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 2024-11-05.

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 6% 4 4 4 3 5% 7% 50%	101226
	Night	9% 5% 2 5% 3 3 3 3 5% 7% 53%	97933
Transparency	Day	32% 5% 12% 26% 20% 4	104936
	Night	29% 5% 12% 26% 22% 5%	102340
Seeing	Day	12% 8% 30% 31% 16% 3	15679
	Night	11% 7% 33% 37% 11% 1	25541

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	8% 10% 3 9% 4 4 4 4 6% 7% 40%	2847
	11th to 20th	Day	7% 12% 4 10% 5% 4 3 3 5% 7% 41%	2850
	21st to end	Day	7% 14% 5% 8% 3 3 3 3 4 6% 43%	3135
February	1st to 10th	Day	8% 11% 3 7% 4 4 4 3 5% 7% 43%	1719
	11th to 20th	Day	6% 6% 2 6% 5% 4 5% 3 6% 8% 47%	1849
	21st to end	Day	6% 6% 2 7% 4 4 4 4 5% 7% 51%	1570
March	1st to 10th	Day	12% 5% 2 4 3 3 3 3 4 6% 56%	3268
	11th to 20th	Day	10% 7% 2 4 3 3 3 2 4 5% 58%	3480
	21st to end	Day	14% 9% 3 5% 4 3 3 3 4 6% 45%	3814
April	1st to 10th	Day	11% 5% 2 5% 4 3 4 4 5% 7% 51%	2122
	11th to 20th	Day	12% 7% 3 6% 5% 4 4 4 5% 6% 44%	2378
	21st to end	Day	16% 7% 2 6% 4 4 4 2 5% 7% 43%	2470
May	1st to 10th	Day	14% 6% 2 5% 4 4 4 3 6% 7% 45%	4082
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 4 7% 46%	4082
	21st to end	Day	13% 5% 2 6% 4 4 4 4 6% 7% 44%	4477
June	1st to 10th	Day	5% 3 1 4 4 3 4 4 6% 8% 57%	2470
	11th to 20th	Day	6% 3 1 5% 4 5% 4 4 7% 9% 52%	2470
	21st to end	Day	4 4 2 4 4 3 4 4 6% 8% 57%	2470
July	1st to 10th	Day	9% 5% 2 5% 4 5% 4 5% 6% 9% 47%	4120
	11th to 20th	Day	4 3 2 5% 4 5% 4 4 7% 10% 52%	4120
	21st to end	Day	6% 3 1 4 3 4 4 4 6% 8% 55%	4465
August	1st to 10th	Day	9% 5% 2 5% 4 4 3 3 5% 8% 51%	4104
	11th to 20th	Day	11% 4 2 5% 4 4 4 4 6% 7% 49%	4041
	21st to end	Day	4 3 1 4 4 3 4 4 5% 8% 60%	4405
September	1st to 10th	Day	11% 5% 1 5% 2 2 3 2 4 6% 57%	2119
	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 2 3 3 5% 7% 60%	1709
	11th to 20th	Day	8% 6% 2 4 3 3 3 2 4 5% 60%	1710
	21st to end	Day	11% 7% 2 4 3 3 3 2 3 6% 55%	1710
December	1st to 10th	Day	7% 7% 3 5% 5% 3 5% 3 5% 8% 49%	1710
	11th to 20th	Day	7% 8% 3 7% 4 4 3 3 5% 6% 50%	2161
	21st to end	Day	12% 10% 4 12% 5% 4 4 3 5% 6% 35%	3135
January	1st to 10th	Night	7% 10% 5% 13% 4 3 4 3 4 5% 42%	4108
	11th to 20th	Night	7% 7% 2 7% 4 4 4 3 4 6% 52%	4120
	21st to end	Night	12% 9% 4 9% 4 4 4 3 4 5% 43%	4532
February	1st to 10th	Night	9% 9% 3 6% 5% 4 4 4 5% 6% 45%	2470
	11th to 20th	Night	6% 5% 3 6% 5% 4 4 5% 6% 7% 48%	2223
	21st to end	Night	8% 5% 2 6% 4 3 4 3 5% 4 57%	1884
