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 2024-11-06.

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	3 6% 3 8% 6% 5% 5% 4 6% 7% 46%	105198
	Night	1 4 2 7% 5% 4 5% 4 6% 8% 53%	102594
Transparency	Day	33% 8% 13% 20% 19% 7%	104951
	Night	23% 8% 16% 25% 21% 6%	102432
Seeing	Day	15% 24% 30% 19% 9% 2	15493
	Night	8% 19% 31% 27% 14% 1	25548

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	2 3 10% 7% 7% 5% 4 6% 9% 47%	2538
	11th to 20th	Day	2 4 12% 8% 9% 6% 4 5% 5% 45%	2803
	21st to end	Day	6% 5% 11% 6% 5% 5% 4 5% 7% 46%	3146
February	1st to 10th	Day	4 3 11% 7% 7% 6% 4 7% 6% 44%	1710
	11th to 20th	Day	4 3 10% 7% 5% 4 4 6% 8% 48%	1767
	21st to end	Day	3 4 13% 7% 6% 5% 5% 5% 6% 47%	1570
March	1st to 10th	Day	5% 6% 15% 6% 5% 4 3 5% 7% 46%	3248
	11th to 20th	Day	7% 8% 14% 5% 4 5% 4 6% 7% 41%	3490
	21st to end	Day	9% 8% 12% 7% 6% 5% 5% 6% 7% 35%	3965
April	1st to 10th	Day	1 7% 3 8% 7% 5% 4 4 6% 7% 47%	2280
	11th to 20th	Day	1 9% 5% 12% 7% 6% 6% 4 6% 6% 39%	2251
	21st to end	Day	1 10% 5% 8% 7% 6% 6% 4 6% 8% 39%	2280
May	1st to 10th	Day	2 11% 7% 10% 6% 5% 5% 5% 8% 8% 34%	3933
	11th to 20th	Day	3 10% 3 7% 5% 5% 6% 6% 6% 8% 41%	4388
	21st to end	Day	3 7% 2 5% 4 5% 5% 5% 8% 8% 49%	4825
June	1st to 10th	Day	3 5% 2 7% 5% 5% 4 4 7% 9% 48%	2660
	11th to 20th	Day	4 5% 2 5% 5% 4 5% 3 6% 7% 55%	2660
	21st to end	Day	4 5% 2 6% 6% 5% 4 4 6% 8% 49%	2660
July	1st to 10th	Day	4 5% 2 6% 5% 4 4 3 5% 5% 56%	4430
	11th to 20th	Day	4 4 2 7% 7% 7% 8% 6% 10% 10% 35%	4430
	21st to end	Day	7% 6% 3 7% 7% 6% 6% 4 6% 7% 40%	4472
August	1st to 10th	Day	8% 7% 3 10% 7% 8% 8% 6% 8% 8% 29%	4092
	11th to 20th	Day	6% 6% 3 8% 6% 6% 7% 5% 7% 8% 39%	3814
	21st to end	Day	5% 6% 3 10% 7% 6% 7% 5% 7% 8% 36%	4180
September	1st to 10th	Day	6% 6% 3 7% 5% 4 5% 4 5% 7% 49%	2280
	11th to 20th	Day	6% 6% 2 7% 5% 4 4 3 5% 6% 51%	2280
	21st to end	Day	4 7% 2 6% 4 4 4 3 4 5% 58%	2044
October	1st to 10th	Day	3 5% 1 4 2 2 2 2 3 4 70%	3170
	11th to 20th	Day	2 5% 2 7% 6% 5% 5% 4 7% 8% 50%	3150
	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 3 4 3 5% 6% 66%	1740
	11th to 20th	Day	3 2 5% 4 3 3 3 4 7% 66%	1710
	21st to end	Day	4 3 6% 5% 4 3 2 4 8% 60%	1554
December	1st to 10th	Day	3 3 8% 6% 5% 5% 4 5% 7% 54%	1520
	11th to 20th	Day	4 2 8% 5% 5% 4 4 7% 11% 49%	1926
	21st to end	Day	3 3 11% 6% 5% 5% 4 7% 9% 48%	2794
January	1st to 10th	Night	1 2 10% 9% 6% 6% 4 8% 10% 44%	4417
	11th to 20th	Night	2 3 12% 8% 6% 7% 4 5% 7% 47%	4197
	21st to end	Night	6% 4 10% 5% 6% 6% 5% 8% 7% 42%	4521
February	1st to 10th	Night	2 2 9% 6% 5% 5% 3 7% 8% 52%	2470
	11th to 20th	Night	2 2 7% 6% 5% 5% 6% 8% 8% 50%	2298
	21st to end	Night	3 2 8% 6% 5% 5% 4 7% 9% 50%	1884
March	1st to 10th	Night	4 6% 10% 5% 4 5% 4 6% 8% 49%	3616
	11th to 20th	Night	6% 5% 12% 5% 5% 5% 4 8% 7% 42%	3490
