Tujunga Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tujunga Observatory clear sky chart from 2005-11-06 to 2024-04-25.

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% 3 1 3 3 2 3 3 4 6% 67%	83813
	Night	6% 4 1 3 3 2 3 3 4 6% 67%	85395
Transparency	Day	19% 4 8% 18% 27% 25%	83537
	Night	19% 4 8% 17% 26% 26%	85192
Seeing	Day	7% 8% 23% 36% 21% 5%	14837
	Night	7% 4 12% 36% 34% 7%	24763

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	7% 2 1 3 3 1 3 3 4 6% 66%	2539
	11th to 20th	Day	9% 4 2 3 3 2 2 3 6% 9% 56%	2796
	21st to end	Day	13% 5% 2 4 3 2 3 4 5% 7% 53%	3469
February	1st to 10th	Day	11% 4 2 3 2 2 2 3 4 6% 61%	1900
	11th to 20th	Day	9% 3 1 3 3 2 3 4 5% 6% 60%	1850
	21st to end	Day	9% 7% 2 4 4 4 3 3 4 6% 55%	1570
March	1st to 10th	Day	14% 7% 2 4 3 3 3 5% 6% 8% 46%	3138
	11th to 20th	Day	6% 4 1 3 2 2 3 4 5% 8% 62%	3285
	21st to end	Day	10% 5% 2 4 4 2 4 5% 6% 8% 51%	4165
April	1st to 10th	Day	7% 4 2 3 3 2 2 4 4 8% 60%	2280
	11th to 20th	Day	4 3 2 3 3 2 3 3 3 6% 68%	2249
	21st to end	Day	4 3 1 2 3 2 3 3 4 6% 68%	2210
May	1st to 10th	Day	5% 5% 1 3 3 2 3 4 4 7% 63%	2209
	11th to 20th	Day	4 3 1 2 2 1 3 4 4 7% 68%	2301
	21st to end	Day	4 4 1 3 3 2 3 3 4 6% 67%	2530
June	1st to 10th	Day	3 3 1 2 2 2 3 3 4 8% 69%	2388
	11th to 20th	Day	1 2 1 2 2 1 2 2 3 5% 80%	2520
	21st to end	Day	1 2 1 1 1 2 2 2 4 84%	2520
July	1st to 10th	Day	1 1 1 2 2 1 3 3 3 4 79%	2520
	11th to 20th	Day	2 2 1 2 2 1 2 2 3 4 80%	2520
	21st to end	Day	2 1 1 2 3 1 3 3 5% 6% 74%	2374
August	1st to 10th	Day	1 1 1 2 1 2 2 2 3 3 83%	2149
	11th to 20th	Day	1 1 1 1 2 2 2 4 86%	2122
	21st to end	Day	1 1 2 1 1 2 3 3 4 83%	2376
September	1st to 10th	Day	2 1 2 2 1 2 3 3 4 81%	2160
	11th to 20th	Day	1 1 1 2 1 1 2 3 4 84%	2036
	21st to end	Day	2 2 1 1 2 1 2 2 2 4 81%	1980
October	1st to 10th	Day	4 2 1 2 3 1 2 3 4 5% 72%	1980
	11th to 20th	Day	3 2 1 2 2 2 2 3 3 3 77%	1847
	21st to end	Day	3 2 1 3 2 1 3 3 3 4 75%	1949
November	1st to 10th	Day	4 3 2 3 3 1 2 3 3 5% 72%	1674
	11th to 20th	Day	5% 3 2 4 3 1 3 3 3 6% 69%	1710
	21st to end	Day	5% 4 2 4 4 2 3 3 6% 7% 62%	1710
December	1st to 10th	Day	7% 5% 1 4 3 3 3 4 4 8% 57%	1710
	11th to 20th	Day	17% 6% 2 5% 4 2 4 4 5% 8% 43%	2169
	21st to end	Day	13% 7% 2 5% 6% 3 4 4 4 6% 46%	2908
January	1st to 10th	Night	8% 4 2 2 2 2 3 3 4 5% 66%	4406
	11th to 20th	Night	7% 5% 2 4 3 2 3 4 4 8% 57%	4168
	21st to end	Night	14% 6% 2 3 2 1 3 4 5% 6% 54%	4203
February	1st to 10th	Night	9% 5% 2 4 3 2 2 3 4 6% 61%	2280
	11th to 20th	Night	9% 4 1 3 3 1 3 3 4 5% 64%	2218
	21st to end	Night	11% 6% 2 4 4 2 3 3 4 5% 55%	1884
March	1st to 10th	Night	11% 9% 2 3 3 2 4 3 5% 8% 48%	3736
	11th to 20th	Night	5% 3 1 3 3 1 3 4 5% 8% 64%	3690
