Clyde Sachtleben Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Clyde Sachtleben Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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%	136379
	Night	9% 6% 2 6% 4 3 4 3 5% 7% 52%	131617
Transparency	Day	33% 6% 13% 26% 19% 4	131115
	Night	31% 5% 12% 26% 21% 4	127889
Seeing	Day	12% 9% 32% 30% 14% 2	17216
	Night	11% 9% 36% 34% 9% 1	27113

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	9% 12% 4 10% 5% 5% 4 3 4 6% 37%	3657
	11th to 20th	Day	5% 9% 4 9% 5% 5% 4 3 5% 5% 46%	4080
	21st to end	Day	5% 7% 5% 10% 5% 5% 4 3 5% 5% 48%	4488
February	1st to 10th	Day	9% 11% 4 8% 6% 4 4 4 5% 7% 39%	1800
	11th to 20th	Day	6% 8% 3 8% 5% 4 4 3 5% 8% 47%	1944
	21st to end	Day	8% 7% 2 6% 4 3 3 4 4 7% 53%	1650
March	1st to 10th	Day	10% 8% 2 4 3 2 3 2 4 4 57%	4632
	11th to 20th	Day	10% 7% 2 4 3 3 3 2 4 7% 55%	4980
	21st to end	Day	13% 9% 2 7% 4 3 4 3 6% 8% 42%	5464
April	1st to 10th	Day	11% 6% 3 5% 4 4 3 4 6% 6% 48%	2275
	11th to 20th	Day	13% 7% 2 6% 4 5% 4 3 5% 7% 44%	2505
	21st to end	Day	19% 7% 2 6% 3 4 4 3 5% 7% 41%	2600
May	1st to 10th	Day	11% 5% 2 5% 4 3 4 4 6% 7% 49%	5900
	11th to 20th	Day	11% 5% 2 5% 3 3 3 3 4 6% 54%	5862
	21st to end	Day	18% 6% 2 6% 4 4 4 3 5% 6% 40%	6446
June	1st to 10th	Day	6% 4 1 5% 3 4 5% 4 6% 7% 55%	2600
	11th to 20th	Day	8% 4 1 5% 4 3 4 4 7% 8% 52%	2787
	21st to end	Day	6% 4 2 5% 4 3 4 4 7% 9% 53%	2800
July	1st to 10th	Day	8% 4 2 5% 4 4 5% 3 5% 8% 52%	6339
	11th to 20th	Day	7% 4 2 5% 4 5% 5% 4 6% 9% 49%	6076
	21st to end	Day	8% 5% 2 5% 3 4 5% 4 6% 8% 50%	6424
August	1st to 10th	Day	8% 5% 2 6% 4 4 4 4 7% 9% 46%	5801
	11th to 20th	Day	9% 5% 2 5% 3 4 4 4 7% 9% 49%	5821
	21st to end	Day	11% 5% 2 6% 4 4 4 3 5% 7% 48%	6314
September	1st to 10th	Day	12% 5% 2 4 4 3 3 2 4 5% 55%	2255
	11th to 20th	Day	9% 4 1 5% 3 3 3 3 6% 7% 56%	2200
	21st to end	Day	9% 4 1 4 3 3 3 3 5% 7% 60%	2200
October	1st to 10th	Day	8% 4 1 3 2 2 2 2 3 4 69%	3525
	11th to 20th	Day	6% 4 1 3 2 2 3 3 4 7% 65%	3240
	21st to end	Day	10% 5% 2 5% 3 3 3 2 4 6% 57%	3359
November	1st to 10th	Day	9% 4 1 4 3 3 4 3 5% 7% 57%	1853
	11th to 20th	Day	7% 6% 3 5% 3 3 3 3 5% 5% 57%	1890
	21st to end	Day	9% 9% 3 5% 3 3 3 3 4 7% 51%	1890
December	1st to 10th	Day	4 5% 2 5% 4 3 4 3 5% 6% 58%	2958
	11th to 20th	Day	6% 7% 2 7% 4 4 4 3 5% 6% 51%	3391
	21st to end	Day	13% 10% 4 11% 4 4 4 3 4 5% 38%	4373
January	1st to 10th	Night	8% 12% 4 12% 4 4 4 4 6% 7% 37%	6271
	11th to 20th	Night	5% 8% 2 6% 5% 5% 5% 3 4 6% 51%	5900
	21st to end	Night	6% 9% 4 11% 4 4 3 3 4 5% 47%	6490
February	1st to 10th	Night	9% 10% 3 7% 4 4 4 4 6% 6% 43%	2600
	11th to 20th	Night	6% 8% 3 7% 5% 5% 4 4 6% 7% 46%	2358
	21st to end	Night	6% 7% 2 6% 5% 3 3 3 4 5% 55%	1980
March	1st to 10th	Night	9% 6% 2 5% 3 3 3 3 4 5% 57%	5236
