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 2026-02-16.

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	14% 18% 6% 12% 7% 6% 6% 4 6% 6% 15%	134335
	Night	9% 16% 6% 13% 7% 7% 7% 5% 6% 6% 18%	135449
Transparency	Day	65% 10% 10% 9% 4 1	134005
	Night	59% 11% 9% 10% 7% 3	135362
Seeing	Day	25% 15% 22% 20% 13% 4	20610
	Night	22% 16% 24% 22% 13% 3	24882

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	3 11% 4 9% 5% 6% 8% 8% 11% 10% 24%	2957
	11th to 20th	Day	2 9% 4 13% 6% 7% 8% 7% 8% 7% 28%	3403
	21st to end	Day	2 9% 4 11% 7% 7% 7% 6% 7% 8% 33%	4494
February	1st to 10th	Day	5% 11% 5% 10% 7% 7% 6% 7% 8% 9% 26%	1470
	11th to 20th	Day	5% 19% 7% 13% 9% 7% 8% 6% 7% 6% 12%	1722
	21st to end	Day	6% 16% 7% 15% 7% 6% 6% 4 6% 7% 19%	1485
March	1st to 10th	Day	4 16% 7% 12% 7% 7% 9% 6% 7% 6% 18%	4301
	11th to 20th	Day	1 9% 5% 13% 9% 8% 12% 8% 6% 7% 22%	4980
	21st to end	Day	4 19% 9% 21% 10% 6% 6% 4 4 4 12%	5570
April	1st to 10th	Day	6% 21% 7% 14% 8% 6% 6% 4 6% 5% 16%	2561
	11th to 20th	Day	6% 20% 7% 13% 9% 6% 6% 4 4 5% 20%	2579
	21st to end	Day	7% 19% 7% 13% 8% 6% 5% 5% 6% 6% 18%	2830
May	1st to 10th	Day	10% 25% 8% 14% 6% 4 4 3 5% 5% 15%	6820
	11th to 20th	Day	10% 22% 9% 15% 8% 6% 5% 4 5% 4 12%	7027
	21st to end	Day	9% 23% 6% 12% 7% 6% 6% 4 6% 8% 14%	8103
June	1st to 10th	Day	16% 19% 5% 10% 7% 5% 5% 4 6% 6% 18%	3400
	11th to 20th	Day	20% 15% 4 10% 6% 6% 6% 4 6% 7% 18%	3400
	21st to end	Day	25% 16% 5% 10% 7% 5% 5% 5% 6% 5% 12%	3400
July	1st to 10th	Day	31% 18% 5% 10% 6% 5% 5% 4 6% 5% 7%	7720
	11th to 20th	Day	25% 18% 5% 10% 6% 5% 5% 3 5% 6% 12%	7720
	21st to end	Day	28% 18% 5% 10% 5% 5% 4 3 4 4 13%	7514
August	1st to 10th	Day	23% 15% 5% 12% 7% 6% 5% 4 5% 7% 12%	4887
	11th to 20th	Day	23% 21% 6% 11% 6% 5% 5% 3 5% 5% 10%	4660
	21st to end	Day	30% 22% 5% 11% 5% 5% 5% 4 4 3 5%	4675
September	1st to 10th	Day	29% 21% 5% 11% 7% 5% 4 3 3 3 9%	2539
	11th to 20th	Day	25% 21% 6% 12% 6% 5% 4 3 4 3 11%	2325
	21st to end	Day	26% 23% 6% 9% 5% 5% 4 3 3 3 13%	2158
October	1st to 10th	Day	12% 24% 8% 13% 7% 6% 5% 4 4 4 14%	3260
	11th to 20th	Day	7% 15% 6% 16% 9% 7% 6% 4 5% 4 21%	2996
	21st to end	Day	9% 19% 10% 17% 9% 7% 5% 3 5% 4 11%	2831
November	1st to 10th	Day	10% 19% 7% 15% 9% 8% 6% 4 5% 5% 12%	1593
	11th to 20th	Day	7% 15% 5% 13% 9% 10% 8% 4 6% 6% 17%	1324
	21st to end	Day	6% 19% 5% 10% 7% 10% 9% 4 6% 8% 16%	1260
December	1st to 10th	Day	6% 12% 5% 13% 9% 9% 9% 8% 8% 5% 15%	1871
	11th to 20th	Day	4 18% 8% 12% 7% 7% 8% 7% 7% 9% 13%	1937
	21st to end	Day	4 12% 5% 13% 7% 6% 7% 6% 9% 9% 22%	2563
January	1st to 10th	Night	3 9% 4 13% 7% 8% 9% 7% 10% 8% 23%	9998
	11th to 20th	Night	2 10% 5% 12% 7% 7% 8% 6% 7% 7% 28%	9541
	21st to end	Night	2 8% 4 10% 8% 7% 7% 7% 10% 11% 26%	9719
February	1st to 10th	Night	4 12% 5% 11% 6% 6% 7% 7% 8% 9% 26%	3148
	11th to 20th	Night	5% 18% 6% 12% 7% 7% 8% 8% 8% 7% 14%	2675
