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 2024-03-01.

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 5% 5% 3 4 5% 5% 7% 40%	78393
	Night	17% 6% 2 5% 4 3 4 4 5% 6% 42%	80993
Transparency	Day	42% 7% 12% 22% 15% 2	78098
	Night	42% 6% 11% 23% 16% 3	80822
Seeing	Day	20% 15% 31% 22% 10% 3	14019
	Night	20% 11% 31% 28% 9% 1	23330

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	15% 7% 2 5% 4 2 4 4 4 5% 48%	2517
	11th to 20th	Day	15% 9% 3 5% 6% 5% 5% 4 5% 7% 36%	2520
	21st to end	Day	22% 11% 4 6% 5% 3 4 3 4 4 33%	3010
February	1st to 10th	Day	18% 8% 3 5% 5% 3 4 3 6% 6% 39%	1717
	11th to 20th	Day	15% 6% 4 5% 5% 3 4 4 4 6% 45%	1848
	21st to end	Day	20% 8% 2 5% 4 3 4 4 5% 6% 38%	1570
March	1st to 10th	Day	14% 6% 3 6% 5% 2 4 4 5% 7% 44%	1797
	11th to 20th	Day	16% 7% 2 4 5% 3 6% 5% 5% 7% 39%	1898
	21st to end	Day	20% 7% 3 5% 5% 2 4 4 4 6% 40%	2354
April	1st to 10th	Day	18% 8% 3 5% 4 3 4 4 5% 6% 40%	2160
	11th to 20th	Day	18% 5% 2 5% 5% 3 4 4 5% 7% 42%	2134
	21st to end	Day	20% 5% 2 5% 4 3 4 4 4 8% 39%	2178
May	1st to 10th	Day	23% 7% 2 5% 4 3 4 4 5% 7% 35%	2340
	11th to 20th	Day	21% 5% 2 5% 5% 3 4 5% 6% 8% 36%	2460
	21st to end	Day	16% 5% 2 5% 4 3 4 5% 5% 8% 42%	2725
June	1st to 10th	Day	17% 5% 2 5% 5% 3 4 6% 7% 9% 38%	2520
	11th to 20th	Day	16% 6% 3 5% 6% 3 4 6% 6% 9% 36%	2520
	21st to end	Day	11% 5% 3 5% 6% 3 5% 5% 7% 9% 40%	2520
July	1st to 10th	Day	11% 4 3 5% 7% 4 6% 6% 7% 9% 38%	2520
	11th to 20th	Day	10% 5% 2 4 5% 3 5% 6% 7% 11% 41%	2520
	21st to end	Day	12% 5% 3 6% 7% 3 6% 7% 6% 8% 36%	2744
August	1st to 10th	Day	10% 5% 2 6% 6% 4 6% 6% 7% 10% 38%	2366
	11th to 20th	Day	13% 5% 2 5% 6% 3 5% 6% 7% 7% 40%	2126
	21st to end	Day	13% 6% 2 5% 6% 3 5% 5% 5% 8% 42%	2376
September	1st to 10th	Day	15% 6% 2 4 5% 3 5% 4 6% 7% 44%	2160
	11th to 20th	Day	14% 5% 2 4 4 2 3 4 4 7% 50%	2084
	21st to end	Day	17% 6% 2 5% 6% 3 4 4 5% 5% 42%	1980
October	1st to 10th	Day	21% 6% 2 4 4 2 3 4 4 6% 42%	1877
	11th to 20th	Day	16% 4 2 5% 5% 3 4 3 5% 7% 46%	1800
	21st to end	Day	22% 5% 2 4 4 3 4 4 4 5% 43%	1841
November	1st to 10th	Day	15% 5% 2 4 4 3 2 4 3 5% 52%	1671
	11th to 20th	Day	19% 5% 2 4 4 2 4 3 5% 5% 47%	1710
	21st to end	Day	15% 6% 3 4 4 3 4 4 5% 7% 45%	1632
December	1st to 10th	Day	19% 7% 2 5% 5% 3 4 3 4 8% 39%	1520
	11th to 20th	Day	23% 5% 2 3 3 2 3 4 4 6% 45%	1906
	21st to end	Day	18% 8% 3 7% 6% 3 5% 5% 6% 7% 32%	2772
January	1st to 10th	Night	17% 7% 2 4 4 2 3 4 4 5% 48%	4429
	11th to 20th	Night	17% 9% 3 5% 4 3 4 4 4 5% 43%	4440
	21st to end	Night	25% 9% 3 5% 4 2 3 4 4 5% 35%	4628
February	1st to 10th	Night	17% 8% 3 5% 4 3 4 3 4 6% 43%	2461
	11th to 20th	Night	17% 8% 3 5% 5% 4 4 4 4 5% 42%	2218
	21st to end	Night	20% 7% 3 4 5% 2 4 5% 5% 5% 42%	1884
March	1st to 10th	Night	17% 6% 2 5% 4 2 4 4 5% 7% 45%	2157
