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 2025-03-12.

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% 65%	112517
	Night	4 4 1 3 3 3 3 3 5% 7% 64%	115345
Transparency	Day	18% 3 8% 17% 29% 24%	112280
	Night	20% 3 7% 15% 30% 25%	115172
Seeing	Day	6% 10% 22% 30% 24% 8%	15121
	Night	6% 5% 25% 38% 23% 3	25501

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 9% 3 5% 3 3 3 3 4 4 59%	4098
	11th to 20th	Day	3 4 1 3 1 2 2 2 3 5% 74%	4100
	21st to end	Day	7% 6% 1 4 2 3 3 3 4 5% 61%	4510
February	1st to 10th	Day	4 5% 1 4 2 2 2 3 4 6% 67%	1909
	11th to 20th	Day	6% 4 1 3 2 2 3 3 4 6% 65%	1949
	21st to end	Day	5% 4 2 3 3 2 2 3 4 5% 68%	1650
March	1st to 10th	Day	4 4 1 3 2 2 3 3 5% 7% 65%	3449
	11th to 20th	Day	3 5% 2 5% 3 4 4 3 5% 6% 60%	3502
	21st to end	Day	2 5% 2 4 4 4 4 3 6% 7% 58%	3939
April	1st to 10th	Day	2 2 1 2 3 2 3 3 4 7% 71%	2280
	11th to 20th	Day	4 3 1 3 2 2 2 2 3 5% 73%	2251
	21st to end	Day	3 3 1 2 2 1 2 3 4 5% 75%	2280
May	1st to 10th	Day	1 2 2 1 2 2 2 2 4 82%	3800
	11th to 20th	Day	1 2 1 2 3 3 3 3 5% 6% 70%	3765
	21st to end	Day	1 1 1 1 1 2 2 2 4 84%	4486
June	1st to 10th	Day	2 2 1 2 3 3 4 4 6% 10% 65%	2660
	11th to 20th	Day	2 2 1 2 2 2 2 3 5% 6% 74%	2660
	21st to end	Day	4 3 1 5% 4 4 4 5% 6% 9% 55%	2586
July	1st to 10th	Day	3 2 1 4 4 5% 6% 5% 8% 9% 53%	4110
	11th to 20th	Day	2 2 1 3 4 4 5% 5% 8% 12% 54%	4110
	21st to end	Day	4 3 1 4 4 5% 6% 7% 11% 12% 42%	4454
August	1st to 10th	Day	2 2 1 4 4 5% 6% 6% 9% 11% 50%	3845
	11th to 20th	Day	3 2 1 3 3 3 4 4 7% 9% 60%	3726
	21st to end	Day	3 3 1 5% 5% 5% 6% 5% 8% 12% 48%	4180
September	1st to 10th	Day	6% 5% 2 5% 4 4 6% 4 7% 8% 50%	2280
	11th to 20th	Day	6% 3 2 3 3 4 4 3 5% 7% 61%	2280
	21st to end	Day	2 1 1 4 2 2 2 2 5% 6% 72%	2080
October	1st to 10th	Day	3 2 1 3 2 2 2 2 3 6% 75%	3170
	11th to 20th	Day	3 2 1 3 2 2 2 2 3 6% 74%	3150
	21st to end	Day	2 1 1 1 1 1 1 1 2 3 86%	3438
November	1st to 10th	Day	3 1 1 3 2 2 3 3 4 6% 73%	1959
	11th to 20th	Day	3 2 1 2 2 2 3 2 4 6% 73%	1847
	21st to end	Day	5% 4 2 4 2 3 3 3 5% 6% 63%	1800
December	1st to 10th	Day	4 4 1 2 2 2 2 2 4 7% 71%	2908
	11th to 20th	Day	3 3 2 4 3 3 3 2 3 6% 67%	3026
	21st to end	Day	4 3 1 2 2 2 3 2 4 5% 72%	4280
January	1st to 10th	Night	5% 11% 2 4 2 2 3 3 4 5% 59%	5867
	11th to 20th	Night	3 3 1 2 2 2 3 2 3 4 75%	5880
	21st to end	Night	7% 7% 1 4 3 3 3 3 5% 6% 59%	6468
February	1st to 10th	Night	4 5% 2 3 2 2 2 2 4 7% 66%	2489
	11th to 20th	Night	9% 5% 1 3 2 2 2 2 4 4 64%	2338
	21st to end	Night	7% 5% 2 3 2 3 3 3 4 6% 63%	1980
March	1st to 10th	Night	6% 5% 1 4 2 3 3 2 3 5% 67%	3639
	11th to 20th	Night	10% 6% 2 4 3 2 3 3 5% 6% 57%	3506
	21st to end	Night	4 7% 2 4 3 3 3 3 5% 7% 57%	3708
