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 2024-04-03.

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 4 2 4 4 5% 7% 62%	78912
	Night	2 4 2 4 4 2 4 4 5% 8% 60%	81250
Transparency	Day	22% 5% 10% 21% 23% 18%	78619
	Night	23% 6% 13% 23% 22% 13%	81065
Seeing	Day	8% 16% 27% 27% 18% 4	14178
	Night	5% 9% 23% 36% 23% 4	23670

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 5% 2 6% 6% 4 6% 6% 8% 11% 46%	2539
	11th to 20th	Day	2 5% 3 7% 7% 4 6% 6% 6% 7% 47%	2576
	21st to end	Day	4 9% 3 6% 8% 4 6% 6% 7% 9% 38%	3146
February	1st to 10th	Day	2 4 3 5% 5% 3 4 5% 6% 9% 53%	1716
	11th to 20th	Day	2 4 4 6% 5% 2 4 4 7% 10% 52%	1850
	21st to end	Day	1 4 2 4 4 2 4 6% 7% 9% 56%	1570
March	1st to 10th	Day	2 5% 3 6% 5% 3 6% 6% 8% 10% 45%	1900
	11th to 20th	Day	1 6% 3 5% 4 4 5% 6% 7% 9% 51%	2042
	21st to end	Day	1 4 2 4 5% 4 5% 5% 7% 9% 54%	2456
April	1st to 10th	Day	2 5% 3 5% 5% 3 4 5% 5% 9% 54%	2185
	11th to 20th	Day	2 3 2 4 5% 3 4 5% 6% 7% 58%	2129
	21st to end	Day	3 4 2 4 4 2 3 4 4 7% 63%	2160
May	1st to 10th	Day	3 3 2 4 4 2 4 4 4 7% 61%	2281
	11th to 20th	Day	4 5% 2 3 4 2 4 4 5% 7% 60%	2430
	21st to end	Day	5% 5% 3 5% 6% 3 5% 5% 5% 6% 52%	2723
June	1st to 10th	Day	3 3 1 3 3 3 4 4 6% 8% 61%	2520
	11th to 20th	Day	2 2 1 2 3 2 2 3 3 5% 75%	2520
	21st to end	Day	2 2 1 3 2 2 3 3 4 4 75%	2520
July	1st to 10th	Day	1 1 1 2 2 1 2 3 3 5% 79%	2520
	11th to 20th	Day	2 1 1 2 2 2 3 2 4 4 79%	2520
	21st to end	Day	2 2 1 3 3 2 3 3 4 5% 71%	2702
August	1st to 10th	Day	2 2 1 3 3 2 2 4 4 4 72%	2303
	11th to 20th	Day	1 1 1 1 2 2 3 3 4 4 79%	2122
	21st to end	Day	1 1 1 1 2 1 2 2 2 4 84%	2376
September	1st to 10th	Day	1 1 1 1 2 1 2 2 2 5% 82%	2160
	11th to 20th	Day	2 2 1 2 3 2 3 2 4 4 75%	2124
	21st to end	Day	2 3 1 2 2 1 2 3 3 6% 76%	1980
October	1st to 10th	Day	4 3 2 3 3 2 3 3 4 5% 69%	1841
	11th to 20th	Day	2 2 2 3 2 3 3 4 6% 73%	1800
	21st to end	Day	1 2 1 3 3 2 3 3 5% 8% 70%	1922
November	1st to 10th	Day	3 4 2 4 4 2 4 5% 6% 7% 60%	1674
	11th to 20th	Day	2 2 1 3 3 2 5% 4 7% 11% 60%	1710
	21st to end	Day	1 4 2 4 4 3 4 5% 5% 9% 59%	1653
December	1st to 10th	Day	3 5% 2 4 5% 3 5% 4 6% 11% 53%	1520
	11th to 20th	Day	3 7% 4 7% 7% 4 6% 7% 9% 9% 37%	1928
	21st to end	Day	2 7% 4 6% 6% 3 6% 6% 7% 13% 41%	2794
January	1st to 10th	Night	1 4 2 5% 6% 3 5% 5% 7% 10% 50%	4406
	11th to 20th	Night	1 6% 3 7% 7% 4 6% 6% 7% 8% 44%	4382
	21st to end	Night	4 8% 3 7% 6% 4 6% 5% 7% 8% 41%	4521
February	1st to 10th	Night	2 4 3 5% 4 3 4 5% 6% 8% 55%	2462
	11th to 20th	Night	1 4 2 5% 4 3 4 5% 7% 9% 55%	2218
	21st to end	Night	2 4 2 4 4 2 4 5% 7% 10% 57%	1884
March	1st to 10th	Night	2 6% 2 6% 4 3 4 5% 6% 10% 50%	2280
	11th to 20th	Night	1 4 2 5% 4 2 5% 5% 7% 10% 55%	2134
	21st to end	Night	2 4 3 4 4 2 4 4 6% 9% 58%	2142
