Mammoth Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mammoth Observatory clear sky chart from 2005-11-06 to 2024-12-24.

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	1 3 1 4 3 3 3 3 5% 7% 67%	105418
	Night	1 2 1 4 3 3 3 3 5% 7% 69%	103413
Transparency	Day	19% 5% 10% 19% 26% 21%	105162
	Night	17% 5% 11% 21% 28% 18%	103258
Seeing	Day	10% 21% 31% 22% 12% 4	15302
	Night	5% 12% 25% 31% 21% 6%	24853

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	3 2 5% 3 3 4 4 7% 16% 54%	2539
	11th to 20th	Day	2 2 6% 5% 4 6% 5% 8% 10% 51%	2609
	21st to end	Day	1 5% 2 7% 4 5% 6% 5% 7% 11% 48%	3146
February	1st to 10th	Day	1 3 2 5% 3 2 4 4 6% 8% 62%	1831
	11th to 20th	Day	1 3 2 5% 6% 2 4 5% 7% 10% 56%	1850
	21st to end	Day	3 2 5% 4 4 5% 5% 7% 7% 57%	1570
March	1st to 10th	Day	4 2 7% 6% 6% 6% 5% 9% 10% 46%	3138
	11th to 20th	Day	4 4 8% 4 3 3 3 6% 10% 56%	3330
	21st to end	Day	4 3 7% 4 4 5% 5% 7% 9% 50%	4148
April	1st to 10th	Day	1 4 2 6% 4 3 4 5% 6% 8% 57%	2280
	11th to 20th	Day	1 4 1 5% 3 3 3 4 6% 8% 60%	2249
	21st to end	Day	1 5% 2 3 3 3 3 4 6% 7% 63%	2280
May	1st to 10th	Day	1 3 1 3 3 2 3 3 5% 7% 70%	4007
	11th to 20th	Day	1 3 1 3 4 3 3 3 5% 6% 67%	4071
	21st to end	Day	2 5% 1 3 3 3 2 2 3 5% 70%	4782
June	1st to 10th	Day	3 4 1 4 3 3 4 4 5% 6% 63%	2660
	11th to 20th	Day	2 1 1 2 1 2 2 2 3 5% 78%	2660
	21st to end	Day	2 3 1 3 2 2 3 3 4 4 74%	2660
July	1st to 10th	Day	1 1 1 2 2 2 3 2 3 4 79%	4420
	11th to 20th	Day	3 4 1 5% 4 3 3 3 5% 6% 63%	4420
	21st to end	Day	3 3 1 3 3 3 3 3 4 5% 70%	4684
August	1st to 10th	Day	2 2 1 3 2 2 3 3 3 5% 75%	3849
	11th to 20th	Day	1 1 1 2 1 2 2 1 2 4 83%	3724
	21st to end	Day	1 1 1 2 1 1 2 1 2 4 84%	4180
September	1st to 10th	Day	2 2 1 3 2 2 3 2 3 4 75%	2280
	11th to 20th	Day	2 2 1 2 3 2 2 2 3 4 75%	2199
	21st to end	Day	1 2 1 3 2 2 2 2 3 5% 77%	2090
October	1st to 10th	Day	1 3 1 3 1 2 2 2 4 7% 73%	3378
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 4 80%	3161
	21st to end	Day	1 1 3 1 1 2 2 4 8% 77%	3328
November	1st to 10th	Day	1 2 2 4 2 2 3 3 6% 9% 67%	1764
	11th to 20th	Day	1 1 2 2 3 2 4 6% 9% 71%	1800
	21st to end	Day	1 3 1 2 2 3 4 2 5% 8% 69%	1800
December	1st to 10th	Day	3 1 2 2 3 4 4 6% 10% 65%	1705
	11th to 20th	Day	3 2 5% 3 4 5% 6% 9% 10% 53%	2008
	21st to end	Day	5% 3 6% 4 4 5% 5% 7% 11% 50%	2818
January	1st to 10th	Night	3 2 6% 4 3 3 3 7% 9% 59%	4406
	11th to 20th	Night	2 1 4 4 4 5% 5% 8% 13% 53%	4349
	21st to end	Night	1 4 2 5% 4 4 4 5% 8% 11% 51%	4521
February	1st to 10th	Night	3 2 5% 4 4 3 4 6% 9% 61%	2347
	11th to 20th	Night	4 1 5% 4 2 4 4 6% 10% 60%	2218
	21st to end	Night	3 1 4 5% 4 3 4 7% 9% 61%	1884
March	1st to 10th	Night	3 3 7% 5% 4 4 5% 8% 12% 50%	3736
	11th to 20th	Night	4 5% 6% 3 4 4 4 6% 8% 57%	3647
