La Mesa Urban Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the La Mesa Urban Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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% 7% 3 6% 4 4 4 3 6% 9% 49%	135782
	Night	7% 11% 3 7% 4 3 3 3 4 7% 47%	133720
Transparency	Day	27% 3 7% 18% 24% 22%	130740
	Night	35% 3 7% 15% 21% 20%	129747
Seeing	Day	9% 9% 24% 33% 20% 6%	17093
	Night	11% 5% 10% 30% 34% 10%	26614

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	10% 6% 2 5% 4 4 4 2 5% 7% 52%	4072
	11th to 20th	Day	9% 5% 2 6% 4 4 4 3 4 5% 54%	4190
	21st to end	Day	12% 9% 4 7% 5% 4 4 3 5% 6% 41%	4609
February	1st to 10th	Day	13% 6% 2 5% 3 4 5% 4 5% 6% 46%	1801
	11th to 20th	Day	11% 7% 3 6% 5% 3 4 3 6% 7% 43%	1886
	21st to end	Day	9% 8% 3 6% 5% 4 4 3 5% 7% 46%	1650
March	1st to 10th	Day	11% 9% 3 7% 5% 6% 5% 6% 7% 10% 31%	4860
	11th to 20th	Day	10% 6% 2 6% 4 4 4 3 5% 7% 49%	5000
	21st to end	Day	9% 8% 3 6% 5% 5% 5% 5% 7% 11% 36%	5588
April	1st to 10th	Day	9% 8% 3 5% 4 4 4 4 6% 8% 47%	2400
	11th to 20th	Day	7% 9% 3 7% 4 4 3 3 6% 7% 48%	2369
	21st to end	Day	5% 9% 2 6% 3 4 3 4 6% 8% 50%	2400
May	1st to 10th	Day	5% 5% 1 4 2 2 2 2 4 9% 64%	5440
	11th to 20th	Day	4 6% 1 5% 3 3 3 3 6% 9% 57%	5879
	21st to end	Day	4 14% 4 6% 3 3 4 3 5% 8% 44%	6902
June	1st to 10th	Day	4 8% 4 7% 5% 4 4 4 6% 9% 47%	2800
	11th to 20th	Day	3 4 2 5% 3 3 3 3 6% 12% 57%	2800
	21st to end	Day	2 6% 3 5% 4 4 3 3 6% 8% 55%	2800
July	1st to 10th	Day	4 3 6% 3 3 4 3 6% 12% 57%	6215
	11th to 20th	Day	1 6% 5% 10% 4 4 3 4 7% 12% 45%	5880
	21st to end	Day	1 21% 4 9% 4 4 4 4 7% 11% 30%	6402
August	1st to 10th	Day	2 7% 5% 9% 4 4 5% 5% 9% 12% 38%	5777
	11th to 20th	Day	2 7% 3 7% 3 3 3 3 6% 11% 51%	5411
	21st to end	Day	1 5% 3 7% 4 3 4 3 7% 11% 53%	5984
September	1st to 10th	Day	2 5% 2 3 3 3 3 2 5% 9% 63%	2400
	11th to 20th	Day	4 7% 2 6% 3 3 3 3 5% 10% 52%	2400
	21st to end	Day	3 3 2 4 3 3 3 3 5% 8% 64%	2134
October	1st to 10th	Day	2 3 1 4 3 2 3 2 5% 7% 68%	3280
	11th to 20th	Day	3 10% 3 6% 2 3 3 3 4 7% 57%	3261
	21st to end	Day	4 5% 2 3 3 4 4 3 5% 8% 60%	3558
November	1st to 10th	Day	6% 4 1 4 4 4 4 4 5% 7% 56%	2043
	11th to 20th	Day	8% 7% 2 6% 4 4 4 4 6% 6% 51%	1890
	21st to end	Day	9% 5% 1 4 3 3 3 3 5% 8% 55%	1801
December	1st to 10th	Day	6% 5% 2 5% 4 3 4 3 6% 9% 54%	2700
	11th to 20th	Day	14% 7% 3 7% 5% 5% 5% 3 5% 7% 40%	3108
	21st to end	Day	18% 9% 3 7% 5% 4 4 3 5% 7% 34%	4092
January	1st to 10th	Night	9% 6% 2 6% 4 3 3 3 5% 9% 51%	6104
	11th to 20th	Night	9% 7% 3 6% 4 3 4 3 5% 6% 50%	6010
	21st to end	Night	13% 11% 3 7% 4 4 3 3 5% 7% 41%	6611
February	1st to 10th	Night	13% 8% 3 6% 4 3 3 3 4 6% 47%	2600
	11th to 20th	Night	11% 9% 2 6% 4 4 4 4 5% 8% 41%	2399
	21st to end	Night	11% 9% 3 5% 4 3 3 3 6% 8% 47%	1980
March	1st to 10th	Night	18% 13% 3 7% 5% 4 4 3 5% 5% 31%	5031
