Charles City observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Charles City observatory 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	13% 6% 2 6% 4 4 4 4 6% 8% 43%	119732
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	119030
Transparency	Day	39% 5% 12% 22% 18% 4	123369
	Night	39% 5% 10% 23% 19% 5%	123531
Seeing	Day	18% 12% 28% 24% 14% 5%	16020
	Night	17% 10% 28% 30% 13% 1	25648

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 2 3 3 3 4 4 56%	3891
	11th to 20th	Day	12% 7% 2 5% 3 3 3 3 4 5% 52%	4060
	21st to end	Day	14% 7% 3 7% 5% 4 4 3 5% 5% 45%	4466
February	1st to 10th	Day	14% 7% 2 6% 4 5% 5% 4 5% 8% 40%	1800
	11th to 20th	Day	21% 6% 2 4 4 4 3 3 5% 6% 43%	1860
	21st to end	Day	17% 8% 2 6% 5% 5% 4 3 5% 5% 39%	1650
March	1st to 10th	Day	19% 7% 3 6% 4 4 4 4 5% 6% 39%	4863
	11th to 20th	Day	11% 6% 2 5% 3 3 3 3 5% 7% 53%	4980
	21st to end	Day	16% 6% 2 6% 4 4 4 3 5% 6% 44%	5556
April	1st to 10th	Day	12% 6% 2 5% 4 3 4 3 5% 7% 50%	2400
	11th to 20th	Day	11% 6% 2 6% 5% 4 4 3 5% 6% 49%	2374
	21st to end	Day	14% 7% 2 6% 4 4 4 3 5% 7% 44%	2400
May	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 5% 6% 10% 33%	5558
	11th to 20th	Day	19% 7% 3 7% 5% 5% 5% 4 6% 9% 31%	5993
	21st to end	Day	15% 5% 2 5% 3 4 4 4 6% 9% 42%	6934
June	1st to 10th	Day	11% 5% 1 5% 4 5% 5% 5% 6% 10% 44%	2800
	11th to 20th	Day	11% 5% 2 5% 5% 5% 5% 5% 7% 9% 42%	2800
	21st to end	Day	7% 4 2 6% 6% 5% 6% 4 8% 11% 41%	2800
July	1st to 10th	Day	6% 4 2 7% 5% 5% 6% 6% 9% 11% 39%	4510
	11th to 20th	Day	12% 6% 2 6% 6% 5% 6% 5% 7% 11% 33%	4137
	21st to end	Day	11% 6% 2 8% 6% 6% 6% 6% 8% 10% 32%	4466
August	1st to 10th	Day	15% 8% 3 8% 5% 5% 6% 5% 8% 10% 26%	4104
	11th to 20th	Day	10% 5% 3 7% 6% 6% 6% 4 6% 9% 38%	3773
	21st to end	Day	9% 4 2 6% 3 4 4 3 6% 11% 48%	4180
September	1st to 10th	Day	13% 5% 2 6% 5% 5% 5% 4 5% 8% 42%	2280
	11th to 20th	Day	13% 6% 3 7% 4 5% 5% 3 6% 7% 41%	2280
	21st to end	Day	19% 8% 3 8% 5% 4 5% 3 4 6% 36%	2008
October	1st to 10th	Day	15% 6% 3 7% 4 4 4 2 4 6% 45%	1900
	11th to 20th	Day	15% 4 2 5% 3 3 3 3 4 5% 52%	1900
	21st to end	Day	12% 5% 1 5% 4 4 3 3 4 6% 53%	2090
November	1st to 10th	Day	12% 5% 2 6% 2 3 3 4 5% 5% 52%	1897
	11th to 20th	Day	14% 7% 2 5% 3 3 3 3 5% 6% 50%	1800
	21st to end	Day	14% 4 2 5% 3 3 3 3 4 7% 52%	1672
December	1st to 10th	Day	11% 5% 2 5% 4 3 3 3 5% 6% 54%	2600
	11th to 20th	Day	19% 6% 2 5% 3 3 2 2 4 6% 47%	2990
	21st to end	Day	13% 6% 1 5% 4 3 4 3 5% 8% 48%	3960
January	1st to 10th	Night	13% 6% 3 5% 3 3 3 2 4 5% 53%	6050
	11th to 20th	Night	13% 6% 2 6% 3 3 3 3 4 4 54%	5900
	21st to end	Night	17% 7% 2 5% 3 3 3 2 4 5% 50%	6490
February	1st to 10th	Night	16% 8% 2 6% 4 4 4 3 5% 6% 43%	2600
	11th to 20th	Night	21% 8% 2 5% 3 4 4 3 4 6% 42%	2425
	21st to end	Night	17% 7% 2 5% 3 2 3 3 4 6% 47%	1980
