Annville
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Annville clear sky chart from 2005-11-06 to 2026-02-16.

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	16% 6% 2 6% 4 5% 5% 4 6% 8% 38%	131068
	Night	16% 7% 2 6% 4 4 4 4 5% 7% 40%	129779
Transparency	Day	42% 7% 12% 22% 15% 2	134659
	Night	41% 6% 12% 23% 16% 3	134274
Seeing	Day	21% 15% 28% 21% 11% 3	16448
	Night	19% 13% 29% 28% 11% 1	25929

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	13% 6% 2 10% 6% 6% 5% 4 5% 5% 38%	4661
	11th to 20th	Day	12% 10% 4 9% 5% 5% 5% 3 5% 6% 34%	5021
	21st to end	Day	15% 7% 3 8% 6% 5% 5% 4 5% 6% 36%	5780
February	1st to 10th	Day	15% 9% 2 7% 5% 5% 5% 4 6% 6% 36%	1890
	11th to 20th	Day	14% 8% 4 7% 5% 5% 5% 3 5% 7% 37%	1867
	21st to end	Day	18% 9% 3 7% 5% 5% 5% 4 4 6% 36%	1650
March	1st to 10th	Day	20% 4 2 5% 4 4 4 4 4 6% 42%	4696
	11th to 20th	Day	11% 7% 3 7% 5% 5% 4 3 6% 7% 43%	4980
	21st to end	Day	17% 7% 2 5% 4 4 5% 4 5% 6% 41%	5647
April	1st to 10th	Day	16% 8% 3 6% 5% 4 4 4 6% 7% 37%	2400
	11th to 20th	Day	16% 6% 2 6% 4 5% 4 4 6% 6% 41%	2374
	21st to end	Day	18% 5% 2 6% 5% 4 4 4 6% 7% 40%	2400
May	1st to 10th	Day	25% 7% 2 6% 3 4 4 3 6% 8% 30%	5895
	11th to 20th	Day	24% 8% 3 6% 4 4 4 4 5% 8% 30%	6318
	21st to end	Day	19% 5% 2 6% 4 4 5% 4 6% 10% 36%	6949
June	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 5% 7% 9% 39%	2800
	11th to 20th	Day	15% 5% 2 6% 5% 5% 5% 5% 6% 9% 37%	2800
	21st to end	Day	10% 4 2 5% 5% 5% 5% 5% 7% 11% 42%	2800
July	1st to 10th	Day	6% 5% 2 8% 6% 6% 6% 5% 7% 10% 39%	6360
	11th to 20th	Day	10% 7% 3 8% 6% 5% 6% 5% 8% 11% 31%	6360
	21st to end	Day	9% 7% 3 7% 5% 5% 5% 4 6% 9% 40%	6574
August	1st to 10th	Day	19% 5% 2 6% 4 6% 4 5% 7% 9% 32%	4234
	11th to 20th	Day	15% 5% 2 6% 3 4 5% 4 7% 9% 40%	3985
	21st to end	Day	14% 5% 2 4 3 3 3 3 6% 9% 48%	4312
September	1st to 10th	Day	15% 6% 2 5% 3 3 3 4 4 6% 47%	2400
	11th to 20th	Day	13% 4 2 5% 3 3 4 3 5% 7% 50%	2400
	21st to end	Day	22% 7% 2 6% 4 4 4 3 5% 6% 37%	2127
October	1st to 10th	Day	17% 6% 2 4 3 3 4 3 6% 7% 45%	2000
	11th to 20th	Day	16% 5% 2 6% 3 3 4 4 5% 8% 44%	2000
	21st to end	Day	17% 6% 2 5% 3 4 4 3 6% 8% 42%	2200
November	1st to 10th	Day	12% 6% 2 4 3 3 4 3 5% 8% 50%	1856
	11th to 20th	Day	17% 5% 1 5% 4 4 5% 3 6% 7% 42%	1858
	21st to end	Day	16% 6% 3 6% 4 4 5% 4 6% 7% 39%	1680
December	1st to 10th	Day	15% 8% 2 6% 5% 5% 5% 4 5% 6% 39%	2680
	11th to 20th	Day	21% 8% 2 5% 3 4 5% 3 5% 7% 37%	3066
	21st to end	Day	18% 7% 2 8% 5% 5% 5% 4 7% 7% 31%	4048
January	1st to 10th	Night	11% 8% 3 9% 5% 4 5% 4 6% 8% 36%	8234
	11th to 20th	Night	13% 8% 3 7% 5% 4 5% 4 6% 7% 38%	7861
	21st to end	Night	15% 7% 2 6% 4 4 4 4 5% 7% 42%	8401
February	1st to 10th	Night	16% 8% 3 6% 5% 4 5% 4 5% 7% 38%	2730
	11th to 20th	Night	16% 8% 3 7% 4 4 4 3 4 7% 39%	2529
	21st to end	Night	17% 7% 2 7% 5% 5% 4 3 5% 7% 38%	1980
