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 2024-02-25.

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% 3 5% 5% 3 5% 5% 6% 8% 40%	78428
	Night	15% 6% 3 5% 5% 3 4 5% 5% 7% 42%	80873
Transparency	Day	41% 7% 13% 22% 14% 2	78136
	Night	41% 7% 12% 23% 15% 2	80685
Seeing	Day	18% 12% 26% 23% 14% 5%	13904
	Night	17% 10% 30% 30% 12% 1	23456

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% 7% 2 5% 5% 3 5% 5% 4 6% 42%	2517
	11th to 20th	Day	13% 11% 4 8% 6% 4 5% 5% 5% 6% 33%	2753
	21st to end	Day	18% 10% 3 8% 6% 3 5% 5% 5% 6% 32%	3124
February	1st to 10th	Day	14% 9% 3 5% 5% 3 4 6% 6% 7% 37%	1710
	11th to 20th	Day	15% 7% 3 5% 6% 5% 4 5% 5% 5% 41%	1724
	21st to end	Day	19% 8% 4 6% 5% 3 4 5% 5% 5% 37%	1520
March	1st to 10th	Day	13% 5% 2 5% 5% 4 5% 4 5% 7% 46%	1898
	11th to 20th	Day	16% 7% 2 5% 5% 3 5% 5% 6% 8% 38%	1980
	21st to end	Day	20% 7% 3 6% 5% 2 4 4 4 7% 37%	2238
April	1st to 10th	Day	14% 7% 3 6% 5% 3 5% 5% 5% 7% 39%	2160
	11th to 20th	Day	15% 5% 3 5% 4 3 5% 5% 5% 7% 43%	2134
	21st to end	Day	18% 6% 3 5% 5% 3 4 5% 5% 8% 39%	2160
May	1st to 10th	Day	17% 7% 3 6% 5% 4 5% 5% 6% 7% 36%	2312
	11th to 20th	Day	19% 7% 3 6% 5% 3 5% 5% 5% 7% 36%	2478
	21st to end	Day	15% 6% 2 5% 5% 3 5% 5% 6% 9% 39%	2725
June	1st to 10th	Day	13% 5% 3 5% 5% 3 5% 5% 7% 10% 40%	2520
	11th to 20th	Day	11% 6% 3 6% 6% 3 5% 6% 6% 9% 39%	2520
	21st to end	Day	10% 3 2 4 6% 3 5% 6% 6% 11% 43%	2520
July	1st to 10th	Day	9% 5% 2 5% 7% 4 6% 5% 6% 10% 42%	2520
	11th to 20th	Day	6% 4 2 5% 5% 4 5% 6% 8% 10% 43%	2520
	21st to end	Day	9% 5% 3 6% 7% 3 7% 7% 7% 9% 38%	2580
August	1st to 10th	Day	10% 5% 2 6% 6% 4 7% 6% 8% 8% 38%	2327
	11th to 20th	Day	11% 6% 3 6% 5% 3 5% 6% 7% 9% 39%	2206
	21st to end	Day	13% 4 2 5% 5% 3 5% 6% 6% 8% 42%	2376
September	1st to 10th	Day	16% 6% 2 5% 5% 3 4 4 4 6% 45%	2160
	11th to 20th	Day	13% 4 2 4 4 3 4 4 5% 8% 49%	2160
	21st to end	Day	17% 7% 3 5% 5% 3 4 5% 5% 7% 40%	1909
October	1st to 10th	Day	17% 6% 2 5% 4 3 3 4 5% 7% 44%	1800
	11th to 20th	Day	16% 6% 3 5% 4 3 3 3 5% 7% 45%	1800
	21st to end	Day	16% 6% 3 4 5% 3 3 5% 5% 7% 43%	1980
November	1st to 10th	Day	13% 5% 2 4 4 2 3 4 4 7% 51%	1694
	11th to 20th	Day	17% 4 2 5% 4 2 4 4 5% 6% 46%	1681
	21st to end	Day	16% 4 3 5% 5% 3 4 5% 6% 6% 43%	1520
December	1st to 10th	Day	18% 7% 3 6% 6% 4 4 4 7% 8% 34%	1520
	11th to 20th	Day	17% 7% 3 5% 4 2 5% 5% 6% 8% 37%	1910
	21st to end	Day	17% 7% 3 7% 7% 3 5% 5% 6% 7% 32%	2772
January	1st to 10th	Night	14% 8% 3 6% 6% 4 5% 5% 5% 6% 38%	4429
	11th to 20th	Night	17% 8% 3 6% 5% 3 4 4 4 6% 40%	4202
	21st to end	Night	21% 9% 3 6% 5% 3 4 4 4 6% 35%	4532
February	1st to 10th	Night	17% 7% 3 6% 4 3 4 5% 4 6% 40%	2470
	11th to 20th	Night	17% 7% 4 5% 4 3 4 5% 4 6% 40%	2342
	21st to end	Night	16% 7% 4 5% 6% 3 5% 5% 5% 5% 39%	1831
March	1st to 10th	Night	14% 6% 2 4 4 3 4 5% 5% 7% 47%	2061
