Ursae Majoris Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ursae Majoris 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	19% 9% 3 7% 5% 5% 4 4 5% 6% 33%	136408
	Night	21% 9% 3 7% 5% 5% 4 3 5% 6% 31%	134145
Transparency	Day	50% 8% 14% 17% 8% 1	136127
	Night	53% 8% 13% 16% 7% 1	134005
Seeing	Day	25% 11% 20% 26% 15% 4	16708
	Night	28% 9% 17% 26% 18% 2	25166

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	34% 14% 4 11% 7% 6% 5% 3 4 4 8%	4723
	11th to 20th	Day	18% 9% 5% 9% 4 5% 6% 5% 6% 7% 27%	4716
	21st to end	Day	29% 9% 3 8% 4 4 3 2 4 5% 28%	5380
February	1st to 10th	Day	29% 13% 3 7% 6% 4 4 4 5% 7% 17%	1890
	11th to 20th	Day	31% 13% 3 8% 6% 5% 3 2 4 6% 19%	1845
	21st to end	Day	28% 13% 5% 8% 6% 5% 5% 3 4 5% 19%	1636
March	1st to 10th	Day	19% 13% 3 9% 6% 6% 6% 5% 6% 7% 20%	4725
	11th to 20th	Day	21% 13% 4 8% 6% 5% 5% 3 4 5% 25%	5110
	21st to end	Day	25% 13% 4 9% 6% 6% 6% 5% 6% 7% 13%	5799
April	1st to 10th	Day	26% 13% 3 7% 6% 5% 6% 4 6% 6% 18%	2413
	11th to 20th	Day	22% 10% 3 8% 6% 5% 5% 4 5% 7% 26%	2382
	21st to end	Day	20% 10% 4 9% 7% 6% 4 4 6% 6% 24%	2639
May	1st to 10th	Day	16% 8% 2 6% 4 4 5% 4 5% 7% 39%	6320
	11th to 20th	Day	19% 11% 4 8% 5% 5% 5% 4 5% 6% 28%	6277
	21st to end	Day	19% 9% 3 8% 5% 5% 5% 4 6% 8% 29%	6903
June	1st to 10th	Day	20% 9% 3 7% 6% 5% 4 4 6% 8% 30%	2800
	11th to 20th	Day	21% 10% 3 9% 6% 6% 5% 5% 6% 7% 23%	2987
	21st to end	Day	12% 7% 3 8% 5% 5% 5% 3 6% 7% 39%	3000
July	1st to 10th	Day	7% 4 2 4 3 3 3 3 3 5% 63%	6780
	11th to 20th	Day	5% 4 1 4 2 3 2 2 3 4 70%	6366
	21st to end	Day	8% 5% 1 5% 3 3 3 3 5% 6% 58%	6874
August	1st to 10th	Day	5% 3 2 5% 3 3 3 3 4 7% 62%	4449
	11th to 20th	Day	10% 6% 3 7% 5% 4 5% 4 6% 8% 42%	4156
	21st to end	Day	15% 8% 2 7% 4 4 4 4 4 6% 41%	4450
September	1st to 10th	Day	9% 6% 2 6% 4 4 4 3 5% 7% 50%	2400
	11th to 20th	Day	14% 7% 3 7% 5% 5% 4 3 6% 7% 39%	2400
	21st to end	Day	17% 7% 2 6% 4 4 4 4 5% 7% 40%	2115
October	1st to 10th	Day	16% 7% 2 6% 4 4 4 4 6% 10% 37%	3280
	11th to 20th	Day	26% 9% 3 7% 4 4 4 3 5% 8% 26%	3261
	21st to end	Day	27% 10% 3 10% 5% 5% 5% 4 6% 6% 21%	3367
November	1st to 10th	Day	31% 10% 3 8% 4 5% 4 4 5% 6% 19%	1708
	11th to 20th	Day	31% 12% 3 7% 5% 4 5% 3 4 6% 19%	1680
	21st to end	Day	29% 12% 3 8% 6% 5% 4 3 6% 6% 20%	1680
December	1st to 10th	Day	23% 7% 3 7% 5% 6% 6% 5% 7% 7% 25%	2700
	11th to 20th	Day	32% 12% 3 9% 7% 6% 6% 4 4 5% 11%	3105
	21st to end	Day	31% 13% 4 10% 6% 5% 5% 3 4 5% 13%	4092
January	1st to 10th	Night	33% 15% 5% 11% 6% 6% 5% 3 3 3 9%	8172
	11th to 20th	Night	19% 9% 4 8% 4 5% 5% 4 5% 7% 30%	8182
	21st to end	Night	26% 8% 3 8% 4 4 4 3 4 5% 29%	8806
February	1st to 10th	Night	29% 12% 4 8% 6% 5% 5% 3 4 6% 20%	2730
	11th to 20th	Night	28% 11% 3 7% 5% 4 5% 4 4 5% 24%	2662
	21st to end	Night	27% 12% 3 7% 5% 4 4 4 5% 5% 23%	2169
