Dick Clark Memorial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dick Clark Memorial Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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	5% 5% 2 5% 4 4 5% 4 6% 8% 52%	135432
	Night	5% 5% 2 5% 3 3 3 3 5% 7% 57%	131749
Transparency	Day	26% 6% 10% 24% 23% 10%	130369
	Night	25% 5% 10% 25% 25% 10%	127816
Seeing	Day	8% 6% 24% 35% 20% 6%	16957
	Night	7% 5% 24% 44% 18% 3	26908

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	1 6% 3 8% 4 3 3 3 5% 7% 56%	3638
	11th to 20th	Day	1 5% 2 10% 6% 5% 5% 4 5% 7% 51%	3709
	21st to end	Day	1 4 2 5% 4 3 3 3 6% 7% 62%	4510
February	1st to 10th	Day	2 6% 3 7% 4 4 4 3 6% 8% 53%	1800
	11th to 20th	Day	1 5% 2 5% 5% 3 4 3 6% 9% 57%	1899
	21st to end	Day	2 7% 2 5% 3 3 3 4 5% 6% 59%	1650
March	1st to 10th	Day	3 8% 3 4 3 2 3 3 5% 9% 57%	4500
	11th to 20th	Day	4 7% 3 7% 5% 5% 5% 4 6% 8% 46%	4950
	21st to end	Day	4 8% 3 5% 4 3 5% 4 7% 9% 48%	5810
April	1st to 10th	Day	5% 7% 2 4 3 3 3 4 5% 10% 54%	2400
	11th to 20th	Day	8% 9% 3 6% 4 4 4 4 6% 7% 45%	2371
	21st to end	Day	9% 9% 2 6% 4 4 4 4 7% 7% 43%	2400
May	1st to 10th	Day	10% 7% 3 6% 5% 4 4 3 5% 8% 44%	5835
	11th to 20th	Day	7% 5% 2 5% 5% 5% 6% 5% 7% 8% 43%	6298
	21st to end	Day	8% 5% 2 5% 4 4 6% 5% 8% 9% 45%	6925
June	1st to 10th	Day	5% 4 2 4 4 5% 5% 6% 8% 10% 48%	2800
	11th to 20th	Day	6% 2 1 3 4 5% 5% 5% 7% 9% 52%	2800
	21st to end	Day	3 3 1 4 4 5% 6% 7% 8% 9% 50%	2800
July	1st to 10th	Day	3 3 1 4 4 4 5% 5% 7% 10% 54%	6339
	11th to 20th	Day	3 3 1 6% 5% 6% 7% 7% 9% 10% 43%	6340
	21st to end	Day	5% 3 1 6% 5% 6% 6% 6% 7% 8% 48%	6822
August	1st to 10th	Day	7% 4 1 5% 5% 6% 6% 5% 9% 10% 42%	5779
	11th to 20th	Day	2 2 1 4 3 4 6% 5% 9% 10% 54%	5416
	21st to end	Day	7% 5% 2 5% 4 5% 6% 4 7% 8% 47%	5984
September	1st to 10th	Day	5% 4 1 4 2 4 4 4 5% 7% 58%	2400
	11th to 20th	Day	6% 2 1 4 3 3 4 4 6% 8% 59%	2385
	21st to end	Day	7% 5% 2 3 3 3 2 3 4 6% 64%	2200
October	1st to 10th	Day	5% 4 1 3 2 2 3 2 3 5% 69%	3270
	11th to 20th	Day	6% 4 1 4 3 4 4 4 5% 8% 57%	3230
	21st to end	Day	5% 5% 2 4 2 2 2 2 4 7% 63%	3300
November	1st to 10th	Day	3 5% 1 3 3 2 3 2 5% 9% 63%	1763
	11th to 20th	Day	3 5% 2 3 3 3 3 3 6% 7% 63%	1800
	21st to end	Day	2 5% 2 5% 3 3 3 3 5% 8% 59%	1659
December	1st to 10th	Day	1 4 2 4 2 2 3 3 4 6% 70%	2620
	11th to 20th	Day	2 4 1 3 3 2 3 2 4 6% 69%	3026
	21st to end	Day	3 7% 2 7% 4 3 4 3 6% 8% 53%	4004
January	1st to 10th	Night	2 8% 4 7% 3 3 3 2 4 8% 55%	6327
	11th to 20th	Night	1 4 3 10% 5% 4 4 3 5% 8% 56%	6312
	21st to end	Night	2 5% 3 8% 4 3 3 2 4 5% 60%	6468
February	1st to 10th	Night	1 7% 3 8% 3 4 4 3 5% 8% 54%	2600
	11th to 20th	Night	2 7% 3 7% 6% 4 4 3 5% 7% 53%	2386
	21st to end	Night	2 8% 3 5% 5% 3 3 3 4 6% 59%	1980
March	1st to 10th	Night	3 9% 3 5% 3 4 3 3 5% 6% 55%	5374