April	1st to 10th	Night	3 3 1 3 3 3 2 3 5% 7% 68%	1900
	11th to 20th	Night	5% 3 1 4 3 3 3 3 4 5% 66%	1877
	21st to end	Night	5% 3 1 3 2 2 3 3 5% 6% 67%	1900
May	1st to 10th	Night	2 2 2 2 2 2 2 3 5% 79%	3170
	11th to 20th	Night	3 3 1 3 2 3 3 3 4 6% 68%	3140
	21st to end	Night	1 1 1 2 1 1 2 2 3 4 82%	3092
June	1st to 10th	Night	2 2 1 4 4 4 4 5% 7% 9% 57%	1520
	11th to 20th	Night	3 1 1 2 2 2 3 3 6% 7% 69%	1520
	21st to end	Night	4 4 2 4 4 4 6% 6% 8% 9% 50%	1585
July	1st to 10th	Night	3 2 1 5% 5% 5% 5% 5% 10% 10% 49%	2860
	11th to 20th	Night	2 3 1 6% 4 5% 5% 5% 6% 11% 52%	2860
	21st to end	Night	5% 3 2 6% 5% 6% 7% 7% 11% 13% 36%	3109
August	1st to 10th	Night	2 3 1 4 4 5% 6% 6% 10% 15% 46%	3091
	11th to 20th	Night	3 3 1 4 3 4 5% 4 7% 13% 53%	3113
	21st to end	Night	3 3 1 6% 6% 6% 7% 6% 10% 12% 40%	3487
September	1st to 10th	Night	6% 3 2 6% 4 4 4 5% 6% 8% 53%	1900
	11th to 20th	Night	5% 3 2 5% 3 4 5% 3 6% 8% 56%	1900
	21st to end	Night	3 3 1 3 2 3 3 3 5% 6% 67%	2095
October	1st to 10th	Night	3 2 1 3 2 2 2 2 4 4 75%	3800
	11th to 20th	Night	6% 3 1 4 3 2 3 2 3 4 69%	3778
	21st to end	Night	4 1 1 2 1 2 2 2 3 6% 78%	4120
November	1st to 10th	Night	4 2 1 3 2 2 2 2 4 7% 73%	2340
	11th to 20th	Night	3 2 1 2 2 3 3 2 4 5% 74%	2530
	21st to end	Night	7% 3 1 4 3 3 3 3 4 6% 63%	2600
December	1st to 10th	Night	5% 4 1 2 2 2 2 2 3 6% 72%	4277
	11th to 20th	Night	4 3 1 3 3 3 3 3 4 7% 66%	5181
	21st to end	Night	6% 3 1 3 2 2 2 2 4 6% 68%	6725

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% 1 7% 10% 41% 11%	4099
	11th to 20th	Day	15% 2 5% 16% 44% 18%	4100
	21st to end	Day	25% 3 6% 14% 30% 22%	4510
February	1st to 10th	Day	20% 1 5% 16% 31% 26%	1909
	11th to 20th	Day	21% 3 5% 18% 30% 23%	1959
	21st to end	Day	19% 2 4 9% 32% 34%	1650
March	1st to 10th	Day	19% 2 4 10% 39% 26%	3449
	11th to 20th	Day	23% 4 5% 20% 29% 19%	3502
	21st to end	Day	23% 2 8% 14% 31% 22%	3939
April	1st to 10th	Day	15% 2 4 10% 33% 35%	2280
	11th to 20th	Day	16% 2 5% 10% 28% 41%	2254
	21st to end	Day	13% 2 4 13% 34% 34%	2280
May	1st to 10th	Day	9% 1 3 7% 27% 53%	3800
	11th to 20th	Day	14% 3 7% 13% 29% 35%	3768
	21st to end	Day	7% 1 3 8% 16% 65%	4486
June	1st to 10th	Day	14% 4 8% 18% 25% 31%	2660
	11th to 20th	Day	11% 2 5% 10% 23% 49%	2660
	21st to end	Day	24% 5% 10% 18% 21% 23%	2586
July	1st to 10th	Day	23% 6% 15% 17% 20% 18%	4046
	11th to 20th	Day	19% 7% 16% 28% 25% 4	4041
	21st to end	Day	27% 9% 17% 31% 14% 2	4314
August	1st to 10th	Day	23% 7% 14% 32% 21% 4	3831
	11th to 20th	Day	19% 6% 11% 21% 35% 8%	3762
	21st to end	Day	26% 7% 15% 28% 20% 4	4179
September	1st to 10th	Day	30% 5% 11% 18% 22% 14%	2280
	11th to 20th	Day	22% 4 7% 22% 26% 19%	2280
	21st to end	Day	15% 3 6% 20% 34% 22%	2080
October	1st to 10th	Day	13% 3 5% 10% 31% 38%	3170
