Windsong Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Windsong Observatory clear sky chart from 2005-11-06 to 2026-10-14.

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% 7% 2 6% 4 4 4 4 5% 7% 39%	148967
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 40%	142770
Transparency	Day	44% 5% 12% 22% 15% 3	152515
	Night	44% 5% 12% 21% 16% 3	147311
Seeing	Day	23% 18% 32% 18% 7% 2	17597
	Night	22% 15% 34% 22% 6%	26437

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	10% 9% 3 7% 4 5% 4 4 5% 6% 44%	4661
	11th to 20th	Day	15% 9% 3 6% 4 3 4 3 4 4 47%	4650
	21st to end	Day	15% 9% 2 5% 3 3 4 3 4 6% 46%	5124
February	1st to 10th	Day	18% 7% 2 6% 4 5% 5% 4 6% 6% 37%	1991
	11th to 20th	Day	14% 8% 3 6% 6% 3 5% 4 5% 6% 41%	2048
	21st to end	Day	18% 11% 2 6% 4 4 4 3 4 5% 38%	1730
March	1st to 10th	Day	21% 11% 2 6% 3 3 3 3 5% 5% 36%	4576
	11th to 20th	Day	15% 8% 2 6% 4 4 4 4 5% 7% 41%	4870
	21st to end	Day	24% 10% 2 6% 4 4 4 4 5% 6% 32%	5909
April	1st to 10th	Day	20% 10% 3 6% 4 4 3 3 5% 6% 38%	2520
	11th to 20th	Day	18% 6% 2 5% 4 3 4 4 6% 7% 40%	2494
	21st to end	Day	20% 7% 2 7% 4 4 4 4 4 5% 39%	2730
May	1st to 10th	Day	26% 10% 3 6% 4 4 4 3 5% 7% 29%	7639
	11th to 20th	Day	20% 9% 3 7% 4 4 4 3 4 7% 36%	7702
	21st to end	Day	22% 8% 2 7% 4 4 5% 4 6% 7% 31%	9060
June	1st to 10th	Day	18% 6% 2 6% 5% 4 6% 4 6% 9% 35%	2940
	11th to 20th	Day	16% 6% 2 6% 5% 5% 5% 4 6% 8% 37%	2940
	21st to end	Day	16% 6% 3 6% 4 5% 4 4 6% 9% 36%	2940
July	1st to 10th	Day	19% 6% 2 6% 5% 5% 5% 4 6% 9% 34%	8280
	11th to 20th	Day	9% 6% 2 6% 4 4 5% 4 7% 11% 41%	8280
	21st to end	Day	16% 6% 2 6% 4 4 4 3 5% 7% 42%	8993
August	1st to 10th	Day	14% 7% 3 8% 5% 5% 6% 5% 7% 9% 32%	6062
	11th to 20th	Day	13% 6% 2 6% 4 5% 5% 4 5% 8% 43%	5819
	21st to end	Day	11% 4 2 5% 5% 5% 6% 5% 6% 9% 42%	6080
September	1st to 10th	Day	14% 6% 2 5% 4 4 4 4 5% 6% 45%	2517
	11th to 20th	Day	14% 5% 2 4 4 4 3 3 4 6% 51%	2310
	21st to end	Day	20% 6% 3 6% 3 4 4 3 5% 5% 42%	2310
October	1st to 10th	Day	21% 6% 2 4 3 3 3 3 4 5% 45%	2310
	11th to 20th	Day	16% 5% 2 6% 3 3 4 3 4 7% 47%	1998
	21st to end	Day	19% 6% 2 6% 3 4 4 4 4 6% 43%	1980
November	1st to 10th	Day	16% 7% 1 5% 4 4 4 3 4 5% 49%	1851
	11th to 20th	Day	17% 6% 1 4 4 3 3 3 5% 6% 47%	1890
	21st to end	Day	18% 7% 2 5% 4 4 3 3 5% 6% 43%	1890
December	1st to 10th	Day	15% 9% 2 4 3 3 3 3 4 6% 47%	2759
	11th to 20th	Day	18% 6% 2 4 3 3 3 2 4 5% 49%	3066
	21st to end	Day	20% 11% 3 7% 3 3 3 3 4 5% 38%	4048
January	1st to 10th	Night	15% 7% 2 5% 3 4 3 3 4 6% 48%	8234
	11th to 20th	Night	15% 11% 4 8% 4 4 4 3 4 5% 40%	8235
	21st to end	Night	15% 9% 3 7% 4 4 4 3 4 5% 41%	9057
February	1st to 10th	Night	17% 8% 3 6% 4 3 4 3 5% 7% 40%	2625
	11th to 20th	Night	16% 8% 2 7% 4 4 4 5% 5% 6% 40%	2458
	21st to end	Night	17% 10% 2 7% 4 3 4 3 5% 5% 41%	2076
