Big M Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Big M Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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 8% 5% 4 4 4 6% 7% 38%	127816
	Night	12% 9% 4 8% 5% 4 4 3 5% 6% 39%	124149
Transparency	Day	44% 5% 12% 24% 13% 1	131314
	Night	45% 5% 11% 23% 15% 2	128724
Seeing	Day	16% 11% 29% 26% 14% 4	17504
	Night	17% 9% 31% 31% 11% 1	27443

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	7% 11% 8% 17% 8% 7% 5% 3 4 4 26%	3697
	11th to 20th	Day	7% 9% 7% 13% 7% 4 4 7% 9% 7% 27%	3668
	21st to end	Day	11% 14% 5% 11% 5% 4 3 2 4 6% 35%	4349
February	1st to 10th	Day	10% 13% 5% 9% 5% 5% 5% 5% 6% 7% 31%	1800
	11th to 20th	Day	13% 11% 4 7% 5% 3 5% 4 6% 6% 36%	1834
	21st to end	Day	12% 11% 3 9% 6% 4 4 3 7% 6% 35%	1650
March	1st to 10th	Day	16% 7% 3 6% 3 4 3 2 4 6% 46%	4480
	11th to 20th	Day	11% 6% 3 7% 4 4 4 4 5% 8% 44%	4971
	21st to end	Day	21% 12% 4 10% 5% 4 4 3 4 5% 28%	5783
April	1st to 10th	Day	18% 12% 4 8% 5% 4 4 4 5% 7% 30%	2400
	11th to 20th	Day	17% 11% 4 8% 5% 4 5% 3 5% 5% 33%	2374
	21st to end	Day	20% 9% 3 7% 5% 5% 4 3 6% 6% 33%	2400
May	1st to 10th	Day	14% 8% 3 6% 5% 4 5% 4 6% 6% 39%	6214
	11th to 20th	Day	13% 6% 2 6% 5% 5% 4 4 6% 8% 42%	6319
	21st to end	Day	15% 7% 2 6% 5% 4 5% 4 6% 8% 38%	6935
June	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 6% 8% 45%	2800
	11th to 20th	Day	12% 5% 3 6% 5% 4 5% 4 6% 9% 41%	2800
	21st to end	Day	11% 6% 2 7% 5% 4 5% 4 7% 9% 39%	2800
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 5% 5% 7% 10% 38%	6360
	11th to 20th	Day	8% 6% 2 7% 4 4 4 4 5% 8% 48%	6360
	21st to end	Day	10% 6% 3 8% 6% 5% 6% 5% 6% 9% 37%	6924
August	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 4 7% 8% 42%	4345
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 4 6% 8% 46%	4015
	21st to end	Day	9% 5% 2 6% 4 4 4 4 7% 9% 47%	4312
September	1st to 10th	Day	13% 5% 2 5% 5% 4 4 3 6% 8% 45%	2400
	11th to 20th	Day	11% 5% 2 5% 3 4 4 3 6% 8% 49%	2380
	21st to end	Day	13% 6% 3 5% 5% 4 4 4 5% 7% 46%	2170
October	1st to 10th	Day	12% 7% 2 5% 5% 3 4 3 5% 6% 48%	2000
	11th to 20th	Day	18% 9% 3 6% 4 4 4 3 5% 6% 39%	2000
	21st to end	Day	17% 9% 3 7% 4 3 3 2 4 6% 41%	2074
November	1st to 10th	Day	12% 8% 3 6% 5% 5% 4 4 4 6% 44%	1852
	11th to 20th	Day	14% 13% 3 8% 5% 5% 4 3 5% 7% 33%	1822
	21st to end	Day	15% 12% 4 9% 4 3 4 3 3 6% 35%	1680
December	1st to 10th	Day	7% 9% 4 10% 5% 5% 5% 4 6% 7% 38%	2690
	11th to 20th	Day	12% 14% 5% 11% 7% 6% 5% 4 5% 7% 23%	3088
	21st to end	Day	17% 22% 9% 14% 6% 4 4 3 4 4 13%	4070
January	1st to 10th	Night	7% 11% 6% 16% 8% 5% 4 3 4 5% 31%	6488
	11th to 20th	Night	9% 13% 5% 13% 7% 5% 5% 5% 8% 6% 24%	6445
	21st to end	Night	12% 15% 6% 10% 5% 5% 3 3 4 5% 31%	6614
February	1st to 10th	Night	10% 13% 4 11% 5% 6% 5% 4 4 5% 33%	2600
	11th to 20th	Night	10% 11% 4 8% 6% 4 5% 3 4 6% 40%	2453
