The Mount Academy
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Mount Academy clear sky chart from 2005-11-06 to 2026-09-29.

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% 7% 3 7% 5% 4 5% 4 6% 8% 38%	149352
	Night	17% 8% 3 7% 5% 4 4 4 5% 7% 37%	142058
Transparency	Day	43% 7% 12% 22% 14% 2	152855
	Night	44% 6% 11% 21% 15% 2	146644
Seeing	Day	21% 14% 28% 23% 11% 3	17341
	Night	20% 10% 30% 29% 9% 1	26368

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% 4 9% 6% 5% 5% 4 5% 6% 36%	4661
	11th to 20th	Day	14% 9% 3 8% 5% 4 4 4 5% 6% 37%	4650
	21st to end	Day	13% 8% 3 6% 4 5% 5% 4 5% 7% 40%	5441
February	1st to 10th	Day	14% 9% 3 8% 6% 5% 5% 4 6% 6% 34%	1890
	11th to 20th	Day	13% 9% 4 9% 5% 4 5% 3 5% 7% 37%	2034
	21st to end	Day	19% 9% 3 5% 6% 4 4 4 5% 6% 36%	1730
March	1st to 10th	Day	22% 9% 3 7% 4 3 4 2 4 4 38%	4576
	11th to 20th	Day	13% 8% 3 6% 4 4 4 3 5% 7% 43%	4850
	21st to end	Day	20% 8% 2 7% 5% 4 4 3 5% 6% 36%	6073
April	1st to 10th	Day	19% 9% 4 7% 4 3 4 3 5% 6% 36%	2520
	11th to 20th	Day	16% 7% 2 6% 5% 4 4 3 5% 6% 40%	2494
	21st to end	Day	20% 6% 2 5% 4 4 5% 3 4 5% 40%	2581
May	1st to 10th	Day	26% 7% 2 6% 4 3 4 3 5% 7% 32%	7697
	11th to 20th	Day	18% 8% 2 6% 4 4 4 4 5% 7% 39%	8221
	21st to end	Day	21% 7% 2 6% 4 4 5% 5% 6% 9% 31%	9060
June	1st to 10th	Day	15% 6% 1 6% 5% 5% 6% 4 7% 10% 36%	2940
	11th to 20th	Day	14% 6% 2 6% 5% 4 5% 4 7% 10% 36%	2940
	21st to end	Day	13% 6% 3 7% 5% 5% 5% 5% 6% 10% 37%	2940
July	1st to 10th	Day	13% 6% 3 7% 5% 5% 6% 4 6% 9% 36%	8280
	11th to 20th	Day	7% 5% 2 6% 4 5% 6% 4 8% 12% 41%	8280
	21st to end	Day	13% 6% 3 6% 5% 4 5% 4 5% 8% 41%	8993
August	1st to 10th	Day	15% 6% 3 8% 6% 6% 6% 5% 7% 9% 29%	6303
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 4 8% 10% 40%	5488
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 4 6% 10% 40%	6080
September	1st to 10th	Day	13% 7% 2 6% 4 5% 5% 4 5% 7% 43%	2520
	11th to 20th	Day	13% 5% 2 5% 3 4 4 3 6% 7% 49%	2428
	21st to end	Day	17% 6% 1 6% 4 4 5% 4 6% 7% 40%	2288
October	1st to 10th	Day	19% 6% 2 5% 4 4 3 3 4 6% 42%	2085
	11th to 20th	Day	14% 7% 2 6% 4 4 4 4 5% 7% 42%	2000
	21st to end	Day	21% 7% 2 6% 4 4 5% 3 5% 7% 35%	2015
November	1st to 10th	Day	13% 7% 2 6% 4 4 4 3 4 7% 47%	1851
	11th to 20th	Day	16% 8% 2 6% 4 4 4 3 5% 5% 42%	1890
	21st to end	Day	18% 10% 2 6% 4 3 4 3 5% 6% 38%	1759
December	1st to 10th	Day	17% 8% 3 6% 5% 4 5% 4 5% 7% 36%	2680
	11th to 20th	Day	22% 8% 2 5% 3 4 3 4 5% 6% 39%	3066
	21st to end	Day	19% 9% 3 9% 5% 4 4 3 4 6% 33%	4048
January	1st to 10th	Night	15% 8% 3 10% 5% 4 5% 3 6% 6% 34%	8234
	11th to 20th	Night	13% 9% 4 11% 7% 6% 6% 4 5% 6% 30%	8235
	21st to end	Night	14% 9% 3 7% 5% 5% 5% 4 5% 6% 37%	8725
February	1st to 10th	Night	15% 9% 3 7% 5% 5% 5% 4 4 6% 38%	2730
	11th to 20th	Night	15% 9% 3 7% 5% 4 5% 4 6% 6% 36%	2470
