Ursa Major Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ursa Major Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	4 6% 2 5% 4 4 4 4 6% 7% 54%	140183
	Night	6% 6% 2 6% 4 4 5% 4 6% 7% 50%	138329
Transparency	Day	26% 7% 12% 25% 20% 9%	134978
	Night	28% 8% 15% 26% 18% 6%	134530
Seeing	Day	10% 17% 26% 26% 16% 6%	16225
	Night	8% 11% 23% 36% 19% 3	25400

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	9% 12% 4 10% 7% 8% 7% 5% 8% 8% 22%	4722
	11th to 20th	Day	6% 7% 2 5% 5% 5% 5% 4 6% 8% 48%	4740
	21st to end	Day	8% 9% 3 7% 5% 5% 5% 4 7% 9% 37%	5603
February	1st to 10th	Day	4 8% 4 9% 6% 7% 6% 5% 7% 10% 34%	1890
	11th to 20th	Day	6% 8% 3 7% 5% 7% 7% 5% 8% 8% 37%	1845
	21st to end	Day	4 9% 3 9% 6% 6% 6% 6% 8% 8% 37%	1725
March	1st to 10th	Day	5% 9% 3 8% 5% 5% 6% 5% 8% 10% 37%	4617
	11th to 20th	Day	5% 12% 4 7% 5% 5% 5% 4 6% 7% 40%	5000
	21st to end	Day	5% 9% 3 8% 7% 7% 7% 6% 9% 9% 29%	5705
April	1st to 10th	Day	7% 9% 3 8% 6% 6% 6% 6% 8% 7% 35%	2400
	11th to 20th	Day	5% 8% 3 7% 5% 4 5% 4 7% 7% 43%	2369
	21st to end	Day	4 5% 2 6% 4 4 5% 4 7% 10% 48%	2477
May	1st to 10th	Day	7% 6% 2 4 3 3 4 4 5% 6% 56%	6310
	11th to 20th	Day	4 4 2 6% 4 5% 5% 4 6% 7% 53%	6277
	21st to end	Day	5% 4 1 4 4 3 4 4 7% 8% 56%	6902
June	1st to 10th	Day	8% 5% 2 5% 5% 4 4 4 7% 7% 49%	2800
	11th to 20th	Day	4 4 1 5% 4 4 4 4 6% 8% 57%	2800
	21st to end	Day	3 3 1 3 3 3 3 3 5% 6% 68%	2800
July	1st to 10th	Day	1 1 2 1 2 2 2 3 4 83%	6319
	11th to 20th	Day	1 1 1 1 2 2 2 3 3 85%	6320
	21st to end	Day	1 2 1 1 1 2 2 4 5% 80%	6880
August	1st to 10th	Day	2 2 1 3 2 2 3 3 5% 6% 72%	5912
	11th to 20th	Day	2 1 1 2 2 2 3 3 4 5% 74%	5782
	21st to end	Day	4 3 1 3 2 3 3 3 4 6% 67%	5984
September	1st to 10th	Day	2 2 1 2 2 2 3 2 4 5% 77%	2400
	11th to 20th	Day	3 3 2 4 3 3 4 3 5% 8% 61%	2400
	21st to end	Day	6% 4 1 4 3 3 3 3 5% 6% 61%	2135
October	1st to 10th	Day	4 4 1 4 3 3 4 3 4 5% 65%	3280
	11th to 20th	Day	4 5% 2 5% 4 5% 5% 4 8% 10% 50%	3261
	21st to end	Day	5% 7% 3 8% 5% 5% 5% 4 7% 8% 42%	3409
November	1st to 10th	Day	5% 7% 3 9% 6% 6% 5% 5% 7% 9% 38%	1854
	11th to 20th	Day	6% 7% 2 9% 6% 6% 8% 7% 9% 9% 31%	1685
	21st to end	Day	6% 7% 4 5% 6% 6% 5% 6% 8% 9% 39%	1680
December	1st to 10th	Day	3 6% 3 6% 4 4 5% 5% 8% 8% 49%	2700
	11th to 20th	Day	9% 13% 5% 10% 6% 6% 6% 5% 6% 9% 25%	3108
	21st to end	Day	7% 11% 4 10% 6% 6% 6% 5% 8% 9% 28%	4092
January	1st to 10th	Night	10% 11% 4 10% 6% 6% 6% 6% 8% 10% 23%	8172
	11th to 20th	Night	7% 6% 2 6% 4 4 5% 3 5% 6% 51%	8220
	21st to end	Night	9% 10% 4 9% 5% 5% 5% 4 5% 6% 38%	8585
February	1st to 10th	Night	5% 9% 4 9% 6% 5% 6% 5% 6% 9% 35%	2730
	11th to 20th	Night	5% 11% 3 7% 6% 4 5% 5% 7% 7% 39%	2662
	21st to end	Night	4 9% 4 9% 5% 5% 6% 5% 7% 9% 37%	2081
March	1st to 10th	Night	4 8% 2 6% 5% 5% 6% 6% 9% 11% 37%	5320
