Cabrillo College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cabrillo College Observatory clear sky chart from 2005-11-06 to 2024-11-10.

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 4 3 5% 7% 57%	105016
	Night	9% 6% 2 6% 4 4 4 3 5% 6% 51%	102896
Transparency	Day	27% 5% 8% 20% 24% 17%	104775
	Night	33% 5% 8% 18% 21% 15%	102727
Seeing	Day	9% 5% 18% 32% 26% 9%	15506
	Night	11% 3 9% 32% 34% 10%	24884

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 6% 5% 4 4 3 6% 6% 43%	2739
	11th to 20th	Day	17% 7% 3 7% 5% 5% 4 4 5% 7% 35%	2860
	21st to end	Day	20% 9% 3 6% 4 5% 4 4 6% 7% 32%	3146
February	1st to 10th	Day	19% 5% 2 5% 4 3 3 3 5% 6% 46%	1710
	11th to 20th	Day	13% 5% 2 6% 4 4 5% 3 5% 7% 45%	1779
	21st to end	Day	14% 5% 2 6% 6% 4 3 4 4 6% 47%	1570
March	1st to 10th	Day	14% 10% 3 9% 6% 5% 6% 4 7% 6% 28%	3374
	11th to 20th	Day	10% 6% 2 4 3 3 3 4 5% 8% 53%	3490
	21st to end	Day	12% 8% 3 7% 6% 6% 6% 5% 6% 8% 33%	3901
April	1st to 10th	Day	11% 6% 2 4 4 4 4 3 5% 8% 47%	2280
	11th to 20th	Day	8% 5% 2 5% 3 4 4 3 5% 8% 53%	2249
	21st to end	Day	6% 4 2 5% 4 4 4 4 5% 8% 54%	2280
May	1st to 10th	Day	6% 3 1 3 2 2 2 2 4 6% 69%	3847
	11th to 20th	Day	3 6% 2 6% 4 3 3 3 5% 8% 57%	4130
	21st to end	Day	3 7% 2 6% 3 3 3 2 4 6% 61%	4813
June	1st to 10th	Day	4 5% 2 5% 4 4 4 3 6% 9% 56%	2660
	11th to 20th	Day	2 4 2 3 3 3 3 3 5% 7% 66%	2660
	21st to end	Day	2 3 2 4 3 3 3 3 4 8% 65%	2660
July	1st to 10th	Day	1 2 1 3 2 2 2 3 3 5% 76%	4396
	11th to 20th	Day	1 3 1 4 3 4 3 2 4 7% 66%	4110
	21st to end	Day	3 1 4 3 3 3 2 5% 8% 68%	4455
August	1st to 10th	Day	1 4 1 4 3 3 5% 4 6% 8% 61%	4091
	11th to 20th	Day	2 1 3 3 3 3 3 3 7% 71%	3782
	21st to end	Day	1 3 2 4 2 3 4 3 4 7% 69%	4180
September	1st to 10th	Day	1 3 1 3 2 3 2 2 4 6% 73%	2280
	11th to 20th	Day	3 5% 2 5% 3 3 3 3 5% 9% 60%	2280
	21st to end	Day	2 2 1 3 2 2 3 2 4 5% 74%	2025
October	1st to 10th	Day	2 3 1 3 2 2 3 3 5% 6% 71%	3180
	11th to 20th	Day	3 3 2 4 3 3 4 3 4 6% 67%	3161
	21st to end	Day	5% 3 1 3 3 3 4 3 5% 8% 62%	3448
November	1st to 10th	Day	8% 5% 2 4 3 3 4 3 5% 7% 56%	1922
	11th to 20th	Day	9% 4 2 4 4 3 4 4 6% 6% 53%	1710
	21st to end	Day	11% 3 1 3 3 2 3 3 4 7% 57%	1606
December	1st to 10th	Day	14% 6% 2 6% 3 3 3 4 5% 7% 46%	1520
	11th to 20th	Day	27% 8% 2 6% 3 3 3 3 5% 6% 33%	1928
	21st to end	Day	18% 9% 2 6% 4 4 5% 4 5% 7% 37%	2794
January	1st to 10th	Night	13% 6% 2 5% 4 3 4 3 5% 6% 48%	4209
	11th to 20th	Night	19% 7% 3 6% 3 5% 5% 4 6% 7% 35%	4110
	21st to end	Night	21% 8% 3 6% 4 4 5% 4 6% 7% 34%	4521
February	1st to 10th	Night	17% 7% 1 4 3 3 3 3 4 5% 49%	2470
	11th to 20th	Night	13% 6% 2 5% 3 4 4 3 6% 7% 47%	2287
	21st to end	Night	13% 6% 1 4 3 2 3 3 5% 7% 52%	1884
March	1st to 10th	Night	17% 11% 3 8% 5% 4 4 3 6% 6% 33%	3495
	11th to 20th	Night	12% 5% 2 6% 4 4 3 3 4 7% 51%	3490
