Three Forks Visitor's Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Three Forks Visitor's Center clear sky chart from 2005-11-06 to 2024-01-15.

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	17% 16% 6% 10% 8% 4 6% 6% 5% 6% 16%	74476
	Night	11% 15% 6% 10% 9% 5% 7% 7% 6% 6% 19%	75331
Transparency	Day	67% 11% 9% 8% 4 1	74128
	Night	62% 13% 9% 9% 5% 2	75200
Seeing	Day	25% 15% 22% 20% 13% 4	17786
	Night	21% 16% 24% 21% 14% 3	22053

By Month

Percentage Cloud Cover NW (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 11% 4 9% 8% 4 8% 9% 8% 9% 27%	949
	11th to 20th	Day	3 14% 5% 14% 9% 5% 7% 8% 8% 9% 20%	1058
	21st to end	Day	4 12% 5% 8% 7% 4 9% 9% 9% 8% 25%	1386
February	1st to 10th	Day	5% 12% 5% 9% 8% 5% 7% 7% 8% 9% 26%	1260
	11th to 20th	Day	5% 19% 8% 11% 11% 5% 9% 7% 7% 6% 12%	1507
	21st to end	Day	7% 15% 8% 13% 8% 4 6% 5% 5% 7% 21%	1332
March	1st to 10th	Day	5% 15% 7% 9% 9% 4 6% 8% 7% 7% 23%	1721
	11th to 20th	Day	3 11% 6% 13% 11% 4 9% 8% 6% 6% 23%	1980
	21st to end	Day	6% 19% 8% 12% 10% 4 8% 5% 5% 4 17%	2215
April	1st to 10th	Day	6% 20% 9% 12% 9% 5% 6% 5% 6% 5% 17%	2304
	11th to 20th	Day	7% 17% 8% 10% 11% 5% 6% 6% 4 5% 21%	2317
	21st to end	Day	7% 16% 9% 12% 8% 5% 5% 6% 6% 6% 20%	2546
May	1st to 10th	Day	13% 22% 8% 11% 8% 4 5% 5% 4 5% 16%	2700
	11th to 20th	Day	10% 17% 7% 10% 8% 4 6% 5% 5% 6% 22%	2751
	21st to end	Day	11% 15% 6% 9% 7% 4 6% 6% 7% 9% 20%	3173
June	1st to 10th	Day	16% 16% 5% 9% 8% 3 6% 5% 6% 7% 19%	3060
	11th to 20th	Day	21% 14% 6% 9% 8% 4 6% 5% 5% 7% 16%	3060
	21st to end	Day	26% 15% 6% 9% 7% 4 5% 5% 6% 5% 12%	3060
July	1st to 10th	Day	28% 15% 5% 9% 7% 4 5% 5% 5% 6% 10%	3060
	11th to 20th	Day	31% 15% 5% 9% 8% 4 5% 4 4 5% 10%	3060
	21st to end	Day	35% 15% 5% 8% 6% 4 5% 4 4 4 8%	2983
August	1st to 10th	Day	32% 16% 5% 8% 7% 4 5% 4 4 5% 9%	2686
	11th to 20th	Day	34% 16% 6% 8% 6% 4 4 4 4 4 11%	2576
	21st to end	Day	37% 16% 4 9% 6% 3 5% 5% 4 4 8%	2574
September	1st to 10th	Day	30% 20% 6% 9% 8% 4 4 4 3 3 9%	2286
	11th to 20th	Day	25% 19% 6% 11% 7% 4 6% 4 4 3 11%	2093
	21st to end	Day	28% 21% 6% 9% 6% 4 4 4 3 3 13%	1943
October	1st to 10th	Day	18% 23% 9% 11% 7% 4 5% 4 4 3 12%	1800
	11th to 20th	Day	11% 17% 7% 15% 9% 4 6% 6% 6% 4 14%	1669
	21st to end	Day	10% 18% 7% 15% 9% 5% 5% 5% 6% 5% 15%	1584
