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 2024-11-05.

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% 4 9% 5% 5% 5% 4 6% 7% 31%	101871
	Night	13% 12% 5% 10% 6% 5% 5% 4 5% 6% 28%	97343
Transparency	Day	52% 4 13% 22% 8% 1	105303
	Night	56% 3 12% 20% 8%	102001
Seeing	Day	19% 13% 33% 23% 9% 2	16257
	Night	19% 10% 31% 28% 10% 1	26150

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% 13% 7% 19% 8% 10% 6% 3 6% 5% 17%	2218
	11th to 20th	Day	6% 11% 6% 14% 9% 10% 11% 7% 6% 5% 15%	2220
	21st to end	Day	9% 18% 9% 22% 7% 6% 6% 5% 5% 4 9%	2716
February	1st to 10th	Day	8% 13% 5% 11% 8% 6% 10% 6% 7% 5% 20%	1710
	11th to 20th	Day	8% 12% 5% 11% 9% 7% 5% 6% 6% 7% 25%	1664
	21st to end	Day	7% 13% 5% 16% 9% 5% 5% 5% 6% 6% 25%	1426
March	1st to 10th	Day	9% 11% 4 8% 5% 5% 5% 4 5% 6% 39%	3236
	11th to 20th	Day	7% 11% 6% 15% 9% 6% 5% 4 5% 5% 27%	3480
	21st to end	Day	12% 15% 6% 10% 5% 5% 5% 3 4 5% 30%	3931
April	1st to 10th	Day	14% 12% 4 9% 4 4 4 3 5% 6% 34%	2280
	11th to 20th	Day	14% 12% 4 8% 5% 4 5% 3 5% 6% 33%	2352
	21st to end	Day	15% 12% 4 8% 4 4 5% 3 5% 6% 35%	2546
May	1st to 10th	Day	15% 11% 3 7% 5% 4 4 4 6% 8% 33%	4428
	11th to 20th	Day	16% 9% 3 9% 6% 5% 5% 3 6% 7% 33%	4399
	21st to end	Day	23% 10% 3 6% 4 4 3 4 4 5% 36%	4979
June	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 5% 7% 9% 31%	2850
	11th to 20th	Day	17% 8% 2 7% 5% 5% 4 5% 6% 8% 33%	2850
	21st to end	Day	17% 6% 2 7% 6% 5% 6% 5% 7% 9% 30%	2850
July	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 4 7% 9% 39%	4750
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 6% 9% 47%	4750
	21st to end	Day	13% 7% 2 6% 4 5% 5% 4 6% 9% 39%	4941
August	1st to 10th	Day	16% 6% 2 7% 4 4 6% 5% 6% 8% 35%	4423
	11th to 20th	Day	11% 4 2 5% 4 5% 5% 4 7% 10% 42%	4050
	21st to end	Day	15% 8% 3 6% 4 4 4 4 5% 9% 38%	4437
September	1st to 10th	Day	16% 8% 3 6% 4 4 4 4 6% 7% 39%	2280
	11th to 20th	Day	18% 10% 3 7% 5% 4 5% 5% 6% 8% 30%	2280
	21st to end	Day	22% 11% 3 6% 4 4 4 3 4 6% 34%	1986
October	1st to 10th	Day	19% 16% 4 7% 5% 5% 5% 3 4 5% 27%	1900
	11th to 20th	Day	18% 19% 4 10% 5% 5% 4 4 4 5% 23%	1900
	21st to end	Day	20% 20% 5% 11% 5% 4 4 3 4 6% 19%	1891
November	1st to 10th	Day	16% 19% 7% 11% 6% 3 4 4 5% 4 19%	1519
	11th to 20th	Day	16% 20% 7% 14% 8% 6% 4 3 4 4 15%	1520
	21st to end	Day	15% 19% 6% 20% 8% 3 3 4 4 5% 14%	1520
December	1st to 10th	Day	9% 16% 6% 19% 10% 8% 6% 4 5% 5% 13%	1468
	11th to 20th	Day	9% 18% 8% 15% 6% 7% 5% 3 3 5% 20%	1679
	21st to end	Day	18% 17% 8% 12% 6% 6% 4 3 5% 4 16%	2442
January	1st to 10th	Night	6% 16% 7% 15% 10% 8% 6% 4 7% 5% 16%	4737
	11th to 20th	Night	6% 12% 5% 16% 11% 13% 10% 6% 5% 4 12%	4750
	21st to end	Night	10% 19% 10% 17% 7% 5% 5% 5% 4 4 14%	4949
February	1st to 10th	Night	8% 12% 6% 12% 6% 8% 9% 6% 6% 6% 21%	2470
	11th to 20th	Night	7% 12% 4 9% 6% 7% 8% 6% 6% 6% 29%	2403
	21st to end	Night	5% 14% 6% 12% 7% 6% 7% 5% 7% 6% 25%	2027
