UND Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the UND Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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% 8% 3 8% 5% 5% 5% 4 6% 8% 38%	140037
	Night	9% 9% 4 9% 6% 5% 5% 4 6% 7% 37%	139159
Transparency	Day	45% 5% 15% 26% 10% 1	134663
	Night	47% 5% 14% 24% 10% 1	135541
Seeing	Day	14% 10% 31% 28% 14% 3	18007
	Night	14% 8% 33% 33% 11% 1	28113

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	5% 10% 5% 17% 10% 9% 8% 5% 6% 7% 17%	4123
	11th to 20th	Day	4 7% 3 12% 10% 11% 14% 9% 7% 6% 16%	4458
	21st to end	Day	4 9% 6% 14% 7% 7% 5% 5% 8% 8% 28%	5188
February	1st to 10th	Day	8% 14% 5% 10% 7% 6% 8% 5% 6% 5% 25%	1870
	11th to 20th	Day	6% 12% 4 11% 8% 7% 8% 6% 7% 7% 25%	1844
	21st to end	Day	6% 10% 4 13% 9% 8% 6% 5% 5% 7% 27%	1688
March	1st to 10th	Day	11% 10% 3 7% 6% 5% 5% 4 7% 9% 34%	4729
	11th to 20th	Day	8% 9% 5% 14% 9% 5% 5% 5% 7% 8% 25%	5090
	21st to end	Day	15% 14% 5% 11% 5% 4 4 3 5% 7% 27%	5585
April	1st to 10th	Day	11% 12% 3 9% 6% 5% 5% 5% 5% 6% 34%	2404
	11th to 20th	Day	17% 11% 4 8% 4 4 5% 4 5% 7% 32%	2569
	21st to end	Day	20% 9% 3 8% 5% 4 4 4 6% 6% 31%	2600
May	1st to 10th	Day	13% 6% 2 4 4 4 4 4 5% 7% 47%	5900
	11th to 20th	Day	16% 9% 3 6% 4 4 5% 4 6% 8% 37%	6413
	21st to end	Day	14% 7% 2 6% 4 5% 4 4 5% 7% 43%	7427
June	1st to 10th	Day	14% 6% 2 6% 5% 4 5% 4 7% 9% 40%	3000
	11th to 20th	Day	14% 5% 2 6% 5% 4 4 4 6% 8% 41%	3000
	21st to end	Day	11% 5% 3 7% 5% 5% 5% 5% 7% 9% 40%	3000
July	1st to 10th	Day	6% 4 2 5% 4 5% 5% 4 7% 11% 48%	6799
	11th to 20th	Day	8% 5% 2 5% 4 5% 4 4 6% 8% 49%	6800
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 7% 10% 46%	7049
August	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 6% 8% 45%	5871
	11th to 20th	Day	13% 5% 2 5% 4 4 4 3 6% 9% 44%	5821
	21st to end	Day	7% 4 2 5% 3 3 4 3 5% 8% 55%	6490
September	1st to 10th	Day	11% 5% 2 4 4 3 4 3 5% 7% 50%	2432
	11th to 20th	Day	13% 6% 2 7% 4 4 5% 3 5% 7% 44%	2200
	21st to end	Day	11% 6% 2 5% 4 3 4 4 6% 9% 47%	2200
October	1st to 10th	Day	11% 6% 2 4 3 3 4 3 4 6% 55%	3341
	11th to 20th	Day	14% 8% 2 6% 4 3 4 3 4 5% 46%	3224
	21st to end	Day	11% 10% 3 9% 5% 4 5% 3 5% 8% 37%	3190
November	1st to 10th	Day	13% 13% 5% 11% 4 4 4 3 4 5% 34%	1749
	11th to 20th	Day	10% 15% 5% 11% 6% 6% 5% 3 5% 6% 29%	1680
	21st to end	Day	10% 13% 5% 13% 8% 4 5% 3 3 5% 31%	1655
December	1st to 10th	Day	9% 11% 6% 15% 12% 8% 6% 4 5% 6% 17%	2360
	11th to 20th	Day	9% 15% 6% 12% 6% 5% 4 5% 6% 7% 24%	2713
	21st to end	Day	11% 15% 6% 17% 7% 7% 5% 3 4 4 21%	3575
January	1st to 10th	Night	5% 10% 6% 16% 9% 8% 7% 5% 7% 7% 20%	8799
	11th to 20th	Night	4 11% 5% 12% 9% 9% 11% 7% 7% 6% 20%	8514
	21st to end	Night	4 9% 6% 13% 6% 6% 6% 4 7% 9% 31%	9026
February	1st to 10th	Night	8% 12% 4 13% 6% 6% 5% 4 5% 6% 29%	2748
	11th to 20th	Night	7% 10% 3 9% 7% 7% 7% 5% 6% 8% 32%	2663