	21st to end	Night	10% 5% 1 3 4 2 3 3 5% 7% 58%	3488
April	1st to 10th	Night	6% 4 2 3 3 2 3 2 4 6% 64%	1900
	11th to 20th	Night	4 4 1 2 3 2 3 3 4 6% 69%	1879
	21st to end	Night	3 3 2 2 2 2 2 3 4 6% 70%	1850
May	1st to 10th	Night	4 4 1 3 3 3 3 3 6% 7% 63%	1747
	11th to 20th	Night	3 3 1 3 3 2 4 3 2 4 72%	1594
	21st to end	Night	2 2 1 3 2 2 3 2 3 7% 73%	1746
June	1st to 10th	Night	1 3 1 2 2 1 3 3 4 8% 72%	1566
	11th to 20th	Night	1 1 1 1 1 2 2 3 3 84%	1440
	21st to end	Night	2 1 1 1 1 1 2 2 3 87%	1440
July	1st to 10th	Night	1 2 1 1 2 1 3 4 85%	1440
	11th to 20th	Night	1 1 1 2 2 1 2 3 3 5% 79%	1440
	21st to end	Night	3 2 1 2 2 2 2 2 4 6% 75%	1936
August	1st to 10th	Night	1 1 1 2 2 1 2 2 3 3 81%	1790
	11th to 20th	Night	1 1 1 1 1 1 1 4 3 86%	1780
	21st to end	Night	2 1 1 1 1 2 2 3 4 85%	1980
September	1st to 10th	Night	2 1 1 1 1 2 2 4 5% 81%	1800
	11th to 20th	Night	1 1 1 1 1 1 1 2 2 2 87%	1922
	21st to end	Night	1 1 1 1 1 1 1 2 2 4 84%	1980
October	1st to 10th	Night	4 2 3 3 2 3 3 3 3 75%	1980
	11th to 20th	Night	3 3 1 2 2 1 2 2 2 4 78%	2109
	21st to end	Night	3 3 1 2 3 2 2 2 3 4 76%	2405
November	1st to 10th	Night	4 3 1 2 2 2 2 3 3 5% 72%	2405
	11th to 20th	Night	4 3 2 3 3 1 2 3 4 6% 71%	2470
	21st to end	Night	5% 3 2 4 3 2 4 4 4 6% 62%	2470
December	1st to 10th	Night	8% 4 2 4 3 2 3 4 4 7% 59%	2470
	11th to 20th	Night	17% 5% 3 4 4 2 3 4 4 7% 46%	3041
	21st to end	Night	11% 6% 3 6% 5% 3 4 4 5% 7% 47%	4742

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	19% 3 5% 23% 32% 18%	2539
	11th to 20th	Day	24% 6% 15% 19% 20% 16%	2796
	21st to end	Day	31% 5% 6% 17% 25% 16%	3469
February	1st to 10th	Day	25% 4 5% 13% 25% 27%	1900
	11th to 20th	Day	23% 4 5% 16% 27% 24%	1860
	21st to end	Day	29% 5% 7% 16% 23% 20%	1570
March	1st to 10th	Day	33% 8% 10% 19% 15% 16%	3138
	11th to 20th	Day	18% 6% 14% 30% 17% 15%	3285
	21st to end	Day	28% 8% 14% 28% 15% 8%	4165
April	1st to 10th	Day	23% 5% 11% 24% 24% 13%	2280
	11th to 20th	Day	19% 4 8% 22% 29% 18%	2252
	21st to end	Day	17% 4 9% 23% 35% 12%	2210
May	1st to 10th	Day	21% 4 11% 21% 29% 14%	2209
	11th to 20th	Day	17% 4 9% 24% 32% 13%	2304
	21st to end	Day	18% 5% 11% 27% 30% 9%	2530
June	1st to 10th	Day	14% 4 11% 20% 33% 18%	2388
	11th to 20th	Day	9% 3 7% 14% 34% 33%	2520
	21st to end	Day	8% 2 4 9% 34% 42%	2520
July	1st to 10th	Day	9% 2 4 11% 26% 48%	2451
	11th to 20th	Day	11% 3 5% 17% 32% 33%	2445
	21st to end	Day	11% 4 7% 21% 34% 23%	2241
August	1st to 10th	Day	8% 2 5% 11% 31% 44%	2135
	11th to 20th	Day	6% 1 3 13% 37% 40%	2122
	21st to end	Day	7% 2 4 10% 37% 40%	2376
September	1st to 10th	Day	9% 2 5% 9% 33% 42%	2160
	11th to 20th	Day	6% 1 5% 13% 36% 39%	2036
	21st to end	Day	9% 2 4 14% 38% 33%	1980
