Site "M"
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Site "M" clear sky chart from 2005-11-06 to 2025-12-03.

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% 8% 3 7% 5% 4 5% 4 6% 8% 39%	127176
	Night	13% 9% 3 7% 4 4 4 3 5% 6% 40%	123776
Transparency	Day	43% 4 12% 24% 15% 2	130699
	Night	43% 4 11% 24% 16% 2	128304
Seeing	Day	16% 10% 27% 26% 16% 5%	17184
	Night	17% 10% 30% 30% 12% 1	26999

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% 11% 6% 15% 6% 5% 4 4 4 6% 26%	3617
	11th to 20th	Day	10% 10% 5% 11% 7% 7% 7% 6% 6% 6% 26%	3694
	21st to end	Day	16% 17% 8% 9% 5% 3 3 2 3 4 31%	4488
February	1st to 10th	Day	15% 12% 6% 9% 6% 6% 3 3 3 7% 31%	1800
	11th to 20th	Day	12% 11% 3 8% 7% 5% 5% 3 5% 7% 33%	1916
	21st to end	Day	14% 12% 4 9% 5% 4 3 3 4 6% 35%	1650
March	1st to 10th	Day	17% 13% 3 6% 3 3 3 2 3 4 43%	4480
	11th to 20th	Day	7% 8% 2 5% 4 4 4 4 6% 8% 47%	4968
	21st to end	Day	15% 11% 3 9% 6% 4 5% 4 5% 7% 30%	5819
April	1st to 10th	Day	15% 8% 3 7% 5% 4 5% 4 6% 6% 37%	2400
	11th to 20th	Day	15% 9% 3 7% 4 4 4 3 5% 6% 38%	2374
	21st to end	Day	18% 7% 2 7% 5% 5% 5% 5% 5% 6% 35%	2400
May	1st to 10th	Day	16% 9% 3 7% 5% 5% 5% 4 6% 9% 32%	5733
	11th to 20th	Day	14% 7% 2 7% 4 4 5% 4 5% 8% 40%	6319
	21st to end	Day	12% 7% 3 7% 5% 5% 5% 4 7% 7% 37%	6935
June	1st to 10th	Day	8% 5% 2 7% 4 4 5% 5% 7% 9% 46%	2800
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 4 5% 9% 47%	2800
	21st to end	Day	7% 6% 2 6% 5% 5% 6% 5% 6% 9% 43%	2800
July	1st to 10th	Day	10% 6% 2 5% 4 4 5% 4 7% 11% 42%	6360
	11th to 20th	Day	7% 7% 3 8% 5% 5% 6% 5% 6% 10% 38%	6360
	21st to end	Day	10% 6% 2 6% 5% 4 5% 5% 8% 11% 36%	6838
August	1st to 10th	Day	6% 4 2 5% 4 5% 4 4 7% 10% 48%	4234
	11th to 20th	Day	10% 6% 2 6% 4 5% 5% 4 6% 10% 43%	3858
	21st to end	Day	5% 4 2 6% 4 5% 5% 4 7% 10% 48%	4312
September	1st to 10th	Day	10% 5% 2 5% 3 4 5% 3 5% 8% 51%	2400
	11th to 20th	Day	8% 5% 2 5% 4 4 4 4 6% 7% 52%	2358
	21st to end	Day	9% 6% 2 5% 4 4 4 3 5% 8% 50%	2200
October	1st to 10th	Day	11% 5% 2 5% 3 3 4 3 5% 6% 52%	2016
	11th to 20th	Day	15% 6% 2 7% 4 3 4 3 4 6% 46%	2000
	21st to end	Day	18% 8% 3 6% 4 4 4 3 4 6% 41%	2119
November	1st to 10th	Day	10% 7% 3 5% 3 3 4 4 5% 7% 49%	1852
	11th to 20th	Day	15% 9% 3 7% 4 2 3 4 5% 6% 41%	1890
	21st to end	Day	16% 11% 4 9% 5% 3 3 3 4 5% 37%	1770
December	1st to 10th	Day	7% 9% 3 6% 4 3 3 3 5% 7% 48%	2626
	11th to 20th	Day	14% 13% 6% 8% 5% 5% 5% 3 5% 7% 30%	3008
	21st to end	Day	25% 19% 6% 12% 4 3 3 3 3 4 17%	3982
January	1st to 10th	Night	13% 13% 6% 11% 5% 4 3 3 5% 5% 31%	6348
	11th to 20th	Night	11% 14% 6% 13% 6% 5% 6% 4 4 3 27%	6268
	21st to end	Night	17% 15% 5% 11% 4 3 3 2 3 5% 31%	6490
February	1st to 10th	Night	16% 12% 4 9% 6% 4 4 3 5% 6% 33%	2600
	11th to 20th	Night	14% 11% 4 8% 5% 4 3 3 4 6% 38%	2368
