Mount Diablo
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Diablo clear sky chart from 2005-11-06 to 2024-04-03.

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	7% 4 2 4 3 2 3 4 5% 7% 59%	78989
	Night	8% 4 2 4 4 3 4 4 5% 8% 53%	81174
Transparency	Day	25% 2 8% 20% 25% 20%	78706
	Night	28% 2 10% 20% 22% 18%	80978
Seeing	Day	9% 12% 27% 31% 17% 4	14504
	Night	9% 6% 18% 36% 26% 5%	23976

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	14% 8% 3 7% 6% 4 5% 5% 6% 6% 35%	2644
	11th to 20th	Day	13% 11% 5% 10% 7% 4 4 4 4 6% 33%	2860
	21st to end	Day	18% 9% 4 8% 6% 3 5% 5% 5% 7% 30%	3146
February	1st to 10th	Day	14% 7% 2 5% 4 4 4 5% 5% 6% 44%	1710
	11th to 20th	Day	11% 6% 4 6% 5% 3 5% 4 5% 9% 43%	1759
	21st to end	Day	12% 5% 2 5% 6% 2 5% 5% 5% 6% 48%	1570
March	1st to 10th	Day	13% 7% 3 7% 7% 4 5% 5% 6% 8% 36%	2028
	11th to 20th	Day	12% 6% 3 6% 6% 3 5% 4 6% 7% 41%	2090
	21st to end	Day	11% 6% 2 4 5% 3 5% 5% 6% 8% 45%	2345
April	1st to 10th	Day	12% 6% 3 5% 5% 3 5% 4 5% 8% 45%	2185
	11th to 20th	Day	8% 4 2 3 3 2 4 4 5% 7% 57%	2129
	21st to end	Day	4 3 2 3 3 3 3 4 4 10% 61%	2160
May	1st to 10th	Day	3 3 1 3 3 2 3 3 6% 7% 64%	2217
	11th to 20th	Day	4 2 1 2 3 2 3 4 4 8% 66%	2383
	21st to end	Day	3 2 1 3 3 2 3 4 5% 9% 65%	2723
June	1st to 10th	Day	3 3 1 3 3 2 3 4 6% 8% 64%	2520
	11th to 20th	Day	1 1 1 1 1 2 4 4 7% 78%	2520
	21st to end	Day	2 1 1 1 1 2 2 3 6% 80%	2520
July	1st to 10th	Day	1 1 1 1 2 2 3 7% 82%	2520
	11th to 20th	Day	1 1 1 1 1 2 2 4 7% 80%	2396
	21st to end	Day	1 1 1 1 2 2 3 7% 81%	2546
August	1st to 10th	Day	1 1 1 2 1 1 2 2 4 7% 78%	2328
	11th to 20th	Day	1 1 1 2 3 4 6% 82%	2178
	21st to end	Day	1 1 1 2 2 3 6% 82%	2376
September	1st to 10th	Day	1 1 1 1 1 1 2 2 5% 84%	2160
	11th to 20th	Day	2 1 1 2 3 2 3 3 5% 8% 71%	2160
	21st to end	Day	2 2 1 2 2 1 2 3 3 6% 77%	1918
October	1st to 10th	Day	2 2 2 3 3 2 2 3 3 7% 72%	1800
	11th to 20th	Day	4 2 1 3 3 2 2 4 4 6% 68%	1800
	21st to end	Day	6% 3 1 4 3 2 3 3 5% 6% 64%	1980
November	1st to 10th	Day	7% 4 2 4 3 2 4 4 6% 9% 55%	1798
	11th to 20th	Day	8% 5% 3 5% 5% 3 5% 4 4 8% 50%	1710
	21st to end	Day	10% 6% 3 5% 3 2 3 3 5% 7% 54%	1568
December	1st to 10th	Day	11% 7% 3 6% 4 3 4 5% 5% 8% 45%	1520
	11th to 20th	Day	20% 12% 4 7% 5% 3 4 5% 5% 5% 30%	1928
	21st to end	Day	15% 8% 4 7% 6% 4 5% 5% 6% 8% 34%	2794
January	1st to 10th	Night	13% 7% 3 5% 4 3 4 4 5% 7% 45%	4299
	11th to 20th	Night	15% 11% 4 8% 6% 4 4 5% 6% 6% 32%	4110
	21st to end	Night	17% 8% 4 7% 7% 5% 5% 5% 4 5% 32%	4521
February	1st to 10th	Night	14% 6% 3 5% 4 2 4 3 5% 5% 50%	2470
	11th to 20th	Night	9% 5% 3 5% 5% 3 6% 5% 5% 8% 47%	2305
	21st to end	Night	11% 5% 2 4 4 3 3 4 5% 7% 53%	1884
March	1st to 10th	Night	13% 7% 3 5% 5% 3 4 5% 5% 7% 42%	2147
	11th to 20th	Night	12% 6% 3 6% 5% 3 4 4 6% 8% 43%	2090