	21st to end	Night	4 2 6% 4 3 4 4 7% 10% 56%	3503
April	1st to 10th	Night	1 4 2 5% 3 4 4 4 5% 8% 61%	1900
	11th to 20th	Night	1 3 2 4 3 4 4 2 5% 9% 64%	1879
	21st to end	Night	1 4 2 4 3 2 3 3 4 6% 68%	1900
May	1st to 10th	Night	1 2 3 2 2 2 3 4 7% 74%	2964
	11th to 20th	Night	2 2 1 3 2 2 2 2 4 5% 74%	2834
	21st to end	Night	2 4 1 2 1 3 2 2 3 5% 75%	2777
June	1st to 10th	Night	1 3 1 3 3 3 4 4 5% 8% 66%	1520
	11th to 20th	Night	2 1 1 2 1 2 2 4 5% 80%	1520
	21st to end	Night	1 1 1 2 2 1 1 2 2 4 83%	1520
July	1st to 10th	Night	1 1 1 1 1 3 3 89%	2540
	11th to 20th	Night	1 3 1 3 3 3 3 3 4 5% 73%	2540
	21st to end	Night	1 2 1 2 1 2 2 2 3 4 81%	2854
August	1st to 10th	Night	1 1 1 2 1 1 2 2 3 5% 81%	3096
	11th to 20th	Night	1 1 1 1 1 1 2 2 2 2 86%	3126
	21st to end	Night	1 1 1 1 1 1 2 2 90%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 2 3 88%	1900
	11th to 20th	Night	1 2 1 2 2 1 2 2 3 4 81%	1981
	21st to end	Night	1 1 2 1 1 2 3 3 5% 81%	2090
October	1st to 10th	Night	1 2 1 2 1 1 1 2 3 3 83%	3599
	11th to 20th	Night	1 2 1 2 1 1 2 2 3 5% 81%	3777
	21st to end	Night	2 1 2 1 2 2 2 4 6% 78%	4236
November	1st to 10th	Night	1 1 3 2 2 2 4 5% 8% 71%	2535
	11th to 20th	Night	1 1 3 2 2 3 3 5% 8% 72%	2600
	21st to end	Night	2 1 4 2 2 3 3 5% 9% 68%	2600
December	1st to 10th	Night	2 1 4 3 2 4 3 6% 10% 65%	2690
	11th to 20th	Night	2 2 5% 3 4 4 5% 8% 13% 54%	3416
	21st to end	Night	5% 3 8% 5% 5% 5% 5% 8% 11% 46%	4910

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	20% 8% 24% 21% 22% 4	2539
	11th to 20th	Day	28% 12% 15% 21% 17% 8%	2609
	21st to end	Day	31% 7% 15% 20% 21% 6%	3146
February	1st to 10th	Day	23% 6% 14% 21% 23% 13%	1831
	11th to 20th	Day	27% 4 11% 24% 25% 10%	1860
	21st to end	Day	31% 5% 14% 24% 21% 7%	1570
March	1st to 10th	Day	34% 5% 17% 17% 19% 8%	3138
	11th to 20th	Day	21% 6% 24% 32% 13% 4	3330
	21st to end	Day	32% 6% 22% 21% 17% 3	4148
April	1st to 10th	Day	28% 6% 16% 25% 17% 8%	2280
	11th to 20th	Day	25% 7% 13% 27% 20% 8%	2252
	21st to end	Day	23% 6% 17% 31% 19% 4	2280
May	1st to 10th	Day	16% 5% 9% 30% 29% 11%	4007
	11th to 20th	Day	16% 6% 9% 26% 34% 9%	4074
	21st to end	Day	19% 3 8% 27% 38% 4	4782
June	1st to 10th	Day	22% 6% 9% 24% 30% 9%	2660
	11th to 20th	Day	11% 4 7% 21% 36% 21%	2660
	21st to end	Day	15% 2 4 13% 35% 31%	2660
July	1st to 10th	Day	9% 3 3 7% 16% 62%	4351
	11th to 20th	Day	19% 6% 7% 11% 21% 36%	4345
	21st to end	Day	16% 5% 6% 13% 18% 43%	4536
August	1st to 10th	Day	13% 3 4 12% 28% 40%	3834
	11th to 20th	Day	9% 2 3 6% 22% 58%	3760
	21st to end	Day	8% 2 3 9% 32% 47%	4179
September	1st to 10th	Day	15% 2 5% 11% 30% 38%	2280
	11th to 20th	Day	14% 2 7% 17% 31% 29%	2199
	21st to end	Day	13% 1 4 18% 36% 28%	2090
October	1st to 10th	Day	12% 3 6% 15% 37% 27%	3378
	11th to 20th	Day	12% 2 4 13% 43% 25%	3161
