McMahan MP Observatory
Clear Sky Chart
History

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

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	12% 6% 2 6% 4 4 4 4 6% 7% 45%	131408
	Night	14% 6% 2 5% 3 3 3 3 5% 7% 49%	130099
Transparency	Day	37% 5% 11% 20% 17% 9%	135164
	Night	35% 5% 11% 20% 18% 11%	134407
Seeing	Day	13% 6% 19% 28% 25% 9%	16491
	Night	15% 6% 25% 36% 17% 1	26535

By Month

Percentage Cloud Cover SE (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	16% 10% 3 6% 3 3 2 3 4 4 47%	5289
	11th to 20th	Day	15% 5% 2 5% 3 3 3 2 4 5% 53%	5310
	21st to end	Day	26% 8% 3 6% 4 2 2 2 4 6% 39%	6109
February	1st to 10th	Day	20% 8% 3 6% 3 3 3 2 4 5% 44%	2100
	11th to 20th	Day	21% 7% 2 5% 3 4 3 3 3 6% 42%	2049
	21st to end	Day	18% 6% 3 5% 3 3 4 3 4 5% 47%	1749
March	1st to 10th	Day	13% 6% 2 7% 4 4 3 3 5% 6% 48%	4958
	11th to 20th	Day	14% 6% 2 6% 4 4 3 3 4 5% 49%	4980
	21st to end	Day	18% 8% 2 6% 4 3 3 3 4 5% 43%	5462
April	1st to 10th	Day	14% 9% 3 6% 4 4 4 3 6% 5% 42%	2200
	11th to 20th	Day	15% 8% 2 6% 6% 4 4 4 5% 5% 41%	2176
	21st to end	Day	15% 7% 2 5% 4 4 4 4 6% 7% 42%	2504
May	1st to 10th	Day	19% 7% 2 7% 5% 5% 4 4 6% 7% 34%	5862
	11th to 20th	Day	11% 5% 2 7% 4 5% 4 4 6% 8% 45%	5862
	21st to end	Day	11% 6% 2 6% 5% 6% 6% 5% 8% 9% 36%	6434
June	1st to 10th	Day	5% 4 1 5% 5% 5% 5% 5% 8% 10% 48%	2600
	11th to 20th	Day	7% 5% 1 5% 5% 5% 5% 4 7% 10% 47%	2600
	21st to end	Day	5% 6% 2 6% 6% 6% 6% 5% 8% 11% 38%	2600
July	1st to 10th	Day	10% 7% 3 8% 5% 5% 5% 5% 7% 9% 38%	5900
	11th to 20th	Day	7% 4 2 5% 5% 5% 6% 5% 8% 13% 40%	5900
	21st to end	Day	8% 6% 2 7% 5% 5% 5% 5% 8% 11% 39%	6422
August	1st to 10th	Day	1 2 1 3 2 3 4 4 5% 9% 67%	4234
	11th to 20th	Day	3 2 1 4 3 4 5% 5% 8% 13% 53%	3946
	21st to end	Day	4 3 2 5% 5% 5% 6% 4 8% 11% 46%	4053
September	1st to 10th	Day	7% 6% 2 7% 6% 6% 6% 4 6% 6% 45%	2200
	11th to 20th	Day	10% 5% 3 7% 6% 5% 6% 4 6% 8% 41%	2200
	21st to end	Day	7% 4 2 5% 4 5% 5% 4 7% 8% 49%	2200
October	1st to 10th	Day	6% 4 2 5% 3 3 5% 3 6% 8% 56%	2200
	11th to 20th	Day	12% 5% 2 4 4 3 3 3 4 6% 54%	2027
	21st to end	Day	10% 5% 2 4 3 3 4 3 4 6% 56%	2200
November	1st to 10th	Day	12% 7% 2 5% 4 4 5% 3 4 6% 48%	2058
	11th to 20th	Day	11% 6% 2 5% 3 3 4 3 4 5% 53%	2036
