Cherry Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cherry Hill Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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% 7% 3 6% 4 4 4 4 5% 7% 45%	132229
	Night	11% 7% 2 6% 4 3 3 3 5% 6% 50%	130891
Transparency	Day	38% 6% 10% 22% 19% 6%	135838
	Night	33% 5% 10% 23% 21% 8%	135359
Seeing	Day	16% 13% 26% 25% 15% 4	17327
	Night	15% 12% 26% 29% 17% 2	27224

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% 10% 4 7% 3 3 3 2 3 5% 46%	5289
	11th to 20th	Day	6% 6% 3 7% 3 4 4 3 4 5% 57%	5310
	21st to end	Day	20% 7% 2 6% 3 2 2 3 4 5% 47%	5812
February	1st to 10th	Day	20% 9% 3 6% 4 4 3 3 4 3 41%	1890
	11th to 20th	Day	16% 8% 4 7% 5% 3 3 3 4 5% 41%	1903
	21st to end	Day	18% 10% 3 6% 3 3 3 3 3 5% 42%	1903
March	1st to 10th	Day	18% 9% 3 7% 3 3 3 3 4 5% 41%	5046
	11th to 20th	Day	10% 6% 2 6% 4 4 4 4 5% 6% 50%	5090
	21st to end	Day	13% 8% 3 8% 5% 4 4 3 5% 6% 40%	5583
April	1st to 10th	Day	15% 7% 2 7% 4 4 4 3 5% 7% 43%	2420
	11th to 20th	Day	17% 7% 2 7% 4 3 4 3 4 6% 41%	2494
	21st to end	Day	16% 9% 4 9% 5% 4 4 3 5% 5% 37%	2520
May	1st to 10th	Day	17% 9% 3 8% 4 4 4 3 5% 6% 37%	5800
	11th to 20th	Day	12% 7% 3 8% 5% 5% 6% 4 6% 6% 37%	5941
	21st to end	Day	15% 9% 3 8% 5% 5% 5% 3 5% 6% 35%	6434
June	1st to 10th	Day	9% 6% 2 6% 6% 5% 5% 5% 7% 9% 41%	2600
	11th to 20th	Day	6% 6% 3 7% 6% 5% 7% 5% 7% 7% 40%	2600
	21st to end	Day	4 5% 2 4 5% 5% 5% 5% 5% 9% 50%	2600
July	1st to 10th	Day	8% 6% 2 7% 5% 6% 6% 5% 8% 11% 36%	5900
	11th to 20th	Day	4 4 2 4 3 4 4 4 5% 8% 58%	5900
	21st to end	Day	7% 5% 2 8% 5% 6% 6% 5% 7% 9% 38%	6422
August	1st to 10th	Day	3 3 1 4 4 4 5% 4 6% 8% 57%	4234
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 7% 10% 45%	4040
	21st to end	Day	5% 4 2 6% 5% 5% 6% 5% 7% 10% 47%	4312
September	1st to 10th	Day	10% 5% 2 7% 5% 5% 4 4 6% 6% 47%	2400
	11th to 20th	Day	12% 4 2 5% 3 4 5% 5% 6% 8% 45%	2235
	21st to end	Day	10% 4 1 4 4 4 4 3 6% 7% 53%	2167
October	1st to 10th	Day	8% 4 1 5% 4 4 4 3 4 7% 56%	2000
	11th to 20th	Day	12% 5% 2 5% 3 2 3 2 4 4 57%	2000
	21st to end	Day	18% 7% 2 4 3 3 3 2 3 5% 49%	2200
November	1st to 10th	Day	12% 7% 2 5% 3 3 3 2 3 7% 53%	2058
	11th to 20th	Day	10% 5% 2 5% 4 3 3 3 5% 5% 54%	2100
	21st to end	Day	16% 8% 3 5% 3 3 3 3 4 6% 48%	1928
December	1st to 10th	Day	17% 7% 2 6% 3 2 2 2 4 5% 50%	3030
	11th to 20th	Day	14% 12% 3 7% 4 3 3 2 4 4 43%	3481
	21st to end	Day	18% 11% 4 7% 4 4 3 3 4 4 38%	4587
January	1st to 10th	Night	13% 8% 3 8% 3 3 3 3 4 5% 46%	7634
	11th to 20th	Night	8% 5% 2 4 2 2 2 2 3 4 67%	7680
	21st to end	Night	21% 9% 2 6% 3 3 3 2 3 5% 43%	8402
February	1st to 10th	Night	17% 9% 3 6% 4 3 4 4 4 5% 43%	2730
	11th to 20th	Night	16% 10% 3 6% 4 3 3 2 4 5% 43%	2620
	21st to end	Night	15% 7% 2 6% 3 3 3 3 4 5% 48%	1903
