Williston
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Williston clear sky chart from 2005-11-06 to 2025-12-03.

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	9% 7% 3 7% 5% 4 5% 4 6% 8% 43%	135795
	Night	7% 8% 3 8% 5% 5% 5% 4 6% 8% 42%	132299
Transparency	Day	41% 3 15% 28% 13% 1	130421
	Night	41% 3 16% 26% 12% 2	128705
Seeing	Day	12% 9% 31% 30% 15% 3	17623
	Night	11% 7% 32% 37% 12% 1	27615

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	3 11% 5% 12% 8% 9% 7% 5% 6% 8% 25%	3638
	11th to 20th	Day	3 8% 4 14% 8% 7% 11% 7% 9% 8% 22%	3640
	21st to end	Day	4 9% 5% 10% 7% 5% 4 4 5% 7% 41%	4004
February	1st to 10th	Day	6% 13% 5% 13% 8% 6% 5% 4 6% 6% 28%	1669
	11th to 20th	Day	7% 10% 6% 11% 7% 6% 5% 5% 7% 7% 30%	1809
	21st to end	Day	5% 11% 4 11% 7% 4 6% 4 6% 8% 32%	1650
March	1st to 10th	Day	6% 10% 5% 13% 6% 4 4 3 4 6% 39%	4500
	11th to 20th	Day	9% 9% 3 8% 6% 5% 5% 4 7% 10% 35%	4665
	21st to end	Day	11% 16% 5% 11% 6% 5% 4 4 6% 6% 26%	5969
April	1st to 10th	Day	7% 9% 3 7% 6% 5% 5% 4 6% 8% 39%	2400
	11th to 20th	Day	14% 12% 4 9% 5% 5% 4 3 5% 7% 32%	2394
	21st to end	Day	13% 9% 2 7% 5% 5% 5% 4 6% 7% 36%	2630
May	1st to 10th	Day	13% 7% 2 5% 4 4 4 3 5% 6% 47%	6340
	11th to 20th	Day	20% 6% 2 6% 4 4 5% 4 6% 8% 35%	6298
	21st to end	Day	13% 5% 2 5% 4 4 4 4 5% 8% 48%	6925
June	1st to 10th	Day	13% 5% 2 4 4 4 5% 4 6% 8% 44%	2880
	11th to 20th	Day	11% 5% 2 7% 5% 5% 5% 4 7% 8% 42%	3000
	21st to end	Day	7% 4 2 6% 4 4 5% 5% 6% 8% 49%	2892
July	1st to 10th	Day	5% 4 2 6% 5% 5% 5% 4 6% 8% 52%	6339
	11th to 20th	Day	6% 4 2 5% 3 3 4 4 8% 9% 52%	6340
	21st to end	Day	5% 4 2 6% 4 5% 5% 4 7% 10% 49%	6903
August	1st to 10th	Day	11% 6% 3 7% 4 3 4 3 5% 6% 48%	6232
	11th to 20th	Day	5% 4 2 5% 4 4 4 4 6% 9% 52%	6102
	21st to end	Day	6% 3 1 4 3 4 4 4 5% 7% 59%	5984
September	1st to 10th	Day	9% 5% 2 5% 4 3 4 3 5% 7% 54%	2400
	11th to 20th	Day	12% 5% 2 5% 4 4 4 4 5% 7% 49%	2340
	21st to end	Day	13% 6% 2 4 3 2 3 3 4 6% 55%	2200
October	1st to 10th	Day	9% 4 2 4 3 4 3 2 4 6% 58%	3339
	11th to 20th	Day	7% 7% 3 5% 3 3 3 3 5% 8% 52%	3097
	21st to end	Day	7% 8% 3 6% 4 3 3 4 6% 7% 48%	3190
November	1st to 10th	Day	11% 12% 4 6% 4 4 6% 4 6% 6% 38%	1853
	11th to 20th	Day	6% 10% 4 9% 5% 4 6% 5% 7% 7% 37%	1697
	21st to end	Day	8% 12% 4 9% 7% 5% 5% 4 6% 7% 34%	1641
December	1st to 10th	Day	5% 12% 6% 13% 5% 5% 4 3 5% 6% 35%	2304
	11th to 20th	Day	8% 13% 5% 11% 6% 8% 7% 4 6% 6% 26%	2643
	21st to end	Day	5% 9% 4 11% 6% 6% 4 3 6% 7% 39%	3888
January	1st to 10th	Night	3 11% 5% 12% 9% 7% 8% 5% 6% 6% 29%	6327
	11th to 20th	Night	4 8% 4 11% 9% 10% 8% 6% 8% 5% 27%	6340
	21st to end	Night	3 9% 4 11% 7% 5% 5% 4 6% 7% 37%	6974
February	1st to 10th	Night	7% 13% 5% 11% 6% 6% 4 5% 7% 7% 29%	2750
	11th to 20th	Night	6% 11% 5% 10% 5% 6% 5% 4 6% 6% 35%	2477
