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 2024-11-30.

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	15% 18% 6% 12% 7% 6% 6% 4 5% 5% 15%	106324
	Night	10% 17% 5% 12% 7% 7% 6% 5% 6% 6% 19%	101922
Transparency	Day	66% 11% 9% 9% 4 1	106037
	Night	61% 12% 9% 9% 6% 3	101859
Seeing	Day	25% 15% 22% 20% 13% 4	19074
	Night	21% 16% 24% 21% 13% 3	23127

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 12% 5% 12% 7% 8% 10% 7% 9% 8% 19%	1589
	11th to 20th	Day	2 10% 3 13% 6% 5% 8% 5% 7% 7% 34%	1802
	21st to end	Day	2 9% 3 7% 5% 4 5% 7% 7% 7% 43%	2431
February	1st to 10th	Day	5% 12% 5% 10% 7% 7% 6% 6% 8% 9% 25%	1330
	11th to 20th	Day	5% 20% 7% 12% 10% 7% 9% 6% 7% 6% 11%	1593
	21st to end	Day	6% 16% 7% 15% 8% 6% 6% 4 6% 7% 20%	1413
March	1st to 10th	Day	4 16% 6% 10% 7% 8% 10% 7% 7% 6% 19%	2997
	11th to 20th	Day	1 8% 4 12% 8% 8% 14% 10% 7% 8% 20%	3480
	21st to end	Day	5% 19% 8% 15% 8% 6% 7% 5% 5% 5% 16%	3892
April	1st to 10th	Day	6% 22% 7% 14% 8% 6% 6% 4 6% 5% 16%	2433
	11th to 20th	Day	6% 19% 6% 13% 9% 6% 6% 4 4 5% 20%	2448
	21st to end	Day	7% 18% 7% 13% 7% 6% 5% 5% 6% 6% 19%	2688
May	1st to 10th	Day	12% 28% 9% 16% 7% 5% 4 3 3 3 10%	4760
	11th to 20th	Day	8% 19% 8% 16% 8% 5% 6% 4 5% 5% 15%	4889
	21st to end	Day	10% 21% 6% 11% 7% 6% 6% 4 6% 7% 16%	5638
June	1st to 10th	Day	15% 18% 4 10% 7% 5% 5% 4 6% 6% 18%	3230
	11th to 20th	Day	20% 15% 4 10% 7% 6% 6% 4 5% 7% 16%	3230
	21st to end	Day	26% 16% 5% 10% 7% 5% 5% 4 6% 5% 12%	3230
July	1st to 10th	Day	29% 19% 5% 10% 6% 5% 5% 4 6% 5% 7%	5390
	11th to 20th	Day	28% 20% 5% 10% 6% 5% 4 3 4 4 9%	5390
	21st to end	Day	29% 17% 4 8% 4 4 4 3 4 4 18%	5231
August	1st to 10th	Day	23% 15% 5% 12% 7% 6% 5% 4 6% 7% 11%	4737
	11th to 20th	Day	23% 21% 6% 11% 6% 5% 5% 3 5% 5% 10%	4515
	21st to end	Day	31% 22% 5% 11% 5% 5% 5% 4 4 3 5%	4532
September	1st to 10th	Day	30% 21% 5% 11% 7% 5% 4 3 3 3 9%	2412
	11th to 20th	Day	25% 20% 6% 11% 6% 5% 4 4 4 3 11%	2209
	21st to end	Day	26% 22% 6% 9% 5% 5% 4 3 3 3 13%	2050
October	1st to 10th	Day	12% 24% 8% 13% 7% 6% 5% 4 4 4 14%	3160
	11th to 20th	Day	7% 15% 6% 16% 9% 7% 6% 4 5% 4 21%	2904
	21st to end	Day	9% 19% 10% 17% 9% 7% 5% 3 5% 4 10%	2743
November	1st to 10th	Day	11% 19% 7% 13% 9% 8% 6% 4 5% 5% 12%	1516
	11th to 20th	Day	7% 15% 5% 13% 9% 10% 8% 4 5% 6% 17%	1261
	21st to end	Day	6% 19% 5% 10% 7% 11% 9% 4 6% 8% 15%	1195
December	1st to 10th	Day	5% 14% 5% 12% 9% 8% 8% 6% 7% 5% 20%	1060
	11th to 20th	Day	6% 17% 6% 10% 7% 8% 8% 7% 8% 8% 16%	1197
	21st to end	Day	4 11% 5% 12% 8% 7% 8% 5% 8% 7% 25%	1749
January	1st to 10th	Night	2 10% 5% 14% 8% 9% 10% 8% 9% 8% 17%	5377
	11th to 20th	Night	2 12% 5% 12% 6% 6% 6% 5% 6% 6% 35%	5110
	21st to end	Night	3 9% 3 7% 6% 5% 6% 6% 8% 12% 34%	5236