	21st to end	Night	15% 10% 3 7% 4 4 4 2 4 6% 40%	1980
March	1st to 10th	Night	18% 12% 4 6% 3 3 3 2 4 5% 40%	5373
	11th to 20th	Night	11% 8% 2 5% 4 3 3 3 4 6% 50%	5036
	21st to end	Night	16% 11% 4 9% 4 5% 5% 4 5% 6% 31%	5110
April	1st to 10th	Night	12% 8% 2 7% 4 5% 4 4 5% 7% 41%	2000
	11th to 20th	Night	15% 7% 2 5% 4 5% 4 4 5% 7% 43%	1975
	21st to end	Night	19% 8% 3 7% 4 4 4 4 5% 5% 39%	2000
May	1st to 10th	Night	18% 9% 3 7% 4 4 5% 4 6% 7% 34%	4157
	11th to 20th	Night	13% 6% 2 6% 4 4 4 4 7% 9% 41%	3596
	21st to end	Night	12% 7% 2 7% 5% 4 4 4 6% 7% 42%	3942
June	1st to 10th	Night	9% 5% 2 4 4 3 3 5% 6% 10% 51%	1600
	11th to 20th	Night	7% 6% 2 5% 5% 4 4 4 7% 8% 50%	1600
	21st to end	Night	5% 5% 2 5% 5% 5% 4 4 6% 8% 50%	1600
July	1st to 10th	Night	9% 5% 3 6% 5% 5% 5% 3 6% 10% 43%	3618
	11th to 20th	Night	4 5% 2 6% 5% 5% 6% 5% 8% 10% 45%	3617
	21st to end	Night	9% 6% 2 7% 4 4 5% 4 6% 10% 43%	4029
August	1st to 10th	Night	5% 4 2 5% 3 3 4 4 6% 9% 56%	2926
	11th to 20th	Night	7% 4 2 4 4 4 4 4 6% 8% 52%	3191
	21st to end	Night	5% 3 1 4 4 4 5% 4 7% 10% 53%	3586
September	1st to 10th	Night	11% 5% 1 4 3 3 3 2 5% 6% 57%	2000
	11th to 20th	Night	7% 5% 2 5% 3 4 4 3 6% 7% 54%	2020
	21st to end	Night	8% 6% 2 5% 5% 4 4 3 5% 7% 52%	2200
October	1st to 10th	Night	10% 4 2 4 4 3 4 3 6% 6% 55%	2380
	11th to 20th	Night	14% 6% 2 4 3 3 3 3 5% 6% 51%	2400
	21st to end	Night	17% 9% 2 6% 3 4 4 2 5% 5% 42%	2720
November	1st to 10th	Night	10% 7% 3 5% 3 3 3 3 5% 6% 53%	2666
	11th to 20th	Night	12% 8% 3 6% 4 4 3 3 5% 6% 46%	2730
	21st to end	Night	17% 10% 3 7% 5% 3 3 3 4 5% 40%	2846
December	1st to 10th	Night	11% 10% 3 8% 4 3 4 3 4 6% 46%	4609
	11th to 20th	Night	15% 10% 4 10% 5% 4 4 3 5% 6% 35%	5199
	21st to end	Night	24% 17% 6% 11% 5% 4 4 3 4 4 17%	6996

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	60% 3 9% 19% 8% 1	3618
	11th to 20th	Day	58% 4 10% 14% 11% 3	3694
	21st to end	Day	60% 2 8% 20% 9% 1	4488
February	1st to 10th	Day	56% 3 12% 20% 8% 1	1800
	11th to 20th	Day	50% 5% 11% 20% 12% 1	1926
	21st to end	Day	51% 3 10% 21% 11% 2	1650
March	1st to 10th	Day	47% 2 7% 22% 19% 2	4480
	11th to 20th	Day	35% 4 10% 30% 20% 1	4968
	21st to end	Day	52% 5% 14% 21% 6% 1	5821
April	1st to 10th	Day	46% 5% 12% 23% 13%	2400
	11th to 20th	Day	47% 3 9% 25% 14% 2	2374
	21st to end	Day	48% 6% 11% 21% 12% 2	2400
May	1st to 10th	Day	48% 5% 12% 21% 12% 1	5769
	11th to 20th	Day	43% 5% 11% 26% 13% 3	6323
	21st to end	Day	44% 5% 10% 22% 18% 1	6949
June	1st to 10th	Day	32% 8% 13% 28% 16% 3	2800
	11th to 20th	Day	34% 6% 10% 25% 22% 3	2800
	21st to end	Day	35% 7% 15% 22% 18% 3	2800
July	1st to 10th	Day	36% 4 13% 28% 17% 1	6291
	11th to 20th	Day	42% 4 14% 24% 15% 1	6285
	21st to end	Day	40% 4 19% 26% 10% 1	6643
August	1st to 10th	Day	30% 6% 17% 35% 11% 1	4220
	11th to 20th	Day	37% 5% 13% 31% 14% 1	3894
