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 2025-03-12.

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 3 2 4 5% 68%	112731
	Night	7% 4 1 3 2 2 2 2 4 5% 66%	115100
Transparency	Day	20% 5% 7% 18% 27% 24%	112483
	Night	22% 4 8% 20% 26% 20%	114940
Seeing	Day	9% 11% 28% 36% 13% 3	15740
	Night	9% 4 19% 38% 26% 4	25471

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	7% 9% 3 6% 3 3 3 2 5% 7% 51%	3639
	11th to 20th	Day	6% 5% 2 5% 3 3 3 4 5% 6% 57%	3893
	21st to end	Day	12% 8% 2 5% 3 3 4 4 6% 7% 45%	4533
February	1st to 10th	Day	14% 9% 2 4 3 3 3 3 5% 6% 49%	2000
	11th to 20th	Day	12% 9% 1 4 3 3 3 3 5% 7% 49%	1950
	21st to end	Day	14% 10% 3 6% 4 3 2 3 4 5% 44%	1650
March	1st to 10th	Day	14% 14% 3 6% 4 4 4 3 5% 7% 36%	3238
	11th to 20th	Day	6% 6% 2 4 3 3 4 3 5% 8% 56%	3335
	21st to end	Day	14% 11% 2 6% 4 4 4 4 6% 7% 37%	4154
April	1st to 10th	Day	10% 6% 2 5% 4 3 3 3 4 6% 54%	2280
	11th to 20th	Day	10% 5% 1 4 3 3 4 3 6% 6% 56%	2249
	21st to end	Day	9% 5% 1 3 3 3 3 3 4 6% 60%	2280
May	1st to 10th	Day	5% 4 1 2 2 2 3 2 4 5% 72%	3941
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 5% 76%	4071
	21st to end	Day	4 2 1 2 1 1 2 2 3 4 80%	4601
June	1st to 10th	Day	3 3 1 2 2 2 2 3 5% 6% 71%	2660
	11th to 20th	Day	1 1 1 1 1 1 1 2 4 86%	2660
	21st to end	Day	1 1 1 1 1 2 1 1 3 3 86%	2660
July	1st to 10th	Day	1 1 1 1 1 2 2 90%	4420
	11th to 20th	Day	1 1 1 1 2 2 2 3 3 84%	4420
	21st to end	Day	1 1 1 1 1 1 1 2 3 86%	4504
August	1st to 10th	Day	1 1 1 1 1 2 1 2 2 87%	3783
	11th to 20th	Day	1 1 1 1 1 1 1 2 91%	3724
	21st to end	Day	1 1 1 1 1 1 1 2 91%	4180
September	1st to 10th	Day	1 1 1 1 1 2 1 2 4 86%	2280
	11th to 20th	Day	1 1 1 2 1 1 2 1 2 3 86%	2185
	21st to end	Day	1 1 1 1 1 2 2 3 4 83%	2090
October	1st to 10th	Day	3 2 2 2 2 2 2 3 5% 78%	3437
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 3 77%	3161
	21st to end	Day	6% 3 1 3 2 3 3 3 5% 6% 65%	3369
November	1st to 10th	Day	6% 6% 1 4 3 3 2 2 4 6% 63%	1764
	11th to 20th	Day	6% 4 1 3 3 3 3 2 4 7% 63%	1800
	21st to end	Day	10% 6% 1 4 4 3 3 3 5% 6% 55%	1800
December	1st to 10th	Day	6% 3 1 4 3 3 4 3 5% 7% 62%	2950
	11th to 20th	Day	13% 7% 2 5% 3 4 4 4 6% 8% 44%	3066
	21st to end	Day	14% 7% 2 7% 5% 5% 5% 4 6% 9% 37%	4004
January	1st to 10th	Night	10% 9% 2 5% 3 3 3 2 4 6% 52%	6306
	11th to 20th	Night	8% 7% 2 4 2 2 3 2 5% 6% 59%	6073
	21st to end	Night	15% 6% 2 4 3 3 4 3 5% 7% 48%	6443
February	1st to 10th	Night	15% 8% 2 5% 3 3 2 3 5% 6% 48%	2400
	11th to 20th	Night	14% 7% 1 3 2 2 2 3 5% 8% 54%	2338
	21st to end	Night	14% 8% 2 5% 3 3 3 2 4 6% 51%	1980
March	1st to 10th	Night	17% 8% 3 4 3 3 3 3 5% 8% 41%	3856
	11th to 20th	Night	7% 5% 1 3 3 3 4 3 5% 7% 59%	3670
