Grand Forks
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grand Forks 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	11% 8% 3 8% 5% 5% 5% 4 6% 8% 38%	135236
	Night	10% 9% 4 9% 6% 5% 5% 4 6% 7% 37%	130809
Transparency	Day	45% 5% 15% 25% 10% 1	129876
	Night	47% 5% 13% 24% 10% 1	127178
Seeing	Day	14% 9% 31% 29% 14% 4	17375
	Night	14% 7% 32% 33% 12% 1	27200

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	4 11% 7% 16% 10% 7% 8% 6% 7% 8% 16%	3178
	11th to 20th	Day	4 7% 4 12% 10% 11% 12% 9% 7% 7% 16%	3384
	21st to end	Day	5% 14% 7% 16% 7% 7% 5% 3 5% 6% 26%	4004
February	1st to 10th	Day	8% 14% 6% 10% 7% 7% 8% 5% 6% 5% 24%	1800
	11th to 20th	Day	6% 12% 3 10% 9% 8% 7% 5% 9% 6% 24%	1754
	21st to end	Day	7% 11% 5% 11% 9% 8% 6% 6% 5% 7% 26%	1594
March	1st to 10th	Day	11% 10% 3 8% 6% 6% 5% 4 6% 9% 33%	4662
	11th to 20th	Day	8% 9% 4 14% 8% 5% 5% 5% 7% 9% 26%	4980
	21st to end	Day	16% 15% 5% 10% 5% 3 4 3 5% 6% 28%	5464
April	1st to 10th	Day	12% 12% 4 8% 5% 5% 4 4 6% 6% 34%	2388
	11th to 20th	Day	17% 11% 4 7% 5% 4 4 4 6% 6% 32%	2569
	21st to end	Day	20% 10% 3 8% 4 4 4 4 5% 6% 32%	2600
May	1st to 10th	Day	13% 6% 2 5% 4 4 4 4 5% 7% 47%	5936
	11th to 20th	Day	16% 9% 2 6% 4 4 4 4 6% 8% 37%	6379
	21st to end	Day	14% 7% 2 5% 4 4 4 4 5% 8% 42%	7427
June	1st to 10th	Day	14% 6% 2 6% 4 5% 4 4 7% 9% 40%	3000
	11th to 20th	Day	14% 5% 3 5% 5% 3 5% 4 6% 9% 41%	3000
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 4 7% 8% 40%	3000
July	1st to 10th	Day	6% 4 2 5% 4 5% 5% 5% 7% 11% 47%	6799
	11th to 20th	Day	9% 5% 2 6% 4 4 4 4 6% 8% 49%	6800
	21st to end	Day	8% 5% 2 6% 4 4 5% 5% 7% 9% 47%	7096
August	1st to 10th	Day	12% 5% 2 6% 4 4 5% 4 5% 8% 45%	5832
	11th to 20th	Day	13% 5% 2 5% 4 4 4 3 5% 9% 46%	5821
	21st to end	Day	8% 4 2 4 3 3 4 3 5% 8% 55%	6487
September	1st to 10th	Day	12% 5% 2 5% 4 3 3 3 5% 8% 50%	2358
	11th to 20th	Day	13% 5% 2 6% 4 4 4 4 6% 7% 45%	2090
	21st to end	Day	12% 5% 2 5% 4 4 4 4 6% 8% 44%	2090
October	1st to 10th	Day	12% 5% 2 4 3 3 3 3 5% 6% 54%	3229
	11th to 20th	Day	14% 8% 2 6% 4 3 3 3 5% 6% 47%	3140
	21st to end	Day	12% 10% 3 9% 6% 4 5% 3 6% 7% 36%	3101
November	1st to 10th	Day	13% 14% 5% 9% 5% 4 4 3 5% 5% 34%	1648
	11th to 20th	Day	11% 15% 5% 14% 7% 5% 4 3 5% 4 28%	1600
	21st to end	Day	11% 13% 5% 13% 7% 4 5% 4 4 5% 30%	1595
December	1st to 10th	Day	9% 12% 6% 16% 12% 8% 6% 4 5% 6% 17%	2290
	11th to 20th	Day	9% 16% 6% 12% 5% 5% 4 5% 7% 7% 24%	2643
	21st to end	Day	12% 15% 6% 17% 7% 7% 5% 3 4 3 20%	3498
January	1st to 10th	Night	4 10% 6% 16% 9% 9% 6% 5% 7% 7% 19%	6787
	11th to 20th	Night	5% 11% 4 12% 8% 10% 10% 8% 8% 5% 18%	6593
	21st to end	Night	6% 14% 7% 15% 6% 6% 5% 4 5% 6% 27%	6961
February	1st to 10th	Night	8% 13% 5% 11% 7% 6% 6% 4 5% 5% 30%	2600
	11th to 20th	Night	7% 9% 4 9% 6% 6% 7% 5% 7% 7% 32%	2533
