The Mad Professor's Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Mad Professor's Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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	2 3 1 4 3 4 5% 4 7% 9% 59%	135650
	Night	2 4 2 4 3 3 4 3 6% 8% 62%	132888
Transparency	Day	19% 7% 13% 25% 25% 10%	130612
	Night	19% 6% 13% 26% 26% 10%	128930
Seeing	Day	5% 10% 25% 31% 21% 8%	16831
	Night	3 5% 29% 42% 19% 2	26959

By Month

Percentage Cloud Cover SW (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	4 3 7% 3 3 4 3 6% 8% 60%	3747
	11th to 20th	Day	2 1 5% 3 3 4 3 6% 7% 67%	4100
	21st to end	Day	5% 2 6% 4 3 4 4 5% 8% 59%	4510
February	1st to 10th	Day	1 3 2 5% 3 4 3 3 6% 7% 63%	1800
	11th to 20th	Day	1 2 1 3 3 3 3 5% 5% 9% 65%	1913
	21st to end	Day	3 2 4 3 3 4 3 6% 7% 64%	1650
March	1st to 10th	Day	3 1 3 3 3 3 3 5% 7% 67%	4679
	11th to 20th	Day	2 8% 2 6% 4 5% 5% 4 6% 8% 50%	5000
	21st to end	Day	1 4 2 5% 3 3 4 4 6% 9% 59%	5692
April	1st to 10th	Day	1 4 1 4 3 3 4 3 6% 8% 63%	2400
	11th to 20th	Day	2 5% 1 4 3 4 4 4 7% 8% 59%	2371
	21st to end	Day	3 5% 1 4 4 3 4 4 6% 8% 57%	2400
May	1st to 10th	Day	5% 5% 2 4 3 4 4 4 6% 8% 56%	5440
	11th to 20th	Day	3 4 1 4 3 4 5% 5% 7% 7% 56%	6148
	21st to end	Day	2 2 1 3 3 3 4 4 7% 8% 63%	6925
June	1st to 10th	Day	2 3 1 4 3 3 5% 6% 8% 9% 57%	2800
	11th to 20th	Day	2 1 1 3 2 3 4 4 6% 8% 67%	2800
	21st to end	Day	2 3 1 4 4 5% 5% 6% 8% 8% 55%	2800
July	1st to 10th	Day	2 2 1 3 3 4 5% 4 7% 9% 60%	6339
	11th to 20th	Day	1 2 1 4 4 5% 8% 7% 10% 12% 46%	6163
	21st to end	Day	3 4 1 6% 5% 6% 8% 6% 9% 12% 40%	6401
August	1st to 10th	Day	3 4 1 5% 5% 5% 6% 6% 9% 12% 45%	5779
	11th to 20th	Day	2 2 1 3 4 5% 6% 5% 9% 11% 52%	5369
	21st to end	Day	4 5% 2 6% 4 6% 6% 6% 9% 11% 42%	5984
September	1st to 10th	Day	3 3 1 5% 4 4 5% 4 7% 9% 54%	2400
	11th to 20th	Day	3 3 1 4 2 3 4 5% 7% 8% 60%	2400
	21st to end	Day	3 2 1 3 3 3 3 4 5% 7% 67%	2165
October	1st to 10th	Day	2 2 1 2 2 2 2 3 4 6% 75%	3270
	11th to 20th	Day	4 3 1 4 4 4 4 3 6% 8% 60%	3250
	21st to end	Day	2 2 1 2 1 2 2 2 3 6% 75%	3548
November	1st to 10th	Day	1 2 1 2 2 1 2 3 3 7% 76%	1918
	11th to 20th	Day	1 2 1 2 1 2 2 2 4 7% 77%	1890
	21st to end	Day	1 3 1 4 2 2 3 3 4 7% 69%	1813
December	1st to 10th	Day	2 1 2 2 2 2 2 3 6% 77%	2700
	11th to 20th	Day	2 1 2 2 2 3 4 7% 10% 67%	3082
	21st to end	Day	2 1 4 3 3 4 4 7% 9% 62%	4004
January	1st to 10th	Night	5% 3 7% 3 3 3 3 6% 8% 58%	6262
	11th to 20th	Night	1 2 1 3 2 2 2 3 4 7% 72%	5880
	21st to end	Night	5% 3 6% 3 4 4 3 6% 6% 60%	6468
February	1st to 10th	Night	1 4 2 4 3 3 2 3 4 7% 67%	2600
	11th to 20th	Night	1 4 1 5% 3 3 4 3 5% 6% 66%	2373
	21st to end	Night	4 2 3 2 2 3 2 5% 6% 70%	1980
March	1st to 10th	Night	1 5% 2 5% 2 3 3 3 5% 5% 66%	5198
