Burk Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Burk Astronomical Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	4 5% 2 5% 4 4 5% 4 6% 8% 52%	135379
	Night	4 6% 2 5% 3 3 3 3 5% 7% 58%	131451
Transparency	Day	26% 8% 12% 23% 21% 9%	130292
	Night	23% 7% 13% 26% 23% 8%	127543
Seeing	Day	14% 25% 31% 20% 8% 2	16948
	Night	11% 29% 27% 23% 9% 1	26888

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 3 8% 4 4 4 3 5% 7% 57%	3638
	11th to 20th	Day	3 1 6% 5% 4 5% 4 7% 10% 56%	3686
	21st to end	Day	4 2 5% 3 3 3 2 5% 8% 64%	4510
February	1st to 10th	Day	1 5% 2 7% 4 4 4 4 6% 8% 56%	1800
	11th to 20th	Day	4 2 5% 4 5% 4 4 6% 9% 55%	1877
	21st to end	Day	1 7% 2 6% 4 3 3 2 6% 8% 58%	1650
March	1st to 10th	Day	2 10% 4 7% 3 3 3 2 5% 7% 54%	4500
	11th to 20th	Day	3 9% 4 7% 5% 4 5% 4 6% 7% 47%	4987
	21st to end	Day	3 7% 3 6% 4 4 4 4 6% 10% 48%	5788
April	1st to 10th	Day	4 7% 2 4 4 4 4 4 6% 9% 51%	2400
	11th to 20th	Day	7% 10% 3 7% 4 4 4 4 6% 7% 44%	2371
	21st to end	Day	8% 9% 2 6% 5% 5% 5% 5% 8% 9% 40%	2400
May	1st to 10th	Day	12% 8% 3 6% 4 4 5% 4 6% 7% 42%	5928
	11th to 20th	Day	7% 7% 2 6% 5% 5% 6% 5% 7% 8% 42%	6298
	21st to end	Day	8% 6% 2 6% 5% 4 6% 5% 7% 9% 43%	6925
June	1st to 10th	Day	5% 5% 2 5% 4 5% 6% 6% 8% 9% 46%	2800
	11th to 20th	Day	6% 4 2 5% 3 4 5% 6% 7% 9% 49%	2800
	21st to end	Day	3 4 1 5% 5% 4 6% 5% 8% 9% 51%	2800
July	1st to 10th	Day	4 3 1 4 4 4 5% 5% 7% 9% 52%	6339
	11th to 20th	Day	2 3 1 5% 5% 6% 7% 6% 9% 9% 46%	6340
	21st to end	Day	6% 5% 2 7% 5% 5% 5% 5% 7% 8% 45%	6845
August	1st to 10th	Day	8% 3 1 5% 4 5% 6% 5% 8% 9% 45%	5779
	11th to 20th	Day	3 3 2 5% 5% 5% 5% 4 6% 8% 53%	5476
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 6% 8% 44%	5984
September	1st to 10th	Day	5% 5% 2 4 3 3 4 4 6% 7% 57%	2400
	11th to 20th	Day	6% 3 1 5% 4 4 4 4 6% 7% 56%	2395
	21st to end	Day	6% 5% 1 4 3 3 3 2 3 6% 63%	2182
October	1st to 10th	Day	4 4 1 3 2 2 2 2 3 4 72%	3200
	11th to 20th	Day	5% 4 2 5% 3 4 3 3 5% 7% 59%	3150
	21st to end	Day	4 6% 2 4 3 2 3 3 5% 7% 63%	3298
November	1st to 10th	Day	2 4 1 3 3 2 3 3 6% 8% 63%	1763
	11th to 20th	Day	1 5% 1 2 3 2 3 3 5% 9% 65%	1800
	21st to end	Day	2 5% 2 4 4 4 4 3 6% 8% 59%	1620
December	1st to 10th	Day	1 3 1 3 2 2 3 2 4 6% 73%	2620
	11th to 20th	Day	2 4 2 3 3 2 3 2 5% 8% 66%	3026
	21st to end	Day	1 5% 3 7% 4 4 5% 3 6% 7% 54%	4004
January	1st to 10th	Night	1 6% 3 7% 4 4 4 3 5% 7% 56%	6327
	11th to 20th	Night	4 3 6% 4 4 3 3 5% 7% 62%	6322
	21st to end	Night	1 5% 2 6% 4 3 3 2 4 5% 64%	6468
February	1st to 10th	Night	1 5% 2 6% 3 4 4 4 5% 8% 58%	2600
	11th to 20th	Night	1 5% 3 6% 5% 4 3 4 5% 6% 57%	2409
	21st to end	Night	1 8% 3 5% 4 4 3 3 4 5% 61%	1980
March	1st to 10th	Night	2 10% 4 7% 4 3 3 2 4 6% 56%	5374
	11th to 20th	Night	3 9% 2 5% 3 3 4 3 4 8% 55%	5010
