La Mesa
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the La Mesa clear sky chart from 2005-11-06 to 2026-09-29.

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 4 2 4 3 3 3 3 5% 7% 64%	157268
	Night	7% 8% 2 5% 3 3 3 3 4 6% 56%	151942
Transparency	Day	21% 3 5% 19% 28% 25%	152368
	Night	29% 3 5% 17% 23% 22%	147820
Seeing	Day	8% 6% 24% 36% 18% 6%	18394
	Night	10% 3 8% 34% 35% 10%	28739

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	9% 5% 2 5% 3 3 3 2 3 5% 60%	5318
	11th to 20th	Day	4 3 1 3 2 2 2 2 4 6% 72%	5718
	21st to end	Day	9% 5% 2 5% 4 4 4 4 6% 9% 47%	6500
February	1st to 10th	Day	8% 4 2 5% 4 3 4 4 4 5% 57%	2100
	11th to 20th	Day	9% 6% 2 5% 3 3 3 4 5% 7% 52%	2050
	21st to end	Day	9% 6% 2 5% 4 5% 5% 4 5% 6% 50%	1730
March	1st to 10th	Day	8% 5% 2 6% 3 4 4 4 8% 11% 43%	4618
	11th to 20th	Day	10% 4 2 4 3 3 4 3 4 7% 56%	4880
	21st to end	Day	10% 5% 2 4 3 3 3 3 5% 8% 54%	5891
April	1st to 10th	Day	6% 5% 2 5% 4 3 3 3 6% 7% 55%	2520
	11th to 20th	Day	4 5% 1 4 3 4 4 3 5% 6% 61%	2489
	21st to end	Day	6% 5% 1 3 3 2 3 3 5% 8% 61%	2520
May	1st to 10th	Day	5% 5% 2 5% 3 2 3 2 4 5% 64%	7375
	11th to 20th	Day	3 6% 2 5% 3 3 3 3 5% 7% 60%	7596
	21st to end	Day	4 6% 2 4 3 3 3 3 5% 7% 62%	8370
June	1st to 10th	Day	5% 8% 2 5% 3 3 3 3 4 7% 58%	2730
	11th to 20th	Day	3 4 1 4 2 2 2 3 4 6% 68%	2730
	21st to end	Day	2 6% 2 4 2 2 3 3 4 4 67%	2730
July	1st to 10th	Day	1 3 2 3 2 2 2 2 4 5% 75%	7649
	11th to 20th	Day	1 5% 2 4 3 2 3 3 5% 8% 65%	7650
	21st to end	Day	1 2 1 2 2 2 2 2 5% 7% 74%	7895
August	1st to 10th	Day	1 1 2 1 1 2 2 4 6% 81%	6986
	11th to 20th	Day	1 3 1 2 1 1 2 2 4 7% 75%	7001
	21st to end	Day	2 2 1 3 2 2 2 2 4 6% 74%	7750
September	1st to 10th	Day	3 3 1 3 3 3 3 3 5% 6% 68%	2496
	11th to 20th	Day	3 3 1 3 2 3 2 2 4 5% 71%	2310
	21st to end	Day	3 3 2 3 2 2 2 2 4 5% 71%	2289
October	1st to 10th	Day	2 3 1 2 2 2 2 2 3 4 77%	3600
	11th to 20th	Day	3 5% 1 4 2 2 2 2 3 6% 70%	3579
	21st to end	Day	3 2 1 3 2 2 2 3 5% 7% 71%	3416
November	1st to 10th	Day	4 3 1 4 3 2 4 3 4 7% 66%	1854
	11th to 20th	Day	5% 3 1 3 3 3 4 3 4 6% 64%	1890
	21st to end	Day	6% 5% 1 4 3 2 4 3 5% 8% 58%	1890
December	1st to 10th	Day	4 2 1 3 2 2 3 3 5% 7% 68%	3040
	11th to 20th	Day	7% 3 1 4 4 5% 4 4 6% 8% 53%	3499
	21st to end	Day	8% 5% 2 6% 4 6% 5% 4 7% 10% 42%	4609
January	1st to 10th	Night	11% 6% 2 5% 3 3 3 2 4 4 56%	7605
	11th to 20th	Night	5% 5% 1 3 2 2 3 2 4 6% 67%	7275
	21st to end	Night	12% 7% 2 6% 3 4 4 3 5% 6% 48%	7747
February	1st to 10th	Night	8% 6% 2 6% 3 4 3 3 4 5% 56%	2520
	11th to 20th	Night	11% 9% 2 5% 4 3 4 3 4 5% 50%	2458
	21st to end	Night	9% 7% 3 5% 3 3 3 3 5% 5% 52%	2076
March	1st to 10th	Night	18% 10% 2 6% 3 2 3 2 4 7% 42%	5496
	11th to 20th	Night	12% 8% 3 5% 4 3 4 3 4 6% 47%	5338
