Dog-Star Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dog-Star Observatory clear sky chart from 2005-11-06 to 2026-08-15.

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	15% 9% 4 9% 5% 5% 5% 4 5% 7% 32%	143998
	Night	15% 10% 4 9% 5% 5% 4 3 5% 7% 33%	137877
Transparency	Day	50% 5% 11% 21% 12% 1	147594
	Night	50% 5% 11% 21% 12% 2	142352
Seeing	Day	24% 14% 27% 20% 11% 4	17038
	Night	22% 12% 25% 28% 11% 1	26451

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	14% 17% 5% 19% 6% 5% 4 3 4 4 21%	4679
	11th to 20th	Day	9% 14% 9% 21% 10% 7% 4 3 4 3 16%	5280
	21st to end	Day	15% 14% 6% 11% 8% 6% 4 3 4 4 25%	5780
February	1st to 10th	Day	15% 16% 5% 10% 7% 6% 4 3 4 6% 26%	1890
	11th to 20th	Day	14% 12% 5% 12% 6% 5% 5% 4 6% 5% 26%	1843
	21st to end	Day	17% 13% 4 11% 5% 4 4 4 4 6% 28%	1707
March	1st to 10th	Day	18% 12% 4 8% 5% 4 4 3 4 6% 34%	5033
	11th to 20th	Day	13% 11% 4 9% 5% 5% 4 3 5% 6% 35%	5090
	21st to end	Day	13% 10% 4 9% 6% 5% 5% 4 5% 5% 36%	5621
April	1st to 10th	Day	17% 11% 3 8% 5% 5% 4 3 4 6% 32%	2520
	11th to 20th	Day	15% 11% 3 7% 4 5% 4 3 6% 7% 35%	2494
	21st to end	Day	20% 7% 4 7% 5% 5% 4 3 5% 7% 34%	2604
May	1st to 10th	Day	22% 7% 3 8% 5% 5% 5% 4 6% 7% 28%	7616
	11th to 20th	Day	20% 8% 3 8% 5% 5% 4 4 5% 8% 31%	8093
	21st to end	Day	22% 7% 2 6% 4 4 4 4 6% 6% 33%	9045
June	1st to 10th	Day	13% 7% 3 6% 5% 6% 5% 4 6% 8% 37%	2940
	11th to 20th	Day	15% 5% 2 7% 5% 6% 5% 5% 6% 9% 35%	2940
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 5% 6% 9% 39%	2940
July	1st to 10th	Day	10% 7% 3 7% 6% 5% 5% 4 6% 8% 39%	8280
	11th to 20th	Day	7% 7% 3 8% 6% 6% 6% 5% 8% 10% 35%	8165
	21st to end	Day	11% 7% 3 7% 5% 5% 5% 4 6% 8% 39%	8340
August	1st to 10th	Day	18% 7% 3 7% 5% 5% 6% 4 7% 9% 30%	4364
	11th to 20th	Day	13% 6% 3 7% 5% 6% 7% 5% 7% 7% 34%	4193
	21st to end	Day	13% 7% 2 5% 4 4 5% 4 6% 9% 42%	4312
September	1st to 10th	Day	16% 6% 2 6% 5% 3 4 4 5% 6% 45%	2400
	11th to 20th	Day	14% 5% 2 4 4 3 4 3 5% 8% 48%	2380
	21st to end	Day	23% 8% 2 6% 4 3 3 3 7% 6% 35%	2096
October	1st to 10th	Day	16% 7% 3 6% 5% 4 4 4 4 7% 41%	2000
	11th to 20th	Day	17% 8% 3 7% 5% 5% 5% 4 6% 6% 36%	2000
	21st to end	Day	17% 9% 3 6% 5% 4 5% 4 5% 7% 36%	2200
November	1st to 10th	Day	11% 7% 3 6% 4 4 5% 5% 5% 7% 43%	1940
	11th to 20th	Day	15% 13% 3 7% 6% 5% 5% 4 5% 6% 30%	1735
	21st to end	Day	16% 13% 4 9% 6% 5% 5% 4 6% 6% 28%	1680
December	1st to 10th	Day	14% 13% 6% 17% 6% 5% 5% 4 4 5% 20%	2680
	11th to 20th	Day	21% 16% 4 9% 4 3 4 4 4 5% 26%	3070
	21st to end	Day	19% 17% 7% 11% 6% 5% 5% 4 5% 4 17%	4048
January	1st to 10th	Night	11% 14% 6% 20% 7% 5% 4 3 5% 6% 18%	8196
	11th to 20th	Night	10% 13% 6% 19% 7% 6% 4 3 5% 6% 20%	7680
	21st to end	Night	14% 12% 4 10% 9% 7% 5% 3 4 5% 26%	8401
February	1st to 10th	Night	14% 12% 4 10% 6% 5% 4 4 5% 7% 30%	2730
	11th to 20th	Night	16% 10% 4 10% 6% 5% 6% 4 5% 5% 28%	2663
