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 2024-01-13.

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	8% 5% 2 4 4 2 4 4 5% 8% 55%	74687
	Night	10% 6% 2 5% 4 3 4 4 5% 7% 51%	75049
Transparency	Day	29% 4 8% 21% 24% 14%	74408
	Night	33% 4 9% 19% 21% 14%	74850
Seeing	Day	9% 5% 18% 32% 26% 9%	14333
	Night	11% 3 9% 32% 34% 10%	23822

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	19% 5% 2 5% 6% 3 4 5% 6% 7% 39%	1641
	11th to 20th	Day	12% 4 2 3 5% 3 4 4 5% 8% 51%	1639
	21st to end	Day	14% 7% 3 5% 4 3 4 4 6% 7% 44%	1782
February	1st to 10th	Day	18% 5% 2 4 4 2 3 4 5% 6% 48%	1620
	11th to 20th	Day	12% 4 2 5% 5% 3 5% 4 5% 7% 47%	1683
	21st to end	Day	14% 4 2 5% 5% 3 4 4 4 6% 48%	1480
March	1st to 10th	Day	17% 7% 3 5% 5% 3 5% 5% 6% 8% 37%	1934
	11th to 20th	Day	17% 7% 3 5% 5% 4 4 5% 6% 8% 37%	1980
	21st to end	Day	11% 5% 2 4 5% 4 5% 5% 4 7% 47%	2216
April	1st to 10th	Day	12% 6% 2 4 4 2 5% 4 5% 8% 47%	2160
	11th to 20th	Day	9% 5% 2 5% 4 2 4 3 5% 7% 54%	2129
	21st to end	Day	7% 4 2 5% 4 3 4 5% 5% 8% 55%	2160
May	1st to 10th	Day	5% 5% 2 4 3 2 3 4 5% 9% 57%	2181
	11th to 20th	Day	5% 4 2 4 4 2 3 4 4 9% 59%	2334
	21st to end	Day	5% 8% 3 5% 5% 3 4 4 5% 7% 51%	2723
June	1st to 10th	Day	4 5% 2 4 4 2 4 4 5% 10% 56%	2520
	11th to 20th	Day	2 4 2 3 4 2 3 3 5% 8% 66%	2520
	21st to end	Day	2 3 2 4 4 2 3 3 4 8% 65%	2520
July	1st to 10th	Day	1 4 1 4 4 2 3 4 5% 7% 64%	2509
	11th to 20th	Day	3 3 2 3 4 3 3 4 5% 8% 63%	2340
	21st to end	Day	1 2 2 4 4 3 3 3 5% 8% 66%	2546
August	1st to 10th	Day	1 6% 2 5% 4 3 3 3 4 9% 60%	2328
	11th to 20th	Day	3 2 4 3 2 3 4 4 8% 67%	2156
	21st to end	Day	1 5% 2 4 3 2 4 3 4 9% 63%	2376
September	1st to 10th	Day	2 3 1 3 3 2 2 2 4 6% 73%	2160
	11th to 20th	Day	3 5% 2 4 4 2 3 4 5% 9% 61%	2160
	21st to end	Day	3 2 1 2 2 1 3 2 4 5% 74%	1919
October	1st to 10th	Day	3 2 1 3 4 2 3 4 5% 7% 67%	1800
	11th to 20th	Day	5% 3 1 3 3 2 3 3 4 7% 64%	1800
	21st to end	Day	7% 4 1 3 4 2 3 4 4 7% 61%	1980
November	1st to 10th	Day	8% 4 2 4 4 3 4 4 5% 7% 55%	1833
	11th to 20th	Day	9% 4 2 4 4 2 4 5% 6% 7% 53%	1710
	21st to end	Day	12% 3 2 3 3 2 4 3 4 7% 57%	1606
December	1st to 10th	Day	14% 5% 2 6% 4 2 3 5% 5% 7% 47%	1520
	11th to 20th	Day	27% 8% 2 5% 4 3 3 4 5% 6% 33%	1928
	21st to end	Day	18% 8% 2 5% 5% 3 5% 5% 5% 7% 37%	2794
January	1st to 10th	Night	17% 6% 2 5% 5% 3 4 4 5% 7% 43%	2521
	11th to 20th	Night	13% 5% 2 4 3 3 5% 5% 5% 7% 48%	2366
	21st to end	Night	15% 5% 2 4 3 2 4 5% 5% 7% 47%	2574
February	1st to 10th	Night	17% 6% 2 4 3 2 3 4 4 5% 50%	2340
