Rivers Bridge State Historic Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rivers Bridge State Historic Site clear sky chart from 2005-11-06 to 2025-12-18.

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	11% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	127123
	Night	12% 6% 2 6% 4 4 4 4 6% 8% 44%	123907
Transparency	Day	39% 7% 12% 18% 17% 7%	130795
	Night	37% 7% 12% 18% 19% 8%	128304
Seeing	Day	13% 8% 23% 25% 20% 11%	16886
	Night	13% 7% 23% 33% 21% 3	26958

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	13% 4 2 5% 3 3 3 3 5% 6% 53%	4056
	11th to 20th	Day	13% 5% 2 5% 3 3 3 2 4 6% 54%	4060
	21st to end	Day	13% 8% 3 6% 4 4 3 3 5% 7% 44%	4466
February	1st to 10th	Day	13% 7% 2 6% 5% 4 5% 4 6% 7% 41%	1850
	11th to 20th	Day	19% 7% 2 5% 3 3 3 3 5% 6% 45%	1948
	21st to end	Day	15% 6% 2 5% 4 4 3 4 5% 6% 46%	1724
March	1st to 10th	Day	20% 8% 3 7% 4 3 4 3 5% 6% 37%	4935
	11th to 20th	Day	7% 3 2 5% 4 4 4 4 5% 7% 55%	4980
	21st to end	Day	14% 7% 2 7% 4 4 4 3 4 5% 47%	5465
April	1st to 10th	Day	8% 4 1 5% 4 4 5% 4 6% 7% 51%	2200
	11th to 20th	Day	10% 5% 2 5% 4 4 4 3 5% 7% 50%	2329
	21st to end	Day	7% 4 1 6% 4 4 5% 4 6% 8% 52%	2487
May	1st to 10th	Day	9% 6% 3 7% 5% 6% 6% 5% 7% 9% 38%	5862
	11th to 20th	Day	15% 7% 2 7% 5% 4 6% 4 7% 9% 34%	5861
	21st to end	Day	8% 5% 2 6% 5% 6% 5% 5% 7% 8% 42%	6433
June	1st to 10th	Day	10% 7% 3 9% 8% 5% 6% 5% 8% 9% 30%	2600
	11th to 20th	Day	6% 5% 2 6% 5% 6% 7% 5% 9% 10% 38%	2600
	21st to end	Day	5% 5% 2 7% 6% 6% 7% 7% 8% 11% 35%	2600
July	1st to 10th	Day	11% 9% 3 10% 7% 8% 7% 6% 8% 10% 22%	5900
	11th to 20th	Day	4 4 2 7% 7% 7% 7% 8% 11% 16% 28%	5900
	21st to end	Day	6% 6% 3 10% 7% 6% 8% 6% 9% 12% 27%	6424
August	1st to 10th	Day	19% 8% 3 9% 6% 7% 6% 5% 8% 9% 21%	4234
	11th to 20th	Day	7% 5% 3 8% 6% 6% 7% 6% 9% 11% 32%	4174
	21st to end	Day	7% 5% 2 8% 5% 6% 7% 6% 9% 11% 33%	4240
September	1st to 10th	Day	8% 5% 2 8% 5% 6% 6% 6% 8% 10% 37%	2200
	11th to 20th	Day	12% 7% 3 8% 5% 5% 5% 4 6% 7% 37%	2200
	21st to end	Day	16% 7% 3 6% 4 4 5% 4 5% 6% 39%	2200
October	1st to 10th	Day	12% 7% 3 7% 5% 4 4 3 5% 6% 45%	2100
	11th to 20th	Day	10% 5% 2 4 3 3 4 3 3 5% 59%	2000
	21st to end	Day	10% 5% 2 5% 4 3 3 2 4 5% 57%	2200
November	1st to 10th	Day	12% 5% 2 5% 4 3 3 3 5% 6% 53%	2057
	11th to 20th	Day	14% 6% 2 4 4 2 3 2 4 6% 52%	2026
	21st to end	Day	13% 6% 2 4 3 4 4 3 5% 7% 49%	1890
December	1st to 10th	Day	9% 6% 2 6% 3 4 4 3 5% 7% 50%	3020
	11th to 20th	Day	13% 6% 2 5% 3 4 4 4 4 6% 49%	3436
	21st to end	Day	19% 8% 2 6% 3 3 3 2 4 6% 43%	4466
January	1st to 10th	Night	11% 5% 1 5% 4 3 3 3 4 5% 56%	5889
	11th to 20th	Night	12% 6% 2 6% 4 4 5% 4 5% 7% 44%	5900
	21st to end	Night	13% 7% 2 7% 4 4 4 3 5% 6% 46%	6490
February	1st to 10th	Night	16% 6% 2 5% 4 3 4 4 5% 7% 44%	2545
	11th to 20th	Night	16% 7% 2 6% 4 3 4 3 4 6% 44%	2338