	21st to end	Night	16% 10% 2 5% 3 4 4 3 5% 6% 42%	3498
April	1st to 10th	Night	10% 5% 1 3 3 3 3 3 4 7% 58%	1900
	11th to 20th	Night	9% 4 1 4 2 2 3 3 4 5% 64%	1879
	21st to end	Night	9% 4 1 2 1 1 2 1 4 8% 67%	1900
May	1st to 10th	Night	7% 3 2 1 1 2 1 2 4 76%	3034
	11th to 20th	Night	4 1 1 1 1 2 2 3 5% 79%	2834
	21st to end	Night	4 1 1 1 1 1 1 1 2 3 83%	2964
June	1st to 10th	Night	5% 2 1 2 1 2 1 2 4 6% 74%	1520
	11th to 20th	Night	2 1 1 1 1 1 2 3 86%	1520
	21st to end	Night	1 1 1 1 1 1 2 3 89%	1520
July	1st to 10th	Night	1 1 1 1 2 3 92%	2540
	11th to 20th	Night	1 1 1 1 1 1 1 2 3 86%	2540
	21st to end	Night	1 1 1 2 1 1 1 2 2 87%	3037
August	1st to 10th	Night	1 1 1 1 1 1 1 2 4 85%	3166
	11th to 20th	Night	1 1 1 1 2 2 93%	3126
	21st to end	Night	1 1 1 1 1 1 1 93%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 1 2 90%	1900
	11th to 20th	Night	1 1 1 1 2 2 2 3 3 85%	1994
	21st to end	Night	1 1 1 1 1 2 2 2 3 85%	2090
October	1st to 10th	Night	2 1 1 1 1 1 2 1 3 3 84%	3540
	11th to 20th	Night	2 1 1 2 2 2 2 3 4 4 79%	3777
	21st to end	Night	4 3 1 2 2 2 2 2 2 4 76%	4196
November	1st to 10th	Night	6% 5% 1 3 3 2 2 2 4 6% 65%	2535
	11th to 20th	Night	6% 4 1 4 2 2 3 2 5% 8% 62%	2600
	21st to end	Night	9% 5% 2 5% 3 3 3 3 5% 6% 56%	2600
December	1st to 10th	Night	6% 3 1 4 3 3 3 3 5% 6% 63%	4240
	11th to 20th	Night	12% 6% 2 5% 4 4 4 4 6% 9% 45%	5134
	21st to end	Night	9% 10% 3 7% 6% 5% 5% 4 6% 9% 36%	6952

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	29% 4 16% 27% 19% 5%	3639
	11th to 20th	Day	24% 6% 10% 18% 20% 22%	3893
	21st to end	Day	35% 6% 9% 15% 23% 11%	4533
February	1st to 10th	Day	36% 5% 8% 22% 21% 9%	2000
	11th to 20th	Day	35% 6% 10% 21% 21% 8%	1960
	21st to end	Day	42% 5% 9% 23% 17% 4	1650
March	1st to 10th	Day	44% 10% 9% 16% 15% 6%	3238
	11th to 20th	Day	23% 11% 16% 31% 13% 7%	3335
	21st to end	Day	40% 12% 14% 21% 12% 2	4154
April	1st to 10th	Day	32% 6% 12% 26% 19% 5%	2280
	11th to 20th	Day	28% 7% 12% 25% 21% 6%	2252
	21st to end	Day	25% 6% 10% 34% 22% 2	2280
May	1st to 10th	Day	15% 7% 8% 31% 32% 7%	3941
	11th to 20th	Day	11% 5% 7% 26% 46% 5%	4074
	21st to end	Day	12% 4 5% 34% 43% 3	4601
June	1st to 10th	Day	15% 5% 10% 22% 36% 12%	2660
	11th to 20th	Day	7% 4 4 17% 44% 25%	2660
	21st to end	Day	7% 3 2 9% 39% 40%	2660
July	1st to 10th	Day	4 2 1 6% 16% 71%	4351
	11th to 20th	Day	8% 1 2 7% 20% 61%	4345
	21st to end	Day	7% 2 2 14% 29% 46%	4362
August	1st to 10th	Day	8% 2 2 6% 39% 44%	3770
	11th to 20th	Day	3 2 1 7% 27% 61%	3760
	21st to end	Day	4 1 1 10% 32% 52%	4179
September	1st to 10th	Day	6% 3 3 9% 33% 46%	2280
	11th to 20th	Day	7% 2 2 18% 42% 28%	2185
	21st to end	Day	8% 3 2 20% 39% 29%	2090
October	1st to 10th	Day	12% 3 3 11% 19% 52%	3437
