Delaware Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Delaware Park clear sky chart from 2005-11-06 to 2025-04-26.

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 8% 5% 5% 5% 4 6% 7% 33%	113186
	Night	14% 11% 4 9% 5% 5% 5% 4 5% 7% 33%	115486
Transparency	Day	47% 9% 14% 19% 10% 1	116786
	Night	49% 8% 13% 19% 10% 1	120014
Seeing	Day	16% 7% 23% 31% 18% 5%	15849
	Night	19% 8% 27% 29% 14% 2	25462

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	12% 14% 8% 17% 5% 5% 5% 3 6% 5% 21%	3614
	11th to 20th	Day	12% 17% 7% 14% 9% 8% 5% 4 4 6% 14%	3620
	21st to end	Day	18% 14% 7% 12% 6% 5% 6% 4 4 5% 19%	4402
February	1st to 10th	Day	12% 15% 5% 13% 8% 5% 5% 4 6% 6% 21%	1800
	11th to 20th	Day	12% 14% 5% 11% 7% 6% 6% 5% 5% 6% 23%	1818
	21st to end	Day	14% 12% 3 12% 7% 5% 6% 4 6% 6% 26%	1650
March	1st to 10th	Day	13% 10% 4 11% 6% 5% 5% 4 5% 5% 32%	4658
	11th to 20th	Day	12% 11% 5% 10% 6% 5% 5% 4 6% 8% 30%	4980
	21st to end	Day	16% 11% 5% 12% 6% 5% 5% 4 4 5% 27%	5465
April	1st to 10th	Day	14% 14% 4 8% 5% 4 5% 4 5% 6% 31%	2303
	11th to 20th	Day	15% 10% 3 6% 3 4 4 4 5% 7% 38%	2572
	21st to end	Day	15% 9% 3 8% 5% 4 4 4 5% 7% 36%	2535
May	1st to 10th	Day	14% 8% 3 7% 5% 5% 5% 3 5% 6% 38%	4082
	11th to 20th	Day	17% 9% 3 7% 5% 6% 4 4 5% 7% 33%	4081
	21st to end	Day	10% 6% 2 6% 5% 5% 4 4 6% 7% 46%	4688
June	1st to 10th	Day	10% 7% 3 6% 4 4 5% 5% 7% 8% 42%	2848
	11th to 20th	Day	9% 5% 2 5% 5% 5% 5% 4 7% 10% 44%	2850
	21st to end	Day	10% 6% 2 7% 4 5% 5% 4 7% 8% 41%	2850
July	1st to 10th	Day	10% 5% 2 7% 6% 5% 6% 5% 7% 8% 39%	4616
	11th to 20th	Day	8% 4 1 5% 4 4 5% 5% 7% 9% 48%	4440
	21st to end	Day	7% 5% 2 6% 4 5% 5% 5% 7% 10% 45%	4539
August	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 5% 6% 8% 40%	4104
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 5% 8% 9% 40%	4044
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 4 7% 9% 43%	4532
September	1st to 10th	Day	10% 7% 2 8% 4 5% 4 4 7% 8% 41%	2175
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 5% 8% 47%	2090
	21st to end	Day	16% 7% 2 6% 4 4 6% 4 6% 8% 36%	2090
October	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 4 5% 7% 40%	1994
	11th to 20th	Day	19% 10% 3 8% 5% 5% 4 4 5% 6% 31%	1900
	21st to end	Day	22% 11% 3 8% 5% 5% 5% 4 6% 7% 25%	1957
November	1st to 10th	Day	14% 11% 4 8% 6% 5% 5% 4 5% 6% 31%	1762
	11th to 20th	Day	14% 13% 5% 9% 6% 5% 6% 4 6% 7% 26%	1720
	21st to end	Day	16% 14% 5% 8% 6% 6% 6% 5% 6% 7% 22%	1600
December	1st to 10th	Day	17% 14% 5% 10% 6% 5% 6% 4 7% 8% 18%	2600
	11th to 20th	Day	19% 18% 5% 9% 6% 4 4 4 5% 5% 21%	2731
	21st to end	Day	23% 24% 6% 10% 5% 4 5% 3 5% 5% 10%	3476
January	1st to 10th	Night	15% 11% 5% 12% 6% 5% 5% 4 5% 7% 27%	6348
	11th to 20th	Night	11% 15% 6% 13% 8% 8% 5% 4 4 6% 19%	6360
	21st to end	Night	14% 14% 6% 10% 5% 5% 5% 4 5% 6% 25%	6554
February	1st to 10th	Night	14% 16% 5% 11% 6% 5% 5% 4 5% 6% 24%	2600