	11th to 20th	Night	6% 9% 2 4 3 3 4 3 5% 7% 54%	5044
	21st to end	Night	7% 10% 2 6% 4 3 3 3 5% 7% 50%	5150
April	1st to 10th	Night	5% 7% 2 5% 3 4 4 3 5% 7% 55%	2000
	11th to 20th	Night	10% 7% 2 5% 3 4 3 3 5% 7% 51%	1977
	21st to end	Night	13% 8% 2 5% 3 3 3 3 4 8% 47%	2000
May	1st to 10th	Night	14% 6% 2 5% 3 4 4 4 5% 7% 46%	4139
	11th to 20th	Night	9% 6% 3 7% 5% 4 5% 3 5% 8% 46%	3617
	21st to end	Night	11% 5% 2 5% 3 3 3 3 4 6% 54%	3972
June	1st to 10th	Night	6% 3 1 4 3 4 3 4 5% 9% 58%	1600
	11th to 20th	Night	6% 3 2 3 3 3 3 3 5% 7% 62%	1600
	21st to end	Night	4 2 1 3 2 3 4 5% 6% 8% 61%	1600
July	1st to 10th	Night	4 2 1 4 3 3 4 4 7% 8% 61%	3640
	11th to 20th	Night	4 2 1 3 3 3 4 4 8% 12% 55%	3640
	21st to end	Night	4 3 1 4 3 4 5% 5% 7% 9% 53%	4048
August	1st to 10th	Night	7% 4 2 5% 4 5% 5% 4 8% 8% 49%	4033
	11th to 20th	Night	3 2 1 4 3 4 5% 4 8% 10% 58%	4432
	21st to end	Night	7% 5% 2 5% 3 4 4 4 7% 9% 50%	4993
September	1st to 10th	Night	6% 3 2 3 3 3 4 4 5% 6% 62%	2000
	11th to 20th	Night	7% 2 1 3 2 3 3 3 5% 6% 66%	2015
	21st to end	Night	7% 3 1 3 3 3 3 2 5% 6% 64%	2200
October	1st to 10th	Night	6% 3 1 2 1 2 2 2 3 5% 73%	3915
	11th to 20th	Night	6% 5% 2 5% 3 3 3 2 5% 6% 60%	3874
	21st to end	Night	6% 6% 2 3 2 2 2 2 4 6% 63%	4213
November	1st to 10th	Night	3 4 2 4 2 3 3 3 5% 6% 64%	2535
	11th to 20th	Night	2 5% 2 3 4 3 2 2 5% 7% 66%	2600
	21st to end	Night	3 6% 2 4 2 3 3 3 6% 7% 61%	2740
December	1st to 10th	Night	1 3 2 4 3 2 3 2 3 5% 72%	4570
	11th to 20th	Night	1 4 2 4 3 3 3 2 5% 9% 64%	5181
	21st to end	Night	4 9% 3 6% 3 4 3 3 5% 6% 53%	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	26% 5% 8% 33% 20% 8%	3639
	11th to 20th	Day	29% 7% 10% 22% 22% 10%	3709
	21st to end	Day	20% 4 10% 29% 25% 12%	4510
February	1st to 10th	Day	28% 4 13% 27% 21% 7%	1800
	11th to 20th	Day	23% 5% 9% 31% 27% 5%	1909
	21st to end	Day	24% 4 10% 24% 27% 11%	1650
March	1st to 10th	Day	24% 3 9% 22% 31% 11%	4500
	11th to 20th	Day	32% 6% 10% 23% 24% 4	4950
	21st to end	Day	27% 7% 13% 36% 15% 2	5810
April	1st to 10th	Day	25% 5% 12% 28% 22% 9%	2400
	11th to 20th	Day	34% 7% 11% 22% 19% 7%	2374
	21st to end	Day	35% 7% 11% 25% 20% 2	2400
May	1st to 10th	Day	35% 7% 12% 27% 15% 5%	5835
	11th to 20th	Day	32% 9% 12% 20% 18% 10%	6302
	21st to end	Day	29% 8% 14% 26% 18% 4	6926
June	1st to 10th	Day	25% 10% 16% 24% 18% 7%	2800
	11th to 20th	Day	25% 7% 12% 19% 23% 14%	2800
	21st to end	Day	24% 8% 11% 21% 24% 12%	2800
July	1st to 10th	Day	22% 6% 7% 18% 28% 19%	6271
	11th to 20th	Day	28% 9% 12% 22% 17% 11%	6265
	21st to end	Day	30% 8% 9% 17% 19% 17%	6625
August	1st to 10th	Day	35% 9% 14% 17% 16% 9%	4208
	11th to 20th	Day	23% 8% 14% 20% 21% 14%	3920
	21st to end	Day	23% 7% 10% 20% 27% 13%	4311
September	1st to 10th	Day	23% 5% 8% 21% 26% 17%	2400