	21st to end	Day	10% 3 5% 24% 40% 18%	3328
November	1st to 10th	Day	18% 3 12% 22% 29% 16%	1764
	11th to 20th	Day	14% 4 12% 25% 31% 13%	1800
	21st to end	Day	19% 4 10% 23% 29% 15%	1800
December	1st to 10th	Day	19% 7% 13% 22% 25% 14%	1705
	11th to 20th	Day	28% 13% 16% 18% 16% 10%	2008
	21st to end	Day	28% 8% 15% 19% 20% 10%	2818
January	1st to 10th	Night	21% 6% 17% 28% 22% 6%	4406
	11th to 20th	Night	23% 11% 19% 22% 17% 8%	4349
	21st to end	Night	27% 8% 16% 22% 19% 7%	4521
February	1st to 10th	Night	23% 7% 12% 23% 22% 14%	2347
	11th to 20th	Night	24% 5% 12% 22% 26% 12%	2219
	21st to end	Night	24% 5% 13% 24% 26% 8%	1884
March	1st to 10th	Night	25% 5% 22% 19% 19% 9%	3736
	11th to 20th	Night	20% 8% 21% 28% 18% 5%	3647
	21st to end	Night	25% 6% 24% 24% 18% 3	3503
April	1st to 10th	Night	26% 6% 15% 27% 18% 8%	1900
	11th to 20th	Night	22% 4 15% 27% 21% 10%	1879
	21st to end	Night	20% 5% 20% 33% 19% 4	1900
May	1st to 10th	Night	13% 3 11% 35% 32% 6%	2964
	11th to 20th	Night	15% 3 12% 29% 34% 7%	2835
	21st to end	Night	14% 4 9% 34% 36% 3	2777
June	1st to 10th	Night	16% 4 13% 27% 32% 7%	1520
	11th to 20th	Night	9% 3 8% 25% 37% 18%	1520
	21st to end	Night	9% 1 4 15% 39% 31%	1520
July	1st to 10th	Night	4 1 2 7% 19% 66%	2500
	11th to 20th	Night	15% 2 5% 11% 32% 35%	2492
	21st to end	Night	9% 2 5% 18% 25% 41%	2764
August	1st to 10th	Night	8% 2 5% 17% 34% 34%	3087
	11th to 20th	Night	6% 3 2 10% 24% 55%	3156
	21st to end	Night	5% 1 2 12% 38% 43%	3498
September	1st to 10th	Night	6% 2 3 15% 41% 34%	1900
	11th to 20th	Night	10% 3 5% 20% 39% 23%	1981
	21st to end	Night	9% 3 4 21% 38% 24%	2090
October	1st to 10th	Night	10% 2 5% 14% 45% 25%	3599
	11th to 20th	Night	9% 3 5% 15% 42% 26%	3777
	21st to end	Night	11% 3 5% 19% 39% 23%	4236
November	1st to 10th	Night	15% 4 12% 25% 29% 16%	2535
	11th to 20th	Night	14% 4 11% 24% 33% 14%	2600
	21st to end	Night	18% 6% 11% 22% 30% 14%	2600
December	1st to 10th	Night	19% 6% 16% 22% 24% 13%	2690
	11th to 20th	Night	24% 13% 15% 19% 20% 9%	3416
	21st to end	Night	30% 11% 13% 16% 21% 10%	4910

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	16% 31% 25% 17% 7% 3	180
	11th to 20th	Day	11% 27% 23% 19% 15% 5%	223
	21st to end	Day	9% 20% 31% 24% 12% 3	407
February	1st to 10th	Day	8% 32% 25% 20% 10% 4	428
	11th to 20th	Day	12% 26% 34% 21% 7% 1	455
	21st to end	Day	14% 29% 31% 19% 7% 1	382
March	1st to 10th	Day	16% 30% 32% 15% 6% 1	460
	11th to 20th	Day	18% 24% 28% 18% 9% 2	460
	21st to end	Day	13% 34% 32% 15% 6%	506
April	1st to 10th	Day	14% 27% 31% 18% 8% 2	460
	11th to 20th	Day	18% 27% 31% 16% 7% 2	452
	21st to end	Day	11% 22% 35% 18% 10% 3	460
May	1st to 10th	Day	16% 21% 32% 17% 10% 3	463
	11th to 20th	Day	14% 22% 28% 21% 9% 5%	488
	21st to end	Day	15% 17% 27% 22% 16% 5%	529
