MAS Eagle Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the MAS Eagle Lake Observatory clear sky chart from 2005-11-06 to 2025-09-04.

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 5% 4 6% 7% 39%	127139
	Night	11% 9% 3 8% 5% 4 4 4 5% 7% 40%	121864
Transparency	Day	44% 4 13% 26% 12% 1	130729
	Night	44% 3 12% 26% 13% 2	126350
Seeing	Day	16% 9% 31% 27% 13% 3	17213
	Night	15% 8% 33% 32% 11% 1	27014

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	8% 14% 5% 15% 10% 6% 7% 4 4 6% 21%	3617
	11th to 20th	Day	6% 10% 6% 13% 7% 7% 8% 8% 10% 6% 20%	3774
	21st to end	Day	5% 14% 7% 14% 7% 5% 5% 3 5% 5% 29%	4488
February	1st to 10th	Day	8% 12% 5% 10% 7% 6% 5% 5% 5% 6% 31%	1800
	11th to 20th	Day	8% 10% 4 10% 6% 5% 6% 4 6% 6% 33%	1754
	21st to end	Day	10% 12% 3 9% 7% 5% 5% 4 6% 6% 34%	1515
March	1st to 10th	Day	12% 8% 3 6% 4 3 3 3 5% 8% 45%	4926
	11th to 20th	Day	9% 10% 3 7% 4 4 4 3 6% 8% 42%	4980
	21st to end	Day	22% 11% 4 9% 6% 5% 4 3 5% 6% 25%	5462
April	1st to 10th	Day	17% 10% 3 7% 5% 4 4 3 5% 7% 36%	2394
	11th to 20th	Day	19% 12% 4 7% 4 4 4 3 4 6% 33%	2422
	21st to end	Day	21% 11% 3 6% 5% 4 4 3 4 6% 32%	2600
May	1st to 10th	Day	13% 5% 2 5% 4 3 4 3 5% 6% 49%	6014
	11th to 20th	Day	17% 8% 2 5% 3 3 3 3 4 7% 45%	6319
	21st to end	Day	18% 7% 2 7% 4 5% 4 4 5% 8% 37%	6935
June	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 7% 8% 46%	2800
	11th to 20th	Day	14% 6% 3 6% 5% 4 4 4 7% 8% 38%	2862
	21st to end	Day	10% 5% 2 6% 5% 5% 6% 4 7% 9% 42%	3000
July	1st to 10th	Day	7% 5% 2 6% 4 4 5% 4 7% 11% 44%	6625
	11th to 20th	Day	8% 4 2 4 3 3 3 3 5% 9% 56%	6360
	21st to end	Day	7% 4 2 6% 4 4 5% 4 7% 9% 47%	6707
August	1st to 10th	Day	12% 6% 2 6% 4 4 5% 4 7% 8% 42%	4234
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 7% 9% 41%	4171
	21st to end	Day	9% 6% 3 6% 4 4 4 3 5% 7% 50%	4515
September	1st to 10th	Day	11% 5% 2 6% 4 3 4 3 6% 7% 49%	2316
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 5% 7% 48%	2209
	21st to end	Day	15% 6% 2 5% 4 4 4 3 5% 7% 45%	2025
October	1st to 10th	Day	16% 7% 2 6% 5% 4 3 3 5% 5% 43%	1900
	11th to 20th	Day	14% 9% 2 6% 4 4 4 3 4 7% 43%	1900
	21st to end	Day	17% 10% 3 7% 4 3 4 3 4 6% 38%	2090
November	1st to 10th	Day	13% 10% 4 7% 5% 4 4 3 5% 7% 38%	1625
	11th to 20th	Day	10% 12% 6% 10% 5% 5% 4 2 5% 6% 33%	1600
	21st to end	Day	12% 10% 4 8% 5% 4 4 4 5% 9% 35%	1600
December	1st to 10th	Day	7% 9% 3 12% 10% 8% 6% 5% 6% 7% 27%	2610
	11th to 20th	Day	10% 16% 5% 11% 6% 6% 5% 4 5% 7% 26%	3008
	21st to end	Day	19% 17% 7% 20% 5% 4 3 2 4 4 15%	3982
January	1st to 10th	Night	7% 12% 5% 16% 8% 7% 5% 3 5% 5% 28%	6348
	11th to 20th	Night	6% 10% 4 11% 7% 7% 10% 7% 8% 6% 23%	6204
	21st to end	Night	6% 15% 7% 10% 5% 5% 5% 4 6% 5% 34%	6490
February	1st to 10th	Night	7% 13% 4 10% 6% 5% 5% 3 6% 7% 35%	2600
