MacPeak Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the MacPeak Observatory clear sky chart from 2005-11-06 to 2026-03-18.

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 4 2 4 3 4 4 4 6% 8% 58%	139972
	Night	2 5% 2 4 3 3 3 3 5% 7% 63%	139057
Transparency	Day	24% 5% 12% 25% 25% 10%	134935
	Night	21% 5% 14% 27% 25% 8%	135102
Seeing	Day	11% 16% 32% 27% 12% 2	17220
	Night	6% 9% 36% 35% 12% 1	27427

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	5% 3 5% 2 3 3 3 4 7% 65%	4720
	11th to 20th	Day	1 1 4 3 3 3 2 4 6% 72%	5257
	21st to end	Day	4 2 4 3 3 3 2 4 7% 67%	5844
February	1st to 10th	Day	6% 2 5% 2 2 3 3 5% 8% 64%	1890
	11th to 20th	Day	1 3 1 3 3 3 4 3 7% 9% 64%	2007
	21st to end	Day	1 3 2 6% 4 3 3 4 5% 7% 63%	1730
March	1st to 10th	Day	4 2 4 3 3 3 3 5% 9% 64%	4709
	11th to 20th	Day	1 10% 4 7% 4 4 4 4 6% 8% 50%	5078
	21st to end	Day	1 5% 2 4 4 4 4 4 7% 9% 57%	5735
April	1st to 10th	Day	2 5% 2 5% 4 4 5% 4 7% 8% 56%	2400
	11th to 20th	Day	4 7% 2 6% 4 4 4 4 6% 7% 51%	2371
	21st to end	Day	4 7% 1 5% 4 5% 5% 5% 6% 8% 50%	2400
May	1st to 10th	Day	8% 8% 2 5% 4 4 5% 4 6% 8% 45%	5481
	11th to 20th	Day	4 5% 2 5% 4 5% 5% 4 6% 9% 52%	6291
	21st to end	Day	3 5% 1 4 3 4 6% 5% 7% 9% 53%	6925
June	1st to 10th	Day	2 4 1 5% 4 4 6% 6% 8% 11% 48%	2800
	11th to 20th	Day	3 3 2 4 3 4 4 4 6% 7% 59%	2800
	21st to end	Day	3 3 1 5% 4 5% 5% 5% 8% 9% 52%	2800
July	1st to 10th	Day	2 2 1 3 3 4 4 4 6% 8% 61%	6339
	11th to 20th	Day	4 3 1 5% 5% 5% 7% 6% 9% 11% 44%	6340
	21st to end	Day	4 4 1 6% 6% 5% 7% 5% 9% 9% 45%	6432
August	1st to 10th	Day	4 3 2 5% 5% 5% 7% 5% 9% 11% 44%	5779
	11th to 20th	Day	2 2 1 4 4 5% 6% 5% 8% 12% 52%	5375
	21st to end	Day	4 5% 2 6% 5% 5% 6% 5% 9% 9% 43%	5984
September	1st to 10th	Day	4 4 1 6% 4 5% 5% 4 7% 7% 54%	2400
	11th to 20th	Day	4 3 1 4 3 3 4 4 6% 7% 58%	2400
	21st to end	Day	4 3 1 4 3 3 3 3 5% 6% 65%	2172
October	1st to 10th	Day	2 2 1 3 2 2 2 3 4 5% 74%	3270
	11th to 20th	Day	4 3 1 2 2 2 3 3 5% 8% 66%	3250
	21st to end	Day	2 3 1 2 2 2 2 2 3 6% 74%	3548
November	1st to 10th	Day	1 3 1 2 1 1 2 2 4 6% 75%	1867
	11th to 20th	Day	2 1 3 2 1 2 2 3 7% 76%	1890
	21st to end	Day	4 1 4 3 3 3 2 4 7% 68%	1790
December	1st to 10th	Day	3 1 3 2 2 2 2 3 5% 77%	2700
	11th to 20th	Day	2 1 2 2 2 2 2 5% 9% 72%	3106
	21st to end	Day	3 2 4 2 3 3 3 5% 9% 66%	4092
January	1st to 10th	Night	4 4 6% 3 2 3 3 4 6% 64%	8201
	11th to 20th	Night	1 1 4 2 2 2 2 3 5% 77%	7788
	21st to end	Night	4 2 5% 3 4 4 2 4 5% 68%	8370
February	1st to 10th	Night	4 3 5% 2 3 2 2 4 6% 69%	2730
	11th to 20th	Night	1 4 2 6% 3 3 3 2 4 6% 66%	2499
	21st to end	Night	4 2 6% 3 2 3 3 5% 6% 66%	2076
March	1st to 10th	Night	7% 3 5% 3 2 3 3 5% 6% 64%	5387
	11th to 20th	Night	2 9% 3 5% 3 3 4 3 4 6% 59%	5077
