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Chabot Space and Science Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chabot Space and Science Center clear sky chart from 2005-11-06 to 2024-03-23.

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	9% 5% 2 4 4 2 3 4 5% 7% 55%	78881
	Night	11% 7% 3 5% 5% 3 4 4 5% 7% 47%	81046
Transparency	Day	31% 3 7% 20% 23% 16%	78600
	Night	38% 4 8% 18% 19% 14%	80849
Seeing	Day	11% 10% 27% 32% 15% 4	14496
	Night	13% 5% 12% 36% 28% 6%	23957

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	17% 9% 3 6% 4 3 4 5% 6% 6% 38%	2699
	11th to 20th	Day	18% 10% 5% 8% 6% 4 4 4 4 6% 33%	2860
	21st to end	Day	22% 10% 4 6% 5% 3 5% 4 5% 7% 29%	3146
February	1st to 10th	Day	17% 7% 3 4 4 3 3 3 5% 6% 45%	1710
	11th to 20th	Day	14% 8% 3 5% 5% 3 4 4 5% 8% 42%	1745
	21st to end	Day	14% 6% 3 4 5% 2 3 3 4 6% 48%	1570
March	1st to 10th	Day	16% 9% 3 7% 5% 3 4 4 6% 8% 35%	2051
	11th to 20th	Day	15% 8% 3 5% 5% 3 4 5% 5% 7% 39%	2090
	21st to end	Day	12% 6% 3 5% 4 3 5% 5% 5% 8% 43%	2231
April	1st to 10th	Day	15% 6% 3 5% 4 2 4 4 5% 8% 43%	2160
	11th to 20th	Day	10% 5% 2 4 4 2 3 4 5% 6% 56%	2129
	21st to end	Day	6% 4 2 3 3 2 3 4 5% 9% 58%	2160
May	1st to 10th	Day	6% 5% 1 4 3 2 3 4 5% 7% 61%	2236
	11th to 20th	Day	6% 3 2 3 3 3 3 4 4 8% 60%	2356
	21st to end	Day	6% 5% 2 5% 4 2 3 4 4 8% 57%	2723
June	1st to 10th	Day	5% 5% 2 3 3 2 4 4 5% 8% 59%	2520
	11th to 20th	Day	2 2 1 2 2 2 2 3 5% 7% 73%	2520
	21st to end	Day	2 2 1 4 3 2 3 2 3 6% 70%	2520
July	1st to 10th	Day	1 2 1 3 3 1 3 3 4 7% 70%	2520
	11th to 20th	Day	1 2 1 3 3 2 3 3 5% 8% 68%	2364
	21st to end	Day	1 2 1 3 2 2 3 3 4 7% 72%	2546
August	1st to 10th	Day	2 4 2 4 3 2 3 4 4 7% 65%	2328
	11th to 20th	Day	1 2 1 3 2 2 3 3 4 8% 71%	2191
	21st to end	Day	1 3 2 4 3 2 2 2 3 8% 71%	2376
September	1st to 10th	Day	1 3 2 2 2 1 2 3 3 5% 76%	2160
	11th to 20th	Day	3 5% 2 4 4 2 3 3 5% 8% 63%	2160
	21st to end	Day	3 3 1 2 2 2 3 2 4 6% 74%	1908
October	1st to 10th	Day	4 3 2 2 2 2 3 3 4 6% 69%	1800
	11th to 20th	Day	6% 4 2 3 4 2 3 3 5% 6% 63%	1800
	21st to end	Day	7% 4 1 3 4 2 3 3 3 8% 61%	1980
November	1st to 10th	Day	9% 5% 3 4 3 2 4 4 6% 7% 55%	1821
	11th to 20th	Day	11% 7% 3 5% 5% 3 4 4 4 7% 49%	1710
	21st to end	Day	12% 5% 2 4 4 2 3 4 5% 7% 53%	1549
December	1st to 10th	Day	14% 7% 3 4 4 2 4 4 5% 7% 45%	1520
	11th to 20th	Day	24% 10% 4 6% 5% 2 4 4 5% 5% 31%	1928
	21st to end	Day	17% 11% 3 6% 5% 3 4 4 5% 8% 34%	2794
January	1st to 10th	Night	14% 8% 2 5% 4 3 4 5% 5% 6% 44%	4247
	11th to 20th	Night	18% 9% 3 7% 7% 4 5% 4 5% 5% 32%	4110
	21st to end	Night	22% 13% 3 7% 5% 3 4 3 4 6% 30%	4521
February	1st to 10th	Night	17% 7% 3 4 3 2 3 4 4 5% 49%	2470
	11th to 20th	Night	13% 7% 3 6% 5% 2 5% 4 5% 7% 44%	2321
	21st to end	Night	13% 6% 2 3 5% 3 4 3 4 7% 51%	1884
