Oakley Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Oakley 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	15% 7% 3 6% 5% 3 5% 5% 5% 7% 39%	74980
	Night	15% 8% 3 6% 5% 3 4 4 5% 7% 40%	75167
Transparency	Day	44% 6% 12% 22% 13% 2	74683
	Night	44% 6% 11% 21% 15% 3	74985
Seeing	Day	17% 9% 25% 25% 17% 7%	14493
	Night	18% 8% 27% 31% 14% 1	24473

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	18% 12% 6% 9% 6% 3 4 4 3 6% 30%	1517
	11th to 20th	Day	19% 13% 5% 8% 7% 3 5% 3 4 5% 28%	1520
	21st to end	Day	18% 14% 6% 7% 5% 3 4 4 5% 5% 29%	1748
February	1st to 10th	Day	19% 11% 8% 7% 6% 3 3 4 5% 5% 30%	1791
	11th to 20th	Day	17% 10% 4 10% 6% 4 4 4 5% 6% 31%	1748
	21st to end	Day	18% 13% 4 8% 5% 3 3 4 4 5% 32%	1480
March	1st to 10th	Day	18% 9% 4 6% 5% 3 4 3 5% 6% 38%	1800
	11th to 20th	Day	19% 10% 4 6% 5% 2 4 5% 4 7% 35%	1858
	21st to end	Day	22% 11% 5% 7% 6% 4 4 4 4 6% 28%	2353
April	1st to 10th	Day	17% 8% 4 6% 5% 2 4 4 5% 8% 39%	2160
	11th to 20th	Day	18% 7% 3 5% 5% 3 5% 5% 5% 5% 40%	2134
	21st to end	Day	20% 7% 3 6% 6% 3 5% 4 5% 7% 35%	2245
May	1st to 10th	Day	19% 7% 3 7% 5% 3 5% 4 5% 7% 34%	2340
	11th to 20th	Day	20% 7% 3 5% 5% 3 4 4 5% 7% 37%	2301
	21st to end	Day	10% 6% 3 7% 7% 4 6% 7% 6% 9% 37%	2663
June	1st to 10th	Day	11% 5% 2 5% 5% 3 5% 5% 6% 9% 44%	2520
	11th to 20th	Day	9% 4 2 5% 5% 3 6% 6% 6% 9% 46%	2520
	21st to end	Day	11% 4 2 5% 6% 3 5% 5% 6% 10% 43%	2520
July	1st to 10th	Day	10% 5% 2 5% 5% 3 6% 6% 6% 11% 41%	2520
	11th to 20th	Day	7% 5% 2 4 5% 4 6% 6% 7% 9% 45%	2520
	21st to end	Day	10% 3 1 4 5% 4 7% 5% 7% 9% 44%	2739
August	1st to 10th	Day	8% 5% 2 4 5% 3 5% 5% 7% 11% 46%	2310
	11th to 20th	Day	11% 4 3 5% 5% 4 6% 6% 6% 9% 41%	2125
	21st to end	Day	6% 4 2 4 6% 3 5% 5% 6% 10% 50%	2376
September	1st to 10th	Day	12% 5% 2 5% 6% 3 4 5% 6% 7% 46%	2160
	11th to 20th	Day	10% 4 3 4 5% 3 5% 6% 6% 8% 48%	2000
	21st to end	Day	11% 4 2 4 5% 3 4 4 5% 7% 52%	1980
October	1st to 10th	Day	13% 4 2 5% 5% 3 3 3 5% 6% 50%	1980
	11th to 20th	Day	16% 5% 2 5% 5% 4 3 5% 5% 8% 42%	1826
	21st to end	Day	20% 8% 3 7% 4 3 4 3 5% 5% 38%	1782
November	1st to 10th	Day	11% 5% 3 4 4 2 4 4 4 7% 52%	1672
	11th to 20th	Day	18% 8% 3 5% 4 3 3 3 4 6% 41%	1710
	21st to end	Day	19% 9% 3 5% 4 2 5% 4 3 6% 38%	1710
December	1st to 10th	Day	19% 12% 4 8% 6% 2 4 4 4 5% 32%	1651
	11th to 20th	Day	17% 14% 6% 7% 5% 3 4 3 4 5% 31%	1918
	21st to end	Day	25% 17% 7% 8% 6% 4 4 4 4 4 18%	2783
January	1st to 10th	Night	17% 11% 4 8% 5% 3 5% 4 4 6% 32%	2648
	11th to 20th	Night	18% 14% 5% 9% 6% 3 4 3 4 4 29%	2660
	21st to end	Night	19% 12% 5% 7% 5% 3 4 4 5% 5% 30%	2831
February	1st to 10th	Night	20% 11% 6% 8% 5% 3 4 4 4 5% 28%	2167
	11th to 20th	Night	17% 9% 5% 8% 6% 3 4 5% 4 6% 33%	2098
	21st to end	Night	20% 12% 3 7% 3 2 5% 4 4 5% 34%	1776