	21st to end	Night	6% 16% 6% 12% 8% 6% 6% 5% 6% 7% 22%	2145
March	1st to 10th	Night	5% 16% 7% 16% 8% 7% 7% 6% 6% 5% 17%	5552
	11th to 20th	Night	2 10% 5% 12% 8% 9% 12% 8% 6% 7% 21%	4978
	21st to end	Night	4 17% 9% 23% 10% 6% 5% 4 4 4 14%	5361
April	1st to 10th	Night	6% 18% 6% 16% 9% 7% 6% 4 7% 6% 15%	1835
	11th to 20th	Night	5% 18% 6% 12% 9% 6% 6% 5% 5% 6% 23%	1771
	21st to end	Night	7% 15% 6% 12% 8% 7% 5% 5% 7% 6% 23%	1569
May	1st to 10th	Night	6% 22% 7% 12% 7% 6% 5% 5% 5% 6% 19%	3160
	11th to 20th	Night	7% 24% 7% 13% 7% 6% 6% 5% 5% 5% 15%	2887
	21st to end	Night	12% 20% 6% 11% 6% 6% 7% 5% 6% 7% 13%	2800
June	1st to 10th	Night	17% 16% 4 10% 9% 6% 5% 4 6% 8% 17%	1000
	11th to 20th	Night	19% 13% 4 9% 8% 5% 5% 5% 5% 7% 20%	1000
	21st to end	Night	20% 15% 3 11% 9% 7% 5% 4 6% 6% 15%	1000
July	1st to 10th	Night	28% 16% 5% 9% 7% 5% 5% 4 4 5% 12%	2280
	11th to 20th	Night	24% 18% 6% 12% 7% 5% 4 3 4 5% 13%	2280
	21st to end	Night	30% 19% 5% 9% 5% 4 4 3 3 4 16%	3350
August	1st to 10th	Night	21% 17% 5% 13% 7% 5% 5% 3 4 4 15%	2273
	11th to 20th	Night	24% 27% 4 8% 5% 6% 4 4 4 5% 10%	2394
	21st to end	Night	27% 26% 5% 10% 6% 6% 4 3 4 3 7%	3223
September	1st to 10th	Night	26% 22% 6% 11% 7% 4 3 4 3 4 10%	1860
	11th to 20th	Night	26% 19% 5% 9% 7% 5% 4 3 6% 5% 11%	2072
	21st to end	Night	26% 21% 4 9% 5% 5% 4 3 3 4 15%	2242
October	1st to 10th	Night	10% 22% 7% 12% 7% 6% 5% 4 5% 6% 15%	3920
	11th to 20th	Night	8% 17% 7% 18% 7% 6% 5% 3 4 6% 19%	4140
	21st to end	Night	13% 24% 7% 15% 8% 5% 4 3 4 4 13%	4970
November	1st to 10th	Night	11% 19% 6% 13% 9% 7% 5% 4 6% 6% 16%	2921
	11th to 20th	Night	5% 16% 6% 12% 9% 8% 8% 7% 6% 6% 17%	3295
	21st to end	Night	6% 15% 6% 12% 9% 8% 8% 5% 6% 7% 17%	3360
December	1st to 10th	Night	6% 14% 5% 13% 8% 8% 9% 7% 8% 7% 16%	5540
	11th to 20th	Night	5% 17% 7% 13% 9% 7% 7% 5% 6% 7% 17%	6511
	21st to end	Night	4 12% 5% 13% 7% 7% 9% 5% 8% 8% 22%	8679

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	43% 12% 11% 15% 15% 3	2958
	11th to 20th	Day	41% 12% 10% 11% 19% 6%	3419
	21st to end	Day	43% 10% 8% 12% 15% 13%	4495
February	1st to 10th	Day	50% 15% 9% 12% 9% 6%	1470
	11th to 20th	Day	67% 12% 7% 8% 4 2	1728
	21st to end	Day	61% 15% 8% 8% 7% 1	1485
March	1st to 10th	Day	50% 8% 9% 19% 9% 5%	4301
	11th to 20th	Day	44% 12% 12% 15% 14% 3	4980
	21st to end	Day	69% 11% 6% 6% 6% 2	5570
April	1st to 10th	Day	66% 10% 8% 10% 5% 1	2561
	11th to 20th	Day	63% 11% 9% 11% 6% 1	2579
	21st to end	Day	63% 13% 8% 10% 4 1	2830
May	1st to 10th	Day	68% 7% 7% 14% 4	6820
	11th to 20th	Day	68% 11% 8% 9% 3	7030
	21st to end	Day	65% 13% 13% 8% 1	8104
June	1st to 10th	Day	64% 14% 11% 10% 1	3400
	11th to 20th	Day	64% 13% 13% 9% 2	3400
	21st to end	Day	72% 12% 13% 3	3400
July	1st to 10th	Day	74% 10% 11% 5%	7636
	11th to 20th	Day	70% 8% 14% 8%	7627
	21st to end	Day	74% 7% 13% 5%	7303
August	1st to 10th	Day	69% 11% 11% 8% 1	4869
