Mount Wilson Observatory
Clear Sky Chart
History

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

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	5% 3 1 3 2 2 3 2 4 5% 71%	105110
	Night	5% 3 1 3 2 2 3 2 4 5% 70%	102705
Transparency	Day	16% 4 7% 16% 27% 30%	104871
	Night	17% 4 8% 16% 25% 30%	102533
Seeing	Day	8% 12% 32% 34% 11% 3	15622
	Night	6% 5% 20% 36% 27% 6%	25464

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% 3 1 3 2 2 3 3 4 6% 67%	2539
	11th to 20th	Day	10% 5% 1 3 2 2 3 2 5% 8% 57%	2826
	21st to end	Day	14% 5% 2 4 3 3 5% 3 4 7% 52%	3498
February	1st to 10th	Day	12% 4 1 3 2 2 2 2 4 5% 62%	1900
	11th to 20th	Day	11% 5% 1 3 2 2 2 3 5% 6% 60%	1850
	21st to end	Day	10% 8% 2 5% 2 2 3 2 4 6% 55%	1570
March	1st to 10th	Day	14% 7% 2 6% 4 3 3 4 5% 7% 44%	3138
	11th to 20th	Day	6% 4 2 4 3 3 3 2 6% 8% 60%	3261
	21st to end	Day	11% 6% 1 4 3 3 5% 4 6% 8% 50%	4165
April	1st to 10th	Day	8% 5% 2 3 3 2 2 3 4 7% 60%	2280
	11th to 20th	Day	5% 5% 1 3 3 2 4 2 4 6% 66%	2249
	21st to end	Day	6% 4 1 2 2 2 3 3 4 6% 68%	2280
May	1st to 10th	Day	4 3 1 2 2 2 3 2 3 5% 74%	3919
	11th to 20th	Day	2 2 1 2 2 2 5% 3 5% 5% 71%	4071
	21st to end	Day	3 2 1 3 2 3 4 3 3 5% 70%	4477
June	1st to 10th	Day	4 3 1 2 2 2 3 3 4 6% 72%	2478
	11th to 20th	Day	2 2 1 1 1 1 2 2 3 5% 81%	2660
	21st to end	Day	1 1 1 2 1 1 2 2 2 3 84%	2660
July	1st to 10th	Day	1 1 1 1 1 1 1 2 2 89%	4420
	11th to 20th	Day	2 2 1 2 2 1 2 1 3 3 82%	4362
	21st to end	Day	2 1 1 1 1 1 1 3 4 84%	4187
August	1st to 10th	Day	1 1 1 1 1 2 1 2 3 86%	3783
	11th to 20th	Day	1 1 1 1 1 1 1 3 91%	3724
	21st to end	Day	1 1 1 1 1 1 1 2 90%	4180
September	1st to 10th	Day	2 1 2 1 1 2 2 3 4 82%	2280
	11th to 20th	Day	1 1 1 1 1 2 1 3 3 85%	2137
	21st to end	Day	2 2 1 1 1 2 2 2 3 4 81%	2090
October	1st to 10th	Day	3 2 1 1 1 2 2 2 4 81%	3490
	11th to 20th	Day	2 1 1 1 1 1 2 6% 6% 78%	3270
	21st to end	Day	4 3 1 3 2 2 3 3 4 5% 69%	3362
November	1st to 10th	Day	5% 3 1 3 2 3 2 2 3 4 72%	1719
	11th to 20th	Day	5% 4 1 3 2 2 3 2 4 5% 69%	1710
	21st to end	Day	6% 3 2 4 3 4 3 3 5% 6% 62%	1710