	11th to 20th	Day	14% 3 6% 12% 33% 31%	3150
	21st to end	Day	8% 1 3 12% 34% 42%	3438
November	1st to 10th	Day	13% 2 5% 20% 35% 25%	1959
	11th to 20th	Day	14% 3 5% 19% 32% 26%	1847
	21st to end	Day	22% 3 6% 18% 31% 20%	1800
December	1st to 10th	Day	15% 2 4 25% 37% 17%	2908
	11th to 20th	Day	20% 2 6% 14% 41% 16%	3024
	21st to end	Day	17% 1 4 21% 39% 18%	4280
January	1st to 10th	Night	28% 2 6% 11% 40% 13%	5867
	11th to 20th	Night	14% 2 4 14% 46% 20%	5880
	21st to end	Night	26% 3 6% 12% 29% 25%	6468
February	1st to 10th	Night	19% 2 6% 16% 27% 30%	2489
	11th to 20th	Night	24% 2 5% 16% 30% 23%	2338
	21st to end	Night	23% 2 4 9% 29% 33%	1980
March	1st to 10th	Night	22% 2 4 10% 38% 24%	3639
	11th to 20th	Night	27% 5% 7% 16% 28% 18%	3506
	21st to end	Night	24% 2 9% 12% 33% 19%	3708
April	1st to 10th	Night	16% 3 5% 10% 36% 30%	1900
	11th to 20th	Night	20% 3 6% 10% 25% 36%	1877
	21st to end	Night	17% 2 5% 12% 33% 30%	1900
May	1st to 10th	Night	10% 1 3 6% 22% 58%	3170
	11th to 20th	Night	16% 3 9% 12% 23% 36%	3141
	21st to end	Night	8% 2 4 8% 15% 63%	3093
June	1st to 10th	Night	20% 6% 9% 16% 19% 30%	1520
	11th to 20th	Night	13% 2 7% 12% 18% 48%	1520
	21st to end	Night	26% 7% 11% 16% 17% 23%	1585
July	1st to 10th	Night	26% 5% 14% 20% 18% 16%	2815
	11th to 20th	Night	25% 6% 13% 23% 28% 7%	2806
	21st to end	Night	32% 9% 17% 28% 11% 2	3011
August	1st to 10th	Night	21% 9% 12% 29% 24% 6%	3082
	11th to 20th	Night	19% 8% 10% 19% 36% 7%	3143
	21st to end	Night	31% 7% 13% 26% 20% 4	3487
September	1st to 10th	Night	26% 6% 13% 17% 22% 16%	1900
	11th to 20th	Night	25% 6% 10% 18% 21% 21%	1900
	21st to end	Night	17% 5% 6% 16% 32% 24%	2095
October	1st to 10th	Night	14% 3 5% 9% 28% 41%	3800
	11th to 20th	Night	20% 3 4 10% 27% 36%	3778
	21st to end	Night	12% 1 3 13% 30% 41%	4120
November	1st to 10th	Night	13% 2 5% 16% 36% 28%	2340
	11th to 20th	Night	14% 2 4 17% 32% 31%	2530
	21st to end	Night	23% 3 4 17% 32% 21%	2600
December	1st to 10th	Night	15% 3 4 22% 36% 20%	4277
	11th to 20th	Night	19% 2 5% 15% 36% 22%	5182
	21st to end	Night	19% 2 3 18% 40% 18%	6725

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% 32% 13% 1	300
	11th to 20th	Day	10% 7% 34% 33% 14% 2	300
	21st to end	Day	8% 9% 37% 29% 14% 2	330
February	1st to 10th	Day	8% 10% 35% 30% 14% 3	300
	11th to 20th	Day	14% 15% 35% 29% 7% 1	300
	21st to end	Day	13% 21% 31% 26% 8% 1	249
March	1st to 10th	Day	12% 22% 34% 26% 5% 2	429
	11th to 20th	Day	11% 18% 31% 26% 12% 2	465
	21st to end	Day	11% 27% 33% 20% 8% 1	506
April	1st to 10th	Day	6% 21% 37% 25% 9% 2	460
	11th to 20th	Day	11% 22% 35% 25% 6% 1	452
	21st to end	Day	8% 22% 35% 25% 9% 1	460
May	1st to 10th	Day	5% 19% 39% 27% 9% 2	463
