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 2024-11-04.

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%	101071
	Night	15% 10% 4 8% 5% 4 4 3 5% 7% 35%	97917
Transparency	Day	45% 6% 13% 22% 12% 1	104699
	Night	47% 6% 12% 22% 13% 1	102428
Seeing	Day	16% 8% 27% 27% 15% 6%	15331
	Night	18% 8% 28% 31% 13% 2	25005

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	16% 16% 7% 11% 6% 5% 5% 4 4 5% 21%	2527
	11th to 20th	Day	15% 17% 6% 13% 7% 8% 7% 5% 5% 6% 12%	2596
	21st to end	Day	27% 18% 7% 12% 5% 4 3 2 4 4 13%	3124
February	1st to 10th	Day	15% 15% 4 10% 8% 6% 5% 4 5% 6% 22%	1710
	11th to 20th	Day	13% 13% 5% 12% 7% 6% 5% 4 6% 7% 24%	1775
	21st to end	Day	17% 12% 4 11% 5% 5% 4 3 5% 6% 27%	1570
March	1st to 10th	Day	18% 14% 5% 9% 5% 4 4 3 4 6% 27%	3236
	11th to 20th	Day	13% 13% 4 8% 5% 5% 5% 3 6% 7% 32%	3480
	21st to end	Day	18% 13% 4 9% 6% 4 4 3 5% 6% 28%	3815
April	1st to 10th	Day	15% 12% 3 6% 5% 5% 5% 4 6% 7% 32%	2175
	11th to 20th	Day	17% 9% 2 8% 4 5% 4 4 6% 6% 34%	2427
	21st to end	Day	18% 9% 3 7% 5% 4 4 3 5% 7% 34%	2470
May	1st to 10th	Day	17% 8% 3 8% 5% 4 4 4 5% 7% 35%	4082
	11th to 20th	Day	17% 8% 2 6% 5% 5% 5% 4 6% 8% 34%	4081
	21st to end	Day	9% 5% 2 6% 5% 4 5% 4 6% 8% 46%	4476
June	1st to 10th	Day	9% 6% 3 6% 5% 5% 5% 5% 7% 8% 40%	2689
	11th to 20th	Day	10% 6% 2 6% 5% 5% 5% 5% 6% 8% 42%	2850
	21st to end	Day	9% 5% 3 5% 5% 6% 5% 5% 8% 10% 38%	2731
July	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 5% 6% 8% 42%	4440
	11th to 20th	Day	8% 3 2 5% 5% 5% 5% 5% 7% 10% 45%	4414
	21st to end	Day	7% 5% 2 7% 5% 5% 6% 5% 7% 8% 43%	4466
August	1st to 10th	Day	13% 4 2 6% 4 5% 5% 4 6% 8% 42%	4104
	11th to 20th	Day	10% 5% 2 6% 4 6% 6% 5% 8% 11% 35%	4044
	21st to end	Day	9% 5% 2 7% 5% 6% 5% 5% 7% 9% 40%	4532
September	1st to 10th	Day	10% 6% 2 7% 4 5% 5% 4 6% 8% 44%	2126
	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	10% 8% 3 7% 5% 4 6% 4 6% 7% 40%	1699
	11th to 20th	Day	13% 12% 3 7% 5% 5% 5% 4 6% 6% 34%	1710
	21st to end	Day	18% 13% 4 7% 6% 5% 5% 3 6% 6% 28%	1577
December	1st to 10th	Day	15% 14% 5% 12% 6% 6% 6% 4 6% 6% 20%	1520
	11th to 20th	Day	16% 14% 6% 8% 6% 5% 5% 4 6% 7% 23%	1868
	21st to end	Day	27% 18% 7% 12% 6% 5% 4 3 4 5% 10%	2687
January	1st to 10th	Night	15% 16% 6% 9% 5% 5% 4 3 5% 6% 25%	4428
	11th to 20th	Night	14% 18% 8% 12% 5% 5% 6% 4 5% 6% 16%	4369
	21st to end	Night	24% 18% 7% 11% 5% 4 4 3 3 4 18%	4532
February	1st to 10th	Night	16% 16% 4 10% 7% 5% 4 3 5% 4 24%	2470