December	1st to 10th	Day	8% 5% 1 5% 3 3 3 3 5% 6% 59%	1710
	11th to 20th	Day	19% 5% 2 5% 3 3 3 4 5% 8% 43%	2169
	21st to end	Day	13% 7% 2 6% 5% 4 4 3 5% 5% 47%	2986
January	1st to 10th	Night	8% 4 1 3 2 2 2 3 3 4 66%	4406
	11th to 20th	Night	6% 4 2 5% 3 2 4 3 5% 7% 59%	4133
	21st to end	Night	13% 6% 2 4 2 2 3 4 4 6% 54%	4180
February	1st to 10th	Night	10% 5% 1 4 3 2 2 2 3 5% 62%	2280
	11th to 20th	Night	10% 4 2 3 2 2 2 3 4 5% 63%	2218
	21st to end	Night	11% 6% 2 4 3 4 4 2 5% 5% 55%	1884
March	1st to 10th	Night	11% 10% 2 3 3 4 4 3 6% 8% 48%	3736
	11th to 20th	Night	4 3 1 3 2 2 4 3 5% 8% 64%	3699
	21st to end	Night	10% 6% 1 4 2 3 4 2 5% 6% 57%	3488
April	1st to 10th	Night	7% 4 1 4 3 3 3 3 4 7% 63%	1900
	11th to 20th	Night	4 5% 1 2 2 2 3 3 3 5% 69%	1879
	21st to end	Night	4 5% 1 2 2 2 2 2 4 6% 70%	1900
May	1st to 10th	Night	5% 4 1 3 2 1 2 2 3 4 75%	3054
	11th to 20th	Night	1 2 2 2 2 4 3 3 5% 76%	2834
	21st to end	Night	1 3 1 3 1 2 5% 3 3 4 73%	3110
June	1st to 10th	Night	1 3 1 2 2 2 2 3 4 7% 74%	1696
	11th to 20th	Night	1 1 1 1 1 1 2 3 4 84%	1520
	21st to end	Night	1 1 1 1 1 1 2 2 2 87%	1520
July	1st to 10th	Night	1 1 1 1 1 1 1 2 92%	2540
	11th to 20th	Night	1 1 2 1 1 2 1 2 3 85%	2595
	21st to end	Night	2 1 1 1 1 1 1 3 4 85%	3381
August	1st to 10th	Night	1 1 1 2 1 1 1 3 3 85%	3166
	11th to 20th	Night	1 1 1 1 1 1 2 3 90%	3126
	21st to end	Night	1 1 1 1 1 1 1 2 91%	3498
September	1st to 10th	Night	3 1 1 1 1 1 1 2 2 3 84%	1900
	11th to 20th	Night	1 1 1 1 1 2 1 3 2 87%	2042
	21st to end	Night	1 2 1 1 1 1 1 1 2 3 86%	2090
October	1st to 10th	Night	2 2 1 2 2 1 1 1 1 2 84%	3490
	11th to 20th	Night	2 1 2 1 1 2 2 9% 5% 75%	3652
	21st to end	Night	3 3 1 3 2 2 2 1 3 4 77%	4201
November	1st to 10th	Night	4 3 1 3 2 2 2 2 3 4 73%	2482
	11th to 20th	Night	5% 3 1 3 2 2 3 2 4 5% 70%	2470
	21st to end	Night	6% 4 2 4 4 3 3 3 4 6% 61%	2470
December	1st to 10th	Night	8% 4 1 5% 3 3 3 2 5% 6% 59%	2470
	11th to 20th	Night	17% 7% 2 4 3 3 4 3 5% 6% 46%	3041
	21st to end	Night	11% 6% 2 5% 4 4 6% 3 5% 7% 48%	4654

