YCAS Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the YCAS Observatory clear sky chart from 2005-11-06 to 2026-08-30.

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	14% 6% 2 6% 4 4 5% 4 6% 8% 39%	144204
	Night	14% 7% 3 6% 4 4 4 4 5% 7% 41%	138043
Transparency	Day	40% 7% 12% 23% 15% 2	147797
	Night	40% 6% 12% 23% 16% 3	142523
Seeing	Day	19% 13% 27% 23% 13% 5%	17247
	Night	18% 10% 30% 30% 12% 1	26587

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	13% 6% 2 7% 5% 5% 6% 4 6% 6% 40%	4661
	11th to 20th	Day	12% 10% 4 8% 5% 4 4 3 4 6% 40%	5137
	21st to end	Day	13% 6% 2 7% 5% 6% 6% 4 7% 7% 36%	5780
February	1st to 10th	Day	14% 9% 3 6% 5% 5% 4 5% 7% 6% 38%	1890
	11th to 20th	Day	15% 8% 3 6% 5% 6% 4 4 6% 5% 39%	1911
	21st to end	Day	18% 9% 4 6% 4 4 5% 3 5% 5% 37%	1730
March	1st to 10th	Day	19% 5% 2 5% 4 5% 4 3 4 6% 42%	4850
	11th to 20th	Day	10% 7% 2 7% 4 4 5% 3 6% 7% 43%	5090
	21st to end	Day	16% 7% 2 6% 4 4 5% 3 5% 7% 41%	5760
April	1st to 10th	Day	16% 7% 3 7% 4 4 4 4 5% 7% 38%	2520
	11th to 20th	Day	14% 6% 2 6% 4 4 5% 4 6% 7% 42%	2494
	21st to end	Day	18% 7% 2 5% 4 4 4 4 5% 7% 40%	2520
May	1st to 10th	Day	21% 7% 2 6% 4 4 4 4 6% 8% 33%	7546
	11th to 20th	Day	19% 6% 2 7% 4 5% 5% 4 6% 8% 36%	8221
	21st to end	Day	23% 6% 2 5% 4 4 4 4 6% 7% 36%	9045
June	1st to 10th	Day	12% 5% 2 6% 4 4 4 5% 7% 9% 41%	2940
	11th to 20th	Day	12% 6% 3 6% 5% 4 4 5% 6% 9% 40%	2940
	21st to end	Day	10% 4 2 6% 5% 5% 5% 4 6% 10% 43%	2940
July	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 4 6% 10% 40%	8280
	11th to 20th	Day	7% 6% 3 7% 5% 5% 6% 5% 9% 11% 35%	8280
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 4 7% 10% 39%	8442
August	1st to 10th	Day	17% 6% 2 7% 4 5% 6% 5% 7% 8% 34%	4364
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 5% 7% 9% 40%	4112
	21st to end	Day	12% 5% 2 5% 3 4 4 3 5% 7% 50%	4420
September	1st to 10th	Day	15% 7% 2 5% 4 4 4 3 4 6% 47%	2400
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 5% 8% 49%	2400
	21st to end	Day	19% 8% 3 6% 4 4 4 3 4 6% 37%	2120
October	1st to 10th	Day	16% 6% 2 5% 3 3 4 3 5% 7% 46%	2000
	11th to 20th	Day	15% 6% 2 5% 4 4 3 3 5% 6% 47%	2000
	21st to end	Day	15% 6% 3 5% 4 3 3 4 5% 7% 46%	2200
November	1st to 10th	Day	12% 5% 2 4 3 3 4 3 5% 8% 52%	1876
	11th to 20th	Day	15% 5% 2 5% 4 4 4 4 6% 7% 45%	1857
	21st to end	Day	15% 6% 3 5% 4 4 4 4 5% 6% 43%	1680
December	1st to 10th	Day	13% 5% 2 7% 5% 5% 4 4 7% 7% 41%	2680
	11th to 20th	Day	18% 10% 2 6% 3 3 4 3 5% 8% 37%	3070
	21st to end	Day	15% 7% 3 9% 6% 5% 5% 4 6% 7% 33%	4048
January	1st to 10th	Night	10% 8% 3 8% 5% 5% 5% 4 6% 7% 37%	8233
	11th to 20th	Night	12% 9% 3 7% 4 4 4 3 5% 6% 44%	7818
	21st to end	Night	14% 7% 2 5% 4 4 4 3 6% 7% 43%	8401
February	1st to 10th	Night	16% 8% 3 6% 5% 4 4 4 5% 6% 39%	2730
	11th to 20th	Night	17% 8% 3 6% 3 4 4 4 5% 6% 39%	2595