	21st to end	Night	1 5% 2 4 3 3 3 3 4 7% 66%	5227
April	1st to 10th	Night	1 5% 2 5% 3 2 3 3 5% 7% 63%	2000
	11th to 20th	Night	5% 8% 2 5% 3 3 3 3 4 6% 57%	1977
	21st to end	Night	4 9% 2 5% 3 3 3 3 5% 7% 55%	2000
May	1st to 10th	Night	7% 9% 3 5% 3 3 4 3 5% 7% 52%	4496
	11th to 20th	Night	7% 6% 2 4 3 3 3 3 5% 7% 58%	3620
	21st to end	Night	6% 8% 2 4 2 3 3 3 4 5% 60%	3972
June	1st to 10th	Night	4 4 1 3 3 3 5% 4 6% 9% 59%	1600
	11th to 20th	Night	2 4 1 3 2 3 4 3 5% 7% 67%	1600
	21st to end	Night	3 4 1 3 4 4 5% 5% 7% 9% 56%	1600
July	1st to 10th	Night	6% 5% 2 4 3 4 4 4 6% 8% 55%	3640
	11th to 20th	Night	4 4 1 4 3 4 5% 5% 9% 13% 48%	3640
	21st to end	Night	4 4 2 4 4 4 5% 5% 9% 10% 48%	4423
August	1st to 10th	Night	4 4 1 5% 3 4 4 4 7% 11% 51%	4033
	11th to 20th	Night	3 3 1 3 2 3 5% 5% 8% 14% 52%	4473
	21st to end	Night	5% 6% 3 7% 6% 5% 5% 5% 8% 10% 40%	4993
September	1st to 10th	Night	5% 5% 2 5% 5% 4 6% 4 6% 10% 51%	2000
	11th to 20th	Night	5% 5% 1 3 2 3 4 3 7% 7% 60%	2000
	21st to end	Night	4 4 2 4 2 3 3 3 5% 6% 64%	2221
October	1st to 10th	Night	3 3 1 4 2 2 3 2 3 5% 72%	3920
	11th to 20th	Night	6% 4 1 2 2 2 2 2 4 7% 68%	3898
	21st to end	Night	2 3 1 3 2 2 2 2 3 5% 76%	4252
November	1st to 10th	Night	1 3 1 2 2 1 2 2 4 7% 74%	2630
	11th to 20th	Night	2 1 2 2 2 2 2 3 5% 77%	2730
	21st to end	Night	2 6% 2 3 2 2 2 3 4 8% 68%	2825
December	1st to 10th	Night	2 1 3 2 2 2 2 3 6% 77%	4710
	11th to 20th	Night	1 2 1 2 2 2 2 3 4 7% 74%	5321
	21st to end	Night	4 2 4 3 3 4 3 5% 8% 64%	7128

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	21% 2 13% 33% 26% 5%	4723
	11th to 20th	Day	16% 3 13% 24% 32% 13%	5257
	21st to end	Day	18% 5% 8% 27% 28% 13%	5844
February	1st to 10th	Day	20% 4 17% 28% 22% 8%	1890
	11th to 20th	Day	17% 5% 14% 30% 27% 6%	2017
	21st to end	Day	22% 4 10% 22% 30% 12%	1730
March	1st to 10th	Day	18% 3 8% 15% 40% 16%	4709
	11th to 20th	Day	32% 5% 8% 27% 25% 4	5078
	21st to end	Day	22% 5% 12% 43% 16% 2	5735
April	1st to 10th	Day	25% 7% 12% 23% 23% 11%	2400
	11th to 20th	Day	31% 5% 11% 23% 22% 8%	2374
	21st to end	Day	30% 6% 13% 24% 22% 4	2400
May	1st to 10th	Day	35% 4 14% 23% 15% 9%	5481
	11th to 20th	Day	27% 7% 9% 27% 20% 10%	6295
	21st to end	Day	26% 5% 13% 20% 28% 8%	6926
June	1st to 10th	Day	26% 10% 15% 20% 21% 8%	2800
	11th to 20th	Day	22% 6% 10% 17% 24% 21%	2800
	21st to end	Day	25% 6% 11% 22% 21% 15%	2800
July	1st to 10th	Day	21% 4 9% 14% 22% 30%	6271
	11th to 20th	Day	30% 5% 20% 28% 13% 4	6265
	21st to end	Day	32% 6% 19% 18% 15% 10%	6247
August	1st to 10th	Day	37% 7% 21% 18% 12% 5%	4208
	11th to 20th	Day	27% 6% 18% 17% 23% 9%	3891
	21st to end	Day	33% 6% 16% 19% 21% 5%	4311
September	1st to 10th	Day	28% 5% 13% 20% 23% 12%	2400
	11th to 20th	Day	23% 6% 12% 19% 28% 13%	2400
	21st to end	Day	20% 5% 7% 24% 35% 10%	2172