	11th to 20th	Day	20% 6% 8% 22% 29% 15%	2385
	21st to end	Day	22% 4 6% 21% 35% 12%	2200
October	1st to 10th	Day	19% 3 6% 16% 33% 23%	3270
	11th to 20th	Day	25% 4 9% 26% 28% 8%	3230
	21st to end	Day	22% 3 7% 25% 34% 10%	3300
November	1st to 10th	Day	18% 4 10% 28% 28% 12%	1763
	11th to 20th	Day	18% 5% 8% 28% 33% 9%	1800
	21st to end	Day	23% 3 9% 24% 32% 10%	1659
December	1st to 10th	Day	16% 4 8% 34% 28% 10%	2620
	11th to 20th	Day	16% 4 7% 35% 33% 5%	3024
	21st to end	Day	28% 4 9% 27% 24% 9%	4004
January	1st to 10th	Night	28% 4 8% 29% 23% 8%	6327
	11th to 20th	Night	25% 6% 11% 23% 22% 14%	6312
	21st to end	Night	26% 5% 9% 24% 25% 11%	6468
February	1st to 10th	Night	28% 4 11% 27% 24% 6%	2600
	11th to 20th	Night	31% 4 9% 28% 22% 6%	2386
	21st to end	Night	27% 3 6% 24% 27% 13%	1980
March	1st to 10th	Night	28% 3 7% 17% 36% 9%	5374
	11th to 20th	Night	28% 4 9% 17% 29% 13%	5044
	21st to end	Night	33% 5% 9% 31% 19% 3	5150
April	1st to 10th	Night	28% 4 10% 26% 22% 10%	2000
	11th to 20th	Night	32% 4 10% 25% 21% 7%	1977
	21st to end	Night	35% 5% 8% 25% 23% 3	2000
May	1st to 10th	Night	35% 7% 13% 26% 13% 6%	4136
	11th to 20th	Night	33% 8% 13% 21% 16% 8%	3618
	21st to end	Night	29% 6% 15% 26% 18% 5%	3972
June	1st to 10th	Night	22% 7% 18% 29% 15% 8%	1600
	11th to 20th	Night	21% 5% 11% 22% 22% 19%	1600
	21st to end	Night	19% 6% 11% 23% 30% 13%	1600
July	1st to 10th	Night	18% 5% 9% 21% 32% 14%	3600
	11th to 20th	Night	19% 7% 18% 28% 19% 9%	3592
	21st to end	Night	24% 7% 13% 23% 20% 13%	3933
August	1st to 10th	Night	31% 8% 14% 24% 18% 6%	2928
	11th to 20th	Night	20% 9% 17% 23% 19% 12%	3204
	21st to end	Night	21% 6% 12% 24% 26% 11%	3597
September	1st to 10th	Night	21% 5% 9% 23% 22% 19%	2000
	11th to 20th	Night	18% 5% 10% 26% 24% 18%	2015
	21st to end	Night	21% 4 7% 23% 31% 15%	2200
October	1st to 10th	Night	16% 3 6% 19% 31% 25%	3915
	11th to 20th	Night	24% 4 9% 25% 30% 10%	3874
	21st to end	Night	23% 3 5% 24% 34% 11%	4213
November	1st to 10th	Night	20% 4 9% 26% 28% 14%	2535
	11th to 20th	Night	19% 3 8% 28% 33% 10%	2600
	21st to end	Night	21% 4 9% 24% 31% 10%	2740
December	1st to 10th	Night	16% 3 7% 34% 32% 8%	4570
	11th to 20th	Night	19% 3 7% 33% 33% 6%	5182
	21st to end	Night	31% 4 8% 24% 27% 8%	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	6% 8% 19% 46% 16% 6%	300
	11th to 20th	Day	5% 6% 31% 45% 13% 1	300
	21st to end	Day	3 5% 30% 45% 15% 2	330
February	1st to 10th	Day	4 11% 39% 39% 6% 1	300
	11th to 20th	Day	3 8% 40% 39% 9% 1	300
	21st to end	Day	6% 12% 35% 39% 7% 1	249
March	1st to 10th	Day	6% 9% 34% 33% 14% 4	300
	11th to 20th	Day	12% 10% 32% 33% 10% 3	476
	21st to end	Day	10% 13% 41% 23% 9% 3	550
April	1st to 10th	Day	10% 13% 33% 31% 11% 1	500
	11th to 20th	Day	13% 17% 34% 27% 7% 2	492
	21st to end	Day	13% 12% 27% 33% 13% 2	500
