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 2024-10-25.

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	15% 7% 2 6% 4 4 5% 4 6% 7% 39%	101203
	Night	15% 7% 2 6% 4 4 4 4 5% 7% 41%	97550
Transparency	Day	41% 7% 12% 23% 14% 2	101001
	Night	41% 7% 12% 23% 15% 2	97424
Seeing	Day	18% 13% 26% 24% 14% 5%	14879
	Night	17% 10% 30% 30% 12% 1	24251

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 6% 5% 5% 5% 4 5% 6% 42%	2517
	11th to 20th	Day	13% 12% 4 9% 5% 5% 5% 4 5% 6% 32%	2753
	21st to end	Day	18% 10% 3 8% 5% 4 5% 3 5% 6% 32%	3124
February	1st to 10th	Day	14% 9% 2 6% 5% 5% 4 5% 6% 7% 37%	1710
	11th to 20th	Day	15% 7% 2 6% 5% 6% 4 4 6% 5% 41%	1724
	21st to end	Day	19% 9% 4 6% 4 4 4 4 4 5% 37%	1570
March	1st to 10th	Day	23% 7% 2 5% 4 5% 4 3 5% 6% 36%	3301
	11th to 20th	Day	10% 6% 2 7% 4 5% 5% 3 6% 7% 44%	3480
	21st to end	Day	21% 8% 2 5% 3 3 3 2 4 6% 42%	3930
April	1st to 10th	Day	15% 7% 3 7% 5% 5% 5% 4 6% 7% 38%	2280
	11th to 20th	Day	14% 6% 2 6% 4 4 5% 4 5% 7% 43%	2254
	21st to end	Day	18% 6% 2 5% 4 4 4 4 5% 7% 40%	2280
May	1st to 10th	Day	27% 7% 2 6% 4 4 4 4 6% 7% 30%	4041
	11th to 20th	Day	23% 8% 3 7% 5% 5% 5% 3 5% 7% 29%	4398
	21st to end	Day	11% 5% 2 6% 4 4 4 4 7% 9% 44%	4823
June	1st to 10th	Day	12% 6% 2 6% 4 4 4 5% 7% 10% 40%	2660
	11th to 20th	Day	11% 6% 3 7% 5% 4 5% 5% 6% 9% 39%	2660
	21st to end	Day	9% 3 2 6% 5% 5% 5% 4 7% 10% 43%	2660
July	1st to 10th	Day	6% 4 2 6% 6% 6% 6% 5% 8% 11% 40%	4440
	11th to 20th	Day	8% 6% 2 6% 5% 5% 6% 5% 9% 10% 40%	4440
	21st to end	Day	10% 7% 3 8% 5% 5% 5% 5% 6% 8% 36%	4513
August	1st to 10th	Day	17% 6% 2 6% 4 5% 6% 5% 7% 8% 33%	4104
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 5% 7% 8% 41%	3863
	21st to end	Day	12% 5% 2 5% 4 4 4 3 5% 7% 49%	4180
September	1st to 10th	Day	15% 7% 2 6% 4 4 4 3 4 5% 46%	2280
	11th to 20th	Day	12% 5% 1 5% 3 4 4 3 5% 8% 49%	2280
	21st to end	Day	20% 8% 3 6% 4 3 4 3 4 7% 38%	2014
October	1st to 10th	Day	17% 6% 2 5% 3 4 4 3 5% 6% 45%	1900
	11th to 20th	Day	15% 6% 2 5% 3 4 3 3 5% 6% 47%	1900
	21st to end	Day	16% 6% 3 5% 4 3 3 4 5% 6% 44%	2027
November	1st to 10th	Day	13% 5% 2 4 3 3 4 3 4 7% 51%	1694
	11th to 20th	Day	16% 5% 2 5% 4 3 4 4 5% 6% 46%	1681
	21st to end	Day	16% 6% 3 5% 4 4 4 4 6% 6% 43%	1520
December	1st to 10th	Day	17% 8% 2 7% 5% 5% 4 3 7% 8% 34%	1520
	11th to 20th	Day	17% 8% 3 6% 4 3 5% 4 6% 8% 36%	1910
	21st to end	Day	17% 7% 3 8% 6% 5% 5% 4 6% 7% 32%	2772
January	1st to 10th	Night	14% 9% 3 7% 5% 5% 4 4 6% 6% 37%	4429
	11th to 20th	Night	17% 9% 3 7% 4 3 4 3 5% 6% 40%	4202
	21st to end	Night	21% 9% 3 7% 5% 4 4 3 4 5% 35%	4532
February	1st to 10th	Night	17% 8% 3 6% 4 4 4 4 4 6% 40%	2470
	11th to 20th	Night	16% 8% 3 6% 3 4 4 4 5% 6% 40%	2342
	21st to end	Night	16% 8% 3 6% 5% 4 4 4 5% 6% 38%	1884
March	1st to 10th	Night	21% 10% 2 6% 4 3 3 3 5% 6% 38%	3551
	11th to 20th	Night	11% 5% 2 6% 4 4 4 4 5% 5% 49%	3480
	21st to end	Night	14% 8% 2 7% 4 3 4 2 4 5% 46%	3700
April	1st to 10th	Night	16% 8% 3 7% 5% 4 5% 4 6% 7% 37%	1900
	11th to 20th	Night	15% 5% 2 6% 4 4 4 4 5% 8% 43%	1875
	21st to end	Night	19% 5% 2 5% 4 3 4 3 5% 7% 43%	1900
