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 2024-04-03.

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	4 8% 5% 10% 8% 4 5% 4 5% 7% 42%	78940
	Night	1 5% 4 10% 8% 4 4 4 5% 7% 49%	81251
Transparency	Day	39% 7% 11% 21% 17% 5%	78651
	Night	30% 8% 14% 25% 18% 5%	81062
Seeing	Day	16% 19% 27% 23% 11% 3	14649
	Night	9% 13% 27% 35% 15% 2	24883

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	3 5% 16% 14% 9% 4 3 4 6% 36%	2538
	11th to 20th	Day	7% 7% 16% 12% 8% 6% 3 4 4 33%	2736
	21st to end	Day	7% 6% 17% 8% 3 3 4 4 6% 42%	3146
February	1st to 10th	Day	5% 8% 18% 11% 4 5% 3 4 7% 35%	1710
	11th to 20th	Day	5% 6% 17% 9% 5% 4 3 4 5% 42%	1774
	21st to end	Day	3 6% 17% 12% 5% 3 4 3 6% 39%	1570
March	1st to 10th	Day	1 9% 6% 13% 9% 3 4 3 4 6% 42%	1943
	11th to 20th	Day	1 9% 6% 14% 8% 2 4 4 4 7% 40%	2090
	21st to end	Day	1 10% 9% 14% 9% 4 5% 4 4 7% 34%	2395
April	1st to 10th	Day	1 8% 7% 12% 6% 3 5% 5% 5% 7% 40%	2185
	11th to 20th	Day	2 14% 9% 14% 7% 4 4 4 5% 5% 33%	2131
	21st to end	Day	2 14% 9% 12% 7% 3 5% 4 5% 7% 32%	2160
May	1st to 10th	Day	4 14% 7% 10% 8% 3 5% 5% 5% 7% 32%	2259
	11th to 20th	Day	5% 14% 4 8% 8% 4 6% 5% 6% 6% 33%	2478
	21st to end	Day	6% 10% 4 9% 8% 4 6% 6% 6% 7% 35%	2724
June	1st to 10th	Day	5% 6% 3 5% 5% 3 5% 6% 7% 9% 45%	2520
	11th to 20th	Day	6% 7% 2 5% 5% 4 5% 5% 5% 7% 51%	2520
	21st to end	Day	4 5% 2 5% 6% 4 5% 5% 7% 8% 49%	2520
July	1st to 10th	Day	7% 6% 3 6% 7% 4 5% 5% 6% 8% 42%	2520
	11th to 20th	Day	7% 5% 3 6% 7% 5% 6% 6% 7% 8% 40%	2520
	21st to end	Day	10% 8% 4 7% 8% 5% 7% 6% 6% 8% 31%	2611
August	1st to 10th	Day	12% 7% 4 8% 8% 4 7% 5% 6% 8% 32%	2328
	11th to 20th	Day	8% 6% 3 7% 6% 4 5% 5% 6% 7% 44%	2170
	21st to end	Day	7% 7% 3 7% 6% 5% 5% 6% 6% 7% 40%	2376
September	1st to 10th	Day	7% 6% 3 6% 6% 3 3 3 5% 6% 52%	2160
	11th to 20th	Day	7% 7% 3 6% 5% 3 5% 4 4 6% 51%	2160
	21st to end	Day	5% 8% 3 5% 5% 2 3 4 4 5% 56%	1947
October	1st to 10th	Day	4 7% 5% 7% 5% 3 4 4 4 6% 49%	1800
	11th to 20th	Day	3 7% 4 5% 4 2 3 3 4 6% 59%	1800
	21st to end	Day	2 10% 5% 10% 4 2 4 3 4 5% 53%	1972
November	1st to 10th	Day	6% 4 7% 5% 2 3 3 5% 5% 59%	1673
	11th to 20th	Day	7% 5% 10% 6% 2 2 2 3 8% 54%	1710
	21st to end	Day	6% 4 10% 8% 3 2 3 5% 5% 54%	1554
December	1st to 10th	Day	5% 6% 14% 10% 6% 4 2 3 7% 41%	1520
	11th to 20th	Day	5% 6% 11% 8% 3 3 4 5% 9% 44%	1926
	21st to end	Day	5% 6% 14% 12% 4 4 4 4 8% 38%	2794
January	1st to 10th	Night	2 3 15% 16% 5% 4 4 5% 7% 38%	4417
	11th to 20th	Night	3 7% 17% 14% 4 8% 3 3 4 36%	4264
	21st to end	Night	5% 7% 16% 12% 5% 4 4 4 6% 38%	4521
February	1st to 10th	Night	3 4 16% 13% 6% 5% 4 4 7% 39%	2470
	11th to 20th	Night	3 4 14% 11% 4 6% 4 5% 6% 42%	2291
	21st to end	Night	3 6% 15% 12% 6% 5% 4 3 6% 41%	1884
March	1st to 10th	Night	4 4 11% 8% 5% 4 3 5% 7% 48%	2236
	11th to 20th	Night	1 6% 6% 10% 6% 3 4 3 4 6% 52%	2090
	21st to end	Night	1 7% 7% 12% 7% 3 4 4 5% 7% 42%	2201
April	1st to 10th	Night	1 6% 4 9% 7% 3 4 4 5% 7% 49%	1820
	11th to 20th	Night	1 9% 5% 10% 8% 3 4 5% 5% 7% 42%	1777
