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 2024-03-28.

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	12% 8% 4 7% 6% 4 5% 5% 6% 7% 36%	78314
	Night	10% 9% 4 8% 7% 4 5% 5% 5% 7% 36%	81735
Transparency	Day	32% 38% 10% 14% 6%	78001
	Night	33% 36% 10% 14% 6% 1	81570
Seeing	Day	14% 9% 30% 29% 14% 4	15189
	Night	14% 7% 32% 33% 12% 1	25345

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	6% 12% 8% 14% 9% 5% 7% 6% 6% 5% 22%	2218
	11th to 20th	Day	4 7% 5% 11% 15% 10% 14% 9% 5% 5% 14%	2354
	21st to end	Day	7% 14% 8% 16% 8% 4 5% 4 5% 5% 23%	2794
February	1st to 10th	Day	8% 13% 6% 9% 8% 5% 8% 6% 6% 5% 24%	1710
	11th to 20th	Day	7% 11% 5% 9% 11% 6% 8% 7% 8% 6% 24%	1664
	21st to end	Day	8% 9% 6% 10% 10% 6% 7% 6% 5% 7% 27%	1516
March	1st to 10th	Day	12% 14% 5% 10% 7% 5% 6% 4 4 7% 27%	1962
	11th to 20th	Day	11% 9% 4 10% 7% 3 5% 5% 7% 9% 30%	2090
	21st to end	Day	14% 12% 5% 9% 6% 3 4 5% 6% 7% 28%	2255
April	1st to 10th	Day	13% 11% 4 7% 7% 3 5% 5% 6% 6% 33%	2148
	11th to 20th	Day	17% 11% 4 6% 5% 3 5% 5% 5% 6% 32%	2309
	21st to end	Day	20% 9% 3 7% 6% 3 4 5% 5% 6% 33%	2340
May	1st to 10th	Day	17% 9% 3 5% 5% 3 4 5% 5% 7% 38%	2348
	11th to 20th	Day	16% 6% 2 6% 4 3 6% 5% 6% 9% 38%	2500
	21st to end	Day	17% 6% 3 5% 6% 3 5% 5% 5% 8% 37%	2918
June	1st to 10th	Day	14% 5% 3 5% 5% 4 5% 5% 6% 9% 40%	2700
	11th to 20th	Day	15% 5% 3 5% 5% 3 5% 5% 5% 9% 41%	2700
	21st to end	Day	11% 4 3 5% 6% 4 5% 5% 6% 9% 41%	2700
July	1st to 10th	Day	6% 3 1 5% 4 3 5% 6% 6% 10% 50%	2700
	11th to 20th	Day	8% 5% 2 5% 5% 3 5% 5% 6% 8% 47%	2700
	21st to end	Day	4 3 2 4 4 3 4 6% 7% 8% 55%	2823
August	1st to 10th	Day	7% 4 2 5% 5% 4 5% 6% 6% 9% 46%	2346
	11th to 20th	Day	11% 5% 2 4 5% 2 4 5% 5% 9% 49%	2301
	21st to end	Day	11% 5% 2 4 4 3 3 5% 5% 6% 53%	2574
September	1st to 10th	Day	13% 5% 2 4 5% 2 3 4 5% 7% 50%	2196
	11th to 20th	Day	13% 5% 2 5% 5% 3 3 5% 5% 7% 45%	1980
	21st to end	Day	13% 5% 2 4 5% 3 5% 5% 6% 8% 43%	1980
October	1st to 10th	Day	19% 9% 3 6% 5% 2 4 3 4 5% 40%	1841
	11th to 20th	Day	15% 11% 3 5% 6% 2 4 5% 5% 6% 38%	1800
	21st to end	Day	14% 12% 4 9% 6% 3 5% 4 6% 6% 32%	1792
November	1st to 10th	Day	13% 12% 6% 9% 5% 3 4 4 5% 5% 34%	1564
	11th to 20th	Day	10% 13% 6% 13% 8% 3 5% 4 5% 4 28%	1520
	21st to end	Day	12% 12% 5% 10% 9% 3 5% 5% 4 5% 30%	1516
December	1st to 10th	Day	9% 12% 6% 12% 9% 5% 7% 6% 5% 5% 23%	1330
	11th to 20th	Day	10% 13% 7% 11% 8% 5% 6% 5% 7% 6% 22%	1683
	21st to end	Day	16% 13% 6% 10% 9% 6% 6% 4 4 4 22%	2442
January	1st to 10th	Night	5% 11% 7% 12% 10% 6% 7% 6% 6% 7% 23%	4737
	11th to 20th	Night	5% 10% 5% 13% 12% 8% 11% 7% 7% 5% 15%	4613
	21st to end	Night	8% 14% 7% 14% 8% 4 5% 5% 5% 5% 25%	4873
February	1st to 10th	Night	8% 12% 4 10% 8% 4 7% 6% 5% 6% 30%	2470
	11th to 20th	Night	7% 9% 5% 9% 7% 5% 8% 7% 6% 7% 31%	2403
