William D. McDowell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the William D. McDowell Observatory clear sky chart from 2005-11-06 to 2026-09-14.

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	16% 6% 2 6% 4 4 4 4 5% 7% 40%	149082
	Night	17% 7% 2 6% 4 4 4 3 5% 6% 42%	141983
Transparency	Day	41% 6% 12% 22% 17% 2	152611
	Night	42% 5% 11% 22% 17% 3	146543
Seeing	Day	20% 15% 31% 22% 10% 3	17392
	Night	20% 11% 32% 28% 9% 1	26477

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	12% 6% 2 6% 4 3 4 4 5% 6% 49%	4661
	11th to 20th	Day	15% 8% 2 6% 4 4 3 3 4 6% 45%	4650
	21st to end	Day	16% 8% 3 6% 4 4 4 3 5% 6% 42%	5582
February	1st to 10th	Day	16% 8% 3 6% 4 4 3 4 6% 6% 41%	1899
	11th to 20th	Day	15% 8% 3 6% 4 4 4 3 4 5% 44%	2048
	21st to end	Day	19% 9% 2 5% 3 4 4 3 5% 6% 39%	1730
March	1st to 10th	Day	21% 7% 2 5% 4 3 3 3 4 5% 42%	4576
	11th to 20th	Day	13% 8% 2 6% 4 4 5% 4 5% 7% 42%	4891
	21st to end	Day	18% 6% 2 6% 4 3 4 3 5% 7% 43%	6051
April	1st to 10th	Day	19% 8% 2 6% 4 4 3 4 6% 6% 38%	2520
	11th to 20th	Day	17% 6% 2 6% 5% 4 4 3 5% 7% 41%	2494
	21st to end	Day	19% 5% 2 6% 4 4 4 3 5% 9% 40%	2541
May	1st to 10th	Day	26% 7% 2 6% 4 4 4 3 5% 7% 33%	7639
	11th to 20th	Day	20% 8% 3 6% 4 4 4 3 4 7% 38%	8162
	21st to end	Day	24% 6% 2 6% 4 4 4 4 5% 8% 34%	9060
June	1st to 10th	Day	16% 5% 2 5% 4 4 5% 5% 7% 8% 39%	2940
	11th to 20th	Day	16% 6% 2 6% 5% 4 5% 4 6% 9% 37%	2940
	21st to end	Day	12% 6% 3 6% 5% 5% 5% 4 7% 9% 39%	2940
July	1st to 10th	Day	13% 5% 2 7% 4 5% 5% 4 6% 9% 38%	8280
	11th to 20th	Day	9% 6% 3 7% 4 5% 5% 5% 7% 11% 38%	8280
	21st to end	Day	13% 6% 2 6% 5% 4 5% 4 6% 9% 39%	8993
August	1st to 10th	Day	16% 6% 2 7% 5% 6% 5% 5% 7% 10% 30%	6098
	11th to 20th	Day	12% 5% 2 5% 4 4 4 4 7% 8% 45%	5488
	21st to end	Day	11% 6% 2 6% 5% 4 5% 4 6% 8% 43%	6080
September	1st to 10th	Day	14% 6% 2 5% 4 5% 5% 3 6% 7% 45%	2520
	11th to 20th	Day	14% 5% 2 4 3 3 3 3 5% 7% 49%	2351
	21st to end	Day	18% 6% 2 7% 5% 4 4 4 5% 5% 39%	2200
October	1st to 10th	Day	19% 6% 2 5% 3 3 3 3 4 7% 43%	2085
	11th to 20th	Day	16% 4 2 5% 4 4 3 3 4 7% 48%	2000
	21st to end	Day	20% 5% 1 5% 3 4 4 3 4 5% 46%	2045
November	1st to 10th	Day	14% 5% 2 5% 3 3 2 3 4 5% 52%	1851
	11th to 20th	Day	17% 5% 1 5% 3 4 4 3 5% 6% 48%	1890
	21st to end	Day	16% 7% 2 4 4 3 4 3 5% 7% 44%	1803
December	1st to 10th	Day	16% 6% 2 5% 4 3 4 2 5% 8% 45%	2680
	11th to 20th	Day	23% 6% 2 4 3 3 3 2 4 6% 45%	3066
	21st to end	Day	18% 7% 2 7% 4 4 4 4 5% 6% 38%	4048
January	1st to 10th	Night	12% 8% 2 6% 4 3 4 3 5% 6% 46%	8234
	11th to 20th	Night	16% 9% 2 6% 3 4 4 2 4 5% 45%	8235
	21st to end	Night	16% 8% 2 5% 4 4 4 3 5% 6% 44%	8581
February	1st to 10th	Night	17% 9% 3 5% 4 4 4 3 5% 6% 42%	2719
	11th to 20th	Night	18% 8% 3 5% 4 4 3 3 5% 6% 40%	2458
	21st to end	Night	18% 9% 2 6% 4 3 4 4 5% 5% 41%	2076