March	1st to 10th	Night	19% 13% 3 8% 6% 5% 5% 5% 7% 8% 22%	5373
	11th to 20th	Night	24% 10% 3 6% 5% 5% 5% 3 4 5% 31%	5109
	21st to end	Night	30% 13% 3 9% 6% 5% 5% 4 5% 5% 15%	5417
April	1st to 10th	Night	22% 10% 3 9% 6% 5% 6% 6% 5% 7% 21%	2010
	11th to 20th	Night	17% 11% 3 7% 6% 5% 4 4 6% 7% 30%	1965
	21st to end	Night	20% 8% 4 8% 6% 5% 4 4 5% 8% 28%	1760
May	1st to 10th	Night	17% 8% 3 7% 4 4 4 3 4 4 43%	3640
	11th to 20th	Night	17% 8% 3 8% 6% 5% 6% 4 5% 7% 32%	3616
	21st to end	Night	14% 10% 3 8% 5% 6% 5% 5% 6% 8% 31%	3972
June	1st to 10th	Night	15% 7% 2 7% 5% 5% 6% 4 5% 6% 37%	1600
	11th to 20th	Night	16% 8% 3 8% 6% 5% 5% 3 6% 10% 31%	1408
	21st to end	Night	11% 6% 3 6% 4 5% 4 3 6% 7% 46%	1400
July	1st to 10th	Night	4 4 2 4 2 3 4 3 4 5% 66%	3180
	11th to 20th	Night	3 2 1 3 2 2 3 2 2 4 76%	3591
	21st to end	Night	6% 3 1 4 2 2 3 3 5% 7% 65%	3969
August	1st to 10th	Night	4 3 1 4 2 2 3 2 4 6% 69%	2701
	11th to 20th	Night	10% 5% 2 6% 4 4 4 4 4 6% 50%	2902
	21st to end	Night	14% 7% 2 5% 4 4 4 3 4 6% 48%	3465
September	1st to 10th	Night	9% 4 2 4 3 3 4 4 4 7% 55%	2000
	11th to 20th	Night	15% 7% 2 6% 5% 5% 4 2 5% 6% 45%	2000
	21st to end	Night	16% 6% 2 6% 4 4 4 2 5% 7% 43%	2281
October	1st to 10th	Night	15% 6% 2 7% 5% 4 3 4 5% 8% 41%	3920
	11th to 20th	Night	27% 8% 3 6% 4 4 5% 3 4 6% 31%	3897
	21st to end	Night	25% 11% 3 7% 5% 4 5% 3 5% 6% 26%	4437
November	1st to 10th	Night	33% 8% 2 7% 5% 5% 5% 4 5% 5% 21%	2805
	11th to 20th	Night	31% 10% 4 8% 5% 4 4 3 4 4 21%	2940
	21st to end	Night	28% 12% 3 8% 5% 4 5% 4 5% 5% 21%	2940
December	1st to 10th	Night	22% 9% 2 7% 6% 5% 5% 5% 6% 6% 27%	4700
	11th to 20th	Night	33% 14% 4 10% 6% 6% 5% 3 4 4 12%	5320
	21st to end	Night	30% 13% 5% 10% 6% 6% 4 3 4 5% 14%	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	76% 7% 7% 7% 2	4723
	11th to 20th	Day	50% 8% 12% 18% 11% 1	4740
	21st to end	Day	57% 5% 8% 17% 14%	5381
February	1st to 10th	Day	66% 9% 11% 9% 4 1	1890
	11th to 20th	Day	67% 9% 8% 9% 6% 1	1854
	21st to end	Day	68% 7% 10% 10% 4	1636
March	1st to 10th	Day	58% 10% 17% 11% 5%	4725
	11th to 20th	Day	61% 7% 10% 20% 2	5110
	21st to end	Day	68% 10% 13% 8% 1	5799
April	1st to 10th	Day	66% 11% 14% 8% 1	2413
	11th to 20th	Day	59% 9% 12% 13% 6% 1	2382
	21st to end	Day	62% 10% 13% 11% 4 1	2639
May	1st to 10th	Day	44% 9% 16% 23% 9%	6320
	11th to 20th	Day	53% 12% 19% 14% 2	6280
	21st to end	Day	51% 11% 22% 14% 2	6904
June	1st to 10th	Day	55% 10% 17% 13% 5%	2800
	11th to 20th	Day	62% 10% 13% 11% 3	2987
	21st to end	Day	45% 10% 19% 18% 8% 1	3000
July	1st to 10th	Day	25% 6% 18% 22% 17% 13%	6706
	11th to 20th	Day	20% 4 12% 29% 31% 4	6290
	21st to end	Day	26% 6% 22% 33% 12% 2	6681
August	1st to 10th	Day	24% 6% 13% 35% 19% 3	4432
	11th to 20th	Day	40% 9% 18% 23% 9% 2	4195
	21st to end	Day	43% 6% 13% 25% 12% 1	4449
