Cosmic Obsession Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cosmic Obsession Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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	13% 6% 2 6% 5% 5% 5% 4 6% 7% 41%	120279
	Night	15% 7% 2 5% 4 3 4 3 5% 7% 45%	119054
Transparency	Day	40% 6% 11% 19% 17% 8%	123983
	Night	37% 5% 11% 20% 18% 9%	123480
Seeing	Day	13% 8% 20% 26% 23% 10%	15807
	Night	15% 6% 23% 33% 20% 2	25647

By Month

Percentage Cloud Cover SE (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	20% 9% 3 6% 4 3 3 3 3 4 42%	4077
	11th to 20th	Day	16% 6% 2 5% 4 3 3 2 3 5% 51%	4080
	21st to end	Day	31% 8% 2 5% 3 2 2 2 4 6% 36%	4488
February	1st to 10th	Day	23% 7% 3 6% 3 4 3 3 5% 5% 38%	1910
	11th to 20th	Day	22% 9% 3 6% 4 3 3 3 4 4 38%	1949
	21st to end	Day	19% 9% 2 6% 4 4 4 4 3 6% 40%	1814
March	1st to 10th	Day	16% 7% 2 7% 5% 4 4 4 5% 7% 39%	4936
	11th to 20th	Day	14% 6% 2 7% 4 4 3 3 5% 6% 46%	4980
	21st to end	Day	18% 7% 2 7% 4 4 4 3 4 5% 40%	5462
April	1st to 10th	Day	15% 8% 3 8% 5% 6% 4 3 4 6% 38%	2200
	11th to 20th	Day	18% 6% 3 6% 5% 5% 4 4 5% 6% 39%	2316
	21st to end	Day	12% 7% 3 7% 5% 5% 4 4 6% 6% 41%	2400
May	1st to 10th	Day	19% 7% 3 7% 5% 5% 5% 4 6% 7% 31%	5588
	11th to 20th	Day	11% 6% 2 7% 4 5% 5% 4 6% 8% 43%	5862
	21st to end	Day	10% 7% 3 7% 5% 5% 6% 5% 7% 10% 35%	6434
June	1st to 10th	Day	6% 5% 2 6% 6% 5% 6% 5% 8% 9% 44%	2600
	11th to 20th	Day	6% 3 2 5% 5% 6% 7% 6% 8% 11% 42%	2600
	21st to end	Day	6% 5% 3 8% 8% 7% 8% 7% 8% 9% 30%	2600
July	1st to 10th	Day	9% 4 2 6% 5% 6% 7% 6% 8% 12% 36%	4250
	11th to 20th	Day	6% 7% 3 8% 6% 6% 6% 6% 8% 12% 32%	4250
	21st to end	Day	14% 9% 3 8% 6% 5% 6% 5% 8% 10% 26%	4466
August	1st to 10th	Day	2 1 1 3 3 4 5% 4 6% 10% 61%	4104
	11th to 20th	Day	3 2 2 4 4 5% 6% 6% 10% 13% 45%	3916
	21st to end	Day	6% 4 2 8% 7% 6% 7% 5% 7% 9% 38%	4060
September	1st to 10th	Day	6% 5% 3 8% 4 6% 6% 5% 7% 8% 40%	2090
	11th to 20th	Day	11% 7% 2 8% 7% 6% 5% 4 6% 7% 37%	2090
	21st to end	Day	7% 4 2 7% 5% 4 5% 5% 7% 7% 47%	2090
October	1st to 10th	Day	7% 4 2 4 3 4 4 4 5% 7% 57%	1987
	11th to 20th	Day	9% 5% 3 5% 5% 4 4 3 4 6% 52%	1900
	21st to end	Day	13% 5% 2 5% 3 3 3 3 4 5% 53%	2090
November	1st to 10th	Day	10% 6% 3 6% 4 5% 5% 4 5% 7% 45%	1958
	11th to 20th	Day	12% 6% 2 6% 3 3 4 4 6% 5% 48%	2000
	21st to end	Day	16% 6% 2 6% 5% 4 4 3 4 5% 43%	1913
December	1st to 10th	Day	22% 8% 2 7% 4 3 3 3 4 6% 38%	2940
	11th to 20th	Day	15% 8% 3 6% 4 3 4 3 5% 7% 42%	3391
	21st to end	Day	16% 6% 2 6% 3 3 4 3 4 5% 47%	4488
January	1st to 10th	Night	20% 6% 2 5% 3 3 3 3 4 5% 47%	5888
	11th to 20th	Night	16% 6% 2 4 3 3 3 2 3 5% 52%	5900
	21st to end	Night	30% 8% 2 5% 3 2 3 2 4 6% 35%	6490
February	1st to 10th	Night	21% 8% 3 6% 4 3 3 2 4 6% 39%	2505
	11th to 20th	Night	22% 8% 2 5% 3 3 3 2 3 5% 43%	2337