March	1st to 10th	Night	19% 6% 2 5% 4 3 3 3 5% 6% 44%	4973
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 5% 6% 50%	4980
	21st to end	Night	14% 7% 2 5% 4 4 3 3 5% 6% 48%	5362
April	1st to 10th	Night	14% 7% 3 6% 4 4 4 4 5% 7% 43%	2000
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 5% 8% 49%	1975
	21st to end	Night	14% 6% 2 5% 4 4 4 4 5% 7% 45%	2000
May	1st to 10th	Night	15% 8% 2 8% 5% 5% 5% 4 6% 9% 33%	4366
	11th to 20th	Night	20% 9% 3 6% 4 4 4 3 5% 7% 34%	3895
	21st to end	Night	17% 7% 2 6% 4 4 4 4 6% 8% 39%	3920
June	1st to 10th	Night	11% 5% 2 6% 4 6% 5% 5% 8% 8% 40%	1600
	11th to 20th	Night	11% 5% 3 6% 4 5% 5% 5% 7% 10% 40%	1600
	21st to end	Night	8% 5% 3 6% 6% 5% 4 5% 7% 9% 43%	1600
July	1st to 10th	Night	9% 4 2 6% 5% 6% 6% 5% 8% 10% 39%	2560
	11th to 20th	Night	12% 5% 3 6% 5% 5% 5% 5% 8% 10% 36%	2805
	21st to end	Night	14% 8% 3 9% 6% 6% 6% 5% 7% 7% 30%	3087
August	1st to 10th	Night	17% 8% 3 7% 5% 4 5% 5% 8% 10% 29%	2825
	11th to 20th	Night	9% 6% 2 7% 5% 5% 5% 4 6% 9% 40%	3023
	21st to end	Night	10% 5% 2 5% 4 4 4 3 6% 8% 49%	3476
September	1st to 10th	Night	12% 6% 2 5% 4 4 5% 4 5% 7% 45%	1900
	11th to 20th	Night	11% 6% 3 6% 4 4 5% 3 6% 6% 45%	1900
	21st to end	Night	21% 8% 3 6% 4 4 3 4 5% 7% 36%	2167
October	1st to 10th	Night	14% 5% 2 7% 4 3 4 3 5% 6% 47%	2280
	11th to 20th	Night	14% 7% 2 6% 4 3 3 3 4 6% 50%	2280
	21st to end	Night	14% 5% 1 5% 3 4 3 2 4 6% 53%	2508
November	1st to 10th	Night	13% 7% 2 5% 4 4 3 3 5% 5% 51%	2397
	11th to 20th	Night	16% 7% 2 5% 3 3 4 3 5% 5% 48%	2600
	21st to end	Night	12% 5% 2 5% 4 4 4 3 5% 6% 51%	2725
December	1st to 10th	Night	10% 5% 1 4 4 3 4 3 5% 6% 55%	4580
	11th to 20th	Night	19% 7% 2 7% 4 5% 4 3 4 5% 39%	5205
	21st to end	Night	13% 7% 3 5% 4 3 3 3 4 7% 47%	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	32% 3 4 27% 24% 10%	3892
	11th to 20th	Day	35% 3 5% 20% 27% 8%	4060
	21st to end	Day	43% 4 8% 26% 16% 3	4466
February	1st to 10th	Day	42% 4 11% 26% 14% 3	1800
	11th to 20th	Day	43% 3 6% 22% 20% 5%	1869
	21st to end	Day	46% 5% 9% 17% 17% 5%	1650
March	1st to 10th	Day	46% 4 7% 17% 20% 7%	4863
	11th to 20th	Day	32% 4 9% 23% 24% 9%	4980
	21st to end	Day	42% 4 11% 17% 20% 6%	5556
April	1st to 10th	Day	35% 4 10% 22% 20% 7%	2400
	11th to 20th	Day	36% 4 10% 23% 21% 6%	2374
	21st to end	Day	40% 5% 11% 22% 16% 6%	2400
May	1st to 10th	Day	45% 7% 16% 19% 11% 1	5590
	11th to 20th	Day	48% 6% 14% 20% 10% 1	5997
	21st to end	Day	38% 6% 14% 23% 17% 2	6949
June	1st to 10th	Day	35% 7% 16% 25% 15% 2	2800
	11th to 20th	Day	36% 7% 16% 24% 16% 1	2800
	21st to end	Day	34% 8% 16% 23% 16% 2	2800
July	1st to 10th	Day	34% 10% 23% 19% 11% 4	4440
	11th to 20th	Day	43% 7% 23% 19% 6% 1	4067
	21st to end	Day	45% 8% 14% 17% 15%	4324
August	1st to 10th	Day	50% 8% 16% 20% 6%	4091
	11th to 20th	Day	41% 8% 14% 25% 12%	3810
