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 2024-11-10.

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% 9% 3 8% 5% 4 5% 4 5% 7% 37%	101581
	Night	13% 10% 4 8% 5% 4 4 3 5% 6% 38%	97651
Transparency	Day	44% 5% 12% 25% 13% 1	105098
	Night	45% 5% 11% 23% 14% 2	102274
Seeing	Day	17% 10% 29% 25% 14% 4	15966
	Night	18% 9% 30% 31% 11% 1	25820

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	10% 16% 9% 15% 7% 5% 3 3 3 5% 24%	2527
	11th to 20th	Day	10% 14% 4 14% 6% 5% 8% 8% 7% 5% 19%	2530
	21st to end	Day	15% 18% 8% 13% 5% 3 3 3 4 5% 22%	2915
February	1st to 10th	Day	9% 13% 5% 9% 5% 5% 6% 4 5% 8% 32%	1710
	11th to 20th	Day	11% 12% 4 8% 5% 4 4 3 6% 7% 35%	1744
	21st to end	Day	13% 11% 4 10% 6% 6% 5% 4 5% 5% 31%	1570
March	1st to 10th	Day	15% 8% 3 5% 4 3 3 3 4 6% 47%	3120
	11th to 20th	Day	10% 7% 3 8% 5% 4 4 3 5% 6% 44%	3406
	21st to end	Day	23% 12% 3 9% 4 4 3 2 4 6% 29%	4087
April	1st to 10th	Day	17% 12% 3 9% 5% 4 4 4 4 5% 32%	2280
	11th to 20th	Day	19% 10% 3 8% 5% 4 4 3 5% 6% 32%	2254
	21st to end	Day	19% 10% 3 8% 5% 5% 4 3 6% 5% 32%	2337
May	1st to 10th	Day	15% 8% 3 7% 4 4 5% 4 5% 6% 38%	4326
	11th to 20th	Day	14% 6% 3 6% 4 4 4 3 5% 7% 44%	4399
	21st to end	Day	16% 6% 2 7% 5% 4 5% 4 5% 8% 37%	4824
June	1st to 10th	Day	12% 6% 2 6% 4 5% 5% 4 6% 7% 44%	2660
	11th to 20th	Day	12% 5% 2 7% 6% 4 5% 4 7% 8% 40%	2660
	21st to end	Day	13% 5% 2 6% 5% 4 5% 4 7% 9% 40%	2660
July	1st to 10th	Day	11% 6% 2 7% 5% 6% 6% 5% 8% 9% 35%	4440
	11th to 20th	Day	5% 3 1 4 4 5% 5% 5% 8% 11% 49%	4440
	21st to end	Day	9% 6% 2 7% 6% 4 5% 4 7% 8% 42%	4813
August	1st to 10th	Day	9% 6% 3 7% 5% 5% 6% 5% 7% 8% 40%	4343
	11th to 20th	Day	12% 6% 2 5% 4 4 5% 4 5% 8% 46%	3888
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 6% 9% 42%	4180
September	1st to 10th	Day	13% 5% 3 6% 4 4 4 4 5% 7% 45%	2280
	11th to 20th	Day	12% 5% 2 6% 4 4 3 4 5% 7% 47%	2242
	21st to end	Day	14% 8% 2 5% 4 4 4 4 6% 7% 44%	2090
October	1st to 10th	Day	13% 6% 2 6% 4 4 5% 3 4 7% 45%	1900
	11th to 20th	Day	18% 10% 3 6% 4 4 5% 4 5% 8% 34%	1900
	21st to end	Day	16% 11% 3 8% 5% 4 3 3 4 5% 39%	1924
November	1st to 10th	Day	13% 9% 3 7% 4 4 4 3 4 7% 42%	1753
	11th to 20th	Day	14% 13% 5% 10% 5% 5% 4 4 5% 6% 29%	1638
	21st to end	Day	14% 11% 4 7% 5% 4 5% 3 4 7% 36%	1520
December	1st to 10th	Day	13% 13% 5% 11% 6% 5% 5% 4 4 5% 29%	1520
	11th to 20th	Day	13% 15% 5% 9% 6% 4 4 4 5% 6% 28%	1918
	21st to end	Day	21% 19% 7% 12% 5% 5% 3 3 4 4 16%	2783
January	1st to 10th	Night	11% 13% 8% 13% 6% 5% 4 2 3 4 32%	4428
	11th to 20th	Night	11% 13% 6% 15% 6% 5% 5% 5% 7% 5% 20%	4440
	21st to end	Night	15% 20% 8% 11% 5% 4 3 3 4 5% 23%	4747
February	1st to 10th	Night	8% 15% 6% 11% 6% 5% 4 3 5% 5% 34%	2470
	11th to 20th	Night	10% 10% 3 8% 6% 6% 4 4 5% 6% 40%	2323
	21st to end	Night	16% 10% 4 8% 5% 4 4 4 4 6% 35%	1884
