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 2025-08-05.

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 7% 4 11% 6% 5% 5% 4 5% 7% 43%	129790
	Night	1 5% 4 10% 6% 5% 4 4 5% 7% 50%	127212
Transparency	Day	36% 7% 10% 22% 19% 6%	129497
	Night	29% 8% 13% 26% 19% 5%	127020
Seeing	Day	16% 20% 28% 22% 11% 3	16591
	Night	9% 13% 28% 35% 14% 1	26605

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% 24% 9% 8% 2 2 3 5% 38%	3638
	11th to 20th	Day	5% 6% 15% 11% 11% 7% 2 3 4 35%	3934
	21st to end	Day	5% 7% 15% 4 7% 3 3 4 5% 47%	4510
February	1st to 10th	Day	6% 7% 19% 9% 5% 4 3 5% 6% 35%	1800
	11th to 20th	Day	6% 6% 19% 8% 6% 3 2 5% 5% 40%	1870
	21st to end	Day	4 6% 19% 9% 6% 3 3 4 6% 40%	1650
March	1st to 10th	Day	5% 7% 18% 6% 4 4 2 4 4 46%	4610
	11th to 20th	Day	9% 7% 16% 6% 4 5% 3 5% 7% 36%	5000
	21st to end	Day	1 13% 8% 17% 7% 5% 4 3 4 6% 33%	5718
April	1st to 10th	Day	1 9% 6% 13% 5% 5% 4 4 5% 6% 41%	2400
	11th to 20th	Day	1 16% 8% 14% 6% 5% 3 3 5% 5% 35%	2371
	21st to end	Day	2 15% 8% 12% 6% 4 5% 4 5% 7% 32%	2400
May	1st to 10th	Day	2 12% 8% 13% 7% 6% 6% 5% 6% 6% 28%	5714
	11th to 20th	Day	3 14% 3 9% 6% 6% 7% 5% 6% 6% 35%	6298
	21st to end	Day	3 7% 3 7% 5% 6% 6% 5% 7% 9% 43%	6925
June	1st to 10th	Day	5% 7% 3 6% 4 5% 5% 5% 7% 8% 45%	2800
	11th to 20th	Day	5% 7% 2 5% 4 5% 4 4 6% 7% 51%	2800
	21st to end	Day	4 6% 2 6% 5% 5% 5% 4 7% 7% 49%	2800
July	1st to 10th	Day	4 4 2 6% 5% 4 5% 4 5% 7% 54%	6339
	11th to 20th	Day	4 4 2 6% 6% 7% 8% 7% 8% 9% 40%	6340
	21st to end	Day	7% 6% 2 8% 7% 6% 6% 5% 7% 7% 40%	6551
August	1st to 10th	Day	8% 6% 3 9% 7% 6% 7% 5% 7% 8% 34%	4145
	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 9% 4 3 3 3 5% 5% 57%	1763
	11th to 20th	Day	8% 4 10% 5% 3 2 2 3 8% 55%	1800
	21st to end	Day	7% 3 12% 6% 3 2 2 5% 5% 54%	1635
December	1st to 10th	Day	3 3 12% 10% 5% 3 1 2 5% 56%	2620
	11th to 20th	Day	6% 5% 11% 5% 3 3 3 4 7% 54%	3026
	21st to end	Day	8% 5% 17% 8% 5% 4 3 4 7% 38%	4004
January	1st to 10th	Night	2 4 23% 11% 5% 3 3 4 6% 39%	6327
	11th to 20th	Night	3 5% 16% 14% 8% 7% 2 4 4 38%	6090
	21st to end	Night	4 6% 15% 8% 6% 4 3 4 6% 44%	6468
February	1st to 10th	Night	3 4 18% 11% 7% 4 4 4 7% 38%	2600
	11th to 20th	Night	4 4 18% 8% 5% 5% 3 5% 6% 41%	2415
	21st to end	Night	4 5% 16% 10% 7% 4 3 3 6% 42%	1980
March	1st to 10th	Night	4 7% 19% 6% 5% 4 3 4 5% 43%	5255
	11th to 20th	Night	5% 6% 16% 4 4 3 3 5% 6% 49%	5000
	21st to end	Night	1 11% 9% 15% 7% 5% 4 3 5% 6% 35%	5244
April	1st to 10th	Night	1 6% 4 11% 6% 3 5% 4 6% 7% 49%	2000
	11th to 20th	Night	1 10% 5% 11% 6% 5% 4 4 5% 6% 44%	1977
	21st to end	Night	2 11% 4 8% 6% 3 3 2 5% 7% 48%	2000
May	1st to 10th	Night	1 8% 5% 9% 5% 4 5% 5% 7% 8% 42%	4246
	11th to 20th	Night	2 10% 2 8% 5% 5% 5% 4 7% 9% 42%	3617
	21st to end	Night	2 6% 3 5% 3 4 4 4 5% 8% 57%	3972
June	1st to 10th	Night	3 4 1 4 3 4 4 4 7% 8% 59%	1600
	11th to 20th	Night	2 3 1 4 3 4 3 3 5% 7% 65%	1600
	21st to end	Night	2 3 1 5% 3 3 3 3 6% 10% 61%	1600