	21st to end	Night	17% 9% 2 7% 5% 5% 4 4 5% 6% 37%	2076
March	1st to 10th	Night	20% 8% 2 5% 4 4 4 3 5% 6% 39%	5484
	11th to 20th	Night	15% 10% 3 7% 4 4 4 4 5% 6% 40%	5324
	21st to end	Night	19% 10% 3 7% 5% 4 4 3 4 4 36%	5101
April	1st to 10th	Night	19% 11% 2 7% 5% 4 4 4 5% 7% 31%	2100
	11th to 20th	Night	19% 7% 3 6% 5% 4 4 4 4 7% 37%	2075
	21st to end	Night	21% 8% 3 6% 4 3 4 3 4 6% 40%	2037
May	1st to 10th	Night	28% 7% 2 6% 4 3 3 3 5% 6% 31%	5244
	11th to 20th	Night	18% 8% 3 8% 4 4 4 4 5% 8% 35%	4639
	21st to end	Night	21% 8% 3 7% 4 5% 4 4 5% 7% 32%	5114
June	1st to 10th	Night	15% 6% 2 6% 4 4 4 4 5% 8% 42%	1680
	11th to 20th	Night	15% 7% 1 6% 4 4 4 3 7% 10% 38%	1680
	21st to end	Night	13% 4 2 5% 3 4 4 5% 8% 9% 42%	1680
July	1st to 10th	Night	15% 6% 2 6% 4 5% 4 4 7% 10% 37%	4680
	11th to 20th	Night	10% 6% 2 8% 5% 5% 5% 5% 7% 9% 39%	4680
	21st to end	Night	12% 7% 3 6% 4 4 5% 3 5% 7% 45%	5084
August	1st to 10th	Night	17% 7% 3 8% 6% 5% 4 4 7% 9% 31%	3797
	11th to 20th	Night	13% 6% 3 7% 4 4 5% 4 7% 9% 38%	4557
	21st to end	Night	13% 5% 2 6% 4 4 4 3 5% 8% 45%	5045
September	1st to 10th	Night	15% 6% 2 6% 4 4 4 3 4 6% 46%	2100
	11th to 20th	Night	12% 5% 2 5% 3 4 3 2 5% 7% 52%	2191
	21st to end	Night	18% 7% 3 5% 3 3 4 3 5% 6% 44%	2289
October	1st to 10th	Night	20% 6% 2 6% 4 3 4 3 4 5% 42%	2310
	11th to 20th	Night	16% 7% 2 6% 4 4 4 3 4 6% 45%	2400
	21st to end	Night	23% 7% 2 6% 4 3 3 3 5% 6% 38%	2825
November	1st to 10th	Night	14% 6% 3 6% 4 3 4 4 5% 6% 45%	2667
	11th to 20th	Night	18% 8% 2 6% 3 3 4 4 4 5% 41%	2730
	21st to end	Night	15% 7% 2 6% 5% 4 4 3 6% 7% 40%	2857
December	1st to 10th	Night	17% 9% 3 7% 4 4 3 3 5% 7% 38%	4720
	11th to 20th	Night	26% 9% 3 6% 3 3 3 3 5% 5% 34%	5348
	21st to end	Night	21% 11% 3 7% 4 4 4 3 5% 6% 33%	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	46% 5% 11% 24% 12% 2	4662
	11th to 20th	Day	45% 5% 10% 19% 14% 7%	4680
	21st to end	Day	42% 5% 11% 24% 17% 1	5441
February	1st to 10th	Day	49% 6% 12% 19% 14% 1	1890
	11th to 20th	Day	46% 6% 10% 21% 14% 2	2043
	21st to end	Day	48% 5% 12% 21% 13% 2	1730
March	1st to 10th	Day	50% 5% 10% 17% 10% 8%	4579
	11th to 20th	Day	39% 6% 14% 28% 9% 4	4850
	21st to end	Day	49% 5% 7% 13% 21% 6%	6073
April	1st to 10th	Day	48% 6% 9% 20% 13% 4	2520
	11th to 20th	Day	44% 5% 11% 21% 16% 2	2494
	21st to end	Day	45% 6% 9% 21% 16% 4	2581
May	1st to 10th	Day	51% 7% 11% 17% 13% 1	7709
	11th to 20th	Day	44% 7% 12% 24% 13% 1	8225
	21st to end	Day	48% 7% 12% 21% 11% 1	9061
June	1st to 10th	Day	42% 8% 13% 21% 14% 1	2940
	11th to 20th	Day	41% 8% 14% 24% 11% 1	2940
	21st to end	Day	41% 9% 13% 23% 13% 1	2940
July	1st to 10th	Day	44% 9% 13% 19% 14% 1	8210
	11th to 20th	Day	34% 8% 15% 30% 13%	8205
	21st to end	Day	40% 6% 11% 24% 17% 1	8795
August	1st to 10th	Day	49% 9% 14% 16% 10% 1	6289
	11th to 20th	Day	35% 9% 20% 29% 7%	5524