May	1st to 10th	Day	14% 9% 25% 35% 13% 3	638
	11th to 20th	Day	19% 6% 20% 31% 19% 6%	725
	21st to end	Day	11% 6% 25% 30% 23% 5%	789
June	1st to 10th	Day	5% 4 17% 36% 27% 10%	730
	11th to 20th	Day	6% 4 26% 36% 23% 5%	730
	21st to end	Day	4 4 23% 36% 25% 8%	730
July	1st to 10th	Day	7% 2 15% 31% 32% 12%	721
	11th to 20th	Day	4 3 13% 34% 33% 12%	718
	21st to end	Day	6% 2 12% 30% 38% 13%	763
August	1st to 10th	Day	6% 4 20% 37% 24% 8%	527
	11th to 20th	Day	3 4 19% 35% 30% 10%	517
	21st to end	Day	8% 3 19% 36% 26% 9%	583
September	1st to 10th	Day	7% 3 21% 35% 25% 11%	530
	11th to 20th	Day	5% 3 22% 37% 24% 10%	515
	21st to end	Day	8% 2 19% 31% 25% 15%	330
October	1st to 10th	Day	11% 4 20% 28% 25% 13%	330
	11th to 20th	Day	2 8% 27% 40% 16% 7%	324
	21st to end	Day	8% 5% 29% 37% 17% 4	330
November	1st to 10th	Day	7% 8% 23% 37% 19% 6%	300
	11th to 20th	Day	6% 5% 27% 45% 13% 4	300
	21st to end	Day	7% 5% 26% 48% 12% 2	300
December	1st to 10th	Day	4 10% 22% 43% 16% 5%	300
	11th to 20th	Day	4 6% 27% 51% 10% 2	300
	21st to end	Day	7% 8% 27% 48% 7% 3	330
January	1st to 10th	Night	4 11% 30% 41% 11% 4	1000
	11th to 20th	Night	4 10% 35% 39% 11% 1	985
	21st to end	Night	5% 7% 30% 48% 9% 1	880
February	1st to 10th	Night	5% 9% 38% 40% 7% 1	800
	11th to 20th	Night	4 8% 36% 43% 10%	784
	21st to end	Night	5% 6% 39% 45% 5%	660
March	1st to 10th	Night	5% 7% 32% 47% 10% 1	800
	11th to 20th	Night	10% 5% 28% 44% 12% 2	800
	21st to end	Night	8% 8% 29% 45% 8% 1	735
April	1st to 10th	Night	9% 7% 32% 46% 6%	600
	11th to 20th	Night	13% 6% 31% 39% 11% 1	593
	21st to end	Night	17% 5% 24% 40% 13% 2	600
May	1st to 10th	Night	16% 5% 23% 40% 15% 2	600
	11th to 20th	Night	19% 3 15% 39% 22% 3	591
	21st to end	Night	10% 4 19% 38% 25% 3	650
June	1st to 10th	Night	7% 2 13% 48% 26% 4	600
	11th to 20th	Night	7% 3 15% 53% 21% 2	600
	21st to end	Night	6% 2 13% 47% 29% 4	600
July	1st to 10th	Night	7% 2 9% 38% 39% 6%	585
	11th to 20th	Night	3 1 7% 41% 42% 7%	582
	21st to end	Night	7% 2 6% 34% 45% 7%	657
August	1st to 10th	Night	6% 2 7% 40% 40% 5%	597
	11th to 20th	Night	4 1 6% 42% 41% 5%	594
	21st to end	Night	5% 2 5% 42% 38% 7%	660
September	1st to 10th	Night	7% 3 9% 43% 32% 7%	600
	11th to 20th	Night	7% 2 12% 47% 26% 7%	600
	21st to end	Night	8% 2 14% 48% 23% 5%	600
October	1st to 10th	Night	10% 3 16% 46% 21% 4	795
	11th to 20th	Night	7% 3 21% 49% 17% 3	792
	21st to end	Night	10% 5% 23% 46% 13% 3	893
November	1st to 10th	Night	5% 4 27% 47% 14% 3	975
	11th to 20th	Night	4 8% 31% 47% 8% 2	1000
	21st to end	Night	6% 5% 27% 48% 13% 1	1000
December	1st to 10th	Night	5% 8% 28% 48% 11%	1000
	11th to 20th	Night	4 6% 33% 47% 9% 1	1000
	21st to end	Night	6% 7% 34% 43% 9% 1	1100

Page generated at 2025-10-15 23:50:29. Version ac07

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