	11th to 20th	Night	16% 6% 3 5% 5% 3 4 5% 4 5% 43%	1980
	21st to end	Night	17% 7% 3 6% 5% 2 4 4 5% 6% 40%	2116
April	1st to 10th	Night	15% 8% 3 6% 6% 3 5% 5% 5% 7% 37%	1800
	11th to 20th	Night	15% 4 2 5% 5% 3 4 5% 5% 8% 44%	1775
	21st to end	Night	20% 5% 2 4 5% 2 4 4 5% 7% 42%	1800
May	1st to 10th	Night	17% 6% 3 6% 5% 3 5% 5% 5% 7% 38%	1659
	11th to 20th	Night	17% 7% 2 6% 5% 2 5% 5% 5% 8% 38%	1415
	21st to end	Night	12% 6% 3 6% 6% 3 4 5% 5% 9% 41%	1550
June	1st to 10th	Night	13% 5% 2 4 4 2 5% 5% 6% 8% 45%	1440
	11th to 20th	Night	11% 5% 3 5% 6% 4 4 5% 8% 10% 40%	1440
	21st to end	Night	9% 3 3 5% 4 3 6% 6% 6% 11% 44%	1440
July	1st to 10th	Night	9% 5% 3 6% 8% 5% 4 7% 6% 9% 38%	1440
	11th to 20th	Night	6% 3 2 4 5% 3 6% 6% 7% 11% 47%	1440
	21st to end	Night	10% 3 2 4 5% 3 5% 6% 7% 9% 45%	1735
August	1st to 10th	Night	8% 4 2 5% 5% 3 5% 7% 8% 9% 42%	1611
	11th to 20th	Night	9% 6% 3 7% 6% 4 6% 5% 6% 7% 41%	1675
	21st to end	Night	13% 5% 2 5% 6% 3 5% 5% 6% 8% 45%	1980
September	1st to 10th	Night	14% 5% 3 5% 6% 2 4 4 5% 6% 46%	1800
	11th to 20th	Night	12% 4 2 4 3 2 3 4 5% 7% 53%	1800
	21st to end	Night	16% 6% 2 5% 5% 3 4 4 5% 7% 42%	2046
October	1st to 10th	Night	19% 5% 3 5% 4 3 3 3 5% 6% 45%	2160
	11th to 20th	Night	15% 4 2 4 4 3 4 4 5% 6% 48%	2160
	21st to end	Night	18% 5% 2 4 4 2 4 3 5% 7% 47%	2376
November	1st to 10th	Night	12% 5% 2 5% 3 2 4 4 5% 6% 52%	2382
	11th to 20th	Night	14% 7% 3 5% 4 2 4 4 5% 7% 46%	2497
	21st to end	Night	14% 4 3 5% 5% 3 5% 4 6% 8% 44%	2660
December	1st to 10th	Night	18% 7% 2 6% 5% 3 4 5% 5% 6% 38%	2660
	11th to 20th	Night	17% 6% 2 5% 5% 3 5% 5% 6% 6% 40%	3285
	21st to end	Night	16% 8% 3 6% 6% 4 5% 6% 6% 7% 32%	4884

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	39% 7% 15% 26% 11% 3	2518
	11th to 20th	Day	50% 6% 12% 17% 13% 3	2753
	21st to end	Day	52% 6% 11% 22% 7% 1	3124
February	1st to 10th	Day	44% 7% 13% 24% 11% 2	1710
	11th to 20th	Day	43% 6% 10% 25% 13% 2	1733
	21st to end	Day	49% 3 11% 21% 12% 4	1520
March	1st to 10th	Day	39% 5% 13% 26% 14% 2	1898
	11th to 20th	Day	43% 6% 12% 19% 16% 5%	1980
	21st to end	Day	47% 6% 9% 20% 16% 3	2238
April	1st to 10th	Day	43% 6% 12% 19% 16% 4	2160
	11th to 20th	Day	39% 7% 11% 22% 16% 4	2134
	21st to end	Day	43% 7% 10% 22% 16% 3	2160
May	1st to 10th	Day	44% 7% 11% 20% 15% 3	2312
	11th to 20th	Day	48% 7% 12% 19% 13% 2	2482
	21st to end	Day	40% 9% 16% 22% 12% 2	2725
June	1st to 10th	Day	38% 10% 13% 23% 15% 1	2520
	11th to 20th	Day	40% 9% 15% 21% 12% 2	2520
	21st to end	Day	34% 9% 16% 26% 15% 1	2520
July	1st to 10th	Day	37% 10% 13% 21% 16% 3	2450
	11th to 20th	Day	32% 9% 16% 25% 16% 2	2445
	21st to end	Day	39% 11% 12% 20% 15% 2	2436
August	1st to 10th	Day	39% 11% 14% 23% 13%	2314
	11th to 20th	Day	39% 9% 14% 22% 15% 2	2206
	21st to end	Day	38% 8% 13% 24% 14% 4	2376
