Mount Olson Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Olson 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	17% 11% 3 8% 5% 5% 4 3 5% 6% 32%	127122
	Night	17% 12% 4 9% 5% 4 4 3 5% 6% 30%	122885
Transparency	Day	52% 5% 11% 21% 10% 1	130600
	Night	54% 5% 11% 19% 10% 1	127494
Seeing	Day	25% 15% 30% 19% 8% 2	15793
	Night	25% 13% 29% 24% 8% 1	24854

By Month

Percentage Cloud Cover NE (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	15% 18% 6% 13% 5% 5% 4 2 4 5% 22%	3597
	11th to 20th	Day	12% 18% 7% 15% 7% 6% 4 3 4 4 20%	3570
	21st to end	Day	14% 18% 7% 13% 5% 7% 4 3 4 4 20%	4172
February	1st to 10th	Day	17% 15% 6% 12% 6% 4 5% 4 4 4 23%	1800
	11th to 20th	Day	14% 16% 6% 10% 7% 5% 4 3 4 5% 26%	1773
	21st to end	Day	21% 16% 6% 9% 6% 6% 4 3 4 4 22%	1650
March	1st to 10th	Day	19% 15% 4 10% 5% 5% 4 3 4 5% 26%	4476
	11th to 20th	Day	10% 14% 4 9% 6% 5% 5% 4 5% 6% 32%	4977
	21st to end	Day	19% 11% 3 8% 5% 4 3 3 3 5% 37%	5776
April	1st to 10th	Day	21% 13% 3 7% 4 4 4 3 5% 5% 32%	2400
	11th to 20th	Day	22% 11% 3 6% 4 3 4 3 4 5% 36%	2374
	21st to end	Day	23% 9% 3 8% 5% 3 4 3 5% 4 32%	2501
May	1st to 10th	Day	25% 11% 4 8% 5% 4 4 3 4 6% 24%	6346
	11th to 20th	Day	17% 9% 3 10% 5% 5% 5% 4 5% 6% 31%	6318
	21st to end	Day	21% 9% 3 6% 4 4 5% 4 6% 7% 33%	6949
June	1st to 10th	Day	21% 7% 3 7% 5% 4 5% 4 6% 8% 30%	2800
	11th to 20th	Day	15% 6% 3 7% 5% 5% 4 4 6% 8% 38%	2800
	21st to end	Day	19% 8% 3 7% 5% 5% 5% 4 6% 7% 32%	2800
July	1st to 10th	Day	13% 7% 3 7% 5% 5% 6% 5% 7% 9% 35%	6360
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 4 7% 8% 43%	6360
	21st to end	Day	10% 5% 2 5% 4 4 5% 5% 7% 9% 44%	6926
August	1st to 10th	Day	15% 8% 3 7% 4 5% 4 3 6% 8% 36%	4450
	11th to 20th	Day	16% 6% 2 6% 4 4 4 3 6% 9% 39%	4090
	21st to end	Day	16% 6% 2 5% 4 4 4 4 6% 8% 39%	4312
September	1st to 10th	Day	14% 5% 2 7% 4 5% 4 4 5% 7% 43%	2400
	11th to 20th	Day	14% 5% 2 5% 4 3 4 3 6% 7% 47%	2397
	21st to end	Day	19% 6% 2 6% 4 5% 4 4 5% 8% 38%	2140
October	1st to 10th	Day	20% 8% 2 7% 4 4 3 3 5% 5% 38%	2000
	11th to 20th	Day	19% 10% 3 8% 5% 5% 4 3 5% 7% 30%	2000
	21st to end	Day	26% 10% 3 7% 5% 4 4 4 4 5% 28%	2048
November	1st to 10th	Day	15% 10% 3 8% 5% 5% 5% 4 6% 5% 35%	1779
	11th to 20th	Day	16% 12% 3 10% 6% 5% 4 3 5% 5% 30%	1635
	21st to end	Day	22% 15% 4 9% 6% 4 4 4 5% 6% 21%	1600
December	1st to 10th	Day	21% 19% 5% 12% 5% 4 5% 4 4 5% 16%	2600
	11th to 20th	Day	19% 17% 5% 12% 6% 4 4 3 4 3 23%	2986
	21st to end	Day	22% 16% 5% 13% 5% 4 4 3 3 4 22%	3960
January	1st to 10th	Night	15% 20% 7% 13% 5% 4 3 3 4 4 22%	6349
	11th to 20th	Night	11% 18% 7% 16% 8% 6% 6% 3 4 4 17%	6315
	21st to end	Night	17% 16% 7% 13% 6% 6% 4 3 4 4 19%	6766
February	1st to 10th	Night	17% 16% 6% 11% 6% 5% 4 4 5% 5% 21%	2600
	11th to 20th	Night	14% 14% 6% 11% 6% 5% 5% 3 4 5% 27%	2512
