Calaveras Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Calaveras Hill Observatory clear sky chart from 2005-11-06 to 2024-03-08.

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	12% 6% 2 5% 5% 3 5% 5% 5% 7% 45%	79063
	Night	13% 5% 2 4 4 2 3 4 5% 7% 51%	80564
Transparency	Day	37% 6% 10% 20% 18% 9%	78788
	Night	33% 5% 12% 20% 18% 11%	80359
Seeing	Day	13% 5% 18% 29% 26% 10%	13763
	Night	15% 5% 24% 38% 17% 1	23886

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	16% 6% 2 5% 5% 2 3 4 4 5% 49%	2847
	11th to 20th	Day	18% 5% 2 3 3 1 2 2 4 6% 55%	2850
	21st to end	Day	21% 7% 2 4 3 2 3 4 6% 7% 41%	3432
February	1st to 10th	Day	20% 8% 2 4 4 2 3 4 4 5% 44%	1900
	11th to 20th	Day	19% 8% 3 5% 4 2 3 3 4 6% 44%	1849
	21st to end	Day	17% 6% 2 5% 4 3 4 3 4 6% 47%	1570
March	1st to 10th	Day	18% 8% 3 5% 4 2 4 4 5% 7% 40%	2030
	11th to 20th	Day	15% 7% 3 5% 5% 3 5% 4 4 5% 44%	1980
	21st to end	Day	16% 7% 3 5% 4 3 3 3 4 7% 46%	2178
April	1st to 10th	Day	13% 7% 3 5% 5% 3 5% 4 5% 7% 42%	1980
	11th to 20th	Day	17% 6% 2 5% 5% 3 4 4 5% 6% 43%	1956
	21st to end	Day	13% 7% 3 6% 5% 3 4 3 4 5% 47%	2250
May	1st to 10th	Day	12% 6% 3 6% 6% 3 5% 5% 6% 7% 42%	2340
	11th to 20th	Day	12% 6% 3 7% 6% 4 5% 5% 6% 8% 40%	2302
	21st to end	Day	11% 6% 2 6% 6% 3 7% 7% 7% 9% 37%	2531
June	1st to 10th	Day	4 3 2 4 6% 4 7% 7% 9% 12% 44%	2340
	11th to 20th	Day	4 3 3 5% 7% 3 6% 6% 7% 10% 46%	2340
	21st to end	Day	7% 5% 4 6% 7% 4 7% 6% 8% 10% 35%	2340
July	1st to 10th	Day	10% 6% 4 6% 7% 4 7% 6% 7% 8% 37%	2340
	11th to 20th	Day	4 3 2 5% 6% 4 7% 6% 6% 10% 48%	2340
	21st to end	Day	4 3 2 4 5% 4 6% 6% 7% 12% 46%	2546
August	1st to 10th	Day	2 2 1 3 5% 3 5% 5% 7% 11% 56%	2271
	11th to 20th	Day	4 2 2 5% 6% 3 5% 6% 7% 11% 48%	2123
	21st to end	Day	6% 3 2 5% 6% 5% 6% 7% 7% 11% 41%	2273
September	1st to 10th	Day	8% 6% 3 7% 7% 4 6% 6% 6% 7% 40%	1980
	11th to 20th	Day	9% 6% 3 7% 7% 4 5% 5% 6% 7% 42%	1980
	21st to end	Day	8% 3 2 5% 7% 4 5% 5% 6% 7% 48%	1980
October	1st to 10th	Day	7% 4 2 3 5% 4 5% 5% 5% 7% 53%	1980
	11th to 20th	Day	12% 5% 2 4 5% 3 4 4 5% 7% 48%	1880
	21st to end	Day	10% 5% 2 5% 4 2 2 3 4 5% 59%	1980
November	1st to 10th	Day	13% 9% 4 5% 6% 2 5% 3 4 7% 43%	1858
	11th to 20th	Day	13% 6% 3 4 4 2 4 4 5% 6% 49%	1801
	21st to end	Day	17% 8% 3 5% 4 2 4 4 4 5% 44%	1710
December	1st to 10th	Day	18% 7% 2 6% 4 2 4 4 4 6% 43%	1710
	11th to 20th	Day	17% 8% 2 5% 5% 3 4 4 4 6% 42%	2161
	21st to end	Day	13% 7% 3 5% 4 2 3 3 4 5% 51%	3135
January	1st to 10th	Night	19% 6% 3 4 3 2 3 3 4 6% 49%	4108
	11th to 20th	Night	15% 3 2 2 3 2 2 2 2 4 64%	4120
	21st to end	Night	24% 6% 2 6% 4 2 3 3 4 6% 40%	4263
February	1st to 10th	Night	18% 7% 3 3 4 2 4 4 4 5% 46%	2280
	11th to 20th	Night	21% 6% 2 3 3 2 2 3 4 5% 48%	2218
	21st to end	Night	18% 6% 2 5% 3 3 3 4 4 5% 47%	1884