	21st to end	Night	12% 12% 3 7% 3 3 3 2 4 6% 44%	5309
April	1st to 10th	Night	10% 10% 2 5% 4 3 3 3 5% 8% 49%	2100
	11th to 20th	Night	9% 11% 2 6% 3 3 3 3 3 6% 52%	2079
	21st to end	Night	8% 11% 3 7% 3 3 4 3 5% 6% 47%	2100
May	1st to 10th	Night	9% 16% 3 5% 3 2 2 2 3 5% 51%	5568
	11th to 20th	Night	9% 21% 4 6% 3 2 3 2 3 6% 40%	5296
	21st to end	Night	9% 18% 3 6% 3 3 2 2 4 5% 45%	5837
June	1st to 10th	Night	7% 16% 4 8% 3 3 4 3 6% 7% 37%	1890
	11th to 20th	Night	5% 11% 4 6% 4 3 3 3 5% 8% 51%	1890
	21st to end	Night	5% 10% 3 6% 3 3 4 3 5% 7% 52%	1890
July	1st to 10th	Night	1 7% 3 5% 3 2 2 2 4 6% 65%	5340
	11th to 20th	Night	1 10% 3 5% 3 2 3 3 5% 8% 59%	5340
	21st to end	Night	3 7% 2 4 2 2 2 2 5% 7% 65%	6219
August	1st to 10th	Night	1 3 1 3 1 1 2 2 5% 9% 73%	5866
	11th to 20th	Night	2 9% 2 3 2 2 2 2 4 7% 64%	5886
	21st to end	Night	3 4 1 4 2 2 2 2 4 5% 72%	6499
September	1st to 10th	Night	4 6% 2 5% 3 3 3 3 5% 6% 59%	2121
	11th to 20th	Night	6% 7% 2 5% 3 2 2 2 3 6% 63%	2310
	21st to end	Night	5% 7% 3 4 3 2 2 3 4 7% 61%	2288
October	1st to 10th	Night	3 4 1 3 2 2 2 2 5% 10% 66%	3600
	11th to 20th	Night	7% 13% 2 4 2 2 2 2 4 5% 56%	3579
	21st to end	Night	4 6% 1 3 3 2 3 2 4 6% 66%	4373
November	1st to 10th	Night	5% 5% 2 4 3 2 3 3 4 7% 63%	2665
	11th to 20th	Night	4 6% 2 4 3 3 3 3 3 6% 62%	2730
	21st to end	Night	6% 5% 2 5% 3 4 4 4 6% 7% 54%	2730
December	1st to 10th	Night	5% 4 2 3 2 3 3 4 4 6% 64%	4370
	11th to 20th	Night	9% 5% 2 5% 3 3 4 4 5% 8% 51%	4941
	21st to end	Night	9% 6% 2 6% 4 4 4 4 7% 10% 44%	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	28% 3 5% 14% 21% 29%	5321
	11th to 20th	Day	16% 2 5% 16% 20% 41%	5718
	21st to end	Day	33% 4 4 10% 23% 26%	6500
February	1st to 10th	Day	29% 3 5% 15% 22% 28%	2100
	11th to 20th	Day	31% 4 7% 17% 23% 19%	2060
	21st to end	Day	32% 7% 5% 14% 22% 20%	1730
March	1st to 10th	Day	29% 8% 6% 23% 19% 14%	4618
	11th to 20th	Day	26% 4 8% 26% 25% 10%	4880
	21st to end	Day	29% 4 8% 31% 17% 10%	5891
April	1st to 10th	Day	27% 4 8% 22% 27% 12%	2520
	11th to 20th	Day	22% 4 7% 19% 29% 18%	2492
	21st to end	Day	22% 3 6% 28% 29% 12%	2520
May	1st to 10th	Day	23% 3 4 22% 36% 13%	7375
	11th to 20th	Day	21% 3 5% 20% 38% 13%	7599
	21st to end	Day	21% 3 5% 24% 38% 10%	8371
June	1st to 10th	Day	25% 3 6% 22% 29% 15%	2730
	11th to 20th	Day	17% 2 3 13% 32% 33%	2730
	21st to end	Day	18% 2 3 8% 29% 40%	2730
July	1st to 10th	Day	12% 2 2 8% 19% 56%	7586
	11th to 20th	Day	18% 2 6% 19% 40% 15%	7581
	21st to end	Day	11% 2 5% 27% 31% 24%	7721
August	1st to 10th	Day	9% 1 6% 29% 38% 16%	5530
	11th to 20th	Day	12% 1 7% 24% 28% 28%	5517
	21st to end	Day	12% 2 2 15% 34% 35%	6077
September	1st to 10th	Day	17% 3 4 12% 32% 32%	2496
	11th to 20th	Day	15% 3 3 11% 35% 34%	2310
	21st to end	Day	16% 2 4 14% 32% 32%	2289