	21st to end	Night	17% 15% 5% 10% 5% 5% 5% 2 3 5% 27%	1980
March	1st to 10th	Night	21% 14% 3 9% 5% 4 4 3 3 4 29%	5364
	11th to 20th	Night	14% 13% 4 7% 4 4 4 4 7% 9% 31%	4979
	21st to end	Night	19% 11% 4 9% 5% 4 4 3 4 5% 33%	5156
April	1st to 10th	Night	21% 10% 4 8% 4 4 5% 3 4 6% 31%	2000
	11th to 20th	Night	22% 10% 2 7% 4 4 3 3 4 5% 36%	1975
	21st to end	Night	22% 9% 3 7% 4 4 4 3 4 5% 34%	1914
May	1st to 10th	Night	26% 11% 3 8% 5% 4 5% 4 5% 6% 24%	3594
	11th to 20th	Night	17% 8% 3 8% 4 4 4 3 5% 8% 37%	3575
	21st to end	Night	18% 9% 3 7% 4 5% 4 4 6% 7% 33%	3926
June	1st to 10th	Night	17% 7% 2 8% 5% 5% 6% 4 6% 7% 33%	1600
	11th to 20th	Night	15% 6% 2 6% 4 4 4 5% 5% 8% 39%	1600
	21st to end	Night	18% 8% 2 6% 5% 4 5% 5% 6% 7% 35%	1600
July	1st to 10th	Night	12% 6% 3 7% 5% 5% 4 4 7% 9% 36%	3600
	11th to 20th	Night	14% 6% 2 6% 4 4 4 3 6% 8% 43%	3600
	21st to end	Night	9% 6% 2 7% 5% 4 4 3 6% 8% 46%	3926
August	1st to 10th	Night	13% 7% 3 7% 4 4 4 4 6% 8% 41%	2682
	11th to 20th	Night	13% 8% 3 7% 4 4 4 3 5% 7% 41%	2926
	21st to end	Night	10% 6% 2 7% 5% 4 5% 4 6% 9% 42%	3586
September	1st to 10th	Night	13% 6% 2 7% 5% 3 4 3 5% 6% 46%	2000
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 5% 6% 51%	2002
	21st to end	Night	16% 6% 2 7% 4 5% 4 4 5% 6% 42%	2256
October	1st to 10th	Night	17% 7% 3 6% 4 4 4 4 5% 7% 38%	2400
	11th to 20th	Night	19% 8% 3 8% 5% 5% 5% 4 6% 7% 33%	2400
	21st to end	Night	25% 10% 3 7% 4 4 4 3 5% 7% 29%	2790
November	1st to 10th	Night	19% 10% 3 7% 4 4 3 4 5% 5% 36%	2564
	11th to 20th	Night	17% 11% 4 7% 6% 4 5% 3 5% 5% 33%	2764
	21st to end	Night	21% 14% 4 9% 5% 5% 4 4 5% 6% 24%	2800
December	1st to 10th	Night	18% 18% 6% 13% 7% 5% 5% 3 4 5% 16%	4580
	11th to 20th	Night	24% 14% 4 9% 5% 3 3 3 4 4 26%	5208
	21st to end	Night	26% 15% 4 12% 5% 4 3 3 4 4 19%	6996

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	65% 6% 10% 14% 4 1	3598
	11th to 20th	Day	68% 4 10% 10% 7% 1	3600
	21st to end	Day	68% 4 9% 9% 8% 1	4172
February	1st to 10th	Day	64% 4 7% 19% 6% 1	1800
	11th to 20th	Day	61% 4 9% 16% 8% 1	1782
	21st to end	Day	66% 4 9% 12% 9% 1	1650
March	1st to 10th	Day	61% 4 9% 14% 10% 3	4479
	11th to 20th	Day	53% 5% 15% 15% 11% 1	4977
	21st to end	Day	52% 3 6% 19% 18% 3	5776
April	1st to 10th	Day	56% 3 6% 19% 13% 3	2400
	11th to 20th	Day	52% 4 6% 20% 16% 2	2374
	21st to end	Day	54% 4 8% 16% 14% 3	2501
May	1st to 10th	Day	61% 5% 8% 16% 8% 1	6357
	11th to 20th	Day	52% 6% 12% 19% 10% 1	6322
	21st to end	Day	50% 5% 12% 23% 10% 1	6949
June	1st to 10th	Day	51% 5% 11% 20% 12% 1	2800
	11th to 20th	Day	43% 6% 11% 26% 13% 2	2800
	21st to end	Day	49% 7% 12% 20% 12% 1	2800
July	1st to 10th	Day	43% 6% 13% 27% 11%	6290
	11th to 20th	Day	38% 6% 14% 28% 15%	6285
	21st to end	Day	34% 7% 13% 37% 9%	6728
August	1st to 10th	Day	45% 5% 15% 24% 10%	4436