February	1st to 10th	Night	4 13% 5% 11% 6% 6% 7% 7% 8% 9% 25%	2849
	11th to 20th	Night	5% 19% 7% 11% 8% 7% 8% 8% 7% 7% 13%	2473
	21st to end	Night	6% 16% 6% 12% 8% 6% 6% 5% 6% 7% 23%	2041
March	1st to 10th	Night	5% 14% 6% 15% 8% 8% 7% 7% 7% 5% 20%	3856
	11th to 20th	Night	2 9% 4 9% 6% 9% 15% 10% 7% 9% 22%	3478
	21st to end	Night	5% 16% 8% 17% 9% 6% 6% 4 5% 5% 19%	3739
April	1st to 10th	Night	6% 19% 6% 16% 9% 7% 6% 3 7% 6% 15%	1743
	11th to 20th	Night	5% 18% 5% 12% 9% 6% 5% 5% 5% 6% 24%	1682
	21st to end	Night	7% 15% 5% 11% 7% 7% 5% 6% 7% 6% 24%	1491
May	1st to 10th	Night	7% 25% 7% 12% 7% 7% 6% 4 5% 5% 15%	2210
	11th to 20th	Night	8% 26% 8% 13% 6% 4 6% 4 4 5% 17%	2015
	21st to end	Night	12% 20% 6% 10% 6% 7% 7% 4 5% 7% 16%	1950
June	1st to 10th	Night	16% 14% 4 10% 9% 6% 5% 4 7% 8% 18%	950
	11th to 20th	Night	20% 13% 4 9% 8% 5% 5% 5% 5% 6% 19%	950
	21st to end	Night	19% 15% 3 10% 8% 7% 5% 4 6% 6% 15%	950
July	1st to 10th	Night	27% 16% 5% 10% 7% 6% 5% 4 5% 5% 10%	1590
	11th to 20th	Night	27% 21% 7% 12% 5% 4 3 3 3 3 11%	1590
	21st to end	Night	27% 18% 5% 7% 4 4 3 3 3 4 22%	2331
August	1st to 10th	Night	21% 16% 5% 13% 7% 5% 5% 4 4 4 15%	2203
	11th to 20th	Night	24% 27% 4 8% 5% 6% 4 4 4 5% 9%	2319
	21st to end	Night	27% 27% 4 10% 6% 6% 4 3 4 3 7%	3124
September	1st to 10th	Night	26% 22% 6% 11% 7% 4 3 4 3 4 11%	1767
	11th to 20th	Night	25% 19% 5% 9% 7% 5% 4 4 6% 5% 11%	1968
	21st to end	Night	26% 21% 4 9% 5% 5% 4 3 3 4 15%	2130
October	1st to 10th	Night	10% 22% 7% 12% 7% 6% 5% 4 5% 6% 15%	3800
	11th to 20th	Night	8% 17% 7% 18% 7% 6% 5% 3 4 6% 20%	4012
	21st to end	Night	13% 24% 6% 15% 8% 5% 4 3 4 4 13%	4816
November	1st to 10th	Night	12% 19% 5% 12% 9% 7% 5% 4 6% 6% 16%	2778
	11th to 20th	Night	5% 16% 6% 12% 10% 8% 9% 7% 6% 5% 17%	3138
	21st to end	Night	7% 15% 6% 12% 9% 8% 8% 5% 6% 7% 17%	3200
December	1st to 10th	Night	5% 15% 5% 13% 8% 8% 8% 6% 7% 6% 18%	3116
	11th to 20th	Night	4 18% 7% 12% 9% 8% 8% 6% 6% 7% 15%	4011
	21st to end	Night	3 11% 5% 13% 7% 7% 9% 5% 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	53% 15% 13% 10% 7% 2	1589
	11th to 20th	Day	44% 12% 9% 9% 20% 6%	1818
	21st to end	Day	36% 10% 7% 12% 15% 21%	2431
February	1st to 10th	Day	52% 15% 8% 10% 9% 6%	1330
	11th to 20th	Day	70% 12% 7% 7% 3 2	1599
	21st to end	Day	61% 15% 8% 8% 7% 1	1413
March	1st to 10th	Day	45% 9% 11% 20% 7% 7%	2997
	11th to 20th	Day	38% 13% 17% 20% 9% 2	3480
	21st to end	Day	62% 13% 7% 8% 8% 2	3892
April	1st to 10th	Day	67% 10% 7% 9% 5% 1	2433
	11th to 20th	Day	63% 11% 9% 11% 6% 1	2448
	21st to end	Day	63% 14% 9% 10% 4 1	2688
May	1st to 10th	Day	78% 9% 6% 6% 1	4760
	11th to 20th	Day	63% 13% 9% 12% 2	4892