March	1st to 10th	Night	20% 9% 2 5% 3 3 3 3 4 6% 43%	5484
	11th to 20th	Night	14% 9% 2 6% 4 3 3 3 5% 6% 45%	5283
	21st to end	Night	19% 9% 3 6% 4 4 4 3 4 6% 40%	5122
April	1st to 10th	Night	20% 7% 2 6% 4 4 4 4 5% 6% 38%	2100
	11th to 20th	Night	19% 8% 1 6% 4 4 4 3 5% 6% 40%	2075
	21st to end	Night	19% 7% 3 5% 3 3 3 3 4 6% 43%	2079
May	1st to 10th	Night	24% 8% 3 7% 4 4 4 3 5% 7% 31%	5259
	11th to 20th	Night	20% 8% 3 6% 4 4 4 3 5% 6% 37%	4740
	21st to end	Night	25% 8% 3 6% 4 4 4 3 5% 7% 32%	5114
June	1st to 10th	Night	17% 5% 2 5% 4 4 4 4 6% 8% 42%	1680
	11th to 20th	Night	15% 6% 1 5% 5% 4 5% 4 6% 9% 40%	1680
	21st to end	Night	11% 4 2 6% 5% 5% 5% 4 7% 9% 42%	1680
July	1st to 10th	Night	13% 6% 2 8% 5% 5% 5% 4 6% 8% 38%	4680
	11th to 20th	Night	13% 8% 3 7% 5% 4 5% 5% 7% 9% 35%	4680
	21st to end	Night	14% 6% 2 6% 4 4 4 4 6% 8% 42%	5084
August	1st to 10th	Night	19% 7% 3 7% 4 5% 5% 4 6% 8% 33%	3988
	11th to 20th	Night	13% 6% 3 7% 4 4 5% 4 7% 8% 39%	4557
	21st to end	Night	12% 5% 2 5% 4 4 4 3 5% 7% 49%	5045
September	1st to 10th	Night	15% 5% 3 6% 4 4 4 3 5% 6% 44%	2100
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 4 7% 51%	2116
	21st to end	Night	18% 7% 3 5% 4 3 3 3 4 6% 43%	2200
October	1st to 10th	Night	19% 6% 2 6% 4 4 3 3 5% 6% 43%	2310
	11th to 20th	Night	16% 5% 2 5% 3 3 3 3 4 6% 50%	2400
	21st to end	Night	20% 6% 2 5% 3 3 3 3 5% 5% 46%	2795
November	1st to 10th	Night	12% 5% 2 5% 3 4 3 4 4 6% 52%	2667
	11th to 20th	Night	18% 6% 2 5% 3 3 3 3 4 5% 49%	2730
	21st to end	Night	15% 6% 2 5% 4 3 3 4 5% 6% 46%	2814
December	1st to 10th	Night	15% 7% 3 5% 3 3 3 3 5% 6% 48%	4720
	11th to 20th	Night	25% 7% 2 4 3 3 3 2 4 4 43%	5348
	21st to end	Night	19% 9% 2 6% 4 3 3 3 4 5% 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	36% 3 11% 30% 19% 2	4662
	11th to 20th	Day	41% 3 10% 22% 18% 6%	4680
	21st to end	Day	44% 3 11% 24% 17% 1	5582
February	1st to 10th	Day	44% 5% 13% 24% 13% 2	1899
	11th to 20th	Day	43% 4 10% 25% 16% 2	2057
	21st to end	Day	45% 5% 11% 22% 14% 3	1730
March	1st to 10th	Day	42% 5% 12% 20% 14% 8%	4579
	11th to 20th	Day	38% 7% 13% 27% 11% 4	4891
	21st to end	Day	42% 5% 11% 13% 25% 5%	6051
April	1st to 10th	Day	44% 5% 11% 20% 16% 4	2520
	11th to 20th	Day	41% 5% 13% 21% 17% 3	2494
	21st to end	Day	42% 5% 11% 21% 17% 4	2541
May	1st to 10th	Day	50% 5% 13% 16% 14% 1	7651
	11th to 20th	Day	45% 6% 10% 23% 15% 1	8166
	21st to end	Day	48% 6% 11% 20% 14% 1	9061
June	1st to 10th	Day	39% 9% 13% 21% 17% 1	2940
	11th to 20th	Day	42% 8% 13% 22% 13% 2	2940
	21st to end	Day	38% 9% 14% 21% 16% 1	2940
July	1st to 10th	Day	42% 8% 13% 20% 16% 1	8210
	11th to 20th	Day	38% 7% 17% 24% 13% 1	8205
	21st to end	Day	40% 6% 11% 21% 20% 1	8795
August	1st to 10th	Day	46% 9% 17% 18% 9% 1	6084
	11th to 20th	Day	35% 6% 16% 32% 10% 1	5524
	21st to end	Day	38% 6% 11% 27% 17% 2	6080