March	1st to 10th	Night	12% 13% 4 9% 5% 5% 6% 4 4 6% 32%	3633
	11th to 20th	Night	8% 14% 6% 11% 8% 6% 5% 4 4 5% 29%	3479
	21st to end	Night	16% 17% 4 8% 5% 5% 3 3 4 5% 31%	3700
April	1st to 10th	Night	15% 10% 3 8% 5% 4 3 3 5% 6% 38%	1900
	11th to 20th	Night	14% 12% 5% 7% 4 3 4 3 5% 6% 37%	1777
	21st to end	Night	13% 11% 4 7% 7% 5% 5% 3 5% 6% 34%	1634
May	1st to 10th	Night	15% 11% 3 7% 5% 4 4 4 5% 7% 36%	2523
	11th to 20th	Night	15% 9% 3 7% 5% 5% 4 4 5% 7% 36%	2506
	21st to end	Night	28% 9% 2 5% 3 3 3 3 4 6% 34%	2605
June	1st to 10th	Night	19% 8% 2 5% 4 5% 5% 4 6% 7% 35%	1330
	11th to 20th	Night	14% 6% 2 5% 5% 4 6% 4 6% 8% 41%	1330
	21st to end	Night	15% 6% 2 5% 4 3 6% 3 6% 9% 41%	1330
July	1st to 10th	Night	10% 6% 2 6% 4 5% 4 4 7% 9% 43%	2220
	11th to 20th	Night	7% 4 2 6% 3 4 5% 3 7% 9% 49%	2220
	21st to end	Night	11% 5% 1 5% 4 3 4 4 5% 9% 49%	2623
August	1st to 10th	Night	14% 7% 3 5% 4 3 4 4 5% 8% 44%	2517
	11th to 20th	Night	12% 5% 2 5% 3 4 4 4 6% 9% 46%	2785
	21st to end	Night	16% 6% 2 6% 4 4 4 3 5% 8% 42%	3226
September	1st to 10th	Night	14% 6% 3 6% 3 4 3 4 5% 7% 44%	1900
	11th to 20th	Night	19% 7% 2 6% 4 4 5% 3 6% 7% 38%	1900
	21st to end	Night	19% 10% 2 5% 4 4 3 3 4 7% 38%	2173
October	1st to 10th	Night	17% 11% 3 7% 4 5% 5% 4 6% 7% 31%	2280
	11th to 20th	Night	20% 14% 4 8% 5% 4 5% 3 5% 6% 28%	2280
	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% 5% 4 4 4 5% 22%	2660
	11th to 20th	Night	16% 17% 6% 16% 8% 6% 5% 3 4 4 16%	2660
	21st to end	Night	15% 18% 7% 14% 8% 5% 4 3 4 4 15%	2660
December	1st to 10th	Night	7% 16% 6% 19% 12% 8% 7% 4 4 4 13%	2708
	11th to 20th	Night	11% 15% 5% 17% 9% 6% 6% 4 5% 6% 16%	3518
	21st to end	Night	14% 18% 8% 14% 10% 7% 5% 4 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	50% 3 8% 22% 15% 2	2280
	11th to 20th	Day	53% 3 9% 22% 12% 1	2352
	21st to end	Day	50% 2 9% 26% 11% 2	2546
May	1st to 10th	Day	49% 3 11% 26% 11%	4440
	11th to 20th	Day	51% 3 13% 19% 13% 1	4403
	21st to end	Day	52% 3 10% 22% 13% 1	4979
June	1st to 10th	Day	48% 5% 14% 22% 11%	2850
	11th to 20th	Day	49% 4 12% 22% 13%	2850
	21st to end	Day	47% 4 18% 23% 7%	2850
July	1st to 10th	Day	39% 6% 19% 28% 7%	4676
	11th to 20th	Day	32% 5% 21% 35% 7%	4669
	21st to end	Day	42% 5% 16% 32% 5%	4784
August	1st to 10th	Day	46% 4 14% 26% 10%	4408
	11th to 20th	Day	35% 5% 25% 30% 4	4089
	21st to end	Day	43% 4 21% 27% 5%	4436
September	1st to 10th	Day	43% 4 15% 31% 7%	2280
	11th to 20th	Day	52% 4 18% 22% 4	2280
	21st to end	Day	51% 4 15% 23% 6%	1986
October	1st to 10th	Day	48% 4 16% 23% 9%	3160
	11th to 20th	Day	55% 3 16% 18% 7%	3140
	21st to end	Day	59% 5% 19% 15% 2	3083
November	1st to 10th	Day	67% 3 13% 11% 5% 1	1519
	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% 21% 19% 2	1900
	11th to 20th	Night	48% 3 9% 22% 16% 2	1777
	21st to end	Night	51% 3 8% 23% 13% 1	1634
May	1st to 10th	Night	49% 3 7% 27% 14%	2530
	11th to 20th	Night	48% 3 14% 23% 10% 1	2507
	21st to end	Night	53% 2 12% 20% 12% 1	2606