	11th to 20th	Night	10% 8% 3 7% 4 4 4 3 4 6% 46%	5000
	21st to end	Night	13% 12% 3 7% 5% 4 4 3 5% 8% 36%	5385
April	1st to 10th	Night	10% 9% 2 6% 4 3 4 4 6% 8% 45%	2000
	11th to 20th	Night	9% 9% 3 6% 3 3 3 3 4 8% 49%	1979
	21st to end	Night	6% 8% 2 6% 3 3 3 3 6% 11% 49%	2000
May	1st to 10th	Night	4 9% 3 5% 2 2 2 2 3 8% 59%	4542
	11th to 20th	Night	4 14% 4 11% 5% 3 2 3 3 6% 45%	4028
	21st to end	Night	3 18% 4 4 2 2 2 2 3 7% 51%	3972
June	1st to 10th	Night	6% 12% 4 6% 3 3 4 3 4 8% 47%	1600
	11th to 20th	Night	2 8% 3 7% 4 3 3 2 4 7% 57%	1600
	21st to end	Night	2 10% 3 6% 4 4 2 3 3 7% 55%	1600
July	1st to 10th	Night	8% 6% 11% 3 3 2 1 3 6% 56%	3730
	11th to 20th	Night	1 16% 8% 14% 5% 4 3 2 4 6% 38%	4100
	21st to end	Night	2 42% 8% 7% 3 2 2 2 3 5% 24%	4472
August	1st to 10th	Night	2 15% 6% 19% 4 4 3 3 4 7% 33%	4035
	11th to 20th	Night	1 15% 6% 10% 3 3 2 2 3 5% 51%	4457
	21st to end	Night	1 10% 5% 11% 4 3 2 2 4 7% 50%	5015
September	1st to 10th	Night	3 7% 2 6% 4 3 4 2 5% 7% 58%	2000
	11th to 20th	Night	4 10% 4 7% 4 3 3 2 4 8% 52%	2000
	21st to end	Night	4 6% 3 4 2 3 2 3 4 8% 62%	2262
October	1st to 10th	Night	3 4 1 5% 3 2 3 2 4 5% 69%	3920
	11th to 20th	Night	4 15% 4 5% 3 3 3 2 3 6% 52%	3897
	21st to end	Night	5% 8% 2 4 3 3 3 2 4 7% 59%	4253
November	1st to 10th	Night	7% 6% 2 5% 4 3 3 4 4 9% 54%	2472
	11th to 20th	Night	9% 7% 2 5% 3 3 4 4 5% 8% 50%	2730
	21st to end	Night	7% 6% 2 4 3 3 4 3 5% 6% 57%	2814
December	1st to 10th	Night	6% 4 1 4 3 3 4 3 5% 7% 59%	4700
	11th to 20th	Night	14% 9% 2 7% 5% 4 5% 4 7% 8% 35%	5316
	21st to end	Night	17% 9% 3 9% 5% 5% 5% 4 5% 7% 31%	7106

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	36% 2 14% 26% 17% 6%	4072
	11th to 20th	Day	34% 2 8% 14% 19% 22%	4190
	21st to end	Day	44% 2 7% 12% 20% 14%	4609
February	1st to 10th	Day	40% 3 6% 18% 21% 12%	1801
	11th to 20th	Day	41% 4 8% 18% 20% 8%	1896
	21st to end	Day	41% 4 6% 23% 19% 7%	1650
March	1st to 10th	Day	53% 3 11% 15% 16% 2	4860
	11th to 20th	Day	36% 4 11% 28% 14% 8%	5000
	21st to end	Day	44% 5% 12% 25% 12% 1	5588
April	1st to 10th	Day	39% 5% 10% 25% 17% 4	2400
	11th to 20th	Day	36% 3 10% 25% 19% 6%	2372
	21st to end	Day	32% 4 9% 28% 21% 6%	2400
May	1st to 10th	Day	20% 2 7% 26% 36% 10%	5440
	11th to 20th	Day	20% 2 5% 26% 35% 12%	5882
	21st to end	Day	28% 1 9% 22% 31% 9%	6904
June	1st to 10th	Day	26% 4 9% 23% 28% 11%	2800
	11th to 20th	Day	15% 2 6% 19% 34% 23%	2800
	21st to end	Day	15% 2 2 12% 31% 39%	2800
July	1st to 10th	Day	8% 1 2 5% 21% 63%	6149
	11th to 20th	Day	10% 1 4 8% 17% 60%	5811
	21st to end	Day	21% 1 9% 16% 24% 29%	6217
August	1st to 10th	Day	14% 2 2 9% 28% 45%	4206
	11th to 20th	Day	9% 1 3 8% 33% 47%	3915
	21st to end	Day	12% 2 3 10% 30% 42%	4311
September	1st to 10th	Day	13% 1 1 9% 29% 47%	2400
	11th to 20th	Day	20% 3 3 19% 33% 22%	2400