November	1st to 10th	Day	11% 19% 7% 12% 10% 5% 8% 5% 5% 5% 13%	1439
	11th to 20th	Day	8% 14% 7% 11% 9% 7% 11% 6% 5% 6% 17%	1198
	21st to end	Day	7% 19% 6% 9% 8% 7% 11% 6% 6% 7% 14%	1140
December	1st to 10th	Day	5% 13% 6% 10% 10% 5% 9% 8% 7% 6% 20%	1060
	11th to 20th	Day	6% 15% 8% 9% 7% 6% 8% 9% 8% 9% 16%	1197
	21st to end	Day	4 10% 5% 11% 9% 5% 8% 7% 8% 8% 25%	1749
January	1st to 10th	Night	3 10% 5% 9% 8% 5% 9% 9% 8% 8% 26%	3217
	11th to 20th	Night	3 12% 7% 11% 8% 6% 8% 8% 9% 7% 22%	2989
	21st to end	Night	5% 14% 5% 9% 9% 5% 8% 10% 9% 7% 18%	2970
February	1st to 10th	Night	4 12% 5% 9% 7% 4 8% 8% 8% 9% 26%	2699
	11th to 20th	Night	5% 18% 7% 11% 8% 5% 8% 10% 7% 7% 14%	2339
	21st to end	Night	6% 15% 6% 10% 9% 5% 6% 6% 6% 7% 24%	1924
March	1st to 10th	Night	5% 13% 7% 12% 8% 5% 7% 8% 6% 6% 24%	2238
	11th to 20th	Night	3 13% 6% 11% 8% 4 9% 9% 7% 7% 23%	1978
	21st to end	Night	4 16% 7% 14% 11% 5% 7% 5% 5% 5% 22%	2141
April	1st to 10th	Night	7% 17% 7% 13% 11% 5% 6% 5% 7% 6% 16%	1652
	11th to 20th	Night	6% 16% 6% 10% 10% 5% 6% 6% 5% 6% 24%	1593
	21st to end	Night	7% 13% 6% 10% 9% 5% 6% 7% 7% 6% 24%	1413
May	1st to 10th	Night	10% 22% 6% 9% 6% 4 5% 6% 5% 7% 20%	1260
	11th to 20th	Night	10% 18% 6% 9% 7% 3 4 6% 5% 7% 25%	1143
	21st to end	Night	13% 12% 4 8% 7% 4 6% 7% 7% 10% 22%	1100
June	1st to 10th	Night	16% 12% 4 9% 10% 4 6% 5% 6% 9% 19%	900
	11th to 20th	Night	21% 12% 5% 7% 9% 4 6% 6% 5% 6% 19%	900
	21st to end	Night	20% 14% 4 8% 10% 4 7% 5% 6% 6% 15%	900
July	1st to 10th	Night	24% 14% 5% 7% 6% 4 7% 6% 6% 6% 15%	900
	11th to 20th	Night	29% 14% 5% 9% 6% 4 3 4 4 5% 15%	900
	21st to end	Night	35% 15% 5% 7% 6% 4 5% 4 4 4 11%	1333
August	1st to 10th	Night	28% 17% 4 8% 7% 3 5% 4 5% 5% 14%	1253
	11th to 20th	Night	32% 18% 5% 7% 6% 4 5% 5% 4 4 11%	1321
	21st to end	Night	34% 16% 4 8% 8% 4 4 4 4 3 11%	1782
September	1st to 10th	Night	26% 21% 5% 10% 8% 3 4 5% 3 4 11%	1674
	11th to 20th	Night	25% 18% 5% 8% 8% 4 4 5% 6% 5% 12%	1864
	21st to end	Night	27% 20% 4 8% 6% 3 5% 4 3 4 15%	2017
October	1st to 10th	Night	17% 22% 7% 10% 7% 4 5% 5% 4 6% 14%	2160
	11th to 20th	Night	13% 19% 7% 13% 7% 4 5% 5% 6% 7% 13%	2287
	21st to end	Night	11% 18% 6% 13% 9% 4 6% 5% 5% 5% 17%	2772
November	1st to 10th	Night	12% 18% 5% 10% 11% 5% 6% 5% 6% 6% 16%	2635
	11th to 20th	Night	6% 15% 7% 10% 9% 6% 9% 8% 6% 5% 17%	2981
