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 2026-01-17.

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% 10% 3 8% 5% 5% 5% 4 5% 7% 39%	131036
	Night	12% 12% 4 8% 5% 4 5% 4 5% 6% 35%	129380
Transparency	Day	25% 41% 9% 14% 9% 3	130762
	Night	29% 40% 10% 12% 7% 2	129231
Seeing	Day	16% 15% 28% 27% 12% 2	16258
	Night	19% 7% 24% 33% 16% 1	25414

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	14% 25% 8% 15% 7% 5% 4 3 3 4 13%	3719
	11th to 20th	Day	13% 9% 3 8% 5% 5% 5% 4 5% 8% 35%	3664
	21st to end	Day	19% 21% 4 8% 4 4 4 3 5% 5% 23%	4004
February	1st to 10th	Day	14% 15% 5% 9% 5% 5% 6% 5% 5% 5% 27%	1615
	11th to 20th	Day	12% 16% 5% 10% 5% 4 5% 4 7% 7% 27%	1798
	21st to end	Day	12% 15% 3 12% 7% 5% 6% 4 6% 6% 23%	1650
March	1st to 10th	Day	8% 13% 4 9% 6% 7% 6% 6% 7% 7% 28%	4518
	11th to 20th	Day	9% 13% 4 8% 5% 5% 4 4 5% 7% 36%	4677
	21st to end	Day	13% 16% 5% 11% 7% 6% 6% 4 6% 7% 19%	5969
April	1st to 10th	Day	13% 13% 4 9% 6% 6% 5% 4 6% 7% 25%	2400
	11th to 20th	Day	10% 11% 4 10% 6% 5% 6% 5% 6% 7% 29%	2421
	21st to end	Day	10% 12% 4 9% 6% 5% 5% 4 7% 8% 31%	2654
May	1st to 10th	Day	9% 7% 3 8% 5% 5% 6% 4 5% 8% 41%	6320
	11th to 20th	Day	10% 11% 4 8% 6% 6% 7% 4 7% 9% 28%	6277
	21st to end	Day	10% 8% 3 9% 6% 6% 5% 5% 6% 7% 35%	6994
June	1st to 10th	Day	17% 9% 3 7% 6% 5% 5% 4 6% 6% 32%	3000
	11th to 20th	Day	13% 8% 4 9% 7% 7% 7% 5% 7% 7% 28%	3000
	21st to end	Day	8% 6% 2 7% 5% 5% 5% 4 6% 7% 44%	3000
July	1st to 10th	Day	3 2 1 4 2 3 4 3 5% 6% 68%	6462
	11th to 20th	Day	2 2 1 3 3 2 2 3 4 5% 73%	6320
	21st to end	Day	4 3 1 4 3 3 4 3 6% 8% 61%	6874
August	1st to 10th	Day	2 3 1 4 3 4 4 4 5% 8% 63%	4541
	11th to 20th	Day	3 3 2 5% 3 4 5% 4 6% 8% 57%	4415
	21st to end	Day	6% 4 2 6% 4 4 5% 4 6% 7% 52%	4353
September	1st to 10th	Day	4 4 2 5% 4 3 3 3 5% 7% 59%	2400
	11th to 20th	Day	9% 5% 2 5% 5% 5% 4 4 6% 7% 47%	2335
	21st to end	Day	11% 7% 2 7% 4 3 4 3 5% 7% 47%	2200
October	1st to 10th	Day	7% 6% 3 6% 4 4 4 4 6% 8% 48%	3349
	11th to 20th	Day	11% 8% 2 8% 5% 6% 5% 4 5% 7% 39%	3106
	21st to end	Day	14% 10% 3 8% 5% 6% 6% 5% 6% 6% 30%	3200
November	1st to 10th	Day	22% 11% 4 7% 5% 4 4 4 6% 8% 24%	1843
	11th to 20th	Day	20% 13% 5% 9% 6% 5% 4 4 5% 5% 24%	1680
	21st to end	Day	19% 19% 4 10% 7% 5% 5% 3 5% 5% 19%	1522
December	1st to 10th	Day	13% 11% 4 11% 6% 6% 6% 5% 6% 7% 25%	2360
	11th to 20th	Day	18% 25% 6% 14% 5% 4 4 3 4 4 14%	2712
	21st to end	Day	18% 25% 8% 17% 5% 5% 3 2 3 3 10%	3684
January	1st to 10th	Night	19% 28% 7% 11% 5% 5% 4 3 3 3 11%	6446
	11th to 20th	Night	12% 11% 4 7% 4 4 4 4 6% 8% 35%	6366
	21st to end	Night	20% 22% 4 7% 3 4 4 3 6% 6% 21%	6952
February	1st to 10th	Night	14% 16% 5% 9% 6% 5% 4 4 6% 6% 26%	2785
	11th to 20th	Night	13% 16% 4 8% 6% 5% 4 3 5% 6% 31%	2488
	21st to end	Night	12% 14% 4 10% 6% 4 5% 3 6% 8% 26%	1980
