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 2025-04-11.

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	5% 5% 2 4 3 3 3 3 5% 8% 60%	118100
	Night	7% 8% 3 6% 4 3 4 3 5% 7% 51%	119214
Transparency	Day	13% 42% 4 11% 16% 15%	117873
	Night	18% 41% 4 9% 14% 13%	119032
Seeing	Day	7% 5% 20% 37% 24% 7%	16353
	Night	10% 3 8% 30% 37% 12%	26789

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	5% 3 1 3 3 3 3 2 3 5% 68%	4098
	11th to 20th	Day	5% 5% 1 4 2 2 3 3 4 6% 64%	4418
	21st to end	Day	9% 4 2 5% 4 4 5% 4 6% 9% 49%	5016
February	1st to 10th	Day	9% 4 2 5% 4 4 4 3 4 5% 56%	2000
	11th to 20th	Day	7% 5% 2 4 4 3 5% 4 5% 8% 52%	1950
	21st to end	Day	8% 5% 2 6% 5% 4 4 4 5% 8% 51%	1650
March	1st to 10th	Day	7% 4 2 6% 4 4 5% 4 8% 10% 46%	4518
	11th to 20th	Day	9% 4 2 5% 3 3 3 3 5% 8% 55%	4751
	21st to end	Day	10% 6% 1 5% 4 3 4 3 5% 8% 51%	5824
April	1st to 10th	Day	6% 5% 2 5% 4 3 3 4 4 9% 55%	2400
	11th to 20th	Day	4 5% 1 4 3 3 3 3 5% 8% 61%	2251
	21st to end	Day	5% 5% 2 4 4 3 3 3 5% 9% 58%	2280
May	1st to 10th	Day	4 5% 2 4 3 3 3 2 4 6% 65%	3950
	11th to 20th	Day	5% 8% 3 6% 3 3 4 3 5% 7% 53%	4071
	21st to end	Day	6% 7% 2 6% 4 3 4 3 5% 8% 53%	4477
June	1st to 10th	Day	6% 9% 3 6% 4 3 3 3 5% 8% 51%	2470
	11th to 20th	Day	4 6% 2 5% 3 3 3 3 4 8% 60%	2470
	21st to end	Day	3 7% 2 5% 3 3 3 3 4 6% 61%	2470
July	1st to 10th	Day	2 5% 2 5% 2 2 3 3 5% 7% 64%	4110
	11th to 20th	Day	1 6% 2 5% 3 3 3 3 5% 8% 60%	4110
	21st to end	Day	2 3 1 3 2 2 3 3 4 7% 70%	4197
August	1st to 10th	Day	1 2 1 3 2 2 3 3 4 8% 70%	3783
	11th to 20th	Day	1 3 1 3 2 2 3 2 3 7% 73%	3724
	21st to end	Day	1 4 1 4 2 2 3 2 4 7% 69%	4180
September	1st to 10th	Day	4 3 1 4 3 2 3 3 5% 8% 64%	2271
	11th to 20th	Day	3 4 1 5% 3 2 3 2 4 7% 65%	2090
	21st to end	Day	3 4 2 4 2 3 3 3 4 7% 66%	2090
October	1st to 10th	Day	2 3 1 3 2 2 2 3 4 7% 71%	3490
	11th to 20th	Day	3 4 2 5% 3 3 3 2 4 6% 65%	3450
	21st to end	Day	3 3 1 3 3 2 3 2 5% 8% 69%	3308
November	1st to 10th	Day	4 4 2 3 4 3 3 2 4 7% 64%	1764
	11th to 20th	Day	4 4 2 4 2 2 4 3 5% 6% 64%	1800
	21st to end	Day	5% 5% 2 5% 3 3 3 5% 5% 8% 58%	1800
December	1st to 10th	Day	4 3 1 3 2 2 3 3 5% 8% 65%	2950
	11th to 20th	Day	7% 4 1 4 4 5% 5% 5% 6% 8% 52%	3409
	21st to end	Day	7% 6% 3 7% 5% 6% 6% 4 6% 9% 40%	4510
January	1st to 10th	Night	5% 4 1 4 3 3 3 3 4 6% 65%	5867
	11th to 20th	Night	6% 6% 2 5% 3 3 3 3 3 6% 59%	5572
	21st to end	Night	13% 6% 2 5% 4 4 4 4 5% 7% 48%	5984
February	1st to 10th	Night	8% 7% 2 6% 5% 3 3 3 5% 5% 54%	2400
	11th to 20th	Night	10% 8% 2 5% 3 4 4 4 4 5% 51%	2338
	21st to end	Night	9% 7% 2 6% 4 3 3 3 6% 6% 51%	1980
March	1st to 10th	Night	16% 8% 3 6% 4 4 4 3 4 7% 41%	5376
	11th to 20th	Night	11% 9% 3 6% 4 4 4 3 5% 6% 46%	5247
