Yamada Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Yamada Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	6% 3 1 3 2 2 3 2 4 5% 69%	139937
	Night	8% 4 1 3 2 2 2 2 4 5% 66%	139691
Transparency	Day	19% 4 8% 18% 27% 24%	134984
	Night	21% 4 9% 20% 26% 20%	135619
Seeing	Day	8% 7% 24% 35% 22% 5%	17142
	Night	9% 2 15% 39% 31% 4	26777

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	13% 7% 2 6% 4 4 5% 4 5% 7% 44%	4722
	11th to 20th	Day	6% 4 2 5% 4 3 3 3 4 5% 61%	5082
	21st to end	Day	10% 5% 2 6% 4 4 5% 3 6% 8% 47%	5843
February	1st to 10th	Day	14% 8% 2 4 3 3 4 3 6% 6% 48%	2031
	11th to 20th	Day	14% 7% 2 5% 3 4 3 4 5% 7% 46%	2050
	21st to end	Day	15% 7% 2 5% 5% 3 4 3 6% 6% 44%	1730
March	1st to 10th	Day	15% 7% 2 6% 4 4 5% 4 7% 9% 36%	4618
	11th to 20th	Day	13% 4 1 4 3 4 3 3 4 8% 51%	4961
	21st to end	Day	15% 6% 1 5% 3 4 4 3 5% 8% 45%	6028
April	1st to 10th	Day	9% 5% 1 4 3 3 4 3 5% 8% 56%	2520
	11th to 20th	Day	10% 4 1 5% 4 3 3 3 5% 6% 55%	2442
	21st to end	Day	8% 4 1 3 3 3 4 3 5% 7% 60%	2400
May	1st to 10th	Day	5% 3 1 3 2 2 2 2 3 4 74%	5610
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 5% 78%	5841
	21st to end	Day	3 1 1 1 2 2 2 3 4 82%	6814
June	1st to 10th	Day	2 1 1 2 2 3 3 2 5% 7% 72%	2800
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 87%	2800
	21st to end	Day	1 1 2 2 1 1 2 2 2 87%	2800
July	1st to 10th	Day	1 1 1 1 1 2 94%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 89%	6320
	21st to end	Day	1 1 1 1 1 1 1 3 4 87%	6545
August	1st to 10th	Day	1 1 1 1 1 1 1 1 2 90%	5416
	11th to 20th	Day	1 1 1 1 1 94%	5364
	21st to end	Day	1 1 1 1 1 1 2 2 3 86%	5984
September	1st to 10th	Day	2 1 1 1 1 1 1 1 2 3 86%	2400
	11th to 20th	Day	2 1 1 1 1 2 2 1 3 4 83%	2329
	21st to end	Day	2 1 1 2 2 2 2 2 3 4 81%	2200
October	1st to 10th	Day	2 2 1 3 2 2 2 2 4 5% 76%	3455
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 4 78%	3261
	21st to end	Day	4 3 1 3 3 3 3 4 5% 7% 64%	3504
November	1st to 10th	Day	7% 4 1 4 3 3 3 3 5% 6% 60%	1854
	11th to 20th	Day	8% 3 1 3 3 3 3 3 5% 8% 59%	1890
	21st to end	Day	9% 4 1 3 3 4 4 3 5% 6% 58%	1890
December	1st to 10th	Day	7% 4 1 4 3 3 4 3 5% 6% 61%	2914
	11th to 20th	Day	16% 6% 2 5% 4 4 3 3 6% 8% 43%	3108
	21st to end	Day	15% 7% 3 7% 6% 5% 6% 4 7% 9% 30%	4092
January	1st to 10th	Night	16% 7% 2 5% 4 3 3 3 5% 7% 45%	8172
	11th to 20th	Night	8% 5% 2 4 3 2 3 2 4 6% 61%	7881
	21st to end	Night	12% 6% 2 6% 4 3 4 3 6% 7% 47%	8371
February	1st to 10th	Night	16% 6% 1 4 3 3 2 3 4 7% 51%	2587
	11th to 20th	Night	14% 5% 1 4 3 3 3 3 4 7% 53%	2458
	21st to end	Night	16% 5% 2 4 3 3 3 3 4 7% 51%	2076
March	1st to 10th	Night	17% 7% 2 6% 3 3 3 4 6% 7% 43%	5496