	21st to end	Day	15% 3 3 19% 38% 22%	2134
October	1st to 10th	Day	15% 2 4 12% 27% 40%	3280
	11th to 20th	Day	22% 3 6% 20% 30% 19%	3261
	21st to end	Day	23% 2 6% 18% 31% 20%	3558
November	1st to 10th	Day	27% 4 9% 21% 25% 14%	2043
	11th to 20th	Day	33% 3 10% 19% 24% 11%	1890
	21st to end	Day	30% 4 7% 18% 27% 14%	1801
December	1st to 10th	Day	28% 3 6% 18% 31% 13%	2700
	11th to 20th	Day	46% 4 9% 16% 16% 9%	3108
	21st to end	Day	52% 4 13% 18% 10% 5%	4092
January	1st to 10th	Night	37% 2 14% 22% 19% 5%	6104
	11th to 20th	Night	37% 2 10% 11% 22% 18%	6010
	21st to end	Night	47% 2 6% 10% 19% 17%	6611
February	1st to 10th	Night	41% 3 7% 15% 19% 15%	2600
	11th to 20th	Night	44% 4 7% 16% 21% 9%	2400
	21st to end	Night	42% 4 7% 24% 16% 7%	1980
March	1st to 10th	Night	58% 2 9% 12% 16% 4	5031
	11th to 20th	Night	40% 3 9% 21% 17% 8%	5000
	21st to end	Night	53% 4 10% 19% 12% 3	5385
April	1st to 10th	Night	40% 5% 11% 23% 16% 6%	2000
	11th to 20th	Night	38% 4 9% 23% 17% 8%	1979
	21st to end	Night	34% 5% 8% 25% 20% 7%	2000
May	1st to 10th	Night	34% 1 4 18% 29% 14%	4539
	11th to 20th	Night	33% 2 4 12% 32% 16%	4029
	21st to end	Night	40% 1 6% 14% 29% 10%	3972
June	1st to 10th	Night	31% 5% 7% 20% 22% 15%	1600
	11th to 20th	Night	21% 3 7% 15% 26% 28%	1600
	21st to end	Night	15% 2 3 11% 25% 44%	1600
July	1st to 10th	Night	13% 1 7% 9% 13% 57%	3689
	11th to 20th	Night	21% 2 6% 9% 12% 50%	4046
	21st to end	Night	47% 2 5% 8% 15% 23%	4345
August	1st to 10th	Night	18% 3 3 15% 15% 46%	2929
	11th to 20th	Night	15% 2 3 10% 24% 46%	3220
	21st to end	Night	19% 2 3 8% 26% 43%	3608
September	1st to 10th	Night	16% 2 2 6% 24% 50%	2000
	11th to 20th	Night	24% 3 2 16% 26% 29%	2000
	21st to end	Night	18% 2 2 15% 35% 27%	2262
October	1st to 10th	Night	15% 2 4 11% 23% 44%	3920
	11th to 20th	Night	30% 3 6% 16% 25% 20%	3897
	21st to end	Night	26% 2 4 16% 27% 24%	4253
November	1st to 10th	Night	28% 3 6% 20% 24% 18%	2472
	11th to 20th	Night	32% 3 9% 20% 23% 12%	2730
	21st to end	Night	31% 4 7% 18% 24% 18%	2814
December	1st to 10th	Night	27% 2 6% 19% 31% 16%	4700
	11th to 20th	Night	48% 4 9% 14% 15% 10%	5316
	21st to end	Night	57% 4 11% 15% 9% 5%	7106

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	26% 15% 22% 23% 12% 1	378
	11th to 20th	Day	13% 11% 26% 28% 20% 3	430
	21st to end	Day	15% 12% 26% 30% 15% 4	473
February	1st to 10th	Day	16% 16% 26% 21% 14% 6%	401
	11th to 20th	Day	19% 15% 30% 25% 10% 1	462
	21st to end	Day	13% 15% 30% 30% 11% 2	414
March	1st to 10th	Day	21% 16% 30% 24% 8% 1	500
	11th to 20th	Day	19% 15% 29% 24% 11% 3	500
	21st to end	Day	17% 18% 33% 23% 8% 1	550
April	1st to 10th	Day	16% 14% 30% 26% 13% 2	500
	11th to 20th	Day	12% 18% 28% 26% 12% 3	492
	21st to end	Day	10% 14% 31% 27% 12% 6%	500
May	1st to 10th	Day	9% 11% 30% 32% 14% 4	503
