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 2024-11-06.

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% 10% 3 8% 5% 5% 4 3 5% 7% 33%	101559
	Night	18% 11% 4 8% 5% 4 4 3 5% 6% 31%	97482
Transparency	Day	50% 5% 12% 22% 10% 1	105055
	Night	53% 5% 12% 20% 9% 1	102075
Seeing	Day	25% 15% 30% 19% 8% 2	14538
	Night	25% 13% 29% 24% 8% 1	23623

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	16% 17% 6% 11% 6% 5% 5% 3 4 5% 23%	2517
	11th to 20th	Day	14% 17% 6% 13% 7% 6% 4 3 4 4 21%	2520
	21st to end	Day	18% 20% 7% 11% 6% 7% 4 3 4 5% 16%	2914
February	1st to 10th	Day	17% 15% 6% 12% 6% 4 5% 4 4 4 23%	1710
	11th to 20th	Day	13% 17% 6% 10% 6% 5% 5% 3 4 5% 26%	1682
	21st to end	Day	21% 16% 5% 9% 5% 6% 4 3 4 4 22%	1570
March	1st to 10th	Day	20% 14% 4 9% 5% 5% 4 3 4 5% 27%	3116
	11th to 20th	Day	13% 18% 4 9% 6% 5% 4 3 4 5% 29%	3477
	21st to end	Day	14% 10% 3 8% 5% 4 4 3 4 6% 40%	4028
April	1st to 10th	Day	21% 13% 3 6% 4 4 4 3 5% 5% 32%	2280
	11th to 20th	Day	22% 10% 3 6% 4 4 4 3 4 5% 36%	2254
	21st to end	Day	23% 9% 3 8% 5% 4 4 3 5% 5% 31%	2376
May	1st to 10th	Day	21% 9% 3 8% 5% 4 4 4 4 7% 31%	4426
	11th to 20th	Day	18% 9% 3 9% 5% 4 4 4 5% 7% 33%	4398
	21st to end	Day	15% 6% 2 6% 4 4 5% 4 6% 9% 39%	4837
June	1st to 10th	Day	22% 8% 3 7% 5% 4 5% 4 6% 8% 30%	2660
	11th to 20th	Day	15% 6% 3 7% 4 5% 4 4 6% 8% 38%	2660
	21st to end	Day	19% 8% 3 7% 5% 5% 5% 4 6% 7% 32%	2660
July	1st to 10th	Day	13% 6% 2 6% 4 5% 6% 5% 7% 9% 37%	4440
	11th to 20th	Day	13% 6% 2 6% 4 5% 5% 4 7% 9% 40%	4440
	21st to end	Day	11% 6% 2 5% 4 5% 5% 5% 7% 9% 41%	4814
August	1st to 10th	Day	16% 9% 3 8% 4 5% 4 3 6% 8% 35%	4313
	11th to 20th	Day	17% 6% 2 6% 4 4 5% 3 6% 9% 38%	3962
	21st to end	Day	16% 6% 2 5% 4 4 4 4 6% 9% 39%	4180
September	1st to 10th	Day	14% 5% 2 7% 4 5% 4 4 5% 7% 42%	2280
	11th to 20th	Day	14% 5% 2 5% 4 4 4 3 6% 7% 46%	2277
	21st to end	Day	18% 6% 2 6% 4 5% 4 4 5% 8% 39%	2033
October	1st to 10th	Day	21% 9% 2 7% 4 4 3 3 5% 5% 36%	1900
	11th to 20th	Day	20% 10% 3 8% 5% 5% 4 3 5% 6% 30%	1900
	21st to end	Day	26% 11% 2 7% 5% 4 5% 4 4 5% 27%	1946
November	1st to 10th	Day	16% 9% 2 8% 5% 5% 5% 4 6% 6% 35%	1717
	11th to 20th	Day	17% 13% 3 9% 6% 5% 4 3 5% 5% 29%	1554
	21st to end	Day	22% 15% 4 9% 6% 4 4 4 5% 6% 21%	1520
December	1st to 10th	Day	24% 17% 5% 11% 5% 3 4 4 4 6% 17%	1520
	11th to 20th	Day	22% 17% 4 10% 6% 4 3 3 4 4 23%	1906
	21st to end	Day	27% 17% 5% 12% 4 4 4 2 3 3 20%	2772
January	1st to 10th	Night	19% 17% 6% 11% 5% 4 4 3 4 5% 23%	4429
	11th to 20th	Night	14% 17% 6% 14% 7% 6% 6% 3 5% 4 18%	4440
