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 2024-04-03.

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	14% 6% 2 5% 5% 3 4 5% 5% 8% 43%	78919
	Night	14% 6% 2 5% 5% 3 4 4 5% 7% 44%	81206
Transparency	Day	39% 5% 11% 23% 17% 4	78634
	Night	40% 5% 10% 23% 18% 4	81011
Seeing	Day	18% 12% 28% 24% 14% 5%	14324
	Night	17% 10% 28% 30% 13% 1	24073

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 3 3 3 3 4 4 5% 53%	2713
	11th to 20th	Day	14% 8% 2 5% 4 3 4 4 5% 6% 46%	2840
	21st to end	Day	18% 9% 3 6% 5% 3 3 4 5% 6% 39%	3124
February	1st to 10th	Day	13% 7% 3 5% 5% 4 5% 4 5% 8% 40%	1710
	11th to 20th	Day	21% 6% 2 3 4 3 3 4 5% 6% 44%	1764
	21st to end	Day	18% 8% 2 5% 6% 3 5% 4 5% 5% 38%	1570
March	1st to 10th	Day	15% 5% 2 5% 4 2 3 4 4 6% 50%	2057
	11th to 20th	Day	17% 6% 2 5% 5% 2 4 4 5% 7% 45%	2090
	21st to end	Day	18% 6% 2 5% 6% 3 4 4 5% 6% 40%	2337
April	1st to 10th	Day	13% 5% 2 4 4 2 3 4 4 7% 51%	2184
	11th to 20th	Day	12% 5% 2 5% 5% 3 3 4 5% 6% 49%	2134
	21st to end	Day	15% 7% 2 5% 5% 2 4 4 5% 7% 44%	2160
May	1st to 10th	Day	14% 5% 3 6% 4 3 5% 5% 5% 8% 41%	2215
	11th to 20th	Day	17% 5% 3 5% 6% 3 4 5% 5% 8% 39%	2349
	21st to end	Day	14% 5% 2 5% 5% 3 4 5% 5% 8% 42%	2725
June	1st to 10th	Day	11% 4 2 5% 5% 4 6% 5% 6% 10% 43%	2520
	11th to 20th	Day	11% 5% 3 5% 5% 4 5% 6% 7% 9% 41%	2520
	21st to end	Day	8% 4 2 5% 7% 3 7% 6% 7% 12% 39%	2520
July	1st to 10th	Day	7% 5% 2 7% 6% 4 6% 7% 8% 10% 39%	2520
	11th to 20th	Day	6% 4 2 5% 7% 4 7% 7% 8% 13% 39%	2357
	21st to end	Day	7% 3 2 7% 6% 4 8% 7% 8% 11% 35%	2547
August	1st to 10th	Day	10% 6% 3 7% 6% 4 6% 7% 9% 10% 34%	2327
	11th to 20th	Day	11% 5% 3 6% 6% 4 7% 6% 7% 9% 37%	2156
	21st to end	Day	10% 5% 3 6% 6% 4 5% 5% 6% 11% 38%	2376
September	1st to 10th	Day	13% 5% 2 5% 5% 3 5% 5% 6% 8% 42%	2160
	11th to 20th	Day	12% 5% 3 6% 5% 4 5% 4 6% 7% 42%	2160
	21st to end	Day	18% 7% 3 7% 6% 3 5% 4 4 6% 37%	1903
October	1st to 10th	Day	15% 6% 3 6% 5% 3 4 3 4 6% 45%	1800
	11th to 20th	Day	16% 4 3 4 4 3 3 3 5% 5% 51%	1800
	21st to end	Day	13% 5% 2 5% 4 3 3 4 4 6% 52%	1980
November	1st to 10th	Day	13% 5% 2 5% 3 2 3 4 4 5% 53%	1800
	11th to 20th	Day	13% 6% 2 5% 4 2 3 3 4 6% 51%	1710
	21st to end	Day	14% 4 2 4 4 2 3 3 4 7% 52%	1589
December	1st to 10th	Day	16% 6% 2 5% 5% 2 3 4 6% 7% 43%	1520
	11th to 20th	Day	16% 5% 2 4 4 2 3 2 4 5% 52%	1910
	21st to end	Day	17% 7% 2 5% 5% 2 4 4 4 6% 44%	2772
January	1st to 10th	Night	14% 6% 3 5% 5% 3 3 4 4 5% 47%	4228
	11th to 20th	Night	16% 6% 3 6% 5% 3 4 4 4 5% 44%	4120
	21st to end	Night	21% 8% 3 4 4 3 3 3 3 4 44%	4532
February	1st to 10th	Night	16% 8% 2 5% 4 3 4 4 5% 6% 44%	2470
	11th to 20th	Night	20% 7% 2 4 4 3 4 4 3 6% 42%	2301
	21st to end	Night	18% 7% 2 5% 4 2 3 4 3 6% 47%	1884
March	1st to 10th	Night	15% 5% 3 5% 4 3 4 4 4 6% 48%	2120
	11th to 20th	Night	18% 6% 2 4 4 3 4 4 6% 7% 42%	2090
	21st to end	Night	15% 7% 3 5% 5% 3 4 4 5% 6% 43%	2260
April	1st to 10th	Night	14% 6% 3 5% 5% 3 4 5% 5% 7% 44%	1821
	11th to 20th	Night	13% 5% 3 5% 4 2 4 4 5% 8% 49%	1775