October	1st to 10th	Day	14% 4 6% 18% 38% 21%	3270
	11th to 20th	Day	18% 3 9% 34% 27% 9%	3250
	21st to end	Day	15% 2 6% 29% 37% 12%	3548
November	1st to 10th	Day	12% 3 10% 26% 41% 8%	1867
	11th to 20th	Day	12% 3 8% 39% 32% 7%	1890
	21st to end	Day	18% 4 9% 32% 29% 9%	1790
December	1st to 10th	Day	12% 4 10% 35% 30% 9%	2700
	11th to 20th	Day	12% 3 11% 37% 31% 6%	3104
	21st to end	Day	16% 4 14% 28% 29% 10%	4092
January	1st to 10th	Night	22% 3 11% 37% 23% 3	8202
	11th to 20th	Night	12% 3 14% 28% 28% 15%	7788
	21st to end	Night	19% 5% 11% 26% 27% 12%	8370
February	1st to 10th	Night	19% 4 16% 30% 24% 7%	2730
	11th to 20th	Night	21% 5% 14% 31% 26% 5%	2499
	21st to end	Night	19% 4 11% 27% 31% 9%	2076
March	1st to 10th	Night	21% 3 9% 19% 40% 8%	5387
	11th to 20th	Night	24% 6% 8% 24% 32% 5%	5077
	21st to end	Night	21% 4 15% 42% 16% 1	5227
April	1st to 10th	Night	22% 5% 13% 28% 26% 7%	2000
	11th to 20th	Night	29% 4 12% 26% 23% 6%	1977
	21st to end	Night	27% 7% 14% 28% 20% 4	2000
May	1st to 10th	Night	32% 4 17% 23% 18% 6%	4493
	11th to 20th	Night	26% 6% 11% 31% 17% 9%	3621
	21st to end	Night	26% 5% 15% 19% 27% 9%	3972
June	1st to 10th	Night	22% 7% 18% 24% 17% 12%	1600
	11th to 20th	Night	17% 5% 11% 21% 25% 21%	1600
	21st to end	Night	23% 6% 14% 21% 22% 13%	1600
July	1st to 10th	Night	26% 6% 12% 17% 23% 16%	3600
	11th to 20th	Night	25% 6% 31% 26% 9% 4	3592
	21st to end	Night	28% 8% 24% 19% 12% 9%	4297
August	1st to 10th	Night	34% 11% 26% 14% 9% 5%	2928
	11th to 20th	Night	22% 9% 27% 17% 18% 6%	3233
	21st to end	Night	33% 8% 18% 23% 14% 3	3597
September	1st to 10th	Night	28% 7% 15% 22% 20% 9%	2000
	11th to 20th	Night	22% 7% 13% 21% 25% 13%	2000
	21st to end	Night	21% 6% 9% 25% 31% 8%	2221
October	1st to 10th	Night	17% 4 7% 26% 30% 16%	3920
	11th to 20th	Night	18% 2 11% 33% 27% 8%	3898
	21st to end	Night	13% 2 6% 31% 37% 10%	4252
November	1st to 10th	Night	13% 3 11% 31% 34% 8%	2630
	11th to 20th	Night	11% 4 8% 38% 31% 8%	2730
	21st to end	Night	17% 5% 10% 30% 31% 8%	2825
December	1st to 10th	Night	12% 3 10% 40% 29% 5%	4710
	11th to 20th	Night	12% 4 10% 34% 36% 4	5322
	21st to end	Night	18% 4 16% 24% 32% 5%	7128

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	9% 14% 44% 25% 8% 1	330
	11th to 20th	Day	13% 13% 45% 22% 5% 1	330
	21st to end	Day	3 16% 43% 28% 10% 1	363
February	1st to 10th	Day	12% 24% 43% 17% 3 1	330
	11th to 20th	Day	14% 25% 38% 18% 5% 1	330
	21st to end	Day	19% 21% 42% 16% 1	273
March	1st to 10th	Day	15% 26% 31% 22% 5%	374
	11th to 20th	Day	19% 24% 33% 17% 7% 1	529
	21st to end	Day	19% 29% 34% 15% 2	550
April	1st to 10th	Day	18% 31% 29% 17% 5%	500
	11th to 20th	Day	22% 30% 31% 13% 4	492
	21st to end	Day	20% 28% 28% 18% 6%	500
May	1st to 10th	Day	18% 24% 35% 16% 6%	518