	21st to end	Night	12% 12% 3 7% 4 3 4 3 4 5% 44%	5147
April	1st to 10th	Night	10% 9% 3 6% 4 4 2 3 4 8% 47%	2000
	11th to 20th	Night	11% 10% 3 5% 4 3 2 3 4 6% 51%	1883
	21st to end	Night	9% 12% 4 6% 4 3 4 3 5% 6% 44%	1900
May	1st to 10th	Night	7% 10% 3 5% 4 3 3 2 4 6% 52%	3021
	11th to 20th	Night	9% 25% 4 6% 4 3 4 2 3 6% 33%	2834
	21st to end	Night	13% 19% 3 6% 4 3 5% 2 4 6% 36%	3110
June	1st to 10th	Night	10% 17% 5% 8% 5% 5% 4 3 6% 9% 27%	1710
	11th to 20th	Night	5% 13% 3 7% 5% 4 4 3 6% 11% 39%	1710
	21st to end	Night	5% 14% 3 9% 4 4 4 3 5% 9% 40%	1710
July	1st to 10th	Night	3 9% 4 7% 3 2 3 3 6% 8% 52%	2860
	11th to 20th	Night	1 12% 3 6% 3 3 4 3 5% 8% 50%	2860
	21st to end	Night	3 7% 3 5% 4 4 4 2 5% 7% 56%	3357
August	1st to 10th	Night	2 6% 3 5% 3 3 4 3 6% 9% 58%	3166
	11th to 20th	Night	2 7% 2 5% 2 3 4 3 5% 9% 58%	3126
	21st to end	Night	3 7% 1 5% 3 3 4 3 4 7% 59%	3498
September	1st to 10th	Night	5% 7% 2 6% 4 4 3 3 5% 7% 54%	1907
	11th to 20th	Night	6% 6% 2 5% 4 3 3 2 4 7% 59%	2090
	21st to end	Night	3 9% 2 5% 3 3 3 3 5% 7% 57%	2090
October	1st to 10th	Night	2 3 2 6% 4 3 4 4 6% 7% 58%	3490
	11th to 20th	Night	7% 13% 4 5% 3 3 3 3 3 5% 51%	3485
	21st to end	Night	3 5% 1 4 3 2 3 2 4 6% 65%	4242
November	1st to 10th	Night	5% 6% 2 4 3 3 4 3 4 6% 61%	2535
	11th to 20th	Night	3 6% 2 4 3 3 3 3 4 5% 63%	2600
	21st to end	Night	6% 5% 3 5% 4 5% 4 4 5% 7% 52%	2600
December	1st to 10th	Night	5% 5% 2 4 3 3 3 3 5% 6% 61%	4240
	11th to 20th	Night	8% 6% 2 5% 4 3 4 4 5% 8% 50%	4811
	21st to end	Night	8% 7% 4 8% 5% 5% 5% 4 6% 8% 40%	6468

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	12% 26% 3 16% 18% 25%	4099
	11th to 20th	Day	15% 28% 8% 16% 15% 19%	4418
	21st to end	Day	21% 30% 4 10% 21% 14%	5016
February	1st to 10th	Day	12% 62% 4 6% 7% 9%	2000
	11th to 20th	Day	11% 61% 3 7% 11% 8%	1960
	21st to end	Day	12% 61% 3 7% 10% 7%	1650
March	1st to 10th	Day	20% 31% 8% 18% 15% 8%	4518
	11th to 20th	Day	18% 30% 8% 23% 16% 6%	4751
	21st to end	Day	22% 30% 9% 24% 10% 5%	5824
April	1st to 10th	Day	9% 62% 4 9% 11% 6%	2400
	11th to 20th	Day	6% 65% 2 6% 12% 8%	2254
	21st to end	Day	8% 65% 3 11% 8% 4	2280
May	1st to 10th	Day	11% 42% 4 11% 30% 2	3950
	11th to 20th	Day	16% 45% 4 9% 22% 4	4074
	21st to end	Day	17% 45% 4 10% 23% 2	4477
June	1st to 10th	Day	10% 65% 2 8% 9% 5%	2470
	11th to 20th	Day	8% 65% 1 4 12% 9%	2470
	21st to end	Day	10% 63% 2 3 10% 12%	2470
July	1st to 10th	Day	10% 36% 1 3 8% 43%	4046
	11th to 20th	Day	14% 37% 3 6% 24% 16%	4041
	21st to end	Day	7% 35% 3 17% 15% 23%	4065
August	1st to 10th	Day	7% 37% 5% 15% 26% 11%	3770
	11th to 20th	Day	6% 37% 4 6% 18% 30%	3760
	21st to end	Day	7% 37% 1 4 19% 31%	4179
September	1st to 10th	Day	7% 60% 3 5% 14% 11%	2271
	11th to 20th	Day	9% 60% 1 5% 14% 11%	2090