	21st to end	Night	23% 8% 2 5% 3 3 3 3 4 6% 40%	1815
March	1st to 10th	Night	20% 9% 3 6% 3 4 3 4 4 6% 38%	4919
	11th to 20th	Night	18% 8% 2 6% 3 3 3 2 5% 5% 45%	4980
	21st to end	Night	20% 9% 2 6% 4 3 3 3 3 5% 41%	5467
April	1st to 10th	Night	15% 7% 3 6% 5% 3 4 4 5% 7% 41%	2200
	11th to 20th	Night	17% 6% 2 5% 3 3 3 3 5% 7% 44%	2030
	21st to end	Night	15% 7% 2 5% 4 4 4 4 5% 7% 44%	2000
May	1st to 10th	Night	24% 9% 3 6% 4 4 4 3 5% 7% 29%	4297
	11th to 20th	Night	12% 7% 3 6% 4 4 5% 4 6% 9% 41%	4053
	21st to end	Night	13% 8% 3 7% 4 4 5% 5% 8% 10% 33%	4443
June	1st to 10th	Night	6% 3 1 4 3 4 4 4 7% 10% 54%	1800
	11th to 20th	Night	4 4 1 3 3 3 4 4 7% 11% 56%	1800
	21st to end	Night	5% 4 2 5% 5% 5% 5% 6% 8% 11% 45%	1800
July	1st to 10th	Night	6% 4 2 5% 4 4 5% 5% 8% 12% 44%	2940
	11th to 20th	Night	4 5% 2 6% 5% 5% 5% 4 6% 12% 46%	2940
	21st to end	Night	9% 8% 3 8% 5% 5% 5% 4 7% 9% 38%	3098
August	1st to 10th	Night	1 1 1 3 2 2 3 3 6% 9% 69%	2836
	11th to 20th	Night	2 2 1 3 2 3 4 4 8% 13% 59%	2917
	21st to end	Night	4 3 2 5% 4 5% 6% 5% 8% 12% 47%	3590
September	1st to 10th	Night	4 5% 2 6% 5% 5% 4 4 8% 9% 47%	2090
	11th to 20th	Night	10% 6% 2 7% 5% 4 4 3 7% 7% 44%	2090
	21st to end	Night	8% 4 2 4 3 3 3 3 5% 9% 55%	2090
October	1st to 10th	Night	4 4 2 4 3 3 3 3 6% 6% 62%	2171
	11th to 20th	Night	10% 4 1 3 3 2 3 3 4 5% 60%	2280
	21st to end	Night	11% 5% 2 4 3 2 2 2 3 4 60%	2508
November	1st to 10th	Night	11% 6% 2 5% 4 4 4 3 5% 6% 52%	2340
	11th to 20th	Night	13% 6% 2 6% 3 4 3 3 5% 5% 51%	2400
	21st to end	Night	16% 6% 2 7% 3 3 4 4 5% 6% 45%	2484
December	1st to 10th	Night	21% 9% 4 8% 4 3 3 3 4 6% 35%	4250
	11th to 20th	Night	16% 10% 3 6% 4 3 3 3 3 5% 44%	4816
	21st to end	Night	15% 7% 2 5% 3 3 3 3 4 5% 49%	6490

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	48% 2 4 20% 18% 7%	4078
	11th to 20th	Day	38% 3 4 18% 24% 13%	4080
	21st to end	Day	53% 2 3 12% 16% 14%	4488
February	1st to 10th	Day	50% 4 5% 13% 17% 12%	1910
	11th to 20th	Day	50% 4 6% 14% 17% 10%	1959
	21st to end	Day	47% 5% 7% 16% 14% 11%	1814
March	1st to 10th	Day	45% 5% 7% 15% 17% 12%	4936
	11th to 20th	Day	39% 5% 6% 11% 20% 20%	4980
	21st to end	Day	46% 4 7% 19% 18% 6%	5464
April	1st to 10th	Day	48% 5% 6% 16% 15% 9%	2200
	11th to 20th	Day	45% 4 9% 15% 17% 9%	2316
	21st to end	Day	40% 6% 8% 20% 19% 6%	2400
May	1st to 10th	Day	50% 6% 10% 20% 11% 3	5623
	11th to 20th	Day	38% 5% 12% 22% 20% 2	5866
	21st to end	Day	40% 8% 12% 28% 11% 1	6447
June	1st to 10th	Day	31% 9% 18% 24% 17% 1	2600
	11th to 20th	Day	31% 10% 17% 24% 15% 3	2600
	21st to end	Day	44% 12% 15% 20% 8%	2600
July	1st to 10th	Day	39% 9% 28% 21% 3	4186
	11th to 20th	Day	43% 8% 20% 21% 8%	4181
	21st to end	Day	49% 10% 19% 19% 3 1	4326
August	1st to 10th	Day	18% 7% 16% 39% 20% 1	4090
	11th to 20th	Day	26% 8% 32% 27% 6% 1	3955
	21st to end	Day	40% 10% 15% 20% 13% 1	4059
