Dryden Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dryden Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	13% 8% 3 8% 5% 4 5% 4 5% 7% 38%	127294
	Night	12% 10% 4 8% 5% 4 4 3 5% 6% 38%	123338
Transparency	Day	44% 5% 12% 25% 13% 1	130766
	Night	45% 5% 11% 23% 14% 2	127939
Seeing	Day	17% 10% 29% 25% 14% 4	17381
	Night	18% 9% 30% 31% 11% 1	27212

By Month

Percentage Cloud Cover SE (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	7% 11% 7% 17% 9% 7% 4 3 3 6% 26%	3617
	11th to 20th	Day	8% 12% 7% 11% 5% 4 7% 7% 8% 6% 25%	3620
	21st to end	Day	11% 15% 7% 10% 4 4 4 3 4 5% 35%	4184
February	1st to 10th	Day	9% 13% 5% 9% 5% 5% 6% 4 5% 8% 31%	1800
	11th to 20th	Day	11% 12% 4 8% 5% 4 4 4 6% 7% 35%	1839
	21st to end	Day	13% 12% 4 10% 6% 6% 5% 4 4 5% 31%	1650
March	1st to 10th	Day	14% 7% 2 5% 4 4 3 3 4 6% 48%	4480
	11th to 20th	Day	11% 6% 2 7% 5% 4 4 3 5% 7% 44%	4878
	21st to end	Day	25% 11% 3 8% 4 4 4 2 4 5% 30%	5847
April	1st to 10th	Day	17% 12% 3 9% 5% 4 4 4 4 5% 32%	2400
	11th to 20th	Day	19% 10% 3 8% 5% 4 4 3 5% 6% 32%	2374
	21st to end	Day	19% 10% 3 8% 5% 5% 4 3 5% 5% 32%	2460
May	1st to 10th	Day	13% 8% 3 6% 4 4 4 3 5% 6% 44%	6218
	11th to 20th	Day	14% 6% 2 7% 5% 4 4 3 5% 7% 41%	6319
	21st to end	Day	17% 6% 2 6% 4 4 5% 4 6% 8% 38%	6935
June	1st to 10th	Day	11% 6% 2 6% 4 5% 5% 4 6% 7% 44%	2800
	11th to 20th	Day	13% 6% 2 7% 6% 4 5% 4 7% 8% 39%	2800
	21st to end	Day	13% 6% 2 6% 5% 4 5% 4 7% 9% 40%	2800
July	1st to 10th	Day	9% 5% 2 6% 4 5% 6% 5% 8% 10% 39%	6360
	11th to 20th	Day	7% 5% 2 4 4 4 5% 5% 7% 10% 47%	6360
	21st to end	Day	9% 6% 2 7% 5% 4 5% 4 6% 8% 41%	6924
August	1st to 10th	Day	9% 6% 3 7% 5% 5% 6% 5% 7% 8% 40%	4481
	11th to 20th	Day	12% 6% 2 5% 4 4 5% 4 5% 8% 46%	4013
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 6% 9% 42%	4312
September	1st to 10th	Day	13% 5% 3 6% 4 4 4 4 5% 7% 45%	2400
	11th to 20th	Day	12% 5% 2 6% 4 4 4 4 5% 7% 47%	2360
	21st to end	Day	13% 8% 2 5% 3 4 4 4 6% 7% 44%	2200
October	1st to 10th	Day	13% 6% 2 6% 4 4 5% 3 4 7% 46%	2000
	11th to 20th	Day	18% 10% 3 6% 4 4 5% 4 5% 8% 34%	2000
	21st to end	Day	16% 11% 3 8% 5% 4 3 3 4 5% 39%	2025
November	1st to 10th	Day	12% 9% 3 7% 4 4 4 4 4 7% 41%	1852
	11th to 20th	Day	14% 13% 5% 10% 5% 5% 4 4 5% 6% 30%	1786
	21st to end	Day	15% 11% 4 8% 5% 4 5% 3 4 7% 35%	1600
December	1st to 10th	Day	8% 9% 5% 12% 5% 8% 5% 4 6% 7% 30%	2610
	11th to 20th	Day	13% 16% 5% 11% 7% 4 4 4 5% 6% 26%	3008
	21st to end	Day	19% 20% 8% 13% 6% 5% 4 3 4 4 15%	3982
January	1st to 10th	Night	8% 10% 6% 14% 8% 5% 4 3 4 5% 34%	6348
	11th to 20th	Night	10% 12% 6% 15% 6% 5% 6% 5% 7% 6% 22%	6360
	21st to end	Night	11% 16% 7% 11% 5% 4 4 3 4 6% 29%	6789
February	1st to 10th	Night	9% 14% 5% 11% 6% 5% 4 3 5% 5% 33%	2600
	11th to 20th	Night	10% 10% 4 8% 6% 6% 4 4 4 6% 39%	2448
