Dryden
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dryden 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	13% 10% 4 10% 5% 5% 5% 4 6% 7% 32%	127143
	Night	12% 12% 5% 11% 6% 6% 5% 4 5% 6% 28%	122864
Transparency	Day	51% 4 13% 22% 9% 1	130555
	Night	57% 3 11% 20% 8% 1	127541
Seeing	Day	19% 13% 34% 23% 9% 2	17613
	Night	19% 11% 31% 28% 10% 1	27451

By Month

Percentage Cloud Cover NE (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	6% 13% 7% 18% 8% 9% 7% 4 5% 4 19%	3178
	11th to 20th	Day	5% 12% 6% 19% 8% 7% 8% 9% 9% 5% 12%	3152
	21st to end	Day	6% 16% 10% 24% 7% 5% 6% 4 4 4 14%	3903
February	1st to 10th	Day	8% 13% 5% 12% 8% 6% 10% 6% 7% 5% 20%	1800
	11th to 20th	Day	8% 11% 5% 10% 8% 7% 6% 7% 6% 7% 25%	1754
	21st to end	Day	6% 13% 5% 16% 9% 5% 5% 5% 6% 6% 25%	1499
March	1st to 10th	Day	8% 12% 4 10% 5% 4 4 4 5% 5% 39%	4652
	11th to 20th	Day	10% 10% 5% 13% 8% 6% 5% 3 4 6% 29%	4980
	21st to end	Day	12% 13% 6% 11% 6% 5% 5% 3 5% 5% 29%	5637
April	1st to 10th	Day	14% 12% 4 9% 4 4 4 3 5% 6% 34%	2400
	11th to 20th	Day	14% 13% 4 8% 5% 4 5% 3 5% 6% 32%	2477
	21st to end	Day	15% 11% 4 8% 4 4 5% 4 5% 6% 34%	2680
May	1st to 10th	Day	14% 9% 2 6% 4 4 4 3 5% 7% 43%	6348
	11th to 20th	Day	14% 9% 3 7% 5% 5% 5% 4 6% 7% 36%	6319
	21st to end	Day	16% 7% 2 5% 4 4 4 4 5% 7% 43%	7156
June	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 5% 7% 9% 31%	3000
	11th to 20th	Day	16% 8% 2 7% 5% 5% 4 5% 6% 8% 33%	3000
	21st to end	Day	16% 7% 2 7% 6% 5% 6% 5% 7% 9% 31%	3000
July	1st to 10th	Day	9% 6% 2 6% 5% 5% 5% 4 7% 9% 42%	6799
	11th to 20th	Day	13% 6% 2 5% 4 5% 4 4 6% 9% 40%	6800
	21st to end	Day	13% 7% 3 7% 4 5% 5% 4 6% 9% 37%	7105
August	1st to 10th	Day	16% 6% 2 7% 4 4 6% 4 6% 8% 36%	4563
	11th to 20th	Day	11% 4 2 6% 4 5% 5% 5% 7% 10% 42%	4180
	21st to end	Day	15% 7% 3 6% 4 4 4 4 6% 9% 38%	4577
September	1st to 10th	Day	15% 8% 3 7% 4 4 4 4 6% 7% 38%	2400
	11th to 20th	Day	18% 10% 3 7% 5% 4 5% 4 6% 8% 30%	2400
	21st to end	Day	21% 10% 3 6% 4 4 4 3 4 7% 36%	2090
October	1st to 10th	Day	18% 15% 4 8% 5% 5% 5% 3 4 6% 27%	2000
	11th to 20th	Day	18% 19% 4 9% 5% 4 3 4 4 5% 23%	1980
	21st to end	Day	20% 20% 5% 11% 5% 4 4 3 4 6% 19%	1891
November	1st to 10th	Day	16% 18% 7% 11% 6% 3 4 4 5% 5% 20%	1567
	11th to 20th	Day	16% 20% 7% 14% 8% 6% 4 3 4 4 15%	1600
	21st to end	Day	14% 20% 6% 21% 8% 3 3 4 4 4 13%	1600
December	1st to 10th	Day	13% 17% 6% 20% 12% 8% 4 3 4 4 8%	2519
	11th to 20th	Day	8% 15% 7% 17% 6% 6% 7% 4 5% 5% 21%	2639
	21st to end	Day	15% 19% 8% 16% 7% 7% 5% 3 5% 4 11%	3498
January	1st to 10th	Night	5% 15% 7% 18% 9% 7% 6% 5% 6% 5% 17%	6787
	11th to 20th	Night	6% 11% 5% 19% 10% 11% 10% 7% 8% 3 11%	6753
	21st to end	Night	8% 19% 10% 17% 8% 6% 5% 5% 5% 5% 12%	7073
February	1st to 10th	Night	8% 12% 6% 12% 6% 8% 10% 6% 5% 6% 21%	2600