September	1st to 10th	Day	38% 7% 11% 22% 19% 3	2520
	11th to 20th	Day	33% 5% 11% 24% 23% 3	2351
	21st to end	Day	45% 6% 10% 18% 18% 3	2200
October	1st to 10th	Day	37% 4 11% 19% 23% 6%	3397
	11th to 20th	Day	26% 4 11% 23% 31% 5%	3240
	21st to end	Day	28% 4 13% 26% 26% 4	3283
November	1st to 10th	Day	34% 4 12% 23% 24% 3	1851
	11th to 20th	Day	38% 4 9% 24% 21% 4	1890
	21st to end	Day	40% 4 14% 28% 11% 2	1803
December	1st to 10th	Day	39% 3 15% 28% 13% 2	2680
	11th to 20th	Day	42% 3 13% 23% 16% 2	3066
	21st to end	Day	44% 7% 10% 22% 15% 2	4048
January	1st to 10th	Night	39% 4 12% 23% 19% 3	8234
	11th to 20th	Night	42% 3 12% 21% 17% 6%	8280
	21st to end	Night	42% 4 9% 21% 20% 4	8581
February	1st to 10th	Night	44% 5% 12% 24% 13% 2	2719
	11th to 20th	Night	45% 4 9% 19% 18% 4	2460
	21st to end	Night	44% 4 8% 25% 16% 3	2076
March	1st to 10th	Night	43% 5% 10% 21% 13% 9%	5494
	11th to 20th	Night	39% 6% 14% 24% 13% 4	5284
	21st to end	Night	46% 5% 7% 14% 23% 5%	5122
April	1st to 10th	Night	46% 5% 11% 18% 16% 4	2100
	11th to 20th	Night	45% 5% 10% 19% 19% 2	2075
	21st to end	Night	42% 6% 8% 21% 19% 4	2079
May	1st to 10th	Night	53% 5% 11% 17% 12% 1	5265
	11th to 20th	Night	47% 6% 13% 19% 13% 2	4741
	21st to end	Night	51% 5% 11% 20% 10% 2	5115
June	1st to 10th	Night	40% 7% 10% 21% 21% 2	1680
	11th to 20th	Night	40% 8% 13% 25% 12% 1	1680
	21st to end	Night	36% 9% 14% 24% 15% 2	1680
July	1st to 10th	Night	44% 8% 12% 21% 14% 2	4640
	11th to 20th	Night	43% 7% 13% 24% 12% 1	4632
	21st to end	Night	39% 6% 15% 20% 19% 1	4967
August	1st to 10th	Night	49% 8% 15% 18% 10% 1	3978
	11th to 20th	Night	41% 7% 15% 29% 8%	4587
	21st to end	Night	35% 5% 12% 28% 18% 1	5045
September	1st to 10th	Night	41% 6% 10% 23% 18% 1	2100
	11th to 20th	Night	33% 5% 11% 23% 25% 3	2116
	21st to end	Night	42% 5% 9% 21% 21% 2	2200
October	1st to 10th	Night	37% 6% 12% 23% 20% 2	3778
	11th to 20th	Night	25% 4 9% 27% 30% 4	3899
	21st to end	Night	30% 4 11% 29% 23% 2	4507
November	1st to 10th	Night	34% 5% 10% 24% 22% 4	2667
	11th to 20th	Night	39% 4 8% 24% 20% 5%	2730
	21st to end	Night	39% 4 12% 28% 16% 2	2814
December	1st to 10th	Night	39% 4 12% 30% 13% 2	4720
	11th to 20th	Night	46% 3 9% 22% 16% 4	5348
	21st to end	Night	45% 5% 10% 18% 17% 4	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	23% 28% 31% 15% 2	220
	11th to 20th	Day	24% 34% 31% 9% 2	220
	21st to end	Day	20% 29% 33% 13% 5% 1	327
February	1st to 10th	Day	24% 23% 36% 15% 1	339
	11th to 20th	Day	26% 21% 36% 15% 2	536
	21st to end	Day	26% 19% 37% 15% 3	446
March	1st to 10th	Day	24% 22% 31% 18% 4	540
	11th to 20th	Day	24% 19% 36% 15% 5% 1	540
	21st to end	Day	26% 24% 32% 13% 4 1	594
April	1st to 10th	Day	27% 22% 32% 12% 6% 1	540
	11th to 20th	Day	23% 21% 33% 15% 8% 1	532
	21st to end	Day	25% 18% 33% 17% 5% 2	540