September	1st to 10th	Day	33% 8% 17% 24% 13% 5%	2400
	11th to 20th	Day	42% 11% 13% 18% 13% 2	2400
	21st to end	Day	44% 9% 17% 18% 11% 2	2115
October	1st to 10th	Day	40% 7% 15% 29% 8%	3280
	11th to 20th	Day	56% 7% 13% 15% 8% 1	3261
	21st to end	Day	62% 6% 13% 10% 8% 1	3367
November	1st to 10th	Day	66% 8% 11% 11% 3	1708
	11th to 20th	Day	67% 9% 8% 8% 6% 2	1680
	21st to end	Day	64% 9% 8% 10% 8% 1	1680
December	1st to 10th	Day	54% 6% 12% 17% 11% 1	2700
	11th to 20th	Day	70% 7% 12% 9% 2	3108
	21st to end	Day	71% 9% 10% 8% 2	4092
January	1st to 10th	Night	76% 7% 8% 6% 1 1	8172
	11th to 20th	Night	48% 9% 14% 19% 10% 1	8220
	21st to end	Night	56% 6% 10% 18% 10% 1	8806
February	1st to 10th	Night	67% 7% 11% 9% 6% 1	2730
	11th to 20th	Night	62% 9% 11% 9% 7% 2	2664
	21st to end	Night	63% 9% 11% 11% 5%	2169
March	1st to 10th	Night	56% 10% 14% 13% 6%	5373
	11th to 20th	Night	55% 8% 15% 21% 2 1	5110
	21st to end	Night	68% 12% 10% 9%	5417
April	1st to 10th	Night	63% 12% 14% 9% 1	2010
	11th to 20th	Night	54% 10% 14% 14% 7% 1	1965
	21st to end	Night	59% 11% 13% 11% 4 1	1760
May	1st to 10th	Night	44% 8% 14% 25% 8%	3637
	11th to 20th	Night	50% 14% 20% 13% 3	3616
	21st to end	Night	53% 15% 18% 14% 1	3972
June	1st to 10th	Night	51% 10% 17% 16% 5%	1600
	11th to 20th	Night	54% 12% 18% 13% 2	1408
	21st to end	Night	40% 8% 22% 20% 8% 2	1400
July	1st to 10th	Night	25% 7% 16% 22% 16% 14%	3145
	11th to 20th	Night	16% 4 12% 30% 35% 3	3545
	21st to end	Night	22% 5% 22% 34% 14% 2	3857
August	1st to 10th	Night	20% 4 13% 39% 22% 2	2693
	11th to 20th	Night	37% 9% 22% 20% 9% 2	2929
	21st to end	Night	39% 6% 15% 29% 10% 1	3465
September	1st to 10th	Night	30% 9% 17% 25% 15% 5%	2000
	11th to 20th	Night	43% 10% 11% 19% 13% 5%	2000
	21st to end	Night	41% 10% 15% 20% 11% 2	2281
October	1st to 10th	Night	40% 7% 20% 28% 5% 1	3920
	11th to 20th	Night	54% 6% 13% 19% 7%	3897
	21st to end	Night	59% 7% 12% 14% 8%	4437
November	1st to 10th	Night	64% 9% 12% 10% 4	2805
	11th to 20th	Night	66% 8% 10% 9% 5% 2	2940
	21st to end	Night	64% 9% 9% 10% 8% 1	2940
December	1st to 10th	Night	53% 6% 13% 19% 8% 1	4700
	11th to 20th	Night	72% 7% 10% 8% 2	5316
	21st to end	Night	69% 11% 11% 7% 2	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	37% 15% 17% 15% 13% 2	220
	11th to 20th	Day	29% 13% 20% 22% 13% 4	220
	21st to end	Day	38% 11% 16% 17% 13% 5%	306
February	1st to 10th	Day	39% 10% 20% 17% 11% 2	430
	11th to 20th	Day	38% 16% 20% 16% 9% 1	425
	21st to end	Day	37% 16% 21% 19% 6%	397
March	1st to 10th	Day	31% 17% 24% 20% 8% 1	540
	11th to 20th	Day	35% 15% 19% 21% 9% 2	540
	21st to end	Day	33% 14% 25% 19% 8% 1	594
April	1st to 10th	Day	34% 16% 21% 19% 9% 2	505
	11th to 20th	Day	25% 19% 27% 17% 8% 4	492
	21st to end	Day	24% 15% 26% 23% 10% 3	500
May	1st to 10th	Day	21% 12% 24% 29% 11% 3	503
	11th to 20th	Day	27% 10% 23% 24% 12% 4	528