March	1st to 10th	Night	7% 11% 4 8% 6% 6% 6% 5% 8% 7% 32%	5376
	11th to 20th	Night	9% 14% 3 7% 5% 4 4 6% 6% 6% 35%	5319
	21st to end	Night	14% 16% 5% 11% 7% 7% 7% 5% 5% 6% 18%	5006
April	1st to 10th	Night	11% 10% 3 6% 5% 5% 5% 4 7% 8% 35%	2000
	11th to 20th	Night	11% 9% 3 9% 5% 5% 6% 4 6% 6% 36%	1930
	21st to end	Night	7% 10% 3 7% 6% 4 5% 5% 7% 8% 38%	1744
May	1st to 10th	Night	7% 6% 2 5% 4 5% 6% 5% 6% 8% 46%	3640
	11th to 20th	Night	8% 9% 3 9% 6% 5% 6% 4 5% 7% 39%	3616
	21st to end	Night	10% 7% 3 7% 5% 5% 5% 4 6% 8% 41%	3880
June	1st to 10th	Night	13% 6% 4 7% 5% 4 4 4 6% 9% 38%	1400
	11th to 20th	Night	10% 6% 3 8% 6% 6% 7% 6% 6% 8% 34%	1400
	21st to end	Night	8% 4 2 5% 5% 5% 6% 4 6% 9% 45%	1400
July	1st to 10th	Night	2 3 1 3 2 2 3 3 3 5% 73%	3493
	11th to 20th	Night	3 2 1 3 2 3 3 2 4 5% 72%	3640
	21st to end	Night	3 2 1 3 2 4 3 3 5% 6% 67%	3969
August	1st to 10th	Night	2 3 1 5% 3 3 4 4 4 6% 65%	2609
	11th to 20th	Night	3 2 1 4 3 3 4 4 7% 9% 60%	2633
	21st to end	Night	5% 3 2 5% 4 5% 4 4 6% 8% 55%	3564
September	1st to 10th	Night	3 3 2 4 3 3 4 4 5% 6% 62%	2000
	11th to 20th	Night	8% 7% 2 5% 3 4 4 3 5% 6% 53%	2065
	21st to end	Night	9% 5% 2 5% 3 4 4 3 5% 8% 52%	2200
October	1st to 10th	Night	6% 4 2 5% 4 4 5% 4 6% 8% 52%	3846
	11th to 20th	Night	14% 8% 3 7% 4 4 4 3 5% 6% 41%	4046
	21st to end	Night	16% 8% 3 8% 5% 6% 5% 4 5% 5% 35%	4600
November	1st to 10th	Night	22% 14% 4 7% 5% 4 5% 4 5% 7% 24%	2674
	11th to 20th	Night	20% 17% 4 7% 5% 4 5% 4 6% 6% 22%	2940
	21st to end	Night	19% 20% 5% 9% 5% 4 3 3 4 5% 21%	3075
December	1st to 10th	Night	12% 12% 4 12% 5% 5% 6% 5% 8% 6% 24%	5040
	11th to 20th	Night	20% 22% 6% 14% 6% 5% 4 4 4 3 11%	5700
	21st to end	Night	22% 23% 6% 12% 7% 5% 4 3 4 3 11%	7558

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% 30% 8% 8% 1	3719
	11th to 20th	Day	28% 31% 22% 11% 5% 2	3688
	21st to end	Day	44% 32% 11% 10% 3	4004
February	1st to 10th	Day	22% 63% 3 5% 5% 1	1615
	11th to 20th	Day	24% 62% 6% 6% 3	1807
	21st to end	Day	21% 63% 7% 6% 4	1650
March	1st to 10th	Day	34% 33% 9% 11% 12%	4518
	11th to 20th	Day	29% 31% 14% 20% 5%	4677
	21st to end	Day	43% 37% 8% 8% 4	5969
April	1st to 10th	Day	18% 64% 8% 7% 3	2400
	11th to 20th	Day	17% 64% 4 11% 3 1	2421
	21st to end	Day	17% 63% 5% 9% 5% 1	2654
May	1st to 10th	Day	23% 33% 7% 17% 18% 1	6320
	11th to 20th	Day	35% 35% 10% 16% 5%	6280
	21st to end	Day	28% 34% 14% 19% 5%	6995
June	1st to 10th	Day	15% 63% 5% 9% 7% 1	3000
	11th to 20th	Day	16% 64% 5% 8% 6% 1	3000
	21st to end	Day	15% 62% 5% 10% 7% 1	3000
July	1st to 10th	Day	10% 27% 8% 16% 23% 15%	6393
	11th to 20th	Day	9% 26% 7% 20% 20% 18%	6245
	21st to end	Day	16% 28% 8% 23% 16% 9%	6681
August	1st to 10th	Day	9% 37% 5% 19% 24% 6%	4524
	11th to 20th	Day	15% 41% 10% 25% 8% 1	4457