	11th to 20th	Day	42% 5% 15% 28% 9% 1	4129
	21st to end	Day	41% 5% 16% 27% 11%	4312
September	1st to 10th	Day	40% 6% 14% 28% 12%	2400
	11th to 20th	Day	36% 5% 12% 33% 15%	2397
	21st to end	Day	44% 6% 13% 26% 11%	2140
October	1st to 10th	Day	45% 4 12% 29% 9%	3260
	11th to 20th	Day	47% 5% 16% 25% 7% 1	3240
	21st to end	Day	48% 4 16% 24% 8%	3286
November	1st to 10th	Day	50% 5% 13% 24% 7% 1	1779
	11th to 20th	Day	56% 6% 10% 17% 10% 1	1635
	21st to end	Day	64% 6% 12% 13% 5%	1600
December	1st to 10th	Day	70% 5% 12% 10% 1	2600
	11th to 20th	Day	67% 4 9% 14% 6%	2986
	21st to end	Day	67% 4 8% 16% 5%	3960
January	1st to 10th	Night	67% 4 12% 12% 4	6349
	11th to 20th	Night	71% 5% 9% 10% 5%	6360
	21st to end	Night	69% 4 8% 9% 9% 1	6766
February	1st to 10th	Night	66% 4 9% 16% 6% 1	2600
	11th to 20th	Night	60% 4 9% 20% 7%	2514
	21st to end	Night	62% 3 8% 17% 9% 1	1980
March	1st to 10th	Night	60% 4 7% 16% 10% 3	5374
	11th to 20th	Night	50% 5% 18% 16% 8% 3	4980
	21st to end	Night	54% 4 5% 22% 13% 2	5156
April	1st to 10th	Night	54% 4 8% 19% 13% 2	2000
	11th to 20th	Night	52% 4 8% 18% 16% 1	1975
	21st to end	Night	53% 4 8% 18% 14% 4	1914
May	1st to 10th	Night	60% 5% 9% 16% 8% 2	3600
	11th to 20th	Night	47% 5% 14% 19% 14% 1	3576
	21st to end	Night	49% 6% 12% 19% 13% 1	3927
June	1st to 10th	Night	48% 7% 12% 21% 11% 1	1600
	11th to 20th	Night	42% 5% 13% 22% 17% 2	1600
	21st to end	Night	48% 6% 12% 21% 13% 1	1600
July	1st to 10th	Night	42% 5% 17% 21% 14% 1	3560
	11th to 20th	Night	38% 6% 14% 30% 12%	3552
	21st to end	Night	36% 6% 12% 31% 14%	3809
August	1st to 10th	Night	42% 6% 14% 27% 11%	2672
	11th to 20th	Night	42% 5% 14% 29% 8% 1	2953
	21st to end	Night	37% 7% 15% 28% 11% 1	3586
September	1st to 10th	Night	39% 5% 12% 29% 14% 1	2000
	11th to 20th	Night	33% 5% 14% 34% 13% 1	2002
	21st to end	Night	42% 5% 12% 27% 13% 1	2256
October	1st to 10th	Night	42% 4 13% 29% 12% 1	3920
	11th to 20th	Night	44% 3 16% 27% 10%	3899
	21st to end	Night	48% 4 17% 22% 8%	4502
November	1st to 10th	Night	50% 4 13% 21% 10% 1	2564
	11th to 20th	Night	53% 5% 11% 20% 10% 1	2764
	21st to end	Night	62% 5% 12% 15% 5%	2800
December	1st to 10th	Night	71% 5% 11% 11% 2	4580
	11th to 20th	Night	63% 5% 8% 14% 9%	5208
	21st to end	Night	69% 3 8% 12% 8%	6996

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	28% 21% 35% 13% 4	200
	11th to 20th	Day	35% 18% 31% 14% 3 1	200
	21st to end	Day	25% 24% 32% 13% 5%	267
February	1st to 10th	Day	34% 26% 30% 9% 1	300
	11th to 20th	Day	33% 29% 30% 8%	320
	21st to end	Day	35% 18% 34% 11% 1	414
March	1st to 10th	Day	31% 19% 31% 16% 3	500
	11th to 20th	Day	31% 19% 35% 11% 3 1	500
	21st to end	Day	35% 20% 25% 14% 5% 1	550
April	1st to 10th	Day	37% 18% 27% 13% 5%	500
	11th to 20th	Day	34% 21% 24% 11% 7% 3	492
	21st to end	Day	33% 17% 32% 11% 5% 2	500