May	1st to 10th	Day	31% 19% 30% 14% 4 2	543
	11th to 20th	Day	22% 14% 36% 20% 6% 1	567
	21st to end	Day	23% 9% 30% 24% 13% 2	617
June	1st to 10th	Day	16% 13% 29% 28% 12% 3	570
	11th to 20th	Day	18% 13% 32% 25% 10% 2	570
	21st to end	Day	14% 12% 30% 27% 15% 2	570
July	1st to 10th	Day	13% 5% 27% 33% 18% 4	565
	11th to 20th	Day	10% 5% 29% 32% 20% 4	564
	21st to end	Day	14% 7% 29% 31% 16% 3	620
August	1st to 10th	Day	12% 6% 26% 32% 17% 6%	566
	11th to 20th	Day	14% 5% 24% 33% 18% 4	559
	21st to end	Day	11% 8% 30% 30% 14% 6%	627
September	1st to 10th	Day	18% 8% 31% 23% 14% 7%	570
	11th to 20th	Day	15% 7% 27% 28% 14% 9%	542
	21st to end	Day	22% 11% 28% 24% 11% 4	530
October	1st to 10th	Day	23% 8% 28% 21% 15% 5%	530
	11th to 20th	Day	20% 16% 29% 23% 10% 2	530
	21st to end	Day	22% 14% 34% 21% 7% 2	498
November	1st to 10th	Day	15% 14% 35% 22% 11% 3	425
	11th to 20th	Day	23% 19% 30% 22% 5%	430
	21st to end	Day	20% 19% 36% 19% 4 1	343
December	1st to 10th	Day	24% 24% 33% 13% 7%	220
	11th to 20th	Day	30% 23% 33% 11% 3	220
	21st to end	Day	27% 18% 37% 11% 7%	242
January	1st to 10th	Night	18% 17% 40% 22% 3	840
	11th to 20th	Night	25% 18% 31% 23% 3	840
	21st to end	Night	19% 17% 41% 21% 2	924
February	1st to 10th	Night	21% 15% 37% 25% 3	840
	11th to 20th	Night	23% 18% 39% 19% 1	823
	21st to end	Night	21% 19% 37% 21% 3	692
March	1st to 10th	Night	22% 15% 37% 22% 5%	840
	11th to 20th	Night	22% 16% 37% 21% 4 1	840
	21st to end	Night	22% 16% 38% 20% 5%	924
April	1st to 10th	Night	24% 16% 35% 21% 5%	840
	11th to 20th	Night	22% 15% 35% 23% 5%	831
	21st to end	Night	21% 12% 35% 25% 5% 1	819
May	1st to 10th	Night	23% 13% 35% 22% 7%	630
	11th to 20th	Night	21% 11% 37% 25% 7%	452
	21st to end	Night	21% 5% 24% 37% 12% 1	455
June	1st to 10th	Night	18% 6% 29% 36% 11%	420
	11th to 20th	Night	16% 8% 26% 37% 13%	420
	21st to end	Night	11% 4 24% 50% 10%	420
July	1st to 10th	Night	13% 5% 23% 41% 17% 1	410
	11th to 20th	Night	13% 3 22% 42% 18% 1	408
	21st to end	Night	14% 5% 20% 44% 16% 1	460
August	1st to 10th	Night	13% 3 19% 44% 20% 1	558
	11th to 20th	Night	15% 3 19% 39% 23% 1	830
	21st to end	Night	15% 4 20% 40% 19% 2	924
September	1st to 10th	Night	15% 5% 25% 36% 18% 2	840
	11th to 20th	Night	16% 3 23% 34% 19% 4	813
	21st to end	Night	22% 5% 25% 32% 15% 2	800
October	1st to 10th	Night	22% 8% 25% 27% 16% 3	800
	11th to 20th	Night	18% 11% 30% 29% 11% 1	800
	21st to end	Night	23% 9% 35% 24% 8%	880
November	1st to 10th	Night	15% 11% 33% 29% 11% 1	820
	11th to 20th	Night	21% 15% 33% 24% 7%	840
	21st to end	Night	18% 16% 35% 24% 6% 1	840
December	1st to 10th	Night	23% 14% 33% 24% 5% 1	840
	11th to 20th	Night	24% 14% 36% 24% 3	840
	21st to end	Night	23% 13% 37% 24% 3	924

Page generated at 2026-09-11 04:11:55. Version ac07

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