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 2024-01-16.

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% 5% 7% 37%	74668
	Night	11% 9% 4 7% 6% 3 5% 5% 5% 7% 38%	75161
Transparency	Day	46% 6% 15% 24% 9% 1	74355
	Night	46% 5% 14% 23% 10% 1	74996
Seeing	Day	14% 9% 31% 29% 14% 3	14974
	Night	14% 8% 32% 33% 12% 1	25045

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% 7% 12% 10% 5% 7% 5% 8% 7% 25%	1328
	11th to 20th	Day	7% 13% 7% 12% 11% 5% 8% 6% 6% 5% 19%	1393
	21st to end	Day	10% 13% 7% 13% 8% 4 6% 6% 7% 7% 18%	1584
February	1st to 10th	Day	8% 13% 5% 9% 8% 5% 8% 7% 6% 5% 26%	1602
	11th to 20th	Day	7% 11% 5% 10% 10% 5% 8% 7% 6% 7% 24%	1574
	21st to end	Day	8% 9% 6% 11% 11% 6% 6% 6% 5% 6% 26%	1444
March	1st to 10th	Day	11% 12% 6% 10% 8% 5% 6% 5% 4 7% 26%	1848
	11th to 20th	Day	10% 9% 4 9% 8% 4 5% 6% 6% 8% 31%	1980
	21st to end	Day	14% 10% 5% 10% 6% 3 5% 6% 5% 7% 29%	2178
April	1st to 10th	Day	12% 11% 3 8% 7% 4 5% 6% 5% 6% 33%	2153
	11th to 20th	Day	17% 10% 4 7% 5% 3 5% 4 5% 6% 32%	2309
	21st to end	Day	20% 9% 3 7% 6% 3 5% 5% 6% 6% 31%	2340
May	1st to 10th	Day	17% 8% 3 4 6% 3 4 5% 5% 7% 38%	2340
	11th to 20th	Day	15% 7% 2 5% 4 3 5% 5% 5% 8% 40%	2508
	21st to end	Day	16% 6% 3 6% 5% 4 4 5% 5% 8% 37%	2918
June	1st to 10th	Day	14% 5% 2 5% 5% 3 5% 5% 7% 9% 40%	2700
	11th to 20th	Day	14% 5% 2 6% 5% 3 5% 5% 6% 9% 41%	2700
	21st to end	Day	11% 4 3 6% 6% 4 5% 5% 7% 9% 41%	2700
July	1st to 10th	Day	6% 4 1 4 4 3 6% 5% 6% 10% 50%	2700
	11th to 20th	Day	8% 5% 2 5% 5% 3 5% 6% 6% 8% 47%	2700
	21st to end	Day	4 3 1 4 4 3 4 6% 7% 9% 55%	2804
August	1st to 10th	Day	8% 4 2 5% 5% 3 5% 6% 7% 9% 46%	2360
	11th to 20th	Day	11% 5% 2 4 5% 4 4 4 6% 9% 48%	2301
	21st to end	Day	9% 4 2 4 4 3 4 4 5% 7% 53%	2574
September	1st to 10th	Day	12% 5% 2 4 5% 3 4 4 4 7% 50%	2190
	11th to 20th	Day	13% 6% 2 5% 5% 3 4 4 4 7% 46%	1980
	21st to end	Day	12% 6% 2 4 5% 3 4 5% 6% 9% 44%	1980
October	1st to 10th	Day	18% 9% 3 6% 4 3 4 4 3 5% 41%	1850
	11th to 20th	Day	15% 10% 3 7% 5% 3 4 5% 5% 5% 38%	1796
	21st to end	Day	13% 12% 4 8% 8% 3 5% 3 5% 7% 32%	1782
November	1st to 10th	Day	13% 12% 6% 9% 5% 2 5% 3 4 6% 35%	1579
	11th to 20th	Day	10% 14% 7% 11% 7% 5% 6% 4 4 6% 27%	1520
	21st to end	Day	11% 12% 5% 10% 9% 3 5% 5% 3 5% 31%	1498
December	1st to 10th	Day	10% 11% 6% 11% 9% 5% 7% 6% 5% 6% 24%	1330
	11th to 20th	Day	9% 13% 7% 11% 8% 5% 6% 6% 7% 7% 22%	1683
	21st to end	Day	14% 14% 6% 10% 8% 6% 7% 4 4 4 23%	2442
January	1st to 10th	Night	7% 11% 6% 9% 8% 4 7% 6% 6% 8% 29%	2837
	11th to 20th	Night	9% 14% 6% 13% 9% 4 7% 6% 5% 6% 22%	2691
	21st to end	Night	9% 11% 7% 11% 10% 5% 6% 6% 5% 7% 24%	2772
February	1st to 10th	Night	8% 9% 4 12% 8% 4 6% 6% 5% 6% 31%	2356
	11th to 20th	Night	7% 10% 4 8% 8% 5% 8% 6% 6% 7% 30%	2273
	21st to end	Night	7% 11% 5% 9% 7% 4 5% 4 5% 7% 35%	1830