	21st to end	Night	12% 6% 2 6% 4 4 4 3 6% 7% 46%	1905
March	1st to 10th	Night	18% 8% 3 6% 4 3 4 3 5% 6% 39%	4918
	11th to 20th	Night	10% 4 2 4 2 3 3 3 6% 8% 54%	4980
	21st to end	Night	12% 5% 2 5% 4 4 4 3 4 7% 50%	5467
April	1st to 10th	Night	10% 5% 2 6% 4 3 4 4 6% 7% 50%	2200
	11th to 20th	Night	10% 4 2 5% 3 3 3 3 5% 8% 52%	2019
	21st to end	Night	7% 4 2 5% 4 3 4 4 7% 7% 53%	1929
May	1st to 10th	Night	7% 7% 2 7% 5% 5% 5% 5% 7% 10% 40%	4033
	11th to 20th	Night	15% 5% 2 6% 4 4 5% 4 7% 10% 38%	4032
	21st to end	Night	10% 5% 2 6% 4 4 4 4 6% 9% 46%	4421
June	1st to 10th	Night	10% 6% 2 7% 5% 5% 6% 5% 9% 12% 33%	1800
	11th to 20th	Night	7% 5% 2 7% 6% 6% 6% 6% 8% 12% 36%	1800
	21st to end	Night	6% 5% 2 6% 6% 5% 5% 7% 10% 12% 37%	1800
July	1st to 10th	Night	11% 8% 4 10% 6% 7% 7% 5% 8% 10% 24%	4058
	11th to 20th	Night	4 5% 2 8% 6% 6% 7% 5% 9% 15% 32%	4057
	21st to end	Night	8% 7% 3 9% 7% 7% 7% 6% 9% 11% 26%	4427
August	1st to 10th	Night	20% 8% 2 7% 5% 6% 6% 6% 8% 10% 22%	2915
	11th to 20th	Night	7% 6% 3 7% 6% 5% 6% 5% 9% 11% 34%	2875
	21st to end	Night	6% 4 2 5% 4 5% 6% 4 8% 11% 44%	3605
September	1st to 10th	Night	9% 6% 2 7% 6% 4 5% 5% 7% 10% 40%	2200
	11th to 20th	Night	9% 7% 3 9% 5% 5% 5% 4 7% 8% 38%	2200
	21st to end	Night	13% 8% 3 6% 5% 4 5% 4 6% 8% 39%	2200
October	1st to 10th	Night	12% 7% 2 6% 3 4 4 3 5% 6% 48%	2299
	11th to 20th	Night	12% 4 2 5% 3 2 2 3 4 5% 59%	2400
	21st to end	Night	10% 5% 2 5% 4 4 3 3 5% 5% 56%	2640
November	1st to 10th	Night	11% 5% 2 5% 3 3 4 3 4 6% 53%	2461
	11th to 20th	Night	13% 7% 2 4 3 2 3 3 5% 5% 53%	2590
	21st to end	Night	11% 5% 3 5% 4 4 4 3 5% 8% 49%	2730
December	1st to 10th	Night	13% 6% 2 5% 3 3 3 2 4 5% 55%	4380
	11th to 20th	Night	16% 7% 3 7% 4 4 3 3 4 5% 45%	4914
	21st to end	Night	17% 7% 2 5% 4 4 4 3 5% 6% 44%	6490

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	31% 5% 6% 18% 31% 9%	4057
	11th to 20th	Day	33% 3 5% 17% 31% 11%	4060
	21st to end	Day	40% 4 4 19% 20% 13%	4466
February	1st to 10th	Day	40% 5% 9% 19% 20% 7%	1850
	11th to 20th	Day	41% 5% 6% 18% 23% 8%	1958
	21st to end	Day	38% 4 9% 17% 21% 11%	1724
March	1st to 10th	Day	47% 5% 6% 11% 21% 10%	4935
	11th to 20th	Day	27% 5% 8% 17% 23% 19%	4980
	21st to end	Day	40% 4 7% 16% 21% 12%	5465
April	1st to 10th	Day	31% 6% 10% 21% 24% 9%	2200
	11th to 20th	Day	34% 6% 8% 21% 19% 13%	2329
	21st to end	Day	28% 6% 11% 25% 24% 5%	2487
May	1st to 10th	Day	40% 8% 15% 23% 12% 2	5897
	11th to 20th	Day	44% 7% 17% 19% 12% 1	5865
	21st to end	Day	35% 8% 14% 18% 22% 2	6447
June	1st to 10th	Day	46% 10% 16% 17% 10% 1	2600
	11th to 20th	Day	35% 11% 20% 23% 10% 1	2600
	21st to end	Day	37% 13% 20% 19% 10% 1	2600
July	1st to 10th	Day	54% 10% 18% 15% 3 1	5835
	11th to 20th	Day	38% 15% 34% 11% 2	5831
	21st to end	Day	44% 12% 21% 18% 5%	6240
August	1st to 10th	Day	57% 10% 19% 11% 3	4221
	11th to 20th	Day	39% 11% 17% 22% 10% 1	4213