March	1st to 10th	Night	18% 8% 3 5% 4 3 5% 5% 5% 6% 39%	2127
	11th to 20th	Night	16% 7% 3 5% 5% 3 4 4 6% 8% 41%	2090
	21st to end	Night	14% 8% 3 5% 4 2 3 3 4 6% 48%	2169
April	1st to 10th	Night	11% 8% 3 5% 5% 2 3 5% 5% 7% 47%	1800
	11th to 20th	Night	10% 8% 2 4 4 2 3 3 4 6% 55%	1779
	21st to end	Night	6% 4 2 4 4 2 4 4 6% 8% 58%	1800
May	1st to 10th	Night	5% 6% 2 6% 4 3 3 3 5% 6% 56%	1720
	11th to 20th	Night	4 6% 4 6% 6% 3 4 4 5% 7% 50%	1536
	21st to end	Night	5% 7% 4 7% 6% 2 4 3 4 6% 50%	1552
June	1st to 10th	Night	5% 7% 2 5% 5% 2 4 5% 6% 7% 52%	1440
	11th to 20th	Night	1 3 2 5% 4 2 3 3 5% 7% 65%	1440
	21st to end	Night	2 4 3 6% 7% 3 3 4 3 5% 59%	1440
July	1st to 10th	Night	1 3 3 6% 5% 3 4 3 4 8% 59%	1440
	11th to 20th	Night	2 3 2 6% 6% 4 5% 4 6% 9% 54%	1592
	21st to end	Night	1 4 4 8% 7% 4 7% 6% 8% 9% 42%	1773
August	1st to 10th	Night	3 8% 3 10% 8% 3 5% 5% 5% 9% 40%	1611
	11th to 20th	Night	3 3 7% 7% 4 4 6% 6% 10% 49%	1709
	21st to end	Night	4 3 9% 7% 4 4 6% 6% 8% 50%	1980
September	1st to 10th	Night	1 2 2 6% 4 2 4 4 5% 8% 62%	1800
	11th to 20th	Night	4 6% 4 7% 6% 4 4 3 5% 7% 51%	1800
	21st to end	Night	2 4 2 3 5% 2 2 3 5% 6% 67%	2048
October	1st to 10th	Night	5% 5% 1 3 3 2 3 4 4 7% 62%	2160
	11th to 20th	Night	7% 5% 2 5% 4 3 4 4 4 5% 57%	2160
	21st to end	Night	7% 4 1 4 3 3 4 4 5% 6% 58%	2376
November	1st to 10th	Night	9% 5% 3 4 4 3 4 4 4 8% 53%	2257
	11th to 20th	Night	11% 6% 2 5% 4 2 3 4 5% 7% 51%	2470
	21st to end	Night	10% 4 1 3 4 3 3 4 4 7% 56%	2626
December	1st to 10th	Night	15% 7% 2 5% 4 3 4 3 4 6% 47%	2660
	11th to 20th	Night	27% 11% 3 4 3 2 3 4 5% 6% 31%	3276
	21st to end	Night	17% 8% 3 7% 6% 3 5% 4 5% 6% 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	47% 2 10% 30% 8% 3	2699
	11th to 20th	Day	55% 2 11% 15% 10% 7%	2860
	21st to end	Day	57% 2 7% 17% 13% 4	3146
February	1st to 10th	Day	41% 3 6% 21% 20% 8%	1710
	11th to 20th	Day	41% 3 9% 19% 22% 6%	1754
	21st to end	Day	38% 3 7% 25% 20% 6%	1570
March	1st to 10th	Day	48% 2 9% 21% 15% 4	2051
	11th to 20th	Day	44% 3 8% 23% 16% 5%	2090
	21st to end	Day	39% 5% 13% 21% 19% 4	2231
April	1st to 10th	Day	40% 3 13% 20% 19% 5%	2160
	11th to 20th	Day	31% 3 13% 25% 22% 6%	2132
	21st to end	Day	25% 3 12% 30% 26% 4	2160
May	1st to 10th	Day	25% 3 7% 29% 29% 7%	2236
	11th to 20th	Day	25% 3 6% 32% 25% 9%	2359
	21st to end	Day	27% 4 11% 30% 23% 4	2724
June	1st to 10th	Day	26% 4 9% 26% 26% 9%	2520
	11th to 20th	Day	15% 2 7% 22% 36% 18%	2520
	21st to end	Day	19% 3 2 13% 33% 31%	2520
July	1st to 10th	Day	17% 2 1 9% 25% 46%	2451
	11th to 20th	Day	17% 4 3 10% 23% 43%	2293
	21st to end	Day	14% 3 1 9% 29% 44%	2405
August	1st to 10th	Day	22% 4 2 13% 29% 31%	2312
	11th to 20th	Day	15% 3 1 10% 36% 35%	2191
	21st to end	Day	17% 3 1 12% 34% 33%	2376
September	1st to 10th	Day	14% 3 1 8% 32% 42%	2160
