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 2026-04-17.

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% 69%	139993
	Night	1 2 1 3 2 2 3 3 5% 7% 71%	139611
Transparency	Day	16% 5% 10% 20% 27% 22%	135033
	Night	15% 5% 11% 21% 29% 19%	135546
Seeing	Day	11% 21% 31% 22% 11% 4	17010
	Night	5% 13% 26% 31% 20% 5%	26608

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	1 3 1 4 3 3 3 3 5% 11% 64%	4722
	11th to 20th	Day	1 1 3 3 2 4 3 5% 6% 71%	4893
	21st to end	Day	1 4 2 8% 4 4 4 3 6% 9% 56%	5843
February	1st to 10th	Day	1 3 2 4 3 2 4 3 6% 8% 63%	2025
	11th to 20th	Day	3 2 5% 6% 2 4 4 7% 10% 56%	2050
	21st to end	Day	3 1 5% 4 4 5% 4 7% 8% 58%	1730
March	1st to 10th	Day	3 2 8% 5% 5% 6% 5% 8% 10% 47%	4618
	11th to 20th	Day	3 3 6% 3 3 3 3 6% 11% 59%	4880
	21st to end	Day	3 2 5% 3 3 4 5% 6% 8% 59%	6070
April	1st to 10th	Day	1 3 2 6% 4 3 4 4 6% 9% 58%	2520
	11th to 20th	Day	1 4 1 5% 3 3 3 4 7% 8% 61%	2442
	21st to end	Day	1 6% 2 3 3 3 3 4 6% 8% 62%	2400
May	1st to 10th	Day	1 4 1 3 2 2 3 3 4 6% 70%	5738
	11th to 20th	Day	1 3 1 3 3 3 3 3 4 6% 69%	5841
	21st to end	Day	1 4 1 3 3 3 3 2 3 6% 72%	6858
June	1st to 10th	Day	3 4 1 4 3 3 4 4 6% 6% 62%	2800
	11th to 20th	Day	2 1 1 2 1 2 2 2 3 5% 79%	2800
	21st to end	Day	2 3 1 3 2 2 3 3 4 4 74%	2800
July	1st to 10th	Day	1 1 1 2 2 2 2 2 3 4 81%	6319
	11th to 20th	Day	2 3 1 4 3 3 3 3 4 5% 70%	6320
	21st to end	Day	2 2 1 2 2 2 3 3 4 6% 74%	6735
August	1st to 10th	Day	2 1 2 1 2 2 2 2 3 82%	5438
	11th to 20th	Day	1 1 1 2 1 2 2 1 2 4 84%	5364
	21st to end	Day	2 2 1 3 3 3 3 3 4 5% 72%	5984
September	1st to 10th	Day	2 2 1 3 2 2 3 2 3 4 75%	2400
	11th to 20th	Day	2 3 1 2 3 2 2 2 4 4 74%	2315
	21st to end	Day	1 2 1 3 2 2 2 2 4 5% 77%	2200
October	1st to 10th	Day	1 3 1 2 1 2 2 2 4 7% 73%	3484
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 4 80%	3261
	21st to end	Day	1 1 3 1 1 2 2 4 7% 77%	3434
November	1st to 10th	Day	1 2 2 4 2 2 3 3 6% 8% 68%	1854
	11th to 20th	Day	1 2 1 3 2 3 2 4 6% 9% 69%	1890
	21st to end	Day	1 3 1 2 2 3 4 2 5% 8% 69%	1890
December	1st to 10th	Day	2 1 2 2 2 4 3 5% 9% 70%	2875
	11th to 20th	Day	2 1 3 2 2 3 4 7% 10% 63%	3108
	21st to end	Day	4 2 5% 3 3 4 4 7% 12% 55%	4092
January	1st to 10th	Night	3 2 5% 3 2 3 2 5% 8% 68%	8172
	11th to 20th	Night	1 1 2 2 3 3 3 5% 8% 72%	8071
	21st to end	Night	3 2 8% 4 4 4 4 6% 9% 57%	8371
February	1st to 10th	Night	3 2 5% 3 4 3 4 5% 9% 62%	2593
	11th to 20th	Night	3 1 5% 3 2 4 4 6% 11% 60%	2458
	21st to end	Night	2 1 4 4 4 3 4 6% 9% 62%	2076
March	1st to 10th	Night	2 2 7% 4 4 4 4 7% 12% 53%	5496
	11th to 20th	Night	4 4 5% 3 3 3 3 5% 8% 61%	5337
	21st to end	Night	4 2 5% 3 3 3 3 6% 10% 61%	5145
April	1st to 10th	Night	1 3 2 5% 3 3 4 3 5% 8% 62%	2100