	21st to end	Night	5% 9% 4 8% 6% 5% 6% 4 6% 8% 39%	1980
March	1st to 10th	Night	7% 8% 3 10% 5% 3 4 4 6% 9% 41%	5374
	11th to 20th	Night	5% 7% 3 9% 6% 5% 6% 5% 7% 9% 38%	5317
	21st to end	Night	9% 15% 5% 11% 6% 5% 4 4 6% 7% 26%	4984
April	1st to 10th	Night	7% 8% 3 7% 6% 5% 4 5% 5% 8% 43%	2000
	11th to 20th	Night	12% 11% 4 8% 5% 4 5% 5% 6% 7% 34%	1953
	21st to end	Night	12% 7% 2 6% 4 5% 5% 4 6% 6% 43%	1766
May	1st to 10th	Night	9% 6% 2 7% 4 4 4 3 5% 7% 48%	3640
	11th to 20th	Night	19% 6% 2 5% 4 5% 4 4 6% 8% 35%	3617
	21st to end	Night	15% 7% 2 5% 3 3 4 3 4 7% 47%	3972
June	1st to 10th	Night	10% 4 2 6% 3 4 4 4 6% 8% 49%	1515
	11th to 20th	Night	10% 6% 1 5% 4 4 4 5% 6% 8% 46%	1400
	21st to end	Night	6% 3 2 4 4 3 4 5% 8% 10% 52%	1499
July	1st to 10th	Night	6% 4 1 5% 5% 5% 5% 4 7% 9% 50%	3640
	11th to 20th	Night	7% 4 1 5% 4 3 4 4 5% 8% 54%	3640
	21st to end	Night	5% 3 1 4 4 5% 4 4 7% 9% 54%	3970
August	1st to 10th	Night	7% 5% 2 5% 4 4 4 3 6% 8% 51%	3580
	11th to 20th	Night	4 3 1 5% 3 3 4 3 6% 11% 58%	3719
	21st to end	Night	5% 3 1 4 3 3 4 3 4 7% 62%	4993
September	1st to 10th	Night	7% 4 2 4 3 2 3 3 4 6% 62%	2000
	11th to 20th	Night	9% 4 1 5% 3 3 3 3 5% 7% 55%	2060
	21st to end	Night	11% 6% 2 4 3 4 3 3 4 7% 53%	2200
October	1st to 10th	Night	8% 4 2 4 3 3 3 3 4 7% 59%	3845
	11th to 20th	Night	7% 5% 2 6% 4 4 4 4 6% 8% 51%	4019
	21st to end	Night	8% 6% 2 6% 4 4 4 4 5% 8% 48%	4610
November	1st to 10th	Night	10% 10% 4 7% 5% 5% 4 4 6% 7% 40%	2665
	11th to 20th	Night	7% 9% 4 9% 6% 5% 4 4 5% 8% 38%	2919
	21st to end	Night	6% 8% 3 9% 7% 5% 5% 4 6% 8% 38%	2975
December	1st to 10th	Night	5% 12% 5% 12% 6% 6% 5% 4 5% 7% 32%	4931
	11th to 20th	Night	6% 11% 4 11% 7% 6% 7% 5% 7% 7% 29%	5564
	21st to end	Night	5% 9% 4 10% 6% 6% 5% 4 6% 6% 38%	7084

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	56% 3 16% 20% 5%	3639
	11th to 20th	Day	58% 3 14% 13% 12%	3640
	21st to end	Day	44% 3 26% 24% 3	4004
February	1st to 10th	Day	56% 4 11% 19% 10%	1669
	11th to 20th	Day	52% 5% 14% 18% 10% 1	1818
	21st to end	Day	49% 4 16% 20% 10%	1650
March	1st to 10th	Day	47% 3 14% 27% 10%	4500
	11th to 20th	Day	46% 3 19% 27% 6%	4665
	21st to end	Day	60% 3 16% 15% 6%	5969
April	1st to 10th	Day	42% 5% 15% 24% 12% 1	2400
	11th to 20th	Day	53% 3 12% 22% 9%	2397
	21st to end	Day	46% 3 14% 25% 11%	2630
May	1st to 10th	Day	39% 2 12% 28% 17% 1	6340
	11th to 20th	Day	47% 3 10% 27% 13%	6302
	21st to end	Day	36% 2 11% 31% 19%	6926
June	1st to 10th	Day	37% 4 18% 28% 12% 2	2880
	11th to 20th	Day	39% 5% 16% 28% 11% 1	3000
	21st to end	Day	32% 5% 18% 31% 12% 1	2892
July	1st to 10th	Day	30% 2 18% 32% 14% 3	6271
	11th to 20th	Day	28% 3 16% 32% 18% 2	6265
	21st to end	Day	30% 4 14% 34% 16% 3	6705
August	1st to 10th	Day	31% 3 11% 32% 18% 5%	4536
	11th to 20th	Day	31% 3 18% 34% 13% 2	4423
	21st to end	Day	26% 3 12% 39% 18% 2	4311