	21st to end	Night	15% 11% 3 11% 5% 6% 4 4 5% 5% 31%	2098
March	1st to 10th	Night	18% 12% 5% 7% 5% 4 3 3 4 5% 33%	5038
	11th to 20th	Night	11% 9% 5% 7% 4 5% 4 3 5% 6% 42%	5090
	21st to end	Night	13% 9% 4 8% 5% 4 4 3 5% 5% 39%	5551
April	1st to 10th	Night	18% 12% 4 9% 5% 4 4 4 4 5% 31%	2100
	11th to 20th	Night	16% 9% 3 7% 5% 4 4 4 5% 7% 37%	2075
	21st to end	Night	17% 7% 3 7% 4 5% 5% 4 5% 7% 37%	2033
May	1st to 10th	Night	23% 8% 3 7% 5% 5% 5% 3 5% 6% 30%	5237
	11th to 20th	Night	21% 10% 3 6% 3 4 4 3 5% 6% 35%	4763
	21st to end	Night	24% 8% 2 6% 4 4 4 3 5% 7% 34%	5108
June	1st to 10th	Night	12% 6% 2 7% 4 5% 3 3 5% 8% 44%	1680
	11th to 20th	Night	12% 6% 3 7% 5% 3 5% 5% 6% 8% 39%	1680
	21st to end	Night	11% 6% 1 5% 4 5% 5% 4 7% 10% 42%	1680
July	1st to 10th	Night	11% 8% 3 7% 5% 5% 5% 4 6% 9% 39%	4678
	11th to 20th	Night	8% 6% 3 8% 5% 5% 5% 5% 7% 11% 36%	4787
	21st to end	Night	11% 6% 3 7% 5% 4 4 4 5% 8% 44%	5736
August	1st to 10th	Night	16% 7% 3 6% 4 4 5% 4 6% 8% 36%	3005
	11th to 20th	Night	14% 8% 3 6% 4 4 5% 3 6% 7% 39%	2911
	21st to end	Night	14% 5% 2 5% 4 4 5% 4 5% 8% 46%	3586
September	1st to 10th	Night	16% 5% 2 5% 5% 4 4 3 5% 7% 46%	2000
	11th to 20th	Night	11% 5% 2 5% 3 3 4 3 6% 8% 50%	2015
	21st to end	Night	20% 7% 2 6% 4 4 4 3 4 7% 39%	2302
October	1st to 10th	Night	17% 6% 2 5% 3 3 4 3 4 7% 46%	2400
	11th to 20th	Night	15% 8% 2 6% 5% 4 3 3 5% 6% 43%	2400
	21st to end	Night	19% 9% 3 5% 3 4 4 3 5% 6% 39%	2640
November	1st to 10th	Night	10% 7% 2 6% 4 4 4 3 4 6% 49%	2577
	11th to 20th	Night	17% 10% 3 6% 5% 4 4 3 5% 6% 34%	2882
	21st to end	Night	17% 11% 5% 7% 5% 4 4 4 5% 6% 32%	2940
December	1st to 10th	Night	13% 11% 7% 13% 5% 5% 5% 4 5% 7% 24%	4720
	11th to 20th	Night	20% 12% 5% 10% 5% 5% 4 3 4 6% 27%	5345
	21st to end	Night	18% 14% 5% 11% 6% 5% 5% 3 5% 5% 22%	7150

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	66% 4 10% 13% 5% 1	4680
	11th to 20th	Day	72% 3 6% 11% 7% 2	5280
	21st to end	Day	62% 3 10% 18% 6%	5780
February	1st to 10th	Day	59% 5% 11% 17% 7% 1	1890
	11th to 20th	Day	58% 6% 9% 20% 7% 1	1852
	21st to end	Day	56% 6% 11% 19% 7% 3	1707
March	1st to 10th	Day	52% 4 12% 20% 10% 2	5033
	11th to 20th	Day	48% 5% 14% 22% 8% 2	5090
	21st to end	Day	49% 5% 11% 19% 15% 3	5621
April	1st to 10th	Day	52% 5% 11% 18% 12% 2	2520
	11th to 20th	Day	47% 6% 11% 18% 15% 2	2494
	21st to end	Day	49% 5% 9% 21% 13% 2	2604
May	1st to 10th	Day	56% 4 10% 19% 9% 2	7651
	11th to 20th	Day	52% 5% 11% 22% 10% 1	8097
	21st to end	Day	50% 5% 9% 20% 13% 1	9061
June	1st to 10th	Day	43% 7% 11% 22% 15% 1	2940
	11th to 20th	Day	44% 8% 13% 20% 13% 2	2940
	21st to end	Day	39% 7% 15% 25% 13% 1	2940
July	1st to 10th	Day	45% 5% 12% 24% 13% 1	8210
	11th to 20th	Day	43% 4 13% 30% 10%	8090
	21st to end	Day	43% 3 9% 26% 18% 1	8156
August	1st to 10th	Day	50% 6% 15% 19% 10%	4351