	21st to end	Night	16% 10% 4 8% 5% 4 5% 4 4 6% 35%	1980
March	1st to 10th	Night	14% 8% 2 7% 4 4 4 3 5% 5% 44%	5373
	11th to 20th	Night	10% 8% 3 6% 4 4 5% 4 6% 7% 43%	5123
	21st to end	Night	22% 12% 3 10% 4 4 3 3 4 5% 30%	5068
April	1st to 10th	Night	19% 11% 3 6% 4 4 4 4 4 5% 36%	2000
	11th to 20th	Night	19% 10% 3 5% 5% 3 4 4 6% 5% 37%	1975
	21st to end	Night	17% 8% 2 7% 4 4 4 4 6% 7% 38%	1940
May	1st to 10th	Night	16% 7% 3 6% 4 4 4 4 5% 7% 41%	3689
	11th to 20th	Night	9% 5% 2 6% 4 4 4 4 6% 8% 48%	3596
	21st to end	Night	16% 8% 3 8% 6% 4 4 3 5% 7% 37%	3942
June	1st to 10th	Night	9% 5% 3 7% 5% 3 4 5% 6% 8% 46%	1600
	11th to 20th	Night	10% 6% 3 7% 5% 4 4 3 5% 8% 44%	1600
	21st to end	Night	10% 6% 2 6% 4 4 5% 4 6% 7% 47%	1600
July	1st to 10th	Night	12% 6% 2 5% 3 4 5% 4 6% 10% 44%	3618
	11th to 20th	Night	7% 5% 1 5% 4 4 4 3 6% 8% 53%	3617
	21st to end	Night	6% 5% 2 6% 4 4 5% 4 6% 8% 49%	3947
August	1st to 10th	Night	11% 4 2 5% 5% 4 5% 4 6% 8% 47%	2662
	11th to 20th	Night	11% 6% 3 5% 4 4 4 3 6% 6% 48%	3039
	21st to end	Night	9% 5% 2 5% 4 4 4 4 5% 7% 50%	3586
September	1st to 10th	Night	10% 5% 2 5% 3 3 4 3 5% 6% 54%	2000
	11th to 20th	Night	13% 5% 2 5% 3 4 4 3 5% 7% 51%	2020
	21st to end	Night	11% 7% 2 5% 4 4 4 3 5% 6% 50%	2200
October	1st to 10th	Night	11% 6% 2 7% 3 4 3 3 5% 6% 50%	2395
	11th to 20th	Night	17% 7% 2 6% 4 5% 4 3 5% 7% 41%	2400
	21st to end	Night	17% 10% 3 7% 4 4 5% 3 5% 6% 35%	2815
November	1st to 10th	Night	15% 10% 3 7% 4 3 4 3 5% 5% 43%	2666
	11th to 20th	Night	13% 13% 4 9% 5% 4 5% 3 4 6% 34%	2767
	21st to end	Night	16% 11% 4 9% 5% 4 4 3 4 6% 35%	2800
December	1st to 10th	Night	7% 9% 4 9% 5% 6% 5% 5% 7% 8% 36%	4580
	11th to 20th	Night	13% 18% 7% 12% 6% 5% 4 3 4 5% 24%	5199
	21st to end	Night	20% 18% 7% 12% 5% 4 4 3 4 5% 19%	6996

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	61% 3 12% 17% 7%	3618
	11th to 20th	Day	55% 4 10% 14% 16% 1	3620
	21st to end	Day	52% 5% 12% 19% 12%	4184
February	1st to 10th	Day	52% 5% 13% 18% 10% 2	1800
	11th to 20th	Day	47% 5% 12% 23% 11% 2	1848
	21st to end	Day	55% 4 10% 19% 10% 2	1650
March	1st to 10th	Day	39% 3 11% 29% 13% 5%	4480
	11th to 20th	Day	39% 3 14% 28% 14% 2	4878
	21st to end	Day	57% 4 9% 16% 13% 1	5849
April	1st to 10th	Day	54% 5% 9% 20% 11% 1	2400
	11th to 20th	Day	52% 5% 9% 21% 12% 2	2374
	21st to end	Day	52% 6% 11% 18% 12% 1	2460
May	1st to 10th	Day	40% 4 10% 25% 17% 3	6255
	11th to 20th	Day	41% 6% 10% 26% 17% 1	6323
	21st to end	Day	43% 6% 9% 27% 14% 1	6949
June	1st to 10th	Day	38% 7% 11% 27% 15% 2	2800
	11th to 20th	Day	42% 7% 11% 24% 15% 1	2800
	21st to end	Day	39% 8% 14% 23% 14% 2	2800
July	1st to 10th	Day	38% 6% 15% 32% 8% 1	6291
	11th to 20th	Day	30% 7% 15% 33% 15% 1	6285
	21st to end	Day	40% 6% 13% 23% 17% 2	6728
August	1st to 10th	Day	39% 7% 15% 26% 12% 1	4466
	11th to 20th	Day	36% 5% 13% 33% 13% 1	4052