March	1st to 10th	Night	10% 5% 2 4 2 3 3 3 4 6% 60%	3589
	11th to 20th	Night	9% 5% 1 4 2 3 3 3 5% 8% 57%	3480
	21st to end	Night	14% 8% 2 4 3 3 4 3 4 6% 48%	3817
April	1st to 10th	Night	9% 6% 2 5% 4 4 4 4 4 7% 53%	2055
	11th to 20th	Night	12% 6% 2 5% 3 3 4 4 5% 7% 49%	1751
	21st to end	Night	13% 6% 3 5% 4 4 3 4 5% 7% 48%	1710
May	1st to 10th	Night	18% 7% 2 5% 3 3 3 3 4 6% 47%	2823
	11th to 20th	Night	12% 4 2 5% 3 4 4 3 6% 8% 49%	2823
	21st to end	Night	12% 4 1 5% 3 4 5% 4 6% 9% 47%	3090
June	1st to 10th	Night	5% 3 1 3 2 4 4 3 6% 10% 59%	1710
	11th to 20th	Night	6% 3 2 3 4 3 4 4 7% 8% 56%	1710
	21st to end	Night	3 4 1 4 4 3 4 4 8% 11% 55%	1710
July	1st to 10th	Night	8% 4 2 6% 5% 4 4 4 6% 9% 49%	2850
	11th to 20th	Night	4 3 1 4 4 4 4 5% 8% 10% 55%	2850
	21st to end	Night	5% 3 1 4 3 3 4 5% 6% 11% 57%	3098
August	1st to 10th	Night	4 4 2 5% 4 3 4 4 6% 11% 53%	2836
	11th to 20th	Night	9% 5% 2 6% 4 4 4 4 6% 8% 50%	2798
	21st to end	Night	4 3 1 4 3 3 4 4 5% 9% 61%	3258
September	1st to 10th	Night	8% 3 1 4 2 2 3 2 5% 7% 63%	2061
	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 3 2 2 3 2 4 7% 64%	2470
	11th to 20th	Night	8% 5% 1 4 3 3 3 3 4 6% 60%	2470
	21st to end	Night	11% 5% 1 4 3 3 3 2 4 6% 57%	2470
December	1st to 10th	Night	6% 6% 2 5% 4 4 3 3 4 7% 56%	2470
	11th to 20th	Night	9% 5% 2 5% 3 3 3 3 6% 6% 54%	3036
	21st to end	Night	15% 10% 3 7% 3 3 3 3 4 6% 42%	4532

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% 7% 14% 26% 11% 2	2848
	11th to 20th	Day	44% 6% 14% 22% 12% 2	2850
	21st to end	Day	43% 4 14% 25% 12% 1	3135
February	1st to 10th	Day	41% 6% 15% 23% 11% 4	1719
	11th to 20th	Day	34% 5% 16% 26% 15% 4	1859
	21st to end	Day	32% 4 10% 25% 23% 6%	1570
March	1st to 10th	Day	30% 5% 8% 24% 27% 6%	3268
	11th to 20th	Day	29% 4 11% 30% 24% 3	3480
	21st to end	Day	39% 6% 19% 27% 7% 1	3814
April	1st to 10th	Day	33% 5% 13% 24% 21% 4	2122
	11th to 20th	Day	40% 5% 11% 25% 15% 3	2378
	21st to end	Day	41% 4 12% 26% 16% 1	2470
May	1st to 10th	Day	38% 6% 11% 24% 14% 7%	4120
	11th to 20th	Day	38% 6% 12% 26% 17%	4086
	21st to end	Day	37% 8% 12% 27% 14% 2	4489
June	1st to 10th	Day	23% 8% 17% 34% 15% 3	2470
	11th to 20th	Day	27% 7% 14% 28% 20% 4	2470
	21st to end	Day	24% 6% 13% 29% 23% 5%	2470
July	1st to 10th	Day	32% 7% 13% 24% 20% 4	4056
	11th to 20th	Day	26% 6% 14% 28% 23% 3	4051
	21st to end	Day	23% 7% 13% 24% 26% 7%	4325
August	1st to 10th	Day	32% 5% 13% 25% 21% 4	4090