March	1st to 10th	Night	11% 11% 6% 9% 6% 4 5% 6% 5% 7% 30%	2111
	11th to 20th	Night	9% 9% 4 7% 7% 4 5% 4 5% 7% 39%	1980
	21st to end	Night	12% 10% 4 8% 6% 3 4 4 5% 8% 35%	2178
April	1st to 10th	Night	11% 9% 3 7% 5% 3 4 6% 6% 6% 41%	1805
	11th to 20th	Night	16% 11% 4 6% 4 3 3 4 5% 7% 36%	1599
	21st to end	Night	16% 8% 3 6% 6% 4 4 4 5% 7% 37%	1620
May	1st to 10th	Night	15% 7% 3 6% 5% 3 4 5% 4 6% 40%	1620
	11th to 20th	Night	15% 4 2 3 4 3 6% 4 6% 8% 44%	1382
	21st to end	Night	13% 6% 2 3 4 3 4 5% 5% 8% 45%	1358
June	1st to 10th	Night	10% 5% 1 4 3 3 5% 5% 6% 9% 47%	1260
	11th to 20th	Night	10% 5% 2 5% 5% 2 4 4 6% 10% 47%	1260
	21st to end	Night	7% 3 2 5% 4 3 5% 6% 5% 10% 50%	1260
July	1st to 10th	Night	4 3 2 3 3 2 3 5% 8% 8% 60%	1260
	11th to 20th	Night	4 3 1 3 3 3 4 5% 7% 9% 58%	1260
	21st to end	Night	5% 2 1 3 4 2 4 5% 6% 9% 57%	1512
August	1st to 10th	Night	5% 4 1 5% 4 3 4 4 6% 7% 57%	1577
	11th to 20th	Night	7% 5% 1 3 4 2 3 5% 6% 9% 54%	1600
	21st to end	Night	9% 4 2 4 5% 2 3 3 5% 7% 57%	1782
September	1st to 10th	Night	9% 4 2 3 4 2 4 4 5% 7% 57%	1765
	11th to 20th	Night	9% 4 2 4 3 2 4 4 5% 8% 54%	1980
	21st to end	Night	13% 5% 2 4 4 2 4 4 4 6% 51%	1980
October	1st to 10th	Night	15% 6% 2 5% 6% 2 4 5% 5% 6% 43%	2091
	11th to 20th	Night	14% 10% 3 5% 5% 3 5% 5% 4 7% 38%	2163
	21st to end	Night	15% 9% 3 6% 5% 3 4 4 6% 8% 36%	2574
November	1st to 10th	Night	11% 9% 5% 9% 7% 4 5% 4 5% 7% 34%	2497
	11th to 20th	Night	9% 13% 7% 10% 7% 4 5% 4 4 6% 31%	2660
	21st to end	Night	12% 11% 4 9% 6% 4 5% 4 5% 6% 34%	2679
December	1st to 10th	Night	9% 12% 5% 11% 10% 5% 7% 5% 5% 6% 25%	2850
	11th to 20th	Night	10% 14% 5% 12% 9% 4 6% 6% 6% 6% 23%	3514
	21st to end	Night	17% 12% 5% 9% 8% 4 5% 4 5% 5% 26%	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	55% 8% 15% 16% 5% 1	1329
	11th to 20th	Day	65% 4 11% 14% 5%	1393
	21st to end	Day	63% 5% 11% 15% 6%	1584
February	1st to 10th	Day	58% 5% 13% 16% 8%	1602
	11th to 20th	Day	55% 6% 12% 17% 8% 1	1583
	21st to end	Day	57% 6% 11% 20% 6%	1444
March	1st to 10th	Day	60% 6% 13% 16% 4	1848
	11th to 20th	Day	51% 5% 15% 22% 6%	1980
	21st to end	Day	54% 8% 15% 19% 5%	2178
April	1st to 10th	Day	50% 6% 12% 21% 9% 1	2153
	11th to 20th	Day	52% 5% 12% 18% 11% 1	2312
	21st to end	Day	51% 6% 15% 21% 6% 1	2340
May	1st to 10th	Day	46% 6% 13% 24% 11% 1	2340
	11th to 20th	Day	42% 5% 13% 28% 11% 1	2512
	21st to end	Day	45% 6% 14% 25% 10% 1	2918
June	1st to 10th	Day	40% 5% 18% 28% 8%	2700
	11th to 20th	Day	39% 6% 18% 26% 9% 1	2700
	21st to end	Day	39% 7% 17% 28% 9% 1	2700
July	1st to 10th	Day	27% 7% 16% 36% 13% 1	2626
	11th to 20th	Day	32% 7% 18% 28% 13% 1	2619
	21st to end	Day	25% 6% 17% 36% 14% 1	2648
August	1st to 10th	Day	33% 7% 18% 30% 11% 2	2344
	11th to 20th	Day	34% 6% 18% 31% 11%	2301
