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 2024-01-31.

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	18% 10% 4 7% 6% 3 5% 4 5% 6% 33%	74882
	Night	19% 10% 4 7% 6% 3 4 4 5% 6% 32%	75242
Transparency	Day	52% 5% 11% 20% 10% 1	74587
	Night	54% 5% 11% 19% 10% 1	75071
Seeing	Day	26% 15% 30% 19% 8% 3	13450
	Night	25% 13% 29% 24% 8% 1	22717

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	17% 18% 6% 10% 7% 4 5% 3 4 5% 21%	1517
	11th to 20th	Day	19% 18% 5% 9% 8% 4 5% 3 4 4 22%	1520
	21st to end	Day	14% 15% 7% 10% 7% 6% 6% 4 5% 5% 22%	1753
February	1st to 10th	Day	18% 14% 6% 11% 7% 3 5% 5% 4 4 22%	1620
	11th to 20th	Day	15% 16% 6% 9% 7% 4 4 4 3 6% 27%	1591
	21st to end	Day	22% 15% 6% 8% 6% 4 4 4 4 4 22%	1480
March	1st to 10th	Day	15% 14% 6% 10% 6% 4 5% 4 4 4 28%	1797
	11th to 20th	Day	18% 14% 4 7% 7% 4 4 3 4 5% 31%	1977
	21st to end	Day	21% 12% 4 5% 5% 2 3 3 3 5% 37%	2294
April	1st to 10th	Day	22% 12% 4 6% 5% 3 4 3 5% 5% 33%	2160
	11th to 20th	Day	21% 9% 3 6% 4 3 4 4 4 5% 37%	2134
	21st to end	Day	24% 9% 3 7% 6% 2 4 4 5% 5% 32%	2250
May	1st to 10th	Day	20% 9% 3 6% 5% 3 5% 4 4 7% 35%	2520
	11th to 20th	Day	17% 7% 3 6% 5% 3 4 4 4 7% 39%	2478
	21st to end	Day	16% 6% 3 5% 5% 3 5% 5% 6% 8% 38%	2725
June	1st to 10th	Day	22% 7% 3 6% 6% 3 4 5% 6% 8% 30%	2520
	11th to 20th	Day	16% 6% 3 6% 5% 4 5% 4 6% 8% 38%	2520
	21st to end	Day	19% 7% 3 6% 6% 3 5% 5% 6% 7% 33%	2520
July	1st to 10th	Day	12% 6% 3 5% 4 3 5% 5% 6% 8% 43%	2520
	11th to 20th	Day	13% 6% 3 5% 5% 3 5% 5% 7% 9% 39%	2520
	21st to end	Day	12% 6% 2 4 5% 4 6% 7% 8% 9% 37%	2744
August	1st to 10th	Day	12% 7% 3 7% 6% 4 5% 4 6% 9% 39%	2452
	11th to 20th	Day	12% 6% 2 6% 5% 4 5% 5% 6% 9% 40%	2263
	21st to end	Day	13% 6% 3 5% 6% 3 5% 5% 6% 8% 40%	2376
September	1st to 10th	Day	14% 5% 2 6% 5% 4 4 5% 5% 8% 41%	2160
	11th to 20th	Day	15% 5% 2 4 4 3 4 4 6% 7% 46%	2158
	21st to end	Day	19% 6% 2 5% 5% 4 4 5% 5% 8% 38%	1926
October	1st to 10th	Day	22% 8% 3 6% 6% 3 4 4 5% 5% 36%	1800
	11th to 20th	Day	21% 10% 3 8% 6% 3 5% 4 5% 6% 29%	1800
	21st to end	Day	28% 10% 3 6% 5% 3 5% 5% 4 5% 27%	1844
November	1st to 10th	Day	16% 8% 3 7% 6% 4 5% 5% 6% 6% 35%	1671
	11th to 20th	Day	17% 12% 4 7% 8% 4 5% 3 5% 5% 29%	1554
	21st to end	Day	22% 14% 5% 8% 7% 3 5% 4 5% 7% 22%	1520
December	1st to 10th	Day	25% 16% 6% 10% 7% 2 4 4 4 6% 17%	1520
	11th to 20th	Day	22% 16% 5% 9% 7% 3 3 4 4 4 23%	1906
	21st to end	Day	27% 16% 5% 11% 5% 3 4 3 3 3 20%	2772
January	1st to 10th	Night	19% 17% 7% 11% 7% 3 4 4 4 4 20%	2649
	11th to 20th	Night	18% 17% 5% 10% 7% 4 6% 5% 4 4 20%	2660
	21st to end	Night	14% 14% 6% 11% 8% 4 5% 4 4 5% 26%	2825
February	1st to 10th	Night	18% 14% 6% 12% 7% 4 5% 4 5% 5% 20%	2340
	11th to 20th	Night	15% 13% 6% 9% 7% 3 4 4 4 5% 28%	2255
	21st to end	Night	18% 13% 5% 9% 6% 4 5% 3 3 5% 28%	1776