July	1st to 10th	Night	2 3 1 4 4 5% 5% 4 6% 8% 58%	3640
	11th to 20th	Night	1 1 1 3 3 3 6% 5% 10% 13% 54%	3640
	21st to end	Night	2 3 1 4 3 3 5% 5% 9% 10% 54%	4304
August	1st to 10th	Night	3 3 2 6% 4 5% 5% 5% 9% 11% 46%	2883
	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 8% 3 3 2 3 4 5% 62%	2524
	11th to 20th	Night	5% 5% 11% 5% 3 2 2 3 7% 57%	2600
	21st to end	Night	5% 3 11% 6% 4 3 4 5% 7% 52%	2760
December	1st to 10th	Night	2 3 8% 8% 4 3 3 3 4 61%	4570
	11th to 20th	Night	3 4 11% 6% 4 4 4 5% 7% 52%	5181
	21st to end	Night	9% 6% 15% 7% 6% 4 3 5% 8% 35%	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	42% 10% 8% 19% 17% 3	3639
	11th to 20th	Day	47% 9% 12% 14% 16% 3	3934
	21st to end	Day	37% 6% 8% 21% 20% 8%	4510
February	1st to 10th	Day	44% 7% 14% 16% 13% 5%	1800
	11th to 20th	Day	41% 7% 9% 21% 18% 5%	1879
	21st to end	Day	42% 5% 10% 18% 19% 6%	1650
March	1st to 10th	Day	38% 4 6% 20% 22% 10%	4610
	11th to 20th	Day	42% 5% 11% 26% 13% 2	5000
	21st to end	Day	46% 7% 13% 24% 9% 1	5718
April	1st to 10th	Day	39% 6% 13% 21% 15% 6%	2400
	11th to 20th	Day	48% 5% 9% 19% 16% 3	2374
	21st to end	Day	48% 6% 9% 26% 11% 1	2400
May	1st to 10th	Day	46% 9% 11% 18% 15% 1	5714
	11th to 20th	Day	39% 10% 10% 23% 16% 2	6302
	21st to end	Day	29% 9% 11% 25% 24% 3	6926
June	1st to 10th	Day	32% 6% 12% 26% 18% 5%	2800
	11th to 20th	Day	31% 5% 9% 20% 24% 12%	2800
	21st to end	Day	31% 5% 9% 20% 24% 11%	2800
July	1st to 10th	Day	26% 4 7% 16% 21% 27%	6271
	11th to 20th	Day	32% 9% 13% 23% 17% 7%	6265
	21st to end	Day	39% 7% 12% 15% 16% 12%	6363
August	1st to 10th	Day	43% 9% 15% 15% 14% 4	4131
	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	25% 6% 12% 22% 30% 4	1763
	11th to 20th	Day	25% 7% 13% 28% 20% 7%	1800
	21st to end	Day	30% 4 8% 29% 19% 8%	1635
December	1st to 10th	Day	31% 5% 9% 35% 18% 3	2620
	11th to 20th	Day	28% 5% 14% 33% 19% 1	3024
	21st to end	Day	38% 11% 9% 19% 20% 4	4004
January	1st to 10th	Night	41% 9% 10% 20% 16% 4	6327
	11th to 20th	Night	42% 10% 13% 17% 14% 4	6090
	21st to end	Night	33% 10% 9% 18% 24% 5%	6468
February	1st to 10th	Night	38% 9% 13% 19% 16% 4	2600
	11th to 20th	Night	36% 8% 11% 25% 17% 3	2416
	21st to end	Night	36% 8% 9% 20% 20% 6%	1980
March	1st to 10th	Night	36% 7% 7% 22% 22% 6%	5255
	11th to 20th	Night	32% 6% 12% 23% 25% 2	5000
	21st to end	Night	40% 11% 17% 26% 6% 1	5244
April	1st to 10th	Night	30% 8% 13% 27% 18% 4	2000
	11th to 20th	Night	36% 6% 12% 27% 16% 2	1977
	21st to end	Night	32% 6% 15% 32% 14%	2000
May	1st to 10th	Night	31% 10% 17% 28% 12% 1	4243
	11th to 20th	Night	31% 10% 17% 26% 16% 1	3618
	21st to end	Night	19% 8% 14% 33% 23% 2	3972
June	1st to 10th	Night	19% 8% 19% 33% 17% 5%	1600
	11th to 20th	Night	18% 5% 13% 23% 27% 14%	1600
	21st to end	Night	17% 5% 14% 25% 26% 13%	1600
July	1st to 10th	Night	21% 6% 11% 19% 23% 20%	3600
	11th to 20th	Night	16% 9% 25% 30% 14% 5%	3592
	21st to end	Night	20% 9% 22% 27% 11% 10%	4180
August	1st to 10th	Night	27% 11% 23% 22% 12% 5%	2874
