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 2025-05-26.

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 5% 7% 38%	113898
	Night	17% 8% 3 7% 4 4 4 3 5% 6% 38%	115362
Transparency	Day	43% 7% 12% 22% 14% 2	117447
	Night	44% 6% 11% 21% 15% 2	119977
Seeing	Day	21% 14% 28% 23% 11% 3	15551
	Night	20% 10% 30% 29% 9% 1	24750

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% 5% 37%	3597
	11th to 20th	Day	13% 9% 3 9% 6% 4 5% 4 6% 6% 35%	3570
	21st to end	Day	16% 9% 3 6% 4 5% 4 4 5% 6% 37%	4208
February	1st to 10th	Day	15% 10% 3 8% 6% 5% 5% 4 6% 6% 33%	1800
	11th to 20th	Day	12% 9% 3 9% 5% 4 5% 3 5% 7% 37%	1934
	21st to end	Day	19% 8% 3 5% 6% 4 4 3 5% 7% 36%	1650
March	1st to 10th	Day	22% 9% 3 7% 4 3 4 2 4 4 37%	4476
	11th to 20th	Day	13% 8% 3 6% 4 4 4 3 5% 7% 43%	4745
	21st to end	Day	20% 8% 2 7% 5% 4 4 3 5% 6% 36%	5942
April	1st to 10th	Day	18% 10% 4 7% 4 3 4 3 5% 6% 36%	2400
	11th to 20th	Day	16% 8% 2 6% 5% 4 4 3 5% 6% 40%	2374
	21st to end	Day	20% 6% 2 5% 4 4 5% 3 4 5% 40%	2458
May	1st to 10th	Day	26% 8% 2 6% 4 3 4 4 5% 7% 32%	4268
	11th to 20th	Day	19% 8% 2 6% 5% 5% 4 4 5% 8% 33%	4538
	21st to end	Day	13% 5% 2 5% 4 4 4 4 6% 10% 43%	4907
June	1st to 10th	Day	16% 6% 1 6% 5% 5% 5% 4 7% 10% 35%	2660
	11th to 20th	Day	14% 6% 2 6% 5% 4 5% 4 8% 10% 36%	2660
	21st to end	Day	13% 6% 2 7% 5% 5% 5% 4 6% 10% 37%	2660
July	1st to 10th	Day	9% 7% 3 8% 6% 6% 6% 4 6% 9% 37%	4440
	11th to 20th	Day	9% 6% 2 6% 4 5% 6% 5% 7% 11% 38%	4440
	21st to end	Day	12% 7% 3 8% 6% 5% 6% 4 6% 8% 34%	4814
August	1st to 10th	Day	19% 6% 3 7% 5% 5% 5% 5% 6% 8% 31%	4303
	11th to 20th	Day	12% 6% 3 7% 4 4 5% 4 8% 9% 38%	3730
	21st to end	Day	11% 5% 2 6% 5% 4 4 4 6% 10% 42%	4180
September	1st to 10th	Day	13% 7% 2 6% 4 5% 5% 4 5% 7% 43%	2280
	11th to 20th	Day	13% 5% 2 5% 3 4 4 3 6% 6% 49%	2197
	21st to end	Day	17% 6% 1 6% 4 4 5% 4 6% 7% 41%	2090
October	1st to 10th	Day	20% 6% 3 5% 4 4 3 3 4 6% 41%	1981
	11th to 20th	Day	14% 7% 2 6% 5% 4 4 4 5% 7% 43%	1900
	21st to end	Day	22% 7% 2 6% 4 4 5% 3 5% 7% 35%	1914
November	1st to 10th	Day	13% 7% 3 5% 4 3 4 3 4 7% 48%	1761
	11th to 20th	Day	17% 7% 2 5% 4 4 4 3 5% 5% 43%	1800
	21st to end	Day	18% 9% 2 6% 4 3 4 3 5% 7% 39%	1675
December	1st to 10th	Day	17% 8% 3 6% 5% 4 5% 5% 5% 6% 36%	2600
	11th to 20th	Day	22% 8% 2 5% 3 4 3 3 5% 6% 39%	2986
	21st to end	Day	19% 9% 3 9% 5% 4 4 3 4 7% 33%	3960
January	1st to 10th	Night	15% 7% 3 10% 5% 4 4 3 5% 6% 37%	6349
	11th to 20th	Night	11% 8% 4 10% 6% 6% 6% 4 5% 7% 33%	6315
	21st to end	Night	17% 10% 3 7% 5% 4 4 3 5% 6% 36%	6746
February	1st to 10th	Night	16% 10% 3 7% 5% 5% 5% 4 4 6% 37%	2600
	11th to 20th	Night	14% 9% 3 7% 5% 4 5% 4 6% 5% 37%	2350
	21st to end	Night	18% 9% 2 7% 5% 4 4 4 5% 5% 37%	1980
March	1st to 10th	Night	19% 8% 2 5% 4 4 4 3 5% 6% 38%	5364
