Maclean Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Maclean Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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 4 2 4 3 3 3 3 5% 6% 66%	124018
	Night	2 4 2 4 3 3 3 3 5% 7% 64%	123971
Transparency	Day	19% 5% 9% 20% 27% 21%	123763
	Night	20% 5% 10% 23% 25% 16%	123814
Seeing	Day	8% 15% 27% 27% 17% 5%	15916
	Night	5% 9% 24% 36% 22% 4	25244

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	1 3 2 5% 4 5% 5% 4 8% 10% 54%	3639
	11th to 20th	Day	1 4 2 6% 4 4 4 3 4 5% 62%	3704
	21st to end	Day	3 8% 4 7% 5% 4 5% 4 6% 7% 48%	4510
February	1st to 10th	Day	2 5% 2 5% 5% 4 4 4 7% 9% 53%	1807
	11th to 20th	Day	2 5% 3 7% 4 4 4 3 7% 10% 52%	1950
	21st to end	Day	1 4 2 5% 3 3 4 4 8% 9% 57%	1650
March	1st to 10th	Day	2 8% 4 8% 5% 5% 5% 4 8% 10% 42%	4518
	11th to 20th	Day	1 4 1 4 3 3 4 3 6% 8% 62%	4902
	21st to end	Day	2 6% 3 6% 4 5% 5% 4 6% 7% 53%	5854
April	1st to 10th	Day	2 6% 2 5% 4 5% 4 4 5% 8% 55%	2400
	11th to 20th	Day	2 4 2 5% 5% 4 4 4 6% 7% 58%	2369
	21st to end	Day	3 6% 2 4 3 3 3 3 4 7% 62%	2400
May	1st to 10th	Day	3 4 2 4 3 2 3 2 3 5% 69%	5743
	11th to 20th	Day	2 3 2 4 2 3 3 3 4 6% 68%	6178
	21st to end	Day	2 3 1 3 3 3 3 2 4 5% 72%	6902
June	1st to 10th	Day	3 3 1 4 3 4 4 3 6% 8% 62%	2800
	11th to 20th	Day	1 2 1 2 2 2 2 3 3 5% 76%	2800
	21st to end	Day	2 2 1 3 2 2 2 3 4 3 77%	2800
July	1st to 10th	Day	1 1 1 1 1 1 1 2 3 88%	4560
	11th to 20th	Day	2 2 1 3 3 3 3 2 4 4 73%	4560
	21st to end	Day	1 2 1 2 2 2 2 3 4 5% 75%	4716
August	1st to 10th	Day	2 2 1 3 2 2 2 2 4 4 75%	4040
	11th to 20th	Day	1 1 1 1 2 2 2 3 3 85%	3724
	21st to end	Day	1 1 1 2 1 1 1 1 2 3 86%	4180
September	1st to 10th	Day	1 1 1 1 1 1 2 1 3 5% 82%	2280
	11th to 20th	Day	2 2 1 3 3 3 3 2 4 5% 74%	2242
	21st to end	Day	2 2 1 2 2 2 2 2 3 5% 77%	2090
October	1st to 10th	Day	2 2 1 2 2 2 2 2 5% 6% 73%	3247
	11th to 20th	Day	2 3 1 3 2 2 2 2 3 4 76%	3161
	21st to end	Day	1 2 1 3 2 2 3 2 5% 8% 69%	3337
November	1st to 10th	Day	3 4 1 4 4 3 4 4 6% 7% 60%	1764
	11th to 20th	Day	2 2 1 3 3 3 4 3 7% 10% 60%	1800
	21st to end	Day	1 4 1 5% 4 4 4 4 6% 9% 59%	1739
December	1st to 10th	Day	1 3 1 3 3 2 4 3 5% 9% 65%	2620
	11th to 20th	Day	2 6% 3 7% 6% 6% 6% 5% 9% 10% 41%	3028
	21st to end	Day	2 6% 3 7% 5% 4 5% 5% 8% 13% 42%	4004
January	1st to 10th	Night	1 4 2 5% 4 4 5% 4 8% 11% 54%	6306
	11th to 20th	Night	1 4 2 6% 4 4 4 3 6% 6% 59%	6256
	21st to end	Night	3 8% 3 7% 4 4 4 3 6% 7% 52%	6468
February	1st to 10th	Night	2 4 3 6% 4 4 4 4 6% 8% 55%	2591
	11th to 20th	Night	1 5% 2 5% 3 4 4 5% 7% 9% 55%	2338
	21st to end	Night	2 4 2 4 4 3 4 4 7% 9% 58%	1980
March	1st to 10th	Night	1 7% 3 7% 4 5% 5% 4 6% 8% 49%	5376
	11th to 20th	Night	1 4 2 5% 4 3 4 3 6% 8% 60%	5094
