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 2025-12-03.

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 5% 3 3 4 3 5% 7% 58%	135380
	Night	9% 6% 3 6% 4 4 4 3 5% 6% 51%	132803
Transparency	Day	27% 4 7% 21% 25% 16%	130324
	Night	34% 4 8% 19% 22% 14%	128844
Seeing	Day	10% 5% 18% 33% 26% 9%	16890
	Night	12% 3 9% 33% 34% 10%	26286

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	12% 5% 1 5% 3 3 3 3 5% 7% 53%	3937
	11th to 20th	Day	12% 5% 2 5% 4 4 3 3 4 5% 55%	4100
	21st to end	Day	15% 7% 2 6% 4 4 4 3 6% 6% 43%	4510
February	1st to 10th	Day	21% 5% 2 5% 4 3 3 3 5% 5% 45%	1800
	11th to 20th	Day	13% 5% 2 6% 4 4 5% 3 5% 7% 45%	1876
	21st to end	Day	13% 5% 2 7% 6% 4 3 4 4 6% 47%	1650
March	1st to 10th	Day	12% 8% 3 8% 6% 6% 7% 5% 7% 7% 33%	4858
	11th to 20th	Day	13% 5% 2 4 4 4 4 4 5% 9% 48%	5000
	21st to end	Day	13% 9% 3 6% 5% 5% 5% 5% 6% 9% 34%	5588
April	1st to 10th	Day	11% 6% 2 4 4 4 4 3 5% 8% 48%	2400
	11th to 20th	Day	8% 5% 2 5% 4 4 4 3 5% 8% 53%	2369
	21st to end	Day	6% 4 2 5% 4 4 4 4 5% 8% 53%	2400
May	1st to 10th	Day	4 2 1 3 2 2 2 2 4 6% 71%	5500
	11th to 20th	Day	2 4 2 5% 3 3 3 3 4 8% 63%	5926
	21st to end	Day	2 5% 2 5% 3 3 2 2 4 6% 67%	6902
June	1st to 10th	Day	4 5% 2 5% 4 4 4 3 5% 9% 56%	2800
	11th to 20th	Day	2 4 2 3 3 3 3 3 5% 7% 67%	2800
	21st to end	Day	2 3 2 4 3 3 3 2 4 7% 66%	2800
July	1st to 10th	Day	2 1 4 3 3 3 3 3 5% 72%	6282
	11th to 20th	Day	1 2 2 5% 4 4 4 2 4 7% 65%	5880
	21st to end	Day	5% 2 5% 3 3 3 2 4 7% 64%	6402
August	1st to 10th	Day	1 3 1 4 3 3 4 3 5% 7% 67%	5777
	11th to 20th	Day	2 1 3 3 3 4 3 3 6% 71%	5448
	21st to end	Day	2 1 3 2 3 4 3 4 7% 69%	5984
September	1st to 10th	Day	1 3 1 3 2 3 2 2 4 6% 73%	2400
	11th to 20th	Day	3 5% 2 5% 3 3 3 3 5% 9% 60%	2400
	21st to end	Day	4 3 1 3 2 2 3 2 4 5% 72%	2131
October	1st to 10th	Day	2 3 1 3 2 2 3 3 5% 6% 71%	3280
	11th to 20th	Day	3 3 2 4 2 4 4 3 4 6% 67%	3261
	21st to end	Day	5% 3 1 3 3 3 4 3 5% 8% 63%	3558
November	1st to 10th	Day	8% 4 2 4 3 3 4 3 5% 7% 57%	2029
	11th to 20th	Day	10% 4 2 4 4 3 4 4 6% 6% 51%	1890
	21st to end	Day	12% 3 1 3 3 2 3 3 5% 7% 57%	1774
December	1st to 10th	Day	8% 4 1 5% 3 3 3 3 5% 9% 54%	2636
	11th to 20th	Day	22% 6% 2 5% 4 4 4 3 6% 7% 38%	3028
	21st to end	Day	24% 9% 2 5% 3 3 4 4 5% 7% 33%	4004
January	1st to 10th	Night	9% 5% 2 4 3 3 3 3 5% 8% 55%	6018
	11th to 20th	Night	13% 5% 2 4 3 4 4 3 5% 5% 52%	5880
	21st to end	Night	15% 7% 2 6% 4 4 4 3 5% 6% 44%	6468
February	1st to 10th	Night	19% 7% 1 4 3 3 3 3 4 5% 49%	2600
	11th to 20th	Night	13% 6% 2 5% 3 4 4 3 6% 7% 46%	2410
	21st to end	Night	12% 6% 2 5% 3 3 3 3 5% 7% 52%	1980
March	1st to 10th	Night	15% 9% 3 8% 5% 5% 5% 4 6% 6% 35%	5031
	11th to 20th	Night	15% 5% 2 6% 4 4 3 3 4 7% 47%	5000
	21st to end	Night	17% 9% 3 6% 4 3 3 3 4 7% 41%	5385
