Centreville
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Centreville clear sky chart from 2005-11-06 to 2025-07-06.

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	14% 6% 2 6% 4 4 4 4 6% 8% 42%	119966
	Night	15% 6% 2 6% 4 4 4 3 5% 7% 44%	118832
Transparency	Day	38% 6% 12% 24% 16% 4	123604
	Night	38% 5% 11% 24% 18% 4	123332
Seeing	Day	18% 10% 26% 24% 15% 6%	15820
	Night	17% 9% 29% 30% 13% 1	25385

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	15% 4 1 4 3 4 4 3 4 6% 51%	3763
	11th to 20th	Day	14% 8% 3 5% 3 3 3 3 4 4 48%	4060
	21st to end	Day	16% 7% 2 6% 5% 4 4 3 4 5% 44%	4466
February	1st to 10th	Day	15% 8% 2 7% 5% 5% 4 4 5% 6% 40%	1800
	11th to 20th	Day	19% 9% 2 5% 4 3 4 4 5% 7% 40%	1798
	21st to end	Day	19% 7% 2 5% 5% 4 4 3 4 6% 41%	1650
March	1st to 10th	Day	18% 6% 2 5% 4 5% 5% 3 5% 6% 42%	4891
	11th to 20th	Day	11% 6% 2 6% 4 3 4 3 4 6% 51%	4980
	21st to end	Day	17% 6% 2 7% 4 4 4 3 5% 7% 41%	5528
April	1st to 10th	Day	14% 6% 2 6% 4 4 4 3 6% 8% 42%	2400
	11th to 20th	Day	13% 7% 2 6% 5% 4 4 3 4 6% 47%	2374
	21st to end	Day	18% 6% 2 5% 4 4 4 4 5% 7% 41%	2400
May	1st to 10th	Day	19% 7% 3 6% 4 5% 5% 4 7% 9% 31%	5804
	11th to 20th	Day	22% 6% 2 6% 4 4 5% 4 7% 10% 31%	6084
	21st to end	Day	16% 6% 2 6% 4 5% 4 4 6% 8% 39%	6934
June	1st to 10th	Day	10% 5% 2 5% 4 5% 5% 5% 7% 9% 43%	2800
	11th to 20th	Day	11% 5% 3 5% 5% 4 4 4 8% 9% 42%	2800
	21st to end	Day	8% 4 2 5% 4 5% 5% 5% 8% 10% 45%	2800
July	1st to 10th	Day	6% 4 2 6% 5% 5% 5% 6% 8% 10% 43%	4510
	11th to 20th	Day	9% 5% 2 6% 4 5% 6% 5% 7% 12% 39%	4248
	21st to end	Day	11% 6% 3 7% 6% 5% 5% 5% 7% 9% 36%	4466
August	1st to 10th	Day	15% 6% 3 6% 5% 4 5% 5% 8% 10% 34%	4104
	11th to 20th	Day	10% 4 2 7% 5% 4 6% 5% 7% 9% 41%	3889
	21st to end	Day	10% 4 2 5% 4 5% 4 4 6% 9% 48%	4180
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 4 5% 7% 45%	2280
	11th to 20th	Day	12% 5% 2 6% 3 4 4 3 5% 8% 48%	2280
	21st to end	Day	20% 7% 3 6% 4 3 5% 4 5% 6% 38%	1990
October	1st to 10th	Day	18% 5% 2 5% 4 3 3 3 5% 7% 46%	1900
	11th to 20th	Day	17% 5% 1 4 3 3 4 3 4 6% 51%	1900
	21st to end	Day	14% 5% 2 5% 3 3 3 3 4 7% 51%	2090
November	1st to 10th	Day	11% 5% 1 5% 3 4 4 2 5% 7% 51%	1877
	11th to 20th	Day	15% 5% 2 3 3 3 4 4 5% 5% 50%	1770
	21st to end	Day	15% 5% 2 4 4 4 5% 3 5% 5% 48%	1600
December	1st to 10th	Day	12% 5% 3 5% 4 3 4 3 5% 6% 50%	2600
	11th to 20th	Day	19% 8% 2 4 3 3 4 3 5% 6% 43%	2990
	21st to end	Day	18% 8% 2 6% 4 4 4 4 5% 8% 37%	3960
January	1st to 10th	Night	13% 6% 2 5% 3 3 4 3 4 6% 50%	6165
	11th to 20th	Night	13% 7% 2 5% 3 3 3 3 4 5% 52%	5900
	21st to end	Night	15% 8% 2 6% 4 3 3 2 4 5% 49%	6490
February	1st to 10th	Night	16% 7% 2 6% 5% 3 5% 4 5% 6% 41%	2600
	11th to 20th	Night	19% 7% 3 6% 3 3 4 3 4 5% 43%	2488
	21st to end	Night	16% 9% 2 6% 4 3 4 3 5% 6% 42%	1980