May	1st to 10th	Night	24% 7% 3 7% 5% 4 4 3 5% 7% 31%	2878
	11th to 20th	Night	23% 8% 3 8% 5% 4 4 4 6% 6% 29%	2495
	21st to end	Night	11% 5% 2 7% 4 4 5% 4 6% 10% 41%	2732
June	1st to 10th	Night	13% 5% 2 5% 4 4 4 5% 6% 7% 45%	1520
	11th to 20th	Night	10% 5% 2 6% 5% 5% 4 4 8% 10% 41%	1520
	21st to end	Night	9% 4 3 6% 5% 5% 5% 4 6% 11% 43%	1520
July	1st to 10th	Night	6% 5% 2 7% 6% 6% 6% 6% 7% 10% 40%	2520
	11th to 20th	Night	7% 6% 2 7% 6% 6% 6% 5% 8% 11% 38%	2520
	21st to end	Night	13% 6% 3 6% 5% 4 4 4 6% 8% 40%	3023
August	1st to 10th	Night	18% 5% 2 7% 5% 4 4 5% 7% 8% 35%	2825
	11th to 20th	Night	11% 6% 3 8% 5% 4 5% 4 6% 7% 41%	2932
	21st to end	Night	12% 5% 2 5% 4 4 5% 3 6% 8% 46%	3476
September	1st to 10th	Night	13% 5% 3 7% 4 3 4 3 5% 6% 47%	1900
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 5% 7% 52%	1900
	21st to end	Night	19% 7% 2 6% 5% 3 5% 3 5% 6% 40%	2161
October	1st to 10th	Night	19% 6% 3 6% 3 3 3 3 5% 5% 45%	2280
	11th to 20th	Night	14% 4 2 5% 3 3 4 3 5% 6% 49%	2280
	21st to end	Night	17% 5% 2 4 3 3 3 3 5% 6% 48%	2435
November	1st to 10th	Night	12% 5% 2 5% 3 4 4 3 5% 6% 52%	2382
	11th to 20th	Night	14% 7% 2 5% 4 3 4 3 5% 6% 46%	2497
	21st to end	Night	13% 5% 2 5% 5% 4 5% 3 6% 7% 44%	2660
December	1st to 10th	Night	17% 7% 2 7% 4 4 4 4 5% 6% 37%	2660
	11th to 20th	Night	17% 6% 2 6% 4 5% 4 4 6% 6% 40%	3285
	21st to end	Night	16% 8% 3 7% 5% 5% 5% 5% 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% 4 11% 21% 12% 4	1570
March	1st to 10th	Day	46% 7% 11% 25% 10% 2	3301
	11th to 20th	Day	35% 6% 13% 27% 16% 3	3480
	21st to end	Day	44% 5% 9% 22% 17% 3	3930
April	1st to 10th	Day	43% 6% 12% 19% 16% 4	2280
	11th to 20th	Day	39% 8% 11% 22% 16% 4	2254
	21st to end	Day	42% 6% 10% 22% 16% 3	2280
May	1st to 10th	Day	52% 7% 10% 20% 10% 1	4078
	11th to 20th	Day	52% 9% 14% 16% 8% 1	4402
	21st to end	Day	33% 9% 15% 25% 16% 2	4837
June	1st to 10th	Day	38% 9% 13% 23% 15% 1	2660
	11th to 20th	Day	39% 9% 15% 22% 12% 3	2660
	21st to end	Day	34% 8% 16% 26% 16% 1	2660
July	1st to 10th	Day	36% 10% 15% 23% 13% 3	4370
	11th to 20th	Day	35% 8% 17% 25% 13% 1	4365
	21st to end	Day	43% 9% 11% 19% 17% 2	4369
August	1st to 10th	Day	46% 9% 12% 24% 9%	4091
	11th to 20th	Day	37% 8% 12% 28% 14% 1	3902
	21st to end	Day	35% 6% 10% 28% 20% 2	4180
September	1st to 10th	Day	40% 6% 11% 19% 20% 4	2280
	11th to 20th	Day	33% 6% 11% 25% 21% 4	2280
	21st to end	Day	46% 6% 9% 18% 19% 2	2014
October	1st to 10th	Day	39% 6% 12% 24% 17% 2	1900
	11th to 20th	Day	37% 5% 12% 25% 18% 3	1900
	21st to end	Day	39% 6% 10% 25% 18% 2	2027
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	45% 6% 11% 19% 15% 4	1884
March	1st to 10th	Night	46% 7% 10% 24% 12% 1	3551
	11th to 20th	Night	32% 8% 12% 32% 14% 2	3480
	21st to end	Night	41% 5% 8% 21% 21% 4	3700
April	1st to 10th	Night	44% 7% 10% 19% 16% 4	1900
	11th to 20th	Night	39% 7% 11% 20% 19% 4	1875
	21st to end	Night	40% 6% 11% 22% 16% 5%	1900
May	1st to 10th	Night	52% 7% 9% 17% 12% 2	2907
	11th to 20th	Night	53% 10% 13% 13% 10% 1	2496
	21st to end	Night	36% 10% 14% 24% 13% 3	2739
June	1st to 10th	Night	35% 9% 12% 25% 18% 2	1520
