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 2024-12-09.

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	20% 9% 3 7% 5% 4 4 3 5% 6% 33%	106412
	Night	22% 9% 3 7% 5% 4 4 3 4 6% 32%	101979
Transparency	Day	51% 9% 13% 17% 8% 2	106172
	Night	54% 9% 13% 16% 7% 1	101866
Seeing	Day	25% 11% 21% 25% 15% 4	15096
	Night	28% 9% 17% 26% 18% 2	23480

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	36% 14% 4 9% 6% 6% 4 3 4 4 11%	2539
	11th to 20th	Day	25% 11% 5% 8% 5% 4 4 4 5% 7% 23%	2516
	21st to end	Day	40% 11% 4 10% 5% 5% 4 3 3 4 12%	2890
February	1st to 10th	Day	31% 13% 4 7% 5% 4 4 4 5% 7% 17%	1710
	11th to 20th	Day	32% 12% 3 8% 6% 4 3 2 4 6% 19%	1665
	21st to end	Day	27% 14% 4 8% 6% 5% 5% 3 4 5% 20%	1484
March	1st to 10th	Day	18% 13% 3 10% 7% 6% 6% 4 6% 6% 22%	3217
	11th to 20th	Day	21% 9% 3 6% 5% 5% 4 3 4 5% 35%	3490
	21st to end	Day	24% 13% 4 9% 6% 5% 6% 4 6% 7% 16%	3961
April	1st to 10th	Day	26% 13% 3 7% 6% 5% 6% 4 5% 6% 18%	2280
	11th to 20th	Day	23% 10% 3 8% 6% 5% 5% 4 5% 7% 25%	2261
	21st to end	Day	21% 10% 4 9% 7% 6% 5% 4 6% 6% 23%	2507
May	1st to 10th	Day	19% 9% 2 5% 4 4 4 4 5% 7% 38%	4420
	11th to 20th	Day	16% 7% 2 6% 4 4 4 4 6% 8% 39%	4377
	21st to end	Day	25% 10% 3 9% 5% 5% 5% 4 6% 7% 21%	4813
June	1st to 10th	Day	21% 9% 3 7% 6% 5% 4 4 6% 8% 28%	2660
	11th to 20th	Day	21% 10% 3 9% 6% 6% 5% 4 6% 7% 23%	2837
	21st to end	Day	12% 7% 2 7% 5% 5% 5% 3 6% 8% 40%	2850
July	1st to 10th	Day	7% 4 2 5% 3 3 3 3 3 5% 62%	4740
	11th to 20th	Day	6% 4 1 3 2 2 3 2 3 5% 68%	4452
	21st to end	Day	10% 5% 2 5% 3 4 3 3 4 5% 55%	4790
August	1st to 10th	Day	5% 3 1 4 3 3 3 3 4 7% 62%	4312
	11th to 20th	Day	10% 7% 3 7% 5% 5% 5% 4 6% 8% 42%	4026
	21st to end	Day	15% 8% 2 7% 4 4 4 4 4 6% 41%	4314
September	1st to 10th	Day	10% 6% 2 6% 4 4 4 3 5% 7% 51%	2280
	11th to 20th	Day	14% 7% 3 8% 5% 5% 4 3 6% 7% 38%	2280
	21st to end	Day	18% 7% 2 7% 4 4 4 4 5% 7% 39%	2009
October	1st to 10th	Day	17% 7% 2 6% 4 4 4 4 6% 9% 37%	3180
	11th to 20th	Day	27% 9% 3 7% 4 4 4 3 5% 8% 26%	3161
	21st to end	Day	27% 10% 3 9% 5% 5% 5% 4 6% 6% 21%	3263
November	1st to 10th	Day	32% 10% 3 8% 4 5% 4 4 5% 6% 18%	1625
	11th to 20th	Day	30% 12% 3 7% 5% 4 5% 3 4 6% 20%	1600
	21st to end	Day	28% 12% 4 8% 5% 5% 4 3 6% 6% 20%	1600
December	1st to 10th	Day	28% 8% 3 7% 4 5% 5% 4 5% 5% 25%	1584
	11th to 20th	Day	32% 12% 3 8% 7% 6% 6% 4 4 5% 13%	1925
	21st to end	Day	27% 13% 4 11% 7% 6% 5% 3 5% 6% 15%	2794
January	1st to 10th	Night	35% 14% 4 9% 5% 5% 4 3 4 4 12%	4406
	11th to 20th	Night	28% 11% 4 6% 4 4 3 3 4 6% 27%	4382
	21st to end	Night	38% 10% 4 9% 5% 5% 4 3 4 5% 14%	4765
February	1st to 10th	Night	31% 12% 4 7% 5% 5% 4 3 4 6% 19%	2470
	11th to 20th	Night	28% 11% 3 7% 5% 4 5% 4 4 5% 24%	2402
	21st to end	Night	28% 12% 3 7% 5% 4 4 4 5% 5% 23%	1969
