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 2024-06-30.

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	7% 7% 2 6% 4 4 4 4 6% 9% 48%	89563
	Night	8% 9% 3 7% 4 4 4 3 5% 8% 46%	89719
Transparency	Day	30% 3 6% 19% 24% 17%	89287
	Night	36% 3 6% 16% 20% 18%	89518
Seeing	Day	9% 9% 24% 33% 20% 6%	15023
	Night	11% 5% 9% 30% 34% 11%	24485

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	12% 8% 3 6% 5% 5% 5% 3 5% 7% 43%	2772
	11th to 20th	Day	13% 7% 3 9% 6% 5% 5% 4 5% 7% 36%	2860
	21st to end	Day	17% 10% 5% 8% 5% 4 4 4 6% 7% 31%	3146
February	1st to 10th	Day	14% 6% 2 5% 3 4 5% 4 5% 6% 47%	1710
	11th to 20th	Day	11% 7% 3 6% 5% 3 4 3 6% 7% 44%	1789
	21st to end	Day	10% 8% 2 5% 5% 4 4 3 5% 8% 46%	1570
March	1st to 10th	Day	9% 9% 3 8% 6% 6% 6% 6% 8% 10% 29%	3376
	11th to 20th	Day	8% 6% 2 6% 4 3 4 3 5% 7% 54%	3490
	21st to end	Day	10% 10% 3 7% 6% 5% 5% 5% 6% 8% 36%	3901
April	1st to 10th	Day	9% 8% 3 5% 5% 4 4 4 6% 8% 46%	2280
	11th to 20th	Day	7% 8% 3 6% 4 4 3 3 6% 7% 49%	2249
	21st to end	Day	5% 8% 2 6% 3 3 3 4 6% 9% 50%	2280
May	1st to 10th	Day	6% 4 1 4 2 2 2 2 3 8% 65%	3800
	11th to 20th	Day	4 5% 2 5% 4 3 3 3 6% 10% 58%	4096
	21st to end	Day	6% 13% 3 6% 3 3 3 3 5% 8% 45%	4813
June	1st to 10th	Day	4 7% 3 7% 5% 3 4 4 6% 9% 48%	2660
	11th to 20th	Day	3 4 2 5% 3 3 3 2 6% 11% 58%	2660
	21st to end	Day	2 6% 3 6% 4 4 4 3 6% 9% 53%	2647
July	1st to 10th	Day	1 7% 3 6% 3 3 4 4 6% 13% 49%	2481
	11th to 20th	Day	2 7% 2 7% 4 4 5% 5% 8% 13% 43%	2340
	21st to end	Day	1 5% 2 8% 4 4 4 4 7% 14% 47%	2546
August	1st to 10th	Day	3 8% 3 8% 4 3 4 4 9% 15% 39%	2328
	11th to 20th	Day	1 4 2 6% 4 4 4 4 7% 12% 53%	2144
	21st to end	Day	1 6% 3 7% 5% 3 4 4 6% 11% 51%	2376
September	1st to 10th	Day	2 4 1 3 3 3 3 2 5% 9% 63%	2160
	11th to 20th	Day	3 6% 2 6% 3 3 3 4 5% 11% 53%	2160
	21st to end	Day	2 3 2 4 3 3 2 3 5% 8% 65%	1922
October	1st to 10th	Day	3 4 1 4 4 3 3 3 6% 7% 63%	1800
	11th to 20th	Day	4 5% 2 5% 3 3 4 4 5% 8% 60%	1800
	21st to end	Day	5% 5% 2 4 2 3 4 4 5% 7% 60%	1980
November	1st to 10th	Day	6% 4 1 5% 4 4 4 4 6% 7% 54%	1845
	11th to 20th	Day	7% 7% 2 6% 4 3 4 4 6% 6% 53%	1710
	21st to end	Day	9% 5% 2 4 3 3 3 3 5% 8% 55%	1630
December	1st to 10th	Day	9% 6% 3 7% 5% 4 4 3 7% 9% 45%	1520
	11th to 20th	Day	16% 9% 3 8% 5% 5% 5% 3 5% 7% 34%	1928
	21st to end	Day	13% 9% 3 7% 5% 4 4 3 5% 8% 39%	2794
January	1st to 10th	Night	11% 7% 2 7% 4 3 4 3 5% 7% 46%	4176
	11th to 20th	Night	13% 9% 3 8% 5% 4 5% 4 6% 8% 34%	4110
	21st to end	Night	19% 11% 3 7% 4 4 4 3 5% 7% 32%	4521
February	1st to 10th	Night	13% 8% 2 6% 4 3 3 3 4 6% 48%	2470
	11th to 20th	Night	10% 9% 3 6% 4 4 4 4 5% 8% 42%	2276
	21st to end	Night	11% 9% 3 5% 4 3 3 3 6% 8% 47%	1884
March	1st to 10th	Night	14% 14% 3 8% 6% 5% 5% 4 5% 6% 31%	3495
	11th to 20th	Night	7% 8% 2 7% 4 3 4 3 4 6% 51%	3490