	21st to end	Day	38% 5% 13% 31% 13% 1	4311
September	1st to 10th	Day	38% 6% 12% 27% 17% 1	2400
	11th to 20th	Day	36% 6% 15% 31% 12% 1	2360
	21st to end	Day	39% 6% 11% 30% 14% 1	2200
October	1st to 10th	Day	25% 3 6% 41% 24% 1	3260
	11th to 20th	Day	39% 4 9% 29% 17% 2	3240
	21st to end	Day	43% 3 12% 29% 13% 1	3250
November	1st to 10th	Day	42% 5% 13% 23% 15% 2	1852
	11th to 20th	Day	53% 4 12% 20% 9% 1	1786
	21st to end	Day	51% 5% 15% 22% 8% 1	1600
December	1st to 10th	Day	52% 7% 15% 19% 6% 1	2610
	11th to 20th	Day	59% 3 10% 18% 8% 2	3005
	21st to end	Day	73% 4 7% 10% 6%	3982
January	1st to 10th	Night	53% 4 10% 22% 11% 1	6348
	11th to 20th	Night	60% 5% 8% 12% 16% 1	6360
	21st to end	Night	56% 4 10% 17% 12%	6789
February	1st to 10th	Night	53% 4 11% 20% 10% 2	2600
	11th to 20th	Night	46% 5% 12% 25% 12% 2	2449
	21st to end	Night	50% 4 12% 21% 12% 2	1980
March	1st to 10th	Night	42% 4 13% 24% 10% 7%	5373
	11th to 20th	Night	39% 4 14% 27% 14% 4	5123
	21st to end	Night	58% 3 8% 17% 11% 1	5068
April	1st to 10th	Night	49% 5% 9% 19% 16% 2	2000
	11th to 20th	Night	47% 5% 8% 21% 16% 3	1975
	21st to end	Night	44% 5% 12% 22% 14% 2	1940
May	1st to 10th	Night	41% 6% 11% 19% 19% 2	3714
	11th to 20th	Night	32% 7% 15% 23% 21% 2	3597
	21st to end	Night	47% 6% 8% 24% 14% 2	3950
June	1st to 10th	Night	34% 8% 12% 28% 17% 1	1600
	11th to 20th	Night	38% 5% 11% 25% 20% 2	1600
	21st to end	Night	33% 9% 13% 26% 14% 4	1600
July	1st to 10th	Night	36% 4 15% 31% 11% 2	3580
	11th to 20th	Night	30% 5% 14% 32% 17% 2	3572
	21st to end	Night	32% 6% 15% 25% 19% 3	3832
August	1st to 10th	Night	35% 7% 13% 25% 19% 2	2653
	11th to 20th	Night	36% 6% 13% 31% 14% 1	3066
	21st to end	Night	33% 5% 15% 26% 19% 2	3586
September	1st to 10th	Night	31% 6% 10% 30% 22% 1	2000
	11th to 20th	Night	33% 5% 12% 34% 14% 1	2020
	21st to end	Night	34% 7% 9% 31% 19% 1	2200
October	1st to 10th	Night	28% 3 5% 35% 27% 2	3915
	11th to 20th	Night	32% 4 11% 30% 22% 2	3899
	21st to end	Night	45% 5% 9% 25% 16%	4540
November	1st to 10th	Night	44% 5% 10% 25% 15% 2	2666
	11th to 20th	Night	52% 4 13% 19% 10% 2	2767
	21st to end	Night	51% 4 13% 23% 7% 1	2800
December	1st to 10th	Night	45% 6% 18% 24% 7% 1	4580
	11th to 20th	Night	63% 4 9% 15% 8% 1	5201
	21st to end	Night	68% 4 8% 13% 7%	6996

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	16% 12% 33% 25% 11% 3	300
	11th to 20th	Day	20% 13% 39% 21% 6% 2	300
	21st to end	Day	22% 14% 34% 22% 6%	330
February	1st to 10th	Day	15% 17% 40% 20% 6% 1	300
	11th to 20th	Day	18% 20% 37% 18% 6% 1	300
	21st to end	Day	20% 16% 43% 18% 2	249
March	1st to 10th	Day	19% 15% 41% 17% 8%	300
	11th to 20th	Day	23% 15% 34% 17% 6% 4	300
	21st to end	Day	29% 13% 27% 20% 9% 1	495
April	1st to 10th	Day	27% 15% 34% 19% 5%	500
	11th to 20th	Day	26% 17% 30% 17% 9% 2	492