April	1st to 10th	Night	2 3 2 4 4 2 5% 5% 7% 10% 55%	1820
	11th to 20th	Night	2 3 1 4 4 2 4 5% 5% 8% 63%	1779
	21st to end	Night	3 3 1 4 3 2 4 4 4 8% 64%	1800
May	1st to 10th	Night	5% 4 2 4 3 2 4 3 4 7% 63%	1679
	11th to 20th	Night	2 4 2 4 4 2 4 5% 5% 7% 62%	1460
	21st to end	Night	5% 4 2 4 4 3 3 4 6% 7% 57%	1552
June	1st to 10th	Night	5% 3 1 3 4 2 3 3 4 7% 65%	1440
	11th to 20th	Night	1 1 1 2 2 2 2 3 3 5% 78%	1440
	21st to end	Night	2 2 1 2 2 2 2 2 3 4 78%	1440
July	1st to 10th	Night	1 1 2 2 1 2 2 4 5% 80%	1440
	11th to 20th	Night	1 1 1 2 2 1 3 3 4 4 78%	1440
	21st to end	Night	2 2 1 2 2 2 3 3 4 5% 72%	1611
August	1st to 10th	Night	3 2 1 2 3 2 4 4 5% 5% 69%	1636
	11th to 20th	Night	2 1 1 1 2 2 3 3 4 4 78%	1780
	21st to end	Night	2 2 2 2 1 2 2 2 4 82%	1980
September	1st to 10th	Night	1 1 1 1 1 1 2 2 2 4 85%	1800
	11th to 20th	Night	2 2 1 2 2 1 2 3 3 5% 77%	1836
	21st to end	Night	2 2 1 2 2 1 2 3 3 5% 76%	1980
October	1st to 10th	Night	4 3 1 2 3 2 3 3 3 5% 71%	2115
	11th to 20th	Night	2 3 1 2 2 2 2 2 4 5% 76%	2160
	21st to end	Night	2 3 1 2 2 2 2 3 4 6% 73%	2434
November	1st to 10th	Night	2 3 2 4 4 2 3 4 5% 8% 63%	2405
	11th to 20th	Night	1 3 1 3 4 2 4 4 5% 9% 64%	2470
	21st to end	Night	2 3 2 4 4 2 4 5% 6% 8% 60%	2526
December	1st to 10th	Night	3 5% 2 5% 5% 2 4 4 6% 9% 56%	2660
	11th to 20th	Night	4 7% 3 6% 6% 3 5% 5% 8% 11% 42%	3276
	21st to end	Night	2 7% 3 7% 6% 4 6% 6% 8% 9% 41%	4862

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	32% 8% 19% 31% 10%	2539
	11th to 20th	Day	36% 7% 16% 28% 11% 3	2576
	21st to end	Day	42% 8% 14% 25% 11% 1	3146
February	1st to 10th	Day	28% 6% 16% 29% 16% 6%	1716
	11th to 20th	Day	28% 4 15% 32% 18% 4	1860
	21st to end	Day	22% 8% 18% 30% 18% 4	1570
March	1st to 10th	Day	32% 9% 15% 27% 15% 2	1900
	11th to 20th	Day	28% 8% 16% 24% 20% 5%	2042
	21st to end	Day	26% 8% 23% 27% 13% 3	2456
April	1st to 10th	Day	27% 7% 15% 25% 21% 6%	2185
	11th to 20th	Day	22% 6% 12% 30% 23% 7%	2132
	21st to end	Day	22% 4 12% 30% 26% 5%	2160
May	1st to 10th	Day	23% 4 8% 29% 30% 6%	2281
	11th to 20th	Day	24% 4 8% 26% 30% 8%	2433
	21st to end	Day	32% 5% 9% 23% 24% 6%	2724
June	1st to 10th	Day	21% 4 8% 22% 31% 13%	2520
	11th to 20th	Day	13% 2 6% 20% 33% 26%	2520
	21st to end	Day	13% 3 2 10% 30% 41%	2520
July	1st to 10th	Day	8% 2 3 8% 24% 54%	2451
	11th to 20th	Day	10% 2 2 10% 24% 51%	2445
	21st to end	Day	16% 2 4 7% 22% 49%	2552
August	1st to 10th	Day	16% 3 4 10% 21% 47%	2287
	11th to 20th	Day	9% 3 4 6% 28% 50%	2122
	21st to end	Day	8% 2 2 6% 31% 51%	2376
September	1st to 10th	Day	8% 2 2 8% 28% 52%	2160
	11th to 20th	Day	13% 3 5% 15% 35% 29%	2124
	21st to end	Day	13% 2 5% 18% 36% 27%	1980
October	1st to 10th	Day	19% 2 7% 19% 32% 21%	1841
