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 2024-01-15.

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	3 4 2 4 4 3 5% 5% 6% 8% 55%	74763
	Night	3 4 2 4 3 2 4 4 5% 8% 62%	75041
Transparency	Day	25% 6% 13% 24% 23% 8%	74478
	Night	21% 6% 15% 27% 24% 7%	74849
Seeing	Day	11% 16% 31% 28% 13% 2	14295
	Night	6% 9% 35% 36% 13% 1	24490

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	3 1 2 2 2 3 4 4 9% 69%	1518
	11th to 20th	Day	1 2 1 3 4 2 3 3 6% 8% 66%	1631
	21st to end	Day	5% 4 4 3 1 3 4 4 8% 63%	1782
February	1st to 10th	Day	5% 2 5% 3 1 3 4 4 8% 64%	1620
	11th to 20th	Day	1 2 1 3 3 3 4 4 7% 9% 62%	1713
	21st to end	Day	1 3 2 6% 6% 3 4 4 5% 8% 60%	1480
March	1st to 10th	Day	1 4 2 4 4 2 3 4 5% 11% 60%	1837
	11th to 20th	Day	2 7% 2 5% 4 3 5% 4 5% 8% 55%	1980
	21st to end	Day	2 5% 3 4 5% 3 5% 5% 7% 10% 52%	2272
April	1st to 10th	Day	2 4 2 4 4 3 4 5% 6% 9% 56%	2160
	11th to 20th	Day	5% 7% 2 5% 6% 3 5% 5% 6% 8% 49%	2131
	21st to end	Day	5% 7% 2 4 5% 3 6% 5% 6% 8% 49%	2160
May	1st to 10th	Day	6% 6% 3 4 5% 4 5% 6% 6% 10% 46%	2173
	11th to 20th	Day	6% 8% 2 6% 6% 4 7% 6% 6% 9% 41%	2471
	21st to end	Day	5% 4 2 5% 5% 3 6% 6% 6% 9% 49%	2724
June	1st to 10th	Day	2 4 2 4 5% 3 7% 7% 7% 12% 49%	2520
	11th to 20th	Day	3 3 2 4 4 3 4 6% 6% 8% 58%	2520
	21st to end	Day	3 2 1 4 5% 4 6% 6% 9% 9% 50%	2520
July	1st to 10th	Day	5% 3 3 5% 6% 4 7% 8% 8% 8% 45%	2520
	11th to 20th	Day	5% 3 2 4 6% 4 6% 7% 8% 10% 46%	2520
	21st to end	Day	5% 5% 3 7% 8% 4 8% 8% 9% 10% 33%	2562
August	1st to 10th	Day	6% 4 3 6% 7% 4 7% 6% 9% 9% 38%	2328
	11th to 20th	Day	5% 3 2 5% 6% 4 6% 6% 8% 9% 46%	2132
	21st to end	Day	5% 5% 2 6% 6% 3 5% 6% 8% 8% 46%	2376
September	1st to 10th	Day	4 4 2 5% 5% 4 5% 5% 7% 7% 52%	2160
	11th to 20th	Day	4 3 1 4 4 3 4 5% 6% 8% 58%	2160
	21st to end	Day	4 3 1 3 3 2 3 4 5% 6% 66%	1956
October	1st to 10th	Day	4 3 1 4 4 2 4 5% 5% 7% 60%	1800
	11th to 20th	Day	3 2 1 2 3 2 3 4 4 6% 72%	1800
	21st to end	Day	3 5% 2 3 3 1 3 3 3 6% 67%	1980
November	1st to 10th	Day	1 2 1 2 2 1 2 3 4 6% 76%	1687
	11th to 20th	Day	2 1 3 3 1 2 2 3 7% 76%	1710
	21st to end	Day	4 2 4 4 2 3 3 4 8% 67%	1620
December	1st to 10th	Day	1 3 1 4 3 2 3 4 4 7% 69%	1520
	11th to 20th	Day	1 3 1 3 2 2 3 3 6% 12% 64%	1926
	21st to end	Day	1 4 2 5% 3 2 3 4 5% 9% 63%	2794
January	1st to 10th	Night	3 2 3 3 1 3 4 4 6% 71%	2647
	11th to 20th	Night	1 3 1 3 3 1 3 3 4 6% 73%	2448
	21st to end	Night	1 3 2 3 3 2 3 3 4 7% 70%	2574
February	1st to 10th	Night	3 3 5% 3 2 3 3 3 6% 69%	2340
	11th to 20th	Night	1 4 2 6% 4 2 3 3 4 6% 65%	2134
	21st to end	Night	1 4 3 5% 4 1 3 3 5% 6% 64%	1776
March	1st to 10th	Night	1 4 2 3 3 2 2 3 5% 7% 68%	2120
	11th to 20th	Night	2 6% 3 3 4 2 4 4 4 8% 59%	1980
	21st to end	Night	2 5% 2 3 3 2 4 4 4 8% 62%	2083
