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 2026-06-16.

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%	137744
	Night	12% 6% 2 5% 3 3 3 3 5% 7% 51%	134599
Transparency	Day	35% 6% 11% 22% 19% 6%	141383
	Night	32% 5% 10% 21% 22% 10%	139027
Seeing	Day	15% 8% 23% 27% 21% 6%	17287
	Night	15% 8% 25% 32% 18% 2	27147

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	13% 9% 3 5% 3 3 3 2 4 6% 49%	5289
	11th to 20th	Day	9% 4 1 3 2 3 4 3 5% 6% 59%	5310
	21st to end	Day	24% 9% 3 6% 3 2 2 2 2 3 42%	5812
February	1st to 10th	Day	21% 9% 3 6% 3 3 3 3 3 4 42%	1890
	11th to 20th	Day	21% 9% 2 6% 4 3 3 2 4 4 42%	2039
	21st to end	Day	21% 7% 2 5% 3 2 2 2 4 3 48%	1874
March	1st to 10th	Day	17% 10% 3 6% 3 3 3 2 4 6% 43%	5046
	11th to 20th	Day	14% 6% 2 5% 3 3 3 3 4 5% 53%	5090
	21st to end	Day	15% 8% 2 6% 4 4 4 3 4 6% 43%	5583
April	1st to 10th	Day	15% 7% 2 6% 4 4 3 2 5% 7% 44%	2315
	11th to 20th	Day	15% 9% 2 5% 4 4 4 4 5% 7% 41%	2494
	21st to end	Day	16% 6% 2 6% 5% 4 5% 4 5% 7% 41%	2520
May	1st to 10th	Day	22% 9% 3 7% 4 4 4 3 5% 5% 36%	7531
	11th to 20th	Day	13% 8% 3 7% 5% 4 4 4 5% 6% 40%	7627
	21st to end	Day	13% 8% 3 8% 5% 5% 5% 5% 7% 9% 33%	8392
June	1st to 10th	Day	7% 6% 2 5% 5% 6% 5% 5% 8% 9% 42%	2730
	11th to 20th	Day	6% 4 2 6% 5% 5% 6% 3 7% 10% 45%	2666
	21st to end	Day	5% 3 1 5% 5% 4 5% 5% 7% 10% 50%	2600
July	1st to 10th	Day	8% 5% 3 6% 6% 6% 6% 5% 8% 11% 36%	5900
	11th to 20th	Day	3 3 1 4 3 3 4 4 7% 10% 58%	5900
	21st to end	Day	6% 6% 3 7% 5% 5% 5% 4 6% 9% 45%	6422
August	1st to 10th	Day	2 1 1 2 3 3 3 3 5% 8% 68%	4234
	11th to 20th	Day	4 2 1 4 3 4 4 5% 7% 10% 57%	4110
	21st to end	Day	4 4 2 6% 5% 5% 6% 6% 8% 10% 44%	4312
September	1st to 10th	Day	10% 5% 2 5% 4 4 4 4 5% 7% 50%	2249
	11th to 20th	Day	11% 6% 2 5% 4 5% 5% 3 6% 8% 45%	2200
	21st to end	Day	8% 4 1 4 3 4 4 4 5% 7% 56%	2200
October	1st to 10th	Day	6% 4 2 4 3 3 3 3 4 7% 61%	2051
	11th to 20th	Day	13% 4 1 4 2 3 3 3 4 5% 58%	2000
	21st to end	Day	16% 6% 2 4 2 2 2 2 3 4 57%	2200
November	1st to 10th	Day	13% 6% 2 4 3 3 3 3 4 6% 54%	2058
	11th to 20th	Day	11% 5% 2 5% 3 3 3 3 4 6% 55%	2100
	21st to end	Day	18% 7% 2 5% 3 2 3 3 4 5% 49%	1902
December	1st to 10th	Day	19% 6% 2 5% 2 3 2 2 4 5% 50%	3030
	11th to 20th	Day	17% 8% 2 6% 5% 3 4 3 4 6% 42%	3481
	21st to end	Day	16% 10% 4 6% 3 3 3 2 3 5% 44%	4587
January	1st to 10th	Night	14% 7% 2 5% 3 3 3 2 4 5% 52%	7634
	11th to 20th	Night	9% 4 1 3 2 2 2 2 4 5% 66%	7680
	21st to end	Night	25% 9% 2 6% 3 3 2 2 4 4 39%	8402
February	1st to 10th	Night	17% 7% 2 6% 3 2 3 3 4 5% 48%	2730
	11th to 20th	Night	20% 8% 2 6% 3 3 3 2 3 4 46%	2484
	21st to end	Night	19% 6% 2 5% 3 2 4 2 2 5% 50%	1930
March	1st to 10th	Night	21% 9% 2 5% 3 3 3 2 4 6% 42%	5029
	11th to 20th	Night	12% 5% 2 5% 4 4 4 3 4 6% 52%	5090
