M2M Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the M2M Observatory clear sky chart from 2005-11-06 to 2024-09-29.

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	13% 6% 2 5% 4 4 4 3 5% 7% 46%	100826
	Night	12% 6% 2 5% 3 3 3 3 5% 7% 52%	97498
Transparency	Day	36% 7% 11% 22% 19% 5%	100640
	Night	32% 6% 11% 21% 22% 8%	97355
Seeing	Day	14% 7% 22% 28% 22% 7%	15047
	Night	14% 8% 25% 32% 18% 2	24828

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% 11% 3 5% 3 3 2 2 4 4 48%	2847
	11th to 20th	Day	16% 5% 2 3 3 2 3 2 5% 5% 54%	2850
	21st to end	Day	24% 9% 2 5% 3 2 2 1 3 3 45%	3135
February	1st to 10th	Day	22% 9% 3 7% 3 3 3 3 3 4 41%	1710
	11th to 20th	Day	20% 9% 2 6% 4 3 3 2 4 4 42%	1839
	21st to end	Day	23% 7% 2 6% 3 2 2 2 4 3 46%	1700
March	1st to 10th	Day	17% 11% 2 5% 4 3 3 3 4 7% 40%	3436
	11th to 20th	Day	20% 8% 2 6% 4 3 3 4 5% 6% 38%	3480
	21st to end	Day	17% 9% 2 7% 5% 4 4 3 5% 7% 37%	3814
April	1st to 10th	Day	15% 7% 2 6% 4 4 4 2 5% 7% 45%	2095
	11th to 20th	Day	16% 8% 2 5% 4 3 3 4 5% 7% 41%	2254
	21st to end	Day	14% 5% 2 6% 5% 4 5% 4 5% 7% 43%	2280
May	1st to 10th	Day	22% 9% 3 7% 4 4 4 3 5% 6% 35%	4038
	11th to 20th	Day	15% 6% 2 6% 4 4 4 4 5% 6% 45%	4082
	21st to end	Day	14% 8% 3 8% 5% 5% 5% 4 7% 8% 34%	4477
June	1st to 10th	Day	7% 6% 2 5% 5% 5% 5% 5% 8% 9% 43%	2470
	11th to 20th	Day	5% 4 2 6% 5% 5% 6% 3 7% 10% 47%	2470
	21st to end	Day	5% 3 1 5% 5% 4 5% 5% 7% 10% 50%	2470
July	1st to 10th	Day	8% 5% 2 5% 5% 5% 5% 5% 7% 11% 41%	4120
	11th to 20th	Day	3 3 1 4 3 4 4 4 7% 10% 55%	4120
	21st to end	Day	9% 8% 3 8% 6% 6% 6% 5% 6% 9% 35%	4465
August	1st to 10th	Day	2 1 1 2 3 3 3 3 5% 8% 69%	4104
	11th to 20th	Day	4 2 1 4 3 4 4 5% 7% 10% 56%	3982
	21st to end	Day	4 4 2 6% 5% 5% 6% 6% 9% 10% 44%	4180
September	1st to 10th	Day	10% 5% 2 5% 4 4 4 4 5% 7% 50%	2137
	11th to 20th	Day	11% 6% 2 5% 4 5% 5% 3 6% 8% 44%	2090
	21st to end	Day	8% 4 1 4 3 4 4 4 5% 7% 56%	2068
October	1st to 10th	Day	7% 4 2 4 3 3 4 3 4 7% 58%	1847
	11th to 20th	Day	15% 4 2 4 2 3 3 2 4 5% 56%	1800
	21st to end	Day	16% 6% 2 4 3 2 2 2 3 4 57%	1980
November	1st to 10th	Day	12% 5% 2 4 3 3 3 3 4 6% 53%	1858
	11th to 20th	Day	11% 5% 2 5% 3 3 3 4 4 6% 54%	1900
	21st to end	Day	19% 7% 2 5% 3 2 3 3 3 5% 48%	1722
December	1st to 10th	Day	16% 7% 2 5% 3 4 3 2 5% 5% 48%	1710
	11th to 20th	Day	16% 8% 2 6% 5% 4 4 3 5% 6% 44%	2161
	21st to end	Day	16% 10% 3 7% 3 3 3 2 4 6% 43%	3135
January	1st to 10th	Night	14% 9% 2 6% 3 2 3 2 4 4 50%	4108
	11th to 20th	Night	16% 7% 2 4 2 2 2 1 3 4 57%	4120
	21st to end	Night	24% 10% 2 4 3 2 2 2 4 4 44%	4532
February	1st to 10th	Night	17% 8% 2 6% 3 2 3 3 4 5% 47%	2470
	11th to 20th	Night	19% 8% 2 6% 3 2 2 2 3 4 46%	2242
	21st to end	Night	20% 6% 2 5% 3 2 4 2 3 5% 48%	1752
March	1st to 10th	Night	21% 10% 2 6% 4 3 3 3 4 6% 38%	3419
