Mount Massive Lakes Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Massive Lakes Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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	2 5% 3 8% 6% 6% 5% 4 6% 7% 46%	135403
	Night	1 4 2 7% 5% 5% 5% 4 6% 8% 54%	131783
Transparency	Day	34% 6% 12% 20% 21% 7%	130329
	Night	25% 7% 15% 25% 22% 6%	127862
Seeing	Day	16% 24% 30% 19% 9% 2	16807
	Night	8% 18% 32% 27% 13% 1	26815

By Month

Percentage Cloud Cover SW (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	1 3 11% 7% 9% 6% 4 6% 8% 46%	3638
	11th to 20th	Day	1 3 9% 6% 9% 7% 4 6% 7% 48%	4029
	21st to end	Day	4 5% 9% 5% 7% 5% 3 4 6% 51%	4510
February	1st to 10th	Day	4 3 11% 8% 7% 6% 4 7% 6% 44%	1800
	11th to 20th	Day	4 4 10% 7% 6% 4 4 6% 8% 47%	1863
	21st to end	Day	3 4 13% 7% 6% 5% 4 5% 6% 48%	1650
March	1st to 10th	Day	4 5% 15% 7% 5% 4 3 4 6% 48%	4674
	11th to 20th	Day	6% 6% 15% 7% 5% 5% 3 6% 7% 39%	5000
	21st to end	Day	8% 6% 11% 6% 6% 5% 4 5% 7% 40%	5678
April	1st to 10th	Day	1 7% 3 8% 6% 5% 4 4 6% 7% 47%	2400
	11th to 20th	Day	1 8% 5% 11% 7% 6% 6% 4 6% 6% 40%	2371
	21st to end	Day	1 10% 4 8% 7% 6% 6% 4 6% 8% 39%	2400
May	1st to 10th	Day	2 9% 6% 9% 7% 6% 6% 6% 8% 8% 34%	5639
	11th to 20th	Day	2 9% 3 8% 6% 6% 6% 6% 6% 8% 41%	6298
	21st to end	Day	2 5% 2 5% 4 5% 6% 6% 8% 8% 48%	6925
June	1st to 10th	Day	3 5% 2 6% 5% 5% 4 5% 7% 8% 48%	2800
	11th to 20th	Day	4 5% 2 5% 5% 4 5% 3 6% 7% 55%	2800
	21st to end	Day	4 5% 2 6% 6% 4 5% 4 6% 8% 50%	2800
July	1st to 10th	Day	3 4 2 6% 5% 4 5% 4 6% 6% 55%	6339
	11th to 20th	Day	4 4 2 8% 7% 8% 8% 6% 9% 9% 34%	6340
	21st to end	Day	7% 6% 3 9% 8% 7% 6% 5% 7% 7% 36%	6433
August	1st to 10th	Day	6% 5% 2 8% 6% 7% 8% 5% 8% 9% 36%	5779
	11th to 20th	Day	5% 6% 3 8% 7% 6% 6% 5% 6% 7% 40%	5493
	21st to end	Day	6% 6% 4 11% 8% 7% 7% 5% 7% 8% 33%	5984
September	1st to 10th	Day	6% 6% 3 7% 5% 4 5% 4 5% 7% 49%	2400
	11th to 20th	Day	6% 6% 2 7% 5% 4 5% 3 5% 6% 50%	2400
	21st to end	Day	4 6% 2 7% 4 4 4 3 4 6% 57%	2152
October	1st to 10th	Day	3 5% 1 4 3 3 2 2 3 4 70%	3270
	11th to 20th	Day	2 5% 2 7% 6% 5% 5% 4 7% 8% 50%	3230
	21st to end	Day	1 3 2 5% 5% 4 3 2 4 6% 65%	3438
November	1st to 10th	Day	1 4 3 4 3 4 4 3 5% 6% 64%	1785
	11th to 20th	Day	3 2 5% 4 3 3 3 4 7% 67%	1800
	21st to end	Day	5% 3 6% 5% 4 3 2 4 8% 60%	1635
December	1st to 10th	Day	2 2 5% 4 4 7% 4 4 5% 64%	2620
	11th to 20th	Day	3 2 6% 4 5% 3 3 5% 9% 60%	3026
	21st to end	Day	3 2 10% 6% 5% 5% 4 6% 9% 48%	4004
January	1st to 10th	Night	1 3 10% 8% 7% 6% 4 7% 9% 45%	6327
	11th to 20th	Night	1 2 9% 6% 5% 7% 5% 6% 7% 52%	5995
	21st to end	Night	4 3 9% 6% 6% 5% 4 7% 6% 49%	6468
February	1st to 10th	Night	2 3 9% 6% 5% 5% 4 7% 8% 52%	2600
	11th to 20th	Night	2 2 8% 6% 6% 5% 6% 8% 8% 49%	2422
	21st to end	Night	3 2 8% 6% 5% 5% 4 7% 9% 51%	1980