	11th to 20th	Day	15% 2 3 24% 33% 24%	3161
	21st to end	Day	22% 3 3 13% 33% 27%	3369
November	1st to 10th	Day	24% 5% 7% 19% 28% 18%	1764
	11th to 20th	Day	23% 4 9% 23% 27% 15%	1800
	21st to end	Day	30% 5% 6% 16% 26% 17%	1800
December	1st to 10th	Day	23% 3 7% 24% 27% 16%	2950
	11th to 20th	Day	36% 5% 11% 17% 22% 8%	3066
	21st to end	Day	40% 5% 15% 21% 14% 4	4004
January	1st to 10th	Night	31% 3 15% 23% 22% 6%	6306
	11th to 20th	Night	25% 6% 10% 18% 26% 15%	6073
	21st to end	Night	35% 6% 7% 18% 22% 12%	6443
February	1st to 10th	Night	38% 5% 6% 21% 20% 10%	2400
	11th to 20th	Night	30% 6% 10% 21% 24% 9%	2339
	21st to end	Night	36% 6% 11% 25% 18% 5%	1980
March	1st to 10th	Night	38% 10% 10% 18% 19% 4	3856
	11th to 20th	Night	22% 10% 16% 31% 13% 7%	3670
	21st to end	Night	41% 9% 16% 23% 10% 1	3498
April	1st to 10th	Night	29% 7% 13% 28% 17% 6%	1900
	11th to 20th	Night	23% 8% 13% 29% 21% 6%	1879
	21st to end	Night	21% 5% 12% 38% 22% 2	1900
May	1st to 10th	Night	16% 5% 6% 37% 29% 7%	3034
	11th to 20th	Night	10% 4 5% 34% 44% 2	2835
	21st to end	Night	10% 3 6% 36% 43% 2	2964
June	1st to 10th	Night	14% 4 11% 23% 38% 10%	1520
	11th to 20th	Night	7% 3 6% 18% 41% 25%	1520
	21st to end	Night	5% 2 2 12% 44% 35%	1520
July	1st to 10th	Night	3 1 1 6% 18% 71%	2500
	11th to 20th	Night	7% 1 2 8% 21% 61%	2492
	21st to end	Night	7% 2 3 14% 25% 50%	2943
August	1st to 10th	Night	8% 1 2 9% 39% 42%	3156
	11th to 20th	Night	2 1 2 9% 24% 62%	3156
	21st to end	Night	4 1 2 9% 34% 51%	3498
September	1st to 10th	Night	5% 2 1 11% 31% 50%	1900
	11th to 20th	Night	7% 2 5% 16% 45% 26%	1994
	21st to end	Night	8% 2 3 21% 35% 31%	2090
October	1st to 10th	Night	9% 3 3 11% 23% 52%	3540
	11th to 20th	Night	11% 2 3 25% 39% 20%	3777
	21st to end	Night	16% 2 3 16% 37% 26%	4196
November	1st to 10th	Night	22% 5% 8% 18% 27% 19%	2535
	11th to 20th	Night	22% 4 10% 19% 26% 20%	2600
	21st to end	Night	30% 4 7% 16% 26% 17%	2600
December	1st to 10th	Night	23% 3 6% 27% 25% 16%	4240
	11th to 20th	Night	34% 5% 11% 22% 19% 9%	5134
	21st to end	Night	42% 7% 15% 22% 12% 4	6952

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% 13% 28% 29% 10% 1	200
	11th to 20th	Day	14% 9% 33% 29% 12% 3	310
	21st to end	Day	16% 9% 29% 31% 14% 1	445
February	1st to 10th	Day	18% 13% 29% 28% 11% 2	500
	11th to 20th	Day	16% 16% 35% 25% 7% 2	495
	21st to end	Day	15% 14% 38% 25% 6%	414
March	1st to 10th	Day	19% 17% 33% 24% 6% 1	500
	11th to 20th	Day	15% 19% 32% 27% 7% 1	465
	21st to end	Day	19% 22% 33% 22% 4	506
April	1st to 10th	Day	14% 17% 30% 27% 10% 1	460
	11th to 20th	Day	15% 22% 29% 26% 7% 1	452
	21st to end	Day	10% 19% 29% 28% 12% 2	460
May	1st to 10th	Day	9% 21% 31% 28% 10% 1	463
	11th to 20th	Day	9% 10% 32% 33% 14% 2	488