	21st to end	Night	5% 8% 3 6% 4 4 4 3 5% 8% 51%	5172
April	1st to 10th	Night	3 8% 2 5% 3 3 4 3 6% 8% 55%	2000
	11th to 20th	Night	8% 8% 3 6% 3 3 3 4 4 7% 52%	1977
	21st to end	Night	10% 11% 2 5% 3 4 2 3 5% 7% 48%	2000
May	1st to 10th	Night	12% 8% 2 5% 4 4 4 3 5% 6% 47%	4034
	11th to 20th	Night	7% 6% 2 6% 4 4 4 5% 6% 9% 48%	3617
	21st to end	Night	11% 7% 2 5% 2 3 3 3 5% 6% 53%	3972
June	1st to 10th	Night	6% 5% 1 4 3 2 4 4 6% 10% 55%	1600
	11th to 20th	Night	5% 3 1 4 3 3 4 3 6% 7% 62%	1600
	21st to end	Night	5% 4 1 4 2 2 3 3 6% 8% 63%	1600
July	1st to 10th	Night	4 4 2 4 3 3 4 4 6% 8% 58%	3640
	11th to 20th	Night	4 3 1 3 3 4 5% 4 8% 11% 53%	3640
	21st to end	Night	5% 3 1 4 3 4 6% 4 7% 8% 55%	4012
August	1st to 10th	Night	9% 5% 1 4 3 4 4 4 6% 9% 51%	4033
	11th to 20th	Night	3 3 2 4 4 4 5% 5% 6% 9% 56%	4373
	21st to end	Night	8% 5% 1 4 3 4 4 3 6% 9% 51%	4993
September	1st to 10th	Night	6% 4 1 3 3 4 4 2 4 7% 60%	2000
	11th to 20th	Night	6% 4 1 3 3 2 3 3 4 7% 63%	2005
	21st to end	Night	6% 4 1 3 3 3 3 2 5% 6% 63%	2215
October	1st to 10th	Night	5% 4 1 2 2 2 2 2 3 5% 73%	3836
	11th to 20th	Night	6% 6% 2 4 3 3 3 3 4 6% 59%	3778
	21st to end	Night	4 6% 2 4 2 2 3 3 4 7% 64%	4227
November	1st to 10th	Night	2 4 1 4 2 3 3 3 5% 7% 65%	2535
	11th to 20th	Night	2 4 1 3 3 3 3 3 5% 8% 65%	2600
	21st to end	Night	1 5% 3 5% 3 3 3 3 6% 7% 61%	2777
December	1st to 10th	Night	1 3 1 3 2 2 2 2 4 5% 76%	4570
	11th to 20th	Night	1 4 2 4 3 2 3 2 5% 8% 67%	5181
	21st to end	Night	2 6% 3 6% 4 4 4 3 5% 8% 55%	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	25% 7% 13% 30% 19% 6%	3639
	11th to 20th	Day	20% 9% 16% 23% 26% 7%	3686
	21st to end	Day	17% 7% 14% 27% 27% 9%	4510
February	1st to 10th	Day	25% 8% 18% 25% 18% 6%	1800
	11th to 20th	Day	24% 8% 15% 28% 20% 5%	1886
	21st to end	Day	24% 7% 13% 26% 23% 7%	1650
March	1st to 10th	Day	27% 7% 10% 20% 29% 7%	4500
	11th to 20th	Day	29% 11% 12% 25% 20% 4	4987
	21st to end	Day	27% 10% 16% 35% 12% 1	5788
April	1st to 10th	Day	27% 8% 14% 26% 17% 8%	2400
	11th to 20th	Day	36% 9% 14% 19% 16% 5%	2374
	21st to end	Day	35% 10% 14% 26% 14% 1	2400
May	1st to 10th	Day	35% 10% 11% 24% 15% 4	5928
	11th to 20th	Day	31% 12% 12% 19% 20% 5%	6302
	21st to end	Day	29% 12% 15% 23% 18% 3	6926
June	1st to 10th	Day	27% 14% 17% 21% 16% 5%	2800
	11th to 20th	Day	27% 9% 13% 18% 21% 12%	2800
	21st to end	Day	24% 10% 12% 21% 20% 13%	2800
July	1st to 10th	Day	24% 8% 7% 16% 24% 21%	6271
	11th to 20th	Day	28% 7% 12% 22% 17% 13%	6265
	21st to end	Day	34% 7% 9% 15% 18% 16%	6648
August	1st to 10th	Day	34% 9% 13% 19% 16% 9%	4208
	11th to 20th	Day	28% 7% 12% 16% 20% 16%	3957
	21st to end	Day	27% 7% 12% 17% 25% 12%	4311
September	1st to 10th	Day	24% 7% 10% 19% 27% 14%	2400
	11th to 20th	Day	24% 7% 12% 21% 25% 12%	2395
