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 2025-06-10.

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	18% 8% 2 6% 4 4 4 4 5% 7% 38%	119128
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 40%	119093
Transparency	Day	44% 6% 12% 21% 15% 3	122719
	Night	44% 6% 12% 21% 16% 3	123663
Seeing	Day	23% 18% 32% 18% 7% 2	15806
	Night	22% 16% 34% 22% 6%	24791

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	11% 8% 3 7% 4 4 3 3 5% 6% 46%	3597
	11th to 20th	Day	14% 9% 2 5% 3 3 4 3 4 4 48%	3570
	21st to end	Day	18% 9% 2 6% 3 3 4 3 4 5% 42%	3960
February	1st to 10th	Day	19% 7% 2 6% 4 5% 5% 4 6% 6% 36%	1896
	11th to 20th	Day	14% 8% 3 6% 6% 3 5% 4 5% 6% 41%	1948
	21st to end	Day	19% 10% 2 7% 4 4 4 3 4 5% 38%	1650
March	1st to 10th	Day	21% 11% 2 6% 4 3 3 3 5% 6% 36%	4476
	11th to 20th	Day	15% 8% 2 7% 4 4 4 4 5% 7% 41%	4765
	21st to end	Day	25% 10% 2 6% 4 4 4 4 5% 6% 32%	5781
April	1st to 10th	Day	19% 10% 3 6% 4 4 3 3 5% 6% 37%	2400
	11th to 20th	Day	18% 6% 2 5% 4 3 4 4 6% 7% 40%	2374
	21st to end	Day	21% 7% 2 7% 4 4 4 4 4 5% 38%	2600
May	1st to 10th	Day	29% 10% 3 7% 4 4 4 3 5% 7% 25%	5886
	11th to 20th	Day	21% 9% 3 7% 5% 5% 4 3 5% 6% 32%	5924
	21st to end	Day	20% 7% 2 6% 4 4 5% 4 6% 7% 35%	6949
June	1st to 10th	Day	18% 6% 2 6% 5% 5% 6% 3 6% 9% 34%	2786
	11th to 20th	Day	16% 6% 2 6% 5% 5% 5% 4 6% 8% 36%	2660
	21st to end	Day	16% 7% 3 6% 4 5% 5% 4 6% 10% 35%	2660
July	1st to 10th	Day	13% 6% 2 6% 6% 7% 6% 5% 7% 9% 34%	4440
	11th to 20th	Day	13% 6% 2 7% 4 4 5% 4 7% 9% 38%	4440
	21st to end	Day	19% 8% 3 7% 5% 4 5% 4 6% 7% 33%	4814
August	1st to 10th	Day	17% 7% 3 8% 5% 5% 5% 4 6% 8% 31%	4137
	11th to 20th	Day	13% 7% 3 7% 4 6% 5% 4 5% 7% 39%	3956
	21st to end	Day	7% 4 2 6% 5% 5% 6% 5% 7% 9% 45%	4180
September	1st to 10th	Day	14% 6% 2 5% 4 4 4 4 5% 6% 46%	2277
	11th to 20th	Day	14% 6% 2 4 4 4 3 3 4 5% 51%	2090
	21st to end	Day	19% 6% 3 6% 3 4 4 3 5% 5% 43%	2090
October	1st to 10th	Day	23% 7% 2 5% 4 3 3 3 4 5% 42%	2090
	11th to 20th	Day	15% 5% 2 6% 3 3 4 3 4 7% 48%	1867
	21st to end	Day	19% 6% 2 6% 3 4 4 4 4 6% 42%	1881
November	1st to 10th	Day	16% 7% 2 5% 3 4 4 2 4 4 49%	1761
	11th to 20th	Day	18% 6% 1 4 4 3 3 3 5% 6% 47%	1800
	21st to end	Day	18% 6% 2 5% 4 4 3 4 5% 6% 44%	1800
December	1st to 10th	Day	15% 9% 2 4 3 3 3 3 4 6% 47%	2677
	11th to 20th	Day	19% 7% 2 4 3 3 3 2 4 5% 49%	2986
	21st to end	Day	20% 11% 3 7% 3 3 3 3 4 5% 38%	3960
January	1st to 10th	Night	14% 6% 2 4 3 4 3 3 5% 7% 50%	6349
	11th to 20th	Night	14% 10% 4 8% 4 3 3 3 4 5% 42%	6315
	21st to end	Night	17% 10% 3 7% 4 4 4 4 4 5% 38%	6996
February	1st to 10th	Night	17% 8% 3 6% 4 3 4 3 5% 6% 39%	2501
	11th to 20th	Night	16% 8% 2 7% 4 4 4 5% 5% 6% 39%	2338
	21st to end	Night	17% 10% 2 7% 4 4 4 3 5% 5% 41%	1980
March	1st to 10th	Night	24% 9% 2 4 4 3 4 3 5% 5% 38%	5364
