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 2024-11-25.

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% 10% 3 8% 5% 5% 5% 4 5% 7% 38%	105397
	Night	13% 12% 3 7% 5% 4 4 4 5% 6% 36%	102673
Transparency	Day	22% 49% 7% 11% 8% 3	105165
	Night	25% 48% 7% 11% 6% 2	102551
Seeing	Day	17% 15% 29% 26% 12% 2	14874
	Night	19% 7% 24% 33% 16% 1	23912

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	15% 27% 7% 14% 7% 5% 4 2 3 3 13%	2539
	11th to 20th	Day	19% 13% 4 8% 5% 5% 5% 4 5% 6% 26%	2516
	21st to end	Day	26% 27% 5% 10% 5% 3 3 3 3 3 13%	2794
February	1st to 10th	Day	15% 15% 5% 9% 5% 5% 6% 5% 5% 5% 25%	1534
	11th to 20th	Day	13% 15% 5% 10% 5% 4 5% 4 7% 7% 27%	1705
	21st to end	Day	12% 14% 3 11% 7% 6% 6% 4 6% 6% 23%	1570
March	1st to 10th	Day	11% 16% 5% 10% 6% 7% 5% 5% 6% 6% 24%	3138
	11th to 20th	Day	8% 12% 2 7% 4 3 4 4 5% 7% 43%	3258
	21st to end	Day	13% 16% 5% 11% 7% 6% 7% 5% 5% 6% 19%	4165
April	1st to 10th	Day	14% 13% 4 9% 6% 6% 6% 4 6% 7% 25%	2280
	11th to 20th	Day	11% 11% 4 10% 7% 6% 6% 5% 6% 7% 28%	2298
	21st to end	Day	10% 12% 4 9% 6% 5% 5% 4 7% 8% 29%	2521
May	1st to 10th	Day	10% 8% 3 9% 6% 5% 6% 4 5% 9% 34%	4420
	11th to 20th	Day	9% 8% 3 7% 5% 5% 6% 4 7% 11% 35%	4377
	21st to end	Day	14% 11% 4 9% 7% 6% 6% 6% 6% 7% 25%	4876
June	1st to 10th	Day	18% 9% 3 8% 6% 5% 5% 4 6% 6% 30%	2850
	11th to 20th	Day	13% 8% 4 9% 8% 7% 6% 4 7% 6% 27%	2850
	21st to end	Day	8% 5% 2 7% 4 5% 5% 4 6% 7% 45%	2850
July	1st to 10th	Day	3 2 1 3 3 3 4 3 5% 5% 68%	4520
	11th to 20th	Day	2 3 1 3 3 3 2 2 4 4 73%	4420
	21st to end	Day	3 2 1 3 3 3 4 3 5% 7% 66%	4790
August	1st to 10th	Day	2 3 1 4 3 3 4 4 5% 8% 64%	4401
	11th to 20th	Day	3 3 2 5% 4 5% 5% 4 6% 8% 56%	4277
	21st to end	Day	6% 4 2 6% 4 4 5% 4 6% 7% 52%	4220
September	1st to 10th	Day	4 4 2 5% 4 3 3 3 5% 7% 59%	2280
	11th to 20th	Day	9% 6% 2 5% 5% 5% 4 4 6% 7% 46%	2218
	21st to end	Day	11% 6% 2 6% 4 3 4 4 5% 7% 47%	2090
October	1st to 10th	Day	7% 6% 3 6% 4 4 4 4 6% 8% 48%	3247
	11th to 20th	Day	11% 8% 2 8% 5% 6% 5% 4 5% 7% 38%	3011
	21st to end	Day	15% 10% 3 8% 5% 6% 6% 5% 6% 6% 30%	3101
November	1st to 10th	Day	22% 12% 4 7% 5% 4 4 4 6% 8% 23%	1754
	11th to 20th	Day	19% 14% 5% 9% 6% 5% 4 4 5% 5% 25%	1600
	21st to end	Day	19% 18% 4 9% 7% 5% 5% 3 5% 5% 19%	1408
December	1st to 10th	Day	16% 16% 5% 10% 6% 5% 3 4 5% 4 26%	1330
	11th to 20th	Day	18% 25% 5% 13% 6% 3 3 3 4 4 15%	1682
	21st to end	Day	16% 23% 5% 14% 6% 6% 4 3 4 4 14%	2507
January	1st to 10th	Night	21% 28% 7% 11% 5% 4 3 2 3 3 11%	4406
	11th to 20th	Night	17% 16% 5% 9% 4 4 3 4 4 7% 26%	4382
	21st to end	Night	27% 29% 5% 9% 4 3 3 2 3 4 11%	4862
February	1st to 10th	Night	15% 17% 5% 9% 6% 5% 4 4 6% 5% 25%	2646