March	1st to 10th	Night	17% 7% 3 5% 4 2 4 4 5% 6% 44%	2087
	11th to 20th	Night	15% 8% 3 4 4 3 4 3 4 6% 44%	1980
	21st to end	Night	15% 5% 2 4 3 2 3 3 5% 7% 51%	2178
April	1st to 10th	Night	12% 6% 2 5% 4 2 4 4 6% 9% 46%	1980
	11th to 20th	Night	15% 6% 2 6% 5% 2 4 4 4 6% 45%	1953
	21st to end	Night	16% 7% 3 4 4 3 4 4 4 5% 46%	1702
May	1st to 10th	Night	10% 6% 2 5% 6% 4 5% 4 4 8% 46%	1620
	11th to 20th	Night	11% 6% 3 5% 3 3 5% 4 5% 9% 46%	1593
	21st to end	Night	9% 4 3 6% 6% 3 4 5% 6% 10% 44%	1745
June	1st to 10th	Night	5% 3 2 3 3 3 3 4 6% 9% 60%	1620
	11th to 20th	Night	4 3 1 4 3 2 4 4 5% 11% 59%	1620
	21st to end	Night	5% 4 3 5% 5% 3 5% 5% 6% 10% 50%	1620
July	1st to 10th	Night	7% 5% 2 6% 4 3 4 5% 6% 10% 47%	1620
	11th to 20th	Night	2 2 1 3 3 2 5% 5% 5% 10% 62%	1620
	21st to end	Night	4 2 1 3 3 2 4 5% 6% 11% 60%	1772
August	1st to 10th	Night	2 1 1 2 2 2 3 4 6% 8% 70%	1663
	11th to 20th	Night	4 2 1 3 4 3 5% 6% 7% 11% 54%	1778
	21st to end	Night	6% 3 2 3 5% 3 4 6% 7% 11% 49%	2081
September	1st to 10th	Night	7% 5% 3 5% 6% 3 4 5% 6% 10% 47%	1980
	11th to 20th	Night	7% 6% 3 6% 6% 3 4 6% 5% 8% 47%	1980
	21st to end	Night	6% 3 2 5% 5% 4 4 5% 6% 10% 51%	1980
October	1st to 10th	Night	5% 3 2 3 3 3 3 4 5% 8% 61%	1980
	11th to 20th	Night	12% 5% 2 4 3 2 2 3 4 7% 56%	2060
	21st to end	Night	12% 4 2 4 3 2 2 2 3 5% 61%	2376
November	1st to 10th	Night	11% 5% 2 4 5% 2 4 3 4 6% 53%	2220
	11th to 20th	Night	11% 5% 2 4 4 2 3 4 5% 6% 54%	2375
	21st to end	Night	17% 6% 2 4 4 2 3 3 4 6% 47%	2470
December	1st to 10th	Night	17% 6% 2 4 3 2 3 2 4 7% 50%	2470
	11th to 20th	Night	19% 7% 3 5% 4 2 3 3 4 5% 45%	3036
	21st to end	Night	16% 6% 2 3 3 2 2 3 4 6% 51%	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	38% 4 5% 13% 25% 15%	2848
	11th to 20th	Day	33% 3 5% 19% 22% 18%	2850
	21st to end	Day	43% 3 5% 13% 28% 9%	3432
February	1st to 10th	Day	43% 4 5% 14% 19% 15%	1900
	11th to 20th	Day	42% 4 7% 16% 18% 13%	1859
	21st to end	Day	39% 5% 8% 16% 18% 14%	1570
March	1st to 10th	Day	44% 5% 9% 16% 16% 10%	2030
	11th to 20th	Day	43% 5% 8% 14% 16% 15%	1980
	21st to end	Day	39% 3 9% 18% 17% 14%	2178
April	1st to 10th	Day	41% 5% 10% 17% 17% 11%	1980
	11th to 20th	Day	42% 5% 7% 17% 17% 13%	1956
	21st to end	Day	40% 4 6% 20% 19% 11%	2250
May	1st to 10th	Day	41% 6% 10% 19% 17% 8%	2340
	11th to 20th	Day	41% 7% 9% 20% 19% 5%	2306
	21st to end	Day	41% 8% 12% 21% 16% 2	2531
June	1st to 10th	Day	28% 8% 16% 25% 20% 2	2340
	11th to 20th	Day	32% 7% 13% 25% 18% 4	2340
	21st to end	Day	41% 8% 13% 23% 11% 3	2340
July	1st to 10th	Day	43% 8% 14% 23% 11% 2	2276
	11th to 20th	Day	30% 8% 18% 24% 18% 2	2271
	21st to end	Day	25% 10% 20% 33% 12%	2405
August	1st to 10th	Day	21% 7% 17% 35% 17% 1	2257
	11th to 20th	Day	27% 9% 18% 30% 13% 3	2123
	21st to end	Day	34% 10% 18% 24% 12% 3	2273
