Three Rivers
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Three Rivers clear sky chart from 2005-11-06 to 2026-07-16.

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	3 3 1 3 3 3 3 3 5% 7% 66%	145382
	Night	4 4 1 4 3 3 3 3 5% 7% 63%	144819
Transparency	Day	18% 3 8% 17% 29% 26%	140445
	Night	20% 3 7% 15% 29% 26%	140764
Seeing	Day	6% 9% 22% 31% 24% 8%	16979
	Night	6% 5% 25% 38% 23% 3	27209

By Month

Percentage Cloud Cover SW (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	4 8% 3 5% 3 3 3 3 5% 5% 57%	5318
	11th to 20th	Day	2 3 1 2 1 1 2 2 3 4 78%	5340
	21st to end	Day	8% 5% 1 4 2 3 4 3 4 5% 60%	5844
February	1st to 10th	Day	4 5% 1 4 2 1 2 3 4 6% 68%	2005
	11th to 20th	Day	6% 4 1 3 2 2 3 3 4 6% 64%	2049
	21st to end	Day	4 4 2 3 3 2 2 3 4 5% 69%	1730
March	1st to 10th	Day	3 3 1 3 2 3 3 3 5% 7% 68%	4916
	11th to 20th	Day	2 4 1 4 3 3 4 3 5% 6% 65%	5110
	21st to end	Day	2 5% 2 4 4 3 4 3 5% 7% 62%	5777
April	1st to 10th	Day	3 2 1 3 3 2 3 3 4 7% 70%	2520
	11th to 20th	Day	3 3 1 3 2 2 2 3 3 5% 72%	2491
	21st to end	Day	2 3 1 2 2 2 2 2 4 5% 75%	2520
May	1st to 10th	Day	2 2 1 3 2 2 3 3 4 6% 73%	7057
	11th to 20th	Day	2 2 1 2 2 2 3 3 4 5% 75%	7032
	21st to end	Day	1 1 1 2 2 2 2 2 4 6% 79%	8372
June	1st to 10th	Day	2 2 1 2 3 3 4 4 6% 9% 65%	2940
	11th to 20th	Day	2 2 1 2 2 2 2 3 5% 6% 75%	2940
	21st to end	Day	4 3 1 5% 4 4 4 4 6% 9% 55%	2858
July	1st to 10th	Day	2 2 1 4 3 4 5% 5% 8% 9% 56%	6009
	11th to 20th	Day	1 1 1 2 3 4 5% 6% 9% 13% 56%	5946
	21st to end	Day	4 3 1 4 4 5% 7% 6% 10% 12% 43%	6401
August	1st to 10th	Day	2 2 1 3 3 4 5% 5% 8% 10% 57%	5435
	11th to 20th	Day	3 2 1 3 3 4 4 5% 8% 11% 56%	5366
	21st to end	Day	2 2 1 4 4 4 5% 5% 8% 12% 53%	5984
September	1st to 10th	Day	6% 5% 2 5% 4 4 6% 4 7% 8% 50%	2400
	11th to 20th	Day	6% 3 2 3 3 4 4 3 5% 7% 60%	2400
	21st to end	Day	3 1 1 4 2 2 2 2 5% 6% 72%	2189
October	1st to 10th	Day	3 2 1 3 2 2 3 2 3 6% 75%	3270
	11th to 20th	Day	3 2 1 3 2 2 2 2 3 6% 74%	3250
	21st to end	Day	2 1 1 1 1 1 1 1 2 3 86%	3548
November	1st to 10th	Day	3 1 3 2 2 3 3 4 6% 75%	2059
	11th to 20th	Day	3 2 1 2 2 2 3 2 3 6% 73%	1939
