Yamabushi Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Yamabushi Observatory clear sky chart from 2005-11-06 to 2024-11-30.

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% 7% 42%	101246
	Night	14% 6% 2 6% 4 4 4 4 6% 7% 43%	98307
Transparency	Day	38% 7% 11% 20% 18% 6%	105040
	Night	37% 6% 10% 19% 20% 7%	102652
Seeing	Day	18% 9% 22% 24% 19% 8%	15081
	Night	16% 9% 22% 29% 20% 3	25128

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	17% 6% 2 6% 4 4 4 4 6% 6% 41%	2836
	11th to 20th	Day	14% 5% 2 4 3 3 4 3 4 5% 52%	2840
	21st to end	Day	28% 6% 2 5% 2 2 3 2 3 4 44%	3124
February	1st to 10th	Day	16% 7% 2 5% 5% 4 5% 4 4 5% 41%	1897
	11th to 20th	Day	22% 7% 2 5% 3 3 2 3 4 4 43%	1848
	21st to end	Day	21% 7% 2 7% 4 3 3 2 5% 6% 40%	1570
March	1st to 10th	Day	24% 8% 3 6% 3 3 3 3 5% 5% 38%	3304
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 4 6% 52%	3480
	21st to end	Day	19% 6% 2 6% 4 4 4 3 5% 6% 41%	3815
April	1st to 10th	Day	12% 5% 2 5% 4 5% 4 4 6% 7% 47%	2090
	11th to 20th	Day	12% 6% 2 6% 5% 4 4 3 5% 7% 47%	2244
	21st to end	Day	11% 4 2 4 4 3 5% 4 6% 7% 51%	2470
May	1st to 10th	Day	11% 7% 3 7% 5% 4 5% 3 6% 8% 41%	4082
	11th to 20th	Day	18% 8% 3 6% 4 4 4 4 6% 7% 37%	4081
	21st to end	Day	8% 4 2 6% 4 5% 6% 4 7% 10% 43%	4476
June	1st to 10th	Day	9% 6% 3 7% 6% 6% 6% 5% 7% 9% 34%	2470
	11th to 20th	Day	6% 5% 1 6% 6% 5% 5% 5% 9% 11% 40%	2470
	21st to end	Day	6% 4 2 7% 6% 7% 7% 6% 8% 11% 37%	2470
July	1st to 10th	Day	8% 6% 3 8% 6% 7% 7% 6% 9% 11% 29%	4120
	11th to 20th	Day	8% 5% 2 7% 6% 7% 7% 5% 8% 11% 34%	4120
	21st to end	Day	11% 8% 3 10% 7% 8% 7% 6% 9% 10% 21%	4466
August	1st to 10th	Day	11% 6% 2 7% 5% 6% 6% 6% 9% 10% 32%	4104
	11th to 20th	Day	9% 5% 2 7% 5% 4 6% 5% 8% 10% 38%	3961
	21st to end	Day	8% 5% 2 6% 4 4 4 4 6% 8% 49%	4098
September	1st to 10th	Day	10% 6% 2 7% 5% 6% 6% 4 6% 8% 40%	2090
	11th to 20th	Day	16% 6% 3 6% 4 4 5% 3 6% 6% 41%	2090
	21st to end	Day	16% 6% 2 5% 3 3 4 4 5% 6% 46%	2090
October	1st to 10th	Day	13% 6% 2 5% 3 3 3 2 4 6% 54%	2090
	11th to 20th	Day	12% 5% 2 5% 3 3 3 2 3 5% 58%	1928
	21st to end	Day	11% 4 3 5% 3 2 3 2 4 5% 57%	2090
November	1st to 10th	Day	14% 5% 2 4 3 3 3 3 4 4 55%	1848
	11th to 20th	Day	14% 5% 2 3 3 3 3 3 5% 6% 52%	1800
	21st to end	Day	14% 6% 2 5% 4 3 3 3 5% 5% 51%	1797
December	1st to 10th	Day	21% 8% 2 6% 4 3 4 4 5% 5% 38%	1710
	11th to 20th	Day	18% 5% 2 5% 4 3 3 3 4 5% 48%	2153
	21st to end	Day	24% 6% 2 4 3 3 3 3 5% 7% 40%	3124
January	1st to 10th	Night	19% 6% 2 5% 3 3 4 3 4 6% 45%	4109
	11th to 20th	Night	13% 7% 2 5% 5% 4 5% 3 5% 5% 47%	4120
	21st to end	Night	25% 8% 2 5% 3 2 3 2 3 4 43%	4532
February	1st to 10th	Night	16% 6% 2 5% 4 3 4 3 4 6% 46%	2280
	11th to 20th	Night	20% 7% 2 6% 3 3 3 3 4 4 44%	2218
	21st to end	Night	20% 6% 2 7% 3 3 4 3 4 6% 42%	1884