April	1st to 10th	Night	1 5% 2 4 4 2 3 4 5% 7% 64%	1800
	11th to 20th	Night	5% 8% 2 4 4 2 4 4 4 6% 56%	1777
	21st to end	Night	4 7% 2 5% 4 2 4 4 5% 8% 56%	1800
May	1st to 10th	Night	7% 7% 2 4 3 2 4 4 5% 7% 55%	1784
	11th to 20th	Night	10% 9% 3 5% 5% 2 4 5% 4 8% 45%	1420
	21st to end	Night	7% 5% 1 3 3 1 3 5% 6% 8% 57%	1552
June	1st to 10th	Night	3 3 1 3 3 2 5% 5% 6% 9% 61%	1440
	11th to 20th	Night	2 3 1 2 3 2 4 4 5% 8% 66%	1440
	21st to end	Night	3 3 1 3 4 2 5% 6% 6% 10% 56%	1440
July	1st to 10th	Night	8% 5% 2 5% 6% 3 6% 6% 6% 10% 44%	1440
	11th to 20th	Night	5% 3 2 4 5% 3 5% 6% 8% 8% 53%	1440
	21st to end	Night	6% 4 3 5% 6% 4 7% 6% 10% 12% 37%	1752
August	1st to 10th	Night	6% 4 2 5% 5% 4 5% 6% 8% 11% 44%	1611
	11th to 20th	Night	5% 4 2 5% 4 3 5% 6% 7% 10% 50%	1768
	21st to end	Night	4 5% 3 5% 6% 3 4 6% 7% 9% 48%	1980
September	1st to 10th	Night	5% 4 2 5% 5% 3 5% 5% 6% 10% 49%	1800
	11th to 20th	Night	5% 4 2 3 2 3 4 4 6% 7% 60%	1800
	21st to end	Night	4 4 2 4 3 2 3 4 5% 6% 64%	1997
October	1st to 10th	Night	5% 4 2 4 3 2 4 4 5% 7% 60%	2160
	11th to 20th	Night	2 2 2 2 1 2 3 4 6% 76%	2160
	21st to end	Night	4 4 1 3 3 1 3 3 4 6% 66%	2376
November	1st to 10th	Night	1 2 1 2 2 1 3 2 4 8% 74%	2373
	11th to 20th	Night	1 2 1 2 2 1 3 3 3 5% 76%	2470
	21st to end	Night	2 5% 2 2 2 1 2 3 4 7% 68%	2555
December	1st to 10th	Night	3 1 3 3 2 3 3 5% 8% 68%	2660
	11th to 20th	Night	1 3 1 3 2 2 3 4 6% 10% 63%	3271
	21st to end	Night	1 5% 2 4 4 2 4 4 5% 9% 60%	4873

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	14% 4 20% 35% 23% 4	1519
	11th to 20th	Day	17% 5% 18% 31% 26% 4	1631
	21st to end	Day	21% 5% 13% 36% 20% 5%	1782
February	1st to 10th	Day	21% 5% 19% 30% 19% 7%	1620
	11th to 20th	Day	18% 5% 16% 30% 26% 5%	1723
	21st to end	Day	25% 4 11% 24% 27% 9%	1480
March	1st to 10th	Day	22% 4 13% 27% 29% 5%	1837
	11th to 20th	Day	27% 5% 11% 28% 24% 5%	1980
	21st to end	Day	27% 7% 11% 31% 20% 5%	2272
April	1st to 10th	Day	24% 8% 13% 23% 22% 11%	2160
	11th to 20th	Day	33% 5% 12% 24% 21% 6%	2134
	21st to end	Day	31% 6% 13% 23% 22% 4	2160
May	1st to 10th	Day	33% 8% 12% 23% 20% 5%	2173
	11th to 20th	Day	38% 9% 13% 22% 15% 3	2475
	21st to end	Day	29% 8% 13% 20% 23% 8%	2724
June	1st to 10th	Day	25% 10% 15% 19% 22% 9%	2520
	11th to 20th	Day	22% 7% 10% 17% 24% 20%	2520
	21st to end	Day	26% 7% 11% 22% 21% 13%	2520
July	1st to 10th	Day	34% 8% 13% 17% 20% 9%	2451
	11th to 20th	Day	29% 8% 17% 26% 14% 6%	2445
	21st to end	Day	40% 12% 22% 16% 8% 2	2420
August	1st to 10th	Day	39% 10% 15% 15% 14% 7%	2313
	11th to 20th	Day	32% 8% 13% 20% 19% 8%	2132
	21st to end	Day	31% 8% 13% 22% 20% 5%	2376
September	1st to 10th	Day	29% 6% 13% 18% 22% 12%	2160
	11th to 20th	Day	24% 7% 12% 18% 26% 13%	2160
