Coma Via Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Coma Via Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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	6% 4 1 3 2 2 2 2 4 5% 69%	146100
	Night	7% 4 1 3 2 2 2 2 4 5% 67%	144108
Transparency	Day	19% 4 6% 18% 29% 23%	141175
	Night	22% 4 8% 20% 27% 20%	140011
Seeing	Day	9% 11% 29% 35% 13% 3	17577
	Night	9% 4 20% 38% 24% 4	27172

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	11% 9% 3 6% 3 3 4 3 5% 6% 47%	4722
	11th to 20th	Day	5% 4 2 4 3 3 3 3 5% 6% 63%	5077
	21st to end	Day	10% 6% 2 5% 4 3 4 4 6% 8% 50%	5880
February	1st to 10th	Day	13% 9% 2 4 3 3 3 3 4 6% 49%	2100
	11th to 20th	Day	13% 8% 1 4 3 3 3 3 6% 7% 48%	2050
	21st to end	Day	14% 10% 3 5% 4 3 2 3 4 6% 46%	1730
March	1st to 10th	Day	15% 12% 2 6% 4 4 4 3 5% 7% 37%	4618
	11th to 20th	Day	11% 7% 2 5% 4 4 5% 4 4 7% 49%	4873
	21st to end	Day	13% 9% 2 6% 4 4 4 4 5% 7% 43%	6089
April	1st to 10th	Day	10% 6% 2 5% 4 3 3 3 4 6% 55%	2520
	11th to 20th	Day	10% 5% 1 4 3 3 3 3 6% 6% 56%	2489
	21st to end	Day	9% 5% 1 4 3 3 3 3 4 6% 59%	2520
May	1st to 10th	Day	6% 4 1 2 2 2 2 2 3 4 72%	7301
	11th to 20th	Day	3 2 1 2 1 1 2 2 3 4 79%	7596
	21st to end	Day	3 2 1 2 1 2 2 2 3 4 78%	8598
June	1st to 10th	Day	3 3 1 2 2 2 2 3 5% 6% 73%	2940
	11th to 20th	Day	1 1 1 1 1 1 1 2 4 87%	2940
	21st to end	Day	1 1 1 1 1 1 1 2 3 86%	2940
July	1st to 10th	Day	1 1 1 1 1 2 93%	6459
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 88%	6391
	21st to end	Day	1 1 1 1 1 1 1 2 3 89%	6477
August	1st to 10th	Day	1 1 1 1 1 2 2 2 2 87%	5346
	11th to 20th	Day	1 1 1 1 1 2 94%	5364
	21st to end	Day	1 1 1 1 1 2 1 2 3 87%	5984
September	1st to 10th	Day	1 1 1 1 1 1 1 2 4 86%	2400
	11th to 20th	Day	1 1 1 2 1 1 2 1 2 3 85%	2300
	21st to end	Day	1 1 1 1 1 1 2 2 3 4 83%	2200
October	1st to 10th	Day	3 2 2 2 2 1 2 3 5% 78%	3545
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 3 76%	3261
	21st to end	Day	6% 3 1 3 2 2 3 3 5% 6% 65%	3477
November	1st to 10th	Day	6% 6% 1 4 3 3 3 2 4 6% 63%	1854
	11th to 20th	Day	8% 5% 1 3 3 3 3 2 4 6% 61%	1890
	21st to end	Day	9% 5% 1 3 4 3 3 3 5% 6% 56%	1890
December	1st to 10th	Day	6% 3 1 4 3 3 4 3 5% 7% 63%	3040
	11th to 20th	Day	13% 7% 1 5% 3 3 4 4 6% 8% 46%	3147
	21st to end	Day	14% 7% 2 7% 5% 5% 5% 4 6% 9% 36%	4092
January	1st to 10th	Night	14% 9% 3 6% 3 3 3 2 4 5% 49%	8172
	11th to 20th	Night	6% 5% 2 3 2 2 2 2 5% 6% 65%	7895
	21st to end	Night	12% 6% 2 5% 4 3 4 3 5% 7% 49%	8333
February	1st to 10th	Night	14% 8% 2 5% 3 3 2 2 4 6% 49%	2520
	11th to 20th	Night	14% 7% 1 3 2 2 2 3 5% 7% 53%	2458
	21st to end	Night	13% 8% 2 5% 3 3 3 2 4 6% 51%	2076
March	1st to 10th	Night	17% 9% 2 5% 3 3 3 3 5% 7% 43%	5496
