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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 2025-11-03.

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	6% 5% 2 5% 3 3 3 3 5% 7% 59%	135279
	Night	9% 7% 2 8% 5% 4 4 3 4 6% 48%	132209
Transparency	Day	27% 3 7% 19% 25% 19%	130199
	Night	35% 6% 7% 17% 20% 15%	128276
Seeing	Day	11% 10% 28% 32% 15% 4	16711
	Night	12% 5% 13% 37% 27% 6%	26004

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% 7% 2 5% 3 3 4 4 6% 8% 45%	3869
	11th to 20th	Day	12% 7% 3 7% 4 3 3 2 3 5% 51%	4100
	21st to end	Day	17% 8% 3 8% 4 4 4 3 4 6% 40%	4510
February	1st to 10th	Day	18% 8% 2 5% 4 3 3 3 6% 5% 44%	1800
	11th to 20th	Day	14% 9% 3 6% 4 4 4 3 5% 8% 40%	1840
	21st to end	Day	14% 7% 2 5% 4 3 3 3 5% 6% 47%	1650
March	1st to 10th	Day	11% 11% 4 10% 5% 5% 5% 4 6% 7% 34%	4867
	11th to 20th	Day	11% 8% 2 5% 4 4 4 3 6% 7% 47%	5000
	21st to end	Day	10% 8% 3 7% 5% 6% 6% 5% 7% 9% 35%	5567
April	1st to 10th	Day	13% 7% 2 6% 4 4 4 3 6% 7% 45%	2400
	11th to 20th	Day	9% 5% 2 5% 4 3 4 3 5% 6% 55%	2369
	21st to end	Day	5% 5% 2 4 3 2 3 3 5% 9% 57%	2400
May	1st to 10th	Day	4 3 1 2 1 2 2 2 3 5% 75%	5633
	11th to 20th	Day	3 2 1 2 2 2 2 2 4 6% 73%	5987
	21st to end	Day	2 3 1 4 2 2 2 2 4 6% 70%	6902
June	1st to 10th	Day	5% 5% 1 4 3 3 4 3 5% 8% 60%	2800
	11th to 20th	Day	1 2 1 2 2 2 2 3 5% 7% 72%	2800
	21st to end	Day	2 2 1 4 2 3 2 2 3 6% 72%	2800
July	1st to 10th	Day	1 1 1 3 2 2 2 2 3 5% 79%	6319
	11th to 20th	Day	1 1 1 4 3 3 3 2 3 6% 74%	5930
	21st to end	Day	3 3 6% 3 3 4 4 5% 7% 62%	6402
August	1st to 10th	Day	1 2 1 3 3 4 4 3 5% 7% 67%	5777
	11th to 20th	Day	2 2 4 2 3 3 2 3 6% 72%	5533
	21st to end	Day	1 2 1 4 3 3 3 2 4 8% 69%	5984
September	1st to 10th	Day	1 3 2 3 2 2 2 3 3 6% 74%	2400
	11th to 20th	Day	3 5% 2 5% 4 3 3 2 5% 8% 61%	2400
	21st to end	Day	3 3 1 3 2 2 3 2 4 5% 73%	2120
October	1st to 10th	Day	3 3 1 3 2 2 3 3 4 6% 71%	3280
	11th to 20th	Day	4 4 2 5% 3 2 2 2 4 6% 64%	3261
	21st to end	Day	5% 5% 2 4 3 3 3 3 5% 7% 58%	3558
November	1st to 10th	Day	8% 5% 2 4 3 3 3 3 6% 7% 56%	1939
	11th to 20th	Day	11% 7% 2 6% 4 4 4 4 4 7% 48%	1800
	21st to end	Day	12% 6% 1 4 3 3 2 3 6% 7% 52%	1630
December	1st to 10th	Day	8% 5% 2 4 2 3 3 3 6% 10% 54%	2620
	11th to 20th	Day	21% 8% 3 7% 5% 4 4 4 6% 9% 28%	3028
	21st to end	Day	19% 15% 3 7% 4 4 5% 3 5% 7% 28%	4004
January	1st to 10th	Night	10% 7% 2 5% 3 3 4 4 6% 9% 48%	6069
	11th to 20th	Night	13% 7% 3 7% 4 4 4 3 5% 4 47%	5880
	21st to end	Night	15% 10% 3 8% 4 4 3 2 4 5% 41%	6468
February	1st to 10th	Night	18% 7% 2 4 3 3 3 3 4 5% 48%	2600
	11th to 20th	Night	13% 7% 3 6% 4 3 4 3 5% 7% 43%	2446
	21st to end	Night	12% 7% 2 4 4 4 3 3 4 7% 51%	1980
March	1st to 10th	Night	14% 12% 4 9% 4 4 4 3 5% 6% 35%	5015
	11th to 20th	Night	16% 8% 2 7% 3 3 3 2 5% 6% 46%	5000
