Mountain Meadows Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mountain Meadows Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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	12% 7% 3 7% 5% 5% 5% 4 6% 7% 38%	126410
	Night	12% 7% 3 7% 5% 5% 5% 4 6% 7% 39%	123615
Transparency	Day	41% 7% 12% 22% 14% 3	130015
	Night	39% 7% 13% 23% 15% 3	128079
Seeing	Day	21% 15% 25% 21% 13% 4	16221
	Night	18% 13% 25% 30% 13% 1	25905

By Month

Percentage Cloud Cover SE (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	11% 10% 4 8% 5% 5% 5% 3 4 5% 41%	4056
	11th to 20th	Day	9% 11% 4 10% 6% 4 4 3 4 4 40%	4060
	21st to end	Day	13% 8% 3 7% 4 4 5% 4 6% 5% 41%	4466
February	1st to 10th	Day	12% 11% 4 7% 6% 5% 5% 4 5% 5% 36%	1800
	11th to 20th	Day	16% 9% 4 8% 5% 4 5% 3 6% 6% 34%	1753
	21st to end	Day	14% 9% 3 7% 5% 3 4 4 6% 6% 39%	1692
March	1st to 10th	Day	14% 9% 3 7% 5% 4 3 3 5% 7% 40%	4935
	11th to 20th	Day	11% 7% 3 6% 4 3 4 3 5% 7% 48%	4980
	21st to end	Day	14% 9% 3 8% 6% 5% 5% 4 5% 6% 36%	5465
April	1st to 10th	Day	12% 11% 4 8% 5% 5% 4 3 5% 7% 36%	2348
	11th to 20th	Day	14% 7% 3 7% 4 4 5% 4 5% 6% 43%	2374
	21st to end	Day	17% 9% 2 7% 5% 4 4 3 5% 6% 37%	2496
May	1st to 10th	Day	16% 7% 3 8% 6% 6% 6% 5% 7% 8% 29%	5862
	11th to 20th	Day	20% 8% 3 7% 5% 5% 6% 4 7% 9% 28%	5861
	21st to end	Day	14% 6% 3 7% 5% 6% 6% 5% 7% 9% 32%	6816
June	1st to 10th	Day	12% 6% 2 5% 5% 5% 6% 4 6% 9% 40%	2800
	11th to 20th	Day	11% 6% 2 7% 6% 5% 6% 5% 8% 10% 36%	2800
	21st to end	Day	9% 4 2 6% 6% 6% 5% 5% 7% 9% 40%	2800
July	1st to 10th	Day	5% 5% 2 7% 5% 6% 5% 5% 7% 10% 43%	6137
	11th to 20th	Day	7% 6% 3 8% 6% 7% 7% 6% 9% 11% 31%	5900
	21st to end	Day	9% 6% 2 8% 5% 6% 6% 5% 7% 10% 36%	6424
August	1st to 10th	Day	14% 6% 2 8% 6% 6% 7% 6% 7% 9% 31%	4234
	11th to 20th	Day	8% 5% 2 7% 6% 6% 6% 5% 7% 8% 37%	4156
	21st to end	Day	7% 4 2 6% 5% 5% 5% 4 6% 9% 46%	4312
September	1st to 10th	Day	13% 6% 2 7% 3 5% 5% 5% 5% 5% 45%	2400
	11th to 20th	Day	12% 6% 2 5% 4 4 5% 3 6% 8% 43%	2286
	21st to end	Day	20% 9% 3 6% 5% 4 4 4 5% 5% 35%	2150
October	1st to 10th	Day	17% 6% 2 5% 4 3 4 3 5% 7% 45%	2000
	11th to 20th	Day	15% 8% 2 6% 4 4 4 3 4 5% 45%	2000
	21st to end	Day	14% 6% 2 6% 4 3 4 4 5% 5% 45%	2200
November	1st to 10th	Day	10% 6% 2 6% 4 3 5% 3 5% 6% 51%	1977
	11th to 20th	Day	13% 7% 4 6% 5% 4 4 4 7% 7% 41%	1702
	21st to end	Day	14% 9% 2 6% 5% 4 4 3 5% 6% 41%	1600
December	1st to 10th	Day	11% 9% 3 6% 5% 5% 4 3 5% 6% 45%	2600
	11th to 20th	Day	17% 9% 3 7% 5% 4 4 4 4 5% 37%	2990
	21st to end	Day	15% 9% 3 8% 5% 5% 5% 5% 7% 8% 31%	3978
January	1st to 10th	Night	9% 10% 3 10% 6% 6% 4 4 6% 6% 37%	5889
	11th to 20th	Night	10% 9% 3 9% 5% 5% 4 4 5% 5% 41%	5900
	21st to end	Night	13% 8% 3 6% 4 4 5% 4 4 5% 43%	6490
February	1st to 10th	Night	11% 8% 4 8% 6% 5% 5% 4 6% 6% 37%	2600
	11th to 20th	Night	15% 10% 3 8% 5% 6% 5% 4 5% 6% 33%	2533