	21st to end	Day	17% 7% 3 5% 4 4 3 2 5% 6% 43%	1890
December	1st to 10th	Day	26% 9% 3 6% 3 3 3 3 4 5% 36%	3030
	11th to 20th	Day	17% 8% 3 6% 4 4 3 3 4 5% 43%	3481
	21st to end	Day	14% 8% 2 4 3 3 3 2 3 5% 53%	4587
January	1st to 10th	Night	17% 8% 2 4 3 3 3 2 3 5% 49%	7634
	11th to 20th	Night	11% 4 1 3 2 3 4 3 4 5% 60%	7680
	21st to end	Night	29% 6% 2 5% 3 2 3 2 4 5% 38%	8149
February	1st to 10th	Night	19% 7% 2 5% 3 2 3 3 4 5% 47%	2520
	11th to 20th	Night	20% 7% 2 4 3 3 2 2 3 6% 48%	2458
	21st to end	Night	21% 7% 2 5% 3 3 3 3 4 6% 44%	2055
March	1st to 10th	Night	16% 8% 2 5% 4 4 3 3 4 6% 44%	4941
	11th to 20th	Night	16% 7% 2 5% 3 3 4 3 4 5% 47%	4980
	21st to end	Night	18% 10% 3 5% 4 3 3 3 4 5% 42%	5467
April	1st to 10th	Night	14% 7% 2 6% 4 3 4 3 6% 7% 45%	2200
	11th to 20th	Night	15% 7% 2 6% 4 3 4 3 5% 7% 44%	2173
	21st to end	Night	16% 6% 3 6% 4 3 3 3 5% 6% 46%	1889
May	1st to 10th	Night	22% 9% 3 7% 4 3 4 3 5% 6% 33%	4053
	11th to 20th	Night	10% 5% 2 6% 4 4 5% 4 5% 9% 46%	4053
	21st to end	Night	13% 6% 3 7% 4 4 5% 4 7% 10% 36%	4443
June	1st to 10th	Night	5% 3 2 4 2 3 4 3 6% 9% 59%	1800
	11th to 20th	Night	4 3 1 3 4 5% 4 4 6% 10% 56%	1800
	21st to end	Night	6% 4 2 6% 5% 4 5% 4 6% 9% 50%	1800
July	1st to 10th	Night	8% 7% 3 7% 5% 4 5% 4 6% 9% 43%	4078
	11th to 20th	Night	5% 4 2 5% 4 4 5% 4 7% 11% 49%	4077
	21st to end	Night	7% 5% 2 6% 4 5% 5% 4 7% 10% 45%	4449
August	1st to 10th	Night	1 1 1 2 2 2 3 3 5% 7% 73%	2926
	11th to 20th	Night	3 2 1 3 2 3 4 5% 8% 12% 57%	3108
	21st to end	Night	4 2 1 4 3 4 4 4 7% 11% 56%	3847
September	1st to 10th	Night	8% 5% 2 6% 4 5% 5% 3 6% 8% 49%	2200
	11th to 20th	Night	8% 5% 2 6% 4 4 4 4 5% 8% 49%	2200
	21st to end	Night	7% 4 2 5% 4 3 5% 4 5% 7% 54%	2200
October	1st to 10th	Night	4 4 1 4 3 3 3 2 5% 8% 63%	2200
	11th to 20th	Night	11% 5% 2 5% 3 2 3 2 4 5% 58%	2350
	21st to end	Night	12% 5% 2 4 2 3 3 2 3 4 62%	2640
November	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 4 5% 55%	2460
	11th to 20th	Night	11% 5% 2 5% 3 3 4 3 4 5% 55%	2580
	21st to end	Night	17% 6% 2 5% 3 3 3 3 4 7% 46%	2730
December	1st to 10th	Night	26% 9% 3 6% 4 3 3 2 4 5% 36%	4380
	11th to 20th	Night	17% 9% 3 6% 3 3 3 2 3 4 47%	4946
	21st to end	Night	15% 6% 2 4 2 2 2 2 3 5% 56%	6633