	21st to end	Day	5% 4 2 4 2 3 3 3 5% 6% 64%	1890
December	1st to 10th	Day	4 4 1 2 2 2 2 2 4 6% 71%	2997
	11th to 20th	Day	3 3 2 4 3 3 3 2 3 7% 67%	3106
	21st to end	Day	5% 3 1 2 2 2 3 2 4 5% 72%	4374
January	1st to 10th	Night	6% 9% 2 4 3 3 3 3 5% 6% 56%	7605
	11th to 20th	Night	3 2 1 1 1 2 2 2 3 4 79%	7650
	21st to end	Night	9% 6% 1 3 3 3 3 3 5% 7% 58%	8370
February	1st to 10th	Night	4 5% 2 3 2 2 2 2 4 7% 67%	2613
	11th to 20th	Night	8% 5% 1 3 2 2 3 2 4 5% 64%	2458
	21st to end	Night	6% 4 2 3 2 3 3 3 4 6% 64%	2076
March	1st to 10th	Night	5% 4 1 4 2 2 3 2 3 5% 68%	5188
	11th to 20th	Night	8% 6% 1 3 3 2 3 2 4 5% 63%	5110
	21st to end	Night	3 5% 1 4 3 3 3 3 5% 6% 64%	5430
April	1st to 10th	Night	4 3 1 3 3 3 2 3 5% 6% 68%	2100
	11th to 20th	Night	5% 3 1 4 3 3 3 3 4 5% 66%	2077
	21st to end	Night	4 3 1 3 2 2 3 3 5% 5% 68%	2100
May	1st to 10th	Night	6% 4 1 4 3 2 3 3 4 5% 65%	5890
	11th to 20th	Night	2 2 1 3 2 2 3 3 4 7% 71%	5860
	21st to end	Night	1 2 1 2 2 2 3 2 4 5% 76%	5799
June	1st to 10th	Night	2 2 1 4 5% 4 4 5% 7% 9% 57%	1680
	11th to 20th	Night	3 1 1 3 2 2 3 3 6% 8% 68%	1680
	21st to end	Night	5% 4 2 4 4 4 6% 6% 7% 9% 49%	1752
July	1st to 10th	Night	3 2 1 5% 5% 5% 5% 4 8% 8% 52%	4190
	11th to 20th	Night	2 2 1 5% 4 5% 5% 6% 8% 12% 50%	4145
	21st to end	Night	4 3 2 7% 5% 6% 8% 7% 12% 13% 34%	4472
August	1st to 10th	Night	2 3 1 4 3 4 5% 5% 10% 14% 48%	4374
	11th to 20th	Night	2 3 1 5% 4 5% 5% 6% 10% 15% 45%	4483
	21st to end	Night	3 3 2 6% 5% 6% 6% 6% 9% 12% 43%	4993
September	1st to 10th	Night	6% 3 2 5% 4 4 4 5% 6% 8% 52%	2000
	11th to 20th	Night	5% 4 2 5% 3 4 5% 3 6% 9% 55%	2000
	21st to end	Night	3 3 1 3 2 3 4 3 5% 6% 65%	2205
October	1st to 10th	Night	3 2 1 3 2 2 2 2 4 4 75%	3920
	11th to 20th	Night	6% 3 1 4 2 2 3 2 3 4 69%	3898
	21st to end	Night	4 1 1 2 1 2 2 2 3 5% 78%	4252
November	1st to 10th	Night	3 2 1 3 2 2 2 2 4 6% 74%	2460
	11th to 20th	Night	3 2 1 2 2 3 3 3 4 5% 73%	2657
	21st to end	Night	7% 3 1 4 3 3 3 3 4 6% 64%	2730
December	1st to 10th	Night	5% 4 1 2 2 2 2 2 3 5% 73%	4408
	11th to 20th	Night	3 3 1 3 2 3 3 3 4 7% 66%	5321
	21st to end	Night	6% 3 1 3 2 2 2 2 4 6% 68%	6873