	11th to 20th	Night	6% 9% 3 9% 5% 5% 5% 4 6% 8% 39%	2533
	21st to end	Night	10% 9% 3 9% 5% 5% 5% 4 5% 7% 38%	2114
March	1st to 10th	Night	10% 6% 2 6% 4 4 4 3 5% 7% 49%	4968
	11th to 20th	Night	10% 6% 2 6% 5% 5% 5% 4 6% 9% 42%	4980
	21st to end	Night	23% 12% 4 9% 4 4 3 3 4 5% 30%	5467
April	1st to 10th	Night	14% 11% 3 6% 4 4 5% 3 6% 6% 38%	2005
	11th to 20th	Night	19% 9% 3 9% 4 3 3 4 5% 6% 35%	1924
	21st to end	Night	19% 9% 3 7% 4 4 4 3 5% 6% 36%	1800
May	1st to 10th	Night	11% 6% 2 5% 4 4 3 3 4 5% 53%	3899
	11th to 20th	Night	15% 6% 2 4 3 3 4 3 5% 7% 50%	3596
	21st to end	Night	17% 9% 2 6% 4 3 4 3 5% 8% 41%	3942
June	1st to 10th	Night	10% 4 1 5% 4 3 4 4 7% 8% 50%	1600
	11th to 20th	Night	9% 6% 3 5% 4 4 5% 4 7% 8% 45%	1530
	21st to end	Night	8% 4 1 4 3 4 4 3 7% 8% 55%	1400
July	1st to 10th	Night	8% 4 1 4 4 4 5% 4 7% 10% 49%	3344
	11th to 20th	Night	5% 3 2 4 4 3 4 4 5% 8% 57%	3617
	21st to end	Night	8% 5% 2 6% 4 4 4 4 6% 8% 50%	4160
August	1st to 10th	Night	12% 6% 3 5% 4 3 4 3 5% 7% 48%	2926
	11th to 20th	Night	13% 5% 1 4 3 3 4 4 6% 8% 49%	2888
	21st to end	Night	8% 6% 2 6% 4 4 4 3 6% 7% 51%	3388
September	1st to 10th	Night	8% 4 2 5% 3 2 4 3 4 8% 55%	1930
	11th to 20th	Night	10% 4 1 4 3 3 4 4 6% 7% 55%	1967
	21st to end	Night	11% 5% 2 4 4 3 4 3 4 6% 53%	2135
October	1st to 10th	Night	14% 6% 2 5% 3 3 3 2 4 7% 50%	2280
	11th to 20th	Night	14% 9% 2 6% 4 4 4 3 5% 7% 42%	2280
	21st to end	Night	17% 10% 3 7% 3 3 4 3 5% 6% 39%	2508
November	1st to 10th	Night	13% 8% 3 6% 4 5% 4 3 6% 6% 44%	2666
	11th to 20th	Night	13% 11% 4 10% 6% 4 4 3 4 5% 36%	2800
	21st to end	Night	13% 9% 4 8% 5% 4 4 4 4 6% 39%	2800
December	1st to 10th	Night	7% 7% 4 13% 7% 5% 5% 4 7% 8% 33%	4580
	11th to 20th	Night	10% 15% 7% 12% 7% 5% 5% 4 5% 5% 26%	5199
	21st to end	Night	20% 16% 6% 17% 5% 4 4 3 3 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	65% 2 10% 16% 6%	3618
	11th to 20th	Day	60% 2 9% 19% 9%	3774
	21st to end	Day	57% 3 15% 18% 7%	4488
February	1st to 10th	Day	54% 3 13% 20% 9% 1	1800
	11th to 20th	Day	50% 5% 12% 24% 8% 1	1763
	21st to end	Day	50% 3 13% 21% 12%	1515
March	1st to 10th	Day	40% 2 16% 28% 11% 2	4926
	11th to 20th	Day	42% 4 12% 31% 11% 2	4980
	21st to end	Day	61% 3 10% 18% 7%	5464
April	1st to 10th	Day	48% 3 12% 23% 13% 1	2394
	11th to 20th	Day	54% 3 10% 20% 12% 1	2422
	21st to end	Day	55% 3 11% 21% 9% 1	2600
May	1st to 10th	Day	37% 3 11% 29% 18% 2	6052
	11th to 20th	Day	41% 3 8% 25% 18% 5%	6323
	21st to end	Day	48% 3 9% 31% 9% 1	6949
June	1st to 10th	Day	35% 4 14% 31% 14% 1	2800
	11th to 20th	Day	43% 5% 14% 22% 15% 1	2862
	21st to end	Day	38% 5% 17% 27% 11% 2	3000
July	1st to 10th	Day	33% 6% 18% 33% 10%	6554
	11th to 20th	Day	26% 4 15% 38% 15% 2	6285
	21st to end	Day	32% 6% 13% 32% 16% 1	6516
August	1st to 10th	Day	39% 5% 12% 29% 13% 2	4220
	11th to 20th	Day	38% 5% 13% 33% 10%	4210
