Appomattox Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Appomattox Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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% 6% 2 6% 4 4 5% 4 6% 8% 43%	130629
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 45%	130516
Transparency	Day	38% 6% 12% 22% 17% 5%	134235
	Night	38% 6% 10% 22% 19% 6%	134980
Seeing	Day	19% 11% 26% 24% 14% 5%	16775
	Night	17% 10% 28% 30% 14% 2	26818

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	16% 7% 2 4 3 3 3 3 5% 5% 49%	5258
	11th to 20th	Day	12% 5% 1 5% 3 3 3 3 4 5% 56%	5280
	21st to end	Day	16% 6% 3 7% 4 4 3 3 3 5% 47%	5780
February	1st to 10th	Day	12% 8% 2 6% 4 5% 4 4 6% 7% 42%	1890
	11th to 20th	Day	21% 6% 2 6% 3 3 4 3 5% 7% 42%	1843
	21st to end	Day	19% 7% 2 6% 5% 5% 4 3 6% 5% 38%	1835
March	1st to 10th	Day	17% 7% 2 6% 5% 4 5% 4 5% 6% 40%	4957
	11th to 20th	Day	11% 5% 2 5% 4 4 4 3 5% 7% 51%	4980
	21st to end	Day	16% 6% 3 6% 5% 4 5% 4 5% 6% 41%	5465
April	1st to 10th	Day	11% 6% 2 5% 4 4 4 4 5% 7% 50%	2338
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 5% 8% 47%	2374
	21st to end	Day	16% 5% 3 6% 5% 4 4 3 6% 6% 42%	2434
May	1st to 10th	Day	12% 6% 2 6% 5% 5% 6% 5% 8% 11% 33%	5862
	11th to 20th	Day	20% 8% 2 7% 5% 4 4 4 6% 8% 30%	5861
	21st to end	Day	14% 5% 2 6% 4 4 5% 4 6% 9% 43%	6468
June	1st to 10th	Day	10% 4 2 5% 4 5% 6% 5% 6% 10% 44%	2800
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 5% 7% 11% 40%	2800
	21st to end	Day	7% 5% 2 5% 4 5% 6% 6% 8% 10% 42%	2718
July	1st to 10th	Day	7% 5% 2 6% 5% 5% 6% 5% 7% 12% 41%	5900
	11th to 20th	Day	7% 6% 3 8% 6% 5% 7% 6% 10% 14% 29%	5900
	21st to end	Day	9% 6% 2 7% 5% 6% 5% 5% 9% 11% 36%	6424
August	1st to 10th	Day	15% 6% 2 6% 5% 5% 6% 5% 8% 11% 30%	4234
	11th to 20th	Day	8% 6% 2 6% 4 5% 6% 5% 8% 10% 39%	4093
	21st to end	Day	8% 5% 1 5% 3 4 4 4 6% 10% 49%	4312
September	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 5% 7% 49%	2400
	11th to 20th	Day	14% 6% 3 5% 5% 4 4 3 6% 7% 44%	2279
	21st to end	Day	23% 7% 2 6% 4 4 4 3 5% 7% 35%	2150
October	1st to 10th	Day	14% 7% 2 5% 3 3 4 2 3 6% 52%	2000
	11th to 20th	Day	15% 4 1 4 4 4 3 3 3 5% 54%	2000
	21st to end	Day	13% 5% 2 6% 4 3 3 2 4 5% 54%	2200
November	1st to 10th	Day	12% 4 2 4 3 3 3 3 4 6% 55%	2057
	11th to 20th	Day	14% 5% 2 5% 3 3 4 3 4 6% 50%	1954
	21st to end	Day	14% 4 2 5% 3 2 4 3 5% 8% 50%	1680
December	1st to 10th	Day	13% 6% 2 5% 3 4 4 4 5% 6% 48%	2680
	11th to 20th	Day	20% 6% 2 4 3 3 3 3 5% 6% 46%	3070
	21st to end	Day	19% 6% 2 6% 4 4 4 3 5% 7% 39%	4353
January	1st to 10th	Night	13% 5% 2 5% 4 3 4 3 5% 5% 52%	7635
	11th to 20th	Night	12% 6% 2 5% 3 3 3 2 3 4 57%	7680
	21st to end	Night	15% 6% 2 5% 3 3 3 2 4 6% 50%	8401
February	1st to 10th	Night	14% 7% 2 5% 4 3 4 4 5% 8% 44%	2730
	11th to 20th	Night	19% 8% 3 5% 4 3 3 2 5% 6% 42%	2663