	11th to 20th	Night	14% 13% 4 11% 6% 5% 5% 4 5% 6% 27%	2288
	21st to end	Night	18% 12% 4 10% 5% 4 5% 3 4 5% 30%	1884
March	1st to 10th	Night	17% 13% 4 8% 4 4 3 3 4 5% 35%	3616
	11th to 20th	Night	9% 8% 3 6% 5% 4 5% 5% 6% 8% 41%	3480
	21st to end	Night	15% 8% 3 8% 4 5% 5% 3 5% 6% 37%	3817
April	1st to 10th	Night	15% 10% 3 6% 5% 5% 4 3 5% 7% 36%	2022
	11th to 20th	Night	17% 8% 2 6% 4 4 4 3 7% 8% 36%	1701
	21st to end	Night	16% 8% 3 6% 4 4 4 4 6% 6% 39%	1710
May	1st to 10th	Night	17% 8% 3 7% 5% 4 4 3 5% 7% 36%	2813
	11th to 20th	Night	15% 7% 3 7% 5% 5% 5% 3 6% 7% 38%	2812
	21st to end	Night	9% 6% 2 6% 4 4 4 3 5% 8% 49%	3079
June	1st to 10th	Night	12% 6% 2 4 4 3 4 3 6% 8% 47%	1475
	11th to 20th	Night	9% 5% 2 6% 5% 5% 5% 4 7% 8% 44%	1330
	21st to end	Night	10% 5% 2 6% 5% 4 5% 3 6% 8% 48%	1439
July	1st to 10th	Night	9% 4 2 4 3 3 4 4 6% 11% 50%	2530
	11th to 20th	Night	10% 3 1 5% 3 4 5% 3 6% 8% 52%	2551
	21st to end	Night	5% 3 2 6% 4 5% 6% 4 8% 9% 48%	3087
August	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 5% 7% 46%	2825
	11th to 20th	Night	9% 5% 2 6% 4 4 5% 4 5% 7% 49%	2785
	21st to end	Night	8% 4 2 7% 5% 4 6% 4 6% 8% 48%	3124
September	1st to 10th	Night	10% 5% 2 6% 5% 4 4 4 5% 6% 49%	2029
	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% 8% 3 7% 4 4 4 4 6% 6% 44%	2452
	11th to 20th	Night	16% 10% 3 8% 5% 5% 4 5% 6% 6% 32%	2470
	21st to end	Night	19% 13% 4 8% 4 4 4 3 5% 6% 29%	2599
December	1st to 10th	Night	17% 14% 5% 10% 7% 5% 4 4 5% 6% 22%	2660
	11th to 20th	Night	15% 15% 4 9% 5% 5% 6% 4 5% 6% 26%	3321
	21st to end	Night	28% 20% 6% 11% 5% 4 3 3 4 4 13%	4973

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	64% 8% 10% 12% 6%	2528
	11th to 20th	Day	72% 5% 12% 8% 3	2596
	21st to end	Day	75% 5% 9% 8% 4	3124
February	1st to 10th	Day	60% 6% 10% 15% 7% 1	1710
	11th to 20th	Day	58% 5% 10% 18% 8% 1	1784
	21st to end	Day	56% 5% 12% 18% 7% 1	1570
March	1st to 10th	Day	56% 6% 13% 19% 5% 1	3236
	11th to 20th	Day	49% 7% 15% 20% 8% 1	3480
	21st to end	Day	55% 5% 13% 16% 10% 1	3815
April	1st to 10th	Day	49% 6% 13% 18% 13% 1	2175
	11th to 20th	Day	48% 6% 12% 20% 14% 2	2427
	21st to end	Day	48% 6% 13% 19% 13% 2	2470
May	1st to 10th	Day	47% 6% 12% 24% 10% 2	4120
	11th to 20th	Day	44% 8% 14% 23% 9% 1	4085
	21st to end	Day	34% 6% 12% 19% 21% 7%	4489
June	1st to 10th	Day	37% 8% 12% 27% 13% 2	2689
	11th to 20th	Day	37% 7% 14% 22% 17% 3	2850
	21st to end	Day	36% 9% 14% 23% 17% 1	2731
July	1st to 10th	Day	38% 6% 12% 23% 18% 2	4371
