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 2024-01-14.

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% 6% 3 5% 5% 3 4 4 6% 9% 49%	74685
	Night	8% 8% 3 6% 5% 3 4 4 5% 8% 47%	75085
Transparency	Day	19% 36% 5% 13% 16% 10%	74409
	Night	22% 38% 5% 11% 14% 10%	74884
Seeing	Day	9% 9% 23% 33% 20% 6%	14383
	Night	10% 5% 9% 30% 35% 11%	23909

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	15% 6% 3 4 5% 4 5% 4 5% 8% 40%	1661
	11th to 20th	Day	9% 5% 1 4 5% 2 4 6% 5% 7% 52%	1648
	21st to end	Day	12% 7% 3 6% 6% 3 4 4 5% 6% 44%	1782
February	1st to 10th	Day	14% 5% 2 4 4 2 5% 6% 5% 6% 48%	1620
	11th to 20th	Day	11% 6% 3 5% 6% 3 4 4 6% 7% 45%	1693
	21st to end	Day	10% 8% 3 5% 6% 3 4 4 5% 8% 46%	1480
March	1st to 10th	Day	12% 8% 3 7% 5% 3 4 6% 5% 10% 37%	1936
	11th to 20th	Day	11% 7% 3 7% 6% 3 6% 5% 7% 9% 37%	1980
	21st to end	Day	9% 7% 3 4 5% 4 4 5% 6% 8% 45%	2216
April	1st to 10th	Day	9% 7% 3 5% 5% 3 4 5% 6% 8% 45%	2160
	11th to 20th	Day	8% 8% 3 6% 4 3 3 3 5% 8% 49%	2129
	21st to end	Day	5% 7% 3 5% 4 2 3 4 6% 9% 51%	2160
May	1st to 10th	Day	5% 6% 3 5% 4 3 4 5% 5% 11% 50%	2160
	11th to 20th	Day	6% 7% 2 5% 4 2 4 4 6% 10% 50%	2313
	21st to end	Day	8% 12% 3 6% 4 3 5% 4 6% 9% 41%	2723
June	1st to 10th	Day	4 7% 3 6% 5% 3 4 4 6% 9% 48%	2520
	11th to 20th	Day	3 4 2 4 4 2 3 3 5% 11% 58%	2520
	21st to end	Day	3 6% 3 5% 5% 3 4 4 6% 8% 54%	2520
July	1st to 10th	Day	1 7% 3 5% 4 2 4 5% 6% 13% 49%	2481
	11th to 20th	Day	3 6% 2 6% 5% 3 4 6% 8% 13% 44%	2340
	21st to end	Day	1 4 3 7% 4 3 4 5% 7% 14% 48%	2546
August	1st to 10th	Day	3 8% 4 7% 5% 2 4 4 8% 15% 40%	2328
	11th to 20th	Day	1 4 2 5% 5% 3 4 5% 7% 12% 54%	2144
	21st to end	Day	1 6% 3 6% 5% 3 4 5% 6% 11% 51%	2376
September	1st to 10th	Day	2 4 2 3 3 3 3 3 5% 9% 64%	2160
	11th to 20th	Day	3 6% 2 5% 4 2 4 4 5% 11% 54%	2160
	21st to end	Day	2 2 2 4 4 2 3 4 5% 8% 65%	1922
October	1st to 10th	Day	3 3 1 3 4 2 4 3 6% 7% 64%	1800
	11th to 20th	Day	4 4 2 4 3 3 3 4 5% 8% 60%	1800
	21st to end	Day	5% 5% 2 3 3 2 4 4 5% 7% 60%	1980
November	1st to 10th	Day	7% 3 2 4 5% 3 4 5% 6% 7% 55%	1845
	11th to 20th	Day	7% 6% 2 5% 4 2 4 4 5% 7% 53%	1710
	21st to end	Day	9% 4 2 4 3 3 3 4 4 9% 55%	1630
December	1st to 10th	Day	9% 5% 3 6% 6% 3 4 4 6% 9% 45%	1520
	11th to 20th	Day	16% 8% 4 7% 7% 4 5% 4 5% 6% 34%	1928
	21st to end	Day	13% 8% 3 6% 6% 3 4 4 5% 8% 39%	2794
January	1st to 10th	Night	12% 7% 3 6% 6% 3 4 4 5% 8% 41%	2500
