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 2026-08-30.

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	12% 9% 3 8% 5% 5% 5% 4 6% 7% 35%	143464
	Night	13% 10% 4 8% 5% 5% 4 4 5% 7% 34%	138859
Transparency	Day	45% 8% 13% 21% 11% 1	147020
	Night	48% 8% 13% 19% 11% 1	143362
Seeing	Day	15% 7% 22% 31% 19% 5%	17740
	Night	19% 8% 27% 29% 15% 2	27101

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	13% 13% 7% 15% 5% 5% 5% 3 5% 5% 22%	4688
	11th to 20th	Day	11% 18% 7% 16% 9% 8% 6% 4 4 5% 14%	4710
	21st to end	Day	15% 13% 6% 10% 8% 6% 6% 4 4 6% 21%	5703
February	1st to 10th	Day	12% 15% 5% 13% 7% 5% 5% 4 6% 6% 21%	1890
	11th to 20th	Day	12% 14% 5% 10% 7% 6% 6% 5% 5% 6% 23%	1911
	21st to end	Day	14% 12% 4 12% 7% 5% 6% 3 6% 6% 25%	1730
March	1st to 10th	Day	13% 10% 4 11% 6% 5% 5% 4 5% 5% 32%	4762
	11th to 20th	Day	12% 11% 5% 10% 6% 5% 5% 4 6% 8% 30%	5090
	21st to end	Day	16% 11% 5% 12% 6% 5% 5% 4 4 5% 27%	5586
April	1st to 10th	Day	15% 13% 4 8% 5% 4 5% 4 5% 6% 30%	2418
	11th to 20th	Day	16% 9% 3 7% 4 4 4 4 5% 7% 37%	2702
	21st to end	Day	14% 9% 3 8% 5% 5% 4 3 5% 8% 36%	2730
May	1st to 10th	Day	14% 8% 3 7% 5% 5% 5% 4 6% 7% 37%	7616
	11th to 20th	Day	13% 8% 3 7% 5% 6% 5% 4 6% 8% 36%	7626
	21st to end	Day	14% 7% 2 6% 4 4 4 3 5% 6% 44%	8785
June	1st to 10th	Day	10% 7% 2 5% 4 4 5% 5% 7% 8% 43%	3148
	11th to 20th	Day	9% 5% 2 5% 5% 5% 5% 4 7% 10% 43%	3150
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 4 7% 9% 42%	3150
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 4 7% 9% 40%	8624
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 6% 10% 50%	8280
	21st to end	Day	7% 4 2 5% 4 4 5% 4 6% 9% 48%	8481
August	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 8% 41%	4364
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 5% 7% 9% 40%	4304
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 4 7% 9% 43%	4792
September	1st to 10th	Day	9% 6% 2 7% 4 5% 4 4 7% 9% 42%	2290
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 5% 9% 48%	2200
	21st to end	Day	16% 7% 2 6% 4 4 6% 4 7% 8% 37%	2200
October	1st to 10th	Day	14% 6% 3 6% 4 4 5% 4 5% 7% 41%	2099
	11th to 20th	Day	19% 10% 2 8% 5% 5% 4 4 6% 6% 32%	2000
	21st to end	Day	21% 11% 3 8% 5% 5% 5% 4 6% 7% 26%	2060
November	1st to 10th	Day	14% 11% 4 8% 6% 5% 5% 4 5% 6% 31%	1852
	11th to 20th	Day	14% 13% 5% 9% 6% 5% 6% 4 6% 7% 26%	1806
	21st to end	Day	16% 14% 5% 8% 6% 6% 6% 5% 5% 7% 22%	1680
December	1st to 10th	Day	17% 14% 5% 10% 6% 5% 6% 4 7% 8% 18%	2680
	11th to 20th	Day	19% 18% 5% 9% 6% 4 4 4 5% 6% 21%	2804
	21st to end	Day	23% 23% 6% 10% 5% 4 5% 3 5% 5% 10%	3553
January	1st to 10th	Night	15% 13% 5% 11% 5% 5% 5% 3 5% 6% 28%	8232
	11th to 20th	Night	9% 16% 6% 14% 8% 8% 5% 4 5% 6% 20%	8280
	21st to end	Night	11% 14% 6% 11% 6% 6% 5% 4 5% 5% 27%	8479
February	1st to 10th	Night	13% 16% 5% 11% 6% 5% 5% 3 5% 5% 25%	2730