	21st to end	Night	14% 10% 3 8% 6% 5% 4 4 5% 6% 35%	1933
March	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 4 6% 44%	4918
	11th to 20th	Night	9% 7% 2 7% 4 4 5% 3 5% 7% 47%	4980
	21st to end	Night	10% 7% 3 9% 5% 5% 4 4 5% 6% 42%	5467
April	1st to 10th	Night	13% 9% 3 8% 6% 6% 4 4 5% 6% 36%	2050
	11th to 20th	Night	12% 8% 3 6% 5% 5% 4 4 6% 7% 41%	1975
	21st to end	Night	15% 6% 3 6% 5% 4 5% 3 5% 8% 41%	1920
May	1st to 10th	Night	18% 8% 3 8% 6% 5% 4 4 7% 8% 29%	4033
	11th to 20th	Night	22% 9% 3 6% 5% 4 5% 4 6% 6% 29%	4032
	21st to end	Night	14% 7% 3 9% 5% 6% 5% 4 6% 8% 32%	4022
June	1st to 10th	Night	10% 5% 2 6% 4 5% 5% 3 7% 7% 45%	1600
	11th to 20th	Night	11% 6% 3 6% 4 5% 5% 5% 7% 9% 41%	1600
	21st to end	Night	9% 5% 2 6% 5% 4 6% 4 8% 9% 43%	1600
July	1st to 10th	Night	6% 5% 3 7% 5% 6% 6% 5% 7% 10% 41%	3816
	11th to 20th	Night	6% 7% 3 10% 7% 7% 7% 6% 7% 9% 31%	4057
	21st to end	Night	6% 6% 3 7% 5% 5% 6% 6% 8% 11% 36%	4427
August	1st to 10th	Night	13% 5% 2 7% 5% 5% 6% 5% 8% 8% 37%	2915
	11th to 20th	Night	9% 7% 3 7% 4 5% 5% 5% 7% 9% 40%	2875
	21st to end	Night	7% 6% 3 7% 3 4 5% 3 6% 9% 48%	3569
September	1st to 10th	Night	13% 5% 2 5% 4 3 4 3 6% 7% 48%	2000
	11th to 20th	Night	11% 5% 2 6% 4 4 4 3 6% 7% 49%	2110
	21st to end	Night	20% 7% 3 5% 4 5% 4 4 4 6% 39%	2250
October	1st to 10th	Night	17% 5% 2 5% 3 4 4 2 4 6% 47%	2400
	11th to 20th	Night	13% 6% 2 5% 3 4 4 3 5% 6% 48%	2400
	21st to end	Night	14% 6% 3 6% 4 3 4 3 4 6% 46%	2640
November	1st to 10th	Night	9% 8% 3 7% 4 4 4 3 5% 5% 48%	2366
	11th to 20th	Night	12% 7% 3 7% 5% 4 4 4 6% 7% 42%	2690
	21st to end	Night	12% 7% 3 6% 5% 6% 5% 4 5% 7% 41%	2800
December	1st to 10th	Night	9% 8% 3 7% 5% 4 3 4 5% 8% 43%	4580
	11th to 20th	Night	14% 8% 3 9% 6% 4 5% 3 5% 6% 35%	5205
	21st to end	Night	13% 8% 4 9% 7% 6% 6% 5% 7% 8% 29%	6973

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	44% 5% 6% 20% 19% 5%	4057
	11th to 20th	Day	44% 6% 9% 16% 18% 7%	4060
	21st to end	Day	40% 6% 10% 24% 16% 4	4466
February	1st to 10th	Day	47% 7% 11% 21% 12% 2	1800
	11th to 20th	Day	48% 5% 12% 19% 14% 2	1762
	21st to end	Day	44% 7% 13% 18% 14% 4	1692
March	1st to 10th	Day	40% 7% 10% 22% 15% 7%	4935
	11th to 20th	Day	34% 5% 10% 27% 19% 5%	4980
	21st to end	Day	46% 6% 10% 17% 17% 4	5465
April	1st to 10th	Day	45% 7% 11% 17% 15% 5%	2348
	11th to 20th	Day	39% 6% 10% 20% 19% 5%	2374
	21st to end	Day	45% 7% 10% 22% 13% 3	2496
May	1st to 10th	Day	49% 9% 12% 21% 8% 2	5897
	11th to 20th	Day	50% 8% 12% 23% 7% 1	5865
	21st to end	Day	43% 10% 13% 20% 12% 2	6831
June	1st to 10th	Day	38% 10% 14% 22% 16% 1	2800
	11th to 20th	Day	40% 11% 15% 21% 12% 2	2800
	21st to end	Day	37% 9% 15% 24% 14% 2	2800
July	1st to 10th	Day	35% 10% 14% 25% 12% 3	6071
	11th to 20th	Day	43% 9% 17% 24% 6% 1	5831
	21st to end	Day	42% 7% 11% 25% 15% 1	6240
August	1st to 10th	Day	47% 8% 15% 22% 8%	4221