	21st to end	Night	14% 12% 3 8% 4 4 4 3 4 6% 37%	1980
March	1st to 10th	Night	15% 9% 4 7% 4 4 3 3 4 4 43%	5373
	11th to 20th	Night	10% 7% 2 5% 4 4 3 3 5% 8% 49%	5031
	21st to end	Night	23% 12% 5% 11% 5% 4 4 3 3 5% 25%	5146
April	1st to 10th	Night	17% 10% 3 7% 4 4 3 3 5% 6% 39%	2000
	11th to 20th	Night	18% 9% 3 6% 5% 4 4 4 4 6% 38%	1975
	21st to end	Night	16% 8% 3 6% 5% 5% 4 4 5% 7% 36%	1998
May	1st to 10th	Night	16% 7% 3 7% 4 4 4 3 5% 6% 40%	3693
	11th to 20th	Night	10% 6% 2 7% 4 4 5% 3 5% 7% 48%	3596
	21st to end	Night	14% 6% 2 7% 3 3 4 4 6% 8% 43%	3942
June	1st to 10th	Night	9% 5% 2 6% 4 4 5% 4 7% 8% 46%	1600
	11th to 20th	Night	11% 5% 3 5% 4 3 3 5% 6% 10% 45%	1600
	21st to end	Night	11% 6% 2 5% 4 3 4 4 7% 7% 47%	1600
July	1st to 10th	Night	11% 7% 2 7% 4 4 4 4 7% 9% 41%	3618
	11th to 20th	Night	5% 5% 2 6% 4 4 4 3 6% 8% 53%	3617
	21st to end	Night	7% 5% 2 6% 4 4 5% 4 6% 8% 48%	3947
August	1st to 10th	Night	8% 4 2 5% 3 4 5% 4 5% 8% 51%	2811
	11th to 20th	Night	10% 5% 2 6% 3 4 4 3 5% 7% 51%	3037
	21st to end	Night	10% 4 2 6% 3 4 4 3 5% 8% 51%	3586
September	1st to 10th	Night	9% 5% 2 4 3 3 3 3 5% 7% 55%	2000
	11th to 20th	Night	10% 5% 1 5% 4 4 4 3 5% 8% 51%	2000
	21st to end	Night	9% 5% 2 5% 4 4 4 4 5% 6% 53%	2225
October	1st to 10th	Night	12% 5% 2 4 4 4 4 3 5% 5% 53%	2400
	11th to 20th	Night	16% 7% 2 6% 3 3 4 3 5% 7% 44%	2400
	21st to end	Night	17% 8% 3 7% 5% 4 4 3 5% 6% 39%	2765
November	1st to 10th	Night	12% 8% 2 6% 3 3 3 3 4 6% 48%	2666
	11th to 20th	Night	14% 11% 4 9% 5% 4 4 3 5% 6% 35%	2794
	21st to end	Night	16% 10% 4 8% 5% 4 3 3 5% 5% 36%	2940
December	1st to 10th	Night	8% 7% 3 8% 6% 5% 4 5% 6% 8% 40%	4720
	11th to 20th	Night	14% 15% 5% 10% 5% 5% 5% 3 5% 4 28%	5339
	21st to end	Night	20% 19% 6% 13% 6% 5% 5% 3 4 4 15%	7150

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	63% 3 10% 17% 7%	3698
	11th to 20th	Day	51% 5% 13% 11% 20% 1	3668
	21st to end	Day	52% 4 14% 19% 11%	4349
February	1st to 10th	Day	53% 5% 12% 20% 10% 1	1800
	11th to 20th	Day	47% 5% 11% 24% 10% 2	1843
	21st to end	Day	49% 6% 11% 21% 11% 2	1650
March	1st to 10th	Day	41% 4 11% 30% 11% 4	4480
	11th to 20th	Day	37% 4 13% 33% 12% 2	4971
	21st to end	Day	59% 5% 11% 16% 8% 1	5785
April	1st to 10th	Day	53% 6% 10% 21% 10%	2400
	11th to 20th	Day	51% 5% 10% 20% 11% 2	2374
	21st to end	Day	51% 5% 10% 20% 12% 2	2400
May	1st to 10th	Day	44% 5% 9% 23% 19% 1	6251
	11th to 20th	Day	40% 7% 10% 29% 13% 1	6323
	21st to end	Day	42% 7% 11% 26% 11% 2	6949
June	1st to 10th	Day	35% 8% 13% 27% 16% 1	2800
	11th to 20th	Day	38% 7% 14% 22% 18% 1	2800
	21st to end	Day	40% 8% 12% 25% 13% 2	2800
July	1st to 10th	Day	39% 7% 15% 29% 9% 1	6291
	11th to 20th	Day	35% 6% 14% 31% 14% 1	6285
	21st to end	Day	41% 8% 14% 20% 15% 1	6728
August	1st to 10th	Day	37% 8% 14% 27% 13% 1	4330
	11th to 20th	Day	35% 5% 14% 30% 14% 1	4054