	21st to end	Day	40% 10% 13% 29% 8% 1	4240
September	1st to 10th	Day	40% 11% 15% 15% 13% 6%	2200
	11th to 20th	Day	44% 8% 12% 15% 16% 5%	2200
	21st to end	Day	44% 6% 11% 18% 16% 5%	2200
October	1st to 10th	Day	40% 5% 6% 14% 26% 9%	3425
	11th to 20th	Day	18% 3 5% 13% 32% 30%	3240
	21st to end	Day	24% 3 5% 17% 36% 15%	3538
November	1st to 10th	Day	32% 5% 8% 18% 23% 15%	2057
	11th to 20th	Day	35% 3 6% 18% 25% 13%	2026
	21st to end	Day	34% 4 9% 21% 23% 9%	1890
December	1st to 10th	Day	34% 4 9% 18% 25% 10%	3020
	11th to 20th	Day	37% 3 8% 26% 21% 6%	3433
	21st to end	Day	44% 3 5% 22% 23% 3	4466
January	1st to 10th	Night	30% 5% 5% 17% 34% 9%	5889
	11th to 20th	Night	38% 4 6% 16% 27% 8%	5900
	21st to end	Night	40% 5% 6% 12% 26% 11%	6490
February	1st to 10th	Night	39% 5% 8% 19% 20% 9%	2545
	11th to 20th	Night	41% 5% 8% 16% 20% 10%	2339
	21st to end	Night	37% 5% 9% 17% 18% 14%	1905
March	1st to 10th	Night	46% 6% 7% 12% 18% 11%	4918
	11th to 20th	Night	28% 5% 8% 13% 27% 19%	4980
	21st to end	Night	35% 5% 9% 18% 21% 13%	5467
April	1st to 10th	Night	32% 6% 12% 21% 20% 10%	2200
	11th to 20th	Night	30% 5% 9% 23% 21% 12%	2019
	21st to end	Night	27% 6% 13% 26% 21% 5%	1929
May	1st to 10th	Night	35% 8% 18% 26% 11% 2	4060
	11th to 20th	Night	40% 8% 20% 20% 12% 1	4033
	21st to end	Night	33% 7% 14% 25% 18% 2	4429
June	1st to 10th	Night	38% 13% 18% 20% 10% 1	1800
	11th to 20th	Night	36% 12% 20% 24% 8% 1	1800
	21st to end	Night	34% 14% 24% 19% 9% 2	1800
July	1st to 10th	Night	51% 11% 19% 15% 3 1	4015
	11th to 20th	Night	38% 13% 35% 11% 3	4006
	21st to end	Night	47% 13% 20% 15% 5%	4297
August	1st to 10th	Night	53% 12% 22% 11% 2	2904
	11th to 20th	Night	39% 12% 21% 21% 6% 1	2902
	21st to end	Night	31% 10% 18% 34% 7% 1	3605
September	1st to 10th	Night	37% 12% 17% 19% 10% 6%	2200
	11th to 20th	Night	41% 10% 14% 18% 14% 3	2200
	21st to end	Night	42% 9% 13% 18% 14% 3	2200
October	1st to 10th	Night	37% 4 8% 17% 24% 9%	3754
	11th to 20th	Night	19% 2 4 13% 25% 36%	3899
	21st to end	Night	24% 4 6% 19% 30% 18%	4252
November	1st to 10th	Night	32% 5% 9% 15% 25% 14%	2461
	11th to 20th	Night	34% 4 7% 17% 25% 13%	2590
	21st to end	Night	35% 4 9% 18% 23% 11%	2730
December	1st to 10th	Night	34% 4 8% 13% 32% 10%	4380
	11th to 20th	Night	43% 4 8% 18% 20% 8%	4916
	21st to end	Night	41% 5% 6% 21% 24% 3	6490

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	15% 15% 49% 17% 4	300
	11th to 20th	Day	15% 18% 40% 19% 6% 1	300
	21st to end	Day	25% 22% 34% 14% 4	330
February	1st to 10th	Day	18% 18% 36% 22% 6%	300
	11th to 20th	Day	28% 19% 36% 16% 2	300
	21st to end	Day	16% 18% 39% 21% 6%	323
March	1st to 10th	Day	19% 16% 29% 24% 12% 1	500
	11th to 20th	Day	12% 15% 38% 25% 8% 1	500
	21st to end	Day	17% 13% 37% 21% 10% 1	550
April	1st to 10th	Day	13% 17% 31% 25% 11% 2	500
	11th to 20th	Day	10% 13% 29% 25% 16% 7%	492