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	43% 2 7% 19% 21% 8%	5290
	11th to 20th	Day	36% 2 4 17% 23% 19%	5310
	21st to end	Day	50% 2 4 10% 24% 10%	6109
February	1st to 10th	Day	44% 4 6% 12% 20% 15%	2100
	11th to 20th	Day	45% 3 8% 17% 17% 11%	2059
	21st to end	Day	40% 5% 7% 16% 19% 13%	1749
March	1st to 10th	Day	38% 4 6% 20% 14% 18%	4958
	11th to 20th	Day	39% 5% 4 8% 17% 26%	4980
	21st to end	Day	44% 4 7% 19% 18% 8%	5464
April	1st to 10th	Day	42% 5% 8% 17% 16% 13%	2200
	11th to 20th	Day	45% 4 6% 15% 19% 11%	2176
	21st to end	Day	41% 6% 7% 20% 18% 9%	2504
May	1st to 10th	Day	47% 6% 10% 20% 13% 4	5900
	11th to 20th	Day	37% 6% 12% 20% 21% 6%	5866
	21st to end	Day	38% 7% 13% 28% 13% 1	6447
June	1st to 10th	Day	28% 8% 15% 27% 19% 2	2600
	11th to 20th	Day	30% 9% 13% 25% 19% 4	2600
	21st to end	Day	36% 10% 15% 24% 12% 2	2600
July	1st to 10th	Day	42% 6% 22% 25% 4 1	5836
	11th to 20th	Day	34% 6% 23% 27% 10%	5831
	21st to end	Day	37% 7% 18% 25% 11% 3	6240
August	1st to 10th	Day	15% 6% 12% 43% 22% 1	4220
	11th to 20th	Day	21% 7% 28% 33% 9% 2	3985
	21st to end	Day	30% 10% 20% 24% 15% 1	4052
September	1st to 10th	Day	38% 8% 11% 19% 16% 7%	2200
	11th to 20th	Day	39% 9% 11% 18% 17% 5%	2200
	21st to end	Day	29% 7% 11% 23% 15% 14%	2200
October	1st to 10th	Day	17% 4 7% 13% 24% 36%	3590
	11th to 20th	Day	22% 3 8% 19% 29% 18%	3280
	21st to end	Day	27% 4 5% 20% 28% 16%	3538
November	1st to 10th	Day	36% 5% 8% 18% 21% 12%	2058
	11th to 20th	Day	33% 5% 10% 17% 20% 16%	2036
	21st to end	Day	43% 4 7% 15% 19% 12%	1890
December	1st to 10th	Day	52% 4 5% 15% 16% 8%	3030
	11th to 20th	Day	43% 4 8% 11% 22% 12%	3479
	21st to end	Day	35% 3 5% 19% 23% 15%	4587
January	1st to 10th	Night	39% 3 7% 18% 20% 13%	7635
	11th to 20th	Night	29% 3 4 17% 29% 19%	7680
	21st to end	Night	50% 2 3 12% 21% 12%	8149
February	1st to 10th	Night	41% 4 6% 13% 18% 17%	2520
	11th to 20th	Night	41% 3 7% 17% 18% 13%	2458
	21st to end	Night	43% 4 6% 16% 17% 14%	2055
March	1st to 10th	Night	43% 4 7% 16% 16% 14%	4941
	11th to 20th	Night	40% 5% 8% 10% 14% 24%	4980
	21st to end	Night	44% 5% 7% 16% 17% 10%	5467
April	1st to 10th	Night	38% 5% 10% 18% 16% 13%	2200
	11th to 20th	Night	40% 4 10% 15% 15% 15%	2173
	21st to end	Night	40% 5% 10% 20% 16% 9%	1889
May	1st to 10th	Night	51% 6% 8% 19% 12% 5%	4080
	11th to 20th	Night	34% 6% 12% 22% 18% 8%	4054
	21st to end	Night	40% 7% 16% 27% 9%	4452
June	1st to 10th	Night	22% 7% 19% 31% 21% 2	1800
	11th to 20th	Night	22% 7% 15% 32% 20% 4	1800
	21st to end	Night	30% 8% 18% 27% 14% 2	1800
July	1st to 10th	Night	38% 7% 25% 27% 4	4035
	11th to 20th	Night	29% 6% 24% 31% 9%	4026
	21st to end	Night	34% 7% 20% 26% 10% 3	4320
August	1st to 10th	Night	10% 5% 18% 46% 20%	2916
	11th to 20th	Night	17% 8% 35% 32% 8% 1	3135
	21st to end	Night	21% 10% 26% 28% 13% 2	3847
September	1st to 10th	Night	32% 7% 18% 22% 14% 7%	2200
	11th to 20th	Night	32% 9% 14% 24% 15% 5%	2200
	21st to end	Night	29% 5% 15% 21% 16% 13%	2200
October	1st to 10th	Night	14% 4 7% 16% 21% 37%	3590
	11th to 20th	Night	18% 3 9% 19% 29% 21%	3823
	21st to end	Night	23% 3 5% 24% 29% 17%	4252
November	1st to 10th	Night	32% 5% 9% 18% 22% 14%	2460
	11th to 20th	Night	31% 5% 10% 18% 21% 16%	2580
	21st to end	Night	39% 4 8% 18% 17% 14%	2730
December	1st to 10th	Night	53% 3 5% 15% 14% 9%	4380
	11th to 20th	Night	42% 3 8% 10% 21% 16%	4947
	21st to end	Night	34% 3 5% 15% 29% 14%	6633