	11th to 20th	Day	73% 10% 9% 7%	4705
	21st to end	Day	81% 6% 8% 4	4674
September	1st to 10th	Day	80% 8% 5% 6% 1	2539
	11th to 20th	Day	77% 7% 5% 8% 3	2325
	21st to end	Day	74% 6% 7% 10% 3	2158
October	1st to 10th	Day	70% 9% 5% 11% 4	3260
	11th to 20th	Day	64% 10% 8% 13% 4	2996
	21st to end	Day	74% 8% 4 8% 6%	2831
November	1st to 10th	Day	70% 11% 7% 7% 3 1	1593
	11th to 20th	Day	61% 10% 10% 11% 6% 1	1324
	21st to end	Day	63% 10% 10% 9% 6% 2	1260
December	1st to 10th	Day	56% 15% 14% 8% 6% 2	1871
	11th to 20th	Day	59% 10% 13% 9% 6% 3	1941
	21st to end	Day	50% 14% 10% 12% 8% 5%	2563
January	1st to 10th	Night	48% 14% 9% 13% 13% 4	9998
	11th to 20th	Night	45% 12% 8% 12% 18% 5%	9588
	21st to end	Night	40% 13% 12% 11% 13% 12%	9719
February	1st to 10th	Night	53% 13% 10% 11% 10% 4	3148
	11th to 20th	Night	64% 12% 7% 8% 6% 2	2680
	21st to end	Night	57% 15% 10% 8% 9%	2145
March	1st to 10th	Night	55% 11% 6% 15% 9% 5%	5552
	11th to 20th	Night	45% 9% 13% 17% 12% 4	4980
	21st to end	Night	70% 11% 7% 6% 4 2	5361
April	1st to 10th	Night	67% 9% 9% 9% 5% 1	1835
	11th to 20th	Night	59% 15% 8% 12% 7%	1771
	21st to end	Night	60% 14% 9% 10% 6% 1	1569
May	1st to 10th	Night	63% 13% 6% 13% 5%	3157
	11th to 20th	Night	67% 11% 10% 11% 1	2888
	21st to end	Night	67% 14% 13% 5% 1	2800
June	1st to 10th	Night	63% 17% 10% 9% 1	1000
	11th to 20th	Night	64% 13% 10% 11% 2	1000
	21st to end	Night	71% 14% 10% 5%	1000
July	1st to 10th	Night	71% 11% 12% 7%	2255
	11th to 20th	Night	73% 8% 14% 5%	2250
	21st to end	Night	74% 7% 13% 6%	3256
August	1st to 10th	Night	71% 9% 6% 12% 1	2266
	11th to 20th	Night	74% 11% 10% 5%	2415
	21st to end	Night	83% 6% 6% 4 1	3223
September	1st to 10th	Night	78% 10% 6% 5% 1	1860
	11th to 20th	Night	75% 10% 7% 5% 3	2072
	21st to end	Night	73% 8% 8% 8% 4	2242
October	1st to 10th	Night	68% 9% 6% 13% 5%	3920
	11th to 20th	Night	66% 10% 8% 13% 4	4140
	21st to end	Night	74% 9% 5% 4 6% 1	4970
November	1st to 10th	Night	66% 12% 7% 10% 4 1	2921
	11th to 20th	Night	59% 11% 11% 11% 7% 1	3295
	21st to end	Night	60% 11% 10% 10% 6% 3	3360
December	1st to 10th	Night	57% 14% 12% 9% 7% 2	5540
	11th to 20th	Night	59% 9% 10% 14% 6% 2	6507
	21st to end	Night	50% 15% 10% 12% 8% 5%	8679

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% 13% 24% 23% 19% 6%	320
	11th to 20th	Day	14% 18% 25% 23% 15% 5%	320
	21st to end	Day	13% 23% 29% 19% 10% 5%	352
February	1st to 10th	Day	14% 15% 27% 22% 12% 10%	320
	11th to 20th	Day	18% 26% 28% 15% 10% 2	370
	21st to end	Day	21% 24% 26% 19% 7% 3	331
March	1st to 10th	Day	16% 22% 29% 21% 12%	469
	11th to 20th	Day	8% 18% 30% 30% 13% 2	600
	21st to end	Day	18% 19% 27% 22% 12% 2	660
April	1st to 10th	Day	16% 24% 26% 19% 12% 3	600
	11th to 20th	Day	19% 17% 28% 22% 12% 3	598
	21st to end	Day	15% 19% 29% 20% 13% 4	700
May	1st to 10th	Day	22% 16% 23% 23% 13% 3	703