	21st to end	Night	6% 11% 4 10% 7% 6% 5% 3 5% 8% 35%	2138
March	1st to 10th	Night	7% 9% 3 8% 6% 5% 5% 4 5% 7% 42%	5358
	11th to 20th	Night	8% 10% 3 10% 8% 6% 5% 4 6% 7% 33%	5090
	21st to end	Night	13% 14% 5% 10% 5% 4 4 3 5% 6% 30%	5588
April	1st to 10th	Night	11% 10% 3 7% 4 4 4 4 6% 6% 41%	2015
	11th to 20th	Night	15% 11% 4 7% 4 5% 3 3 5% 6% 36%	1779
	21st to end	Night	17% 8% 3 7% 5% 4 4 3 5% 7% 38%	1800
May	1st to 10th	Night	13% 7% 3 5% 4 3 4 4 4 6% 47%	4080
	11th to 20th	Night	15% 6% 2 6% 4 4 5% 3 6% 7% 41%	3484
	21st to end	Night	12% 7% 2 4 3 3 4 4 6% 7% 48%	3470
June	1st to 10th	Night	10% 5% 1 5% 3 4 4 4 6% 9% 48%	1400
	11th to 20th	Night	10% 6% 2 5% 4 3 5% 4 7% 9% 46%	1400
	21st to end	Night	8% 4 2 5% 4 4 6% 4 6% 9% 49%	1400
July	1st to 10th	Night	7% 4 1 4 3 3 4 4 7% 8% 54%	3180
	11th to 20th	Night	5% 4 2 4 3 4 4 4 6% 9% 57%	3180
	21st to end	Night	7% 4 2 6% 4 4 5% 4 8% 10% 45%	3823
August	1st to 10th	Night	10% 4 2 6% 3 4 4 4 6% 7% 50%	3939
	11th to 20th	Night	11% 5% 2 4 4 4 4 4 6% 10% 48%	4028
	21st to end	Night	8% 5% 1 4 3 3 3 2 5% 7% 58%	4487
September	1st to 10th	Night	8% 4 2 4 3 3 4 3 5% 7% 57%	1963
	11th to 20th	Night	10% 5% 2 5% 3 3 4 3 5% 8% 52%	2200
	21st to end	Night	12% 5% 2 5% 4 3 4 3 4 6% 52%	2200
October	1st to 10th	Night	10% 4 1 6% 4 3 4 3 5% 7% 53%	3805
	11th to 20th	Night	10% 8% 2 6% 4 4 4 4 5% 7% 44%	3915
	21st to end	Night	14% 9% 3 8% 4 4 4 3 5% 7% 40%	4610
November	1st to 10th	Night	11% 10% 4 11% 6% 5% 4 4 5% 7% 33%	2767
	11th to 20th	Night	9% 14% 6% 11% 6% 5% 4 4 4 6% 31%	2940
	21st to end	Night	11% 12% 4 11% 6% 5% 4 4 5% 5% 33%	2961
December	1st to 10th	Night	6% 12% 4 17% 11% 7% 6% 4 6% 6% 20%	5050
	11th to 20th	Night	8% 14% 5% 14% 9% 6% 6% 5% 5% 6% 23%	5714
	21st to end	Night	13% 13% 5% 16% 8% 6% 5% 3 5% 5% 22%	7645

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	64% 5% 11% 17% 4	4125
	11th to 20th	Day	65% 4 11% 15% 5%	4458
	21st to end	Day	52% 4 13% 23% 8%	5188
February	1st to 10th	Day	59% 4 13% 15% 9%	1870
	11th to 20th	Day	54% 6% 12% 20% 8% 1	1853
	21st to end	Day	56% 6% 14% 19% 5%	1688
March	1st to 10th	Day	47% 4 18% 22% 9%	4729
	11th to 20th	Day	55% 4 15% 21% 5%	5090
	21st to end	Day	57% 5% 13% 21% 4	5585
April	1st to 10th	Day	50% 6% 12% 22% 10% 1	2404
	11th to 20th	Day	52% 5% 12% 19% 11% 1	2572
	21st to end	Day	52% 5% 13% 22% 6% 1	2600
May	1st to 10th	Day	36% 5% 13% 30% 16% 1	5900
	11th to 20th	Day	45% 4 11% 23% 16% 1	6417
	21st to end	Day	39% 4 10% 29% 16% 2	7428
June	1st to 10th	Day	39% 5% 18% 27% 9% 1	3000
	11th to 20th	Day	39% 6% 19% 26% 10% 1	3000
	21st to end	Day	39% 6% 17% 28% 9% 1	3000
July	1st to 10th	Day	30% 6% 25% 32% 7%	6726
	11th to 20th	Day	32% 5% 18% 36% 9%	6719
	21st to end	Day	33% 5% 22% 30% 10%	6847
August	1st to 10th	Day	33% 5% 16% 28% 15% 1	4264
	11th to 20th	Day	36% 5% 16% 33% 9%	4210