	21st to end	Day	30% 5% 16% 28% 19% 1	4180
September	1st to 10th	Day	39% 7% 14% 16% 20% 4	2280
	11th to 20th	Day	40% 5% 9% 23% 19% 3	2280
	21st to end	Day	50% 4 7% 21% 14% 3	2008
October	1st to 10th	Day	37% 5% 7% 19% 26% 5%	3160
	11th to 20th	Day	24% 3 6% 20% 37% 10%	3140
	21st to end	Day	22% 2 8% 28% 28% 12%	3428
November	1st to 10th	Day	34% 4 8% 26% 23% 4	1897
	11th to 20th	Day	36% 3 10% 21% 23% 7%	1800
	21st to end	Day	34% 4 7% 28% 23% 5%	1672
December	1st to 10th	Day	33% 3 8% 28% 21% 7%	2600
	11th to 20th	Day	42% 3 12% 22% 17% 5%	2986
	21st to end	Day	36% 4 12% 29% 17% 2	3960
January	1st to 10th	Night	35% 2 5% 21% 26% 10%	6050
	11th to 20th	Night	35% 3 5% 23% 28% 6%	5900
	21st to end	Night	39% 3 6% 27% 21% 4	6490
February	1st to 10th	Night	42% 3 11% 23% 17% 3	2600
	11th to 20th	Night	45% 3 6% 21% 19% 6%	2427
	21st to end	Night	40% 3 9% 21% 19% 8%	1980
March	1st to 10th	Night	42% 3 7% 20% 21% 7%	4973
	11th to 20th	Night	35% 4 7% 19% 24% 10%	4980
	21st to end	Night	39% 4 8% 24% 19% 6%	5362
April	1st to 10th	Night	40% 6% 9% 18% 21% 6%	2000
	11th to 20th	Night	35% 5% 10% 23% 21% 6%	1975
	21st to end	Night	38% 6% 12% 23% 18% 4	2000
May	1st to 10th	Night	47% 6% 17% 18% 10% 1	4396
	11th to 20th	Night	49% 6% 11% 24% 9% 1	3896
	21st to end	Night	43% 6% 14% 24% 13% 1	3927
June	1st to 10th	Night	38% 8% 14% 23% 16% 1	1600
	11th to 20th	Night	38% 8% 16% 22% 14% 1	1600
	21st to end	Night	35% 7% 14% 24% 16% 4	1600
July	1st to 10th	Night	37% 13% 17% 20% 10% 3	2520
	11th to 20th	Night	40% 10% 22% 22% 5% 1	2753
	21st to end	Night	51% 8% 11% 17% 13%	2988
August	1st to 10th	Night	49% 7% 17% 22% 6%	2814
	11th to 20th	Night	39% 7% 17% 24% 13%	3052
	21st to end	Night	32% 5% 12% 26% 23% 1	3476
September	1st to 10th	Night	37% 7% 17% 17% 17% 5%	1900
	11th to 20th	Night	37% 6% 11% 22% 22% 2	1900
	21st to end	Night	48% 6% 9% 17% 17% 4	2167
October	1st to 10th	Night	37% 6% 11% 20% 22% 5%	3800
	11th to 20th	Night	25% 2 6% 22% 30% 14%	3779
	21st to end	Night	22% 2 5% 32% 27% 13%	4120
November	1st to 10th	Night	36% 4 9% 23% 23% 5%	2397
	11th to 20th	Night	39% 3 8% 20% 20% 10%	2600
	21st to end	Night	34% 4 7% 26% 24% 5%	2725
December	1st to 10th	Night	31% 3 9% 27% 25% 5%	4580
	11th to 20th	Night	47% 4 10% 18% 14% 6%	5208
	21st to end	Night	38% 3 11% 29% 17% 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	23% 27% 33% 13% 3	263
	11th to 20th	Day	22% 34% 31% 11% 2	300
	21st to end	Day	20% 23% 44% 11% 2	330
February	1st to 10th	Day	20% 27% 38% 12% 3	300
	11th to 20th	Day	34% 25% 28% 12% 1	407
	21st to end	Day	28% 18% 34% 16% 4	414
March	1st to 10th	Day	24% 20% 36% 17% 4	500
	11th to 20th	Day	24% 21% 34% 16% 4 1	500
	21st to end	Day	23% 19% 37% 15% 5% 1	550
April	1st to 10th	Day	23% 21% 34% 14% 7% 1	500
	11th to 20th	Day	16% 14% 38% 21% 8% 3	492
	21st to end	Day	19% 15% 35% 24% 6% 1	500
