University of California San Diego
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the University of California San Diego clear sky chart from 2005-11-06 to 2026-08-15.

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	4 5% 2 5% 3 3 3 3 5% 8% 60%	152047
	Night	7% 9% 3 6% 3 3 4 3 5% 7% 51%	147611
Transparency	Day	15% 34% 4 12% 18% 16%	147148
	Night	21% 35% 5% 11% 15% 14%	143488
Seeing	Day	7% 5% 20% 37% 24% 7%	18239
	Night	10% 3 8% 30% 37% 12%	28593

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 2 5% 3 3 3 3 3 5% 61%	5318
	11th to 20th	Day	4 4 1 3 2 2 3 2 4 6% 70%	5756
	21st to end	Day	8% 4 2 6% 4 4 5% 4 6% 9% 49%	6500
February	1st to 10th	Day	8% 4 2 5% 4 4 4 3 4 5% 57%	2100
	11th to 20th	Day	8% 5% 2 4 4 3 5% 4 5% 8% 51%	2050
	21st to end	Day	8% 5% 2 5% 4 4 4 4 5% 8% 52%	1730
March	1st to 10th	Day	7% 4 2 6% 4 4 5% 4 8% 10% 46%	4618
	11th to 20th	Day	9% 4 2 4 3 3 3 3 5% 8% 56%	4855
	21st to end	Day	9% 6% 1 5% 4 3 4 3 5% 8% 50%	5952
April	1st to 10th	Day	6% 5% 2 5% 4 3 3 4 4 9% 54%	2520
	11th to 20th	Day	4 5% 1 5% 3 3 3 3 5% 8% 60%	2489
	21st to end	Day	5% 5% 2 4 3 3 3 3 5% 9% 57%	2520
May	1st to 10th	Day	4 6% 2 5% 3 3 3 2 5% 7% 59%	7336
	11th to 20th	Day	4 7% 3 7% 4 4 5% 3 5% 8% 52%	7596
	21st to end	Day	5% 6% 2 6% 3 3 4 3 5% 8% 54%	8370
June	1st to 10th	Day	6% 9% 3 6% 4 3 3 3 5% 8% 51%	2730
	11th to 20th	Day	4 6% 3 5% 3 2 3 3 4 8% 60%	2730
	21st to end	Day	3 7% 2 5% 3 3 3 3 4 6% 61%	2730
July	1st to 10th	Day	1 5% 2 5% 2 2 3 2 4 6% 66%	7649
	11th to 20th	Day	1 6% 2 5% 3 3 4 3 6% 9% 58%	7650
	21st to end	Day	1 3 1 3 3 2 3 2 5% 8% 69%	7840
August	1st to 10th	Day	1 2 1 3 2 2 3 2 4 7% 74%	5466
	11th to 20th	Day	1 3 2 4 2 2 2 2 3 7% 72%	5413
	21st to end	Day	2 3 1 4 2 2 3 2 4 7% 68%	5984
September	1st to 10th	Day	3 3 1 4 3 2 3 3 5% 8% 65%	2390
	11th to 20th	Day	4 4 1 5% 3 2 3 2 4 7% 64%	2200
	21st to end	Day	3 4 2 4 2 3 3 3 4 7% 65%	2200
October	1st to 10th	Day	2 3 1 3 2 2 3 3 4 7% 70%	3600
	11th to 20th	Day	3 4 2 5% 3 3 3 2 4 6% 65%	3559
	21st to end	Day	3 3 1 3 2 2 3 2 5% 8% 69%	3414
November	1st to 10th	Day	4 4 2 3 4 3 3 2 4 7% 64%	1854
	11th to 20th	Day	5% 4 1 4 3 3 4 3 5% 6% 62%	1890
	21st to end	Day	6% 4 2 5% 3 3 3 4 5% 8% 58%	1890
December	1st to 10th	Day	4 3 1 3 2 2 3 3 5% 8% 66%	3040
	11th to 20th	Day	7% 3 1 4 4 4 5% 5% 6% 8% 52%	3499
	21st to end	Day	7% 6% 3 7% 5% 6% 6% 4 6% 9% 40%	4609
January	1st to 10th	Night	10% 6% 2 5% 3 3 3 2 4 5% 56%	7605
	11th to 20th	Night	5% 5% 2 4 2 2 3 2 3 6% 66%	7251
	21st to end	Night	11% 6% 3 6% 4 4 4 4 5% 7% 48%	7747
February	1st to 10th	Night	7% 7% 2 6% 4 3 3 3 5% 5% 55%	2520
	11th to 20th	Night	11% 9% 2 5% 4 4 4 4 4 5% 50%	2458
	21st to end	Night	9% 6% 2 6% 4 3 3 3 5% 6% 52%	2076
March	1st to 10th	Night	15% 8% 3 6% 4 4 4 3 4 7% 42%	5496