March	1st to 10th	Night	19% 13% 3 9% 6% 6% 5% 5% 7% 7% 20%	3654
	11th to 20th	Night	20% 7% 2 5% 5% 4 5% 3 4 6% 39%	3489
	21st to end	Night	29% 14% 3 9% 6% 5% 5% 4 5% 5% 16%	3691
April	1st to 10th	Night	23% 10% 3 9% 6% 5% 6% 6% 5% 7% 21%	1900
	11th to 20th	Night	17% 11% 3 7% 6% 6% 4 4 6% 7% 29%	1866
	21st to end	Night	20% 9% 4 8% 6% 5% 4 4 5% 8% 26%	1672
May	1st to 10th	Night	20% 9% 3 7% 4 4 3 3 4 4 39%	2540
	11th to 20th	Night	14% 5% 2 7% 6% 5% 5% 3 5% 7% 40%	2516
	21st to end	Night	18% 11% 3 9% 6% 6% 4 5% 6% 6% 27%	2762
June	1st to 10th	Night	16% 8% 2 8% 6% 5% 6% 3 5% 6% 35%	1520
	11th to 20th	Night	16% 8% 3 8% 6% 5% 5% 3 6% 9% 31%	1338
	21st to end	Night	10% 6% 3 6% 4 5% 4 3 6% 7% 46%	1330
July	1st to 10th	Night	5% 4 2 4 2 3 4 3 4 5% 66%	2220
	11th to 20th	Night	4 2 1 3 2 2 3 2 3 4 73%	2505
	21st to end	Night	8% 4 2 3 2 2 3 3 4 6% 63%	2762
August	1st to 10th	Night	3 2 1 4 2 2 3 2 4 6% 70%	2618
	11th to 20th	Night	10% 5% 2 6% 4 4 4 4 4 6% 51%	2812
	21st to end	Night	15% 7% 2 5% 4 3 3 2 4 6% 49%	3359
September	1st to 10th	Night	9% 4 2 4 3 3 4 4 4 7% 56%	1900
	11th to 20th	Night	16% 7% 2 6% 5% 5% 4 2 4 6% 44%	1900
	21st to end	Night	16% 7% 2 6% 4 4 4 2 4 7% 43%	2167
October	1st to 10th	Night	15% 6% 2 7% 5% 4 4 4 5% 8% 41%	3800
	11th to 20th	Night	27% 8% 3 6% 4 4 5% 3 4 6% 31%	3777
	21st to end	Night	25% 11% 3 7% 5% 4 4 3 5% 6% 26%	4299
November	1st to 10th	Night	33% 8% 2 7% 5% 5% 5% 4 5% 4 22%	2668
	11th to 20th	Night	31% 10% 3 8% 6% 4 4 3 4 4 22%	2800
	21st to end	Night	28% 11% 3 8% 5% 4 5% 4 5% 5% 22%	2800
December	1st to 10th	Night	27% 11% 3 7% 6% 4 4 3 4 5% 25%	2778
	11th to 20th	Night	29% 12% 4 9% 6% 6% 5% 3 5% 4 16%	3280
	21st to end	Night	29% 12% 5% 10% 6% 7% 4 3 4 4 15%	4862

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	75% 10% 6% 6% 3 1	2539
	11th to 20th	Day	59% 11% 12% 12% 5% 1	2540
	21st to end	Day	74% 8% 7% 7% 3	2890
February	1st to 10th	Day	66% 9% 11% 8% 4 1	1710
	11th to 20th	Day	67% 9% 9% 9% 5% 1	1674
	21st to end	Day	68% 8% 11% 10% 4	1484
March	1st to 10th	Day	57% 12% 16% 12% 3	3217
	11th to 20th	Day	51% 8% 12% 27% 1 1	3490
	21st to end	Day	67% 12% 12% 9%	3961
April	1st to 10th	Day	66% 11% 14% 8% 1	2280
	11th to 20th	Day	61% 9% 12% 11% 6% 1	2261
	21st to end	Day	64% 10% 12% 10% 4 1	2507
May	1st to 10th	Day	47% 11% 15% 18% 9%	4420
	11th to 20th	Day	42% 14% 25% 17% 3	4380
	21st to end	Day	63% 13% 15% 7% 1	4814
June	1st to 10th	Day	58% 10% 17% 11% 5%	2660
	11th to 20th	Day	63% 10% 14% 10% 3	2837
	21st to end	Day	44% 10% 19% 18% 8% 1	2850
July	1st to 10th	Day	26% 6% 10% 19% 20% 18%	4666
	11th to 20th	Day	21% 4 9% 25% 35% 6%	4376
	21st to end	Day	30% 6% 13% 33% 16% 2	4639
August	1st to 10th	Day	23% 6% 13% 36% 20% 3	4295
	11th to 20th	Day	40% 9% 18% 23% 8% 2	4065
	21st to end	Day	43% 6% 13% 25% 12% 1	4313
September	1st to 10th	Day	32% 8% 16% 25% 13% 5%	2280