	11th to 20th	Night	10% 7% 3 5% 5% 3 5% 5% 4 8% 46%	2390
	21st to end	Night	12% 7% 3 4 5% 3 4 4 5% 8% 45%	2574
February	1st to 10th	Night	12% 6% 3 5% 4 2 4 4 4 6% 49%	2340
	11th to 20th	Night	11% 8% 3 5% 5% 3 4 5% 5% 8% 43%	2153
	21st to end	Night	11% 8% 3 4 4 2 3 4 6% 7% 47%	1776
March	1st to 10th	Night	13% 10% 4 5% 6% 3 4 5% 5% 6% 38%	2021
	11th to 20th	Night	11% 9% 3 7% 5% 2 5% 5% 4 9% 40%	1980
	21st to end	Night	10% 9% 3 5% 4 3 4 5% 4 9% 45%	2138
April	1st to 10th	Night	10% 8% 3 6% 4 2 5% 5% 5% 8% 45%	1800
	11th to 20th	Night	10% 8% 4 5% 4 2 3 4 4 7% 50%	1779
	21st to end	Night	6% 7% 3 5% 4 2 3 4 5% 11% 50%	1800
May	1st to 10th	Night	5% 9% 4 6% 5% 2 4 4 5% 8% 49%	1796
	11th to 20th	Night	8% 10% 4 7% 6% 3 3 5% 4 8% 43%	1576
	21st to end	Night	9% 14% 4 5% 4 2 4 3 4 8% 41%	1552
June	1st to 10th	Night	7% 9% 3 5% 4 3 3 4 4 8% 48%	1440
	11th to 20th	Night	2 7% 3 6% 6% 3 3 3 4 8% 56%	1440
	21st to end	Night	2 10% 2 5% 5% 3 4 4 3 7% 54%	1440
July	1st to 10th	Night	1 13% 4 6% 5% 3 4 3 5% 8% 48%	1474
	11th to 20th	Night	2 10% 4 6% 6% 4 5% 4 5% 10% 45%	1620
	21st to end	Night	2 14% 5% 9% 7% 4 5% 4 6% 9% 34%	1773
August	1st to 10th	Night	3 16% 5% 10% 5% 3 4 5% 5% 7% 37%	1611
	11th to 20th	Night	2 7% 4 10% 6% 3 4 4 4 8% 48%	1758
	21st to end	Night	2 10% 4 9% 6% 3 4 3 4 9% 47%	1980
September	1st to 10th	Night	3 5% 2 5% 5% 3 4 3 5% 8% 59%	1800
	11th to 20th	Night	3 7% 3 6% 5% 2 4 3 4 7% 55%	1800
	21st to end	Night	3 4 3 4 3 2 3 3 4 9% 63%	2034
October	1st to 10th	Night	5% 5% 2 5% 4 2 3 3 4 5% 61%	2160
	11th to 20th	Night	5% 5% 3 5% 5% 3 4 4 5% 7% 55%	2160
	21st to end	Night	5% 7% 2 4 4 3 3 3 4 7% 59%	2376
November	1st to 10th	Night	7% 6% 3 5% 4 3 3 5% 4 9% 53%	2230
	11th to 20th	Night	8% 6% 3 5% 3 2 4 4 5% 9% 52%	2470
	21st to end	Night	7% 5% 2 3 4 2 4 4 5% 6% 58%	2546
December	1st to 10th	Night	11% 7% 2 5% 4 2 4 4 5% 7% 49%	2660
	11th to 20th	Night	16% 9% 3 7% 6% 3 5% 5% 6% 7% 33%	3276
	21st to end	Night	14% 7% 3 6% 5% 4 5% 5% 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	24% 41% 8% 16% 9% 3	1661
	11th to 20th	Day	17% 42% 5% 12% 15% 9%	1648
	21st to end	Day	30% 37% 6% 6% 15% 6%	1782
February	1st to 10th	Day	23% 36% 5% 13% 15% 9%	1620
	11th to 20th	Day	26% 38% 5% 13% 13% 5%	1703
	21st to end	Day	29% 38% 5% 12% 13% 4	1480
March	1st to 10th	Day	28% 39% 6% 14% 11% 3	1936
	11th to 20th	Day	29% 39% 8% 12% 9% 2	1980