	21st to end	Night	11% 5% 3 5% 5% 2 4 4 5% 7% 49%	2252
April	1st to 10th	Night	8% 5% 3 5% 6% 3 4 4 7% 8% 48%	1820
	11th to 20th	Night	7% 3 2 3 4 3 5% 4 6% 7% 57%	1779
	21st to end	Night	4 2 1 2 3 2 3 4 7% 10% 61%	1800
May	1st to 10th	Night	4 2 1 3 3 3 3 5% 8% 8% 61%	1740
	11th to 20th	Night	2 3 1 2 3 2 4 5% 6% 15% 58%	1509
	21st to end	Night	3 1 2 2 2 5% 5% 7% 12% 61%	1552
June	1st to 10th	Night	3 2 1 4 3 3 3 4 5% 9% 63%	1440
	11th to 20th	Night	1 1 1 1 1 1 2 4 4 7% 77%	1440
	21st to end	Night	1 1 1 2 1 2 4 4 10% 73%	1440
July	1st to 10th	Night	1 1 1 1 2 3 5% 12% 75%	1440
	11th to 20th	Night	1 1 2 1 1 1 3 4 8% 76%	1558
	21st to end	Night	1 2 1 1 1 2 3 4 6% 15% 65%	1773
August	1st to 10th	Night	2 1 1 2 1 3 6% 7% 15% 60%	1611
	11th to 20th	Night	1 1 1 1 2 5% 7% 12% 70%	1723
	21st to end	Night	1 1 1 1 1 1 2 5% 7% 10% 71%	1980
September	1st to 10th	Night	1 1 1 1 1 3 4 5% 8% 76%	1800
	11th to 20th	Night	2 2 1 2 3 1 3 5% 6% 11% 64%	1800
	21st to end	Night	1 1 1 1 2 2 3 4 5% 7% 74%	2036
October	1st to 10th	Night	3 2 1 2 3 1 3 3 5% 7% 70%	2160
	11th to 20th	Night	3 2 1 3 4 2 4 5% 6% 7% 63%	2160
	21st to end	Night	5% 3 1 3 3 2 3 3 6% 7% 65%	2376
November	1st to 10th	Night	5% 3 2 4 4 3 4 4 6% 8% 57%	2281
	11th to 20th	Night	8% 5% 2 4 4 3 4 5% 6% 8% 52%	2470
	21st to end	Night	8% 4 2 3 3 2 4 3 5% 8% 57%	2610
December	1st to 10th	Night	12% 6% 2 6% 5% 3 4 4 4 6% 48%	2660
	11th to 20th	Night	20% 9% 4 7% 6% 3 4 4 6% 6% 31%	3276
	21st to end	Night	14% 8% 4 7% 6% 3 6% 5% 6% 7% 35%	4862

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	45% 5% 12% 28% 8% 2	2644
	11th to 20th	Day	54% 2 11% 16% 12% 5%	2860
	21st to end	Day	54% 2 10% 17% 13% 3	3146
February	1st to 10th	Day	40% 3 8% 23% 20% 7%	1710
	11th to 20th	Day	38% 3 10% 19% 24% 6%	1768
	21st to end	Day	36% 4 9% 25% 21% 7%	1570
March	1st to 10th	Day	45% 3 10% 21% 17% 4	2028
	11th to 20th	Day	40% 4 9% 23% 17% 6%	2090
	21st to end	Day	35% 4 15% 23% 19% 5%	2345
April	1st to 10th	Day	37% 3 12% 21% 21% 5%	2185
	11th to 20th	Day	26% 3 14% 26% 25% 7%	2132
	21st to end	Day	20% 2 12% 30% 29% 6%	2160
May	1st to 10th	Day	19% 3 9% 28% 33% 9%	2217
	11th to 20th	Day	16% 2 7% 30% 33% 11%	2386
	21st to end	Day	16% 2 14% 34% 26% 7%	2724
June	1st to 10th	Day	17% 3 10% 26% 30% 14%	2520
	11th to 20th	Day	7% 1 8% 22% 36% 26%	2520
	21st to end	Day	8% 1 3 13% 33% 43%	2520
July	1st to 10th	Day	5% 1 3 8% 25% 59%	2451
	11th to 20th	Day	6% 1 4 12% 23% 54%	2323
	21st to end	Day	5% 1 1 7% 29% 56%	2405
August	1st to 10th	Day	9% 1 2 13% 31% 45%	2312
	11th to 20th	Day	5% 1 1 12% 34% 47%	2178
	21st to end	Day	5% 1 1 13% 36% 44%	2376
September	1st to 10th	Day	6% 1 1 8% 31% 52%	2160
	11th to 20th	Day	12% 1 4 22% 38% 22%	2160
	21st to end	Day	11% 1 3 21% 37% 27%	1918