	21st to end	Night	7% 11% 4 10% 7% 5% 5% 3 6% 8% 34%	2055
March	1st to 10th	Night	7% 9% 3 8% 7% 5% 4 3 5% 7% 42%	5203
	11th to 20th	Night	8% 11% 4 11% 7% 6% 6% 4 5% 7% 33%	4980
	21st to end	Night	14% 14% 5% 11% 5% 4 4 3 4 6% 30%	5467
April	1st to 10th	Night	12% 10% 3 7% 4 5% 4 4 6% 7% 41%	2007
	11th to 20th	Night	16% 11% 3 7% 3 4 4 3 5% 6% 36%	1779
	21st to end	Night	17% 9% 3 6% 5% 3 5% 3 4 7% 38%	1800
May	1st to 10th	Night	14% 7% 2 5% 4 4 4 3 5% 6% 47%	4044
	11th to 20th	Night	15% 8% 2 6% 4 4 4 3 6% 7% 41%	3519
	21st to end	Night	13% 6% 2 5% 3 3 4 4 6% 7% 48%	3470
June	1st to 10th	Night	10% 5% 2 5% 3 4 5% 4 6% 9% 47%	1400
	11th to 20th	Night	11% 5% 2 5% 4 4 4 4 7% 9% 46%	1400
	21st to end	Night	8% 4 2 6% 4 3 5% 4 6% 10% 48%	1400
July	1st to 10th	Night	8% 5% 2 3 3 3 4 4 7% 9% 53%	3180
	11th to 20th	Night	6% 4 1 4 3 3 4 3 6% 8% 58%	3180
	21st to end	Night	7% 5% 2 6% 4 4 5% 4 7% 10% 45%	3775
August	1st to 10th	Night	11% 4 2 5% 4 4 5% 4 5% 7% 51%	3968
	11th to 20th	Night	11% 5% 1 4 4 4 4 4 6% 9% 49%	4028
	21st to end	Night	8% 4 2 5% 3 3 3 3 5% 8% 57%	4487
September	1st to 10th	Night	8% 4 2 4 3 3 4 3 5% 7% 57%	1887
	11th to 20th	Night	9% 5% 2 5% 3 3 3 4 6% 8% 53%	2090
	21st to end	Night	13% 5% 2 5% 4 4 3 3 4 7% 50%	2090
October	1st to 10th	Night	11% 4 2 6% 4 4 4 3 4 7% 52%	3699
	11th to 20th	Night	11% 8% 2 6% 4 4 4 4 5% 7% 44%	3779
	21st to end	Night	15% 9% 3 7% 5% 4 4 3 5% 6% 40%	4454
November	1st to 10th	Night	11% 12% 4 9% 5% 5% 5% 3 6% 7% 33%	2650
	11th to 20th	Night	11% 14% 6% 11% 6% 5% 5% 4 5% 6% 29%	2800
	21st to end	Night	12% 12% 4 10% 6% 5% 5% 3 5% 6% 32%	2800
December	1st to 10th	Night	6% 13% 4 18% 11% 7% 6% 4 5% 6% 19%	4900
	11th to 20th	Night	8% 14% 5% 14% 9% 7% 6% 4 5% 5% 22%	5564
	21st to end	Night	13% 13% 6% 16% 8% 6% 5% 3 4 4 20%	7480

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	62% 6% 9% 19% 4	3179
	11th to 20th	Day	64% 5% 10% 15% 6%	3384
	21st to end	Day	58% 5% 14% 19% 4	4004
February	1st to 10th	Day	59% 5% 12% 15% 9%	1800
	11th to 20th	Day	53% 7% 12% 19% 8% 1	1763
	21st to end	Day	55% 7% 14% 19% 5%	1594
March	1st to 10th	Day	46% 5% 18% 22% 9%	4662
	11th to 20th	Day	55% 5% 15% 20% 5%	4980
	21st to end	Day	57% 6% 12% 22% 4	5464
April	1st to 10th	Day	49% 7% 12% 22% 9% 1	2388
	11th to 20th	Day	52% 6% 13% 18% 10% 1	2572
	21st to end	Day	52% 7% 13% 21% 7% 1	2600
May	1st to 10th	Day	35% 6% 14% 28% 16% 1	5936
	11th to 20th	Day	45% 5% 10% 22% 16% 1	6383
	21st to end	Day	39% 6% 10% 27% 17% 2	7428
June	1st to 10th	Day	39% 8% 17% 27% 9% 1	3000
	11th to 20th	Day	39% 8% 18% 25% 10% 1	3000
	21st to end	Day	39% 8% 17% 26% 9% 1	3000
July	1st to 10th	Day	31% 5% 26% 31% 7%	6726
	11th to 20th	Day	33% 4 18% 36% 9%	6719
	21st to end	Day	34% 4 21% 30% 9%	6893
August	1st to 10th	Day	35% 5% 15% 28% 15% 1	4238
	11th to 20th	Day	37% 4 15% 34% 9%	4210
