Lake Dillon Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lake Dillon Observatory clear sky chart from 2005-11-06 to 2024-11-04.

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	3 8% 4 11% 6% 5% 4 4 5% 7% 43%	105212
	Night	1 5% 4 10% 6% 4 4 3 5% 7% 50%	102535
Transparency	Day	36% 7% 11% 21% 19% 6%	104961
	Night	28% 8% 14% 26% 19% 5%	102376
Seeing	Day	16% 19% 28% 23% 11% 3	15542
	Night	9% 13% 27% 35% 15% 2	25571

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	4 4 20% 11% 10% 3 2 4 6% 36%	2538
	11th to 20th	Day	8% 7% 18% 10% 10% 4 2 4 4 33%	2736
	21st to end	Day	7% 6% 18% 5% 4 3 3 4 6% 42%	3146
February	1st to 10th	Day	6% 7% 20% 10% 5% 4 3 5% 6% 34%	1710
	11th to 20th	Day	6% 6% 19% 8% 6% 3 2 4 5% 42%	1774
	21st to end	Day	4 6% 19% 10% 6% 3 3 4 6% 39%	1570
March	1st to 10th	Day	7% 7% 21% 6% 4 3 2 3 4 42%	3212
	11th to 20th	Day	10% 7% 15% 6% 4 4 3 5% 7% 39%	3490
	21st to end	Day	1 13% 10% 20% 7% 5% 4 3 4 5% 30%	3992
April	1st to 10th	Day	1 10% 6% 14% 5% 5% 4 4 5% 6% 41%	2280
	11th to 20th	Day	1 17% 8% 15% 6% 5% 3 3 5% 5% 33%	2251
	21st to end	Day	2 16% 8% 13% 6% 4 4 3 5% 6% 32%	2280
May	1st to 10th	Day	2 15% 11% 15% 7% 5% 5% 4 5% 5% 26%	3993
	11th to 20th	Day	3 13% 3 9% 6% 6% 6% 5% 6% 7% 36%	4388
	21st to end	Day	3 9% 3 7% 5% 6% 6% 4 7% 8% 41%	4825
June	1st to 10th	Day	5% 7% 3 6% 4 5% 5% 5% 7% 8% 45%	2660
	11th to 20th	Day	6% 7% 2 5% 4 5% 4 4 6% 7% 51%	2660
	21st to end	Day	4 5% 2 6% 5% 5% 5% 4 7% 7% 48%	2660
July	1st to 10th	Day	5% 6% 2 6% 5% 4 4 4 5% 7% 55%	4430
	11th to 20th	Day	4 4 2 6% 5% 6% 7% 6% 8% 9% 43%	4430
	21st to end	Day	6% 6% 2 7% 6% 5% 6% 5% 6% 7% 45%	4562
August	1st to 10th	Day	8% 6% 3 9% 7% 6% 7% 5% 7% 8% 34%	4092
	11th to 20th	Day	6% 5% 2 8% 6% 6% 6% 5% 7% 7% 43%	3797
	21st to end	Day	6% 6% 2 8% 6% 6% 6% 5% 7% 7% 42%	4180
September	1st to 10th	Day	6% 6% 3 7% 5% 3 3 3 5% 6% 52%	2280
	11th to 20th	Day	6% 7% 3 6% 4 4 4 3 4 6% 51%	2280
	21st to end	Day	5% 8% 2 6% 4 3 4 3 4 5% 57%	2055
October	1st to 10th	Day	2 5% 2 5% 3 3 2 2 3 5% 68%	3170
	11th to 20th	Day	3 6% 2 6% 5% 5% 5% 4 6% 9% 50%	3150
	21st to end	Day	2 9% 3 8% 3 3 3 2 4 5% 59%	3417
November	1st to 10th	Day	6% 4 8% 4 3 3 3 5% 5% 58%	1700
	11th to 20th	Day	8% 4 11% 5% 3 2 2 3 8% 54%	1710
	21st to end	Day	7% 4 12% 6% 3 2 2 5% 5% 54%	1554
December	1st to 10th	Day	5% 6% 17% 9% 7% 3 2 3 7% 41%	1520
	11th to 20th	Day	7% 5% 12% 7% 4 3 3 6% 9% 43%	1926
	21st to end	Day	6% 6% 16% 10% 6% 4 3 4 7% 38%	2794
January	1st to 10th	Night	2 3 19% 13% 6% 4 3 5% 7% 38%	4417
	11th to 20th	Night	4 7% 20% 11% 7% 6% 2 3 4 36%	4264
	21st to end	Night	6% 7% 19% 9% 6% 4 3 4 6% 37%	4521
February	1st to 10th	Night	3 4 19% 11% 7% 4 3 4 7% 38%	2470
	11th to 20th	Night	4 4 17% 9% 5% 5% 3 6% 6% 42%	2291
	21st to end	Night	3 5% 17% 10% 7% 4 3 3 6% 41%	1884