	21st to end	Night	13% 14% 3 7% 5% 5% 4 4 5% 8% 33%	3750
April	1st to 10th	Night	10% 9% 2 6% 4 3 4 4 6% 8% 44%	1900
	11th to 20th	Night	9% 9% 3 6% 3 3 3 3 4 8% 49%	1879
	21st to end	Night	6% 7% 2 5% 3 3 3 3 6% 11% 50%	1900
May	1st to 10th	Night	4 9% 3 4 3 2 3 3 4 9% 57%	3162
	11th to 20th	Night	4 11% 4 13% 5% 3 2 3 3 7% 44%	2802
	21st to end	Night	5% 18% 3 5% 2 2 3 2 3 7% 51%	2762
June	1st to 10th	Night	6% 11% 3 6% 3 3 4 3 4 8% 48%	1520
	11th to 20th	Night	2 7% 3 7% 4 3 3 2 5% 7% 56%	1520
	21st to end	Night	2 10% 2 6% 4 4 2 3 4 7% 54%	1512
July	1st to 10th	Night	1 13% 4 7% 4 4 4 2 5% 8% 48%	1474
	11th to 20th	Night	1 11% 4 7% 6% 5% 5% 2 6% 9% 44%	1620
	21st to end	Night	1 16% 5% 11% 6% 5% 5% 4 6% 9% 34%	1773
August	1st to 10th	Night	2 17% 5% 11% 4 4 4 4 5% 8% 36%	1611
	11th to 20th	Night	2 8% 3 11% 5% 4 4 3 4 7% 48%	1758
	21st to end	Night	2 11% 4 10% 5% 4 2 2 5% 9% 47%	1980
September	1st to 10th	Night	3 6% 2 5% 4 3 4 2 6% 7% 58%	1800
	11th to 20th	Night	3 8% 3 7% 4 3 3 3 4 7% 55%	1800
	21st to end	Night	3 5% 2 4 2 3 3 3 4 9% 63%	2034
October	1st to 10th	Night	5% 6% 2 6% 4 2 3 2 4 5% 61%	2160
	11th to 20th	Night	5% 5% 2 6% 4 4 4 3 4 7% 55%	2160
	21st to end	Night	5% 7% 2 5% 3 4 3 2 4 7% 59%	2376
November	1st to 10th	Night	7% 6% 2 5% 4 3 3 4 5% 8% 53%	2230
	11th to 20th	Night	8% 6% 2 5% 3 3 4 4 5% 8% 52%	2470
	21st to end	Night	7% 5% 2 4 3 3 4 3 5% 6% 58%	2546
December	1st to 10th	Night	10% 7% 2 6% 3 3 4 3 5% 7% 49%	2660
	11th to 20th	Night	15% 10% 3 8% 6% 4 5% 4 6% 7% 33%	3276
	21st to end	Night	14% 8% 3 7% 5% 5% 5% 4 6% 8% 36%	4862

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	46% 2 9% 29% 11% 3	2772
	11th to 20th	Day	50% 3 11% 17% 12% 7%	2860
	21st to end	Day	54% 3 7% 17% 14% 6%	3146
February	1st to 10th	Day	40% 3 6% 18% 21% 12%	1710
	11th to 20th	Day	41% 4 7% 19% 21% 9%	1799
	21st to end	Day	41% 5% 7% 23% 19% 6%	1570
March	1st to 10th	Day	55% 3 11% 15% 13% 3	3376
	11th to 20th	Day	32% 3 9% 25% 18% 12%	3490
	21st to end	Day	51% 3 11% 22% 11% 2	3901
April	1st to 10th	Day	39% 5% 10% 25% 17% 4	2280
	11th to 20th	Day	36% 3 10% 26% 19% 6%	2252
	21st to end	Day	32% 4 9% 28% 21% 6%	2280
May	1st to 10th	Day	23% 2 8% 28% 30% 10%	3800
	11th to 20th	Day	18% 2 3 24% 37% 17%	4099
	21st to end	Day	30% 2 6% 25% 31% 7%	4814
June	1st to 10th	Day	27% 4 9% 22% 27% 11%	2660
	11th to 20th	Day	16% 3 6% 20% 34% 21%	2660
	21st to end	Day	15% 2 2 12% 30% 38%	2647
July	1st to 10th	Day	15% 1 2 9% 26% 47%	2414
	11th to 20th	Day	15% 2 4 9% 24% 45%	2271
	21st to end	Day	10% 2 2 10% 30% 45%	2405
August	1st to 10th	Day	19% 3 1 10% 28% 39%	2312
	11th to 20th	Day	12% 2 1 13% 32% 41%	2144
	21st to end	Day	13% 2 1 13% 32% 38%	2376
September	1st to 10th	Day	12% 1 1 8% 29% 48%	2160
	11th to 20th	Day	18% 3 2 20% 33% 24%	2160