	11th to 20th	Day	44% 11% 12% 17% 13% 2	2280
	21st to end	Day	45% 9% 17% 16% 11% 2	2009
October	1st to 10th	Day	41% 7% 15% 29% 8%	3180
	11th to 20th	Day	56% 7% 13% 15% 7% 1	3161
	21st to end	Day	62% 6% 13% 10% 8% 1	3263
November	1st to 10th	Day	67% 8% 11% 11% 4	1625
	11th to 20th	Day	66% 9% 8% 8% 7% 2	1600
	21st to end	Day	64% 9% 8% 9% 8% 1	1600
December	1st to 10th	Day	60% 8% 11% 11% 9% 1	1584
	11th to 20th	Day	67% 10% 12% 9% 2 1	1928
	21st to end	Day	67% 11% 11% 8% 3	2794
January	1st to 10th	Night	75% 10% 7% 5% 2 1	4406
	11th to 20th	Night	58% 10% 9% 17% 5% 1	4420
	21st to end	Night	71% 9% 8% 7% 4 1	4765
February	1st to 10th	Night	68% 7% 11% 9% 5% 1	2470
	11th to 20th	Night	62% 10% 11% 9% 7% 1	2404
	21st to end	Night	63% 10% 11% 11% 5%	1969
March	1st to 10th	Night	58% 12% 13% 13% 4	3654
	11th to 20th	Night	45% 9% 18% 25% 2 1	3490
	21st to end	Night	68% 14% 10% 8%	3691
April	1st to 10th	Night	64% 12% 14% 9% 1	1900
	11th to 20th	Night	55% 10% 14% 13% 7% 1	1866
	21st to end	Night	61% 12% 12% 10% 4 1	1672
May	1st to 10th	Night	48% 10% 12% 21% 8%	2540
	11th to 20th	Night	41% 15% 22% 17% 4 1	2516
	21st to end	Night	59% 18% 13% 8% 1	2762
June	1st to 10th	Night	53% 10% 16% 15% 5%	1520
	11th to 20th	Night	55% 12% 19% 12% 3	1338
	21st to end	Night	39% 8% 22% 20% 9% 2	1330
July	1st to 10th	Night	27% 7% 9% 20% 17% 20%	2185
	11th to 20th	Night	19% 4 8% 27% 39% 3	2459
	21st to end	Night	26% 6% 15% 30% 20% 3	2674
August	1st to 10th	Night	19% 4 13% 39% 23% 2	2610
	11th to 20th	Night	37% 10% 22% 21% 9% 2	2839
	21st to end	Night	39% 6% 15% 29% 10% 1	3359
September	1st to 10th	Night	30% 9% 16% 26% 15% 5%	1900
	11th to 20th	Night	44% 10% 10% 18% 13% 5%	1900
	21st to end	Night	42% 10% 14% 20% 12% 2	2167
October	1st to 10th	Night	39% 7% 19% 29% 5% 1	3800
	11th to 20th	Night	54% 6% 14% 19% 7%	3777
	21st to end	Night	59% 7% 12% 14% 8%	4299
November	1st to 10th	Night	64% 9% 12% 11% 4	2668
	11th to 20th	Night	66% 8% 10% 9% 6% 2	2800
	21st to end	Night	63% 9% 9% 10% 8% 1	2800
December	1st to 10th	Night	62% 7% 11% 12% 8% 1	2778
	11th to 20th	Night	66% 10% 13% 8% 3	3276
	21st to end	Night	67% 13% 10% 7% 2	4862

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% 17% 15% 15% 14% 3	180
	11th to 20th	Day	32% 16% 20% 21% 9% 3	180
	21st to end	Day	40% 14% 18% 15% 11% 3	255
February	1st to 10th	Day	42% 10% 20% 16% 10% 2	370
	11th to 20th	Day	41% 15% 19% 15% 8% 1	365
	21st to end	Day	37% 16% 21% 18% 7%	341
March	1st to 10th	Day	30% 17% 25% 20% 7% 1	460
	11th to 20th	Day	31% 15% 20% 21% 10% 3	460
	21st to end	Day	34% 15% 25% 18% 8%	506
April	1st to 10th	Day	35% 17% 20% 18% 10% 1	460
	11th to 20th	Day	27% 19% 27% 16% 6% 4	452
	21st to end	Day	25% 16% 27% 22% 8% 2	460
May	1st to 10th	Day	22% 12% 25% 28% 11% 2	463
	11th to 20th	Day	26% 9% 23% 25% 13% 4	488