	11th to 20th	Day	40% 8% 11% 28% 12%	4195
	21st to end	Day	34% 7% 15% 28% 15% 1	4312
September	1st to 10th	Day	39% 7% 13% 18% 20% 3	2400
	11th to 20th	Day	38% 6% 9% 26% 18% 3	2286
	21st to end	Day	49% 7% 9% 18% 15% 2	2150
October	1st to 10th	Day	31% 4 9% 24% 26% 6%	3260
	11th to 20th	Day	32% 4 8% 26% 18% 12%	3240
	21st to end	Day	28% 4 9% 29% 23% 7%	3538
November	1st to 10th	Day	33% 5% 11% 24% 22% 5%	1977
	11th to 20th	Day	39% 6% 11% 18% 22% 4	1702
	21st to end	Day	41% 6% 13% 22% 17% 2	1600
December	1st to 10th	Day	39% 6% 11% 24% 17% 4	2600
	11th to 20th	Day	46% 6% 14% 21% 11% 2	2986
	21st to end	Day	45% 10% 16% 17% 12% 1	3978
January	1st to 10th	Night	38% 10% 11% 20% 20% 2	5889
	11th to 20th	Night	39% 9% 9% 22% 15% 7%	5900
	21st to end	Night	39% 7% 11% 26% 15% 2	6490
February	1st to 10th	Night	44% 6% 14% 22% 12% 1	2600
	11th to 20th	Night	49% 6% 12% 18% 13% 3	2535
	21st to end	Night	46% 7% 12% 19% 12% 4	1933
March	1st to 10th	Night	38% 7% 10% 24% 14% 7%	4918
	11th to 20th	Night	30% 8% 13% 28% 17% 4	4980
	21st to end	Night	39% 7% 12% 22% 17% 3	5467
April	1st to 10th	Night	45% 8% 12% 18% 15% 3	2050
	11th to 20th	Night	38% 8% 12% 22% 15% 5%	1975
	21st to end	Night	41% 7% 11% 26% 13% 3	1920
May	1st to 10th	Night	49% 10% 14% 16% 9% 2	4060
	11th to 20th	Night	50% 8% 11% 22% 7%	4033
	21st to end	Night	44% 10% 16% 21% 6% 2	4029
June	1st to 10th	Night	35% 10% 13% 27% 16% 1	1600
	11th to 20th	Night	37% 11% 16% 23% 13% 1	1600
	21st to end	Night	33% 10% 17% 22% 16% 2	1600
July	1st to 10th	Night	36% 10% 20% 21% 11% 2	3774
	11th to 20th	Night	47% 9% 18% 20% 7% 1	4006
	21st to end	Night	37% 8% 16% 26% 13%	4297
August	1st to 10th	Night	42% 8% 18% 24% 7%	2904
	11th to 20th	Night	38% 7% 16% 24% 14%	2902
	21st to end	Night	33% 7% 16% 31% 12% 1	3569
September	1st to 10th	Night	35% 8% 14% 21% 18% 4	2000
	11th to 20th	Night	33% 6% 12% 29% 18% 2	2110
	21st to end	Night	45% 7% 10% 21% 16% 1	2250
October	1st to 10th	Night	36% 3 11% 26% 22% 2	3920
	11th to 20th	Night	29% 4 11% 22% 24% 9%	3899
	21st to end	Night	25% 5% 12% 29% 24% 6%	4252
November	1st to 10th	Night	36% 5% 13% 22% 19% 5%	2366
	11th to 20th	Night	39% 6% 11% 23% 18% 4	2690
	21st to end	Night	41% 7% 13% 20% 16% 3	2800
December	1st to 10th	Night	37% 6% 12% 28% 16% 1	4580
	11th to 20th	Night	46% 5% 15% 17% 15% 2	5208
	21st to end	Night	46% 9% 17% 16% 11%	6973

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	33% 29% 22% 12% 3 1	300
	11th to 20th	Day	36% 22% 28% 12% 3	300
	21st to end	Day	30% 29% 29% 9% 2	330
February	1st to 10th	Day	34% 29% 25% 11% 1	300
	11th to 20th	Day	47% 23% 25% 5%	300
	21st to end	Day	35% 22% 26% 16% 1	366
March	1st to 10th	Day	31% 22% 25% 19% 4	500
	11th to 20th	Day	28% 26% 23% 16% 5% 1	500
	21st to end	Day	29% 23% 28% 14% 5% 1	550
April	1st to 10th	Day	26% 26% 26% 16% 5% 1	500
	11th to 20th	Day	24% 20% 27% 17% 9% 3	492