	11th to 20th	Night	15% 10% 3 7% 4 4 4 4 5% 6% 40%	5209
	21st to end	Night	19% 10% 3 7% 4 4 4 3 4 4 37%	4990
April	1st to 10th	Night	18% 11% 2 7% 5% 4 4 4 5% 7% 31%	2000
	11th to 20th	Night	19% 7% 3 6% 5% 4 4 4 4 8% 37%	1975
	21st to end	Night	20% 8% 3 6% 4 3 3 3 4 6% 40%	1940
May	1st to 10th	Night	27% 9% 3 6% 4 3 4 3 5% 6% 30%	2909
	11th to 20th	Night	20% 9% 3 8% 6% 3 5% 3 4 6% 33%	2575
	21st to end	Night	14% 7% 2 6% 4 4 4 4 5% 8% 41%	2784
June	1st to 10th	Night	16% 6% 1 6% 4 4 4 3 5% 8% 41%	1520
	11th to 20th	Night	15% 7% 1 5% 4 4 4 3 7% 10% 40%	1520
	21st to end	Night	13% 4 2 5% 3 5% 4 5% 8% 9% 42%	1520
July	1st to 10th	Night	11% 5% 3 6% 5% 5% 6% 5% 8% 9% 39%	2520
	11th to 20th	Night	12% 6% 2 8% 5% 4 5% 5% 7% 9% 38%	2520
	21st to end	Night	15% 8% 3 6% 5% 4 5% 3 5% 6% 40%	2739
August	1st to 10th	Night	20% 6% 2 8% 6% 5% 4 4 6% 8% 31%	2592
	11th to 20th	Night	13% 7% 3 7% 4 4 5% 4 6% 8% 38%	3097
	21st to end	Night	12% 4 2 5% 4 4 4 3 5% 8% 49%	3476
September	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 4 6% 46%	1900
	11th to 20th	Night	12% 5% 2 5% 3 3 3 2 5% 7% 52%	1982
	21st to end	Night	16% 6% 3 5% 3 4 4 3 5% 6% 45%	2090
October	1st to 10th	Night	21% 6% 2 7% 4 3 4 3 4 5% 40%	2194
	11th to 20th	Night	15% 7% 2 6% 5% 4 4 3 4 6% 45%	2280
	21st to end	Night	23% 7% 2 6% 4 3 3 3 5% 6% 37%	2684
November	1st to 10th	Night	14% 6% 3 6% 4 4 4 4 5% 6% 46%	2537
	11th to 20th	Night	19% 8% 2 5% 3 3 4 3 4 5% 41%	2600
	21st to end	Night	16% 7% 3 6% 4 4 4 3 6% 7% 41%	2721
December	1st to 10th	Night	18% 9% 3 7% 4 4 3 3 5% 7% 38%	4580
	11th to 20th	Night	26% 9% 3 6% 3 3 3 3 5% 5% 34%	5208
	21st to end	Night	21% 11% 3 7% 4 3 4 3 4 6% 33%	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	44% 5% 10% 26% 12% 3	3598
	11th to 20th	Day	47% 6% 10% 20% 12% 5%	3600
	21st to end	Day	46% 6% 10% 23% 14% 1	4208
February	1st to 10th	Day	49% 7% 12% 18% 13% 1	1800
	11th to 20th	Day	45% 6% 10% 21% 15% 3	1943
	21st to end	Day	48% 5% 11% 21% 13% 2	1650
March	1st to 10th	Day	50% 5% 10% 17% 10% 9%	4479
	11th to 20th	Day	39% 6% 14% 28% 9% 4	4745
	21st to end	Day	49% 5% 7% 13% 21% 6%	5942
April	1st to 10th	Day	49% 6% 9% 20% 13% 4	2400
	11th to 20th	Day	45% 5% 10% 21% 16% 3	2374
	21st to end	Day	45% 6% 9% 20% 16% 4	2458
May	1st to 10th	Day	51% 7% 12% 16% 12% 2	4282
	11th to 20th	Day	47% 9% 13% 20% 10% 1	4542
	21st to end	Day	34% 9% 13% 27% 15% 2	4907
June	1st to 10th	Day	42% 9% 14% 21% 13% 1	2660
	11th to 20th	Day	41% 8% 15% 24% 10% 2	2660
	21st to end	Day	41% 9% 13% 24% 13% 1	2660
July	1st to 10th	Day	42% 8% 13% 20% 16% 1	4370
	11th to 20th	Day	37% 9% 17% 24% 13% 1	4365
	21st to end	Day	46% 7% 13% 18% 15% 1	4661
August	1st to 10th	Day	49% 9% 13% 20% 8% 1	4289
	11th to 20th	Day	37% 9% 16% 28% 9%	3766
	21st to end	Day	36% 7% 12% 30% 13% 1	4180
September	1st to 10th	Day	39% 7% 11% 23% 17% 2	2280