September	1st to 10th	Day	40% 9% 12% 17% 15% 6%	1980
	11th to 20th	Day	40% 7% 12% 20% 16% 4	1980
	21st to end	Day	33% 7% 12% 21% 17% 11%	1980
October	1st to 10th	Day	29% 6% 10% 21% 24% 10%	1980
	11th to 20th	Day	35% 5% 9% 17% 19% 15%	1880
	21st to end	Day	29% 3 7% 16% 24% 22%	1980
November	1st to 10th	Day	42% 4 8% 17% 18% 11%	1858
	11th to 20th	Day	36% 4 10% 16% 19% 14%	1801
	21st to end	Day	43% 4 7% 14% 19% 13%	1710
December	1st to 10th	Day	43% 4 7% 18% 19% 9%	1710
	11th to 20th	Day	43% 4 8% 10% 20% 15%	2159
	21st to end	Day	37% 3 6% 19% 24% 12%	3135
January	1st to 10th	Night	36% 4 7% 14% 18% 21%	4108
	11th to 20th	Night	28% 3 5% 16% 28% 20%	4120
	21st to end	Night	46% 3 5% 12% 24% 10%	4263
February	1st to 10th	Night	41% 4 8% 12% 18% 18%	2280
	11th to 20th	Night	39% 3 6% 18% 19% 15%	2218
	21st to end	Night	40% 5% 6% 17% 18% 15%	1884
March	1st to 10th	Night	41% 5% 12% 15% 16% 11%	2087
	11th to 20th	Night	42% 4 10% 13% 14% 17%	1980
	21st to end	Night	34% 4 13% 16% 18% 14%	2178
April	1st to 10th	Night	35% 5% 13% 19% 16% 12%	1980
	11th to 20th	Night	39% 5% 10% 14% 15% 17%	1953
	21st to end	Night	42% 5% 9% 18% 16% 11%	1702
May	1st to 10th	Night	37% 6% 11% 20% 16% 10%	1620
	11th to 20th	Night	34% 7% 11% 25% 19% 5%	1594
	21st to end	Night	34% 8% 18% 25% 14% 1	1746
June	1st to 10th	Night	21% 6% 18% 31% 23% 2	1620
	11th to 20th	Night	21% 6% 14% 32% 22% 5%	1620
	21st to end	Night	30% 8% 19% 27% 14% 2	1620
July	1st to 10th	Night	31% 8% 21% 29% 10% 1	1575
	11th to 20th	Night	17% 7% 25% 33% 17% 1	1566
	21st to end	Night	16% 8% 29% 36% 10% 1	1673
August	1st to 10th	Night	12% 7% 23% 41% 16%	1653
	11th to 20th	Night	21% 12% 23% 33% 10% 2	1778
	21st to end	Night	26% 10% 24% 28% 10% 2	2081
September	1st to 10th	Night	33% 7% 20% 20% 14% 7%	1980
	11th to 20th	Night	33% 10% 13% 25% 14% 4	1980
	21st to end	Night	27% 8% 16% 23% 17% 10%	1980
October	1st to 10th	Night	22% 5% 11% 29% 20% 12%	1980
	11th to 20th	Night	29% 5% 10% 21% 20% 16%	2060
	21st to end	Night	28% 3 7% 17% 23% 22%	2376
November	1st to 10th	Night	33% 4 9% 19% 20% 14%	2220
	11th to 20th	Night	32% 4 12% 19% 18% 15%	2375
	21st to end	Night	39% 4 9% 16% 19% 13%	2470
December	1st to 10th	Night	37% 3 8% 20% 19% 12%	2470
	11th to 20th	Night	42% 4 6% 13% 17% 18%	3037
	21st to end	Night	35% 4 7% 14% 27% 13%	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	21% 8% 35% 27% 7% 2	270
	11th to 20th	Day	24% 10% 28% 26% 11% 1	270
	21st to end	Day	21% 8% 32% 29% 10% 1	297
February	1st to 10th	Day	24% 11% 26% 26% 11% 2	270
	11th to 20th	Day	25% 9% 31% 25% 10% 1	270
	21st to end	Day	22% 9% 45% 20% 4	225
March	1st to 10th	Day	22% 8% 38% 26% 7%	261
	11th to 20th	Day	14% 10% 41% 28% 8%	240
	21st to end	Day	16% 13% 34% 29% 9%	264
April	1st to 10th	Day	20% 7% 33% 31% 8%	240
	11th to 20th	Day	17% 11% 31% 28% 14%	234