October	1st to 10th	Day	15% 2 6% 18% 30% 29%	1800
	11th to 20th	Day	17% 2 9% 22% 25% 26%	1800
	21st to end	Day	21% 1 8% 22% 27% 20%	1980
November	1st to 10th	Day	28% 3 8% 27% 22% 12%	1798
	11th to 20th	Day	33% 3 10% 26% 23% 6%	1710
	21st to end	Day	31% 2 9% 20% 28% 9%	1568
December	1st to 10th	Day	38% 3 12% 22% 18% 8%	1520
	11th to 20th	Day	56% 4 7% 15% 15% 3	1928
	21st to end	Day	49% 2 15% 20% 12% 2	2794
January	1st to 10th	Night	38% 2 20% 26% 11% 3	4299
	11th to 20th	Night	52% 2 15% 14% 10% 7%	4110
	21st to end	Night	52% 2 13% 15% 13% 5%	4521
February	1st to 10th	Night	37% 2 8% 21% 21% 11%	2470
	11th to 20th	Night	33% 3 10% 22% 22% 9%	2307
	21st to end	Night	30% 3 11% 29% 21% 6%	1884
March	1st to 10th	Night	40% 4 14% 20% 17% 6%	2147
	11th to 20th	Night	38% 3 7% 24% 19% 8%	2090
	21st to end	Night	33% 3 14% 23% 19% 7%	2252
April	1st to 10th	Night	32% 3 16% 21% 21% 8%	1820
	11th to 20th	Night	24% 4 13% 28% 20% 11%	1779
	21st to end	Night	17% 3 12% 31% 29% 8%	1800
May	1st to 10th	Night	18% 2 10% 25% 32% 13%	1740
	11th to 20th	Night	14% 2 12% 28% 32% 13%	1509
	21st to end	Night	12% 2 17% 33% 28% 8%	1552
June	1st to 10th	Night	16% 3 10% 25% 30% 16%	1440
	11th to 20th	Night	8% 1 8% 17% 34% 31%	1440
	21st to end	Night	8% 2 2 13% 30% 45%	1440
July	1st to 10th	Night	5% 2 9% 24% 60%	1400
	11th to 20th	Night	6% 1 3 10% 21% 58%	1508
	21st to end	Night	8% 2 1 8% 26% 56%	1674
August	1st to 10th	Night	10% 1 1 14% 27% 46%	1602
	11th to 20th	Night	6% 2 1 11% 31% 49%	1723
	21st to end	Night	6% 1 2 9% 36% 46%	1980
September	1st to 10th	Night	6% 1 2 9% 27% 55%	1800
	11th to 20th	Night	12% 1 4 20% 35% 27%	1800
	21st to end	Night	9% 2 4 20% 32% 33%	2036
October	1st to 10th	Night	14% 2 8% 19% 26% 32%	2160
	11th to 20th	Night	17% 2 8% 20% 24% 29%	2160
	21st to end	Night	19% 1 6% 21% 29% 25%	2376
November	1st to 10th	Night	23% 3 10% 28% 20% 16%	2281
	11th to 20th	Night	29% 3 9% 27% 24% 8%	2470
	21st to end	Night	28% 2 9% 22% 27% 12%	2610
December	1st to 10th	Night	36% 3 10% 21% 17% 12%	2660
	11th to 20th	Night	51% 4 10% 15% 17% 3	3276
	21st to end	Night	48% 2 16% 18% 12% 4	4862

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% 14% 26% 20% 13% 1	246
	11th to 20th	Day	18% 11% 21% 32% 15% 3	370
	21st to end	Day	16% 10% 23% 29% 19% 3	407
February	1st to 10th	Day	19% 14% 21% 28% 16% 2	370
	11th to 20th	Day	19% 19% 26% 21% 14% 1	410
	21st to end	Day	15% 15% 28% 28% 11% 2	382
March	1st to 10th	Day	21% 18% 29% 21% 9% 2	460
	11th to 20th	Day	19% 18% 24% 21% 15% 3	460
	21st to end	Day	15% 22% 31% 23% 9% 1	506
April	1st to 10th	Day	17% 19% 28% 21% 12% 3	429
	11th to 20th	Day	15% 20% 31% 22% 9% 3	412
	21st to end	Day	7% 23% 26% 28% 11% 5%	420
May	1st to 10th	Day	9% 18% 36% 24% 12% 1	423