	21st to end	Night	14% 7% 2 6% 3 4 3 3 5% 6% 47%	5588
April	1st to 10th	Night	12% 6% 1 5% 4 3 4 3 6% 8% 49%	2305
	11th to 20th	Night	12% 6% 2 5% 4 3 4 3 6% 7% 47%	2075
	21st to end	Night	13% 6% 2 5% 3 3 3 3 5% 7% 50%	2100
May	1st to 10th	Night	19% 10% 3 7% 4 4 4 3 5% 7% 34%	5331
	11th to 20th	Night	13% 7% 3 7% 4 4 4 3 4 8% 44%	5265
	21st to end	Night	12% 7% 3 6% 4 4 4 4 6% 9% 41%	5793
June	1st to 10th	Night	5% 3 2 5% 4 3 3 4 6% 11% 53%	1890
	11th to 20th	Night	5% 3 2 4 2 2 2 4 5% 8% 63%	1845
	21st to end	Night	5% 4 1 3 3 3 4 4 6% 10% 58%	1800
July	1st to 10th	Night	6% 4 3 6% 4 5% 5% 4 7% 13% 42%	4078
	11th to 20th	Night	3 2 1 3 2 3 3 3 5% 11% 64%	4077
	21st to end	Night	4 4 1 5% 4 5% 5% 4 6% 9% 52%	4449
August	1st to 10th	Night	2 1 1 2 2 2 3 3 4 8% 74%	2926
	11th to 20th	Night	4 2 1 3 2 3 3 3 6% 11% 62%	2946
	21st to end	Night	3 3 1 4 3 4 4 4 8% 10% 55%	3586
September	1st to 10th	Night	7% 4 1 5% 4 4 4 3 6% 7% 55%	2146
	11th to 20th	Night	10% 4 2 4 3 3 3 3 5% 7% 54%	2200
	21st to end	Night	6% 3 1 4 3 3 3 4 4 8% 62%	2200
October	1st to 10th	Night	6% 3 1 4 2 2 2 3 4 6% 67%	2327
	11th to 20th	Night	11% 3 1 4 2 2 3 2 3 6% 63%	2400
	21st to end	Night	15% 4 1 4 2 2 2 2 3 4 60%	2640
November	1st to 10th	Night	11% 6% 2 3 3 3 2 2 3 5% 60%	2460
	11th to 20th	Night	9% 5% 2 5% 3 3 2 3 4 6% 57%	2520
	21st to end	Night	16% 7% 2 4 4 3 3 2 3 5% 51%	2714
December	1st to 10th	Night	17% 7% 3 5% 3 3 2 2 3 4 51%	4380
	11th to 20th	Night	17% 8% 2 6% 4 3 3 2 3 4 47%	4946
	21st to end	Night	15% 10% 3 5% 3 2 2 2 3 4 50%	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	38% 3 9% 26% 22% 3	5290
	11th to 20th	Day	25% 4 7% 27% 31% 6%	5310
	21st to end	Day	49% 2 4 13% 22% 10%	5812
February	1st to 10th	Day	47% 4 5% 16% 20% 9%	1890
	11th to 20th	Day	47% 4 6% 19% 17% 8%	2049
	21st to end	Day	42% 4 5% 14% 24% 11%	1874
March	1st to 10th	Day	42% 5% 6% 13% 20% 13%	5046
	11th to 20th	Day	33% 5% 6% 17% 18% 20%	5090
	21st to end	Day	41% 6% 9% 21% 19% 5%	5585
April	1st to 10th	Day	41% 5% 9% 20% 17% 8%	2315
	11th to 20th	Day	41% 7% 9% 16% 19% 7%	2494
	21st to end	Day	41% 7% 9% 19% 20% 4	2520
May	1st to 10th	Day	48% 7% 10% 21% 12% 3	7569
	11th to 20th	Day	43% 7% 9% 18% 20% 2	7631
	21st to end	Day	44% 9% 14% 18% 13% 2	8405
June	1st to 10th	Day	35% 10% 14% 26% 14% 2	2730
	11th to 20th	Day	32% 8% 17% 24% 16% 3	2666
	21st to end	Day	27% 8% 15% 29% 18% 3	2600
July	1st to 10th	Day	38% 11% 22% 23% 6%	5836
	11th to 20th	Day	20% 7% 19% 37% 16% 1	5831
	21st to end	Day	34% 8% 14% 33% 11% 1	6240
August	1st to 10th	Day	14% 6% 15% 32% 30% 4	4220
	11th to 20th	Day	21% 7% 19% 39% 13% 1	4149
	21st to end	Day	30% 11% 19% 26% 13% 1	4311
September	1st to 10th	Day	33% 7% 11% 21% 19% 10%	2249
	11th to 20th	Day	35% 9% 13% 18% 19% 7%	2200