	11th to 20th	Night	13% 13% 5% 10% 5% 5% 5% 4 6% 7% 28%	2464
	21st to end	Night	16% 12% 4 9% 6% 5% 6% 3 6% 7% 26%	1980
March	1st to 10th	Night	13% 11% 4 8% 5% 4 4 3 5% 7% 37%	5194
	11th to 20th	Night	10% 8% 4 7% 5% 4 5% 4 7% 9% 37%	4980
	21st to end	Night	16% 10% 4 9% 4 5% 4 3 5% 5% 36%	5467
April	1st to 10th	Night	19% 11% 4 7% 5% 4 4 3 6% 6% 33%	2111
	11th to 20th	Night	18% 7% 2 5% 5% 5% 4 4 5% 7% 38%	1777
	21st to end	Night	19% 9% 3 8% 5% 4 4 3 5% 7% 35%	1761
May	1st to 10th	Night	15% 9% 3 8% 4 5% 4 4 6% 6% 37%	2813
	11th to 20th	Night	14% 10% 3 8% 5% 5% 4 3 5% 7% 38%	2812
	21st to end	Night	10% 5% 2 4 4 5% 5% 3 5% 8% 50%	2849
June	1st to 10th	Night	13% 6% 2 5% 4 5% 5% 5% 7% 6% 42%	1330
	11th to 20th	Night	11% 5% 3 5% 4 4 5% 5% 6% 9% 44%	1330
	21st to end	Night	8% 6% 2 5% 5% 5% 5% 3 7% 8% 46%	1330
July	1st to 10th	Night	7% 6% 2 7% 4 4 4 4 7% 9% 46%	2335
	11th to 20th	Night	9% 4 1 4 3 3 4 3 6% 8% 53%	2530
	21st to end	Night	6% 4 2 5% 4 5% 6% 5% 7% 9% 47%	3015
August	1st to 10th	Night	10% 6% 2 7% 4 4 4 3 6% 8% 46%	2825
	11th to 20th	Night	11% 6% 3 6% 4 4 4 4 6% 8% 45%	2785
	21st to end	Night	8% 5% 2 6% 4 5% 5% 4 6% 8% 47%	3124
September	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 5% 8% 45%	1981
	11th to 20th	Night	11% 4 1 4 3 4 3 3 5% 6% 53%	2090
	21st to end	Night	16% 7% 3 6% 4 4 4 3 4 6% 40%	2090
October	1st to 10th	Night	15% 6% 3 7% 5% 3 3 3 6% 7% 41%	2185
	11th to 20th	Night	20% 9% 3 7% 5% 4 4 3 5% 6% 34%	2280
	21st to end	Night	21% 10% 3 9% 5% 5% 5% 4 4 6% 28%	2636
November	1st to 10th	Night	14% 10% 2 7% 5% 4 5% 4 6% 7% 37%	2536
	11th to 20th	Night	14% 14% 3 8% 5% 4 5% 5% 6% 7% 28%	2678
	21st to end	Night	19% 12% 4 9% 5% 5% 4 4 6% 7% 25%	2800
December	1st to 10th	Night	15% 15% 5% 9% 5% 6% 6% 5% 7% 8% 20%	4580
	11th to 20th	Night	14% 16% 6% 11% 6% 4 5% 4 5% 6% 24%	5468
	21st to end	Night	24% 20% 7% 12% 5% 4 4 3 4 5% 14%	7488

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% 11% 11% 10% 4	3615
	11th to 20th	Day	72% 8% 9% 6% 5%	3620
	21st to end	Day	66% 7% 13% 8% 5%	4402
February	1st to 10th	Day	62% 8% 11% 14% 4	1800
	11th to 20th	Day	60% 6% 13% 15% 6% 1	1827
	21st to end	Day	57% 7% 14% 13% 8% 1	1650
March	1st to 10th	Day	51% 7% 17% 18% 7% 1	4658
	11th to 20th	Day	49% 8% 16% 19% 7% 1	4980
	21st to end	Day	54% 10% 13% 15% 8% 1	5465
April	1st to 10th	Day	51% 8% 12% 20% 9% 1	2303
	11th to 20th	Day	44% 7% 15% 18% 14% 2	2572
	21st to end	Day	46% 8% 15% 18% 12% 2	2535
May	1st to 10th	Day	42% 10% 12% 18% 16% 3	4120
	11th to 20th	Day	44% 13% 13% 19% 9% 2	4085
	21st to end	Day	34% 9% 11% 18% 21% 6%	4702
June	1st to 10th	Day	37% 8% 14% 25% 14% 2	2848
	11th to 20th	Day	33% 8% 14% 26% 17% 2	2850
	21st to end	Day	37% 9% 14% 25% 15% 1	2850
July	1st to 10th	Day	37% 10% 15% 21% 15% 2	4546
	11th to 20th	Day	28% 9% 14% 36% 12% 1	4365
	21st to end	Day	30% 9% 18% 21% 20% 1	4395