	11th to 20th	Night	7% 11% 4 9% 6% 7% 8% 7% 6% 6% 28%	2533
	21st to end	Night	6% 15% 6% 12% 7% 6% 7% 5% 7% 6% 24%	2130
March	1st to 10th	Night	10% 13% 4 10% 6% 5% 6% 4 5% 6% 32%	5231
	11th to 20th	Night	8% 13% 5% 12% 9% 8% 6% 4 5% 6% 25%	4979
	21st to end	Night	16% 18% 5% 9% 5% 4 3 3 4 5% 29%	5294
April	1st to 10th	Night	15% 10% 3 8% 5% 4 4 3 5% 6% 38%	2000
	11th to 20th	Night	14% 13% 5% 7% 4 3 4 3 5% 6% 37%	1871
	21st to end	Night	13% 12% 4 7% 6% 5% 5% 3 5% 6% 34%	1720
May	1st to 10th	Night	14% 9% 2 5% 4 3 3 3 4 5% 47%	3613
	11th to 20th	Night	16% 8% 2 6% 4 4 4 4 5% 7% 39%	3596
	21st to end	Night	20% 7% 1 5% 3 4 4 3 5% 7% 41%	3739
June	1st to 10th	Night	19% 8% 2 5% 4 5% 4 4 6% 7% 36%	1400
	11th to 20th	Night	13% 6% 2 5% 5% 4 5% 4 6% 8% 42%	1400
	21st to end	Night	14% 7% 2 5% 4 3 6% 3 6% 9% 40%	1400
July	1st to 10th	Night	10% 5% 2 5% 4 4 4 4 6% 8% 47%	3180
	11th to 20th	Night	10% 5% 3 5% 4 5% 5% 4 7% 9% 44%	3180
	21st to end	Night	11% 6% 2 6% 4 3 4 4 6% 9% 45%	3769
August	1st to 10th	Night	14% 7% 2 5% 4 3 4 4 5% 8% 44%	2597
	11th to 20th	Night	12% 5% 2 5% 3 4 4 4 6% 9% 46%	2875
	21st to end	Night	15% 6% 2 6% 4 4 4 3 5% 8% 42%	3328
September	1st to 10th	Night	14% 6% 3 6% 4 4 3 4 5% 8% 44%	2000
	11th to 20th	Night	19% 7% 2 6% 4 4 5% 3 6% 6% 39%	2000
	21st to end	Night	18% 9% 2 5% 4 4 3 3 4 7% 39%	2288
October	1st to 10th	Night	17% 11% 3 7% 4 5% 5% 4 6% 7% 31%	2400
	11th to 20th	Night	21% 14% 3 8% 5% 3 4 3 4 6% 27%	2377
	21st to end	Night	20% 15% 4 10% 5% 5% 4 3 4 6% 23%	2705
November	1st to 10th	Night	16% 18% 6% 11% 5% 4 4 4 4 5% 23%	2731
	11th to 20th	Night	16% 17% 6% 15% 8% 6% 4 4 4 4 16%	2800
	21st to end	Night	15% 19% 8% 15% 8% 5% 4 3 4 4 15%	2800
December	1st to 10th	Night	10% 20% 9% 23% 11% 7% 5% 3 3 3 8%	4667
	11th to 20th	Night	11% 13% 5% 16% 8% 8% 7% 5% 5% 6% 16%	5568
	21st to end	Night	13% 19% 8% 18% 10% 6% 5% 3 4 3 13%	7480

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	69% 3 6% 12% 7% 3	3179
	11th to 20th	Day	68% 2 7% 20% 3	3180
	21st to end	Day	74% 2 5% 11% 8%	3903
February	1st to 10th	Day	63% 3 10% 13% 9% 1	1800
	11th to 20th	Day	57% 3 10% 22% 7% 1	1763
	21st to end	Day	58% 4 9% 18% 9% 1	1499
March	1st to 10th	Day	47% 3 13% 17% 15% 6%	4654
	11th to 20th	Day	57% 3 9% 20% 11%	4980
	21st to end	Day	55% 4 8% 19% 13% 1	5637
April	1st to 10th	Day	51% 3 8% 22% 15% 2	2400
	11th to 20th	Day	53% 3 10% 21% 12% 1	2477
	21st to end	Day	51% 2 9% 26% 11% 2	2680
May	1st to 10th	Day	41% 4 11% 28% 15% 1	6357
	11th to 20th	Day	46% 3 12% 20% 18% 1	6323
	21st to end	Day	41% 3 8% 28% 17% 3	7156
June	1st to 10th	Day	47% 5% 14% 23% 11%	3000
	11th to 20th	Day	49% 4 12% 22% 12%	3000
	21st to end	Day	47% 4 18% 23% 8%	3000
July	1st to 10th	Day	37% 6% 19% 29% 9%	6726
	11th to 20th	Day	38% 5% 20% 32% 5%	6719
	21st to end	Day	44% 5% 15% 30% 6%	6902
August	1st to 10th	Day	45% 5% 14% 27% 10%	4548
