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 2024-11-11.

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%	101194
	Night	12% 6% 2 6% 4 4 5% 4 6% 8% 43%	97916
Transparency	Day	39% 8% 12% 18% 17% 6%	104910
	Night	37% 7% 12% 18% 17% 8%	102339
Seeing	Day	13% 8% 22% 25% 21% 11%	15409
	Night	13% 7% 23% 33% 21% 3	25469

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% 5% 2 6% 4 4 4 4 6% 8% 44%	2836
	11th to 20th	Day	12% 5% 2 5% 4 3 3 3 5% 7% 50%	2840
	21st to end	Day	14% 8% 3 6% 5% 4 4 3 4 6% 43%	3124
February	1st to 10th	Day	14% 7% 2 6% 5% 4 4 4 6% 7% 40%	1757
	11th to 20th	Day	18% 6% 2 5% 4 3 3 3 5% 6% 46%	1848
	21st to end	Day	15% 6% 2 6% 4 4 3 4 5% 6% 44%	1640
March	1st to 10th	Day	21% 9% 3 8% 4 3 4 3 5% 5% 35%	3435
	11th to 20th	Day	8% 4 2 5% 4 4 4 4 6% 7% 53%	3480
	21st to end	Day	17% 7% 2 6% 4 4 3 3 4 5% 43%	3815
April	1st to 10th	Day	8% 4 1 5% 5% 5% 5% 4 6% 7% 51%	2090
	11th to 20th	Day	10% 5% 2 6% 4 5% 4 3 5% 7% 48%	2211
	21st to end	Day	7% 4 1 5% 4 4 5% 4 6% 8% 52%	2362
May	1st to 10th	Day	7% 6% 2 7% 5% 6% 6% 4 6% 8% 42%	4082
	11th to 20th	Day	14% 8% 3 7% 5% 4 5% 4 6% 6% 38%	4081
	21st to end	Day	6% 4 2 6% 5% 6% 5% 5% 7% 9% 46%	4476
June	1st to 10th	Day	10% 7% 3 9% 8% 6% 6% 6% 8% 9% 30%	2470
	11th to 20th	Day	7% 5% 2 6% 5% 5% 7% 5% 9% 10% 39%	2470
	21st to end	Day	6% 6% 2 7% 6% 6% 7% 7% 8% 11% 35%	2470
July	1st to 10th	Day	12% 9% 3 9% 6% 6% 6% 5% 8% 10% 25%	4120
	11th to 20th	Day	5% 5% 2 8% 8% 8% 7% 8% 11% 14% 25%	4120
	21st to end	Day	7% 7% 4 11% 8% 7% 8% 7% 10% 12% 21%	4466
August	1st to 10th	Day	19% 7% 3 9% 6% 7% 6% 5% 8% 9% 21%	4104
	11th to 20th	Day	7% 5% 3 8% 5% 6% 7% 6% 9% 11% 33%	4044
	21st to end	Day	7% 5% 2 8% 5% 6% 7% 6% 9% 11% 33%	4110
September	1st to 10th	Day	8% 6% 2 8% 5% 6% 6% 6% 8% 10% 35%	2090
	11th to 20th	Day	12% 7% 3 8% 5% 6% 5% 4 6% 7% 36%	2090
	21st to end	Day	16% 7% 3 6% 4 4 5% 4 5% 7% 39%	2090
October	1st to 10th	Day	12% 6% 2 7% 5% 4 4 3 5% 6% 47%	1995
	11th to 20th	Day	9% 4 2 4 3 3 4 3 3 5% 59%	1900
	21st to end	Day	9% 4 2 5% 3 3 3 3 4 5% 57%	2090
November	1st to 10th	Day	12% 5% 2 5% 4 3 3 4 5% 5% 51%	1957
	11th to 20th	Day	15% 6% 2 5% 4 3 3 3 4 6% 50%	1834
	21st to end	Day	14% 6% 2 4 3 3 4 3 5% 7% 49%	1710
December	1st to 10th	Day	14% 7% 2 6% 3 4 5% 4 5% 7% 42%	1710
	11th to 20th	Day	15% 6% 2 4 3 3 3 3 4 6% 51%	2153
	21st to end	Day	17% 9% 3 6% 3 3 3 2 4 6% 44%	3124
January	1st to 10th	Night	15% 6% 2 6% 4 4 4 3 5% 6% 44%	4109
	11th to 20th	Night	10% 6% 2 7% 5% 5% 5% 4 6% 7% 43%	4120
	21st to end	Night	14% 7% 2 7% 4 4 4 4 4 5% 44%	4532
February	1st to 10th	Night	16% 6% 2 5% 4 4 4 4 5% 7% 43%	2418
	11th to 20th	Night	16% 7% 2 6% 4 3 4 3 4 6% 45%	2218
	21st to end	Night	13% 6% 2 6% 4 4 4 3 6% 7% 45%	1813
March	1st to 10th	Night	21% 11% 4 7% 4 4 3 3 4 5% 34%	3418