	21st to end	Night	15% 12% 3 6% 4 3 3 3 5% 7% 39%	5407
April	1st to 10th	Night	9% 9% 3 5% 5% 3 3 4 5% 7% 48%	2000
	11th to 20th	Night	9% 8% 2 6% 3 3 3 2 4 6% 53%	1979
	21st to end	Night	5% 6% 2 4 4 3 3 4 6% 8% 56%	2000
May	1st to 10th	Night	4 4 1 4 4 3 2 2 3 4 69%	4340
	11th to 20th	Night	2 6% 2 8% 8% 5% 4 3 5% 5% 52%	3918
	21st to end	Night	2 4 2 10% 9% 4 3 2 3 4 57%	3972
June	1st to 10th	Night	4 7% 1 7% 4 3 4 4 7% 8% 51%	1600
	11th to 20th	Night	1 3 1 6% 3 3 3 2 5% 7% 66%	1600
	21st to end	Night	2 4 2 7% 6% 5% 3 4 3 5% 60%	1600
July	1st to 10th	Night	2 1 8% 8% 5% 3 3 3 5% 63%	3640
	11th to 20th	Night	1 3 2 16% 10% 6% 4 2 4 5% 47%	4044
	21st to end	Night	1 12% 6% 21% 7% 5% 4 3 5% 5% 31%	4472
August	1st to 10th	Night	2 4 1 14% 17% 8% 4 4 4 6% 37%	4035
	11th to 20th	Night	3 2 14% 8% 6% 4 3 4 5% 51%	4327
	21st to end	Night	1 3 2 10% 11% 7% 4 4 4 5% 48%	5015
September	1st to 10th	Night	1 3 2 7% 4 3 4 3 5% 8% 60%	2000
	11th to 20th	Night	4 7% 4 9% 5% 5% 4 2 4 6% 50%	2000
	21st to end	Night	2 4 1 5% 4 3 2 2 5% 6% 65%	2276
October	1st to 10th	Night	3 3 1 3 3 3 3 3 4 6% 67%	3920
	11th to 20th	Night	5% 7% 3 12% 5% 3 3 2 3 4 52%	3897
	21st to end	Night	6% 5% 2 5% 4 4 4 3 5% 5% 57%	4253
November	1st to 10th	Night	8% 6% 2 4 4 3 4 3 5% 8% 53%	2403
	11th to 20th	Night	10% 7% 2 5% 4 3 3 3 5% 7% 51%	2600
	21st to end	Night	11% 5% 1 4 4 3 3 4 4 7% 54%	2765
December	1st to 10th	Night	9% 4 2 5% 3 4 4 3 4 6% 58%	4560
	11th to 20th	Night	22% 11% 3 6% 4 4 4 3 7% 7% 29%	5176
	21st to end	Night	21% 11% 3 8% 5% 4 4 3 5% 5% 31%	6952

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% 3 16% 23% 15% 5%	3869
	11th to 20th	Day	39% 2 8% 14% 21% 15%	4100
	21st to end	Day	48% 2 6% 14% 15% 17%	4510
February	1st to 10th	Day	41% 3 6% 21% 21% 8%	1800
	11th to 20th	Day	42% 3 9% 19% 21% 6%	1849
	21st to end	Day	38% 3 7% 25% 20% 7%	1650
March	1st to 10th	Day	50% 2 8% 19% 19% 2	4867
	11th to 20th	Day	36% 3 14% 28% 10% 8%	5000
	21st to end	Day	44% 5% 14% 25% 11% 2	5567
April	1st to 10th	Day	39% 3 12% 22% 20% 4	2400
	11th to 20th	Day	31% 3 13% 24% 21% 6%	2372
	21st to end	Day	26% 3 11% 30% 25% 4	2400
May	1st to 10th	Day	15% 2 7% 28% 31% 17%	5633
	11th to 20th	Day	14% 2 6% 31% 38% 9%	5990
	21st to end	Day	17% 2 7% 30% 40% 4	6904
June	1st to 10th	Day	25% 4 9% 27% 27% 9%	2800
	11th to 20th	Day	14% 2 7% 21% 37% 19%	2800
	21st to end	Day	17% 3 2 12% 33% 33%	2800
July	1st to 10th	Day	10% 3 2 6% 21% 58%	6251
	11th to 20th	Day	11% 5% 3 6% 12% 64%	5859
	21st to end	Day	17% 5% 6% 13% 24% 34%	6217
August	1st to 10th	Day	17% 5% 4 10% 26% 38%	4206
	11th to 20th	Day	12% 4 3 8% 41% 32%	4004
	21st to end	Day	15% 4 3 11% 35% 33%	4311
September	1st to 10th	Day	14% 3 2 9% 31% 41%	2400
	11th to 20th	Day	25% 3 3 22% 32% 16%	2400