	11th to 20th	Day	34% 6% 15% 25% 18% 2	4080
	21st to end	Day	23% 5% 16% 26% 22% 8%	4404
September	1st to 10th	Day	29% 4 10% 26% 23% 9%	2119
	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	24% 4 11% 31% 25% 5%	1709
	11th to 20th	Day	27% 4 9% 28% 29% 4	1710
	21st to end	Day	32% 3 10% 28% 23% 4	1710
December	1st to 10th	Day	33% 5% 14% 29% 18% 1	1710
	11th to 20th	Day	34% 5% 12% 33% 15% 1	2159
	21st to end	Day	49% 6% 10% 23% 12% 1	3135
January	1st to 10th	Night	44% 5% 13% 24% 13% 1	4108
	11th to 20th	Night	32% 6% 14% 26% 20% 1	4120
	21st to end	Night	43% 5% 14% 22% 15% 2	4532
February	1st to 10th	Night	40% 6% 12% 22% 15% 5%	2470
	11th to 20th	Night	34% 6% 12% 29% 17% 3	2223
	21st to end	Night	31% 4 10% 23% 23% 9%	1884
March	1st to 10th	Night	26% 4 9% 26% 28% 8%	3589
	11th to 20th	Night	26% 4 10% 26% 30% 4	3480
	21st to end	Night	37% 6% 19% 29% 8% 1	3817
April	1st to 10th	Night	32% 5% 13% 24% 23% 4	2055
	11th to 20th	Night	33% 6% 14% 25% 19% 3	1751
	21st to end	Night	36% 5% 12% 26% 17% 3	1710
May	1st to 10th	Night	39% 6% 11% 24% 14% 7%	2850
	11th to 20th	Night	31% 7% 16% 25% 20% 1	2824
	21st to end	Night	31% 9% 17% 27% 14% 1	3099
June	1st to 10th	Night	21% 7% 17% 32% 19% 4	1710
	11th to 20th	Night	24% 7% 12% 32% 19% 6%	1710
	21st to end	Night	21% 7% 13% 30% 23% 6%	1710
July	1st to 10th	Night	31% 6% 13% 21% 24% 5%	2805
	11th to 20th	Night	21% 7% 14% 30% 24% 5%	2796
	21st to end	Night	20% 7% 15% 28% 24% 7%	2999
August	1st to 10th	Night	24% 6% 17% 27% 20% 6%	2826
	11th to 20th	Night	33% 5% 15% 26% 19% 2	2825
	21st to end	Night	21% 5% 15% 29% 22% 8%	3258
September	1st to 10th	Night	22% 4 12% 25% 27% 10%	2061
	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%	2470
	11th to 20th	Night	25% 4 9% 29% 28% 5%	2470
	21st to end	Night	30% 4 10% 28% 24% 5%	2470
December	1st to 10th	Night	28% 5% 14% 29% 22% 2	2470
	11th to 20th	Night	30% 4 10% 37% 18% 1	3037
	21st to end	Night	43% 5% 12% 27% 12% 2	4532

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	13% 11% 37% 24% 12% 2	270
	11th to 20th	Day	18% 16% 32% 24% 10%	270
	21st to end	Day	11% 9% 48% 24% 7% 1	297
February	1st to 10th	Day	17% 13% 43% 23% 4	270
	11th to 20th	Day	10% 9% 46% 27% 7% 1	270
	21st to end	Day	12% 15% 51% 18% 4	225
March	1st to 10th	Day	13% 13% 43% 22% 8%	270
	11th to 20th	Day	21% 11% 34% 24% 9% 1	270
	21st to end	Day	20% 18% 42% 16% 4 1	297
April	1st to 10th	Day	14% 16% 40% 24% 6% 1	302
	11th to 20th	Day	15% 11% 44% 25% 5%	576
