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 2024-12-02.

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%	101290
	Night	13% 6% 2 5% 3 3 3 3 5% 7% 49%	98460
Transparency	Day	36% 6% 11% 21% 18% 8%	105085
	Night	33% 5% 12% 21% 18% 10%	102792
Seeing	Day	13% 6% 18% 28% 26% 9%	14887
	Night	15% 6% 25% 36% 17% 2	24819

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	15% 9% 3 6% 3 3 2 3 5% 4 46%	2847
	11th to 20th	Day	18% 5% 2 3 2 2 2 2 5% 6% 53%	2850
	21st to end	Day	21% 8% 2 5% 3 3 3 2 5% 7% 41%	3293
February	1st to 10th	Day	21% 8% 3 6% 3 3 3 2 4 5% 43%	1900
	11th to 20th	Day	21% 7% 2 5% 3 3 3 3 3 6% 43%	1849
	21st to end	Day	18% 7% 3 5% 3 3 4 3 4 6% 45%	1587
March	1st to 10th	Day	15% 8% 3 8% 5% 5% 3 3 5% 7% 38%	3436
	11th to 20th	Day	20% 8% 3 8% 6% 6% 4 3 5% 5% 31%	3480
	21st to end	Day	17% 7% 2 6% 4 3 4 3 5% 6% 44%	3814
April	1st to 10th	Day	14% 9% 3 6% 4 4 4 3 6% 5% 42%	2090
	11th to 20th	Day	16% 8% 2 6% 6% 4 4 4 5% 5% 40%	2066
	21st to end	Day	15% 7% 2 5% 4 4 4 3 6% 7% 43%	2378
May	1st to 10th	Day	20% 8% 2 8% 6% 5% 4 4 5% 6% 31%	4082
	11th to 20th	Day	15% 6% 2 7% 4 5% 4 4 6% 6% 41%	4082
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 5% 8% 9% 39%	4477
June	1st to 10th	Day	5% 4 1 5% 4 5% 5% 5% 8% 10% 48%	2470
	11th to 20th	Day	7% 5% 1 5% 5% 5% 5% 4 7% 10% 47%	2470
	21st to end	Day	6% 6% 2 6% 6% 6% 6% 5% 8% 10% 38%	2470
July	1st to 10th	Day	7% 4 2 6% 5% 4 4 5% 6% 9% 47%	4120
	11th to 20th	Day	6% 4 2 5% 5% 5% 6% 5% 8% 11% 41%	4120
	21st to end	Day	10% 7% 3 7% 5% 6% 5% 5% 7% 10% 35%	4465
August	1st to 10th	Day	2 2 1 3 2 3 4 4 5% 9% 67%	4104
	11th to 20th	Day	3 2 1 4 3 4 5% 5% 8% 13% 53%	3823
	21st to end	Day	4 3 2 5% 5% 5% 6% 4 8% 11% 46%	3929
September	1st to 10th	Day	7% 6% 2 7% 6% 6% 6% 4 6% 6% 44%	2090
	11th to 20th	Day	10% 5% 3 7% 6% 5% 6% 4 6% 7% 40%	2090
	21st to end	Day	8% 4 2 6% 4 5% 5% 4 6% 7% 49%	2090
October	1st to 10th	Day	6% 4 2 5% 3 3 5% 3 6% 8% 55%	2090
	11th to 20th	Day	13% 5% 2 4 4 3 4 3 4 6% 53%	1926
	21st to end	Day	10% 5% 2 4 3 3 4 2 4 6% 57%	2090
November	1st to 10th	Day	12% 7% 2 5% 4 4 5% 3 4 6% 46%	1958
	11th to 20th	Day	11% 6% 2 5% 3 3 4 3 4 5% 52%	1939
	21st to end	Day	17% 7% 3 5% 4 4 3 2 5% 6% 44%	1800
December	1st to 10th	Day	17% 9% 2 5% 3 4 3 3 5% 6% 43%	1719
	11th to 20th	Day	18% 7% 3 6% 4 4 3 3 3 5% 44%	2161
	21st to end	Day	16% 8% 2 4 3 3 3 2 3 5% 50%	3135
January	1st to 10th	Night	20% 8% 1 4 2 3 3 2 4 5% 49%	4108
	11th to 20th	Night	15% 4 1 3 2 2 2 2 3 4 62%	4120
	21st to end	Night	23% 7% 2 6% 4 3 3 2 4 6% 41%	4404
February	1st to 10th	Night	20% 7% 2 5% 3 2 3 3 4 5% 45%	2280
	11th to 20th	Night	20% 7% 2 4 2 2 2 2 3 6% 48%	2218
	21st to end	Night	21% 7% 3 5% 3 3 3 3 4 6% 43%	1865
March	1st to 10th	Night	19% 10% 2 5% 4 4 3 3 5% 6% 37%	3419