	11th to 20th	Night	12% 5% 1 5% 4 3 5% 4 6% 7% 48%	2163
	21st to end	Night	14% 6% 2 4 3 1 3 4 5% 7% 52%	1776
March	1st to 10th	Night	17% 8% 3 6% 5% 3 4 4 5% 5% 39%	2021
	11th to 20th	Night	17% 5% 2 4 5% 3 4 4 5% 10% 41%	1980
	21st to end	Night	14% 6% 3 4 4 2 3 4 4 8% 49%	2138
April	1st to 10th	Night	12% 5% 2 5% 4 3 4 5% 5% 7% 47%	1800
	11th to 20th	Night	10% 5% 2 5% 4 3 4 4 5% 7% 51%	1779
	21st to end	Night	6% 4 2 2 3 3 4 4 6% 10% 56%	1800
May	1st to 10th	Night	5% 6% 2 5% 4 3 4 4 5% 8% 55%	1775
	11th to 20th	Night	3 5% 3 5% 5% 3 5% 5% 6% 9% 50%	1558
	21st to end	Night	5% 10% 3 6% 4 3 4 4 5% 8% 49%	1552
June	1st to 10th	Night	3 5% 3 4 6% 3 5% 4 4 7% 55%	1440
	11th to 20th	Night	1 5% 2 6% 5% 3 4 4 4 7% 59%	1440
	21st to end	Night	2 5% 3 6% 6% 2 4 4 4 8% 55%	1440
July	1st to 10th	Night	1 5% 3 7% 6% 3 4 4 3 7% 57%	1446
	11th to 20th	Night	1 4 4 7% 5% 3 4 5% 7% 9% 52%	1620
	21st to end	Night	1 7% 4 8% 8% 5% 6% 6% 7% 9% 38%	1773
August	1st to 10th	Night	2 9% 4 11% 7% 4 4 5% 5% 7% 41%	1611
	11th to 20th	Night	1 6% 3 8% 6% 2 3 5% 7% 8% 52%	1745
	21st to end	Night	1 8% 5% 8% 6% 2 3 5% 5% 7% 51%	1980
September	1st to 10th	Night	1 3 2 4 4 3 4 4 4 7% 65%	1800
	11th to 20th	Night	4 5% 2 5% 5% 3 4 3 4 7% 59%	1800
	21st to end	Night	2 4 1 3 3 2 3 3 3 6% 69%	2035
October	1st to 10th	Night	4 5% 2 4 4 1 3 3 3 5% 64%	2160
	11th to 20th	Night	5% 5% 2 4 4 2 4 4 5% 7% 58%	2160
	21st to end	Night	6% 4 2 4 4 2 3 3 4 6% 61%	2376
November	1st to 10th	Night	8% 5% 2 4 3 2 3 3 5% 7% 57%	2243
	11th to 20th	Night	9% 4 2 4 3 2 3 4 6% 9% 53%	2470
	21st to end	Night	8% 4 2 4 4 3 4 4 4 5% 59%	2569
December	1st to 10th	Night	15% 5% 2 4 4 2 4 4 4 6% 51%	2660
	11th to 20th	Night	28% 8% 3 4 4 2 3 3 5% 6% 35%	3276
	21st to end	Night	19% 6% 2 5% 5% 3 4 5% 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	42% 6% 13% 21% 12% 6%	1641
	11th to 20th	Day	32% 6% 11% 19% 18% 14%	1639
	21st to end	Day	38% 7% 10% 13% 23% 9%	1782
February	1st to 10th	Day	37% 4 7% 21% 21% 11%	1620
	11th to 20th	Day	36% 5% 8% 20% 22% 9%	1692
	21st to end	Day	36% 6% 9% 23% 18% 7%	1480
March	1st to 10th	Day	43% 7% 8% 21% 16% 4	1934
	11th to 20th	Day	42% 6% 10% 19% 18% 4	1980
	21st to end	Day	35% 8% 14% 22% 18% 4	2216
April	1st to 10th	Day	34% 6% 13% 21% 20% 5%	2160
	11th to 20th	Day	28% 6% 14% 27% 19% 7%	2132
	21st to end	Day	28% 5% 13% 28% 21% 5%	2160
May	1st to 10th	Day	25% 5% 11% 27% 27% 5%	2181
	11th to 20th	Day	24% 4 11% 31% 22% 8%	2337
	21st to end	Day	32% 6% 15% 24% 20% 4	2724
June	1st to 10th	Day	25% 7% 10% 27% 24% 7%	2520
	11th to 20th	Day	20% 3 9% 24% 30% 13%	2520