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 7% 22% 34% 15%	2539
	11th to 20th	Day	24% 7% 14% 17% 22% 16%	2826
	21st to end	Day	31% 5% 8% 15% 26% 15%	3498
February	1st to 10th	Day	25% 4 6% 14% 24% 27%	1900
	11th to 20th	Day	24% 4 6% 16% 27% 23%	1860
	21st to end	Day	29% 5% 7% 17% 22% 18%	1570
March	1st to 10th	Day	32% 9% 11% 18% 14% 16%	3138
	11th to 20th	Day	19% 7% 13% 29% 18% 14%	3261
	21st to end	Day	28% 8% 14% 26% 17% 8%	4165
April	1st to 10th	Day	23% 6% 12% 24% 23% 12%	2280
	11th to 20th	Day	19% 5% 8% 22% 28% 17%	2252
	21st to end	Day	17% 4 10% 24% 34% 11%	2280
May	1st to 10th	Day	13% 4 9% 19% 35% 18%	3919
	11th to 20th	Day	12% 3 15% 24% 35% 10%	4074
	21st to end	Day	13% 3 14% 28% 33% 9%	4477
June	1st to 10th	Day	15% 5% 10% 22% 32% 17%	2478
	11th to 20th	Day	10% 2 6% 15% 31% 35%	2660
	21st to end	Day	10% 2 3 10% 29% 45%	2660
July	1st to 10th	Day	6% 2 2 7% 13% 70%	4351
	11th to 20th	Day	9% 2 4 11% 23% 50%	4287
	21st to end	Day	6% 2 5% 17% 30% 40%	4055
August	1st to 10th	Day	7% 2 3 11% 38% 40%	3770
	11th to 20th	Day	4 1 2 10% 30% 53%	3760
	21st to end	Day	5% 1 2 7% 25% 59%	4179
September	1st to 10th	Day	9% 2 5% 9% 32% 43%	2280
	11th to 20th	Day	7% 1 4 14% 34% 39%	2137
	21st to end	Day	9% 3 4 15% 33% 36%	2090
October	1st to 10th	Day	10% 2 4 10% 17% 57%	3490
	11th to 20th	Day	8% 4 4 14% 36% 34%	3270
	21st to end	Day	16% 3 4 10% 26% 40%	3362
November	1st to 10th	Day	16% 3 6% 16% 29% 30%	1719
	11th to 20th	Day	18% 4 5% 13% 22% 37%	1710
	21st to end	Day	22% 4 8% 11% 24% 31%	1710
December	1st to 10th	Day	25% 6% 6% 16% 21% 25%	1710
	11th to 20th	Day	37% 6% 9% 13% 19% 16%	2168
	21st to end	Day	36% 6% 9% 18% 18% 13%	2986
January	1st to 10th	Night	21% 4 7% 13% 36% 19%	4406
	11th to 20th	Night	24% 6% 11% 22% 21% 17%	4133
	21st to end	Night	30% 5% 12% 13% 23% 16%	4180
February	1st to 10th	Night	27% 3 7% 13% 23% 27%	2280
	11th to 20th	Night	24% 5% 6% 16% 25% 25%	2219
	21st to end	Night	30% 5% 8% 14% 21% 22%	1884
March	1st to 10th	Night	29% 8% 10% 19% 18% 16%	3736
	11th to 20th	Night	17% 8% 14% 25% 21% 15%	3699
	21st to end	Night	23% 7% 16% 23% 23% 8%	3488
April	1st to 10th	Night	24% 5% 9% 23% 21% 17%	1900
	11th to 20th	Night	18% 5% 10% 21% 26% 20%	1879
	21st to end	Night	18% 4 12% 22% 32% 12%	1900
May	1st to 10th	Night	15% 5% 11% 16% 32% 21%	3054
	11th to 20th	Night	12% 3 17% 25% 30% 12%	2835
	21st to end	Night	9% 3 16% 26% 32% 13%	3110
June	1st to 10th	Night	11% 5% 13% 25% 26% 20%	1696
	11th to 20th	Night	9% 3 10% 16% 27% 36%	1520
	21st to end	Night	9% 2 5% 11% 26% 48%	1520
July	1st to 10th	Night	4 1 2 8% 12% 73%	2500
	11th to 20th	Night	7% 2 5% 10% 20% 57%	2547
	21st to end	Night	7% 2 5% 19% 26% 41%	3275
August	1st to 10th	Night	7% 2 4 10% 37% 41%	3156
	11th to 20th	Night	4 1 4 13% 31% 48%	3156
	21st to end	Night	4 1 3 8% 25% 58%	3498
September	1st to 10th	Night	10% 2 4 12% 25% 47%	1900
	11th to 20th	Night	7% 2 6% 12% 33% 40%	2042
	21st to end	Night	8% 2 5% 15% 30% 40%	2090
October	1st to 10th	Night	10% 2 3 9% 14% 62%	3490
	11th to 20th	Night	9% 5% 8% 12% 31% 35%	3652
	21st to end	Night	14% 2 3 7% 25% 49%	4201
November	1st to 10th	Night	17% 3 6% 14% 27% 33%	2482
	11th to 20th	Night	17% 4 6% 12% 22% 39%	2470
	21st to end	Night	23% 4 7% 15% 19% 31%	2470
December	1st to 10th	Night	26% 4 7% 14% 20% 29%	2470
	11th to 20th	Night	37% 5% 7% 12% 22% 16%	3041
	21st to end	Night	32% 6% 13% 18% 19% 12%	4654