	11th to 20th	Day	35% 6% 25% 30% 4	4219
	21st to end	Day	43% 4 21% 28% 5%	4576
September	1st to 10th	Day	44% 4 15% 30% 7%	2400
	11th to 20th	Day	52% 3 18% 22% 4	2400
	21st to end	Day	49% 4 16% 25% 6%	2090
October	1st to 10th	Day	48% 4 16% 23% 8%	3260
	11th to 20th	Day	55% 3 16% 18% 7%	3220
	21st to end	Day	59% 5% 19% 15% 2	3083
November	1st to 10th	Day	66% 3 13% 11% 5% 1	1567
	11th to 20th	Day	74% 3 9% 11% 4	1600
	21st to end	Day	75% 3 10% 10% 2	1600
December	1st to 10th	Day	81% 2 8% 7% 2	2519
	11th to 20th	Day	66% 3 10% 13% 8%	2640
	21st to end	Day	77% 2 5% 12% 4	3498
January	1st to 10th	Night	69% 2 8% 13% 8%	6787
	11th to 20th	Night	74% 2 7% 13% 4	6800
	21st to end	Night	74% 2 9% 11% 4	7073
February	1st to 10th	Night	62% 3 11% 15% 8% 1	2600
	11th to 20th	Night	54% 4 12% 22% 9%	2534
	21st to end	Night	58% 3 9% 20% 8% 1	2130
March	1st to 10th	Night	53% 3 9% 15% 17% 4	5242
	11th to 20th	Night	61% 2 10% 17% 8% 1	4980
	21st to end	Night	59% 2 6% 18% 14% 1	5294
April	1st to 10th	Night	48% 2 8% 22% 19% 2	2000
	11th to 20th	Night	48% 3 9% 23% 16% 2	1871
	21st to end	Night	51% 3 8% 23% 13% 1	1720
May	1st to 10th	Night	40% 2 7% 32% 18% 1	3620
	11th to 20th	Night	45% 3 13% 24% 14% 1	3597
	21st to end	Night	43% 3 10% 24% 18% 3	3740
June	1st to 10th	Night	47% 3 13% 24% 14%	1400
	11th to 20th	Night	40% 4 13% 27% 16% 1	1400
	21st to end	Night	40% 5% 19% 27% 9% 1	1400
July	1st to 10th	Night	33% 6% 21% 30% 9%	3145
	11th to 20th	Night	35% 5% 21% 33% 7%	3138
	21st to end	Night	37% 5% 18% 32% 8%	3658
August	1st to 10th	Night	38% 4 14% 34% 9% 1	2588
	11th to 20th	Night	34% 6% 22% 32% 6%	2902
	21st to end	Night	42% 4 21% 28% 5%	3328
September	1st to 10th	Night	39% 3 17% 30% 10%	2000
	11th to 20th	Night	46% 4 18% 26% 6%	2000
	21st to end	Night	46% 4 14% 29% 6%	2288
October	1st to 10th	Night	45% 5% 14% 28% 8%	3920
	11th to 20th	Night	52% 3 16% 24% 5%	3876
	21st to end	Night	58% 4 16% 19% 4	4466
November	1st to 10th	Night	66% 3 11% 16% 4	2731
	11th to 20th	Night	73% 3 10% 12% 3	2800
	21st to end	Night	73% 3 11% 8% 3	2800
December	1st to 10th	Night	84% 2 6% 6% 2	4667
	11th to 20th	Night	69% 3 10% 12% 7%	5566
	21st to end	Night	77% 2 5% 13% 3	7480

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	13% 11% 39% 26% 10% 1	300
	11th to 20th	Day	12% 9% 39% 31% 8% 1	300
	21st to end	Day	16% 11% 38% 25% 8% 1	330
February	1st to 10th	Day	17% 15% 44% 16% 6% 2	300
	11th to 20th	Day	13% 19% 35% 21% 8% 3	300
	21st to end	Day	16% 20% 41% 16% 4 2	249
March	1st to 10th	Day	13% 14% 43% 23% 6% 1	300
	11th to 20th	Day	14% 19% 38% 21% 5% 2	300
	21st to end	Day	16% 17% 34% 22% 8% 3	400
April	1st to 10th	Day	20% 13% 33% 23% 8% 2	500
	11th to 20th	Day	24% 13% 31% 23% 8% 1	595
	21st to end	Day	20% 13% 33% 24% 9% 1	700
May	1st to 10th	Day	20% 17% 34% 20% 6% 2	703