March	1st to 10th	Night	24% 7% 2 6% 4 3 3 3 4 5% 39%	3546
	11th to 20th	Night	11% 4 1 4 3 3 4 4 6% 8% 50%	3480
	21st to end	Night	19% 6% 2 6% 4 3 3 3 5% 6% 42%	3817
April	1st to 10th	Night	12% 5% 2 4 4 4 4 4 6% 6% 50%	2090
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 6% 7% 49%	1897
	21st to end	Night	10% 5% 1 4 5% 3 5% 3 6% 6% 51%	1710
May	1st to 10th	Night	11% 6% 2 7% 5% 5% 5% 4 7% 9% 40%	2813
	11th to 20th	Night	16% 6% 2 6% 4 4 4 3 6% 7% 40%	2812
	21st to end	Night	7% 4 2 5% 5% 5% 5% 4 7% 10% 46%	3079
June	1st to 10th	Night	8% 5% 3 7% 5% 5% 6% 4 8% 12% 36%	1710
	11th to 20th	Night	6% 5% 3 5% 5% 4 7% 5% 8% 11% 41%	1710
	21st to end	Night	7% 4 2 6% 5% 5% 6% 5% 9% 11% 40%	1710
July	1st to 10th	Night	10% 9% 3 9% 6% 5% 6% 4 8% 9% 32%	2840
	11th to 20th	Night	7% 5% 3 7% 6% 7% 7% 6% 9% 14% 31%	2840
	21st to end	Night	12% 9% 3 10% 7% 7% 6% 5% 8% 11% 22%	3087
August	1st to 10th	Night	12% 7% 2 6% 4 6% 6% 5% 9% 13% 31%	2825
	11th to 20th	Night	11% 6% 3 8% 5% 4 6% 5% 7% 10% 37%	2864
	21st to end	Night	8% 5% 2 7% 4 5% 5% 4 7% 8% 45%	3526
September	1st to 10th	Night	10% 6% 2 7% 5% 5% 5% 4 7% 8% 41%	2090
	11th to 20th	Night	17% 6% 2 5% 3 5% 5% 4 7% 8% 39%	2090
	21st to end	Night	13% 4 2 6% 3 3 5% 3 6% 7% 47%	2090
October	1st to 10th	Night	13% 4 2 4 3 4 3 4 5% 6% 53%	2090
	11th to 20th	Night	12% 4 2 5% 3 3 3 3 4 4 57%	2251
	21st to end	Night	12% 3 2 3 3 2 4 3 4 5% 59%	2508
November	1st to 10th	Night	14% 6% 2 4 3 3 3 3 4 4 55%	2446
	11th to 20th	Night	15% 5% 1 4 3 3 3 2 5% 6% 53%	2600
	21st to end	Night	15% 4 1 5% 4 3 4 3 4 6% 49%	2599
December	1st to 10th	Night	18% 6% 3 6% 4 4 3 3 4 5% 43%	2470
	11th to 20th	Night	18% 5% 2 4 3 3 3 2 3 5% 52%	3042
	21st to end	Night	20% 8% 2 7% 4 5% 5% 3 5% 7% 36%	4532

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% 4 8% 18% 20% 8%	2837
	11th to 20th	Day	34% 4 7% 20% 27% 9%	2840
	21st to end	Day	46% 2 4 18% 22% 9%	3124
February	1st to 10th	Day	44% 4 8% 18% 21% 6%	1897
	11th to 20th	Day	43% 3 6% 18% 22% 8%	1858
	21st to end	Day	45% 5% 6% 15% 20% 9%	1570
March	1st to 10th	Day	49% 3 8% 15% 13% 11%	3304
	11th to 20th	Day	35% 5% 6% 17% 25% 12%	3480
	21st to end	Day	44% 5% 7% 20% 20% 4	3815
April	1st to 10th	Day	36% 5% 9% 20% 23% 6%	2090
	11th to 20th	Day	37% 4 9% 17% 23% 10%	2244
	21st to end	Day	29% 6% 10% 28% 22% 4	2470
May	1st to 10th	Day	38% 8% 13% 22% 16% 3	4120
	11th to 20th	Day	44% 7% 11% 22% 14% 1	4085
	21st to end	Day	32% 10% 14% 23% 21% 1	4489
June	1st to 10th	Day	41% 10% 17% 20% 10% 2	2470
	11th to 20th	Day	32% 9% 19% 25% 14% 1	2470
	21st to end	Day	36% 11% 17% 19% 13% 4	2470
July	1st to 10th	Day	42% 14% 19% 19% 5% 1	4055
	11th to 20th	Day	39% 12% 24% 19% 4 2	4051
	21st to end	Day	53% 13% 15% 13% 5%	4324
August	1st to 10th	Day	41% 9% 16% 27% 6%	4091
	11th to 20th	Day	36% 10% 19% 20% 14% 2	4000
	21st to end	Day	32% 8% 14% 23% 19% 4	4098