	11th to 20th	Night	9% 5% 2 4 3 3 3 2 4 6% 59%	4980
	21st to end	Night	11% 8% 2 6% 4 5% 5% 4 6% 8% 41%	5467
April	1st to 10th	Night	10% 7% 2 5% 4 4 4 3 5% 7% 48%	2122
	11th to 20th	Night	12% 5% 2 5% 4 4 5% 3 6% 7% 47%	1841
	21st to end	Night	13% 6% 3 5% 3 3 3 3 4 9% 48%	1800
May	1st to 10th	Night	17% 6% 2 4 3 3 4 3 5% 6% 48%	4080
	11th to 20th	Night	11% 5% 2 4 3 3 3 3 5% 6% 54%	4053
	21st to end	Night	17% 6% 2 5% 4 3 4 3 5% 8% 42%	4451
June	1st to 10th	Night	6% 3 1 4 3 3 4 4 7% 9% 55%	1800
	11th to 20th	Night	8% 3 1 4 4 3 3 3 6% 8% 54%	1607
	21st to end	Night	4 4 1 4 3 3 5% 4 6% 9% 58%	1600
July	1st to 10th	Night	7% 4 2 5% 3 3 4 4 7% 7% 55%	3640
	11th to 20th	Night	7% 4 2 5% 4 4 5% 5% 7% 10% 46%	3899
	21st to end	Night	6% 5% 1 5% 3 3 3 3 5% 9% 56%	4449
August	1st to 10th	Night	4 3 2 4 4 4 4 4 7% 12% 53%	4011
	11th to 20th	Night	8% 4 1 5% 3 3 4 4 6% 9% 53%	4028
	21st to end	Night	10% 5% 2 5% 4 4 4 4 6% 8% 47%	4660
September	1st to 10th	Night	9% 3 1 3 2 3 3 2 5% 6% 63%	2143
	11th to 20th	Night	8% 3 2 5% 3 3 4 3 5% 6% 59%	2200
	21st to end	Night	7% 4 1 4 3 3 3 3 4 7% 61%	2200
October	1st to 10th	Night	8% 2 1 3 2 2 2 1 3 5% 70%	3621
	11th to 20th	Night	6% 3 1 3 3 3 2 2 4 5% 66%	3899
	21st to end	Night	9% 4 2 4 3 3 3 2 4 6% 60%	4433
November	1st to 10th	Night	6% 3 1 4 3 3 3 3 4 6% 63%	2665
	11th to 20th	Night	7% 5% 1 4 3 3 3 4 5% 6% 59%	2730
	21st to end	Night	9% 6% 2 5% 3 3 3 3 5% 7% 56%	2730
December	1st to 10th	Night	4 4 1 3 4 3 3 2 5% 7% 64%	4277
	11th to 20th	Night	7% 4 1 5% 3 4 4 4 5% 8% 54%	4816
	21st to end	Night	13% 11% 3 8% 5% 4 3 3 4 6% 38%	6580

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	46% 5% 12% 23% 13% 1	3658
	11th to 20th	Day	39% 5% 13% 21% 19% 3	4080
	21st to end	Day	38% 5% 12% 26% 16% 3	4488
February	1st to 10th	Day	44% 6% 15% 22% 11% 3	1800
	11th to 20th	Day	36% 5% 16% 23% 16% 4	1954
	21st to end	Day	31% 5% 13% 27% 18% 6%	1650
March	1st to 10th	Day	31% 4 8% 22% 26% 9%	4632
	11th to 20th	Day	30% 4 9% 23% 26% 8%	4980
	21st to end	Day	39% 7% 21% 25% 7% 1	5464
April	1st to 10th	Day	35% 5% 15% 26% 17% 2	2275
	11th to 20th	Day	39% 6% 13% 24% 15% 3	2508
	21st to end	Day	42% 8% 11% 25% 14% 1	2600
May	1st to 10th	Day	32% 7% 13% 26% 19% 4	5900
	11th to 20th	Day	31% 6% 11% 25% 20% 7%	5866
	21st to end	Day	42% 7% 10% 26% 14% 1	6447
June	1st to 10th	Day	25% 7% 19% 32% 15% 2	2600
	11th to 20th	Day	28% 7% 15% 29% 18% 3	2787
	21st to end	Day	25% 8% 14% 29% 20% 4	2800
July	1st to 10th	Day	31% 5% 12% 28% 23% 2	6271
	11th to 20th	Day	30% 8% 16% 27% 18% 2	6003
	21st to end	Day	29% 8% 14% 24% 21% 5%	6240
August	1st to 10th	Day	32% 7% 17% 23% 17% 3	4220
	11th to 20th	Day	36% 7% 15% 23% 18% 1	4210
	21st to end	Day	26% 6% 14% 24% 23% 8%	4550
September	1st to 10th	Day	31% 5% 12% 23% 22% 6%	2255