	11th to 20th	Night	4 7% 3 6% 6% 5% 4 4 7% 8% 46%	5000
	21st to end	Night	6% 9% 4 9% 6% 6% 7% 6% 8% 10% 31%	5270
April	1st to 10th	Night	6% 9% 2 7% 5% 5% 5% 5% 7% 9% 41%	2000
	11th to 20th	Night	4 8% 3 6% 5% 4 5% 5% 6% 9% 44%	1979
	21st to end	Night	6% 7% 3 4 5% 5% 5% 4 7% 8% 46%	1920
May	1st to 10th	Night	9% 5% 1 4 3 3 4 3 5% 6% 58%	3650
	11th to 20th	Night	4 4 2 5% 4 4 4 4 5% 6% 57%	3616
	21st to end	Night	6% 4 1 4 3 2 4 4 5% 7% 60%	3972
June	1st to 10th	Night	9% 5% 2 6% 4 3 4 3 6% 6% 51%	1600
	11th to 20th	Night	5% 3 1 4 3 4 4 4 5% 8% 60%	1600
	21st to end	Night	3 3 1 3 2 2 3 3 5% 5% 71%	1600
July	1st to 10th	Night	1 1 1 2 1 2 2 2 3 3 80%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 3 3 86%	3640
	21st to end	Night	2 1 1 2 2 2 2 2 3 4 79%	3970
August	1st to 10th	Night	2 2 1 3 2 3 4 3 6% 6% 69%	3890
	11th to 20th	Night	1 2 1 2 2 3 3 3 4 6% 73%	4066
	21st to end	Night	4 3 1 3 2 2 2 2 4 6% 70%	5015
September	1st to 10th	Night	1 1 1 2 1 2 2 2 4 5% 79%	2000
	11th to 20th	Night	3 3 1 3 3 4 4 4 4 7% 65%	2000
	21st to end	Night	5% 3 1 3 3 3 3 4 4 5% 65%	2261
October	1st to 10th	Night	3 4 1 4 2 2 3 3 4 4 70%	3920
	11th to 20th	Night	4 5% 1 4 4 4 4 4 6% 9% 54%	3897
	21st to end	Night	6% 6% 3 7% 5% 5% 5% 4 6% 8% 43%	4395
November	1st to 10th	Night	8% 8% 2 8% 5% 6% 5% 6% 7% 7% 38%	2665
	11th to 20th	Night	7% 8% 4 8% 6% 5% 6% 5% 7% 8% 36%	2931
	21st to end	Night	5% 7% 3 6% 5% 5% 6% 5% 7% 9% 44%	2940
December	1st to 10th	Night	4 6% 2 6% 4 3 5% 3 6% 8% 54%	4700
	11th to 20th	Night	9% 12% 5% 10% 7% 6% 7% 5% 7% 8% 24%	5316
	21st to end	Night	10% 11% 3 8% 6% 5% 6% 5% 6% 8% 32%	7106

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	47% 11% 17% 18% 8%	4723
	11th to 20th	Day	27% 8% 17% 25% 19% 5%	4740
	21st to end	Day	34% 11% 13% 22% 17% 4	5603
February	1st to 10th	Day	42% 11% 15% 22% 9% 1	1890
	11th to 20th	Day	35% 12% 17% 22% 12% 2	1854
	21st to end	Day	38% 12% 17% 17% 13% 2	1725
March	1st to 10th	Day	32% 7% 11% 28% 21% 1	4617
	11th to 20th	Day	36% 9% 13% 25% 15%	5000
	21st to end	Day	38% 12% 20% 23% 7%	5705
April	1st to 10th	Day	41% 8% 18% 20% 10% 2	2400
	11th to 20th	Day	36% 8% 15% 22% 16% 3	2372
	21st to end	Day	30% 8% 19% 27% 14% 2	2477
May	1st to 10th	Day	25% 6% 9% 32% 26% 3	6310
	11th to 20th	Day	24% 7% 14% 28% 23% 4	6280
	21st to end	Day	23% 6% 20% 37% 13%	6904
June	1st to 10th	Day	33% 7% 14% 32% 13% 2	2800
	11th to 20th	Day	24% 7% 13% 33% 20% 3	2800
	21st to end	Day	17% 5% 11% 34% 25% 8%	2800
July	1st to 10th	Day	9% 3 4 18% 29% 38%	6251
	11th to 20th	Day	6% 2 4 9% 25% 53%	6245
	21st to end	Day	9% 3 4 29% 36% 20%	6681
August	1st to 10th	Day	14% 4 5% 18% 39% 20%	4305
	11th to 20th	Day	13% 5% 7% 32% 31% 13%	4181
	21st to end	Day	15% 3 7% 28% 32% 16%	4311
September	1st to 10th	Day	11% 4 5% 27% 34% 18%	2400
	11th to 20th	Day	21% 7% 10% 27% 24% 10%	2400