September	1st to 10th	Day	41% 6% 12% 19% 18% 4	2160
	11th to 20th	Day	33% 7% 11% 25% 21% 4	2160
	21st to end	Day	43% 6% 9% 19% 20% 3	1909
October	1st to 10th	Day	40% 6% 12% 24% 16% 2	1800
	11th to 20th	Day	39% 5% 12% 24% 18% 2	1800
	21st to end	Day	40% 6% 11% 25% 17% 2	1980
November	1st to 10th	Day	34% 4 14% 25% 18% 4	1694
	11th to 20th	Day	38% 6% 12% 24% 19% 2	1681
	21st to end	Day	41% 5% 12% 29% 10% 2	1520
December	1st to 10th	Day	47% 7% 15% 20% 11% 1	1520
	11th to 20th	Day	43% 6% 14% 29% 7% 1	1906
	21st to end	Day	48% 8% 16% 19% 9% 1	2772
January	1st to 10th	Night	43% 6% 14% 25% 10% 3	4429
	11th to 20th	Night	43% 5% 14% 25% 11% 2	4202
	21st to end	Night	48% 5% 10% 26% 9% 2	4532
February	1st to 10th	Night	44% 6% 13% 23% 12% 1	2470
	11th to 20th	Night	44% 5% 11% 21% 17% 2	2344
	21st to end	Night	46% 6% 10% 19% 15% 4	1831
March	1st to 10th	Night	36% 6% 12% 25% 19% 2	2061
	11th to 20th	Night	42% 6% 11% 21% 16% 4	1980
	21st to end	Night	44% 5% 11% 20% 17% 4	2116
April	1st to 10th	Night	44% 7% 10% 19% 16% 4	1800
	11th to 20th	Night	38% 6% 11% 21% 19% 4	1775
	21st to end	Night	41% 6% 11% 21% 16% 5%	1800
May	1st to 10th	Night	45% 7% 11% 19% 14% 4	1659
	11th to 20th	Night	44% 8% 12% 16% 18% 2	1416
	21st to end	Night	39% 10% 15% 21% 14% 2	1551
June	1st to 10th	Night	36% 9% 12% 24% 18% 2	1440
	11th to 20th	Night	37% 11% 15% 19% 16% 2	1440
	21st to end	Night	34% 9% 14% 26% 16% 2	1440
July	1st to 10th	Night	41% 10% 15% 18% 12% 3	1400
	11th to 20th	Night	29% 10% 17% 24% 17% 2	1392
	21st to end	Night	31% 11% 18% 21% 17% 2	1639
August	1st to 10th	Night	34% 10% 18% 24% 13% 1	1600
	11th to 20th	Night	39% 10% 14% 19% 16% 1	1675
	21st to end	Night	37% 8% 13% 22% 17% 3	1980
September	1st to 10th	Night	38% 7% 11% 21% 19% 4	1800
	11th to 20th	Night	31% 6% 10% 24% 25% 4	1800
	21st to end	Night	42% 7% 9% 20% 20% 3	2046
October	1st to 10th	Night	42% 6% 11% 23% 16% 2	2160
	11th to 20th	Night	36% 6% 11% 28% 17% 2	2160
	21st to end	Night	37% 5% 11% 27% 18% 2	2376
November	1st to 10th	Night	33% 5% 11% 26% 20% 5%	2382
	11th to 20th	Night	39% 4 10% 24% 18% 5%	2497
	21st to end	Night	37% 5% 13% 27% 15% 2	2660
December	1st to 10th	Night	46% 6% 12% 23% 12%	2660
	11th to 20th	Night	42% 6% 13% 23% 14% 2	3288
	21st to end	Night	45% 10% 15% 21% 8% 1	4884

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	24% 26% 31% 14% 4 1	180
	11th to 20th	Day	30% 24% 31% 13% 2	247
	21st to end	Day	22% 23% 36% 14% 4 1	297
February	1st to 10th	Day	25% 22% 35% 15% 3	270
	11th to 20th	Day	25% 26% 37% 10% 2	331
	21st to end	Day	30% 18% 32% 18% 2	362
March	1st to 10th	Day	20% 23% 30% 22% 5%	420
	11th to 20th	Day	22% 21% 34% 18% 4 1	420
	21st to end	Day	24% 21% 29% 19% 6% 1	462
April	1st to 10th	Day	21% 20% 34% 16% 8% 1	420
	11th to 20th	Day	18% 18% 29% 18% 12% 5%	412
	21st to end	Day	25% 14% 30% 20% 8% 2	420