	11th to 20th	Day	27% 5% 12% 30% 23% 3	2200
	21st to end	Day	25% 4 10% 32% 26% 3	2200
October	1st to 10th	Day	21% 3 6% 21% 43% 7%	3525
	11th to 20th	Day	20% 4 10% 36% 21% 8%	3240
	21st to end	Day	29% 3 9% 29% 27% 3	3359
November	1st to 10th	Day	27% 5% 12% 30% 22% 3	1853
	11th to 20th	Day	28% 5% 11% 26% 26% 4	1890
	21st to end	Day	34% 5% 12% 26% 19% 3	1890
December	1st to 10th	Day	27% 3 11% 35% 20% 3	2958
	11th to 20th	Day	34% 4 18% 30% 13% 1	3389
	21st to end	Day	48% 5% 9% 25% 12% 1	4373
January	1st to 10th	Night	46% 4 11% 23% 15% 1	6271
	11th to 20th	Night	33% 4 12% 26% 22% 2	5900
	21st to end	Night	40% 5% 10% 21% 19% 6%	6490
February	1st to 10th	Night	41% 7% 13% 22% 14% 4	2600
	11th to 20th	Night	36% 6% 13% 27% 16% 3	2358
	21st to end	Night	31% 4 12% 22% 23% 8%	1980
March	1st to 10th	Night	29% 4 8% 25% 27% 7%	5236
	11th to 20th	Night	27% 4 8% 20% 30% 12%	4980
	21st to end	Night	39% 9% 20% 24% 7% 1	5467
April	1st to 10th	Night	34% 6% 14% 23% 20% 2	2122
	11th to 20th	Night	35% 6% 15% 25% 17% 2	1841
	21st to end	Night	34% 7% 13% 27% 17% 2	1800
May	1st to 10th	Night	36% 6% 11% 27% 18% 2	4080
	11th to 20th	Night	30% 6% 12% 23% 21% 7%	4054
	21st to end	Night	39% 7% 14% 23% 16% 1	4452
June	1st to 10th	Night	23% 9% 18% 30% 18% 2	1800
	11th to 20th	Night	26% 8% 14% 29% 18% 6%	1607
	21st to end	Night	22% 6% 17% 30% 21% 4	1600
July	1st to 10th	Night	27% 7% 13% 32% 19% 3	3600
	11th to 20th	Night	32% 8% 14% 28% 16% 3	3850
	21st to end	Night	25% 6% 14% 25% 23% 6%	4320
August	1st to 10th	Night	25% 7% 18% 26% 19% 5%	2916
	11th to 20th	Night	32% 6% 18% 27% 15% 2	2915
	21st to end	Night	25% 6% 16% 27% 18% 9%	3355
September	1st to 10th	Night	22% 5% 11% 27% 26% 9%	2143
	11th to 20th	Night	26% 5% 11% 30% 24% 5%	2200
	21st to end	Night	25% 5% 10% 27% 29% 5%	2200
October	1st to 10th	Night	20% 3 6% 19% 44% 8%	3621
	11th to 20th	Night	21% 3 9% 29% 26% 11%	3899
	21st to end	Night	26% 3 10% 28% 29% 4	4433
November	1st to 10th	Night	23% 4 11% 30% 26% 6%	2665
	11th to 20th	Night	25% 5% 11% 29% 25% 5%	2730
	21st to end	Night	29% 4 13% 27% 23% 4	2730
December	1st to 10th	Night	21% 3 15% 37% 21% 3	4277
	11th to 20th	Night	28% 4 15% 34% 17% 2	4817
	21st to end	Night	46% 6% 10% 26% 12% 1	6580

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	15% 12% 38% 26% 8% 1	300
	11th to 20th	Day	15% 15% 36% 25% 8%	300
	21st to end	Day	12% 9% 45% 25% 8% 1	330
February	1st to 10th	Day	14% 15% 45% 22% 4	300
	11th to 20th	Day	11% 13% 42% 27% 6% 1	300
	21st to end	Day	10% 18% 48% 20% 4	249
March	1st to 10th	Day	14% 15% 43% 23% 4 1	300
	11th to 20th	Day	22% 10% 36% 22% 9%	300
	21st to end	Day	22% 16% 39% 18% 5%	330
April	1st to 10th	Day	15% 20% 42% 18% 4	375
	11th to 20th	Day	16% 17% 38% 25% 4	626
	21st to end	Day	22% 14% 35% 19% 9% 2	700