	21st to end	Night	2 7% 2 6% 4 4 4 4 7% 9% 50%	5121
April	1st to 10th	Night	2 4 2 4 4 4 4 4 7% 9% 57%	2000
	11th to 20th	Night	2 4 2 4 3 3 4 4 5% 8% 62%	1979
	21st to end	Night	3 4 1 5% 3 3 3 4 4 7% 63%	2000
May	1st to 10th	Night	3 3 1 3 2 2 2 2 3 5% 74%	4243
	11th to 20th	Night	1 2 1 4 2 2 3 3 4 5% 72%	3716
	21st to end	Night	2 2 1 3 2 2 2 2 3 5% 75%	3972
June	1st to 10th	Night	5% 3 1 4 4 3 3 3 5% 7% 64%	1600
	11th to 20th	Night	1 1 1 2 2 2 2 2 3 5% 79%	1600
	21st to end	Night	2 2 1 2 2 1 2 2 3 4 79%	1600
July	1st to 10th	Night	1 1 1 1 1 1 2 3 88%	2620
	11th to 20th	Night	2 3 2 3 3 2 2 2 3 4 74%	2620
	21st to end	Night	2 2 1 2 2 3 2 2 4 5% 75%	2822
August	1st to 10th	Night	2 3 1 3 2 2 3 3 4 5% 73%	2901
	11th to 20th	Night	1 1 1 1 2 2 1 2 3 86%	3126
	21st to end	Night	2 3 2 1 1 1 1 1 3 85%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 2 4 84%	1900
	11th to 20th	Night	2 3 1 2 2 2 2 2 3 5% 76%	1938
	21st to end	Night	2 2 1 2 2 2 2 3 3 5% 77%	2090
October	1st to 10th	Night	2 2 1 2 2 2 2 2 3 5% 79%	3729
	11th to 20th	Night	1 2 1 2 1 2 2 2 3 5% 80%	3777
	21st to end	Night	2 3 2 4 2 2 3 3 5% 6% 69%	4228
November	1st to 10th	Night	2 3 2 4 3 3 3 4 5% 8% 64%	2535
	11th to 20th	Night	1 4 1 4 3 3 4 3 6% 9% 64%	2600
	21st to end	Night	2 3 2 4 4 3 4 4 6% 8% 60%	2659
December	1st to 10th	Night	2 3 1 4 3 3 3 3 6% 7% 67%	4560
	11th to 20th	Night	2 5% 2 6% 5% 4 5% 4 9% 13% 44%	5176
	21st to end	Night	2 7% 3 9% 6% 6% 6% 5% 8% 10% 40%	6952

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	25% 8% 23% 29% 15%	3639
	11th to 20th	Day	25% 5% 11% 26% 28% 5%	3704
	21st to end	Day	35% 7% 10% 20% 20% 8%	4510
February	1st to 10th	Day	27% 6% 15% 28% 16% 7%	1807
	11th to 20th	Day	27% 4 14% 32% 18% 4	1960
	21st to end	Day	22% 8% 18% 30% 17% 6%	1650
March	1st to 10th	Day	34% 8% 11% 24% 19% 4	4518
	11th to 20th	Day	18% 7% 13% 32% 26% 4	4902
	21st to end	Day	26% 9% 14% 24% 23% 3	5854
April	1st to 10th	Day	27% 7% 14% 26% 21% 5%	2400
	11th to 20th	Day	22% 6% 12% 30% 23% 8%	2372
	21st to end	Day	23% 5% 12% 30% 27% 5%	2400
May	1st to 10th	Day	19% 3 5% 24% 40% 9%	5743
	11th to 20th	Day	15% 4 7% 22% 40% 12%	6181
	21st to end	Day	15% 4 5% 19% 42% 15%	6904
June	1st to 10th	Day	20% 5% 8% 22% 33% 13%	2800
	11th to 20th	Day	12% 2 6% 18% 34% 28%	2800
	21st to end	Day	12% 3 2 10% 31% 42%	2800
July	1st to 10th	Day	5% 1 2 4 14% 73%	4491
	11th to 20th	Day	14% 3 2 6% 16% 59%	4485
	21st to end	Day	11% 3 3 6% 23% 54%	4567
August	1st to 10th	Day	14% 2 3 6% 20% 55%	4025
	11th to 20th	Day	7% 2 3 3 20% 65%	3760
	21st to end	Day	7% 2 2 7% 34% 49%	4179
September	1st to 10th	Day	8% 2 2 8% 28% 52%	2280
	11th to 20th	Day	13% 3 4 16% 35% 28%	2242
	21st to end	Day	12% 2 4 17% 36% 28%	2090
October	1st to 10th	Day	12% 3 5% 12% 37% 31%	3247