	21st to end	Day	13% 11% 37% 29% 8% 2	510
May	1st to 10th	Day	15% 7% 35% 31% 12% 1	603
	11th to 20th	Day	15% 6% 24% 37% 16% 3	625
	21st to end	Day	12% 4 21% 41% 20% 3	679
June	1st to 10th	Day	6% 1 9% 35% 38% 10%	630
	11th to 20th	Day	5% 2 7% 42% 36% 8%	630
	21st to end	Day	9% 2 8% 28% 38% 15%	630
July	1st to 10th	Day	11% 2 6% 27% 39% 16%	621
	11th to 20th	Day	4 2 4 22% 48% 21%	618
	21st to end	Day	3 1 5% 24% 49% 18%	684
August	1st to 10th	Day	2 1 2 27% 45% 23%	569
	11th to 20th	Day	5% 3 27% 43% 23%	437
	21st to end	Day	6% 2 3 22% 38% 29%	392
September	1st to 10th	Day	10% 1 2 13% 47% 28%	270
	11th to 20th	Day	7% 1 3 16% 40% 33%	270
	21st to end	Day	10% 1 3 22% 38% 26%	270
October	1st to 10th	Day	6% 1 9% 26% 39% 18%	270
	11th to 20th	Day	13% 2 11% 35% 27% 12%	270
	21st to end	Day	15% 8% 20% 29% 20% 8%	297
November	1st to 10th	Day	19% 4 14% 33% 23% 7%	270
	11th to 20th	Day	23% 4 17% 29% 22% 6%	270
	21st to end	Day	25% 9% 31% 20% 11% 4	270
December	1st to 10th	Day	20% 7% 23% 30% 17% 4	270
	11th to 20th	Day	22% 11% 23% 33% 9% 2	270
	21st to end	Day	16% 7% 30% 32% 13% 1	297
January	1st to 10th	Night	20% 5% 27% 36% 10% 1	950
	11th to 20th	Night	21% 7% 27% 36% 8%	950
	21st to end	Night	23% 6% 28% 36% 7%	880
February	1st to 10th	Night	23% 10% 29% 32% 6%	760
	11th to 20th	Night	26% 8% 31% 30% 4	744
	21st to end	Night	22% 11% 35% 29% 2	628
March	1st to 10th	Night	21% 10% 37% 28% 3	587
	11th to 20th	Night	19% 8% 36% 31% 5%	540
	21st to end	Night	17% 9% 40% 30% 4	594
April	1st to 10th	Night	14% 7% 45% 30% 4	540
	11th to 20th	Night	18% 8% 38% 31% 5%	533
	21st to end	Night	17% 7% 42% 25% 7% 1	540
May	1st to 10th	Night	12% 6% 46% 31% 5%	540
	11th to 20th	Night	14% 3 36% 37% 9% 1	531
	21st to end	Night	12% 2 33% 42% 10%	584
June	1st to 10th	Night	6% 1 19% 53% 21% 1	540
	11th to 20th	Night	5% 1 22% 54% 17% 1	540
	21st to end	Night	6% 1 16% 47% 27% 3	540
July	1st to 10th	Night	6% 2 11% 49% 30% 2	525
	11th to 20th	Night	3 2 7% 51% 34% 2	522
	21st to end	Night	4 2 7% 51% 35% 2	591
August	1st to 10th	Night	1 7% 54% 35% 2	537
	11th to 20th	Night	4 1 6% 48% 36% 4	534
	21st to end	Night	6% 1 4 42% 42% 5%	594
September	1st to 10th	Night	8% 1 7% 40% 39% 5%	540
	11th to 20th	Night	10% 1 6% 37% 43% 3	540
	21st to end	Night	7% 1 10% 38% 40% 4	540
October	1st to 10th	Night	5% 1 12% 48% 30% 4	540
	11th to 20th	Night	14% 2 18% 43% 21% 1	620
	21st to end	Night	12% 4 26% 35% 21% 2	792
November	1st to 10th	Night	13% 4 19% 40% 22% 1	740
	11th to 20th	Night	14% 4 23% 41% 16% 2	855
	21st to end	Night	20% 7% 24% 34% 15% 1	950
December	1st to 10th	Night	21% 5% 21% 38% 14% 2	950
	11th to 20th	Night	25% 6% 25% 34% 10% 1	950
	21st to end	Night	20% 7% 26% 37% 11%	1045

Page generated at 2024-03-05 06:24:01. Version ac07

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