April	1st to 10th	Night	11% 6% 1 5% 4 3 4 4 5% 8% 49%	2000
	11th to 20th	Night	9% 7% 2 5% 4 4 4 4 6% 6% 49%	1979
	21st to end	Night	6% 5% 2 3 3 4 3 4 6% 9% 55%	2000
May	1st to 10th	Night	2 4 3 4 3 3 3 2 4 5% 66%	4479
	11th to 20th	Night	1 11% 5% 11% 5% 5% 4 4 6% 6% 42%	3982
	21st to end	Night	2 5% 2 7% 3 3 5% 4 6% 7% 56%	3972
June	1st to 10th	Night	3 5% 3 6% 6% 5% 4 3 5% 7% 54%	1600
	11th to 20th	Night	1 5% 2 7% 4 4 4 3 4 7% 59%	1600
	21st to end	Night	2 4 3 7% 5% 3 5% 3 4 7% 57%	1600
July	1st to 10th	Night	3 3 10% 5% 4 5% 3 3 4 59%	3674
	11th to 20th	Night	4 6% 14% 5% 6% 6% 3 5% 6% 45%	4100
	21st to end	Night	1 15% 6% 10% 5% 5% 5% 5% 6% 7% 35%	4472
August	1st to 10th	Night	1 5% 4 8% 4 4 5% 4 6% 6% 53%	4035
	11th to 20th	Night	4 3 9% 5% 5% 6% 4 5% 5% 53%	4417
	21st to end	Night	1 4 3 8% 4 5% 4 3 4 5% 59%	5015
September	1st to 10th	Night	1 3 2 5% 3 4 4 3 4 6% 65%	2000
	11th to 20th	Night	4 6% 3 7% 4 4 4 2 4 7% 57%	2000
	21st to end	Night	3 5% 2 4 3 2 3 2 3 5% 67%	2263
October	1st to 10th	Night	3 3 2 4 3 2 2 2 3 6% 70%	3920
	11th to 20th	Night	3 5% 3 7% 4 4 3 3 4 5% 58%	3897
	21st to end	Night	5% 4 2 4 2 2 3 3 4 6% 65%	4253
November	1st to 10th	Night	7% 5% 2 4 3 3 3 3 5% 7% 59%	2487
	11th to 20th	Night	10% 5% 2 5% 3 3 4 4 6% 8% 51%	2730
	21st to end	Night	8% 5% 2 4 3 4 4 3 4 6% 57%	2840
December	1st to 10th	Night	9% 4 1 3 3 3 4 3 5% 6% 59%	4588
	11th to 20th	Night	21% 8% 2 5% 3 3 3 4 7% 8% 35%	5176
	21st to end	Night	24% 8% 3 7% 5% 4 4 3 5% 5% 33%	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	30% 5% 18% 25% 17% 5%	3937
	11th to 20th	Day	32% 6% 8% 15% 18% 20%	4100
	21st to end	Day	42% 5% 6% 15% 19% 14%	4510
February	1st to 10th	Day	39% 4 6% 20% 20% 10%	1800
	11th to 20th	Day	36% 6% 10% 20% 19% 8%	1885
	21st to end	Day	37% 6% 8% 24% 18% 7%	1650
March	1st to 10th	Day	43% 10% 7% 18% 20% 2	4858
	11th to 20th	Day	33% 4 15% 29% 11% 8%	5000
	21st to end	Day	41% 10% 15% 21% 12% 2	5588
April	1st to 10th	Day	33% 6% 13% 24% 20% 5%	2400
	11th to 20th	Day	29% 6% 14% 27% 19% 6%	2372
	21st to end	Day	28% 6% 14% 28% 20% 4	2400
May	1st to 10th	Day	17% 2 8% 31% 30% 12%	5500
	11th to 20th	Day	19% 5% 8% 34% 27% 6%	5929
	21st to end	Day	20% 4 9% 32% 33% 2	6904
June	1st to 10th	Day	25% 7% 10% 27% 24% 6%	2800
	11th to 20th	Day	20% 3 9% 24% 32% 13%	2800
	21st to end	Day	22% 2 4 17% 31% 24%	2800
July	1st to 10th	Day	19% 1 1 8% 21% 50%	6214
	11th to 20th	Day	25% 1 2 6% 19% 47%	5811
	21st to end	Day	24% 1 2 20% 30% 24%	6217
August	1st to 10th	Day	23% 1 2 13% 32% 28%	4206
	11th to 20th	Day	18% 1 1 12% 49% 19%	3941
	21st to end	Day	20% 1 3 12% 38% 25%	4311
September	1st to 10th	Day	17% 1 3 12% 31% 35%	2400
	11th to 20th	Day	24% 3 6% 24% 30% 14%	2400
	21st to end	Day	17% 2 5% 22% 36% 18%	2131
