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 2024-11-04.

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%	101023
	Night	15% 7% 2 6% 4 4 4 3 5% 7% 43%	97921
Transparency	Day	39% 6% 12% 22% 17% 4	104662
	Night	39% 5% 10% 22% 18% 4	102422
Seeing	Day	18% 11% 28% 24% 14% 5%	15184
	Night	17% 10% 28% 31% 13% 1	24783

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	17% 4 2 4 2 3 3 3 4 5% 52%	2713
	11th to 20th	Day	14% 8% 2 6% 3 3 4 3 5% 6% 46%	2840
	21st to end	Day	18% 9% 3 7% 4 4 3 3 5% 5% 39%	3124
February	1st to 10th	Day	13% 7% 3 6% 4 5% 5% 4 5% 8% 40%	1710
	11th to 20th	Day	20% 6% 2 4 4 4 3 3 5% 6% 44%	1764
	21st to end	Day	18% 8% 2 6% 5% 5% 5% 3 5% 5% 37%	1570
March	1st to 10th	Day	22% 10% 3 6% 3 3 3 3 4 5% 37%	3391
	11th to 20th	Day	10% 5% 1 4 4 3 4 3 5% 6% 54%	3480
	21st to end	Day	21% 6% 2 5% 4 4 3 3 5% 6% 41%	3878
April	1st to 10th	Day	12% 5% 1 5% 4 3 4 3 5% 6% 51%	2280
	11th to 20th	Day	11% 6% 2 6% 5% 4 3 3 5% 6% 49%	2254
	21st to end	Day	15% 7% 2 6% 4 3 4 3 5% 7% 44%	2280
May	1st to 10th	Day	16% 7% 3 7% 5% 5% 5% 5% 6% 9% 33%	3876
	11th to 20th	Day	20% 6% 3 7% 5% 5% 5% 4 6% 8% 32%	4171
	21st to end	Day	10% 4 2 5% 3 4 4 5% 6% 10% 47%	4823
June	1st to 10th	Day	11% 5% 1 5% 4 5% 5% 5% 6% 10% 43%	2660
	11th to 20th	Day	11% 5% 2 5% 5% 5% 5% 5% 7% 9% 42%	2660
	21st to end	Day	8% 4 2 6% 6% 5% 6% 4 8% 11% 40%	2660
July	1st to 10th	Day	6% 4 2 7% 5% 5% 6% 6% 9% 11% 39%	4440
	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	13% 5% 2 5% 2 3 3 4 4 5% 52%	1830
	11th to 20th	Day	13% 7% 2 5% 4 3 3 3 4 6% 50%	1710
	21st to end	Day	14% 5% 2 5% 3 3 3 3 4 7% 52%	1589
December	1st to 10th	Day	16% 7% 2 6% 4 3 4 3 6% 7% 43%	1520
	11th to 20th	Day	16% 6% 2 5% 4 3 2 2 4 5% 52%	1910
	21st to end	Day	17% 7% 2 6% 5% 3 3 3 4 6% 44%	2772
January	1st to 10th	Night	14% 7% 3 6% 4 4 3 3 4 5% 47%	4228
	11th to 20th	Night	16% 7% 3 7% 4 4 3 3 4 5% 44%	4120
	21st to end	Night	21% 8% 2 5% 3 3 3 3 3 4 43%	4532
February	1st to 10th	Night	16% 8% 2 6% 4 4 4 3 5% 6% 44%	2470
	11th to 20th	Night	20% 8% 2 5% 3 4 4 3 4 6% 42%	2301
	21st to end	Night	18% 7% 2 6% 3 2 3 3 4 6% 47%	1884
March	1st to 10th	Night	24% 8% 2 6% 4 3 3 3 5% 6% 36%	3459
	11th to 20th	Night	15% 5% 2 4 3 3 3 3 5% 6% 51%	3480
	21st to end	Night	17% 8% 2 5% 3 3 2 2 4 5% 48%	3740
April	1st to 10th	Night	14% 7% 2 6% 4 4 4 4 5% 7% 44%	1900
	11th to 20th	Night	12% 5% 2 6% 4 3 3 3 5% 7% 49%	1875
	21st to end	Night	14% 6% 2 5% 4 4 4 4 6% 7% 45%	1900
May	1st to 10th	Night	17% 8% 2 8% 5% 5% 4 3 5% 8% 34%	3048
	11th to 20th	Night	20% 9% 3 6% 4 4 4 3 5% 6% 37%	2717
	21st to end	Night	12% 5% 2 5% 4 4 4 4 6% 9% 44%	2732
June	1st to 10th	Night	12% 5% 1 6% 4 6% 5% 5% 8% 8% 40%	1520
	11th to 20th	Night	10% 5% 3 6% 4 5% 5% 5% 7% 10% 40%	1520
	21st to end	Night	8% 5% 3 6% 6% 5% 4 5% 7% 9% 43%	1520
July	1st to 10th	Night	8% 4 2 6% 5% 6% 6% 5% 8% 10% 39%	2520
	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 3 3 3 5% 5% 51%	2317
	11th to 20th	Night	15% 7% 2 5% 3 3 4 3 5% 5% 49%	2470
	21st to end	Night	12% 5% 2 5% 4 4 4 3 5% 6% 50%	2588
December	1st to 10th	Night	17% 8% 2 6% 5% 3 4 3 4 6% 42%	2660
	11th to 20th	Night	17% 6% 2 5% 3 4 3 3 4 6% 46%	3285
	21st to end	Night	15% 8% 3 5% 4 3 4 3 5% 7% 43%	4884