	11th to 20th	Night	23% 10% 3 7% 4 4 4 3 5% 5% 31%	3480
	21st to end	Night	15% 8% 3 6% 4 4 3 3 5% 5% 45%	3817
April	1st to 10th	Night	13% 7% 2 6% 4 3 4 3 6% 7% 45%	2090
	11th to 20th	Night	16% 7% 2 6% 4 3 3 2 5% 7% 43%	2063
	21st to end	Night	16% 6% 3 6% 4 3 3 3 5% 5% 46%	1795
May	1st to 10th	Night	23% 9% 2 6% 4 3 3 3 5% 6% 35%	2823
	11th to 20th	Night	13% 6% 2 6% 4 4 4 4 4 7% 47%	2823
	21st to end	Night	12% 6% 2 6% 3 4 5% 4 6% 10% 42%	3090
June	1st to 10th	Night	5% 3 1 4 2 3 4 3 6% 9% 59%	1710
	11th to 20th	Night	4 3 1 3 4 5% 4 4 6% 10% 57%	1710
	21st to end	Night	6% 4 2 6% 5% 4 5% 4 6% 9% 49%	1710
July	1st to 10th	Night	4 4 2 4 4 4 4 4 6% 9% 54%	2850
	11th to 20th	Night	4 4 2 5% 4 4 4 4 6% 11% 51%	2850
	21st to end	Night	8% 6% 2 7% 4 4 4 4 8% 10% 42%	3098
August	1st to 10th	Night	1 1 1 2 2 2 3 3 5% 7% 73%	2836
	11th to 20th	Night	3 2 1 3 2 3 4 5% 8% 12% 57%	3011
	21st to end	Night	4 2 1 4 3 4 4 4 7% 11% 56%	3729
September	1st to 10th	Night	8% 5% 2 6% 4 5% 5% 3 6% 8% 49%	2090
	11th to 20th	Night	9% 5% 2 7% 4 4 4 4 5% 8% 48%	2090
	21st to end	Night	7% 4 2 5% 4 3 5% 4 5% 7% 53%	2090
October	1st to 10th	Night	4 4 1 4 3 2 3 2 5% 8% 63%	2090
	11th to 20th	Night	12% 5% 2 5% 3 2 3 3 4 5% 57%	2232
	21st to end	Night	12% 5% 2 4 2 3 3 2 3 4 61%	2508
November	1st to 10th	Night	13% 5% 2 5% 3 3 3 3 4 6% 53%	2340
	11th to 20th	Night	10% 5% 2 5% 3 3 4 3 4 5% 55%	2457
	21st to end	Night	17% 6% 2 5% 3 3 3 3 4 7% 47%	2600
December	1st to 10th	Night	16% 7% 2 5% 4 3 3 3 4 6% 48%	2496
	11th to 20th	Night	18% 7% 3 6% 4 3 3 3 4 5% 44%	3036
	21st to end	Night	17% 7% 2 4 3 3 2 2 4 5% 52%	4532

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	42% 3 5% 14% 24% 12%	2848
	11th to 20th	Day	34% 2 7% 18% 24% 14%	2850
	21st to end	Day	45% 3 4 11% 28% 10%	3293
February	1st to 10th	Day	45% 4 6% 12% 20% 14%	1900
	11th to 20th	Day	44% 4 7% 16% 17% 12%	1859
	21st to end	Day	42% 5% 7% 15% 17% 13%	1587
March	1st to 10th	Day	46% 6% 8% 17% 16% 8%	3436
	11th to 20th	Day	54% 7% 6% 8% 17% 8%	3480
	21st to end	Day	40% 5% 8% 17% 20% 9%	3814
April	1st to 10th	Day	42% 6% 8% 17% 15% 12%	2090
	11th to 20th	Day	45% 4 6% 15% 18% 11%	2066
	21st to end	Day	40% 6% 7% 20% 19% 9%	2378
May	1st to 10th	Day	50% 8% 10% 15% 11% 5%	4120
	11th to 20th	Day	41% 7% 9% 17% 20% 5%	4086
	21st to end	Day	35% 9% 15% 31% 11% 1	4489
June	1st to 10th	Day	28% 9% 15% 26% 20% 2	2470
	11th to 20th	Day	30% 9% 14% 25% 18% 4	2470
	21st to end	Day	37% 10% 14% 25% 12% 2	2470
July	1st to 10th	Day	32% 6% 21% 34% 6% 1	4056
	11th to 20th	Day	35% 7% 20% 25% 13% 1	4051
	21st to end	Day	41% 8% 18% 27% 6% 1	4325
August	1st to 10th	Day	16% 7% 12% 43% 22% 1	4090
	11th to 20th	Day	21% 7% 28% 33% 9% 2	3862
	21st to end	Day	30% 10% 20% 24% 15% 1	3928
September	1st to 10th	Day	38% 8% 12% 19% 15% 7%	2090