	21st to end	Night	15% 8% 3 6% 5% 4 4 4 5% 6% 39%	2076
March	1st to 10th	Night	16% 9% 2 6% 3 3 3 3 4 6% 44%	5221
	11th to 20th	Night	9% 5% 2 6% 4 4 4 4 4 6% 50%	5090
	21st to end	Night	13% 8% 2 7% 5% 4 5% 3 5% 6% 43%	5412
April	1st to 10th	Night	17% 8% 3 7% 5% 4 4 4 5% 7% 37%	2100
	11th to 20th	Night	15% 5% 2 6% 4 4 4 4 6% 8% 42%	2075
	21st to end	Night	18% 5% 2 5% 4 3 4 3 5% 7% 43%	2100
May	1st to 10th	Night	19% 7% 3 7% 5% 5% 5% 3 6% 8% 32%	5359
	11th to 20th	Night	18% 7% 3 6% 4 3 4 3 7% 9% 35%	4639
	21st to end	Night	23% 7% 3 7% 4 4 4 3 5% 7% 34%	5108
June	1st to 10th	Night	12% 5% 2 5% 4 4 4 5% 6% 8% 45%	1680
	11th to 20th	Night	11% 5% 2 6% 5% 5% 4 4 8% 10% 40%	1680
	21st to end	Night	9% 4 3 6% 5% 5% 5% 4 5% 11% 42%	1680
July	1st to 10th	Night	10% 6% 3 7% 5% 5% 5% 5% 6% 10% 39%	4678
	11th to 20th	Night	7% 7% 3 9% 6% 6% 6% 5% 7% 11% 33%	4677
	21st to end	Night	10% 5% 2 6% 5% 5% 5% 4 6% 9% 43%	5617
August	1st to 10th	Night	18% 5% 2 7% 5% 4 4 5% 7% 9% 35%	3005
	11th to 20th	Night	11% 6% 3 8% 5% 4 5% 4 6% 8% 41%	3121
	21st to end	Night	12% 5% 2 6% 4 4 5% 4 6% 8% 46%	3677
September	1st to 10th	Night	13% 5% 3 7% 4 3 4 3 5% 6% 47%	2000
	11th to 20th	Night	12% 5% 2 5% 3 3 4 3 5% 7% 51%	2000
	21st to end	Night	19% 7% 2 6% 5% 4 5% 3 5% 7% 38%	2275
October	1st to 10th	Night	18% 6% 2 5% 3 3 3 3 5% 5% 46%	2400
	11th to 20th	Night	15% 4 2 5% 3 3 4 3 5% 6% 50%	2400
	21st to end	Night	16% 5% 2 4 3 3 3 3 5% 7% 49%	2640
November	1st to 10th	Night	11% 5% 2 5% 3 4 4 3 5% 6% 53%	2640
	11th to 20th	Night	14% 7% 2 5% 4 4 4 3 5% 6% 46%	2761
	21st to end	Night	13% 5% 2 5% 5% 4 5% 3 6% 7% 44%	2940
December	1st to 10th	Night	11% 6% 2 7% 5% 4 5% 5% 6% 7% 42%	4720
	11th to 20th	Night	20% 7% 2 5% 4 4 4 4 5% 7% 39%	5345
	21st to end	Night	16% 9% 3 7% 5% 5% 5% 4 6% 6% 33%	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	41% 5% 12% 25% 14% 2	4662
	11th to 20th	Day	44% 4 10% 19% 17% 7%	5137
	21st to end	Day	43% 7% 12% 25% 12% 1	5780
February	1st to 10th	Day	43% 7% 13% 24% 11% 2	1890
	11th to 20th	Day	45% 6% 11% 24% 13% 2	1920
	21st to end	Day	48% 3 11% 22% 12% 4	1730
March	1st to 10th	Day	41% 6% 10% 23% 15% 6%	4850
	11th to 20th	Day	37% 6% 13% 26% 13% 5%	5090
	21st to end	Day	42% 5% 10% 19% 18% 5%	5760
April	1st to 10th	Day	44% 6% 12% 19% 15% 4	2520
	11th to 20th	Day	38% 7% 12% 22% 16% 4	2494
	21st to end	Day	42% 6% 9% 23% 16% 3	2520
May	1st to 10th	Day	46% 6% 12% 22% 13% 1	7580
	11th to 20th	Day	44% 7% 11% 24% 12% 1	8225
	21st to end	Day	46% 7% 11% 21% 15% 1	9061
June	1st to 10th	Day	37% 9% 12% 22% 17% 2	2940
	11th to 20th	Day	39% 8% 15% 23% 13% 3	2940
	21st to end	Day	35% 8% 15% 25% 16% 1	2940
July	1st to 10th	Day	39% 9% 13% 22% 16% 2	8210
	11th to 20th	Day	39% 8% 15% 27% 11% 1	8205
	21st to end	Day	38% 7% 11% 23% 20% 1	8256
August	1st to 10th	Day	46% 9% 12% 24% 9%	4351
	11th to 20th	Day	38% 8% 12% 28% 13% 1	4151