	21st to end	Day	25% 14% 27% 21% 10% 3	500
May	1st to 10th	Day	18% 12% 31% 28% 9% 2	653
	11th to 20th	Day	20% 13% 27% 24% 12% 4	725
	21st to end	Day	19% 9% 27% 28% 12% 6%	789
June	1st to 10th	Day	13% 9% 28% 28% 18% 3	730
	11th to 20th	Day	16% 7% 32% 25% 15% 4	730
	21st to end	Day	15% 8% 28% 30% 15% 4	730
July	1st to 10th	Day	9% 4 20% 30% 26% 11%	721
	11th to 20th	Day	9% 6% 25% 31% 20% 9%	718
	21st to end	Day	9% 3 25% 35% 21% 7%	794
August	1st to 10th	Day	9% 4 21% 32% 24% 9%	726
	11th to 20th	Day	13% 5% 18% 30% 23% 11%	622
	21st to end	Day	13% 7% 21% 30% 22% 8%	583
September	1st to 10th	Day	16% 8% 21% 27% 18% 9%	530
	11th to 20th	Day	12% 4 22% 32% 23% 6%	530
	21st to end	Day	18% 9% 23% 26% 19% 5%	530
October	1st to 10th	Day	16% 11% 29% 27% 15% 2	330
	11th to 20th	Day	25% 18% 32% 15% 9% 1	330
	21st to end	Day	25% 13% 32% 22% 6% 1	363
November	1st to 10th	Day	16% 15% 40% 20% 8% 1	330
	11th to 20th	Day	18% 19% 38% 19% 6% 1	321
	21st to end	Day	24% 13% 42% 15% 6%	300
December	1st to 10th	Day	17% 14% 40% 21% 7% 1	300
	11th to 20th	Day	22% 14% 36% 21% 7% 1	300
	21st to end	Day	28% 14% 36% 18% 4 1	330
January	1st to 10th	Night	16% 12% 36% 27% 8% 1	1000
	11th to 20th	Night	20% 10% 36% 28% 5% 1	1000
	21st to end	Night	20% 11% 35% 27% 6%	1100
February	1st to 10th	Night	17% 12% 39% 26% 6%	1000
	11th to 20th	Night	16% 14% 37% 27% 5%	894
	21st to end	Night	20% 13% 38% 25% 4	660
March	1st to 10th	Night	20% 11% 35% 26% 7% 1	800
	11th to 20th	Night	20% 11% 35% 28% 7%	665
	21st to end	Night	25% 8% 27% 31% 7% 2	660
April	1st to 10th	Night	23% 12% 35% 25% 5%	600
	11th to 20th	Night	25% 10% 29% 30% 5% 1	593
	21st to end	Night	22% 10% 31% 30% 8% 1	600
May	1st to 10th	Night	23% 8% 32% 31% 6% 1	600
	11th to 20th	Night	16% 8% 30% 36% 9% 1	591
	21st to end	Night	15% 5% 26% 37% 15% 1	650
June	1st to 10th	Night	11% 6% 29% 42% 12%	600
	11th to 20th	Night	13% 4 31% 40% 12% 1	600
	21st to end	Night	12% 4 27% 40% 16% 1	600
July	1st to 10th	Night	9% 2 15% 44% 28% 2	585
	11th to 20th	Night	9% 4 20% 43% 22% 2	582
	21st to end	Night	7% 2 17% 48% 23% 3	657
August	1st to 10th	Night	8% 2 19% 43% 27% 2	597
	11th to 20th	Night	12% 3 15% 39% 28% 4	594
	21st to end	Night	11% 2 19% 43% 22% 3	660
September	1st to 10th	Night	11% 4 24% 38% 19% 5%	600
	11th to 20th	Night	15% 4 21% 33% 23% 4	620
	21st to end	Night	15% 7% 22% 37% 17% 1	800
October	1st to 10th	Night	15% 8% 26% 32% 18% 1	800
	11th to 20th	Night	19% 11% 33% 29% 8%	800
	21st to end	Night	22% 10% 30% 29% 8% 1	880
November	1st to 10th	Night	18% 10% 36% 26% 9%	820
	11th to 20th	Night	19% 14% 39% 22% 5%	904
	21st to end	Night	21% 13% 35% 24% 6%	1000
December	1st to 10th	Night	19% 12% 40% 23% 6% 1	1000
	11th to 20th	Night	23% 13% 33% 25% 6% 1	1000
	21st to end	Night	27% 12% 31% 24% 5%	1100

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