	21st to end	Day	16% 2 3 21% 34% 24%	2120
October	1st to 10th	Day	16% 2 5% 12% 28% 38%	3280
	11th to 20th	Day	22% 3 5% 21% 29% 19%	3261
	21st to end	Day	25% 2 7% 23% 29% 14%	3558
November	1st to 10th	Day	28% 3 9% 27% 21% 12%	1939
	11th to 20th	Day	38% 2 8% 24% 22% 6%	1800
	21st to end	Day	32% 3 9% 19% 26% 11%	1630
December	1st to 10th	Day	27% 2 7% 28% 29% 6%	2620
	11th to 20th	Day	52% 4 9% 16% 17% 3	3028
	21st to end	Day	56% 3 12% 18% 10% 2	4004
January	1st to 10th	Night	34% 4 20% 23% 15% 4	6069
	11th to 20th	Night	42% 2 10% 13% 20% 13%	5880
	21st to end	Night	48% 1 7% 11% 18% 16%	6468
February	1st to 10th	Night	40% 2 7% 20% 20% 11%	2600
	11th to 20th	Night	42% 3 8% 19% 20% 8%	2448
	21st to end	Night	35% 3 9% 29% 19% 6%	1980
March	1st to 10th	Night	50% 3 9% 18% 17% 3	5015
	11th to 20th	Night	40% 3 10% 24% 15% 8%	5000
	21st to end	Night	47% 4 12% 23% 11% 2	5407
April	1st to 10th	Night	37% 3 13% 22% 20% 5%	2000
	11th to 20th	Night	35% 3 11% 25% 19% 8%	1979
	21st to end	Night	27% 4 11% 31% 22% 6%	2000
May	1st to 10th	Night	20% 3 6% 25% 29% 16%	4337
	11th to 20th	Night	30% 7% 5% 22% 25% 10%	3919
	21st to end	Night	28% 7% 6% 23% 32% 4	3972
June	1st to 10th	Night	31% 6% 7% 21% 26% 9%	1600
	11th to 20th	Night	20% 4 6% 17% 34% 19%	1600
	21st to end	Night	26% 6% 2 11% 28% 29%	1600
July	1st to 10th	Night	13% 14% 2 6% 18% 47%	3600
	11th to 20th	Night	20% 22% 4 5% 8% 41%	3992
	21st to end	Night	38% 21% 1 5% 14% 21%	4345
August	1st to 10th	Night	32% 26% 2 8% 16% 17%	2929
	11th to 20th	Night	22% 12% 2 6% 31% 27%	3126
	21st to end	Night	28% 11% 3 7% 26% 26%	3608
September	1st to 10th	Night	23% 6% 2 6% 25% 39%	2000
	11th to 20th	Night	36% 5% 2 17% 24% 16%	2000
	21st to end	Night	21% 3 2 19% 28% 26%	2276
October	1st to 10th	Night	18% 3 4 11% 20% 44%	3920
	11th to 20th	Night	34% 5% 4 17% 18% 22%	3897
	21st to end	Night	28% 4 4 19% 24% 20%	4253
November	1st to 10th	Night	31% 4 8% 25% 19% 14%	2403
	11th to 20th	Night	34% 3 9% 26% 21% 8%	2600
	21st to end	Night	31% 3 9% 21% 24% 13%	2765
December	1st to 10th	Night	29% 2 6% 22% 29% 12%	4560
	11th to 20th	Night	54% 3 8% 15% 17% 3	5176
	21st to end	Night	56% 3 12% 16% 9% 3	6952

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	27% 9% 28% 22% 12% 2	300
	11th to 20th	Day	19% 11% 24% 30% 15% 2	400
	21st to end	Day	18% 8% 28% 27% 16% 3	440
February	1st to 10th	Day	25% 10% 23% 25% 16% 3	400
	11th to 20th	Day	22% 17% 25% 24% 11% 1	438
	21st to end	Day	16% 12% 30% 29% 11% 2	414
March	1st to 10th	Day	26% 14% 30% 20% 9% 2	500
	11th to 20th	Day	24% 14% 25% 20% 13% 4	500
	21st to end	Day	19% 18% 31% 22% 9% 1	550
April	1st to 10th	Day	18% 16% 31% 22% 10% 3	500
	11th to 20th	Day	16% 17% 32% 23% 8% 3	492
	21st to end	Day	8% 20% 30% 26% 10% 5%	500
May	1st to 10th	Day	12% 13% 35% 27% 11% 2	503