	21st to end	Day	23% 6% 7% 20% 35% 9%	2182
October	1st to 10th	Day	17% 4 6% 16% 35% 22%	3200
	11th to 20th	Day	25% 5% 9% 31% 23% 7%	3150
	21st to end	Day	21% 5% 8% 33% 27% 6%	3298
November	1st to 10th	Day	18% 7% 14% 27% 27% 7%	1763
	11th to 20th	Day	18% 6% 12% 31% 26% 8%	1800
	21st to end	Day	21% 7% 13% 27% 24% 8%	1620
December	1st to 10th	Day	13% 4 11% 34% 31% 6%	2620
	11th to 20th	Day	18% 4 12% 37% 27% 2	3024
	21st to end	Day	26% 7% 13% 27% 19% 9%	4004
January	1st to 10th	Night	25% 7% 12% 28% 20% 8%	6327
	11th to 20th	Night	19% 9% 14% 22% 26% 11%	6322
	21st to end	Night	21% 6% 13% 24% 26% 9%	6468
February	1st to 10th	Night	23% 7% 19% 28% 19% 5%	2600
	11th to 20th	Night	25% 8% 14% 30% 20% 4	2410
	21st to end	Night	25% 4 11% 25% 26% 9%	1980
March	1st to 10th	Night	27% 6% 8% 20% 30% 8%	5374
	11th to 20th	Night	25% 8% 11% 19% 29% 8%	5010
	21st to end	Night	30% 8% 17% 30% 13% 1	5172
April	1st to 10th	Night	26% 8% 13% 27% 20% 6%	2000
	11th to 20th	Night	31% 7% 15% 25% 17% 5%	1977
	21st to end	Night	35% 7% 12% 27% 18% 1	2000
May	1st to 10th	Night	33% 9% 13% 28% 12% 5%	4031
	11th to 20th	Night	28% 11% 16% 24% 17% 4	3618
	21st to end	Night	29% 9% 16% 26% 18% 3	3972
June	1st to 10th	Night	22% 9% 21% 26% 15% 6%	1600
	11th to 20th	Night	20% 7% 14% 23% 22% 15%	1600
	21st to end	Night	17% 7% 11% 27% 25% 13%	1600
July	1st to 10th	Night	22% 6% 10% 22% 27% 14%	3600
	11th to 20th	Night	23% 5% 19% 26% 17% 10%	3592
	21st to end	Night	25% 6% 14% 23% 19% 14%	3898
August	1st to 10th	Night	30% 7% 17% 21% 19% 5%	2928
	11th to 20th	Night	26% 7% 16% 22% 18% 11%	3168
	21st to end	Night	20% 5% 16% 24% 26% 10%	3597
September	1st to 10th	Night	23% 6% 11% 19% 25% 16%	2000
	11th to 20th	Night	21% 5% 10% 26% 25% 13%	2005
	21st to end	Night	22% 5% 8% 24% 31% 10%	2215
October	1st to 10th	Night	16% 4 7% 20% 31% 21%	3836
	11th to 20th	Night	27% 4 9% 29% 24% 7%	3778
	21st to end	Night	22% 4 8% 29% 29% 8%	4227
November	1st to 10th	Night	19% 6% 16% 25% 27% 8%	2535
	11th to 20th	Night	17% 6% 13% 29% 27% 8%	2600
	21st to end	Night	21% 6% 11% 28% 27% 7%	2777
December	1st to 10th	Night	13% 4 12% 35% 30% 6%	4570
	11th to 20th	Night	16% 4 12% 38% 27% 3	5182
	21st to end	Night	24% 8% 14% 25% 24% 6%	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% 35% 24% 15% 4 1	300
	11th to 20th	Day	20% 47% 17% 16%	300
	21st to end	Day	16% 47% 26% 9% 2	330
February	1st to 10th	Day	20% 53% 20% 6% 1 1	300
	11th to 20th	Day	15% 51% 26% 5% 2	300
	21st to end	Day	26% 41% 26% 7%	249
March	1st to 10th	Day	18% 38% 29% 13% 2	300
	11th to 20th	Day	17% 36% 31% 12% 3	487
	21st to end	Day	18% 38% 32% 9% 2	550
April	1st to 10th	Day	21% 36% 30% 11% 2	500
	11th to 20th	Day	24% 36% 29% 9% 2 1	492
	21st to end	Day	21% 30% 30% 15% 3 1	500
May	1st to 10th	Day	23% 26% 28% 16% 6% 1	643