March	1st to 10th	Night	4 5% 11% 6% 5% 5% 4 6% 7% 48%	5204
	11th to 20th	Night	5% 4 12% 6% 5% 5% 4 6% 7% 45%	5000
	21st to end	Night	6% 4 8% 7% 6% 5% 4 6% 8% 44%	5273
April	1st to 10th	Night	1 3 2 4 4 4 5% 4 6% 10% 57%	2000
	11th to 20th	Night	1 7% 2 7% 5% 5% 4 4 6% 8% 50%	1977
	21st to end	Night	1 7% 3 5% 5% 4 4 3 5% 9% 55%	2000
May	1st to 10th	Night	1 7% 3 8% 5% 5% 7% 5% 7% 8% 43%	4334
	11th to 20th	Night	2 8% 3 7% 6% 6% 6% 4 7% 8% 44%	3617
	21st to end	Night	2 4 2 4 3 4 4 4 5% 7% 61%	3972
June	1st to 10th	Night	2 4 2 4 2 2 3 3 7% 9% 62%	1600
	11th to 20th	Night	3 4 1 3 2 3 4 3 5% 5% 66%	1600
	21st to end	Night	2 3 1 3 3 4 4 4 6% 10% 60%	1600
July	1st to 10th	Night	2 4 2 5% 3 4 5% 4 6% 7% 58%	3640
	11th to 20th	Night	1 2 1 4 4 5% 6% 6% 10% 12% 49%	3640
	21st to end	Night	3 4 2 6% 4 5% 6% 6% 8% 10% 48%	4422
August	1st to 10th	Night	3 3 2 6% 4 4 5% 5% 9% 10% 51%	4033
	11th to 20th	Night	2 3 1 3 3 4 5% 5% 10% 11% 52%	4353
	21st to end	Night	3 4 2 8% 6% 6% 7% 6% 7% 9% 42%	4993
September	1st to 10th	Night	3 4 2 5% 3 4 5% 4 6% 8% 56%	2000
	11th to 20th	Night	4 5% 1 5% 4 3 3 4 5% 7% 60%	2000
	21st to end	Night	2 4 1 4 3 3 4 4 5% 6% 62%	2242
October	1st to 10th	Night	2 3 1 3 2 2 3 2 3 4 75%	3920
	11th to 20th	Night	1 3 1 6% 5% 3 4 3 6% 6% 61%	3874
	21st to end	Night	1 4 1 4 3 3 3 2 3 7% 68%	4120
November	1st to 10th	Night	3 2 5% 2 3 3 3 5% 7% 67%	2492
	11th to 20th	Night	1 1 4 4 3 4 3 6% 8% 66%	2600
	21st to end	Night	4 3 5% 4 3 4 4 6% 8% 59%	2760
December	1st to 10th	Night	1 1 4 3 3 4 3 4 7% 71%	4570
	11th to 20th	Night	2 1 4 4 4 4 4 6% 9% 62%	5181
	21st to end	Night	4 2 10% 7% 8% 5% 4 8% 9% 44%	6974

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	33% 7% 15% 20% 19% 6%	3639
	11th to 20th	Day	24% 10% 19% 15% 26% 7%	4029
	21st to end	Day	30% 7% 9% 22% 23% 10%	4510
February	1st to 10th	Day	34% 10% 17% 17% 15% 7%	1800
	11th to 20th	Day	31% 9% 13% 22% 19% 7%	1872
	21st to end	Day	33% 6% 14% 19% 20% 8%	1650
March	1st to 10th	Day	36% 3 9% 16% 24% 12%	4674
	11th to 20th	Day	42% 7% 10% 24% 15% 2	5000
	21st to end	Day	37% 10% 16% 25% 11% 1	5678
April	1st to 10th	Day	32% 8% 14% 22% 18% 6%	2400
	11th to 20th	Day	40% 7% 15% 18% 16% 3	2374
	21st to end	Day	39% 9% 12% 26% 13% 1	2400
May	1st to 10th	Day	41% 6% 16% 18% 17% 2	5639
	11th to 20th	Day	38% 7% 11% 22% 20% 2	6302
	21st to end	Day	27% 5% 14% 24% 25% 4	6926
June	1st to 10th	Day	30% 8% 14% 22% 20% 5%	2800
	11th to 20th	Day	28% 6% 9% 18% 25% 14%	2800
	21st to end	Day	32% 7% 9% 18% 22% 12%	2800
July	1st to 10th	Day	30% 3 5% 13% 22% 28%	6271
	11th to 20th	Day	43% 5% 13% 21% 15% 5%	6265
	21st to end	Day	46% 5% 10% 13% 14% 11%	6248
August	1st to 10th	Day	51% 6% 15% 12% 12% 4	4208
	11th to 20th	Day	44% 6% 11% 12% 20% 6%	3975
	21st to end	Day	45% 6% 9% 15% 20% 5%	4311