September	1st to 10th	Day	39% 9% 14% 19% 14% 5%	2090
	11th to 20th	Day	43% 8% 12% 17% 16% 4	2090
	21st to end	Day	33% 7% 12% 18% 17% 12%	2090
October	1st to 10th	Day	18% 4 5% 15% 18% 40%	3299
	11th to 20th	Day	23% 4 5% 16% 34% 18%	3140
	21st to end	Day	29% 3 4 16% 35% 13%	3428
November	1st to 10th	Day	38% 5% 9% 18% 20% 10%	1958
	11th to 20th	Day	37% 4 10% 13% 23% 14%	2000
	21st to end	Day	43% 5% 7% 16% 17% 11%	1913
December	1st to 10th	Day	49% 3 6% 16% 21% 5%	2940
	11th to 20th	Day	41% 5% 7% 15% 21% 11%	3389
	21st to end	Day	40% 4 5% 16% 25% 11%	4488
January	1st to 10th	Night	41% 3 5% 22% 20% 9%	5888
	11th to 20th	Night	37% 3 4 19% 26% 12%	5900
	21st to end	Night	52% 2 3 13% 17% 12%	6490
February	1st to 10th	Night	47% 4 5% 12% 16% 14%	2505
	11th to 20th	Night	45% 3 7% 15% 18% 12%	2337
	21st to end	Night	46% 5% 6% 14% 16% 13%	1815
March	1st to 10th	Night	48% 4 6% 14% 19% 9%	4919
	11th to 20th	Night	42% 4 6% 14% 16% 18%	4980
	21st to end	Night	46% 4 7% 15% 19% 8%	5467
April	1st to 10th	Night	42% 6% 9% 18% 15% 10%	2200
	11th to 20th	Night	39% 5% 11% 17% 15% 13%	2030
	21st to end	Night	40% 5% 9% 20% 18% 7%	2000
May	1st to 10th	Night	54% 5% 9% 20% 9% 3	4327
	11th to 20th	Night	37% 6% 14% 21% 18% 4	4054
	21st to end	Night	44% 7% 16% 25% 7% 1	4452
June	1st to 10th	Night	22% 8% 19% 32% 18% 1	1800
	11th to 20th	Night	22% 7% 19% 31% 17% 4	1800
	21st to end	Night	30% 9% 21% 27% 11% 1	1800
July	1st to 10th	Night	30% 8% 33% 25% 4	2895
	11th to 20th	Night	31% 9% 25% 26% 9%	2886
	21st to end	Night	41% 10% 21% 23% 4	2999
August	1st to 10th	Night	12% 5% 19% 44% 19% 1	2826
	11th to 20th	Night	16% 7% 35% 34% 7% 1	2944
	21st to end	Night	27% 13% 21% 28% 9% 1	3590
September	1st to 10th	Night	30% 10% 21% 21% 13% 5%	2090
	11th to 20th	Night	36% 8% 14% 21% 16% 5%	2090
	21st to end	Night	27% 5% 15% 22% 18% 12%	2090
October	1st to 10th	Night	16% 4 7% 18% 16% 40%	3626
	11th to 20th	Night	16% 3 5% 22% 32% 22%	3779
	21st to end	Night	22% 3 5% 18% 36% 16%	4120
November	1st to 10th	Night	33% 5% 11% 20% 21% 10%	2340
	11th to 20th	Night	34% 5% 9% 18% 21% 14%	2400
	21st to end	Night	40% 4 8% 18% 18% 11%	2484
December	1st to 10th	Night	52% 3 5% 15% 18% 6%	4250
	11th to 20th	Night	44% 3 9% 9% 21% 14%	4817
	21st to end	Night	38% 4 6% 14% 30% 9%	6490

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	24% 14% 34% 21% 5% 1	300
	11th to 20th	Day	26% 14% 28% 25% 6% 1	300
	21st to end	Day	25% 18% 33% 17% 6%	330
February	1st to 10th	Day	27% 17% 26% 23% 6% 1	300
	11th to 20th	Day	30% 18% 33% 16% 4 1	300
	21st to end	Day	26% 20% 39% 11% 4	249
March	1st to 10th	Day	23% 20% 36% 15% 6%	300
	11th to 20th	Day	17% 25% 40% 14% 4 1	300
	21st to end	Day	23% 12% 37% 20% 8%	330
April	1st to 10th	Day	17% 20% 36% 21% 6%	300
	11th to 20th	Day	25% 15% 28% 21% 11%	434