	21st to end	Day	23% 5% 11% 27% 24% 11%	2135
October	1st to 10th	Day	21% 5% 12% 23% 26% 14%	3280
	11th to 20th	Day	25% 7% 19% 27% 20% 3	3261
	21st to end	Day	32% 9% 11% 27% 20% 2	3409
November	1st to 10th	Day	36% 9% 18% 23% 13% 1	1854
	11th to 20th	Day	36% 14% 22% 17% 10% 1	1685
	21st to end	Day	35% 11% 17% 20% 15% 2	1680
December	1st to 10th	Day	26% 10% 14% 31% 18% 1	2700
	11th to 20th	Day	48% 9% 19% 16% 7% 1	3108
	21st to end	Day	45% 10% 14% 23% 8% 1	4092
January	1st to 10th	Night	42% 12% 21% 19% 6%	8172
	11th to 20th	Night	27% 8% 13% 28% 20% 5%	8220
	21st to end	Night	41% 9% 11% 19% 16% 5%	8585
February	1st to 10th	Night	39% 10% 20% 19% 11% 1	2730
	11th to 20th	Night	37% 9% 20% 22% 11% 2	2664
	21st to end	Night	37% 9% 18% 22% 12% 2	2081
March	1st to 10th	Night	30% 10% 13% 28% 18% 1	5320
	11th to 20th	Night	27% 9% 17% 30% 16%	5000
	21st to end	Night	37% 13% 21% 23% 6%	5270
April	1st to 10th	Night	35% 9% 22% 22% 10% 1	2000
	11th to 20th	Night	31% 10% 19% 26% 12% 2	1979
	21st to end	Night	31% 9% 18% 27% 12% 2	1920
May	1st to 10th	Night	25% 5% 9% 33% 28% 1	3647
	11th to 20th	Night	21% 7% 18% 30% 22% 2	3616
	21st to end	Night	21% 6% 30% 35% 8%	3972
June	1st to 10th	Night	34% 7% 14% 30% 14% 1	1600
	11th to 20th	Night	22% 5% 16% 37% 17% 2	1600
	21st to end	Night	16% 4 11% 37% 25% 6%	1600
July	1st to 10th	Night	11% 2 5% 24% 25% 34%	3600
	11th to 20th	Night	6% 2 4 14% 25% 49%	3592
	21st to end	Night	10% 3 4 32% 33% 19%	3857
August	1st to 10th	Night	12% 4 4 21% 42% 17%	2823
	11th to 20th	Night	11% 4 8% 38% 31% 8%	2943
	21st to end	Night	12% 3 9% 30% 33% 13%	3608
September	1st to 10th	Night	11% 3 6% 29% 36% 16%	2000
	11th to 20th	Night	19% 6% 11% 28% 27% 9%	2000
	21st to end	Night	20% 4 10% 28% 26% 11%	2261
October	1st to 10th	Night	19% 4 9% 26% 33% 9%	3920
	11th to 20th	Night	24% 6% 22% 29% 16% 3	3897
	21st to end	Night	31% 10% 12% 30% 14% 2	4395
November	1st to 10th	Night	37% 9% 18% 21% 13% 3	2665
	11th to 20th	Night	37% 12% 19% 21% 9% 1	2931
	21st to end	Night	31% 10% 19% 20% 17% 2	2940
December	1st to 10th	Night	24% 7% 13% 33% 21% 1	4700
	11th to 20th	Night	47% 10% 18% 17% 8% 1	5316
	21st to end	Night	41% 10% 18% 22% 8% 1	7106

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	20% 15% 27% 26% 11% 1	220
	11th to 20th	Day	16% 10% 22% 25% 17% 10%	220
	21st to end	Day	13% 19% 21% 25% 17% 5%	389
February	1st to 10th	Day	14% 24% 27% 21% 10% 3	430
	11th to 20th	Day	12% 30% 27% 21% 9% 2	425
	21st to end	Day	16% 24% 29% 20% 10% 1	443
March	1st to 10th	Day	15% 24% 28% 25% 8% 1	509
	11th to 20th	Day	14% 24% 27% 24% 9% 2	500
	21st to end	Day	17% 26% 29% 20% 7% 1	550
April	1st to 10th	Day	17% 21% 28% 19% 11% 3	500
	11th to 20th	Day	15% 22% 31% 18% 8% 5%	492
	21st to end	Day	9% 28% 28% 20% 11% 4	500
May	1st to 10th	Day	12% 21% 29% 22% 12% 5%	503
