Bismarck
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bismarck clear sky chart from 2005-11-06 to 2025-09-04.

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	10% 7% 3 8% 5% 4 5% 4 6% 7% 42%	134798
	Night	8% 8% 3 8% 5% 5% 5% 4 6% 7% 42%	131308
Transparency	Day	40% 7% 14% 26% 12% 1	129429
	Night	40% 6% 15% 26% 13% 1	127687
Seeing	Day	14% 10% 30% 30% 14% 3	17294
	Night	12% 8% 31% 37% 12% 1	27151

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 10% 6% 17% 11% 8% 7% 5% 7% 6% 20%	3638
	11th to 20th	Day	3 7% 4 12% 7% 8% 11% 9% 12% 8% 20%	3640
	21st to end	Day	4 9% 6% 14% 9% 6% 5% 3 5% 5% 35%	4004
February	1st to 10th	Day	8% 15% 5% 10% 8% 7% 5% 3 5% 7% 28%	1625
	11th to 20th	Day	5% 13% 4 11% 7% 7% 5% 5% 7% 6% 30%	1914
	21st to end	Day	6% 12% 4 12% 7% 6% 5% 4 5% 7% 31%	1650
March	1st to 10th	Day	7% 9% 4 9% 5% 4 4 3 5% 8% 41%	4500
	11th to 20th	Day	5% 8% 3 9% 6% 5% 6% 6% 7% 9% 35%	4802
	21st to end	Day	15% 16% 5% 11% 5% 4 4 3 4 6% 26%	5708
April	1st to 10th	Day	10% 9% 3 6% 6% 5% 5% 4 5% 8% 38%	2400
	11th to 20th	Day	15% 10% 3 8% 4 5% 5% 3 5% 7% 34%	2489
	21st to end	Day	18% 10% 3 6% 4 4 4 4 5% 7% 36%	2600
May	1st to 10th	Day	12% 5% 2 5% 3 4 4 3 5% 7% 50%	5934
	11th to 20th	Day	21% 6% 2 6% 4 3 4 3 5% 6% 40%	6298
	21st to end	Day	13% 5% 2 6% 4 4 4 3 5% 7% 46%	7106
June	1st to 10th	Day	13% 5% 2 5% 4 4 4 4 5% 9% 45%	3000
	11th to 20th	Day	13% 6% 2 5% 4 4 5% 3 8% 8% 42%	3000
	21st to end	Day	9% 4 2 5% 3 4 4 4 6% 9% 50%	3000
July	1st to 10th	Day	8% 4 2 6% 4 5% 6% 4 7% 9% 46%	6531
	11th to 20th	Day	10% 4 2 4 3 3 4 3 5% 7% 55%	6340
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 7% 8% 47%	6903
August	1st to 10th	Day	12% 6% 2 6% 3 4 4 4 5% 8% 46%	6190
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 6% 8% 42%	5821
	21st to end	Day	7% 5% 2 5% 3 3 3 3 4 6% 60%	6263
September	1st to 10th	Day	10% 4 1 4 3 3 3 3 4 6% 58%	2317
	11th to 20th	Day	10% 4 1 5% 3 4 4 4 6% 7% 53%	2110
	21st to end	Day	12% 6% 2 4 3 3 4 3 4 6% 54%	2090
October	1st to 10th	Day	10% 4 1 4 2 3 4 3 4 6% 58%	3480
	11th to 20th	Day	10% 5% 2 5% 3 2 3 2 4 8% 54%	2875
	21st to end	Day	10% 8% 3 8% 4 4 5% 4 6% 7% 42%	3091
November	1st to 10th	Day	11% 10% 4 8% 6% 4 4 4 5% 8% 37%	1763
	11th to 20th	Day	8% 10% 4 10% 6% 5% 5% 4 6% 6% 37%	1795
	21st to end	Day	5% 12% 4 10% 7% 5% 6% 4 7% 7% 36%	1430
December	1st to 10th	Day	6% 12% 6% 13% 7% 6% 4 3 4 7% 32%	2290
	11th to 20th	Day	6% 12% 7% 15% 7% 6% 5% 4 6% 7% 24%	2643
	21st to end	Day	8% 12% 4 13% 6% 5% 5% 3 5% 6% 34%	3558
January	1st to 10th	Night	4 12% 5% 17% 9% 6% 6% 5% 6% 6% 24%	6327
	11th to 20th	Night	4 7% 3 10% 8% 8% 10% 6% 10% 8% 26%	6340
	21st to end	Night	3 9% 5% 12% 6% 6% 5% 4 5% 6% 38%	6974
February	1st to 10th	Night	8% 13% 4 10% 6% 6% 5% 5% 6% 7% 29%	2774
	11th to 20th	Night	5% 12% 3 8% 6% 6% 5% 4 6% 6% 39%	2389
