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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 2024-11-06.

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% 5% 2 5% 3 3 3 3 5% 7% 58%	105074
	Night	9% 7% 2 7% 5% 4 4 3 5% 6% 49%	102743
Transparency	Day	28% 3 7% 19% 24% 20%	104833
	Night	35% 5% 7% 17% 19% 16%	102574
Seeing	Day	11% 10% 28% 32% 15% 4	15408
	Night	12% 5% 12% 36% 28% 6%	24716

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% 10% 3 6% 3 4 4 4 6% 6% 37%	2699
	11th to 20th	Day	18% 10% 5% 10% 5% 5% 4 3 4 6% 33%	2860
	21st to end	Day	22% 10% 3 8% 4 4 5% 3 5% 6% 29%	3146
February	1st to 10th	Day	17% 7% 2 5% 4 3 3 3 6% 5% 45%	1710
	11th to 20th	Day	14% 9% 3 6% 4 3 4 3 5% 7% 41%	1745
	21st to end	Day	14% 7% 2 5% 5% 3 3 3 5% 6% 48%	1570
March	1st to 10th	Day	13% 13% 4 11% 6% 5% 4 4 5% 7% 29%	3383
	11th to 20th	Day	10% 6% 2 4 3 3 4 3 5% 7% 53%	3490
	21st to end	Day	11% 9% 3 8% 5% 5% 5% 5% 7% 8% 35%	3880
April	1st to 10th	Day	13% 7% 2 6% 4 4 4 3 6% 7% 44%	2280
	11th to 20th	Day	10% 5% 2 5% 4 2 3 3 5% 6% 55%	2249
	21st to end	Day	5% 5% 2 4 3 2 3 3 5% 9% 58%	2280
May	1st to 10th	Day	6% 3 1 3 1 2 2 2 3 5% 72%	3941
	11th to 20th	Day	3 2 1 3 2 3 2 2 4 6% 71%	4178
	21st to end	Day	3 4 1 5% 3 2 2 2 4 6% 67%	4813
June	1st to 10th	Day	5% 5% 1 4 3 3 4 3 5% 8% 59%	2660
	11th to 20th	Day	1 2 1 2 2 2 2 3 5% 7% 72%	2660
	21st to end	Day	2 2 1 4 2 3 2 2 3 6% 71%	2660
July	1st to 10th	Day	1 1 1 2 2 2 2 1 3 5% 80%	4420
	11th to 20th	Day	1 2 1 3 2 3 3 2 4 7% 72%	4147
	21st to end	Day	2 1 4 2 2 3 3 4 7% 71%	4455
August	1st to 10th	Day	1 3 1 4 3 4 5% 3 6% 9% 60%	4091
	11th to 20th	Day	1 1 4 2 3 3 2 4 7% 72%	3841
	21st to end	Day	1 3 1 4 3 3 3 2 4 7% 70%	4180
September	1st to 10th	Day	1 3 2 3 2 2 2 2 3 5% 76%	2280
	11th to 20th	Day	3 5% 2 4 3 3 3 2 5% 8% 62%	2280
	21st to end	Day	3 3 1 2 2 2 3 2 4 5% 74%	2014
October	1st to 10th	Day	2 3 1 3 2 2 3 3 4 6% 71%	3180
	11th to 20th	Day	4 4 2 5% 4 2 2 2 4 6% 64%	3161
	21st to end	Day	5% 5% 2 4 3 3 3 3 5% 8% 59%	3448
November	1st to 10th	Day	8% 5% 2 4 2 3 3 3 6% 7% 56%	1872
	11th to 20th	Day	11% 7% 2 5% 5% 4 3 4 4 6% 48%	1710
	21st to end	Day	11% 6% 1 4 4 3 2 3 6% 7% 53%	1549
December	1st to 10th	Day	14% 7% 3 6% 3 3 4 3 5% 7% 45%	1520
	11th to 20th	Day	24% 10% 4 7% 4 3 3 3 5% 5% 31%	1928
	21st to end	Day	17% 11% 3 7% 4 4 5% 3 5% 8% 34%	2794
January	1st to 10th	Night	14% 8% 2 6% 4 3 4 4 5% 6% 44%	4247
	11th to 20th	Night	18% 9% 3 8% 5% 5% 5% 3 5% 5% 32%	4110
	21st to end	Night	22% 13% 3 8% 4 4 4 3 4 6% 30%	4521
February	1st to 10th	Night	17% 7% 2 4 3 3 3 3 4 5% 48%	2470
	11th to 20th	Night	13% 7% 2 6% 4 3 5% 3 5% 7% 43%	2321
	21st to end	Night	12% 7% 2 4 4 4 3 3 4 7% 51%	1884
March	1st to 10th	Night	16% 13% 4 8% 4 4 4 4 5% 5% 33%	3479
	11th to 20th	Night	13% 6% 2 7% 3 3 3 2 5% 6% 50%	3490
	21st to end	Night	14% 13% 4 8% 4 4 4 3 4 5% 37%	3772
April	1st to 10th	Night	10% 9% 3 5% 5% 3 3 4 5% 7% 48%	1900
	11th to 20th	Night	9% 8% 1 5% 3 3 3 2 4 6% 54%	1879