	21st to end	Day	26% 6% 10% 21% 27% 11%	2200
October	1st to 10th	Day	15% 3 7% 12% 28% 37%	3337
	11th to 20th	Day	18% 4 4 20% 31% 22%	3240
	21st to end	Day	25% 4 5% 25% 32% 9%	3538
November	1st to 10th	Day	33% 5% 8% 19% 26% 9%	2058
	11th to 20th	Day	30% 4 9% 18% 27% 11%	2100
	21st to end	Day	39% 4 6% 18% 25% 8%	1902
December	1st to 10th	Day	39% 3 5% 25% 23% 5%	3030
	11th to 20th	Day	43% 4 6% 14% 23% 9%	3479
	21st to end	Day	43% 4 6% 19% 23% 5%	4587
January	1st to 10th	Night	36% 4 8% 21% 27% 5%	7635
	11th to 20th	Night	22% 3 6% 19% 35% 15%	7680
	21st to end	Night	50% 3 4 14% 18% 11%	8402
February	1st to 10th	Night	39% 4 7% 18% 21% 11%	2730
	11th to 20th	Night	44% 3 6% 17% 22% 8%	2484
	21st to end	Night	39% 4 7% 15% 22% 14%	1930
March	1st to 10th	Night	44% 5% 5% 11% 19% 16%	5029
	11th to 20th	Night	32% 6% 7% 14% 18% 22%	5090
	21st to end	Night	37% 5% 9% 20% 22% 6%	5588
April	1st to 10th	Night	33% 5% 11% 20% 18% 11%	2305
	11th to 20th	Night	36% 5% 10% 18% 21% 10%	2075
	21st to end	Night	33% 6% 11% 20% 25% 5%	2100
May	1st to 10th	Night	47% 8% 10% 19% 12% 4	5358
	11th to 20th	Night	39% 7% 13% 17% 21% 3	5266
	21st to end	Night	37% 9% 18% 22% 9% 6%	5805
June	1st to 10th	Night	23% 9% 17% 27% 21% 3	1890
	11th to 20th	Night	20% 7% 16% 32% 20% 5%	1845
	21st to end	Night	21% 8% 15% 27% 25% 3	1800
July	1st to 10th	Night	31% 10% 23% 29% 8%	4035
	11th to 20th	Night	15% 6% 21% 38% 17% 3	4026
	21st to end	Night	27% 8% 16% 33% 13% 3	4320
August	1st to 10th	Night	9% 4 14% 32% 35% 6%	2916
	11th to 20th	Night	16% 5% 15% 43% 16% 3	2973
	21st to end	Night	21% 10% 18% 28% 21% 2	3586
September	1st to 10th	Night	27% 7% 13% 22% 21% 10%	2146
	11th to 20th	Night	28% 6% 12% 25% 20% 9%	2200
	21st to end	Night	21% 5% 10% 22% 29% 14%	2200
October	1st to 10th	Night	12% 2 5% 14% 22% 44%	3808
	11th to 20th	Night	16% 2 5% 16% 34% 27%	3899
	21st to end	Night	25% 3 5% 21% 29% 17%	4252
November	1st to 10th	Night	28% 5% 7% 19% 27% 14%	2460
	11th to 20th	Night	29% 4 8% 17% 27% 15%	2520
	21st to end	Night	37% 4 6% 17% 25% 10%	2714
December	1st to 10th	Night	38% 4 6% 18% 27% 7%	4380
	11th to 20th	Night	42% 4 6% 13% 23% 11%	4947
	21st to end	Night	39% 4 6% 18% 27% 7%	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	24% 15% 39% 17% 5% 1	330
	11th to 20th	Day	23% 20% 36% 15% 6%	330
	21st to end	Day	26% 17% 37% 17% 4	363
February	1st to 10th	Day	28% 16% 38% 14% 3	330
	11th to 20th	Day	35% 20% 31% 12% 2	330
	21st to end	Day	32% 23% 33% 10% 3	273
March	1st to 10th	Day	25% 17% 37% 18% 3	330
	11th to 20th	Day	26% 23% 34% 13% 3 1	330
	21st to end	Day	19% 17% 40% 18% 5%	363
April	1st to 10th	Day	26% 13% 36% 20% 5%	335
	11th to 20th	Day	23% 16% 32% 20% 8% 1	532
	21st to end	Day	21% 13% 31% 25% 8% 2	540
May	1st to 10th	Day	17% 11% 37% 27% 8%	719
