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 2024-11-02.

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	11% 6% 2 6% 4 4 4 4 6% 8% 45%	100958
	Night	13% 6% 2 5% 3 3 3 3 5% 7% 50%	98115
Transparency	Day	36% 6% 11% 22% 17% 8%	100769
	Night	34% 6% 12% 21% 17% 10%	97975
Seeing	Day	12% 5% 18% 29% 26% 10%	14711
	Night	15% 5% 24% 38% 17% 1	24622

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 6% 4 3 4 2 4 5% 48%	2847
	11th to 20th	Day	17% 5% 1 4 2 2 2 2 4 5% 55%	2850
	21st to end	Day	20% 8% 2 4 3 3 3 3 6% 7% 41%	3432
February	1st to 10th	Day	20% 8% 2 5% 4 2 3 3 4 5% 44%	1900
	11th to 20th	Day	19% 8% 2 6% 4 3 3 3 4 6% 44%	1849
	21st to end	Day	17% 6% 2 5% 3 4 3 2 4 6% 47%	1570
March	1st to 10th	Day	16% 8% 3 7% 4 4 5% 3 5% 7% 39%	3399
	11th to 20th	Day	20% 10% 4 9% 5% 5% 4 3 4 4 33%	3480
	21st to end	Day	15% 8% 2 5% 4 3 3 2 5% 7% 47%	3814
April	1st to 10th	Day	13% 7% 2 6% 4 5% 4 3 5% 6% 43%	2090
	11th to 20th	Day	17% 7% 2 6% 5% 4 4 3 5% 6% 43%	2066
	21st to end	Day	12% 8% 2 7% 5% 4 4 2 5% 5% 45%	2376
May	1st to 10th	Day	19% 9% 3 7% 5% 5% 4 4 5% 6% 33%	4082
	11th to 20th	Day	11% 6% 3 7% 4 5% 4 4 6% 7% 42%	4082
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 9% 44%	4477
June	1st to 10th	Day	4 3 2 4 5% 5% 6% 5% 9% 11% 44%	2470
	11th to 20th	Day	5% 4 2 6% 6% 5% 5% 5% 7% 10% 45%	2470
	21st to end	Day	7% 6% 3 7% 7% 6% 7% 5% 8% 10% 36%	2470
July	1st to 10th	Day	7% 5% 2 5% 5% 5% 5% 4 7% 9% 46%	4120
	11th to 20th	Day	6% 5% 2 7% 5% 5% 6% 5% 8% 10% 41%	4120
	21st to end	Day	8% 6% 3 7% 6% 6% 6% 5% 7% 11% 35%	4465
August	1st to 10th	Day	2 1 1 3 3 3 4 3 6% 9% 65%	3991
	11th to 20th	Day	3 2 1 4 4 3 5% 5% 8% 12% 52%	3726
	21st to end	Day	5% 3 2 5% 5% 5% 6% 5% 7% 11% 45%	3993
September	1st to 10th	Day	8% 7% 3 8% 6% 6% 6% 5% 7% 7% 40%	2090
	11th to 20th	Day	9% 6% 2 8% 6% 5% 5% 4 6% 7% 42%	2090
	21st to end	Day	7% 4 2 6% 6% 5% 4 4 6% 8% 49%	2090
October	1st to 10th	Day	6% 4 2 4 4 5% 5% 4 5% 7% 55%	2090
	11th to 20th	Day	12% 5% 2 5% 5% 4 4 3 5% 7% 50%	1984
	21st to end	Day	9% 5% 2 5% 3 3 2 3 4 5% 59%	2090
November	1st to 10th	Day	13% 10% 3 6% 5% 3 4 3 4 7% 42%	1868
	11th to 20th	Day	13% 6% 3 5% 3 3 3 3 5% 6% 49%	1801
	21st to end	Day	17% 8% 3 6% 4 3 4 3 4 5% 44%	1710
December	1st to 10th	Day	18% 8% 2 7% 4 3 3 3 4 6% 43%	1710
	11th to 20th	Day	16% 8% 2 6% 5% 4 3 3 5% 5% 42%	2161
	21st to end	Day	13% 7% 3 6% 3 3 3 3 4 5% 51%	3135
January	1st to 10th	Night	19% 6% 2 4 3 2 3 2 4 6% 49%	4108
	11th to 20th	Night	14% 4 1 3 2 2 2 1 2 4 64%	4120
	21st to end	Night	24% 6% 2 6% 3 2 3 3 4 6% 40%	4263
February	1st to 10th	Night	18% 7% 3 4 3 3 3 3 5% 5% 46%	2280
	11th to 20th	Night	21% 7% 2 4 3 2 2 2 4 5% 48%	2218
	21st to end	Night	18% 7% 2 6% 3 4 3 3 4 5% 46%	1884
March	1st to 10th	Night	19% 8% 2 7% 4 4 4 3 4 5% 39%	3454
	11th to 20th	Night	22% 9% 4 7% 4 4 4 3 4 5% 33%	3480
	21st to end	Night	16% 6% 2 4 4 3 3 3 5% 6% 47%	3817
April	1st to 10th	Night	12% 6% 2 5% 4 3 3 4 6% 9% 45%	2090
	11th to 20th	Night	16% 7% 2 6% 5% 3 3 3 4 6% 44%	2063
	21st to end	Night	17% 8% 3 4 4 4 4 4 4 5% 45%	1795