March	1st to 10th	Night	18% 9% 2 6% 4 4 3 3 4 6% 41%	5139
	11th to 20th	Night	10% 6% 2 6% 4 4 3 3 5% 6% 49%	4979
	21st to end	Night	15% 8% 2 6% 5% 4 4 3 5% 6% 43%	5284
April	1st to 10th	Night	19% 8% 2 6% 5% 4 4 4 5% 7% 35%	2000
	11th to 20th	Night	15% 6% 2 6% 4 4 4 4 6% 8% 40%	1975
	21st to end	Night	19% 6% 2 5% 3 3 4 3 5% 6% 44%	2000
May	1st to 10th	Night	26% 7% 3 6% 4 4 4 4 5% 7% 28%	4086
	11th to 20th	Night	25% 8% 3 6% 4 3 4 3 5% 7% 32%	3575
	21st to end	Night	21% 6% 2 6% 4 4 4 3 4 7% 38%	3926
June	1st to 10th	Night	14% 5% 2 5% 5% 4 5% 3 6% 8% 43%	1600
	11th to 20th	Night	13% 5% 2 6% 5% 4 4 5% 6% 9% 41%	1600
	21st to end	Night	11% 4 2 5% 4 5% 6% 5% 6% 11% 42%	1600
July	1st to 10th	Night	8% 6% 3 8% 6% 5% 6% 5% 7% 10% 39%	3600
	11th to 20th	Night	11% 7% 3 8% 6% 6% 6% 5% 7% 9% 31%	3600
	21st to end	Night	12% 6% 2 6% 5% 4 5% 4 6% 8% 42%	4261
August	1st to 10th	Night	20% 6% 2 6% 5% 4 4 4 6% 8% 35%	2915
	11th to 20th	Night	14% 7% 2 8% 4 4 4 3 5% 7% 40%	3029
	21st to end	Night	13% 5% 2 5% 4 4 4 4 6% 8% 46%	3586
September	1st to 10th	Night	14% 6% 2 5% 4 4 3 3 4 7% 48%	2000
	11th to 20th	Night	12% 4 2 5% 4 4 4 4 5% 7% 50%	2000
	21st to end	Night	21% 6% 2 6% 4 4 4 4 5% 6% 38%	2269
October	1st to 10th	Night	20% 6% 1 5% 3 3 3 3 4 6% 46%	2400
	11th to 20th	Night	17% 6% 2 4 3 4 4 2 4 6% 47%	2400
	21st to end	Night	17% 5% 2 4 3 3 4 4 6% 7% 46%	2640
November	1st to 10th	Night	11% 5% 2 5% 4 3 4 3 5% 6% 52%	2662
	11th to 20th	Night	14% 8% 3 5% 4 4 4 3 5% 6% 44%	2760
	21st to end	Night	15% 6% 2 5% 5% 4 4 4 5% 7% 42%	2940
December	1st to 10th	Night	13% 9% 3 6% 4 4 5% 4 6% 7% 40%	4720
	11th to 20th	Night	22% 7% 2 5% 3 3 4 3 5% 6% 38%	5348
	21st to end	Night	19% 8% 3 7% 5% 4 5% 4 6% 6% 33%	7150

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	45% 6% 12% 23% 13% 1	4662
	11th to 20th	Day	47% 5% 9% 18% 15% 5%	5055
	21st to end	Day	47% 5% 12% 22% 13% 1	5780
February	1st to 10th	Day	46% 6% 12% 22% 11% 2	1890
	11th to 20th	Day	45% 7% 10% 23% 13% 2	1876
	21st to end	Day	49% 5% 10% 21% 11% 3	1650
March	1st to 10th	Day	41% 5% 11% 24% 14% 6%	4699
	11th to 20th	Day	38% 7% 13% 26% 11% 5%	4980
	21st to end	Day	43% 5% 10% 18% 20% 4	5647
April	1st to 10th	Day	45% 6% 11% 19% 14% 5%	2400
	11th to 20th	Day	41% 7% 11% 21% 15% 4	2374
	21st to end	Day	41% 7% 10% 22% 16% 4	2400
May	1st to 10th	Day	51% 7% 12% 18% 10% 2	5904
	11th to 20th	Day	51% 8% 11% 20% 10% 1	6322
	21st to end	Day	42% 7% 12% 23% 14% 2	6949
June	1st to 10th	Day	38% 9% 13% 23% 16% 1	2800
	11th to 20th	Day	40% 9% 14% 23% 11% 3	2800
	21st to end	Day	34% 9% 15% 25% 15% 1	2800
July	1st to 10th	Day	38% 9% 14% 22% 16% 2	6290
	11th to 20th	Day	43% 8% 16% 21% 10% 1	6285
	21st to end	Day	40% 6% 11% 20% 22% 1	6386
August	1st to 10th	Day	46% 8% 13% 23% 10%	4221
	11th to 20th	Day	38% 7% 11% 28% 14% 1	4024