	21st to end	Day	31% 4 15% 35% 14% 1	2574
September	1st to 10th	Day	35% 4 15% 32% 13% 1	2190
	11th to 20th	Day	39% 5% 18% 28% 9% 1	1980
	21st to end	Day	36% 6% 17% 33% 7% 1	1980
October	1st to 10th	Day	47% 4 15% 24% 9%	1850
	11th to 20th	Day	47% 4 14% 25% 10%	1796
	21st to end	Day	52% 5% 16% 20% 6%	1782
November	1st to 10th	Day	52% 4 15% 18% 10% 1	1579
	11th to 20th	Day	59% 5% 14% 16% 6% 1	1520
	21st to end	Day	56% 6% 14% 17% 6%	1498
December	1st to 10th	Day	61% 5% 14% 16% 4	1330
	11th to 20th	Day	60% 5% 15% 14% 5%	1680
	21st to end	Day	66% 4 6% 19% 5%	2442
January	1st to 10th	Night	52% 6% 13% 22% 7%	2837
	11th to 20th	Night	62% 5% 12% 14% 7%	2691
	21st to end	Night	59% 5% 12% 16% 7%	2772
February	1st to 10th	Night	53% 4 15% 20% 9%	2356
	11th to 20th	Night	51% 7% 12% 18% 12%	2274
	21st to end	Night	49% 4 13% 26% 8% 2	1830
March	1st to 10th	Night	52% 7% 15% 20% 7%	2111
	11th to 20th	Night	45% 5% 13% 25% 11%	1980
	21st to end	Night	48% 5% 17% 19% 9% 1	2178
April	1st to 10th	Night	42% 5% 12% 25% 13% 2	1805
	11th to 20th	Night	49% 4 12% 19% 13% 3	1599
	21st to end	Night	48% 4 13% 24% 9% 2	1620
May	1st to 10th	Night	44% 5% 12% 23% 14% 2	1620
	11th to 20th	Night	38% 5% 15% 25% 15% 3	1383
	21st to end	Night	37% 6% 14% 28% 14% 1	1358
June	1st to 10th	Night	33% 6% 17% 32% 10% 2	1260
	11th to 20th	Night	33% 5% 16% 28% 14% 3	1260
	21st to end	Night	30% 7% 20% 34% 8% 2	1260
July	1st to 10th	Night	20% 5% 18% 38% 17% 2	1225
	11th to 20th	Night	22% 6% 18% 35% 16% 2	1218
	21st to end	Night	23% 6% 14% 34% 21% 2	1428
August	1st to 10th	Night	26% 5% 15% 34% 17% 3	1569
	11th to 20th	Night	26% 6% 20% 32% 15% 1	1600
	21st to end	Night	28% 4 13% 34% 20% 1	1782
September	1st to 10th	Night	28% 5% 14% 35% 17% 1	1765
	11th to 20th	Night	28% 5% 18% 32% 14% 1	1980
	21st to end	Night	36% 3 13% 32% 14% 1	1980
October	1st to 10th	Night	41% 5% 17% 25% 12% 1	2091
	11th to 20th	Night	45% 4 12% 26% 11% 1	2163
	21st to end	Night	46% 5% 16% 24% 8% 1	2574
November	1st to 10th	Night	49% 6% 13% 20% 10% 2	2497
	11th to 20th	Night	56% 5% 14% 17% 7% 1	2660
	21st to end	Night	51% 5% 17% 20% 7%	2679
December	1st to 10th	Night	59% 4 16% 17% 4	2850
	11th to 20th	Night	61% 6% 16% 14% 4	3516
	21st to end	Night	62% 3 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% 9% 39% 30% 10% 3	270
	11th to 20th	Day	13% 8% 34% 33% 11% 1	257
	21st to end	Day	17% 7% 38% 27% 9% 2	264
February	1st to 10th	Day	15% 13% 38% 29% 5% 1	240
	11th to 20th	Day	13% 11% 43% 23% 9% 1	240
	21st to end	Day	8% 9% 44% 28% 12%	198
March	1st to 10th	Day	15% 14% 29% 30% 11% 1	240
	11th to 20th	Day	16% 8% 35% 24% 13% 4	240
	21st to end	Day	13% 12% 31% 29% 12% 3	264
April	1st to 10th	Day	19% 17% 32% 22% 9% 1	413
	11th to 20th	Day	25% 13% 28% 23% 9% 2	590
	21st to end	Day	22% 10% 29% 25% 12% 2	600