	21st to end	Day	22% 2 5% 18% 31% 22%	2520
July	1st to 10th	Day	21% 1 3 13% 29% 32%	2440
	11th to 20th	Day	24% 2 6% 15% 25% 28%	2271
	21st to end	Day	20% 2 3 17% 30% 28%	2405
August	1st to 10th	Day	25% 2 3 17% 30% 23%	2312
	11th to 20th	Day	20% 2 2 18% 30% 28%	2156
	21st to end	Day	24% 2 3 17% 31% 23%	2376
September	1st to 10th	Day	17% 1 2 12% 31% 36%	2160
	11th to 20th	Day	23% 3 6% 23% 30% 15%	2160
	21st to end	Day	15% 3 5% 23% 36% 18%	1919
October	1st to 10th	Day	17% 5% 8% 20% 27% 24%	1800
	11th to 20th	Day	22% 4 8% 22% 21% 23%	1800
	21st to end	Day	24% 4 8% 22% 22% 20%	1980
November	1st to 10th	Day	27% 6% 7% 24% 25% 11%	1833
	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	40% 5% 13% 22% 15% 6%	2521
	11th to 20th	Night	35% 5% 13% 17% 16% 14%	2366
	21st to end	Night	36% 6% 12% 16% 18% 13%	2574
February	1st to 10th	Night	38% 3 7% 18% 20% 14%	2340
	11th to 20th	Night	35% 4 10% 19% 23% 9%	2165
	21st to end	Night	31% 4 10% 30% 18% 7%	1776
March	1st to 10th	Night	45% 5% 11% 17% 16% 5%	2021
	11th to 20th	Night	39% 4 9% 22% 20% 6%	1980
	21st to end	Night	35% 5% 13% 22% 18% 6%	2138
April	1st to 10th	Night	34% 5% 14% 21% 20% 6%	1800
	11th to 20th	Night	32% 5% 11% 28% 17% 8%	1779
	21st to end	Night	25% 6% 12% 32% 20% 6%	1800
May	1st to 10th	Night	29% 3 9% 25% 24% 10%	1775
	11th to 20th	Night	29% 5% 12% 25% 22% 7%	1559
	21st to end	Night	34% 6% 13% 23% 21% 4	1552
June	1st to 10th	Night	28% 6% 9% 23% 24% 9%	1440
	11th to 20th	Night	26% 4 8% 15% 29% 18%	1440
	21st to end	Night	27% 5% 4 15% 27% 23%	1440
July	1st to 10th	Night	27% 3 1 10% 26% 32%	1406
	11th to 20th	Night	30% 3 5% 12% 23% 27%	1566
	21st to end	Night	41% 4 3 11% 20% 21%	1674
August	1st to 10th	Night	42% 4 3 12% 20% 20%	1602
	11th to 20th	Night	30% 3 2 12% 26% 27%	1745
	21st to end	Night	34% 3 3 11% 24% 24%	1980
September	1st to 10th	Night	21% 3 3 10% 31% 33%	1800
	11th to 20th	Night	28% 3 4 20% 30% 15%	1800
	21st to end	Night	19% 2 3 20% 34% 20%	2035
October	1st to 10th	Night	25% 3 7% 17% 22% 27%	2160
	11th to 20th	Night	28% 3 6% 20% 21% 22%	2160
	21st to end	Night	26% 3 5% 21% 22% 22%	2376
November	1st to 10th	Night	27% 4 8% 24% 23% 14%	2243
	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	17% 5% 21% 25% 27% 5%	347
	21st to end	Day	15% 3 20% 30% 24% 7%	374
February	1st to 10th	Day	19% 8% 22% 20% 21% 11%	340
	11th to 20th	Day	12% 14% 23% 30% 17% 5%	385
	21st to end	Day	12% 11% 26% 31% 17% 3	346
March	1st to 10th	Day	25% 10% 22% 27% 14% 3	420
	11th to 20th	Day	23% 7% 20% 26% 18% 6%	420