	11th to 20th	Night	11% 4 2 6% 3 4 4 3 5% 8% 51%	3480
	21st to end	Night	16% 5% 2 6% 4 4 4 3 4 7% 45%	3817
April	1st to 10th	Night	10% 5% 2 6% 4 3 4 4 6% 7% 50%	2090
	11th to 20th	Night	11% 4 2 5% 3 3 3 3 5% 8% 51%	1917
	21st to end	Night	7% 4 2 4 3 3 4 4 7% 7% 54%	1833
May	1st to 10th	Night	7% 6% 2 6% 5% 5% 5% 4 7% 10% 44%	2813
	11th to 20th	Night	11% 6% 2 6% 5% 4 4 4 6% 9% 42%	2812
	21st to end	Night	6% 4 2 5% 4 4 4 4 6% 9% 52%	3079
June	1st to 10th	Night	10% 6% 2 7% 5% 5% 6% 5% 9% 12% 33%	1710
	11th to 20th	Night	7% 5% 2 6% 6% 6% 6% 6% 8% 12% 37%	1710
	21st to end	Night	6% 5% 2 7% 7% 5% 5% 6% 10% 12% 36%	1710
July	1st to 10th	Night	12% 9% 4 10% 5% 6% 6% 4 7% 9% 28%	2840
	11th to 20th	Night	5% 6% 3 8% 6% 6% 7% 6% 9% 13% 29%	2840
	21st to end	Night	9% 8% 3 9% 7% 7% 7% 7% 9% 11% 23%	3087
August	1st to 10th	Night	20% 8% 2 6% 5% 6% 6% 6% 8% 10% 23%	2825
	11th to 20th	Night	7% 6% 3 7% 5% 5% 6% 5% 9% 11% 35%	2785
	21st to end	Night	6% 4 2 5% 4 5% 6% 4 8% 12% 44%	3494
September	1st to 10th	Night	9% 6% 2 8% 6% 4 5% 5% 7% 10% 38%	2090
	11th to 20th	Night	10% 7% 3 9% 6% 5% 5% 4 7% 8% 38%	2090
	21st to end	Night	13% 7% 3 6% 5% 4 5% 4 6% 8% 40%	2090
October	1st to 10th	Night	12% 7% 2 6% 3 4 4 3 5% 6% 49%	2184
	11th to 20th	Night	11% 4 2 5% 3 2 2 3 4 5% 59%	2280
	21st to end	Night	9% 5% 2 5% 4 4 3 3 5% 5% 56%	2508
November	1st to 10th	Night	12% 5% 2 5% 3 3 4 3 4 6% 52%	2341
	11th to 20th	Night	14% 7% 2 5% 3 2 3 3 5% 5% 51%	2349
	21st to end	Night	12% 5% 3 5% 4 4 4 3 5% 7% 48%	2470
December	1st to 10th	Night	15% 6% 2 6% 4 4 4 3 5% 7% 43%	2470
	11th to 20th	Night	18% 6% 2 5% 3 3 3 2 4 5% 49%	3042
	21st to end	Night	18% 7% 2 6% 4 4 4 4 5% 6% 40%	4532

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% 6% 8% 20% 20% 10%	2837
	11th to 20th	Day	33% 4 6% 19% 27% 11%	2840
	21st to end	Day	42% 5% 5% 16% 21% 11%	3124
February	1st to 10th	Day	42% 5% 9% 18% 20% 7%	1757
	11th to 20th	Day	39% 5% 6% 18% 24% 8%	1858
	21st to end	Day	40% 4 9% 17% 20% 11%	1640
March	1st to 10th	Day	50% 6% 8% 13% 14% 9%	3435
	11th to 20th	Day	28% 7% 9% 18% 25% 13%	3480
	21st to end	Day	42% 5% 7% 17% 22% 7%	3815
April	1st to 10th	Day	32% 6% 10% 19% 24% 9%	2090
	11th to 20th	Day	35% 6% 9% 19% 19% 13%	2211
	21st to end	Day	28% 6% 11% 25% 25% 5%	2362
May	1st to 10th	Day	36% 9% 14% 24% 13% 3	4120
	11th to 20th	Day	43% 8% 11% 20% 16% 1	4085
	21st to end	Day	29% 10% 16% 20% 22% 2	4489
June	1st to 10th	Day	46% 10% 16% 16% 10% 1	2470
	11th to 20th	Day	35% 11% 19% 24% 10% 1	2470
	21st to end	Day	38% 13% 20% 18% 10% 1	2470
July	1st to 10th	Day	50% 10% 18% 16% 4 1	4055
	11th to 20th	Day	42% 15% 28% 11% 3 1	4051
	21st to end	Day	50% 14% 20% 12% 3	4324
August	1st to 10th	Day	57% 10% 19% 11% 3	4091
	11th to 20th	Day	39% 11% 17% 22% 10% 1	4083
	21st to end	Day	39% 10% 13% 29% 8% 1	4110