August	1st to 10th	Day	38% 10% 15% 23% 13% 1	4091
	11th to 20th	Day	36% 9% 19% 26% 10% 1	4083
	21st to end	Day	33% 8% 15% 30% 13%	4531
September	1st to 10th	Day	37% 10% 16% 24% 11% 1	2175
	11th to 20th	Day	35% 7% 15% 30% 13% 1	2090
	21st to end	Day	43% 8% 16% 23% 9%	2090
October	1st to 10th	Day	33% 7% 13% 34% 11% 2	3306
	11th to 20th	Day	46% 7% 12% 21% 13%	3140
	21st to end	Day	51% 7% 13% 22% 6%	3207
November	1st to 10th	Day	52% 8% 12% 20% 7% 1	1762
	11th to 20th	Day	56% 8% 16% 12% 8%	1720
	21st to end	Day	63% 7% 15% 12% 3	1600
December	1st to 10th	Day	64% 11% 15% 8% 2	2600
	11th to 20th	Day	65% 8% 12% 10% 6%	2727
	21st to end	Day	76% 8% 8% 6% 2	3476
January	1st to 10th	Night	57% 11% 14% 14% 4	6348
	11th to 20th	Night	66% 8% 12% 11% 3 1	6360
	21st to end	Night	59% 8% 14% 12% 7%	6554
February	1st to 10th	Night	61% 7% 11% 14% 5% 1	2600
	11th to 20th	Night	55% 7% 15% 18% 6%	2466
	21st to end	Night	57% 7% 13% 16% 7%	1980
March	1st to 10th	Night	47% 6% 16% 22% 7% 1	5194
	11th to 20th	Night	42% 7% 19% 22% 10% 1	4980
	21st to end	Night	49% 7% 13% 19% 12% 1	5467
April	1st to 10th	Night	51% 6% 10% 19% 12% 2	2111
	11th to 20th	Night	44% 6% 12% 18% 18% 2	1777
	21st to end	Night	49% 6% 10% 18% 14% 3	1761
May	1st to 10th	Night	44% 8% 10% 20% 14% 4	2840
	11th to 20th	Night	43% 9% 12% 21% 13% 3	2813
	21st to end	Night	31% 9% 12% 23% 19% 7%	2856
June	1st to 10th	Night	39% 8% 12% 23% 16% 2	1330
	11th to 20th	Night	35% 8% 12% 23% 19% 3	1330
	21st to end	Night	34% 8% 14% 24% 17% 2	1330
July	1st to 10th	Night	34% 8% 14% 24% 15% 6%	2297
	11th to 20th	Night	27% 7% 14% 33% 17% 1	2482
	21st to end	Night	29% 10% 18% 19% 21% 2	2919
August	1st to 10th	Night	35% 8% 13% 28% 15% 1	2814
	11th to 20th	Night	35% 8% 16% 25% 13% 2	2812
	21st to end	Night	32% 10% 13% 31% 13% 1	3124
September	1st to 10th	Night	35% 9% 15% 25% 15% 1	1981
	11th to 20th	Night	31% 6% 13% 32% 17% 1	2090
	21st to end	Night	43% 8% 11% 27% 10% 1	2090
October	1st to 10th	Night	29% 7% 13% 28% 23% 1	3640
	11th to 20th	Night	47% 5% 11% 22% 14%	3779
	21st to end	Night	47% 7% 13% 25% 8% 1	4336
November	1st to 10th	Night	47% 8% 15% 21% 9%	2536
	11th to 20th	Night	55% 9% 16% 14% 6% 1	2678
	21st to end	Night	61% 8% 17% 12% 2	2800
December	1st to 10th	Night	62% 11% 18% 7% 2	4580
	11th to 20th	Night	62% 8% 12% 10% 7%	5471
	21st to end	Night	73% 8% 10% 7% 2	7488

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	17% 12% 39% 21% 10% 1	299
	11th to 20th	Day	22% 8% 42% 20% 8%	300
	21st to end	Day	20% 8% 35% 28% 8% 1	330
February	1st to 10th	Day	19% 13% 38% 24% 6% 1	300
	11th to 20th	Day	16% 15% 35% 31% 2	300
	21st to end	Day	15% 18% 40% 22% 6%	249
March	1st to 10th	Day	18% 13% 34% 27% 7% 1	374
	11th to 20th	Day	16% 9% 33% 30% 10% 1	500
	21st to end	Day	21% 8% 30% 31% 9% 1	550
April	1st to 10th	Day	17% 9% 32% 31% 10% 1	500
	11th to 20th	Day	22% 9% 25% 28% 11% 5%	492