	11th to 20th	Night	18% 6% 3 5% 4 3 4 4 4 7% 42%	2057
	21st to end	Night	18% 7% 2 5% 3 3 4 4 5% 6% 42%	2001
April	1st to 10th	Night	19% 6% 3 5% 5% 2 5% 4 5% 6% 39%	1800
	11th to 20th	Night	18% 7% 2 6% 4 3 5% 4 4 6% 41%	1775
	21st to end	Night	20% 7% 3 4 4 2 4 4 4 6% 42%	1782
May	1st to 10th	Night	22% 7% 3 6% 5% 3 3 5% 4 5% 36%	1620
	11th to 20th	Night	19% 7% 3 6% 5% 3 4 4 5% 6% 39%	1448
	21st to end	Night	17% 6% 2 4 4 3 5% 3 6% 8% 41%	1550
June	1st to 10th	Night	18% 5% 2 5% 4 3 5% 5% 6% 8% 40%	1440
	11th to 20th	Night	15% 5% 2 4 6% 3 5% 4 6% 9% 41%	1440
	21st to end	Night	10% 4 2 5% 6% 3 5% 5% 7% 10% 45%	1440
July	1st to 10th	Night	13% 4 3 6% 6% 4 5% 5% 8% 8% 40%	1440
	11th to 20th	Night	12% 5% 1 5% 6% 3 6% 7% 7% 9% 40%	1440
	21st to end	Night	11% 5% 2 4 5% 3 5% 6% 8% 9% 42%	1574
August	1st to 10th	Night	9% 5% 3 5% 5% 4 5% 6% 6% 9% 44%	1551
	11th to 20th	Night	11% 5% 3 7% 7% 4 5% 6% 7% 8% 37%	1774
	21st to end	Night	14% 5% 3 5% 5% 3 5% 5% 5% 8% 42%	1980
September	1st to 10th	Night	15% 5% 3 6% 5% 3 5% 5% 5% 6% 43%	1800
	11th to 20th	Night	16% 4 2 4 4 2 3 3 4 7% 52%	1876
	21st to end	Night	17% 6% 3 4 5% 2 3 4 4 6% 45%	1980
October	1st to 10th	Night	21% 6% 2 5% 4 3 4 3 5% 5% 42%	2078
	11th to 20th	Night	16% 4 2 5% 5% 2 3 4 4 7% 48%	2160
	21st to end	Night	21% 7% 2 5% 3 2 3 3 5% 5% 44%	2515
November	1st to 10th	Night	13% 5% 2 4 4 3 3 5% 4 5% 51%	2407
	11th to 20th	Night	20% 5% 3 5% 3 2 3 3 3 5% 48%	2470
	21st to end	Night	15% 5% 2 4 5% 2 4 5% 5% 7% 47%	2546
December	1st to 10th	Night	20% 6% 3 5% 4 3 3 4 5% 6% 40%	2660
	11th to 20th	Night	19% 7% 2 4 4 3 4 4 5% 6% 42%	3288
	21st to end	Night	20% 9% 3 5% 5% 3 4 4 4 6% 36%	4884

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% 3 11% 31% 14% 3	2518
	11th to 20th	Day	45% 6% 13% 23% 12% 2	2520
	21st to end	Day	54% 4 9% 21% 10% 2	3010
February	1st to 10th	Day	45% 5% 13% 23% 12% 2	1717
	11th to 20th	Day	42% 4 10% 25% 17% 3	1857
	21st to end	Day	46% 5% 10% 21% 14% 4	1570
March	1st to 10th	Day	39% 6% 14% 24% 15% 3	1800
	11th to 20th	Day	42% 6% 12% 23% 15% 2	1898
	21st to end	Day	46% 5% 9% 18% 18% 5%	2354
April	1st to 10th	Day	43% 5% 11% 20% 16% 4	2160
	11th to 20th	Day	40% 6% 11% 22% 17% 3	2134
	21st to end	Day	44% 5% 10% 20% 17% 4	2178
May	1st to 10th	Day	49% 5% 10% 18% 16% 2	2340
	11th to 20th	Day	44% 7% 11% 21% 15% 2	2464
	21st to end	Day	40% 7% 16% 22% 14% 2	2725
June	1st to 10th	Day	40% 9% 14% 21% 14% 2	2520
	11th to 20th	Day	44% 8% 13% 21% 12% 2	2520
	21st to end	Day	38% 9% 13% 23% 16% 1	2520
July	1st to 10th	Day	39% 10% 12% 21% 16% 1	2450
	11th to 20th	Day	33% 10% 16% 24% 15% 2	2445
	21st to end	Day	40% 10% 14% 18% 16% 3	2591
August	1st to 10th	Day	37% 11% 17% 22% 14% 1	2352
	11th to 20th	Day	38% 9% 15% 22% 14% 1	2126
	21st to end	Day	39% 7% 11% 21% 18% 3	2376
September	1st to 10th	Day	39% 8% 12% 21% 18% 3	2160