	11th to 20th	Day	40% 9% 11% 17% 17% 5%	2090
	21st to end	Day	30% 7% 11% 22% 15% 14%	2090
October	1st to 10th	Day	17% 4 7% 13% 23% 36%	3480
	11th to 20th	Day	22% 3 9% 19% 29% 18%	3179
	21st to end	Day	27% 4 5% 20% 28% 15%	3428
November	1st to 10th	Day	38% 6% 9% 18% 19% 11%	1958
	11th to 20th	Day	34% 5% 9% 17% 20% 16%	1939
	21st to end	Day	42% 4 7% 16% 18% 13%	1800
December	1st to 10th	Day	42% 5% 8% 18% 19% 8%	1719
	11th to 20th	Day	43% 5% 7% 10% 22% 13%	2159
	21st to end	Day	38% 4 6% 17% 24% 13%	3135
January	1st to 10th	Night	38% 4 7% 14% 20% 17%	4108
	11th to 20th	Night	29% 3 5% 17% 30% 16%	4120
	21st to end	Night	46% 3 4 12% 24% 10%	4404
February	1st to 10th	Night	43% 4 6% 13% 18% 16%	2280
	11th to 20th	Night	40% 3 7% 16% 19% 14%	2218
	21st to end	Night	44% 4 6% 16% 16% 14%	1865
March	1st to 10th	Night	48% 6% 10% 15% 14% 7%	3419
	11th to 20th	Night	54% 6% 10% 11% 12% 7%	3480
	21st to end	Night	40% 6% 9% 15% 20% 10%	3817
April	1st to 10th	Night	38% 6% 10% 18% 16% 12%	2090
	11th to 20th	Night	40% 4 10% 15% 15% 15%	2063
	21st to end	Night	40% 5% 9% 20% 17% 9%	1795
May	1st to 10th	Night	50% 6% 9% 18% 11% 5%	2850
	11th to 20th	Night	36% 7% 12% 20% 18% 7%	2824
	21st to end	Night	35% 8% 16% 30% 10%	3099
June	1st to 10th	Night	21% 7% 19% 30% 21% 2	1710
	11th to 20th	Night	21% 7% 15% 32% 20% 4	1710
	21st to end	Night	30% 9% 17% 28% 15% 2	1710
July	1st to 10th	Night	26% 7% 24% 38% 5%	2805
	11th to 20th	Night	28% 8% 21% 30% 13%	2796
	21st to end	Night	37% 7% 23% 27% 6%	2999
August	1st to 10th	Night	10% 5% 18% 46% 20%	2826
	11th to 20th	Night	17% 8% 34% 32% 8% 1	3038
	21st to end	Night	21% 11% 26% 28% 13% 2	3729
September	1st to 10th	Night	31% 7% 18% 22% 14% 7%	2090
	11th to 20th	Night	33% 9% 14% 23% 15% 5%	2090
	21st to end	Night	29% 6% 16% 20% 16% 13%	2090
October	1st to 10th	Night	14% 4 7% 16% 20% 38%	3480
	11th to 20th	Night	19% 3 10% 19% 29% 22%	3705
	21st to end	Night	23% 3 5% 24% 29% 17%	4120
November	1st to 10th	Night	34% 6% 10% 18% 20% 13%	2340
	11th to 20th	Night	32% 5% 10% 18% 20% 16%	2457
	21st to end	Night	38% 4 8% 18% 18% 14%	2600
December	1st to 10th	Night	40% 4 7% 21% 17% 11%	2496
	11th to 20th	Night	42% 4 6% 12% 19% 16%	3037
	21st to end	Night	36% 4 6% 14% 28% 12%	4532

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	21% 10% 34% 26% 8% 1	270
	11th to 20th	Day	23% 13% 26% 25% 10% 2	270
	21st to end	Day	22% 11% 33% 24% 10%	297
February	1st to 10th	Day	32% 12% 27% 17% 11% 1	270
	11th to 20th	Day	26% 13% 33% 21% 6% 1	270
	21st to end	Day	25% 11% 42% 19% 3	225
March	1st to 10th	Day	20% 13% 36% 24% 6% 1	270
	11th to 20th	Day	17% 15% 38% 22% 7% 1	270
	21st to end	Day	18% 11% 36% 26% 8% 1	297
April	1st to 10th	Day	21% 13% 37% 23% 6%	270
	11th to 20th	Day	20% 13% 33% 23% 9% 1	264
	21st to end	Day	20% 9% 35% 26% 9% 1	558