	11th to 20th	Day	14% 3 7% 26% 31% 19%	1800
	21st to end	Day	15% 3 9% 28% 31% 13%	1922
November	1st to 10th	Day	23% 5% 12% 26% 27% 8%	1674
	11th to 20th	Day	19% 6% 17% 28% 26% 4	1710
	21st to end	Day	23% 6% 13% 27% 26% 5%	1653
December	1st to 10th	Day	26% 8% 15% 29% 19% 4	1520
	11th to 20th	Day	39% 11% 13% 24% 11% 3	1928
	21st to end	Day	34% 8% 19% 26% 11% 1	2794
January	1st to 10th	Night	29% 6% 23% 30% 11% 1	4406
	11th to 20th	Night	37% 8% 18% 23% 10% 3	4382
	21st to end	Night	40% 8% 17% 23% 11% 1	4521
February	1st to 10th	Night	27% 7% 15% 28% 18% 5%	2462
	11th to 20th	Night	24% 5% 16% 33% 20% 2	2219
	21st to end	Night	23% 7% 16% 32% 16% 5%	1884
March	1st to 10th	Night	28% 7% 16% 28% 16% 4	2280
	11th to 20th	Night	23% 8% 17% 28% 20% 5%	2134
	21st to end	Night	24% 8% 22% 27% 14% 3	2142
April	1st to 10th	Night	23% 7% 18% 29% 18% 5%	1820
	11th to 20th	Night	19% 7% 16% 30% 20% 8%	1779
	21st to end	Night	20% 6% 15% 34% 22% 4	1800
May	1st to 10th	Night	23% 4 10% 27% 30% 6%	1679
	11th to 20th	Night	21% 5% 12% 29% 25% 8%	1460
	21st to end	Night	26% 5% 14% 28% 21% 5%	1552
June	1st to 10th	Night	22% 4 10% 23% 29% 13%	1440
	11th to 20th	Night	11% 3 6% 22% 34% 24%	1440
	21st to end	Night	12% 3 3 14% 33% 35%	1440
July	1st to 10th	Night	8% 2 3 7% 32% 48%	1400
	11th to 20th	Night	10% 4 3 10% 25% 48%	1392
	21st to end	Night	15% 3 4 7% 25% 47%	1522
August	1st to 10th	Night	16% 4 5% 14% 22% 39%	1627
	11th to 20th	Night	11% 2 2 10% 29% 45%	1780
	21st to end	Night	9% 2 2 8% 34% 45%	1980
September	1st to 10th	Night	7% 2 3 7% 33% 49%	1800
	11th to 20th	Night	11% 3 5% 19% 35% 27%	1836
	21st to end	Night	12% 3 5% 20% 35% 26%	1980
October	1st to 10th	Night	18% 2 8% 21% 29% 22%	2115
	11th to 20th	Night	13% 3 8% 23% 35% 18%	2160
	21st to end	Night	15% 2 9% 27% 34% 13%	2434
November	1st to 10th	Night	21% 5% 12% 28% 23% 11%	2405
	11th to 20th	Night	19% 6% 13% 30% 27% 5%	2470
	21st to end	Night	21% 7% 13% 28% 25% 6%	2526
December	1st to 10th	Night	27% 8% 15% 26% 19% 4	2660
	11th to 20th	Night	34% 11% 14% 24% 14% 2	3276
	21st to end	Night	37% 9% 18% 22% 12% 3	4862

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% 17% 32% 22% 12% 2	180
	11th to 20th	Day	10% 18% 28% 24% 17% 2	203
	21st to end	Day	7% 12% 32% 29% 16% 4	407
February	1st to 10th	Day	11% 16% 28% 27% 15% 3	374
	11th to 20th	Day	7% 21% 29% 29% 13% 1	455
	21st to end	Day	9% 20% 27% 33% 10% 1	382
March	1st to 10th	Day	12% 24% 31% 22% 8% 2	460
	11th to 20th	Day	9% 25% 28% 21% 15% 2	460
	21st to end	Day	11% 26% 29% 25% 8%	506
April	1st to 10th	Day	11% 25% 29% 22% 10% 3	429
	11th to 20th	Day	15% 26% 25% 20% 11% 2	412
	21st to end	Day	13% 20% 27% 24% 13% 4	420
May	1st to 10th	Day	12% 21% 28% 25% 10% 3	423
	11th to 20th	Day	15% 23% 23% 23% 10% 6%	448