	11th to 20th	Night	13% 15% 4 8% 6% 5% 4 4 5% 6% 31%	2361
	21st to end	Night	11% 14% 4 11% 6% 4 5% 3 6% 9% 27%	1884
March	1st to 10th	Night	9% 13% 4 8% 6% 6% 6% 5% 8% 8% 27%	3736
	11th to 20th	Night	9% 10% 2 5% 4 3 4 7% 7% 6% 43%	3718
	21st to end	Night	14% 15% 4 10% 6% 6% 7% 5% 5% 6% 22%	3488
April	1st to 10th	Night	12% 11% 3 6% 5% 5% 5% 4 7% 8% 34%	1900
	11th to 20th	Night	11% 9% 3 9% 5% 5% 6% 4 6% 6% 35%	1833
	21st to end	Night	7% 11% 3 7% 6% 5% 5% 5% 7% 8% 37%	1657
May	1st to 10th	Night	8% 7% 2 6% 4 5% 6% 5% 7% 9% 40%	2540
	11th to 20th	Night	6% 6% 2 6% 4 5% 5% 4 6% 8% 47%	2516
	21st to end	Night	13% 9% 4 8% 5% 6% 5% 5% 6% 7% 32%	2698
June	1st to 10th	Night	13% 6% 4 7% 5% 5% 5% 4 6% 8% 36%	1330
	11th to 20th	Night	11% 6% 3 8% 6% 6% 7% 6% 6% 8% 32%	1330
	21st to end	Night	8% 4 2 5% 5% 5% 6% 4 6% 9% 46%	1330
July	1st to 10th	Night	2 2 1 3 2 2 3 3 3 5% 74%	2435
	11th to 20th	Night	2 2 1 2 2 2 2 2 4 5% 75%	2540
	21st to end	Night	2 2 1 3 2 3 3 3 5% 6% 70%	2762
August	1st to 10th	Night	2 3 1 5% 3 3 4 4 4 6% 65%	2529
	11th to 20th	Night	2 2 1 4 3 3 5% 4 7% 9% 60%	2551
	21st to end	Night	6% 3 2 5% 4 5% 4 4 6% 8% 54%	3455
September	1st to 10th	Night	3 3 2 4 3 3 4 4 6% 6% 62%	1900
	11th to 20th	Night	8% 7% 2 5% 4 4 4 3 4 6% 52%	1962
	21st to end	Night	9% 6% 2 4 3 3 4 3 5% 8% 52%	2090
October	1st to 10th	Night	7% 4 2 5% 3 4 5% 4 6% 8% 52%	3728
	11th to 20th	Night	14% 8% 3 7% 4 4 5% 3 5% 7% 41%	3921
	21st to end	Night	16% 8% 3 8% 5% 6% 5% 4 5% 5% 35%	4457
November	1st to 10th	Night	22% 14% 4 7% 5% 4 5% 4 5% 7% 24%	2543
	11th to 20th	Night	19% 17% 4 7% 5% 4 5% 5% 6% 6% 23%	2800
	21st to end	Night	20% 19% 5% 9% 5% 4 3 3 4 5% 21%	2845
December	1st to 10th	Night	18% 16% 5% 9% 6% 4 4 3 6% 5% 23%	2850
	11th to 20th	Night	21% 24% 6% 15% 6% 3 4 3 3 3 13%	3510
	21st to end	Night	20% 22% 5% 11% 6% 5% 4 4 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	46% 41% 6% 6% 2	2539
	11th to 20th	Day	35% 39% 12% 10% 1 3	2540
	21st to end	Day	53% 41% 2 3 1	2794
February	1st to 10th	Day	22% 67% 3 5% 2 1	1534
	11th to 20th	Day	22% 65% 5% 5% 2	1714
	21st to end	Day	18% 66% 6% 6% 4	1570
March	1st to 10th	Day	35% 44% 5% 9% 8%	3138
	11th to 20th	Day	16% 41% 16% 23% 4	3258
	21st to end	Day	37% 48% 6% 5% 3	4165
April	1st to 10th	Day	18% 67% 7% 7% 2	2280
	11th to 20th	Day	17% 67% 4 10% 2	2298
	21st to end	Day	17% 66% 4 7% 5% 1	2521
May	1st to 10th	Day	22% 45% 8% 14% 10%	4420
	11th to 20th	Day	20% 44% 11% 18% 6%	4380
	21st to end	Day	32% 46% 9% 11% 3	4877
June	1st to 10th	Day	16% 66% 5% 8% 5% 1	2850
	11th to 20th	Day	16% 67% 5% 7% 4 1	2850
	21st to end	Day	14% 64% 5% 9% 7%	2850
July	1st to 10th	Day	8% 37% 4 9% 23% 19%	4451
	11th to 20th	Day	8% 36% 3 6% 22% 26%	4345
	21st to end	Day	11% 36% 6% 15% 19% 13%	4639