	11th to 20th	Night	16% 10% 3 6% 4 4 4 3 5% 6% 39%	5193
	21st to end	Night	24% 10% 3 6% 3 3 4 3 5% 5% 34%	5146
April	1st to 10th	Night	18% 8% 3 7% 4 5% 4 3 5% 6% 36%	2000
	11th to 20th	Night	19% 8% 2 6% 6% 4 4 4 5% 5% 38%	1975
	21st to end	Night	20% 8% 3 6% 4 3 4 3 4 5% 40%	1800
May	1st to 10th	Night	29% 9% 3 7% 3 4 4 3 4 7% 26%	4054
	11th to 20th	Night	19% 9% 3 8% 6% 6% 5% 4 6% 7% 28%	3997
	21st to end	Night	20% 7% 3 7% 6% 5% 5% 3 6% 6% 32%	3926
June	1st to 10th	Night	15% 8% 3 6% 6% 5% 4 4 6% 7% 37%	1598
	11th to 20th	Night	19% 7% 3 5% 5% 4 4 4 4 7% 38%	1520
	21st to end	Night	15% 6% 2 6% 3 4 4 3 7% 9% 40%	1520
July	1st to 10th	Night	14% 7% 3 7% 4 5% 5% 5% 7% 10% 34%	2520
	11th to 20th	Night	14% 7% 2 6% 5% 4 5% 3 6% 9% 39%	2520
	21st to end	Night	17% 10% 3 7% 5% 4 5% 5% 5% 7% 32%	2739
August	1st to 10th	Night	20% 8% 3 7% 5% 5% 5% 4 7% 7% 29%	2759
	11th to 20th	Night	11% 7% 3 7% 6% 6% 7% 4 7% 7% 35%	2868
	21st to end	Night	8% 4 2 5% 3 3 3 3 5% 7% 56%	3476
September	1st to 10th	Night	12% 6% 2 7% 5% 5% 5% 4 5% 6% 44%	1902
	11th to 20th	Night	15% 5% 1 5% 3 3 3 3 5% 6% 52%	2090
	21st to end	Night	19% 7% 3 7% 3 3 4 4 4 5% 42%	2090
October	1st to 10th	Night	21% 7% 2 6% 3 3 3 3 5% 5% 42%	2090
	11th to 20th	Night	15% 5% 2 4 3 3 4 4 5% 7% 48%	2310
	21st to end	Night	20% 8% 3 6% 4 4 4 3 4 5% 40%	2717
November	1st to 10th	Night	14% 5% 2 6% 4 4 4 2 4 6% 49%	2537
	11th to 20th	Night	18% 7% 2 5% 4 3 3 2 4 5% 47%	2600
	21st to end	Night	16% 7% 2 4 4 3 3 4 5% 7% 45%	2600
December	1st to 10th	Night	17% 8% 2 5% 4 3 3 3 4 5% 47%	4499
	11th to 20th	Night	23% 8% 2 5% 3 3 3 3 5% 5% 41%	5208
	21st to end	Night	16% 11% 3 7% 3 4 3 3 4 6% 40%	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	38% 4 7% 27% 21% 3	3598
	11th to 20th	Day	39% 5% 10% 21% 18% 7%	3600
	21st to end	Day	43% 6% 9% 21% 19% 2	3960
February	1st to 10th	Day	46% 6% 13% 18% 14% 3	1896
	11th to 20th	Day	43% 6% 10% 21% 16% 4	1957
	21st to end	Day	48% 4 11% 19% 15% 3	1650
March	1st to 10th	Day	48% 4 11% 14% 12% 11%	4479
	11th to 20th	Day	40% 7% 14% 27% 10% 2	4765
	21st to end	Day	53% 5% 6% 15% 15% 7%	5781
April	1st to 10th	Day	47% 5% 11% 18% 16% 3	2400
	11th to 20th	Day	42% 6% 11% 20% 18% 3	2374
	21st to end	Day	46% 6% 9% 19% 17% 4	2600
May	1st to 10th	Day	58% 6% 14% 12% 8% 1	5897
	11th to 20th	Day	52% 6% 14% 16% 11% 2	5928
	21st to end	Day	46% 7% 12% 21% 13% 1	6949
June	1st to 10th	Day	45% 8% 13% 21% 12% 1	2786
	11th to 20th	Day	43% 8% 13% 20% 14% 2	2660
	21st to end	Day	44% 7% 15% 20% 14% 1	2660
July	1st to 10th	Day	46% 7% 12% 20% 14% 2	4370
	11th to 20th	Day	42% 6% 17% 22% 12% 2	4365
	21st to end	Day	49% 6% 12% 19% 12% 2	4661
August	1st to 10th	Day	50% 7% 16% 19% 8%	4123
	11th to 20th	Day	44% 6% 14% 27% 10%	3995
	21st to end	Day	33% 6% 14% 27% 18% 1	4180
