University of Califoria San Diego
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the University of Califoria San Diego clear sky chart from 2005-11-06 to 2024-02-20.

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 4 4 2 4 4 5% 8% 57%	79005
	Night	8% 8% 3 5% 5% 3 4 4 5% 7% 49%	80271
Transparency	Day	25% 5% 8% 17% 24% 21%	78741
	Night	32% 6% 8% 15% 19% 19%	80058
Seeing	Day	7% 4 20% 37% 25% 7%	14768
	Night	10% 3 8% 30% 37% 12%	25204

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	8% 4 1 3 3 2 3 3 4 7% 63%	2858
	11th to 20th	Day	7% 6% 2 4 4 2 4 4 5% 8% 55%	3080
	21st to end	Day	12% 4 2 5% 5% 2 4 4 5% 8% 49%	3498
February	1st to 10th	Day	9% 4 2 5% 4 3 4 4 4 6% 57%	1900
	11th to 20th	Day	6% 5% 3 4 4 3 4 5% 5% 9% 54%	1841
	21st to end	Day	9% 4 3 5% 5% 3 4 5% 5% 8% 51%	1480
March	1st to 10th	Day	7% 4 2 5% 5% 2 4 4 6% 10% 50%	1800
	11th to 20th	Day	7% 4 3 4 4 2 5% 4 5% 9% 53%	1874
	21st to end	Day	5% 4 2 4 4 2 4 4 4 8% 58%	2309
April	1st to 10th	Day	7% 5% 3 4 4 2 4 5% 5% 9% 52%	2160
	11th to 20th	Day	4 4 2 4 3 2 3 4 5% 8% 61%	2129
	21st to end	Day	6% 4 2 4 4 2 3 3 5% 9% 59%	2160
May	1st to 10th	Day	7% 6% 2 4 4 3 4 4 5% 7% 53%	2245
	11th to 20th	Day	6% 7% 3 4 3 2 4 3 4 8% 57%	2301
	21st to end	Day	6% 6% 2 4 4 3 3 4 5% 8% 55%	2530
June	1st to 10th	Day	7% 8% 3 5% 4 2 4 4 4 8% 52%	2340
	11th to 20th	Day	4 5% 3 4 3 2 3 4 4 8% 61%	2340
	21st to end	Day	3 7% 2 5% 4 2 3 4 4 6% 61%	2340
July	1st to 10th	Day	3 7% 3 5% 3 2 4 5% 5% 9% 54%	2340
	11th to 20th	Day	2 5% 2 5% 3 2 4 4 6% 10% 58%	2340
	21st to end	Day	3 5% 2 4 3 2 4 5% 5% 9% 58%	2402
August	1st to 10th	Day	2 3 1 4 3 2 3 4 4 7% 65%	2149
	11th to 20th	Day	2 4 2 3 3 2 2 3 3 7% 67%	2122
	21st to end	Day	2 4 1 4 4 2 4 3 5% 8% 63%	2376
September	1st to 10th	Day	4 3 1 3 4 2 3 4 5% 8% 63%	2152
	11th to 20th	Day	3 4 1 5% 3 1 3 3 3 7% 66%	1980
	21st to end	Day	3 3 2 4 2 2 3 3 4 7% 67%	1980
October	1st to 10th	Day	4 4 2 4 3 2 3 4 4 7% 62%	1980
	11th to 20th	Day	4 4 2 3 4 2 4 4 4 6% 64%	1973
	21st to end	Day	3 4 2 3 3 1 3 2 3 7% 69%	1907
November	1st to 10th	Day	4 4 2 3 4 2 4 3 3 8% 63%	1674
	11th to 20th	Day	4 4 2 4 3 2 3 4 5% 7% 63%	1710
	21st to end	Day	6% 4 2 4 3 2 3 5% 5% 8% 58%	1710
December	1st to 10th	Day	6% 4 2 4 4 3 4 5% 5% 7% 55%	1710
	11th to 20th	Day	11% 5% 2 5% 5% 3 5% 5% 6% 7% 45%	2169
	21st to end	Day	9% 6% 3 5% 6% 5% 6% 6% 6% 8% 42%	3146
January	1st to 10th	Night	7% 4 2 4 4 2 3 3 4 6% 62%	4097
	11th to 20th	Night	9% 6% 3 6% 4 3 4 4 4 8% 49%	3893
	21st to end	Night	15% 6% 2 4 4 3 4 4 4 7% 48%	4180
February	1st to 10th	Night	8% 6% 2 5% 5% 2 3 3 4 6% 56%	2280
	11th to 20th	Night	10% 8% 3 4 4 3 4 4 4 5% 52%	2211
	21st to end	Night	10% 6% 2 5% 4 3 3 4 5% 6% 51%	1776
March	1st to 10th	Night	9% 7% 4 5% 5% 2 4 4 5% 8% 46%	2160
	11th to 20th	Night	10% 9% 3 6% 4 3 4 3 6% 8% 44%	2084