	11th to 20th	Night	2 8% 2 5% 4 3 3 3 5% 6% 59%	5000
	21st to end	Night	1 4 2 4 3 3 3 2 5% 6% 67%	5271
April	1st to 10th	Night	1 4 1 3 3 3 3 3 5% 7% 67%	2000
	11th to 20th	Night	3 4 2 5% 3 4 3 3 5% 6% 62%	1977
	21st to end	Night	4 5% 1 4 3 3 4 3 4 7% 63%	2000
May	1st to 10th	Night	7% 5% 2 5% 4 4 4 3 5% 7% 56%	4540
	11th to 20th	Night	6% 4 1 3 3 3 4 3 6% 7% 60%	3763
	21st to end	Night	3 3 1 3 3 3 3 3 5% 7% 67%	3972
June	1st to 10th	Night	2 1 1 3 4 4 3 4 6% 8% 65%	1600
	11th to 20th	Night	1 2 3 2 3 3 3 4 6% 73%	1600
	21st to end	Night	3 3 2 4 4 4 5% 5% 6% 9% 56%	1600
July	1st to 10th	Night	3 2 1 4 4 4 5% 5% 6% 9% 56%	3640
	11th to 20th	Night	2 2 1 5% 4 5% 6% 6% 10% 14% 44%	3813
	21st to end	Night	3 4 2 5% 5% 5% 6% 6% 10% 12% 41%	4472
August	1st to 10th	Night	3 4 2 6% 5% 5% 5% 5% 8% 9% 50%	4033
	11th to 20th	Night	3 3 1 5% 4 4 6% 6% 10% 15% 43%	4478
	21st to end	Night	4 5% 2 7% 5% 6% 7% 6% 10% 12% 35%	4993
September	1st to 10th	Night	6% 3 2 5% 3 4 5% 4 7% 9% 52%	2000
	11th to 20th	Night	4 3 1 3 2 3 4 3 5% 8% 63%	2000
	21st to end	Night	4 3 1 4 3 3 3 3 5% 6% 65%	2229
October	1st to 10th	Night	2 2 1 2 2 2 3 2 4 6% 74%	3920
	11th to 20th	Night	5% 4 1 3 3 3 4 3 5% 8% 61%	3898
	21st to end	Night	2 3 1 3 2 2 2 1 3 6% 76%	4252
November	1st to 10th	Night	1 2 1 2 2 2 2 2 4 7% 76%	2578
	11th to 20th	Night	1 2 1 2 1 3 2 2 4 5% 78%	2730
	21st to end	Night	2 4 1 3 2 2 3 3 4 6% 70%	2804
December	1st to 10th	Night	2 1 3 1 2 2 2 3 5% 78%	4710
	11th to 20th	Night	1 2 1 3 2 2 3 3 5% 8% 70%	5280
	21st to end	Night	1 3 1 4 3 4 3 3 6% 8% 63%	6974

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	19% 4 15% 33% 25% 4	3748
	11th to 20th	Day	14% 4 12% 24% 38% 8%	4100
	21st to end	Day	21% 7% 10% 27% 28% 7%	4510
February	1st to 10th	Day	17% 6% 16% 29% 24% 9%	1800
	11th to 20th	Day	16% 6% 12% 30% 27% 8%	1923
	21st to end	Day	16% 6% 11% 22% 32% 14%	1650
March	1st to 10th	Day	15% 4 9% 14% 41% 18%	4679
	11th to 20th	Day	27% 6% 10% 26% 25% 6%	5000
	21st to end	Day	18% 6% 11% 42% 20% 3	5692
April	1st to 10th	Day	18% 5% 12% 27% 27% 12%	2400
	11th to 20th	Day	20% 7% 10% 23% 28% 11%	2374
	21st to end	Day	22% 6% 13% 25% 26% 7%	2400
May	1st to 10th	Day	24% 6% 12% 21% 23% 14%	5440
	11th to 20th	Day	19% 8% 14% 21% 23% 15%	6152
	21st to end	Day	15% 6% 12% 19% 32% 16%	6926
June	1st to 10th	Day	19% 8% 14% 23% 23% 14%	2800
	11th to 20th	Day	13% 5% 10% 18% 26% 27%	2800
	21st to end	Day	21% 7% 12% 20% 23% 16%	2800
July	1st to 10th	Day	17% 8% 11% 15% 24% 26%	6271
	11th to 20th	Day	23% 11% 23% 26% 10% 6%	6088
	21st to end	Day	30% 12% 18% 22% 13% 4	6217
August	1st to 10th	Day	34% 13% 20% 17% 11% 5%	4208
	11th to 20th	Day	25% 13% 17% 18% 18% 8%	3885
	21st to end	Day	27% 13% 17% 18% 20% 4	4311
September	1st to 10th	Day	25% 10% 13% 22% 21% 10%	2400
