SSC Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SSC Observatory clear sky chart from 2005-11-06 to 2025-09-04.

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	9% 6% 2 5% 4 4 4 3 5% 7% 51%	126153
	Night	9% 5% 2 5% 4 3 4 3 5% 7% 53%	122869
Transparency	Day	32% 5% 12% 27% 20% 4	129824
	Night	30% 5% 12% 26% 22% 5%	127251
Seeing	Day	12% 8% 30% 31% 16% 3	16870
	Night	11% 8% 33% 37% 11% 1	26665

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	10% 9% 3 8% 4 4 4 4 7% 8% 38%	4077
	11th to 20th	Day	6% 10% 4 10% 6% 4 3 3 4 6% 44%	4080
	21st to end	Day	6% 11% 4 8% 4 3 4 3 4 5% 48%	4488
February	1st to 10th	Day	8% 12% 3 7% 4 4 4 3 5% 7% 43%	1810
	11th to 20th	Day	7% 7% 3 6% 5% 4 5% 3 5% 8% 46%	1949
	21st to end	Day	6% 6% 2 7% 4 3 4 4 5% 7% 52%	1650
March	1st to 10th	Day	10% 7% 2 4 3 3 3 3 3 5% 57%	4690
	11th to 20th	Day	9% 7% 2 4 3 3 3 3 4 6% 58%	4980
	21st to end	Day	11% 7% 2 5% 4 4 4 4 5% 7% 47%	5462
April	1st to 10th	Day	11% 5% 2 5% 4 3 4 3 5% 7% 51%	2234
	11th to 20th	Day	12% 7% 3 6% 5% 4 4 4 5% 6% 45%	2505
	21st to end	Day	16% 7% 2 6% 4 4 4 2 5% 7% 43%	2600
May	1st to 10th	Day	13% 5% 2 4 4 4 4 3 5% 6% 49%	5862
	11th to 20th	Day	11% 5% 2 5% 3 3 3 3 4 6% 56%	5862
	21st to end	Day	19% 6% 2 5% 4 3 4 3 5% 7% 41%	6434
June	1st to 10th	Day	6% 3 1 4 3 3 4 4 6% 8% 57%	2600
	11th to 20th	Day	6% 3 1 4 4 5% 4 4 7% 9% 53%	2600
	21st to end	Day	5% 4 2 4 4 3 4 4 6% 8% 57%	2600
July	1st to 10th	Day	8% 4 2 4 3 4 4 4 6% 9% 52%	5900
	11th to 20th	Day	7% 3 2 6% 4 5% 5% 4 7% 9% 48%	5900
	21st to end	Day	7% 4 2 5% 4 4 4 4 6% 8% 53%	6422
August	1st to 10th	Day	9% 5% 2 5% 4 4 3 3 6% 8% 51%	4234
	11th to 20th	Day	11% 4 2 5% 4 4 4 4 6% 8% 50%	4171
	21st to end	Day	4 3 2 5% 4 3 4 4 5% 8% 59%	4544
September	1st to 10th	Day	11% 5% 1 5% 2 2 3 2 4 6% 57%	2153
	11th to 20th	Day	10% 4 2 5% 2 3 3 3 5% 7% 56%	2090
	21st to end	Day	9% 3 1 4 3 3 3 3 6% 7% 58%	2090
October	1st to 10th	Day	15% 4 2 5% 2 2 3 2 4 5% 56%	2051
	11th to 20th	Day	10% 4 1 4 3 2 3 2 4 5% 63%	1900
	21st to end	Day	11% 6% 2 4 3 2 3 3 4 4 58%	2033
November	1st to 10th	Day	8% 4 2 3 3 3 3 3 5% 7% 60%	1763
	11th to 20th	Day	8% 6% 2 4 3 3 3 2 4 5% 60%	1800
	21st to end	Day	11% 7% 2 4 3 3 3 2 4 6% 56%	1800
December	1st to 10th	Day	5% 4 2 4 4 3 4 3 5% 7% 61%	2940
	11th to 20th	Day	6% 6% 2 6% 3 4 3 2 5% 5% 58%	3391
	21st to end	Day	14% 9% 3 10% 4 4 4 3 5% 5% 38%	4488
January	1st to 10th	Night	10% 9% 4 11% 4 4 4 3 5% 6% 40%	5888
	11th to 20th	Night	5% 7% 3 7% 5% 4 4 3 4 6% 53%	5900
	21st to end	Night	10% 8% 4 9% 4 4 4 3 4 5% 47%	6490
February	1st to 10th	Night	9% 9% 3 6% 5% 4 4 4 5% 6% 45%	2600
	11th to 20th	Night	7% 6% 3 7% 5% 4 4 5% 5% 7% 47%	2343
	21st to end	Night	7% 5% 2 6% 4 3 4 3 5% 4 57%	1980
March	1st to 10th	Night	8% 4 1 4 2 3 3 2 4 6% 63%	5173