	11th to 20th	Day	17% 7% 28% 31% 15% 2	775
	21st to end	Day	14% 6% 25% 35% 17% 3	844
June	1st to 10th	Day	9% 4 16% 37% 27% 8%	780
	11th to 20th	Day	8% 2 16% 42% 26% 6%	756
	21st to end	Day	4 2 12% 34% 37% 11%	730
July	1st to 10th	Day	8% 2 9% 33% 37% 11%	721
	11th to 20th	Day	3 1 7% 36% 40% 14%	718
	21st to end	Day	5% 1 7% 32% 41% 14%	794
August	1st to 10th	Day	4 5% 28% 49% 13%	726
	11th to 20th	Day	8% 1 5% 31% 45% 11%	680
	21st to end	Day	9% 1 8% 23% 39% 20%	583
September	1st to 10th	Day	8% 3 11% 27% 35% 16%	379
	11th to 20th	Day	4 1 11% 28% 35% 21%	330
	21st to end	Day	11% 4 11% 32% 29% 13%	330
October	1st to 10th	Day	6% 1 18% 40% 28% 7%	330
	11th to 20th	Day	11% 6% 32% 30% 16% 6%	330
	21st to end	Day	25% 9% 25% 24% 13% 4	363
November	1st to 10th	Day	19% 8% 29% 28% 13% 2	330
	11th to 20th	Day	14% 10% 35% 31% 7% 2	330
	21st to end	Day	25% 14% 30% 21% 9% 1	330
December	1st to 10th	Day	23% 13% 35% 20% 8% 2	330
	11th to 20th	Day	23% 14% 37% 18% 6% 1	330
	21st to end	Day	26% 15% 35% 19% 4 1	363
January	1st to 10th	Night	20% 12% 32% 29% 6%	1050
	11th to 20th	Night	21% 16% 31% 26% 6%	1050
	21st to end	Night	22% 13% 32% 29% 4	1155
February	1st to 10th	Night	21% 14% 36% 25% 5%	1050
	11th to 20th	Night	27% 14% 33% 23% 2	850
	21st to end	Night	23% 18% 37% 18% 4	546
March	1st to 10th	Night	23% 11% 36% 25% 6%	630
	11th to 20th	Night	18% 13% 36% 28% 5%	630
	21st to end	Night	16% 11% 38% 28% 6%	693
April	1st to 10th	Night	14% 13% 40% 28% 6%	630
	11th to 20th	Night	15% 16% 35% 26% 8% 1	623
	21st to end	Night	15% 10% 37% 27% 10% 1	630
May	1st to 10th	Night	12% 8% 38% 33% 8% 1	630
	11th to 20th	Night	13% 6% 31% 33% 16% 2	621
	21st to end	Night	8% 4 26% 42% 18% 2	683
June	1st to 10th	Night	6% 2 14% 41% 34% 3	630
	11th to 20th	Night	5% 2 16% 48% 27% 2	615
	21st to end	Night	5% 1 11% 37% 40% 7%	600
July	1st to 10th	Night	5% 1 9% 38% 42% 5%	585
	11th to 20th	Night	3 7% 38% 45% 6%	582
	21st to end	Night	4 1 6% 40% 42% 7%	657
August	1st to 10th	Night	2 1 4 39% 48% 6%	597
	11th to 20th	Night	7% 1 5% 34% 47% 6%	594
	21st to end	Night	5% 1 7% 36% 44% 6%	660
September	1st to 10th	Night	9% 1 9% 34% 42% 5%	600
	11th to 20th	Night	12% 1 8% 33% 42% 4	600
	21st to end	Night	7% 2 14% 38% 36% 4	600
October	1st to 10th	Night	7% 2 15% 45% 28% 4	727
	11th to 20th	Night	13% 5% 25% 38% 18% 2	800
	21st to end	Night	17% 8% 27% 29% 16% 2	880
November	1st to 10th	Night	14% 5% 26% 38% 16% 1	820
	11th to 20th	Night	12% 9% 31% 36% 10% 1	840
	21st to end	Night	20% 12% 29% 27% 11%	1034
December	1st to 10th	Night	19% 11% 33% 30% 6%	1050
	11th to 20th	Night	21% 12% 28% 33% 5%	1050
	21st to end	Night	22% 12% 31% 30% 6%	1155

Page generated at 2026-06-13 01:31:44. Version ac07

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