May	1st to 10th	Day	17% 10% 33% 29% 10%	653
	11th to 20th	Day	16% 7% 24% 33% 18% 2	675
	21st to end	Day	12% 6% 22% 38% 20% 2	734
June	1st to 10th	Day	7% 1 10% 40% 33% 8%	680
	11th to 20th	Day	7% 2 12% 39% 33% 6%	680
	21st to end	Day	6% 1 9% 35% 35% 14%	680
July	1st to 10th	Day	7% 2 6% 30% 39% 16%	671
	11th to 20th	Day	4 1 5% 25% 49% 16%	668
	21st to end	Day	6% 2 4 25% 46% 17%	739
August	1st to 10th	Day	3 1 3 22% 50% 21%	676
	11th to 20th	Day	4 1 3 24% 46% 23%	538
	21st to end	Day	6% 2 5% 17% 34% 36%	392
September	1st to 10th	Day	10% 1 4 19% 40% 26%	300
	11th to 20th	Day	6% 2 23% 43% 25%	300
	21st to end	Day	11% 3 6% 20% 37% 25%	300
October	1st to 10th	Day	7% 9% 28% 40% 16%	300
	11th to 20th	Day	11% 2 15% 31% 29% 11%	300
	21st to end	Day	16% 10% 20% 27% 20% 8%	330
November	1st to 10th	Day	18% 5% 18% 35% 18% 6%	300
	11th to 20th	Day	18% 5% 22% 29% 21% 4	300
	21st to end	Day	23% 13% 28% 19% 14% 3	300
December	1st to 10th	Day	18% 8% 28% 29% 11% 5%	273
	11th to 20th	Day	24% 12% 26% 26% 11% 1	270
	21st to end	Day	21% 9% 34% 24% 11% 2	297
January	1st to 10th	Night	21% 8% 30% 32% 8% 1	950
	11th to 20th	Night	21% 10% 30% 29% 9%	950
	21st to end	Night	23% 8% 30% 33% 7%	969
February	1st to 10th	Night	26% 10% 30% 29% 5%	760
	11th to 20th	Night	25% 9% 35% 27% 3	744
	21st to end	Night	25% 12% 40% 21% 2	609
March	1st to 10th	Night	22% 11% 36% 26% 4	570
	11th to 20th	Night	23% 8% 36% 28% 5%	570
	21st to end	Night	18% 9% 42% 27% 4 1	627
April	1st to 10th	Night	17% 8% 45% 27% 3	570
	11th to 20th	Night	18% 10% 38% 28% 5%	563
	21st to end	Night	18% 10% 40% 25% 8%	570
May	1st to 10th	Night	18% 6% 42% 29% 5%	570
	11th to 20th	Night	14% 4 34% 37% 10%	561
	21st to end	Night	12% 3 34% 39% 13%	617
June	1st to 10th	Night	5% 1 20% 50% 22% 1	570
	11th to 20th	Night	6% 1 27% 50% 16% 1	570
	21st to end	Night	6% 2 14% 45% 29% 4	570
July	1st to 10th	Night	6% 2 10% 47% 31% 4	555
	11th to 20th	Night	4 1 9% 48% 35% 3	552
	21st to end	Night	4 1 7% 49% 35% 3	624
August	1st to 10th	Night	2 5% 53% 36% 4	567
	11th to 20th	Night	4 1 5% 49% 35% 5%	564
	21st to end	Night	6% 1 5% 38% 45% 6%	627
September	1st to 10th	Night	10% 1 6% 38% 39% 6%	570
	11th to 20th	Night	10% 3 4 38% 41% 4	570
	21st to end	Night	8% 2 12% 35% 39% 4	570
October	1st to 10th	Night	5% 2 12% 47% 30% 5%	570
	11th to 20th	Night	13% 3 18% 43% 20% 2	712
	21st to end	Night	12% 7% 26% 34% 19% 2	836
November	1st to 10th	Night	14% 5% 20% 39% 21% 1	780
	11th to 20th	Night	13% 6% 25% 38% 17% 1	857
	21st to end	Night	19% 8% 28% 31% 13% 1	1000
December	1st to 10th	Night	20% 8% 24% 36% 11% 1	960
	11th to 20th	Night	25% 9% 27% 31% 8%	950
	21st to end	Night	20% 8% 28% 36% 8%	1045

Page generated at 2024-11-29 09:43:09. Version ac07

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