	21st to end	Day	9% 9% 25% 26% 19% 12%	500
May	1st to 10th	Day	9% 13% 27% 26% 17% 9%	503
	11th to 20th	Day	11% 5% 22% 27% 24% 11%	527
	21st to end	Day	11% 6% 18% 25% 23% 17%	573
June	1st to 10th	Day	12% 4 17% 25% 27% 16%	530
	11th to 20th	Day	8% 4 14% 26% 26% 22%	530
	21st to end	Day	7% 2 10% 30% 31% 22%	530
July	1st to 10th	Day	9% 2 7% 21% 33% 28%	525
	11th to 20th	Day	6% 1 5% 19% 39% 30%	524
	21st to end	Day	6% 2 7% 24% 39% 22%	576
August	1st to 10th	Day	12% 2 12% 23% 29% 21%	526
	11th to 20th	Day	9% 2 8% 19% 34% 28%	519
	21st to end	Day	13% 3 10% 25% 28% 21%	583
September	1st to 10th	Day	10% 3 10% 31% 28% 18%	530
	11th to 20th	Day	15% 4 12% 29% 30% 10%	530
	21st to end	Day	17% 2 13% 29% 28% 11%	530
October	1st to 10th	Day	15% 5% 21% 30% 20% 9%	530
	11th to 20th	Day	12% 4 21% 33% 22% 9%	530
	21st to end	Day	12% 7% 23% 31% 21% 7%	583
November	1st to 10th	Day	15% 6% 28% 26% 19% 6%	535
	11th to 20th	Day	19% 9% 36% 23% 12% 2	466
	21st to end	Day	13% 14% 31% 30% 11% 1	330
December	1st to 10th	Day	21% 14% 34% 24% 7% 1	330
	11th to 20th	Day	17% 15% 36% 21% 10% 1	321
	21st to end	Day	28% 10% 32% 19% 8% 2	330
January	1st to 10th	Night	16% 13% 34% 30% 6% 1	800
	11th to 20th	Night	16% 12% 33% 30% 9%	800
	21st to end	Night	18% 11% 39% 28% 3	880
February	1st to 10th	Night	18% 11% 31% 34% 7%	800
	11th to 20th	Night	20% 12% 38% 27% 3	783
	21st to end	Night	14% 13% 37% 32% 3	660
March	1st to 10th	Night	14% 12% 35% 34% 7%	800
	11th to 20th	Night	15% 12% 35% 32% 7%	800
	21st to end	Night	12% 12% 36% 33% 8%	880
April	1st to 10th	Night	12% 12% 32% 35% 9%	800
	11th to 20th	Night	11% 10% 30% 34% 14% 1	791
	21st to end	Night	8% 8% 28% 42% 15%	729
May	1st to 10th	Night	7% 7% 27% 42% 17%	600
	11th to 20th	Night	9% 4 23% 40% 22% 3	591
	21st to end	Night	9% 2 16% 38% 31% 3	650
June	1st to 10th	Night	11% 2 14% 41% 28% 4	600
	11th to 20th	Night	9% 2 9% 35% 35% 10%	600
	21st to end	Night	7% 2 10% 36% 39% 7%	600
July	1st to 10th	Night	9% 2 7% 32% 44% 7%	585
	11th to 20th	Night	8% 1 4 30% 49% 7%	582
	21st to end	Night	5% 2 7% 35% 44% 7%	657
August	1st to 10th	Night	11% 3 5% 31% 43% 8%	597
	11th to 20th	Night	8% 3 6% 26% 47% 9%	594
	21st to end	Night	12% 2 9% 30% 38% 8%	850
September	1st to 10th	Night	10% 2 10% 33% 40% 6%	800
	11th to 20th	Night	11% 3 12% 33% 38% 4	800
	21st to end	Night	17% 3 13% 33% 30% 4	800
October	1st to 10th	Night	14% 4 17% 38% 23% 5%	800
	11th to 20th	Night	13% 5% 20% 33% 26% 3	800
	21st to end	Night	12% 5% 21% 36% 24% 2	880
November	1st to 10th	Night	13% 7% 22% 37% 19% 3	820
	11th to 20th	Night	17% 8% 27% 33% 15% 1	840
	21st to end	Night	13% 8% 28% 33% 16% 1	840
December	1st to 10th	Night	18% 8% 30% 32% 11%	840
	11th to 20th	Night	19% 7% 29% 32% 11% 1	829
	21st to end	Night	22% 11% 27% 31% 10%	880

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