	11th to 20th	Night	10% 7% 1 4 3 3 4 3 4 6% 54%	5343
	21st to end	Night	13% 8% 2 5% 4 4 4 3 4 7% 47%	5127
April	1st to 10th	Night	10% 6% 1 3 3 3 3 3 4 7% 58%	2100
	11th to 20th	Night	9% 4 1 4 1 2 3 3 4 5% 63%	2079
	21st to end	Night	10% 4 1 2 2 1 2 1 4 7% 65%	2100
May	1st to 10th	Night	7% 3 1 2 1 2 2 2 2 5% 74%	5663
	11th to 20th	Night	4 1 1 1 1 1 1 3 4 82%	5296
	21st to end	Night	4 2 1 1 1 1 1 1 2 2 84%	5579
June	1st to 10th	Night	4 2 1 2 1 2 1 1 3 6% 76%	1680
	11th to 20th	Night	1 1 1 1 1 1 2 3 87%	1680
	21st to end	Night	1 1 1 1 1 1 2 2 90%	1680
July	1st to 10th	Night	1 1 2 94%	3720
	11th to 20th	Night	1 1 1 1 1 1 1 2 2 90%	3680
	21st to end	Night	1 1 1 1 1 1 1 2 3 88%	4372
August	1st to 10th	Night	1 1 1 1 1 1 1 2 4 87%	4486
	11th to 20th	Night	1 1 1 95%	4506
	21st to end	Night	1 1 1 1 2 2 2 3 88%	5015
September	1st to 10th	Night	1 1 1 1 1 1 1 1 2 90%	2000
	11th to 20th	Night	1 2 1 1 1 2 2 3 3 84%	2099
	21st to end	Night	1 1 2 1 1 2 2 2 3 84%	2200
October	1st to 10th	Night	2 1 1 1 1 2 1 3 3 84%	3652
	11th to 20th	Night	2 1 1 2 2 1 2 3 4 4 79%	3897
	21st to end	Night	4 3 1 2 2 2 2 2 2 4 76%	4330
November	1st to 10th	Night	6% 5% 1 3 3 2 2 2 4 6% 66%	2665
	11th to 20th	Night	7% 5% 1 4 2 2 3 2 5% 8% 60%	2730
	21st to end	Night	9% 5% 2 5% 3 3 3 3 4 6% 57%	2730
December	1st to 10th	Night	6% 3 1 3 3 3 3 3 5% 6% 64%	4370
	11th to 20th	Night	12% 6% 2 5% 4 4 4 4 6% 9% 46%	5273
	21st to end	Night	9% 10% 3 7% 6% 5% 5% 4 6% 9% 36%	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	35% 3 14% 26% 18% 4	4723
	11th to 20th	Day	20% 5% 8% 15% 23% 29%	5077
	21st to end	Day	32% 4 7% 18% 27% 12%	5880
February	1st to 10th	Day	35% 4 7% 21% 22% 10%	2100
	11th to 20th	Day	35% 5% 9% 21% 21% 8%	2060
	21st to end	Day	41% 5% 10% 23% 17% 4	1730
March	1st to 10th	Day	44% 7% 8% 14% 19% 7%	4618
	11th to 20th	Day	31% 8% 15% 30% 11% 5%	4873
	21st to end	Day	38% 9% 14% 23% 14% 3	6089
April	1st to 10th	Day	31% 6% 12% 25% 20% 5%	2520
	11th to 20th	Day	28% 7% 12% 25% 22% 6%	2492
	21st to end	Day	27% 5% 10% 34% 22% 2	2520
May	1st to 10th	Day	19% 4 8% 27% 38% 5%	7301
	11th to 20th	Day	11% 3 4 28% 44% 9%	7599
	21st to end	Day	13% 2 4 26% 51% 5%	8599
June	1st to 10th	Day	14% 5% 9% 22% 38% 12%	2940
	11th to 20th	Day	6% 3 4 16% 44% 27%	2940
	21st to end	Day	7% 2 2 9% 40% 41%	2940
July	1st to 10th	Day	3 1 1 5% 20% 70%	6391
	11th to 20th	Day	6% 1 1 6% 27% 59%	6316
	21st to end	Day	5% 2 2 13% 40% 39%	6290
August	1st to 10th	Day	8% 1 2 6% 39% 44%	3890
	11th to 20th	Day	3 2 1 6% 28% 60%	3880
	21st to end	Day	5% 1 1 10% 32% 51%	4311
September	1st to 10th	Day	6% 2 3 10% 34% 45%	2400
	11th to 20th	Day	8% 2 2 18% 43% 27%	2300
	21st to end	Day	9% 3 3 20% 38% 28%	2200
October	1st to 10th	Day	12% 3 3 11% 19% 51%	3545