	21st to end	Night	15% 10% 3 7% 4 4 4 4 5% 7% 37%	3750
April	1st to 10th	Night	11% 6% 1 5% 4 3 4 4 5% 8% 48%	1900
	11th to 20th	Night	10% 6% 2 5% 4 4 4 4 6% 6% 50%	1879
	21st to end	Night	6% 5% 2 3 3 4 4 3 6% 9% 56%	1900
May	1st to 10th	Night	3 4 1 3 2 2 3 2 4 6% 70%	3127
	11th to 20th	Night	2 15% 6% 14% 7% 5% 4 3 5% 6% 34%	2770
	21st to end	Night	3 7% 3 7% 4 4 4 3 5% 7% 54%	2762
June	1st to 10th	Night	3 5% 3 5% 6% 5% 4 3 5% 7% 55%	1520
	11th to 20th	Night	1 5% 2 7% 4 4 4 3 5% 7% 59%	1520
	21st to end	Night	2 5% 3 7% 5% 3 5% 3 4 7% 56%	1520
July	1st to 10th	Night	3 2 7% 4 3 3 3 3 4 69%	2560
	11th to 20th	Night	1 4 4 10% 4 4 4 3 7% 7% 53%	2860
	21st to end	Night	1 10% 4 9% 5% 5% 5% 5% 7% 8% 41%	3109
August	1st to 10th	Night	2 6% 3 8% 4 4 5% 4 6% 8% 49%	2848
	11th to 20th	Night	4 2 8% 5% 4 6% 4 6% 6% 55%	3065
	21st to end	Night	1 5% 3 8% 4 5% 5% 3 4 5% 58%	3498
September	1st to 10th	Night	1 3 2 4 3 4 4 3 4 6% 66%	1900
	11th to 20th	Night	4 6% 3 6% 4 4 4 2 4 7% 57%	1900
	21st to end	Night	2 5% 1 4 3 2 3 2 3 6% 69%	2149
October	1st to 10th	Night	3 3 1 4 3 2 2 2 3 6% 71%	3800
	11th to 20th	Night	3 5% 3 7% 4 4 3 3 4 6% 58%	3777
	21st to end	Night	5% 4 2 4 2 2 3 3 4 6% 65%	4121
November	1st to 10th	Night	8% 5% 2 4 3 3 3 3 5% 7% 58%	2358
	11th to 20th	Night	9% 4 2 4 3 3 4 4 6% 9% 52%	2470
	21st to end	Night	8% 5% 2 4 3 4 4 3 4 5% 58%	2569
December	1st to 10th	Night	15% 6% 2 4 3 3 4 3 4 6% 50%	2660
	11th to 20th	Night	28% 9% 2 4 3 3 2 3 5% 6% 35%	3276
	21st to end	Night	19% 7% 2 6% 4 4 4 3 5% 7% 38%	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	38% 6% 13% 28% 11% 3	2739
	11th to 20th	Day	46% 8% 11% 15% 11% 8%	2860
	21st to end	Day	49% 7% 8% 18% 13% 5%	3146
February	1st to 10th	Day	38% 5% 6% 21% 20% 10%	1710
	11th to 20th	Day	36% 6% 9% 20% 20% 9%	1788
	21st to end	Day	36% 7% 9% 24% 18% 7%	1570
March	1st to 10th	Day	46% 14% 9% 15% 14% 3	3374
	11th to 20th	Day	29% 6% 13% 29% 13% 11%	3490
	21st to end	Day	41% 12% 15% 20% 11% 2	3901
April	1st to 10th	Day	33% 7% 13% 22% 20% 5%	2280
	11th to 20th	Day	28% 6% 14% 27% 19% 6%	2252
	21st to end	Day	27% 6% 14% 28% 21% 5%	2280
May	1st to 10th	Day	18% 4 11% 28% 27% 12%	3847
	11th to 20th	Day	22% 8% 7% 30% 24% 9%	4133
	21st to end	Day	24% 5% 12% 29% 27% 3	4814
June	1st to 10th	Day	25% 7% 10% 27% 24% 7%	2660
	11th to 20th	Day	20% 3 9% 25% 30% 13%	2660
	21st to end	Day	22% 2 4 17% 30% 24%	2660
July	1st to 10th	Day	15% 1 2 8% 16% 59%	4327
	11th to 20th	Day	23% 1 3 9% 15% 49%	4041
	21st to end	Day	20% 1 2 11% 33% 34%	4315
August	1st to 10th	Day	24% 1 2 13% 31% 29%	4076
	11th to 20th	Day	18% 1 1 12% 50% 18%	3819
	21st to end	Day	20% 1 3 12% 38% 26%	4179
September	1st to 10th	Day	17% 1 2 11% 32% 37%	2280
	11th to 20th	Day	24% 3 6% 24% 29% 14%	2280