	21st to end	Night	7% 5% 9% 6% 5% 5% 5% 7% 8% 42%	3682
April	1st to 10th	Night	1 3 2 4 4 4 5% 4 6% 10% 57%	1900
	11th to 20th	Night	1 7% 3 7% 5% 5% 4 4 6% 8% 50%	1877
	21st to end	Night	2 7% 3 6% 5% 4 4 3 6% 8% 53%	1900
May	1st to 10th	Night	2 8% 3 8% 4 4 4 5% 6% 8% 47%	3030
	11th to 20th	Night	2 11% 4 7% 4 5% 6% 3 6% 7% 45%	2517
	21st to end	Night	2 5% 2 4 4 3 4 4 5% 7% 61%	2762
June	1st to 10th	Night	1 3 2 4 2 2 3 3 6% 10% 62%	1520
	11th to 20th	Night	3 4 1 3 3 3 4 3 5% 5% 65%	1520
	21st to end	Night	2 3 1 3 3 4 4 4 6% 10% 59%	1520
July	1st to 10th	Night	3 5% 3 6% 3 4 4 3 5% 6% 58%	2540
	11th to 20th	Night	1 2 1 4 4 5% 6% 6% 9% 11% 51%	2540
	21st to end	Night	3 4 2 6% 4 5% 5% 5% 7% 9% 50%	3073
August	1st to 10th	Night	4 4 2 7% 5% 5% 6% 6% 11% 11% 40%	2847
	11th to 20th	Night	3 4 1 4 3 4 6% 5% 10% 11% 49%	3022
	21st to end	Night	3 4 2 7% 5% 6% 7% 5% 8% 10% 45%	3487
September	1st to 10th	Night	3 4 2 5% 3 5% 5% 4 6% 9% 56%	1900
	11th to 20th	Night	4 5% 1 5% 4 2 3 4 5% 7% 61%	1900
	21st to end	Night	2 4 1 4 4 3 4 3 5% 6% 63%	2130
October	1st to 10th	Night	2 3 1 3 2 2 3 2 3 4 75%	3800
	11th to 20th	Night	1 3 1 6% 5% 3 4 3 6% 6% 61%	3778
	21st to end	Night	1 4 1 4 3 3 3 2 3 7% 68%	4120
November	1st to 10th	Night	3 2 4 2 2 3 3 5% 7% 68%	2440
	11th to 20th	Night	1 1 4 4 3 4 3 6% 8% 66%	2470
	21st to end	Night	3 3 6% 4 4 4 4 6% 8% 59%	2622
December	1st to 10th	Night	2 2 6% 5% 5% 4 4 5% 8% 60%	2660
	11th to 20th	Night	4 2 5% 5% 5% 5% 5% 7% 10% 52%	3271
	21st to end	Night	3 2 9% 5% 5% 4 4 8% 9% 50%	4873

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	26% 10% 21% 18% 20% 4	2539
	11th to 20th	Day	26% 13% 27% 20% 12% 3	2803
	21st to end	Day	32% 9% 12% 26% 15% 5%	3146
February	1st to 10th	Day	34% 10% 18% 17% 15% 6%	1710
	11th to 20th	Day	30% 10% 13% 21% 19% 7%	1776
	21st to end	Day	34% 6% 15% 19% 19% 7%	1570
March	1st to 10th	Day	35% 5% 13% 21% 21% 6%	3248
	11th to 20th	Day	37% 10% 14% 28% 9% 2	3490
	21st to end	Day	37% 14% 16% 19% 13% 1	3965
April	1st to 10th	Day	33% 9% 14% 21% 17% 6%	2280
	11th to 20th	Day	41% 7% 16% 17% 16% 2	2254
	21st to end	Day	39% 9% 13% 25% 13% 1	2280
May	1st to 10th	Day	39% 8% 18% 16% 17% 2	3933
	11th to 20th	Day	36% 10% 14% 23% 16% 1	4392
	21st to end	Day	24% 7% 16% 21% 26% 5%	4825
June	1st to 10th	Day	30% 9% 14% 22% 20% 6%	2660
	11th to 20th	Day	28% 7% 9% 18% 24% 14%	2660
	21st to end	Day	32% 7% 10% 18% 22% 12%	2660
July	1st to 10th	Day	31% 5% 6% 11% 20% 28%	4361
	11th to 20th	Day	40% 6% 11% 21% 15% 7%	4355
	21st to end	Day	42% 6% 9% 14% 12% 16%	4331
August	1st to 10th	Day	51% 7% 15% 12% 11% 4	4078
	11th to 20th	Day	44% 6% 11% 12% 20% 7%	3852
	21st to end	Day	45% 6% 9% 15% 20% 5%	4179
September	1st to 10th	Day	36% 6% 10% 13% 23% 11%	2280