	21st to end	Day	37% 7% 14% 28% 12% 1	6080
September	1st to 10th	Day	39% 7% 11% 24% 17% 2	2520
	11th to 20th	Day	34% 6% 10% 25% 22% 2	2428
	21st to end	Day	41% 7% 12% 21% 17% 2	2288
October	1st to 10th	Day	39% 6% 8% 21% 23% 3	3397
	11th to 20th	Day	33% 5% 9% 27% 24% 2	3240
	21st to end	Day	33% 5% 14% 26% 18% 3	3227
November	1st to 10th	Day	38% 6% 12% 25% 17% 2	1851
	11th to 20th	Day	42% 6% 11% 22% 18% 2	1890
	21st to end	Day	46% 5% 13% 25% 9% 2	1759
December	1st to 10th	Day	46% 6% 14% 25% 8% 1	2680
	11th to 20th	Day	45% 5% 14% 21% 12% 3	3066
	21st to end	Day	50% 6% 9% 20% 14% 1	4048
January	1st to 10th	Night	48% 6% 10% 19% 15% 2	8234
	11th to 20th	Night	52% 4 12% 15% 11% 5%	8280
	21st to end	Night	45% 5% 9% 20% 17% 3	8725
February	1st to 10th	Night	46% 6% 13% 21% 12% 2	2730
	11th to 20th	Night	47% 7% 10% 17% 16% 3	2472
	21st to end	Night	46% 6% 8% 22% 13% 4	2076
March	1st to 10th	Night	45% 6% 12% 18% 14% 5%	5494
	11th to 20th	Night	42% 7% 14% 23% 10% 3	5325
	21st to end	Night	49% 5% 6% 13% 23% 4	5101
April	1st to 10th	Night	51% 6% 9% 19% 13% 2	2100
	11th to 20th	Night	46% 7% 9% 18% 19% 1	2075
	21st to end	Night	45% 6% 8% 21% 17% 3	2037
May	1st to 10th	Night	52% 7% 12% 16% 12% 1	5250
	11th to 20th	Night	47% 7% 12% 21% 12% 1	4640
	21st to end	Night	49% 7% 11% 22% 9% 1	5115
June	1st to 10th	Night	40% 7% 10% 24% 17% 2	1680
	11th to 20th	Night	39% 10% 15% 26% 10% 1	1680
	21st to end	Night	34% 10% 14% 26% 14% 1	1680
July	1st to 10th	Night	41% 8% 14% 21% 14% 1	4640
	11th to 20th	Night	39% 8% 13% 25% 13% 1	4632
	21st to end	Night	38% 6% 11% 27% 17% 1	4967
August	1st to 10th	Night	49% 9% 13% 17% 11% 1	3787
	11th to 20th	Night	40% 8% 17% 27% 7%	4587
	21st to end	Night	36% 7% 13% 30% 13% 1	5045
September	1st to 10th	Night	39% 7% 11% 26% 17% 1	2100
	11th to 20th	Night	33% 6% 10% 25% 25% 2	2191
	21st to end	Night	41% 7% 9% 21% 21% 1	2289
October	1st to 10th	Night	43% 6% 9% 22% 18% 2	3778
	11th to 20th	Night	30% 5% 8% 30% 26% 1	3899
	21st to end	Night	35% 5% 12% 30% 17% 1	4563
November	1st to 10th	Night	37% 7% 10% 24% 19% 3	2667
	11th to 20th	Night	44% 4 9% 21% 17% 3	2730
	21st to end	Night	42% 6% 14% 27% 10% 1	2857
December	1st to 10th	Night	46% 5% 12% 28% 8% 1	4720
	11th to 20th	Night	51% 5% 12% 15% 15% 2	5348
	21st to end	Night	51% 6% 9% 18% 13% 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	25% 21% 27% 19% 8%	220
	11th to 20th	Day	27% 25% 33% 12% 3	220
	21st to end	Day	22% 21% 36% 15% 4 1	300
February	1st to 10th	Day	27% 21% 35% 15% 3	330
	11th to 20th	Day	25% 21% 35% 18% 2	522
	21st to end	Day	26% 22% 33% 17% 3	446
March	1st to 10th	Day	27% 18% 30% 21% 4	540
	11th to 20th	Day	25% 23% 30% 17% 4 1	540
	21st to end	Day	30% 20% 28% 17% 4 2	594
April	1st to 10th	Day	31% 19% 27% 15% 6% 2	540
	11th to 20th	Day	25% 23% 28% 14% 7% 2	532
	21st to end	Day	28% 16% 31% 18% 6% 2	540
