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 2024-01-14.

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% 10% 4 7% 6% 4 5% 5% 5% 7% 35%	74570
	Night	14% 11% 4 7% 6% 3 5% 5% 5% 6% 34%	75205
Transparency	Day	47% 8% 13% 18% 11% 2	74272
	Night	48% 8% 13% 18% 10% 2	75021
Seeing	Day	17% 15% 29% 26% 11% 2	13694
	Night	19% 7% 24% 32% 16% 1	22887

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	21% 21% 7% 11% 8% 3 5% 4 3 4 13%	1519
	11th to 20th	Day	18% 16% 7% 10% 7% 3 4 4 4 5% 22%	1464
	21st to end	Day	16% 17% 5% 10% 6% 3 4 5% 5% 5% 23%	1584
February	1st to 10th	Day	15% 13% 5% 8% 6% 4 6% 7% 5% 5% 26%	1454
	11th to 20th	Day	13% 15% 5% 9% 6% 3 4 5% 6% 7% 27%	1613
	21st to end	Day	11% 13% 4 10% 8% 4 7% 6% 6% 6% 24%	1480
March	1st to 10th	Day	14% 13% 6% 9% 6% 5% 5% 6% 6% 7% 24%	1800
	11th to 20th	Day	15% 14% 4 8% 7% 4 5% 5% 5% 8% 27%	1839
	21st to end	Day	15% 14% 6% 9% 8% 4 6% 5% 6% 6% 21%	2376
April	1st to 10th	Day	14% 12% 5% 8% 8% 4 7% 5% 6% 7% 24%	2160
	11th to 20th	Day	12% 10% 5% 10% 8% 4 6% 6% 6% 7% 26%	2175
	21st to end	Day	11% 11% 4 7% 7% 4 5% 5% 7% 8% 30%	2388
May	1st to 10th	Day	11% 7% 3 8% 6% 4 6% 6% 5% 7% 35%	2520
	11th to 20th	Day	12% 9% 4 7% 7% 4 6% 5% 7% 9% 30%	2477
	21st to end	Day	13% 8% 4 9% 8% 4 6% 6% 6% 8% 28%	2758
June	1st to 10th	Day	18% 8% 4 7% 7% 4 5% 5% 6% 6% 30%	2700
	11th to 20th	Day	14% 7% 4 8% 9% 5% 7% 6% 7% 6% 27%	2700
	21st to end	Day	8% 5% 3 6% 5% 4 6% 5% 5% 8% 45%	2700
July	1st to 10th	Day	6% 3 2 5% 5% 3 5% 5% 6% 8% 52%	2578
	11th to 20th	Day	3 4 2 4 5% 3 4 5% 5% 6% 60%	2520
	21st to end	Day	3 2 1 3 3 2 3 4 4 6% 66%	2741
August	1st to 10th	Day	3 3 1 5% 4 4 4 5% 5% 7% 59%	2507
	11th to 20th	Day	4 3 1 3 3 2 4 4 5% 8% 64%	2448
	21st to end	Day	5% 4 2 5% 5% 3 5% 5% 6% 8% 52%	2403
September	1st to 10th	Day	5% 4 2 5% 4 2 4 4 5% 7% 58%	2160
	11th to 20th	Day	10% 5% 3 5% 5% 3 5% 5% 6% 7% 46%	2102
	21st to end	Day	12% 7% 2 6% 5% 3 4 4 5% 7% 46%	1980
October	1st to 10th	Day	11% 7% 4 6% 6% 4 4 5% 6% 8% 39%	1841
	11th to 20th	Day	13% 8% 3 7% 6% 4 6% 5% 6% 6% 37%	1719
	21st to end	Day	19% 11% 3 6% 5% 3 4 5% 5% 7% 31%	1782
November	1st to 10th	Day	22% 10% 4 7% 6% 3 5% 5% 6% 9% 23%	1665
	11th to 20th	Day	19% 12% 5% 8% 7% 3 5% 5% 5% 5% 26%	1520
	21st to end	Day	19% 16% 6% 9% 7% 4 6% 5% 5% 5% 20%	1378
December	1st to 10th	Day	16% 14% 5% 10% 7% 3 4 4 5% 4 26%	1330
	11th to 20th	Day	18% 24% 6% 11% 7% 3 3 4 4 5% 15%	1682
	21st to end	Day	17% 22% 6% 13% 7% 5% 5% 4 4 4 14%	2507
January	1st to 10th	Night	27% 19% 6% 9% 6% 4 4 4 4 4 13%	2646
	11th to 20th	Night	20% 17% 6% 11% 7% 3 4 4 4 6% 18%	2574
	21st to end	Night	15% 19% 7% 11% 7% 4 5% 4 5% 6% 18%	2772
February	1st to 10th	Night	15% 15% 5% 7% 7% 4 5% 4 6% 6% 27%	2506
	11th to 20th	Night	14% 14% 4 8% 6% 4 4 4 5% 6% 31%	2233
	21st to end	Night	10% 13% 5% 10% 7% 4 5% 4 6% 9% 28%	1776