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	22% 11% 33% 24% 9% 1	330
	11th to 20th	Day	22% 13% 28% 25% 10% 2	330
	21st to end	Day	25% 12% 32% 23% 9%	363
February	1st to 10th	Day	28% 12% 27% 22% 11% 1	330
	11th to 20th	Day	26% 14% 32% 21% 5% 1	330
	21st to end	Day	23% 10% 41% 21% 5%	273
March	1st to 10th	Day	18% 14% 36% 25% 6% 1	306
	11th to 20th	Day	16% 16% 39% 22% 6% 1	300
	21st to end	Day	18% 12% 36% 26% 8% 1	330
April	1st to 10th	Day	21% 13% 35% 25% 6%	300
	11th to 20th	Day	20% 14% 32% 24% 10% 1	294
	21st to end	Day	20% 10% 34% 26% 9% 1	604
May	1st to 10th	Day	17% 10% 34% 28% 10%	703
	11th to 20th	Day	15% 8% 25% 33% 17% 2	725
	21st to end	Day	12% 6% 22% 38% 20% 2	789
June	1st to 10th	Day	6% 2 11% 41% 32% 8%	730
	11th to 20th	Day	8% 3 13% 39% 32% 6%	730
	21st to end	Day	6% 1 8% 35% 35% 14%	730
July	1st to 10th	Day	8% 2 7% 31% 39% 15%	721
	11th to 20th	Day	5% 2 5% 26% 47% 15%	718
	21st to end	Day	6% 2 4 25% 46% 17%	794
August	1st to 10th	Day	3 1 3 22% 50% 21%	726
	11th to 20th	Day	3 1 3 24% 46% 23%	581
	21st to end	Day	6% 2 4 17% 35% 36%	428
September	1st to 10th	Day	10% 1 5% 19% 41% 25%	330
	11th to 20th	Day	5% 2 23% 43% 26%	330
	21st to end	Day	10% 2 6% 20% 38% 24%	330
October	1st to 10th	Day	7% 8% 27% 40% 18%	330
	11th to 20th	Day	11% 2 16% 30% 28% 14%	330
	21st to end	Day	14% 10% 20% 26% 22% 7%	363
November	1st to 10th	Day	16% 6% 17% 34% 19% 8%	330
	11th to 20th	Day	17% 5% 22% 31% 21% 4	330
	21st to end	Day	22% 13% 29% 20% 13% 3	330
December	1st to 10th	Day	21% 8% 30% 27% 10% 5%	330
	11th to 20th	Day	22% 12% 25% 27% 13% 2	330
	21st to end	Day	19% 10% 31% 25% 13% 2	363
January	1st to 10th	Night	21% 9% 30% 31% 8% 1	1050
	11th to 20th	Night	20% 11% 32% 29% 8%	1050
	21st to end	Night	24% 8% 29% 32% 6%	1070
February	1st to 10th	Night	25% 9% 31% 30% 5%	840
	11th to 20th	Night	25% 9% 36% 26% 4	824
	21st to end	Night	24% 11% 40% 22% 3	671
March	1st to 10th	Night	22% 12% 37% 26% 4	606
	11th to 20th	Night	22% 10% 37% 27% 5%	600
	21st to end	Night	18% 9% 41% 27% 3 1	660
April	1st to 10th	Night	17% 9% 45% 27% 3	600
	11th to 20th	Night	18% 10% 38% 28% 5%	593
	21st to end	Night	17% 10% 40% 25% 7%	600
May	1st to 10th	Night	18% 7% 42% 28% 5%	600
	11th to 20th	Night	14% 5% 35% 36% 10%	591
	21st to end	Night	11% 3 34% 38% 12%	650
June	1st to 10th	Night	6% 2 20% 50% 22% 1	600
	11th to 20th	Night	6% 1 27% 50% 16% 1	600
	21st to end	Night	6% 2 14% 45% 30% 4	600
July	1st to 10th	Night	7% 2 10% 46% 32% 3	585
	11th to 20th	Night	4 1 9% 48% 34% 3	582
	21st to end	Night	5% 1 8% 49% 35% 3	657
August	1st to 10th	Night	2 6% 52% 36% 4	597
	11th to 20th	Night	4 1 5% 49% 36% 5%	594
	21st to end	Night	6% 1 5% 37% 45% 7%	660
September	1st to 10th	Night	10% 1 6% 38% 39% 6%	600
	11th to 20th	Night	9% 3 5% 38% 42% 4	600
	21st to end	Night	9% 2 11% 36% 39% 4	600
October	1st to 10th	Night	4 2 12% 47% 31% 5%	600
	11th to 20th	Night	13% 3 18% 43% 21% 2	750
	21st to end	Night	12% 7% 26% 34% 19% 1	880
November	1st to 10th	Night	14% 5% 20% 40% 21% 1	820
	11th to 20th	Night	13% 6% 25% 38% 17% 1	900
	21st to end	Night	19% 8% 28% 31% 12% 1	1050
December	1st to 10th	Night	21% 8% 24% 36% 10% 1	1050
	11th to 20th	Night	24% 9% 27% 32% 8%	1050
	21st to end	Night	20% 8% 28% 36% 8%	1155

Page generated at 2026-02-28 00:40:21. Version ac07

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