	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	23% 6% 12% 28% 27% 5%	2524
	11th to 20th	Night	25% 7% 10% 30% 22% 5%	2600
	21st to end	Night	28% 5% 11% 29% 20% 7%	2760
December	1st to 10th	Night	23% 7% 9% 42% 17% 3	4570
	11th to 20th	Night	26% 7% 13% 30% 23% 1	5182
	21st to end	Night	41% 10% 9% 19% 18% 3	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	20% 23% 24% 22% 9% 3	300
	11th to 20th	Day	19% 26% 28% 21% 4 1	300
	21st to end	Day	14% 28% 30% 21% 5% 2	330
February	1st to 10th	Day	30% 31% 22% 14% 2 1	300
	11th to 20th	Day	29% 27% 27% 13% 2 1	300
	21st to end	Day	29% 29% 22% 17% 2	249
March	1st to 10th	Day	25% 24% 26% 18% 7%	345
	11th to 20th	Day	21% 24% 32% 18% 4 1	500
	21st to end	Day	17% 36% 31% 12% 2 1	550
April	1st to 10th	Day	22% 32% 31% 10% 4 1	500
	11th to 20th	Day	26% 31% 29% 10% 3 1	492
	21st to end	Day	20% 29% 36% 10% 4 1	500
May	1st to 10th	Day	19% 27% 32% 16% 6% 1	553
	11th to 20th	Day	18% 14% 30% 26% 10% 2	725
	21st to end	Day	13% 20% 26% 26% 13% 2	789
June	1st to 10th	Day	11% 13% 28% 26% 18% 4	730
	11th to 20th	Day	13% 15% 28% 28% 12% 3	730
	21st to end	Day	7% 15% 30% 28% 17% 4	730
July	1st to 10th	Day	10% 8% 23% 30% 23% 5%	721
	11th to 20th	Day	7% 9% 20% 32% 23% 8%	718
	21st to end	Day	9% 9% 23% 30% 21% 6%	647
August	1st to 10th	Day	16% 11% 28% 26% 15% 4	504
	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	19% 26% 28% 16% 10% 1	300
	11th to 20th	Day	22% 24% 28% 17% 7% 2	300
	21st to end	Day	13% 23% 33% 26% 4 1	300
December	1st to 10th	Day	21% 24% 24% 19% 9% 2	300
	11th to 20th	Day	20% 24% 32% 17% 6% 1	300
	21st to end	Day	23% 26% 28% 16% 6% 1	330
January	1st to 10th	Night	15% 19% 27% 27% 10% 2	1000
	11th to 20th	Night	16% 18% 28% 28% 9% 2	893
	21st to end	Night	9% 16% 35% 32% 7% 1	880
February	1st to 10th	Night	12% 22% 38% 22% 7%	800
	11th to 20th	Night	11% 22% 37% 25% 5%	784
	21st to end	Night	10% 21% 37% 30% 3	660
March	1st to 10th	Night	9% 18% 34% 32% 6% 1	800
	11th to 20th	Night	7% 15% 35% 35% 9% 1	800
	21st to end	Night	7% 19% 40% 30% 4	777
April	1st to 10th	Night	10% 18% 39% 31% 3	600
	11th to 20th	Night	10% 18% 35% 31% 6% 1	593
	21st to end	Night	11% 13% 33% 33% 8% 1	600
May	1st to 10th	Night	9% 10% 36% 35% 11% 1	600
	11th to 20th	Night	10% 6% 27% 43% 15%	591
	21st to end	Night	10% 9% 25% 38% 16% 2	650
June	1st to 10th	Night	6% 5% 23% 42% 22% 2	600
	11th to 20th	Night	5% 7% 28% 46% 14% 1	600
	21st to end	Night	4 3 20% 46% 27% 1	600
July	1st to 10th	Night	6% 2 11% 42% 36% 3	585
	11th to 20th	Night	2 1 8% 46% 40% 3	582
	21st to end	Night	5% 3 7% 39% 41% 4	657
August	1st to 10th	Night	7% 3 10% 41% 37% 2	579
	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% 33% 34% 10% 1	964
	11th to 20th	Night	13% 18% 32% 30% 7% 1	1000
	21st to end	Night	9% 15% 32% 35% 8% 1	1000
December	1st to 10th	Night	9% 20% 36% 28% 7%	1000
	11th to 20th	Night	12% 21% 33% 26% 8% 1	1000
	21st to end	Night	15% 20% 33% 26% 6% 1	1100

Page generated at 2025-08-01 23:32:24. Version ac07

Copyright © 2025 Attilla Danko.