	21st to end	Day	10% 14% 26% 35% 12% 4	529
June	1st to 10th	Day	2 11% 28% 38% 16% 5%	490
	11th to 20th	Day	2 12% 28% 39% 15% 4	490
	21st to end	Day	2 8% 29% 41% 16% 4	490
July	1st to 10th	Day	1 2 23% 49% 18% 7%	486
	11th to 20th	Day	1 2 14% 50% 25% 8%	484
	21st to end	Day	2 2 15% 55% 20% 6%	531
August	1st to 10th	Day	3 5% 28% 43% 16% 6%	487
	11th to 20th	Day	1 4 25% 48% 16% 5%	477
	21st to end	Day	1 2 23% 50% 19% 4	539
September	1st to 10th	Day	3 3 27% 46% 17% 4	490
	11th to 20th	Day	2 10% 31% 43% 12% 1	490
	21st to end	Day	3 10% 30% 39% 18% 1	490
October	1st to 10th	Day	4 12% 30% 38% 16% 1	490
	11th to 20th	Day	4 10% 32% 40% 12% 1	490
	21st to end	Day	6% 11% 27% 39% 17% 1	496
November	1st to 10th	Day	6% 13% 27% 37% 14% 2	300
	11th to 20th	Day	7% 11% 30% 30% 17% 4	300
	21st to end	Day	16% 10% 25% 33% 13% 4	300
December	1st to 10th	Day	13% 10% 32% 33% 11% 1	300
	11th to 20th	Day	15% 9% 25% 29% 21% 2	213
	21st to end	Day	23% 12% 29% 25% 10% 1	220
January	1st to 10th	Night	16% 7% 26% 33% 14% 3	800
	11th to 20th	Night	9% 6% 31% 38% 14% 2	800
	21st to end	Night	17% 5% 28% 35% 14% 2	880
February	1st to 10th	Night	18% 6% 26% 34% 15% 2	800
	11th to 20th	Night	16% 5% 26% 32% 16% 4	784
	21st to end	Night	18% 7% 26% 33% 14% 2	660
March	1st to 10th	Night	16% 9% 24% 36% 14% 2	800
	11th to 20th	Night	12% 5% 24% 33% 21% 4	766
	21st to end	Night	17% 8% 19% 38% 18% 1	836
April	1st to 10th	Night	12% 5% 22% 41% 18% 2	760
	11th to 20th	Night	10% 6% 24% 34% 24% 2	751
	21st to end	Night	10% 6% 22% 36% 23% 3	760
May	1st to 10th	Night	8% 5% 23% 39% 24% 1	679
	11th to 20th	Night	6% 5% 18% 41% 27% 3	561
	21st to end	Night	9% 6% 19% 38% 25% 2	536
June	1st to 10th	Night	5% 3 13% 46% 32% 1	380
	11th to 20th	Night	2 2 14% 47% 32% 4	380
	21st to end	Night	2 12% 43% 38% 5%	380
July	1st to 10th	Night	1 6% 38% 50% 5%	370
	11th to 20th	Night	1 3 34% 54% 8%	368
	21st to end	Night	1 2 6% 31% 48% 13%	580
August	1st to 10th	Night	2 1 7% 43% 42% 6%	756
	11th to 20th	Night	1 8% 38% 43% 11%	752
	21st to end	Night	1 6% 32% 51% 10%	836
September	1st to 10th	Night	1 1 7% 38% 43% 10%	760
	11th to 20th	Night	2 1 13% 45% 33% 7%	760
	21st to end	Night	1 1 16% 39% 37% 6%	760
October	1st to 10th	Night	3 2 17% 48% 27% 4	760
	11th to 20th	Night	4 3 17% 42% 29% 6%	760
	21st to end	Night	5% 2 20% 38% 28% 6%	836
November	1st to 10th	Night	9% 5% 17% 41% 24% 4	780
	11th to 20th	Night	9% 3 21% 40% 23% 3	800
	21st to end	Night	12% 3 25% 39% 19% 3	800
December	1st to 10th	Night	11% 4 29% 37% 17% 3	800
	11th to 20th	Night	17% 5% 24% 38% 14% 2	800
	21st to end	Night	18% 6% 29% 34% 11% 2	880

Page generated at 2025-03-09 05:08:47. Version ac07

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