	21st to end	Day	64% 16% 11% 7% 1	5639
June	1st to 10th	Day	64% 14% 11% 10% 1	3230
	11th to 20th	Day	65% 13% 12% 9% 2	3230
	21st to end	Day	72% 12% 13% 3	3230
July	1st to 10th	Day	75% 10% 9% 6%	5306
	11th to 20th	Day	75% 8% 10% 7%	5297
	21st to end	Day	69% 7% 17% 7%	5065
August	1st to 10th	Day	69% 11% 11% 8% 1	4719
	11th to 20th	Day	73% 10% 9% 7%	4560
	21st to end	Day	81% 6% 8% 4	4531
September	1st to 10th	Day	81% 8% 4 6% 1	2412
	11th to 20th	Day	77% 7% 5% 7% 3	2209
	21st to end	Day	74% 7% 7% 9% 3	2050
October	1st to 10th	Day	70% 9% 5% 11% 4	3160
	11th to 20th	Day	64% 10% 9% 13% 4	2904
	21st to end	Day	74% 8% 4 8% 6% 1	2743
November	1st to 10th	Day	70% 12% 8% 7% 3 1	1516
	11th to 20th	Day	61% 11% 11% 12% 5% 1	1261
	21st to end	Day	64% 10% 10% 8% 6% 2	1195
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	54% 19% 11% 7% 6% 2	5377
	11th to 20th	Night	46% 13% 6% 9% 20% 5%	5157
	21st to end	Night	38% 12% 7% 10% 12% 21%	5236
February	1st to 10th	Night	54% 13% 9% 10% 10% 4	2849
	11th to 20th	Night	66% 13% 7% 8% 4 2	2478
	21st to end	Night	57% 16% 10% 8% 9%	2041
March	1st to 10th	Night	49% 12% 7% 19% 6% 7%	3856
	11th to 20th	Night	37% 11% 18% 23% 9% 3	3480
	21st to end	Night	63% 14% 8% 7% 6% 3	3739
April	1st to 10th	Night	67% 9% 9% 8% 5% 1	1743
	11th to 20th	Night	58% 15% 8% 12% 7%	1682
	21st to end	Night	59% 15% 9% 10% 6% 1	1491
May	1st to 10th	Night	71% 14% 7% 6% 2	2210
	11th to 20th	Night	69% 12% 7% 11% 1	2016
	21st to end	Night	66% 17% 10% 5% 1	1950
June	1st to 10th	Night	62% 17% 10% 10% 1	950
	11th to 20th	Night	65% 14% 10% 10% 2	950
	21st to end	Night	71% 14% 10% 5%	950
July	1st to 10th	Night	73% 12% 11% 5%	1565
	11th to 20th	Night	79% 8% 8% 5%	1560
	21st to end	Night	67% 8% 17% 8%	2258
August	1st to 10th	Night	71% 9% 6% 13% 1	2196
	11th to 20th	Night	74% 11% 9% 5%	2340
	21st to end	Night	83% 6% 6% 4 1	3124
September	1st to 10th	Night	78% 10% 6% 5% 1	1767
	11th to 20th	Night	75% 10% 8% 5% 3	1968
	21st to end	Night	72% 9% 8% 7% 4	2130
October	1st to 10th	Night	68% 9% 5% 13% 5%	3800
	11th to 20th	Night	65% 10% 8% 13% 4	4012
	21st to end	Night	75% 10% 5% 4 6% 1	4816
November	1st to 10th	Night	65% 12% 8% 10% 4 1	2778
	11th to 20th	Night	59% 12% 11% 11% 6% 1	3138
	21st to end	Night	60% 11% 10% 9% 6% 3	3200
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	12% 20% 25% 23% 15% 6%	280
	21st to end	Day	11% 25% 31% 20% 9% 4	308
February	1st to 10th	Day	15% 16% 29% 23% 10% 8%	280
	11th to 20th	Day	17% 29% 29% 14% 10% 2	331
	21st to end	Day	23% 25% 26% 18% 5% 3	307
March	1st to 10th	Day	17% 22% 28% 21% 12%	435
	11th to 20th	Day	8% 19% 31% 29% 12% 2	560
	21st to end	Day	19% 18% 28% 22% 12% 2	616
April	1st to 10th	Day	17% 24% 25% 19% 12% 4	560