March	1st to 10th	Night	14% 9% 2 6% 3 4 4 3 5% 5% 43%	3733
	11th to 20th	Night	10% 8% 2 7% 5% 4 5% 4 6% 7% 42%	3581
	21st to end	Night	23% 11% 3 10% 5% 4 3 3 4 4 30%	3544
April	1st to 10th	Night	19% 12% 3 6% 4 3 4 4 4 5% 35%	1900
	11th to 20th	Night	19% 9% 3 5% 5% 3 4 4 6% 5% 37%	1875
	21st to end	Night	17% 8% 2 7% 4 4 4 4 6% 8% 37%	1843
May	1st to 10th	Night	16% 8% 3 7% 4 4 5% 4 7% 8% 34%	2573
	11th to 20th	Night	11% 6% 3 6% 4 4 4 3 6% 8% 47%	2506
	21st to end	Night	13% 7% 2 7% 5% 4 4 3 5% 7% 41%	2743
June	1st to 10th	Night	8% 5% 3 7% 5% 4 4 5% 6% 8% 45%	1520
	11th to 20th	Night	10% 6% 3 7% 5% 3 4 3 5% 8% 45%	1520
	21st to end	Night	9% 6% 2 6% 4 4 4 4 6% 7% 48%	1520
July	1st to 10th	Night	13% 6% 2 5% 3 4 4 4 6% 9% 44%	2530
	11th to 20th	Night	4 4 1 5% 4 4 4 4 6% 8% 55%	2530
	21st to end	Night	6% 5% 2 6% 4 4 5% 5% 6% 9% 48%	2750
August	1st to 10th	Night	11% 4 2 5% 5% 4 4 4 6% 8% 47%	2580
	11th to 20th	Night	10% 6% 2 5% 4 4 4 3 6% 7% 49%	2944
	21st to end	Night	9% 5% 2 5% 4 4 4 4 6% 7% 49%	3476
September	1st to 10th	Night	10% 5% 2 5% 3 3 4 3 5% 6% 54%	1900
	11th to 20th	Night	14% 5% 2 5% 2 3 3 3 4 7% 51%	1919
	21st to end	Night	11% 7% 2 5% 4 4 4 3 5% 6% 49%	2090
October	1st to 10th	Night	12% 6% 3 6% 3 3 3 2 5% 6% 50%	2275
	11th to 20th	Night	16% 7% 2 5% 4 5% 4 3 6% 7% 41%	2280
	21st to end	Night	17% 10% 3 7% 4 4 5% 3 5% 6% 34%	2674
November	1st to 10th	Night	15% 9% 3 7% 4 3 4 3 5% 5% 43%	2530
	11th to 20th	Night	14% 12% 4 9% 5% 4 5% 3 4 6% 33%	2540
	21st to end	Night	16% 11% 4 8% 5% 4 4 3 4 6% 36%	2660
December	1st to 10th	Night	12% 14% 5% 9% 5% 4 4 4 6% 6% 29%	2660
	11th to 20th	Night	13% 15% 6% 11% 7% 5% 4 3 4 5% 27%	3279
	21st to end	Night	21% 17% 6% 11% 5% 4 4 3 4 5% 20%	4884

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	64% 4 13% 14% 6%	2528
	11th to 20th	Day	62% 5% 12% 16% 5%	2530
	21st to end	Day	65% 5% 10% 15% 5%	2915
February	1st to 10th	Day	52% 5% 13% 19% 10% 1	1710
	11th to 20th	Day	47% 5% 13% 23% 11% 2	1753
	21st to end	Day	55% 4 10% 18% 10% 3	1570
March	1st to 10th	Day	39% 4 10% 29% 15% 3	3120
	11th to 20th	Day	40% 4 12% 30% 13% 2	3406
	21st to end	Day	57% 5% 10% 18% 9% 1	4087
April	1st to 10th	Day	54% 5% 9% 19% 11% 1	2280
	11th to 20th	Day	52% 5% 8% 21% 12% 2	2254
	21st to end	Day	51% 6% 11% 18% 12% 1	2337
May	1st to 10th	Day	44% 6% 11% 21% 17% 1	4366
	11th to 20th	Day	38% 6% 9% 31% 14% 1	4403
	21st to end	Day	43% 6% 10% 22% 16% 2	4837
June	1st to 10th	Day	37% 7% 12% 27% 15% 2	2660
	11th to 20th	Day	41% 7% 11% 24% 16% 1	2660
	21st to end	Day	38% 8% 14% 24% 14% 2	2660
July	1st to 10th	Day	43% 7% 14% 27% 8% 2	4371
	11th to 20th	Day	25% 9% 15% 32% 17% 1	4365
	21st to end	Day	39% 7% 13% 22% 18% 2	4662
August	1st to 10th	Day	39% 7% 14% 26% 12% 1	4328
	11th to 20th	Day	36% 5% 12% 33% 13% 1	3927