	11th to 20th	Day	24% 3 3 22% 31% 17%	2160
	21st to end	Day	15% 2 3 22% 34% 24%	1908
October	1st to 10th	Day	20% 2 5% 18% 31% 25%	1800
	11th to 20th	Day	23% 3 7% 20% 24% 24%	1800
	21st to end	Day	26% 2 7% 21% 25% 20%	1980
November	1st to 10th	Day	29% 3 9% 26% 20% 13%	1821
	11th to 20th	Day	38% 2 7% 23% 23% 7%	1710
	21st to end	Day	31% 3 9% 19% 26% 11%	1549
December	1st to 10th	Day	40% 2 10% 21% 18% 9%	1520
	11th to 20th	Day	56% 2 8% 14% 16% 4	1928
	21st to end	Day	51% 2 12% 20% 12% 3	2794
January	1st to 10th	Night	41% 3 18% 25% 10% 3	4247
	11th to 20th	Night	55% 2 13% 13% 9% 8%	4110
	21st to end	Night	59% 2 8% 13% 12% 6%	4521
February	1st to 10th	Night	39% 2 7% 21% 20% 11%	2470
	11th to 20th	Night	41% 3 7% 19% 21% 8%	2323
	21st to end	Night	36% 3 9% 29% 19% 5%	1884
March	1st to 10th	Night	48% 4 10% 18% 17% 5%	2127
	11th to 20th	Night	43% 3 5% 23% 19% 6%	2090
	21st to end	Night	40% 4 12% 21% 18% 5%	2169
April	1st to 10th	Night	38% 3 14% 20% 20% 5%	1800
	11th to 20th	Night	33% 3 11% 25% 20% 8%	1779
	21st to end	Night	26% 4 11% 30% 23% 6%	1800
May	1st to 10th	Night	30% 3 5% 24% 27% 10%	1720
	11th to 20th	Night	35% 3 7% 24% 24% 7%	1537
	21st to end	Night	37% 4 10% 22% 22% 4	1552
June	1st to 10th	Night	31% 7% 7% 20% 26% 10%	1440
	11th to 20th	Night	21% 3 6% 18% 33% 18%	1440
	21st to end	Night	28% 5% 1 11% 28% 27%	1440
July	1st to 10th	Night	25% 6% 6% 23% 40%	1400
	11th to 20th	Night	26% 7% 3 7% 17% 39%	1540
	21st to end	Night	33% 9% 1 6% 19% 33%	1674
August	1st to 10th	Night	41% 6% 1 8% 20% 24%	1602
	11th to 20th	Night	28% 7% 1 7% 24% 33%	1709
	21st to end	Night	31% 8% 1 8% 24% 29%	1980
September	1st to 10th	Night	22% 6% 1 5% 26% 41%	1800
	11th to 20th	Night	35% 5% 2 17% 24% 17%	1800
	21st to end	Night	19% 3 2 21% 29% 26%	2048
October	1st to 10th	Night	24% 3 6% 17% 25% 25%	2160
	11th to 20th	Night	29% 3 4 18% 21% 24%	2160
	21st to end	Night	28% 4 4 20% 22% 22%	2376
November	1st to 10th	Night	31% 4 8% 25% 17% 14%	2257
	11th to 20th	Night	34% 3 8% 25% 21% 9%	2470
	21st to end	Night	30% 3 9% 21% 25% 13%	2626
December	1st to 10th	Night	40% 2 8% 19% 17% 12%	2660
	11th to 20th	Night	54% 2 8% 15% 16% 4	3276
	21st to end	Night	51% 2 14% 17% 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	28% 9% 28% 21% 13% 2	275
	11th to 20th	Day	20% 10% 23% 31% 15% 2	370
	21st to end	Day	19% 8% 28% 27% 16% 3	407
February	1st to 10th	Day	22% 10% 23% 26% 16% 3	370
	11th to 20th	Day	22% 16% 26% 24% 11% 1	403
	21st to end	Day	17% 13% 29% 28% 11% 2	382
March	1st to 10th	Day	27% 13% 29% 20% 9% 2	460
	11th to 20th	Day	23% 14% 25% 21% 13% 4	460
	21st to end	Day	17% 17% 31% 24% 8% 1	471
April	1st to 10th	Day	20% 16% 31% 21% 10% 2	420
	11th to 20th	Day	17% 18% 32% 23% 7% 3	412
	21st to end	Day	8% 21% 29% 27% 10% 5%	420
May	1st to 10th	Day	13% 14% 34% 28% 10% 2	423