October	1st to 10th	Day	15% 3 5% 13% 30% 35%	3280
	11th to 20th	Day	22% 2 6% 26% 27% 17%	3261
	21st to end	Day	21% 2 6% 23% 32% 16%	3558
November	1st to 10th	Day	26% 5% 8% 24% 26% 11%	2029
	11th to 20th	Day	31% 5% 11% 22% 24% 8%	1890
	21st to end	Day	27% 4 10% 19% 29% 11%	1774
December	1st to 10th	Day	28% 3 6% 22% 33% 8%	2636
	11th to 20th	Day	45% 4 10% 19% 15% 8%	3028
	21st to end	Day	50% 4 11% 20% 11% 3	4004
January	1st to 10th	Night	28% 4 19% 27% 16% 5%	6018
	11th to 20th	Night	34% 5% 10% 16% 21% 14%	5880
	21st to end	Night	40% 4 8% 13% 18% 16%	6468
February	1st to 10th	Night	40% 3 7% 17% 20% 13%	2600
	11th to 20th	Night	36% 4 11% 19% 21% 8%	2412
	21st to end	Night	31% 4 11% 29% 18% 7%	1980
March	1st to 10th	Night	47% 7% 9% 16% 19% 3	5031
	11th to 20th	Night	35% 5% 11% 24% 17% 8%	5000
	21st to end	Night	42% 7% 12% 24% 13% 2	5385
April	1st to 10th	Night	32% 5% 13% 24% 20% 6%	2000
	11th to 20th	Night	33% 6% 10% 28% 16% 7%	1979
	21st to end	Night	26% 6% 11% 32% 19% 6%	2000
May	1st to 10th	Night	23% 2 8% 28% 29% 11%	4476
	11th to 20th	Night	35% 9% 8% 21% 20% 7%	3983
	21st to end	Night	24% 7% 9% 22% 36% 2	3972
June	1st to 10th	Night	28% 7% 9% 24% 24% 8%	1600
	11th to 20th	Night	26% 4 7% 16% 30% 18%	1600
	21st to end	Night	25% 5% 3 13% 28% 26%	1600
July	1st to 10th	Night	31% 1 6% 16% 46%	3634
	11th to 20th	Night	43% 1 2 5% 14% 35%	4046
	21st to end	Night	44% 2 2 11% 21% 21%	4345
August	1st to 10th	Night	35% 2 2 11% 27% 24%	2929
	11th to 20th	Night	33% 1 1 8% 36% 21%	3192
	21st to end	Night	30% 2 4 10% 37% 18%	3608
September	1st to 10th	Night	22% 2 3 11% 31% 33%	2000
	11th to 20th	Night	29% 3 4 21% 29% 14%	2000
	21st to end	Night	22% 2 3 19% 34% 20%	2263
October	1st to 10th	Night	19% 1 4 12% 25% 39%	3920
	11th to 20th	Night	30% 2 4 23% 22% 19%	3897
	21st to end	Night	24% 2 5% 22% 28% 19%	4253
November	1st to 10th	Night	26% 4 8% 25% 24% 14%	2487
	11th to 20th	Night	31% 4 11% 22% 23% 9%	2730
	21st to end	Night	29% 3 7% 21% 24% 15%	2840
December	1st to 10th	Night	27% 2 6% 24% 27% 13%	4588
	11th to 20th	Night	45% 4 11% 16% 18% 6%	5176
	21st to end	Night	52% 5% 11% 18% 10% 4	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	26% 7% 16% 29% 18% 5%	329
	11th to 20th	Day	17% 4 21% 28% 26% 5%	400
	21st to end	Day	15% 3 21% 31% 23% 7%	440
February	1st to 10th	Day	23% 7% 22% 20% 19% 9%	400
	11th to 20th	Day	14% 13% 23% 30% 16% 4	458
	21st to end	Day	12% 9% 25% 33% 18% 3	414
March	1st to 10th	Day	24% 10% 22% 27% 14% 2	500
	11th to 20th	Day	23% 8% 20% 26% 18% 6%	500
	21st to end	Day	16% 11% 27% 28% 15% 3	550
April	1st to 10th	Day	13% 10% 25% 30% 16% 5%	500
	11th to 20th	Day	10% 12% 25% 32% 14% 6%	492
	21st to end	Day	6% 9% 26% 31% 21% 7%	500
May	1st to 10th	Day	8% 8% 25% 35% 17% 6%	503