	11th to 20th	Night	1 2 2 4 3 3 4 2 4 8% 66%	2039
	21st to end	Night	1 5% 2 4 3 2 3 3 4 6% 67%	2000
May	1st to 10th	Night	2 5% 1 3 1 2 2 2 3 6% 73%	4246
	11th to 20th	Night	1 2 1 3 2 2 2 2 4 5% 76%	4074
	21st to end	Night	2 3 1 2 1 2 2 1 3 5% 79%	4001
June	1st to 10th	Night	1 3 1 3 3 3 4 4 5% 8% 66%	1600
	11th to 20th	Night	2 1 1 2 1 2 2 4 5% 81%	1600
	21st to end	Night	1 1 1 2 2 1 1 2 2 4 84%	1600
July	1st to 10th	Night	1 1 1 1 1 2 3 90%	3640
	11th to 20th	Night	1 2 1 2 2 2 3 2 3 5% 78%	3640
	21st to end	Night	1 1 1 1 1 1 2 2 3 86%	4105
August	1st to 10th	Night	1 1 1 1 1 1 1 2 4 87%	4388
	11th to 20th	Night	1 1 1 1 1 1 1 2 3 87%	4506
	21st to end	Night	1 1 1 2 1 2 2 2 4 84%	5015
September	1st to 10th	Night	1 1 1 1 1 1 2 2 3 87%	2000
	11th to 20th	Night	1 2 1 2 2 1 2 2 3 4 80%	2085
	21st to end	Night	1 1 2 1 1 2 3 4 5% 80%	2200
October	1st to 10th	Night	1 2 1 2 1 1 1 2 3 3 83%	3712
	11th to 20th	Night	1 2 1 2 1 1 2 2 3 4 81%	3897
	21st to end	Night	2 1 2 1 2 2 2 4 6% 78%	4372
November	1st to 10th	Night	1 1 3 2 2 2 3 5% 8% 73%	2665
	11th to 20th	Night	1 2 1 3 2 2 3 3 5% 8% 70%	2730
	21st to end	Night	3 1 4 2 2 3 3 4 9% 68%	2730
December	1st to 10th	Night	1 1 3 2 2 3 3 5% 8% 73%	4525
	11th to 20th	Night	1 1 3 2 2 3 4 7% 14% 61%	5316
	21st to end	Night	4 2 6% 4 4 5% 5% 8% 14% 48%	7106

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	15% 7% 22% 24% 26% 5%	4723
	11th to 20th	Day	15% 7% 8% 13% 25% 32%	4893
	21st to end	Day	25% 6% 11% 19% 27% 11%	5843
February	1st to 10th	Day	22% 6% 13% 21% 23% 15%	2025
	11th to 20th	Day	26% 4 11% 25% 24% 10%	2060
	21st to end	Day	28% 6% 15% 24% 20% 8%	1730
March	1st to 10th	Day	31% 5% 14% 18% 22% 9%	4618
	11th to 20th	Day	16% 7% 25% 34% 14% 5%	4880
	21st to end	Day	24% 8% 20% 24% 20% 4	6070
April	1st to 10th	Day	26% 6% 16% 25% 18% 9%	2520
	11th to 20th	Day	24% 7% 13% 27% 21% 8%	2445
	21st to end	Day	23% 6% 16% 31% 19% 4	2400
May	1st to 10th	Day	17% 5% 8% 32% 29% 9%	5738
	11th to 20th	Day	14% 6% 10% 29% 33% 8%	5844
	21st to end	Day	16% 3 7% 26% 44% 4	6859
June	1st to 10th	Day	22% 6% 9% 25% 29% 9%	2800
	11th to 20th	Day	10% 4 7% 20% 35% 24%	2800
	21st to end	Day	15% 2 4 13% 35% 32%	2800
July	1st to 10th	Day	8% 3 2 6% 16% 65%	6251
	11th to 20th	Day	14% 5% 5% 9% 19% 48%	6245
	21st to end	Day	11% 4 6% 12% 23% 43%	6539
August	1st to 10th	Day	12% 3 4 12% 28% 41%	3956
	11th to 20th	Day	8% 2 3 7% 23% 58%	3880
	21st to end	Day	9% 2 3 9% 31% 46%	4311
September	1st to 10th	Day	14% 3 5% 12% 31% 36%	2400
	11th to 20th	Day	15% 3 7% 16% 32% 28%	2315
	21st to end	Day	13% 2 4 19% 35% 27%	2200
October	1st to 10th	Day	12% 3 6% 15% 37% 27%	3484
	11th to 20th	Day	12% 2 4 14% 43% 25%	3261
	21st to end	Day	10% 3 5% 24% 41% 18%	3434