	11th to 20th	Night	35% 10% 15% 21% 17% 2	1520
	21st to end	Night	34% 9% 14% 26% 16% 2	1520
July	1st to 10th	Night	37% 10% 18% 18% 13% 5%	2480
	11th to 20th	Night	38% 10% 15% 22% 14% 1	2472
	21st to end	Night	40% 8% 14% 18% 18% 1	2927
August	1st to 10th	Night	44% 8% 18% 21% 8% 1	2814
	11th to 20th	Night	40% 7% 11% 26% 16% 1	2959
	21st to end	Night	35% 7% 11% 29% 16% 2	3476
September	1st to 10th	Night	37% 6% 11% 21% 21% 4	1900
	11th to 20th	Night	31% 6% 10% 24% 25% 3	1900
	21st to end	Night	44% 6% 9% 19% 19% 3	2161
October	1st to 10th	Night	42% 6% 10% 23% 17% 2	2280
	11th to 20th	Night	35% 6% 11% 28% 18% 3	2280
	21st to end	Night	36% 5% 10% 27% 19% 2	2435
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	31% 18% 31% 18% 3	382
March	1st to 10th	Day	22% 22% 30% 20% 5%	460
	11th to 20th	Day	22% 22% 33% 18% 4 1	460
	21st to end	Day	24% 21% 29% 19% 5% 1	506
April	1st to 10th	Day	22% 20% 34% 16% 8% 1	460
	11th to 20th	Day	19% 19% 29% 18% 11% 4	452
	21st to end	Day	24% 15% 30% 21% 7% 3	460
May	1st to 10th	Day	25% 16% 27% 19% 10% 3	463
	11th to 20th	Day	19% 13% 28% 23% 11% 5%	487
	21st to end	Day	17% 6% 21% 27% 20% 9%	529
June	1st to 10th	Day	14% 11% 28% 25% 16% 7%	490
	11th to 20th	Day	15% 8% 24% 28% 19% 6%	490
	21st to end	Day	11% 6% 28% 31% 19% 6%	490
July	1st to 10th	Day	11% 3 19% 30% 25% 12%	485
	11th to 20th	Day	6% 4 19% 34% 28% 10%	484
	21st to end	Day	11% 3 24% 34% 21% 7%	532
August	1st to 10th	Day	12% 3 20% 27% 28% 10%	486
	11th to 20th	Day	13% 4 18% 29% 23% 12%	479
	21st to end	Day	12% 4 18% 31% 22% 13%	539
September	1st to 10th	Day	17% 8% 17% 23% 23% 12%	490
	11th to 20th	Day	13% 4 18% 30% 22% 12%	490
	21st to end	Day	28% 10% 19% 24% 14% 6%	490
October	1st to 10th	Day	21% 7% 24% 23% 18% 7%	490
	11th to 20th	Day	18% 13% 26% 28% 13% 3	490
	21st to end	Day	17% 11% 36% 24% 9% 3	514
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% 19% 39% 18% 3	628
March	1st to 10th	Night	18% 16% 38% 23% 4	760
	11th to 20th	Night	21% 17% 35% 22% 5%	760
	21st to end	Night	21% 12% 38% 23% 6%	836
April	1st to 10th	Night	21% 16% 37% 22% 4	760
	11th to 20th	Night	17% 11% 35% 28% 8%	751
	21st to end	Night	20% 11% 33% 28% 8%	760
May	1st to 10th	Night	19% 9% 29% 32% 11%	611
	11th to 20th	Night	20% 6% 29% 34% 10%	374
	21st to end	Night	14% 4 20% 41% 20% 1	411
June	1st to 10th	Night	14% 2 26% 38% 20% 1	380
	11th to 20th	Night	11% 4 28% 39% 18% 1	380
	21st to end	Night	10% 2 25% 48% 15%	380
July	1st to 10th	Night	10% 4 18% 41% 25% 2	370
	11th to 20th	Night	5% 2 16% 46% 29% 2	368
	21st to end	Night	12% 1 17% 44% 24% 2	586
August	1st to 10th	Night	11% 1 11% 48% 27% 2	567
	11th to 20th	Night	12% 3 12% 41% 28% 3	646
	21st to end	Night	12% 3 15% 38% 29% 3	836
September	1st to 10th	Night	15% 3 21% 34% 25% 3	760
	11th to 20th	Night	12% 4 17% 36% 27% 4	760
	21st to end	Night	22% 3 22% 31% 20% 2	760
October	1st to 10th	Night	21% 6% 23% 27% 21% 3	760
	11th to 20th	Night	16% 9% 29% 32% 13%	760
	21st to end	Night	19% 8% 33% 28% 12%	812
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 2024-10-22 20:21:01. Version ac07

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