	11th to 20th	Night	11% 9% 3 6% 4 4 4 3 5% 6% 47%	5363
	21st to end	Night	11% 12% 3 7% 4 3 4 3 4 5% 44%	5260
April	1st to 10th	Night	10% 10% 3 6% 4 4 2 2 4 8% 47%	2100
	11th to 20th	Night	10% 10% 3 5% 3 3 3 3 4 6% 50%	2079
	21st to end	Night	10% 12% 4 6% 4 3 4 3 5% 7% 43%	2100
May	1st to 10th	Night	9% 16% 3 6% 3 3 3 2 4 6% 45%	5622
	11th to 20th	Night	8% 20% 4 6% 4 3 4 2 4 6% 37%	5296
	21st to end	Night	11% 16% 3 6% 4 3 5% 2 4 6% 39%	5837
June	1st to 10th	Night	10% 18% 5% 8% 5% 5% 4 3 6% 9% 28%	1890
	11th to 20th	Night	5% 14% 3 6% 4 4 4 3 6% 11% 41%	1890
	21st to end	Night	6% 13% 3 8% 4 4 4 3 5% 8% 41%	1890
July	1st to 10th	Night	2 11% 4 6% 3 3 4 3 5% 7% 54%	5340
	11th to 20th	Night	1 12% 3 5% 3 3 4 3 5% 9% 53%	5340
	21st to end	Night	3 9% 3 5% 3 3 4 2 5% 7% 56%	6276
August	1st to 10th	Night	1 7% 4 5% 2 2 4 3 5% 8% 59%	4586
	11th to 20th	Night	2 12% 2 5% 2 2 3 2 4 8% 56%	4546
	21st to end	Night	4 5% 1 4 2 3 4 3 5% 7% 63%	5015
September	1st to 10th	Night	5% 7% 2 6% 4 4 3 3 5% 7% 54%	2008
	11th to 20th	Night	6% 6% 2 5% 4 3 2 3 4 7% 58%	2200
	21st to end	Night	4 9% 3 5% 3 3 3 3 5% 7% 56%	2200
October	1st to 10th	Night	2 4 2 6% 4 3 4 4 6% 7% 58%	3600
	11th to 20th	Night	6% 13% 4 5% 3 3 3 3 3 5% 51%	3595
	21st to end	Night	3 6% 1 4 3 2 3 2 4 6% 65%	4378
November	1st to 10th	Night	5% 6% 2 4 3 3 4 3 4 6% 60%	2665
	11th to 20th	Night	5% 6% 2 4 3 3 3 2 4 6% 61%	2730
	21st to end	Night	6% 5% 3 5% 4 4 5% 4 5% 7% 52%	2730
December	1st to 10th	Night	5% 5% 2 4 3 3 3 3 5% 6% 62%	4370
	11th to 20th	Night	8% 6% 2 5% 4 3 5% 4 5% 8% 50%	4941
	21st to end	Night	9% 7% 4 8% 5% 5% 5% 4 6% 8% 39%	6611

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	21% 21% 5% 13% 17% 23%	5321
	11th to 20th	Day	12% 21% 6% 12% 15% 33%	5756
	21st to end	Day	25% 24% 4 10% 18% 20%	6500
February	1st to 10th	Day	13% 59% 4 6% 8% 10%	2100
	11th to 20th	Day	13% 58% 3 8% 11% 8%	2060
	21st to end	Day	11% 59% 3 8% 12% 7%	1730
March	1st to 10th	Day	20% 31% 8% 18% 15% 8%	4618
	11th to 20th	Day	18% 29% 8% 22% 16% 7%	4855
	21st to end	Day	22% 30% 9% 24% 11% 5%	5952
April	1st to 10th	Day	10% 59% 4 9% 12% 6%	2520
	11th to 20th	Day	8% 60% 3 8% 14% 8%	2492
	21st to end	Day	10% 59% 4 14% 9% 4	2520
May	1st to 10th	Day	20% 24% 5% 16% 28% 8%	7336
	11th to 20th	Day	20% 26% 5% 14% 26% 9%	7599
	21st to end	Day	19% 26% 5% 18% 25% 7%	8371
June	1st to 10th	Day	13% 59% 2 9% 12% 5%	2730
	11th to 20th	Day	9% 59% 1 5% 14% 12%	2730
	21st to end	Day	11% 57% 2 3 13% 13%	2730
July	1st to 10th	Day	13% 20% 2 6% 16% 43%	7586
	11th to 20th	Day	18% 21% 5% 14% 32% 10%	7581
	21st to end	Day	9% 20% 4 21% 25% 21%	7667
August	1st to 10th	Day	8% 35% 4 16% 26% 11%	4010
	11th to 20th	Day	7% 36% 4 6% 18% 29%	3929
	21st to end	Day	8% 36% 1 4 20% 30%	4311
September	1st to 10th	Day	7% 57% 3 6% 16% 12%	2390
	11th to 20th	Day	10% 57% 1 5% 16% 11%	2200
