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 2024-11-04.

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% 7% 38%	105461
	Night	10% 9% 3 9% 5% 5% 5% 4 5% 7% 37%	102203
Transparency	Day	44% 6% 15% 25% 9% 1	105188
	Night	45% 6% 14% 24% 10% 1	102065
Seeing	Day	14% 9% 30% 29% 14% 4	16160
	Night	14% 7% 32% 33% 12% 1	26056

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% 13% 7% 15% 7% 6% 6% 5% 6% 5% 22%	2218
	11th to 20th	Day	4 8% 5% 13% 13% 14% 13% 6% 6% 5% 13%	2354
	21st to end	Day	7% 15% 8% 18% 6% 5% 5% 3 5% 5% 22%	2794
February	1st to 10th	Day	8% 14% 6% 10% 7% 6% 8% 5% 6% 5% 24%	1710
	11th to 20th	Day	6% 12% 4 10% 9% 9% 7% 5% 8% 6% 24%	1664
	21st to end	Day	7% 11% 5% 11% 9% 8% 6% 5% 5% 7% 26%	1516
March	1st to 10th	Day	12% 12% 4 9% 7% 6% 5% 3 5% 7% 30%	3240
	11th to 20th	Day	8% 9% 4 13% 8% 4 6% 4 7% 10% 27%	3480
	21st to end	Day	15% 13% 5% 11% 5% 4 4 4 5% 6% 27%	3814
April	1st to 10th	Day	12% 12% 4 8% 6% 5% 4 4 6% 6% 34%	2268
	11th to 20th	Day	17% 11% 4 7% 5% 4 4 4 6% 6% 32%	2439
	21st to end	Day	20% 9% 3 8% 4 4 4 4 5% 6% 33%	2470
May	1st to 10th	Day	17% 8% 2 6% 5% 4 5% 4 6% 8% 35%	4142
	11th to 20th	Day	15% 8% 2 7% 4 5% 5% 4 6% 8% 36%	4446
	21st to end	Day	18% 8% 3 7% 5% 4 4 4 5% 8% 35%	5173
June	1st to 10th	Day	14% 6% 2 6% 5% 5% 4 4 7% 9% 40%	2850
	11th to 20th	Day	14% 5% 3 5% 5% 4 4 4 6% 9% 41%	2850
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 4 7% 8% 41%	2850
July	1st to 10th	Day	7% 3 1 5% 4 5% 6% 5% 7% 11% 46%	4750
	11th to 20th	Day	5% 3 1 5% 3 4 4 4 6% 9% 55%	4750
	21st to end	Day	5% 4 1 5% 4 4 5% 5% 7% 9% 51%	4939
August	1st to 10th	Day	9% 5% 2 5% 4 4 4 4 6% 8% 47%	4121
	11th to 20th	Day	13% 5% 2 5% 4 3 4 4 5% 9% 45%	4041
	21st to end	Day	11% 5% 2 5% 4 4 4 3 5% 7% 51%	4532
September	1st to 10th	Day	12% 5% 2 5% 4 3 3 3 5% 7% 51%	2318
	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	14% 13% 5% 9% 5% 4 4 3 5% 5% 34%	1591
	11th to 20th	Day	10% 15% 5% 14% 8% 5% 4 3 5% 4 28%	1520
	21st to end	Day	11% 13% 4 12% 7% 4 5% 4 4 5% 30%	1516
December	1st to 10th	Day	8% 14% 5% 13% 9% 7% 7% 4 5% 5% 23%	1330
	11th to 20th	Day	9% 15% 6% 12% 6% 6% 6% 4 7% 6% 22%	1683
	21st to end	Day	15% 14% 6% 12% 7% 8% 5% 3 4 4 22%	2442
January	1st to 10th	Night	5% 12% 6% 14% 8% 9% 6% 5% 6% 7% 23%	4737
	11th to 20th	Night	5% 11% 5% 15% 10% 11% 10% 6% 7% 5% 15%	4613
	21st to end	Night	8% 14% 7% 16% 6% 6% 5% 4 5% 5% 25%	4873
February	1st to 10th	Night	8% 12% 4 11% 7% 6% 6% 4 5% 5% 30%	2470
	11th to 20th	Night	7% 10% 4 10% 6% 7% 7% 5% 6% 7% 31%	2403
	21st to end	Night	7% 11% 4 10% 7% 5% 4 3 6% 8% 34%	1956
March	1st to 10th	Night	8% 10% 3 9% 8% 6% 5% 4 5% 5% 37%	3619
	11th to 20th	Night	7% 11% 3 9% 7% 5% 5% 4 5% 7% 37%	3480
	21st to end	Night	14% 13% 5% 12% 6% 4 4 3 5% 6% 29%	3817
April	1st to 10th	Night	11% 10% 2 6% 4 5% 4 4 6% 7% 41%	1907
	11th to 20th	Night	16% 12% 3 7% 4 4 4 3 5% 6% 36%	1689
	21st to end	Night	17% 9% 3 7% 5% 3 5% 3 4 7% 38%	1710