	11th to 20th	Day	3 13% 32% 32% 17% 4	488
	21st to end	Day	3 15% 32% 31% 17% 2	591
June	1st to 10th	Day	3 4 19% 38% 31% 5%	680
	11th to 20th	Day	3 4 20% 37% 31% 4	680
	21st to end	Day	5% 2 13% 33% 36% 11%	606
July	1st to 10th	Day	4 4 33% 44% 16%	486
	11th to 20th	Day	2 3 23% 51% 21%	484
	21st to end	Day	3 1 4 21% 46% 26%	531
August	1st to 10th	Day	3 4 27% 41% 24%	487
	11th to 20th	Day	5% 3 5% 26% 45% 16%	477
	21st to end	Day	3 2 5% 26% 43% 20%	539
September	1st to 10th	Day	7% 1 11% 32% 37% 13%	490
	11th to 20th	Day	4 1 14% 39% 31% 10%	490
	21st to end	Day	3 3 15% 37% 29% 14%	318
October	1st to 10th	Day	6% 6% 16% 29% 27% 16%	300
	11th to 20th	Day	7% 5% 19% 28% 33% 8%	300
	21st to end	Day	7% 10% 18% 34% 22% 10%	330
November	1st to 10th	Day	7% 8% 26% 35% 20% 3	300
	11th to 20th	Day	4 9% 21% 39% 21% 6%	300
	21st to end	Day	8% 11% 33% 29% 17% 1	300
December	1st to 10th	Day	11% 8% 28% 35% 17% 1	300
	11th to 20th	Day	7% 13% 30% 35% 14% 1	300
	21st to end	Day	10% 15% 29% 33% 13%	330
January	1st to 10th	Night	8% 5% 31% 39% 15% 3	800
	11th to 20th	Night	8% 5% 32% 39% 14% 2	800
	21st to end	Night	9% 5% 34% 35% 16% 1	880
February	1st to 10th	Night	6% 6% 35% 36% 15% 2	800
	11th to 20th	Night	12% 7% 39% 34% 8% 1	784
	21st to end	Night	8% 11% 43% 31% 6%	660
March	1st to 10th	Night	8% 12% 36% 33% 11% 1	800
	11th to 20th	Night	7% 10% 32% 34% 17% 1	766
	21st to end	Night	7% 12% 34% 39% 8% 1	767
April	1st to 10th	Night	4 9% 43% 36% 8%	570
	11th to 20th	Night	6% 11% 38% 34% 10% 1	563
	21st to end	Night	7% 7% 39% 38% 10%	570
May	1st to 10th	Night	4 7% 39% 42% 9%	570
	11th to 20th	Night	5% 4 27% 48% 16%	561
	21st to end	Night	2 4 33% 47% 12% 1	617
June	1st to 10th	Night	2 3 22% 50% 22% 1	570
	11th to 20th	Night	2 2 20% 55% 21% 1	570
	21st to end	Night	5% 2 12% 40% 39% 3	570
July	1st to 10th	Night	5% 6% 37% 47% 5%	555
	11th to 20th	Night	2 1 3 31% 56% 7%	552
	21st to end	Night	3 1 3 24% 58% 12%	624
August	1st to 10th	Night	3 1 3 23% 57% 12%	567
	11th to 20th	Night	4 1 5% 29% 52% 9%	564
	21st to end	Night	3 1 4 32% 51% 9%	627
September	1st to 10th	Night	6% 1 6% 36% 46% 5%	570
	11th to 20th	Night	6% 1 9% 42% 36% 4	570
	21st to end	Night	5% 1 15% 46% 30% 4	594
October	1st to 10th	Night	5% 2 19% 44% 27% 3	760
	11th to 20th	Night	4 3 22% 44% 23% 3	760
	21st to end	Night	6% 5% 22% 36% 25% 7%	836
November	1st to 10th	Night	5% 4 25% 39% 23% 4	780
	11th to 20th	Night	4 4 26% 39% 23% 4	930
	21st to end	Night	9% 7% 29% 31% 21% 3	1000
December	1st to 10th	Night	10% 6% 25% 40% 18% 1	1000
	11th to 20th	Night	7% 7% 32% 39% 15% 1	1000
	21st to end	Night	10% 8% 28% 40% 13% 2	994

Page generated at 2025-03-09 06:08:45. Version ac07

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