	21st to end	Day	9% 57% 2 4 17% 10%	2200
October	1st to 10th	Day	5% 35% 1 4 14% 41%	3600
	11th to 20th	Day	13% 36% 3 8% 18% 23%	3559
	21st to end	Day	10% 35% 1 7% 25% 22%	3414
November	1st to 10th	Day	6% 53% 2 6% 10% 23%	1854
	11th to 20th	Day	13% 54% 3 7% 9% 14%	1890
	21st to end	Day	13% 54% 4 9% 7% 14%	1890
December	1st to 10th	Day	9% 35% 3 13% 18% 22%	3040
	11th to 20th	Day	17% 31% 6% 12% 21% 13%	3498
	21st to end	Day	29% 28% 10% 18% 11% 4	4609
January	1st to 10th	Night	26% 21% 4 12% 20% 17%	7605
	11th to 20th	Night	16% 22% 6% 11% 12% 34%	7251
	21st to end	Night	29% 25% 4 7% 17% 19%	7747
February	1st to 10th	Night	13% 59% 3 6% 9% 9%	2520
	11th to 20th	Night	17% 58% 3 6% 8% 8%	2459
	21st to end	Night	13% 59% 2 7% 13% 7%	2076
March	1st to 10th	Night	29% 31% 9% 10% 13% 8%	5496
	11th to 20th	Night	25% 31% 5% 20% 15% 5%	5363
	21st to end	Night	30% 32% 9% 12% 13% 4	5260
April	1st to 10th	Night	14% 60% 3 8% 10% 5%	2100
	11th to 20th	Night	12% 59% 3 7% 12% 6%	2079
	21st to end	Night	16% 60% 3 9% 7% 4	2100
May	1st to 10th	Night	30% 24% 5% 17% 19% 4	5620
	11th to 20th	Night	34% 25% 5% 13% 17% 5%	5297
	21st to end	Night	34% 25% 6% 13% 16% 6%	5838
June	1st to 10th	Night	22% 61% 2 4 7% 4	1890
	11th to 20th	Night	15% 60% 1 5% 10% 9%	1890
	21st to end	Night	17% 59% 1 4 9% 10%	1890
July	1st to 10th	Night	18% 21% 2 9% 15% 35%	5295
	11th to 20th	Night	21% 21% 7% 17% 28% 7%	5286
	21st to end	Night	17% 21% 4 24% 23% 12%	6138
August	1st to 10th	Night	10% 36% 6% 15% 25% 8%	3356
	11th to 20th	Night	11% 36% 5% 8% 16% 24%	3296
	21st to end	Night	11% 36% 2 5% 20% 26%	3608
September	1st to 10th	Night	15% 58% 2 5% 11% 8%	2008
	11th to 20th	Night	12% 57% 3 4 16% 8%	2200
	21st to end	Night	12% 58% 2 6% 16% 7%	2200
October	1st to 10th	Night	10% 36% 2 7% 16% 29%	3600
	11th to 20th	Night	24% 37% 2 5% 13% 19%	3595
	21st to end	Night	12% 36% 2 7% 22% 21%	4378
November	1st to 10th	Night	7% 53% 2 6% 11% 21%	2665
	11th to 20th	Night	15% 54% 3 6% 8% 14%	2730
	21st to end	Night	16% 55% 3 8% 7% 11%	2730
December	1st to 10th	Night	13% 35% 3 11% 18% 20%	4370
	11th to 20th	Night	21% 32% 5% 8% 18% 17%	4941
	21st to end	Night	29% 30% 11% 17% 7% 5%	6611

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	14% 7% 17% 35% 22% 4	330
	11th to 20th	Day	8% 5% 21% 39% 22% 4	473
	21st to end	Day	9% 7% 26% 33% 20% 5%	594
February	1st to 10th	Day	11% 7% 24% 33% 20% 4	540
	11th to 20th	Day	10% 10% 29% 33% 15% 3	535
	21st to end	Day	8% 9% 33% 33% 14% 3	446
March	1st to 10th	Day	10% 10% 33% 31% 13% 2	540
	11th to 20th	Day	11% 6% 28% 34% 16% 5%	540
	21st to end	Day	7% 11% 33% 32% 13% 4	594
April	1st to 10th	Day	8% 7% 30% 33% 16% 6%	540
	11th to 20th	Day	5% 8% 35% 34% 13% 5%	532
	21st to end	Day	6% 10% 26% 34% 18% 6%	540
May	1st to 10th	Day	10% 6% 27% 35% 16% 5%	543