	21st to end	Night	8% 10% 5% 8% 8% 3 5% 4 6% 8% 34%	1956
March	1st to 10th	Night	11% 11% 6% 8% 7% 4 5% 5% 6% 6% 31%	2217
	11th to 20th	Night	8% 10% 3 7% 7% 4 5% 5% 5% 6% 40%	2090
	21st to end	Night	13% 10% 3 10% 7% 3 3 4 5% 7% 34%	2255
April	1st to 10th	Night	12% 9% 3 6% 5% 4 4 5% 5% 7% 41%	1807
	11th to 20th	Night	17% 11% 4 6% 5% 3 4 4 5% 6% 37%	1599
	21st to end	Night	17% 8% 3 5% 6% 2 4 5% 5% 7% 38%	1620
May	1st to 10th	Night	16% 7% 3 5% 6% 3 4 4 4 7% 40%	1612
	11th to 20th	Night	15% 6% 2 4 5% 3 4 5% 5% 8% 44%	1391
	21st to end	Night	13% 4 2 5% 4 2 5% 5% 6% 7% 46%	1358
June	1st to 10th	Night	11% 5% 2 4 4 3 4 6% 6% 10% 46%	1260
	11th to 20th	Night	10% 4 2 5% 4 3 5% 4 7% 9% 47%	1260
	21st to end	Night	7% 3 2 5% 5% 2 5% 5% 6% 10% 50%	1260
July	1st to 10th	Night	5% 3 1 3 3 2 3 5% 6% 8% 61%	1260
	11th to 20th	Night	4 3 1 3 4 2 4 5% 7% 9% 59%	1260
	21st to end	Night	5% 3 2 4 4 2 4 4 7% 9% 57%	1492
August	1st to 10th	Night	5% 4 2 4 5% 3 5% 4 5% 6% 57%	1593
	11th to 20th	Night	7% 5% 1 4 4 3 4 5% 6% 9% 54%	1600
	21st to end	Night	8% 4 2 5% 4 2 4 3 4 8% 56%	1782
September	1st to 10th	Night	9% 3 2 4 4 2 4 4 4 7% 57%	1762
	11th to 20th	Night	9% 5% 2 4 3 2 3 4 6% 8% 53%	1980
	21st to end	Night	14% 5% 2 4 5% 3 4 4 4 7% 49%	1980
October	1st to 10th	Night	16% 6% 2 6% 5% 3 4 4 4 7% 42%	2101
	11th to 20th	Night	16% 10% 3 4 5% 3 4 5% 4 7% 38%	2160
	21st to end	Night	17% 9% 3 7% 5% 3 4 4 5% 7% 36%	2561
November	1st to 10th	Night	11% 10% 5% 9% 5% 4 5% 4 6% 7% 33%	2514
	11th to 20th	Night	10% 13% 7% 10% 7% 4 5% 5% 5% 5% 29%	2660
	21st to end	Night	12% 11% 4 8% 7% 4 6% 4 5% 7% 33%	2660
December	1st to 10th	Night	9% 13% 5% 12% 9% 5% 6% 5% 6% 6% 24%	2850
	11th to 20th	Night	10% 13% 6% 12% 9% 5% 6% 5% 5% 5% 23%	3514
	21st to end	Night	18% 13% 5% 9% 9% 4 5% 4 5% 4 25%	5225

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	47% 29% 10% 12% 2	2219
	11th to 20th	Day	56% 28% 9% 5% 2	2354
	21st to end	Day	49% 25% 6% 15% 4	2794
February	1st to 10th	Day	40% 36% 9% 9% 5%	1710
	11th to 20th	Day	36% 38% 9% 11% 5% 1	1673
	21st to end	Day	39% 37% 10% 10% 3	1516
March	1st to 10th	Day	40% 38% 11% 8% 3	1962
	11th to 20th	Day	34% 38% 11% 12% 3	2090
	21st to end	Day	39% 38% 10% 11% 2	2255
April	1st to 10th	Day	33% 39% 8% 13% 6% 1	2148
	11th to 20th	Day	34% 37% 9% 12% 7%	2312
	21st to end	Day	36% 39% 10% 11% 4	2340
May	1st to 10th	Day	32% 38% 10% 13% 7% 1	2348
	11th to 20th	Day	26% 39% 10% 16% 8% 1	2504
	21st to end	Day	30% 40% 9% 13% 8% 1	2918
June	1st to 10th	Day	29% 40% 11% 15% 5%	2700
	11th to 20th	Day	25% 40% 12% 17% 7% 1	2700
	21st to end	Day	24% 39% 13% 17% 6% 1	2700
July	1st to 10th	Day	17% 41% 12% 22% 8%	2626
	11th to 20th	Day	21% 41% 12% 18% 9% 1	2619
	21st to end	Day	17% 41% 13% 20% 8% 1	2666
August	1st to 10th	Day	24% 40% 11% 18% 7% 1	2331
	11th to 20th	Day	21% 37% 13% 19% 9%	2301