September	1st to 10th	Day	36% 6% 11% 14% 23% 11%	2400
	11th to 20th	Day	33% 7% 11% 17% 24% 8%	2400
	21st to end	Day	29% 5% 8% 23% 28% 7%	2152
October	1st to 10th	Day	19% 5% 5% 22% 37% 12%	3270
	11th to 20th	Day	32% 3 9% 30% 20% 6%	3230
	21st to end	Day	22% 3 6% 33% 31% 4	3438
November	1st to 10th	Day	20% 5% 14% 23% 32% 6%	1785
	11th to 20th	Day	17% 6% 13% 29% 26% 8%	1800
	21st to end	Day	24% 6% 12% 27% 21% 11%	1635
December	1st to 10th	Day	21% 6% 12% 37% 19% 6%	2620
	11th to 20th	Day	20% 7% 16% 32% 23% 2	3024
	21st to end	Day	30% 8% 13% 19% 25% 6%	4004
January	1st to 10th	Night	31% 8% 12% 24% 18% 7%	6327
	11th to 20th	Night	21% 10% 18% 19% 23% 9%	5995
	21st to end	Night	26% 8% 10% 20% 27% 8%	6468
February	1st to 10th	Night	26% 8% 19% 24% 18% 5%	2600
	11th to 20th	Night	25% 10% 16% 23% 21% 5%	2423
	21st to end	Night	25% 8% 15% 22% 22% 8%	1980
March	1st to 10th	Night	30% 6% 13% 15% 27% 8%	5204
	11th to 20th	Night	34% 6% 10% 20% 25% 4	5000
	21st to end	Night	30% 11% 21% 28% 9% 1	5273
April	1st to 10th	Night	20% 10% 17% 29% 20% 5%	2000
	11th to 20th	Night	28% 7% 16% 28% 18% 3	1977
	21st to end	Night	25% 9% 18% 31% 17% 1	2000
May	1st to 10th	Night	30% 6% 20% 26% 16% 2	4331
	11th to 20th	Night	34% 6% 12% 29% 18%	3618
	21st to end	Night	20% 5% 15% 30% 28% 3	3972
June	1st to 10th	Night	18% 9% 15% 32% 21% 6%	1600
	11th to 20th	Night	18% 7% 11% 20% 30% 15%	1600
	21st to end	Night	18% 6% 13% 22% 27% 14%	1600
July	1st to 10th	Night	25% 4 9% 17% 23% 22%	3600
	11th to 20th	Night	25% 5% 24% 26% 14% 6%	3592
	21st to end	Night	28% 7% 20% 26% 11% 9%	4296
August	1st to 10th	Night	32% 11% 25% 16% 13% 4	2928
	11th to 20th	Night	27% 8% 24% 14% 20% 7%	3147
	21st to end	Night	33% 8% 17% 18% 21% 3	3597
September	1st to 10th	Night	25% 7% 14% 21% 24% 9%	2000
	11th to 20th	Night	23% 5% 14% 24% 24% 9%	2000
	21st to end	Night	21% 5% 12% 31% 27% 5%	2242
October	1st to 10th	Night	14% 5% 7% 29% 36% 9%	3920
	11th to 20th	Night	22% 3 13% 37% 20% 6%	3874
	21st to end	Night	17% 4 7% 33% 33% 6%	4120
November	1st to 10th	Night	16% 6% 14% 27% 30% 7%	2492
	11th to 20th	Night	13% 7% 15% 30% 26% 8%	2600
	21st to end	Night	21% 7% 12% 28% 23% 9%	2760
December	1st to 10th	Night	14% 6% 12% 39% 24% 5%	4570
	11th to 20th	Night	18% 7% 13% 33% 27% 3	5182
	21st to end	Night	34% 8% 12% 19% 20% 8%	6974

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	22% 28% 24% 19% 6% 1	300
	11th to 20th	Day	23% 34% 24% 14% 4	300
	21st to end	Day	15% 35% 30% 16% 4 1	330
February	1st to 10th	Day	32% 39% 21% 6% 1 1	300
	11th to 20th	Day	32% 33% 23% 10% 3	300
	21st to end	Day	32% 37% 22% 8% 1	249
March	1st to 10th	Day	22% 36% 25% 12% 4	370
	11th to 20th	Day	23% 31% 30% 13% 4	500
	21st to end	Day	22% 41% 28% 8% 1	550
April	1st to 10th	Day	19% 40% 31% 9% 2 1	500
	11th to 20th	Day	26% 37% 26% 9% 2 1	492