	21st to end	Night	8% 6% 2 5% 5% 2 5% 4 5% 7% 49%	2043
April	1st to 10th	Night	10% 8% 4 6% 4 3 3 3 4 9% 46%	1800
	11th to 20th	Night	11% 8% 3 5% 4 2 2 3 3 6% 51%	1779
	21st to end	Night	9% 11% 4 6% 4 2 4 4 5% 6% 44%	1800
May	1st to 10th	Night	9% 11% 4 6% 6% 3 4 4 5% 10% 38%	1711
	11th to 20th	Night	10% 15% 6% 7% 6% 3 4 3 4 7% 36%	1594
	21st to end	Night	12% 15% 3 6% 5% 2 3 3 5% 8% 38%	1746
June	1st to 10th	Night	11% 15% 6% 7% 6% 3 5% 4 6% 9% 29%	1620
	11th to 20th	Night	5% 13% 4 6% 5% 3 5% 4 6% 11% 39%	1620
	21st to end	Night	5% 14% 4 8% 5% 3 5% 4 5% 10% 38%	1620
July	1st to 10th	Night	6% 13% 5% 8% 4 2 4 3 7% 9% 40%	1620
	11th to 20th	Night	3 9% 4 8% 4 3 5% 5% 7% 9% 44%	1620
	21st to end	Night	5% 10% 4 6% 6% 4 5% 5% 6% 10% 41%	1912
August	1st to 10th	Night	3 8% 4 6% 4 2 3 4 5% 9% 50%	1790
	11th to 20th	Night	4 10% 4 7% 5% 2 4 5% 5% 8% 46%	1780
	21st to end	Night	4 7% 2 6% 5% 3 5% 4 5% 9% 51%	1980
September	1st to 10th	Night	5% 8% 2 6% 4 3 3 4 5% 8% 52%	1806
	11th to 20th	Night	5% 6% 2 4 4 2 3 3 4 7% 59%	1980
	21st to end	Night	4 7% 3 4 3 2 3 3 5% 7% 58%	1980
October	1st to 10th	Night	4 6% 2 4 5% 3 3 3 5% 8% 57%	1980
	11th to 20th	Night	7% 7% 2 4 4 2 4 4 4 6% 56%	1984
	21st to end	Night	5% 6% 2 3 4 2 3 3 3 7% 61%	2448
November	1st to 10th	Night	5% 6% 2 4 4 2 4 4 3 6% 60%	2405
	11th to 20th	Night	3 6% 2 3 4 2 4 3 4 6% 63%	2470
	21st to end	Night	6% 5% 2 5% 4 3 5% 5% 5% 7% 53%	2470
December	1st to 10th	Night	9% 5% 3 4 5% 2 3 4 5% 7% 52%	2470
	11th to 20th	Night	11% 7% 3 5% 5% 2 4 5% 5% 8% 45%	3041
	21st to end	Night	11% 6% 2 6% 6% 4 6% 5% 6% 6% 42%	4521

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	22% 5% 6% 25% 23% 20%	2859
	11th to 20th	Day	28% 6% 14% 23% 15% 15%	3080
	21st to end	Day	34% 4 8% 16% 23% 16%	3498
February	1st to 10th	Day	29% 4 6% 15% 20% 26%	1900
	11th to 20th	Day	28% 5% 9% 14% 26% 19%	1851
	21st to end	Day	31% 4 9% 15% 23% 16%	1480
March	1st to 10th	Day	29% 5% 10% 19% 18% 20%	1800
	11th to 20th	Day	28% 5% 10% 18% 23% 15%	1874
	21st to end	Day	25% 5% 11% 26% 20% 13%	2309
April	1st to 10th	Day	29% 6% 10% 25% 22% 9%	2160
	11th to 20th	Day	22% 5% 9% 20% 27% 17%	2132
	21st to end	Day	23% 5% 8% 24% 28% 12%	2160
May	1st to 10th	Day	29% 6% 8% 19% 23% 14%	2245
	11th to 20th	Day	27% 5% 7% 24% 26% 11%	2304
	21st to end	Day	28% 6% 8% 25% 27% 6%	2530
June	1st to 10th	Day	30% 7% 7% 19% 25% 12%	2340
	11th to 20th	Day	22% 6% 6% 11% 30% 26%	2340
	21st to end	Day	24% 5% 6% 7% 23% 36%	2340
July	1st to 10th	Day	24% 8% 6% 8% 22% 32%	2276
	11th to 20th	Day	20% 7% 12% 20% 23% 18%	2271
	21st to end	Day	21% 8% 13% 22% 26% 9%	2269
August	1st to 10th	Day	18% 6% 6% 16% 29% 26%	2135
	11th to 20th	Day	17% 6% 6% 17% 29% 25%	2122
	21st to end	Day	19% 5% 6% 13% 32% 25%	2376
September	1st to 10th	Day	19% 5% 7% 11% 31% 28%	2152