	21st to end	Day	34% 6% 10% 28% 20% 2	4420
September	1st to 10th	Day	39% 6% 11% 20% 20% 4	2400
	11th to 20th	Day	33% 6% 10% 25% 21% 4	2400
	21st to end	Day	47% 6% 9% 17% 19% 2	2120
October	1st to 10th	Day	35% 5% 8% 21% 26% 5%	3260
	11th to 20th	Day	28% 4 12% 28% 20% 7%	3240
	21st to end	Day	25% 4 11% 28% 28% 3	3538
November	1st to 10th	Day	33% 4 14% 25% 20% 4	1876
	11th to 20th	Day	37% 6% 11% 24% 19% 3	1857
	21st to end	Day	41% 5% 12% 28% 11% 2	1680
December	1st to 10th	Day	40% 6% 17% 25% 10% 2	2680
	11th to 20th	Day	44% 5% 15% 27% 8% 1	3066
	21st to end	Day	46% 7% 16% 18% 11% 1	4048
January	1st to 10th	Night	42% 5% 14% 23% 13% 2	8234
	11th to 20th	Night	39% 4 11% 22% 18% 6%	7818
	21st to end	Night	37% 4 10% 29% 17% 2	8401
February	1st to 10th	Night	44% 6% 13% 23% 12% 1	2730
	11th to 20th	Night	45% 4 11% 21% 17% 2	2597
	21st to end	Night	45% 5% 11% 20% 15% 4	2076
March	1st to 10th	Night	41% 5% 9% 25% 12% 8%	5221
	11th to 20th	Night	32% 7% 13% 30% 12% 6%	5090
	21st to end	Night	41% 5% 9% 18% 24% 3	5412
April	1st to 10th	Night	44% 7% 11% 19% 16% 4	2100
	11th to 20th	Night	39% 6% 12% 20% 19% 4	2075
	21st to end	Night	40% 6% 11% 22% 17% 4	2100
May	1st to 10th	Night	50% 6% 11% 18% 13% 3	5388
	11th to 20th	Night	45% 8% 13% 24% 10% 1	4640
	21st to end	Night	50% 6% 9% 21% 12% 2	5115
June	1st to 10th	Night	35% 8% 11% 25% 20% 2	1680
	11th to 20th	Night	37% 10% 14% 21% 17% 1	1680
	21st to end	Night	35% 8% 13% 25% 16% 2	1680
July	1st to 10th	Night	41% 9% 14% 19% 14% 3	4640
	11th to 20th	Night	44% 9% 13% 22% 11% 1	4632
	21st to end	Night	36% 8% 14% 23% 19% 1	5490
August	1st to 10th	Night	44% 8% 17% 21% 9% 1	2994
	11th to 20th	Night	40% 7% 11% 26% 15% 1	3148
	21st to end	Night	35% 7% 10% 29% 16% 2	3677
September	1st to 10th	Night	37% 6% 11% 21% 21% 4	2000
	11th to 20th	Night	32% 6% 10% 24% 24% 3	2000
	21st to end	Night	45% 6% 9% 18% 18% 3	2275
October	1st to 10th	Night	41% 6% 9% 21% 22% 2	3920
	11th to 20th	Night	26% 5% 11% 28% 27% 4	3899
	21st to end	Night	25% 4 11% 33% 24% 3	4252
November	1st to 10th	Night	32% 5% 11% 26% 22% 4	2640
	11th to 20th	Night	39% 4 9% 24% 18% 5%	2761
	21st to end	Night	38% 5% 13% 27% 15% 2	2940
December	1st to 10th	Night	39% 5% 15% 30% 11% 1	4720
	11th to 20th	Night	44% 5% 14% 23% 12% 2	5348
	21st to end	Night	47% 8% 12% 18% 14% 1	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	25% 27% 30% 13% 4	220
	11th to 20th	Day	32% 25% 30% 11% 2	303
	21st to end	Day	24% 25% 35% 12% 4 1	363
February	1st to 10th	Day	25% 23% 34% 15% 3	330
	11th to 20th	Day	30% 24% 35% 10% 1	398
	21st to end	Day	29% 19% 33% 17% 3	446
March	1st to 10th	Day	24% 22% 31% 18% 5%	540
	11th to 20th	Day	25% 20% 32% 17% 4 1	540
	21st to end	Day	25% 23% 29% 18% 5% 1	594
April	1st to 10th	Day	26% 19% 31% 16% 7% 1	540
	11th to 20th	Day	21% 19% 30% 17% 10% 4	532