	11th to 20th	Day	33% 6% 12% 23% 24% 3	2084
	21st to end	Day	42% 6% 10% 19% 20% 3	1980
October	1st to 10th	Day	42% 6% 12% 22% 16% 2	1877
	11th to 20th	Day	37% 6% 12% 25% 19% 2	1800
	21st to end	Day	43% 5% 10% 22% 17% 2	1841
November	1st to 10th	Day	36% 4 11% 23% 23% 3	1671
	11th to 20th	Day	39% 5% 9% 23% 21% 4	1710
	21st to end	Day	39% 5% 14% 29% 12% 3	1632
December	1st to 10th	Day	45% 4 15% 21% 14% 1	1520
	11th to 20th	Day	42% 4 14% 24% 14% 2	1906
	21st to end	Day	48% 9% 12% 22% 8%	2772
January	1st to 10th	Night	39% 4 10% 30% 12% 4	4429
	11th to 20th	Night	44% 5% 12% 25% 12% 2	4440
	21st to end	Night	51% 5% 9% 22% 12% 2	4628
February	1st to 10th	Night	44% 5% 12% 24% 13% 2	2461
	11th to 20th	Night	45% 4 9% 19% 18% 5%	2220
	21st to end	Night	44% 4 8% 25% 16% 4	1884
March	1st to 10th	Night	40% 5% 13% 23% 15% 5%	2167
	11th to 20th	Night	42% 6% 10% 21% 17% 4	2058
	21st to end	Night	44% 5% 9% 17% 19% 6%	2001
April	1st to 10th	Night	44% 5% 11% 19% 17% 3	1800
	11th to 20th	Night	44% 5% 9% 20% 19% 2	1775
	21st to end	Night	43% 6% 8% 21% 18% 4	1782
May	1st to 10th	Night	50% 5% 10% 17% 15% 2	1620
	11th to 20th	Night	46% 7% 10% 15% 18% 3	1449
	21st to end	Night	42% 7% 16% 20% 12% 3	1551
June	1st to 10th	Night	42% 7% 10% 21% 19% 2	1440
	11th to 20th	Night	39% 9% 14% 25% 12% 1	1440
	21st to end	Night	34% 9% 15% 25% 16% 1	1440
July	1st to 10th	Night	40% 10% 13% 20% 16% 2	1400
	11th to 20th	Night	37% 10% 14% 25% 12% 2	1392
	21st to end	Night	32% 10% 19% 20% 16% 3	1486
August	1st to 10th	Night	35% 11% 16% 22% 15% 2	1541
	11th to 20th	Night	41% 10% 14% 23% 10% 1	1774
	21st to end	Night	39% 8% 13% 19% 19% 2	1980
September	1st to 10th	Night	41% 7% 10% 23% 17% 1	1800
	11th to 20th	Night	33% 5% 11% 23% 24% 3	1876
	21st to end	Night	40% 6% 8% 22% 23% 2	1980
October	1st to 10th	Night	43% 6% 12% 22% 14% 3	2078
	11th to 20th	Night	36% 5% 13% 27% 17% 2	2160
	21st to end	Night	43% 5% 10% 23% 17% 1	2515
November	1st to 10th	Night	35% 6% 10% 23% 22% 4	2407
	11th to 20th	Night	40% 4 9% 24% 19% 4	2470
	21st to end	Night	38% 4 13% 28% 15% 2	2546
December	1st to 10th	Night	44% 6% 12% 23% 13% 2	2660
	11th to 20th	Night	43% 5% 12% 20% 17% 4	3288
	21st to end	Night	47% 7% 12% 21% 12%	4884

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	21% 28% 30% 18% 3 1	180
	11th to 20th	Day	23% 34% 29% 11% 3	180
	21st to end	Day	20% 27% 32% 13% 6% 1	267
February	1st to 10th	Day	26% 25% 34% 13% 2	277
	11th to 20th	Day	24% 21% 37% 16% 2	456
	21st to end	Day	28% 18% 34% 15% 3	382
March	1st to 10th	Day	20% 25% 31% 20% 4	420
	11th to 20th	Day	23% 19% 38% 14% 5% 1	420
	21st to end	Day	27% 21% 33% 13% 5% 1	462
April	1st to 10th	Day	24% 22% 33% 13% 7% 1	420
	11th to 20th	Day	21% 21% 31% 17% 9% 1	412
	21st to end	Day	28% 17% 34% 15% 4 2	420