	21st to end	Day	34% 5% 13% 32% 15% 2	4311
September	1st to 10th	Day	37% 6% 12% 25% 18% 2	2400
	11th to 20th	Day	32% 6% 11% 33% 16% 2	2380
	21st to end	Day	38% 5% 10% 28% 18% 1	2170
October	1st to 10th	Day	24% 4 7% 39% 26% 1	3260
	11th to 20th	Day	34% 4 9% 26% 24% 3	3240
	21st to end	Day	41% 4 11% 28% 14% 2	3325
November	1st to 10th	Day	42% 4 13% 26% 13% 2	1852
	11th to 20th	Day	51% 4 13% 21% 10% 1	1822
	21st to end	Day	51% 4 15% 22% 7% 1	1680
December	1st to 10th	Day	43% 5% 18% 26% 7% 1	2690
	11th to 20th	Day	60% 5% 13% 15% 7% 1	3085
	21st to end	Day	75% 4 8% 9% 4	4070
January	1st to 10th	Night	56% 4 11% 19% 9% 1	6488
	11th to 20th	Night	58% 4 11% 12% 15% 1	6445
	21st to end	Night	55% 4 12% 18% 11% 1	6614
February	1st to 10th	Night	54% 4 11% 18% 12% 2	2600
	11th to 20th	Night	47% 4 12% 23% 13% 1	2454
	21st to end	Night	49% 4 12% 21% 11% 3	1980
March	1st to 10th	Night	46% 3 11% 22% 12% 6%	5373
	11th to 20th	Night	34% 4 11% 33% 15% 3	5031
	21st to end	Night	63% 4 8% 16% 9% 1	5146
April	1st to 10th	Night	46% 4 11% 23% 14% 1	2000
	11th to 20th	Night	47% 5% 8% 19% 17% 3	1975
	21st to end	Night	46% 6% 12% 20% 13% 2	1998
May	1st to 10th	Night	43% 6% 11% 18% 20% 2	3718
	11th to 20th	Night	36% 7% 13% 24% 18% 3	3597
	21st to end	Night	38% 6% 11% 28% 15% 2	3950
June	1st to 10th	Night	33% 7% 13% 26% 19% 2	1600
	11th to 20th	Night	34% 6% 12% 24% 21% 3	1600
	21st to end	Night	34% 8% 13% 24% 17% 4	1600
July	1st to 10th	Night	38% 6% 15% 26% 14% 2	3580
	11th to 20th	Night	29% 5% 14% 31% 18% 2	3572
	21st to end	Night	33% 7% 19% 21% 18% 2	3832
August	1st to 10th	Night	30% 6% 14% 30% 19% 1	2802
	11th to 20th	Night	32% 6% 13% 32% 16% 1	3064
	21st to end	Night	32% 6% 12% 28% 17% 4	3586
September	1st to 10th	Night	28% 6% 11% 28% 25% 2	2000
	11th to 20th	Night	32% 6% 11% 32% 17% 2	2000
	21st to end	Night	31% 5% 10% 29% 23% 1	2225
October	1st to 10th	Night	24% 4 6% 36% 29% 2	3920
	11th to 20th	Night	31% 4 10% 24% 28% 3	3899
	21st to end	Night	44% 4 9% 26% 16% 2	4464
November	1st to 10th	Night	38% 4 12% 26% 18% 2	2666
	11th to 20th	Night	50% 4 11% 21% 12% 1	2794
	21st to end	Night	49% 5% 12% 24% 10% 1	2940
December	1st to 10th	Night	42% 4 17% 28% 8% 1	4720
	11th to 20th	Night	58% 4 13% 16% 8% 1	5341
	21st to end	Night	72% 4 8% 11% 5%	7150

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	15% 14% 32% 27% 10% 1	330
	11th to 20th	Day	17% 14% 45% 19% 5% 1	318
	21st to end	Day	20% 15% 38% 23% 3 1	330
February	1st to 10th	Day	15% 19% 43% 18% 4	300
	11th to 20th	Day	20% 19% 37% 18% 5% 1	300
	21st to end	Day	19% 22% 41% 15% 3	249
March	1st to 10th	Day	17% 16% 44% 16% 6%	300
	11th to 20th	Day	18% 21% 35% 18% 5% 2	300
	21st to end	Day	26% 15% 36% 16% 7% 1	470
April	1st to 10th	Day	27% 15% 33% 21% 5%	500
	11th to 20th	Day	25% 17% 27% 20% 9% 1	492
	21st to end	Day	23% 14% 31% 22% 9% 2	500