March	1st to 10th	Night	15% 12% 5% 7% 6% 4 6% 5% 6% 7% 27%	2160
	11th to 20th	Night	14% 11% 4 7% 6% 4 5% 4 6% 7% 33%	2117
	21st to end	Night	12% 11% 4 7% 7% 4 7% 6% 5% 7% 29%	1980
April	1st to 10th	Night	12% 10% 3 4 6% 3 6% 5% 7% 9% 35%	1800
	11th to 20th	Night	12% 8% 3 8% 7% 4 5% 6% 6% 6% 34%	1736
	21st to end	Night	7% 11% 3 6% 6% 3 5% 6% 7% 8% 37%	1570
May	1st to 10th	Night	9% 5% 2 6% 5% 3 7% 6% 7% 8% 41%	1440
	11th to 20th	Night	10% 7% 2 6% 6% 4 5% 7% 6% 9% 39%	1416
	21st to end	Night	12% 8% 4 7% 6% 4 6% 6% 6% 7% 34%	1516
June	1st to 10th	Night	14% 6% 5% 6% 6% 4 5% 5% 6% 9% 36%	1260
	11th to 20th	Night	11% 6% 3 7% 7% 4 7% 7% 7% 9% 33%	1260
	21st to end	Night	7% 3 3 5% 5% 4 5% 6% 7% 9% 46%	1260
July	1st to 10th	Night	4 3 1 5% 4 2 5% 6% 5% 8% 57%	1377
	11th to 20th	Night	3 2 1 3 4 2 4 4 5% 7% 64%	1440
	21st to end	Night	2 1 2 2 2 4 4 5% 6% 73%	1576
August	1st to 10th	Night	2 3 1 4 4 3 3 4 5% 5% 65%	1432
	11th to 20th	Night	3 2 1 2 3 2 4 4 5% 8% 67%	1450
	21st to end	Night	4 3 1 3 3 3 4 4 6% 9% 61%	1951
September	1st to 10th	Night	4 3 2 3 4 3 4 5% 5% 7% 61%	1800
	11th to 20th	Night	9% 7% 3 5% 4 3 4 4 4 7% 52%	1858
	21st to end	Night	10% 6% 2 4 4 3 4 4 5% 8% 51%	1980
October	1st to 10th	Night	11% 6% 3 6% 4 2 5% 5% 6% 8% 46%	2114
	11th to 20th	Night	13% 8% 3 6% 6% 3 5% 5% 6% 7% 40%	2240
	21st to end	Night	18% 8% 3 7% 5% 3 4 5% 5% 6% 36%	2574
November	1st to 10th	Night	23% 13% 4 6% 5% 3 5% 5% 5% 7% 24%	2412
	11th to 20th	Night	19% 15% 4 7% 5% 3 5% 5% 6% 6% 24%	2660
	21st to end	Night	20% 16% 8% 8% 6% 3 4 4 4 5% 22%	2781
December	1st to 10th	Night	19% 14% 6% 8% 6% 3 5% 4 6% 5% 24%	2850
	11th to 20th	Night	21% 23% 7% 14% 6% 3 4 4 3 3 13%	3510
	21st to end	Night	21% 21% 6% 9% 7% 4 5% 5% 5% 4 14%	5178

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	74% 8% 8% 6% 4	1519
	11th to 20th	Day	63% 8% 14% 12% 2 1	1464
	21st to end	Day	60% 8% 12% 15% 4 1	1584
February	1st to 10th	Day	58% 9% 15% 11% 5% 2	1454
	11th to 20th	Day	55% 10% 16% 13% 6%	1622
	21st to end	Day	59% 9% 12% 13% 7% 1	1480
March	1st to 10th	Day	59% 10% 14% 12% 6%	1800
	11th to 20th	Day	55% 10% 14% 15% 6%	1839
	21st to end	Day	63% 9% 15% 10% 3	2376
April	1st to 10th	Day	57% 8% 14% 15% 5% 1	2160
	11th to 20th	Day	55% 9% 14% 15% 7%	2175
	21st to end	Day	51% 8% 13% 19% 7% 2	2388
May	1st to 10th	Day	46% 7% 16% 20% 9% 1	2520
	11th to 20th	Day	48% 10% 16% 20% 6% 1	2480
	21st to end	Day	53% 10% 14% 16% 7%	2759
June	1st to 10th	Day	53% 8% 11% 15% 10% 4	2700
	11th to 20th	Day	53% 11% 13% 16% 7% 1	2700
	21st to end	Day	36% 8% 14% 22% 17% 3	2700
July	1st to 10th	Day	29% 8% 12% 23% 20% 8%	2509
	11th to 20th	Day	25% 6% 10% 21% 28% 10%	2445
	21st to end	Day	18% 6% 8% 23% 32% 12%	2589
August	1st to 10th	Day	23% 7% 13% 22% 25% 9%	2490
	11th to 20th	Day	20% 5% 11% 30% 27% 6%	2448