March	1st to 10th	Night	16% 15% 5% 8% 7% 3 5% 4 4 5% 30%	2150
	11th to 20th	Night	17% 13% 5% 7% 5% 2 3 4 4 5% 33%	1979
	21st to end	Night	20% 8% 4 7% 5% 2 4 3 4 5% 37%	2062
April	1st to 10th	Night	21% 9% 4 7% 5% 3 4 4 4 6% 32%	1800
	11th to 20th	Night	21% 8% 3 7% 5% 3 4 3 4 5% 37%	1775
	21st to end	Night	23% 10% 3 6% 5% 3 4 4 5% 5% 32%	1724
May	1st to 10th	Night	23% 8% 2 5% 4 2 5% 5% 5% 6% 36%	1440
	11th to 20th	Night	15% 7% 3 7% 5% 3 4 5% 4 7% 41%	1415
	21st to end	Night	13% 6% 2 6% 4 3 4 5% 5% 7% 45%	1550
June	1st to 10th	Night	18% 6% 2 6% 7% 3 5% 6% 6% 7% 33%	1440
	11th to 20th	Night	16% 6% 2 5% 5% 2 4 5% 5% 8% 41%	1440
	21st to end	Night	17% 7% 2 5% 5% 3 5% 6% 6% 7% 36%	1440
July	1st to 10th	Night	10% 4 2 6% 6% 4 4 4 5% 8% 46%	1440
	11th to 20th	Night	12% 5% 2 5% 5% 3 5% 5% 7% 10% 42%	1440
	21st to end	Night	10% 4 2 6% 5% 3 5% 5% 7% 9% 44%	1574
August	1st to 10th	Night	11% 6% 3 5% 5% 4 5% 5% 6% 9% 41%	1483
	11th to 20th	Night	12% 6% 3 6% 5% 3 5% 4 5% 7% 44%	1620
	21st to end	Night	12% 6% 2 5% 6% 2 5% 4 5% 8% 44%	1980
September	1st to 10th	Night	14% 6% 3 6% 6% 2 4 3 4 7% 45%	1800
	11th to 20th	Night	15% 5% 2 4 4 2 3 4 5% 6% 50%	1801
	21st to end	Night	16% 6% 2 6% 4 3 4 4 4 7% 42%	2030
October	1st to 10th	Night	19% 7% 4 6% 5% 4 4 5% 5% 8% 35%	2160
	11th to 20th	Night	21% 8% 3 7% 6% 4 4 5% 6% 7% 31%	2160
	21st to end	Night	26% 10% 3 6% 5% 3 4 4 5% 7% 28%	2510
November	1st to 10th	Night	19% 9% 4 5% 5% 3 3 4 5% 5% 37%	2407
	11th to 20th	Night	18% 11% 4 6% 6% 3 5% 4 4 6% 33%	2625
	21st to end	Night	22% 12% 5% 7% 6% 4 5% 4 5% 6% 24%	2660
December	1st to 10th	Night	21% 18% 5% 9% 7% 3 4 4 5% 5% 18%	2660
	11th to 20th	Night	23% 15% 4 8% 6% 3 3 3 3 4 27%	3288
	21st to end	Night	31% 14% 5% 11% 5% 3 4 3 3 4 17%	4884

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	69% 3 10% 12% 5% 1	1518
	11th to 20th	Day	67% 4 8% 11% 9% 1	1520
	21st to end	Day	64% 5% 11% 13% 6% 1	1753
February	1st to 10th	Day	65% 4 7% 18% 5% 1	1620
	11th to 20th	Day	60% 4 9% 17% 9% 1	1600
	21st to end	Day	66% 4 8% 12% 9% 1	1480
March	1st to 10th	Day	60% 4 10% 16% 10% 1	1800
	11th to 20th	Day	58% 3 10% 14% 12% 3	1977
	21st to end	Day	53% 2 7% 18% 15% 5%	2294
April	1st to 10th	Day	55% 3 6% 19% 13% 3	2160
	11th to 20th	Day	51% 4 6% 21% 16% 2	2134
	21st to end	Day	54% 4 8% 16% 14% 4	2250
May	1st to 10th	Day	50% 5% 8% 19% 16% 3	2520
	11th to 20th	Day	45% 5% 9% 24% 15% 2	2482
	21st to end	Day	43% 7% 14% 22% 13% 2	2725
June	1st to 10th	Day	51% 5% 11% 20% 12% 1	2520
	11th to 20th	Day	43% 6% 10% 26% 13% 2	2520
	21st to end	Day	49% 7% 12% 19% 11% 1	2520
July	1st to 10th	Day	37% 6% 11% 30% 15% 1	2450
	11th to 20th	Day	40% 6% 17% 28% 10%	2445
	21st to end	Day	39% 9% 16% 25% 10% 1	2591
August	1st to 10th	Day	41% 7% 15% 26% 10%	2438