	11th to 20th	Day	21% 16% 33% 20% 9% 2	725
	21st to end	Day	23% 11% 31% 24% 10% 1	789
June	1st to 10th	Day	20% 14% 33% 23% 7% 3	730
	11th to 20th	Day	20% 14% 38% 19% 7% 2	730
	21st to end	Day	19% 10% 33% 24% 10% 4	730
July	1st to 10th	Day	11% 11% 36% 23% 14% 6%	721
	11th to 20th	Day	16% 12% 28% 27% 14% 4	718
	21st to end	Day	12% 9% 31% 29% 14% 6%	794
August	1st to 10th	Day	15% 8% 31% 30% 13% 4	726
	11th to 20th	Day	15% 7% 30% 28% 14% 6%	715
	21st to end	Day	18% 11% 28% 27% 14% 1	744
September	1st to 10th	Day	17% 14% 33% 23% 11% 2	530
	11th to 20th	Day	22% 14% 29% 25% 7% 2	530
	21st to end	Day	27% 13% 29% 21% 8% 1	340
October	1st to 10th	Day	27% 13% 31% 24% 5% 1	330
	11th to 20th	Day	27% 15% 35% 18% 5% 1	324
	21st to end	Day	32% 17% 34% 13% 4	330
November	1st to 10th	Day	26% 18% 32% 17% 7% 1	300
	11th to 20th	Day	28% 16% 35% 15% 6%	300
	21st to end	Day	23% 26% 34% 11% 5%	300
December	1st to 10th	Day	18% 14% 39% 23% 5% 2	300
	11th to 20th	Day	17% 16% 42% 22% 4	300
	21st to end	Day	20% 11% 37% 22% 7% 3	330
January	1st to 10th	Night	13% 12% 36% 28% 9% 1	1000
	11th to 20th	Night	17% 10% 38% 29% 6% 1	1000
	21st to end	Night	17% 12% 35% 28% 7% 1	1100
February	1st to 10th	Night	14% 11% 36% 29% 10% 1	1000
	11th to 20th	Night	12% 11% 34% 28% 12% 2	979
	21st to end	Night	13% 13% 35% 26% 11% 3	810
March	1st to 10th	Night	15% 12% 34% 28% 9% 1	747
	11th to 20th	Night	15% 16% 35% 25% 8% 1	600
	21st to end	Night	22% 11% 27% 26% 10% 3	660
April	1st to 10th	Night	20% 10% 30% 31% 10%	600
	11th to 20th	Night	21% 10% 28% 30% 10%	593
	21st to end	Night	19% 11% 26% 32% 13%	600
May	1st to 10th	Night	22% 11% 32% 28% 7% 1	600
	11th to 20th	Night	25% 9% 29% 26% 9% 1	591
	21st to end	Night	23% 8% 27% 31% 10% 1	650
June	1st to 10th	Night	22% 8% 27% 31% 11% 1	600
	11th to 20th	Night	16% 8% 34% 28% 13%	600
	21st to end	Night	17% 7% 23% 35% 16% 1	600
July	1st to 10th	Night	9% 6% 29% 35% 19% 2	585
	11th to 20th	Night	11% 7% 28% 36% 15% 3	582
	21st to end	Night	11% 4 24% 37% 22% 2	657
August	1st to 10th	Night	11% 6% 27% 34% 20% 2	597
	11th to 20th	Night	16% 7% 22% 31% 20% 3	594
	21st to end	Night	18% 7% 28% 31% 16% 1	660
September	1st to 10th	Night	17% 8% 30% 30% 14% 2	600
	11th to 20th	Night	23% 7% 29% 26% 14% 1	600
	21st to end	Night	24% 10% 26% 27% 11% 2	700
October	1st to 10th	Night	26% 12% 30% 26% 5% 1	800
	11th to 20th	Night	29% 13% 26% 23% 7% 1	793
	21st to end	Night	28% 11% 31% 23% 6% 1	878
November	1st to 10th	Night	24% 14% 30% 23% 7% 2	975
	11th to 20th	Night	25% 11% 33% 23% 7% 1	1000
	21st to end	Night	26% 16% 33% 20% 4	1000
December	1st to 10th	Night	16% 14% 38% 24% 7% 2	1000
	11th to 20th	Night	18% 14% 37% 23% 7% 1	1000
	21st to end	Night	22% 11% 32% 25% 10% 2	1100

Page generated at 2025-10-16 00:50:28. Version ac07

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