November	1st to 10th	Day	17% 3 12% 22% 29% 16%	1854
	11th to 20th	Day	16% 4 13% 24% 31% 12%	1890
	21st to end	Day	19% 4 10% 23% 29% 15%	1890
December	1st to 10th	Day	15% 5% 9% 24% 29% 18%	2875
	11th to 20th	Day	19% 10% 20% 23% 19% 9%	3108
	21st to end	Day	22% 9% 21% 21% 19% 8%	4092
January	1st to 10th	Night	16% 6% 18% 28% 26% 7%	8172
	11th to 20th	Night	12% 6% 10% 14% 30% 27%	8071
	21st to end	Night	25% 7% 10% 19% 26% 14%	8371
February	1st to 10th	Night	22% 7% 11% 23% 21% 16%	2593
	11th to 20th	Night	23% 5% 12% 23% 25% 12%	2459
	21st to end	Night	22% 6% 14% 24% 25% 9%	2076
March	1st to 10th	Night	23% 5% 17% 19% 25% 11%	5496
	11th to 20th	Night	18% 7% 18% 35% 18% 5%	5337
	21st to end	Night	20% 7% 25% 25% 19% 4	5145
April	1st to 10th	Night	24% 5% 15% 27% 19% 9%	2100
	11th to 20th	Night	21% 4 15% 28% 22% 9%	2039
	21st to end	Night	21% 5% 20% 33% 18% 4	2000
May	1st to 10th	Night	16% 3 10% 36% 30% 4	4243
	11th to 20th	Night	13% 5% 13% 33% 32% 6%	4075
	21st to end	Night	11% 3 7% 29% 45% 5%	4001
June	1st to 10th	Night	17% 4 13% 30% 30% 6%	1600
	11th to 20th	Night	8% 3 8% 24% 36% 21%	1600
	21st to end	Night	9% 1 4 15% 40% 32%	1600
July	1st to 10th	Night	4 1 2 7% 17% 69%	3600
	11th to 20th	Night	11% 2 4 9% 32% 43%	3592
	21st to end	Night	6% 2 3 14% 35% 40%	3989
August	1st to 10th	Night	8% 2 5% 16% 34% 34%	3185
	11th to 20th	Night	6% 3 2 10% 25% 55%	3256
	21st to end	Night	5% 2 2 12% 38% 42%	3608
September	1st to 10th	Night	6% 2 3 16% 41% 33%	2000
	11th to 20th	Night	10% 3 6% 21% 39% 21%	2085
	21st to end	Night	9% 3 5% 22% 38% 23%	2200
October	1st to 10th	Night	10% 2 5% 15% 44% 25%	3712
	11th to 20th	Night	9% 3 4 15% 42% 27%	3897
	21st to end	Night	10% 3 6% 19% 40% 22%	4372
November	1st to 10th	Night	14% 4 11% 25% 29% 16%	2665
	11th to 20th	Night	15% 4 12% 24% 32% 14%	2730
	21st to end	Night	18% 5% 11% 22% 30% 14%	2730
December	1st to 10th	Night	13% 5% 12% 24% 27% 19%	4525
	11th to 20th	Night	16% 13% 16% 23% 23% 8%	5316
	21st to end	Night	25% 13% 16% 16% 21% 10%	7106

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	19% 29% 25% 17% 7% 3	220
	11th to 20th	Day	9% 26% 24% 21% 14% 5%	269
	21st to end	Day	8% 21% 32% 23% 12% 3	473
February	1st to 10th	Day	13% 29% 24% 21% 9% 4	502
	11th to 20th	Day	17% 26% 32% 19% 6% 1	535
	21st to end	Day	15% 28% 33% 18% 6%	446
March	1st to 10th	Day	17% 31% 30% 15% 6% 1	540
	11th to 20th	Day	20% 23% 29% 18% 9% 2	540
	21st to end	Day	15% 33% 30% 15% 6%	594
April	1st to 10th	Day	14% 27% 30% 19% 8% 2	540
	11th to 20th	Day	19% 26% 30% 17% 6% 1	517
	21st to end	Day	12% 24% 34% 18% 10% 3	500
May	1st to 10th	Day	16% 21% 32% 17% 11% 3	503
	11th to 20th	Day	16% 22% 28% 20% 9% 5%	528
	21st to end	Day	14% 17% 27% 22% 15% 4	573