May	1st to 10th	Day	18% 11% 30% 28% 12% 2	603
	11th to 20th	Day	18% 9% 28% 28% 12% 4	625
	21st to end	Day	15% 9% 29% 27% 18% 3	679
June	1st to 10th	Day	12% 9% 29% 30% 15% 5%	630
	11th to 20th	Day	14% 8% 39% 23% 14% 2	630
	21st to end	Day	11% 10% 27% 27% 19% 6%	630
July	1st to 10th	Day	6% 4 28% 33% 21% 8%	621
	11th to 20th	Day	11% 7% 30% 29% 20% 4	618
	21st to end	Day	5% 3 25% 37% 23% 7%	684
August	1st to 10th	Day	6% 4 26% 36% 22% 6%	626
	11th to 20th	Day	12% 5% 23% 32% 19% 9%	615
	21st to end	Day	11% 8% 26% 31% 20% 4	693
September	1st to 10th	Day	12% 8% 31% 31% 15% 3	480
	11th to 20th	Day	16% 10% 30% 29% 13% 3	270
	21st to end	Day	15% 13% 37% 25% 9% 1	270
October	1st to 10th	Day	20% 10% 30% 27% 10% 3	270
	11th to 20th	Day	14% 11% 37% 26% 10% 1	270
	21st to end	Day	19% 15% 36% 22% 7% 1	297
November	1st to 10th	Day	22% 12% 26% 30% 10%	270
	11th to 20th	Day	14% 10% 40% 27% 9%	270
	21st to end	Day	17% 15% 37% 24% 7%	270
December	1st to 10th	Day	11% 9% 35% 33% 10% 3	270
	11th to 20th	Day	16% 10% 40% 30% 4	270
	21st to end	Day	17% 7% 35% 27% 12% 2	297
January	1st to 10th	Night	12% 9% 34% 33% 11% 1	950
	11th to 20th	Night	14% 8% 35% 32% 9% 1	930
	21st to end	Night	15% 8% 37% 32% 8%	990
February	1st to 10th	Night	13% 9% 33% 34% 9% 1	900
	11th to 20th	Night	12% 9% 36% 31% 10% 2	879
	21st to end	Night	12% 8% 34% 33% 12% 2	646
March	1st to 10th	Night	14% 8% 36% 28% 12% 2	671
	11th to 20th	Night	14% 7% 38% 30% 10% 1	540
	21st to end	Night	15% 8% 32% 30% 13% 2	594
April	1st to 10th	Night	16% 9% 34% 31% 9%	540
	11th to 20th	Night	19% 9% 29% 31% 12%	533
	21st to end	Night	21% 6% 25% 36% 11% 1	540
May	1st to 10th	Night	19% 7% 28% 33% 11% 1	540
	11th to 20th	Night	15% 8% 31% 30% 15% 1	531
	21st to end	Night	15% 7% 33% 32% 13% 1	584
June	1st to 10th	Night	13% 5% 27% 39% 15% 1	540
	11th to 20th	Night	11% 5% 34% 36% 13%	540
	21st to end	Night	7% 4 29% 43% 16% 1	540
July	1st to 10th	Night	5% 4 24% 46% 20% 1	525
	11th to 20th	Night	7% 3 28% 44% 17% 1	522
	21st to end	Night	6% 3 22% 49% 19% 2	591
August	1st to 10th	Night	6% 3 23% 47% 20% 1	537
	11th to 20th	Night	8% 3 25% 40% 21% 3	534
	21st to end	Night	10% 5% 26% 38% 20% 1	594
September	1st to 10th	Night	10% 4 29% 39% 16% 2	540
	11th to 20th	Night	11% 5% 30% 38% 15% 1	540
	21st to end	Night	15% 6% 30% 36% 12%	540
October	1st to 10th	Night	19% 8% 33% 32% 8%	651
	11th to 20th	Night	19% 10% 34% 29% 7%	723
	21st to end	Night	21% 10% 33% 27% 8%	990
November	1st to 10th	Night	14% 10% 38% 28% 10%	925
	11th to 20th	Night	16% 10% 36% 28% 10% 1	950
	21st to end	Night	17% 13% 39% 25% 6% 1	950
December	1st to 10th	Night	15% 8% 34% 33% 9% 1	950
	11th to 20th	Night	18% 11% 33% 30% 7% 1	950
	21st to end	Night	22% 8% 33% 27% 9% 1	1045

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