October	1st to 10th	Day	15% 4 5% 17% 25% 34%	1980
	11th to 20th	Day	13% 3 4 14% 29% 37%	1847
	21st to end	Day	13% 5% 4 12% 23% 44%	1949
November	1st to 10th	Day	17% 3 5% 16% 28% 31%	1674
	11th to 20th	Day	19% 3 5% 13% 21% 39%	1710
	21st to end	Day	23% 3 6% 12% 24% 31%	1710
December	1st to 10th	Day	26% 5% 6% 15% 23% 26%	1710
	11th to 20th	Day	38% 5% 8% 13% 20% 16%	2168
	21st to end	Day	37% 5% 7% 23% 16% 12%	2908
January	1st to 10th	Night	22% 4 6% 13% 34% 21%	4406
	11th to 20th	Night	25% 5% 11% 23% 19% 17%	4168
	21st to end	Night	30% 5% 7% 16% 25% 16%	4203
February	1st to 10th	Night	26% 3 6% 13% 24% 28%	2280
	11th to 20th	Night	24% 3 7% 15% 26% 26%	2219
	21st to end	Night	30% 4 8% 15% 20% 23%	1884
March	1st to 10th	Night	30% 7% 9% 21% 18% 15%	3736
	11th to 20th	Night	17% 7% 14% 28% 18% 16%	3690
	21st to end	Night	24% 6% 12% 28% 22% 9%	3488
April	1st to 10th	Night	23% 4 10% 24% 22% 17%	1900
	11th to 20th	Night	16% 6% 8% 24% 26% 20%	1879
	21st to end	Night	16% 3 12% 22% 34% 13%	1850
May	1st to 10th	Night	19% 7% 11% 21% 27% 16%	1747
	11th to 20th	Night	17% 4 10% 25% 31% 13%	1595
	21st to end	Night	13% 4 13% 28% 30% 12%	1746
June	1st to 10th	Night	11% 4 15% 21% 27% 22%	1566
	11th to 20th	Night	6% 3 9% 16% 32% 33%	1440
	21st to end	Night	7% 1 5% 8% 33% 46%	1440
July	1st to 10th	Night	6% 1 4 10% 27% 51%	1400
	11th to 20th	Night	9% 3 5% 18% 28% 36%	1392
	21st to end	Night	11% 3 6% 21% 34% 24%	1829
August	1st to 10th	Night	9% 2 4 8% 34% 42%	1780
	11th to 20th	Night	5% 1 4 10% 41% 39%	1780
	21st to end	Night	7% 2 3 10% 38% 41%	1980
September	1st to 10th	Night	8% 1 6% 10% 28% 47%	1800
	11th to 20th	Night	6% 2 5% 10% 35% 43%	1922
	21st to end	Night	8% 2 3 13% 35% 38%	1980
October	1st to 10th	Night	16% 2 4 17% 25% 36%	1980
	11th to 20th	Night	12% 3 6% 14% 27% 38%	2109
	21st to end	Night	14% 4 3 9% 24% 46%	2405
November	1st to 10th	Night	17% 2 5% 14% 27% 34%	2405
	11th to 20th	Night	17% 3 6% 11% 24% 39%	2470
	21st to end	Night	23% 3 6% 16% 19% 33%	2470
December	1st to 10th	Night	26% 4 6% 14% 20% 30%	2470
	11th to 20th	Night	37% 5% 7% 12% 22% 16%	3041
	21st to end	Night	32% 7% 10% 22% 16% 13%	4742

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	18% 5% 29% 31% 14% 3	180
	11th to 20th	Day	13% 10% 33% 29% 13% 2	332
	21st to end	Day	10% 17% 30% 31% 11% 1	499
February	1st to 10th	Day	15% 13% 29% 28% 12% 3	460
	11th to 20th	Day	15% 14% 32% 29% 9% 1	455
	21st to end	Day	13% 18% 34% 28% 7%	382
March	1st to 10th	Day	15% 17% 34% 22% 11% 1	460
	11th to 20th	Day	15% 15% 31% 29% 8% 2	460
	21st to end	Day	11% 18% 33% 27% 9%	506
April	1st to 10th	Day	11% 12% 31% 29% 16% 2	460
	11th to 20th	Day	9% 15% 34% 31% 9% 2	452
	21st to end	Day	5% 16% 30% 30% 14% 5%	437