	11th to 20th	Day	40% 6% 17% 26% 11% 1	2263
	21st to end	Day	40% 5% 14% 28% 12%	2376
September	1st to 10th	Day	41% 6% 14% 26% 13%	2160
	11th to 20th	Day	37% 5% 12% 32% 14%	2158
	21st to end	Day	44% 6% 13% 28% 9%	1926
October	1st to 10th	Day	50% 4 11% 23% 11%	1800
	11th to 20th	Day	56% 5% 13% 18% 7% 1	1800
	21st to end	Day	60% 5% 10% 16% 8% 1	1844
November	1st to 10th	Day	49% 5% 14% 24% 7% 1	1671
	11th to 20th	Day	57% 6% 10% 16% 9% 1	1554
	21st to end	Day	63% 6% 12% 13% 5%	1520
December	1st to 10th	Day	69% 5% 11% 12% 2	1520
	11th to 20th	Day	67% 4 12% 13% 3	1906
	21st to end	Day	72% 4 7% 14% 4	2772
January	1st to 10th	Night	69% 4 11% 12% 5%	2649
	11th to 20th	Night	68% 5% 10% 10% 7% 1	2660
	21st to end	Night	61% 4 9% 16% 8% 1	2825
February	1st to 10th	Night	67% 4 9% 14% 5% 1	2340
	11th to 20th	Night	59% 4 9% 21% 7%	2257
	21st to end	Night	61% 3 8% 17% 10% 1	1776
March	1st to 10th	Night	59% 3 11% 16% 10% 1	2160
	11th to 20th	Night	55% 4 11% 17% 12% 2	1980
	21st to end	Night	51% 3 6% 20% 15% 4	2062
April	1st to 10th	Night	53% 4 8% 20% 14% 2	1800
	11th to 20th	Night	51% 4 8% 18% 17% 2	1775
	21st to end	Night	54% 4 8% 17% 13% 4	1724
May	1st to 10th	Night	48% 5% 8% 19% 16% 5%	1440
	11th to 20th	Night	43% 6% 8% 22% 18% 3	1416
	21st to end	Night	39% 6% 15% 21% 16% 3	1551
June	1st to 10th	Night	47% 8% 13% 21% 10% 1	1440
	11th to 20th	Night	42% 5% 12% 22% 17% 2	1440
	21st to end	Night	46% 6% 13% 21% 13% 1	1440
July	1st to 10th	Night	35% 7% 12% 28% 17% 2	1400
	11th to 20th	Night	36% 6% 14% 31% 13%	1392
	21st to end	Night	36% 8% 15% 27% 13% 1	1486
August	1st to 10th	Night	39% 7% 16% 24% 13%	1473
	11th to 20th	Night	40% 5% 14% 27% 13% 1	1620
	21st to end	Night	38% 6% 14% 29% 13%	1980
September	1st to 10th	Night	40% 5% 13% 27% 14% 1	1800
	11th to 20th	Night	35% 5% 15% 32% 12% 1	1801
	21st to end	Night	42% 5% 12% 30% 12% 1	2030
October	1st to 10th	Night	49% 4 11% 22% 12% 1	2160
	11th to 20th	Night	52% 4 14% 21% 9% 1	2160
	21st to end	Night	58% 4 14% 15% 9% 1	2510
November	1st to 10th	Night	50% 4 13% 21% 10% 1	2407
	11th to 20th	Night	53% 5% 12% 20% 9% 2	2625
	21st to end	Night	62% 5% 13% 15% 5%	2660
December	1st to 10th	Night	69% 5% 10% 13% 3	2660
	11th to 20th	Night	63% 4 10% 16% 7% 1	3288
	21st to end	Night	72% 4 9% 12% 2	4884

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	29% 21% 32% 14% 4	180
	11th to 20th	Day	37% 17% 29% 14% 2 1	180
	21st to end	Day	26% 22% 32% 14% 6%	237
February	1st to 10th	Day	37% 28% 30% 5%	240
	11th to 20th	Day	33% 28% 33% 5%	258
	21st to end	Day	36% 19% 31% 12% 2	346
March	1st to 10th	Day	29% 17% 33% 17% 3	420
	11th to 20th	Day	32% 18% 35% 11% 3 1	420
	21st to end	Day	35% 19% 24% 15% 6% 1	462
April	1st to 10th	Day	35% 16% 28% 15% 5%	420
	11th to 20th	Day	33% 19% 25% 13% 7% 4	412