September	1st to 10th	Day	30% 3 12% 32% 19% 3	2400
	11th to 20th	Day	36% 4 14% 29% 15% 2	2340
	21st to end	Day	32% 3 11% 33% 19% 2	2200
October	1st to 10th	Day	29% 2 10% 29% 26% 4	3339
	11th to 20th	Day	33% 2 13% 38% 13%	3097
	21st to end	Day	36% 3 13% 36% 12% 1	3190
November	1st to 10th	Day	47% 4 11% 26% 11% 2	1853
	11th to 20th	Day	44% 4 20% 23% 9% 1	1697
	21st to end	Day	50% 4 22% 18% 7%	1641
December	1st to 10th	Day	51% 3 21% 19% 7%	2304
	11th to 20th	Day	59% 3 22% 13% 2	2640
	21st to end	Day	45% 2 11% 32% 10% 1	3888
January	1st to 10th	Night	55% 4 16% 19% 6%	6327
	11th to 20th	Night	55% 3 15% 15% 11%	6340
	21st to end	Night	45% 4 24% 23% 4	6974
February	1st to 10th	Night	53% 5% 13% 20% 9%	2750
	11th to 20th	Night	48% 5% 14% 22% 11% 1	2478
	21st to end	Night	43% 3 15% 26% 12% 1	1980
March	1st to 10th	Night	40% 2 14% 32% 11%	5374
	11th to 20th	Night	42% 2 19% 29% 8%	5317
	21st to end	Night	57% 3 15% 19% 5%	4984
April	1st to 10th	Night	41% 4 13% 25% 15% 2	2000
	11th to 20th	Night	49% 3 10% 25% 12% 2	1953
	21st to end	Night	41% 4 12% 28% 14% 1	1766
May	1st to 10th	Night	37% 3 9% 29% 21% 2	3640
	11th to 20th	Night	48% 2 16% 22% 12%	3618
	21st to end	Night	39% 2 10% 28% 19% 1	3972
June	1st to 10th	Night	33% 4 15% 29% 15% 3	1515
	11th to 20th	Night	35% 6% 17% 27% 13% 2	1400
	21st to end	Night	27% 5% 20% 32% 14% 2	1499
July	1st to 10th	Night	31% 4 20% 31% 11% 2	3600
	11th to 20th	Night	29% 3 15% 33% 17% 3	3592
	21st to end	Night	27% 3 19% 30% 16% 5%	3856
August	1st to 10th	Night	29% 2 14% 30% 20% 5%	2600
	11th to 20th	Night	26% 2 21% 36% 13% 2	2699
	21st to end	Night	24% 2 13% 35% 21% 4	3597
September	1st to 10th	Night	25% 3 10% 34% 24% 5%	2000
	11th to 20th	Night	29% 4 15% 28% 20% 4	2060
	21st to end	Night	32% 3 10% 31% 21% 2	2200
October	1st to 10th	Night	27% 3 9% 31% 24% 5%	3845
	11th to 20th	Night	32% 2 10% 39% 17% 1	4019
	21st to end	Night	35% 3 11% 36% 13% 2	4610
November	1st to 10th	Night	44% 4 12% 25% 13% 2	2665
	11th to 20th	Night	45% 3 18% 25% 7% 1	2919
	21st to end	Night	46% 4 20% 23% 8%	2975
December	1st to 10th	Night	53% 4 24% 15% 4	4931
	11th to 20th	Night	53% 3 23% 17% 4	5566
	21st to end	Night	46% 3 11% 26% 12% 2	7084

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	8% 6% 39% 30% 12% 5%	300
	11th to 20th	Day	8% 11% 41% 27% 13% 1	300
	21st to end	Day	12% 13% 33% 31% 12%	330
February	1st to 10th	Day	10% 10% 41% 34% 5%	300
	11th to 20th	Day	11% 6% 42% 29% 10% 2	300
	21st to end	Day	8% 12% 37% 30% 12% 1	249
March	1st to 10th	Day	17% 9% 34% 25% 13% 2	300
	11th to 20th	Day	13% 13% 34% 26% 12% 2	343
	21st to end	Day	13% 11% 36% 26% 13% 1	550
April	1st to 10th	Day	11% 16% 39% 27% 6%	500
	11th to 20th	Day	22% 15% 35% 20% 7% 1	515
	21st to end	Day	17% 12% 28% 28% 13% 2	700
May	1st to 10th	Day	14% 8% 29% 29% 16% 3	703