August	1st to 10th	Day	8% 38% 4 18% 24% 6%	4384
	11th to 20th	Day	14% 42% 10% 25% 7% 1	4319
	21st to end	Day	18% 41% 11% 17% 11% 2	4219
September	1st to 10th	Day	12% 61% 6% 12% 7% 2	2280
	11th to 20th	Day	16% 62% 5% 10% 6% 1	2218
	21st to end	Day	16% 61% 8% 9% 6% 1	2090
October	1st to 10th	Day	17% 39% 16% 23% 6%	3247
	11th to 20th	Day	28% 40% 7% 18% 6%	3011
	21st to end	Day	31% 43% 11% 11% 4	3101
November	1st to 10th	Day	25% 59% 8% 7% 1	1754
	11th to 20th	Day	26% 59% 6% 5% 3 1	1600
	21st to end	Day	25% 60% 6% 5% 3	1408
December	1st to 10th	Day	27% 60% 6% 5% 1	1330
	11th to 20th	Day	38% 52% 4 4 2	1684
	21st to end	Day	44% 43% 5% 5% 3	2507
January	1st to 10th	Night	50% 38% 6% 4 2	4406
	11th to 20th	Night	34% 38% 7% 13% 4 4	4420
	21st to end	Night	54% 41% 2 1 1	4862
February	1st to 10th	Night	22% 66% 4 4 3 1	2646
	11th to 20th	Night	22% 64% 4 7% 2 1	2363
	21st to end	Night	16% 66% 5% 7% 6%	1884
March	1st to 10th	Night	26% 46% 10% 11% 7%	3736
	11th to 20th	Night	15% 41% 17% 22% 5%	3719
	21st to end	Night	34% 48% 7% 9% 1	3488
April	1st to 10th	Night	15% 68% 6% 8% 3	1900
	11th to 20th	Night	16% 66% 5% 9% 4	1833
	21st to end	Night	14% 66% 5% 9% 7%	1657
May	1st to 10th	Night	19% 44% 10% 16% 11%	2540
	11th to 20th	Night	15% 44% 11% 21% 8% 2	2516
	21st to end	Night	25% 48% 9% 13% 5%	2698
June	1st to 10th	Night	12% 65% 5% 11% 5% 1	1330
	11th to 20th	Night	17% 66% 5% 7% 5% 1	1330
	21st to end	Night	11% 65% 5% 9% 8% 1	1330
July	1st to 10th	Night	6% 35% 2 11% 27% 19%	2395
	11th to 20th	Night	7% 36% 4 5% 17% 32%	2492
	21st to end	Night	11% 35% 5% 16% 21% 13%	2674
August	1st to 10th	Night	11% 38% 4 17% 23% 7%	2521
	11th to 20th	Night	12% 41% 12% 24% 10% 1	2575
	21st to end	Night	19% 40% 10% 18% 11% 2	3455
September	1st to 10th	Night	11% 61% 6% 11% 10% 1	1900
	11th to 20th	Night	15% 61% 5% 11% 7% 1	1962
	21st to end	Night	16% 60% 7% 12% 5%	2090
October	1st to 10th	Night	16% 39% 16% 21% 7%	3728
	11th to 20th	Night	28% 39% 8% 19% 6%	3921
	21st to end	Night	31% 39% 14% 13% 2	4457
November	1st to 10th	Night	26% 59% 8% 6% 1	2543
	11th to 20th	Night	25% 59% 7% 5% 3 1	2800
	21st to end	Night	25% 60% 5% 6% 4	2845
December	1st to 10th	Night	27% 60% 5% 7% 2	2850
	11th to 20th	Night	40% 53% 2 3 2	3507
	21st to end	Night	43% 42% 6% 5% 3	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	29% 11% 25% 18% 13% 3	180
	21st to end	Day	34% 9% 22% 28% 7% 1	198
February	1st to 10th	Day	20% 18% 33% 24% 6%	194
	11th to 20th	Day	23% 14% 32% 23% 7% 1	385
	21st to end	Day	18% 16% 32% 25% 8% 1	382
March	1st to 10th	Day	25% 20% 30% 16% 7% 2	460
	11th to 20th	Day	19% 20% 24% 25% 12%	460
	21st to end	Day	24% 19% 27% 19% 9% 1	506
April	1st to 10th	Day	26% 20% 29% 18% 6% 1	460