May	1st to 10th	Day	13% 13% 32% 29% 12% 1	703
	11th to 20th	Day	17% 11% 32% 25% 13% 2	725
	21st to end	Day	17% 10% 32% 26% 13% 2	789
June	1st to 10th	Day	6% 7% 27% 35% 19% 6%	730
	11th to 20th	Day	9% 7% 36% 32% 16% 1	730
	21st to end	Day	8% 7% 34% 33% 16% 2	730
July	1st to 10th	Day	8% 3 20% 38% 26% 5%	721
	11th to 20th	Day	4 3 20% 44% 23% 5%	718
	21st to end	Day	9% 2 19% 40% 25% 6%	794
August	1st to 10th	Day	9% 3 19% 43% 23% 3	726
	11th to 20th	Day	7% 2 23% 41% 25% 2	715
	21st to end	Day	9% 2 23% 40% 21% 5%	731
September	1st to 10th	Day	13% 4 24% 38% 17% 4	385
	11th to 20th	Day	7% 5% 28% 37% 17% 5%	330
	21st to end	Day	12% 8% 30% 27% 18% 5%	330
October	1st to 10th	Day	20% 7% 24% 30% 15% 4	330
	11th to 20th	Day	10% 14% 43% 22% 10% 1	330
	21st to end	Day	18% 17% 34% 23% 8% 1	363
November	1st to 10th	Day	12% 16% 36% 26% 8% 2	330
	11th to 20th	Day	11% 17% 43% 21% 8% 1	330
	21st to end	Day	19% 13% 37% 21% 9% 1	330
December	1st to 10th	Day	7% 10% 44% 27% 10% 1	306
	11th to 20th	Day	15% 9% 42% 29% 6%	300
	21st to end	Day	17% 13% 44% 23% 3	330
January	1st to 10th	Night	12% 11% 38% 32% 6% 1	1000
	11th to 20th	Night	11% 11% 40% 31% 7% 1	1000
	21st to end	Night	10% 10% 42% 30% 7%	1100
February	1st to 10th	Night	14% 11% 43% 28% 4	1000
	11th to 20th	Night	10% 11% 44% 31% 4	804
	21st to end	Night	10% 12% 45% 28% 4	660
March	1st to 10th	Night	12% 14% 42% 29% 3 1	740
	11th to 20th	Night	15% 11% 41% 28% 5% 1	600
	21st to end	Night	16% 13% 44% 22% 5%	660
April	1st to 10th	Night	14% 14% 42% 27% 3	600
	11th to 20th	Night	15% 15% 42% 24% 4	593
	21st to end	Night	16% 13% 35% 30% 6% 1	600
May	1st to 10th	Night	18% 9% 34% 31% 7%	600
	11th to 20th	Night	15% 9% 35% 33% 8% 1	591
	21st to end	Night	16% 9% 31% 33% 11%	650
June	1st to 10th	Night	7% 6% 26% 46% 14% 2	600
	11th to 20th	Night	8% 5% 37% 39% 11%	600
	21st to end	Night	4 7% 36% 42% 11%	600
July	1st to 10th	Night	6% 3 22% 44% 24%	585
	11th to 20th	Night	3 2 22% 52% 20% 1	582
	21st to end	Night	3 2 17% 55% 23% 1	657
August	1st to 10th	Night	5% 3 20% 51% 19% 2	597
	11th to 20th	Night	8% 2 23% 49% 17% 1	594
	21st to end	Night	7% 3 24% 43% 22% 1	660
September	1st to 10th	Night	11% 3 25% 45% 15% 1	600
	11th to 20th	Night	10% 3 35% 38% 14% 1	600
	21st to end	Night	8% 6% 34% 37% 15% 1	600
October	1st to 10th	Night	15% 8% 32% 32% 13%	618
	11th to 20th	Night	13% 10% 39% 30% 8%	800
	21st to end	Night	15% 12% 32% 31% 10%	986
November	1st to 10th	Night	8% 10% 38% 37% 7%	1025
	11th to 20th	Night	10% 11% 42% 31% 6%	1050
	21st to end	Night	11% 12% 34% 32% 10% 1	1050
December	1st to 10th	Night	9% 10% 43% 33% 4	1011
	11th to 20th	Night	13% 11% 40% 29% 7%	1000
	21st to end	Night	16% 11% 38% 31% 5%	1100

Page generated at 2025-11-30 00:00:50. Version ac07

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