	21st to end	Night	7% 14% 7% 10% 10% 6% 9% 7% 6% 7% 15%	3040
December	1st to 10th	Night	6% 14% 6% 12% 9% 5% 9% 8% 7% 6% 19%	3116
	11th to 20th	Night	5% 16% 8% 10% 11% 5% 9% 8% 6% 7% 15%	4011
	21st to end	Night	3 10% 5% 11% 8% 5% 9% 7% 8% 7% 25%	5929

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	48% 14% 12% 12% 11% 3	949
	11th to 20th	Day	57% 13% 10% 9% 7% 4	1058
	21st to end	Day	52% 15% 10% 11% 7% 4	1386
February	1st to 10th	Day	52% 15% 8% 10% 9% 6%	1260
	11th to 20th	Day	69% 11% 7% 7% 3 2	1513
	21st to end	Day	61% 15% 8% 7% 8% 1	1332
March	1st to 10th	Day	53% 12% 8% 13% 9% 5%	1721
	11th to 20th	Day	55% 8% 11% 11% 11% 4	1980
	21st to end	Day	65% 8% 8% 9% 6% 4	2215
April	1st to 10th	Day	66% 10% 7% 10% 5% 1	2304
	11th to 20th	Day	63% 10% 9% 12% 6% 1	2317
	21st to end	Day	63% 14% 8% 10% 4 1	2546
May	1st to 10th	Day	70% 11% 7% 10% 2	2700
	11th to 20th	Day	61% 12% 9% 15% 3	2754
	21st to end	Day	60% 15% 13% 9% 2	3174
June	1st to 10th	Day	63% 14% 12% 10% 1	3060
	11th to 20th	Day	66% 13% 11% 9% 2	3060
	21st to end	Day	73% 12% 12% 2	3060
July	1st to 10th	Day	75% 11% 9% 5%	2976
	11th to 20th	Day	73% 10% 11% 5%	2967
	21st to end	Day	78% 9% 7% 5%	2816
August	1st to 10th	Day	78% 11% 6% 4 1	2668
	11th to 20th	Day	75% 11% 7% 6% 1	2576
	21st to end	Day	79% 8% 8% 4 1	2574
September	1st to 10th	Day	81% 8% 4 6% 1	2286
	11th to 20th	Day	77% 7% 5% 7% 3	2093
	21st to end	Day	74% 7% 7% 9% 3	1943
October	1st to 10th	Day	75% 9% 5% 7% 4	1800
	11th to 20th	Day	72% 12% 7% 6% 3	1669
	21st to end	Day	69% 11% 6% 7% 6% 1	1584
November	1st to 10th	Day	70% 12% 8% 7% 3 1	1439
	11th to 20th	Day	61% 11% 11% 11% 5% 1	1198
	21st to end	Day	66% 11% 11% 8% 4 1	1140
December	1st to 10th	Day	60% 14% 10% 7% 7% 2	1060
	11th to 20th	Day	59% 10% 10% 9% 8% 5%	1201
	21st to end	Day	50% 16% 10% 11% 7% 6%	1749
January	1st to 10th	Night	50% 15% 11% 11% 10% 3	3217
	11th to 20th	Night	58% 15% 7% 8% 8% 4	2989
	21st to end	Night	56% 15% 9% 10% 6% 4	2970
February	1st to 10th	Night	54% 13% 9% 10% 10% 4	2699
	11th to 20th	Night	67% 12% 7% 8% 4 2	2344
	21st to end	Night	57% 15% 10% 7% 9%	1924
March	1st to 10th	Night	55% 12% 8% 12% 9% 4	2238
	11th to 20th	Night	52% 9% 10% 15% 10% 4	1980
	21st to end	Night	61% 10% 11% 9% 6% 4	2141
April	1st to 10th	Night	67% 9% 9% 8% 5% 1	1652