	11th to 20th	Night	17% 7% 2 6% 4 4 4 3 5% 6% 41%	3480
	21st to end	Night	15% 7% 2 6% 4 4 4 3 5% 7% 44%	3817
April	1st to 10th	Night	11% 5% 1 5% 4 3 4 3 5% 8% 50%	2085
	11th to 20th	Night	12% 6% 2 5% 4 3 4 4 6% 7% 47%	1875
	21st to end	Night	13% 5% 2 5% 3 3 3 3 5% 7% 52%	1900
May	1st to 10th	Night	17% 9% 3 6% 5% 4 4 3 5% 8% 37%	2862
	11th to 20th	Night	13% 6% 2 6% 4 4 3 3 5% 8% 47%	2823
	21st to end	Night	12% 6% 3 6% 4 4 4 4 6% 9% 43%	3090
June	1st to 10th	Night	4 3 2 5% 4 3 3 4 6% 11% 54%	1710
	11th to 20th	Night	4 3 1 4 2 2 3 4 5% 8% 64%	1710
	21st to end	Night	5% 4 1 3 2 3 4 4 6% 10% 58%	1710
July	1st to 10th	Night	7% 4 2 4 3 4 4 3 6% 12% 52%	2850
	11th to 20th	Night	3 3 1 3 3 3 3 3 6% 11% 63%	2850
	21st to end	Night	6% 6% 2 6% 5% 6% 6% 5% 6% 9% 43%	3098
August	1st to 10th	Night	2 1 1 1 2 2 3 3 4 8% 74%	2836
	11th to 20th	Night	4 2 1 3 2 3 3 3 5% 11% 62%	2854
	21st to end	Night	2 3 1 4 3 4 4 4 8% 9% 56%	3476
September	1st to 10th	Night	7% 4 1 4 4 4 4 3 6% 7% 55%	2038
	11th to 20th	Night	10% 4 2 4 3 3 3 3 5% 7% 55%	2090
	21st to end	Night	6% 3 1 4 3 3 3 3 4 8% 62%	2079
October	1st to 10th	Night	6% 4 1 4 2 2 2 3 5% 6% 64%	2093
	11th to 20th	Night	12% 3 1 4 3 2 3 2 3 6% 61%	2160
	21st to end	Night	16% 5% 1 4 2 2 2 2 3 4 59%	2376
November	1st to 10th	Night	10% 6% 2 4 3 3 2 3 4 4 60%	2220
	11th to 20th	Night	9% 5% 2 5% 3 3 3 3 4 6% 55%	2280
	21st to end	Night	17% 7% 1 4 4 3 3 2 3 4 51%	2455
December	1st to 10th	Night	15% 7% 2 5% 2 3 3 3 5% 6% 50%	2470
	11th to 20th	Night	15% 8% 3 6% 4 3 3 3 4 5% 48%	3036
	21st to end	Night	16% 8% 2 5% 3 3 3 2 3 4 51%	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	41% 3 6% 19% 26% 5%	2848
	11th to 20th	Day	33% 4 8% 24% 27% 4	2850
	21st to end	Day	47% 2 5% 15% 22% 9%	3135
February	1st to 10th	Day	48% 4 5% 16% 19% 8%	1710
	11th to 20th	Day	47% 4 5% 18% 18% 9%	1849
	21st to end	Day	44% 4 5% 13% 23% 11%	1700
March	1st to 10th	Day	43% 6% 8% 14% 21% 7%	3436
	11th to 20th	Day	45% 6% 8% 21% 16% 4	3480
	21st to end	Day	45% 6% 9% 17% 19% 3	3814
April	1st to 10th	Day	40% 5% 10% 21% 17% 8%	2095
	11th to 20th	Day	40% 8% 9% 16% 19% 8%	2254
	21st to end	Day	38% 7% 9% 20% 22% 4	2280
May	1st to 10th	Day	48% 8% 10% 21% 10% 3	4076
	11th to 20th	Day	38% 7% 10% 22% 22% 1	4086
	21st to end	Day	45% 10% 13% 18% 11% 3	4489
June	1st to 10th	Day	33% 10% 15% 26% 14% 2	2470
	11th to 20th	Day	31% 8% 16% 25% 16% 3	2470
	21st to end	Day	28% 8% 14% 29% 19% 3	2470
July	1st to 10th	Day	34% 10% 24% 23% 8%	4056
	11th to 20th	Day	21% 7% 16% 37% 17% 1	4051
	21st to end	Day	42% 9% 14% 26% 9%	4325
August	1st to 10th	Day	13% 6% 15% 32% 30% 4	4090
	11th to 20th	Day	21% 7% 19% 39% 13% 1	4021
	21st to end	Day	29% 11% 19% 27% 12% 1	4179
September	1st to 10th	Day	33% 6% 11% 21% 18% 10%	2137