March	1st to 10th	Night	4 7% 19% 6% 5% 3 3 5% 5% 44%	3654
	11th to 20th	Night	6% 7% 12% 4 4 3 3 5% 7% 49%	3490
	21st to end	Night	1 10% 8% 15% 6% 4 4 3 5% 6% 37%	3653
April	1st to 10th	Night	1 6% 4 11% 6% 3 4 4 6% 7% 48%	1900
	11th to 20th	Night	1 10% 5% 11% 6% 5% 4 4 5% 6% 43%	1877
	21st to end	Night	2 11% 4 9% 6% 3 3 2 5% 7% 47%	1900
May	1st to 10th	Night	1 9% 7% 10% 5% 4 5% 4 6% 7% 42%	2970
	11th to 20th	Night	3 9% 2 7% 5% 6% 5% 5% 9% 10% 39%	2517
	21st to end	Night	2 9% 3 7% 4 4 4 4 5% 7% 53%	2762
June	1st to 10th	Night	3 3 1 4 3 4 4 4 7% 8% 59%	1520
	11th to 20th	Night	2 3 1 4 3 4 3 3 5% 7% 64%	1520
	21st to end	Night	2 3 1 4 3 3 3 3 6% 10% 61%	1520
July	1st to 10th	Night	2 3 1 4 4 5% 4 4 6% 8% 60%	2540
	11th to 20th	Night	1 2 1 3 3 3 6% 5% 10% 11% 55%	2540
	21st to end	Night	2 3 1 4 3 3 5% 4 8% 10% 58%	2997
August	1st to 10th	Night	3 3 2 6% 4 5% 5% 5% 9% 11% 46%	2847
	11th to 20th	Night	2 3 1 4 3 4 5% 6% 7% 11% 54%	3040
	21st to end	Night	3 3 1 4 4 4 6% 6% 8% 11% 49%	3487
September	1st to 10th	Night	3 4 1 5% 3 4 4 4 6% 8% 59%	1900
	11th to 20th	Night	3 4 1 3 4 3 3 3 5% 7% 63%	1900
	21st to end	Night	3 5% 2 4 3 3 4 4 4 7% 62%	2120
October	1st to 10th	Night	1 3 1 3 2 2 3 2 4 6% 73%	3800
	11th to 20th	Night	1 4 2 4 2 3 4 4 6% 8% 61%	3778
	21st to end	Night	1 6% 2 7% 2 3 3 2 4 6% 65%	4120
November	1st to 10th	Night	5% 3 7% 3 3 2 3 4 5% 63%	2440
	11th to 20th	Night	5% 4 10% 5% 3 2 3 4 7% 56%	2470
	21st to end	Night	5% 3 11% 6% 4 3 4 5% 7% 52%	2622
December	1st to 10th	Night	4 4 12% 8% 6% 4 4 4 6% 48%	2660
	11th to 20th	Night	5% 5% 12% 7% 5% 5% 4 5% 8% 43%	3271
	21st to end	Night	5% 4 15% 8% 7% 5% 3 4 8% 41%	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	37% 14% 12% 18% 17% 3	2539
	11th to 20th	Day	46% 11% 16% 15% 10% 2	2736
	21st to end	Day	39% 9% 11% 23% 13% 4	3146
February	1st to 10th	Day	46% 7% 15% 16% 12% 4	1710
	11th to 20th	Day	40% 7% 9% 21% 18% 5%	1783
	21st to end	Day	43% 5% 10% 18% 18% 5%	1570
March	1st to 10th	Day	42% 5% 8% 22% 19% 4	3212
	11th to 20th	Day	41% 6% 15% 27% 9% 2	3490
	21st to end	Day	50% 7% 12% 19% 9% 1	3992
April	1st to 10th	Day	40% 6% 13% 19% 16% 6%	2280
	11th to 20th	Day	50% 5% 9% 18% 15% 2	2254
	21st to end	Day	49% 6% 9% 25% 11% 1	2280
May	1st to 10th	Day	51% 8% 11% 14% 14% 1	3993
	11th to 20th	Day	38% 11% 12% 24% 14% 1	4392
	21st to end	Day	31% 9% 11% 23% 23% 4	4825
June	1st to 10th	Day	32% 6% 12% 26% 18% 6%	2660
	11th to 20th	Day	32% 5% 9% 20% 23% 12%	2660
	21st to end	Day	31% 5% 9% 20% 24% 11%	2660
July	1st to 10th	Day	28% 4 8% 13% 21% 26%	4361
	11th to 20th	Day	31% 8% 12% 22% 18% 9%	4355
	21st to end	Day	33% 6% 11% 17% 16% 17%	4418
August	1st to 10th	Day	43% 9% 15% 15% 14% 4	4078
	11th to 20th	Day	36% 9% 11% 15% 21% 8%	3834
	21st to end	Day	37% 6% 10% 18% 23% 6%	4179
September	1st to 10th	Day	33% 4 7% 18% 27% 10%	2280