September	1st to 10th	Day	38% 7% 11% 22% 20% 2	2277
	11th to 20th	Day	35% 5% 10% 23% 24% 3	2090
	21st to end	Day	42% 5% 9% 19% 22% 2	2090
October	1st to 10th	Day	41% 4 9% 21% 21% 4	3480
	11th to 20th	Day	30% 4 7% 25% 29% 4	3083
	21st to end	Day	32% 4 13% 28% 20% 3	3081
November	1st to 10th	Day	38% 5% 13% 22% 19% 2	1761
	11th to 20th	Day	38% 5% 10% 22% 22% 3	1800
	21st to end	Day	41% 5% 12% 27% 12% 3	1800
December	1st to 10th	Day	39% 4 14% 27% 14% 1	2677
	11th to 20th	Day	40% 3 14% 17% 19% 6%	2986
	21st to end	Day	48% 6% 8% 19% 15% 4	3960
January	1st to 10th	Night	35% 3 10% 33% 17% 2	6349
	11th to 20th	Night	45% 5% 11% 20% 14% 5%	6360
	21st to end	Night	48% 5% 9% 17% 17% 5%	6996
February	1st to 10th	Night	45% 6% 13% 21% 13% 2	2501
	11th to 20th	Night	44% 6% 10% 19% 17% 4	2340
	21st to end	Night	45% 4 9% 22% 16% 4	1980
March	1st to 10th	Night	47% 5% 9% 16% 16% 7%	5374
	11th to 20th	Night	44% 6% 17% 18% 12% 3	5194
	21st to end	Night	52% 4 6% 12% 20% 6%	5146
April	1st to 10th	Night	48% 5% 10% 19% 15% 3	2000
	11th to 20th	Night	46% 6% 9% 18% 18% 3	1975
	21st to end	Night	46% 6% 10% 18% 16% 4	1800
May	1st to 10th	Night	57% 8% 15% 11% 8% 1	4060
	11th to 20th	Night	53% 8% 15% 14% 8% 2	3998
	21st to end	Night	50% 8% 14% 18% 10% 1	3927
June	1st to 10th	Night	44% 9% 13% 20% 13% 1	1598
	11th to 20th	Night	44% 7% 13% 21% 13% 1	1520
	21st to end	Night	39% 9% 16% 23% 12% 1	1520
July	1st to 10th	Night	44% 9% 13% 21% 13% 1	2480
	11th to 20th	Night	42% 6% 18% 23% 11% 1	2472
	21st to end	Night	51% 7% 11% 16% 14% 2	2651
August	1st to 10th	Night	53% 7% 14% 17% 8% 1	2749
	11th to 20th	Night	46% 7% 14% 24% 9%	2895
	21st to end	Night	28% 5% 15% 29% 22% 1	3476
September	1st to 10th	Night	40% 7% 11% 22% 18% 2	1902
	11th to 20th	Night	34% 7% 9% 22% 26% 2	2090
	21st to end	Night	44% 6% 8% 20% 19% 2	2090
October	1st to 10th	Night	40% 5% 13% 23% 16% 3	3480
	11th to 20th	Night	26% 4 9% 27% 31% 3	3833
	21st to end	Night	34% 3 12% 29% 20% 3	4467
November	1st to 10th	Night	36% 7% 11% 22% 19% 4	2537
	11th to 20th	Night	41% 3 10% 22% 21% 4	2600
	21st to end	Night	39% 5% 13% 27% 14% 2	2600
December	1st to 10th	Night	42% 3 12% 28% 14% 1	4499
	11th to 20th	Night	46% 4 15% 14% 19% 2	5208
	21st to end	Night	45% 5% 8% 21% 17% 3	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	29% 28% 29% 13% 2	200
	11th to 20th	Day	30% 28% 34% 6% 4	200
	21st to end	Day	25% 30% 32% 10% 2 1	220
February	1st to 10th	Day	33% 24% 31% 11% 1	425
	11th to 20th	Day	31% 33% 27% 8% 1	496
	21st to end	Day	30% 25% 30% 12% 3	414
March	1st to 10th	Day	32% 21% 32% 12% 3	500
	11th to 20th	Day	29% 26% 30% 12% 3	500
	21st to end	Day	31% 23% 32% 10% 4 2	550
April	1st to 10th	Day	35% 23% 26% 11% 5% 1	500
	11th to 20th	Day	29% 26% 29% 12% 3 1	492
	21st to end	Day	28% 22% 31% 15% 2 2	500