October	1st to 10th	Day	13% 2 3 9% 19% 54%	3600
	11th to 20th	Day	18% 2 2 14% 26% 38%	3579
	21st to end	Day	14% 2 2 12% 33% 36%	3416
November	1st to 10th	Day	19% 3 5% 17% 21% 36%	1854
	11th to 20th	Day	22% 3 5% 14% 20% 36%	1890
	21st to end	Day	24% 4 6% 15% 19% 32%	1890
December	1st to 10th	Day	16% 3 5% 17% 26% 34%	3040
	11th to 20th	Day	28% 4 7% 16% 24% 20%	3498
	21st to end	Day	34% 5% 9% 24% 20% 8%	4609
January	1st to 10th	Night	32% 3 4 14% 23% 24%	7605
	11th to 20th	Night	20% 3 6% 14% 17% 41%	7275
	21st to end	Night	37% 3 4 10% 20% 25%	7747
February	1st to 10th	Night	30% 3 6% 13% 21% 27%	2520
	11th to 20th	Night	36% 4 5% 15% 18% 21%	2459
	21st to end	Night	33% 4 5% 14% 24% 20%	2076
March	1st to 10th	Night	41% 6% 8% 15% 17% 13%	5496
	11th to 20th	Night	36% 5% 7% 23% 19% 9%	5338
	21st to end	Night	39% 6% 9% 19% 19% 8%	5309
April	1st to 10th	Night	33% 4 7% 22% 21% 13%	2100
	11th to 20th	Night	34% 4 5% 20% 22% 16%	2079
	21st to end	Night	37% 4 7% 21% 19% 11%	2100
May	1st to 10th	Night	37% 3 5% 24% 23% 8%	5566
	11th to 20th	Night	45% 3 5% 19% 21% 8%	5297
	21st to end	Night	41% 4 5% 21% 22% 8%	5838
June	1st to 10th	Night	43% 3 8% 18% 18% 10%	1890
	11th to 20th	Night	31% 2 3 13% 27% 23%	1890
	21st to end	Night	29% 1 3 12% 24% 31%	1890
July	1st to 10th	Night	18% 4 2 9% 18% 48%	5295
	11th to 20th	Night	23% 3 7% 22% 36% 10%	5286
	21st to end	Night	19% 2 4 30% 28% 16%	6082
August	1st to 10th	Night	11% 1 9% 29% 39% 11%	4636
	11th to 20th	Night	19% 1 7% 26% 23% 23%	4636
	21st to end	Night	17% 3 2 16% 35% 27%	5092
September	1st to 10th	Night	25% 4 3 13% 25% 30%	2121
	11th to 20th	Night	23% 3 3 10% 32% 28%	2310
	21st to end	Night	22% 3 3 14% 35% 24%	2288
October	1st to 10th	Night	16% 3 3 10% 21% 47%	3600
	11th to 20th	Night	31% 2 3 10% 21% 33%	3579
	21st to end	Night	21% 3 2 11% 30% 35%	4373
November	1st to 10th	Night	22% 3 5% 17% 18% 35%	2665
	11th to 20th	Night	24% 3 5% 14% 17% 38%	2730
	21st to end	Night	28% 3 6% 18% 16% 29%	2730
December	1st to 10th	Night	22% 3 5% 17% 22% 31%	4370
	11th to 20th	Night	31% 3 7% 12% 25% 22%	4941
	21st to end	Night	33% 6% 13% 23% 15% 10%	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	17% 5% 22% 33% 19% 3	330
	11th to 20th	Day	12% 5% 25% 38% 17% 4	461
	21st to end	Day	10% 10% 29% 33% 15% 3	594
February	1st to 10th	Day	10% 10% 28% 32% 15% 5%	540
	11th to 20th	Day	13% 12% 34% 28% 11% 2	535
	21st to end	Day	12% 11% 36% 29% 10% 2	446
March	1st to 10th	Day	13% 15% 36% 25% 11% 1	540
	11th to 20th	Day	13% 11% 30% 32% 11% 3	540
	21st to end	Day	10% 16% 35% 25% 10% 4	594
April	1st to 10th	Day	11% 11% 36% 24% 13% 6%	540
	11th to 20th	Day	7% 13% 37% 29% 10% 4	532
	21st to end	Day	8% 13% 31% 30% 14% 5%	540
May	1st to 10th	Day	12% 10% 31% 31% 12% 5%	543