	11th to 20th	Night	8% 4 1 4 2 3 3 3 5% 8% 59%	4980
	21st to end	Night	11% 6% 2 5% 4 4 5% 4 5% 7% 47%	5467
April	1st to 10th	Night	9% 6% 2 5% 4 4 4 4 4 7% 52%	2163
	11th to 20th	Night	12% 6% 2 5% 3 3 4 4 5% 7% 49%	1844
	21st to end	Night	14% 6% 3 5% 4 4 3 3 4 6% 47%	1800
May	1st to 10th	Night	16% 6% 2 4 3 3 3 3 4 7% 49%	4053
	11th to 20th	Night	10% 4 2 4 3 3 3 3 5% 8% 56%	4053
	21st to end	Night	18% 5% 2 4 3 4 4 3 5% 7% 45%	4443
June	1st to 10th	Night	6% 3 1 3 2 4 4 3 6% 9% 60%	1800
	11th to 20th	Night	6% 3 2 3 4 3 4 4 6% 8% 57%	1800
	21st to end	Night	3 4 1 4 4 3 4 4 8% 11% 55%	1800
July	1st to 10th	Night	7% 4 2 6% 4 3 3 3 6% 9% 52%	4078
	11th to 20th	Night	6% 4 2 5% 4 4 4 5% 7% 10% 49%	4077
	21st to end	Night	6% 4 1 5% 3 3 4 4 5% 9% 55%	4449
August	1st to 10th	Night	4 4 2 5% 4 3 4 4 7% 11% 53%	2926
	11th to 20th	Night	9% 5% 2 6% 4 4 4 4 6% 8% 50%	2888
	21st to end	Night	5% 3 1 4 3 3 4 4 5% 9% 59%	3361
September	1st to 10th	Night	8% 3 1 4 2 2 2 2 5% 7% 62%	2093
	11th to 20th	Night	8% 4 2 4 4 4 3 4 4 6% 59%	2090
	21st to end	Night	7% 3 1 4 3 3 4 3 4 6% 62%	2090
October	1st to 10th	Night	12% 4 1 4 3 3 3 3 5% 6% 57%	2109
	11th to 20th	Night	9% 5% 1 4 3 2 3 3 4 5% 61%	2280
	21st to end	Night	10% 5% 1 4 3 3 3 2 3 5% 60%	2560
November	1st to 10th	Night	8% 3 1 4 2 2 3 2 4 7% 64%	2535
	11th to 20th	Night	8% 5% 1 4 3 3 3 3 4 6% 60%	2600
	21st to end	Night	10% 5% 2 4 3 3 3 2 4 6% 57%	2600
December	1st to 10th	Night	4 4 1 3 3 3 3 3 4 6% 66%	4250
	11th to 20th	Night	7% 4 2 5% 3 3 4 3 6% 7% 56%	4816
	21st to end	Night	14% 10% 4 8% 4 4 3 3 4 6% 41%	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	41% 6% 12% 24% 15% 2	4078
	11th to 20th	Day	42% 5% 10% 23% 17% 2	4080
	21st to end	Day	39% 4 12% 25% 18% 2	4488
February	1st to 10th	Day	41% 5% 15% 23% 11% 4	1810
	11th to 20th	Day	36% 5% 16% 25% 15% 4	1959
	21st to end	Day	31% 4 10% 26% 23% 6%	1650
March	1st to 10th	Day	30% 4 7% 22% 26% 11%	4690
	11th to 20th	Day	29% 3 8% 23% 28% 10%	4980
	21st to end	Day	36% 6% 18% 31% 8% 1	5464
April	1st to 10th	Day	33% 5% 12% 24% 21% 4	2234
	11th to 20th	Day	39% 5% 11% 25% 16% 3	2505
	21st to end	Day	42% 4 11% 26% 15% 1	2600
May	1st to 10th	Day	34% 6% 11% 28% 16% 5%	5900
	11th to 20th	Day	30% 5% 9% 24% 25% 8%	5866
	21st to end	Day	41% 7% 10% 26% 14% 2	6447
June	1st to 10th	Day	23% 8% 16% 33% 16% 3	2600
	11th to 20th	Day	26% 7% 14% 28% 21% 4	2600
	21st to end	Day	24% 6% 13% 29% 23% 5%	2600
July	1st to 10th	Day	27% 6% 13% 27% 23% 4	5836
	11th to 20th	Day	30% 6% 16% 25% 20% 3	5831
	21st to end	Day	27% 6% 12% 27% 23% 5%	6240
August	1st to 10th	Day	31% 5% 13% 26% 21% 4	4220
	11th to 20th	Day	33% 6% 15% 26% 18% 2	4210
	21st to end	Day	24% 5% 15% 26% 22% 8%	4543
September	1st to 10th	Day	29% 4 10% 26% 23% 9%	2153