	11th to 20th	Day	46% 5% 10% 27% 10% 1	4232
	21st to end	Day	39% 4 11% 28% 15% 2	4312
September	1st to 10th	Day	40% 6% 9% 23% 18% 3	2400
	11th to 20th	Day	34% 6% 11% 28% 19% 2	2380
	21st to end	Day	48% 6% 11% 18% 16% 1	2096
October	1st to 10th	Day	40% 4 7% 27% 18% 4	3260
	11th to 20th	Day	44% 4 8% 24% 16% 4	3240
	21st to end	Day	33% 4 14% 31% 18% 1	3538
November	1st to 10th	Day	39% 5% 14% 21% 17% 3	1940
	11th to 20th	Day	54% 5% 9% 19% 10% 2	1735
	21st to end	Day	55% 6% 12% 20% 7%	1680
December	1st to 10th	Day	66% 4 10% 13% 5% 1	2680
	11th to 20th	Day	60% 4 12% 19% 5%	3066
	21st to end	Day	69% 5% 10% 10% 5% 1	4048
January	1st to 10th	Night	66% 4 10% 14% 5%	8197
	11th to 20th	Night	64% 4 11% 11% 9% 2	7680
	21st to end	Night	61% 3 8% 19% 8% 1	8401
February	1st to 10th	Night	53% 6% 11% 21% 9% 1	2730
	11th to 20th	Night	55% 6% 10% 18% 10% 1	2665
	21st to end	Night	54% 6% 9% 20% 10% 1	2098
March	1st to 10th	Night	54% 4 11% 19% 10% 2	5038
	11th to 20th	Night	40% 7% 16% 25% 8% 4	5090
	21st to end	Night	48% 3 9% 19% 19% 2	5551
April	1st to 10th	Night	53% 5% 10% 18% 13% 1	2100
	11th to 20th	Night	46% 7% 11% 20% 15% 2	2075
	21st to end	Night	45% 5% 11% 23% 14% 3	2033
May	1st to 10th	Night	55% 4 10% 17% 12% 2	5264
	11th to 20th	Night	49% 5% 13% 22% 10% 1	4764
	21st to end	Night	50% 5% 11% 20% 12% 2	5115
June	1st to 10th	Night	38% 7% 12% 25% 17% 2	1680
	11th to 20th	Night	39% 9% 14% 22% 15% 2	1680
	21st to end	Night	36% 8% 14% 27% 15%	1680
July	1st to 10th	Night	43% 5% 13% 22% 15% 2	4640
	11th to 20th	Night	42% 5% 15% 27% 11% 1	4742
	21st to end	Night	40% 4 11% 23% 21% 1	5606
August	1st to 10th	Night	46% 7% 14% 23% 9% 1	2994
	11th to 20th	Night	44% 4 11% 27% 13% 1	2938
	21st to end	Night	37% 5% 12% 32% 13% 2	3586
September	1st to 10th	Night	38% 6% 11% 24% 18% 3	2000
	11th to 20th	Night	31% 6% 11% 30% 19% 2	2015
	21st to end	Night	45% 5% 11% 21% 17% 1	2302
October	1st to 10th	Night	38% 4 10% 26% 21% 1	3920
	11th to 20th	Night	35% 4 10% 26% 20% 5%	3899
	21st to end	Night	34% 3 13% 28% 20% 2	4252
November	1st to 10th	Night	36% 5% 14% 25% 18% 2	2577
	11th to 20th	Night	48% 5% 10% 21% 13% 3	2882
	21st to end	Night	52% 5% 12% 22% 8% 1	2940
December	1st to 10th	Night	59% 5% 13% 17% 4	4720
	11th to 20th	Night	59% 4 13% 18% 6%	5348
	21st to end	Night	62% 6% 10% 15% 7%	7150

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	32% 26% 26% 10% 4 1	223
	11th to 20th	Day	42% 21% 27% 9% 1	330
	21st to end	Day	31% 25% 33% 9% 3 1	363
February	1st to 10th	Day	32% 22% 31% 12% 4	330
	11th to 20th	Day	39% 24% 31% 6% 1	330
	21st to end	Day	34% 21% 28% 16% 1	423
March	1st to 10th	Day	32% 20% 27% 17% 3	540
	11th to 20th	Day	32% 21% 27% 14% 4 1	540
	21st to end	Day	30% 22% 29% 14% 4 2	594
April	1st to 10th	Day	31% 19% 27% 17% 4 1	540
	11th to 20th	Day	25% 22% 26% 16% 8% 3	532
	21st to end	Day	29% 16% 31% 15% 7% 1	540