	11th to 20th	Day	16% 17% 26% 24% 11% 5%	528
	21st to end	Day	10% 15% 24% 27% 17% 6%	573
June	1st to 10th	Day	10% 19% 22% 26% 15% 8%	530
	11th to 20th	Day	11% 20% 29% 24% 13% 3	530
	21st to end	Day	5% 12% 25% 29% 20% 9%	530
July	1st to 10th	Day	3 9% 31% 31% 18% 9%	526
	11th to 20th	Day	2 9% 31% 27% 21% 10%	524
	21st to end	Day	2 5% 30% 31% 20% 12%	575
August	1st to 10th	Day	2 7% 26% 33% 20% 13%	527
	11th to 20th	Day	4 6% 21% 33% 21% 14%	517
	21st to end	Day	4 10% 26% 31% 19% 11%	583
September	1st to 10th	Day	4 8% 22% 31% 23% 12%	530
	11th to 20th	Day	5% 11% 22% 31% 22% 8%	530
	21st to end	Day	9% 15% 21% 27% 22% 7%	530
October	1st to 10th	Day	9% 17% 21% 28% 21% 4	530
	11th to 20th	Day	9% 18% 22% 24% 22% 5%	530
	21st to end	Day	13% 20% 24% 21% 19% 4	496
November	1st to 10th	Day	11% 16% 26% 28% 16% 4	330
	11th to 20th	Day	10% 20% 24% 23% 17% 6%	223
	21st to end	Day	14% 14% 21% 25% 20% 6%	220
December	1st to 10th	Day	15% 18% 19% 20% 15% 12%	220
	11th to 20th	Day	14% 21% 27% 25% 12% 2	220
	21st to end	Day	17% 21% 21% 24% 14% 3	242
January	1st to 10th	Night	13% 18% 24% 28% 14% 3	840
	11th to 20th	Night	9% 16% 21% 30% 19% 4	840
	21st to end	Night	12% 10% 22% 37% 14% 4	924
February	1st to 10th	Night	10% 15% 25% 33% 14% 2	840
	11th to 20th	Night	11% 14% 28% 34% 12% 2	824
	21st to end	Night	10% 18% 27% 36% 8% 1	692
March	1st to 10th	Night	10% 19% 29% 29% 12% 1	808
	11th to 20th	Night	8% 15% 26% 35% 15% 1	800
	21st to end	Night	10% 19% 30% 29% 13% 1	880
April	1st to 10th	Night	11% 17% 25% 32% 15% 1	800
	11th to 20th	Night	8% 17% 29% 28% 16% 3	791
	21st to end	Night	8% 13% 29% 33% 16% 2	720
May	1st to 10th	Night	8% 11% 24% 35% 20% 2	410
	11th to 20th	Night	8% 8% 22% 39% 21% 1	394
	21st to end	Night	7% 6% 23% 40% 23% 1	433
June	1st to 10th	Night	13% 6% 24% 37% 19% 2	400
	11th to 20th	Night	6% 7% 28% 42% 17% 1	400
	21st to end	Night	4 4 21% 47% 23% 1	400
July	1st to 10th	Night	2 2 16% 54% 25% 1	390
	11th to 20th	Night	1 18% 59% 20% 1	388
	21st to end	Night	1 1 13% 51% 32% 2	438
August	1st to 10th	Night	1 1 11% 47% 35% 5%	529
	11th to 20th	Night	3 1 9% 40% 40% 8%	595
	21st to end	Night	3 3 19% 44% 28% 4	880
September	1st to 10th	Night	2 3 18% 40% 33% 4	800
	11th to 20th	Night	5% 2 16% 43% 29% 5%	800
	21st to end	Night	8% 6% 17% 36% 28% 6%	800
October	1st to 10th	Night	8% 4 20% 40% 24% 5%	800
	11th to 20th	Night	6% 11% 19% 37% 23% 4	800
	21st to end	Night	11% 10% 25% 33% 16% 6%	880
November	1st to 10th	Night	12% 10% 24% 38% 14% 2	820
	11th to 20th	Night	13% 16% 25% 31% 13% 2	840
	21st to end	Night	9% 13% 23% 35% 17% 3	840
December	1st to 10th	Night	10% 13% 23% 32% 18% 3	840
	11th to 20th	Night	11% 15% 25% 33% 15% 1	840
	21st to end	Night	12% 21% 26% 29% 10% 1	924

Page generated at 2026-02-27 22:40:25. Version ac07

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