May	1st to 10th	Day	29% 17% 28% 18% 7% 1	503
	11th to 20th	Day	20% 17% 34% 19% 7% 2	527
	21st to end	Day	21% 17% 29% 22% 10% 1	573
June	1st to 10th	Day	25% 15% 32% 19% 8% 1	530
	11th to 20th	Day	17% 15% 34% 21% 10% 2	530
	21st to end	Day	25% 13% 32% 19% 9% 2	530
July	1st to 10th	Day	14% 8% 34% 29% 14% 2	525
	11th to 20th	Day	15% 10% 30% 29% 13% 2	524
	21st to end	Day	14% 9% 33% 31% 9% 3	576
August	1st to 10th	Day	17% 8% 25% 33% 14% 3	526
	11th to 20th	Day	16% 5% 28% 29% 14% 7%	519
	21st to end	Day	17% 10% 31% 22% 13% 7%	583
September	1st to 10th	Day	17% 10% 30% 23% 13% 7%	530
	11th to 20th	Day	13% 7% 25% 28% 17% 10%	530
	21st to end	Day	20% 11% 28% 24% 13% 3	530
October	1st to 10th	Day	25% 14% 26% 17% 12% 5%	530
	11th to 20th	Day	28% 17% 32% 16% 5% 1	530
	21st to end	Day	34% 14% 27% 20% 4	498
November	1st to 10th	Day	29% 18% 29% 18% 5% 1	401
	11th to 20th	Day	29% 15% 33% 14% 7% 2	235
	21st to end	Day	33% 18% 35% 11% 4	200
December	1st to 10th	Day	36% 17% 28% 16% 5%	200
	11th to 20th	Day	36% 27% 26% 9% 3	200
	21st to end	Day	35% 16% 31% 12% 5% 1	220
January	1st to 10th	Night	31% 17% 30% 18% 4	800
	11th to 20th	Night	31% 18% 27% 21% 4	800
	21st to end	Night	23% 17% 33% 23% 4	880
February	1st to 10th	Night	27% 19% 33% 19% 2	800
	11th to 20th	Night	24% 20% 37% 16% 2	783
	21st to end	Night	29% 17% 30% 20% 3	660
March	1st to 10th	Night	29% 16% 30% 22% 4	800
	11th to 20th	Night	28% 18% 33% 18% 4	800
	21st to end	Night	28% 15% 30% 22% 5%	880
April	1st to 10th	Night	28% 15% 31% 22% 3	800
	11th to 20th	Night	31% 18% 27% 18% 6%	791
	21st to end	Night	29% 11% 29% 23% 7%	714
May	1st to 10th	Night	27% 11% 27% 26% 9% 1	400
	11th to 20th	Night	16% 12% 34% 30% 7%	394
	21st to end	Night	15% 10% 36% 27% 10%	433
June	1st to 10th	Night	19% 7% 30% 37% 6% 1	400
	11th to 20th	Night	18% 10% 30% 31% 12% 1	400
	21st to end	Night	21% 7% 32% 28% 13%	400
July	1st to 10th	Night	11% 6% 31% 41% 11% 1	390
	11th to 20th	Night	14% 5% 26% 40% 13% 1	388
	21st to end	Night	10% 5% 33% 39% 12% 1	438
August	1st to 10th	Night	14% 4 25% 38% 19%	455
	11th to 20th	Night	15% 3 24% 34% 22% 2	619
	21st to end	Night	13% 7% 29% 35% 15% 2	880
September	1st to 10th	Night	16% 7% 28% 33% 15% 2	800
	11th to 20th	Night	16% 5% 23% 28% 24% 5%	800
	21st to end	Night	20% 9% 26% 27% 17% 2	800
October	1st to 10th	Night	22% 10% 29% 22% 15% 2	800
	11th to 20th	Night	24% 13% 32% 24% 7% 1	800
	21st to end	Night	31% 12% 29% 24% 5%	880
November	1st to 10th	Night	26% 16% 28% 22% 7%	789
	11th to 20th	Night	27% 15% 31% 19% 9%	800
	21st to end	Night	32% 18% 27% 17% 6%	800
December	1st to 10th	Night	35% 16% 28% 18% 4	800
	11th to 20th	Night	32% 18% 31% 15% 3 1	800
	21st to end	Night	36% 16% 26% 19% 3	880

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

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