	21st to end	Night	2 10% 5% 8% 6% 3 4 3 5% 8% 47%	1800
May	1st to 10th	Night	2 8% 4 7% 5% 3 4 5% 7% 9% 46%	1694
	11th to 20th	Night	4 9% 2 5% 6% 3 6% 5% 8% 9% 44%	1417
	21st to end	Night	3 6% 2 5% 5% 3 4 5% 5% 10% 52%	1552
June	1st to 10th	Night	3 3 1 3 3 2 4 5% 7% 9% 60%	1440
	11th to 20th	Night	3 3 1 4 4 3 4 3 5% 8% 64%	1440
	21st to end	Night	2 3 1 4 3 3 3 3 6% 11% 61%	1440
July	1st to 10th	Night	4 3 2 3 4 4 5% 6% 7% 11% 50%	1440
	11th to 20th	Night	2 2 2 3 4 2 5% 6% 9% 11% 54%	1440
	21st to end	Night	4 4 2 5% 5% 3 5% 7% 9% 11% 44%	1703
August	1st to 10th	Night	5% 4 3 6% 5% 3 4 5% 6% 11% 47%	1611
	11th to 20th	Night	4 4 1 4 5% 2 4 7% 5% 8% 56%	1729
	21st to end	Night	4 4 2 4 5% 3 5% 6% 6% 9% 51%	1980
September	1st to 10th	Night	3 4 1 4 4 2 5% 5% 6% 8% 58%	1800
	11th to 20th	Night	3 4 2 3 5% 2 3 4 5% 7% 62%	1800
	21st to end	Night	3 5% 2 3 4 2 4 4 4 7% 61%	2008
October	1st to 10th	Night	2 5% 2 5% 5% 2 4 4 6% 8% 57%	2160
	11th to 20th	Night	1 6% 3 5% 4 1 2 3 4 5% 66%	2160
	21st to end	Night	1 8% 3 9% 4 3 3 3 4 7% 55%	2376
November	1st to 10th	Night	5% 3 7% 4 2 3 4 4 5% 63%	2394
	11th to 20th	Night	5% 4 10% 6% 3 3 3 4 6% 57%	2470
	21st to end	Night	4 4 9% 7% 3 4 4 4 8% 52%	2622
December	1st to 10th	Night	4 4 10% 10% 4 5% 4 4 6% 49%	2660
	11th to 20th	Night	4 6% 11% 8% 4 5% 5% 5% 8% 44%	3271
	21st to end	Night	5% 4 13% 10% 5% 5% 5% 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	41% 5% 9% 25% 19% 1	1943
	11th to 20th	Day	42% 5% 14% 23% 14% 3	2090
	21st to end	Day	47% 7% 10% 23% 11% 2	2395
April	1st to 10th	Day	41% 6% 13% 19% 15% 6%	2185
	11th to 20th	Day	51% 5% 10% 18% 14% 2	2134
	21st to end	Day	50% 5% 9% 25% 10% 1	2160
May	1st to 10th	Day	48% 6% 10% 21% 14% 1	2259
	11th to 20th	Day	47% 7% 13% 23% 9% 1	2482
	21st to end	Day	45% 7% 11% 21% 14% 3	2724
June	1st to 10th	Day	32% 6% 13% 25% 18% 6%	2520
	11th to 20th	Day	32% 5% 9% 19% 23% 12%	2520
	21st to end	Day	31% 5% 9% 20% 24% 11%	2520
July	1st to 10th	Day	38% 5% 11% 17% 20% 8%	2451
	11th to 20th	Day	39% 8% 14% 19% 13% 7%	2445
	21st to end	Day	47% 8% 16% 15% 11% 2	2466
August	1st to 10th	Day	49% 7% 12% 14% 15% 3	2313
	11th to 20th	Day	38% 6% 11% 20% 19% 7%	2170
	21st to end	Day	42% 6% 9% 21% 17% 5%	2376
September	1st to 10th	Day	34% 4 7% 17% 28% 11%	2160
	11th to 20th	Day	34% 4 9% 21% 24% 8%	2160
	21st to end	Day	29% 4 7% 28% 26% 5%	1947
October	1st to 10th	Day	34% 5% 9% 21% 24% 6%	1800
	11th to 20th	Day	26% 4 8% 30% 25% 7%	1800
	21st to end	Day	33% 4 10% 27% 19% 6%	1972
November	1st to 10th	Day	24% 6% 12% 23% 31% 4	1673
	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	32% 7% 9% 29% 21% 2	2236
	11th to 20th	Night	29% 8% 15% 30% 15% 4	2090
	21st to end	Night	36% 9% 16% 27% 11% 2	2201
April	1st to 10th	Night	30% 8% 14% 25% 19% 4	1820
	11th to 20th	Night	38% 6% 13% 27% 15% 2	1777
	21st to end	Night	33% 6% 16% 31% 14% 1	1800
May	1st to 10th	Night	32% 7% 17% 29% 14% 1	1694
	11th to 20th	Night	32% 8% 21% 27% 12%	1418