	11th to 20th	Day	15% 2 3 24% 33% 23%	3261
	21st to end	Day	22% 3 3 13% 33% 27%	3477
November	1st to 10th	Day	23% 5% 7% 18% 28% 19%	1854
	11th to 20th	Day	25% 4 8% 23% 26% 14%	1890
	21st to end	Day	29% 5% 6% 18% 25% 17%	1890
December	1st to 10th	Day	23% 3 7% 24% 27% 16%	3040
	11th to 20th	Day	35% 5% 11% 18% 22% 8%	3147
	21st to end	Day	41% 5% 15% 21% 14% 4	4092
January	1st to 10th	Night	37% 2 15% 23% 18% 6%	8172
	11th to 20th	Night	20% 5% 8% 16% 28% 23%	7895
	21st to end	Night	34% 5% 5% 21% 24% 11%	8333
February	1st to 10th	Night	38% 4 6% 21% 20% 11%	2520
	11th to 20th	Night	32% 5% 9% 21% 24% 9%	2459
	21st to end	Night	35% 6% 11% 25% 18% 5%	2076
March	1st to 10th	Night	40% 7% 9% 17% 21% 6%	5496
	11th to 20th	Night	30% 7% 16% 31% 10% 6%	5343
	21st to end	Night	36% 7% 21% 21% 13% 2	5127
April	1st to 10th	Night	29% 6% 12% 28% 18% 7%	2100
	11th to 20th	Night	24% 7% 12% 30% 21% 6%	2079
	21st to end	Night	23% 4 12% 38% 21% 2	2100
May	1st to 10th	Night	18% 3 8% 33% 33% 5%	5661
	11th to 20th	Night	10% 2 3 31% 47% 7%	5297
	21st to end	Night	11% 2 3 31% 51% 3	5580
June	1st to 10th	Night	13% 4 10% 23% 40% 11%	1680
	11th to 20th	Night	7% 3 5% 17% 40% 29%	1680
	21st to end	Night	5% 2 2 11% 44% 37%	1680
July	1st to 10th	Night	2 1 1 4 22% 70%	3680
	11th to 20th	Night	5% 1 1 6% 24% 62%	3632
	21st to end	Night	6% 1 2 14% 36% 41%	4249
August	1st to 10th	Night	8% 1 2 9% 38% 43%	3256
	11th to 20th	Night	2 1 2 9% 25% 62%	3256
	21st to end	Night	4 1 2 9% 35% 50%	3608
September	1st to 10th	Night	5% 2 2 12% 32% 48%	2000
	11th to 20th	Night	8% 2 5% 16% 45% 25%	2099
	21st to end	Night	9% 2 3 21% 35% 30%	2200
October	1st to 10th	Night	9% 3 3 11% 23% 51%	3652
	11th to 20th	Night	11% 2 3 25% 39% 20%	3897
	21st to end	Night	16% 2 3 16% 36% 26%	4330
November	1st to 10th	Night	21% 5% 8% 18% 28% 20%	2665
	11th to 20th	Night	24% 4 10% 19% 25% 19%	2730
	21st to end	Night	29% 4 7% 17% 27% 16%	2730
December	1st to 10th	Night	23% 3 6% 28% 26% 16%	4370
	11th to 20th	Night	34% 5% 11% 22% 19% 9%	5273
	21st to end	Night	42% 7% 15% 21% 11% 4	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	21% 13% 27% 28% 10%	220
	11th to 20th	Day	13% 8% 32% 30% 13% 4	336
	21st to end	Day	15% 10% 29% 31% 14% 2	479
February	1st to 10th	Day	18% 13% 29% 28% 11% 2	540
	11th to 20th	Day	17% 17% 34% 24% 7% 1	535
	21st to end	Day	15% 15% 38% 25% 6% 1	446
March	1st to 10th	Day	18% 18% 34% 23% 6% 1	540
	11th to 20th	Day	15% 17% 34% 25% 7% 1	540
	21st to end	Day	18% 21% 34% 22% 6% 1	594
April	1st to 10th	Day	13% 17% 31% 26% 10% 2	540
	11th to 20th	Day	15% 21% 30% 26% 7% 1	532
	21st to end	Day	11% 19% 29% 26% 11% 2	540
May	1st to 10th	Day	11% 20% 32% 27% 10% 1	543
	11th to 20th	Day	11% 10% 33% 31% 13% 2	568