	21st to end	Night	20% 18% 6% 11% 6% 6% 4 3 3 4 19%	4724
February	1st to 10th	Night	17% 15% 6% 12% 6% 5% 4 4 5% 5% 21%	2470
	11th to 20th	Night	14% 14% 6% 11% 6% 4 4 3 4 5% 28%	2384
	21st to end	Night	17% 14% 5% 10% 6% 6% 5% 2 3 5% 28%	1884
March	1st to 10th	Night	20% 12% 3 8% 5% 4 4 3 3 4 33%	3724
	11th to 20th	Night	19% 17% 5% 9% 4 3 4 2 4 6% 26%	3479
	21st to end	Night	17% 10% 3 8% 4 4 4 3 4 5% 39%	3604
April	1st to 10th	Night	21% 10% 4 8% 4 4 4 3 4 6% 32%	1900
	11th to 20th	Night	21% 9% 3 7% 5% 4 3 3 4 5% 36%	1875
	21st to end	Night	22% 10% 3 7% 4 4 4 3 5% 5% 33%	1818
May	1st to 10th	Night	23% 10% 2 7% 4 4 5% 4 6% 6% 29%	2514
	11th to 20th	Night	21% 8% 3 7% 4 3 4 4 4 6% 36%	2495
	21st to end	Night	14% 7% 2 6% 4 5% 4 3 6% 8% 41%	2738
June	1st to 10th	Night	17% 7% 2 8% 5% 5% 6% 4 6% 7% 33%	1520
	11th to 20th	Night	16% 6% 2 6% 4 4 4 5% 5% 8% 40%	1520
	21st to end	Night	18% 8% 2 6% 5% 4 5% 5% 6% 7% 35%	1520
July	1st to 10th	Night	12% 7% 3 8% 6% 6% 4 4 6% 8% 37%	2520
	11th to 20th	Night	15% 6% 2 6% 4 4 4 4 6% 9% 43%	2520
	21st to end	Night	11% 6% 3 7% 5% 5% 5% 4 6% 8% 42%	2739
August	1st to 10th	Night	13% 8% 3 7% 5% 4 4 4 6% 8% 40%	2599
	11th to 20th	Night	13% 8% 3 7% 4 4 4 3 5% 7% 41%	2835
	21st to end	Night	9% 6% 2 7% 5% 4 5% 4 6% 9% 42%	3476
September	1st to 10th	Night	13% 6% 2 7% 5% 3 4 3 4 6% 46%	1900
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 5% 6% 51%	1902
	21st to end	Night	16% 6% 2 7% 4 5% 3 4 5% 7% 42%	2143
October	1st to 10th	Night	18% 7% 3 6% 5% 5% 4 4 5% 7% 36%	2280
	11th to 20th	Night	20% 8% 3 8% 5% 4 4 3 6% 6% 33%	2280
	21st to end	Night	25% 10% 3 7% 4 4 4 3 5% 7% 29%	2650
November	1st to 10th	Night	19% 10% 4 7% 4 4 3 4 5% 5% 36%	2483
	11th to 20th	Night	18% 11% 3 7% 6% 4 5% 3 5% 6% 33%	2625
	21st to end	Night	22% 13% 4 8% 5% 5% 4 4 5% 6% 24%	2660
December	1st to 10th	Night	20% 19% 5% 10% 6% 4 4 3 5% 5% 18%	2660
	11th to 20th	Night	23% 16% 4 9% 5% 3 3 3 3 4 27%	3288
	21st to end	Night	31% 15% 4 12% 5% 4 3 2 4 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	64% 7% 12% 11% 5% 1	2518
	11th to 20th	Day	67% 5% 11% 10% 7% 1	2520
	21st to end	Day	72% 5% 10% 9% 4 1	2914
February	1st to 10th	Day	64% 4 7% 19% 5% 1	1710
	11th to 20th	Day	60% 4 9% 16% 9% 1	1691
	21st to end	Day	65% 4 9% 12% 9% 1	1570
March	1st to 10th	Day	59% 4 9% 19% 8% 1	3119
	11th to 20th	Day	58% 5% 12% 14% 10% 2	3477
	21st to end	Day	45% 4 7% 23% 17% 3	4028
April	1st to 10th	Day	55% 3 7% 19% 13% 3	2280
	11th to 20th	Day	52% 4 6% 20% 16% 2	2254
	21st to end	Day	54% 4 8% 16% 14% 3	2376