	21st to end	Day	27% 39% 9% 13% 10% 2	2216
April	1st to 10th	Day	24% 38% 9% 14% 11% 3	2160
	11th to 20th	Day	26% 37% 8% 14% 11% 5%	2132
	21st to end	Day	21% 37% 7% 19% 13% 3	2160
May	1st to 10th	Day	16% 37% 7% 16% 19% 5%	2160
	11th to 20th	Day	14% 36% 5% 22% 17% 6%	2316
	21st to end	Day	21% 37% 11% 17% 13% 2	2724
June	1st to 10th	Day	21% 38% 7% 16% 15% 3	2520
	11th to 20th	Day	13% 35% 7% 15% 19% 12%	2520
	21st to end	Day	12% 34% 2 14% 22% 16%	2520
July	1st to 10th	Day	9% 36% 1 7% 21% 27%	2414
	11th to 20th	Day	12% 35% 1 9% 21% 22%	2271
	21st to end	Day	10% 35% 2 10% 23% 19%	2405
August	1st to 10th	Day	15% 34% 1 8% 24% 18%	2312
	11th to 20th	Day	10% 33% 2 11% 21% 24%	2144
	21st to end	Day	10% 34% 1 9% 25% 20%	2376
September	1st to 10th	Day	7% 34% 1 8% 19% 30%	2160
	11th to 20th	Day	14% 35% 3 16% 22% 10%	2160
	21st to end	Day	8% 35% 4 15% 24% 14%	1922
October	1st to 10th	Day	15% 36% 6% 13% 18% 12%	1800
	11th to 20th	Day	13% 35% 4 18% 15% 15%	1800
	21st to end	Day	15% 37% 5% 13% 15% 14%	1980
November	1st to 10th	Day	21% 42% 7% 12% 12% 6%	1845
	11th to 20th	Day	18% 39% 7% 12% 15% 9%	1710
	21st to end	Day	18% 40% 4 10% 16% 12%	1630
December	1st to 10th	Day	20% 40% 5% 11% 15% 8%	1520
	11th to 20th	Day	39% 34% 6% 10% 8% 2	1928
	21st to end	Day	39% 28% 9% 16% 7% 1	2794
January	1st to 10th	Night	24% 41% 8% 14% 10% 3	2500
	11th to 20th	Night	19% 42% 7% 9% 13% 11%	2390
	21st to end	Night	26% 39% 7% 8% 12% 9%	2574
February	1st to 10th	Night	23% 36% 4 12% 14% 11%	2340
	11th to 20th	Night	26% 39% 5% 10% 15% 6%	2154
	21st to end	Night	27% 39% 6% 15% 9% 5%	1776
March	1st to 10th	Night	30% 37% 8% 11% 10% 4	2021
	11th to 20th	Night	31% 39% 6% 11% 10% 3	1980
	21st to end	Night	27% 41% 7% 11% 10% 4	2138
April	1st to 10th	Night	24% 41% 9% 13% 9% 4	1800
	11th to 20th	Night	26% 39% 7% 12% 11% 5%	1779
	21st to end	Night	19% 39% 5% 18% 14% 4	1800
May	1st to 10th	Night	17% 39% 5% 13% 18% 7%	1796
	11th to 20th	Night	20% 39% 4 15% 18% 5%	1577
	21st to end	Night	21% 41% 7% 13% 15% 3	1552
June	1st to 10th	Night	22% 39% 6% 15% 13% 4	1440
	11th to 20th	Night	15% 37% 4 9% 18% 16%	1440
	21st to end	Night	15% 35% 2 10% 21% 18%	1440
July	1st to 10th	Night	13% 38% 4 18% 27%	1433
	11th to 20th	Night	14% 36% 1 7% 20% 22%	1566
	21st to end	Night	22% 36% 2 7% 17% 17%	1674
August	1st to 10th	Night	23% 35% 1 7% 14% 19%	1602
	11th to 20th	Night	15% 33% 1 9% 19% 23%	1758