	21st to end	Day	38% 5% 13% 30% 12% 1	4179
September	1st to 10th	Day	38% 6% 12% 26% 18% 1	2280
	11th to 20th	Day	36% 6% 14% 31% 13% 1	2242
	21st to end	Day	39% 6% 12% 29% 14% 1	2090
October	1st to 10th	Day	25% 3 6% 42% 23% 1	3160
	11th to 20th	Day	39% 4 9% 29% 18% 2	3140
	21st to end	Day	43% 3 12% 29% 12% 1	3149
November	1st to 10th	Day	42% 5% 14% 23% 15% 2	1753
	11th to 20th	Day	55% 4 12% 19% 9% 1	1638
	21st to end	Day	49% 5% 15% 22% 8% 1	1520
December	1st to 10th	Day	56% 6% 13% 17% 7% 1	1520
	11th to 20th	Day	55% 4 15% 15% 7% 3	1915
	21st to end	Day	73% 4 8% 10% 5%	2783
January	1st to 10th	Night	57% 5% 10% 20% 8% 1	4428
	11th to 20th	Night	62% 5% 10% 14% 9%	4440
	21st to end	Night	65% 4 9% 16% 6%	4747
February	1st to 10th	Night	53% 4 11% 21% 10% 2	2470
	11th to 20th	Night	45% 5% 12% 25% 12% 2	2324
	21st to end	Night	50% 4 11% 21% 11% 2	1884
March	1st to 10th	Night	41% 5% 12% 27% 9% 5%	3733
	11th to 20th	Night	39% 4 10% 26% 17% 3	3581
	21st to end	Night	58% 4 7% 20% 10% 2	3544
April	1st to 10th	Night	50% 6% 9% 19% 15% 2	1900
	11th to 20th	Night	47% 5% 8% 20% 17% 4	1875
	21st to end	Night	44% 5% 12% 22% 14% 2	1843
May	1st to 10th	Night	44% 8% 14% 15% 18%	2598
	11th to 20th	Night	34% 8% 11% 26% 20% 1	2507
	21st to end	Night	41% 7% 10% 21% 19% 2	2751
June	1st to 10th	Night	34% 8% 12% 28% 17% 1	1520
	11th to 20th	Night	36% 5% 10% 25% 21% 2	1520
	21st to end	Night	33% 9% 14% 27% 14% 4	1520
July	1st to 10th	Night	38% 4 17% 24% 13% 3	2490
	11th to 20th	Night	25% 6% 15% 33% 19% 2	2482
	21st to end	Night	31% 7% 15% 23% 20% 4	2662
August	1st to 10th	Night	35% 7% 13% 25% 18% 2	2571
	11th to 20th	Night	35% 6% 13% 31% 15% 1	2971
	21st to end	Night	33% 5% 15% 26% 19% 3	3476
September	1st to 10th	Night	31% 6% 10% 29% 22% 2	1900
	11th to 20th	Night	33% 5% 12% 34% 15% 2	1919
	21st to end	Night	34% 7% 9% 30% 18% 1	2090
October	1st to 10th	Night	27% 3 6% 35% 27% 2	3795
	11th to 20th	Night	32% 4 11% 30% 22% 1	3779
	21st to end	Night	46% 5% 9% 25% 15%	4399
November	1st to 10th	Night	43% 5% 10% 24% 15% 2	2530
	11th to 20th	Night	52% 4 14% 18% 11% 1	2540
	21st to end	Night	50% 4 14% 24% 7% 1	2660
December	1st to 10th	Night	53% 6% 13% 21% 7%	2660
	11th to 20th	Night	59% 5% 11% 17% 7% 1	3281
	21st to end	Night	66% 5% 9% 14% 5%	4884

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	18% 11% 33% 24% 11% 3	270
	11th to 20th	Day	21% 11% 39% 21% 6% 2	270
	21st to end	Day	23% 13% 33% 24% 7%	297
February	1st to 10th	Day	14% 17% 39% 21% 7% 1	270
	11th to 20th	Day	18% 20% 36% 17% 7% 1	270
	21st to end	Day	20% 16% 42% 20% 2	225
March	1st to 10th	Day	19% 14% 40% 18% 8%	270
	11th to 20th	Day	23% 14% 35% 17% 7% 4	270
	21st to end	Day	28% 14% 27% 20% 10% 2	454
April	1st to 10th	Day	28% 14% 34% 19% 5%	460
	11th to 20th	Day	26% 16% 31% 17% 10% 2	452