	11th to 20th	Day	18% 17% 23% 22% 15% 4	725
	21st to end	Day	18% 13% 25% 23% 14% 6%	860
June	1st to 10th	Day	22% 11% 19% 21% 21% 7%	930
	11th to 20th	Day	21% 10% 18% 22% 22% 6%	930
	21st to end	Day	27% 10% 16% 23% 19% 5%	930
July	1st to 10th	Day	35% 12% 16% 18% 16% 3	916
	11th to 20th	Day	34% 9% 17% 22% 13% 4	912
	21st to end	Day	35% 9% 19% 19% 14% 5%	798
August	1st to 10th	Day	37% 10% 18% 19% 13% 2	726
	11th to 20th	Day	36% 11% 19% 23% 9% 2	709
	21st to end	Day	45% 11% 15% 16% 10% 3	682
September	1st to 10th	Day	40% 14% 20% 14% 10% 2	620
	11th to 20th	Day	42% 15% 17% 14% 9% 3	620
	21st to end	Day	38% 12% 22% 17% 7% 4	578
October	1st to 10th	Day	35% 20% 25% 13% 5% 3	420
	11th to 20th	Day	22% 19% 25% 17% 12% 5%	420
	21st to end	Day	25% 18% 25% 20% 9% 3	462
November	1st to 10th	Day	25% 20% 24% 19% 9% 4	397
	11th to 20th	Day	18% 22% 29% 16% 10% 6%	320
	21st to end	Day	18% 20% 26% 22% 13% 2	320
December	1st to 10th	Day	19% 18% 29% 18% 13% 2	320
	11th to 20th	Day	22% 18% 23% 24% 9% 3	320
	21st to end	Day	19% 21% 25% 19% 11% 5%	352
January	1st to 10th	Night	10% 15% 28% 28% 16% 3	1060
	11th to 20th	Night	13% 18% 28% 24% 14% 3	1021
	21st to end	Night	14% 19% 30% 24% 10% 2	1045
February	1st to 10th	Night	12% 18% 25% 26% 16% 3	948
	11th to 20th	Night	19% 23% 31% 17% 7% 2	708
	21st to end	Night	18% 23% 25% 20% 11% 2	578
March	1st to 10th	Night	15% 20% 28% 23% 12% 2	700
	11th to 20th	Night	8% 15% 28% 32% 15% 2	700
	21st to end	Night	11% 14% 29% 26% 16% 4	748
April	1st to 10th	Night	18% 18% 26% 22% 14% 3	600
	11th to 20th	Night	16% 15% 23% 28% 15% 3	593
	21st to end	Night	12% 14% 31% 24% 15% 4	569
May	1st to 10th	Night	18% 10% 26% 23% 21% 2	400
	11th to 20th	Night	18% 15% 23% 23% 16% 5%	394
	21st to end	Night	18% 8% 24% 24% 20% 5%	433
June	1st to 10th	Night	23% 11% 16% 23% 21% 6%	400
	11th to 20th	Night	24% 7% 19% 23% 23% 4	400
	21st to end	Night	26% 9% 14% 18% 28% 5%	400
July	1st to 10th	Night	32% 9% 15% 19% 20% 5%	390
	11th to 20th	Night	32% 7% 18% 21% 17% 5%	388
	21st to end	Night	40% 9% 13% 18% 16% 4	438
August	1st to 10th	Night	37% 8% 19% 19% 14% 3	398
	11th to 20th	Night	39% 11% 16% 18% 13% 3	476
	21st to end	Night	41% 11% 16% 16% 13% 3	660
September	1st to 10th	Night	38% 13% 20% 16% 11% 2	600
	11th to 20th	Night	40% 12% 15% 15% 14% 3	641
	21st to end	Night	38% 12% 21% 16% 11% 2	710
October	1st to 10th	Night	34% 20% 20% 16% 9% 1	710
	11th to 20th	Night	24% 18% 21% 21% 12% 3	722
	21st to end	Night	26% 18% 22% 21% 11% 2	1001
November	1st to 10th	Night	25% 19% 21% 19% 12% 4	930
	11th to 20th	Night	15% 22% 31% 19% 11% 2	950
	21st to end	Night	16% 16% 30% 24% 11% 3	950
December	1st to 10th	Night	17% 21% 28% 21% 11% 2	995
	11th to 20th	Night	19% 20% 29% 21% 8% 2	1060
	21st to end	Night	18% 19% 26% 22% 12% 3	1166

Page generated at 2026-02-12 21:42:16. Version ac07

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