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% 32% 20% 3	2714
	11th to 20th	Day	39% 5% 7% 21% 24% 5%	2840
	21st to end	Day	47% 4 9% 20% 17% 3	3124
February	1st to 10th	Day	42% 4 11% 26% 14% 3	1710
	11th to 20th	Day	42% 4 6% 23% 21% 5%	1773
	21st to end	Day	48% 5% 10% 15% 16% 5%	1570
March	1st to 10th	Day	50% 4 8% 17% 16% 5%	3391
	11th to 20th	Day	30% 4 8% 24% 25% 8%	3480
	21st to end	Day	45% 4 12% 18% 18% 4	3878
April	1st to 10th	Day	34% 5% 10% 23% 21% 8%	2280
	11th to 20th	Day	36% 4 10% 22% 21% 6%	2254
	21st to end	Day	40% 5% 11% 22% 16% 6%	2280
May	1st to 10th	Day	47% 7% 14% 20% 10% 2	3911
	11th to 20th	Day	48% 6% 13% 22% 9% 1	4175
	21st to end	Day	32% 6% 15% 25% 19% 3	4837
June	1st to 10th	Day	35% 7% 16% 25% 15% 2	2660
	11th to 20th	Day	36% 7% 16% 24% 15% 1	2660
	21st to end	Day	35% 8% 16% 23% 16% 2	2660
July	1st to 10th	Day	34% 10% 23% 19% 11% 4	4370
	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 9% 26% 24% 4	1830
	11th to 20th	Day	36% 3 10% 21% 23% 7%	1710
	21st to end	Day	34% 4 7% 28% 22% 5%	1589
December	1st to 10th	Day	41% 4 11% 24% 17% 4	1520
	11th to 20th	Day	36% 3 7% 29% 18% 7%	1906
	21st to end	Day	42% 4 8% 29% 15% 2	2772
January	1st to 10th	Night	39% 3 7% 25% 21% 5%	4228
	11th to 20th	Night	43% 4 7% 21% 22% 4	4120
	21st to end	Night	45% 3 6% 22% 18% 5%	4532
February	1st to 10th	Night	42% 4 10% 24% 17% 3	2470
	11th to 20th	Night	44% 3 6% 21% 19% 6%	2303
	21st to end	Night	41% 3 9% 21% 19% 6%	1884
March	1st to 10th	Night	50% 3 7% 21% 17% 2	3459
	11th to 20th	Night	34% 4 7% 21% 25% 9%	3480
	21st to end	Night	40% 3 7% 25% 20% 5%	3740
April	1st to 10th	Night	39% 6% 9% 18% 22% 6%	1900
	11th to 20th	Night	35% 5% 10% 23% 22% 6%	1875
	21st to end	Night	38% 6% 11% 24% 17% 4	1900
May	1st to 10th	Night	49% 5% 16% 19% 11% 2	3078
	11th to 20th	Night	47% 7% 10% 26% 9% 1	2718
	21st to end	Night	34% 7% 17% 24% 16% 1	2739
June	1st to 10th	Night	38% 8% 14% 23% 15% 1	1520
	11th to 20th	Night	37% 8% 16% 23% 15% 1	1520
	21st to end	Night	36% 7% 14% 23% 16% 4	1520
July	1st to 10th	Night	37% 13% 16% 20% 10% 3	2480
	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% 3 10% 23% 23% 5%	2317
	11th to 20th	Night	39% 3 8% 20% 21% 10%	2470
	21st to end	Night	35% 4 7% 26% 23% 5%	2588
December	1st to 10th	Night	44% 4 10% 22% 17% 3	2660
	11th to 20th	Night	39% 4 9% 21% 20% 6%	3288
	21st to end	Night	41% 4 11% 26% 17% 2	4884