	21st to end	Day	8% 60% 2 4 17% 10%	2090
October	1st to 10th	Day	4 36% 1 4 13% 42%	3490
	11th to 20th	Day	12% 37% 3 8% 18% 22%	3450
	21st to end	Day	10% 36% 1 7% 24% 22%	3308
November	1st to 10th	Day	5% 56% 2 6% 9% 22%	1764
	11th to 20th	Day	11% 57% 3 7% 9% 14%	1800
	21st to end	Day	12% 56% 4 8% 7% 13%	1800
December	1st to 10th	Day	9% 36% 3 13% 17% 22%	2950
	11th to 20th	Day	18% 32% 6% 12% 20% 13%	3408
	21st to end	Day	29% 28% 10% 18% 11% 4	4510
January	1st to 10th	Night	14% 27% 4 14% 22% 18%	5867
	11th to 20th	Night	20% 28% 8% 14% 10% 20%	5572
	21st to end	Night	26% 31% 4 7% 17% 15%	5984
February	1st to 10th	Night	13% 62% 3 5% 9% 8%	2400
	11th to 20th	Night	14% 61% 3 5% 9% 8%	2339
	21st to end	Night	13% 62% 2 6% 11% 6%	1980
March	1st to 10th	Night	30% 31% 9% 10% 13% 7%	5376
	11th to 20th	Night	25% 32% 5% 20% 15% 4	5247
	21st to end	Night	30% 33% 9% 12% 13% 4	5147
April	1st to 10th	Night	13% 63% 3 7% 10% 4	2000
	11th to 20th	Night	10% 65% 3 5% 11% 6%	1883
	21st to end	Night	15% 65% 2 6% 7% 5%	1900
May	1st to 10th	Night	16% 43% 3 13% 23% 2	3021
	11th to 20th	Night	33% 42% 4 6% 12% 2	2835
	21st to end	Night	31% 44% 4 7% 14% 1	3110
June	1st to 10th	Night	19% 67% 2 3 5% 5%	1710
	11th to 20th	Night	14% 66% 1 4 8% 7%	1710
	21st to end	Night	15% 65% 1 3 7% 8%	1710
July	1st to 10th	Night	14% 36% 1 3 8% 37%	2815
	11th to 20th	Night	18% 37% 3 7% 24% 12%	2806
	21st to end	Night	11% 37% 2 16% 19% 14%	3252
August	1st to 10th	Night	10% 39% 6% 14% 25% 8%	3156
	11th to 20th	Night	10% 38% 5% 7% 15% 25%	3156
	21st to end	Night	10% 37% 2 5% 19% 26%	3498
September	1st to 10th	Night	15% 61% 2 5% 10% 7%	1907
	11th to 20th	Night	11% 60% 2 4 15% 8%	2090
	21st to end	Night	10% 61% 1 6% 15% 7%	2090
October	1st to 10th	Night	10% 37% 2 7% 15% 30%	3490
	11th to 20th	Night	24% 38% 2 5% 12% 19%	3485
	21st to end	Night	11% 37% 1 7% 23% 20%	4242
November	1st to 10th	Night	7% 55% 2 5% 10% 21%	2535
	11th to 20th	Night	13% 57% 3 5% 8% 14%	2600
	21st to end	Night	15% 57% 3 7% 7% 10%	2600
December	1st to 10th	Night	13% 36% 3 11% 17% 20%	4240
	11th to 20th	Night	21% 32% 5% 8% 17% 17%	4811
	21st to end	Night	28% 31% 11% 17% 7% 5%	6468

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% 5% 16% 37% 24% 5%	300
	11th to 20th	Day	9% 6% 22% 38% 22% 4	436
	21st to end	Day	9% 7% 27% 33% 19% 5%	550
February	1st to 10th	Day	12% 7% 25% 31% 20% 5%	500
	11th to 20th	Day	9% 9% 30% 34% 16% 3	495
	21st to end	Day	9% 9% 34% 33% 12% 2	414
March	1st to 10th	Day	11% 10% 33% 32% 13% 2	500
	11th to 20th	Day	12% 7% 29% 33% 15% 3	500
	21st to end	Day	7% 11% 35% 32% 12% 2	550
April	1st to 10th	Day	8% 7% 30% 33% 16% 6%	500
	11th to 20th	Day	4 8% 35% 36% 13% 4	453
	21st to end	Day	7% 9% 25% 35% 18% 6%	460