September	1st to 10th	Day	41% 11% 16% 14% 12% 6%	2090
	11th to 20th	Day	46% 8% 12% 14% 16% 5%	2090
	21st to end	Day	44% 6% 10% 18% 17% 5%	2090
October	1st to 10th	Day	39% 5% 6% 14% 27% 9%	3320
	11th to 20th	Day	18% 3 5% 13% 32% 30%	3140
	21st to end	Day	23% 3 5% 18% 35% 15%	3428
November	1st to 10th	Day	33% 5% 8% 18% 23% 14%	1957
	11th to 20th	Day	36% 3 6% 18% 24% 13%	1834
	21st to end	Day	35% 4 9% 22% 21% 9%	1710
December	1st to 10th	Day	40% 6% 12% 17% 18% 6%	1710
	11th to 20th	Day	36% 4 6% 21% 25% 8%	2150
	21st to end	Day	43% 4 6% 18% 25% 4	3124
January	1st to 10th	Night	38% 6% 7% 17% 21% 11%	4109
	11th to 20th	Night	37% 6% 8% 18% 23% 7%	4120
	21st to end	Night	42% 6% 6% 11% 23% 12%	4532
February	1st to 10th	Night	40% 5% 9% 18% 19% 10%	2418
	11th to 20th	Night	40% 5% 8% 16% 21% 10%	2219
	21st to end	Night	38% 6% 9% 18% 17% 13%	1813
March	1st to 10th	Night	51% 7% 9% 13% 13% 7%	3418
	11th to 20th	Night	31% 6% 9% 14% 24% 15%	3480
	21st to end	Night	39% 6% 9% 19% 18% 9%	3817
April	1st to 10th	Night	32% 6% 12% 20% 21% 10%	2090
	11th to 20th	Night	31% 5% 10% 21% 21% 12%	1917
	21st to end	Night	27% 6% 13% 26% 22% 6%	1833
May	1st to 10th	Night	31% 10% 17% 27% 12% 3	2840
	11th to 20th	Night	36% 10% 14% 22% 17% 1	2813
	21st to end	Night	26% 9% 16% 28% 19% 3	3087
June	1st to 10th	Night	38% 13% 19% 20% 10% 1	1710
	11th to 20th	Night	36% 12% 19% 25% 8% 1	1710
	21st to end	Night	35% 14% 23% 18% 8% 2	1710
July	1st to 10th	Night	50% 10% 16% 19% 4 1	2795
	11th to 20th	Night	41% 14% 28% 13% 4	2786
	21st to end	Night	49% 16% 20% 12% 3	2988
August	1st to 10th	Night	52% 12% 22% 11% 2	2814
	11th to 20th	Night	38% 12% 21% 22% 7% 1	2812
	21st to end	Night	30% 10% 19% 34% 7% 1	3494
September	1st to 10th	Night	37% 12% 18% 17% 10% 6%	2090
	11th to 20th	Night	42% 10% 14% 17% 14% 3	2090
	21st to end	Night	41% 10% 13% 18% 15% 4	2090
October	1st to 10th	Night	36% 4 8% 17% 25% 9%	3639
	11th to 20th	Night	18% 2 5% 13% 25% 37%	3779
	21st to end	Night	23% 4 6% 19% 30% 18%	4120
November	1st to 10th	Night	33% 5% 9% 15% 24% 14%	2341
	11th to 20th	Night	35% 4 7% 17% 23% 13%	2349
	21st to end	Night	36% 4 8% 18% 22% 11%	2470
December	1st to 10th	Night	40% 6% 13% 16% 18% 7%	2470
	11th to 20th	Night	38% 4 6% 18% 27% 7%	3044
	21st to end	Night	43% 5% 7% 18% 22% 4	4532

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	14% 15% 49% 17% 4	270
	11th to 20th	Day	13% 19% 40% 20% 6% 1	270
	21st to end	Day	25% 23% 34% 14% 4	297
February	1st to 10th	Day	20% 19% 34% 22% 6%	270
	11th to 20th	Day	25% 19% 37% 16% 2	270
	21st to end	Day	17% 19% 39% 19% 5%	295
March	1st to 10th	Day	17% 15% 30% 25% 12% 1	460
	11th to 20th	Day	12% 14% 40% 24% 8% 2	460
	21st to end	Day	17% 13% 37% 22% 10% 1	506
April	1st to 10th	Day	14% 17% 30% 24% 12% 2	460
	11th to 20th	Day	11% 13% 29% 24% 16% 7%	452
