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 2024-04-10.

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	14% 11% 5% 9% 7% 4 5% 5% 5% 7% 29%	82676
	Night	13% 12% 5% 9% 8% 4 6% 5% 5% 6% 27%	86160
Transparency	Day	55% 3 12% 21% 8% 1	82366
	Night	59% 3 11% 19% 8% 1	86004
Seeing	Day	19% 13% 33% 23% 9% 2	15331
	Night	19% 10% 31% 28% 10% 1	25474

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	7% 12% 8% 17% 9% 9% 6% 5% 6% 5% 17%	2218
	11th to 20th	Day	6% 10% 6% 13% 11% 7% 11% 9% 6% 5% 15%	2220
	21st to end	Day	9% 17% 9% 21% 9% 5% 5% 7% 5% 4 9%	2716
February	1st to 10th	Day	8% 13% 5% 10% 8% 5% 8% 8% 7% 5% 21%	1710
	11th to 20th	Day	8% 11% 5% 9% 9% 5% 6% 8% 6% 7% 25%	1664
	21st to end	Day	7% 11% 5% 14% 10% 4 6% 6% 6% 6% 25%	1426
March	1st to 10th	Day	9% 11% 4 7% 6% 3 5% 5% 5% 6% 39%	3236
	11th to 20th	Day	7% 10% 6% 13% 11% 4 5% 5% 5% 5% 27%	3480
	21st to end	Day	12% 14% 6% 9% 7% 4 5% 4 4 5% 30%	3931
April	1st to 10th	Day	14% 11% 4 9% 5% 3 4 4 5% 6% 35%	2269
	11th to 20th	Day	14% 11% 4 7% 6% 3 5% 4 5% 6% 34%	2226
	21st to end	Day	15% 11% 4 7% 5% 3 5% 4 4 7% 35%	2412
May	1st to 10th	Day	15% 12% 4 7% 7% 3 5% 5% 5% 6% 31%	2520
	11th to 20th	Day	17% 9% 3 7% 6% 4 5% 4 6% 6% 33%	2479
	21st to end	Day	22% 6% 3 5% 5% 2 4 5% 4 7% 36%	2802
June	1st to 10th	Day	17% 7% 3 6% 6% 3 6% 7% 7% 9% 31%	2700
	11th to 20th	Day	17% 8% 3 7% 6% 3 5% 5% 6% 8% 33%	2700
	21st to end	Day	17% 6% 2 6% 6% 4 6% 6% 7% 10% 30%	2700
July	1st to 10th	Day	11% 5% 3 5% 5% 3 5% 5% 6% 9% 44%	2700
	11th to 20th	Day	15% 6% 2 5% 5% 4 4 5% 6% 8% 39%	2700
	21st to end	Day	12% 6% 2 5% 5% 4 5% 6% 6% 9% 40%	2818
August	1st to 10th	Day	11% 6% 2 7% 5% 4 5% 6% 6% 10% 39%	2507
	11th to 20th	Day	12% 5% 2 6% 5% 4 6% 5% 6% 9% 39%	2310
	21st to end	Day	14% 8% 3 5% 5% 3 4 5% 4 8% 40%	2521
September	1st to 10th	Day	16% 8% 3 5% 5% 2 4 5% 6% 7% 38%	2160
	11th to 20th	Day	19% 9% 3 6% 6% 3 5% 6% 6% 8% 31%	2160
	21st to end	Day	23% 11% 3 5% 5% 3 4 4 4 7% 33%	1882
October	1st to 10th	Day	20% 16% 5% 7% 6% 4 5% 4 4 5% 25%	1800
	11th to 20th	Day	19% 18% 5% 8% 6% 3 4 5% 4 5% 22%	1800
	21st to end	Day	21% 19% 6% 10% 6% 3 4 4 4 6% 18%	1793
November	1st to 10th	Day	16% 17% 9% 10% 8% 2 4 5% 5% 5% 20%	1487
	11th to 20th	Day	16% 17% 9% 12% 10% 4 5% 4 4 4 15%	1520
	21st to end	Day	15% 18% 6% 18% 10% 3 3 4 4 5% 14%	1520
December	1st to 10th	Day	9% 14% 8% 16% 12% 6% 6% 6% 5% 5% 13%	1468
	11th to 20th	Day	9% 17% 9% 14% 7% 4 6% 4 3 5% 21%	1679
	21st to end	Day	18% 16% 9% 11% 8% 5% 5% 4 5% 5% 16%	2442
January	1st to 10th	Night	6% 15% 8% 12% 12% 7% 6% 6% 7% 5% 16%	4737
	11th to 20th	Night	7% 11% 6% 13% 14% 10% 11% 8% 5% 4 12%	4750
	21st to end	Night	10% 18% 11% 15% 8% 4 5% 6% 4 4 14%	4949
February	1st to 10th	Night	8% 12% 7% 11% 7% 5% 10% 8% 5% 6% 22%	2470
	11th to 20th	Night	7% 11% 4 8% 7% 5% 8% 8% 6% 6% 29%	2403