May	1st to 10th	Day	11% 6% 27% 36% 17% 4	463
	11th to 20th	Day	8% 6% 25% 38% 19% 3	488
	21st to end	Day	8% 7% 26% 36% 20% 3	529
June	1st to 10th	Day	6% 5% 17% 36% 29% 6%	490
	11th to 20th	Day	5% 2 18% 42% 26% 7%	490
	21st to end	Day	6% 1 11% 42% 31% 8%	490
July	1st to 10th	Day	5% 1 6% 42% 37% 10%	486
	11th to 20th	Day	3 3 37% 41% 15%	484
	21st to end	Day	4 1 4 36% 39% 17%	531
August	1st to 10th	Day	2 1 6% 40% 38% 14%	487
	11th to 20th	Day	4 6% 40% 36% 13%	477
	21st to end	Day	4 1 7% 40% 39% 10%	539
September	1st to 10th	Day	7% 9% 45% 32% 7%	490
	11th to 20th	Day	7% 2 17% 46% 23% 5%	490
	21st to end	Day	4 3 18% 41% 27% 6%	490
October	1st to 10th	Day	4 6% 19% 41% 24% 6%	490
	11th to 20th	Day	5% 3 21% 41% 23% 6%	490
	21st to end	Day	5% 2 19% 37% 27% 9%	461
November	1st to 10th	Day	4 6% 18% 35% 28% 10%	300
	11th to 20th	Day	7% 4 18% 35% 28% 8%	300
	21st to end	Day	7% 7% 20% 36% 24% 7%	300
December	1st to 10th	Day	8% 2 21% 36% 25% 7%	300
	11th to 20th	Day	5% 5% 17% 35% 27% 12%	300
	21st to end	Day	12% 7% 22% 35% 19% 5%	330
January	1st to 10th	Night	9% 3 14% 37% 28% 9%	800
	11th to 20th	Night	8% 3 10% 40% 33% 7%	800
	21st to end	Night	12% 4 13% 39% 28% 4	880
February	1st to 10th	Night	11% 3 13% 34% 31% 9%	800
	11th to 20th	Night	13% 4 14% 36% 26% 6%	784
	21st to end	Night	12% 5% 12% 43% 26% 3	660
March	1st to 10th	Night	13% 4 13% 41% 26% 4	800
	11th to 20th	Night	13% 5% 11% 33% 29% 8%	800
	21st to end	Night	11% 5% 11% 38% 30% 5%	880
April	1st to 10th	Night	12% 4 11% 36% 31% 6%	800
	11th to 20th	Night	12% 4 14% 34% 29% 7%	753
	21st to end	Night	14% 4 9% 30% 34% 9%	760
May	1st to 10th	Night	12% 5% 11% 30% 36% 6%	666
	11th to 20th	Night	14% 5% 11% 29% 33% 8%	561
	21st to end	Night	16% 6% 11% 25% 36% 6%	617
June	1st to 10th	Night	12% 5% 9% 28% 37% 9%	570
	11th to 20th	Night	8% 3 6% 28% 48% 8%	570
	21st to end	Night	8% 4 4 24% 49% 11%	570
July	1st to 10th	Night	9% 3 3 15% 49% 21%	555
	11th to 20th	Night	3 2 1 12% 55% 26%	552
	21st to end	Night	8% 2 1 11% 48% 30%	771
August	1st to 10th	Night	6% 2 2 15% 47% 27%	756
	11th to 20th	Night	8% 2 3 12% 44% 32%	752
	21st to end	Night	6% 1 2 10% 51% 30%	836
September	1st to 10th	Night	8% 1 3 15% 51% 21%	760
	11th to 20th	Night	8% 2 5% 20% 52% 13%	760
	21st to end	Night	5% 4 5% 26% 46% 15%	760
October	1st to 10th	Night	6% 2 5% 33% 45% 9%	760
	11th to 20th	Night	10% 3 5% 31% 40% 10%	760
	21st to end	Night	7% 2 8% 30% 39% 14%	836
November	1st to 10th	Night	6% 2 8% 34% 38% 12%	780
	11th to 20th	Night	6% 3 7% 35% 36% 14%	800
	21st to end	Night	8% 1 11% 41% 31% 9%	800
December	1st to 10th	Night	10% 2 10% 40% 31% 7%	800
	11th to 20th	Night	13% 4 10% 38% 28% 7%	800
	21st to end	Night	10% 4 14% 37% 27% 8%	880

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