	11th to 20th	Day	8% 6% 26% 37% 19% 4	568
	21st to end	Day	7% 8% 26% 35% 21% 3	617
June	1st to 10th	Day	7% 5% 18% 37% 27% 6%	570
	11th to 20th	Day	5% 2 17% 44% 25% 6%	570
	21st to end	Day	6% 2 11% 44% 30% 8%	570
July	1st to 10th	Day	4 1 7% 43% 36% 10%	566
	11th to 20th	Day	3 1 4 38% 38% 16%	564
	21st to end	Day	3 1 5% 37% 37% 17%	619
August	1st to 10th	Day	2 1 5% 40% 36% 16%	567
	11th to 20th	Day	3 1 6% 41% 34% 15%	534
	21st to end	Day	4 1 8% 39% 37% 11%	583
September	1st to 10th	Day	6% 1 9% 45% 32% 7%	530
	11th to 20th	Day	7% 3 18% 44% 24% 5%	530
	21st to end	Day	5% 3 19% 40% 26% 6%	530
October	1st to 10th	Day	4 5% 20% 42% 23% 6%	530
	11th to 20th	Day	5% 3 23% 40% 23% 6%	530
	21st to end	Day	4 3 20% 36% 27% 10%	501
November	1st to 10th	Day	4 6% 17% 34% 27% 12%	330
	11th to 20th	Day	8% 5% 17% 35% 27% 8%	330
	21st to end	Day	7% 6% 19% 36% 24% 7%	330
December	1st to 10th	Day	8% 2 20% 35% 26% 8%	330
	11th to 20th	Day	5% 5% 15% 35% 28% 12%	330
	21st to end	Day	13% 7% 22% 33% 19% 5%	363
January	1st to 10th	Night	11% 3 14% 37% 27% 9%	840
	11th to 20th	Night	8% 3 10% 41% 32% 7%	840
	21st to end	Night	12% 4 13% 40% 27% 4	924
February	1st to 10th	Night	10% 3 13% 35% 31% 8%	840
	11th to 20th	Night	14% 5% 14% 36% 25% 6%	824
	21st to end	Night	11% 4 11% 44% 26% 3	692
March	1st to 10th	Night	13% 4 14% 41% 25% 3	840
	11th to 20th	Night	13% 5% 11% 33% 30% 9%	840
	21st to end	Night	11% 5% 11% 37% 32% 5%	924
April	1st to 10th	Night	13% 4 11% 37% 30% 5%	840
	11th to 20th	Night	11% 5% 14% 35% 29% 7%	831
	21st to end	Night	14% 4 9% 32% 33% 8%	840
May	1st to 10th	Night	13% 5% 11% 30% 34% 6%	736
	11th to 20th	Night	15% 5% 11% 30% 32% 7%	621
	21st to end	Night	16% 6% 10% 27% 36% 5%	683
June	1st to 10th	Night	13% 5% 9% 29% 36% 8%	630
	11th to 20th	Night	8% 3 6% 28% 47% 7%	630
	21st to end	Night	9% 4 4 25% 47% 10%	630
July	1st to 10th	Night	9% 3 3 18% 49% 19%	615
	11th to 20th	Night	4 2 1 13% 55% 25%	612
	21st to end	Night	8% 2 1 13% 47% 29%	853
August	1st to 10th	Night	6% 2 2 15% 48% 28%	836
	11th to 20th	Night	8% 2 3 12% 44% 31%	808
	21st to end	Night	6% 1 2 10% 51% 30%	880
September	1st to 10th	Night	8% 1 3 15% 51% 22%	800
	11th to 20th	Night	8% 2 5% 21% 51% 13%	800
	21st to end	Night	6% 4 5% 26% 45% 14%	800
October	1st to 10th	Night	7% 2 5% 33% 45% 8%	800
	11th to 20th	Night	10% 3 5% 32% 39% 10%	800
	21st to end	Night	7% 2 8% 30% 39% 14%	880
November	1st to 10th	Night	5% 2 8% 34% 39% 12%	820
	11th to 20th	Night	7% 3 7% 35% 35% 13%	840
	21st to end	Night	8% 1 10% 40% 31% 9%	840
December	1st to 10th	Night	10% 1 10% 40% 31% 7%	840
	11th to 20th	Night	12% 4 10% 38% 29% 7%	840
	21st to end	Night	10% 4 15% 37% 26% 8%	924

Page generated at 2026-08-12 00:37:15. Version ac07

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