May	1st to 10th	Night	17% 7% 3 6% 5% 5% 4 4 5% 7% 37%	2828
	11th to 20th	Night	14% 6% 2 6% 4 4 4 4 6% 7% 42%	2455
	21st to end	Night	16% 7% 2 6% 3 3 3 3 5% 6% 45%	2414
June	1st to 10th	Night	10% 5% 2 5% 3 4 5% 4 6% 10% 46%	1330
	11th to 20th	Night	11% 5% 2 5% 4 4 4 4 7% 8% 46%	1330
	21st to end	Night	8% 3 2 6% 4 3 5% 4 6% 9% 49%	1330
July	1st to 10th	Night	8% 4 2 3 3 3 4 4 7% 10% 53%	2220
	11th to 20th	Night	4 3 1 4 3 3 3 3 5% 8% 64%	2220
	21st to end	Night	6% 3 2 5% 3 4 5% 4 8% 10% 48%	2622
August	1st to 10th	Night	6% 3 2 5% 5% 4 5% 4 6% 7% 54%	2806
	11th to 20th	Night	11% 4 1 4 4 4 4 4 6% 9% 48%	2798
	21st to end	Night	11% 5% 2 6% 3 3 3 3 4 7% 53%	3135
September	1st to 10th	Night	8% 4 2 4 4 3 4 3 5% 7% 57%	1860
	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 10% 5% 5% 5% 3 6% 7% 33%	2560
	11th to 20th	Night	10% 14% 6% 11% 6% 5% 5% 4 5% 6% 29%	2660
	21st to end	Night	11% 12% 4 9% 6% 5% 5% 3 5% 6% 33%	2660
December	1st to 10th	Night	9% 14% 4 14% 8% 6% 5% 4 6% 6% 24%	2850
	11th to 20th	Night	10% 14% 5% 14% 8% 6% 6% 4 5% 5% 23%	3514
	21st to end	Night	18% 13% 4 11% 7% 5% 4 3 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	58% 8% 12% 18% 3	2219
	11th to 20th	Day	68% 7% 11% 10% 3	2354
	21st to end	Day	61% 6% 9% 19% 5%	2794
February	1st to 10th	Day	59% 6% 12% 15% 8%	1710
	11th to 20th	Day	54% 7% 13% 18% 7% 1	1673
	21st to end	Day	55% 7% 14% 19% 5%	1516
March	1st to 10th	Day	51% 7% 14% 22% 6%	3240
	11th to 20th	Day	52% 7% 16% 20% 4	3480
	21st to end	Day	55% 8% 12% 20% 4	3814
April	1st to 10th	Day	49% 7% 12% 22% 9% 1	2268
	11th to 20th	Day	52% 7% 12% 18% 11% 1	2442
	21st to end	Day	51% 7% 13% 21% 6% 1	2470
May	1st to 10th	Day	43% 8% 14% 23% 10% 1	4142
	11th to 20th	Day	43% 8% 13% 25% 10% 1	4450
	21st to end	Day	46% 8% 13% 25% 8% 1	5173
June	1st to 10th	Day	38% 8% 18% 26% 9% 1	2850
	11th to 20th	Day	39% 8% 17% 25% 11% 1	2850
	21st to end	Day	39% 8% 17% 27% 9% 1	2850
July	1st to 10th	Day	30% 7% 24% 31% 8%	4676
	11th to 20th	Day	26% 5% 18% 39% 11% 1	4669
	21st to end	Day	29% 5% 21% 35% 9% 1	4783
August	1st to 10th	Day	35% 5% 15% 28% 15% 1	4107
	11th to 20th	Day	38% 4 15% 33% 10%	4080
	21st to end	Day	34% 4 19% 34% 9%	4531
September	1st to 10th	Day	33% 6% 15% 34% 12% 1	2318
	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	1591
	11th to 20th	Day	59% 6% 13% 15% 6% 1	1520
	21st to end	Day	56% 7% 14% 16% 6%	1516
December	1st to 10th	Day	61% 7% 13% 15% 4	1330
	11th to 20th	Day	59% 8% 14% 13% 5%	1680
	21st to end	Day	66% 5% 7% 18% 5%	2442
January	1st to 10th	Night	57% 8% 13% 17% 5%	4737
	11th to 20th	Night	65% 8% 10% 13% 4	4613
	21st to end	Night	60% 6% 10% 19% 5%	4873
February	1st to 10th	Night	54% 5% 14% 18% 8%	2470
	11th to 20th	Night	49% 8% 14% 18% 11%	2404
	21st to end	Night	49% 5% 13% 24% 8% 1	1956
March	1st to 10th	Night	45% 8% 14% 25% 9%	3619
	11th to 20th	Night	43% 8% 15% 23% 10%	3480