March	1st to 10th	Night	18% 10% 3 6% 3 3 3 2 4 4 43%	5029
	11th to 20th	Night	10% 7% 2 5% 4 3 4 3 5% 6% 52%	5090
	21st to end	Night	12% 8% 3 6% 4 3 4 3 5% 8% 44%	5588
April	1st to 10th	Night	12% 7% 3 5% 4 4 3 3 5% 7% 48%	2199
	11th to 20th	Night	12% 7% 2 6% 4 4 4 2 5% 6% 48%	2075
	21st to end	Night	12% 7% 3 6% 4 4 4 3 4 7% 47%	2100
May	1st to 10th	Night	15% 9% 3 7% 4 4 4 3 5% 6% 39%	4322
	11th to 20th	Night	12% 8% 3 7% 4 5% 4 4 6% 8% 39%	4107
	21st to end	Night	15% 8% 3 6% 5% 3 4 4 5% 7% 41%	4443
June	1st to 10th	Night	4 4 2 6% 3 4 4 3 6% 10% 55%	1800
	11th to 20th	Night	4 5% 2 5% 4 3 3 3 5% 9% 57%	1800
	21st to end	Night	4 3 1 3 2 3 4 4 5% 8% 64%	1800
July	1st to 10th	Night	6% 4 2 5% 4 5% 4 5% 8% 12% 44%	4078
	11th to 20th	Night	4 3 1 3 2 2 3 3 5% 8% 66%	4077
	21st to end	Night	4 5% 2 6% 5% 5% 5% 4 7% 9% 49%	4449
August	1st to 10th	Night	2 2 1 3 3 3 3 3 5% 8% 67%	2926
	11th to 20th	Night	4 3 1 4 3 3 4 4 6% 9% 59%	3015
	21st to end	Night	4 4 1 4 4 3 4 4 6% 9% 58%	3586
September	1st to 10th	Night	7% 4 2 5% 4 3 4 4 4 6% 58%	2000
	11th to 20th	Night	10% 4 2 5% 4 4 4 3 6% 8% 50%	2160
	21st to end	Night	8% 4 1 4 3 3 4 3 5% 7% 57%	2211
October	1st to 10th	Night	6% 5% 1 4 3 3 3 3 4 7% 62%	2400
	11th to 20th	Night	11% 4 2 4 3 3 2 2 4 5% 60%	2400
	21st to end	Night	15% 7% 2 5% 3 3 2 2 3 4 54%	2640
November	1st to 10th	Night	11% 6% 2 5% 2 3 3 2 4 5% 57%	2460
	11th to 20th	Night	9% 6% 2 5% 3 3 2 3 5% 6% 57%	2520
	21st to end	Night	14% 8% 2 5% 3 3 3 3 4 5% 50%	2688
December	1st to 10th	Night	10% 7% 3 8% 4 3 3 3 4 5% 50%	4380
	11th to 20th	Night	12% 10% 4 7% 5% 3 4 2 4 3 45%	4946
	21st to end	Night	15% 11% 4 8% 4 3 3 2 4 5% 41%	6633

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	42% 4 8% 25% 19% 3	5290
	11th to 20th	Day	31% 3 7% 20% 31% 8%	5310
	21st to end	Day	42% 3 3 18% 22% 11%	5812
February	1st to 10th	Day	48% 3 9% 18% 17% 4	1890
	11th to 20th	Day	45% 5% 7% 19% 19% 4	1912
	21st to end	Day	47% 3 7% 13% 21% 9%	1903
March	1st to 10th	Day	45% 5% 6% 15% 16% 13%	5046
	11th to 20th	Day	36% 5% 7% 16% 20% 18%	5090
	21st to end	Day	43% 6% 10% 22% 19% 1	5585
April	1st to 10th	Day	42% 6% 8% 20% 18% 5%	2420
	11th to 20th	Day	44% 5% 10% 18% 18% 5%	2494
	21st to end	Day	47% 7% 8% 19% 16% 3	2520
May	1st to 10th	Day	48% 6% 8% 23% 14% 1	5835
	11th to 20th	Day	42% 9% 11% 19% 17% 2	5945
	21st to end	Day	49% 7% 11% 16% 13% 3	6447
June	1st to 10th	Day	37% 10% 13% 24% 14% 2	2600
	11th to 20th	Day	38% 10% 12% 22% 17% 1	2600
	21st to end	Day	30% 8% 15% 29% 16% 2	2600
July	1st to 10th	Day	37% 10% 19% 26% 8% 1	5836
	11th to 20th	Day	23% 6% 14% 40% 16% 1	5831
	21st to end	Day	37% 9% 15% 28% 10% 1	6240
August	1st to 10th	Day	23% 7% 12% 26% 27% 4	4220
	11th to 20th	Day	34% 7% 16% 33% 9% 1	4079
	21st to end	Day	30% 9% 14% 30% 15% 2	4311