	21st to end	Day	33% 6% 11% 29% 20% 2	4312
September	1st to 10th	Day	38% 6% 11% 21% 20% 4	2400
	11th to 20th	Day	33% 5% 10% 26% 22% 3	2400
	21st to end	Day	48% 5% 9% 17% 19% 2	2127
October	1st to 10th	Day	36% 5% 9% 22% 25% 5%	3260
	11th to 20th	Day	28% 5% 12% 28% 21% 6%	3240
	21st to end	Day	27% 4 13% 28% 26% 3	3538
November	1st to 10th	Day	33% 4 14% 24% 20% 4	1856
	11th to 20th	Day	39% 5% 12% 24% 17% 3	1858
	21st to end	Day	42% 5% 13% 29% 9% 2	1680
December	1st to 10th	Day	43% 6% 17% 23% 10% 1	2680
	11th to 20th	Day	45% 5% 15% 28% 7% 1	3066
	21st to end	Day	47% 8% 16% 18% 10% 1	4048
January	1st to 10th	Night	43% 6% 16% 21% 12% 2	8234
	11th to 20th	Night	41% 6% 12% 20% 16% 5%	7902
	21st to end	Night	39% 5% 11% 27% 17% 1	8401
February	1st to 10th	Night	45% 5% 14% 24% 11% 1	2730
	11th to 20th	Night	46% 5% 10% 21% 17% 2	2531
	21st to end	Night	46% 5% 10% 20% 15% 3	1980
March	1st to 10th	Night	43% 5% 10% 22% 12% 7%	5149
	11th to 20th	Night	33% 6% 16% 28% 11% 5%	4980
	21st to end	Night	42% 5% 8% 17% 24% 3	5284
April	1st to 10th	Night	46% 5% 10% 20% 15% 3	2000
	11th to 20th	Night	41% 6% 12% 20% 19% 3	1975
	21st to end	Night	40% 6% 11% 22% 17% 5%	2000
May	1st to 10th	Night	54% 7% 9% 17% 11% 3	4093
	11th to 20th	Night	50% 8% 12% 18% 11% 1	3576
	21st to end	Night	46% 7% 9% 22% 13% 3	3927
June	1st to 10th	Night	38% 8% 10% 23% 19% 2	1600
	11th to 20th	Night	37% 10% 13% 23% 15% 2	1600
	21st to end	Night	34% 8% 14% 26% 16% 2	1600
July	1st to 10th	Night	41% 8% 14% 19% 15% 3	3560
	11th to 20th	Night	47% 8% 14% 19% 11% 1	3552
	21st to end	Night	37% 7% 12% 22% 21% 1	4134
August	1st to 10th	Night	46% 7% 17% 20% 10% 1	2904
	11th to 20th	Night	42% 6% 11% 25% 16% 1	3056
	21st to end	Night	34% 7% 11% 29% 18% 1	3586
September	1st to 10th	Night	37% 6% 12% 22% 20% 4	2000
	11th to 20th	Night	33% 5% 10% 25% 23% 4	2000
	21st to end	Night	46% 6% 10% 17% 20% 2	2269
October	1st to 10th	Night	42% 4 10% 21% 22% 2	3920
	11th to 20th	Night	29% 4 10% 28% 24% 5%	3899
	21st to end	Night	26% 4 12% 31% 25% 2	4252
November	1st to 10th	Night	32% 5% 12% 27% 20% 4	2662
	11th to 20th	Night	40% 4 10% 24% 18% 4	2760
	21st to end	Night	40% 5% 12% 28% 13% 1	2940
December	1st to 10th	Night	42% 5% 14% 28% 10% 1	4720
	11th to 20th	Night	45% 5% 13% 23% 11% 2	5348
	21st to end	Night	48% 8% 12% 18% 14% 1	7150

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	28% 28% 29% 13% 3	220
	11th to 20th	Day	35% 27% 28% 9% 1	288
	21st to end	Day	25% 30% 30% 12% 2 1	363
February	1st to 10th	Day	25% 26% 33% 14% 2	330
	11th to 20th	Day	31% 26% 33% 9%	384
	21st to end	Day	29% 20% 31% 17% 2	414
March	1st to 10th	Day	25% 23% 32% 15% 4	500
	11th to 20th	Day	23% 28% 31% 14% 4 1	500
	21st to end	Day	27% 23% 31% 14% 5% 1	550
April	1st to 10th	Day	28% 22% 29% 14% 6% 1	500
	11th to 20th	Day	25% 23% 25% 17% 7% 3	492
	21st to end	Day	25% 17% 33% 16% 7% 2	500