	21st to end	Night	6% 12% 7% 11% 8% 4 7% 6% 7% 6% 25%	2027
March	1st to 10th	Night	12% 12% 5% 7% 7% 4 6% 5% 4 6% 32%	3633
	11th to 20th	Night	8% 14% 6% 9% 9% 5% 6% 4 4 5% 29%	3479
	21st to end	Night	16% 16% 5% 7% 6% 4 3 3 4 5% 31%	3700
April	1st to 10th	Night	15% 9% 4 6% 6% 3 3 4 5% 6% 38%	1900
	11th to 20th	Night	14% 12% 5% 6% 4 3 4 3 5% 7% 38%	1683
	21st to end	Night	13% 11% 5% 6% 7% 3 5% 4 5% 6% 34%	1548
May	1st to 10th	Night	17% 10% 4 6% 5% 3 4 5% 5% 7% 33%	1440
	11th to 20th	Night	20% 7% 3 5% 5% 3 5% 3 5% 6% 39%	1416
	21st to end	Night	20% 8% 2 4 4 3 4 4 5% 7% 38%	1471
June	1st to 10th	Night	20% 7% 2 4 5% 4 5% 5% 6% 7% 35%	1260
	11th to 20th	Night	15% 5% 2 5% 4 4 6% 5% 6% 8% 42%	1260
	21st to end	Night	14% 6% 2 4 4 2 6% 4 5% 10% 42%	1260
July	1st to 10th	Night	9% 4 2 4 4 3 4 4 6% 9% 50%	1260
	11th to 20th	Night	11% 4 2 5% 4 3 6% 4 7% 9% 46%	1260
	21st to end	Night	10% 3 1 3 5% 3 4 5% 5% 9% 50%	1500
August	1st to 10th	Night	10% 6% 2 4 4 2 4 5% 5% 8% 49%	1432
	11th to 20th	Night	13% 5% 2 5% 5% 3 4 5% 5% 8% 44%	1587
	21st to end	Night	15% 5% 2 5% 4 4 4 4 4 6% 46%	1834
September	1st to 10th	Night	15% 6% 3 5% 4 3 4 5% 5% 7% 43%	1800
	11th to 20th	Night	19% 7% 2 5% 4 3 4 4 6% 7% 38%	1800
	21st to end	Night	20% 9% 3 5% 5% 3 3 5% 4 7% 36%	2058
October	1st to 10th	Night	18% 11% 3 6% 6% 4 5% 5% 5% 7% 31%	2160
	11th to 20th	Night	21% 13% 4 7% 6% 2 4 5% 5% 6% 27%	2160
	21st to end	Night	21% 15% 5% 8% 6% 4 5% 4 4 6% 22%	2561
November	1st to 10th	Night	16% 17% 7% 10% 6% 3 4 5% 4 5% 23%	2591
	11th to 20th	Night	16% 15% 6% 14% 9% 5% 5% 4 4 5% 16%	2660
	21st to end	Night	16% 17% 8% 12% 10% 4 5% 4 4 4 16%	2660
December	1st to 10th	Night	8% 14% 7% 17% 14% 7% 8% 5% 4 4 13%	2708
	11th to 20th	Night	12% 13% 6% 15% 11% 5% 6% 6% 5% 6% 16%	3518
	21st to end	Night	14% 17% 9% 12% 12% 5% 5% 5% 5% 3 13%	5225

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	71% 3 7% 14% 6%	2219
	11th to 20th	Day	69% 2 9% 17% 3	2220
	21st to end	Day	77% 2 6% 9% 5%	2716
February	1st to 10th	Day	62% 3 11% 14% 10% 1	1710
	11th to 20th	Day	57% 3 9% 22% 7% 1	1673
	21st to end	Day	58% 4 8% 19% 10% 1	1426
March	1st to 10th	Day	46% 3 11% 21% 18% 1	3238
	11th to 20th	Day	59% 3 10% 20% 8%	3480
	21st to end	Day	56% 3 7% 20% 12% 1	3931
April	1st to 10th	Day	51% 3 8% 22% 15% 2	2269
	11th to 20th	Day	53% 3 9% 21% 13% 1	2226
	21st to end	Day	50% 2 9% 26% 11% 2	2412
May	1st to 10th	Day	53% 4 9% 23% 10% 1	2520
	11th to 20th	Day	51% 3 11% 22% 12% 1	2483
	21st to end	Day	49% 3 12% 21% 13% 2	2802
June	1st to 10th	Day	48% 5% 14% 22% 10% 1	2700
	11th to 20th	Day	50% 4 12% 21% 13%	2700
	21st to end	Day	47% 4 17% 24% 7%	2700
July	1st to 10th	Day	36% 4 14% 34% 11% 1	2626
	11th to 20th	Day	42% 4 13% 32% 9%	2619
	21st to end	Day	41% 4 18% 28% 8% 1	2661
August	1st to 10th	Day	41% 4 17% 29% 7%	2492