	21st to end	Day	35% 4 16% 30% 13% 1	4514
September	1st to 10th	Day	35% 5% 13% 33% 14%	2316
	11th to 20th	Day	37% 3 16% 34% 10%	2209
	21st to end	Day	39% 4 12% 33% 12% 1	2025
October	1st to 10th	Day	32% 2 8% 34% 23%	3160
	11th to 20th	Day	38% 4 11% 26% 20% 1	3140
	21st to end	Day	44% 2 14% 29% 10%	3428
November	1st to 10th	Day	47% 4 13% 20% 14% 3	1625
	11th to 20th	Day	53% 4 14% 19% 10% 1	1600
	21st to end	Day	48% 4 18% 22% 7% 1	1600
December	1st to 10th	Day	56% 4 16% 17% 6% 1	2610
	11th to 20th	Day	59% 3 16% 17% 5%	3005
	21st to end	Day	75% 2 8% 10% 4	3982
January	1st to 10th	Night	59% 2 10% 21% 7%	6348
	11th to 20th	Night	60% 3 10% 16% 11%	6204
	21st to end	Night	52% 3 12% 25% 8%	6490
February	1st to 10th	Night	50% 2 13% 23% 11% 1	2600
	11th to 20th	Night	44% 4 14% 26% 10% 2	2534
	21st to end	Night	47% 3 12% 23% 13% 2	2114
March	1st to 10th	Night	36% 2 12% 31% 16% 2	4968
	11th to 20th	Night	39% 3 12% 30% 15% 1	4980
	21st to end	Night	59% 3 9% 18% 11% 1	5467
April	1st to 10th	Night	46% 4 10% 26% 13% 1	2005
	11th to 20th	Night	50% 3 7% 21% 17% 2	1924
	21st to end	Night	50% 4 8% 23% 13% 2	1800
May	1st to 10th	Night	35% 3 9% 29% 21% 3	3926
	11th to 20th	Night	36% 3 9% 24% 20% 9%	3597
	21st to end	Night	44% 3 10% 30% 13% 1	3950
June	1st to 10th	Night	31% 3 14% 32% 18% 3	1600
	11th to 20th	Night	35% 5% 17% 22% 18% 2	1530
	21st to end	Night	27% 4 16% 31% 18% 3	1400
July	1st to 10th	Night	29% 6% 19% 30% 15% 1	3308
	11th to 20th	Night	26% 4 14% 40% 15% 2	3572
	21st to end	Night	33% 5% 15% 30% 15% 2	4040
August	1st to 10th	Night	36% 4 12% 29% 13% 5%	2916
	11th to 20th	Night	32% 5% 15% 33% 14% 1	2915
	21st to end	Night	33% 5% 17% 29% 14% 2	3388
September	1st to 10th	Night	28% 4 14% 34% 20% 1	1930
	11th to 20th	Night	29% 4 17% 33% 18% 1	1967
	21st to end	Night	33% 3 12% 33% 18% 2	2135
October	1st to 10th	Night	24% 2 5% 37% 28% 4	3800
	11th to 20th	Night	31% 3 13% 27% 23% 3	3779
	21st to end	Night	42% 3 12% 30% 13% 1	4120
November	1st to 10th	Night	41% 3 14% 26% 14% 2	2666
	11th to 20th	Night	51% 4 12% 20% 13% 1	2800
	21st to end	Night	48% 3 16% 24% 8% 1	2800
December	1st to 10th	Night	48% 5% 19% 21% 6%	4580
	11th to 20th	Night	60% 4 11% 19% 5% 1	5201
	21st to end	Night	73% 2 8% 14% 3	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	13% 11% 36% 26% 12% 2	300
	11th to 20th	Day	14% 13% 41% 28% 4 1	300
	21st to end	Day	16% 9% 40% 24% 10% 2	330
February	1st to 10th	Day	14% 14% 46% 23% 2	300
	11th to 20th	Day	15% 14% 40% 23% 7% 1	300
	21st to end	Day	16% 12% 43% 27% 2	249
March	1st to 10th	Day	18% 12% 41% 25% 5%	300
	11th to 20th	Day	19% 9% 40% 25% 6% 1	300
	21st to end	Day	26% 13% 28% 23% 8% 2	330
April	1st to 10th	Day	24% 15% 34% 22% 4	494
	11th to 20th	Day	28% 14% 27% 20% 10% 1	540
	21st to end	Day	24% 12% 32% 21% 8% 2	700