	21st to end	Day	28% 8% 14% 29% 18% 2	2403
September	1st to 10th	Day	25% 6% 13% 30% 20% 5%	2160
	11th to 20th	Day	34% 9% 12% 24% 18% 3	2102
	21st to end	Day	37% 8% 17% 22% 13% 2	1980
October	1st to 10th	Day	40% 8% 16% 28% 8%	1841
	11th to 20th	Day	45% 9% 15% 22% 9%	1719
	21st to end	Day	52% 9% 15% 18% 6%	1782
November	1st to 10th	Day	57% 11% 14% 13% 5%	1665
	11th to 20th	Day	59% 9% 13% 14% 4 2	1520
	21st to end	Day	63% 9% 14% 10% 4 1	1378
December	1st to 10th	Day	59% 7% 14% 16% 4	1330
	11th to 20th	Day	68% 10% 9% 9% 4	1684
	21st to end	Day	67% 10% 9% 9% 5%	2507
January	1st to 10th	Night	73% 6% 9% 7% 4	2646
	11th to 20th	Night	67% 5% 12% 11% 3 1	2574
	21st to end	Night	64% 9% 12% 10% 4	2772
February	1st to 10th	Night	56% 8% 15% 14% 6% 2	2506
	11th to 20th	Night	53% 7% 16% 19% 5%	2235
	21st to end	Night	52% 7% 18% 14% 9%	1776
March	1st to 10th	Night	54% 9% 16% 14% 6% 1	2160
	11th to 20th	Night	50% 7% 14% 21% 8%	2118
	21st to end	Night	53% 9% 15% 16% 6%	1980
April	1st to 10th	Night	44% 9% 17% 21% 8% 1	1800
	11th to 20th	Night	46% 9% 16% 20% 8% 1	1736
	21st to end	Night	43% 8% 15% 23% 11% 1	1570
May	1st to 10th	Night	37% 10% 15% 24% 13% 1	1440
	11th to 20th	Night	38% 9% 20% 21% 10% 1	1416
	21st to end	Night	48% 9% 16% 20% 7%	1516
June	1st to 10th	Night	44% 7% 13% 20% 14% 2	1260
	11th to 20th	Night	42% 11% 16% 20% 10% 1	1260
	21st to end	Night	31% 8% 16% 21% 20% 3	1260
July	1st to 10th	Night	23% 6% 12% 24% 24% 9%	1337
	11th to 20th	Night	21% 5% 11% 22% 26% 14%	1392
	21st to end	Night	12% 5% 7% 25% 37% 15%	1488
August	1st to 10th	Night	20% 7% 11% 23% 28% 11%	1424
	11th to 20th	Night	15% 6% 10% 32% 31% 5%	1450
	21st to end	Night	19% 6% 14% 32% 24% 5%	1951
September	1st to 10th	Night	22% 7% 11% 32% 24% 6%	1800
	11th to 20th	Night	32% 6% 11% 24% 23% 4	1858
	21st to end	Night	31% 7% 18% 27% 13% 3	1980
October	1st to 10th	Night	35% 7% 18% 29% 10%	2114
	11th to 20th	Night	42% 7% 17% 23% 11% 1	2240
	21st to end	Night	48% 8% 15% 22% 6%	2574
November	1st to 10th	Night	58% 8% 14% 12% 6%	2412
	11th to 20th	Night	58% 8% 15% 12% 5% 1	2660
	21st to end	Night	63% 7% 13% 12% 5%	2781
December	1st to 10th	Night	60% 7% 13% 16% 4	2850
	11th to 20th	Night	73% 10% 7% 7% 3	3507
	21st to end	Night	67% 9% 9% 10% 5%	5178

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	28% 13% 24% 21% 11% 3	180
	11th to 20th	Day	30% 11% 25% 17% 14% 3	166
	21st to end	Day	33% 8% 22% 30% 7% 1	176
February	1st to 10th	Day	20% 17% 35% 23% 6%	174
	11th to 20th	Day	23% 14% 33% 23% 6% 1	353
	21st to end	Day	16% 15% 34% 26% 8% 1	346
March	1st to 10th	Day	25% 22% 30% 15% 7% 2	420
	11th to 20th	Day	20% 20% 24% 25% 10%	420
	21st to end	Day	24% 19% 28% 18% 9% 2	462
April	1st to 10th	Day	27% 18% 30% 18% 7%	420
	11th to 20th	Day	21% 24% 25% 20% 7% 3	412
	21st to end	Day	18% 20% 32% 19% 9% 2	420