	11th to 20th	Day	11% 13% 34% 30% 11% 1	528
	21st to end	Day	10% 13% 32% 31% 12% 2	573
June	1st to 10th	Day	8% 9% 34% 35% 11% 3	530
	11th to 20th	Day	4 12% 38% 35% 9% 2	530
	21st to end	Day	2 9% 32% 40% 13% 3	530
July	1st to 10th	Day	3 5% 31% 43% 15% 2	526
	11th to 20th	Day	3 27% 50% 17% 3	524
	21st to end	Day	2 3 28% 45% 16% 6%	575
August	1st to 10th	Day	2 8% 28% 41% 16% 4	527
	11th to 20th	Day	2 5% 30% 40% 17% 7%	517
	21st to end	Day	1 4 24% 45% 21% 5%	583
September	1st to 10th	Day	4 6% 23% 42% 19% 7%	530
	11th to 20th	Day	6% 12% 26% 35% 15% 6%	530
	21st to end	Day	5% 8% 25% 34% 24% 5%	530
October	1st to 10th	Day	5% 9% 28% 31% 18% 8%	530
	11th to 20th	Day	8% 9% 22% 33% 21% 7%	530
	21st to end	Day	11% 8% 25% 29% 21% 5%	583
November	1st to 10th	Day	11% 11% 20% 27% 24% 7%	463
	11th to 20th	Day	10% 9% 19% 35% 21% 7%	300
	21st to end	Day	15% 8% 28% 28% 19% 1	215
December	1st to 10th	Day	19% 11% 21% 28% 18% 5%	200
	11th to 20th	Day	19% 12% 24% 24% 20% 2	200
	21st to end	Day	27% 9% 27% 23% 13% 1	220
January	1st to 10th	Night	21% 4 13% 34% 22% 6%	800
	11th to 20th	Night	16% 5% 12% 38% 24% 5%	800
	21st to end	Night	20% 4 11% 37% 24% 4	880
February	1st to 10th	Night	22% 6% 12% 34% 22% 4	800
	11th to 20th	Night	16% 8% 15% 35% 20% 5%	784
	21st to end	Night	15% 7% 17% 38% 20% 2	660
March	1st to 10th	Night	23% 7% 14% 31% 22% 3	800
	11th to 20th	Night	22% 6% 14% 33% 23% 3	800
	21st to end	Night	18% 8% 18% 36% 19% 2	880
April	1st to 10th	Night	13% 9% 17% 38% 20% 2	800
	11th to 20th	Night	14% 10% 18% 34% 21% 3	791
	21st to end	Night	9% 7% 21% 35% 22% 5%	800
May	1st to 10th	Night	9% 5% 18% 41% 23% 3	710
	11th to 20th	Night	8% 6% 12% 42% 27% 5%	527
	21st to end	Night	9% 11% 16% 39% 21% 4	433
June	1st to 10th	Night	7% 6% 17% 43% 25% 2	400
	11th to 20th	Night	2 4 16% 56% 22% 2	400
	21st to end	Night	5% 4 13% 44% 33% 2	400
July	1st to 10th	Night	5% 2 10% 43% 37% 3	390
	11th to 20th	Night	3 2 7% 45% 35% 8%	554
	21st to end	Night	4 4 9% 34% 41% 8%	657
August	1st to 10th	Night	8% 6% 10% 35% 33% 8%	597
	11th to 20th	Night	2 6% 11% 38% 36% 7%	713
	21st to end	Night	3 3 9% 39% 38% 8%	880
September	1st to 10th	Night	4 3 8% 36% 40% 9%	800
	11th to 20th	Night	10% 3 10% 36% 35% 7%	800
	21st to end	Night	5% 4 10% 34% 38% 10%	800
October	1st to 10th	Night	8% 3 9% 38% 34% 8%	800
	11th to 20th	Night	10% 6% 9% 33% 35% 8%	800
	21st to end	Night	11% 3 10% 32% 32% 11%	880
November	1st to 10th	Night	10% 4 11% 34% 32% 9%	788
	11th to 20th	Night	14% 3 12% 36% 28% 7%	800
	21st to end	Night	14% 4 12% 37% 26% 6%	800
December	1st to 10th	Night	18% 5% 12% 37% 23% 6%	800
	11th to 20th	Night	25% 5% 10% 36% 21% 4	800
	21st to end	Night	22% 8% 18% 32% 19% 2	880

Page generated at 2025-10-30 23:02:38. Version ac07

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