May	1st to 10th	Day	19% 11% 35% 23% 10% 2	703
	11th to 20th	Day	21% 8% 30% 24% 15% 3	725
	21st to end	Day	20% 8% 28% 26% 15% 3	789
June	1st to 10th	Day	11% 7% 27% 31% 20% 4	730
	11th to 20th	Day	17% 7% 33% 29% 11% 2	730
	21st to end	Day	13% 7% 33% 28% 15% 4	730
July	1st to 10th	Day	9% 4 24% 32% 24% 8%	721
	11th to 20th	Day	10% 5% 25% 32% 22% 6%	718
	21st to end	Day	6% 4 24% 39% 22% 5%	794
August	1st to 10th	Day	11% 5% 21% 34% 25% 4	726
	11th to 20th	Day	15% 4 21% 30% 25% 6%	715
	21st to end	Day	11% 4 25% 35% 19% 5%	708
September	1st to 10th	Day	13% 8% 28% 30% 16% 6%	502
	11th to 20th	Day	12% 5% 30% 31% 17% 5%	419
	21st to end	Day	19% 8% 30% 23% 12% 7%	300
October	1st to 10th	Day	16% 12% 27% 25% 13% 7%	300
	11th to 20th	Day	15% 19% 34% 22% 9% 1	300
	21st to end	Day	25% 14% 37% 16% 8% 1	330
November	1st to 10th	Day	21% 11% 34% 24% 8% 1	300
	11th to 20th	Day	14% 17% 43% 21% 4	300
	21st to end	Day	20% 14% 40% 20% 5%	300
December	1st to 10th	Day	13% 10% 40% 27% 8% 1	300
	11th to 20th	Day	19% 15% 37% 24% 6%	300
	21st to end	Day	28% 11% 36% 21% 5%	330
January	1st to 10th	Night	11% 10% 40% 30% 9% 1	1000
	11th to 20th	Night	15% 11% 38% 30% 6% 1	1000
	21st to end	Night	16% 10% 37% 29% 7%	1100
February	1st to 10th	Night	14% 11% 39% 29% 6% 1	1000
	11th to 20th	Night	10% 11% 43% 29% 6% 1	979
	21st to end	Night	14% 10% 37% 29% 8% 1	794
March	1st to 10th	Night	17% 8% 40% 26% 9% 1	625
	11th to 20th	Night	17% 10% 39% 28% 6% 1	600
	21st to end	Night	24% 9% 31% 24% 10% 2	660
April	1st to 10th	Night	19% 11% 35% 29% 6%	600
	11th to 20th	Night	23% 11% 30% 27% 8% 1	593
	21st to end	Night	23% 11% 31% 27% 7% 1	600
May	1st to 10th	Night	18% 10% 33% 30% 9%	600
	11th to 20th	Night	17% 8% 32% 32% 11% 1	591
	21st to end	Night	16% 8% 24% 37% 14%	650
June	1st to 10th	Night	12% 4 30% 40% 14% 1	600
	11th to 20th	Night	12% 6% 29% 44% 10%	600
	21st to end	Night	10% 4 26% 45% 15%	600
July	1st to 10th	Night	6% 2 23% 39% 28% 2	585
	11th to 20th	Night	5% 3 24% 47% 20% 2	582
	21st to end	Night	6% 1 22% 49% 20% 2	657
August	1st to 10th	Night	9% 2 18% 47% 23% 1	597
	11th to 20th	Night	12% 2 19% 40% 24% 3	594
	21st to end	Night	7% 4 26% 37% 24% 3	660
September	1st to 10th	Night	10% 2 26% 40% 20% 2	579
	11th to 20th	Night	10% 5% 32% 32% 19% 2	570
	21st to end	Night	12% 9% 31% 32% 15% 1	615
October	1st to 10th	Night	16% 9% 30% 30% 14% 1	760
	11th to 20th	Night	18% 11% 35% 28% 7% 1	760
	21st to end	Night	22% 11% 31% 29% 6%	836
November	1st to 10th	Night	18% 10% 34% 30% 9% 1	927
	11th to 20th	Night	18% 12% 39% 24% 7%	1000
	21st to end	Night	19% 11% 37% 26% 7%	1000
December	1st to 10th	Night	15% 9% 39% 28% 8% 1	1000
	11th to 20th	Night	19% 9% 38% 28% 6% 1	1000
	21st to end	Night	24% 10% 34% 27% 5% 1	1100

Page generated at 2025-09-01 00:51:02. Version ac07

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