	21st to end	Day	24% 14% 29% 21% 8% 3	540
May	1st to 10th	Day	25% 17% 29% 17% 8% 2	543
	11th to 20th	Day	20% 14% 28% 23% 11% 5%	567
	21st to end	Day	22% 6% 21% 25% 18% 7%	617
June	1st to 10th	Day	14% 11% 27% 26% 15% 7%	570
	11th to 20th	Day	18% 9% 25% 26% 16% 5%	570
	21st to end	Day	12% 6% 26% 31% 18% 6%	570
July	1st to 10th	Day	12% 3 18% 31% 25% 11%	565
	11th to 20th	Day	6% 3 21% 33% 27% 10%	564
	21st to end	Day	11% 4 25% 34% 20% 6%	620
August	1st to 10th	Day	12% 3 21% 28% 27% 9%	566
	11th to 20th	Day	13% 4 20% 30% 23% 10%	559
	21st to end	Day	11% 5% 21% 31% 21% 12%	619
September	1st to 10th	Day	16% 8% 18% 23% 23% 12%	530
	11th to 20th	Day	13% 4 18% 31% 22% 12%	530
	21st to end	Day	27% 9% 20% 25% 14% 5%	530
October	1st to 10th	Day	20% 7% 23% 24% 18% 9%	530
	11th to 20th	Day	19% 13% 26% 27% 12% 3	530
	21st to end	Day	17% 12% 36% 24% 9% 2	583
November	1st to 10th	Day	16% 13% 30% 26% 13% 2	439
	11th to 20th	Day	20% 17% 29% 24% 8% 2	397
	21st to end	Day	24% 24% 30% 15% 7% 1	220
December	1st to 10th	Day	25% 21% 35% 14% 3 2	220
	11th to 20th	Day	28% 20% 30% 16% 5%	220
	21st to end	Day	24% 19% 32% 17% 7%	242
January	1st to 10th	Night	18% 16% 43% 21% 3	840
	11th to 20th	Night	24% 16% 32% 25% 3	840
	21st to end	Night	17% 16% 43% 22% 2	924
February	1st to 10th	Night	22% 15% 33% 28% 2	840
	11th to 20th	Night	23% 19% 38% 19% 1	823
	21st to end	Night	21% 18% 39% 18% 3	692
March	1st to 10th	Night	20% 16% 37% 23% 4	840
	11th to 20th	Night	21% 16% 35% 23% 5%	840
	21st to end	Night	21% 13% 38% 23% 5%	924
April	1st to 10th	Night	22% 16% 36% 22% 4	840
	11th to 20th	Night	17% 12% 35% 27% 8%	831
	21st to end	Night	20% 10% 33% 29% 8%	840
May	1st to 10th	Night	19% 10% 30% 31% 10%	675
	11th to 20th	Night	20% 6% 31% 34% 9%	414
	21st to end	Night	17% 4 20% 40% 18% 1	455
June	1st to 10th	Night	13% 2 27% 37% 20%	420
	11th to 20th	Night	12% 5% 29% 38% 16%	420
	21st to end	Night	11% 2 23% 47% 16%	420
July	1st to 10th	Night	11% 3 17% 40% 26% 2	410
	11th to 20th	Night	5% 2 15% 47% 29% 2	408
	21st to end	Night	12% 2 16% 44% 25% 2	648
August	1st to 10th	Night	11% 2 11% 46% 28% 2	627
	11th to 20th	Night	11% 3 13% 41% 29% 3	715
	21st to end	Night	12% 3 16% 38% 29% 3	917
September	1st to 10th	Night	15% 3 21% 34% 25% 3	800
	11th to 20th	Night	13% 4 17% 37% 27% 4	800
	21st to end	Night	22% 3 21% 32% 20% 2	800
October	1st to 10th	Night	20% 5% 23% 27% 22% 3	800
	11th to 20th	Night	17% 9% 28% 32% 13%	800
	21st to end	Night	18% 8% 33% 29% 11%	880
November	1st to 10th	Night	14% 7% 33% 34% 11% 1	820
	11th to 20th	Night	19% 13% 32% 28% 8% 1	840
	21st to end	Night	17% 14% 35% 27% 7%	840
December	1st to 10th	Night	20% 13% 38% 23% 6%	840
	11th to 20th	Night	19% 11% 39% 27% 4	840
	21st to end	Night	20% 13% 34% 30% 3	924

Page generated at 2026-08-27 04:07:15. Version ac07

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