	21st to end	Day	29% 9% 23% 25% 12% 3	529
June	1st to 10th	Day	27% 9% 23% 23% 15% 3	490
	11th to 20th	Day	23% 11% 27% 24% 11% 3	667
	21st to end	Day	14% 8% 21% 32% 23% 3	680
July	1st to 10th	Day	12% 5% 18% 37% 24% 3	671
	11th to 20th	Day	11% 5% 20% 38% 21% 5%	502
	21st to end	Day	9% 4 16% 39% 27% 5%	531
August	1st to 10th	Day	12% 5% 12% 37% 25% 8%	487
	11th to 20th	Day	7% 6% 13% 37% 29% 9%	477
	21st to end	Day	14% 8% 23% 32% 16% 7%	539
September	1st to 10th	Day	14% 6% 24% 36% 15% 5%	490
	11th to 20th	Day	18% 7% 20% 29% 20% 4	490
	21st to end	Day	19% 10% 22% 27% 20% 2	490
October	1st to 10th	Day	24% 10% 20% 22% 20% 4	490
	11th to 20th	Day	31% 11% 14% 23% 15% 5%	490
	21st to end	Day	32% 15% 20% 19% 10% 4	429
November	1st to 10th	Day	37% 14% 17% 18% 13% 1	230
	11th to 20th	Day	40% 14% 18% 11% 13% 5%	200
	21st to end	Day	33% 13% 19% 17% 12% 8%	200
December	1st to 10th	Day	43% 9% 13% 12% 15% 8%	196
	11th to 20th	Day	41% 12% 18% 19% 9% 1	180
	21st to end	Day	39% 13% 15% 21% 11% 2	198
January	1st to 10th	Night	41% 16% 16% 17% 9% 1	760
	11th to 20th	Night	38% 10% 16% 24% 12% 1	760
	21st to end	Night	35% 9% 17% 28% 11%	836
February	1st to 10th	Night	38% 10% 18% 24% 9% 1	760
	11th to 20th	Night	35% 12% 15% 26% 11% 1	744
	21st to end	Night	36% 13% 20% 21% 9% 1	628
March	1st to 10th	Night	29% 14% 21% 23% 13% 1	760
	11th to 20th	Night	31% 13% 18% 26% 11% 1	760
	21st to end	Night	33% 15% 19% 23% 10% 1	836
April	1st to 10th	Night	31% 11% 20% 25% 13% 2	760
	11th to 20th	Night	26% 13% 21% 25% 13% 2	738
	21st to end	Night	26% 12% 19% 28% 13% 1	532
May	1st to 10th	Night	21% 12% 18% 30% 18% 1	380
	11th to 20th	Night	21% 6% 16% 30% 25% 1	374
	21st to end	Night	22% 9% 16% 27% 23% 3	411
June	1st to 10th	Night	21% 9% 19% 27% 23% 2	380
	11th to 20th	Night	23% 8% 21% 28% 19% 1	380
	21st to end	Night	16% 4 17% 34% 27% 2	380
July	1st to 10th	Night	11% 2 15% 33% 36% 4	370
	11th to 20th	Night	11% 2 10% 40% 34% 3	368
	21st to end	Night	7% 3 11% 31% 44% 4	416
August	1st to 10th	Night	11% 2 9% 30% 40% 8%	428
	11th to 20th	Night	10% 2 7% 28% 43% 10%	564
	21st to end	Night	13% 4 13% 30% 35% 5%	749
September	1st to 10th	Night	13% 4 12% 35% 33% 4	760
	11th to 20th	Night	20% 5% 12% 24% 34% 6%	760
	21st to end	Night	20% 5% 15% 30% 27% 2	760
October	1st to 10th	Night	23% 5% 15% 29% 26% 2	760
	11th to 20th	Night	28% 8% 14% 27% 19% 4	760
	21st to end	Night	32% 8% 18% 26% 13% 3	836
November	1st to 10th	Night	36% 11% 21% 24% 8%	780
	11th to 20th	Night	38% 11% 19% 22% 9% 1	800
	21st to end	Night	33% 12% 18% 24% 13% 1	800
December	1st to 10th	Night	30% 9% 22% 25% 13% 2	794
	11th to 20th	Night	39% 10% 19% 23% 8% 1	760
	21st to end	Night	46% 11% 17% 18% 7%	836

Page generated at 2024-12-06 00:54:47. Version ac07

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