	11th to 20th	Day	34% 6% 10% 25% 23% 2	2197
	21st to end	Day	40% 7% 12% 21% 17% 2	2090
October	1st to 10th	Day	40% 6% 8% 20% 23% 2	3293
	11th to 20th	Day	32% 5% 9% 27% 25% 2	3140
	21st to end	Day	33% 5% 14% 26% 18% 4	3126
November	1st to 10th	Day	37% 6% 13% 26% 17% 2	1761
	11th to 20th	Day	41% 6% 11% 22% 18% 2	1800
	21st to end	Day	45% 5% 14% 25% 9% 2	1675
December	1st to 10th	Day	46% 6% 15% 25% 8% 1	2600
	11th to 20th	Day	46% 5% 14% 21% 12% 3	2986
	21st to end	Day	50% 6% 9% 20% 14% 1	3960
January	1st to 10th	Night	45% 6% 10% 21% 17% 2	6349
	11th to 20th	Night	48% 5% 14% 17% 11% 5%	6360
	21st to end	Night	47% 5% 11% 16% 16% 4	6746
February	1st to 10th	Night	47% 6% 14% 21% 11% 2	2600
	11th to 20th	Night	46% 7% 10% 17% 17% 3	2352
	21st to end	Night	46% 6% 9% 22% 14% 4	1980
March	1st to 10th	Night	45% 6% 12% 18% 14% 6%	5374
	11th to 20th	Night	42% 7% 14% 23% 10% 3	5210
	21st to end	Night	49% 5% 6% 13% 23% 4	4990
April	1st to 10th	Night	51% 6% 8% 19% 13% 2	2000
	11th to 20th	Night	46% 7% 9% 17% 19% 1	1975
	21st to end	Night	44% 7% 8% 21% 17% 3	1940
May	1st to 10th	Night	53% 8% 11% 16% 11% 2	2915
	11th to 20th	Night	50% 8% 13% 17% 10% 2	2576
	21st to end	Night	38% 10% 14% 24% 13% 2	2785
June	1st to 10th	Night	41% 7% 11% 24% 16% 2	1520
	11th to 20th	Night	37% 10% 15% 26% 10% 1	1520
	21st to end	Night	34% 11% 13% 27% 14% 1	1520
July	1st to 10th	Night	38% 10% 14% 20% 15% 2	2480
	11th to 20th	Night	40% 8% 14% 25% 13% 1	2472
	21st to end	Night	43% 7% 12% 19% 16% 2	2651
August	1st to 10th	Night	49% 10% 14% 18% 9% 1	2582
	11th to 20th	Night	41% 8% 15% 26% 10%	3127
	21st to end	Night	33% 7% 11% 34% 14% 1	3476
September	1st to 10th	Night	39% 7% 11% 25% 17% 1	1900
	11th to 20th	Night	32% 6% 10% 26% 25% 2	1982
	21st to end	Night	39% 7% 9% 22% 22% 1	2090
October	1st to 10th	Night	44% 6% 9% 22% 18% 1	3662
	11th to 20th	Night	30% 5% 8% 30% 26% 1	3779
	21st to end	Night	35% 5% 12% 30% 17% 1	4422
November	1st to 10th	Night	37% 7% 10% 23% 20% 3	2537
	11th to 20th	Night	44% 4 10% 21% 17% 4	2600
	21st to end	Night	41% 6% 14% 28% 10% 1	2721
December	1st to 10th	Night	46% 5% 12% 28% 8% 1	4580
	11th to 20th	Night	51% 5% 12% 15% 15% 2	5208
	21st to end	Night	50% 6% 9% 18% 13% 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	26% 22% 27% 18% 9%	200
	11th to 20th	Day	24% 25% 34% 14% 4 1	200
	21st to end	Day	22% 19% 37% 15% 5% 1	273
February	1st to 10th	Day	29% 21% 34% 13% 2	300
	11th to 20th	Day	25% 21% 36% 17% 1	482
	21st to end	Day	26% 21% 34% 17% 3	414
March	1st to 10th	Day	26% 18% 31% 21% 4	500
	11th to 20th	Day	23% 24% 31% 17% 4 1	500
	21st to end	Day	29% 19% 29% 17% 4 3	550
April	1st to 10th	Day	30% 19% 27% 15% 6% 2	500
	11th to 20th	Day	26% 23% 28% 14% 7% 2	492
	21st to end	Day	28% 16% 30% 18% 6% 2	500