	11th to 20th	Day	11% 9% 29% 35% 14% 3	568
	21st to end	Day	10% 10% 30% 33% 14% 4	617
June	1st to 10th	Day	6% 5% 24% 39% 20% 6%	570
	11th to 20th	Day	5% 3 25% 42% 19% 6%	570
	21st to end	Day	5% 2 18% 46% 21% 8%	570
July	1st to 10th	Day	5% 1 11% 45% 28% 10%	566
	11th to 20th	Day	3 1 7% 45% 31% 13%	564
	21st to end	Day	4 1 7% 45% 29% 14%	619
August	1st to 10th	Day	2 1 11% 46% 26% 14%	567
	11th to 20th	Day	4 9% 48% 25% 13%	557
	21st to end	Day	4 2 13% 43% 27% 11%	627
September	1st to 10th	Day	7% 2 17% 44% 21% 9%	570
	11th to 20th	Day	6% 4 25% 42% 17% 6%	570
	21st to end	Day	6% 7% 25% 38% 18% 7%	563
October	1st to 10th	Day	5% 7% 28% 38% 18% 6%	530
	11th to 20th	Day	8% 6% 29% 35% 17% 5%	530
	21st to end	Day	5% 4 25% 35% 23% 8%	488
November	1st to 10th	Day	7% 5% 20% 38% 22% 8%	330
	11th to 20th	Day	7% 5% 21% 37% 26% 4	330
	21st to end	Day	8% 6% 22% 37% 22% 5%	330
December	1st to 10th	Day	9% 2 24% 38% 22% 6%	330
	11th to 20th	Day	4 4 20% 37% 28% 7%	330
	21st to end	Day	18% 5% 26% 32% 15% 4	363
January	1st to 10th	Night	13% 2 15% 38% 22% 9%	840
	11th to 20th	Night	9% 2 11% 42% 28% 8%	840
	21st to end	Night	14% 3 14% 41% 24% 4	924
February	1st to 10th	Night	11% 3 11% 40% 28% 8%	840
	11th to 20th	Night	14% 4 14% 40% 23% 5%	824
	21st to end	Night	12% 4 14% 44% 23% 4	692
March	1st to 10th	Night	15% 5% 14% 42% 21% 4	840
	11th to 20th	Night	13% 4 9% 40% 27% 7%	840
	21st to end	Night	13% 3 12% 41% 27% 4	924
April	1st to 10th	Night	14% 3 10% 42% 27% 5%	840
	11th to 20th	Night	13% 5% 13% 39% 25% 6%	831
	21st to end	Night	12% 5% 10% 37% 29% 7%	840
May	1st to 10th	Night	12% 4 12% 36% 29% 6%	721
	11th to 20th	Night	15% 4 11% 34% 30% 6%	621
	21st to end	Night	17% 3 11% 34% 30% 4	683
June	1st to 10th	Night	13% 4 10% 36% 31% 6%	630
	11th to 20th	Night	8% 3 8% 36% 40% 4	630
	21st to end	Night	8% 1 7% 32% 45% 7%	630
July	1st to 10th	Night	7% 1 3 27% 52% 11%	615
	11th to 20th	Night	3 1 1 20% 58% 16%	612
	21st to end	Night	6% 1 1 19% 54% 18%	834
August	1st to 10th	Night	5% 1 1 20% 52% 20%	836
	11th to 20th	Night	6% 2 2 13% 53% 25%	832
	21st to end	Night	6% 1 1 15% 54% 23%	924
September	1st to 10th	Night	7% 1 2 23% 52% 16%	840
	11th to 20th	Night	9% 3 3 26% 46% 13%	840
	21st to end	Night	8% 1 5% 32% 40% 13%	832
October	1st to 10th	Night	8% 3 5% 36% 42% 7%	800
	11th to 20th	Night	11% 3 6% 36% 34% 10%	800
	21st to end	Night	9% 2 5% 34% 36% 14%	880
November	1st to 10th	Night	8% 2 6% 39% 34% 11%	820
	11th to 20th	Night	7% 2 9% 38% 32% 13%	840
	21st to end	Night	8% 2 10% 41% 29% 9%	840
December	1st to 10th	Night	10% 3 10% 42% 28% 8%	840
	11th to 20th	Night	13% 3 9% 42% 25% 8%	840
	21st to end	Night	12% 2 15% 38% 25% 7%	924

Page generated at 2026-09-26 01:18:41. Version ac07

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