	11th to 20th	Day	18% 16% 27% 23% 13% 3	557
	21st to end	Day	15% 19% 28% 21% 13% 4	650
May	1st to 10th	Day	22% 17% 24% 22% 13% 3	653
	11th to 20th	Day	18% 17% 23% 23% 15% 4	675
	21st to end	Day	18% 14% 25% 22% 15% 6%	801
June	1st to 10th	Day	20% 10% 19% 22% 22% 7%	870
	11th to 20th	Day	22% 10% 17% 22% 22% 6%	870
	21st to end	Day	27% 9% 16% 23% 20% 5%	870
July	1st to 10th	Day	35% 12% 15% 19% 16% 4	856
	11th to 20th	Day	35% 9% 16% 22% 13% 4	852
	21st to end	Day	35% 9% 18% 19% 14% 5%	743
August	1st to 10th	Day	38% 10% 18% 19% 13% 2	676
	11th to 20th	Day	35% 12% 19% 23% 9% 2	660
	21st to end	Day	45% 11% 14% 17% 9% 3	638
September	1st to 10th	Day	39% 13% 20% 15% 11% 2	580
	11th to 20th	Day	42% 15% 17% 14% 9% 3	580
	21st to end	Day	39% 11% 21% 17% 7% 5%	540
October	1st to 10th	Day	35% 20% 24% 14% 5% 3	390
	11th to 20th	Day	22% 19% 24% 17% 12% 5%	390
	21st to end	Day	25% 18% 25% 20% 9% 3	429
November	1st to 10th	Day	26% 20% 24% 19% 8% 4	370
	11th to 20th	Day	18% 22% 30% 15% 9% 6%	300
	21st to end	Day	18% 21% 25% 22% 12% 2	299
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	11% 19% 28% 24% 14% 4	909
	21st to end	Night	13% 19% 30% 24% 10% 3	935
February	1st to 10th	Night	12% 19% 26% 24% 15% 3	849
	11th to 20th	Night	20% 23% 31% 16% 7% 2	648
	21st to end	Night	19% 24% 26% 20% 9% 2	546
March	1st to 10th	Night	14% 20% 28% 23% 13% 2	660
	11th to 20th	Night	8% 15% 28% 32% 14% 2	660
	21st to end	Night	11% 15% 28% 26% 15% 4	706
April	1st to 10th	Night	17% 18% 26% 22% 14% 3	570
	11th to 20th	Night	15% 15% 23% 28% 15% 3	563
	21st to end	Night	11% 14% 31% 24% 15% 5%	541
May	1st to 10th	Night	18% 11% 26% 22% 21% 2	380
	11th to 20th	Night	19% 15% 23% 23% 16% 4	374
	21st to end	Night	18% 9% 24% 23% 20% 5%	411
June	1st to 10th	Night	22% 11% 16% 23% 22% 7%	380
	11th to 20th	Night	25% 7% 19% 22% 23% 4	380
	21st to end	Night	26% 9% 14% 17% 28% 5%	380
July	1st to 10th	Night	32% 10% 14% 18% 21% 5%	370
	11th to 20th	Night	33% 8% 17% 20% 17% 5%	368
	21st to end	Night	39% 9% 14% 18% 16% 5%	416
August	1st to 10th	Night	37% 8% 18% 19% 15% 3	378
	11th to 20th	Night	39% 11% 17% 17% 14% 3	452
	21st to end	Night	42% 11% 16% 16% 12% 3	627
September	1st to 10th	Night	38% 12% 20% 17% 11% 2	570
	11th to 20th	Night	40% 12% 16% 14% 14% 3	607
	21st to end	Night	38% 12% 21% 17% 11% 2	670
October	1st to 10th	Night	33% 19% 20% 17% 10% 1	670
	11th to 20th	Night	23% 19% 21% 22% 12% 4	681
	21st to end	Night	26% 18% 23% 20% 10% 2	946
November	1st to 10th	Night	26% 19% 20% 19% 12% 4	880
	11th to 20th	Night	15% 23% 31% 19% 11% 2	900
	21st to end	Night	16% 17% 31% 23% 10% 3	900
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

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