	11th to 20th	Night	59% 14% 8% 12% 7%	1593
	21st to end	Night	59% 15% 9% 10% 7% 1	1413
May	1st to 10th	Night	66% 14% 9% 8% 2	1260
	11th to 20th	Night	61% 13% 10% 13% 2	1144
	21st to end	Night	58% 16% 15% 9% 2	1100
June	1st to 10th	Night	61% 17% 11% 10% 1	900
	11th to 20th	Night	66% 15% 9% 9% 2	900
	21st to end	Night	71% 14% 11% 5%	900
July	1st to 10th	Night	73% 10% 13% 5%	875
	11th to 20th	Night	73% 11% 9% 6%	870
	21st to end	Night	77% 11% 8% 4	1260
August	1st to 10th	Night	74% 12% 8% 5%	1246
	11th to 20th	Night	76% 11% 7% 6%	1321
	21st to end	Night	79% 8% 7% 6%	1782
September	1st to 10th	Night	77% 11% 6% 5% 1	1674
	11th to 20th	Night	74% 10% 8% 5% 3	1864
	21st to end	Night	73% 9% 8% 7% 4	2017
October	1st to 10th	Night	74% 10% 5% 8% 4	2160
	11th to 20th	Night	70% 14% 8% 6% 2	2287
	21st to end	Night	68% 12% 6% 6% 5% 2	2772
November	1st to 10th	Night	65% 12% 8% 10% 4 1	2635
	11th to 20th	Night	59% 12% 12% 10% 6% 1	2981
	21st to end	Night	62% 12% 11% 9% 5% 2	3040
December	1st to 10th	Night	62% 14% 9% 6% 6% 2	3116
	11th to 20th	Night	61% 10% 10% 11% 5% 3	4007
	21st to end	Night	50% 16% 10% 10% 8% 6%	5929

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% 14% 22% 25% 18% 6%	280
	11th to 20th	Day	13% 20% 22% 24% 15% 6%	269
	21st to end	Day	12% 25% 30% 21% 8% 4	286
February	1st to 10th	Day	14% 15% 30% 23% 10% 8%	260
	11th to 20th	Day	14% 28% 30% 15% 10% 2	305
	21st to end	Day	22% 24% 26% 20% 5% 3	280
March	1st to 10th	Day	17% 22% 29% 20% 12%	401
	11th to 20th	Day	8% 20% 31% 28% 12% 2	520
	21st to end	Day	19% 17% 28% 23% 12% 2	572
April	1st to 10th	Day	16% 23% 25% 19% 12% 4	520
	11th to 20th	Day	17% 15% 27% 24% 14% 3	516
	21st to end	Day	15% 18% 29% 21% 13% 4	600
May	1st to 10th	Day	21% 15% 25% 22% 13% 3	603
	11th to 20th	Day	19% 17% 21% 24% 15% 4	625
	21st to end	Day	18% 13% 25% 23% 15% 6%	742
June	1st to 10th	Day	20% 10% 18% 22% 23% 8%	810
	11th to 20th	Day	22% 10% 18% 21% 22% 6%	810
	21st to end	Day	26% 9% 16% 23% 20% 5%	810
July	1st to 10th	Day	34% 12% 14% 19% 18% 4	796
	11th to 20th	Day	34% 9% 16% 22% 15% 4	792
	21st to end	Day	35% 9% 18% 18% 15% 5%	688
August	1st to 10th	Day	38% 9% 18% 19% 13% 2	626
	11th to 20th	Day	34% 11% 19% 24% 10% 2	611
	21st to end	Day	44% 11% 14% 18% 10% 3	594
September	1st to 10th	Day	40% 13% 19% 15% 11% 3	540
	11th to 20th	Day	40% 14% 17% 15% 10% 3	540