	11th to 20th	Day	14% 17% 29% 26% 12% 2	725
	21st to end	Day	11% 16% 29% 29% 13% 1	789
June	1st to 10th	Day	6% 11% 28% 35% 17% 3	730
	11th to 20th	Day	7% 12% 35% 31% 14% 1	730
	21st to end	Day	5% 12% 31% 31% 18% 3	730
July	1st to 10th	Day	5% 7% 24% 39% 21% 3	721
	11th to 20th	Day	5% 3 19% 39% 27% 5%	718
	21st to end	Day	6% 6% 22% 39% 22% 5%	593
August	1st to 10th	Day	6% 7% 30% 38% 14% 5%	527
	11th to 20th	Day	5% 4 32% 40% 15% 3	517
	21st to end	Day	8% 7% 30% 37% 16% 2	583
September	1st to 10th	Day	7% 11% 28% 33% 17% 4	530
	11th to 20th	Day	7% 11% 40% 26% 13% 2	530
	21st to end	Day	9% 9% 28% 33% 18% 3	342
October	1st to 10th	Day	10% 14% 26% 31% 15% 4	330
	11th to 20th	Day	10% 22% 28% 28% 11% 2	330
	21st to end	Day	12% 18% 34% 23% 11% 2	363
November	1st to 10th	Day	12% 18% 33% 24% 13% 1	330
	11th to 20th	Day	9% 17% 35% 28% 8% 2	330
	21st to end	Day	8% 17% 39% 28% 7% 1	330
December	1st to 10th	Day	9% 17% 41% 25% 8% 1	330
	11th to 20th	Day	11% 16% 41% 23% 8% 1	330
	21st to end	Day	14% 22% 40% 18% 5% 1	363
January	1st to 10th	Night	4 14% 43% 29% 9% 1	1050
	11th to 20th	Night	5% 12% 48% 28% 6% 1	887
	21st to end	Night	2 11% 48% 31% 7%	924
February	1st to 10th	Night	4 10% 56% 26% 3	840
	11th to 20th	Night	5% 13% 53% 25% 4	824
	21st to end	Night	6% 12% 56% 25% 1	692
March	1st to 10th	Night	5% 11% 49% 30% 5%	840
	11th to 20th	Night	6% 10% 41% 34% 9%	828
	21st to end	Night	6% 12% 49% 28% 5%	773
April	1st to 10th	Night	7% 15% 42% 33% 4	600
	11th to 20th	Night	13% 13% 36% 32% 6%	593
	21st to end	Night	10% 13% 37% 34% 7%	600
May	1st to 10th	Night	11% 10% 30% 40% 9% 1	600
	11th to 20th	Night	13% 7% 27% 40% 12% 1	591
	21st to end	Night	11% 9% 28% 37% 14% 1	650
June	1st to 10th	Night	6% 6% 22% 44% 21% 1	600
	11th to 20th	Night	4 6% 28% 50% 12% 1	600
	21st to end	Night	4 3 21% 47% 24% 1	600
July	1st to 10th	Night	7% 3 14% 45% 28% 2	585
	11th to 20th	Night	6% 2 8% 44% 37% 3	582
	21st to end	Night	8% 2 9% 39% 40% 3	657
August	1st to 10th	Night	8% 3 12% 43% 32% 2	597
	11th to 20th	Night	6% 3 12% 48% 30% 2	594
	21st to end	Night	6% 3 13% 48% 28% 2	660
September	1st to 10th	Night	6% 3 17% 48% 23% 3	600
	11th to 20th	Night	6% 4 19% 48% 22% 2	600
	21st to end	Night	5% 5% 25% 45% 18% 1	635
October	1st to 10th	Night	6% 5% 34% 41% 13% 1	800
	11th to 20th	Night	5% 6% 37% 39% 13% 1	800
	21st to end	Night	6% 6% 40% 36% 11% 1	880
November	1st to 10th	Night	3 7% 44% 36% 9% 1	990
	11th to 20th	Night	5% 12% 44% 31% 8% 1	1050
	21st to end	Night	6% 11% 42% 33% 7%	1050
December	1st to 10th	Night	4 13% 46% 31% 6%	1050
	11th to 20th	Night	6% 12% 47% 27% 8%	1050
	21st to end	Night	7% 14% 49% 26% 4	1155

Page generated at 2026-03-15 00:23:57. Version ac07

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