	21st to end	Night	54% 7% 13% 18% 7% 1	3817
April	1st to 10th	Night	41% 7% 13% 25% 13% 2	1907
	11th to 20th	Night	48% 5% 12% 19% 13% 3	1689
	21st to end	Night	47% 6% 12% 24% 10% 2	1710
May	1st to 10th	Night	43% 8% 13% 25% 10% 1	2828
	11th to 20th	Night	40% 7% 11% 26% 13% 3	2456
	21st to end	Night	38% 7% 12% 31% 12% 1	2414
June	1st to 10th	Night	31% 8% 18% 30% 11% 2	1330
	11th to 20th	Night	35% 7% 14% 28% 14% 3	1330
	21st to end	Night	31% 8% 19% 34% 8% 2	1330
July	1st to 10th	Night	27% 6% 25% 31% 10% 1	2185
	11th to 20th	Night	19% 5% 15% 48% 11% 1	2178
	21st to end	Night	30% 5% 23% 30% 11% 1	2539
August	1st to 10th	Night	29% 4 14% 35% 16% 1	2797
	11th to 20th	Night	32% 5% 19% 32% 12% 1	2825
	21st to end	Night	33% 4 17% 31% 15% 1	3135
September	1st to 10th	Night	27% 7% 14% 35% 17% 1	1860
	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	2560
	11th to 20th	Night	55% 6% 14% 16% 7% 1	2660
	21st to end	Night	51% 7% 16% 19% 7%	2660
December	1st to 10th	Night	58% 7% 15% 16% 4	2850
	11th to 20th	Night	61% 8% 15% 13% 3	3516
	21st to end	Night	61% 5% 11% 17% 6%	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% 29% 33% 10% 3	297
April	1st to 10th	Day	18% 15% 32% 23% 9% 2	448
	11th to 20th	Day	24% 13% 28% 24% 9% 2	640
	21st to end	Day	22% 11% 28% 24% 12% 2	650
May	1st to 10th	Day	18% 11% 29% 29% 11% 2	653
	11th to 20th	Day	17% 10% 28% 28% 14% 3	675
	21st to end	Day	16% 9% 28% 28% 16% 3	734
June	1st to 10th	Day	13% 9% 28% 29% 15% 6%	680
	11th to 20th	Day	15% 9% 39% 21% 14% 2	680
	21st to end	Day	11% 9% 29% 27% 19% 5%	680
July	1st to 10th	Day	6% 4 26% 34% 20% 10%	671
	11th to 20th	Day	11% 5% 29% 29% 20% 6%	668
	21st to end	Day	4 3 25% 37% 22% 8%	739
August	1st to 10th	Day	6% 4 27% 33% 25% 5%	676
	11th to 20th	Day	12% 5% 23% 29% 21% 10%	665
	21st to end	Day	11% 7% 27% 30% 20% 5%	748
September	1st to 10th	Day	12% 6% 34% 28% 18% 2	528
	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	23% 10% 26% 30% 11%	279
	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	16% 9% 30% 30% 12% 2	627
April	1st to 10th	Night	15% 8% 35% 31% 10%	570
	11th to 20th	Night	19% 10% 29% 29% 11%	563
	21st to end	Night	20% 6% 26% 35% 12% 1	570
May	1st to 10th	Night	19% 8% 29% 32% 10% 1	570
	11th to 20th	Night	16% 9% 30% 31% 13% 1	561
	21st to end	Night	15% 8% 33% 31% 12% 1	617
June	1st to 10th	Night	11% 6% 29% 39% 14% 1	570
	11th to 20th	Night	12% 5% 36% 35% 12%	570
	21st to end	Night	8% 3 28% 44% 16% 1	570
July	1st to 10th	Night	5% 4 24% 44% 21% 2	555
	11th to 20th	Night	6% 2 28% 44% 18% 1	552
	21st to end	Night	6% 3 23% 47% 20% 2	624
August	1st to 10th	Night	6% 2 25% 47% 20% 1	567
	11th to 20th	Night	9% 3 24% 41% 21% 2	564
	21st to end	Night	9% 5% 27% 38% 21%	627
September	1st to 10th	Night	9% 4 31% 36% 17% 2	570
	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%	943
	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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