	21st to end	Day	18% 40% 11% 17% 12% 3	4352
September	1st to 10th	Day	12% 59% 6% 13% 8% 2	2400
	11th to 20th	Day	16% 59% 6% 11% 7% 1	2335
	21st to end	Day	17% 58% 9% 10% 5% 1	2200
October	1st to 10th	Day	18% 38% 16% 23% 5%	3349
	11th to 20th	Day	29% 39% 7% 18% 7%	3106
	21st to end	Day	32% 42% 12% 11% 4	3200
November	1st to 10th	Day	26% 57% 9% 8% 1	1843
	11th to 20th	Day	29% 56% 6% 4 3 1	1680
	21st to end	Day	29% 56% 6% 5% 3	1522
December	1st to 10th	Day	32% 42% 17% 8% 1	2360
	11th to 20th	Day	52% 35% 6% 4 2	2714
	21st to end	Day	60% 30% 4 4 2	3684
January	1st to 10th	Night	56% 29% 10% 4 1	6446
	11th to 20th	Night	28% 31% 15% 16% 7% 3	6404
	21st to end	Night	44% 32% 13% 8% 2	6952
February	1st to 10th	Night	23% 63% 4 4 4 1	2785
	11th to 20th	Night	24% 61% 5% 7% 2 1	2490
	21st to end	Night	19% 63% 6% 6% 6%	1980
March	1st to 10th	Night	28% 34% 14% 13% 10%	5376
	11th to 20th	Night	29% 31% 15% 19% 5%	5320
	21st to end	Night	44% 38% 8% 8% 2	5006
April	1st to 10th	Night	16% 65% 7% 8% 3	2000
	11th to 20th	Night	16% 63% 5% 11% 5%	1930
	21st to end	Night	13% 63% 5% 11% 8%	1744
May	1st to 10th	Night	21% 32% 8% 21% 18% 1	3637
	11th to 20th	Night	30% 33% 10% 20% 6% 1	3616
	21st to end	Night	22% 35% 14% 24% 5%	3880
June	1st to 10th	Night	12% 62% 6% 12% 7% 1	1400
	11th to 20th	Night	17% 63% 5% 7% 6% 1	1400
	21st to end	Night	13% 63% 6% 9% 8% 1	1400
July	1st to 10th	Night	9% 25% 7% 17% 26% 16%	3453
	11th to 20th	Night	11% 27% 7% 18% 15% 22%	3592
	21st to end	Night	14% 26% 6% 24% 20% 9%	3857
August	1st to 10th	Night	12% 37% 5% 17% 22% 7%	2601
	11th to 20th	Night	13% 40% 13% 24% 10% 1	2657
	21st to end	Night	19% 39% 10% 18% 12% 2	3564
September	1st to 10th	Night	12% 58% 7% 12% 10% 1	2000
	11th to 20th	Night	15% 59% 6% 12% 8% 1	2065
	21st to end	Night	16% 58% 8% 13% 5%	2200
October	1st to 10th	Night	16% 38% 17% 22% 7%	3846
	11th to 20th	Night	28% 38% 8% 19% 7%	4046
	21st to end	Night	31% 39% 15% 13% 2	4600
November	1st to 10th	Night	27% 56% 9% 6% 1	2674
	11th to 20th	Night	27% 56% 8% 5% 3 1	2940
	21st to end	Night	28% 56% 6% 6% 4	3075
December	1st to 10th	Night	31% 41% 19% 8% 1	5040
	11th to 20th	Night	52% 38% 6% 3 1	5697
	21st to end	Night	55% 32% 6% 4 3	7558

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	25% 15% 25% 23% 10% 2	220
	11th to 20th	Day	25% 11% 25% 22% 15% 2	212
	21st to end	Day	31% 9% 22% 29% 8%	220
February	1st to 10th	Day	18% 18% 31% 26% 6% 2	215
	11th to 20th	Day	21% 13% 32% 25% 7% 1	418
	21st to end	Day	19% 16% 32% 24% 8% 1	414
March	1st to 10th	Day	23% 20% 30% 17% 9% 2	500
	11th to 20th	Day	19% 20% 24% 25% 11%	500
	21st to end	Day	23% 20% 28% 19% 9% 1	550
April	1st to 10th	Day	25% 20% 29% 19% 7% 1	500
	11th to 20th	Day	20% 24% 25% 22% 7% 2	492
	21st to end	Day	16% 20% 32% 21% 9% 2	500
May	1st to 10th	Day	16% 21% 32% 22% 9% 2	503