	21st to end	Day	15% 12% 37% 25% 10% 1	500
May	1st to 10th	Day	16% 13% 33% 25% 13% 1	582
	11th to 20th	Day	14% 5% 30% 32% 16% 2	725
	21st to end	Day	9% 7% 25% 37% 19% 3	789
June	1st to 10th	Day	7% 2 14% 35% 32% 10%	730
	11th to 20th	Day	7% 2 10% 39% 32% 10%	730
	21st to end	Day	8% 2 8% 29% 34% 19%	730
July	1st to 10th	Day	8% 3 7% 25% 36% 22%	721
	11th to 20th	Day	5% 3 3 22% 45% 22%	718
	21st to end	Day	6% 2 4 21% 46% 20%	740
August	1st to 10th	Day	3 3 19% 50% 25%	676
	11th to 20th	Day	6% 1 2 18% 44% 30%	592
	21st to end	Day	10% 1 5% 19% 34% 31%	471
September	1st to 10th	Day	8% 2 6% 25% 38% 22%	300
	11th to 20th	Day	10% 2 8% 26% 37% 18%	300
	21st to end	Day	9% 1 8% 32% 35% 14%	300
October	1st to 10th	Day	7% 2 12% 44% 28% 7%	300
	11th to 20th	Day	11% 4 19% 37% 21% 8%	300
	21st to end	Day	20% 9% 22% 23% 19% 6%	330
November	1st to 10th	Day	17% 6% 18% 40% 16% 3	300
	11th to 20th	Day	13% 9% 30% 32% 13% 2	300
	21st to end	Day	17% 19% 32% 21% 8% 3	300
December	1st to 10th	Day	20% 7% 37% 24% 9% 3	300
	11th to 20th	Day	20% 18% 30% 26% 5% 1	300
	21st to end	Day	20% 12% 35% 23% 7% 2	330
January	1st to 10th	Night	23% 11% 30% 28% 8%	1000
	11th to 20th	Night	23% 11% 29% 31% 6%	1000
	21st to end	Night	23% 10% 29% 31% 6%	1100
February	1st to 10th	Night	26% 11% 30% 28% 4	905
	11th to 20th	Night	27% 10% 31% 27% 5%	783
	21st to end	Night	22% 15% 37% 24% 3	495
March	1st to 10th	Night	24% 10% 33% 29% 4	600
	11th to 20th	Night	20% 10% 36% 29% 6%	600
	21st to end	Night	17% 11% 36% 29% 7%	660
April	1st to 10th	Night	19% 9% 33% 33% 6%	600
	11th to 20th	Night	21% 8% 31% 31% 8%	593
	21st to end	Night	18% 8% 33% 32% 9%	600
May	1st to 10th	Night	15% 11% 32% 35% 7%	600
	11th to 20th	Night	14% 5% 30% 37% 13% 2	591
	21st to end	Night	9% 6% 24% 39% 21% 2	650
June	1st to 10th	Night	7% 2 12% 45% 31% 3	600
	11th to 20th	Night	4 1 13% 52% 29% 2	600
	21st to end	Night	6% 2 10% 37% 40% 6%	600
July	1st to 10th	Night	5% 2 6% 39% 39% 9%	585
	11th to 20th	Night	5% 3 37% 47% 7%	582
	21st to end	Night	3 1 4 39% 48% 6%	624
August	1st to 10th	Night	1 1 2 39% 51% 6%	567
	11th to 20th	Night	4 1 3 35% 50% 7%	564
	21st to end	Night	4 1 5% 29% 52% 9%	627
September	1st to 10th	Night	5% 1 6% 30% 50% 8%	570
	11th to 20th	Night	12% 1 7% 31% 45% 4	570
	21st to end	Night	8% 2 10% 34% 41% 5%	570
October	1st to 10th	Night	5% 1 12% 40% 37% 5%	651
	11th to 20th	Night	11% 2 23% 39% 24% 2	760
	21st to end	Night	14% 6% 24% 34% 21% 2	836
November	1st to 10th	Night	12% 4 22% 38% 21% 2	780
	11th to 20th	Night	15% 9% 24% 35% 17% 1	800
	21st to end	Night	19% 8% 29% 30% 14% 1	884
December	1st to 10th	Night	22% 7% 27% 31% 12% 1	1000
	11th to 20th	Night	22% 11% 28% 31% 7% 1	1000
	21st to end	Night	21% 9% 31% 30% 9%	1100

Page generated at 2025-07-18 00:10:01. Version ac07

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