	11th to 20th	Day	34% 4 9% 20% 24% 8%	2280
	21st to end	Day	29% 4 7% 27% 28% 5%	2055
October	1st to 10th	Day	19% 3 5% 26% 36% 10%	3170
	11th to 20th	Day	26% 5% 11% 33% 20% 4	3150
	21st to end	Day	27% 3 6% 34% 27% 4	3417
November	1st to 10th	Day	24% 6% 11% 23% 31% 4	1700
	11th to 20th	Day	25% 7% 14% 28% 19% 6%	1710
	21st to end	Day	31% 5% 9% 29% 18% 9%	1554
December	1st to 10th	Day	39% 8% 13% 22% 14% 3	1520
	11th to 20th	Day	34% 7% 11% 30% 17% 1	1924
	21st to end	Day	38% 12% 6% 18% 21% 6%	2794
January	1st to 10th	Night	37% 12% 12% 22% 15% 2	4417
	11th to 20th	Night	43% 11% 17% 17% 11% 3	4264
	21st to end	Night	36% 14% 12% 20% 15% 3	4521
February	1st to 10th	Night	40% 9% 13% 20% 15% 3	2470
	11th to 20th	Night	35% 8% 11% 25% 18% 3	2292
	21st to end	Night	37% 8% 9% 21% 19% 6%	1884
March	1st to 10th	Night	33% 8% 10% 25% 20% 4	3654
	11th to 20th	Night	30% 8% 17% 29% 14% 3	3490
	21st to end	Night	38% 11% 17% 24% 8% 1	3653
April	1st to 10th	Night	30% 9% 13% 25% 19% 4	1900
	11th to 20th	Night	37% 6% 13% 27% 15% 2	1877
	21st to end	Night	33% 6% 15% 31% 14%	1900
May	1st to 10th	Night	34% 9% 14% 27% 15% 2	2970
	11th to 20th	Night	31% 12% 17% 27% 13%	2518
	21st to end	Night	23% 10% 13% 30% 23% 2	2762
June	1st to 10th	Night	18% 8% 20% 31% 18% 5%	1520
	11th to 20th	Night	18% 6% 13% 23% 27% 13%	1520
	21st to end	Night	17% 5% 15% 25% 26% 12%	1520
July	1st to 10th	Night	21% 6% 12% 21% 20% 20%	2500
	11th to 20th	Night	17% 9% 20% 32% 15% 7%	2492
	21st to end	Night	19% 8% 19% 30% 9% 15%	2903
August	1st to 10th	Night	28% 11% 24% 22% 11% 5%	2838
	11th to 20th	Night	20% 11% 25% 17% 18% 8%	3069
	21st to end	Night	23% 9% 20% 22% 22% 4	3487
September	1st to 10th	Night	22% 6% 14% 23% 27% 8%	1900
	11th to 20th	Night	21% 5% 14% 25% 25% 10%	1900
	21st to end	Night	22% 5% 10% 33% 26% 5%	2120
October	1st to 10th	Night	13% 4 9% 33% 33% 9%	3800
	11th to 20th	Night	16% 6% 14% 43% 17% 4	3778
	21st to end	Night	20% 4 8% 36% 28% 4	4120
November	1st to 10th	Night	22% 6% 12% 28% 27% 5%	2440
	11th to 20th	Night	25% 7% 11% 31% 21% 5%	2470
	21st to end	Night	28% 5% 11% 29% 20% 7%	2622
December	1st to 10th	Night	31% 10% 15% 25% 16% 4	2660
	11th to 20th	Night	33% 8% 13% 24% 21% 1	3272
	21st to end	Night	34% 12% 9% 19% 21% 5%	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	20% 23% 21% 24% 9% 3	270
	11th to 20th	Day	21% 25% 27% 21% 4 1	270
	21st to end	Day	13% 28% 31% 21% 6% 2	297
February	1st to 10th	Day	27% 31% 23% 16% 3 1	270
	11th to 20th	Day	29% 27% 27% 14% 3 1	270
	21st to end	Day	31% 29% 23% 16% 2	225
March	1st to 10th	Day	26% 23% 27% 18% 6%	313
	11th to 20th	Day	20% 23% 32% 19% 4 1	460
	21st to end	Day	17% 36% 32% 13% 2 1	506
April	1st to 10th	Day	22% 32% 31% 10% 4 1	460
	11th to 20th	Day	28% 30% 29% 10% 3 1	452
	21st to end	Day	20% 30% 35% 10% 4 1	460