	21st to end	Night	5% 5% 2 4 3 3 3 3 6% 8% 57%	1900
May	1st to 10th	Night	4 5% 1 4 3 2 2 2 4 4 68%	3030
	11th to 20th	Night	2 5% 2 8% 9% 6% 5% 3 4 5% 51%	2720
	21st to end	Night	3 5% 2 10% 5% 3 3 2 3 4 60%	2762
June	1st to 10th	Night	4 7% 1 7% 4 3 4 4 6% 8% 52%	1520
	11th to 20th	Night	1 3 2 6% 3 3 3 2 5% 7% 65%	1520
	21st to end	Night	2 4 2 6% 6% 5% 3 4 3 5% 60%	1520
July	1st to 10th	Night	2 1 6% 4 3 3 2 3 5% 70%	2540
	11th to 20th	Night	1 2 1 10% 8% 6% 4 3 5% 7% 54%	2818
	21st to end	Night	1 7% 4 16% 7% 6% 5% 4 6% 7% 39%	3109
August	1st to 10th	Night	3 5% 2 13% 18% 10% 5% 4 4 7% 28%	2848
	11th to 20th	Night	3 2 11% 7% 4 4 3 4 7% 54%	3003
	21st to end	Night	2 4 2 10% 8% 5% 3 3 4 5% 52%	3498
September	1st to 10th	Night	1 2 2 6% 4 3 4 3 5% 8% 61%	1900
	11th to 20th	Night	4 6% 4 9% 5% 5% 4 2 5% 6% 50%	1900
	21st to end	Night	2 4 1 5% 4 3 2 2 5% 6% 66%	2162
October	1st to 10th	Night	3 3 1 3 3 3 3 2 5% 6% 68%	3800
	11th to 20th	Night	5% 7% 3 12% 5% 3 3 2 3 4 52%	3777
	21st to end	Night	6% 5% 1 5% 4 4 4 3 5% 5% 58%	4121
November	1st to 10th	Night	8% 6% 2 4 4 3 4 3 4 8% 53%	2328
	11th to 20th	Night	11% 7% 2 5% 4 3 3 3 5% 7% 51%	2470
	21st to end	Night	10% 5% 1 4 3 4 3 4 4 7% 56%	2626
December	1st to 10th	Night	15% 7% 2 6% 3 4 3 3 4 6% 47%	2660
	11th to 20th	Night	26% 11% 2 5% 3 3 3 3 5% 6% 31%	3276
	21st to end	Night	17% 8% 3 8% 5% 5% 4 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	57% 2 8% 17% 13% 3	3383
	11th to 20th	Day	33% 2 11% 30% 13% 11%	3490
	21st to end	Day	47% 3 13% 24% 11% 2	3880
April	1st to 10th	Day	39% 3 13% 21% 20% 4	2280
	11th to 20th	Day	31% 3 13% 25% 21% 6%	2252
	21st to end	Day	25% 3 12% 30% 26% 4	2280
May	1st to 10th	Day	18% 2 8% 31% 24% 16%	3941
	11th to 20th	Day	18% 2 4 25% 39% 12%	4181
	21st to end	Day	22% 2 7% 32% 32% 4	4814
June	1st to 10th	Day	25% 4 9% 26% 26% 9%	2660
	11th to 20th	Day	14% 2 7% 22% 37% 17%	2660
	21st to end	Day	18% 3 2 12% 32% 33%	2660
July	1st to 10th	Day	10% 2 1 5% 14% 67%	4351
	11th to 20th	Day	13% 4 3 7% 15% 59%	4076
	21st to end	Day	12% 5% 2 7% 28% 47%	4315
August	1st to 10th	Day	17% 5% 4 10% 25% 38%	4076
	11th to 20th	Day	12% 4 3 8% 41% 32%	3880
	21st to end	Day	15% 4 3 11% 34% 33%	4179
September	1st to 10th	Day	14% 2 1 8% 32% 43%	2280
	11th to 20th	Day	24% 3 3 22% 31% 16%	2280
	21st to end	Day	15% 2 3 21% 35% 25%	2014
October	1st to 10th	Day	15% 2 4 12% 28% 39%	3180
	11th to 20th	Day	22% 3 5% 21% 29% 19%	3161
	21st to end	Day	25% 2 7% 22% 29% 15%	3448
November	1st to 10th	Day	28% 3 9% 27% 20% 12%	1872
	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	55% 2 11% 14% 14% 3	3479
	11th to 20th	Night	37% 2 8% 24% 18% 11%	3490
	21st to end	Night	51% 2 12% 22% 11% 3	3772
April	1st to 10th	Night	37% 3 13% 22% 20% 5%	1900
	11th to 20th	Night	33% 3 11% 26% 19% 8%	1879
	21st to end	Night	26% 4 11% 31% 23% 6%	1900
May	1st to 10th	Night	23% 2 8% 27% 28% 13%	3030
	11th to 20th	Night	32% 8% 4 16% 26% 14%	2721
	21st to end	Night	32% 2 6% 28% 29% 3	2762
June	1st to 10th	Night	30% 6% 7% 21% 26% 9%	1520