	11th to 20th	Day	18% 5% 6% 8% 31% 32%	1980
	21st to end	Day	18% 4 6% 12% 33% 28%	1980
October	1st to 10th	Day	22% 5% 5% 17% 23% 28%	1980
	11th to 20th	Day	22% 4 6% 12% 24% 31%	1973
	21st to end	Day	18% 4 5% 13% 23% 38%	1907
November	1st to 10th	Day	21% 4 6% 19% 18% 32%	1674
	11th to 20th	Day	21% 4 7% 15% 18% 35%	1710
	21st to end	Day	23% 4 9% 15% 20% 30%	1710
December	1st to 10th	Day	27% 5% 10% 16% 18% 24%	1710
	11th to 20th	Day	36% 6% 10% 14% 19% 15%	2168
	21st to end	Day	37% 7% 10% 21% 14% 11%	3146
January	1st to 10th	Night	23% 5% 8% 18% 29% 18%	4097
	11th to 20th	Night	33% 6% 11% 21% 13% 15%	3893
	21st to end	Night	36% 6% 8% 12% 20% 18%	4180
February	1st to 10th	Night	30% 4 7% 14% 19% 25%	2280
	11th to 20th	Night	35% 4 6% 13% 21% 21%	2212
	21st to end	Night	33% 4 6% 15% 25% 17%	1776
March	1st to 10th	Night	36% 5% 8% 17% 16% 18%	2160
	11th to 20th	Night	38% 5% 7% 16% 19% 15%	2084
	21st to end	Night	33% 6% 10% 20% 19% 10%	2043
April	1st to 10th	Night	36% 6% 9% 21% 17% 11%	1800
	11th to 20th	Night	34% 4 8% 16% 22% 16%	1779
	21st to end	Night	38% 7% 8% 17% 19% 11%	1800
May	1st to 10th	Night	41% 8% 9% 15% 14% 13%	1711
	11th to 20th	Night	48% 7% 7% 15% 12% 12%	1595
	21st to end	Night	45% 6% 7% 19% 16% 7%	1746
June	1st to 10th	Night	48% 10% 8% 8% 16% 10%	1620
	11th to 20th	Night	36% 10% 6% 10% 20% 18%	1620
	21st to end	Night	40% 8% 5% 7% 15% 26%	1620
July	1st to 10th	Night	35% 9% 7% 9% 17% 23%	1575
	11th to 20th	Night	30% 10% 7% 17% 23% 13%	1566
	21st to end	Night	35% 10% 9% 20% 19% 6%	1806
August	1st to 10th	Night	27% 8% 9% 14% 24% 18%	1780
	11th to 20th	Night	30% 9% 6% 15% 24% 15%	1780
	21st to end	Night	26% 6% 7% 14% 24% 23%	1980
September	1st to 10th	Night	29% 7% 6% 12% 20% 25%	1806
	11th to 20th	Night	23% 4 6% 10% 31% 27%	1980
	21st to end	Night	24% 5% 5% 12% 32% 22%	1980
October	1st to 10th	Night	25% 5% 7% 17% 20% 26%	1980
	11th to 20th	Night	28% 5% 6% 13% 19% 28%	1984
	21st to end	Night	25% 4 3 13% 21% 35%	2448
November	1st to 10th	Night	23% 6% 7% 16% 17% 32%	2405
	11th to 20th	Night	24% 3 5% 13% 18% 37%	2470
	21st to end	Night	29% 5% 7% 16% 16% 27%	2470
December	1st to 10th	Night	31% 5% 8% 16% 16% 25%	2470
	11th to 20th	Night	36% 6% 9% 15% 20% 15%	3041
	21st to end	Night	38% 9% 13% 17% 11% 13%	4521

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	16% 4 15% 37% 24% 4	270
	11th to 20th	Day	10% 6% 22% 36% 22% 4	399
	21st to end	Day	9% 7% 26% 34% 19% 5%	506
February	1st to 10th	Day	12% 7% 25% 31% 20% 5%	460
	11th to 20th	Day	7% 8% 32% 35% 16% 3	452
	21st to end	Day	10% 10% 33% 33% 13% 1	346
March	1st to 10th	Day	11% 7% 34% 32% 14% 2	420
	11th to 20th	Day	12% 5% 29% 34% 16% 4	420
	21st to end	Day	6% 11% 34% 35% 13% 2	462
April	1st to 10th	Day	9% 5% 30% 35% 16% 5%	420
	11th to 20th	Day	4 8% 35% 36% 12% 5%	412