May	1st to 10th	Day	53% 6% 9% 20% 10% 1	4440
	11th to 20th	Day	50% 6% 12% 22% 9% 1	4402
	21st to end	Day	41% 6% 12% 26% 14% 1	4837
June	1st to 10th	Day	51% 5% 11% 20% 12% 1	2660
	11th to 20th	Day	43% 6% 10% 26% 13% 2	2660
	21st to end	Day	50% 7% 12% 19% 11% 1	2660
July	1st to 10th	Day	40% 6% 13% 29% 12%	4370
	11th to 20th	Day	39% 6% 16% 31% 9%	4365
	21st to end	Day	37% 8% 14% 32% 10%	4661
August	1st to 10th	Day	46% 5% 15% 23% 10% 1	4299
	11th to 20th	Day	42% 5% 15% 28% 9%	4001
	21st to end	Day	41% 5% 17% 26% 11%	4180
September	1st to 10th	Day	40% 6% 14% 27% 13%	2280
	11th to 20th	Day	36% 5% 12% 32% 14%	2277
	21st to end	Day	44% 5% 13% 27% 10%	2033
October	1st to 10th	Day	46% 4 13% 29% 9%	3160
	11th to 20th	Day	47% 5% 16% 25% 7%	3140
	21st to end	Day	48% 4 17% 24% 7%	3184
November	1st to 10th	Day	49% 5% 14% 24% 7% 1	1717
	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	65% 5% 15% 11% 4	4429
	11th to 20th	Night	69% 6% 10% 10% 4	4440
	21st to end	Night	70% 4 9% 11% 5% 1	4724
February	1st to 10th	Night	66% 4 9% 15% 6% 1	2470
	11th to 20th	Night	59% 4 9% 21% 7%	2386
	21st to end	Night	61% 3 8% 17% 10% 1	1884
March	1st to 10th	Night	55% 4 9% 21% 9% 1	3734
	11th to 20th	Night	59% 5% 14% 13% 9% 1	3480
	21st to end	Night	48% 4 5% 24% 16% 3	3604
April	1st to 10th	Night	53% 4 8% 19% 13% 2	1900
	11th to 20th	Night	52% 4 8% 18% 16% 1	1875
	21st to end	Night	53% 4 8% 17% 14% 4	1818
May	1st to 10th	Night	53% 6% 10% 19% 9% 3	2520
	11th to 20th	Night	49% 6% 12% 21% 11% 2	2496
	21st to end	Night	40% 6% 13% 22% 17% 2	2739
June	1st to 10th	Night	48% 8% 12% 21% 11% 1	1520
	11th to 20th	Night	42% 5% 12% 22% 17% 2	1520
	21st to end	Night	48% 6% 12% 20% 13% 1	1520
July	1st to 10th	Night	44% 5% 13% 22% 14% 1	2480
	11th to 20th	Night	38% 6% 14% 34% 8%	2472
	21st to end	Night	40% 7% 14% 24% 14%	2651
August	1st to 10th	Night	43% 6% 14% 26% 11%	2589
	11th to 20th	Night	43% 5% 14% 30% 8% 1	2862
	21st to end	Night	37% 7% 15% 28% 11% 1	3476
September	1st to 10th	Night	39% 5% 12% 28% 14% 1	1900
	11th to 20th	Night	34% 4 15% 33% 13% 1	1902
	21st to end	Night	42% 5% 12% 28% 12% 1	2143
October	1st to 10th	Night	43% 5% 13% 29% 11%	3800
	11th to 20th	Night	44% 3 16% 27% 9%	3779
	21st to end	Night	48% 4 17% 22% 8%	4362
November	1st to 10th	Night	51% 4 13% 21% 10% 1	2483
	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% 21% 33% 14% 6%	240
February	1st to 10th	Day	34% 26% 30% 10%	270
	11th to 20th	Day	31% 28% 33% 7%	289
	21st to end	Day	36% 19% 32% 12% 2	382
March	1st to 10th	Day	30% 18% 32% 17% 3	460
	11th to 20th	Day	32% 18% 36% 11% 3 1	460