May	1st to 10th	Day	19% 27% 33% 15% 5% 1	510
	11th to 20th	Day	17% 14% 30% 26% 11% 2	675
	21st to end	Day	14% 19% 25% 26% 13% 3	734
June	1st to 10th	Day	11% 13% 28% 26% 19% 4	680
	11th to 20th	Day	13% 15% 30% 27% 12% 3	680
	21st to end	Day	7% 14% 30% 28% 17% 4	680
July	1st to 10th	Day	11% 8% 23% 31% 23% 5%	671
	11th to 20th	Day	8% 9% 20% 32% 24% 8%	668
	21st to end	Day	9% 9% 23% 30% 22% 7%	600
August	1st to 10th	Day	17% 10% 28% 26% 15% 4	487
	11th to 20th	Day	8% 11% 30% 29% 15% 7%	477
	21st to end	Day	11% 11% 27% 32% 14% 5%	539
September	1st to 10th	Day	12% 13% 27% 26% 15% 7%	490
	11th to 20th	Day	12% 14% 32% 27% 11% 4	490
	21st to end	Day	11% 19% 26% 22% 15% 7%	322
October	1st to 10th	Day	12% 18% 25% 22% 17% 6%	300
	11th to 20th	Day	16% 26% 24% 21% 10% 4	300
	21st to end	Day	21% 27% 24% 18% 9% 2	330
November	1st to 10th	Day	20% 25% 29% 15% 9% 1	279
	11th to 20th	Day	22% 23% 26% 19% 7% 3	270
	21st to end	Day	13% 22% 33% 26% 4 1	270
December	1st to 10th	Day	21% 26% 25% 17% 8% 2	270
	11th to 20th	Day	19% 25% 32% 17% 6% 1	270
	21st to end	Day	22% 28% 28% 15% 6% 1	297
January	1st to 10th	Night	15% 19% 26% 28% 10% 2	950
	11th to 20th	Night	17% 18% 27% 28% 9% 2	849
	21st to end	Night	9% 17% 34% 32% 7% 1	836
February	1st to 10th	Night	11% 20% 38% 23% 7%	760
	11th to 20th	Night	10% 21% 37% 26% 5%	744
	21st to end	Night	11% 21% 36% 30% 3	628
March	1st to 10th	Night	9% 17% 35% 32% 6% 1	760
	11th to 20th	Night	6% 14% 34% 36% 9% 1	760
	21st to end	Night	6% 19% 40% 30% 5%	738
April	1st to 10th	Night	10% 18% 37% 31% 4	570
	11th to 20th	Night	10% 18% 33% 31% 7% 1	563
	21st to end	Night	12% 14% 32% 32% 9% 1	570
May	1st to 10th	Night	9% 9% 36% 34% 11% 1	570
	11th to 20th	Night	10% 5% 26% 43% 15%	561
	21st to end	Night	10% 9% 25% 38% 17% 2	617
June	1st to 10th	Night	6% 5% 23% 42% 22% 2	570
	11th to 20th	Night	5% 7% 28% 45% 14% 1	570
	21st to end	Night	4 2 19% 46% 28% 1	570
July	1st to 10th	Night	6% 2 10% 42% 37% 3	555
	11th to 20th	Night	3 1 7% 45% 42% 4	552
	21st to end	Night	5% 2 7% 38% 43% 4	624
August	1st to 10th	Night	7% 3 10% 41% 37% 2	567
	11th to 20th	Night	5% 2 12% 49% 29% 4	564
	21st to end	Night	5% 2 9% 45% 36% 4	627
September	1st to 10th	Night	4 2 11% 50% 29% 4	570
	11th to 20th	Night	5% 2 16% 52% 22% 3	570
	21st to end	Night	6% 5% 22% 42% 22% 4	623
October	1st to 10th	Night	5% 9% 28% 39% 16% 3	760
	11th to 20th	Night	9% 13% 26% 35% 14% 2	760
	21st to end	Night	10% 11% 29% 37% 11% 2	836
November	1st to 10th	Night	7% 15% 32% 35% 10% 1	932
	11th to 20th	Night	13% 18% 31% 30% 7% 1	950
	21st to end	Night	9% 15% 30% 36% 9% 1	950
December	1st to 10th	Night	10% 21% 37% 26% 6%	950
	11th to 20th	Night	12% 21% 31% 27% 8% 1	950
	21st to end	Night	15% 20% 32% 26% 6% 1	1045

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