	11th to 20th	Day	19% 13% 27% 27% 12% 2	528
	21st to end	Day	14% 13% 29% 26% 15% 3	612
June	1st to 10th	Day	20% 15% 27% 27% 9% 2	730
	11th to 20th	Day	13% 15% 31% 27% 11% 2	730
	21st to end	Day	11% 9% 29% 32% 16% 3	730
July	1st to 10th	Day	8% 12% 31% 34% 13% 2	590
	11th to 20th	Day	7% 16% 30% 31% 13% 3	524
	21st to end	Day	5% 10% 32% 32% 18% 3	575
August	1st to 10th	Day	5% 8% 32% 33% 17% 4	527
	11th to 20th	Day	7% 8% 26% 37% 18% 4	517
	21st to end	Day	8% 15% 25% 30% 17% 5%	583
September	1st to 10th	Day	10% 12% 24% 31% 17% 5%	530
	11th to 20th	Day	13% 10% 29% 28% 16% 3	530
	21st to end	Day	15% 18% 29% 24% 12% 2	530
October	1st to 10th	Day	14% 14% 32% 27% 13% 2	530
	11th to 20th	Day	18% 18% 28% 24% 11% 2	439
	21st to end	Day	23% 12% 27% 28% 8% 2	363
November	1st to 10th	Day	26% 11% 25% 28% 9%	324
	11th to 20th	Day	29% 16% 25% 19% 10% 2	220
	21st to end	Day	26% 11% 28% 23% 10% 2	220
December	1st to 10th	Day	26% 8% 23% 19% 18% 5%	220
	11th to 20th	Day	25% 12% 28% 27% 6% 1	220
	21st to end	Day	26% 14% 21% 29% 10%	242
January	1st to 10th	Night	33% 11% 24% 25% 7% 1	840
	11th to 20th	Night	28% 6% 25% 31% 9% 1	824
	21st to end	Night	23% 7% 25% 34% 11%	880
February	1st to 10th	Night	21% 8% 30% 30% 10% 1	800
	11th to 20th	Night	20% 7% 29% 35% 9%	784
	21st to end	Night	21% 7% 26% 31% 14% 1	660
March	1st to 10th	Night	21% 8% 24% 32% 15% 1	800
	11th to 20th	Night	18% 9% 27% 33% 13% 1	800
	21st to end	Night	18% 9% 29% 29% 13% 2	880
April	1st to 10th	Night	17% 7% 26% 32% 16% 1	800
	11th to 20th	Night	15% 8% 30% 34% 12% 2	742
	21st to end	Night	13% 12% 25% 33% 16% 1	544
May	1st to 10th	Night	10% 8% 23% 39% 19% 1	400
	11th to 20th	Night	12% 6% 24% 32% 24% 2	394
	21st to end	Night	15% 5% 18% 34% 24% 3	433
June	1st to 10th	Night	14% 8% 26% 34% 17% 2	400
	11th to 20th	Night	15% 6% 23% 40% 17% 1	400
	21st to end	Night	10% 7% 21% 35% 27% 1	400
July	1st to 10th	Night	6% 3 17% 43% 28% 3	390
	11th to 20th	Night	7% 5% 22% 40% 24% 1	388
	21st to end	Night	2 3 18% 41% 33% 2	438
August	1st to 10th	Night	4 3 14% 38% 38% 4	398
	11th to 20th	Night	4 3 15% 38% 36% 4	426
	21st to end	Night	6% 5% 20% 38% 28% 3	849
September	1st to 10th	Night	5% 4 21% 38% 27% 4	800
	11th to 20th	Night	11% 6% 18% 30% 33% 3	800
	21st to end	Night	12% 7% 20% 42% 17% 3	800
October	1st to 10th	Night	14% 4 21% 40% 20% 1	800
	11th to 20th	Night	18% 8% 23% 34% 17% 2	800
	21st to end	Night	22% 9% 25% 32% 10% 2	880
November	1st to 10th	Night	30% 8% 25% 32% 6%	820
	11th to 20th	Night	30% 9% 24% 28% 9%	840
	21st to end	Night	27% 8% 23% 30% 11% 1	975
December	1st to 10th	Night	25% 6% 27% 30% 12% 1	1050
	11th to 20th	Night	30% 7% 27% 29% 6%	1050
	21st to end	Night	30% 9% 29% 27% 5%	1129

Page generated at 2026-01-13 22:14:29. Version ac07

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