	21st to end	Day	19% 15% 37% 19% 8% 2	650
May	1st to 10th	Day	15% 11% 32% 31% 11% 1	653
	11th to 20th	Day	17% 10% 26% 29% 14% 3	675
	21st to end	Day	16% 7% 30% 30% 14% 2	734
June	1st to 10th	Day	4 4 24% 36% 25% 8%	680
	11th to 20th	Day	6% 5% 31% 38% 17% 4	680
	21st to end	Day	6% 7% 32% 35% 18% 2	680
July	1st to 10th	Day	8% 3 19% 35% 28% 7%	671
	11th to 20th	Day	3 2 19% 42% 27% 7%	668
	21st to end	Day	7% 2 15% 39% 31% 5%	739
August	1st to 10th	Day	8% 3 14% 44% 27% 4	676
	11th to 20th	Day	8% 2 19% 40% 27% 3	665
	21st to end	Day	8% 2 18% 37% 30% 6%	673
September	1st to 10th	Day	12% 3 20% 41% 22% 3	329
	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	15% 12% 34% 27% 10% 2	282
	11th to 20th	Day	11% 12% 39% 24% 12% 1	270
	21st to end	Day	21% 9% 33% 23% 12% 2	270
December	1st to 10th	Day	10% 8% 42% 28% 11% 2	270
	11th to 20th	Day	14% 9% 39% 31% 7%	270
	21st to end	Day	21% 12% 38% 25% 3	297
January	1st to 10th	Night	10% 10% 33% 39% 7% 1	950
	11th to 20th	Night	13% 9% 35% 34% 8% 1	950
	21st to end	Night	11% 7% 40% 36% 6%	1045
February	1st to 10th	Night	14% 8% 38% 35% 5%	950
	11th to 20th	Night	10% 11% 41% 32% 5%	749
	21st to end	Night	10% 12% 42% 30% 5%	628
March	1st to 10th	Night	14% 13% 37% 32% 4	680
	11th to 20th	Night	18% 9% 34% 31% 7% 1	570
	21st to end	Night	19% 11% 40% 25% 4	627
April	1st to 10th	Night	11% 14% 42% 27% 5%	570
	11th to 20th	Night	16% 12% 39% 28% 5%	563
	21st to end	Night	16% 9% 36% 29% 8% 1	570
May	1st to 10th	Night	17% 8% 34% 34% 7%	570
	11th to 20th	Night	14% 6% 34% 34% 10% 1	561
	21st to end	Night	13% 7% 30% 37% 12% 2	617
June	1st to 10th	Night	6% 4 29% 44% 16%	570
	11th to 20th	Night	6% 4 37% 38% 15%	570
	21st to end	Night	3 3 39% 44% 11%	570
July	1st to 10th	Night	6% 2 24% 43% 25%	555
	11th to 20th	Night	4 1 22% 51% 21% 1	552
	21st to end	Night	6% 2 17% 48% 25% 1	624
August	1st to 10th	Night	6% 2 17% 54% 21%	567
	11th to 20th	Night	8% 2 18% 53% 19%	564
	21st to end	Night	4 1 21% 48% 24% 1	627
September	1st to 10th	Night	9% 2 23% 47% 17% 2	570
	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% 41% 11%	950
	11th to 20th	Night	11% 9% 38% 34% 8%	950
	21st to end	Night	14% 9% 30% 36% 10% 1	950
December	1st to 10th	Night	9% 7% 37% 41% 5% 1	950
	11th to 20th	Night	14% 9% 35% 35% 8%	950
	21st to end	Night	16% 11% 32% 35% 6%	1045

Page generated at 2024-11-03 01:11:27. Version ac07

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