May	1st to 10th	Day	9% 13% 28% 32% 13% 4	423
	11th to 20th	Day	7% 9% 29% 33% 19% 3	448
	21st to end	Day	8% 8% 30% 34% 16% 5%	485
June	1st to 10th	Day	4 6% 21% 37% 25% 7%	450
	11th to 20th	Day	3 5% 17% 42% 26% 7%	450
	21st to end	Day	4 2 18% 38% 29% 9%	450
July	1st to 10th	Day	1 1 7% 45% 36% 10%	446
	11th to 20th	Day	1 4 37% 43% 15%	444
	21st to end	Day	2 5% 40% 41% 11%	487
August	1st to 10th	Day	2 1 11% 45% 32% 9%	447
	11th to 20th	Day	2 1 6% 43% 36% 11%	437
	21st to end	Day	1 1 8% 47% 35% 8%	495
September	1st to 10th	Day	3 2 16% 44% 28% 7%	450
	11th to 20th	Day	2 3 21% 43% 27% 4	450
	21st to end	Day	2 5% 23% 40% 25% 6%	450
October	1st to 10th	Day	4 8% 24% 38% 23% 3	450
	11th to 20th	Day	3 7% 26% 41% 19% 3	450
	21st to end	Day	5% 10% 25% 33% 23% 4	464
November	1st to 10th	Day	6% 6% 21% 38% 25% 4	270
	11th to 20th	Day	6% 8% 21% 41% 19% 4	270
	21st to end	Day	6% 10% 27% 34% 19% 3	270
December	1st to 10th	Day	13% 4 24% 38% 18% 3	270
	11th to 20th	Day	9% 10% 19% 40% 18% 4	270
	21st to end	Day	22% 11% 23% 33% 10% 1	228
January	1st to 10th	Night	13% 5% 16% 44% 17% 4	760
	11th to 20th	Night	9% 4 20% 43% 21% 3	760
	21st to end	Night	14% 6% 19% 41% 18% 2	836
February	1st to 10th	Night	11% 6% 16% 38% 24% 4	760
	11th to 20th	Night	11% 6% 17% 39% 22% 5%	744
	21st to end	Night	13% 7% 21% 36% 20% 3	628
March	1st to 10th	Night	11% 7% 18% 42% 21% 2	760
	11th to 20th	Night	10% 6% 15% 38% 26% 6%	760
	21st to end	Night	9% 7% 19% 42% 19% 4	836
April	1st to 10th	Night	8% 5% 15% 43% 25% 3	760
	11th to 20th	Night	5% 7% 19% 39% 29% 2	751
	21st to end	Night	4 9% 15% 37% 29% 6%	740
May	1st to 10th	Night	8% 3 18% 38% 29% 4	667
	11th to 20th	Night	5% 4 14% 38% 36% 4	531
	21st to end	Night	3 4 17% 38% 35% 4	584
June	1st to 10th	Night	2 3 10% 38% 44% 3	486
	11th to 20th	Night	1 2 6% 36% 50% 6%	360
	21st to end	Night	2 1 2 33% 56% 6%	360
July	1st to 10th	Night	1 1 17% 70% 11%	350
	11th to 20th	Night	1 13% 72% 13%	348
	21st to end	Night	3 1 21% 61% 14%	754
August	1st to 10th	Night	2 1 3 25% 56% 13%	716
	11th to 20th	Night	2 1 1 16% 61% 19%	712
	21st to end	Night	2 1 21% 58% 17%	792
September	1st to 10th	Night	3 1 3 24% 53% 17%	720
	11th to 20th	Night	2 1 3 31% 51% 13%	720
	21st to end	Night	2 1 6% 33% 48% 10%	720
October	1st to 10th	Night	5% 1 8% 45% 37% 4	720
	11th to 20th	Night	5% 3 10% 36% 38% 8%	720
	21st to end	Night	5% 3 13% 36% 31% 12%	792
November	1st to 10th	Night	5% 5% 12% 39% 31% 8%	740
	11th to 20th	Night	6% 2 14% 43% 28% 8%	760
	21st to end	Night	8% 3 16% 43% 24% 6%	760
December	1st to 10th	Night	11% 5% 16% 38% 26% 4	760
	11th to 20th	Night	13% 6% 18% 41% 20% 2	760
	21st to end	Night	11% 9% 19% 41% 16% 4	836

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