	11th to 20th	Day	21% 8% 11% 18% 29% 12%	2400
	21st to end	Day	17% 6% 10% 23% 37% 8%	2165
October	1st to 10th	Day	12% 5% 7% 22% 36% 19%	3270
	11th to 20th	Day	22% 4 8% 28% 30% 8%	3250
	21st to end	Day	11% 3 6% 32% 42% 6%	3548
November	1st to 10th	Day	10% 4 9% 31% 37% 9%	1918
	11th to 20th	Day	9% 3 11% 38% 32% 7%	1890
	21st to end	Day	15% 5% 11% 31% 31% 8%	1813
December	1st to 10th	Day	10% 3 12% 43% 26% 6%	2700
	11th to 20th	Day	10% 5% 19% 40% 23% 3	3080
	21st to end	Day	14% 4 17% 29% 26% 9%	4004
January	1st to 10th	Night	21% 3 12% 34% 25% 5%	6262
	11th to 20th	Night	11% 4 10% 25% 39% 12%	5880
	21st to end	Night	19% 6% 16% 25% 25% 10%	6468
February	1st to 10th	Night	17% 4 14% 31% 25% 10%	2600
	11th to 20th	Night	17% 4 11% 32% 28% 8%	2373
	21st to end	Night	14% 3 10% 23% 37% 13%	1980
March	1st to 10th	Night	18% 3 6% 17% 43% 13%	5198
	11th to 20th	Night	23% 5% 10% 21% 31% 11%	5000
	21st to end	Night	17% 4 11% 42% 22% 4	5271
April	1st to 10th	Night	15% 4 12% 29% 26% 13%	2000
	11th to 20th	Night	19% 4 12% 24% 29% 12%	1977
	21st to end	Night	20% 5% 13% 26% 27% 9%	2000
May	1st to 10th	Night	27% 5% 12% 20% 22% 14%	4537
	11th to 20th	Night	19% 5% 14% 21% 26% 14%	3764
	21st to end	Night	15% 5% 13% 16% 31% 20%	3972
June	1st to 10th	Night	17% 7% 15% 21% 24% 17%	1600
	11th to 20th	Night	12% 4 8% 19% 29% 29%	1600
	21st to end	Night	22% 8% 14% 17% 21% 18%	1600
July	1st to 10th	Night	22% 9% 14% 15% 14% 26%	3600
	11th to 20th	Night	24% 11% 31% 22% 5% 6%	3765
	21st to end	Night	29% 12% 24% 18% 12% 5%	4344
August	1st to 10th	Night	37% 15% 21% 12% 9% 6%	2928
	11th to 20th	Night	26% 12% 26% 15% 13% 8%	3238
	21st to end	Night	32% 12% 20% 20% 13% 3	3597
September	1st to 10th	Night	26% 9% 15% 21% 20% 9%	2000
	11th to 20th	Night	19% 8% 12% 18% 28% 15%	2000
	21st to end	Night	18% 6% 8% 22% 36% 9%	2229
October	1st to 10th	Night	12% 4 9% 23% 31% 20%	3920
	11th to 20th	Night	21% 5% 8% 27% 30% 9%	3898
	21st to end	Night	11% 3 6% 35% 35% 10%	4252
November	1st to 10th	Night	9% 3 8% 32% 39% 9%	2578
	11th to 20th	Night	9% 3 9% 36% 33% 10%	2730
	21st to end	Night	15% 4 10% 29% 33% 9%	2804
December	1st to 10th	Night	11% 3 11% 46% 24% 5%	4710
	11th to 20th	Night	13% 4 14% 41% 26% 3	5281
	21st to end	Night	17% 4 16% 31% 25% 6%	6974

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	4 6% 31% 42% 16% 1	300
	11th to 20th	Day	4 9% 34% 39% 14% 1	300
	21st to end	Day	1 2 35% 43% 16% 2	330
February	1st to 10th	Day	3 13% 35% 40% 8% 1	300
	11th to 20th	Day	5% 12% 40% 31% 11% 1	300
	21st to end	Day	9% 16% 35% 31% 7% 2	249
March	1st to 10th	Day	7% 19% 33% 29% 11% 1	375
	11th to 20th	Day	7% 20% 33% 26% 12% 2	500
	21st to end	Day	11% 26% 33% 19% 9% 2	550
April	1st to 10th	Day	9% 25% 31% 23% 10% 2	500
	11th to 20th	Day	11% 30% 31% 19% 8% 1	492