September	1st to 10th	Day	37% 7% 10% 21% 17% 8%	2400
	11th to 20th	Day	34% 8% 14% 22% 16% 6%	2235
	21st to end	Day	30% 6% 10% 19% 25% 9%	2167
October	1st to 10th	Day	18% 3 5% 12% 32% 30%	3260
	11th to 20th	Day	21% 3 4 18% 35% 20%	3240
	21st to end	Day	32% 4 6% 29% 23% 7%	3538
November	1st to 10th	Day	34% 4 8% 23% 24% 8%	2058
	11th to 20th	Day	31% 5% 9% 21% 25% 10%	2100
	21st to end	Day	39% 4 6% 18% 26% 7%	1928
December	1st to 10th	Day	40% 3 7% 27% 20% 4	3030
	11th to 20th	Day	45% 4 4 18% 24% 5%	3479
	21st to end	Day	47% 5% 6% 21% 17% 4	4587
January	1st to 10th	Night	39% 4 7% 27% 21% 2	7635
	11th to 20th	Night	23% 3 5% 19% 36% 14%	7680
	21st to end	Night	45% 3 5% 15% 21% 10%	8402
February	1st to 10th	Night	43% 5% 8% 19% 18% 6%	2730
	11th to 20th	Night	45% 3 7% 19% 19% 6%	2621
	21st to end	Night	39% 3 8% 16% 22% 11%	1903
March	1st to 10th	Night	44% 5% 6% 13% 19% 13%	5029
	11th to 20th	Night	33% 4 6% 21% 19% 17%	5090
	21st to end	Night	37% 6% 11% 23% 19% 4	5588
April	1st to 10th	Night	36% 5% 11% 20% 19% 9%	2199
	11th to 20th	Night	37% 7% 8% 20% 20% 9%	2075
	21st to end	Night	37% 7% 11% 21% 21% 3	2100
May	1st to 10th	Night	45% 7% 11% 22% 11% 3	4352
	11th to 20th	Night	40% 9% 14% 21% 15% 1	4108
	21st to end	Night	41% 8% 14% 19% 13% 5%	4452
June	1st to 10th	Night	25% 9% 14% 32% 18% 3	1800
	11th to 20th	Night	25% 6% 14% 31% 21% 3	1800
	21st to end	Night	18% 7% 15% 30% 26% 4	1800
July	1st to 10th	Night	29% 9% 21% 28% 12% 1	4035
	11th to 20th	Night	17% 5% 13% 45% 18% 1	4026
	21st to end	Night	29% 7% 17% 30% 15% 1	4320
August	1st to 10th	Night	14% 7% 15% 29% 28% 6%	2916
	11th to 20th	Night	22% 6% 20% 35% 16% 1	3042
	21st to end	Night	22% 7% 17% 29% 23% 2	3586
September	1st to 10th	Night	27% 6% 10% 25% 23% 9%	2000
	11th to 20th	Night	31% 7% 12% 22% 21% 7%	2160
	21st to end	Night	28% 5% 10% 21% 27% 9%	2211
October	1st to 10th	Night	15% 4 7% 14% 28% 33%	3920
	11th to 20th	Night	18% 2 4 16% 35% 23%	3899
	21st to end	Night	29% 3 5% 25% 27% 11%	4252
November	1st to 10th	Night	32% 4 8% 20% 25% 11%	2460
	11th to 20th	Night	28% 4 9% 22% 26% 12%	2520
	21st to end	Night	37% 4 7% 19% 26% 7%	2688
December	1st to 10th	Night	36% 4 7% 29% 20% 5%	4380
	11th to 20th	Night	45% 3 4 18% 20% 10%	4947
	21st to end	Night	43% 5% 8% 22% 19% 4	6633

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	25% 22% 34% 15% 2 1	330
	11th to 20th	Day	22% 28% 38% 10% 2 1	330
	21st to end	Day	29% 26% 32% 9% 4	363
February	1st to 10th	Day	29% 24% 32% 12% 3	330
	11th to 20th	Day	34% 29% 30% 5% 2	330
	21st to end	Day	37% 26% 27% 8% 1	273
March	1st to 10th	Day	30% 24% 30% 14% 2	330
	11th to 20th	Day	33% 26% 32% 8% 1	330
	21st to end	Day	27% 25% 37% 9% 1	363
April	1st to 10th	Day	27% 22% 31% 15% 6%	440
	11th to 20th	Day	27% 21% 32% 14% 5%	532
	21st to end	Day	22% 24% 30% 17% 5% 1	540