	11th to 20th	Night	13% 13% 4 10% 5% 5% 5% 4 6% 7% 29%	2590
	21st to end	Night	15% 13% 4 10% 6% 5% 5% 3 6% 7% 26%	2076
March	1st to 10th	Night	13% 12% 3 8% 5% 4 4 3 5% 7% 37%	5310
	11th to 20th	Night	10% 9% 4 7% 5% 4 5% 4 7% 9% 37%	5090
	21st to end	Night	16% 10% 4 9% 4 5% 4 3 5% 5% 36%	5588
April	1st to 10th	Night	18% 11% 4 7% 4 4 4 3 6% 6% 33%	2217
	11th to 20th	Night	18% 7% 2 5% 5% 5% 4 4 6% 7% 37%	1867
	21st to end	Night	18% 9% 2 7% 4 4 4 3 5% 7% 37%	1890
May	1st to 10th	Night	15% 8% 3 8% 4 5% 4 4 6% 6% 36%	5237
	11th to 20th	Night	14% 9% 3 8% 5% 5% 4 3 5% 7% 38%	5234
	21st to end	Night	17% 5% 2 5% 4 4 4 3 5% 7% 44%	5337
June	1st to 10th	Night	13% 6% 2 5% 4 5% 5% 5% 7% 6% 43%	1470
	11th to 20th	Night	10% 6% 3 6% 4 4 5% 6% 6% 8% 42%	1470
	21st to end	Night	8% 6% 2 5% 5% 5% 5% 3 7% 8% 46%	1470
July	1st to 10th	Night	8% 7% 2 7% 4 4 4 4 6% 9% 47%	4345
	11th to 20th	Night	7% 3 1 4 4 3 5% 4 7% 11% 52%	4707
	21st to end	Night	8% 5% 2 6% 4 5% 5% 4 6% 8% 48%	5601
August	1st to 10th	Night	10% 6% 2 7% 4 4 4 3 6% 7% 46%	3005
	11th to 20th	Night	10% 6% 3 7% 4 4 4 4 6% 8% 45%	2965
	21st to end	Night	8% 5% 2 6% 4 5% 5% 4 6% 8% 47%	3304
September	1st to 10th	Night	10% 6% 2 6% 4 4 5% 4 5% 8% 46%	2085
	11th to 20th	Night	11% 4 1 4 3 4 3 3 5% 7% 54%	2200
	21st to end	Night	16% 7% 3 6% 4 4 4 3 4 6% 40%	2200
October	1st to 10th	Night	14% 6% 2 6% 5% 3 3 3 6% 7% 43%	2300
	11th to 20th	Night	19% 9% 3 7% 5% 4 4 3 5% 6% 34%	2400
	21st to end	Night	20% 10% 3 8% 5% 5% 5% 4 4 7% 29%	2775
November	1st to 10th	Night	14% 10% 3 6% 5% 4 5% 4 6% 7% 37%	2666
	11th to 20th	Night	14% 14% 3 8% 5% 4 5% 5% 6% 7% 29%	2812
	21st to end	Night	19% 12% 5% 9% 5% 5% 5% 4 6% 7% 25%	2940
December	1st to 10th	Night	14% 15% 5% 9% 5% 6% 5% 5% 7% 8% 21%	4720
	11th to 20th	Night	14% 16% 6% 11% 6% 5% 5% 4 5% 6% 24%	5614
	21st to end	Night	23% 19% 7% 12% 5% 4 4 3 4 5% 14%	7653

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	63% 10% 12% 11% 4	4689
	11th to 20th	Day	74% 7% 8% 6% 4	4710
	21st to end	Day	62% 7% 12% 10% 9%	5703
February	1st to 10th	Day	61% 8% 11% 15% 4	1890
	11th to 20th	Day	60% 6% 13% 15% 6% 1	1920
	21st to end	Day	58% 7% 14% 13% 8% 1	1730
March	1st to 10th	Day	51% 7% 16% 18% 7% 1	4762
	11th to 20th	Day	49% 8% 16% 19% 7% 1	5090
	21st to end	Day	54% 9% 13% 15% 8% 1	5586
April	1st to 10th	Day	51% 8% 12% 20% 9% 1	2418
	11th to 20th	Day	45% 7% 14% 18% 14% 2	2702
	21st to end	Day	45% 7% 15% 20% 12% 2	2730
May	1st to 10th	Day	42% 9% 13% 20% 13% 4	7651
	11th to 20th	Day	42% 10% 11% 22% 13% 2	7630
	21st to end	Day	38% 7% 8% 22% 20% 4	8800
June	1st to 10th	Day	36% 8% 13% 26% 15% 2	3148
	11th to 20th	Day	34% 9% 14% 25% 16% 2	3150
	21st to end	Day	37% 9% 14% 26% 14% 1	3150
July	1st to 10th	Day	38% 9% 14% 23% 15% 1	8554
	11th to 20th	Day	27% 8% 13% 34% 17% 1	8205
	21st to end	Day	28% 8% 16% 26% 20% 1	8295