May	1st to 10th	Day	31% 24% 28% 14% 3 1	503
	11th to 20th	Day	23% 18% 41% 14% 5%	527
	21st to end	Day	19% 16% 36% 22% 6% 2	573
June	1st to 10th	Day	21% 14% 35% 20% 8% 1	526
	11th to 20th	Day	17% 17% 38% 21% 6% 1	490
	21st to end	Day	17% 16% 37% 21% 7% 2	490
July	1st to 10th	Day	17% 9% 34% 25% 13% 1	485
	11th to 20th	Day	16% 7% 34% 27% 14% 2	484
	21st to end	Day	14% 9% 41% 27% 9% 1	532
August	1st to 10th	Day	12% 11% 34% 31% 10% 2	486
	11th to 20th	Day	15% 9% 34% 28% 12% 3	479
	21st to end	Day	11% 11% 34% 26% 13% 6%	539
September	1st to 10th	Day	18% 13% 27% 25% 14% 4	490
	11th to 20th	Day	19% 12% 27% 24% 11% 7%	490
	21st to end	Day	23% 12% 32% 21% 8% 3	490
October	1st to 10th	Day	28% 13% 26% 20% 9% 5%	490
	11th to 20th	Day	19% 17% 34% 17% 9% 3	440
	21st to end	Day	23% 18% 31% 18% 8% 1	429
November	1st to 10th	Day	21% 17% 34% 17% 8% 2	395
	11th to 20th	Day	27% 23% 29% 16% 5%	400
	21st to end	Day	27% 21% 31% 15% 6% 1	400
December	1st to 10th	Day	22% 31% 29% 13% 4	251
	11th to 20th	Day	31% 28% 32% 7% 2 1	200
	21st to end	Day	25% 27% 32% 10% 3 3	220
January	1st to 10th	Night	23% 21% 38% 15% 3	800
	11th to 20th	Night	28% 20% 35% 15% 2	800
	21st to end	Night	21% 23% 35% 18% 3	880
February	1st to 10th	Night	25% 19% 37% 18% 1	800
	11th to 20th	Night	26% 23% 36% 13% 1	783
	21st to end	Night	26% 21% 34% 15% 3	660
March	1st to 10th	Night	26% 21% 34% 16% 3	800
	11th to 20th	Night	21% 24% 36% 17% 3	800
	21st to end	Night	27% 20% 35% 15% 4	880
April	1st to 10th	Night	25% 22% 33% 17% 3	800
	11th to 20th	Night	24% 21% 35% 17% 4	791
	21st to end	Night	24% 19% 39% 16% 3	600
May	1st to 10th	Night	31% 15% 32% 20% 3	600
	11th to 20th	Night	22% 16% 39% 20% 3	582
	21st to end	Night	18% 9% 39% 29% 5%	433
June	1st to 10th	Night	18% 8% 36% 30% 8%	400
	11th to 20th	Night	19% 7% 37% 31% 5%	380
	21st to end	Night	16% 7% 39% 32% 6%	380
July	1st to 10th	Night	14% 5% 34% 38% 9%	370
	11th to 20th	Night	16% 4 31% 39% 10% 1	368
	21st to end	Night	13% 6% 34% 39% 7%	416
August	1st to 10th	Night	13% 5% 35% 38% 8%	527
	11th to 20th	Night	14% 6% 29% 39% 12%	609
	21st to end	Night	14% 6% 27% 38% 14% 2	836
September	1st to 10th	Night	16% 5% 34% 33% 11% 1	760
	11th to 20th	Night	18% 8% 28% 28% 16% 2	760
	21st to end	Night	23% 8% 29% 27% 11% 2	760
October	1st to 10th	Night	26% 10% 28% 24% 11% 2	760
	11th to 20th	Night	19% 12% 34% 23% 12% 1	760
	21st to end	Night	25% 16% 33% 19% 7%	836
November	1st to 10th	Night	17% 17% 34% 23% 9% 1	780
	11th to 20th	Night	23% 19% 35% 17% 6%	800
	21st to end	Night	23% 21% 32% 20% 4 1	800
December	1st to 10th	Night	26% 18% 34% 18% 4	800
	11th to 20th	Night	27% 19% 33% 20% 2	800
	21st to end	Night	25% 20% 35% 18% 3	880

Page generated at 2025-06-07 10:28:19. Version ac07

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