	21st to end	Night	7% 10% 4 8% 6% 5% 4 4 6% 9% 39%	1980
March	1st to 10th	Night	7% 8% 3 10% 4 4 4 4 7% 8% 41%	5374
	11th to 20th	Night	7% 8% 2 9% 5% 5% 5% 4 6% 8% 40%	5165
	21st to end	Night	13% 13% 4 10% 6% 4 4 4 5% 7% 29%	5231
April	1st to 10th	Night	8% 9% 3 7% 5% 4 4 5% 6% 8% 41%	2000
	11th to 20th	Night	12% 8% 4 8% 5% 5% 4 5% 7% 6% 37%	1859
	21st to end	Night	15% 8% 2 5% 5% 3 4 4 5% 7% 41%	1800
May	1st to 10th	Night	9% 6% 2 5% 4 4 4 4 6% 7% 48%	4043
	11th to 20th	Night	18% 7% 3 5% 4 4 4 4 5% 7% 39%	3617
	21st to end	Night	13% 6% 2 6% 3 4 4 3 6% 7% 45%	3788
June	1st to 10th	Night	9% 4 1 4 3 4 5% 3 6% 9% 52%	1400
	11th to 20th	Night	10% 5% 2 5% 5% 4 4 4 7% 8% 47%	1400
	21st to end	Night	5% 3 1 5% 3 3 4 4 6% 9% 57%	1400
July	1st to 10th	Night	5% 4 2 5% 4 4 5% 4 6% 9% 54%	3444
	11th to 20th	Night	6% 3 1 4 3 3 4 3 7% 9% 58%	3640
	21st to end	Night	8% 4 2 6% 4 4 4 4 6% 8% 48%	3970
August	1st to 10th	Night	7% 4 2 5% 3 4 3 4 6% 8% 55%	3620
	11th to 20th	Night	8% 5% 2 5% 3 4 3 3 4 7% 56%	4028
	21st to end	Night	5% 4 1 5% 3 4 4 3 5% 7% 60%	4707
September	1st to 10th	Night	8% 3 1 3 2 3 3 2 4 6% 64%	1930
	11th to 20th	Night	8% 4 1 4 4 4 5% 3 5% 7% 56%	2068
	21st to end	Night	12% 5% 2 4 4 3 3 3 5% 6% 54%	2090
October	1st to 10th	Night	9% 4 1 4 3 3 3 2 4 6% 60%	3480
	11th to 20th	Night	9% 6% 2 6% 4 4 3 3 6% 8% 49%	4018
	21st to end	Night	11% 7% 2 6% 4 4 4 3 5% 7% 47%	4467
November	1st to 10th	Night	9% 8% 2 6% 5% 5% 5% 4 7% 7% 41%	2535
	11th to 20th	Night	7% 10% 4 11% 5% 4 4 4 5% 6% 40%	2604
	21st to end	Night	6% 7% 4 9% 6% 5% 5% 4 7% 8% 40%	2966
December	1st to 10th	Night	5% 10% 5% 11% 6% 6% 6% 4 6% 6% 34%	4900
	11th to 20th	Night	7% 11% 4 11% 6% 7% 7% 5% 7% 7% 30%	5564
	21st to end	Night	10% 11% 4 12% 6% 6% 5% 4 5% 7% 32%	7416

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	59% 8% 15% 14% 4	3639
	11th to 20th	Day	52% 8% 15% 14% 12%	3640
	21st to end	Day	50% 7% 17% 21% 5%	4004
February	1st to 10th	Day	56% 7% 12% 16% 9% 1	1625
	11th to 20th	Day	50% 7% 12% 21% 8% 1	1924
	21st to end	Day	51% 7% 14% 19% 9%	1650
March	1st to 10th	Day	40% 6% 13% 29% 12%	4500
	11th to 20th	Day	40% 8% 16% 25% 10%	4802
	21st to end	Day	59% 6% 14% 15% 6%	5708
April	1st to 10th	Day	44% 6% 15% 23% 11% 1	2400
	11th to 20th	Day	49% 5% 12% 21% 11% 1	2492
	21st to end	Day	47% 6% 13% 25% 9% 1	2600
May	1st to 10th	Day	33% 6% 11% 31% 19% 1	5934
	11th to 20th	Day	43% 6% 10% 24% 14% 2	6302
	21st to end	Day	35% 7% 12% 31% 14%	7107
June	1st to 10th	Day	36% 7% 16% 29% 11% 2	3000
	11th to 20th	Day	37% 7% 16% 27% 12% 2	3000
	21st to end	Day	30% 8% 19% 29% 14% 1	3000
July	1st to 10th	Day	31% 9% 16% 29% 11% 3	6463
	11th to 20th	Day	28% 6% 11% 38% 16% 2	6265
	21st to end	Day	31% 9% 17% 28% 13% 1	6705
August	1st to 10th	Day	31% 6% 14% 29% 17% 3	4505
	11th to 20th	Day	38% 7% 16% 28% 9% 1	4210