	21st to end	Night	14% 6% 2 6% 5% 4 4 3 5% 6% 45%	1966
March	1st to 10th	Night	16% 5% 3 6% 4 4 4 4 5% 5% 45%	4941
	11th to 20th	Night	10% 4 1 4 3 3 4 3 5% 7% 54%	4980
	21st to end	Night	13% 7% 2 6% 4 4 4 3 5% 7% 45%	5467
April	1st to 10th	Night	15% 6% 2 6% 3 3 3 4 6% 6% 46%	2060
	11th to 20th	Night	11% 5% 2 4 3 4 4 4 6% 8% 47%	1975
	21st to end	Night	14% 5% 2 5% 4 4 4 4 7% 7% 44%	1982
May	1st to 10th	Night	15% 7% 3 7% 5% 5% 5% 4 6% 9% 34%	4033
	11th to 20th	Night	22% 8% 3 7% 5% 4 4 3 6% 6% 33%	4032
	21st to end	Night	15% 5% 2 5% 4 4 4 4 6% 9% 41%	4381
June	1st to 10th	Night	9% 6% 2 5% 5% 5% 5% 4 7% 10% 43%	1600
	11th to 20th	Night	10% 6% 2 5% 4 5% 5% 4 7% 10% 42%	1600
	21st to end	Night	7% 5% 2 5% 5% 4 5% 6% 6% 10% 45%	1672
July	1st to 10th	Night	10% 5% 2 7% 6% 6% 5% 5% 8% 9% 37%	4058
	11th to 20th	Night	10% 8% 3 8% 6% 6% 7% 6% 7% 11% 28%	4057
	21st to end	Night	9% 7% 3 8% 6% 6% 6% 6% 8% 8% 32%	4427
August	1st to 10th	Night	16% 7% 3 8% 6% 5% 5% 5% 6% 9% 31%	2915
	11th to 20th	Night	8% 6% 3 7% 5% 5% 6% 5% 8% 10% 36%	2908
	21st to end	Night	9% 5% 2 6% 4 5% 5% 4 6% 10% 43%	3586
September	1st to 10th	Night	11% 6% 2 6% 4 4 4 4 5% 7% 48%	2000
	11th to 20th	Night	14% 6% 2 6% 4 3 4 3 4 7% 45%	2116
	21st to end	Night	24% 8% 2 6% 4 4 4 3 5% 5% 36%	2249
October	1st to 10th	Night	16% 6% 2 5% 4 3 3 2 4 6% 49%	2400
	11th to 20th	Night	14% 5% 2 4 4 4 3 3 5% 6% 51%	2400
	21st to end	Night	13% 5% 2 4 3 3 4 3 4 6% 55%	2640
November	1st to 10th	Night	13% 6% 2 6% 4 3 3 3 3 4 53%	2461
	11th to 20th	Night	13% 5% 2 5% 4 3 4 4 5% 5% 51%	2657
	21st to end	Night	13% 4 2 5% 3 4 4 4 6% 6% 50%	2940
December	1st to 10th	Night	11% 5% 2 5% 3 3 4 3 4 5% 53%	4720
	11th to 20th	Night	19% 8% 2 5% 4 4 3 3 4 5% 41%	5345
	21st to end	Night	17% 5% 2 5% 4 4 4 4 5% 7% 43%	6839

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	37% 3 9% 22% 22% 6%	5259
	11th to 20th	Day	32% 2 5% 18% 29% 14%	5280
	21st to end	Day	41% 3 6% 27% 19% 4	5780
February	1st to 10th	Day	39% 6% 10% 23% 17% 4	1890
	11th to 20th	Day	43% 4 8% 22% 20% 5%	1852
	21st to end	Day	47% 5% 10% 16% 16% 5%	1835
March	1st to 10th	Day	43% 4 9% 18% 17% 10%	4957
	11th to 20th	Day	32% 5% 8% 21% 24% 10%	4980
	21st to end	Day	43% 5% 9% 18% 18% 7%	5465
April	1st to 10th	Day	33% 6% 11% 23% 20% 7%	2338
	11th to 20th	Day	36% 5% 10% 24% 20% 6%	2374
	21st to end	Day	40% 6% 11% 23% 15% 4	2434
May	1st to 10th	Day	42% 9% 15% 22% 10% 1	5897
	11th to 20th	Day	50% 7% 11% 19% 12% 1	5865
	21st to end	Day	36% 8% 15% 19% 18% 3	6483
June	1st to 10th	Day	33% 8% 16% 26% 16% 1	2800
	11th to 20th	Day	36% 9% 17% 21% 15% 2	2800
	21st to end	Day	33% 10% 17% 23% 16% 2	2718
July	1st to 10th	Day	35% 10% 20% 19% 12% 4	5835
	11th to 20th	Day	41% 12% 21% 19% 6% 1	5831
	21st to end	Day	39% 9% 15% 23% 13%	6240
August	1st to 10th	Day	45% 8% 17% 24% 6% 1	4221
	11th to 20th	Day	36% 9% 18% 27% 10%	4132