	11th to 20th	Day	14% 2 5% 20% 46% 14%	3161
	21st to end	Day	15% 3 6% 30% 34% 12%	3337
November	1st to 10th	Day	22% 5% 12% 27% 26% 8%	1764
	11th to 20th	Day	18% 6% 17% 29% 26% 4	1800
	21st to end	Day	23% 6% 13% 27% 26% 5%	1739
December	1st to 10th	Day	18% 5% 10% 32% 29% 6%	2620
	11th to 20th	Day	32% 11% 15% 25% 13% 3	3028
	21st to end	Day	30% 11% 22% 27% 10% 1	4004
January	1st to 10th	Night	23% 8% 25% 28% 16% 1	6306
	11th to 20th	Night	27% 6% 13% 25% 25% 5%	6256
	21st to end	Night	33% 6% 12% 17% 27% 6%	6468
February	1st to 10th	Night	27% 7% 14% 27% 19% 6%	2591
	11th to 20th	Night	24% 5% 16% 32% 21% 2	2339
	21st to end	Night	23% 7% 16% 32% 16% 7%	1980
March	1st to 10th	Night	32% 7% 11% 27% 18% 5%	5376
	11th to 20th	Night	20% 7% 11% 44% 17% 2	5094
	21st to end	Night	28% 9% 19% 29% 12% 3	5121
April	1st to 10th	Night	22% 7% 17% 29% 20% 5%	2000
	11th to 20th	Night	20% 6% 15% 31% 20% 8%	1979
	21st to end	Night	22% 5% 14% 33% 23% 4	2000
May	1st to 10th	Night	15% 3 6% 31% 39% 6%	4240
	11th to 20th	Night	14% 5% 7% 27% 37% 10%	3716
	21st to end	Night	12% 3 7% 22% 44% 12%	3972
June	1st to 10th	Night	21% 4 10% 23% 29% 12%	1600
	11th to 20th	Night	10% 3 5% 20% 35% 27%	1600
	21st to end	Night	11% 3 3 13% 34% 37%	1600
July	1st to 10th	Night	5% 1 2 4 18% 70%	2580
	11th to 20th	Night	16% 3 3 6% 16% 57%	2572
	21st to end	Night	12% 3 4 5% 25% 52%	2734
August	1st to 10th	Night	14% 3 3 8% 24% 48%	2892
	11th to 20th	Night	7% 1 2 6% 23% 61%	3156
	21st to end	Night	8% 2 1 8% 38% 43%	3498
September	1st to 10th	Night	7% 2 3 7% 33% 49%	1900
	11th to 20th	Night	13% 3 5% 19% 35% 26%	1938
	21st to end	Night	12% 3 4 19% 35% 28%	2090
October	1st to 10th	Night	11% 2 5% 13% 33% 36%	3729
	11th to 20th	Night	9% 3 5% 20% 48% 15%	3777
	21st to end	Night	16% 4 6% 27% 34% 12%	4228
November	1st to 10th	Night	20% 5% 12% 29% 23% 10%	2535
	11th to 20th	Night	19% 6% 13% 31% 27% 5%	2600
	21st to end	Night	21% 7% 13% 27% 25% 6%	2659
December	1st to 10th	Night	19% 6% 10% 27% 27% 11%	4560
	11th to 20th	Night	28% 11% 16% 27% 17% 2	5176
	21st to end	Night	35% 11% 17% 23% 12% 2	6952

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	15% 16% 33% 23% 13% 2	200
	11th to 20th	Day	9% 17% 29% 26% 17% 2	225
	21st to end	Day	8% 11% 32% 30% 16% 4	440
February	1st to 10th	Day	14% 16% 27% 26% 14% 2	405
	11th to 20th	Day	8% 21% 31% 27% 12% 1	495
	21st to end	Day	9% 19% 28% 33% 10%	414
March	1st to 10th	Day	12% 24% 31% 22% 9% 2	500
	11th to 20th	Day	11% 24% 28% 20% 14% 2	500
	21st to end	Day	13% 24% 29% 24% 9% 1	550
April	1st to 10th	Day	11% 23% 29% 23% 10% 3	500
	11th to 20th	Day	13% 25% 27% 21% 12% 2	492
	21st to end	Day	13% 19% 28% 24% 13% 4	500
May	1st to 10th	Day	12% 21% 28% 25% 10% 3	503
	11th to 20th	Day	15% 22% 23% 23% 12% 6%	528