	11th to 20th	Day	28% 5% 9% 28% 27% 4	2090
	21st to end	Day	27% 4 9% 30% 26% 4	2090
October	1st to 10th	Day	21% 3 5% 20% 44% 8%	3402
	11th to 20th	Day	18% 3 9% 34% 26% 10%	3140
	21st to end	Day	26% 3 9% 26% 31% 5%	3338
November	1st to 10th	Day	25% 4 11% 30% 25% 5%	1763
	11th to 20th	Day	28% 3 9% 27% 30% 4	1800
	21st to end	Day	32% 3 11% 29% 22% 4	1800
December	1st to 10th	Day	24% 3 12% 36% 22% 3	2940
	11th to 20th	Day	28% 4 14% 40% 13% 1	3389
	21st to end	Day	48% 5% 9% 27% 11% 1	4488
January	1st to 10th	Night	44% 5% 11% 23% 15% 1	5888
	11th to 20th	Night	32% 5% 11% 26% 24% 3	5900
	21st to end	Night	40% 4 11% 21% 17% 8%	6490
February	1st to 10th	Night	40% 6% 12% 23% 15% 5%	2600
	11th to 20th	Night	35% 6% 12% 28% 16% 3	2343
	21st to end	Night	30% 4 10% 23% 24% 9%	1980
March	1st to 10th	Night	24% 3 8% 26% 29% 10%	5173
	11th to 20th	Night	23% 4 7% 20% 32% 13%	4980
	21st to end	Night	36% 6% 19% 29% 8% 1	5467
April	1st to 10th	Night	32% 5% 12% 23% 23% 4	2163
	11th to 20th	Night	33% 6% 14% 25% 19% 3	1844
	21st to end	Night	38% 5% 12% 26% 17% 3	1800
May	1st to 10th	Night	36% 6% 11% 24% 18% 5%	4080
	11th to 20th	Night	27% 6% 12% 22% 23% 10%	4054
	21st to end	Night	38% 7% 14% 25% 15% 1	4452
June	1st to 10th	Night	21% 7% 17% 31% 20% 4	1800
	11th to 20th	Night	24% 7% 12% 32% 20% 6%	1800
	21st to end	Night	21% 7% 13% 30% 23% 6%	1800
July	1st to 10th	Night	28% 6% 13% 25% 23% 4	4035
	11th to 20th	Night	29% 6% 14% 26% 22% 3	4026
	21st to end	Night	25% 6% 13% 29% 23% 5%	4320
August	1st to 10th	Night	24% 6% 17% 27% 20% 6%	2916
	11th to 20th	Night	33% 5% 15% 26% 19% 2	2915
	21st to end	Night	23% 5% 15% 28% 21% 8%	3361
September	1st to 10th	Night	23% 4 12% 25% 26% 10%	2093
	11th to 20th	Night	27% 5% 10% 25% 27% 7%	2090
	21st to end	Night	24% 4 11% 23% 32% 6%	2090
October	1st to 10th	Night	18% 3 5% 18% 49% 8%	3512
	11th to 20th	Night	19% 3 8% 29% 29% 12%	3779
	21st to end	Night	22% 3 8% 28% 34% 5%	4208
November	1st to 10th	Night	23% 3 9% 30% 27% 8%	2535
	11th to 20th	Night	25% 4 9% 29% 27% 5%	2600
	21st to end	Night	30% 4 10% 28% 24% 5%	2600
December	1st to 10th	Night	21% 4 14% 38% 22% 2	4250
	11th to 20th	Night	27% 4 12% 35% 19% 3	4817
	21st to end	Night	45% 4 10% 28% 12% 1	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	14% 11% 37% 26% 11% 2	300
	11th to 20th	Day	17% 16% 32% 27% 9%	300
	21st to end	Day	11% 9% 50% 23% 7% 1	330
February	1st to 10th	Day	17% 13% 42% 23% 4	300
	11th to 20th	Day	12% 12% 43% 26% 7% 1	300
	21st to end	Day	11% 15% 51% 19% 4	249
March	1st to 10th	Day	15% 13% 41% 23% 8%	300
	11th to 20th	Day	20% 12% 34% 24% 9% 1	300
	21st to end	Day	19% 18% 42% 16% 3 1	330
April	1st to 10th	Day	15% 17% 39% 22% 6% 1	334
	11th to 20th	Day	14% 13% 44% 24% 5%	623
	21st to end	Day	20% 15% 36% 19% 8% 2	700