	11th to 20th	Day	20% 8% 25% 25% 18% 5%	725
	21st to end	Day	17% 7% 27% 24% 19% 6%	789
June	1st to 10th	Day	14% 10% 27% 28% 18% 4	730
	11th to 20th	Day	12% 12% 36% 27% 12% 2	730
	21st to end	Day	10% 7% 26% 35% 18% 4	730
July	1st to 10th	Day	6% 5% 26% 37% 23% 3	721
	11th to 20th	Day	7% 6% 26% 40% 18% 4	718
	21st to end	Day	4 5% 28% 36% 22% 6%	794
August	1st to 10th	Day	7% 6% 24% 34% 24% 6%	726
	11th to 20th	Day	7% 5% 25% 36% 21% 5%	708
	21st to end	Day	8% 7% 26% 31% 21% 7%	583
September	1st to 10th	Day	10% 8% 29% 34% 15% 3	530
	11th to 20th	Day	16% 9% 25% 27% 21% 1	470
	21st to end	Day	18% 8% 28% 28% 15% 4	330
October	1st to 10th	Day	21% 6% 28% 26% 15% 3	330
	11th to 20th	Day	15% 12% 35% 23% 12% 3	330
	21st to end	Day	16% 10% 40% 25% 8% 1	363
November	1st to 10th	Day	21% 9% 32% 24% 13% 1	330
	11th to 20th	Day	12% 11% 32% 34% 11% 1	330
	21st to end	Day	13% 8% 39% 30% 10%	330
December	1st to 10th	Day	11% 10% 41% 25% 10% 2	306
	11th to 20th	Day	14% 12% 42% 26% 5% 2	300
	21st to end	Day	9% 5% 38% 32% 14% 3	330
January	1st to 10th	Night	8% 8% 38% 35% 11% 1	1000
	11th to 20th	Night	10% 10% 36% 35% 9% 1	1000
	21st to end	Night	9% 11% 38% 31% 10% 1	1100
February	1st to 10th	Night	12% 9% 35% 33% 9% 1	950
	11th to 20th	Night	11% 5% 39% 31% 12% 1	784
	21st to end	Night	9% 6% 27% 41% 14% 3	660
March	1st to 10th	Night	9% 10% 35% 34% 10% 1	800
	11th to 20th	Night	10% 7% 39% 33% 10% 1	795
	21st to end	Night	13% 9% 30% 36% 12%	660
April	1st to 10th	Night	13% 10% 36% 35% 7%	600
	11th to 20th	Night	18% 10% 35% 31% 6%	593
	21st to end	Night	15% 7% 32% 34% 13%	600
May	1st to 10th	Night	13% 4 29% 38% 15% 1	600
	11th to 20th	Night	16% 5% 25% 36% 16% 1	591
	21st to end	Night	18% 6% 24% 37% 15% 1	650
June	1st to 10th	Night	11% 8% 31% 35% 14% 1	600
	11th to 20th	Night	12% 5% 37% 36% 10%	600
	21st to end	Night	7% 5% 26% 46% 15% 1	600
July	1st to 10th	Night	7% 3 25% 46% 18% 1	585
	11th to 20th	Night	5% 5% 30% 47% 13%	582
	21st to end	Night	5% 3 27% 49% 16%	657
August	1st to 10th	Night	6% 4 26% 49% 14% 1	597
	11th to 20th	Night	8% 3 24% 45% 19% 1	594
	21st to end	Night	6% 4 26% 44% 20% 1	660
September	1st to 10th	Night	9% 4 27% 43% 16% 2	600
	11th to 20th	Night	10% 5% 28% 43% 14% 1	600
	21st to end	Night	13% 6% 29% 40% 12% 1	600
October	1st to 10th	Night	15% 7% 27% 36% 14%	751
	11th to 20th	Night	14% 9% 28% 36% 13%	870
	21st to end	Night	14% 8% 35% 33% 10% 1	1100
November	1st to 10th	Night	14% 8% 33% 36% 7% 1	1025
	11th to 20th	Night	13% 8% 39% 30% 9% 1	1050
	21st to end	Night	10% 9% 36% 37% 7% 1	1050
December	1st to 10th	Night	9% 7% 36% 36% 9% 2	1011
	11th to 20th	Night	11% 11% 33% 35% 9% 1	1000
	21st to end	Night	14% 7% 33% 33% 12%	1100

Page generated at 2025-11-30 00:00:57. Version ac07

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