	21st to end	Day	15% 2 5% 22% 37% 19%	2025
October	1st to 10th	Day	14% 3 5% 13% 30% 36%	3180
	11th to 20th	Day	22% 2 6% 26% 27% 18%	3161
	21st to end	Day	21% 2 6% 23% 31% 16%	3448
November	1st to 10th	Day	26% 6% 7% 25% 25% 11%	1922
	11th to 20th	Day	29% 5% 11% 21% 25% 9%	1710
	21st to end	Day	26% 4 10% 18% 29% 12%	1606
December	1st to 10th	Day	36% 5% 10% 20% 17% 12%	1520
	11th to 20th	Day	50% 7% 8% 15% 14% 6%	1928
	21st to end	Day	44% 5% 12% 22% 13% 4	2794
January	1st to 10th	Night	35% 6% 17% 27% 12% 4	4209
	11th to 20th	Night	46% 7% 14% 14% 10% 8%	4110
	21st to end	Night	48% 6% 11% 17% 12% 7%	4521
February	1st to 10th	Night	39% 3 7% 18% 19% 13%	2470
	11th to 20th	Night	36% 5% 10% 19% 22% 8%	2289
	21st to end	Night	31% 4 10% 30% 18% 6%	1884
March	1st to 10th	Night	48% 9% 11% 14% 13% 3	3495
	11th to 20th	Night	30% 7% 9% 23% 21% 10%	3490
	21st to end	Night	42% 8% 13% 21% 11% 3	3750
April	1st to 10th	Night	33% 6% 14% 22% 20% 6%	1900
	11th to 20th	Night	32% 6% 11% 28% 16% 7%	1879
	21st to end	Night	25% 6% 12% 31% 20% 6%	1900
May	1st to 10th	Night	18% 3 10% 29% 26% 14%	3127
	11th to 20th	Night	41% 13% 7% 15% 14% 10%	2771
	21st to end	Night	24% 11% 11% 22% 29% 3	2762
June	1st to 10th	Night	27% 7% 9% 23% 24% 9%	1520
	11th to 20th	Night	26% 4 8% 16% 29% 18%	1520
	21st to end	Night	26% 5% 3 14% 26% 26%	1520
July	1st to 10th	Night	21% 2 1 6% 14% 57%	2520
	11th to 20th	Night	31% 2 3 7% 14% 44%	2806
	21st to end	Night	38% 2 2 7% 23% 28%	3011
August	1st to 10th	Night	35% 2 2 11% 26% 24%	2839
	11th to 20th	Night	33% 2 1 7% 36% 21%	3094
	21st to end	Night	31% 2 4 10% 36% 18%	3498
September	1st to 10th	Night	20% 2 3 9% 32% 34%	1900
	11th to 20th	Night	29% 3 4 21% 30% 14%	1900
	21st to end	Night	20% 2 3 19% 34% 21%	2149
October	1st to 10th	Night	18% 1 4 11% 25% 41%	3800
	11th to 20th	Night	30% 2 4 23% 22% 20%	3777
	21st to end	Night	24% 2 5% 22% 29% 19%	4121
November	1st to 10th	Night	26% 4 8% 24% 24% 14%	2358
	11th to 20th	Night	29% 4 11% 22% 24% 10%	2470
	21st to end	Night	28% 4 8% 20% 25% 16%	2569
December	1st to 10th	Night	35% 4 9% 20% 18% 13%	2660
	11th to 20th	Night	50% 5% 10% 15% 16% 5%	3276
	21st to end	Night	44% 7% 13% 18% 13% 5%	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	27% 7% 15% 27% 18% 6%	302
	11th to 20th	Day	18% 4 21% 26% 26% 5%	370
	21st to end	Day	16% 3 21% 30% 23% 7%	407
February	1st to 10th	Day	20% 7% 23% 20% 20% 10%	370
	11th to 20th	Day	14% 13% 22% 29% 17% 5%	421
	21st to end	Day	12% 10% 26% 32% 17% 3	382
March	1st to 10th	Day	25% 10% 21% 27% 14% 2	460
	11th to 20th	Day	22% 7% 20% 26% 18% 6%	460
	21st to end	Day	16% 10% 28% 29% 14% 3	506
April	1st to 10th	Day	14% 9% 25% 30% 16% 5%	460
	11th to 20th	Day	10% 13% 24% 32% 15% 6%	452
	21st to end	Day	6% 9% 27% 30% 21% 6%	460
May	1st to 10th	Day	8% 8% 25% 35% 17% 6%	463