September	1st to 10th	Day	40% 9% 13% 16% 18% 4	2090
	11th to 20th	Day	42% 8% 9% 19% 20% 2	2090
	21st to end	Day	37% 5% 11% 21% 19% 6%	2090
October	1st to 10th	Day	26% 3 7% 16% 36% 12%	3480
	11th to 20th	Day	21% 2 4 15% 29% 29%	3181
	21st to end	Day	27% 3 5% 21% 30% 14%	3428
November	1st to 10th	Day	33% 4 8% 15% 28% 12%	1848
	11th to 20th	Day	32% 4 7% 21% 27% 10%	1800
	21st to end	Day	35% 4 8% 19% 28% 7%	1797
December	1st to 10th	Day	47% 4 7% 18% 16% 7%	1710
	11th to 20th	Day	40% 3 5% 18% 22% 11%	2150
	21st to end	Day	43% 5% 8% 20% 22% 3	3124
January	1st to 10th	Night	40% 4 6% 16% 26% 8%	4109
	11th to 20th	Night	37% 4 6% 20% 23% 9%	4120
	21st to end	Night	46% 3 5% 14% 25% 7%	4532
February	1st to 10th	Night	39% 4 8% 19% 22% 8%	2280
	11th to 20th	Night	44% 3 5% 17% 22% 8%	2219
	21st to end	Night	44% 4 8% 17% 19% 9%	1884
March	1st to 10th	Night	47% 5% 9% 15% 16% 8%	3546
	11th to 20th	Night	30% 5% 9% 14% 28% 13%	3480
	21st to end	Night	42% 5% 7% 19% 19% 9%	3817
April	1st to 10th	Night	33% 5% 10% 20% 24% 8%	2090
	11th to 20th	Night	33% 4 6% 22% 24% 11%	1897
	21st to end	Night	32% 5% 9% 27% 21% 6%	1710
May	1st to 10th	Night	36% 10% 14% 21% 17% 2	2840
	11th to 20th	Night	41% 7% 14% 22% 15% 1	2813
	21st to end	Night	27% 9% 15% 26% 20% 3	3087
June	1st to 10th	Night	36% 10% 19% 20% 12% 2	1710
	11th to 20th	Night	32% 9% 18% 27% 14% 1	1710
	21st to end	Night	33% 11% 20% 20% 13% 4	1710
July	1st to 10th	Night	45% 11% 20% 17% 5% 1	2795
	11th to 20th	Night	38% 17% 21% 18% 5% 1	2786
	21st to end	Night	53% 13% 16% 13% 4 1	2988
August	1st to 10th	Night	40% 9% 23% 22% 5%	2814
	11th to 20th	Night	39% 11% 19% 18% 12% 1	2891
	21st to end	Night	33% 9% 13% 22% 20% 2	3526
September	1st to 10th	Night	38% 10% 16% 16% 16% 5%	2090
	11th to 20th	Night	41% 8% 11% 18% 19% 3	2090
	21st to end	Night	35% 6% 12% 20% 19% 7%	2090
October	1st to 10th	Night	26% 5% 8% 16% 36% 9%	3480
	11th to 20th	Night	19% 2 5% 17% 29% 27%	3737
	21st to end	Night	23% 3 5% 22% 29% 18%	4120
November	1st to 10th	Night	33% 4 7% 18% 25% 13%	2446
	11th to 20th	Night	33% 3 6% 18% 28% 12%	2600
	21st to end	Night	36% 4 6% 21% 25% 8%	2599
December	1st to 10th	Night	44% 4 7% 21% 18% 6%	2470
	11th to 20th	Night	36% 3 7% 17% 29% 8%	3044
	21st to end	Night	47% 5% 8% 21% 17% 3	4532

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	28% 16% 40% 13% 2	270
	11th to 20th	Day	28% 19% 30% 19% 3 1	270
	21st to end	Day	28% 22% 34% 11% 3	297
February	1st to 10th	Day	26% 17% 36% 18% 3	270
	11th to 20th	Day	39% 14% 34% 12% 1	270
	21st to end	Day	36% 18% 36% 9% 1	225
March	1st to 10th	Day	24% 14% 29% 24% 8%	376
	11th to 20th	Day	24% 15% 38% 16% 6% 1	460
	21st to end	Day	23% 17% 32% 21% 7%	506
April	1st to 10th	Day	23% 13% 29% 23% 11% 1	460
	11th to 20th	Day	16% 14% 35% 21% 13% 1	452
	21st to end	Day	18% 8% 28% 25% 15% 6%	460