May	1st to 10th	Night	19% 9% 2 6% 4 4 3 3 5% 7% 38%	2823
	11th to 20th	Night	13% 6% 2 5% 3 4 3 3 5% 7% 48%	2823
	21st to end	Night	8% 6% 2 6% 5% 4 4 4 6% 9% 45%	3090
June	1st to 10th	Night	5% 3 2 4 3 3 3 4 6% 9% 59%	1710
	11th to 20th	Night	4 4 1 4 2 4 4 3 5% 10% 59%	1710
	21st to end	Night	5% 4 2 5% 5% 4 5% 4 6% 9% 50%	1710
July	1st to 10th	Night	4 4 1 5% 4 4 5% 5% 7% 10% 51%	2850
	11th to 20th	Night	4 4 2 6% 4 5% 6% 5% 7% 9% 49%	2850
	21st to end	Night	7% 6% 3 7% 5% 4 4 5% 6% 10% 44%	3098
August	1st to 10th	Night	1 1 2 2 2 3 3 4 7% 75%	2930
	11th to 20th	Night	3 2 1 3 3 4 4 4 7% 11% 59%	3113
	21st to end	Night	5% 3 1 4 3 3 4 4 7% 11% 55%	3666
September	1st to 10th	Night	7% 5% 3 6% 5% 4 4 4 6% 9% 46%	2090
	11th to 20th	Night	7% 6% 3 7% 5% 3 4 5% 5% 8% 47%	2090
	21st to end	Night	6% 3 2 5% 4 5% 4 4 6% 10% 51%	2090
October	1st to 10th	Night	5% 3 1 4 3 3 3 3 5% 8% 62%	2090
	11th to 20th	Night	11% 5% 1 4 3 2 3 3 4 6% 58%	2175
	21st to end	Night	12% 4 1 5% 2 2 2 2 3 4 62%	2508
November	1st to 10th	Night	11% 6% 2 5% 5% 3 4 2 5% 6% 52%	2244
	11th to 20th	Night	11% 5% 2 5% 3 3 3 3 5% 6% 54%	2375
	21st to end	Night	17% 6% 2 5% 3 3 3 3 5% 6% 47%	2470
December	1st to 10th	Night	17% 7% 2 4 3 3 3 2 5% 6% 49%	2470
	11th to 20th	Night	19% 7% 2 5% 3 3 3 3 4 5% 45%	3036
	21st to end	Night	16% 6% 2 4 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% 6% 8% 17% 17% 8%	3399
	11th to 20th	Day	54% 5% 7% 9% 16% 9%	3480
	21st to end	Day	37% 4 8% 19% 17% 13%	3814
April	1st to 10th	Day	40% 5% 10% 16% 16% 13%	2090
	11th to 20th	Day	43% 5% 7% 17% 17% 12%	2066
	21st to end	Day	40% 5% 7% 20% 18% 11%	2376
May	1st to 10th	Day	48% 7% 11% 16% 12% 6%	4120
	11th to 20th	Day	37% 8% 9% 20% 19% 7%	4086
	21st to end	Day	32% 8% 12% 35% 12% 1	4489
June	1st to 10th	Day	28% 8% 16% 26% 20% 2	2470
	11th to 20th	Day	32% 7% 13% 25% 19% 4	2470
	21st to end	Day	40% 8% 13% 25% 11% 3	2470
July	1st to 10th	Day	32% 7% 17% 36% 7% 1	4056
	11th to 20th	Day	35% 9% 17% 25% 13% 1	4051
	21st to end	Day	38% 9% 17% 28% 8%	4325
August	1st to 10th	Day	15% 5% 12% 44% 22% 1	3977
	11th to 20th	Day	21% 8% 25% 34% 10% 2	3762
	21st to end	Day	30% 11% 18% 23% 17% 1	3992
September	1st to 10th	Day	41% 8% 12% 17% 16% 7%	2090
	11th to 20th	Day	39% 7% 12% 20% 16% 5%	2090
	21st to end	Day	31% 7% 11% 21% 17% 12%	2090
October	1st to 10th	Day	27% 6% 10% 20% 24% 13%	2090
	11th to 20th	Day	34% 5% 9% 17% 19% 16%	1984
	21st to end	Day	29% 3 6% 17% 24% 21%	2090
November	1st to 10th	Day	42% 4 8% 17% 18% 11%	1868
	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	47% 6% 9% 15% 15% 8%	3454
	11th to 20th	Night	52% 6% 9% 12% 11% 10%	3480
	21st to end	Night	36% 6% 12% 15% 21% 11%	3817
April	1st to 10th	Night	35% 5% 13% 19% 15% 13%	2090
	11th to 20th	Night	40% 5% 10% 14% 14% 17%	2063
	21st to end	Night	42% 5% 9% 18% 16% 10%	1795
May	1st to 10th	Night	45% 6% 10% 19% 13% 7%	2850
	11th to 20th	Night	35% 7% 12% 21% 19% 7%	2824
	21st to end	Night	33% 9% 16% 31% 11% 1	3099
June	1st to 10th	Night	21% 6% 18% 31% 22% 2	1710
	11th to 20th	Night	22% 6% 14% 31% 23% 5%	1710