	21st to end	Day	31% 4 15% 30% 16% 4	4311
September	1st to 10th	Day	33% 6% 9% 27% 20% 6%	2400
	11th to 20th	Day	32% 5% 8% 26% 24% 5%	2358
	21st to end	Day	33% 4 9% 28% 24% 2	2200
October	1st to 10th	Day	24% 3 6% 18% 45% 3	3276
	11th to 20th	Day	28% 2 8% 22% 34% 5%	3240
	21st to end	Day	41% 2 6% 27% 22% 2	3396
November	1st to 10th	Day	35% 4 13% 25% 18% 4	1852
	11th to 20th	Day	44% 3 10% 25% 15% 3	1890
	21st to end	Day	51% 4 11% 23% 11%	1770
December	1st to 10th	Day	38% 2 17% 32% 11%	2626
	11th to 20th	Day	54% 3 10% 22% 8% 2	3005
	21st to end	Day	73% 3 10% 10% 4	3982
January	1st to 10th	Night	55% 3 12% 22% 7%	6348
	11th to 20th	Night	61% 2 8% 15% 11% 2	6268
	21st to end	Night	58% 2 8% 19% 11% 1	6490
February	1st to 10th	Night	54% 3 12% 18% 11% 2	2600
	11th to 20th	Night	48% 4 10% 22% 14% 2	2369
	21st to end	Night	46% 3 11% 23% 14% 3	1980
March	1st to 10th	Night	49% 3 6% 23% 17% 2	5373
	11th to 20th	Night	36% 4 7% 29% 20% 4	5036
	21st to end	Night	54% 4 10% 23% 8% 2	5110
April	1st to 10th	Night	42% 5% 12% 23% 15% 3	2000
	11th to 20th	Night	41% 5% 10% 25% 17% 3	1975
	21st to end	Night	47% 4 10% 20% 15% 3	2000
May	1st to 10th	Night	49% 5% 11% 22% 10% 2	4186
	11th to 20th	Night	39% 5% 13% 26% 13% 4	3597
	21st to end	Night	40% 5% 11% 23% 18% 3	3950
June	1st to 10th	Night	28% 6% 15% 29% 18% 5%	1600
	11th to 20th	Night	31% 5% 12% 24% 23% 5%	1600
	21st to end	Night	32% 7% 15% 23% 20% 4	1600
July	1st to 10th	Night	39% 3 10% 31% 15% 2	3580
	11th to 20th	Night	34% 3 15% 29% 18% 1	3572
	21st to end	Night	38% 4 18% 24% 14% 2	3913
August	1st to 10th	Night	25% 5% 18% 34% 17% 1	2916
	11th to 20th	Night	30% 4 12% 35% 18% 1	3221
	21st to end	Night	26% 5% 17% 33% 17% 4	3586
September	1st to 10th	Night	30% 3 10% 26% 24% 7%	2000
	11th to 20th	Night	29% 4 8% 26% 27% 5%	2020
	21st to end	Night	32% 4 9% 25% 26% 5%	2200
October	1st to 10th	Night	22% 3 5% 22% 44% 4	3887
	11th to 20th	Night	25% 2 9% 23% 36% 6%	3899
	21st to end	Night	37% 2 10% 23% 24% 4	4380
November	1st to 10th	Night	33% 4 11% 27% 19% 5%	2666
	11th to 20th	Night	39% 3 11% 25% 19% 2	2730
	21st to end	Night	48% 3 9% 24% 14% 1	2846
December	1st to 10th	Night	42% 2 11% 31% 13% 1	4609
	11th to 20th	Night	52% 3 11% 22% 12%	5201
	21st to end	Night	71% 3 9% 12% 4	6996

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% 20% 32% 17% 8% 1	300
	11th to 20th	Day	25% 19% 38% 15% 2	300
	21st to end	Day	18% 18% 38% 21% 4	330
February	1st to 10th	Day	26% 16% 38% 17% 4	300
	11th to 20th	Day	21% 21% 33% 20% 5%	300
	21st to end	Day	24% 15% 45% 15% 1	249
March	1st to 10th	Day	23% 19% 41% 16% 2	300
	11th to 20th	Day	28% 16% 35% 16% 5%	300
	21st to end	Day	31% 16% 32% 15% 5% 1	480
April	1st to 10th	Day	22% 14% 33% 25% 5%	500
	11th to 20th	Day	19% 19% 31% 21% 9% 1	492
	21st to end	Day	21% 13% 36% 21% 7% 1	500