	21st to end	Day	29% 8% 20% 27% 16% 1	4312
September	1st to 10th	Day	35% 7% 14% 18% 22% 4	2400
	11th to 20th	Day	39% 6% 10% 22% 19% 3	2279
	21st to end	Day	49% 5% 10% 17% 17% 2	2150
October	1st to 10th	Day	34% 3 8% 19% 31% 6%	3260
	11th to 20th	Day	25% 3 6% 23% 29% 14%	3240
	21st to end	Day	23% 3 12% 26% 25% 12%	3538
November	1st to 10th	Day	31% 3 10% 25% 26% 5%	2057
	11th to 20th	Day	35% 4 10% 23% 21% 7%	1954
	21st to end	Day	33% 5% 10% 24% 24% 4	1680
December	1st to 10th	Day	36% 5% 8% 23% 20% 7%	2680
	11th to 20th	Day	40% 3 12% 21% 19% 5%	3066
	21st to end	Day	43% 5% 12% 24% 15% 1	4353
January	1st to 10th	Night	33% 3 10% 22% 22% 9%	7636
	11th to 20th	Night	33% 3 5% 19% 31% 10%	7680
	21st to end	Night	38% 2 6% 27% 21% 7%	8401
February	1st to 10th	Night	38% 5% 12% 23% 19% 3	2730
	11th to 20th	Night	44% 3 8% 19% 19% 7%	2665
	21st to end	Night	40% 4 9% 22% 17% 7%	1966
March	1st to 10th	Night	39% 4 8% 20% 20% 9%	4941
	11th to 20th	Night	29% 5% 7% 18% 30% 11%	4980
	21st to end	Night	39% 4 9% 21% 21% 6%	5467
April	1st to 10th	Night	38% 6% 12% 18% 21% 6%	2060
	11th to 20th	Night	33% 6% 12% 23% 21% 6%	1975
	21st to end	Night	37% 8% 12% 22% 17% 4	1982
May	1st to 10th	Night	45% 8% 15% 18% 11% 2	4060
	11th to 20th	Night	51% 6% 11% 19% 12% 1	4033
	21st to end	Night	38% 8% 15% 25% 12% 2	4388
June	1st to 10th	Night	33% 11% 14% 26% 16% 1	1600
	11th to 20th	Night	36% 8% 18% 20% 16% 2	1600
	21st to end	Night	32% 11% 15% 21% 17% 4	1672
July	1st to 10th	Night	41% 11% 17% 17% 13% 2	4015
	11th to 20th	Night	48% 12% 19% 15% 5% 1	4006
	21st to end	Night	45% 10% 13% 20% 11%	4297
August	1st to 10th	Night	48% 9% 16% 21% 7%	2904
	11th to 20th	Night	38% 9% 17% 24% 10% 1	2935
	21st to end	Night	36% 7% 14% 32% 10% 2	3586
September	1st to 10th	Night	36% 8% 13% 19% 18% 6%	2000
	11th to 20th	Night	38% 8% 10% 21% 21% 2	2116
	21st to end	Night	50% 5% 9% 17% 15% 3	2249
October	1st to 10th	Night	39% 4 10% 18% 24% 5%	3920
	11th to 20th	Night	25% 4 7% 21% 29% 13%	3899
	21st to end	Night	21% 2 6% 30% 28% 13%	4252
November	1st to 10th	Night	36% 4 7% 20% 25% 7%	2461
	11th to 20th	Night	34% 5% 8% 20% 25% 9%	2657
	21st to end	Night	33% 5% 9% 24% 23% 6%	2940
December	1st to 10th	Night	33% 4 7% 28% 24% 4	4720
	11th to 20th	Night	46% 4 10% 18% 15% 8%	5348
	21st to end	Night	39% 6% 12% 26% 16% 2	6839

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	29% 25% 28% 13% 3 1	330
	11th to 20th	Day	30% 28% 33% 8% 1	330
	21st to end	Day	27% 24% 36% 11% 1	363
February	1st to 10th	Day	22% 22% 41% 12% 2	330
	11th to 20th	Day	41% 24% 26% 6% 3	330
	21st to end	Day	31% 19% 31% 15% 3	429
March	1st to 10th	Day	26% 18% 34% 18% 5% 1	508
	11th to 20th	Day	22% 23% 31% 18% 4 1	500
	21st to end	Day	24% 17% 38% 15% 5% 1	550
April	1st to 10th	Day	23% 20% 31% 18% 6% 1	500
	11th to 20th	Day	21% 15% 30% 22% 9% 3	492
	21st to end	Day	21% 11% 35% 23% 9% 1	500