August	1st to 10th	Day	37% 9% 15% 23% 14% 1	4351
	11th to 20th	Day	35% 10% 19% 26% 9% 1	4343
	21st to end	Day	33% 9% 16% 30% 12%	4791
September	1st to 10th	Day	36% 9% 16% 25% 11% 1	2290
	11th to 20th	Day	33% 6% 16% 31% 13% 1	2200
	21st to end	Day	43% 8% 15% 22% 10%	2200
October	1st to 10th	Day	33% 7% 13% 34% 11% 2	3411
	11th to 20th	Day	46% 7% 12% 22% 13%	3240
	21st to end	Day	51% 7% 13% 22% 7%	3310
November	1st to 10th	Day	53% 8% 11% 20% 7% 1	1852
	11th to 20th	Day	56% 8% 16% 12% 8%	1806
	21st to end	Day	63% 7% 15% 12% 3	1680
December	1st to 10th	Day	64% 11% 15% 8% 2	2680
	11th to 20th	Day	65% 8% 12% 10% 6%	2800
	21st to end	Day	76% 8% 8% 6% 2	3553
January	1st to 10th	Night	57% 10% 14% 14% 4	8233
	11th to 20th	Night	66% 7% 10% 12% 4 1	8280
	21st to end	Night	58% 7% 12% 14% 9%	8479
February	1st to 10th	Night	60% 7% 11% 15% 5% 1	2730
	11th to 20th	Night	54% 7% 15% 18% 7%	2592
	21st to end	Night	58% 7% 13% 15% 7%	2076
March	1st to 10th	Night	47% 6% 16% 22% 7% 1	5310
	11th to 20th	Night	42% 7% 19% 22% 10% 1	5090
	21st to end	Night	49% 7% 13% 19% 12% 1	5588
April	1st to 10th	Night	51% 6% 10% 19% 12% 1	2217
	11th to 20th	Night	45% 7% 12% 18% 17% 2	1867
	21st to end	Night	48% 6% 10% 18% 14% 3	1890
May	1st to 10th	Night	45% 8% 10% 18% 15% 4	5264
	11th to 20th	Night	44% 8% 11% 21% 14% 1	5235
	21st to end	Night	38% 7% 9% 24% 18% 4	5344
June	1st to 10th	Night	38% 8% 11% 23% 19% 2	1470
	11th to 20th	Night	36% 8% 12% 23% 18% 3	1470
	21st to end	Night	34% 8% 14% 24% 18% 2	1470
July	1st to 10th	Night	34% 8% 15% 23% 17% 3	4309
	11th to 20th	Night	25% 8% 13% 34% 19% 1	4662
	21st to end	Night	32% 8% 13% 24% 22% 1	5474
August	1st to 10th	Night	35% 8% 14% 27% 15% 1	2994
	11th to 20th	Night	35% 8% 17% 25% 13% 2	2992
	21st to end	Night	32% 10% 14% 31% 13% 1	3304
September	1st to 10th	Night	35% 9% 15% 26% 14% 1	2085
	11th to 20th	Night	30% 6% 13% 33% 16% 1	2200
	21st to end	Night	43% 7% 12% 27% 10% 1	2200
October	1st to 10th	Night	28% 7% 12% 29% 23% 1	3755
	11th to 20th	Night	47% 5% 11% 22% 15%	3899
	21st to end	Night	46% 7% 13% 25% 9% 1	4475
November	1st to 10th	Night	47% 8% 14% 21% 9%	2666
	11th to 20th	Night	54% 9% 16% 14% 6% 1	2812
	21st to end	Night	61% 8% 17% 12% 2	2940
December	1st to 10th	Night	62% 11% 17% 7% 2	4720
	11th to 20th	Night	61% 8% 12% 11% 7%	5617
	21st to end	Night	73% 8% 10% 7% 2	7653

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	19% 13% 38% 22% 9% 1	329
	11th to 20th	Day	22% 9% 41% 20% 7%	330
	21st to end	Day	20% 9% 35% 28% 8% 1	363
February	1st to 10th	Day	18% 13% 36% 25% 6% 1	330
	11th to 20th	Day	17% 14% 35% 31% 3	330
	21st to end	Day	15% 17% 40% 23% 6%	273
March	1st to 10th	Day	17% 14% 35% 27% 7%	408
	11th to 20th	Day	18% 9% 32% 30% 10% 1	540
	21st to end	Day	21% 8% 31% 31% 8% 1	594
April	1st to 10th	Day	18% 9% 32% 31% 9% 1	540
	11th to 20th	Day	22% 9% 24% 28% 12% 5%	532