	11th to 20th	Day	36% 8% 12% 17% 19% 8%	2090
	21st to end	Day	27% 6% 10% 21% 26% 11%	2068
October	1st to 10th	Day	26% 5% 12% 20% 22% 15%	1847
	11th to 20th	Day	31% 5% 6% 17% 25% 15%	1800
	21st to end	Day	34% 3 5% 17% 26% 14%	1980
November	1st to 10th	Day	33% 5% 8% 20% 25% 9%	1858
	11th to 20th	Day	31% 4 8% 18% 28% 10%	1900
	21st to end	Day	40% 4 6% 17% 24% 8%	1722
December	1st to 10th	Day	38% 5% 9% 19% 22% 8%	1710
	11th to 20th	Day	41% 5% 6% 15% 24% 8%	2159
	21st to end	Day	43% 4 6% 18% 21% 7%	3135
January	1st to 10th	Night	37% 5% 6% 14% 30% 8%	4108
	11th to 20th	Night	33% 3 8% 18% 27% 11%	4120
	21st to end	Night	44% 4 6% 14% 23% 9%	4532
February	1st to 10th	Night	40% 4 8% 18% 20% 11%	2470
	11th to 20th	Night	44% 3 5% 15% 23% 9%	2242
	21st to end	Night	41% 4 7% 15% 20% 13%	1752
March	1st to 10th	Night	46% 6% 7% 13% 16% 11%	3419
	11th to 20th	Night	41% 8% 10% 15% 20% 7%	3480
	21st to end	Night	38% 6% 10% 19% 23% 4	3817
April	1st to 10th	Night	33% 5% 11% 20% 19% 12%	2085
	11th to 20th	Night	36% 5% 9% 18% 20% 11%	1875
	21st to end	Night	32% 6% 11% 20% 27% 5%	1900
May	1st to 10th	Night	43% 10% 10% 20% 13% 5%	2889
	11th to 20th	Night	35% 8% 14% 18% 22% 3	2824
	21st to end	Night	34% 10% 15% 24% 13% 4	3099
June	1st to 10th	Night	22% 9% 17% 28% 20% 3	1710
	11th to 20th	Night	19% 6% 16% 33% 21% 6%	1710
	21st to end	Night	21% 8% 14% 27% 26% 4	1710
July	1st to 10th	Night	25% 8% 24% 34% 9%	2805
	11th to 20th	Night	16% 6% 17% 40% 18% 3	2796
	21st to end	Night	35% 9% 16% 29% 10% 1	2999
August	1st to 10th	Night	9% 4 14% 31% 35% 7%	2826
	11th to 20th	Night	17% 5% 15% 43% 17% 4	2881
	21st to end	Night	21% 10% 18% 28% 21% 2	3476
September	1st to 10th	Night	27% 7% 13% 23% 20% 10%	2038
	11th to 20th	Night	28% 6% 12% 23% 20% 10%	2090
	21st to end	Night	21% 5% 9% 21% 29% 14%	2079
October	1st to 10th	Night	22% 4 10% 25% 22% 17%	2093
	11th to 20th	Night	27% 3 7% 16% 26% 21%	2160
	21st to end	Night	31% 4 5% 17% 25% 18%	2376
November	1st to 10th	Night	28% 5% 8% 18% 28% 13%	2220
	11th to 20th	Night	30% 5% 8% 17% 27% 14%	2280
	21st to end	Night	38% 4 6% 17% 25% 11%	2455
December	1st to 10th	Night	36% 5% 9% 19% 21% 10%	2470
	11th to 20th	Night	40% 4 6% 15% 23% 12%	3037
	21st to end	Night	37% 4 6% 17% 27% 9%	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	23% 15% 37% 20% 5% 1	270
	11th to 20th	Day	26% 19% 36% 13% 7%	270
	21st to end	Day	24% 16% 39% 17% 4	297
February	1st to 10th	Day	30% 16% 35% 16% 4	270
	11th to 20th	Day	35% 18% 32% 13% 1	270
	21st to end	Day	35% 24% 32% 8% 2	225
March	1st to 10th	Day	23% 17% 38% 19% 4	270
	11th to 20th	Day	26% 23% 32% 15% 4 1	270
	21st to end	Day	22% 18% 38% 16% 5%	297
April	1st to 10th	Day	25% 12% 38% 21% 3	275
	11th to 20th	Day	23% 15% 29% 22% 10% 1	452
	21st to end	Day	18% 12% 33% 26% 8% 3	460