	21st to end	Day	19% 38% 10% 21% 11% 1	2574
September	1st to 10th	Day	23% 37% 11% 19% 10% 1	2196
	11th to 20th	Day	24% 39% 11% 20% 6%	1980
	21st to end	Day	22% 39% 12% 22% 5%	1980
October	1st to 10th	Day	30% 38% 10% 15% 7%	1841
	11th to 20th	Day	33% 39% 9% 14% 4	1800
	21st to end	Day	34% 40% 11% 11% 4	1792
November	1st to 10th	Day	32% 43% 9% 9% 6% 1	1564
	11th to 20th	Day	39% 41% 8% 8% 3	1520
	21st to end	Day	32% 43% 10% 12% 3	1516
December	1st to 10th	Day	41% 42% 9% 7% 1	1330
	11th to 20th	Day	41% 37% 11% 8% 2	1680
	21st to end	Day	51% 28% 7% 12% 2	2442
January	1st to 10th	Night	47% 29% 12% 10% 2	4737
	11th to 20th	Night	52% 31% 8% 7% 2	4613
	21st to end	Night	48% 26% 9% 15% 3	4873
February	1st to 10th	Night	36% 36% 10% 12% 6%	2470
	11th to 20th	Night	32% 40% 9% 12% 8%	2404
	21st to end	Night	35% 36% 11% 13% 4 1	1956
March	1st to 10th	Night	37% 38% 12% 10% 3	2217
	11th to 20th	Night	32% 37% 9% 16% 6%	2090
	21st to end	Night	34% 37% 11% 12% 5%	2255
April	1st to 10th	Night	27% 37% 10% 16% 9% 1	1807
	11th to 20th	Night	34% 36% 8% 12% 8% 2	1599
	21st to end	Night	31% 38% 10% 14% 6% 1	1620
May	1st to 10th	Night	30% 38% 10% 13% 8% 1	1612
	11th to 20th	Night	25% 39% 10% 15% 9% 3	1392
	21st to end	Night	24% 39% 10% 17% 9% 1	1358
June	1st to 10th	Night	23% 39% 13% 17% 7% 1	1260
	11th to 20th	Night	21% 39% 11% 20% 7% 3	1260
	21st to end	Night	19% 38% 14% 21% 5% 1	1260
July	1st to 10th	Night	11% 39% 11% 27% 11% 1	1225
	11th to 20th	Night	14% 40% 14% 20% 11% 1	1218
	21st to end	Night	15% 39% 10% 21% 13% 2	1409
August	1st to 10th	Night	18% 38% 11% 20% 11% 2	1584
	11th to 20th	Night	16% 39% 13% 20% 11% 1	1600
	21st to end	Night	19% 37% 9% 18% 16% 1	1782
September	1st to 10th	Night	17% 38% 9% 22% 13% 1	1762
	11th to 20th	Night	19% 38% 11% 20% 11% 1	1980
	21st to end	Night	22% 37% 9% 21% 9% 1	1980
October	1st to 10th	Night	26% 37% 11% 16% 9%	2101
	11th to 20th	Night	33% 39% 10% 14% 5%	2160
	21st to end	Night	31% 39% 12% 14% 4	2561
November	1st to 10th	Night	30% 42% 8% 12% 6% 2	2514
	11th to 20th	Night	36% 41% 9% 9% 4	2660
	21st to end	Night	30% 42% 12% 12% 4	2660
December	1st to 10th	Night	38% 42% 11% 8% 1	2850
	11th to 20th	Night	42% 37% 12% 8% 1	3516
	21st to end	Night	47% 27% 11% 12% 3	5225

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	10% 10% 31% 35% 10% 4	270
	11th to 20th	Day	11% 8% 38% 31% 11% 1	270
	21st to end	Day	16% 7% 33% 29% 13% 2	297
February	1st to 10th	Day	16% 13% 32% 31% 6% 1	270
	11th to 20th	Day	11% 13% 40% 24% 10% 1	270
	21st to end	Day	7% 9% 41% 31% 12%	225
March	1st to 10th	Day	16% 13% 30% 30% 10% 1	270
	11th to 20th	Day	16% 10% 34% 26% 11% 3	270
	21st to end	Day	14% 11% 30% 33% 9% 3	285
April	1st to 10th	Day	19% 16% 32% 23% 8% 2	408
	11th to 20th	Day	24% 12% 27% 24% 10% 2	590