March	1st to 10th	Night	16% 5% 2 5% 4 4 4 3 4 5% 47%	4948
	11th to 20th	Night	11% 6% 3 4 3 3 4 3 6% 7% 50%	4980
	21st to end	Night	12% 7% 2 6% 4 4 5% 3 5% 7% 44%	5404
April	1st to 10th	Night	14% 8% 2 7% 5% 4 4 4 6% 6% 41%	2000
	11th to 20th	Night	14% 4 2 6% 4 4 4 3 6% 7% 46%	1975
	21st to end	Night	16% 5% 1 5% 4 4 5% 5% 6% 7% 44%	2000
May	1st to 10th	Night	21% 8% 3 8% 4 5% 5% 4 5% 7% 30%	4134
	11th to 20th	Night	24% 8% 3 6% 4 5% 4 3 6% 7% 31%	3805
	21st to end	Night	17% 7% 2 7% 4 4 6% 4 6% 8% 36%	3920
June	1st to 10th	Night	10% 5% 2 6% 4 4 5% 4 7% 9% 44%	1600
	11th to 20th	Night	13% 6% 2 6% 4 5% 4 5% 6% 9% 42%	1600
	21st to end	Night	10% 4 2 5% 5% 4 4 4 8% 11% 43%	1600
July	1st to 10th	Night	7% 5% 2 7% 5% 5% 6% 5% 8% 9% 42%	2560
	11th to 20th	Night	8% 5% 2 7% 5% 6% 5% 5% 8% 11% 38%	2707
	21st to end	Night	12% 7% 2 7% 5% 4 6% 4 7% 9% 36%	3087
August	1st to 10th	Night	17% 5% 2 8% 5% 5% 6% 4 7% 9% 32%	2825
	11th to 20th	Night	11% 7% 3 8% 6% 5% 5% 4 7% 8% 38%	2908
	21st to end	Night	10% 5% 2 5% 3 4 4 3 5% 8% 51%	3476
September	1st to 10th	Night	15% 5% 2 6% 4 4 5% 4 4 7% 44%	1900
	11th to 20th	Night	10% 6% 2 6% 4 4 4 3 5% 8% 49%	1900
	21st to end	Night	19% 8% 2 7% 4 4 4 3 5% 6% 38%	2185
October	1st to 10th	Night	20% 5% 2 4 4 4 3 2 5% 5% 47%	2280
	11th to 20th	Night	16% 4 2 5% 3 3 3 3 4 5% 52%	2280
	21st to end	Night	13% 5% 2 4 3 4 4 3 5% 6% 51%	2508
November	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 4 5% 54%	2418
	11th to 20th	Night	15% 6% 2 4 3 3 3 3 5% 6% 50%	2628
	21st to end	Night	14% 4 2 4 4 4 4 4 6% 6% 49%	2800
December	1st to 10th	Night	10% 6% 2 5% 3 3 3 3 4 7% 53%	4580
	11th to 20th	Night	21% 6% 2 5% 4 4 4 3 5% 6% 41%	5205
	21st to end	Night	17% 7% 2 6% 4 4 5% 4 6% 7% 37%	6996

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	35% 4 6% 27% 21% 8%	3764
	11th to 20th	Day	40% 4 7% 22% 18% 9%	4060
	21st to end	Day	43% 4 9% 28% 14% 2	4466
February	1st to 10th	Day	45% 4 12% 24% 13% 2	1800
	11th to 20th	Day	43% 4 9% 24% 17% 3	1807
	21st to end	Day	45% 3 11% 21% 16% 4	1650
March	1st to 10th	Day	41% 5% 8% 21% 16% 8%	4891
	11th to 20th	Day	33% 6% 9% 26% 20% 6%	4980
	21st to end	Day	43% 5% 10% 18% 17% 6%	5528
April	1st to 10th	Day	40% 5% 12% 20% 18% 5%	2400
	11th to 20th	Day	38% 5% 10% 21% 20% 6%	2374
	21st to end	Day	42% 5% 10% 23% 16% 4	2400
May	1st to 10th	Day	46% 8% 15% 19% 10% 1	5838
	11th to 20th	Day	47% 8% 11% 25% 8% 1	6088
	21st to end	Day	40% 8% 13% 22% 15% 2	6949
June	1st to 10th	Day	35% 8% 14% 25% 16% 1	2800
	11th to 20th	Day	37% 8% 13% 26% 15% 2	2800
	21st to end	Day	31% 8% 16% 25% 17% 2	2800
July	1st to 10th	Day	33% 10% 19% 21% 12% 5%	4440
	11th to 20th	Day	35% 8% 17% 29% 10% 1	4175
	21st to end	Day	42% 8% 10% 23% 16% 1	4324
August	1st to 10th	Day	43% 8% 16% 25% 7% 1	4091
	11th to 20th	Day	36% 9% 13% 27% 16% 1	3928
	21st to end	Day	33% 7% 14% 28% 17% 2	4180