	11th to 20th	Day	8% 16% 36% 25% 13% 1	448
	21st to end	Day	6% 15% 30% 35% 12% 2	485
June	1st to 10th	Day	5% 14% 34% 29% 14% 4	450
	11th to 20th	Day	3 14% 32% 39% 11% 1	450
	21st to end	Day	2 7% 32% 40% 17% 2	450
July	1st to 10th	Day	1 4 32% 46% 15% 2	446
	11th to 20th	Day	2 29% 46% 19% 4	444
	21st to end	Day	1 2 30% 41% 18% 7%	487
August	1st to 10th	Day	2 6% 29% 42% 17% 4	447
	11th to 20th	Day	1 5% 27% 40% 17% 10%	437
	21st to end	Day	1 3 23% 45% 23% 6%	495
September	1st to 10th	Day	2 4 21% 42% 24% 8%	450
	11th to 20th	Day	3 11% 26% 36% 18% 6%	450
	21st to end	Day	3 9% 24% 34% 23% 7%	450
October	1st to 10th	Day	3 11% 28% 28% 25% 6%	450
	11th to 20th	Day	6% 9% 21% 28% 28% 8%	450
	21st to end	Day	8% 9% 21% 30% 26% 5%	495
November	1st to 10th	Day	8% 12% 22% 24% 29% 5%	394
	11th to 20th	Day	7% 11% 16% 34% 26% 8%	270
	21st to end	Day	9% 10% 26% 30% 24%	203
December	1st to 10th	Day	17% 15% 19% 24% 19% 6%	180
	11th to 20th	Day	17% 13% 22% 23% 21% 4	180
	21st to end	Day	21% 12% 28% 24% 13% 2	198
January	1st to 10th	Night	18% 5% 17% 31% 21% 8%	760
	11th to 20th	Night	14% 6% 15% 35% 24% 6%	760
	21st to end	Night	18% 6% 13% 35% 24% 4	836
February	1st to 10th	Night	17% 8% 15% 33% 22% 5%	760
	11th to 20th	Night	13% 9% 19% 34% 20% 6%	744
	21st to end	Night	13% 11% 20% 34% 20% 3	628
March	1st to 10th	Night	18% 11% 18% 30% 21% 3	760
	11th to 20th	Night	16% 11% 18% 30% 21% 3	760
	21st to end	Night	13% 11% 24% 33% 16% 2	836
April	1st to 10th	Night	10% 10% 23% 35% 19% 3	728
	11th to 20th	Night	8% 14% 28% 28% 19% 4	711
	21st to end	Night	5% 9% 28% 33% 20% 4	720
May	1st to 10th	Night	5% 7% 24% 39% 21% 3	660
	11th to 20th	Night	4 5% 22% 41% 26% 2	447
	21st to end	Night	3 10% 27% 37% 21% 2	389
June	1st to 10th	Night	3 5% 25% 46% 19% 2	360
	11th to 20th	Night	2 4 23% 51% 19% 1	360
	21st to end	Night	1 5% 19% 46% 27% 2	360
July	1st to 10th	Night	1 2 20% 45% 30% 1	350
	11th to 20th	Night	2 1 15% 45% 32% 6%	464
	21st to end	Night	2 1 17% 37% 37% 6%	591
August	1st to 10th	Night	3 3 21% 34% 32% 7%	537
	11th to 20th	Night	2 18% 44% 32% 5%	655
	21st to end	Night	1 1 14% 44% 34% 6%	792
September	1st to 10th	Night	1 2 15% 37% 37% 9%	720
	11th to 20th	Night	3 5% 16% 39% 32% 5%	720
	21st to end	Night	2 4 13% 37% 35% 9%	720
October	1st to 10th	Night	4 4 16% 37% 32% 8%	720
	11th to 20th	Night	6% 4 14% 35% 33% 8%	720
	21st to end	Night	7% 4 13% 33% 30% 12%	792
November	1st to 10th	Night	7% 5% 16% 32% 31% 9%	740
	11th to 20th	Night	10% 3 14% 37% 29% 7%	760
	21st to end	Night	11% 4 15% 33% 29% 8%	760
December	1st to 10th	Night	15% 6% 16% 34% 24% 5%	760
	11th to 20th	Night	21% 6% 14% 32% 23% 4	760
	21st to end	Night	19% 8% 19% 32% 20% 2	836

Page generated at 2024-03-31 03:08:54. Version ac07

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