	21st to end	Day	33% 4 19% 35% 10%	4674
September	1st to 10th	Day	34% 4 16% 33% 12% 1	2432
	11th to 20th	Day	40% 5% 19% 27% 8% 1	2200
	21st to end	Day	34% 6% 16% 34% 10% 1	2200
October	1st to 10th	Day	32% 3 12% 35% 17% 1	3341
	11th to 20th	Day	40% 4 10% 30% 16%	3224
	21st to end	Day	47% 4 14% 26% 9%	3190
November	1st to 10th	Day	53% 4 14% 19% 9% 1	1749
	11th to 20th	Day	57% 4 14% 17% 6% 1	1680
	21st to end	Day	57% 5% 14% 17% 6%	1655
December	1st to 10th	Day	68% 4 13% 12% 3	2360
	11th to 20th	Day	59% 4 15% 14% 7%	2710
	21st to end	Day	68% 4 6% 18% 4	3575
January	1st to 10th	Night	61% 4 13% 17% 4	8800
	11th to 20th	Night	61% 5% 11% 19% 4	8514
	21st to end	Night	50% 4 14% 24% 9%	9026
February	1st to 10th	Night	54% 4 14% 19% 9%	2748
	11th to 20th	Night	48% 6% 12% 22% 12%	2664
	21st to end	Night	49% 3 13% 26% 8% 1	2138
March	1st to 10th	Night	41% 5% 17% 26% 10% 2	5358
	11th to 20th	Night	50% 5% 12% 23% 10% 1	5090
	21st to end	Night	55% 5% 13% 20% 7%	5588
April	1st to 10th	Night	42% 5% 12% 26% 13% 2	2015
	11th to 20th	Night	49% 4 12% 20% 13% 3	1779
	21st to end	Night	47% 3 13% 24% 10% 2	1800
May	1st to 10th	Night	37% 4 12% 29% 18% 1	4080
	11th to 20th	Night	42% 4 10% 24% 18% 3	3485
	21st to end	Night	35% 4 10% 29% 21% 1	3470
June	1st to 10th	Night	32% 6% 17% 33% 11% 2	1400
	11th to 20th	Night	34% 5% 17% 27% 14% 3	1400
	21st to end	Night	31% 7% 19% 34% 8% 1	1400
July	1st to 10th	Night	27% 6% 23% 35% 8% 1	3145
	11th to 20th	Night	24% 4 19% 41% 10% 1	3138
	21st to end	Night	33% 6% 25% 25% 10% 1	3712
August	1st to 10th	Night	28% 4 15% 35% 17% 2	2870
	11th to 20th	Night	31% 5% 19% 31% 12% 1	2915
	21st to end	Night	32% 4 16% 32% 16% 1	3234
September	1st to 10th	Night	27% 5% 14% 37% 16% 1	1963
	11th to 20th	Night	30% 5% 19% 31% 13% 1	2200
	21st to end	Night	34% 3 13% 32% 16% 1	2200
October	1st to 10th	Night	31% 3 12% 35% 15% 3	3805
	11th to 20th	Night	40% 3 11% 29% 17% 1	3915
	21st to end	Night	45% 4 12% 27% 12%	4610
November	1st to 10th	Night	50% 6% 13% 19% 10% 2	2767
	11th to 20th	Night	55% 4 15% 18% 7% 1	2940
	21st to end	Night	53% 5% 16% 20% 7%	2961
December	1st to 10th	Night	64% 5% 15% 13% 3	5050
	11th to 20th	Night	61% 5% 13% 16% 4	5716
	21st to end	Night	66% 4 10% 16% 5%	7645

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	9% 9% 38% 31% 10% 3	330
	11th to 20th	Day	12% 8% 38% 31% 10% 1	330
	21st to end	Day	14% 9% 39% 27% 9% 2	363
February	1st to 10th	Day	15% 15% 36% 27% 6% 1	330
	11th to 20th	Day	12% 11% 40% 24% 12% 2	330
	21st to end	Day	8% 10% 44% 27% 10%	273
March	1st to 10th	Day	16% 13% 30% 29% 9% 1	330
	11th to 20th	Day	15% 12% 37% 23% 9% 3	330
	21st to end	Day	13% 16% 32% 27% 10% 2	363
April	1st to 10th	Day	18% 16% 30% 24% 9% 2	496
	11th to 20th	Day	23% 15% 29% 23% 8% 2	690
	21st to end	Day	23% 11% 28% 25% 11% 2	700