	11th to 20th	Day	20% 24% 25% 22% 7% 2	452
	21st to end	Day	18% 20% 33% 20% 8% 2	460
May	1st to 10th	Day	16% 21% 32% 21% 9% 1	463
	11th to 20th	Day	19% 12% 27% 27% 12% 2	488
	21st to end	Day	15% 13% 29% 25% 15% 3	566
June	1st to 10th	Day	21% 15% 28% 26% 9% 2	680
	11th to 20th	Day	14% 16% 32% 26% 11% 2	680
	21st to end	Day	11% 9% 28% 32% 16% 3	680
July	1st to 10th	Day	8% 11% 31% 34% 14% 2	547
	11th to 20th	Day	7% 17% 31% 30% 13% 3	484
	21st to end	Day	5% 11% 31% 33% 17% 3	531
August	1st to 10th	Day	5% 8% 33% 33% 17% 4	487
	11th to 20th	Day	6% 7% 26% 38% 18% 5%	477
	21st to end	Day	8% 16% 26% 31% 16% 4	539
September	1st to 10th	Day	10% 13% 26% 31% 16% 3	490
	11th to 20th	Day	14% 11% 31% 28% 14% 2	490
	21st to end	Day	16% 19% 29% 23% 12% 2	490
October	1st to 10th	Day	14% 14% 32% 26% 13% 2	490
	11th to 20th	Day	18% 18% 27% 24% 11% 2	404
	21st to end	Day	25% 12% 26% 28% 8% 2	330
November	1st to 10th	Day	26% 12% 25% 28% 9%	295
	11th to 20th	Day	29% 16% 24% 19% 11% 2	200
	21st to end	Day	27% 12% 27% 23% 10% 2	188
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% 24% 30% 9% 1	760
	21st to end	Night	24% 7% 25% 33% 10%	836
February	1st to 10th	Night	22% 8% 30% 29% 10% 1	760
	11th to 20th	Night	20% 7% 29% 33% 10%	744
	21st to end	Night	20% 7% 26% 31% 15% 1	628
March	1st to 10th	Night	21% 8% 24% 32% 14% 1	760
	11th to 20th	Night	18% 8% 27% 33% 13% 1	760
	21st to end	Night	18% 9% 29% 28% 14% 2	836
April	1st to 10th	Night	18% 8% 26% 32% 16% 1	760
	11th to 20th	Night	16% 8% 29% 34% 12% 2	705
	21st to end	Night	13% 13% 26% 33% 14% 1	517
May	1st to 10th	Night	11% 8% 23% 39% 18% 1	380
	11th to 20th	Night	12% 5% 23% 33% 24% 2	374
	21st to end	Night	16% 5% 19% 34% 23% 3	411
June	1st to 10th	Night	14% 8% 27% 34% 16% 2	380
	11th to 20th	Night	15% 6% 22% 38% 17% 1	380
	21st to end	Night	10% 7% 21% 34% 26% 1	380
July	1st to 10th	Night	6% 4 16% 43% 28% 3	370
	11th to 20th	Night	7% 4 23% 40% 24% 1	368
	21st to end	Night	2 3 18% 41% 33% 2	416
August	1st to 10th	Night	4 3 14% 37% 39% 4	378
	11th to 20th	Night	4 3 16% 36% 37% 4	404
	21st to end	Night	6% 5% 20% 38% 27% 3	806
September	1st to 10th	Night	5% 4 22% 39% 26% 4	760
	11th to 20th	Night	11% 6% 19% 29% 32% 3	760
	21st to end	Night	12% 7% 20% 41% 17% 3	760
October	1st to 10th	Night	15% 4 21% 39% 20% 1	760
	11th to 20th	Night	18% 8% 22% 34% 17% 2	760
	21st to end	Night	22% 9% 26% 31% 10% 2	836
November	1st to 10th	Night	30% 8% 24% 32% 6%	780
	11th to 20th	Night	29% 9% 25% 28% 10%	800
	21st to end	Night	28% 8% 23% 29% 10% 1	901
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

Page generated at 2024-11-22 05:02:23. Version ac07

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