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	29% 2 7% 16% 38% 9%	5321
	11th to 20th	Day	12% 1 4 15% 44% 24%	5340
	21st to end	Day	26% 2 5% 13% 33% 22%	5844
February	1st to 10th	Day	20% 1 5% 16% 32% 25%	2005
	11th to 20th	Day	22% 2 5% 18% 31% 22%	2059
	21st to end	Day	18% 2 3 9% 33% 34%	1730
March	1st to 10th	Day	17% 2 3 8% 33% 36%	4916
	11th to 20th	Day	20% 3 4 15% 28% 30%	5110
	21st to end	Day	22% 2 6% 13% 29% 28%	5777
April	1st to 10th	Day	16% 2 4 11% 33% 34%	2520
	11th to 20th	Day	16% 2 5% 9% 28% 40%	2494
	21st to end	Day	13% 2 4 13% 33% 37%	2520
May	1st to 10th	Day	15% 1 7% 13% 28% 36%	7057
	11th to 20th	Day	13% 2 4 10% 28% 43%	7035
	21st to end	Day	10% 1 5% 11% 15% 57%	8373
June	1st to 10th	Day	14% 4 8% 18% 24% 32%	2940
	11th to 20th	Day	11% 2 5% 11% 23% 48%	2940
	21st to end	Day	24% 5% 10% 17% 21% 24%	2858
July	1st to 10th	Day	22% 6% 13% 18% 23% 18%	5946
	11th to 20th	Day	17% 6% 21% 32% 20% 6%	5877
	21st to end	Day	28% 8% 22% 25% 13% 4	6217
August	1st to 10th	Day	23% 6% 13% 32% 22% 5%	3953
	11th to 20th	Day	19% 6% 11% 21% 34% 8%	3882
	21st to end	Day	25% 7% 16% 29% 20% 4	4311
September	1st to 10th	Day	30% 5% 12% 18% 22% 13%	2400
	11th to 20th	Day	22% 5% 8% 21% 26% 18%	2400
	21st to end	Day	15% 3 6% 21% 34% 22%	2189
October	1st to 10th	Day	14% 3 5% 9% 31% 38%	3270
	11th to 20th	Day	14% 3 6% 12% 33% 32%	3250
	21st to end	Day	8% 1 3 12% 35% 41%	3548
November	1st to 10th	Day	13% 2 4 19% 36% 27%	2059
	11th to 20th	Day	14% 3 5% 19% 32% 26%	1939
	21st to end	Day	21% 3 6% 19% 32% 19%	1890
December	1st to 10th	Day	15% 2 4 24% 37% 17%	2997
	11th to 20th	Day	20% 2 6% 14% 41% 17%	3104
	21st to end	Day	17% 1 4 21% 39% 17%	4374
January	1st to 10th	Night	30% 1 6% 16% 36% 11%	7605
	11th to 20th	Night	11% 2 3 13% 50% 22%	7650
	21st to end	Night	26% 3 5% 12% 31% 24%	8370
February	1st to 10th	Night	18% 2 6% 16% 29% 29%	2613
	11th to 20th	Night	24% 2 5% 16% 31% 22%	2458
	21st to end	Night	22% 2 4 9% 31% 33%	2076
March	1st to 10th	Night	20% 2 3 8% 35% 32%	5188
	11th to 20th	Night	24% 3 5% 13% 30% 26%	5110
	21st to end	Night	21% 2 6% 11% 32% 28%	5430
April	1st to 10th	Night	17% 3 5% 11% 37% 28%	2100
	11th to 20th	Night	21% 3 6% 9% 25% 36%	2077
	21st to end	Night	17% 2 5% 11% 31% 34%	2100
May	1st to 10th	Night	21% 3 5% 12% 19% 40%	5888
	11th to 20th	Night	14% 2 5% 10% 25% 43%	5861
	21st to end	Night	12% 2 5% 8% 14% 59%	5801
June	1st to 10th	Night	20% 6% 8% 16% 20% 30%	1680
	11th to 20th	Night	14% 3 7% 12% 18% 47%	1680
	21st to end	Night	27% 7% 11% 16% 16% 23%	1752
July	1st to 10th	Night	26% 5% 14% 21% 20% 14%	4145
	11th to 20th	Night	24% 5% 17% 26% 22% 5%	4091
	21st to end	Night	33% 8% 19% 24% 12% 4	4344
August	1st to 10th	Night	21% 9% 12% 28% 24% 7%	3180
	11th to 20th	Night	20% 8% 10% 19% 35% 7%	3243
	21st to end	Night	31% 7% 12% 25% 20% 4	3597
September	1st to 10th	Night	27% 6% 13% 17% 22% 15%	2000
	11th to 20th	Night	25% 5% 10% 18% 22% 20%	2000
	21st to end	Night	18% 5% 6% 16% 32% 23%	2205
October	1st to 10th	Night	14% 3 5% 9% 28% 41%	3920
	11th to 20th	Night	21% 3 4 9% 27% 36%	3898
	21st to end	Night	12% 1 3 12% 31% 41%	4252
November	1st to 10th	Night	13% 2 4 15% 36% 29%	2460
	11th to 20th	Night	15% 2 4 17% 32% 30%	2657
	21st to end	Night	22% 3 4 18% 33% 20%	2730
December	1st to 10th	Night	15% 3 4 22% 36% 20%	4408
	11th to 20th	Night	19% 2 5% 14% 36% 23%	5322
	21st to end	Night	19% 2 4 18% 40% 18%	6873