May	1st to 10th	Day	16% 20% 32% 22% 8% 1	423
	11th to 20th	Day	20% 12% 27% 27% 13% 2	448
	21st to end	Day	16% 13% 27% 26% 16% 3	520
June	1st to 10th	Day	20% 15% 27% 26% 9% 2	630
	11th to 20th	Day	14% 16% 32% 26% 10% 2	630
	21st to end	Day	12% 9% 28% 31% 17% 3	630
July	1st to 10th	Day	9% 11% 31% 34% 14% 1	504
	11th to 20th	Day	7% 18% 32% 29% 11% 2	444
	21st to end	Day	4 11% 32% 33% 16% 3	487
August	1st to 10th	Day	5% 8% 34% 34% 16% 3	447
	11th to 20th	Day	6% 7% 26% 38% 18% 4	437
	21st to end	Day	8% 16% 27% 32% 15% 3	495
September	1st to 10th	Day	11% 14% 28% 31% 14% 1	450
	11th to 20th	Day	15% 11% 30% 27% 15% 2	450
	21st to end	Day	16% 20% 28% 23% 12% 2	450
October	1st to 10th	Day	15% 14% 32% 25% 12% 1	450
	11th to 20th	Day	19% 19% 27% 24% 10% 1	369
	21st to end	Day	27% 12% 25% 26% 7% 2	297
November	1st to 10th	Day	26% 12% 25% 29% 8%	266
	11th to 20th	Day	27% 16% 24% 21% 11% 2	180
	21st to end	Day	26% 12% 26% 24% 10% 2	180
December	1st to 10th	Day	27% 8% 24% 19% 16% 6%	180
	11th to 20th	Day	23% 14% 29% 26% 6% 1	180
	21st to end	Day	25% 13% 22% 31% 9%	198
January	1st to 10th	Night	34% 11% 25% 23% 7% 1	760
	11th to 20th	Night	30% 6% 23% 30% 10% 1	736
	21st to end	Night	23% 6% 26% 34% 10%	792
February	1st to 10th	Night	23% 7% 30% 30% 10% 1	720
	11th to 20th	Night	21% 6% 29% 34% 10%	704
	21st to end	Night	19% 6% 27% 32% 15% 2	592
March	1st to 10th	Night	22% 8% 24% 31% 14% 1	720
	11th to 20th	Night	18% 9% 28% 31% 13% 1	720
	21st to end	Night	19% 9% 29% 28% 14% 2	792
April	1st to 10th	Night	18% 7% 26% 32% 17% 1	720
	11th to 20th	Night	16% 8% 28% 35% 12% 2	668
	21st to end	Night	13% 13% 26% 33% 14% 1	490
May	1st to 10th	Night	11% 8% 23% 39% 19% 1	360
	11th to 20th	Night	13% 5% 23% 33% 24% 3	354
	21st to end	Night	16% 5% 19% 34% 24% 3	389
June	1st to 10th	Night	15% 7% 27% 34% 16% 2	360
	11th to 20th	Night	16% 6% 23% 37% 17% 1	360
	21st to end	Night	10% 7% 21% 34% 27% 1	360
July	1st to 10th	Night	6% 4 15% 43% 28% 3	350
	11th to 20th	Night	8% 4 24% 39% 24% 1	348
	21st to end	Night	2 3 17% 40% 34% 3	394
August	1st to 10th	Night	4 3 14% 36% 39% 4	358
	11th to 20th	Night	4 3 15% 37% 37% 4	382
	21st to end	Night	6% 5% 20% 38% 28% 3	763
September	1st to 10th	Night	6% 5% 23% 40% 23% 3	720
	11th to 20th	Night	11% 6% 19% 28% 32% 3	720
	21st to end	Night	13% 7% 20% 41% 17% 3	720
October	1st to 10th	Night	16% 4 21% 38% 20% 1	720
	11th to 20th	Night	18% 8% 22% 34% 17% 2	720
	21st to end	Night	22% 9% 26% 30% 10% 2	792
November	1st to 10th	Night	31% 8% 24% 31% 6%	740
	11th to 20th	Night	28% 8% 25% 28% 10%	760
	21st to end	Night	27% 8% 23% 30% 10% 1	881
December	1st to 10th	Night	26% 6% 28% 29% 11% 1	950
	11th to 20th	Night	31% 7% 27% 28% 6%	950
	21st to end	Night	31% 8% 30% 26% 5%	1022

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