	21st to end	Day	17% 5% 26% 36% 15% 1	540
May	1st to 10th	Day	15% 6% 24% 38% 15% 3	543
	11th to 20th	Day	15% 3 22% 34% 21% 5%	567
	21st to end	Day	13% 4 15% 35% 26% 7%	617
June	1st to 10th	Day	11% 3 17% 39% 25% 5%	778
	11th to 20th	Day	11% 2 18% 38% 25% 5%	780
	21st to end	Day	12% 4 12% 42% 26% 4	780
July	1st to 10th	Day	8% 4 9% 35% 34% 9%	771
	11th to 20th	Day	10% 3 12% 38% 30% 7%	768
	21st to end	Day	8% 1 12% 40% 31% 8%	672
August	1st to 10th	Day	10% 3 11% 33% 33% 11%	566
	11th to 20th	Day	9% 2 9% 37% 33% 10%	559
	21st to end	Day	11% 5% 12% 31% 35% 7%	619
September	1st to 10th	Day	12% 4 19% 28% 25% 11%	530
	11th to 20th	Day	11% 4 14% 24% 27% 21%	530
	21st to end	Day	18% 7% 19% 25% 21% 10%	530
October	1st to 10th	Day	17% 7% 24% 24% 21% 8%	530
	11th to 20th	Day	20% 11% 29% 25% 12% 2	530
	21st to end	Day	25% 11% 29% 26% 8% 1	443
November	1st to 10th	Day	20% 13% 26% 28% 12% 2	330
	11th to 20th	Day	22% 17% 31% 23% 5% 2	286
	21st to end	Day	25% 11% 38% 23% 4	220
December	1st to 10th	Day	23% 15% 35% 19% 8%	220
	11th to 20th	Day	25% 22% 32% 19% 2	220
	21st to end	Day	29% 13% 33% 21% 3	242
January	1st to 10th	Night	22% 13% 38% 21% 6%	1050
	11th to 20th	Night	21% 14% 35% 24% 6%	1050
	21st to end	Night	17% 9% 37% 29% 7%	950
February	1st to 10th	Night	21% 10% 36% 28% 5%	840
	11th to 20th	Night	21% 12% 32% 28% 7%	823
	21st to end	Night	23% 9% 37% 27% 4	692
March	1st to 10th	Night	18% 8% 34% 31% 10%	840
	11th to 20th	Night	17% 10% 34% 31% 8% 1	840
	21st to end	Night	21% 9% 27% 32% 11% 1	924
April	1st to 10th	Night	22% 9% 29% 31% 9% 1	767
	11th to 20th	Night	20% 5% 29% 30% 12% 4	623
	21st to end	Night	20% 7% 22% 35% 15% 1	630
May	1st to 10th	Night	15% 5% 21% 40% 17% 2	630
	11th to 20th	Night	18% 3 20% 36% 21% 3	621
	21st to end	Night	13% 2 15% 37% 28% 5%	543
June	1st to 10th	Night	18% 2 14% 37% 27% 3	420
	11th to 20th	Night	14% 3 15% 40% 26% 3	420
	21st to end	Night	11% 2 14% 42% 29% 2	420
July	1st to 10th	Night	7% 2 9% 42% 35% 5%	488
	11th to 20th	Night	10% 2 11% 36% 36% 6%	612
	21st to end	Night	10% 2 10% 44% 30% 4	690
August	1st to 10th	Night	11% 3 10% 38% 32% 6%	627
	11th to 20th	Night	11% 2 13% 33% 33% 8%	624
	21st to end	Night	12% 2 16% 32% 31% 6%	687
September	1st to 10th	Night	13% 4 20% 29% 24% 10%	744
	11th to 20th	Night	14% 3 14% 28% 30% 11%	800
	21st to end	Night	18% 6% 19% 30% 20% 8%	800
October	1st to 10th	Night	18% 6% 27% 26% 20% 4	800
	11th to 20th	Night	24% 12% 29% 24% 11% 1	800
	21st to end	Night	26% 10% 29% 28% 6%	880
November	1st to 10th	Night	21% 11% 31% 25% 10% 1	820
	11th to 20th	Night	20% 17% 34% 20% 9%	840
	21st to end	Night	26% 14% 35% 20% 4	840
December	1st to 10th	Night	26% 16% 36% 17% 5%	840
	11th to 20th	Night	21% 16% 38% 20% 4	971
	21st to end	Night	29% 13% 34% 21% 3	1155

Page generated at 2026-08-27 04:07:15. Version ac07

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