March	1st to 10th	Night	23% 9% 2 4 3 3 4 3 5% 5% 39%	5484
	11th to 20th	Night	16% 10% 3 6% 4 4 4 3 5% 6% 40%	5308
	21st to end	Night	24% 10% 3 6% 3 3 4 3 5% 5% 34%	5260
April	1st to 10th	Night	19% 8% 3 7% 4 5% 4 3 5% 6% 37%	2100
	11th to 20th	Night	19% 8% 2 5% 5% 4 4 4 5% 5% 38%	2075
	21st to end	Night	19% 8% 3 7% 4 3 4 3 4 5% 40%	1890
May	1st to 10th	Night	27% 9% 3 6% 3 3 4 3 5% 6% 30%	5259
	11th to 20th	Night	18% 8% 3 7% 5% 5% 5% 4 5% 7% 32%	5199
	21st to end	Night	21% 8% 3 7% 5% 5% 5% 3 6% 6% 30%	5114
June	1st to 10th	Night	15% 8% 2 6% 5% 4 4 4 6% 7% 38%	1680
	11th to 20th	Night	18% 8% 3 5% 5% 4 5% 4 5% 7% 37%	1680
	21st to end	Night	15% 6% 2 6% 3 4 4 3 7% 9% 41%	1680
July	1st to 10th	Night	18% 7% 2 6% 4 4 4 4 6% 10% 35%	4680
	11th to 20th	Night	12% 6% 2 6% 4 4 4 3 6% 9% 43%	4680
	21st to end	Night	15% 7% 2 6% 4 4 4 4 5% 7% 43%	5084
August	1st to 10th	Night	16% 6% 3 7% 5% 5% 6% 5% 7% 8% 32%	4039
	11th to 20th	Night	12% 7% 3 7% 5% 5% 6% 4 6% 8% 37%	4209
	21st to end	Night	11% 5% 2 5% 4 3 4 3 6% 7% 48%	5045
September	1st to 10th	Night	12% 6% 2 7% 5% 5% 5% 4 5% 6% 44%	2102
	11th to 20th	Night	14% 5% 1 5% 3 3 3 3 5% 6% 52%	2310
	21st to end	Night	20% 8% 2 7% 3 3 4 4 4 5% 40%	2310
October	1st to 10th	Night	19% 7% 2 5% 3 3 3 3 5% 5% 45%	2310
	11th to 20th	Night	16% 5% 2 4 3 3 4 4 5% 7% 47%	2466
	21st to end	Night	20% 8% 3 6% 4 4 4 3 4 5% 40%	2860
November	1st to 10th	Night	14% 5% 2 6% 4 4 4 2 5% 6% 49%	2667
	11th to 20th	Night	17% 7% 2 5% 4 3 3 2 4 5% 47%	2730
	21st to end	Night	16% 7% 2 4 4 3 4 4 5% 7% 45%	2730
December	1st to 10th	Night	17% 8% 2 5% 4 3 3 3 4 5% 46%	4636
	11th to 20th	Night	22% 8% 3 5% 3 3 3 3 5% 5% 41%	5348
	21st to end	Night	16% 11% 3 7% 3 4 3 3 4 6% 40%	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	41% 3 10% 25% 19% 2	4662
	11th to 20th	Day	41% 4 9% 20% 18% 7%	4680
	21st to end	Day	39% 4 10% 24% 21% 2	5124
February	1st to 10th	Day	45% 6% 13% 18% 15% 3	1991
	11th to 20th	Day	43% 6% 10% 22% 16% 3	2057
	21st to end	Day	48% 4 10% 20% 15% 3	1730
March	1st to 10th	Day	49% 4 11% 14% 12% 10%	4579
	11th to 20th	Day	40% 7% 14% 27% 10% 2	4870
	21st to end	Day	53% 5% 6% 15% 15% 7%	5909
April	1st to 10th	Day	47% 5% 11% 18% 16% 3	2520
	11th to 20th	Day	42% 6% 12% 19% 18% 3	2494
	21st to end	Day	46% 6% 8% 19% 17% 4	2730
May	1st to 10th	Day	55% 5% 13% 15% 11% 1	7651
	11th to 20th	Day	49% 5% 14% 19% 12% 1	7706
	21st to end	Day	50% 6% 11% 20% 12% 1	9061
June	1st to 10th	Day	44% 8% 13% 22% 13% 1	2940
	11th to 20th	Day	43% 7% 13% 20% 14% 2	2940
	21st to end	Day	43% 7% 15% 20% 14% 1	2940
July	1st to 10th	Day	47% 5% 14% 21% 12% 1	8210
	11th to 20th	Day	38% 4 16% 28% 13% 1	8205
	21st to end	Day	42% 4 10% 24% 18% 2	8795
August	1st to 10th	Day	48% 6% 18% 19% 9% 1	6048
	11th to 20th	Day	40% 4 14% 32% 9%	5858