	11th to 20th	Day	32% 7% 11% 17% 24% 9%	2280
	21st to end	Day	29% 5% 8% 23% 29% 6%	2044
October	1st to 10th	Day	19% 5% 5% 22% 37% 12%	3170
	11th to 20th	Day	32% 3 9% 31% 20% 5%	3150
	21st to end	Day	22% 3 6% 33% 31% 4	3438
November	1st to 10th	Day	19% 5% 14% 23% 33% 6%	1740
	11th to 20th	Day	18% 6% 14% 30% 24% 7%	1710
	21st to end	Day	24% 6% 12% 27% 19% 11%	1554
December	1st to 10th	Day	26% 10% 19% 26% 15% 4	1520
	11th to 20th	Day	25% 10% 15% 27% 20% 2	1924
	21st to end	Day	26% 11% 13% 17% 23% 8%	2794
January	1st to 10th	Night	27% 11% 17% 25% 16% 4	4417
	11th to 20th	Night	21% 14% 26% 22% 13% 4	4197
	21st to end	Night	26% 11% 15% 25% 18% 5%	4521
February	1st to 10th	Night	26% 8% 20% 24% 18% 5%	2470
	11th to 20th	Night	23% 10% 17% 24% 22% 5%	2299
	21st to end	Night	25% 8% 16% 23% 21% 7%	1884
March	1st to 10th	Night	24% 8% 19% 21% 22% 6%	3616
	11th to 20th	Night	32% 8% 15% 28% 14% 3	3490
	21st to end	Night	26% 16% 20% 26% 12% 1	3682
April	1st to 10th	Night	20% 10% 18% 27% 20% 5%	1900
	11th to 20th	Night	28% 7% 16% 28% 17% 2	1877
	21st to end	Night	26% 9% 18% 29% 16% 1	1900
May	1st to 10th	Night	27% 8% 18% 24% 20% 2	3030
	11th to 20th	Night	33% 9% 12% 31% 16%	2518
	21st to end	Night	18% 8% 14% 28% 28% 4	2762
June	1st to 10th	Night	17% 9% 15% 31% 22% 7%	1520
	11th to 20th	Night	19% 7% 11% 20% 29% 14%	1520
	21st to end	Night	18% 6% 14% 22% 27% 12%	1520
July	1st to 10th	Night	26% 6% 10% 17% 21% 20%	2500
	11th to 20th	Night	23% 6% 21% 27% 15% 8%	2492
	21st to end	Night	26% 7% 18% 27% 9% 13%	2977
August	1st to 10th	Night	32% 11% 25% 15% 12% 4	2838
	11th to 20th	Night	27% 8% 24% 14% 20% 7%	3050
	21st to end	Night	33% 8% 17% 18% 21% 3	3487
September	1st to 10th	Night	25% 8% 14% 20% 25% 9%	1900
	11th to 20th	Night	22% 6% 15% 23% 25% 9%	1900
	21st to end	Night	21% 5% 12% 31% 27% 4	2130
October	1st to 10th	Night	13% 5% 8% 29% 36% 9%	3800
	11th to 20th	Night	21% 3 13% 38% 19% 6%	3778
	21st to end	Night	17% 4 7% 33% 33% 6%	4120
November	1st to 10th	Night	16% 6% 14% 27% 30% 7%	2440
	11th to 20th	Night	13% 7% 16% 31% 24% 8%	2470
	21st to end	Night	21% 7% 13% 27% 22% 10%	2622
December	1st to 10th	Night	19% 10% 21% 25% 19% 7%	2660
	11th to 20th	Night	21% 11% 16% 24% 23% 5%	3272
	21st to end	Night	24% 11% 15% 18% 20% 11%	4873

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% 29% 21% 20% 7% 1	270
	11th to 20th	Day	24% 35% 23% 15% 3	270
	21st to end	Day	13% 35% 31% 15% 4 1	297
February	1st to 10th	Day	30% 41% 21% 6% 1 1	270
	11th to 20th	Day	31% 34% 23% 9% 3	270
	21st to end	Day	33% 38% 22% 6% 1	225
March	1st to 10th	Day	23% 36% 25% 12% 3	336
	11th to 20th	Day	20% 31% 31% 13% 4	460
	21st to end	Day	22% 42% 28% 8% 1	506
April	1st to 10th	Day	18% 40% 31% 8% 2 1	460
	11th to 20th	Day	25% 38% 26% 9% 2 1	452
	21st to end	Day	16% 40% 32% 9% 2 1	460