May	1st to 10th	Day	24% 16% 28% 20% 10% 3	423
	11th to 20th	Day	19% 14% 28% 22% 12% 4	447
	21st to end	Day	19% 6% 19% 27% 20% 9%	485
June	1st to 10th	Day	14% 9% 28% 26% 16% 7%	450
	11th to 20th	Day	16% 8% 25% 26% 18% 6%	450
	21st to end	Day	10% 5% 27% 31% 20% 7%	450
July	1st to 10th	Day	12% 3 18% 29% 26% 13%	445
	11th to 20th	Day	5% 3 18% 34% 29% 11%	444
	21st to end	Day	11% 3 23% 33% 22% 7%	488
August	1st to 10th	Day	11% 3 20% 26% 30% 11%	446
	11th to 20th	Day	13% 4 17% 29% 24% 13%	439
	21st to end	Day	11% 4 18% 31% 22% 13%	495
September	1st to 10th	Day	18% 7% 16% 22% 24% 13%	450
	11th to 20th	Day	14% 5% 19% 28% 21% 13%	450
	21st to end	Day	23% 10% 20% 26% 15% 6%	450
October	1st to 10th	Day	21% 6% 24% 23% 19% 8%	450
	11th to 20th	Day	18% 12% 26% 28% 13% 3	450
	21st to end	Day	18% 11% 36% 25% 8% 2	495
November	1st to 10th	Day	16% 11% 30% 25% 15% 2	377
	11th to 20th	Day	19% 17% 29% 24% 9% 2	341
	21st to end	Day	20% 24% 30% 17% 8% 1	180
December	1st to 10th	Day	26% 20% 32% 15% 4 3	180
	11th to 20th	Day	27% 21% 30% 16% 6% 1	180
	21st to end	Day	22% 19% 32% 19% 8% 1	198
January	1st to 10th	Night	17% 15% 44% 21% 3	760
	11th to 20th	Night	23% 15% 31% 27% 3	760
	21st to end	Night	16% 16% 43% 23% 2	836
February	1st to 10th	Night	22% 15% 33% 28% 2	760
	11th to 20th	Night	22% 19% 39% 19% 1	743
	21st to end	Night	21% 18% 40% 17% 3	611
March	1st to 10th	Night	18% 16% 38% 24% 4	720
	11th to 20th	Night	21% 15% 35% 23% 5%	720
	21st to end	Night	21% 11% 38% 23% 6%	792
April	1st to 10th	Night	21% 16% 37% 22% 5%	720
	11th to 20th	Night	16% 11% 35% 29% 8%	711
	21st to end	Night	21% 10% 33% 28% 8%	720
May	1st to 10th	Night	19% 8% 29% 32% 11%	579
	11th to 20th	Night	21% 6% 30% 34% 10%	354
	21st to end	Night	15% 4 19% 41% 20% 2	389
June	1st to 10th	Night	14% 1 26% 37% 21% 1	360
	11th to 20th	Night	11% 4 27% 39% 18% 1	360
	21st to end	Night	9% 2 24% 50% 15%	360
July	1st to 10th	Night	11% 3 18% 40% 26% 2	350
	11th to 20th	Night	5% 2 16% 46% 30% 2	348
	21st to end	Night	11% 1 17% 45% 24% 2	555
August	1st to 10th	Night	10% 1 11% 48% 27% 2	537
	11th to 20th	Night	11% 4 12% 40% 30% 3	611
	21st to end	Night	12% 3 15% 37% 30% 3	792
September	1st to 10th	Night	16% 3 20% 34% 25% 3	720
	11th to 20th	Night	12% 4 18% 36% 27% 4	720
	21st to end	Night	19% 3 23% 32% 21% 2	720
October	1st to 10th	Night	20% 5% 23% 28% 21% 3	720
	11th to 20th	Night	17% 9% 29% 32% 13%	720
	21st to end	Night	20% 8% 34% 28% 11%	792
November	1st to 10th	Night	14% 7% 33% 33% 12% 1	740
	11th to 20th	Night	18% 13% 32% 28% 8% 1	760
	21st to end	Night	16% 14% 35% 28% 7%	760
December	1st to 10th	Night	21% 13% 37% 22% 6%	760
	11th to 20th	Night	17% 11% 41% 27% 4	760
	21st to end	Night	19% 13% 35% 31% 2	836

Page generated at 2024-02-22 06:05:44. Version ac07

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