	21st to end	Day	9% 14% 28% 34% 11% 4	617
June	1st to 10th	Day	2 12% 30% 38% 15% 4	570
	11th to 20th	Day	2 12% 31% 38% 13% 4	570
	21st to end	Day	4 8% 29% 40% 14% 4	570
July	1st to 10th	Day	1 4 27% 46% 15% 6%	566
	11th to 20th	Day	1 2 15% 51% 23% 7%	545
	21st to end	Day	2 2 16% 55% 19% 6%	575
August	1st to 10th	Day	3 5% 28% 42% 15% 6%	527
	11th to 20th	Day	1 4 25% 48% 16% 5%	517
	21st to end	Day	1 2 25% 49% 17% 5%	583
September	1st to 10th	Day	2 4 29% 45% 16% 4	530
	11th to 20th	Day	3 10% 31% 42% 12% 2	530
	21st to end	Day	3 11% 31% 36% 17% 1	530
October	1st to 10th	Day	4 13% 30% 37% 15% 1	530
	11th to 20th	Day	5% 10% 33% 38% 12% 2	530
	21st to end	Day	6% 11% 27% 39% 16% 1	538
November	1st to 10th	Day	5% 13% 27% 38% 15% 2	330
	11th to 20th	Day	10% 12% 30% 29% 16% 3	330
	21st to end	Day	14% 10% 26% 33% 13% 3	330
December	1st to 10th	Day	12% 10% 33% 32% 12% 2	330
	11th to 20th	Day	14% 9% 27% 27% 20% 3	234
	21st to end	Day	25% 12% 29% 24% 9% 1	242
January	1st to 10th	Night	17% 8% 26% 32% 14% 3	840
	11th to 20th	Night	9% 6% 30% 40% 13% 2	840
	21st to end	Night	16% 5% 28% 35% 13% 2	924
February	1st to 10th	Night	17% 6% 26% 34% 15% 2	840
	11th to 20th	Night	16% 8% 26% 31% 16% 4	824
	21st to end	Night	18% 9% 26% 31% 13% 2	692
March	1st to 10th	Night	16% 10% 24% 35% 13% 2	840
	11th to 20th	Night	13% 6% 25% 33% 19% 4	840
	21st to end	Night	16% 8% 20% 39% 16% 1	924
April	1st to 10th	Night	13% 5% 23% 40% 16% 2	840
	11th to 20th	Night	11% 6% 26% 33% 22% 2	831
	21st to end	Night	11% 6% 23% 36% 21% 3	840
May	1st to 10th	Night	9% 5% 24% 39% 23% 1	751
	11th to 20th	Night	6% 6% 20% 41% 25% 2	621
	21st to end	Night	9% 6% 20% 39% 24% 2	594
June	1st to 10th	Night	5% 3 17% 45% 30% 1	420
	11th to 20th	Night	2 3 17% 46% 29% 4	420
	21st to end	Night	2 16% 43% 35% 4	420
July	1st to 10th	Night	1 10% 39% 45% 4	410
	11th to 20th	Night	1 3 37% 51% 8%	398
	21st to end	Night	1 1 8% 32% 45% 12%	611
August	1st to 10th	Night	2 1 8% 43% 40% 6%	796
	11th to 20th	Night	1 8% 39% 41% 10%	792
	21st to end	Night	1 7% 34% 49% 9%	880
September	1st to 10th	Night	1 1 8% 40% 41% 10%	800
	11th to 20th	Night	2 1 14% 45% 32% 6%	800
	21st to end	Night	2 2 17% 39% 35% 5%	800
October	1st to 10th	Night	4 2 18% 47% 26% 3	800
	11th to 20th	Night	4 3 19% 41% 27% 5%	800
	21st to end	Night	5% 2 20% 39% 28% 6%	880
November	1st to 10th	Night	9% 5% 17% 41% 25% 4	820
	11th to 20th	Night	10% 4 22% 39% 22% 3	840
	21st to end	Night	12% 3 25% 39% 19% 3	840
December	1st to 10th	Night	10% 4 29% 37% 17% 3	840
	11th to 20th	Night	16% 5% 24% 38% 15% 2	840
	21st to end	Night	18% 7% 30% 33% 10% 2	924

Page generated at 2026-07-13 00:14:48. Version ac07

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