	21st to end	Day	15% 2 2 21% 39% 20%	1922
October	1st to 10th	Day	19% 3 6% 19% 29% 24%	1800
	11th to 20th	Day	21% 3 5% 22% 24% 25%	1800
	21st to end	Day	23% 3 6% 19% 26% 23%	1980
November	1st to 10th	Day	29% 4 8% 21% 24% 15%	1845
	11th to 20th	Day	31% 3 10% 19% 26% 12%	1710
	21st to end	Day	30% 4 7% 17% 27% 14%	1630
December	1st to 10th	Day	37% 4 9% 18% 18% 14%	1520
	11th to 20th	Day	53% 4 9% 15% 14% 6%	1928
	21st to end	Day	48% 3 12% 21% 12% 5%	2794
January	1st to 10th	Night	46% 2 12% 22% 14% 4	4176
	11th to 20th	Night	51% 3 13% 15% 10% 9%	4110
	21st to end	Night	56% 2 6% 13% 14% 8%	4521
February	1st to 10th	Night	40% 3 7% 16% 19% 15%	2470
	11th to 20th	Night	43% 4 6% 16% 22% 9%	2277
	21st to end	Night	42% 5% 7% 24% 16% 6%	1884
March	1st to 10th	Night	59% 2 10% 12% 13% 4	3495
	11th to 20th	Night	35% 3 7% 22% 23% 12%	3490
	21st to end	Night	56% 3 8% 18% 11% 4	3750
April	1st to 10th	Night	41% 5% 11% 22% 16% 6%	1900
	11th to 20th	Night	39% 4 9% 23% 17% 8%	1879
	21st to end	Night	33% 5% 8% 26% 20% 8%	1900
May	1st to 10th	Night	37% 2 4 19% 23% 15%	3162
	11th to 20th	Night	24% 3 3 12% 36% 22%	2803
	21st to end	Night	38% 2 5% 19% 28% 9%	2762
June	1st to 10th	Night	31% 4 7% 20% 22% 15%	1520
	11th to 20th	Night	21% 4 7% 15% 26% 27%	1520
	21st to end	Night	16% 2 2 11% 26% 44%	1512
July	1st to 10th	Night	20% 4 1 6% 23% 47%	1433
	11th to 20th	Night	16% 4 2 9% 23% 47%	1566
	21st to end	Night	21% 4 2 7% 24% 42%	1674
August	1st to 10th	Night	27% 5% 1 10% 18% 40%	1602
	11th to 20th	Night	16% 4 7% 25% 48%	1758
	21st to end	Night	18% 3 1 11% 26% 41%	1980
September	1st to 10th	Night	15% 2 1 6% 25% 51%	1800
	11th to 20th	Night	21% 3 1 17% 27% 32%	1800
	21st to end	Night	17% 2 2 16% 36% 27%	2034
October	1st to 10th	Night	21% 3 5% 16% 25% 30%	2160
	11th to 20th	Night	25% 3 3 18% 24% 26%	2160
	21st to end	Night	23% 3 4 19% 26% 25%	2376
November	1st to 10th	Night	29% 3 6% 20% 23% 19%	2230
	11th to 20th	Night	30% 3 9% 20% 24% 13%	2470
	21st to end	Night	29% 4 7% 18% 24% 18%	2546
December	1st to 10th	Night	36% 4 8% 17% 19% 16%	2660
	11th to 20th	Night	53% 4 9% 14% 13% 7%	3276
	21st to end	Night	51% 2 12% 17% 12% 5%	4862

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	28% 14% 22% 20% 14% 1	322
	11th to 20th	Day	15% 12% 25% 27% 19% 2	370
	21st to end	Day	16% 12% 23% 30% 15% 4	407
February	1st to 10th	Day	16% 16% 25% 21% 15% 7%	370
	11th to 20th	Day	19% 15% 29% 25% 11% 1	425
	21st to end	Day	13% 16% 30% 28% 11% 2	382
March	1st to 10th	Day	20% 15% 30% 24% 8% 1	460
	11th to 20th	Day	17% 15% 29% 24% 12% 3	460
	21st to end	Day	17% 18% 33% 23% 8% 1	506
April	1st to 10th	Day	16% 14% 30% 25% 13% 2	460
	11th to 20th	Day	12% 19% 29% 26% 12% 2	452
	21st to end	Day	10% 14% 31% 27% 12% 6%	460
May	1st to 10th	Day	9% 11% 30% 33% 13% 4	463