June	1st to 10th	Day	8% 18% 34% 20% 13% 6%	490
	11th to 20th	Day	12% 21% 32% 19% 11% 6%	490
	21st to end	Day	7% 16% 38% 24% 10% 5%	490
July	1st to 10th	Day	4 12% 37% 27% 16% 4	486
	11th to 20th	Day	4 10% 35% 30% 15% 6%	484
	21st to end	Day	8% 11% 33% 27% 15% 7%	531
August	1st to 10th	Day	5% 14% 35% 26% 15% 4	487
	11th to 20th	Day	8% 15% 30% 23% 17% 6%	477
	21st to end	Day	4 11% 38% 27% 13% 7%	539
September	1st to 10th	Day	5% 14% 31% 29% 15% 5%	490
	11th to 20th	Day	9% 21% 27% 27% 14% 3	490
	21st to end	Day	6% 16% 36% 28% 11% 3	490
October	1st to 10th	Day	7% 20% 31% 24% 15% 3	490
	11th to 20th	Day	9% 17% 31% 28% 11% 3	490
	21st to end	Day	11% 21% 30% 22% 13% 4	468
November	1st to 10th	Day	11% 22% 23% 18% 20% 6%	300
	11th to 20th	Day	10% 23% 24% 23% 13% 6%	300
	21st to end	Day	10% 19% 25% 26% 17% 3	300
December	1st to 10th	Day	10% 21% 28% 21% 14% 6%	253
	11th to 20th	Day	11% 27% 26% 21% 13% 3	200
	21st to end	Day	19% 22% 28% 17% 9% 5%	204
January	1st to 10th	Night	6% 15% 31% 22% 16% 10%	760
	11th to 20th	Night	3 16% 29% 30% 15% 7%	760
	21st to end	Night	5% 14% 30% 31% 16% 6%	836
February	1st to 10th	Night	4 14% 31% 30% 17% 4	760
	11th to 20th	Night	7% 15% 29% 28% 15% 6%	744
	21st to end	Night	9% 17% 29% 30% 11% 4	628
March	1st to 10th	Night	9% 19% 31% 25% 13% 4	760
	11th to 20th	Night	6% 18% 27% 26% 17% 6%	760
	21st to end	Night	5% 21% 34% 25% 13% 2	836
April	1st to 10th	Night	6% 15% 34% 26% 16% 3	760
	11th to 20th	Night	8% 16% 29% 27% 14% 5%	751
	21st to end	Night	6% 14% 31% 27% 18% 5%	760
May	1st to 10th	Night	7% 12% 30% 30% 17% 4	635
	11th to 20th	Night	6% 12% 26% 35% 19% 3	561
	21st to end	Night	8% 8% 22% 31% 25% 5%	426
June	1st to 10th	Night	3 6% 20% 37% 29% 3	380
	11th to 20th	Night	5% 7% 21% 38% 26% 3	380
	21st to end	Night	3 4 20% 41% 29% 2	380
July	1st to 10th	Night	1 2 14% 44% 35% 3	370
	11th to 20th	Night	3 16% 42% 35% 4	368
	21st to end	Night	3 1 10% 40% 41% 6%	473
August	1st to 10th	Night	2 4 21% 37% 31% 6%	711
	11th to 20th	Night	3 5% 16% 38% 32% 6%	752
	21st to end	Night	3 3 17% 38% 32% 7%	836
September	1st to 10th	Night	2 4 17% 37% 33% 7%	760
	11th to 20th	Night	4 10% 17% 33% 29% 7%	760
	21st to end	Night	1 10% 18% 34% 27% 9%	760
October	1st to 10th	Night	4 7% 21% 37% 25% 6%	760
	11th to 20th	Night	5% 9% 18% 36% 25% 7%	760
	21st to end	Night	5% 11% 23% 27% 24% 11%	836
November	1st to 10th	Night	5% 11% 21% 30% 23% 9%	780
	11th to 20th	Night	4 14% 29% 30% 18% 6%	800
	21st to end	Night	4 15% 27% 30% 19% 6%	800
December	1st to 10th	Night	6% 15% 29% 31% 15% 5%	800
	11th to 20th	Night	7% 20% 31% 26% 13% 3	800
	21st to end	Night	9% 18% 31% 26% 13% 3	850

Page generated at 2024-12-21 01:13:52. Version ac07

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