	21st to end	Day	27% 18% 29% 16% 8% 2	500
May	1st to 10th	Day	23% 22% 25% 16% 10% 4	503
	11th to 20th	Day	24% 15% 28% 19% 11% 3	527
	21st to end	Day	20% 11% 26% 22% 16% 6%	573
June	1st to 10th	Day	15% 12% 28% 25% 14% 6%	530
	11th to 20th	Day	15% 10% 29% 23% 17% 6%	530
	21st to end	Day	9% 12% 29% 29% 16% 6%	530
July	1st to 10th	Day	10% 5% 22% 30% 24% 9%	525
	11th to 20th	Day	7% 5% 23% 31% 25% 10%	524
	21st to end	Day	13% 5% 21% 29% 23% 8%	576
August	1st to 10th	Day	13% 6% 22% 26% 23% 9%	526
	11th to 20th	Day	12% 5% 23% 25% 25% 11%	519
	21st to end	Day	9% 7% 22% 31% 19% 13%	583
September	1st to 10th	Day	15% 8% 21% 23% 22% 10%	530
	11th to 20th	Day	14% 5% 20% 28% 24% 10%	530
	21st to end	Day	29% 10% 23% 22% 13% 3	530
October	1st to 10th	Day	18% 8% 24% 28% 18% 5%	530
	11th to 20th	Day	23% 17% 21% 25% 11% 2	530
	21st to end	Day	21% 17% 28% 22% 8% 3	583
November	1st to 10th	Day	18% 15% 28% 26% 11% 3	503
	11th to 20th	Day	24% 17% 29% 21% 8% 1	278
	21st to end	Day	31% 16% 33% 17% 4 1	200
December	1st to 10th	Day	26% 25% 32% 13% 4 2	200
	11th to 20th	Day	28% 19% 30% 17% 7%	200
	21st to end	Day	24% 23% 27% 20% 6%	223
January	1st to 10th	Night	23% 23% 31% 20% 3	800
	11th to 20th	Night	25% 19% 27% 26% 3	800
	21st to end	Night	20% 21% 36% 22% 2	880
February	1st to 10th	Night	19% 23% 28% 28% 2	800
	11th to 20th	Night	30% 25% 28% 17% 1	783
	21st to end	Night	25% 18% 35% 19% 2	660
March	1st to 10th	Night	23% 19% 32% 24% 3	800
	11th to 20th	Night	24% 22% 26% 22% 5% 1	800
	21st to end	Night	23% 20% 31% 20% 5% 1	880
April	1st to 10th	Night	21% 22% 29% 23% 5%	800
	11th to 20th	Night	18% 18% 28% 27% 9%	791
	21st to end	Night	21% 13% 28% 30% 7%	720
May	1st to 10th	Night	20% 10% 27% 30% 13% 1	600
	11th to 20th	Night	18% 8% 27% 33% 13% 1	591
	21st to end	Night	14% 3 24% 37% 19% 2	484
June	1st to 10th	Night	12% 4 25% 38% 21% 2	400
	11th to 20th	Night	13% 4 20% 44% 18% 2	400
	21st to end	Night	8% 3 20% 49% 20% 1	400
July	1st to 10th	Night	11% 2 15% 38% 29% 5%	474
	11th to 20th	Night	8% 2 10% 40% 36% 4	582
	21st to end	Night	8% 2 15% 40% 31% 4	657
August	1st to 10th	Night	9% 3 11% 43% 32% 2	597
	11th to 20th	Night	11% 2 11% 42% 30% 4	594
	21st to end	Night	12% 2 16% 38% 28% 4	863
September	1st to 10th	Night	15% 3 17% 33% 27% 5%	800
	11th to 20th	Night	12% 4 15% 34% 31% 4	800
	21st to end	Night	23% 5% 22% 31% 19% 1	800
October	1st to 10th	Night	19% 8% 19% 30% 21% 4	800
	11th to 20th	Night	17% 15% 24% 31% 13% 1	800
	21st to end	Night	19% 13% 26% 31% 11% 1	880
November	1st to 10th	Night	14% 12% 26% 36% 11% 1	789
	11th to 20th	Night	19% 17% 26% 29% 9% 1	800
	21st to end	Night	19% 18% 27% 28% 7%	800
December	1st to 10th	Night	23% 18% 30% 24% 6%	800
	11th to 20th	Night	20% 15% 32% 30% 3	800
	21st to end	Night	19% 18% 28% 34% 2	880

Page generated at 2025-10-31 02:02:41. Version ac07

Copyright © 2025 Attilla Danko.