	11th to 20th	Day	10% 15% 34% 29% 11% 1	448
	21st to end	Day	11% 14% 31% 30% 12% 2	485
June	1st to 10th	Day	9% 10% 32% 34% 12% 4	450
	11th to 20th	Day	4 11% 35% 37% 10% 2	450
	21st to end	Day	2 10% 31% 40% 14% 3	450
July	1st to 10th	Day	4 5% 30% 44% 15% 2	446
	11th to 20th	Day	2 26% 50% 18% 3	444
	21st to end	Day	2 1 26% 47% 17% 6%	487
August	1st to 10th	Day	3 7% 28% 42% 17% 4	447
	11th to 20th	Day	1 4 27% 42% 19% 8%	437
	21st to end	Day	1 4 24% 46% 21% 5%	495
September	1st to 10th	Day	4 4 20% 44% 21% 8%	450
	11th to 20th	Day	5% 10% 25% 38% 16% 6%	450
	21st to end	Day	4 8% 24% 34% 25% 6%	450
October	1st to 10th	Day	5% 8% 29% 32% 17% 8%	450
	11th to 20th	Day	9% 8% 22% 33% 21% 7%	450
	21st to end	Day	11% 7% 25% 29% 22% 6%	495
November	1st to 10th	Day	12% 9% 20% 27% 25% 7%	417
	11th to 20th	Day	9% 9% 16% 37% 23% 7%	270
	21st to end	Day	12% 7% 28% 30% 21% 2	194
December	1st to 10th	Day	21% 12% 20% 26% 17% 4	180
	11th to 20th	Day	18% 12% 25% 23% 21% 2	180
	21st to end	Day	24% 10% 28% 23% 15% 1	198
January	1st to 10th	Night	21% 4 13% 33% 23% 7%	760
	11th to 20th	Night	17% 5% 11% 38% 24% 5%	760
	21st to end	Night	21% 4 11% 36% 24% 4	836
February	1st to 10th	Night	21% 6% 12% 34% 23% 4	760
	11th to 20th	Night	15% 9% 15% 35% 21% 5%	744
	21st to end	Night	16% 7% 17% 36% 21% 3	628
March	1st to 10th	Night	23% 6% 14% 31% 22% 3	760
	11th to 20th	Night	21% 6% 13% 33% 24% 3	760
	21st to end	Night	17% 8% 18% 36% 19% 2	800
April	1st to 10th	Night	14% 9% 16% 38% 20% 2	720
	11th to 20th	Night	14% 11% 19% 32% 22% 3	711
	21st to end	Night	8% 8% 21% 35% 23% 6%	720
May	1st to 10th	Night	10% 4 18% 40% 24% 4	640
	11th to 20th	Night	8% 6% 11% 41% 30% 5%	474
	21st to end	Night	10% 12% 14% 38% 22% 4	389
June	1st to 10th	Night	7% 7% 15% 42% 26% 2	360
	11th to 20th	Night	2 4 13% 56% 24% 2	360
	21st to end	Night	5% 4 12% 43% 34% 3	360
July	1st to 10th	Night	5% 2 9% 41% 40% 3	350
	11th to 20th	Night	3 2 6% 43% 37% 9%	496
	21st to end	Night	4 4 6% 34% 44% 9%	591
August	1st to 10th	Night	9% 6% 9% 34% 35% 8%	537
	11th to 20th	Night	2 5% 9% 37% 39% 8%	641
	21st to end	Night	3 3 8% 39% 39% 8%	792
September	1st to 10th	Night	3 2 7% 37% 41% 10%	720
	11th to 20th	Night	9% 3 9% 36% 37% 6%	720
	21st to end	Night	5% 3 9% 34% 39% 11%	720
October	1st to 10th	Night	8% 3 8% 38% 34% 8%	720
	11th to 20th	Night	9% 6% 8% 32% 36% 8%	720
	21st to end	Night	10% 3 9% 32% 33% 13%	792
November	1st to 10th	Night	10% 4 10% 34% 32% 9%	740
	11th to 20th	Night	13% 3 11% 36% 29% 8%	760
	21st to end	Night	13% 4 12% 38% 27% 6%	760
December	1st to 10th	Night	19% 5% 11% 37% 23% 6%	760
	11th to 20th	Night	25% 5% 10% 35% 21% 4	760
	21st to end	Night	22% 7% 18% 32% 20% 2	836

Page generated at 2024-03-19 20:31:02. Version ac07

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