May	1st to 10th	Day	31% 16% 27% 17% 7% 2	543
	11th to 20th	Day	19% 13% 34% 22% 10% 2	567
	21st to end	Day	21% 11% 26% 28% 11% 4	617
June	1st to 10th	Day	18% 13% 26% 26% 15% 3	570
	11th to 20th	Day	15% 13% 30% 25% 14% 2	570
	21st to end	Day	15% 10% 29% 26% 15% 4	570
July	1st to 10th	Day	15% 4 26% 32% 19% 4	565
	11th to 20th	Day	12% 6% 24% 34% 18% 5%	564
	21st to end	Day	13% 7% 27% 31% 17% 4	620
August	1st to 10th	Day	12% 7% 20% 34% 18% 8%	566
	11th to 20th	Day	14% 7% 23% 28% 21% 7%	559
	21st to end	Day	11% 7% 27% 30% 17% 8%	627
September	1st to 10th	Day	16% 11% 20% 27% 18% 7%	570
	11th to 20th	Day	14% 7% 27% 23% 18% 12%	570
	21st to end	Day	21% 10% 27% 27% 11% 5%	562
October	1st to 10th	Day	25% 10% 20% 22% 15% 8%	530
	11th to 20th	Day	18% 14% 31% 25% 10% 3	530
	21st to end	Day	25% 14% 28% 23% 9% 1	481
November	1st to 10th	Day	18% 16% 26% 28% 9% 2	425
	11th to 20th	Day	21% 17% 30% 26% 6% 1	430
	21st to end	Day	22% 17% 30% 23% 6% 1	299
December	1st to 10th	Day	23% 24% 30% 14% 9%	220
	11th to 20th	Day	28% 20% 33% 16% 2	220
	21st to end	Day	22% 14% 38% 20% 5% 1	242
January	1st to 10th	Night	19% 14% 37% 27% 3	840
	11th to 20th	Night	23% 14% 35% 25% 3	840
	21st to end	Night	18% 14% 41% 24% 3	924
February	1st to 10th	Night	21% 15% 35% 27% 2	840
	11th to 20th	Night	21% 15% 41% 22% 1	823
	21st to end	Night	23% 14% 38% 23% 3	692
March	1st to 10th	Night	23% 17% 35% 23% 3	840
	11th to 20th	Night	22% 15% 37% 21% 4	840
	21st to end	Night	22% 17% 35% 20% 6%	924
April	1st to 10th	Night	27% 14% 35% 21% 3	840
	11th to 20th	Night	22% 14% 35% 23% 6%	831
	21st to end	Night	24% 15% 30% 25% 6%	777
May	1st to 10th	Night	25% 14% 32% 23% 6%	628
	11th to 20th	Night	18% 8% 35% 35% 5%	414
	21st to end	Night	17% 6% 23% 40% 13%	455
June	1st to 10th	Night	16% 5% 25% 39% 15%	420
	11th to 20th	Night	17% 6% 24% 38% 15% 1	420
	21st to end	Night	15% 2 24% 42% 18%	420
July	1st to 10th	Night	13% 3 18% 45% 20% 1	410
	11th to 20th	Night	12% 5% 20% 46% 17% 1	408
	21st to end	Night	11% 4 21% 43% 19% 1	460
August	1st to 10th	Night	14% 1 15% 46% 22% 2	471
	11th to 20th	Night	12% 4 17% 41% 23% 3	830
	21st to end	Night	15% 3 19% 37% 23% 3	924
September	1st to 10th	Night	18% 3 23% 37% 17% 2	840
	11th to 20th	Night	15% 4 23% 31% 23% 5%	840
	21st to end	Night	20% 6% 24% 33% 16% 2	833
October	1st to 10th	Night	24% 6% 24% 30% 15% 2	800
	11th to 20th	Night	19% 8% 28% 35% 9%	800
	21st to end	Night	25% 10% 29% 28% 7%	880
November	1st to 10th	Night	17% 9% 33% 31% 9%	820
	11th to 20th	Night	24% 10% 33% 25% 7% 1	840
	21st to end	Night	20% 12% 33% 29% 6% 1	840
December	1st to 10th	Night	25% 13% 34% 23% 5%	840
	11th to 20th	Night	25% 11% 34% 25% 4	840
	21st to end	Night	25% 12% 37% 23% 3	924

Page generated at 2026-09-26 04:18:42. Version ac07

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