	21st to end	Day	9% 23% 34% 24% 10% 1	500
May	1st to 10th	Day	8% 22% 35% 22% 10% 2	503
	11th to 20th	Day	6% 12% 26% 34% 17% 5%	675
	21st to end	Day	5% 12% 26% 33% 18% 5%	789
June	1st to 10th	Day	3 8% 23% 32% 27% 7%	730
	11th to 20th	Day	2 7% 28% 34% 22% 7%	730
	21st to end	Day	3 8% 21% 32% 29% 8%	730
July	1st to 10th	Day	3 3 14% 31% 36% 13%	721
	11th to 20th	Day	4 2 10% 28% 38% 18%	645
	21st to end	Day	6% 2 13% 27% 33% 19%	575
August	1st to 10th	Day	5% 3 16% 28% 31% 16%	527
	11th to 20th	Day	4 2 17% 30% 29% 17%	517
	21st to end	Day	5% 3 17% 30% 29% 16%	583
September	1st to 10th	Day	4 4 16% 29% 31% 15%	530
	11th to 20th	Day	3 5% 26% 33% 25% 8%	530
	21st to end	Day	4 7% 16% 26% 31% 16%	356
October	1st to 10th	Day	4 10% 17% 26% 28% 16%	330
	11th to 20th	Day	5% 12% 22% 28% 25% 9%	330
	21st to end	Day	8% 9% 21% 32% 23% 7%	363
November	1st to 10th	Day	3 11% 24% 37% 21% 4	330
	11th to 20th	Day	2 8% 24% 38% 23% 5%	330
	21st to end	Day	4 7% 25% 40% 19% 5%	330
December	1st to 10th	Day	3 5% 32% 43% 12% 6%	330
	11th to 20th	Day	2 5% 32% 37% 19% 4	321
	21st to end	Day	5% 7% 36% 38% 11% 2	330
January	1st to 10th	Night	1 5% 32% 47% 13% 2	974
	11th to 20th	Night	3 4 34% 44% 14% 1	800
	21st to end	Night	1 2 35% 48% 12% 1	880
February	1st to 10th	Night	2 4 43% 42% 8% 1	800
	11th to 20th	Night	2 5% 43% 41% 8%	784
	21st to end	Night	2 8% 47% 38% 4	660
March	1st to 10th	Night	3 9% 41% 37% 9%	800
	11th to 20th	Night	4 5% 35% 45% 11% 1	800
	21st to end	Night	3 8% 45% 37% 6%	791
April	1st to 10th	Night	4 10% 47% 35% 5%	600
	11th to 20th	Night	5% 12% 41% 34% 8%	593
	21st to end	Night	5% 8% 42% 36% 8% 1	600
May	1st to 10th	Night	8% 4 35% 41% 11% 1	600
	11th to 20th	Night	7% 4 28% 44% 15% 2	591
	21st to end	Night	4 7% 30% 41% 17% 1	650
June	1st to 10th	Night	2 3 22% 49% 24% 2	600
	11th to 20th	Night	2 3 26% 51% 17% 1	600
	21st to end	Night	3 2 18% 45% 30% 2	600
July	1st to 10th	Night	4 1 12% 42% 37% 4	585
	11th to 20th	Night	4 1 6% 35% 47% 7%	582
	21st to end	Night	4 1 7% 27% 53% 8%	657
August	1st to 10th	Night	3 2 10% 31% 48% 6%	597
	11th to 20th	Night	5% 1 9% 41% 40% 4	594
	21st to end	Night	5% 1 10% 42% 38% 5%	660
September	1st to 10th	Night	6% 1 12% 45% 32% 5%	600
	11th to 20th	Night	5% 1 15% 51% 26% 3	600
	21st to end	Night	5% 2 19% 44% 29% 1	657
October	1st to 10th	Night	3 3 25% 44% 23% 2	800
	11th to 20th	Night	3 2 31% 43% 19% 3	800
	21st to end	Night	4 4 27% 42% 20% 3	880
November	1st to 10th	Night	3 5% 27% 49% 16% 1	938
	11th to 20th	Night	1 6% 30% 45% 17% 1	1050
	21st to end	Night	4 5% 30% 42% 17% 1	1050
December	1st to 10th	Night	1 5% 34% 46% 14% 1	1050
	11th to 20th	Night	2 6% 32% 45% 14% 1	1036
	21st to end	Night	3 7% 33% 45% 11%	1100

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