	21st to end	Day	10% 9% 26% 25% 19% 11%	460
May	1st to 10th	Day	8% 13% 27% 24% 17% 10%	463
	11th to 20th	Day	10% 5% 21% 28% 25% 12%	487
	21st to end	Day	9% 5% 16% 26% 24% 19%	529
June	1st to 10th	Day	11% 3 16% 25% 27% 17%	490
	11th to 20th	Day	8% 4 14% 26% 24% 23%	490
	21st to end	Day	7% 2 10% 31% 29% 20%	490
July	1st to 10th	Day	9% 2 6% 21% 34% 28%	485
	11th to 20th	Day	6% 1 5% 19% 38% 29%	484
	21st to end	Day	6% 2 8% 25% 38% 20%	532
August	1st to 10th	Day	11% 2 13% 21% 30% 23%	486
	11th to 20th	Day	9% 3 8% 20% 34% 27%	479
	21st to end	Day	13% 3 10% 25% 27% 23%	539
September	1st to 10th	Day	11% 2 9% 30% 29% 19%	490
	11th to 20th	Day	17% 4 11% 28% 31% 10%	490
	21st to end	Day	17% 2 13% 28% 28% 12%	490
October	1st to 10th	Day	15% 5% 21% 30% 20% 10%	490
	11th to 20th	Day	11% 4 22% 33% 22% 9%	490
	21st to end	Day	11% 6% 22% 31% 22% 7%	539
November	1st to 10th	Day	16% 5% 27% 26% 19% 6%	495
	11th to 20th	Day	21% 9% 34% 24% 11% 2	394
	21st to end	Day	15% 12% 30% 30% 13% 1	270
December	1st to 10th	Day	19% 13% 34% 26% 7% 1	270
	11th to 20th	Day	18% 16% 36% 21% 8% 1	270
	21st to end	Day	28% 10% 31% 20% 9% 2	297
January	1st to 10th	Night	17% 12% 34% 30% 7% 1	760
	11th to 20th	Night	16% 12% 32% 31% 10%	760
	21st to end	Night	19% 11% 39% 27% 3	836
February	1st to 10th	Night	18% 11% 31% 33% 7%	760
	11th to 20th	Night	19% 12% 38% 28% 3	743
	21st to end	Night	15% 13% 37% 31% 4	628
March	1st to 10th	Night	14% 11% 35% 34% 7%	760
	11th to 20th	Night	15% 11% 35% 32% 8%	760
	21st to end	Night	12% 12% 35% 33% 8%	836
April	1st to 10th	Night	12% 12% 31% 35% 9%	760
	11th to 20th	Night	12% 10% 30% 33% 14% 1	751
	21st to end	Night	8% 8% 29% 41% 14%	693
May	1st to 10th	Night	7% 6% 27% 41% 18%	570
	11th to 20th	Night	9% 4 22% 40% 22% 3	561
	21st to end	Night	9% 2 15% 39% 33% 4	617
June	1st to 10th	Night	11% 2 14% 41% 28% 4	570
	11th to 20th	Night	9% 2 10% 34% 34% 11%	570
	21st to end	Night	7% 2 10% 36% 38% 7%	570
July	1st to 10th	Night	9% 1 7% 31% 45% 7%	555
	11th to 20th	Night	8% 1 5% 30% 49% 6%	552
	21st to end	Night	5% 2 8% 36% 43% 6%	624
August	1st to 10th	Night	9% 3 4 32% 44% 8%	567
	11th to 20th	Night	8% 2 7% 27% 48% 8%	564
	21st to end	Night	11% 2 9% 29% 40% 9%	807
September	1st to 10th	Night	10% 2 9% 31% 42% 6%	760
	11th to 20th	Night	11% 3 12% 32% 38% 4	760
	21st to end	Night	16% 3 14% 33% 30% 4	760
October	1st to 10th	Night	14% 3 16% 38% 24% 5%	760
	11th to 20th	Night	13% 5% 21% 33% 25% 3	760
	21st to end	Night	11% 5% 20% 37% 25% 2	836
November	1st to 10th	Night	13% 7% 22% 35% 19% 3	780
	11th to 20th	Night	17% 8% 27% 33% 14% 1	763
	21st to end	Night	14% 7% 27% 34% 17% 1	760
December	1st to 10th	Night	17% 8% 29% 33% 12% 1	760
	11th to 20th	Night	20% 7% 29% 32% 11% 1	760
	21st to end	Night	22% 11% 27% 30% 11%	836

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