September	1st to 10th	Day	37% 6% 14% 19% 20% 5%	2280
	11th to 20th	Day	34% 5% 8% 27% 21% 4	2280
	21st to end	Day	45% 5% 10% 20% 18% 2	1990
October	1st to 10th	Day	37% 4 7% 18% 29% 5%	3160
	11th to 20th	Day	24% 4 7% 30% 26% 9%	3140
	21st to end	Day	22% 3 9% 30% 28% 8%	3428
November	1st to 10th	Day	33% 4 11% 27% 20% 5%	1877
	11th to 20th	Day	34% 5% 11% 24% 21% 5%	1770
	21st to end	Day	38% 5% 10% 27% 18% 2	1600
December	1st to 10th	Day	34% 5% 12% 28% 17% 4	2600
	11th to 20th	Day	42% 4 14% 27% 11% 2	2986
	21st to end	Day	43% 7% 14% 21% 15% 1	3960
January	1st to 10th	Night	35% 4 6% 25% 24% 5%	6165
	11th to 20th	Night	34% 4 8% 24% 22% 7%	5900
	21st to end	Night	39% 3 8% 29% 18% 3	6490
February	1st to 10th	Night	42% 5% 12% 23% 15% 2	2600
	11th to 20th	Night	44% 4 8% 22% 18% 5%	2490
	21st to end	Night	42% 5% 10% 22% 17% 5%	1980
March	1st to 10th	Night	39% 4 8% 25% 16% 8%	4948
	11th to 20th	Night	32% 6% 11% 26% 20% 6%	4980
	21st to end	Night	40% 5% 8% 23% 20% 5%	5404
April	1st to 10th	Night	42% 5% 10% 19% 21% 4	2000
	11th to 20th	Night	37% 5% 11% 22% 19% 5%	1975
	21st to end	Night	38% 6% 12% 24% 15% 5%	2000
May	1st to 10th	Night	52% 8% 12% 17% 8% 4	4162
	11th to 20th	Night	50% 6% 11% 21% 11% 1	3806
	21st to end	Night	44% 7% 14% 23% 10% 2	3927
June	1st to 10th	Night	34% 8% 12% 26% 17% 2	1600
	11th to 20th	Night	39% 7% 14% 24% 13% 2	1600
	21st to end	Night	33% 8% 16% 22% 18% 3	1600
July	1st to 10th	Night	36% 9% 20% 18% 13% 3	2520
	11th to 20th	Night	37% 9% 19% 23% 11% 1	2658
	21st to end	Night	42% 8% 14% 20% 15% 1	2988
August	1st to 10th	Night	46% 8% 16% 22% 8% 1	2814
	11th to 20th	Night	41% 8% 12% 25% 13% 1	2935
	21st to end	Night	31% 6% 13% 30% 18% 2	3476
September	1st to 10th	Night	39% 6% 10% 21% 19% 5%	1900
	11th to 20th	Night	33% 5% 9% 27% 22% 3	1900
	21st to end	Night	45% 6% 9% 20% 16% 3	2185
October	1st to 10th	Night	41% 4 9% 21% 20% 5%	3800
	11th to 20th	Night	24% 4 11% 22% 31% 8%	3779
	21st to end	Night	23% 3 9% 32% 26% 7%	4120
November	1st to 10th	Night	33% 3 9% 26% 22% 6%	2418
	11th to 20th	Night	36% 4 9% 24% 22% 6%	2628
	21st to end	Night	35% 5% 11% 25% 21% 3	2800
December	1st to 10th	Night	32% 4 12% 28% 22% 2	4580
	11th to 20th	Night	44% 4 12% 20% 16% 4	5208
	21st to end	Night	43% 7% 14% 21% 14% 1	6996

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	27% 20% 32% 16% 3 1	237
	11th to 20th	Day	29% 19% 30% 18% 3	300
	21st to end	Day	21% 22% 35% 15% 6% 1	330
February	1st to 10th	Day	26% 20% 37% 13% 4	300
	11th to 20th	Day	34% 23% 31% 10% 2	345
	21st to end	Day	28% 16% 33% 19% 4	414
March	1st to 10th	Day	23% 19% 33% 21% 4	500
	11th to 20th	Day	23% 18% 33% 18% 7% 1	500
	21st to end	Day	22% 15% 35% 20% 7% 1	550
April	1st to 10th	Day	24% 18% 31% 20% 6% 1	500
	11th to 20th	Day	17% 16% 29% 22% 13% 3	492
	21st to end	Day	25% 9% 31% 22% 11% 2	500