	21st to end	Night	29% 8% 18% 29% 14% 2	1710
July	1st to 10th	Night	26% 8% 24% 35% 6% 1	2805
	11th to 20th	Night	29% 9% 21% 27% 14% 1	2796
	21st to end	Night	34% 8% 24% 26% 7%	2999
August	1st to 10th	Night	10% 5% 16% 48% 21%	2920
	11th to 20th	Night	17% 9% 32% 34% 7% 1	3143
	21st to end	Night	21% 11% 25% 27% 13% 1	3666
September	1st to 10th	Night	33% 7% 19% 19% 14% 7%	2090
	11th to 20th	Night	33% 10% 13% 25% 15% 4	2090
	21st to end	Night	27% 7% 15% 22% 17% 11%	2090
October	1st to 10th	Night	21% 5% 11% 28% 20% 15%	2090
	11th to 20th	Night	28% 5% 9% 20% 21% 17%	2175
	21st to end	Night	27% 3 7% 18% 24% 21%	2508
November	1st to 10th	Night	34% 4 9% 19% 20% 14%	2244
	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	21% 9% 37% 27% 6%	270
	11th to 20th	Day	15% 10% 41% 26% 7%	270
	21st to end	Day	15% 12% 36% 28% 8%	297
April	1st to 10th	Day	19% 10% 33% 29% 9%	270
	11th to 20th	Day	16% 10% 32% 29% 13%	264
	21st to end	Day	14% 11% 37% 29% 8% 1	556
May	1st to 10th	Day	17% 7% 34% 30% 11% 1	653
	11th to 20th	Day	15% 6% 25% 36% 16% 2	675
	21st to end	Day	12% 4 22% 41% 19% 3	734
June	1st to 10th	Day	6% 2 9% 36% 36% 10%	680
	11th to 20th	Day	6% 2 8% 41% 35% 8%	680
	21st to end	Day	9% 1 8% 29% 37% 16%	680
July	1st to 10th	Day	10% 2 6% 26% 40% 16%	671
	11th to 20th	Day	5% 2 4 22% 47% 21%	668
	21st to end	Day	5% 1 5% 25% 45% 18%	739
August	1st to 10th	Day	2 1 2 26% 45% 25%	615
	11th to 20th	Day	5% 3 28% 43% 22%	477
	21st to end	Day	5% 2 3 23% 39% 28%	430
September	1st to 10th	Day	10% 1 5% 16% 43% 25%	300
	11th to 20th	Day	6% 1 3 18% 40% 32%	300
	21st to end	Day	10% 1 3 22% 39% 26%	300
October	1st to 10th	Day	6% 1 11% 25% 39% 18%	300
	11th to 20th	Day	12% 2 11% 35% 28% 12%	300
	21st to end	Day	14% 8% 19% 29% 21% 9%	330
November	1st to 10th	Day	20% 4 14% 32% 22% 7%	273
	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% 11% 38% 28% 3	594
	11th to 20th	Night	20% 9% 36% 30% 5%	570
	21st to end	Night	17% 9% 41% 29% 4	627
April	1st to 10th	Night	14% 8% 45% 29% 4	570
	11th to 20th	Night	19% 8% 39% 30% 4	563
	21st to end	Night	19% 8% 41% 26% 7% 1	570
May	1st to 10th	Night	14% 7% 45% 30% 4	570
	11th to 20th	Night	14% 3 36% 37% 9% 1	561
	21st to end	Night	12% 3 35% 40% 10%	617
June	1st to 10th	Night	6% 1 19% 52% 20% 1	570
	11th to 20th	Night	5% 2 23% 53% 16% 1	570
	21st to end	Night	6% 2 16% 46% 27% 3	570
July	1st to 10th	Night	6% 1 11% 49% 30% 2	555
	11th to 20th	Night	4 2 8% 51% 34% 2	552
	21st to end	Night	4 2 8% 50% 34% 2	624
August	1st to 10th	Night	1 7% 53% 36% 2	567
	11th to 20th	Night	4 1 6% 49% 36% 4	564
	21st to end	Night	5% 1 4 41% 43% 4	627
September	1st to 10th	Night	8% 2 7% 40% 38% 5%	570
	11th to 20th	Night	9% 1 6% 38% 42% 3	570
	21st to end	Night	7% 1 11% 38% 40% 4	570
October	1st to 10th	Night	5% 1 12% 48% 31% 4	570
	11th to 20th	Night	13% 2 19% 44% 21% 1	655
	21st to end	Night	12% 4 25% 36% 21% 2	836
November	1st to 10th	Night	13% 4 19% 40% 22% 1	748
	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

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