May	1st to 10th	Day	32% 19% 27% 16% 4 3	423
	11th to 20th	Day	22% 14% 36% 20% 6% 1	447
	21st to end	Day	20% 7% 28% 26% 15% 3	485
June	1st to 10th	Day	16% 13% 30% 26% 12% 3	450
	11th to 20th	Day	17% 13% 32% 26% 9% 2	450
	21st to end	Day	14% 12% 30% 27% 15% 2	450
July	1st to 10th	Day	12% 5% 27% 33% 17% 5%	445
	11th to 20th	Day	10% 4 28% 32% 21% 4	444
	21st to end	Day	14% 7% 29% 31% 15% 3	488
August	1st to 10th	Day	10% 7% 26% 33% 17% 7%	446
	11th to 20th	Day	16% 4 24% 33% 20% 5%	439
	21st to end	Day	12% 8% 28% 30% 16% 6%	495
September	1st to 10th	Day	18% 8% 29% 22% 15% 8%	450
	11th to 20th	Day	15% 7% 29% 26% 14% 10%	450
	21st to end	Day	20% 12% 30% 22% 11% 5%	450
October	1st to 10th	Day	26% 8% 27% 21% 14% 4	450
	11th to 20th	Day	19% 16% 28% 25% 11% 1	450
	21st to end	Day	24% 14% 34% 20% 6% 2	426
November	1st to 10th	Day	16% 14% 32% 23% 12% 3	365
	11th to 20th	Day	23% 18% 31% 21% 6%	370
	21st to end	Day	18% 19% 35% 21% 5% 2	292
December	1st to 10th	Day	23% 26% 32% 12% 8%	180
	11th to 20th	Day	29% 25% 34% 9% 2	180
	21st to end	Day	26% 19% 36% 11% 8%	198
January	1st to 10th	Night	19% 16% 40% 22% 3	760
	11th to 20th	Night	26% 17% 31% 23% 3	760
	21st to end	Night	20% 17% 40% 22% 2	836
February	1st to 10th	Night	22% 15% 36% 25% 3	760
	11th to 20th	Night	23% 18% 40% 18% 1	743
	21st to end	Night	23% 19% 34% 21% 3	628
March	1st to 10th	Night	21% 15% 38% 21% 5%	723
	11th to 20th	Night	23% 14% 38% 21% 4 1	720
	21st to end	Night	21% 15% 37% 21% 6%	792
April	1st to 10th	Night	23% 16% 35% 22% 5%	720
	11th to 20th	Night	22% 14% 34% 24% 6%	711
	21st to end	Night	23% 12% 34% 25% 5% 1	702
May	1st to 10th	Night	24% 12% 34% 23% 7%	540
	11th to 20th	Night	21% 10% 38% 24% 7%	387
	21st to end	Night	20% 5% 23% 38% 14% 1	389
June	1st to 10th	Night	19% 7% 28% 34% 12%	360
	11th to 20th	Night	16% 7% 25% 38% 14%	360
	21st to end	Night	10% 4 23% 52% 10% 1	360
July	1st to 10th	Night	12% 5% 25% 40% 18% 1	350
	11th to 20th	Night	12% 3 22% 43% 19% 1	348
	21st to end	Night	14% 4 21% 45% 15% 1	394
August	1st to 10th	Night	10% 4 21% 44% 20% 1	477
	11th to 20th	Night	15% 3 18% 38% 24% 1	710
	21st to end	Night	15% 4 18% 41% 20% 2	792
September	1st to 10th	Night	16% 5% 24% 35% 18% 2	720
	11th to 20th	Night	18% 3 24% 32% 18% 4	720
	21st to end	Night	21% 5% 26% 32% 15% 2	720
October	1st to 10th	Night	23% 8% 24% 27% 14% 3	720
	11th to 20th	Night	18% 11% 29% 31% 11% 1	720
	21st to end	Night	24% 9% 36% 23% 8%	792
November	1st to 10th	Night	15% 11% 33% 28% 11% 1	740
	11th to 20th	Night	22% 14% 32% 24% 8% 1	760
	21st to end	Night	17% 16% 35% 24% 7% 1	760
December	1st to 10th	Night	24% 14% 32% 24% 5% 1	760
	11th to 20th	Night	23% 13% 37% 25% 3	760
	21st to end	Night	23% 13% 37% 24% 3	836

Page generated at 2024-02-27 06:30:15. Version ac07

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