	21st to end	Day	39% 11% 21% 17% 7% 5%	503
October	1st to 10th	Day	36% 20% 24% 13% 4 3	360
	11th to 20th	Day	23% 20% 24% 16% 12% 5%	360
	21st to end	Day	25% 19% 24% 20% 9% 3	396
November	1st to 10th	Day	26% 19% 23% 20% 8% 4	343
	11th to 20th	Day	19% 21% 30% 14% 10% 6%	280
	21st to end	Day	20% 22% 27% 18% 11% 2	280
December	1st to 10th	Day	19% 19% 30% 18% 13% 2	280
	11th to 20th	Day	19% 21% 24% 23% 10% 3	280
	21st to end	Day	18% 22% 25% 18% 11% 6%	308
January	1st to 10th	Night	10% 15% 27% 28% 17% 3	940
	11th to 20th	Night	12% 18% 28% 24% 14% 4	876
	21st to end	Night	14% 19% 30% 25% 9% 3	880
February	1st to 10th	Night	12% 17% 26% 25% 16% 4	799
	11th to 20th	Night	18% 23% 32% 17% 8% 3	608
	21st to end	Night	18% 23% 26% 21% 10% 2	510
March	1st to 10th	Night	14% 21% 27% 23% 13% 2	620
	11th to 20th	Night	9% 15% 28% 32% 14% 3	620
	21st to end	Night	11% 14% 29% 27% 15% 5%	665
April	1st to 10th	Night	17% 18% 26% 22% 15% 3	540
	11th to 20th	Night	14% 15% 23% 30% 16% 4	533
	21st to end	Night	11% 14% 32% 24% 15% 5%	513
May	1st to 10th	Night	18% 9% 26% 23% 21% 3	360
	11th to 20th	Night	19% 15% 22% 24% 16% 5%	354
	21st to end	Night	18% 8% 24% 24% 21% 5%	389
June	1st to 10th	Night	21% 9% 16% 24% 23% 7%	360
	11th to 20th	Night	26% 7% 20% 21% 23% 4	360
	21st to end	Night	27% 9% 14% 16% 29% 6%	360
July	1st to 10th	Night	33% 9% 13% 18% 22% 5%	350
	11th to 20th	Night	32% 7% 18% 20% 17% 6%	348
	21st to end	Night	40% 9% 14% 17% 15% 5%	394
August	1st to 10th	Night	38% 8% 18% 18% 15% 3	358
	11th to 20th	Night	38% 10% 18% 17% 15% 3	428
	21st to end	Night	42% 10% 16% 17% 13% 3	594
September	1st to 10th	Night	37% 12% 20% 17% 12% 2	540
	11th to 20th	Night	39% 13% 16% 15% 14% 4	573
	21st to end	Night	38% 12% 21% 16% 11% 2	630
October	1st to 10th	Night	34% 20% 20% 15% 9% 1	630
	11th to 20th	Night	24% 19% 21% 20% 12% 4	640
	21st to end	Night	25% 18% 23% 21% 11% 2	891
November	1st to 10th	Night	25% 19% 20% 19% 13% 5%	830
	11th to 20th	Night	15% 23% 30% 18% 11% 2	850
	21st to end	Night	17% 18% 30% 21% 10% 3	850
December	1st to 10th	Night	16% 22% 28% 19% 11% 3	886
	11th to 20th	Night	17% 20% 29% 22% 9% 2	940
	21st to end	Night	17% 20% 25% 23% 12% 3	1034

Page generated at 2024-01-11 18:33:48. Version ac07

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