May	1st to 10th	Day	16% 22% 38% 19% 6%	662
	11th to 20th	Day	15% 15% 34% 26% 10% 2	756
	21st to end	Day	12% 12% 35% 24% 14% 3	789
June	1st to 10th	Day	11% 6% 24% 32% 22% 5%	730
	11th to 20th	Day	6% 7% 28% 32% 23% 5%	730
	21st to end	Day	4 5% 26% 36% 23% 7%	730
July	1st to 10th	Day	7% 4 18% 34% 29% 8%	721
	11th to 20th	Day	4 3 15% 38% 31% 9%	718
	21st to end	Day	6% 2 11% 38% 31% 11%	794
August	1st to 10th	Day	6% 2 14% 42% 30% 6%	726
	11th to 20th	Day	12% 2 11% 35% 30% 9%	636
	21st to end	Day	10% 2 7% 33% 35% 13%	583
September	1st to 10th	Day	7% 3 12% 38% 33% 8%	530
	11th to 20th	Day	7% 4 19% 32% 31% 6%	365
	21st to end	Day	15% 7% 25% 33% 17% 3	330
October	1st to 10th	Day	5% 7% 32% 36% 18% 1	330
	11th to 20th	Day	13% 14% 38% 25% 8% 1	330
	21st to end	Day	29% 15% 32% 18% 5% 1	363
November	1st to 10th	Day	16% 21% 32% 22% 8% 1	330
	11th to 20th	Day	18% 17% 41% 20% 3 1	330
	21st to end	Day	23% 25% 31% 15% 5%	330
December	1st to 10th	Day	25% 24% 30% 17% 3 1	330
	11th to 20th	Day	23% 26% 30% 16% 4	330
	21st to end	Day	26% 25% 34% 13% 3	363
January	1st to 10th	Night	20% 17% 33% 26% 4	1050
	11th to 20th	Night	21% 21% 32% 21% 4	1050
	21st to end	Night	20% 18% 34% 25% 4	1155
February	1st to 10th	Night	23% 19% 31% 23% 4	1050
	11th to 20th	Night	26% 21% 34% 17% 2	986
	21st to end	Night	24% 18% 33% 22% 3	519
March	1st to 10th	Night	23% 15% 31% 26% 5%	630
	11th to 20th	Night	23% 16% 33% 23% 5%	630
	21st to end	Night	16% 18% 35% 26% 5%	693
April	1st to 10th	Night	17% 21% 30% 26% 6%	630
	11th to 20th	Night	15% 21% 31% 23% 10%	623
	21st to end	Night	15% 18% 34% 23% 9% 1	630
May	1st to 10th	Night	13% 11% 32% 34% 10%	630
	11th to 20th	Night	13% 11% 28% 31% 15% 2	609
	21st to end	Night	10% 7% 28% 31% 21% 3	650
June	1st to 10th	Night	6% 4 17% 35% 36% 2	600
	11th to 20th	Night	6% 3 21% 40% 29% 2	600
	21st to end	Night	4 1 16% 37% 38% 5%	600
July	1st to 10th	Night	4 2 11% 33% 43% 6%	585
	11th to 20th	Night	4 1 8% 34% 46% 8%	582
	21st to end	Night	4 7% 37% 44% 8%	657
August	1st to 10th	Night	4 1 9% 35% 46% 5%	597
	11th to 20th	Night	9% 1 9% 33% 42% 6%	594
	21st to end	Night	7% 2 8% 34% 42% 7%	660
September	1st to 10th	Night	8% 2 11% 37% 39% 3	600
	11th to 20th	Night	11% 3 12% 34% 37% 5%	600
	21st to end	Night	10% 2 16% 32% 35% 4	611
October	1st to 10th	Night	8% 4 19% 38% 28% 3	800
	11th to 20th	Night	14% 8% 29% 33% 16% 1	800
	21st to end	Night	19% 11% 27% 26% 15% 1	880
November	1st to 10th	Night	13% 11% 29% 35% 12%	820
	11th to 20th	Night	14% 13% 31% 32% 10%	840
	21st to end	Night	20% 16% 30% 25% 9%	1008
December	1st to 10th	Night	19% 14% 31% 30% 5%	1050
	11th to 20th	Night	22% 16% 30% 27% 5%	1050
	21st to end	Night	22% 16% 30% 25% 6%	1155

Page generated at 2026-05-14 00:58:57. Version ac07

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