	21st to end	Day	12% 15% 23% 26% 19% 6%	573
June	1st to 10th	Day	8% 15% 27% 23% 21% 6%	530
	11th to 20th	Day	8% 18% 32% 24% 14% 4	530
	21st to end	Day	3 11% 26% 32% 21% 7%	530
July	1st to 10th	Day	1 11% 30% 31% 21% 6%	526
	11th to 20th	Day	2 8% 27% 33% 22% 8%	524
	21st to end	Day	2 6% 29% 30% 26% 8%	532
August	1st to 10th	Day	3 9% 28% 28% 25% 7%	487
	11th to 20th	Day	2 11% 25% 29% 23% 10%	477
	21st to end	Day	4 7% 28% 32% 23% 5%	539
September	1st to 10th	Day	2 8% 28% 32% 24% 6%	490
	11th to 20th	Day	5% 15% 25% 29% 21% 4	490
	21st to end	Day	4 12% 26% 37% 17% 4	490
October	1st to 10th	Day	6% 13% 26% 27% 23% 5%	490
	11th to 20th	Day	8% 12% 23% 30% 24% 5%	490
	21st to end	Day	8% 13% 26% 27% 22% 4	477
November	1st to 10th	Day	11% 12% 22% 26% 19% 9%	300
	11th to 20th	Day	6% 12% 26% 29% 21% 7%	300
	21st to end	Day	7% 14% 25% 28% 21% 5%	269
December	1st to 10th	Day	10% 12% 25% 22% 21% 11%	200
	11th to 20th	Day	6% 18% 32% 24% 14% 6%	200
	21st to end	Day	16% 17% 25% 26% 14% 2	220
January	1st to 10th	Night	6% 12% 30% 28% 17% 7%	800
	11th to 20th	Night	5% 12% 23% 39% 17% 5%	800
	21st to end	Night	6% 7% 28% 34% 20% 5%	880
February	1st to 10th	Night	7% 11% 26% 35% 18% 3	800
	11th to 20th	Night	4 12% 28% 34% 17% 5%	784
	21st to end	Night	5% 15% 26% 38% 12% 2	660
March	1st to 10th	Night	7% 14% 33% 28% 15% 3	800
	11th to 20th	Night	4 16% 27% 28% 22% 3	800
	21st to end	Night	6% 17% 31% 30% 15% 1	880
April	1st to 10th	Night	5% 13% 31% 32% 18% 1	800
	11th to 20th	Night	7% 15% 27% 32% 16% 4	791
	21st to end	Night	6% 13% 26% 33% 19% 3	800
May	1st to 10th	Night	8% 14% 24% 34% 18% 3	665
	11th to 20th	Night	6% 8% 23% 40% 22% 2	442
	21st to end	Night	7% 6% 18% 40% 26% 3	433
June	1st to 10th	Night	5% 5% 21% 41% 25% 3	400
	11th to 20th	Night	1 5% 24% 44% 25% 1	400
	21st to end	Night	2 2 18% 48% 28% 2	400
July	1st to 10th	Night	1 2 14% 53% 28% 3	390
	11th to 20th	Night	1 1 16% 53% 28% 1	388
	21st to end	Night	3 1 10% 46% 36% 4	453
August	1st to 10th	Night	4 1 13% 44% 36% 3	594
	11th to 20th	Night	2 3 18% 44% 29% 4	752
	21st to end	Night	2 3 19% 45% 28% 3	836
September	1st to 10th	Night	2 3 16% 40% 35% 5%	760
	11th to 20th	Night	3 5% 18% 33% 36% 6%	760
	21st to end	Night	3 4 21% 34% 29% 8%	760
October	1st to 10th	Night	4 6% 19% 36% 29% 6%	760
	11th to 20th	Night	4 8% 15% 40% 28% 6%	760
	21st to end	Night	4 8% 20% 32% 26% 10%	836
November	1st to 10th	Night	5% 11% 22% 29% 27% 6%	780
	11th to 20th	Night	6% 8% 29% 34% 19% 5%	800
	21st to end	Night	7% 9% 26% 31% 24% 5%	800
December	1st to 10th	Night	6% 11% 25% 36% 17% 6%	800
	11th to 20th	Night	6% 14% 27% 33% 15% 6%	800
	21st to end	Night	8% 12% 34% 31% 13% 2	880

Page generated at 2025-07-17 22:10:05. Version ac07

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