	11th to 20th	Day	41% 5% 19% 30% 5%	2310
	21st to end	Day	44% 4 17% 27% 8%	2521
September	1st to 10th	Day	44% 4 14% 30% 7%	2160
	11th to 20th	Day	52% 3 18% 22% 4	2160
	21st to end	Day	53% 4 16% 23% 5%	1882
October	1st to 10th	Day	62% 3 14% 18% 3	1800
	11th to 20th	Day	65% 3 11% 18% 4	1800
	21st to end	Day	69% 3 12% 13% 3	1793
November	1st to 10th	Day	66% 3 13% 11% 5% 1	1487
	11th to 20th	Day	74% 3 8% 11% 4	1520
	21st to end	Day	74% 3 10% 11% 3	1520
December	1st to 10th	Day	74% 3 8% 11% 3	1468
	11th to 20th	Day	67% 2 11% 17% 3	1680
	21st to end	Day	72% 2 6% 14% 6%	2442
January	1st to 10th	Night	69% 2 10% 16% 3	4737
	11th to 20th	Night	75% 2 10% 10% 3	4750
	21st to end	Night	74% 3 11% 9% 4	4949
February	1st to 10th	Night	62% 3 10% 15% 9% 1	2470
	11th to 20th	Night	54% 3 11% 22% 9%	2404
	21st to end	Night	57% 3 8% 21% 9% 1	2027
March	1st to 10th	Night	53% 3 9% 18% 16% 1	3644
	11th to 20th	Night	59% 2 11% 20% 7% 1	3480
	21st to end	Night	57% 2 6% 18% 14% 2	3700
April	1st to 10th	Night	48% 2 8% 22% 19% 2	1900
	11th to 20th	Night	47% 3 9% 22% 17% 2	1683
	21st to end	Night	51% 3 8% 23% 13% 2	1548
May	1st to 10th	Night	51% 4 7% 24% 14% 1	1440
	11th to 20th	Night	47% 2 10% 23% 16% 2	1417
	21st to end	Night	46% 3 11% 22% 15% 3	1472
June	1st to 10th	Night	48% 3 12% 23% 13%	1260
	11th to 20th	Night	41% 3 12% 25% 18% 1	1260
	21st to end	Night	39% 5% 19% 27% 9% 1	1260
July	1st to 10th	Night	30% 4 16% 36% 13% 1	1225
	11th to 20th	Night	33% 4 15% 35% 11% 1	1218
	21st to end	Night	31% 5% 17% 35% 11% 1	1417
August	1st to 10th	Night	33% 4 18% 34% 10% 1	1423
	11th to 20th	Night	38% 5% 17% 32% 9%	1587
	21st to end	Night	41% 3 17% 29% 9% 1	1834
September	1st to 10th	Night	41% 3 17% 29% 10%	1800
	11th to 20th	Night	46% 4 17% 26% 6%	1800
	21st to end	Night	49% 4 14% 29% 5%	2058
October	1st to 10th	Night	53% 4 14% 23% 6%	2160
	11th to 20th	Night	59% 3 12% 23% 4	2160
	21st to end	Night	64% 4 11% 18% 3	2561
November	1st to 10th	Night	66% 3 11% 16% 4	2591
	11th to 20th	Night	73% 3 9% 11% 3	2660
	21st to end	Night	72% 4 12% 9% 4	2660
December	1st to 10th	Night	75% 3 9% 10% 3	2708
	11th to 20th	Night	69% 3 10% 14% 4	3516
	21st to end	Night	76% 2 6% 12% 3	5225

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	14% 11% 39% 25% 10% 1	270
	11th to 20th	Day	13% 9% 39% 31% 8% 1	270
	21st to end	Day	16% 10% 37% 26% 9% 1	297
February	1st to 10th	Day	17% 13% 44% 17% 7% 2	270
	11th to 20th	Day	14% 21% 35% 19% 8% 3	270
	21st to end	Day	16% 19% 41% 16% 4 2	225
March	1st to 10th	Day	13% 13% 42% 25% 7% 1	270
	11th to 20th	Day	13% 20% 37% 23% 6% 2	270
	21st to end	Day	16% 17% 33% 24% 8% 3	363
April	1st to 10th	Day	20% 13% 33% 23% 9% 1	457
	11th to 20th	Day	23% 12% 30% 25% 9% 1	504
	21st to end	Day	19% 13% 32% 26% 9% 2	600
May	1st to 10th	Day	21% 17% 33% 21% 6% 2	603