	21st to end	Day	37% 5% 14% 27% 16% 2	6080
September	1st to 10th	Day	38% 7% 11% 23% 19% 2	2517
	11th to 20th	Day	35% 5% 10% 24% 24% 3	2310
	21st to end	Day	44% 5% 9% 20% 21% 2	2310
October	1st to 10th	Day	39% 4 9% 22% 22% 4	3700
	11th to 20th	Day	32% 4 7% 25% 28% 4	3214
	21st to end	Day	32% 4 13% 28% 20% 3	3180
November	1st to 10th	Day	39% 5% 13% 22% 20% 2	1851
	11th to 20th	Day	38% 5% 10% 22% 21% 3	1890
	21st to end	Day	42% 5% 12% 27% 12% 3	1890
December	1st to 10th	Day	39% 4 14% 27% 15% 1	2759
	11th to 20th	Day	40% 3 14% 17% 19% 6%	3066
	21st to end	Day	48% 6% 8% 19% 15% 4	4048
January	1st to 10th	Night	38% 3 11% 29% 17% 2	8234
	11th to 20th	Night	47% 4 10% 19% 15% 5%	8280
	21st to end	Night	45% 4 8% 18% 20% 4	9057
February	1st to 10th	Night	44% 6% 12% 21% 15% 2	2625
	11th to 20th	Night	44% 6% 10% 20% 17% 4	2460
	21st to end	Night	46% 4 8% 22% 15% 4	2076
March	1st to 10th	Night	47% 5% 9% 17% 16% 7%	5494
	11th to 20th	Night	44% 6% 17% 18% 12% 3	5309
	21st to end	Night	52% 4 6% 12% 20% 6%	5260
April	1st to 10th	Night	48% 5% 10% 19% 15% 3	2100
	11th to 20th	Night	46% 6% 9% 18% 18% 3	2075
	21st to end	Night	46% 6% 10% 19% 16% 4	1890
May	1st to 10th	Night	54% 6% 14% 14% 11% 1	5265
	11th to 20th	Night	50% 6% 15% 18% 9% 2	5200
	21st to end	Night	53% 6% 12% 20% 8%	5115
June	1st to 10th	Night	43% 9% 13% 21% 14% 1	1680
	11th to 20th	Night	45% 7% 14% 20% 13% 1	1680
	21st to end	Night	39% 9% 16% 24% 12% 1	1680
July	1st to 10th	Night	46% 6% 13% 21% 13%	4640
	11th to 20th	Night	39% 4 16% 25% 15% 1	4632
	21st to end	Night	42% 5% 11% 25% 17% 1	4967
August	1st to 10th	Night	50% 6% 17% 16% 11% 1	4029
	11th to 20th	Night	44% 6% 16% 26% 8%	4236
	21st to end	Night	35% 5% 15% 28% 17% 1	5045
September	1st to 10th	Night	41% 6% 11% 23% 18% 1	2102
	11th to 20th	Night	34% 7% 9% 22% 26% 2	2310
	21st to end	Night	46% 6% 8% 19% 19% 2	2310
October	1st to 10th	Night	39% 5% 12% 24% 17% 3	3700
	11th to 20th	Night	27% 4 9% 27% 30% 3	3989
	21st to end	Night	35% 3 11% 28% 20% 2	4610
November	1st to 10th	Night	36% 7% 11% 23% 19% 3	2667
	11th to 20th	Night	40% 3 10% 22% 21% 4	2730
	21st to end	Night	39% 5% 13% 28% 14% 2	2730
December	1st to 10th	Night	42% 3 11% 28% 14% 1	4636
	11th to 20th	Night	46% 4 14% 15% 19% 2	5348
	21st to end	Night	45% 5% 8% 21% 17% 3	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	29% 28% 30% 12% 2	220
	11th to 20th	Day	32% 26% 33% 5% 3	220
	21st to end	Day	27% 31% 31% 9% 2 1	242
February	1st to 10th	Day	31% 25% 31% 12% 1	460
	11th to 20th	Day	30% 32% 29% 8% 1	536
	21st to end	Day	30% 24% 31% 11% 3	446
March	1st to 10th	Day	31% 20% 32% 13% 3	540
	11th to 20th	Day	29% 25% 30% 12% 3	540
	21st to end	Day	31% 24% 30% 9% 4 2	594
April	1st to 10th	Day	35% 22% 27% 11% 4 1	540
	11th to 20th	Day	30% 25% 29% 12% 3 1	532