	21st to end	Day	22% 11% 28% 25% 12% 2	600
May	1st to 10th	Day	18% 10% 30% 29% 11% 2	603
	11th to 20th	Day	18% 10% 27% 27% 14% 4	625
	21st to end	Day	16% 9% 27% 29% 16% 3	679
June	1st to 10th	Day	12% 8% 28% 31% 15% 7%	630
	11th to 20th	Day	15% 8% 39% 22% 15% 2	630
	21st to end	Day	11% 9% 27% 28% 19% 6%	630
July	1st to 10th	Day	6% 4 27% 36% 20% 8%	621
	11th to 20th	Day	12% 5% 30% 29% 19% 5%	618
	21st to end	Day	4 3 24% 39% 22% 8%	684
August	1st to 10th	Day	5% 4 26% 34% 25% 6%	626
	11th to 20th	Day	11% 5% 24% 30% 20% 10%	615
	21st to end	Day	11% 6% 26% 31% 21% 5%	693
September	1st to 10th	Day	12% 6% 34% 28% 17% 2	486
	11th to 20th	Day	15% 10% 31% 29% 14% 2	270
	21st to end	Day	14% 14% 35% 26% 9% 1	270
October	1st to 10th	Day	21% 8% 30% 27% 10% 4	270
	11th to 20th	Day	14% 11% 36% 27% 10% 1	270
	21st to end	Day	20% 14% 35% 23% 8% 1	297
November	1st to 10th	Day	23% 10% 26% 30% 11%	270
	11th to 20th	Day	15% 13% 36% 27% 9%	270
	21st to end	Day	18% 14% 36% 26% 6%	270
December	1st to 10th	Day	12% 7% 37% 33% 9% 3	270
	11th to 20th	Day	16% 8% 39% 32% 5%	270
	21st to end	Day	16% 9% 32% 31% 10% 3	297
January	1st to 10th	Night	12% 9% 33% 33% 12% 1	950
	11th to 20th	Night	14% 7% 35% 34% 9% 1	950
	21st to end	Night	14% 8% 35% 33% 9%	1045
February	1st to 10th	Night	12% 9% 33% 34% 11% 1	950
	11th to 20th	Night	11% 8% 37% 31% 10% 3	929
	21st to end	Night	12% 8% 32% 34% 12% 2	700
March	1st to 10th	Night	15% 8% 36% 28% 11% 2	697
	11th to 20th	Night	12% 8% 38% 32% 9% 1	570
	21st to end	Night	17% 8% 30% 30% 13% 2	615
April	1st to 10th	Night	15% 8% 35% 31% 10%	540
	11th to 20th	Night	20% 9% 29% 30% 12%	533
	21st to end	Night	20% 7% 26% 34% 12% 1	540
May	1st to 10th	Night	19% 7% 29% 32% 11% 1	540
	11th to 20th	Night	16% 8% 30% 31% 14% 1	531
	21st to end	Night	15% 7% 33% 31% 12% 1	584
June	1st to 10th	Night	11% 6% 28% 40% 15% 1	540
	11th to 20th	Night	11% 5% 35% 36% 13%	540
	21st to end	Night	7% 4 28% 44% 17% 1	540
July	1st to 10th	Night	4 4 24% 45% 21% 1	525
	11th to 20th	Night	6% 2 30% 43% 18% 1	522
	21st to end	Night	6% 3 22% 47% 20% 2	591
August	1st to 10th	Night	6% 3 24% 47% 20% 1	537
	11th to 20th	Night	8% 3 24% 41% 22% 2	534
	21st to end	Night	9% 5% 26% 38% 22% 1	594
September	1st to 10th	Night	10% 4 30% 37% 17% 2	540
	11th to 20th	Night	11% 4 32% 37% 15% 1	540
	21st to end	Night	15% 5% 31% 37% 12%	540
October	1st to 10th	Night	19% 8% 32% 34% 7%	661
	11th to 20th	Night	19% 9% 35% 28% 8%	720
	21st to end	Night	21% 9% 32% 29% 8% 1	977
November	1st to 10th	Night	15% 10% 36% 28% 11%	925
	11th to 20th	Night	17% 10% 35% 28% 10% 1	950
	21st to end	Night	17% 12% 40% 25% 6% 1	950
December	1st to 10th	Night	14% 8% 34% 33% 10% 1	950
	11th to 20th	Night	17% 10% 33% 30% 8% 1	950
	21st to end	Night	22% 9% 33% 27% 9% 1	1045

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