	21st to end	Night	15% 6% 2 4 5% 3 5% 4 5% 7% 45%	1800
May	1st to 10th	Night	13% 7% 2 6% 5% 4 4 5% 5% 10% 39%	1760
	11th to 20th	Night	14% 7% 3 6% 5% 3 5% 5% 5% 7% 40%	1539
	21st to end	Night	14% 6% 2 5% 5% 3 5% 5% 5% 9% 42%	1550
June	1st to 10th	Night	12% 4 2 5% 5% 4 5% 6% 8% 9% 41%	1440
	11th to 20th	Night	11% 5% 3 6% 4 4 5% 6% 6% 10% 40%	1440
	21st to end	Night	8% 4 3 6% 6% 4 5% 6% 6% 10% 42%	1440
July	1st to 10th	Night	9% 4 1 5% 6% 5% 5% 6% 7% 10% 41%	1440
	11th to 20th	Night	5% 3 3 5% 5% 4 5% 6% 9% 10% 45%	1599
	21st to end	Night	6% 5% 3 7% 6% 4 6% 7% 8% 10% 38%	1771
August	1st to 10th	Night	8% 5% 3 6% 7% 4 5% 7% 8% 10% 36%	1611
	11th to 20th	Night	10% 7% 3 7% 7% 4 5% 6% 6% 9% 37%	1726
	21st to end	Night	9% 5% 2 5% 6% 4 6% 5% 7% 10% 40%	1980
September	1st to 10th	Night	12% 6% 2 5% 5% 3 5% 5% 5% 7% 44%	1800
	11th to 20th	Night	11% 6% 3 5% 5% 3 5% 5% 6% 7% 46%	1800
	21st to end	Night	20% 8% 3 5% 5% 3 4 4 4 7% 37%	2052
October	1st to 10th	Night	15% 5% 2 5% 5% 3 3 4 4 7% 47%	2160
	11th to 20th	Night	15% 6% 2 5% 4 2 3 4 4 6% 48%	2160
	21st to end	Night	15% 5% 1 4 4 3 4 3 4 6% 52%	2376
November	1st to 10th	Night	14% 7% 2 5% 4 3 3 3 4 5% 51%	2274
	11th to 20th	Night	15% 6% 2 5% 3 2 4 4 5% 5% 49%	2470
	21st to end	Night	13% 4 3 4 4 3 4 4 5% 6% 51%	2588
December	1st to 10th	Night	17% 7% 2 5% 5% 3 4 4 4 6% 42%	2660
	11th to 20th	Night	17% 6% 2 4 4 2 4 4 4 6% 47%	3285
	21st to end	Night	15% 7% 3 5% 5% 2 4 4 5% 7% 44%	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	36% 5% 10% 23% 22% 5%	2057
	11th to 20th	Day	40% 4 9% 21% 18% 7%	2090
	21st to end	Day	45% 6% 9% 18% 19% 4	2337
April	1st to 10th	Day	34% 4 11% 23% 21% 8%	2184
	11th to 20th	Day	36% 4 10% 22% 22% 6%	2134
	21st to end	Day	41% 5% 11% 20% 16% 6%	2160
May	1st to 10th	Day	40% 6% 15% 20% 16% 3	2215
	11th to 20th	Day	44% 6% 13% 24% 11% 2	2353
	21st to end	Day	40% 6% 13% 26% 15% 1	2725
June	1st to 10th	Day	36% 7% 16% 25% 14% 2	2520
	11th to 20th	Day	37% 7% 16% 23% 15% 1	2520
	21st to end	Day	36% 8% 16% 22% 16% 2	2520
July	1st to 10th	Day	37% 8% 17% 21% 14% 3	2450
	11th to 20th	Day	34% 9% 20% 25% 10% 2	2287
	21st to end	Day	37% 9% 18% 20% 15% 1	2405
August	1st to 10th	Day	42% 8% 16% 24% 9% 1	2314
	11th to 20th	Day	43% 8% 16% 21% 12% 1	2156
	21st to end	Day	39% 7% 15% 23% 14% 2	2376
September	1st to 10th	Day	39% 7% 15% 17% 19% 4	2160
	11th to 20th	Day	39% 5% 9% 23% 20% 3	2160
	21st to end	Day	49% 4 7% 22% 15% 3	1903
October	1st to 10th	Day	41% 5% 8% 22% 21% 4	1800
	11th to 20th	Day	36% 4 8% 23% 24% 5%	1800
	21st to end	Day	35% 3 6% 24% 25% 6%	1980
November	1st to 10th	Day	34% 4 9% 26% 23% 5%	1800
	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	38% 4 8% 25% 22% 3	2120
	11th to 20th	Night	41% 5% 9% 21% 19% 5%	2090
	21st to end	Night	42% 5% 8% 22% 18% 5%	2260
April	1st to 10th	Night	39% 6% 9% 19% 21% 6%	1821
	11th to 20th	Night	35% 5% 10% 23% 22% 6%	1775
	21st to end	Night	39% 6% 11% 23% 17% 4	1800
May	1st to 10th	Night	42% 5% 14% 21% 15% 3	1760
	11th to 20th	Night	42% 7% 12% 22% 15% 1	1540