	11th to 20th	Night	10% 5% 2 5% 3 3 3 3 5% 6% 54%	5256
	21st to end	Night	17% 6% 2 5% 3 3 3 3 4 6% 48%	5189
April	1st to 10th	Night	10% 4 1 3 3 2 3 2 4 5% 61%	2100
	11th to 20th	Night	8% 4 1 4 3 3 3 3 3 6% 62%	2039
	21st to end	Night	5% 4 1 3 3 3 3 2 4 6% 66%	2000
May	1st to 10th	Night	4 2 1 2 2 2 2 2 3 5% 75%	4381
	11th to 20th	Night	3 1 1 1 1 2 2 3 5% 80%	4074
	21st to end	Night	2 1 1 1 1 1 1 2 3 88%	4046
June	1st to 10th	Night	4 1 1 2 2 2 2 2 3 6% 76%	1600
	11th to 20th	Night	1 1 1 1 1 1 1 2 4 87%	1600
	21st to end	Night	1 1 1 1 1 1 1 2 2 90%	1600
July	1st to 10th	Night	1 1 2 95%	3640
	11th to 20th	Night	1 1 1 1 2 1 1 2 2 87%	3640
	21st to end	Night	1 1 1 1 1 1 1 2 3 88%	4304
August	1st to 10th	Night	1 1 1 1 1 1 1 1 2 2 88%	4411
	11th to 20th	Night	1 1 1 1 1 94%	4506
	21st to end	Night	1 1 1 1 2 2 2 2 4 84%	5015
September	1st to 10th	Night	1 1 1 1 1 2 1 2 2 89%	2000
	11th to 20th	Night	1 2 1 2 1 1 2 2 2 3 83%	2070
	21st to end	Night	2 1 1 1 1 1 1 2 4 4 83%	2200
October	1st to 10th	Night	2 1 1 1 1 2 1 3 4 83%	3741
	11th to 20th	Night	3 1 1 1 2 2 2 3 4 81%	3897
	21st to end	Night	3 2 1 2 2 2 2 2 3 5% 77%	4305
November	1st to 10th	Night	7% 4 1 2 2 3 2 2 4 5% 68%	2665
	11th to 20th	Night	7% 3 1 3 2 2 3 3 5% 6% 64%	2730
	21st to end	Night	9% 4 2 4 3 3 3 3 5% 6% 58%	2730
December	1st to 10th	Night	7% 3 1 4 3 3 4 3 4 5% 62%	4489
	11th to 20th	Night	16% 5% 2 5% 3 3 4 4 6% 9% 44%	5316
	21st to end	Night	11% 8% 3 8% 6% 5% 6% 5% 6% 9% 33%	7106

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	37% 7% 21% 18% 15% 2	4723
	11th to 20th	Day	24% 7% 8% 12% 26% 22%	5082
	21st to end	Day	34% 6% 10% 19% 27% 3	5843
February	1st to 10th	Day	34% 7% 9% 24% 19% 7%	2031
	11th to 20th	Day	36% 6% 11% 23% 18% 7%	2060
	21st to end	Day	39% 7% 12% 25% 15% 2	1730
March	1st to 10th	Day	38% 9% 12% 17% 21% 2	4618
	11th to 20th	Day	30% 6% 18% 28% 12% 6%	4961
	21st to end	Day	35% 8% 15% 25% 14% 3	6028
April	1st to 10th	Day	26% 6% 13% 29% 22% 5%	2520
	11th to 20th	Day	27% 6% 12% 26% 21% 8%	2445
	21st to end	Day	22% 7% 10% 30% 27% 4	2400
May	1st to 10th	Day	14% 4 7% 26% 37% 12%	5610
	11th to 20th	Day	10% 3 5% 31% 42% 9%	5844
	21st to end	Day	9% 3 4 24% 53% 8%	6815
June	1st to 10th	Day	13% 4 9% 21% 36% 17%	2800
	11th to 20th	Day	4 2 5% 15% 41% 33%	2800
	21st to end	Day	7% 1 2 8% 32% 50%	2800
July	1st to 10th	Day	2 1 1 3 17% 76%	6251
	11th to 20th	Day	5% 1 2 4 19% 70%	6245
	21st to end	Day	4 1 2 11% 36% 45%	6356
August	1st to 10th	Day	7% 1 2 5% 32% 53%	3934
	11th to 20th	Day	3 1 1 6% 28% 61%	3880
	21st to end	Day	5% 1 3 7% 30% 55%	4311
September	1st to 10th	Day	6% 1 3 7% 33% 49%	2400
	11th to 20th	Day	8% 3 3 18% 39% 28%	2329
	21st to end	Day	9% 2 4 17% 37% 31%	2200