	11th to 20th	Day	9% 8% 25% 30% 23% 6%	528
	21st to end	Day	10% 7% 20% 33% 23% 7%	573
June	1st to 10th	Day	6% 5% 20% 37% 25% 8%	530
	11th to 20th	Day	3 5% 24% 40% 24% 4	530
	21st to end	Day	2 2 18% 40% 29% 9%	530
July	1st to 10th	Day	4 1 13% 39% 33% 10%	526
	11th to 20th	Day	3 1 6% 41% 37% 12%	524
	21st to end	Day	3 9% 40% 36% 12%	575
August	1st to 10th	Day	5% 12% 41% 33% 8%	527
	11th to 20th	Day	2 1 13% 42% 32% 10%	517
	21st to end	Day	4 1 9% 36% 37% 13%	583
September	1st to 10th	Day	3 2 12% 37% 32% 14%	530
	11th to 20th	Day	6% 3 17% 36% 26% 12%	530
	21st to end	Day	5% 4 13% 32% 36% 11%	530
October	1st to 10th	Day	5% 4 17% 30% 30% 14%	530
	11th to 20th	Day	5% 4 18% 27% 31% 14%	530
	21st to end	Day	9% 4 17% 27% 30% 13%	583
November	1st to 10th	Day	10% 4 16% 26% 30% 14%	505
	11th to 20th	Day	12% 4 17% 32% 25% 10%	330
	21st to end	Day	10% 2 17% 34% 28% 8%	269
December	1st to 10th	Day	15% 7% 15% 25% 27% 11%	204
	11th to 20th	Day	17% 7% 17% 24% 25% 11%	200
	21st to end	Day	22% 6% 23% 23% 20% 6%	220
January	1st to 10th	Night	19% 4 11% 36% 25% 6%	800
	11th to 20th	Night	16% 2 12% 36% 27% 7%	800
	21st to end	Night	17% 3 9% 37% 29% 4	880
February	1st to 10th	Night	21% 6% 12% 31% 24% 6%	800
	11th to 20th	Night	17% 4 15% 35% 23% 6%	784
	21st to end	Night	15% 5% 14% 40% 24% 3	660
March	1st to 10th	Night	19% 6% 13% 35% 24% 3	800
	11th to 20th	Night	19% 4 14% 32% 26% 6%	800
	21st to end	Night	17% 4 16% 36% 25% 3	880
April	1st to 10th	Night	13% 4 14% 38% 27% 4	800
	11th to 20th	Night	12% 7% 13% 36% 26% 6%	791
	21st to end	Night	9% 4 18% 33% 28% 9%	800
May	1st to 10th	Night	8% 3 14% 37% 31% 8%	771
	11th to 20th	Night	6% 2 8% 35% 38% 11%	552
	21st to end	Night	8% 5% 11% 33% 36% 7%	433
June	1st to 10th	Night	6% 1 7% 39% 38% 10%	400
	11th to 20th	Night	3 1 9% 40% 42% 5%	400
	21st to end	Night	5% 4 34% 47% 11%	400
July	1st to 10th	Night	2 1 2 23% 60% 13%	398
	11th to 20th	Night	5% 1 18% 53% 23%	582
	21st to end	Night	7% 2 19% 53% 19%	657
August	1st to 10th	Night	8% 1 2 26% 48% 15%	597
	11th to 20th	Night	4 1 2 31% 47% 14%	753
	21st to end	Night	5% 1 2 24% 49% 19%	880
September	1st to 10th	Night	4 4 25% 50% 18%	800
	11th to 20th	Night	6% 1 5% 29% 44% 15%	800
	21st to end	Night	7% 1 6% 25% 45% 15%	800
October	1st to 10th	Night	7% 2 6% 33% 37% 15%	800
	11th to 20th	Night	7% 2 6% 30% 41% 14%	800
	21st to end	Night	8% 1 7% 33% 34% 17%	880
November	1st to 10th	Night	10% 1 10% 29% 38% 12%	820
	11th to 20th	Night	14% 1 8% 36% 34% 6%	840
	21st to end	Night	10% 2 10% 40% 30% 7%	840
December	1st to 10th	Night	17% 2 10% 36% 27% 7%	808
	11th to 20th	Night	22% 3 12% 37% 23% 4	800
	21st to end	Night	21% 5% 15% 38% 18% 4	880

Page generated at 2025-11-29 22:00:53. Version ac07

Copyright © 2025 Attilla Danko.