	11th to 20th	Day	21% 17% 33% 19% 8% 2	725
	21st to end	Day	14% 21% 28% 23% 12% 2	789
June	1st to 10th	Day	9% 14% 32% 26% 15% 3	730
	11th to 20th	Day	10% 17% 39% 23% 10% 2	730
	21st to end	Day	8% 15% 36% 25% 12% 3	730
July	1st to 10th	Day	8% 5% 29% 34% 19% 4	721
	11th to 20th	Day	3 7% 36% 33% 17% 4	718
	21st to end	Day	6% 7% 28% 36% 18% 5%	773
August	1st to 10th	Day	9% 11% 37% 33% 8% 2	527
	11th to 20th	Day	6% 8% 37% 32% 14% 3	517
	21st to end	Day	6% 12% 37% 32% 10% 2	583
September	1st to 10th	Day	7% 13% 38% 29% 10% 3	530
	11th to 20th	Day	9% 18% 35% 27% 10% 2	525
	21st to end	Day	11% 20% 31% 22% 12% 5%	330
October	1st to 10th	Day	11% 20% 30% 22% 13% 3	309
	11th to 20th	Day	12% 34% 32% 13% 7% 3	300
	21st to end	Day	12% 40% 30% 13% 4 1	330
November	1st to 10th	Day	16% 34% 31% 11% 6% 2	300
	11th to 20th	Day	16% 44% 26% 12% 2	300
	21st to end	Day	16% 43% 29% 10% 1 1	300
December	1st to 10th	Day	22% 40% 26% 8% 3	300
	11th to 20th	Day	22% 50% 22% 5% 1	300
	21st to end	Day	23% 44% 19% 11% 2 1	330
January	1st to 10th	Night	12% 46% 23% 13% 5% 1	1000
	11th to 20th	Night	16% 51% 18% 11% 3 1	995
	21st to end	Night	10% 50% 24% 11% 3 1	880
February	1st to 10th	Night	11% 51% 24% 10% 3	800
	11th to 20th	Night	11% 43% 31% 14% 1	784
	21st to end	Night	15% 45% 29% 9% 1	660
March	1st to 10th	Night	9% 40% 33% 15% 4	800
	11th to 20th	Night	12% 32% 28% 23% 5% 1	800
	21st to end	Night	14% 32% 31% 20% 3 1	744
April	1st to 10th	Night	15% 28% 35% 20% 3	600
	11th to 20th	Night	17% 25% 33% 19% 5% 1	593
	21st to end	Night	17% 23% 29% 24% 7% 1	600
May	1st to 10th	Night	17% 17% 34% 23% 9% 1	600
	11th to 20th	Night	17% 12% 28% 28% 14% 1	591
	21st to end	Night	11% 13% 27% 30% 17% 1	650
June	1st to 10th	Night	7% 9% 27% 38% 17% 2	600
	11th to 20th	Night	7% 10% 35% 38% 10%	600
	21st to end	Night	7% 7% 29% 36% 19% 2	600
July	1st to 10th	Night	7% 4 17% 42% 28% 3	585
	11th to 20th	Night	3 3 21% 44% 26% 2	582
	21st to end	Night	5% 3 15% 46% 28% 3	657
August	1st to 10th	Night	6% 3 20% 46% 23% 2	597
	11th to 20th	Night	5% 3 21% 46% 21% 3	594
	21st to end	Night	5% 4 20% 42% 26% 3	660
September	1st to 10th	Night	9% 5% 21% 42% 20% 4	600
	11th to 20th	Night	9% 7% 27% 38% 17% 3	600
	21st to end	Night	8% 13% 27% 35% 14% 2	615
October	1st to 10th	Night	11% 19% 28% 27% 13% 1	772
	11th to 20th	Night	8% 33% 26% 23% 9% 1	760
	21st to end	Night	11% 31% 34% 17% 5% 2	894
November	1st to 10th	Night	8% 34% 34% 17% 5% 1	975
	11th to 20th	Night	12% 44% 27% 14% 2 1	1000
	21st to end	Night	10% 40% 33% 14% 3	1000
December	1st to 10th	Night	13% 47% 25% 13% 2	1000
	11th to 20th	Night	12% 49% 24% 12% 3 1	1000
	21st to end	Night	14% 49% 23% 10% 4	1100

Page generated at 2025-09-30 23:39:04. Version ac07

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