October	1st to 10th	Day	12% 3 4 11% 23% 48%	3455
	11th to 20th	Day	12% 3 4 23% 37% 21%	3261
	21st to end	Day	20% 3 6% 15% 36% 20%	3504
November	1st to 10th	Day	22% 6% 10% 22% 25% 14%	1854
	11th to 20th	Day	24% 5% 14% 25% 24% 9%	1890
	21st to end	Day	27% 5% 11% 21% 25% 12%	1890
December	1st to 10th	Day	24% 4 11% 28% 24% 8%	2914
	11th to 20th	Day	39% 5% 14% 20% 17% 5%	3108
	21st to end	Day	48% 8% 17% 18% 7% 2	4092
January	1st to 10th	Night	36% 7% 20% 20% 15% 1	8172
	11th to 20th	Night	24% 5% 9% 17% 32% 12%	7881
	21st to end	Night	35% 6% 9% 23% 25% 3	8371
February	1st to 10th	Night	34% 4 10% 25% 20% 7%	2587
	11th to 20th	Night	30% 6% 12% 25% 21% 6%	2459
	21st to end	Night	33% 6% 14% 28% 16% 4	2076
March	1st to 10th	Night	36% 7% 13% 20% 21% 3	5496
	11th to 20th	Night	30% 5% 17% 31% 12% 6%	5256
	21st to end	Night	36% 6% 20% 20% 15% 4	5189
April	1st to 10th	Night	26% 5% 12% 30% 20% 7%	2100
	11th to 20th	Night	24% 4 11% 31% 22% 8%	2039
	21st to end	Night	19% 5% 14% 30% 30% 3	2000
May	1st to 10th	Night	13% 3 9% 31% 34% 10%	4378
	11th to 20th	Night	9% 2 5% 29% 46% 9%	4075
	21st to end	Night	6% 2 4 23% 56% 9%	4046
June	1st to 10th	Night	12% 3 8% 25% 34% 18%	1600
	11th to 20th	Night	5% 2 4 15% 38% 36%	1600
	21st to end	Night	4 1 2 7% 33% 52%	1600
July	1st to 10th	Night	2 1 2 17% 78%	3600
	11th to 20th	Night	7% 1 1 4 15% 72%	3592
	21st to end	Night	5% 2 2 8% 37% 48%	4181
August	1st to 10th	Night	8% 1 1 7% 29% 53%	3208
	11th to 20th	Night	3 1 1 5% 22% 68%	3256
	21st to end	Night	6% 1 1 7% 29% 57%	3608
September	1st to 10th	Night	5% 1 2 9% 27% 56%	2000
	11th to 20th	Night	8% 3 3 13% 44% 29%	2070
	21st to end	Night	8% 1 4 16% 38% 33%	2200
October	1st to 10th	Night	9% 2 4 10% 25% 51%	3741
	11th to 20th	Night	10% 2 5% 21% 43% 20%	3897
	21st to end	Night	12% 2 4 21% 39% 21%	4305
November	1st to 10th	Night	19% 4 11% 24% 25% 16%	2665
	11th to 20th	Night	21% 4 13% 24% 25% 13%	2730
	21st to end	Night	27% 4 10% 21% 28% 11%	2730
December	1st to 10th	Night	24% 4 12% 28% 22% 9%	4489
	11th to 20th	Night	36% 5% 17% 21% 16% 5%	5316
	21st to end	Night	43% 9% 16% 22% 7% 2	7106

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	26% 6% 25% 25% 14% 3	220
	11th to 20th	Day	13% 8% 25% 33% 16% 5%	341
	21st to end	Day	13% 8% 25% 33% 18% 3	473
February	1st to 10th	Day	18% 8% 27% 29% 14% 5%	504
	11th to 20th	Day	18% 10% 33% 26% 12% 1	535
	21st to end	Day	15% 8% 35% 30% 11%	446
March	1st to 10th	Day	17% 11% 32% 30% 9% 1	540
	11th to 20th	Day	14% 10% 32% 27% 15% 2	540
	21st to end	Day	15% 13% 30% 29% 10% 2	594
April	1st to 10th	Day	12% 13% 29% 28% 15% 3	540
	11th to 20th	Day	13% 13% 27% 33% 13% 2	517
	21st to end	Day	6% 12% 27% 34% 15% 6%	500