May	1st to 10th	Day	20% 35% 29% 11% 4 1	472
	11th to 20th	Day	12% 21% 37% 19% 9% 2	675
	21st to end	Day	14% 21% 30% 24% 9% 2	734
June	1st to 10th	Day	7% 17% 31% 27% 16% 2	680
	11th to 20th	Day	9% 21% 34% 23% 10% 2	680
	21st to end	Day	5% 20% 34% 23% 17% 2	680
July	1st to 10th	Day	9% 9% 29% 30% 20% 3	671
	11th to 20th	Day	9% 6% 28% 30% 22% 6%	668
	21st to end	Day	12% 10% 28% 32% 15% 2	549
August	1st to 10th	Day	12% 11% 37% 25% 13% 2	487
	11th to 20th	Day	9% 11% 37% 25% 14% 4	477
	21st to end	Day	10% 12% 33% 29% 12% 3	539
September	1st to 10th	Day	11% 15% 32% 24% 13% 5%	490
	11th to 20th	Day	10% 19% 39% 20% 10% 3	490
	21st to end	Day	11% 21% 28% 26% 10% 4	333
October	1st to 10th	Day	12% 23% 26% 21% 16% 3	300
	11th to 20th	Day	22% 25% 28% 14% 8% 2	300
	21st to end	Day	17% 28% 30% 16% 7% 1	330
November	1st to 10th	Day	21% 33% 25% 14% 7% 1	285
	11th to 20th	Day	21% 29% 31% 13% 4 1	270
	21st to end	Day	14% 31% 34% 17% 3	270
December	1st to 10th	Day	24% 33% 24% 12% 5% 1	270
	11th to 20th	Day	22% 38% 25% 9% 4 1	270
	21st to end	Day	24% 35% 27% 9% 4	297
January	1st to 10th	Night	13% 27% 29% 22% 8% 1	950
	11th to 20th	Night	15% 26% 32% 19% 7% 1	808
	21st to end	Night	7% 25% 40% 20% 7% 1	836
February	1st to 10th	Night	12% 29% 40% 16% 4	760
	11th to 20th	Night	12% 30% 35% 19% 4	744
	21st to end	Night	12% 30% 42% 14% 1	628
March	1st to 10th	Night	10% 27% 33% 24% 6%	760
	11th to 20th	Night	5% 20% 42% 23% 10%	760
	21st to end	Night	6% 29% 41% 20% 4	754
April	1st to 10th	Night	8% 27% 39% 22% 4	570
	11th to 20th	Night	8% 28% 36% 21% 7% 1	563
	21st to end	Night	7% 22% 39% 24% 7% 1	570
May	1st to 10th	Night	9% 14% 42% 24% 10% 1	570
	11th to 20th	Night	7% 9% 33% 34% 15% 2	561
	21st to end	Night	8% 13% 27% 35% 16% 2	617
June	1st to 10th	Night	4 8% 28% 37% 23% 1	570
	11th to 20th	Night	5% 11% 29% 40% 14% 1	570
	21st to end	Night	2 6% 24% 39% 28% 2	570
July	1st to 10th	Night	7% 4 12% 40% 33% 4	555
	11th to 20th	Night	3 2 11% 39% 42% 4	552
	21st to end	Night	6% 2 10% 35% 42% 4	624
August	1st to 10th	Night	8% 2 12% 38% 36% 3	567
	11th to 20th	Night	5% 3 16% 39% 33% 4	564
	21st to end	Night	4 2 17% 38% 34% 4	627
September	1st to 10th	Night	5% 4 17% 43% 27% 5%	570
	11th to 20th	Night	6% 4 24% 40% 23% 2	570
	21st to end	Night	4 9% 26% 35% 22% 4	645
October	1st to 10th	Night	5% 15% 32% 28% 17% 3	760
	11th to 20th	Night	7% 18% 33% 28% 13% 2	760
	21st to end	Night	7% 19% 35% 26% 10% 2	836
November	1st to 10th	Night	8% 21% 38% 23% 9% 1	912
	11th to 20th	Night	11% 26% 35% 20% 7% 1	950
	21st to end	Night	8% 23% 35% 28% 7%	950
December	1st to 10th	Night	11% 29% 35% 19% 6%	950
	11th to 20th	Night	12% 28% 35% 18% 6%	950
	21st to end	Night	14% 27% 36% 18% 4 1	1045

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