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 2025-02-21.

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%	109266
	Night	13% 6% 2 5% 3 3 3 3 4 6% 52%	109535
Transparency	Day	35% 6% 10% 22% 20% 6%	112944
	Night	32% 5% 9% 21% 23% 10%	113984
Seeing	Day	14% 8% 22% 28% 22% 7%	15482
	Night	15% 8% 25% 32% 18% 2	25483

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% 11% 3 5% 3 3 2 2 4 4 49%	4077
	11th to 20th	Day	12% 4 1 3 2 2 3 2 5% 6% 58%	4080
	21st to end	Day	24% 9% 3 5% 3 2 2 1 2 3 45%	4488
February	1st to 10th	Day	22% 9% 3 6% 3 3 3 3 3 4 42%	1800
	11th to 20th	Day	21% 9% 2 6% 4 3 3 2 4 4 42%	1939
	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%	2090
October	1st to 10th	Day	7% 4 2 4 3 3 3 3 4 7% 60%	1949
	11th to 20th	Day	14% 4 2 4 2 3 3 2 4 5% 57%	1900
	21st to end	Day	16% 6% 2 4 2 2 2 2 3 4 58%	2090
November	1st to 10th	Day	13% 6% 2 4 4 3 3 3 4 6% 52%	1958
	11th to 20th	Day	11% 5% 2 5% 3 3 3 3 4 6% 55%	2000
	21st to end	Day	18% 7% 2 5% 3 2 3 3 3 5% 49%	1812
December	1st to 10th	Day	19% 6% 2 5% 2 3 2 2 4 5% 50%	2940
	11th to 20th	Day	17% 8% 2 6% 4 4 4 3 5% 6% 41%	3391
	21st to end	Day	17% 10% 4 6% 3 3 3 2 3 5% 44%	4488
January	1st to 10th	Night	16% 7% 2 5% 3 2 3 2 4 4 53%	5888
	11th to 20th	Night	11% 5% 1 3 2 2 2 1 3 5% 64%	5900
	21st to end	Night	26% 10% 2 5% 2 2 2 2 3 3 43%	6490
February	1st to 10th	Night	17% 7% 2 6% 3 2 3 3 4 5% 47%	2600
	11th to 20th	Night	20% 8% 3 6% 3 3 3 2 3 4 45%	2363
	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 7% 62%	2090
October	1st to 10th	Night	6% 3 1 4 2 2 2 3 4 6% 66%	2210
	11th to 20th	Night	12% 3 1 4 2 2 3 2 3 6% 62%	2280
	21st to end	Night	16% 4 1 4 2 2 2 2 3 4 60%	2508
November	1st to 10th	Night	12% 6% 2 4 3 3 3 3 4 4 58%	2340
	11th to 20th	Night	9% 5% 2 5% 3 3 3 3 4 6% 56%	2400
	21st to end	Night	16% 7% 1 4 4 3 3 2 3 4 52%	2585
December	1st to 10th	Night	17% 7% 2 4 3 3 2 2 3 4 51%	4250
	11th to 20th	Night	17% 8% 3 6% 4 3 3 2 3 4 47%	4816
	21st to end	Night	15% 10% 3 5% 3 2 2 2 3 4 50%	6490

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 7% 27% 19% 3	4078
	11th to 20th	Day	27% 4 7% 23% 32% 7%	4080
	21st to end	Day	48% 2 4 12% 22% 12%	4488
February	1st to 10th	Day	48% 4 5% 15% 20% 9%	1800
	11th to 20th	Day	47% 4 5% 19% 17% 8%	1949
	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	26% 6% 10% 21% 27% 11%	2090
October	1st to 10th	Day	15% 3 7% 12% 27% 37%	3235
	11th to 20th	Day	19% 4 4 20% 31% 23%	3140
	21st to end	Day	25% 4 5% 25% 32% 9%	3428
November	1st to 10th	Day	34% 5% 8% 19% 24% 9%	1958
	11th to 20th	Day	31% 4 8% 17% 28% 12%	2000
	21st to end	Day	39% 4 7% 17% 25% 8%	1812
December	1st to 10th	Day	38% 3 5% 25% 23% 5%	2940
	11th to 20th	Day	43% 5% 6% 14% 23% 9%	3389
	21st to end	Day	43% 4 6% 19% 24% 5%	4488
January	1st to 10th	Night	35% 4 6% 23% 26% 5%	5888
	11th to 20th	Night	25% 3 6% 19% 34% 14%	5900
	21st to end	Night	48% 3 5% 13% 18% 13%	6490
February	1st to 10th	Night	40% 4 7% 18% 20% 12%	2600
	11th to 20th	Night	45% 3 6% 16% 22% 9%	2363
	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%	2090
October	1st to 10th	Night	12% 3 5% 14% 21% 45%	3691
	11th to 20th	Night	16% 2 5% 15% 34% 27%	3779
	21st to end	Night	25% 3 4 21% 30% 17%	4120
November	1st to 10th	Night	30% 5% 7% 17% 27% 13%	2340
	11th to 20th	Night	29% 5% 8% 17% 27% 15%	2400
	21st to end	Night	37% 4 6% 17% 25% 11%	2585
December	1st to 10th	Night	38% 4 6% 18% 27% 7%	4250
	11th to 20th	Night	42% 4 6% 13% 23% 12%	4817
	21st to end	Night	39% 4 5% 17% 27% 7%	6490

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% 38% 18% 5% 1	300
	11th to 20th	Day	24% 20% 35% 14% 6%	300
	21st to end	Day	25% 15% 38% 17% 4	330
February	1st to 10th	Day	30% 16% 35% 15% 4	300
	11th to 20th	Day	36% 19% 31% 12% 1	300
	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	11% 4 10% 31% 30% 13%	300
October	1st to 10th	Day	6% 1 19% 39% 27% 7%	300
	11th to 20th	Day	12% 6% 32% 30% 14% 6%	300
	21st to end	Day	25% 9% 26% 23% 13% 4	330
November	1st to 10th	Day	20% 9% 29% 27% 12% 2	300
	11th to 20th	Day	16% 10% 34% 31% 7% 2	300
	21st to end	Day	25% 13% 28% 23% 9% 1	300
December	1st to 10th	Day	22% 13% 34% 20% 9% 2	300
	11th to 20th	Day	23% 14% 38% 19% 6% 1	300
	21st to end	Day	27% 15% 36% 18% 4 1	330
January	1st to 10th	Night	20% 12% 32% 29% 7%	1000
	11th to 20th	Night	22% 15% 31% 26% 6% 1	1000
	21st to end	Night	22% 12% 32% 29% 4	1100
February	1st to 10th	Night	22% 14% 34% 25% 5%	1000
	11th to 20th	Night	28% 14% 33% 23% 2	809
	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	570
October	1st to 10th	Night	7% 2 15% 44% 28% 4	690
	11th to 20th	Night	13% 5% 25% 37% 18% 2	760
	21st to end	Night	17% 8% 27% 29% 17% 2	836
November	1st to 10th	Night	14% 5% 26% 38% 15% 2	780
	11th to 20th	Night	12% 9% 30% 37% 11% 1	800
	21st to end	Night	20% 11% 30% 27% 12%	985
December	1st to 10th	Night	19% 10% 33% 31% 7%	1000
	11th to 20th	Night	21% 12% 28% 34% 6%	1000
	21st to end	Night	22% 12% 31% 30% 6%	1100

Page generated at 2025-02-18 02:26:32. Version ac07

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