	21st to end	Day	27% 6% 14% 33% 18% 2	4515
September	1st to 10th	Day	28% 4 12% 34% 20% 2	2317
	11th to 20th	Day	29% 6% 15% 33% 16% 2	2110
	21st to end	Day	32% 5% 13% 31% 17% 2	2090
October	1st to 10th	Day	27% 4 10% 30% 22% 7%	3480
	11th to 20th	Day	29% 4 12% 39% 15% 1	2875
	21st to end	Day	39% 6% 13% 21% 20% 2	3091
November	1st to 10th	Day	44% 7% 14% 21% 12% 1	1763
	11th to 20th	Day	45% 7% 15% 22% 10% 1	1795
	21st to end	Day	46% 6% 18% 21% 10%	1430
December	1st to 10th	Day	53% 5% 16% 19% 5% 1	2290
	11th to 20th	Day	56% 8% 16% 16% 4	2640
	21st to end	Day	51% 5% 10% 27% 7%	3558
January	1st to 10th	Night	58% 6% 11% 19% 6%	6327
	11th to 20th	Night	48% 6% 17% 17% 11%	6340
	21st to end	Night	46% 7% 17% 23% 8%	6974
February	1st to 10th	Night	52% 6% 12% 19% 11%	2774
	11th to 20th	Night	44% 5% 12% 25% 12% 2	2389
	21st to end	Night	42% 5% 15% 25% 11% 1	1980
March	1st to 10th	Night	38% 6% 13% 30% 13%	5374
	11th to 20th	Night	39% 6% 18% 26% 10%	5165
	21st to end	Night	53% 7% 14% 19% 6%	5231
April	1st to 10th	Night	40% 6% 13% 25% 14% 3	2000
	11th to 20th	Night	46% 4 11% 22% 15% 2	1859
	21st to end	Night	42% 6% 13% 26% 13% 1	1800
May	1st to 10th	Night	32% 7% 11% 31% 16% 3	4043
	11th to 20th	Night	43% 6% 13% 20% 18%	3618
	21st to end	Night	35% 7% 15% 27% 16%	3788
June	1st to 10th	Night	29% 6% 16% 34% 13% 3	1400
	11th to 20th	Night	34% 7% 17% 27% 12% 3	1400
	21st to end	Night	22% 7% 20% 33% 16% 2	1400
July	1st to 10th	Night	25% 9% 18% 35% 11% 2	3404
	11th to 20th	Night	21% 6% 14% 41% 15% 2	3592
	21st to end	Night	32% 7% 22% 26% 11% 2	3856
August	1st to 10th	Night	22% 7% 15% 32% 20% 4	2629
	11th to 20th	Night	29% 6% 17% 32% 14% 2	2915
	21st to end	Night	24% 6% 11% 39% 18% 3	3389
September	1st to 10th	Night	22% 4 11% 35% 23% 3	1930
	11th to 20th	Night	27% 6% 16% 31% 18% 2	2068
	21st to end	Night	30% 5% 11% 34% 18% 2	2090
October	1st to 10th	Night	26% 4 9% 26% 32% 3	3480
	11th to 20th	Night	33% 4 10% 37% 15% 2	4018
	21st to end	Night	37% 4 12% 30% 16% 1	4467
November	1st to 10th	Night	39% 6% 15% 23% 16% 2	2535
	11th to 20th	Night	44% 5% 16% 23% 11% 2	2604
	21st to end	Night	40% 7% 18% 27% 8%	2966
December	1st to 10th	Night	49% 6% 22% 17% 6%	4900
	11th to 20th	Night	50% 8% 20% 16% 6%	5566
	21st to end	Night	51% 5% 12% 21% 9% 1	7416

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	11% 8% 30% 33% 12% 5%	300
	11th to 20th	Day	12% 13% 33% 28% 9% 4	300
	21st to end	Day	10% 12% 36% 26% 13% 3	330
February	1st to 10th	Day	12% 13% 39% 31% 4	300
	11th to 20th	Day	10% 9% 40% 30% 10% 1	300
	21st to end	Day	14% 9% 33% 31% 13%	249
March	1st to 10th	Day	22% 9% 36% 23% 9% 2	300
	11th to 20th	Day	13% 13% 36% 27% 8% 3	300
	21st to end	Day	18% 9% 40% 19% 11% 3	432
April	1st to 10th	Day	17% 17% 35% 24% 7%	500
	11th to 20th	Day	24% 14% 30% 22% 9% 1	610
	21st to end	Day	22% 11% 31% 24% 12% 1	700