	21st to end	Day	20% 5% 7% 24% 34% 9%	1956
October	1st to 10th	Day	24% 6% 10% 23% 27% 10%	1800
	11th to 20th	Day	15% 4 7% 22% 36% 15%	1800
	21st to end	Day	21% 3 9% 28% 29% 10%	1980
November	1st to 10th	Day	11% 4 11% 25% 41% 9%	1687
	11th to 20th	Day	12% 3 8% 38% 32% 6%	1710
	21st to end	Day	19% 4 8% 31% 29% 10%	1620
December	1st to 10th	Day	17% 6% 17% 32% 21% 7%	1520
	11th to 20th	Day	17% 5% 15% 32% 27% 3	1924
	21st to end	Day	19% 5% 17% 24% 25% 10%	2794
January	1st to 10th	Night	16% 4 18% 33% 26% 3	2647
	11th to 20th	Night	15% 3 18% 31% 28% 4	2448
	21st to end	Night	16% 4 14% 36% 24% 5%	2574
February	1st to 10th	Night	19% 4 18% 32% 21% 6%	2340
	11th to 20th	Night	21% 5% 14% 29% 26% 4	2134
	21st to end	Night	20% 4 12% 27% 30% 7%	1776
March	1st to 10th	Night	18% 4 14% 28% 30% 5%	2120
	11th to 20th	Night	23% 6% 13% 33% 22% 3	1980
	21st to end	Night	24% 5% 14% 30% 25% 3	2083
April	1st to 10th	Night	21% 5% 14% 27% 26% 7%	1800
	11th to 20th	Night	29% 4 13% 26% 22% 5%	1777
	21st to end	Night	27% 7% 14% 28% 20% 4	1800
May	1st to 10th	Night	30% 6% 12% 26% 20% 6%	1784
	11th to 20th	Night	37% 10% 15% 20% 15% 3	1421
	21st to end	Night	24% 6% 17% 23% 20% 9%	1552
June	1st to 10th	Night	21% 8% 17% 24% 18% 13%	1440
	11th to 20th	Night	17% 6% 11% 21% 25% 20%	1440
	21st to end	Night	22% 7% 15% 22% 22% 13%	1440
July	1st to 10th	Night	35% 10% 15% 17% 16% 8%	1400
	11th to 20th	Night	26% 9% 20% 26% 11% 8%	1392
	21st to end	Night	35% 17% 25% 17% 6% 1	1655
August	1st to 10th	Night	31% 14% 19% 18% 11% 6%	1602
	11th to 20th	Night	28% 10% 19% 23% 14% 7%	1768
	21st to end	Night	29% 10% 16% 24% 15% 6%	1980
September	1st to 10th	Night	29% 8% 15% 20% 19% 10%	1800
	11th to 20th	Night	22% 8% 13% 20% 24% 13%	1800
	21st to end	Night	21% 6% 9% 25% 31% 8%	1997
October	1st to 10th	Night	25% 7% 11% 23% 24% 10%	2160
	11th to 20th	Night	11% 4 9% 24% 39% 14%	2160
	21st to end	Night	20% 3 8% 29% 30% 9%	2376
November	1st to 10th	Night	11% 4 12% 31% 33% 9%	2373
	11th to 20th	Night	12% 4 9% 37% 30% 8%	2470
	21st to end	Night	17% 5% 9% 29% 31% 8%	2555
December	1st to 10th	Night	16% 6% 18% 31% 24% 6%	2660
	11th to 20th	Night	18% 6% 15% 29% 29% 3	3272
	21st to end	Night	21% 6% 18% 23% 27% 6%	4873

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% 12% 43% 26% 10% 1	270
	11th to 20th	Day	12% 12% 47% 23% 5% 1	254
	21st to end	Day	2 15% 45% 28% 9%	264
February	1st to 10th	Day	10% 24% 45% 18% 3 1	240
	11th to 20th	Day	12% 23% 41% 17% 7% 1	240
	21st to end	Day	17% 22% 42% 17% 2 1	198
March	1st to 10th	Day	14% 28% 31% 23% 4	277
	11th to 20th	Day	19% 23% 33% 18% 7%	420
	21st to end	Day	21% 30% 35% 13% 2	462
April	1st to 10th	Day	18% 32% 28% 16% 5%	420
	11th to 20th	Day	23% 30% 30% 14% 3	412
	21st to end	Day	22% 29% 27% 16% 6%	420