May	1st to 10th	Day	14% 11% 30% 31% 12% 2	703
	11th to 20th	Day	17% 11% 26% 28% 14% 3	725
	21st to end	Day	17% 7% 29% 30% 14% 3	789
June	1st to 10th	Day	5% 4 24% 35% 24% 8%	730
	11th to 20th	Day	6% 5% 30% 38% 18% 3	730
	21st to end	Day	6% 7% 32% 36% 18% 2	730
July	1st to 10th	Day	8% 3 19% 36% 28% 7%	721
	11th to 20th	Day	4 2 19% 42% 26% 7%	718
	21st to end	Day	7% 3 15% 40% 30% 5%	794
August	1st to 10th	Day	8% 3 15% 45% 25% 4	726
	11th to 20th	Day	8% 2 18% 40% 28% 4	715
	21st to end	Day	9% 2 17% 36% 29% 6%	724
September	1st to 10th	Day	12% 3 20% 40% 22% 3	339
	11th to 20th	Day	6% 3 27% 40% 17% 7%	300
	21st to end	Day	14% 5% 29% 29% 19% 4	300
October	1st to 10th	Day	21% 9% 20% 29% 18% 4	300
	11th to 20th	Day	7% 14% 39% 24% 14% 2	300
	21st to end	Day	18% 13% 35% 22% 9% 2	330
November	1st to 10th	Day	16% 13% 34% 27% 9% 2	300
	11th to 20th	Day	11% 14% 37% 24% 12% 1	300
	21st to end	Day	19% 9% 34% 23% 12% 2	300
December	1st to 10th	Day	9% 7% 42% 28% 11% 2	300
	11th to 20th	Day	13% 9% 41% 31% 6%	300
	21st to end	Day	21% 12% 38% 25% 5%	330
January	1st to 10th	Night	11% 10% 34% 38% 7% 1	1000
	11th to 20th	Night	12% 9% 36% 35% 7% 1	1000
	21st to end	Night	10% 7% 41% 36% 5%	1100
February	1st to 10th	Night	14% 8% 39% 34% 4	1000
	11th to 20th	Night	11% 11% 41% 31% 5%	789
	21st to end	Night	10% 12% 42% 32% 5%	660
March	1st to 10th	Night	14% 13% 38% 31% 4	716
	11th to 20th	Night	18% 10% 35% 31% 7% 1	600
	21st to end	Night	18% 12% 40% 25% 4	660
April	1st to 10th	Night	12% 15% 41% 27% 5%	600
	11th to 20th	Night	16% 12% 40% 27% 5%	593
	21st to end	Night	16% 11% 35% 29% 8% 1	600
May	1st to 10th	Night	17% 8% 33% 34% 8%	600
	11th to 20th	Night	14% 7% 35% 33% 11% 1	591
	21st to end	Night	14% 7% 29% 37% 11% 2	650
June	1st to 10th	Night	7% 4 29% 44% 16%	600
	11th to 20th	Night	6% 4 37% 38% 16%	600
	21st to end	Night	3 3 39% 43% 11%	600
July	1st to 10th	Night	6% 2 24% 43% 25%	585
	11th to 20th	Night	4 1 23% 50% 21% 1	582
	21st to end	Night	6% 2 17% 49% 25% 1	657
August	1st to 10th	Night	6% 2 18% 54% 20%	597
	11th to 20th	Night	8% 2 18% 53% 19%	594
	21st to end	Night	6% 1 21% 47% 24% 1	660
September	1st to 10th	Night	9% 2 23% 47% 17% 2	579
	11th to 20th	Night	9% 4 31% 37% 18% 1	570
	21st to end	Night	7% 5% 30% 43% 14% 1	570
October	1st to 10th	Night	13% 7% 28% 35% 15% 1	589
	11th to 20th	Night	13% 8% 36% 34% 8%	760
	21st to end	Night	14% 10% 31% 32% 11% 1	888
November	1st to 10th	Night	9% 8% 32% 40% 10%	975
	11th to 20th	Night	11% 10% 38% 33% 8%	1000
	21st to end	Night	14% 9% 30% 36% 10% 1	1000
December	1st to 10th	Night	9% 7% 37% 41% 5% 1	1000
	11th to 20th	Night	14% 9% 36% 34% 8%	1000
	21st to end	Night	16% 10% 32% 35% 6%	1100

Page generated at 2025-09-01 00:51:02. Version ac07

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