May	1st to 10th	Day	31% 17% 28% 12% 8% 3	543
	11th to 20th	Day	25% 17% 26% 17% 11% 4	567
	21st to end	Day	26% 9% 25% 21% 14% 5%	617
June	1st to 10th	Day	18% 15% 27% 25% 11% 4	570
	11th to 20th	Day	22% 12% 27% 24% 12% 3	570
	21st to end	Day	17% 11% 28% 28% 13% 3	570
July	1st to 10th	Day	15% 6% 24% 29% 17% 9%	565
	11th to 20th	Day	10% 6% 28% 29% 20% 6%	564
	21st to end	Day	15% 6% 29% 29% 16% 5%	620
August	1st to 10th	Day	16% 6% 25% 27% 20% 7%	566
	11th to 20th	Day	16% 5% 24% 26% 20% 9%	535
	21st to end	Day	14% 8% 24% 27% 15% 12%	583
September	1st to 10th	Day	18% 8% 19% 28% 19% 8%	530
	11th to 20th	Day	16% 3 19% 28% 24% 9%	530
	21st to end	Day	30% 9% 21% 22% 14% 5%	530
October	1st to 10th	Day	23% 10% 19% 25% 18% 5%	530
	11th to 20th	Day	22% 17% 28% 22% 10% 2	530
	21st to end	Day	23% 15% 32% 23% 7% 1	583
November	1st to 10th	Day	19% 17% 30% 22% 11% 2	473
	11th to 20th	Day	28% 20% 28% 16% 7% 1	275
	21st to end	Day	27% 21% 36% 13% 2 1	220
December	1st to 10th	Day	32% 26% 29% 9% 3 1	220
	11th to 20th	Day	32% 22% 29% 15% 2	220
	21st to end	Day	32% 21% 29% 14% 5%	242
January	1st to 10th	Night	26% 20% 33% 17% 3	840
	11th to 20th	Night	29% 18% 29% 21% 3	840
	21st to end	Night	24% 21% 31% 21% 2	924
February	1st to 10th	Night	26% 18% 28% 26% 2	840
	11th to 20th	Night	30% 22% 31% 17% 1	823
	21st to end	Night	29% 18% 32% 20% 2	692
March	1st to 10th	Night	26% 17% 31% 23% 3	840
	11th to 20th	Night	27% 18% 29% 22% 5%	840
	21st to end	Night	26% 15% 33% 20% 5% 1	924
April	1st to 10th	Night	28% 17% 29% 22% 4	840
	11th to 20th	Night	23% 14% 30% 26% 7% 1	831
	21st to end	Night	23% 12% 28% 31% 6%	773
May	1st to 10th	Night	24% 9% 25% 31% 10%	630
	11th to 20th	Night	20% 8% 29% 33% 9%	462
	21st to end	Night	18% 7% 19% 36% 18% 1	455
June	1st to 10th	Night	14% 7% 23% 38% 19% 1	420
	11th to 20th	Night	15% 7% 25% 37% 16%	420
	21st to end	Night	13% 5% 21% 43% 18%	420
July	1st to 10th	Night	12% 5% 17% 40% 23% 3	410
	11th to 20th	Night	8% 3 17% 41% 29% 2	440
	21st to end	Night	12% 3 18% 44% 22% 1	690
August	1st to 10th	Night	11% 1 15% 42% 29% 2	627
	11th to 20th	Night	16% 3 13% 38% 27% 4	606
	21st to end	Night	14% 3 17% 35% 28% 3	880
September	1st to 10th	Night	17% 4 17% 35% 24% 3	800
	11th to 20th	Night	14% 4 16% 33% 28% 5%	800
	21st to end	Night	24% 7% 17% 30% 19% 3	800
October	1st to 10th	Night	21% 7% 20% 29% 21% 3	800
	11th to 20th	Night	20% 13% 25% 31% 11% 1	800
	21st to end	Night	24% 13% 24% 32% 6% 1	880
November	1st to 10th	Night	17% 11% 27% 34% 10% 1	820
	11th to 20th	Night	27% 17% 22% 25% 7% 1	840
	21st to end	Night	25% 17% 27% 25% 5% 1	840
December	1st to 10th	Night	28% 17% 30% 20% 5%	840
	11th to 20th	Night	28% 16% 28% 25% 4	840
	21st to end	Night	27% 16% 27% 27% 2	924

Page generated at 2026-08-12 03:37:20. Version ac07

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