	21st to end	Night	15% 35% 1 6% 19% 24%	1980
September	1st to 10th	Night	9% 34% 1 6% 18% 32%	1800
	11th to 20th	Night	16% 37% 3 10% 22% 12%	1800
	21st to end	Night	9% 36% 2 13% 25% 15%	2034
October	1st to 10th	Night	17% 35% 5% 14% 15% 14%	2160
	11th to 20th	Night	17% 35% 3 14% 17% 14%	2160
	21st to end	Night	16% 37% 3 15% 16% 13%	2376
November	1st to 10th	Night	22% 41% 6% 11% 10% 10%	2230
	11th to 20th	Night	18% 40% 7% 12% 15% 8%	2470
	21st to end	Night	16% 40% 5% 10% 14% 14%	2546
December	1st to 10th	Night	21% 40% 4 10% 15% 10%	2660
	11th to 20th	Night	39% 35% 6% 9% 8% 3	3276
	21st to end	Night	39% 32% 10% 11% 5% 2	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	14% 12% 26% 27% 19% 2	350
	21st to end	Day	16% 11% 23% 30% 16% 4	374
February	1st to 10th	Day	15% 15% 24% 22% 17% 7%	340
	11th to 20th	Day	18% 14% 29% 26% 11% 2	389
	21st to end	Day	14% 17% 28% 28% 11% 2	346
March	1st to 10th	Day	20% 14% 31% 25% 9% 1	420
	11th to 20th	Day	18% 15% 29% 24% 11% 3	420
	21st to end	Day	16% 18% 34% 23% 8% 2	462
April	1st to 10th	Day	16% 14% 31% 26% 12% 2	420
	11th to 20th	Day	12% 19% 31% 25% 12% 2	412
	21st to end	Day	9% 14% 30% 27% 13% 6%	420
May	1st to 10th	Day	9% 11% 30% 33% 14% 4	423
	11th to 20th	Day	10% 14% 30% 28% 13% 4	448
	21st to end	Day	12% 11% 28% 29% 15% 5%	485
June	1st to 10th	Day	7% 6% 29% 33% 19% 6%	450
	11th to 20th	Day	3 7% 28% 38% 20% 4	450
	21st to end	Day	2 3 23% 43% 25% 5%	450
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% 32% 28% 9%	736
	21st to end	Night	17% 7% 11% 31% 29% 6%	792
February	1st to 10th	Night	14% 7% 12% 32% 27% 8%	720
	11th to 20th	Night	15% 10% 13% 31% 26% 6%	704
	21st to end	Night	17% 9% 16% 31% 25% 4	592
March	1st to 10th	Night	17% 10% 15% 30% 23% 5%	720
	11th to 20th	Night	15% 8% 13% 31% 26% 7%	720
	21st to end	Night	15% 9% 14% 30% 27% 4	792
April	1st to 10th	Night	14% 7% 15% 30% 29% 5%	720
	11th to 20th	Night	14% 11% 12% 29% 27% 7%	711
	21st to end	Night	11% 6% 16% 28% 29% 11%	720
May	1st to 10th	Night	9% 7% 12% 33% 31% 9%	716
	11th to 20th	Night	12% 5% 10% 25% 36% 12%	514
	21st to end	Night	11% 4 12% 29% 33% 11%	389
June	1st to 10th	Night	8% 5% 8% 31% 39% 10%	360
	11th to 20th	Night	4 3 10% 33% 43% 8%	360
	21st to end	Night	3 2 3 31% 45% 16%	360
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 2024-01-11 10:31:58. Version ac07

Copyright 2024 Attilla Danko