May	1st to 10th	Day	16% 11% 29% 29% 13% 2	703
	11th to 20th	Day	19% 9% 26% 31% 12% 3	725
	21st to end	Day	15% 9% 30% 23% 17% 5%	789
June	1st to 10th	Day	12% 10% 25% 31% 18% 5%	730
	11th to 20th	Day	14% 11% 36% 26% 12% 2	730
	21st to end	Day	12% 9% 28% 35% 15% 2	730
July	1st to 10th	Day	5% 6% 26% 39% 19% 4	721
	11th to 20th	Day	6% 8% 31% 36% 17% 3	718
	21st to end	Day	5% 5% 26% 39% 21% 4	794
August	1st to 10th	Day	9% 5% 22% 38% 21% 5%	726
	11th to 20th	Day	11% 4 22% 35% 22% 6%	715
	21st to end	Day	8% 7% 28% 31% 22% 4	709
September	1st to 10th	Day	12% 11% 29% 28% 17% 3	503
	11th to 20th	Day	11% 9% 28% 33% 16% 3	320
	21st to end	Day	18% 13% 26% 29% 12% 2	300
October	1st to 10th	Day	20% 10% 29% 23% 17% 2	300
	11th to 20th	Day	14% 16% 34% 22% 12% 2	300
	21st to end	Day	21% 12% 37% 21% 6% 2	330
November	1st to 10th	Day	21% 10% 31% 27% 10% 1	300
	11th to 20th	Day	14% 11% 35% 29% 8% 2	300
	21st to end	Day	11% 13% 32% 30% 13% 1	300
December	1st to 10th	Day	14% 8% 37% 28% 10% 3	300
	11th to 20th	Day	17% 10% 35% 31% 6% 1	300
	21st to end	Day	14% 11% 36% 28% 9% 3	330
January	1st to 10th	Night	11% 10% 34% 34% 10% 2	1000
	11th to 20th	Night	12% 11% 33% 33% 11% 1	1000
	21st to end	Night	10% 10% 37% 35% 9%	1100
February	1st to 10th	Night	15% 9% 30% 36% 10% 1	1000
	11th to 20th	Night	11% 8% 34% 35% 11% 1	835
	21st to end	Night	11% 6% 29% 41% 11% 3	660
March	1st to 10th	Night	13% 9% 34% 34% 9% 2	800
	11th to 20th	Night	15% 7% 31% 35% 10% 1	681
	21st to end	Night	16% 6% 31% 36% 10% 1	660
April	1st to 10th	Night	11% 9% 37% 37% 6% 1	600
	11th to 20th	Night	16% 10% 34% 32% 8% 1	593
	21st to end	Night	19% 7% 29% 32% 13%	600
May	1st to 10th	Night	13% 7% 29% 37% 14%	600
	11th to 20th	Night	15% 4 30% 38% 12% 1	591
	21st to end	Night	14% 6% 28% 37% 14%	650
June	1st to 10th	Night	11% 8% 27% 39% 16% 1	600
	11th to 20th	Night	12% 6% 37% 34% 12%	600
	21st to end	Night	7% 4 29% 45% 14% 1	600
July	1st to 10th	Night	6% 2 24% 48% 19%	585
	11th to 20th	Night	4 4 32% 45% 15%	582
	21st to end	Night	6% 2 25% 52% 14% 2	657
August	1st to 10th	Night	6% 3 26% 49% 16%	597
	11th to 20th	Night	7% 3 26% 43% 19% 2	594
	21st to end	Night	6% 5% 27% 44% 18% 1	660
September	1st to 10th	Night	8% 3 25% 45% 17% 2	579
	11th to 20th	Night	8% 5% 28% 42% 16% 1	570
	21st to end	Night	13% 6% 31% 37% 12% 1	570
October	1st to 10th	Night	18% 7% 28% 36% 10%	570
	11th to 20th	Night	16% 9% 28% 35% 11% 1	897
	21st to end	Night	16% 8% 35% 30% 10% 1	1045
November	1st to 10th	Night	12% 10% 34% 33% 11% 1	975
	11th to 20th	Night	12% 11% 32% 36% 10% 1	1000
	21st to end	Night	10% 11% 33% 38% 8%	1000
December	1st to 10th	Night	12% 8% 33% 36% 11% 1	1000
	11th to 20th	Night	14% 11% 30% 34% 10% 1	1000
	21st to end	Night	17% 10% 27% 35% 11%	1100

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