May	1st to 10th	Day	20% 12% 32% 26% 10% 2	649
	11th to 20th	Day	19% 14% 25% 27% 12% 4	725
	21st to end	Day	17% 10% 27% 26% 14% 6%	789
June	1st to 10th	Day	11% 8% 27% 31% 19% 4	730
	11th to 20th	Day	14% 5% 30% 32% 14% 4	730
	21st to end	Day	13% 8% 30% 30% 14% 6%	730
July	1st to 10th	Day	9% 3 18% 34% 23% 13%	721
	11th to 20th	Day	9% 4 26% 29% 23% 8%	718
	21st to end	Day	10% 3 24% 35% 22% 7%	794
August	1st to 10th	Day	8% 3 20% 35% 25% 10%	726
	11th to 20th	Day	12% 6% 20% 27% 23% 13%	624
	21st to end	Day	10% 5% 19% 33% 22% 10%	583
September	1st to 10th	Day	14% 7% 23% 31% 18% 7%	530
	11th to 20th	Day	9% 4 18% 35% 24% 9%	530
	21st to end	Day	15% 9% 26% 26% 19% 5%	500
October	1st to 10th	Day	16% 8% 29% 27% 17% 2	330
	11th to 20th	Day	22% 18% 33% 18% 9%	330
	21st to end	Day	23% 16% 30% 24% 6% 1	363
November	1st to 10th	Day	15% 17% 38% 19% 8% 2	330
	11th to 20th	Day	18% 18% 36% 18% 8% 1	330
	21st to end	Day	24% 13% 35% 23% 4	330
December	1st to 10th	Day	13% 17% 42% 19% 8% 1	330
	11th to 20th	Day	17% 13% 41% 21% 7% 1	330
	21st to end	Day	26% 15% 38% 17% 4	363
January	1st to 10th	Night	14% 15% 39% 25% 7%	1050
	11th to 20th	Night	18% 13% 38% 26% 5%	1031
	21st to end	Night	21% 10% 35% 27% 6%	1100
February	1st to 10th	Night	17% 13% 39% 24% 6%	1000
	11th to 20th	Night	17% 13% 39% 26% 5%	899
	21st to end	Night	23% 12% 39% 22% 4	660
March	1st to 10th	Night	22% 11% 37% 24% 5% 1	800
	11th to 20th	Night	18% 11% 35% 30% 6%	625
	21st to end	Night	24% 10% 35% 25% 6% 1	660
April	1st to 10th	Night	20% 14% 36% 27% 4	600
	11th to 20th	Night	22% 14% 28% 29% 6% 1	593
	21st to end	Night	20% 10% 32% 30% 9% 1	600
May	1st to 10th	Night	19% 7% 35% 31% 7% 1	600
	11th to 20th	Night	18% 7% 30% 34% 12%	591
	21st to end	Night	16% 4 28% 37% 15% 2	650
June	1st to 10th	Night	11% 4 26% 45% 13% 1	600
	11th to 20th	Night	13% 4 31% 39% 13%	600
	21st to end	Night	12% 3 26% 42% 16% 2	600
July	1st to 10th	Night	6% 1 15% 43% 31% 4	585
	11th to 20th	Night	8% 3 21% 41% 26% 2	582
	21st to end	Night	8% 2 17% 44% 27% 2	657
August	1st to 10th	Night	6% 2 16% 47% 27% 3	597
	11th to 20th	Night	10% 2 15% 40% 29% 4	594
	21st to end	Night	11% 2 16% 44% 24% 3	660
September	1st to 10th	Night	11% 4 19% 39% 22% 6%	600
	11th to 20th	Night	11% 2 20% 36% 26% 4	600
	21st to end	Night	11% 7% 25% 36% 19% 1	800
October	1st to 10th	Night	15% 6% 27% 33% 18% 1	800
	11th to 20th	Night	20% 9% 33% 29% 9%	800
	21st to end	Night	21% 11% 33% 28% 7% 1	880
November	1st to 10th	Night	16% 11% 38% 25% 9% 1	820
	11th to 20th	Night	19% 13% 39% 24% 5%	904
	21st to end	Night	20% 12% 37% 23% 7%	1050
December	1st to 10th	Night	19% 11% 40% 25% 5%	1050
	11th to 20th	Night	22% 11% 35% 25% 6%	1050
	21st to end	Night	27% 12% 36% 21% 3	1155

Page generated at 2026-01-14 00:14:30. Version ac07

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