May	1st to 10th	Day	17% 14% 37% 21% 9% 2	503
	11th to 20th	Day	22% 9% 31% 24% 12% 2	527
	21st to end	Day	21% 5% 27% 25% 17% 5%	573
June	1st to 10th	Day	12% 5% 30% 29% 20% 3	530
	11th to 20th	Day	13% 8% 22% 32% 19% 6%	530
	21st to end	Day	9% 4 23% 36% 22% 5%	530
July	1st to 10th	Day	10% 2 17% 33% 29% 9%	505
	11th to 20th	Day	7% 2 14% 39% 30% 9%	484
	21st to end	Day	9% 3 20% 36% 24% 8%	532
August	1st to 10th	Day	13% 4 19% 35% 23% 8%	486
	11th to 20th	Day	14% 3 16% 33% 24% 10%	479
	21st to end	Day	9% 3 18% 30% 25% 14%	539
September	1st to 10th	Day	15% 4 20% 25% 23% 14%	490
	11th to 20th	Day	16% 4 21% 28% 20% 12%	490
	21st to end	Day	26% 7% 26% 23% 12% 5%	490
October	1st to 10th	Day	19% 7% 21% 29% 14% 10%	490
	11th to 20th	Day	16% 13% 23% 29% 17% 3	490
	21st to end	Day	15% 12% 33% 24% 12% 5%	539
November	1st to 10th	Day	18% 9% 32% 23% 14% 3	465
	11th to 20th	Day	20% 15% 34% 21% 10% 1	400
	21st to end	Day	19% 18% 31% 22% 8% 2	272
December	1st to 10th	Day	22% 20% 37% 14% 8% 1	200
	11th to 20th	Day	27% 12% 35% 19% 8%	200
	21st to end	Day	23% 18% 31% 17% 9% 2	220
January	1st to 10th	Night	20% 19% 36% 22% 3	800
	11th to 20th	Night	21% 18% 33% 24% 4	800
	21st to end	Night	20% 15% 38% 25% 1	880
February	1st to 10th	Night	21% 13% 38% 24% 3	800
	11th to 20th	Night	28% 19% 35% 17% 2	783
	21st to end	Night	22% 16% 37% 22% 2	660
March	1st to 10th	Night	19% 16% 37% 24% 4	800
	11th to 20th	Night	21% 17% 35% 22% 5% 1	800
	21st to end	Night	20% 14% 40% 21% 4	880
April	1st to 10th	Night	19% 18% 39% 22% 3	800
	11th to 20th	Night	15% 14% 38% 26% 7% 1	791
	21st to end	Night	17% 10% 35% 31% 7%	800
May	1st to 10th	Night	17% 12% 35% 29% 8%	754
	11th to 20th	Night	17% 5% 30% 37% 11% 1	532
	21st to end	Night	18% 3 21% 39% 19% 1	433
June	1st to 10th	Night	12% 3 21% 46% 18% 2	400
	11th to 20th	Night	13% 4 19% 44% 19% 2	400
	21st to end	Night	9% 3 21% 46% 22% 1	400
July	1st to 10th	Night	10% 4 16% 41% 28% 2	380
	11th to 20th	Night	6% 2 11% 41% 36% 4	532
	21st to end	Night	8% 2 19% 42% 25% 4	624
August	1st to 10th	Night	9% 3 14% 44% 27% 4	567
	11th to 20th	Night	13% 3 12% 42% 25% 4	700
	21st to end	Night	11% 2 15% 36% 31% 5%	836
September	1st to 10th	Night	14% 3 14% 39% 26% 4	760
	11th to 20th	Night	13% 4 17% 33% 29% 4	760
	21st to end	Night	25% 5% 19% 33% 17% 2	760
October	1st to 10th	Night	18% 5% 20% 30% 24% 4	760
	11th to 20th	Night	17% 8% 26% 33% 15% 2	760
	21st to end	Night	17% 9% 29% 28% 15% 2	836
November	1st to 10th	Night	17% 8% 27% 33% 13% 2	780
	11th to 20th	Night	19% 11% 31% 29% 10%	800
	21st to end	Night	14% 14% 31% 30% 11% 1	800
December	1st to 10th	Night	21% 13% 34% 24% 8% 1	800
	11th to 20th	Night	20% 10% 34% 31% 5%	800
	21st to end	Night	20% 12% 32% 30% 6%	880

Page generated at 2025-07-03 01:30:12. Version ac07

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