	21st to end	Night	38% 8% 15% 26% 12% 1	1551
June	1st to 10th	Night	37% 8% 14% 24% 15% 1	1440
	11th to 20th	Night	38% 8% 17% 21% 15% 1	1440
	21st to end	Night	37% 7% 14% 22% 16% 3	1440
July	1st to 10th	Night	37% 9% 15% 22% 13% 4	1400
	11th to 20th	Night	30% 10% 21% 28% 8% 2	1547
	21st to end	Night	35% 10% 16% 26% 12% 1	1672
August	1st to 10th	Night	39% 9% 18% 23% 10%	1600
	11th to 20th	Night	42% 8% 17% 20% 13% 1	1726
	21st to end	Night	37% 8% 18% 23% 13% 2	1980
September	1st to 10th	Night	38% 7% 17% 17% 17% 4	1800
	11th to 20th	Night	37% 7% 10% 22% 22% 2	1800
	21st to end	Night	47% 6% 9% 18% 17% 4	2052
October	1st to 10th	Night	37% 6% 10% 23% 20% 4	2160
	11th to 20th	Night	38% 4 7% 24% 23% 5%	2160
	21st to end	Night	35% 3 7% 24% 25% 6%	2376
November	1st to 10th	Night	36% 4 10% 23% 23% 5%	2274
	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	23% 20% 33% 15% 8% 1	428
	11th to 20th	Day	15% 15% 35% 21% 9% 4	412
	21st to end	Day	20% 15% 35% 23% 6% 2	420
May	1st to 10th	Day	18% 13% 38% 22% 8% 2	423
	11th to 20th	Day	22% 9% 30% 25% 12% 2	447
	21st to end	Day	21% 4 25% 26% 19% 5%	485
June	1st to 10th	Day	14% 4 28% 30% 20% 4	450
	11th to 20th	Day	13% 9% 21% 31% 20% 6%	450
	21st to end	Day	11% 4 24% 36% 21% 4	450
July	1st to 10th	Day	11% 2 17% 33% 29% 9%	445
	11th to 20th	Day	5% 2 14% 39% 31% 9%	444
	21st to end	Day	9% 2 20% 36% 25% 8%	488
August	1st to 10th	Day	12% 3 18% 35% 24% 8%	446
	11th to 20th	Day	14% 3 15% 32% 26% 10%	439
	21st to end	Day	10% 3 18% 30% 24% 14%	495
September	1st to 10th	Day	16% 4 19% 25% 22% 15%	450
	11th to 20th	Day	14% 3 20% 29% 20% 13%	450
	21st to end	Day	25% 7% 26% 24% 13% 5%	450
October	1st to 10th	Day	20% 8% 21% 27% 15% 9%	450
	11th to 20th	Day	16% 12% 23% 30% 17% 2	450
	21st to end	Day	16% 12% 34% 23% 11% 3	495
November	1st to 10th	Day	18% 9% 32% 22% 15% 4	428
	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% 17% 39% 23% 3	729
	11th to 20th	Night	15% 13% 37% 27% 8% 1	711
	21st to end	Night	19% 9% 35% 31% 6%	720
May	1st to 10th	Night	17% 12% 34% 28% 8%	680
	11th to 20th	Night	16% 4 30% 38% 12% 1	478
	21st to end	Night	17% 2 19% 40% 21% 1	389
June	1st to 10th	Night	12% 3 20% 45% 18% 2	360
	11th to 20th	Night	12% 4 19% 43% 20% 3	360
	21st to end	Night	9% 2 21% 45% 22% 1	360
July	1st to 10th	Night	10% 4 17% 39% 27% 2	350
	11th to 20th	Night	5% 2 11% 41% 37% 4	503
	21st to end	Night	8% 2 19% 42% 26% 4	591
August	1st to 10th	Night	7% 3 14% 43% 29% 4	537
	11th to 20th	Night	14% 3 12% 41% 26% 4	662
	21st to end	Night	11% 2 16% 35% 31% 6%	792
September	1st to 10th	Night	14% 3 14% 38% 27% 5%	720
	11th to 20th	Night	13% 4 16% 33% 30% 4	720
	21st to end	Night	24% 5% 19% 33% 17% 2	720
October	1st to 10th	Night	18% 5% 20% 29% 25% 4	720
	11th to 20th	Night	18% 8% 25% 33% 15% 2	720
	21st to end	Night	18% 9% 30% 28% 14% 2	792
November	1st to 10th	Night	17% 8% 27% 33% 14% 2	740
	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 2024-03-31 06:08:51. Version ac07

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