May	1st to 10th	Day	13% 12% 32% 22% 14% 7%	463
	11th to 20th	Day	17% 7% 19% 27% 20% 10%	487
	21st to end	Day	14% 7% 18% 29% 20% 12%	529
June	1st to 10th	Day	12% 3 15% 34% 26% 10%	490
	11th to 20th	Day	8% 4 18% 27% 25% 18%	490
	21st to end	Day	9% 4 19% 29% 24% 15%	490
July	1st to 10th	Day	11% 3 8% 24% 28% 25%	485
	11th to 20th	Day	10% 3 8% 24% 37% 19%	484
	21st to end	Day	6% 2 9% 30% 36% 17%	532
August	1st to 10th	Day	13% 4 13% 23% 30% 17%	486
	11th to 20th	Day	13% 2 9% 30% 33% 14%	479
	21st to end	Day	15% 4 9% 24% 31% 18%	539
September	1st to 10th	Day	10% 4 13% 29% 30% 13%	490
	11th to 20th	Day	19% 4 11% 21% 30% 14%	490
	21st to end	Day	19% 4 15% 25% 27% 9%	490
October	1st to 10th	Day	19% 4 20% 31% 18% 9%	490
	11th to 20th	Day	13% 9% 24% 27% 22% 4	490
	21st to end	Day	16% 9% 25% 26% 21% 3	539
November	1st to 10th	Day	22% 9% 24% 28% 15% 2	386
	11th to 20th	Day	19% 14% 36% 24% 5% 1	300
	21st to end	Day	17% 14% 33% 22% 11% 2	299
December	1st to 10th	Day	33% 13% 30% 18% 5%	270
	11th to 20th	Day	26% 13% 33% 22% 4 1	270
	21st to end	Day	34% 16% 26% 15% 8% 2	297
January	1st to 10th	Night	25% 17% 28% 22% 7%	760
	11th to 20th	Night	22% 15% 31% 25% 7%	760
	21st to end	Night	21% 18% 34% 24% 3	836
February	1st to 10th	Night	19% 15% 30% 31% 5%	760
	11th to 20th	Night	24% 17% 33% 22% 4	743
	21st to end	Night	23% 16% 34% 24% 3	628
March	1st to 10th	Night	19% 14% 31% 31% 5%	760
	11th to 20th	Night	18% 12% 36% 24% 9%	760
	21st to end	Night	19% 14% 31% 28% 9%	836
April	1st to 10th	Night	15% 14% 31% 28% 10% 1	760
	11th to 20th	Night	14% 13% 27% 29% 16% 1	612
	21st to end	Night	12% 7% 28% 34% 19% 1	570
May	1st to 10th	Night	10% 9% 26% 34% 21% 1	570
	11th to 20th	Night	14% 5% 20% 34% 22% 5%	561
	21st to end	Night	12% 2 13% 33% 34% 5%	617
June	1st to 10th	Night	10% 2 13% 38% 30% 7%	570
	11th to 20th	Night	7% 2 15% 31% 34% 11%	570
	21st to end	Night	9% 2 10% 37% 34% 9%	570
July	1st to 10th	Night	12% 3 7% 25% 42% 12%	555
	11th to 20th	Night	9% 1 5% 27% 43% 14%	552
	21st to end	Night	7% 3 9% 31% 41% 9%	624
August	1st to 10th	Night	13% 2 10% 27% 37% 10%	567
	11th to 20th	Night	15% 2 7% 27% 39% 9%	564
	21st to end	Night	14% 2 8% 28% 36% 12%	651
September	1st to 10th	Night	11% 2 10% 30% 38% 9%	760
	11th to 20th	Night	20% 3 11% 27% 34% 5%	760
	21st to end	Night	16% 3 14% 33% 31% 3	760
October	1st to 10th	Night	14% 5% 17% 33% 26% 6%	760
	11th to 20th	Night	14% 6% 21% 33% 22% 3	760
	21st to end	Night	13% 8% 22% 34% 21% 1	836
November	1st to 10th	Night	16% 7% 23% 33% 20% 1	780
	11th to 20th	Night	17% 10% 28% 30% 14% 1	800
	21st to end	Night	17% 11% 27% 30% 14% 1	800
December	1st to 10th	Night	21% 15% 24% 32% 8%	760
	11th to 20th	Night	21% 14% 27% 28% 10% 1	760
	21st to end	Night	26% 14% 25% 29% 6%	836

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