	11th to 20th	Day	29% 8% 15% 32% 16% 1	4339
	21st to end	Day	35% 6% 16% 19% 21% 3	4324
August	1st to 10th	Day	36% 7% 15% 28% 14% 1	4091
	11th to 20th	Day	36% 10% 14% 31% 9% 1	4083
	21st to end	Day	36% 7% 16% 27% 14% 1	4531
September	1st to 10th	Day	37% 6% 15% 25% 15% 2	2126
	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% 13% 24% 12% 2	1699
	11th to 20th	Day	48% 6% 16% 20% 9% 1	1710
	21st to end	Day	55% 6% 13% 18% 7% 1	1577
December	1st to 10th	Day	62% 8% 11% 14% 5%	1520
	11th to 20th	Day	58% 8% 13% 15% 6%	1864
	21st to end	Day	76% 6% 8% 7% 3	2687
January	1st to 10th	Night	60% 6% 11% 16% 6% 1	4428
	11th to 20th	Night	67% 7% 11% 11% 4	4369
	21st to end	Night	70% 5% 9% 12% 4	4532
February	1st to 10th	Night	61% 6% 11% 15% 6% 1	2470
	11th to 20th	Night	56% 5% 9% 19% 9% 1	2290
	21st to end	Night	56% 5% 12% 18% 8% 2	1884
March	1st to 10th	Night	51% 5% 13% 24% 7%	3616
	11th to 20th	Night	37% 8% 15% 20% 18% 1	3480
	21st to end	Night	47% 6% 11% 23% 13% 1	3817
April	1st to 10th	Night	48% 5% 11% 22% 14% 2	2022
	11th to 20th	Night	45% 5% 12% 19% 17% 2	1701
	21st to end	Night	44% 6% 10% 20% 16% 4	1710
May	1st to 10th	Night	47% 5% 10% 20% 15% 3	2840
	11th to 20th	Night	44% 7% 10% 26% 12% 1	2813
	21st to end	Night	33% 7% 11% 23% 21% 5%	3087
June	1st to 10th	Night	35% 5% 11% 29% 18% 3	1475
	11th to 20th	Night	36% 7% 14% 24% 17% 2	1330
	21st to end	Night	34% 6% 13% 26% 21%	1439
July	1st to 10th	Night	29% 5% 13% 32% 16% 6%	2490
	11th to 20th	Night	29% 5% 14% 33% 17% 2	2503
	21st to end	Night	29% 7% 19% 21% 21% 3	2988
August	1st to 10th	Night	36% 7% 13% 25% 17% 2	2814
	11th to 20th	Night	34% 7% 11% 33% 14% 1	2812
	21st to end	Night	33% 7% 13% 31% 15% 1	3124
September	1st to 10th	Night	34% 7% 12% 26% 18% 2	2029
	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	2452
	11th to 20th	Night	50% 6% 12% 21% 10% 1	2470
	21st to end	Night	56% 5% 13% 19% 6%	2599
December	1st to 10th	Night	62% 6% 11% 15% 6%	2660
	11th to 20th	Night	57% 8% 13% 15% 7%	3324
	21st to end	Night	75% 5% 9% 8% 2	4973

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% 16% 37% 19% 7% 1	270
	11th to 20th	Day	19% 13% 41% 20% 5% 1	270
	21st to end	Day	24% 10% 33% 29% 4	297
February	1st to 10th	Day	21% 13% 40% 22% 3 1	270
	11th to 20th	Day	19% 17% 34% 27% 3	270
	21st to end	Day	18% 17% 46% 15% 4	225
March	1st to 10th	Day	21% 15% 34% 25% 5%	334
	11th to 20th	Day	19% 10% 33% 29% 7% 1	460
	21st to end	Day	22% 13% 33% 23% 8% 1	506
April	1st to 10th	Day	16% 11% 33% 29% 9% 1	460