May	1st to 10th	Day	21% 13% 32% 23% 8% 3	503
	11th to 20th	Day	23% 9% 27% 22% 14% 3	527
	21st to end	Day	19% 8% 22% 27% 18% 6%	573
June	1st to 10th	Day	11% 6% 27% 29% 21% 6%	530
	11th to 20th	Day	11% 6% 24% 31% 20% 8%	530
	21st to end	Day	9% 4 22% 33% 24% 7%	530
July	1st to 10th	Day	10% 3 18% 31% 28% 11%	525
	11th to 20th	Day	6% 2 17% 32% 29% 13%	524
	21st to end	Day	11% 3 19% 33% 23% 11%	576
August	1st to 10th	Day	16% 3 19% 26% 24% 11%	526
	11th to 20th	Day	12% 3 17% 31% 26% 12%	519
	21st to end	Day	10% 3 17% 31% 24% 15%	583
September	1st to 10th	Day	14% 4 17% 27% 22% 15%	530
	11th to 20th	Day	17% 4 15% 28% 25% 11%	530
	21st to end	Day	31% 5% 23% 24% 14% 4	530
October	1st to 10th	Day	18% 8% 20% 30% 18% 6%	530
	11th to 20th	Day	15% 13% 24% 27% 18% 3	530
	21st to end	Day	17% 12% 29% 28% 9% 3	583
November	1st to 10th	Day	16% 10% 33% 27% 10% 4	535
	11th to 20th	Day	20% 15% 33% 20% 12%	425
	21st to end	Day	25% 20% 26% 19% 8% 1	220
December	1st to 10th	Day	22% 24% 34% 15% 5%	220
	11th to 20th	Day	26% 14% 33% 22% 3 1	220
	21st to end	Day	29% 16% 32% 16% 6% 1	314
January	1st to 10th	Night	19% 19% 36% 23% 3	840
	11th to 20th	Night	19% 20% 35% 24% 2	840
	21st to end	Night	18% 17% 41% 22% 2	924
February	1st to 10th	Night	18% 15% 40% 24% 3	840
	11th to 20th	Night	26% 19% 37% 17% 1	823
	21st to end	Night	21% 17% 36% 23% 3	692
March	1st to 10th	Night	18% 18% 35% 25% 3	809
	11th to 20th	Night	23% 14% 35% 22% 6% 1	800
	21st to end	Night	18% 15% 37% 25% 5%	880
April	1st to 10th	Night	18% 18% 37% 22% 5%	800
	11th to 20th	Night	15% 14% 34% 28% 9% 1	791
	21st to end	Night	16% 9% 35% 33% 7% 1	782
May	1st to 10th	Night	17% 11% 33% 27% 12% 1	600
	11th to 20th	Night	21% 6% 24% 33% 14% 1	591
	21st to end	Night	17% 2 17% 35% 25% 3	635
June	1st to 10th	Night	9% 3 21% 42% 25% 2	400
	11th to 20th	Night	13% 2 17% 38% 27% 3	400
	21st to end	Night	7% 2 17% 45% 26% 2	472
July	1st to 10th	Night	13% 2 11% 37% 34% 3	585
	11th to 20th	Night	7% 1 11% 37% 38% 6%	582
	21st to end	Night	9% 2 15% 38% 31% 5%	657
August	1st to 10th	Night	14% 3 12% 37% 30% 4	597
	11th to 20th	Night	13% 2 11% 36% 35% 3	614
	21st to end	Night	12% 2 13% 37% 31% 5%	880
September	1st to 10th	Night	14% 2 12% 35% 31% 6%	800
	11th to 20th	Night	17% 3 14% 35% 28% 5%	800
	21st to end	Night	27% 5% 18% 29% 19% 2	800
October	1st to 10th	Night	19% 5% 21% 25% 25% 5%	800
	11th to 20th	Night	15% 9% 26% 34% 16% 1	800
	21st to end	Night	16% 8% 28% 33% 13% 1	880
November	1st to 10th	Night	15% 9% 30% 31% 13% 2	820
	11th to 20th	Night	17% 12% 32% 27% 11%	840
	21st to end	Night	15% 11% 33% 30% 10% 1	840
December	1st to 10th	Night	22% 12% 36% 24% 6%	840
	11th to 20th	Night	20% 10% 36% 29% 5%	840
	21st to end	Night	21% 13% 31% 31% 4	924

Page generated at 2026-02-28 01:40:26. Version ac07

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