	21st to end	Day	35% 20% 25% 14% 5% 1	506
April	1st to 10th	Day	37% 16% 28% 14% 5%	460
	11th to 20th	Day	34% 20% 24% 12% 6% 4	452
	21st to end	Day	33% 16% 31% 12% 5% 2	460
May	1st to 10th	Day	28% 17% 28% 20% 7% 1	463
	11th to 20th	Day	21% 17% 34% 18% 7% 2	487
	21st to end	Day	19% 17% 30% 22% 10% 1	529
June	1st to 10th	Day	26% 14% 31% 20% 8% 1	490
	11th to 20th	Day	17% 14% 33% 22% 10% 2	490
	21st to end	Day	26% 13% 31% 18% 10% 2	490
July	1st to 10th	Day	14% 7% 32% 30% 14% 2	485
	11th to 20th	Day	16% 10% 29% 29% 14% 2	484
	21st to end	Day	15% 9% 31% 33% 9% 3	532
August	1st to 10th	Day	18% 9% 24% 31% 14% 3	486
	11th to 20th	Day	17% 5% 28% 29% 15% 7%	479
	21st to end	Day	17% 10% 31% 23% 13% 7%	539
September	1st to 10th	Day	17% 10% 31% 24% 12% 7%	490
	11th to 20th	Day	14% 7% 24% 26% 17% 11%	490
	21st to end	Day	20% 11% 27% 25% 14% 3	490
October	1st to 10th	Day	27% 14% 26% 17% 12% 5%	490
	11th to 20th	Day	29% 17% 31% 17% 6% 1	490
	21st to end	Day	35% 14% 27% 20% 4	462
November	1st to 10th	Day	28% 19% 29% 18% 5% 1	381
	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% 24% 4	836
February	1st to 10th	Night	27% 19% 33% 19% 2	760
	11th to 20th	Night	23% 20% 38% 17% 2	743
	21st to end	Night	28% 18% 31% 21% 3	628
March	1st to 10th	Night	28% 15% 30% 23% 4	760
	11th to 20th	Night	28% 18% 32% 18% 4	760
	21st to end	Night	27% 15% 30% 22% 6%	836
April	1st to 10th	Night	28% 15% 31% 23% 3	760
	11th to 20th	Night	30% 18% 27% 19% 6%	751
	21st to end	Night	29% 11% 29% 23% 8%	678
May	1st to 10th	Night	26% 10% 27% 27% 9% 1	380
	11th to 20th	Night	16% 11% 34% 31% 7%	374
	21st to end	Night	15% 10% 37% 27% 11%	411
June	1st to 10th	Night	19% 7% 29% 38% 6% 1	380
	11th to 20th	Night	17% 9% 29% 31% 12% 1	380
	21st to end	Night	22% 6% 31% 28% 13%	380
July	1st to 10th	Night	11% 6% 29% 42% 11% 1	370
	11th to 20th	Night	14% 5% 26% 41% 14% 1	368
	21st to end	Night	11% 5% 33% 39% 12% 1	416
August	1st to 10th	Night	15% 4 25% 37% 19%	432
	11th to 20th	Night	15% 3 24% 34% 22% 2	588
	21st to end	Night	13% 7% 29% 35% 15% 2	836
September	1st to 10th	Night	16% 7% 28% 33% 14% 2	760
	11th to 20th	Night	17% 5% 23% 28% 23% 5%	760
	21st to end	Night	20% 9% 26% 26% 18% 2	760
October	1st to 10th	Night	22% 10% 29% 21% 15% 2	760
	11th to 20th	Night	25% 13% 30% 24% 7% 1	760
	21st to end	Night	32% 12% 28% 23% 5%	836
November	1st to 10th	Night	26% 16% 28% 22% 8%	764
	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

Page generated at 2024-11-03 13:18:58. Version ac07

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