	21st to end	Day	34% 15% 31% 13% 5% 2	420
May	1st to 10th	Day	29% 17% 29% 18% 7% 1	423
	11th to 20th	Day	21% 18% 34% 18% 7% 2	447
	21st to end	Day	19% 18% 29% 21% 11% 1	485
June	1st to 10th	Day	26% 14% 31% 20% 8% 1	450
	11th to 20th	Day	18% 14% 32% 23% 10% 3	450
	21st to end	Day	24% 12% 32% 19% 11% 2	450
July	1st to 10th	Day	15% 6% 31% 31% 15% 2	445
	11th to 20th	Day	16% 10% 27% 30% 14% 2	444
	21st to end	Day	15% 8% 33% 32% 9% 3	488
August	1st to 10th	Day	17% 8% 24% 33% 15% 3	446
	11th to 20th	Day	16% 4 28% 29% 15% 7%	439
	21st to end	Day	16% 10% 32% 21% 13% 7%	495
September	1st to 10th	Day	16% 10% 32% 23% 12% 8%	450
	11th to 20th	Day	15% 8% 26% 27% 14% 10%	450
	21st to end	Day	20% 11% 28% 24% 13% 4	450
October	1st to 10th	Day	28% 14% 24% 17% 12% 5%	450
	11th to 20th	Day	30% 16% 31% 17% 5% 1	450
	21st to end	Day	35% 14% 27% 20% 4	426
November	1st to 10th	Day	28% 18% 29% 18% 5% 1	365
	11th to 20th	Day	31% 14% 32% 14% 7% 2	214
	21st to end	Day	31% 18% 37% 11% 4	180
December	1st to 10th	Day	36% 17% 28% 15% 4	180
	11th to 20th	Day	36% 27% 26% 9% 2	180
	21st to end	Day	37% 18% 31% 10% 3 1	198
January	1st to 10th	Night	32% 17% 29% 19% 4	760
	11th to 20th	Night	32% 17% 27% 20% 4	760
	21st to end	Night	23% 16% 33% 23% 4	835
February	1st to 10th	Night	28% 20% 33% 17% 2	720
	11th to 20th	Night	24% 19% 38% 17% 2	703
	21st to end	Night	28% 17% 31% 20% 3	592
March	1st to 10th	Night	27% 16% 30% 23% 4	720
	11th to 20th	Night	28% 18% 31% 18% 4	720
	21st to end	Night	27% 15% 29% 23% 6% 1	792
April	1st to 10th	Night	27% 15% 31% 23% 3	720
	11th to 20th	Night	29% 17% 28% 19% 7%	711
	21st to end	Night	29% 11% 29% 23% 7%	644
May	1st to 10th	Night	26% 9% 26% 27% 10% 1	360
	11th to 20th	Night	16% 12% 35% 29% 7%	354
	21st to end	Night	15% 10% 37% 26% 12% 1	389
June	1st to 10th	Night	19% 7% 29% 37% 6% 1	360
	11th to 20th	Night	18% 10% 29% 31% 13% 1	360
	21st to end	Night	20% 6% 30% 29% 14%	360
July	1st to 10th	Night	10% 6% 29% 42% 12% 1	350
	11th to 20th	Night	14% 5% 26% 41% 14% 1	348
	21st to end	Night	10% 4 34% 39% 11% 1	394
August	1st to 10th	Night	15% 4 25% 36% 20%	409
	11th to 20th	Night	15% 3 24% 35% 21% 2	556
	21st to end	Night	13% 7% 29% 34% 15% 2	792
September	1st to 10th	Night	17% 7% 28% 33% 15% 2	720
	11th to 20th	Night	18% 5% 24% 28% 21% 4	720
	21st to end	Night	19% 9% 26% 26% 18% 2	720
October	1st to 10th	Night	23% 10% 29% 21% 15% 2	720
	11th to 20th	Night	25% 13% 31% 24% 7% 1	720
	21st to end	Night	32% 12% 28% 23% 5%	792
November	1st to 10th	Night	26% 15% 29% 22% 8%	740
	11th to 20th	Night	27% 16% 30% 18% 9%	760
	21st to end	Night	32% 17% 27% 17% 6%	760
December	1st to 10th	Night	35% 16% 28% 19% 3	760
	11th to 20th	Night	32% 17% 31% 15% 3 1	760
	21st to end	Night	36% 16% 26% 19% 2	836

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