	11th to 20th	Day	9% 7% 24% 30% 24% 7%	488
	21st to end	Day	11% 7% 19% 33% 22% 7%	529
June	1st to 10th	Day	6% 5% 20% 36% 24% 9%	490
	11th to 20th	Day	4 5% 23% 40% 24% 4	490
	21st to end	Day	2 2 17% 41% 29% 8%	490
July	1st to 10th	Day	4 1 11% 39% 35% 10%	486
	11th to 20th	Day	3 1 6% 41% 37% 12%	484
	21st to end	Day	2 9% 40% 37% 12%	531
August	1st to 10th	Day	5% 11% 41% 35% 7%	487
	11th to 20th	Day	1 1 13% 42% 33% 10%	477
	21st to end	Day	4 1 9% 36% 37% 13%	539
September	1st to 10th	Day	2 1 11% 37% 33% 15%	490
	11th to 20th	Day	6% 3 18% 36% 27% 11%	490
	21st to end	Day	3 4 12% 31% 38% 11%	490
October	1st to 10th	Day	3 4 17% 31% 31% 14%	490
	11th to 20th	Day	6% 3 18% 27% 31% 14%	490
	21st to end	Day	9% 4 17% 27% 30% 13%	539
November	1st to 10th	Day	10% 4 17% 25% 30% 14%	464
	11th to 20th	Day	9% 4 13% 34% 29% 11%	270
	21st to end	Day	9% 3 17% 33% 30% 8%	221
December	1st to 10th	Day	17% 8% 15% 23% 27% 10%	180
	11th to 20th	Day	17% 7% 17% 24% 25% 10%	180
	21st to end	Day	21% 7% 21% 23% 22% 7%	198
January	1st to 10th	Night	20% 4 11% 34% 25% 6%	760
	11th to 20th	Night	16% 3 12% 34% 27% 8%	760
	21st to end	Night	17% 4 9% 36% 29% 4	836
February	1st to 10th	Night	20% 6% 11% 31% 25% 6%	760
	11th to 20th	Night	16% 4 15% 34% 24% 6%	744
	21st to end	Night	16% 5% 15% 38% 23% 3	628
March	1st to 10th	Night	20% 6% 12% 35% 24% 3	760
	11th to 20th	Night	18% 3 13% 32% 27% 6%	760
	21st to end	Night	17% 4 16% 36% 25% 3	836
April	1st to 10th	Night	13% 4 14% 38% 27% 4	760
	11th to 20th	Night	12% 7% 14% 35% 25% 6%	751
	21st to end	Night	9% 4 18% 33% 28% 9%	760
May	1st to 10th	Night	8% 3 14% 37% 31% 7%	733
	11th to 20th	Night	6% 2 8% 33% 40% 12%	524
	21st to end	Night	8% 5% 11% 32% 36% 7%	411
June	1st to 10th	Night	6% 1 8% 40% 38% 8%	380
	11th to 20th	Night	3 1 9% 39% 42% 5%	380
	21st to end	Night	4 3 34% 48% 11%	380
July	1st to 10th	Night	2 1 1 24% 59% 13%	377
	11th to 20th	Night	4 1 18% 54% 22%	552
	21st to end	Night	6% 2 20% 53% 19%	624
August	1st to 10th	Night	8% 1 2 26% 48% 14%	567
	11th to 20th	Night	4 1 3 31% 47% 14%	715
	21st to end	Night	5% 1 2 24% 48% 20%	836
September	1st to 10th	Night	3 4 25% 51% 18%	760
	11th to 20th	Night	6% 1 5% 30% 44% 14%	760
	21st to end	Night	6% 1 6% 24% 46% 16%	760
October	1st to 10th	Night	6% 2 6% 33% 38% 15%	760
	11th to 20th	Night	7% 2 6% 29% 42% 14%	760
	21st to end	Night	8% 1 8% 32% 34% 17%	836
November	1st to 10th	Night	10% 1 11% 31% 37% 11%	778
	11th to 20th	Night	12% 1 8% 35% 37% 7%	760
	21st to end	Night	9% 3 11% 40% 30% 8%	760
December	1st to 10th	Night	18% 3 10% 36% 27% 7%	760
	11th to 20th	Night	22% 3 11% 38% 22% 4	760
	21st to end	Night	20% 5% 15% 37% 19% 4	836

Page generated at 2024-11-07 03:55:49. Version ac07

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