	11th to 20th	Night	21% 3 6% 18% 34% 18%	1520
	21st to end	Night	26% 6% 1 11% 28% 28%	1520
July	1st to 10th	Night	14% 8% 1 4 13% 59%	2500
	11th to 20th	Night	20% 14% 3 6% 10% 47%	2766
	21st to end	Night	27% 21% 1 5% 16% 30%	3011
August	1st to 10th	Night	33% 26% 2 7% 15% 17%	2839
	11th to 20th	Night	22% 12% 2 5% 31% 27%	3030
	21st to end	Night	27% 11% 3 7% 26% 26%	3498
September	1st to 10th	Night	21% 6% 2 6% 25% 41%	1900
	11th to 20th	Night	35% 5% 2 17% 25% 16%	1900
	21st to end	Night	20% 3 2 20% 29% 26%	2162
October	1st to 10th	Night	18% 3 4 10% 19% 45%	3800
	11th to 20th	Night	34% 5% 4 16% 18% 22%	3777
	21st to end	Night	27% 4 4 20% 24% 21%	4121
November	1st to 10th	Night	31% 4 8% 25% 18% 14%	2328
	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	19% 18% 31% 23% 8% 1	506
April	1st to 10th	Day	19% 16% 31% 22% 10% 3	460
	11th to 20th	Day	17% 17% 31% 23% 8% 4	452
	21st to end	Day	8% 20% 30% 27% 10% 5%	460
May	1st to 10th	Day	12% 14% 34% 27% 10% 2	463
	11th to 20th	Day	10% 14% 33% 31% 11% 1	488
	21st to end	Day	11% 13% 31% 30% 13% 2	529
June	1st to 10th	Day	8% 10% 33% 34% 12% 3	490
	11th to 20th	Day	4 11% 36% 37% 10% 2	490
	21st to end	Day	2 9% 33% 40% 13% 3	490
July	1st to 10th	Day	3 5% 29% 44% 16% 2	486
	11th to 20th	Day	3 27% 50% 17% 3	484
	21st to end	Day	2 2 28% 46% 17% 6%	531
August	1st to 10th	Day	2 7% 28% 42% 17% 4	487
	11th to 20th	Day	2 5% 30% 40% 17% 7%	477
	21st to end	Day	1 4 25% 45% 20% 5%	539
September	1st to 10th	Day	3 5% 21% 43% 20% 7%	490
	11th to 20th	Day	6% 12% 26% 35% 15% 6%	490
	21st to end	Day	3 8% 24% 34% 25% 5%	490
October	1st to 10th	Day	5% 8% 29% 33% 18% 9%	490
	11th to 20th	Day	8% 9% 23% 32% 21% 7%	490
	21st to end	Day	10% 8% 25% 29% 21% 6%	539
November	1st to 10th	Day	12% 11% 21% 26% 24% 6%	438
	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% 19% 36% 19% 2	836
April	1st to 10th	Night	14% 9% 17% 38% 20% 2	760
	11th to 20th	Night	14% 10% 18% 33% 22% 3	751
	21st to end	Night	8% 7% 21% 35% 23% 5%	760
May	1st to 10th	Night	9% 5% 18% 40% 24% 4	675
	11th to 20th	Night	7% 6% 11% 42% 29% 5%	500
	21st to end	Night	10% 11% 14% 39% 22% 4	411
June	1st to 10th	Night	8% 6% 17% 42% 26% 2	380
	11th to 20th	Night	2 4 14% 56% 22% 2	380
	21st to end	Night	5% 4 12% 44% 33% 2	380
July	1st to 10th	Night	5% 2 9% 42% 39% 3	370
	11th to 20th	Night	3 2 7% 44% 35% 9%	525
	21st to end	Night	4 4 7% 34% 43% 9%	624
August	1st to 10th	Night	9% 6% 9% 35% 34% 7%	567
	11th to 20th	Night	2 6% 10% 37% 37% 7%	677
	21st to end	Night	3 3 9% 39% 39% 8%	836
September	1st to 10th	Night	3 2 8% 37% 41% 10%	760
	11th to 20th	Night	9% 3 10% 36% 36% 6%	760
	21st to end	Night	4 3 9% 34% 39% 10%	760
October	1st to 10th	Night	8% 3 8% 39% 34% 8%	760
	11th to 20th	Night	9% 6% 8% 32% 36% 8%	760
	21st to end	Night	10% 3 10% 33% 32% 12%	836
November	1st to 10th	Night	10% 4 11% 34% 32% 9%	764
	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 2024-11-03 10:19:03. Version ac07

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