	11th to 20th	Day	23% 15% 29% 20% 8% 5%	452
	21st to end	Day	22% 8% 33% 23% 11% 2	460
May	1st to 10th	Day	20% 8% 29% 28% 11% 4	463
	11th to 20th	Day	19% 7% 25% 26% 17% 7%	487
	21st to end	Day	13% 4 21% 31% 22% 9%	529
June	1st to 10th	Day	11% 5% 26% 36% 18% 5%	600
	11th to 20th	Day	11% 5% 27% 35% 20% 3	680
	21st to end	Day	10% 6% 29% 31% 19% 6%	680
July	1st to 10th	Day	9% 5% 18% 31% 26% 11%	671
	11th to 20th	Day	11% 2 22% 31% 24% 10%	655
	21st to end	Day	8% 3 17% 33% 27% 11%	532
August	1st to 10th	Day	9% 4 15% 33% 23% 16%	486
	11th to 20th	Day	11% 3 14% 34% 27% 11%	479
	21st to end	Day	11% 3 17% 29% 26% 14%	539
September	1st to 10th	Day	12% 3 23% 26% 23% 13%	490
	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	14% 12% 36% 23% 11% 3	279
	11th to 20th	Day	17% 20% 32% 24% 6% 1	270
	21st to end	Day	23% 12% 39% 22% 4	207
December	1st to 10th	Day	19% 17% 37% 17% 7% 2	180
	11th to 20th	Day	26% 13% 41% 17% 2 1	180
	21st to end	Day	26% 17% 33% 20% 3	270
January	1st to 10th	Night	22% 13% 36% 22% 6% 1	950
	11th to 20th	Night	26% 11% 33% 25% 5%	905
	21st to end	Night	19% 11% 35% 30% 6%	836
February	1st to 10th	Night	24% 11% 36% 26% 4	760
	11th to 20th	Night	20% 10% 35% 30% 5%	743
	21st to end	Night	24% 12% 37% 24% 3	628
March	1st to 10th	Night	21% 9% 33% 29% 7%	760
	11th to 20th	Night	16% 12% 36% 29% 6% 1	760
	21st to end	Night	21% 10% 29% 31% 8% 1	836
April	1st to 10th	Night	19% 11% 33% 31% 6%	692
	11th to 20th	Night	19% 9% 34% 27% 9% 1	563
	21st to end	Night	21% 6% 31% 33% 9%	570
May	1st to 10th	Night	19% 4 30% 33% 12% 1	570
	11th to 20th	Night	19% 3 25% 35% 16% 2	561
	21st to end	Night	11% 3 21% 40% 22% 3	617
June	1st to 10th	Night	13% 2 23% 45% 16% 1	451
	11th to 20th	Night	12% 3 19% 48% 18% 1	380
	21st to end	Night	10% 3 21% 44% 19% 2	489
July	1st to 10th	Night	8% 3 15% 39% 29% 5%	555
	11th to 20th	Night	9% 2 17% 41% 27% 5%	552
	21st to end	Night	6% 2 13% 49% 27% 4	624
August	1st to 10th	Night	9% 2 15% 40% 30% 5%	567
	11th to 20th	Night	10% 3 14% 36% 31% 5%	564
	21st to end	Night	9% 3 16% 36% 31% 5%	627
September	1st to 10th	Night	12% 6% 16% 34% 25% 7%	706
	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	15% 10% 33% 26% 15% 1	755
	11th to 20th	Night	21% 13% 35% 21% 8% 1	760
	21st to end	Night	25% 10% 35% 23% 6%	760
December	1st to 10th	Night	24% 13% 36% 21% 6% 1	760
	11th to 20th	Night	23% 13% 36% 24% 4	783
	21st to end	Night	29% 10% 36% 22% 3	1045

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