	21st to end	Day	33% 3 19% 35% 9%	4674
September	1st to 10th	Day	34% 6% 15% 33% 12% 1	2358
	11th to 20th	Day	38% 7% 18% 28% 8% 1	2090
	21st to end	Day	35% 8% 16% 32% 9%	2090
October	1st to 10th	Day	33% 3 11% 35% 17% 1	3229
	11th to 20th	Day	40% 4 9% 31% 16%	3140
	21st to end	Day	47% 4 14% 25% 9%	3101
November	1st to 10th	Day	52% 6% 14% 18% 9% 1	1648
	11th to 20th	Day	58% 6% 13% 16% 6% 1	1600
	21st to end	Day	57% 7% 14% 16% 6%	1595
December	1st to 10th	Day	69% 5% 12% 12% 3	2290
	11th to 20th	Day	59% 5% 15% 13% 7%	2640
	21st to end	Day	68% 3 7% 18% 4	3498
January	1st to 10th	Night	61% 6% 11% 19% 4	6787
	11th to 20th	Night	61% 6% 9% 18% 5%	6593
	21st to end	Night	58% 5% 13% 20% 5%	6961
February	1st to 10th	Night	54% 5% 14% 18% 9%	2600
	11th to 20th	Night	47% 8% 13% 19% 12%	2534
	21st to end	Night	48% 5% 13% 25% 8% 1	2055
March	1st to 10th	Night	40% 5% 18% 25% 9% 2	5203
	11th to 20th	Night	49% 6% 12% 22% 10%	4980
	21st to end	Night	55% 6% 12% 20% 7%	5467
April	1st to 10th	Night	41% 6% 13% 25% 13% 2	2007
	11th to 20th	Night	49% 4 12% 19% 14% 3	1779
	21st to end	Night	46% 5% 12% 24% 10% 2	1800
May	1st to 10th	Night	36% 6% 12% 27% 18% 1	4044
	11th to 20th	Night	43% 5% 10% 22% 18% 2	3520
	21st to end	Night	35% 5% 9% 28% 23% 1	3470
June	1st to 10th	Night	31% 8% 17% 31% 11% 2	1400
	11th to 20th	Night	35% 7% 15% 27% 14% 3	1400
	21st to end	Night	32% 7% 18% 34% 8% 2	1400
July	1st to 10th	Night	28% 5% 23% 35% 8% 1	3145
	11th to 20th	Night	26% 4 18% 42% 9% 1	3138
	21st to end	Night	34% 4 25% 26% 10% 1	3665
August	1st to 10th	Night	29% 4 14% 35% 16% 1	2886
	11th to 20th	Night	32% 5% 19% 32% 12% 1	2915
	21st to end	Night	32% 3 17% 32% 15% 1	3234
September	1st to 10th	Night	27% 6% 14% 35% 17% 1	1887
	11th to 20th	Night	29% 7% 19% 31% 14% 1	2090
	21st to end	Night	35% 5% 11% 31% 16% 1	2090
October	1st to 10th	Night	34% 3 11% 34% 16% 3	3699
	11th to 20th	Night	40% 4 10% 29% 16% 1	3779
	21st to end	Night	45% 4 11% 27% 12%	4454
November	1st to 10th	Night	50% 6% 14% 19% 10% 2	2650
	11th to 20th	Night	56% 6% 14% 17% 7% 1	2800
	21st to end	Night	52% 7% 16% 19% 6%	2800
December	1st to 10th	Night	65% 6% 14% 13% 3	4900
	11th to 20th	Night	62% 6% 12% 16% 4	5566
	21st to end	Night	66% 4 9% 16% 5%	7480

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	9% 10% 32% 34% 11% 5%	300
	11th to 20th	Day	12% 7% 37% 31% 11% 1	300
	21st to end	Day	15% 9% 34% 29% 12% 2	330
February	1st to 10th	Day	16% 14% 33% 30% 6% 1	300
	11th to 20th	Day	11% 12% 39% 26% 11% 1	300
	21st to end	Day	7% 9% 41% 32% 11%	249
March	1st to 10th	Day	16% 12% 31% 29% 10% 2	300
	11th to 20th	Day	16% 9% 37% 24% 11% 3	300
	21st to end	Day	15% 10% 30% 32% 10% 3	330
April	1st to 10th	Day	18% 15% 32% 24% 9% 2	488
	11th to 20th	Day	24% 13% 28% 24% 9% 2	690
	21st to end	Day	23% 11% 28% 25% 12% 2	700