May	1st to 10th	Day	17% 12% 29% 29% 11% 2	703
	11th to 20th	Day	18% 10% 29% 28% 11% 4	725
	21st to end	Day	15% 9% 28% 26% 19% 3	789
June	1st to 10th	Day	12% 11% 29% 28% 15% 5%	730
	11th to 20th	Day	14% 9% 39% 24% 13% 2	730
	21st to end	Day	12% 11% 29% 26% 18% 5%	730
July	1st to 10th	Day	6% 4 28% 32% 20% 10%	721
	11th to 20th	Day	11% 6% 29% 28% 20% 5%	718
	21st to end	Day	5% 4 27% 36% 22% 7%	794
August	1st to 10th	Day	8% 5% 25% 35% 22% 5%	726
	11th to 20th	Day	13% 5% 22% 31% 20% 9%	715
	21st to end	Day	10% 8% 27% 31% 20% 4	803
September	1st to 10th	Day	11% 7% 31% 30% 17% 3	562
	11th to 20th	Day	16% 9% 31% 30% 12% 3	330
	21st to end	Day	13% 13% 35% 25% 11% 3	330
October	1st to 10th	Day	17% 9% 31% 28% 12% 3	330
	11th to 20th	Day	17% 13% 37% 23% 8% 1	330
	21st to end	Day	19% 15% 35% 22% 8% 2	363
November	1st to 10th	Day	22% 12% 28% 28% 9% 1	330
	11th to 20th	Day	15% 11% 39% 26% 8% 1	330
	21st to end	Day	15% 14% 38% 24% 9%	330
December	1st to 10th	Day	13% 10% 36% 29% 11% 2	330
	11th to 20th	Day	18% 13% 37% 28% 5%	330
	21st to end	Day	17% 7% 35% 26% 11% 3	363
January	1st to 10th	Night	11% 9% 34% 34% 10% 1	1050
	11th to 20th	Night	13% 9% 37% 32% 9%	1050
	21st to end	Night	13% 9% 38% 31% 9%	1155
February	1st to 10th	Night	13% 10% 34% 33% 9% 2	1050
	11th to 20th	Night	11% 8% 37% 31% 10% 2	1029
	21st to end	Night	11% 10% 34% 32% 11% 2	754
March	1st to 10th	Night	13% 10% 37% 28% 10% 2	782
	11th to 20th	Night	13% 8% 39% 30% 8% 1	630
	21st to end	Night	15% 9% 34% 30% 11% 2	693
April	1st to 10th	Night	15% 9% 34% 31% 10%	603
	11th to 20th	Night	18% 10% 31% 29% 11%	593
	21st to end	Night	22% 6% 25% 36% 11% 1	600
May	1st to 10th	Night	19% 7% 29% 34% 10% 1	600
	11th to 20th	Night	15% 8% 32% 30% 14% 1	591
	21st to end	Night	15% 7% 32% 32% 14% 1	650
June	1st to 10th	Night	13% 5% 29% 39% 14% 1	600
	11th to 20th	Night	11% 5% 34% 37% 12%	600
	21st to end	Night	8% 4 29% 42% 15% 1	600
July	1st to 10th	Night	7% 3 24% 45% 20% 2	585
	11th to 20th	Night	6% 4 28% 45% 17% 1	582
	21st to end	Night	5% 2 23% 49% 18% 2	657
August	1st to 10th	Night	6% 4 23% 46% 19% 1	597
	11th to 20th	Night	9% 4 24% 41% 20% 2	594
	21st to end	Night	10% 5% 27% 38% 20% 1	660
September	1st to 10th	Night	9% 4 30% 39% 17% 2	600
	11th to 20th	Night	12% 5% 31% 38% 15% 1	600
	21st to end	Night	14% 6% 30% 36% 13%	600
October	1st to 10th	Night	17% 8% 34% 33% 7%	724
	11th to 20th	Night	19% 11% 33% 29% 7%	804
	21st to end	Night	21% 10% 33% 29% 8%	1100
November	1st to 10th	Night	15% 10% 37% 28% 10% 1	1025
	11th to 20th	Night	16% 11% 36% 27% 10% 1	1050
	21st to end	Night	16% 13% 40% 25% 6% 1	1050
December	1st to 10th	Night	15% 8% 35% 33% 9% 1	1050
	11th to 20th	Night	17% 12% 34% 29% 7% 1	1050
	21st to end	Night	21% 8% 34% 26% 9% 1	1155

Page generated at 2026-03-30 01:18:39. Version ac07

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