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	12% 9% 33% 33% 12% 1	330
	11th to 20th	Day	9% 7% 33% 32% 17% 2	330
	21st to end	Day	9% 9% 35% 30% 15% 2	363
February	1st to 10th	Day	8% 9% 34% 30% 16% 3	330
	11th to 20th	Day	15% 15% 34% 29% 7% 1	330
	21st to end	Day	12% 19% 30% 28% 9% 2	273
March	1st to 10th	Day	11% 20% 35% 26% 6% 2	465
	11th to 20th	Day	10% 18% 31% 26% 12% 3	540
	21st to end	Day	10% 24% 30% 24% 10% 2	594
April	1st to 10th	Day	7% 20% 34% 26% 11% 3	540
	11th to 20th	Day	11% 23% 34% 24% 7% 1	532
	21st to end	Day	8% 22% 34% 25% 9% 1	540
May	1st to 10th	Day	5% 18% 37% 29% 10% 1	543
	11th to 20th	Day	3 15% 32% 31% 16% 4	568
	21st to end	Day	3 13% 31% 34% 17% 2	685
June	1st to 10th	Day	3 4 20% 39% 29% 5%	780
	11th to 20th	Day	3 4 20% 38% 32% 4	780
	21st to end	Day	5% 2 14% 34% 34% 10%	698
July	1st to 10th	Day	3 5% 35% 42% 15%	566
	11th to 20th	Day	2 2 23% 51% 21%	545
	21st to end	Day	3 1 4 21% 46% 25%	575
August	1st to 10th	Day	3 1 4 27% 43% 23%	527
	11th to 20th	Day	5% 3 5% 26% 45% 16%	517
	21st to end	Day	3 2 5% 27% 43% 21%	583
September	1st to 10th	Day	7% 1 11% 33% 35% 14%	530
	11th to 20th	Day	4 1 15% 38% 31% 11%	530
	21st to end	Day	3 3 15% 37% 29% 14%	349
October	1st to 10th	Day	6% 6% 16% 29% 26% 17%	330
	11th to 20th	Day	8% 4 21% 28% 32% 7%	330
	21st to end	Day	6% 9% 18% 34% 23% 10%	363
November	1st to 10th	Day	7% 8% 25% 35% 22% 5%	330
	11th to 20th	Day	4 9% 21% 36% 23% 7%	330
	21st to end	Day	8% 10% 34% 30% 17% 1	330
December	1st to 10th	Day	10% 8% 28% 35% 18% 1	330
	11th to 20th	Day	6% 12% 29% 35% 17% 1	330
	21st to end	Day	10% 14% 28% 32% 15% 1	363
January	1st to 10th	Night	8% 4 31% 39% 14% 3	840
	11th to 20th	Night	8% 5% 31% 40% 15% 2	840
	21st to end	Night	9% 5% 33% 36% 15% 1	924
February	1st to 10th	Night	6% 6% 35% 37% 15% 2	840
	11th to 20th	Night	12% 7% 39% 34% 8%	824
	21st to end	Night	8% 11% 43% 32% 7%	692
March	1st to 10th	Night	8% 12% 37% 33% 10% 1	840
	11th to 20th	Night	7% 9% 35% 33% 15% 1	840
	21st to end	Night	6% 11% 33% 41% 9%	847
April	1st to 10th	Night	6% 8% 41% 38% 7%	630
	11th to 20th	Night	6% 11% 39% 35% 9% 1	623
	21st to end	Night	6% 7% 40% 37% 9%	630
May	1st to 10th	Night	4 7% 39% 42% 8%	630
	11th to 20th	Night	5% 5% 30% 45% 15%	621
	21st to end	Night	2 4 33% 48% 12% 1	683
June	1st to 10th	Night	2 3 22% 51% 20% 1	630
	11th to 20th	Night	3 2 20% 55% 20% 1	630
	21st to end	Night	5% 2 13% 41% 36% 3	630
July	1st to 10th	Night	4 1 6% 38% 46% 5%	615
	11th to 20th	Night	2 1 3 32% 56% 7%	597
	21st to end	Night	3 2 2 25% 57% 11%	657
August	1st to 10th	Night	3 1 3 25% 56% 11%	597
	11th to 20th	Night	4 1 5% 29% 52% 9%	594
	21st to end	Night	3 1 4 32% 52% 9%	660
September	1st to 10th	Night	6% 1 6% 37% 45% 5%	600
	11th to 20th	Night	7% 1 9% 44% 35% 4	600
	21st to end	Night	4 1 15% 47% 30% 4	625
October	1st to 10th	Night	5% 2 19% 45% 27% 3	800
	11th to 20th	Night	5% 3 22% 45% 23% 3	800
	21st to end	Night	5% 5% 22% 37% 24% 7%	880
November	1st to 10th	Night	5% 4 24% 40% 23% 4	820
	11th to 20th	Night	5% 4 26% 40% 23% 3	977
	21st to end	Night	9% 6% 30% 32% 20% 3	1050
December	1st to 10th	Night	9% 6% 27% 40% 17% 1	1050
	11th to 20th	Night	7% 6% 32% 40% 15% 1	1050
	21st to end	Night	9% 8% 28% 41% 13% 2	1043

Page generated at 2026-07-13 01:14:49. Version ac07

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