	11th to 20th	Day	21% 15% 32% 20% 9% 2	625
	21st to end	Day	24% 11% 30% 24% 10% 1	679
June	1st to 10th	Day	20% 12% 33% 24% 8% 3	630
	11th to 20th	Day	20% 14% 37% 20% 7% 2	630
	21st to end	Day	17% 10% 33% 24% 11% 4	630
July	1st to 10th	Day	11% 11% 37% 22% 14% 5%	621
	11th to 20th	Day	17% 12% 27% 26% 14% 4	618
	21st to end	Day	12% 9% 30% 29% 15% 6%	684
August	1st to 10th	Day	13% 8% 31% 30% 14% 4	626
	11th to 20th	Day	15% 7% 30% 28% 15% 5%	615
	21st to end	Day	19% 11% 26% 28% 14% 2	640
September	1st to 10th	Day	18% 14% 34% 21% 11% 2	450
	11th to 20th	Day	23% 13% 30% 25% 7% 1	450
	21st to end	Day	30% 15% 26% 20% 8% 1	280
October	1st to 10th	Day	28% 11% 28% 26% 6% 1	270
	11th to 20th	Day	25% 14% 37% 18% 5% 1	270
	21st to end	Day	33% 16% 32% 14% 4	297
November	1st to 10th	Day	26% 17% 33% 17% 6%	270
	11th to 20th	Day	27% 16% 35% 15% 6%	270
	21st to end	Day	25% 26% 34% 11% 4	270
December	1st to 10th	Day	17% 14% 40% 22% 5% 2	270
	11th to 20th	Day	19% 16% 40% 22% 3	270
	21st to end	Day	19% 12% 37% 23% 7% 3	297
January	1st to 10th	Night	14% 13% 36% 27% 9% 1	950
	11th to 20th	Night	17% 10% 38% 29% 6% 1	950
	21st to end	Night	17% 11% 35% 28% 7% 1	1045
February	1st to 10th	Night	13% 10% 36% 30% 10% 1	950
	11th to 20th	Night	13% 12% 33% 27% 12% 2	929
	21st to end	Night	12% 13% 35% 27% 11% 3	771
March	1st to 10th	Night	15% 12% 34% 28% 10% 1	710
	11th to 20th	Night	15% 16% 34% 25% 8% 1	570
	21st to end	Night	21% 10% 28% 26% 10% 3	627
April	1st to 10th	Night	19% 10% 29% 31% 10%	570
	11th to 20th	Night	21% 9% 29% 31% 11%	533
	21st to end	Night	18% 11% 25% 33% 13%	540
May	1st to 10th	Night	23% 10% 31% 29% 8% 1	540
	11th to 20th	Night	25% 9% 28% 26% 10% 1	531
	21st to end	Night	23% 7% 28% 32% 10%	584
June	1st to 10th	Night	23% 7% 26% 31% 11% 1	540
	11th to 20th	Night	17% 7% 34% 29% 14%	540
	21st to end	Night	17% 6% 21% 37% 18% 1	540
July	1st to 10th	Night	8% 7% 29% 36% 18% 2	525
	11th to 20th	Night	12% 8% 28% 34% 16% 3	522
	21st to end	Night	12% 4 24% 36% 22% 3	591
August	1st to 10th	Night	11% 6% 28% 33% 21% 2	537
	11th to 20th	Night	16% 7% 22% 32% 20% 3	534
	21st to end	Night	18% 6% 27% 31% 17% 1	594
September	1st to 10th	Night	17% 8% 30% 29% 14% 1	540
	11th to 20th	Night	24% 6% 28% 26% 14% 1	540
	21st to end	Night	26% 10% 26% 26% 10% 2	630
October	1st to 10th	Night	26% 11% 30% 27% 6% 1	720
	11th to 20th	Night	29% 12% 27% 23% 8% 1	720
	21st to end	Night	29% 11% 30% 23% 6% 1	831
November	1st to 10th	Night	25% 14% 30% 23% 7% 2	925
	11th to 20th	Night	25% 11% 33% 23% 7% 1	950
	21st to end	Night	27% 16% 33% 20% 4	950
December	1st to 10th	Night	14% 13% 38% 25% 8% 2	950
	11th to 20th	Night	18% 14% 37% 23% 7% 1	950
	21st to end	Night	22% 10% 32% 25% 10% 2	1045

Page generated at 2024-04-07 08:28:14. Version ac07

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