	21st to end	Day	18% 5% 27% 35% 14% 1	480
May	1st to 10th	Day	15% 6% 24% 37% 15% 3	463
	11th to 20th	Day	16% 3 21% 33% 22% 5%	487
	21st to end	Day	12% 4 15% 34% 27% 9%	529
June	1st to 10th	Day	11% 3 19% 37% 25% 5%	678
	11th to 20th	Day	11% 2 18% 37% 26% 6%	680
	21st to end	Day	13% 4 13% 43% 24% 4	680
July	1st to 10th	Day	8% 4 10% 35% 33% 10%	671
	11th to 20th	Day	11% 3 13% 38% 29% 7%	668
	21st to end	Day	8% 1 11% 40% 31% 8%	579
August	1st to 10th	Day	10% 3 12% 34% 31% 11%	486
	11th to 20th	Day	9% 2 9% 36% 33% 11%	479
	21st to end	Day	11% 5% 13% 29% 34% 7%	539
September	1st to 10th	Day	12% 4 20% 29% 25% 10%	490
	11th to 20th	Day	11% 4 15% 25% 27% 18%	490
	21st to end	Day	18% 7% 20% 26% 20% 10%	490
October	1st to 10th	Day	18% 7% 25% 24% 18% 8%	490
	11th to 20th	Day	21% 12% 29% 25% 11% 2	490
	21st to end	Day	26% 12% 29% 25% 6% 1	406
November	1st to 10th	Day	20% 12% 27% 27% 12% 2	300
	11th to 20th	Day	22% 17% 30% 23% 5% 2	260
	21st to end	Day	25% 11% 39% 23% 4	200
December	1st to 10th	Day	23% 15% 37% 18% 8%	200
	11th to 20th	Day	25% 21% 33% 20% 2	200
	21st to end	Day	28% 13% 34% 21% 4	220
January	1st to 10th	Night	22% 13% 38% 21% 6% 1	1000
	11th to 20th	Night	21% 14% 35% 24% 6% 1	1000
	21st to end	Night	18% 9% 36% 29% 7%	905
February	1st to 10th	Night	22% 10% 36% 27% 5%	800
	11th to 20th	Night	21% 12% 33% 27% 7%	783
	21st to end	Night	22% 9% 37% 27% 4	660
March	1st to 10th	Night	18% 8% 34% 30% 10%	800
	11th to 20th	Night	16% 9% 35% 31% 8% 1	800
	21st to end	Night	22% 9% 26% 32% 11% 1	880
April	1st to 10th	Night	22% 8% 29% 31% 9% 1	730
	11th to 20th	Night	20% 5% 30% 30% 12% 4	593
	21st to end	Night	21% 7% 22% 34% 15% 1	587
May	1st to 10th	Night	15% 5% 21% 39% 18% 2	570
	11th to 20th	Night	19% 3 19% 35% 22% 3	561
	21st to end	Night	12% 2 15% 36% 29% 6%	491
June	1st to 10th	Night	18% 2 14% 37% 26% 2	380
	11th to 20th	Night	13% 2 15% 40% 27% 3	380
	21st to end	Night	11% 2 14% 41% 29% 2	380
July	1st to 10th	Night	8% 2 9% 43% 33% 5%	440
	11th to 20th	Night	10% 2 11% 35% 36% 6%	552
	21st to end	Night	9% 2 10% 45% 29% 4	624
August	1st to 10th	Night	11% 2 11% 39% 31% 6%	567
	11th to 20th	Night	11% 2 13% 32% 34% 8%	564
	21st to end	Night	13% 2 17% 32% 30% 6%	627
September	1st to 10th	Night	14% 3 20% 30% 23% 10%	707
	11th to 20th	Night	15% 3 15% 29% 28% 10%	760
	21st to end	Night	18% 6% 19% 29% 20% 8%	760
October	1st to 10th	Night	19% 6% 28% 25% 19% 4	760
	11th to 20th	Night	24% 12% 29% 24% 11% 1	760
	21st to end	Night	27% 11% 29% 27% 6%	836
November	1st to 10th	Night	21% 11% 31% 25% 11% 1	780
	11th to 20th	Night	21% 17% 34% 20% 8%	800
	21st to end	Night	26% 14% 35% 21% 4	800
December	1st to 10th	Night	27% 16% 36% 16% 5%	800
	11th to 20th	Night	21% 16% 38% 20% 4	925
	21st to end	Night	29% 13% 34% 21% 3	1100

Page generated at 2025-04-23 09:25:06. Version ac07

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