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	21% 26% 35% 14% 4	236
	11th to 20th	Day	24% 32% 30% 12% 2	270
	21st to end	Day	20% 24% 44% 10% 2	297
February	1st to 10th	Day	20% 25% 39% 13% 4	270
	11th to 20th	Day	32% 25% 29% 12% 2	371
	21st to end	Day	30% 18% 32% 16% 4	382
March	1st to 10th	Day	23% 19% 37% 17% 4	460
	11th to 20th	Day	23% 22% 34% 15% 4 1	460
	21st to end	Day	23% 18% 37% 16% 5% 1	506
April	1st to 10th	Day	22% 21% 35% 15% 7% 1	460
	11th to 20th	Day	15% 14% 37% 21% 9% 4	452
	21st to end	Day	20% 15% 35% 23% 6% 2	460
May	1st to 10th	Day	17% 13% 36% 22% 10% 2	463
	11th to 20th	Day	22% 9% 30% 25% 12% 2	487
	21st to end	Day	20% 4 26% 27% 18% 5%	529
June	1st to 10th	Day	13% 4 29% 29% 21% 4	490
	11th to 20th	Day	12% 9% 21% 32% 20% 6%	490
	21st to end	Day	10% 4 25% 36% 20% 5%	490
July	1st to 10th	Day	10% 2 16% 33% 30% 9%	485
	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% 31% 23% 15% 4	440
	11th to 20th	Day	19% 16% 32% 22% 10% 1	370
	21st to end	Day	19% 16% 31% 23% 8% 2	249
December	1st to 10th	Day	23% 19% 35% 14% 8% 1	180
	11th to 20th	Day	27% 12% 33% 21% 8%	180
	21st to end	Day	23% 18% 30% 18% 9% 2	198
January	1st to 10th	Night	20% 18% 36% 23% 3	760
	11th to 20th	Night	21% 18% 32% 24% 5%	760
	21st to end	Night	20% 15% 38% 26% 1	836
February	1st to 10th	Night	21% 13% 38% 25% 3	760
	11th to 20th	Night	27% 19% 36% 17% 2	743
	21st to end	Night	23% 16% 36% 22% 2	628
March	1st to 10th	Night	19% 15% 37% 24% 4	760
	11th to 20th	Night	22% 17% 35% 21% 5% 1	760
	21st to end	Night	20% 14% 40% 22% 4	836
April	1st to 10th	Night	18% 18% 39% 22% 3	760
	11th to 20th	Night	15% 13% 37% 26% 8% 1	751
	21st to end	Night	18% 9% 35% 32% 6%	760
May	1st to 10th	Night	17% 12% 34% 29% 8%	717
	11th to 20th	Night	16% 5% 30% 37% 11% 1	505
	21st to end	Night	17% 2 20% 40% 20% 1	411
June	1st to 10th	Night	12% 3 21% 45% 18% 2	380
	11th to 20th	Night	12% 3 19% 44% 19% 2	380
	21st to end	Night	9% 3 21% 46% 21% 1	380
July	1st to 10th	Night	9% 4 16% 40% 28% 2	370
	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% 7% 27% 33% 14% 2	755
	11th to 20th	Night	19% 11% 31% 29% 11%	760
	21st to end	Night	14% 13% 30% 30% 11% 1	760
December	1st to 10th	Night	22% 13% 33% 24% 8% 1	760
	11th to 20th	Night	19% 10% 35% 31% 5%	760
	21st to end	Night	21% 12% 31% 31% 6%	836

Page generated at 2024-11-01 22:47:38. Version ac07

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