	21st to end	Day	17% 39% 31% 10% 3 1	500
May	1st to 10th	Day	20% 34% 29% 12% 4 1	513
	11th to 20th	Day	14% 21% 36% 18% 9% 2	725
	21st to end	Day	15% 21% 30% 24% 8% 2	789
June	1st to 10th	Day	8% 17% 31% 26% 16% 2	730
	11th to 20th	Day	10% 20% 33% 23% 12% 2	730
	21st to end	Day	7% 19% 33% 22% 17% 3	730
July	1st to 10th	Day	9% 10% 28% 30% 20% 3	721
	11th to 20th	Day	9% 7% 27% 30% 21% 6%	718
	21st to end	Day	14% 10% 28% 31% 15% 2	594
August	1st to 10th	Day	12% 13% 36% 24% 13% 2	527
	11th to 20th	Day	10% 11% 35% 26% 14% 4	517
	21st to end	Day	11% 12% 32% 29% 13% 3	583
September	1st to 10th	Day	11% 15% 32% 25% 13% 5%	530
	11th to 20th	Day	10% 19% 39% 19% 10% 2	530
	21st to end	Day	12% 20% 28% 25% 10% 4	365
October	1st to 10th	Day	12% 22% 26% 20% 16% 3	330
	11th to 20th	Day	25% 25% 27% 14% 8% 2	324
	21st to end	Day	17% 28% 30% 16% 7% 1	330
November	1st to 10th	Day	21% 32% 25% 14% 7% 1	300
	11th to 20th	Day	21% 30% 30% 14% 5% 1	300
	21st to end	Day	14% 31% 34% 18% 3	300
December	1st to 10th	Day	23% 32% 24% 13% 7% 1	300
	11th to 20th	Day	23% 37% 25% 9% 4 1	300
	21st to end	Day	25% 34% 27% 9% 4 1	330
January	1st to 10th	Night	13% 27% 30% 21% 8% 1	1000
	11th to 20th	Night	15% 26% 32% 19% 7% 1	850
	21st to end	Night	7% 25% 41% 20% 6% 1	880
February	1st to 10th	Night	11% 28% 41% 16% 4	800
	11th to 20th	Night	13% 30% 36% 18% 4	784
	21st to end	Night	12% 29% 42% 15% 1	660
March	1st to 10th	Night	10% 27% 33% 25% 5%	800
	11th to 20th	Night	5% 20% 42% 23% 9%	800
	21st to end	Night	7% 28% 40% 21% 4	793
April	1st to 10th	Night	8% 26% 40% 22% 4	600
	11th to 20th	Night	7% 26% 38% 21% 7% 1	593
	21st to end	Night	7% 21% 39% 26% 7% 1	600
May	1st to 10th	Night	9% 14% 42% 25% 11% 1	600
	11th to 20th	Night	8% 9% 33% 34% 15% 2	591
	21st to end	Night	8% 12% 27% 36% 15% 2	650
June	1st to 10th	Night	4 8% 28% 38% 22% 1	600
	11th to 20th	Night	5% 10% 29% 41% 14% 1	600
	21st to end	Night	3 6% 24% 39% 27% 2	600
July	1st to 10th	Night	7% 4 13% 41% 32% 4	585
	11th to 20th	Night	3 2 10% 40% 40% 4	582
	21st to end	Night	7% 2 11% 36% 41% 4	657
August	1st to 10th	Night	8% 2 13% 40% 35% 3	597
	11th to 20th	Night	5% 3 16% 39% 33% 4	594
	21st to end	Night	5% 2 18% 38% 33% 4	660
September	1st to 10th	Night	5% 4 17% 44% 26% 5%	600
	11th to 20th	Night	6% 4 24% 41% 22% 2	600
	21st to end	Night	4 9% 25% 36% 22% 4	679
October	1st to 10th	Night	5% 15% 33% 28% 17% 3	800
	11th to 20th	Night	7% 18% 34% 27% 12% 2	792
	21st to end	Night	7% 19% 35% 26% 10% 2	836
November	1st to 10th	Night	8% 21% 38% 24% 9% 1	932
	11th to 20th	Night	11% 26% 36% 20% 6% 1	1000
	21st to end	Night	8% 22% 35% 28% 7%	1000
December	1st to 10th	Night	11% 28% 35% 20% 7%	1000
	11th to 20th	Night	12% 28% 37% 18% 6%	1000
	21st to end	Night	14% 27% 36% 19% 4 1	1100

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