May	1st to 10th	Day	20% 13% 32% 24% 9% 2	504
	11th to 20th	Day	18% 12% 26% 25% 15% 3	725
	21st to end	Day	14% 8% 32% 25% 13% 8%	789
June	1st to 10th	Day	9% 6% 23% 32% 23% 7%	730
	11th to 20th	Day	11% 5% 23% 36% 21% 5%	730
	21st to end	Day	8% 6% 28% 34% 18% 5%	730
July	1st to 10th	Day	11% 3 14% 32% 26% 14%	721
	11th to 20th	Day	8% 2 20% 35% 25% 9%	718
	21st to end	Day	10% 2 17% 35% 29% 7%	794
August	1st to 10th	Day	8% 2 16% 40% 25% 10%	726
	11th to 20th	Day	10% 4 16% 27% 29% 13%	528
	21st to end	Day	7% 2 14% 34% 33% 10%	583
September	1st to 10th	Day	12% 5% 18% 28% 30% 7%	530
	11th to 20th	Day	8% 2 12% 34% 30% 13%	530
	21st to end	Day	12% 10% 21% 25% 25% 9%	530
October	1st to 10th	Day	12% 5% 27% 34% 15% 6%	346
	11th to 20th	Day	15% 20% 28% 27% 9% 1	330
	21st to end	Day	30% 13% 33% 20% 4	363
November	1st to 10th	Day	10% 16% 37% 22% 10% 5%	330
	11th to 20th	Day	22% 16% 33% 17% 10% 2	330
	21st to end	Day	27% 12% 35% 20% 5%	330
December	1st to 10th	Day	14% 22% 38% 17% 7% 3	306
	11th to 20th	Day	21% 17% 38% 20% 4	300
	21st to end	Day	28% 17% 35% 17% 2	330
January	1st to 10th	Night	17% 16% 39% 23% 5%	1000
	11th to 20th	Night	21% 15% 37% 22% 4	1000
	21st to end	Night	21% 15% 36% 26% 3	1100
February	1st to 10th	Night	22% 14% 36% 24% 4	1000
	11th to 20th	Night	21% 15% 35% 26% 3	813
	21st to end	Night	21% 17% 37% 22% 2	660
March	1st to 10th	Night	22% 13% 37% 23% 4	800
	11th to 20th	Night	21% 12% 35% 27% 5%	630
	21st to end	Night	26% 11% 35% 23% 5%	660
April	1st to 10th	Night	17% 14% 37% 27% 5%	600
	11th to 20th	Night	18% 13% 33% 27% 9%	593
	21st to end	Night	22% 9% 35% 27% 7%	600
May	1st to 10th	Night	21% 6% 35% 29% 8% 1	600
	11th to 20th	Night	17% 6% 26% 38% 12% 1	591
	21st to end	Night	11% 4 28% 35% 19% 3	650
June	1st to 10th	Night	10% 3 23% 44% 19%	600
	11th to 20th	Night	10% 4 22% 42% 20% 2	600
	21st to end	Night	9% 2 27% 42% 19% 1	600
July	1st to 10th	Night	10% 3 11% 37% 34% 4	585
	11th to 20th	Night	5% 1 17% 44% 30% 2	582
	21st to end	Night	8% 14% 42% 33% 2	657
August	1st to 10th	Night	7% 2 11% 46% 29% 4	597
	11th to 20th	Night	8% 2 12% 40% 32% 6%	594
	21st to end	Night	6% 1 13% 39% 39% 2	660
September	1st to 10th	Night	12% 3 15% 38% 28% 4	600
	11th to 20th	Night	8% 3 16% 40% 30% 4	620
	21st to end	Night	11% 8% 22% 32% 24% 4	800
October	1st to 10th	Night	11% 5% 28% 34% 20% 2	800
	11th to 20th	Night	17% 10% 31% 30% 11% 1	800
	21st to end	Night	23% 12% 34% 24% 7%	880
November	1st to 10th	Night	14% 11% 39% 28% 8%	820
	11th to 20th	Night	18% 14% 38% 25% 5% 1	840
	21st to end	Night	22% 14% 35% 23% 5%	956
December	1st to 10th	Night	21% 12% 39% 23% 5%	1011
	11th to 20th	Night	23% 11% 34% 27% 5%	1000
	21st to end	Night	27% 16% 34% 22% 2	1100

Page generated at 2025-11-30 00:00:53. Version ac07

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