May	1st to 10th	Day	23% 11% 30% 18% 14% 4	503
	11th to 20th	Day	21% 7% 30% 21% 16% 5%	527
	21st to end	Day	19% 6% 22% 27% 19% 7%	573
June	1st to 10th	Day	12% 4 27% 27% 20% 9%	530
	11th to 20th	Day	13% 7% 22% 33% 18% 8%	530
	21st to end	Day	8% 4 23% 34% 22% 10%	530
July	1st to 10th	Day	11% 3 14% 32% 25% 15%	505
	11th to 20th	Day	5% 3 15% 32% 33% 12%	484
	21st to end	Day	14% 2 18% 29% 28% 10%	532
August	1st to 10th	Day	12% 3 15% 27% 31% 12%	486
	11th to 20th	Day	12% 4 15% 30% 28% 12%	479
	21st to end	Day	8% 2 17% 32% 22% 19%	539
September	1st to 10th	Day	17% 4 17% 24% 22% 17%	490
	11th to 20th	Day	12% 4 18% 28% 23% 15%	490
	21st to end	Day	26% 7% 22% 27% 14% 6%	490
October	1st to 10th	Day	21% 6% 19% 25% 22% 7%	490
	11th to 20th	Day	16% 11% 25% 28% 17% 3	490
	21st to end	Day	17% 8% 33% 27% 12% 4	539
November	1st to 10th	Day	15% 10% 31% 28% 12% 3	455
	11th to 20th	Day	20% 10% 33% 24% 11% 2	370
	21st to end	Day	20% 21% 29% 21% 9% 2	200
December	1st to 10th	Day	20% 14% 39% 21% 5% 2	200
	11th to 20th	Day	27% 12% 32% 21% 9%	200
	21st to end	Day	25% 13% 33% 21% 7% 1	220
January	1st to 10th	Night	21% 17% 41% 18% 3	800
	11th to 20th	Night	22% 14% 34% 26% 4	800
	21st to end	Night	20% 14% 40% 24% 2	880
February	1st to 10th	Night	19% 13% 39% 26% 3	800
	11th to 20th	Night	25% 19% 38% 17% 2	783
	21st to end	Night	24% 16% 36% 21% 3	660
March	1st to 10th	Night	20% 16% 39% 23% 3	800
	11th to 20th	Night	22% 15% 34% 23% 5% 1	800
	21st to end	Night	18% 15% 38% 24% 4 1	880
April	1st to 10th	Night	18% 18% 37% 22% 4	800
	11th to 20th	Night	16% 12% 34% 29% 8% 1	791
	21st to end	Night	20% 8% 32% 33% 8%	800
May	1st to 10th	Night	20% 7% 29% 30% 12% 1	650
	11th to 20th	Night	22% 5% 24% 35% 13% 1	494
	21st to end	Night	14% 3 22% 39% 20% 1	433
June	1st to 10th	Night	10% 3 24% 41% 21% 1	400
	11th to 20th	Night	13% 4 22% 40% 20% 2	400
	21st to end	Night	9% 2 20% 50% 19% 1	400
July	1st to 10th	Night	10% 1 18% 44% 23% 5%	380
	11th to 20th	Night	6% 2 13% 39% 35% 5%	476
	21st to end	Night	9% 3 14% 42% 29% 4	624
August	1st to 10th	Night	11% 3 12% 41% 31% 3	567
	11th to 20th	Night	11% 3 11% 43% 29% 3	635
	21st to end	Night	11% 3 14% 40% 28% 4	836
September	1st to 10th	Night	16% 4 15% 31% 30% 4	760
	11th to 20th	Night	12% 3 13% 38% 32% 3	760
	21st to end	Night	23% 4 20% 32% 21% 1	760
October	1st to 10th	Night	22% 5% 18% 30% 20% 5%	760
	11th to 20th	Night	16% 8% 27% 32% 16%	760
	21st to end	Night	16% 9% 32% 29% 14% 1	836
November	1st to 10th	Night	13% 10% 31% 33% 12% 1	780
	11th to 20th	Night	17% 11% 34% 29% 8% 1	800
	21st to end	Night	15% 12% 33% 30% 8% 1	800
December	1st to 10th	Night	21% 11% 35% 25% 7% 1	800
	11th to 20th	Night	19% 9% 38% 28% 6%	800
	21st to end	Night	20% 11% 34% 32% 3	880

Page generated at 2025-07-03 01:30:13. Version ac07

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