May	1st to 10th	Day	5% 15% 36% 26% 15% 4	503
	11th to 20th	Day	6% 9% 29% 34% 18% 5%	528
	21st to end	Day	7% 8% 24% 36% 20% 5%	573
June	1st to 10th	Day	2 6% 24% 35% 27% 7%	530
	11th to 20th	Day	2 7% 25% 37% 22% 6%	530
	21st to end	Day	1 5% 19% 40% 27% 8%	530
July	1st to 10th	Day	1 16% 42% 32% 9%	526
	11th to 20th	Day	1 7% 41% 37% 14%	524
	21st to end	Day	2 1 11% 39% 37% 11%	575
August	1st to 10th	Day	3 2 18% 44% 21% 11%	527
	11th to 20th	Day	1 2 14% 42% 29% 11%	517
	21st to end	Day	2 1 12% 41% 33% 11%	583
September	1st to 10th	Day	2 2 15% 43% 29% 8%	530
	11th to 20th	Day	3 6% 21% 41% 25% 5%	530
	21st to end	Day	2 6% 21% 37% 30% 4	530
October	1st to 10th	Day	2 4 24% 37% 31% 2	530
	11th to 20th	Day	5% 5% 22% 40% 25% 4	530
	21st to end	Day	5% 6% 20% 38% 27% 4	553
November	1st to 10th	Day	7% 6% 24% 31% 27% 4	330
	11th to 20th	Day	9% 5% 25% 34% 22% 4	330
	21st to end	Day	13% 3 25% 33% 22% 4	330
December	1st to 10th	Day	14% 3 25% 34% 20% 4	291
	11th to 20th	Day	15% 5% 25% 29% 19% 7%	220
	21st to end	Day	26% 4 26% 26% 14% 4	242
January	1st to 10th	Night	20% 2 19% 32% 22% 5%	840
	11th to 20th	Night	12% 3 16% 43% 23% 4	840
	21st to end	Night	18% 2 17% 37% 23% 3	924
February	1st to 10th	Night	18% 3 18% 37% 21% 3	840
	11th to 20th	Night	16% 4 20% 36% 21% 3	824
	21st to end	Night	17% 3 21% 42% 15% 1	692
March	1st to 10th	Night	16% 5% 21% 39% 16% 3	840
	11th to 20th	Night	14% 3 19% 34% 26% 3	840
	21st to end	Night	16% 4 17% 41% 22% 1	924
April	1st to 10th	Night	11% 3 20% 43% 21% 1	840
	11th to 20th	Night	10% 5% 20% 39% 26% 1	815
	21st to end	Night	7% 4 18% 41% 29% 2	800
May	1st to 10th	Night	5% 2 20% 44% 28% 1	725
	11th to 20th	Night	6% 1 17% 42% 32% 1	591
	21st to end	Night	5% 3 18% 41% 30% 2	469
June	1st to 10th	Night	5% 1 12% 45% 36% 3	400
	11th to 20th	Night	1 16% 48% 34% 3	400
	21st to end	Night	1 1 10% 43% 42% 3	400
July	1st to 10th	Night	5% 40% 51% 3	390
	11th to 20th	Night	2 2 38% 52% 6%	388
	21st to end	Night	1 4 29% 57% 8%	580
August	1st to 10th	Night	2 6% 36% 49% 6%	759
	11th to 20th	Night	1 4 36% 54% 6%	792
	21st to end	Night	2 5% 29% 58% 7%	880
September	1st to 10th	Night	1 1 5% 35% 53% 5%	800
	11th to 20th	Night	2 2 11% 41% 39% 6%	800
	21st to end	Night	2 1 10% 42% 41% 4	800
October	1st to 10th	Night	4 1 11% 49% 30% 4	800
	11th to 20th	Night	5% 2 14% 41% 34% 5%	800
	21st to end	Night	5% 1 15% 36% 33% 10%	880
November	1st to 10th	Night	9% 1 13% 36% 33% 8%	820
	11th to 20th	Night	8% 2 18% 38% 28% 6%	840
	21st to end	Night	11% 2 17% 39% 25% 6%	840
December	1st to 10th	Night	14% 2 16% 40% 22% 5%	840
	11th to 20th	Night	17% 1 18% 39% 20% 4	840
	21st to end	Night	19% 3 20% 38% 16% 4	924

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