May	1st to 10th	Day	17% 26% 35% 16% 6%	436
	11th to 20th	Day	14% 17% 28% 26% 12% 2	625
	21st to end	Day	10% 17% 29% 30% 13% 1	679
June	1st to 10th	Day	6% 12% 27% 35% 18% 3	630
	11th to 20th	Day	7% 12% 34% 31% 15% 1	630
	21st to end	Day	5% 12% 30% 31% 20% 3	630
July	1st to 10th	Day	6% 6% 23% 39% 23% 4	621
	11th to 20th	Day	6% 3 18% 37% 30% 6%	618
	21st to end	Day	6% 7% 18% 39% 24% 6%	504
August	1st to 10th	Day	7% 7% 26% 40% 15% 5%	447
	11th to 20th	Day	6% 5% 32% 39% 15% 3	437
	21st to end	Day	8% 8% 29% 35% 17% 3	495
September	1st to 10th	Day	7% 12% 28% 32% 17% 4	450
	11th to 20th	Day	8% 11% 36% 27% 15% 3	450
	21st to end	Day	9% 10% 28% 31% 19% 3	282
October	1st to 10th	Day	11% 14% 24% 30% 16% 4	270
	11th to 20th	Day	9% 19% 27% 31% 12% 2	270
	21st to end	Day	13% 18% 30% 24% 12% 2	297
November	1st to 10th	Day	10% 19% 30% 24% 15% 1	270
	11th to 20th	Day	9% 16% 35% 29% 10% 2	270
	21st to end	Day	9% 17% 37% 28% 9% 1	270
December	1st to 10th	Day	10% 16% 41% 25% 9%	270
	11th to 20th	Day	8% 16% 41% 25% 9% 1	270
	21st to end	Day	14% 21% 39% 20% 5% 1	297
January	1st to 10th	Night	4 14% 41% 30% 10% 1	950
	11th to 20th	Night	5% 12% 48% 28% 7% 1	783
	21st to end	Night	2 11% 47% 33% 7%	792
February	1st to 10th	Night	3 11% 55% 27% 3	720
	11th to 20th	Night	5% 12% 53% 26% 4	704
	21st to end	Night	5% 12% 54% 27% 2	592
March	1st to 10th	Night	4 12% 48% 31% 5%	720
	11th to 20th	Night	5% 10% 39% 35% 10%	720
	21st to end	Night	6% 12% 48% 28% 5%	697
April	1st to 10th	Night	7% 15% 40% 34% 4	540
	11th to 20th	Night	13% 14% 34% 32% 6%	533
	21st to end	Night	9% 14% 36% 34% 7%	540
May	1st to 10th	Night	9% 11% 30% 40% 10% 1	540
	11th to 20th	Night	14% 7% 25% 40% 13% 1	531
	21st to end	Night	10% 9% 29% 38% 14% 1	584
June	1st to 10th	Night	5% 6% 21% 44% 22% 1	540
	11th to 20th	Night	3 6% 27% 50% 13% 1	540
	21st to end	Night	4 3 18% 48% 26% 1	540
July	1st to 10th	Night	7% 3 11% 46% 30% 2	525
	11th to 20th	Night	6% 2 8% 42% 39% 4	522
	21st to end	Night	8% 2 7% 37% 43% 3	591
August	1st to 10th	Night	8% 3 10% 42% 35% 2	537
	11th to 20th	Night	6% 2 11% 48% 31% 2	534
	21st to end	Night	6% 3 12% 47% 30% 2	594
September	1st to 10th	Night	7% 3 19% 44% 24% 3	540
	11th to 20th	Night	6% 4 15% 49% 24% 2	540
	21st to end	Night	5% 6% 24% 45% 19% 2	571
October	1st to 10th	Night	6% 6% 32% 41% 14% 1	720
	11th to 20th	Night	3 6% 36% 41% 14% 1	720
	21st to end	Night	6% 7% 36% 38% 12% 1	792
November	1st to 10th	Night	3 7% 41% 38% 10% 1	893
	11th to 20th	Night	5% 12% 41% 32% 9% 1	950
	21st to end	Night	6% 11% 39% 36% 8%	950
December	1st to 10th	Night	4 14% 46% 31% 5%	950
	11th to 20th	Night	5% 12% 46% 28% 8% 1	950
	21st to end	Night	7% 15% 47% 27% 4	1045

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