	21st to end	Day	24% 14% 26% 22% 11% 3	460
May	1st to 10th	Day	18% 13% 32% 26% 9% 2	605
	11th to 20th	Day	20% 13% 28% 24% 11% 3	675
	21st to end	Day	19% 9% 25% 28% 12% 6%	734
June	1st to 10th	Day	13% 9% 27% 28% 18% 4	680
	11th to 20th	Day	16% 8% 31% 26% 15% 4	680
	21st to end	Day	14% 8% 28% 30% 16% 5%	680
July	1st to 10th	Day	10% 4 20% 30% 26% 11%	671
	11th to 20th	Day	8% 6% 26% 31% 20% 9%	668
	21st to end	Day	9% 3 25% 33% 22% 7%	739
August	1st to 10th	Day	9% 4 22% 33% 24% 8%	676
	11th to 20th	Day	13% 6% 18% 30% 23% 12%	577
	21st to end	Day	13% 6% 21% 29% 22% 8%	539
September	1st to 10th	Day	17% 8% 21% 27% 19% 8%	490
	11th to 20th	Day	13% 4 23% 31% 23% 6%	490
	21st to end	Day	19% 9% 24% 26% 19% 3	490
October	1st to 10th	Day	17% 12% 29% 25% 15% 2	300
	11th to 20th	Day	25% 19% 30% 16% 9% 1	300
	21st to end	Day	25% 15% 34% 20% 5% 1	330
November	1st to 10th	Day	17% 12% 39% 22% 8% 1	297
	11th to 20th	Day	19% 18% 38% 19% 6% 1	270
	21st to end	Day	21% 12% 43% 16% 7%	270
December	1st to 10th	Day	18% 14% 38% 21% 7% 1	270
	11th to 20th	Day	21% 13% 36% 23% 7% 1	270
	21st to end	Day	29% 15% 37% 16% 3	297
January	1st to 10th	Night	17% 12% 36% 27% 8% 1	950
	11th to 20th	Night	20% 10% 35% 28% 5% 1	950
	21st to end	Night	21% 10% 34% 28% 7%	1045
February	1st to 10th	Night	16% 12% 39% 26% 6%	950
	11th to 20th	Night	16% 14% 37% 27% 6%	849
	21st to end	Night	21% 13% 37% 24% 4	628
March	1st to 10th	Night	20% 11% 34% 26% 8% 1	760
	11th to 20th	Night	20% 10% 34% 28% 8%	632
	21st to end	Night	25% 8% 26% 31% 7% 2	627
April	1st to 10th	Night	24% 12% 36% 24% 5%	570
	11th to 20th	Night	25% 10% 29% 31% 5% 1	563
	21st to end	Night	21% 10% 32% 29% 8% 1	570
May	1st to 10th	Night	23% 8% 32% 30% 6% 1	570
	11th to 20th	Night	16% 8% 30% 36% 9% 1	561
	21st to end	Night	15% 5% 27% 37% 16% 1	617
June	1st to 10th	Night	11% 6% 29% 41% 12%	570
	11th to 20th	Night	13% 4 30% 40% 12% 1	570
	21st to end	Night	11% 4 26% 41% 17% 1	570
July	1st to 10th	Night	9% 2 16% 43% 29% 2	555
	11th to 20th	Night	8% 4 20% 43% 22% 2	552
	21st to end	Night	7% 2 16% 47% 24% 3	624
August	1st to 10th	Night	8% 2 19% 42% 27% 2	567
	11th to 20th	Night	11% 3 15% 39% 28% 4	564
	21st to end	Night	11% 2 19% 44% 21% 3	627
September	1st to 10th	Night	11% 4 23% 38% 19% 5%	570
	11th to 20th	Night	16% 4 21% 33% 23% 4	589
	21st to end	Night	16% 8% 23% 36% 16% 1	760
October	1st to 10th	Night	15% 8% 26% 32% 18% 1	760
	11th to 20th	Night	18% 11% 33% 29% 8%	760
	21st to end	Night	22% 11% 30% 29% 7% 1	836
November	1st to 10th	Night	18% 10% 36% 26% 9% 1	779
	11th to 20th	Night	19% 14% 40% 22% 5%	830
	21st to end	Night	21% 14% 35% 24% 7%	950
December	1st to 10th	Night	19% 11% 41% 22% 6% 1	950
	11th to 20th	Night	22% 12% 33% 25% 6% 1	950
	21st to end	Night	27% 13% 32% 24% 4	1045

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