	21st to end	Day	7% 8% 25% 36% 18% 6%	420
May	1st to 10th	Day	11% 5% 27% 35% 18% 4	423
	11th to 20th	Day	9% 6% 24% 38% 20% 4	448
	21st to end	Day	8% 7% 27% 35% 20% 3	485
June	1st to 10th	Day	7% 5% 18% 36% 28% 5%	450
	11th to 20th	Day	5% 2 16% 42% 27% 7%	450
	21st to end	Day	7% 1 11% 42% 32% 7%	450
July	1st to 10th	Day	5% 1 5% 42% 37% 9%	446
	11th to 20th	Day	3 3 36% 43% 15%	444
	21st to end	Day	4 1 4 36% 40% 16%	487
August	1st to 10th	Day	2 1 6% 40% 38% 13%	447
	11th to 20th	Day	4 5% 41% 38% 12%	437
	21st to end	Day	4 6% 39% 40% 10%	495
September	1st to 10th	Day	7% 9% 45% 34% 5%	450
	11th to 20th	Day	6% 2 15% 47% 24% 5%	450
	21st to end	Day	5% 3 17% 41% 28% 6%	450
October	1st to 10th	Day	4 6% 20% 42% 23% 5%	450
	11th to 20th	Day	5% 3 23% 40% 23% 6%	450
	21st to end	Day	5% 2 19% 38% 27% 9%	422
November	1st to 10th	Day	4 6% 16% 36% 29% 9%	270
	11th to 20th	Day	7% 4 16% 36% 29% 8%	270
	21st to end	Day	7% 7% 19% 35% 25% 7%	270
December	1st to 10th	Day	9% 3 20% 37% 24% 7%	270
	11th to 20th	Day	6% 4 18% 35% 27% 10%	270
	21st to end	Day	13% 8% 21% 34% 19% 6%	297
January	1st to 10th	Night	10% 3 12% 37% 29% 9%	760
	11th to 20th	Night	9% 3 9% 40% 33% 7%	760
	21st to end	Night	12% 4 12% 39% 28% 5%	836
February	1st to 10th	Night	10% 3 12% 34% 32% 9%	760
	11th to 20th	Night	12% 5% 14% 36% 27% 6%	742
	21st to end	Night	12% 5% 12% 42% 27% 3	592
March	1st to 10th	Night	13% 3 12% 41% 28% 4	720
	11th to 20th	Night	13% 5% 10% 32% 31% 9%	720
	21st to end	Night	9% 4 11% 38% 32% 5%	792
April	1st to 10th	Night	13% 3 10% 37% 31% 6%	720
	11th to 20th	Night	12% 4 14% 33% 29% 7%	711
	21st to end	Night	13% 4 8% 31% 34% 9%	720
May	1st to 10th	Night	12% 5% 10% 30% 35% 7%	631
	11th to 20th	Night	14% 5% 11% 29% 34% 8%	531
	21st to end	Night	16% 5% 11% 25% 37% 6%	584
June	1st to 10th	Night	12% 5% 9% 29% 36% 8%	540
	11th to 20th	Night	7% 3 6% 26% 49% 8%	540
	21st to end	Night	9% 4 4 23% 49% 11%	540
July	1st to 10th	Night	9% 3 3 15% 48% 21%	525
	11th to 20th	Night	3 2 2 12% 55% 27%	522
	21st to end	Night	8% 2 1 11% 48% 30%	730
August	1st to 10th	Night	6% 2 2 16% 47% 27%	716
	11th to 20th	Night	8% 2 3 11% 44% 33%	712
	21st to end	Night	6% 1 2 9% 51% 31%	792
September	1st to 10th	Night	8% 1 3 15% 50% 22%	720
	11th to 20th	Night	8% 2 4 19% 53% 14%	720
	21st to end	Night	5% 4 5% 25% 46% 15%	720
October	1st to 10th	Night	6% 2 6% 34% 44% 8%	720
	11th to 20th	Night	10% 3 5% 31% 40% 11%	720
	21st to end	Night	7% 2 8% 29% 39% 15%	792
November	1st to 10th	Night	6% 2 8% 34% 38% 12%	740
	11th to 20th	Night	5% 3 6% 35% 37% 14%	760
	21st to end	Night	8% 1 11% 38% 32% 9%	760
December	1st to 10th	Night	10% 2 10% 39% 31% 8%	760
	11th to 20th	Night	13% 4 11% 39% 27% 7%	760
	21st to end	Night	11% 4 14% 36% 27% 8%	836

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