	21st to end	Night	27% 6% 15% 26% 21% 4	1552
June	1st to 10th	Night	18% 7% 20% 31% 18% 6%	1440
	11th to 20th	Night	19% 5% 13% 23% 26% 13%	1440
	21st to end	Night	17% 5% 15% 25% 26% 13%	1440
July	1st to 10th	Night	27% 9% 17% 21% 18% 8%	1400
	11th to 20th	Night	19% 10% 20% 26% 17% 8%	1392
	21st to end	Night	28% 13% 26% 21% 10% 3	1608
August	1st to 10th	Night	31% 9% 21% 21% 13% 5%	1602
	11th to 20th	Night	25% 6% 18% 24% 21% 6%	1729
	21st to end	Night	27% 7% 16% 27% 18% 5%	1980
September	1st to 10th	Night	22% 6% 14% 21% 28% 9%	1800
	11th to 20th	Night	22% 5% 14% 25% 24% 10%	1800
	21st to end	Night	22% 5% 10% 33% 25% 4	2008
October	1st to 10th	Night	23% 7% 13% 28% 22% 8%	2160
	11th to 20th	Night	18% 5% 11% 33% 25% 7%	2160
	21st to end	Night	27% 5% 13% 28% 22% 5%	2376
November	1st to 10th	Night	22% 6% 12% 27% 27% 5%	2394
	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% 30% 32% 10% 3 1	429
	11th to 20th	Day	28% 31% 27% 11% 2 1	412
	21st to end	Day	20% 31% 34% 10% 4 1	420
May	1st to 10th	Day	18% 26% 33% 16% 6% 1	467
	11th to 20th	Day	17% 14% 28% 26% 12% 2	625
	21st to end	Day	14% 18% 25% 26% 14% 3	679
June	1st to 10th	Day	11% 13% 26% 26% 20% 4	630
	11th to 20th	Day	13% 15% 29% 28% 13% 3	630
	21st to end	Day	7% 14% 29% 29% 18% 4	630
July	1st to 10th	Day	11% 6% 22% 31% 25% 5%	621
	11th to 20th	Day	8% 9% 18% 32% 24% 9%	618
	21st to end	Day	10% 8% 22% 29% 23% 7%	553
August	1st to 10th	Day	18% 10% 27% 26% 15% 4	447
	11th to 20th	Day	9% 9% 29% 30% 15% 8%	437
	21st to end	Day	12% 11% 26% 32% 14% 6%	495
September	1st to 10th	Day	13% 13% 28% 25% 14% 7%	450
	11th to 20th	Day	12% 12% 32% 28% 12% 4	450
	21st to end	Day	11% 19% 25% 22% 15% 7%	291
October	1st to 10th	Day	13% 19% 23% 22% 17% 6%	270
	11th to 20th	Day	17% 26% 23% 22% 9% 4	270
	21st to end	Day	22% 25% 24% 19% 9% 2	297
November	1st to 10th	Day	20% 25% 29% 15% 9% 1	270
	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% 32% 3	546
	11th to 20th	Night	10% 19% 33% 31% 7% 1	533
	21st to end	Night	11% 14% 31% 33% 9% 1	540
May	1st to 10th	Night	8% 9% 36% 35% 11% 1	540
	11th to 20th	Night	10% 5% 26% 43% 16%	531
	21st to end	Night	10% 9% 24% 38% 17% 2	584
June	1st to 10th	Night	6% 5% 23% 41% 23% 2	540
	11th to 20th	Night	5% 7% 27% 46% 15% 1	540
	21st to end	Night	4 2 19% 45% 29% 1	540
July	1st to 10th	Night	7% 2 8% 41% 39% 3	525
	11th to 20th	Night	3 1 7% 43% 43% 4	522
	21st to end	Night	6% 3 6% 37% 45% 4	591
August	1st to 10th	Night	7% 3 9% 40% 38% 3	537
	11th to 20th	Night	5% 2 11% 47% 31% 4	534
	21st to end	Night	5% 2 8% 44% 37% 4	594
September	1st to 10th	Night	4 2 11% 50% 28% 4	540
	11th to 20th	Night	5% 2 14% 53% 23% 3	540
	21st to end	Night	6% 5% 22% 42% 22% 4	590
October	1st to 10th	Night	6% 9% 29% 36% 17% 3	720
	11th to 20th	Night	10% 14% 26% 34% 14% 2	720
	21st to end	Night	11% 11% 27% 37% 11% 2	792
November	1st to 10th	Night	7% 15% 31% 35% 10% 1	914
	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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