% 4	1800
May	1st to 10th	Night	53% 5% 9% 15% 15% 3	4060
	11th to 20th	Night	45% 6% 10% 27% 10% 3	4033
	21st to end	Night	39% 6% 12% 23% 17% 4	4429
June	1st to 10th	Night	35% 5% 11% 28% 19% 2	1553
	11th to 20th	Night	36% 7% 14% 25% 16% 2	1400
	21st to end	Night	34% 5% 13% 26% 22% 1	1515
July	1st to 10th	Night	31% 5% 15% 29% 16% 5%	3580
	11th to 20th	Night	33% 5% 15% 28% 17% 1	3606
	21st to end	Night	31% 6% 15% 22% 24% 2	4297
August	1st to 10th	Night	35% 6% 14% 25% 18% 2	2904
	11th to 20th	Night	34% 6% 11% 33% 13% 1	2902
	21st to end	Night	33% 7% 13% 31% 15% 1	3223
September	1st to 10th	Night	34% 7% 12% 27% 18% 2	2060
	11th to 20th	Night	29% 6% 10% 31% 21% 2	2090
	21st to end	Night	41% 6% 12% 25% 14% 1	2090
October	1st to 10th	Night	26% 4 9% 33% 27% 1	3602
	11th to 20th	Night	41% 4 11% 23% 19% 1	3779
	21st to end	Night	41% 4 11% 28% 15% 1	4326
November	1st to 10th	Night	40% 6% 12% 26% 15% 1	2536
	11th to 20th	Night	50% 6% 12% 21% 11% 1	2600
	21st to end	Night	57% 5% 13% 19% 6%	2736
December	1st to 10th	Night	59% 5% 17% 15% 4	4580
	11th to 20th	Night	62% 6% 12% 13% 6% 1	5272
	21st to end	Night	72% 5% 9% 9% 5%	7113

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	19% 16% 39% 19% 7% 1	300
	11th to 20th	Day	19% 14% 43% 19% 4 1	300
	21st to end	Day	23% 13% 34% 27% 4	330
February	1st to 10th	Day	22% 14% 41% 20% 2 1	300
	11th to 20th	Day	19% 18% 35% 26% 2	300
	21st to end	Day	20% 17% 45% 14% 4	249
March	1st to 10th	Day	20% 15% 37% 23% 5%	368
	11th to 20th	Day	19% 12% 33% 28% 7% 1	500
	21st to end	Day	23% 14% 32% 23% 8% 1	550
April	1st to 10th	Day	19% 12% 33% 28% 9% 1	500
	11th to 20th	Day	22% 15% 30% 19% 9% 4	492
	21st to end	Day	22% 8% 33% 24% 11% 2	500
May	1st to 10th	Day	21% 10% 29% 27% 10% 4	503
	11th to 20th	Day	19% 7% 26% 26% 16% 6%	527
	21st to end	Day	14% 5% 23% 29% 21% 8%	573
June	1st to 10th	Day	11% 6% 28% 34% 16% 5%	646
	11th to 20th	Day	11% 5% 28% 34% 18% 3	730
	21st to end	Day	9% 5% 29% 33% 18% 5%	730
July	1st to 10th	Day	9% 6% 19% 32% 25% 11%	721
	11th to 20th	Day	11% 3 23% 30% 23% 10%	704
	21st to end	Day	8% 3 18% 33% 26% 11%	576
August	1st to 10th	Day	8% 4 15% 35% 23% 15%	526
	11th to 20th	Day	11% 2 14% 35% 26% 11%	519
	21st to end	Day	10% 4 17% 31% 25% 14%	583
September	1st to 10th	Day	11% 3 22% 26% 24% 13%	502
	11th to 20th	Day	9% 2 16% 27% 32% 14%	490
	21st to end	Day	19% 7% 21% 24% 19% 9%	490
October	1st to 10th	Day	11% 8% 27% 24% 20% 10%	490
	11th to 20th	Day	18% 14% 28% 28% 9% 3	490
	21st to end	Day	24% 11% 33% 24% 6% 1	420
November	1st to 10th	Day	15% 12% 37% 21% 11% 3	300
	11th to 20th	Day	18% 20% 32% 23% 6% 1	300
	21st to end	Day	22% 13% 40% 21% 4	230
December	1st to 10th	Day	20% 18% 35% 20% 7% 2	200
	11th to 20th	Day	27% 14% 40% 17% 2 1	200
	21st to end	Day	26% 17% 34% 20% 4	301
January	1st to 10th	Night	22% 13% 37% 22% 6% 1	1000
	11th to 20th	Night	25% 12% 34% 24% 4	952
	21st to end	Night	19% 12% 35% 28% 6%	880
February	1st to 10th	Night	23% 12% 36% 26% 4	800
	11th to 20th	Night	20% 11% 35% 29% 5%	783
	21st to end	Night	24% 12% 38% 23% 3	660
March	1st to 10th	Night	21% 10% 34% 29% 7%	800
	11th to 20th	Night	17% 12% 36% 29% 6% 1	800
	21st to end	Night	21% 11% 30% 30% 8% 1	880
April	1st to 10th	Night	20% 12% 32% 30% 5%	728
	11th to 20th	Night	19% 10% 33% 28% 9% 1	593
	21st to end	Night	20% 7% 31% 33% 8%	600
May	1st to 10th	Night	20% 5% 30% 32% 11% 1	600
	11th to 20th	Night	20% 3 26% 34% 15% 2	591
	21st to end	Night	12% 3 21% 40% 21% 3	650
June	1st to 10th	Night	13% 3 24% 44% 16% 1	475
	11th to 20th	Night	12% 3 20% 48% 17% 1	400
	21st to end	Night	9% 3 22% 44% 20% 2	515
July	1st to 10th	Night	8% 3 15% 41% 29% 5%	585
	11th to 20th	Night	9% 3 16% 41% 27% 5%	582
	21st to end	Night	6% 2 13% 49% 26% 4	657
August	1st to 10th	Night	8% 2 15% 41% 30% 5%	597
	11th to 20th	Night	10% 3 14% 36% 31% 5%	594
	21st to end	Night	9% 3 16% 37% 31% 5%	660
September	1st to 10th	Night	12% 6% 16% 34% 26% 6%	716
	11th to 20th	Night	9% 4 16% 33% 31% 5%	760
	21st to end	Night	17% 7% 20% 31% 21% 5%	760
October	1st to 10th	Night	14% 6% 26% 31% 19% 4	760
	11th to 20th	Night	21% 12% 29% 25% 12% 1	760
	21st to end	Night	25% 10% 30% 28% 6%	836
November	1st to 10th	Night	16% 10% 33% 25% 15% 1	780
	11th to 20th	Night	22% 13% 35% 22% 8% 1	800
	21st to end	Night	26% 11% 35% 22% 6%	800
December	1st to 10th	Night	24% 14% 35% 21% 5% 1	800
	11th to 20th	Night	23% 13% 36% 23% 4	825
	21st to end	Night	29% 9% 36% 22% 3	1100

Page generated at 2025-09-01 01:51:02. Version ac07

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