	21st to end	Day	15% 10% 27% 29% 15% 3	462
April	1st to 10th	Day	14% 9% 25% 32% 16% 4	420
	11th to 20th	Day	10% 14% 25% 32% 14% 5%	412
	21st to end	Day	6% 10% 27% 31% 20% 6%	420
May	1st to 10th	Day	8% 9% 24% 35% 17% 6%	423
	11th to 20th	Day	9% 8% 25% 31% 21% 6%	448
	21st to end	Day	11% 8% 20% 32% 21% 7%	485
June	1st to 10th	Day	6% 5% 20% 35% 24% 9%	450
	11th to 20th	Day	4 6% 22% 39% 25% 4	450
	21st to end	Day	3 2 18% 40% 29% 8%	450
July	1st to 10th	Day	5% 1 12% 39% 35% 9%	446
	11th to 20th	Day	3 1 6% 41% 38% 11%	444
	21st to end	Day	2 8% 40% 38% 13%	487
August	1st to 10th	Day	5% 12% 42% 34% 6%	447
	11th to 20th	Day	2 11% 41% 35% 11%	437
	21st to end	Day	4 1 8% 37% 38% 12%	495
September	1st to 10th	Day	2 2 11% 36% 33% 15%	450
	11th to 20th	Day	5% 2 18% 36% 28% 12%	450
	21st to end	Day	4 5% 12% 31% 38% 11%	450
October	1st to 10th	Day	4 4 17% 31% 30% 14%	450
	11th to 20th	Day	6% 3 18% 28% 32% 14%	450
	21st to end	Day	10% 4 16% 26% 30% 14%	495
November	1st to 10th	Day	10% 4 16% 24% 31% 14%	429
	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	15% 3 13% 34% 27% 8%	728
	21st to end	Night	17% 4 9% 36% 30% 5%	792
February	1st to 10th	Night	19% 6% 11% 31% 27% 7%	720
	11th to 20th	Night	15% 5% 16% 34% 24% 7%	704
	21st to end	Night	16% 5% 16% 36% 23% 3	592
March	1st to 10th	Night	19% 6% 12% 34% 25% 3	720
	11th to 20th	Night	19% 3 13% 32% 27% 7%	720
	21st to end	Night	16% 4 16% 36% 25% 3	792
April	1st to 10th	Night	14% 4 13% 38% 27% 3	720
	11th to 20th	Night	12% 7% 15% 35% 25% 6%	711
	21st to end	Night	8% 4 18% 33% 28% 9%	720
May	1st to 10th	Night	8% 3 14% 36% 31% 7%	695
	11th to 20th	Night	5% 2 8% 33% 39% 12%	496
	21st to end	Night	9% 5% 12% 31% 35% 7%	389
June	1st to 10th	Night	6% 1 7% 39% 38% 9%	360
	11th to 20th	Night	3 1 9% 39% 42% 6%	360
	21st to end	Night	5% 3 33% 49% 10%	360
July	1st to 10th	Night	2 1 1 25% 58% 13%	356
	11th to 20th	Night	4 1 19% 53% 23%	522
	21st to end	Night	6% 2 19% 53% 20%	591
August	1st to 10th	Night	9% 1 2 27% 48% 13%	537
	11th to 20th	Night	4 1 2 30% 48% 14%	677
	21st to end	Night	5% 1 2 24% 50% 19%	792
September	1st to 10th	Night	3 4 25% 50% 19%	720
	11th to 20th	Night	6% 1 5% 30% 44% 15%	720
	21st to end	Night	5% 2 6% 25% 46% 16%	720
October	1st to 10th	Night	6% 2 6% 34% 37% 15%	720
	11th to 20th	Night	8% 2 6% 28% 42% 14%	720
	21st to end	Night	8% 1 7% 31% 34% 18%	792
November	1st to 10th	Night	10% 1 10% 31% 37% 12%	740
	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 2024-01-09 22:25:03. Version ac07

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