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% 11% 33% 30% 7% 3	180
	11th to 20th	Day	15% 15% 37% 24% 9%	331
	21st to end	Day	13% 20% 31% 27% 8% 1	506
February	1st to 10th	Day	17% 17% 32% 26% 8% 2	460
	11th to 20th	Day	15% 19% 36% 23% 6% 1	455
	21st to end	Day	15% 22% 37% 21% 5%	382
March	1st to 10th	Day	18% 22% 36% 18% 6%	460
	11th to 20th	Day	17% 20% 35% 21% 7% 1	460
	21st to end	Day	17% 22% 38% 19% 4	506
April	1st to 10th	Day	13% 17% 37% 24% 8% 2	460
	11th to 20th	Day	9% 24% 40% 22% 5% 1	452
	21st to end	Day	7% 20% 32% 30% 11% 2	460
May	1st to 10th	Day	13% 15% 37% 26% 7% 2	463
	11th to 20th	Day	8% 11% 37% 32% 9% 2	488
	21st to end	Day	8% 13% 38% 29% 10% 2	529
June	1st to 10th	Day	4 11% 28% 39% 15% 4	490
	11th to 20th	Day	2 8% 33% 40% 11% 5%	490
	21st to end	Day	2 5% 34% 41% 12% 6%	490
July	1st to 10th	Day	2 1 22% 50% 19% 5%	486
	11th to 20th	Day	2 1 17% 52% 20% 9%	484
	21st to end	Day	1 1 19% 53% 17% 9%	531
August	1st to 10th	Day	2 3 26% 49% 15% 5%	487
	11th to 20th	Day	2 2 23% 49% 19% 5%	477
	21st to end	Day	1 2 26% 49% 18% 5%	539
September	1st to 10th	Day	2 3 34% 42% 15% 4	490
	11th to 20th	Day	2 7% 39% 38% 12% 1	490
	21st to end	Day	3 9% 32% 40% 13% 3	490
October	1st to 10th	Day	4 10% 33% 38% 13% 2	490
	11th to 20th	Day	4 10% 38% 36% 11% 1	490
	21st to end	Day	6% 13% 33% 33% 14% 2	489
November	1st to 10th	Day	6% 14% 26% 40% 13% 2	285
	11th to 20th	Day	9% 9% 37% 29% 14% 3	270
	21st to end	Day	6% 14% 35% 31% 12% 2	270
December	1st to 10th	Day	11% 13% 31% 31% 13% 1	270
	11th to 20th	Day	6% 16% 33% 28% 14% 2	270
	21st to end	Day	19% 19% 31% 20% 8% 2	252
January	1st to 10th	Night	13% 8% 27% 35% 14% 3	760
	11th to 20th	Night	9% 6% 32% 37% 15% 2	760
	21st to end	Night	14% 10% 28% 34% 12% 2	836
February	1st to 10th	Night	11% 8% 26% 35% 18% 3	760
	11th to 20th	Night	11% 8% 27% 32% 18% 4	744
	21st to end	Night	12% 10% 28% 32% 16% 2	628
March	1st to 10th	Night	11% 9% 30% 34% 15% 2	760
	11th to 20th	Night	10% 7% 24% 33% 22% 5%	760
	21st to end	Night	9% 10% 25% 36% 17% 3	836
April	1st to 10th	Night	8% 6% 22% 42% 19% 3	760
	11th to 20th	Night	6% 7% 26% 37% 21% 2	751
	21st to end	Night	5% 7% 24% 37% 21% 6%	760
May	1st to 10th	Night	7% 4 24% 39% 21% 4	686
	11th to 20th	Night	4 5% 18% 38% 31% 3	561
	21st to end	Night	3 4 19% 42% 28% 4	617
June	1st to 10th	Night	2 1 13% 42% 38% 3	556
	11th to 20th	Night	1 1 11% 41% 41% 6%	380
	21st to end	Night	2 8% 39% 45% 6%	380
July	1st to 10th	Night	1 4 26% 57% 12%	370
	11th to 20th	Night	2 1 2 28% 56% 11%	397
	21st to end	Night	2 1 3 33% 50% 12%	782
August	1st to 10th	Night	2 5% 34% 47% 12%	756
	11th to 20th	Night	1 3 29% 51% 15%	752
	21st to end	Night	2 4 31% 48% 15%	836
September	1st to 10th	Night	3 1 6% 34% 43% 14%	760
	11th to 20th	Night	1 1 11% 41% 36% 10%	760
	21st to end	Night	2 1 14% 38% 36% 9%	760
October	1st to 10th	Night	4 2 16% 47% 28% 3	760
	11th to 20th	Night	4 4 18% 38% 30% 6%	760
	21st to end	Night	4 7% 20% 36% 23% 10%	836
November	1st to 10th	Night	6% 4 20% 40% 24% 6%	764
	11th to 20th	Night	6% 4 27% 36% 21% 6%	760
	21st to end	Night	8% 6% 28% 39% 16% 4	760
December	1st to 10th	Night	8% 8% 26% 39% 17% 2	760
	11th to 20th	Night	13% 9% 27% 37% 13% 1	760
	21st to end	Night	11% 12% 30% 35% 11% 3	836

Page generated at 2024-11-03 10:19:06. Version ac07

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