June	1st to 10th	Night	48% 3 12% 24% 13%	1330
	11th to 20th	Night	41% 3 12% 26% 17% 1	1330
	21st to end	Night	40% 5% 19% 27% 9% 1	1330
July	1st to 10th	Night	35% 7% 22% 26% 9% 1	2185
	11th to 20th	Night	29% 5% 20% 37% 8% 1	2178
	21st to end	Night	33% 5% 17% 38% 7% 1	2540
August	1st to 10th	Night	38% 4 14% 34% 9% 1	2508
	11th to 20th	Night	33% 6% 22% 33% 6%	2812
	21st to end	Night	42% 4 21% 27% 6%	3226
September	1st to 10th	Night	40% 3 17% 30% 10%	1900
	11th to 20th	Night	46% 4 18% 26% 6%	1900
	21st to end	Night	47% 3 14% 29% 6%	2173
October	1st to 10th	Night	45% 5% 15% 28% 8%	3800
	11th to 20th	Night	51% 3 16% 24% 5%	3779
	21st to end	Night	58% 4 16% 19% 4	4466
November	1st to 10th	Night	66% 3 11% 16% 4	2660
	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% 2	460
	11th to 20th	Day	23% 13% 31% 24% 8% 1	550
	21st to end	Day	19% 13% 33% 24% 9% 2	650
May	1st to 10th	Day	21% 17% 33% 20% 6% 2	653
	11th to 20th	Day	21% 15% 32% 21% 9% 2	675
	21st to end	Day	25% 11% 31% 23% 9% 1	734
June	1st to 10th	Day	21% 13% 32% 23% 8% 3	680
	11th to 20th	Day	21% 14% 38% 19% 7% 2	680
	21st to end	Day	18% 10% 33% 24% 11% 4	680
July	1st to 10th	Day	12% 11% 34% 22% 15% 6%	671
	11th to 20th	Day	16% 11% 27% 28% 14% 4	668
	21st to end	Day	12% 9% 29% 29% 15% 6%	739
August	1st to 10th	Day	14% 8% 31% 30% 14% 3	676
	11th to 20th	Day	15% 7% 29% 29% 15% 6%	665
	21st to end	Day	19% 12% 27% 27% 13% 2	692
September	1st to 10th	Day	17% 14% 34% 22% 11% 2	490
	11th to 20th	Day	22% 14% 30% 25% 7% 1	490
	21st to end	Day	28% 14% 29% 21% 7% 1	310
October	1st to 10th	Day	29% 13% 28% 25% 5% 1	300
	11th to 20th	Day	25% 16% 36% 18% 5% 1	300
	21st to end	Day	32% 17% 34% 13% 4	330
November	1st to 10th	Day	26% 17% 33% 17% 6% 1	282
	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	20% 9% 30% 31% 10%	570
	11th to 20th	Night	21% 10% 29% 30% 10%	563
	21st to end	Night	18% 11% 26% 32% 13%	570
May	1st to 10th	Night	22% 11% 31% 28% 7% 1	570
	11th to 20th	Night	25% 9% 28% 27% 10% 1	561
	21st to end	Night	24% 7% 28% 31% 10%	617
June	1st to 10th	Night	22% 8% 26% 31% 11% 1	570
	11th to 20th	Night	16% 8% 34% 28% 13%	570
	21st to end	Night	17% 6% 22% 36% 17% 1	570
July	1st to 10th	Night	8% 6% 28% 35% 19% 2	555
	11th to 20th	Night	11% 8% 27% 35% 16% 3	552
	21st to end	Night	12% 4 24% 36% 22% 3	624
August	1st to 10th	Night	11% 5% 28% 34% 20% 2	567
	11th to 20th	Night	16% 6% 22% 32% 20% 3	564
	21st to end	Night	18% 7% 28% 31% 16% 1	627
September	1st to 10th	Night	17% 8% 30% 29% 14% 1	570
	11th to 20th	Night	24% 6% 29% 26% 14% 1	570
	21st to end	Night	25% 11% 26% 27% 11% 2	665
October	1st to 10th	Night	26% 11% 30% 26% 5% 1	760
	11th to 20th	Night	28% 13% 27% 24% 7% 1	760
	21st to end	Night	28% 11% 31% 23% 6% 1	878
November	1st to 10th	Night	25% 14% 30% 23% 7% 2	950
	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 2024-11-03 01:11:29. Version ac07

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