	21st to end	Day	14% 16% 21% 24% 19% 6%	485
June	1st to 10th	Day	9% 17% 28% 22% 19% 5%	450
	11th to 20th	Day	9% 18% 32% 23% 15% 4	450
	21st to end	Day	3 12% 26% 31% 22% 6%	450
July	1st to 10th	Day	2 10% 31% 31% 21% 5%	446
	11th to 20th	Day	2 9% 30% 30% 22% 7%	444
	21st to end	Day	2 7% 28% 31% 25% 7%	487
August	1st to 10th	Day	3 10% 28% 27% 25% 7%	447
	11th to 20th	Day	2 9% 25% 29% 25% 10%	437
	21st to end	Day	3 7% 30% 33% 23% 5%	495
September	1st to 10th	Day	2 7% 27% 32% 25% 6%	450
	11th to 20th	Day	5% 15% 24% 30% 22% 4	450
	21st to end	Day	5% 13% 25% 38% 16% 4	450
October	1st to 10th	Day	6% 14% 27% 25% 23% 5%	450
	11th to 20th	Day	8% 12% 22% 30% 24% 5%	450
	21st to end	Day	8% 13% 24% 27% 24% 4	437
November	1st to 10th	Day	13% 12% 22% 24% 19% 10%	270
	11th to 20th	Day	4 13% 23% 30% 22% 8%	270
	21st to end	Day	4 15% 25% 30% 21% 5%	243
December	1st to 10th	Day	11% 13% 26% 22% 20% 8%	180
	11th to 20th	Day	5% 18% 33% 23% 14% 7%	180
	21st to end	Day	16% 16% 23% 28% 16% 2	198
January	1st to 10th	Night	6% 12% 29% 28% 18% 8%	760
	11th to 20th	Night	5% 13% 22% 38% 17% 5%	760
	21st to end	Night	6% 7% 28% 33% 20% 5%	836
February	1st to 10th	Night	5% 12% 26% 36% 19% 3	760
	11th to 20th	Night	4 12% 27% 35% 17% 5%	744
	21st to end	Night	5% 16% 27% 37% 13% 3	628
March	1st to 10th	Night	7% 15% 33% 28% 15% 3	760
	11th to 20th	Night	4 16% 27% 28% 23% 3	760
	21st to end	Night	5% 18% 32% 30% 14% 1	836
April	1st to 10th	Night	5% 13% 30% 32% 18% 1	728
	11th to 20th	Night	6% 16% 26% 32% 16% 4	711
	21st to end	Night	5% 13% 26% 32% 20% 4	720
May	1st to 10th	Night	8% 14% 23% 34% 19% 3	599
	11th to 20th	Night	5% 8% 22% 40% 23% 2	398
	21st to end	Night	8% 6% 17% 38% 28% 3	389
June	1st to 10th	Night	6% 5% 22% 39% 24% 3	360
	11th to 20th	Night	1 6% 23% 42% 28% 1	360
	21st to end	Night	3 3 18% 46% 29% 2	360
July	1st to 10th	Night	1 2 14% 51% 30% 3	350
	11th to 20th	Night	1 1 17% 54% 26% 1	348
	21st to end	Night	3 1 10% 46% 36% 4	429
August	1st to 10th	Night	4 1 13% 43% 36% 3	562
	11th to 20th	Night	2 3 15% 45% 31% 4	712
	21st to end	Night	2 2 19% 46% 28% 3	792
September	1st to 10th	Night	2 3 16% 38% 36% 5%	720
	11th to 20th	Night	3 5% 16% 33% 37% 6%	720
	21st to end	Night	3 5% 21% 33% 29% 9%	720
October	1st to 10th	Night	5% 6% 20% 36% 27% 6%	720
	11th to 20th	Night	4 8% 14% 39% 28% 7%	720
	21st to end	Night	4 8% 19% 32% 27% 11%	792
November	1st to 10th	Night	5% 11% 21% 29% 28% 6%	740
	11th to 20th	Night	5% 8% 27% 36% 20% 5%	760
	21st to end	Night	6% 9% 25% 31% 24% 5%	760
December	1st to 10th	Night	6% 12% 25% 36% 16% 6%	760
	11th to 20th	Night	6% 14% 26% 32% 16% 6%	760
	21st to end	Night	8% 12% 33% 32% 13% 2	836

Page generated at 2024-03-31 03:08:52. Version ac07

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