May	1st to 10th	Day	17% 11% 29% 30% 11% 2	703
	11th to 20th	Day	18% 10% 28% 27% 13% 3	725
	21st to end	Day	16% 9% 26% 28% 17% 4	789
June	1st to 10th	Day	13% 9% 29% 29% 15% 6%	730
	11th to 20th	Day	15% 9% 38% 23% 14% 2	730
	21st to end	Day	12% 9% 29% 27% 18% 5%	730
July	1st to 10th	Day	6% 3 27% 34% 20% 9%	721
	11th to 20th	Day	12% 5% 30% 28% 20% 6%	718
	21st to end	Day	5% 3 26% 37% 22% 7%	794
August	1st to 10th	Day	7% 4 26% 33% 25% 5%	726
	11th to 20th	Day	12% 5% 23% 29% 21% 10%	715
	21st to end	Day	10% 7% 27% 31% 20% 5%	803
September	1st to 10th	Day	12% 7% 34% 28% 18% 2	544
	11th to 20th	Day	14% 10% 31% 30% 14% 2	300
	21st to end	Day	14% 14% 35% 25% 10% 2	300
October	1st to 10th	Day	19% 9% 30% 27% 11% 4	300
	11th to 20th	Day	14% 12% 36% 27% 9% 1	300
	21st to end	Day	18% 14% 35% 22% 9% 2	330
November	1st to 10th	Day	24% 9% 27% 28% 12% 1	300
	11th to 20th	Day	17% 12% 36% 26% 9%	300
	21st to end	Day	16% 13% 37% 26% 8%	300
December	1st to 10th	Day	13% 8% 37% 31% 9% 3	300
	11th to 20th	Day	18% 8% 38% 31% 6%	300
	21st to end	Day	15% 8% 32% 30% 11% 3	330
January	1st to 10th	Night	11% 9% 34% 33% 11% 1	1000
	11th to 20th	Night	14% 8% 34% 35% 9% 1	1000
	21st to end	Night	14% 8% 36% 33% 9%	1100
February	1st to 10th	Night	13% 10% 33% 34% 10% 1	1000
	11th to 20th	Night	11% 8% 37% 32% 10% 2	979
	21st to end	Night	12% 9% 33% 33% 11% 1	735
March	1st to 10th	Night	14% 9% 37% 28% 10% 2	733
	11th to 20th	Night	12% 8% 38% 33% 9% 1	600
	21st to end	Night	16% 8% 32% 30% 12% 2	660
April	1st to 10th	Night	16% 8% 35% 31% 10%	600
	11th to 20th	Night	19% 10% 31% 29% 11%	593
	21st to end	Night	21% 7% 26% 35% 12% 1	600
May	1st to 10th	Night	19% 8% 29% 33% 10% 1	600
	11th to 20th	Night	16% 9% 31% 30% 13% 1	591
	21st to end	Night	15% 7% 32% 31% 13% 1	650
June	1st to 10th	Night	11% 6% 29% 40% 14% 1	600
	11th to 20th	Night	12% 5% 35% 36% 12%	600
	21st to end	Night	8% 4 29% 43% 16% 1	600
July	1st to 10th	Night	6% 4 24% 44% 21% 2	585
	11th to 20th	Night	6% 2 29% 44% 18% 1	582
	21st to end	Night	6% 3 23% 47% 19% 2	657
August	1st to 10th	Night	7% 3 24% 46% 20% 1	597
	11th to 20th	Night	9% 3 23% 42% 21% 2	594
	21st to end	Night	8% 5% 27% 37% 21% 1	660
September	1st to 10th	Night	9% 4 31% 37% 17% 2	579
	11th to 20th	Night	11% 4 32% 37% 15% 1	570
	21st to end	Night	15% 5% 31% 37% 12%	570
October	1st to 10th	Night	18% 8% 32% 34% 7%	698
	11th to 20th	Night	19% 9% 36% 28% 8%	760
	21st to end	Night	21% 9% 32% 30% 8% 1	1032
November	1st to 10th	Night	15% 9% 36% 28% 11%	975
	11th to 20th	Night	17% 10% 35% 28% 10% 1	1000
	21st to end	Night	17% 12% 40% 25% 6% 1	1000
December	1st to 10th	Night	14% 8% 35% 33% 9% 1	1000
	11th to 20th	Night	17% 11% 34% 30% 8% 1	1000
	21st to end	Night	21% 8% 34% 27% 9% 1	1100

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