	11th to 20th	Day	10% 13% 29% 28% 15% 4	528
	21st to end	Day	10% 10% 28% 31% 16% 5%	573
June	1st to 10th	Day	6% 6% 28% 35% 19% 6%	530
	11th to 20th	Day	3 8% 31% 36% 19% 4	530
	21st to end	Day	2 4 22% 42% 24% 6%	530
July	1st to 10th	Day	2 1 20% 43% 27% 7%	526
	11th to 20th	Day	1 1 12% 47% 28% 11%	524
	21st to end	Day	1 1 15% 47% 26% 11%	575
August	1st to 10th	Day	3 3 20% 45% 20% 9%	527
	11th to 20th	Day	2 1 17% 46% 25% 9%	517
	21st to end	Day	1 1 16% 46% 28% 8%	583
September	1st to 10th	Day	3 5% 16% 40% 27% 9%	530
	11th to 20th	Day	6% 6% 23% 36% 21% 8%	530
	21st to end	Day	3 8% 18% 35% 30% 7%	530
October	1st to 10th	Day	5% 5% 20% 37% 28% 6%	530
	11th to 20th	Day	5% 7% 21% 29% 29% 9%	530
	21st to end	Day	6% 6% 23% 31% 27% 7%	583
November	1st to 10th	Day	5% 12% 18% 30% 26% 9%	519
	11th to 20th	Day	10% 9% 22% 30% 21% 8%	330
	21st to end	Day	9% 10% 21% 32% 22% 6%	283
December	1st to 10th	Day	11% 11% 20% 30% 20% 7%	220
	11th to 20th	Day	13% 13% 21% 22% 24% 8%	220
	21st to end	Day	21% 15% 24% 23% 12% 5%	242
January	1st to 10th	Night	18% 6% 14% 34% 21% 7%	840
	11th to 20th	Night	11% 7% 12% 34% 28% 8%	840
	21st to end	Night	17% 7% 11% 33% 27% 5%	924
February	1st to 10th	Night	17% 7% 14% 31% 24% 7%	800
	11th to 20th	Night	17% 9% 14% 32% 24% 5%	784
	21st to end	Night	16% 8% 16% 33% 23% 3	660
March	1st to 10th	Night	19% 10% 15% 30% 22% 5%	800
	11th to 20th	Night	16% 8% 15% 30% 25% 6%	800
	21st to end	Night	16% 9% 15% 30% 26% 4	880
April	1st to 10th	Night	14% 7% 15% 30% 28% 5%	800
	11th to 20th	Night	13% 11% 12% 30% 28% 6%	791
	21st to end	Night	12% 7% 16% 27% 29% 10%	800
May	1st to 10th	Night	10% 7% 12% 33% 30% 9%	795
	11th to 20th	Night	12% 4 10% 26% 35% 12%	572
	21st to end	Night	10% 3 13% 31% 32% 10%	433
June	1st to 10th	Night	7% 5% 8% 30% 40% 10%	400
	11th to 20th	Night	4 3 12% 34% 41% 7%	400
	21st to end	Night	3 2 4 30% 47% 15%	400
July	1st to 10th	Night	1 1 3 26% 53% 15%	427
	11th to 20th	Night	2 1 18% 50% 29%	582
	21st to end	Night	2 1 2 22% 53% 19%	657
August	1st to 10th	Night	5% 1 3 26% 50% 15%	597
	11th to 20th	Night	2 1 4 30% 49% 15%	768
	21st to end	Night	3 4 24% 54% 16%	880
September	1st to 10th	Night	4 1 4 24% 49% 19%	800
	11th to 20th	Night	6% 3 5% 29% 46% 11%	800
	21st to end	Night	4 2 5% 27% 43% 18%	800
October	1st to 10th	Night	7% 2 4 34% 40% 12%	800
	11th to 20th	Night	7% 4 7% 31% 35% 16%	800
	21st to end	Night	7% 3 7% 31% 34% 17%	880
November	1st to 10th	Night	9% 5% 9% 32% 34% 12%	820
	11th to 20th	Night	12% 5% 12% 30% 33% 9%	840
	21st to end	Night	9% 5% 12% 39% 28% 8%	840
December	1st to 10th	Night	12% 7% 12% 35% 27% 7%	840
	11th to 20th	Night	18% 6% 11% 35% 24% 5%	840
	21st to end	Night	20% 9% 16% 32% 19% 4	924

Page generated at 2026-01-28 22:21:46. Version ac07

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