May	1st to 10th	Day	25% 18% 30% 17% 6% 3	503
	11th to 20th	Day	23% 17% 30% 18% 9% 2	527
	21st to end	Day	20% 9% 26% 24% 16% 5%	573
June	1st to 10th	Day	16% 15% 28% 22% 16% 4	530
	11th to 20th	Day	19% 12% 26% 26% 14% 3	530
	21st to end	Day	12% 10% 31% 29% 14% 4	530
July	1st to 10th	Day	12% 6% 22% 30% 22% 8%	525
	11th to 20th	Day	9% 5% 25% 33% 23% 6%	524
	21st to end	Day	11% 6% 29% 32% 18% 4	576
August	1st to 10th	Day	13% 4 23% 31% 20% 9%	526
	11th to 20th	Day	15% 6% 20% 31% 21% 8%	519
	21st to end	Day	12% 6% 25% 30% 16% 10%	583
September	1st to 10th	Day	18% 11% 19% 23% 22% 8%	530
	11th to 20th	Day	13% 5% 23% 28% 22% 9%	530
	21st to end	Day	28% 12% 22% 21% 14% 4	530
October	1st to 10th	Day	21% 7% 25% 24% 17% 6%	530
	11th to 20th	Day	20% 17% 27% 23% 11% 2	530
	21st to end	Day	19% 17% 31% 22% 8% 2	583
November	1st to 10th	Day	18% 16% 28% 25% 11% 2	428
	11th to 20th	Day	22% 20% 31% 20% 7%	398
	21st to end	Day	27% 22% 32% 14% 5% 1	220
December	1st to 10th	Day	25% 27% 34% 9% 4 1	220
	11th to 20th	Day	30% 22% 30% 15% 3	220
	21st to end	Day	27% 22% 30% 16% 5% 1	242
January	1st to 10th	Night	19% 22% 38% 18% 3	840
	11th to 20th	Night	23% 19% 31% 22% 4	840
	21st to end	Night	18% 21% 37% 22% 2	924
February	1st to 10th	Night	21% 19% 33% 25% 2	840
	11th to 20th	Night	23% 24% 36% 16% 1	804
	21st to end	Night	23% 22% 34% 18% 3	660
March	1st to 10th	Night	21% 20% 33% 22% 4	800
	11th to 20th	Night	23% 20% 33% 20% 5% 1	800
	21st to end	Night	22% 15% 38% 20% 5%	880
April	1st to 10th	Night	24% 20% 33% 19% 5%	800
	11th to 20th	Night	19% 15% 35% 24% 7%	791
	21st to end	Night	22% 13% 32% 25% 7%	800
May	1st to 10th	Night	24% 10% 27% 30% 9%	620
	11th to 20th	Night	20% 8% 34% 29% 9%	394
	21st to end	Night	17% 6% 24% 36% 16% 1	433
June	1st to 10th	Night	15% 6% 26% 34% 20%	400
	11th to 20th	Night	15% 7% 28% 38% 13%	400
	21st to end	Night	11% 3 25% 45% 15% 1	400
July	1st to 10th	Night	11% 4 21% 41% 22% 2	390
	11th to 20th	Night	10% 2 17% 42% 28% 2	388
	21st to end	Night	11% 3 19% 40% 25% 1	581
August	1st to 10th	Night	14% 2 13% 41% 27% 2	597
	11th to 20th	Night	13% 6% 14% 36% 28% 3	683
	21st to end	Night	14% 3 18% 35% 27% 2	880
September	1st to 10th	Night	16% 4 22% 33% 22% 3	800
	11th to 20th	Night	13% 5% 18% 35% 25% 3	800
	21st to end	Night	23% 6% 20% 31% 19% 2	800
October	1st to 10th	Night	22% 7% 21% 28% 19% 3	800
	11th to 20th	Night	20% 10% 28% 30% 12% 1	800
	21st to end	Night	20% 13% 27% 30% 10%	880
November	1st to 10th	Night	15% 12% 32% 30% 11% 1	820
	11th to 20th	Night	19% 19% 28% 27% 7% 1	840
	21st to end	Night	18% 18% 33% 25% 6%	840
December	1st to 10th	Night	23% 16% 33% 22% 5%	840
	11th to 20th	Night	20% 17% 34% 24% 4	840
	21st to end	Night	22% 17% 30% 29% 2	924

Page generated at 2026-02-13 01:42:18. Version ac07

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