June	1st to 10th	Day	7% 18% 33% 22% 13% 6%	530
	11th to 20th	Day	13% 23% 31% 18% 10% 5%	530
	21st to end	Day	7% 17% 37% 24% 10% 5%	530
July	1st to 10th	Day	5% 14% 36% 26% 15% 4	526
	11th to 20th	Day	4 10% 35% 30% 15% 6%	524
	21st to end	Day	7% 13% 32% 27% 15% 6%	575
August	1st to 10th	Day	6% 16% 35% 25% 15% 4	527
	11th to 20th	Day	7% 15% 31% 24% 16% 7%	517
	21st to end	Day	4 12% 38% 27% 13% 7%	583
September	1st to 10th	Day	6% 15% 30% 28% 15% 5%	530
	11th to 20th	Day	9% 20% 28% 27% 13% 3	530
	21st to end	Day	8% 17% 35% 28% 11% 2	530
October	1st to 10th	Day	9% 20% 31% 24% 14% 3	530
	11th to 20th	Day	11% 17% 31% 27% 11% 3	530
	21st to end	Day	11% 20% 30% 23% 12% 3	508
November	1st to 10th	Day	11% 23% 22% 19% 20% 5%	330
	11th to 20th	Day	13% 24% 23% 22% 12% 6%	330
	21st to end	Day	9% 20% 26% 27% 16% 3	330
December	1st to 10th	Day	9% 21% 29% 22% 14% 6%	278
	11th to 20th	Day	13% 25% 25% 20% 13% 3	220
	21st to end	Day	25% 20% 26% 16% 9% 4	242
January	1st to 10th	Night	8% 18% 31% 21% 14% 9%	840
	11th to 20th	Night	3 17% 32% 29% 14% 6%	840
	21st to end	Night	5% 14% 32% 30% 14% 5%	924
February	1st to 10th	Night	6% 15% 31% 29% 15% 4	840
	11th to 20th	Night	9% 17% 29% 25% 14% 5%	824
	21st to end	Night	9% 18% 31% 28% 10% 4	692
March	1st to 10th	Night	8% 21% 33% 23% 11% 4	840
	11th to 20th	Night	7% 19% 28% 25% 16% 5%	840
	21st to end	Night	6% 22% 34% 25% 11% 2	924
April	1st to 10th	Night	7% 16% 34% 26% 15% 3	840
	11th to 20th	Night	8% 17% 30% 26% 13% 5%	815
	21st to end	Night	6% 14% 31% 27% 17% 5%	800
May	1st to 10th	Night	8% 12% 31% 30% 16% 3	668
	11th to 20th	Night	7% 12% 27% 34% 18% 3	591
	21st to end	Night	8% 9% 23% 31% 24% 5%	449
June	1st to 10th	Night	3 6% 20% 39% 28% 3	400
	11th to 20th	Night	5% 10% 21% 36% 25% 3	400
	21st to end	Night	3 5% 22% 40% 28% 2	400
July	1st to 10th	Night	2 3 17% 43% 33% 3	390
	11th to 20th	Night	3 17% 43% 34% 4	388
	21st to end	Night	3 1 12% 40% 39% 6%	498
August	1st to 10th	Night	2 5% 21% 37% 30% 5%	749
	11th to 20th	Night	3 5% 17% 39% 31% 5%	792
	21st to end	Night	3 3 18% 39% 30% 7%	880
September	1st to 10th	Night	3 5% 18% 36% 31% 7%	800
	11th to 20th	Night	4 10% 17% 34% 28% 7%	800
	21st to end	Night	2 10% 20% 34% 26% 8%	800
October	1st to 10th	Night	4 8% 22% 36% 24% 6%	800
	11th to 20th	Night	5% 10% 19% 35% 24% 7%	800
	21st to end	Night	5% 11% 23% 28% 23% 11%	880
November	1st to 10th	Night	6% 11% 21% 31% 23% 9%	820
	11th to 20th	Night	5% 15% 29% 28% 18% 6%	840
	21st to end	Night	4 15% 28% 29% 18% 5%	840
December	1st to 10th	Night	6% 15% 30% 30% 14% 5%	840
	11th to 20th	Night	8% 20% 30% 27% 13% 3	840
	21st to end	Night	11% 20% 31% 24% 12% 3	924

Page generated at 2026-04-13 23:44:23. Version ac07

Copyright © 2026 Attilla Danko.