May	1st to 10th	Day	32% 15% 26% 17% 8% 2	503
	11th to 20th	Day	20% 12% 34% 22% 10% 2	527
	21st to end	Day	19% 10% 25% 30% 13% 4	549
June	1st to 10th	Day	19% 12% 27% 26% 14% 2	490
	11th to 20th	Day	14% 11% 30% 27% 16% 2	490
	21st to end	Day	15% 11% 30% 26% 15% 4	490
July	1st to 10th	Day	15% 4 26% 32% 19% 5%	485
	11th to 20th	Day	13% 5% 23% 35% 19% 5%	484
	21st to end	Day	12% 7% 28% 32% 17% 3	532
August	1st to 10th	Day	13% 7% 21% 35% 17% 7%	486
	11th to 20th	Day	14% 7% 23% 27% 21% 8%	479
	21st to end	Day	11% 7% 25% 31% 17% 9%	539
September	1st to 10th	Day	17% 11% 19% 25% 19% 8%	490
	11th to 20th	Day	14% 7% 28% 20% 18% 13%	490
	21st to end	Day	20% 9% 26% 28% 12% 6%	490
October	1st to 10th	Day	27% 10% 20% 22% 15% 6%	490
	11th to 20th	Day	17% 12% 32% 26% 10% 3	490
	21st to end	Day	26% 13% 27% 23% 9% 1	446
November	1st to 10th	Day	17% 15% 26% 29% 10% 2	395
	11th to 20th	Day	22% 15% 31% 26% 7% 1	400
	21st to end	Day	22% 15% 30% 25% 7% 1	275
December	1st to 10th	Day	23% 24% 32% 14% 9%	200
	11th to 20th	Day	29% 20% 33% 17% 2	200
	21st to end	Day	20% 14% 38% 22% 5% 1	220
January	1st to 10th	Night	19% 15% 37% 27% 3	800
	11th to 20th	Night	22% 13% 35% 26% 4	800
	21st to end	Night	18% 14% 40% 25% 3	880
February	1st to 10th	Night	21% 15% 36% 26% 2	800
	11th to 20th	Night	21% 15% 42% 21% 1	783
	21st to end	Night	23% 14% 38% 23% 3	660
March	1st to 10th	Night	22% 17% 35% 23% 4	800
	11th to 20th	Night	22% 15% 37% 21% 5%	800
	21st to end	Night	22% 17% 35% 20% 6%	880
April	1st to 10th	Night	27% 14% 35% 21% 4	800
	11th to 20th	Night	22% 14% 35% 23% 7%	791
	21st to end	Night	24% 15% 29% 25% 6%	740
May	1st to 10th	Night	25% 14% 31% 24% 6%	598
	11th to 20th	Night	18% 7% 36% 34% 5%	394
	21st to end	Night	16% 6% 22% 41% 14%	423
June	1st to 10th	Night	16% 4 26% 39% 14% 1	380
	11th to 20th	Night	16% 5% 23% 38% 16% 1	380
	21st to end	Night	14% 2 25% 42% 17%	380
July	1st to 10th	Night	13% 3 18% 45% 21% 1	370
	11th to 20th	Night	13% 5% 18% 45% 18% 1	368
	21st to end	Night	12% 4 20% 44% 19% 1	416
August	1st to 10th	Night	14% 2 15% 46% 21% 2	425
	11th to 20th	Night	13% 4 16% 41% 23% 3	750
	21st to end	Night	15% 3 18% 37% 24% 3	836
September	1st to 10th	Night	18% 2 24% 36% 18% 2	760
	11th to 20th	Night	15% 4 23% 30% 23% 5%	760
	21st to end	Night	18% 6% 24% 34% 16% 2	760
October	1st to 10th	Night	24% 6% 23% 30% 14% 2	760
	11th to 20th	Night	18% 8% 29% 35% 10%	760
	21st to end	Night	26% 10% 29% 28% 7%	836
November	1st to 10th	Night	16% 9% 34% 31% 10% 1	780
	11th to 20th	Night	24% 9% 33% 26% 7% 1	800
	21st to end	Night	20% 11% 34% 29% 6% 1	800
December	1st to 10th	Night	26% 12% 34% 23% 5%	800
	11th to 20th	Night	25% 10% 34% 27% 4	800
	21st to end	Night	25% 11% 37% 24% 3	880

Page generated at 2025-05-23 09:21:02. Version ac07

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