	21st to end	Day	28% 22% 31% 15% 2 2	540
May	1st to 10th	Day	30% 25% 28% 13% 2 1	543
	11th to 20th	Day	23% 18% 41% 14% 5%	567
	21st to end	Day	20% 17% 35% 21% 5% 1	617
June	1st to 10th	Day	20% 13% 36% 22% 8% 1	570
	11th to 20th	Day	18% 17% 38% 20% 5% 1	570
	21st to end	Day	17% 15% 36% 23% 7% 1	570
July	1st to 10th	Day	19% 8% 35% 25% 12% 1	565
	11th to 20th	Day	15% 9% 35% 26% 13% 2	564
	21st to end	Day	14% 8% 41% 26% 9% 2	620
August	1st to 10th	Day	11% 10% 34% 31% 11% 2	566
	11th to 20th	Day	14% 9% 34% 29% 11% 3	559
	21st to end	Day	12% 11% 35% 24% 12% 5%	627
September	1st to 10th	Day	17% 14% 29% 24% 12% 4	570
	11th to 20th	Day	18% 11% 28% 25% 11% 7%	570
	21st to end	Day	25% 12% 32% 20% 8% 3	570
October	1st to 10th	Day	26% 13% 27% 20% 10% 5%	570
	11th to 20th	Day	21% 17% 33% 17% 9% 3	487
	21st to end	Day	25% 18% 31% 18% 8% 1	462
November	1st to 10th	Day	23% 17% 34% 16% 8% 2	425
	11th to 20th	Day	27% 24% 29% 16% 5%	430
	21st to end	Day	28% 22% 30% 15% 5%	430
December	1st to 10th	Day	23% 31% 29% 12% 4	273
	11th to 20th	Day	33% 27% 31% 6% 2	220
	21st to end	Day	27% 27% 31% 9% 3 2	242
January	1st to 10th	Night	23% 21% 38% 15% 3	840
	11th to 20th	Night	28% 21% 34% 14% 2	840
	21st to end	Night	21% 24% 35% 18% 3	924
February	1st to 10th	Night	24% 19% 38% 18% 1	840
	11th to 20th	Night	26% 22% 36% 14% 1	823
	21st to end	Night	27% 21% 35% 15% 3	692
March	1st to 10th	Night	26% 20% 34% 16% 3	840
	11th to 20th	Night	22% 23% 35% 17% 3	840
	21st to end	Night	27% 21% 34% 14% 3	924
April	1st to 10th	Night	26% 23% 32% 16% 3	840
	11th to 20th	Night	23% 21% 35% 16% 4	831
	21st to end	Night	23% 18% 39% 16% 3	630
May	1st to 10th	Night	31% 15% 32% 19% 3	630
	11th to 20th	Night	21% 16% 39% 20% 3	612
	21st to end	Night	20% 9% 39% 28% 5%	455
June	1st to 10th	Night	17% 7% 37% 30% 8%	420
	11th to 20th	Night	20% 8% 38% 30% 5%	420
	21st to end	Night	16% 6% 39% 33% 6%	420
July	1st to 10th	Night	15% 5% 34% 38% 8%	410
	11th to 20th	Night	16% 3 32% 38% 11%	408
	21st to end	Night	13% 6% 35% 38% 8%	460
August	1st to 10th	Night	13% 5% 34% 40% 7%	583
	11th to 20th	Night	13% 5% 31% 39% 11%	674
	21st to end	Night	15% 6% 28% 37% 13% 2	924
September	1st to 10th	Night	16% 6% 34% 34% 11%	840
	11th to 20th	Night	17% 8% 29% 28% 17% 2	840
	21st to end	Night	25% 8% 28% 27% 11% 2	840
October	1st to 10th	Night	24% 11% 29% 24% 11% 1	840
	11th to 20th	Night	20% 12% 33% 23% 12% 1	813
	21st to end	Night	26% 16% 33% 19% 7%	880
November	1st to 10th	Night	18% 17% 33% 23% 9% 1	820
	11th to 20th	Night	23% 19% 35% 17% 6%	840
	21st to end	Night	23% 20% 32% 20% 4 1	840
December	1st to 10th	Night	26% 18% 34% 18% 4	840
	11th to 20th	Night	27% 18% 33% 19% 2	840
	21st to end	Night	26% 20% 35% 17% 2	924

Page generated at 2026-10-11 05:06:27. Version ac07

Copyright © 2026 Attilla Danko.