May	1st to 10th	Day	15% 11% 36% 28% 9%	622
	11th to 20th	Day	15% 5% 28% 34% 15% 2	675
	21st to end	Day	14% 6% 25% 34% 17% 4	734
June	1st to 10th	Day	8% 2 14% 37% 30% 8%	680
	11th to 20th	Day	6% 2 15% 43% 27% 7%	680
	21st to end	Day	4 2 11% 33% 37% 12%	680
July	1st to 10th	Day	8% 2 9% 33% 37% 11%	671
	11th to 20th	Day	3 1 7% 34% 41% 14%	668
	21st to end	Day	5% 1 7% 32% 42% 13%	739
August	1st to 10th	Day	4 5% 29% 49% 13%	676
	11th to 20th	Day	8% 1 5% 31% 44% 11%	632
	21st to end	Day	9% 1 7% 22% 40% 21%	539
September	1st to 10th	Day	9% 2 10% 27% 36% 17%	347
	11th to 20th	Day	5% 1 11% 29% 35% 18%	300
	21st to end	Day	12% 4 11% 32% 29% 13%	294
October	1st to 10th	Day	7% 1 19% 37% 29% 6%	270
	11th to 20th	Day	13% 6% 32% 27% 16% 6%	270
	21st to end	Day	27% 9% 26% 22% 12% 4	297
November	1st to 10th	Day	19% 7% 32% 27% 13% 3	270
	11th to 20th	Day	15% 10% 36% 30% 8% 2	270
	21st to end	Day	27% 13% 26% 23% 10% 1	270
December	1st to 10th	Day	21% 14% 33% 20% 9% 2	270
	11th to 20th	Day	22% 13% 38% 20% 6% 1	270
	21st to end	Day	26% 14% 37% 18% 4 1	297
January	1st to 10th	Night	20% 13% 32% 29% 7%	950
	11th to 20th	Night	22% 14% 31% 26% 6% 1	950
	21st to end	Night	22% 12% 33% 29% 4	1045
February	1st to 10th	Night	21% 14% 34% 25% 5%	950
	11th to 20th	Night	26% 14% 33% 24% 2	768
	21st to end	Night	24% 18% 36% 18% 3	496
March	1st to 10th	Night	22% 11% 36% 25% 6%	570
	11th to 20th	Night	17% 12% 36% 28% 6% 1	570
	21st to end	Night	16% 11% 38% 28% 6%	627
April	1st to 10th	Night	13% 13% 41% 27% 6%	570
	11th to 20th	Night	15% 14% 36% 26% 9% 1	563
	21st to end	Night	15% 10% 38% 27% 10% 1	570
May	1st to 10th	Night	11% 8% 39% 33% 9% 1	570
	11th to 20th	Night	12% 4 31% 35% 17% 2	561
	21st to end	Night	8% 4 28% 39% 19% 2	617
June	1st to 10th	Night	5% 1 14% 42% 36% 3	570
	11th to 20th	Night	5% 2 15% 48% 29% 2	570
	21st to end	Night	5% 1 11% 35% 40% 7%	570
July	1st to 10th	Night	5% 1 9% 39% 41% 6%	555
	11th to 20th	Night	3 7% 38% 45% 6%	552
	21st to end	Night	4 1 6% 41% 43% 6%	624
August	1st to 10th	Night	2 1 4 40% 48% 6%	567
	11th to 20th	Night	7% 1 5% 35% 46% 6%	564
	21st to end	Night	5% 7% 36% 46% 7%	627
September	1st to 10th	Night	9% 1 8% 34% 43% 6%	570
	11th to 20th	Night	13% 1 8% 33% 42% 4	570
	21st to end	Night	7% 2 13% 38% 36% 4	567
October	1st to 10th	Night	8% 2 15% 43% 28% 4	653
	11th to 20th	Night	14% 5% 25% 36% 18% 2	720
	21st to end	Night	18% 8% 27% 29% 17% 2	792
November	1st to 10th	Night	13% 4 27% 38% 16% 2	740
	11th to 20th	Night	12% 9% 30% 37% 11% 1	760
	21st to end	Night	21% 11% 29% 27% 12%	935
December	1st to 10th	Night	19% 10% 33% 31% 7%	950
	11th to 20th	Night	21% 12% 28% 34% 6%	950
	21st to end	Night	22% 12% 30% 30% 6%	1045

Page generated at 2024-09-26 08:52:20. Version ac07

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