	11th to 20th	Day	10% 13% 29% 29% 15% 5%	488
	21st to end	Day	11% 11% 27% 30% 16% 5%	529
June	1st to 10th	Day	7% 6% 29% 33% 19% 6%	490
	11th to 20th	Day	3 7% 29% 38% 19% 4	490
	21st to end	Day	1 3 22% 43% 25% 6%	487
July	1st to 10th	Day	3 1 20% 44% 26% 6%	446
	11th to 20th	Day	1 1 12% 47% 29% 10%	444
	21st to end	Day	14% 47% 27% 11%	487
August	1st to 10th	Day	3 2 22% 45% 20% 8%	447
	11th to 20th	Day	1 14% 48% 26% 11%	437
	21st to end	Day	1 1 15% 47% 29% 7%	495
September	1st to 10th	Day	3 4 14% 41% 28% 9%	450
	11th to 20th	Day	5% 5% 24% 37% 22% 7%	450
	21st to end	Day	2 8% 17% 35% 31% 7%	450
October	1st to 10th	Day	5% 5% 21% 38% 26% 5%	450
	11th to 20th	Day	5% 7% 20% 30% 30% 9%	450
	21st to end	Day	6% 5% 22% 30% 28% 8%	495
November	1st to 10th	Day	6% 12% 19% 29% 26% 9%	441
	11th to 20th	Day	6% 9% 19% 33% 24% 10%	270
	21st to end	Day	9% 10% 19% 32% 23% 8%	232
December	1st to 10th	Day	13% 13% 21% 29% 18% 6%	180
	11th to 20th	Day	13% 15% 22% 20% 22% 8%	180
	21st to end	Day	17% 15% 24% 23% 14% 7%	198
January	1st to 10th	Night	18% 6% 13% 34% 22% 8%	760
	11th to 20th	Night	12% 8% 12% 33% 27% 8%	760
	21st to end	Night	18% 7% 11% 31% 28% 6%	836
February	1st to 10th	Night	16% 7% 13% 31% 25% 7%	760
	11th to 20th	Night	15% 10% 13% 31% 25% 5%	744
	21st to end	Night	16% 8% 16% 32% 24% 4	628
March	1st to 10th	Night	18% 11% 14% 30% 22% 5%	760
	11th to 20th	Night	15% 8% 14% 31% 26% 7%	760
	21st to end	Night	16% 9% 14% 31% 26% 4	836
April	1st to 10th	Night	14% 7% 15% 30% 29% 5%	760
	11th to 20th	Night	14% 11% 12% 29% 27% 7%	751
	21st to end	Night	11% 7% 16% 28% 29% 10%	760
May	1st to 10th	Night	9% 7% 12% 33% 30% 8%	755
	11th to 20th	Night	11% 4 9% 26% 37% 12%	543
	21st to end	Night	11% 4 12% 30% 33% 10%	411
June	1st to 10th	Night	7% 5% 8% 31% 39% 9%	380
	11th to 20th	Night	4 3 12% 33% 42% 7%	380
	21st to end	Night	3 2 4 30% 46% 16%	378
July	1st to 10th	Night	1 1 3 25% 52% 16%	383
	11th to 20th	Night	3 18% 50% 28%	522
	21st to end	Night	2 1 21% 55% 20%	591
August	1st to 10th	Night	6% 1 2 27% 49% 14%	537
	11th to 20th	Night	2 2 28% 51% 16%	690
	21st to end	Night	3 3 23% 55% 16%	792
September	1st to 10th	Night	4 1 3 22% 50% 21%	720
	11th to 20th	Night	4 3 4 29% 48% 12%	720
	21st to end	Night	4 2 5% 24% 45% 19%	720
October	1st to 10th	Night	8% 2 4 33% 40% 13%	720
	11th to 20th	Night	7% 4 5% 31% 37% 16%	720
	21st to end	Night	6% 3 7% 30% 35% 19%	792
November	1st to 10th	Night	9% 4 9% 32% 33% 12%	740
	11th to 20th	Night	10% 4 11% 30% 35% 10%	760
	21st to end	Night	8% 5% 11% 39% 29% 8%	760
December	1st to 10th	Night	13% 7% 12% 34% 26% 8%	760
	11th to 20th	Night	19% 7% 11% 34% 24% 6%	760
	21st to end	Night	20% 8% 15% 31% 21% 4	836

Page generated at 2024-06-26 19:18:42. Version ac07

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