	21st to end	Day	28% 8% 23% 25% 12% 3	573
June	1st to 10th	Day	25% 9% 22% 25% 16% 4	530
	11th to 20th	Day	22% 11% 27% 25% 11% 2	717
	21st to end	Day	14% 8% 21% 32% 22% 3	730
July	1st to 10th	Day	12% 5% 18% 38% 24% 3	721
	11th to 20th	Day	10% 5% 20% 39% 22% 5%	543
	21st to end	Day	9% 4 15% 38% 28% 6%	575
August	1st to 10th	Day	12% 4 13% 36% 25% 9%	527
	11th to 20th	Day	7% 6% 14% 37% 28% 8%	517
	21st to end	Day	13% 8% 23% 32% 17% 7%	583
September	1st to 10th	Day	14% 5% 24% 35% 17% 5%	530
	11th to 20th	Day	17% 7% 20% 30% 21% 5%	530
	21st to end	Day	18% 10% 22% 27% 21% 2	530
October	1st to 10th	Day	23% 9% 21% 23% 20% 4	530
	11th to 20th	Day	32% 11% 15% 23% 15% 5%	530
	21st to end	Day	32% 15% 19% 21% 9% 4	467
November	1st to 10th	Day	36% 15% 17% 19% 12% 1	253
	11th to 20th	Day	40% 13% 17% 13% 12% 5%	220
	21st to end	Day	33% 13% 18% 19% 11% 7%	220
December	1st to 10th	Day	45% 8% 13% 13% 13% 8%	220
	11th to 20th	Day	42% 12% 19% 17% 9% 1	220
	21st to end	Day	40% 12% 14% 19% 12% 2	242
January	1st to 10th	Night	40% 15% 17% 18% 9% 1	840
	11th to 20th	Night	36% 10% 15% 26% 13% 1	840
	21st to end	Night	33% 8% 16% 29% 12% 1	924
February	1st to 10th	Night	37% 9% 19% 25% 9% 1	840
	11th to 20th	Night	34% 12% 15% 26% 11% 1	824
	21st to end	Night	35% 13% 21% 22% 9% 1	692
March	1st to 10th	Night	29% 13% 20% 23% 13% 1	840
	11th to 20th	Night	34% 13% 17% 25% 10% 1	840
	21st to end	Night	32% 14% 20% 23% 9% 1	924
April	1st to 10th	Night	30% 10% 20% 25% 13% 2	804
	11th to 20th	Night	25% 13% 20% 26% 13% 2	777
	21st to end	Night	26% 12% 18% 28% 14% 2	560
May	1st to 10th	Night	20% 12% 17% 32% 19% 1	400
	11th to 20th	Night	22% 7% 15% 29% 25% 1	394
	21st to end	Night	21% 9% 16% 27% 24% 3	433
June	1st to 10th	Night	20% 8% 19% 27% 24% 2	400
	11th to 20th	Night	23% 8% 21% 30% 19% 1	400
	21st to end	Night	17% 5% 17% 33% 28% 2	400
July	1st to 10th	Night	11% 2 14% 34% 35% 4	390
	11th to 20th	Night	10% 2 10% 40% 35% 3	388
	21st to end	Night	7% 3 11% 31% 45% 4	438
August	1st to 10th	Night	10% 2 9% 30% 41% 8%	451
	11th to 20th	Night	11% 2 8% 28% 41% 9%	594
	21st to end	Night	12% 4 13% 30% 36% 5%	789
September	1st to 10th	Night	13% 4 12% 34% 33% 5%	800
	11th to 20th	Night	19% 6% 12% 24% 34% 6%	800
	21st to end	Night	20% 5% 15% 30% 28% 3	800
October	1st to 10th	Night	23% 5% 15% 29% 26% 2	800
	11th to 20th	Night	29% 8% 15% 27% 18% 4	800
	21st to end	Night	31% 8% 18% 27% 13% 3	880
November	1st to 10th	Night	36% 12% 20% 24% 8%	820
	11th to 20th	Night	38% 11% 19% 22% 9% 1	840
	21st to end	Night	33% 12% 17% 24% 13% 1	840
December	1st to 10th	Night	32% 9% 20% 24% 13% 2	840
	11th to 20th	Night	40% 10% 19% 23% 7% 1	840
	21st to end	Night	45% 11% 17% 18% 7%	924

Page generated at 2026-03-29 23:18:42. Version ac07

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