March	1st to 10th	Night	18% 10% 4 5% 4 3 4 4 4 5% 39%	2160
	11th to 20th	Night	18% 8% 3 6% 5% 3 4 4 4 7% 39%	2102
	21st to end	Night	22% 8% 4 6% 5% 3 4 4 5% 6% 33%	2018
April	1st to 10th	Night	14% 8% 3 5% 5% 3 4 4 6% 6% 42%	1800
	11th to 20th	Night	17% 7% 3 5% 4 2 4 4 4 7% 43%	1775
	21st to end	Night	18% 6% 3 7% 5% 4 5% 4 6% 6% 36%	1715
May	1st to 10th	Night	16% 9% 3 6% 6% 3 5% 5% 5% 9% 35%	1620
	11th to 20th	Night	15% 5% 3 5% 5% 4 4 4 5% 7% 42%	1592
	21st to end	Night	9% 5% 3 5% 6% 4 5% 6% 7% 10% 41%	1608
June	1st to 10th	Night	9% 6% 3 5% 4 3 5% 4 4 8% 50%	1440
	11th to 20th	Night	8% 4 2 5% 5% 3 3 5% 5% 8% 52%	1440
	21st to end	Night	8% 4 2 4 6% 3 4 6% 7% 11% 46%	1440
July	1st to 10th	Night	8% 3 2 4 4 3 5% 6% 7% 10% 48%	1440
	11th to 20th	Night	7% 4 2 5% 4 2 3 4 7% 8% 53%	1440
	21st to end	Night	6% 4 2 5% 4 2 4 4 4 10% 54%	1577
August	1st to 10th	Night	7% 4 2 4 5% 3 5% 5% 5% 10% 50%	1624
	11th to 20th	Night	6% 5% 2 5% 5% 2 5% 6% 5% 9% 50%	1776
	21st to end	Night	7% 4 1 3 4 3 3 4 5% 9% 56%	1980
September	1st to 10th	Night	11% 5% 2 5% 4 3 4 3 5% 8% 50%	1800
	11th to 20th	Night	7% 4 2 3 5% 3 3 4 4 8% 58%	1941
	21st to end	Night	12% 5% 2 4 4 3 3 4 5% 7% 51%	1980
October	1st to 10th	Night	9% 4 1 5% 4 3 4 4 5% 5% 56%	1980
	11th to 20th	Night	17% 5% 2 4 3 2 3 4 4 6% 49%	2129
	21st to end	Night	18% 8% 2 5% 5% 2 4 5% 4 7% 40%	2574
November	1st to 10th	Night	11% 7% 3 5% 4 3 4 4 4 5% 51%	2406
	11th to 20th	Night	13% 8% 3 6% 4 2 4 4 5% 7% 43%	2470
	21st to end	Night	20% 9% 4 5% 5% 3 4 3 4 5% 37%	2470
December	1st to 10th	Night	18% 11% 5% 7% 6% 2 3 5% 5% 6% 32%	2527
	11th to 20th	Night	19% 11% 4 8% 5% 2 4 4 5% 5% 35%	3279
	21st to end	Night	26% 17% 5% 8% 6% 3 4 4 3 4 20%	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	57% 4 8% 19% 11% 1	1518
	11th to 20th	Day	61% 4 12% 17% 7%	1520
	21st to end	Day	57% 4 12% 20% 5% 1	1748
February	1st to 10th	Day	56% 5% 12% 17% 8% 2	1791
	11th to 20th	Day	56% 4 11% 21% 8% 1	1758
	21st to end	Day	54% 6% 8% 21% 9% 2	1480
March	1st to 10th	Day	49% 4 12% 23% 11% 1	1800
	11th to 20th	Day	50% 5% 13% 20% 11% 1	1858
	21st to end	Day	58% 5% 10% 20% 6% 1	2353
April	1st to 10th	Day	44% 7% 11% 23% 14% 1	2160
	11th to 20th	Day	45% 6% 10% 24% 11% 4	2134
	21st to end	Day	49% 6% 11% 21% 10% 3	2245
May	1st to 10th	Day	50% 7% 13% 20% 10%	2340
	11th to 20th	Day	47% 8% 11% 19% 14% 1	2305
	21st to end	Day	42% 10% 13% 19% 13% 2	2663
June	1st to 10th	Day	35% 8% 13% 25% 16% 3	2520
	11th to 20th	Day	32% 9% 13% 22% 21% 4	2520
	21st to end	Day	36% 8% 17% 19% 16% 4	2520
July	1st to 10th	Day	36% 10% 14% 23% 15% 3	2451
	11th to 20th	Day	31% 10% 15% 26% 16% 2	2445
	21st to end	Day	34% 8% 15% 25% 15% 3	2587
August	1st to 10th	Day	32% 8% 16% 29% 14% 2	2297
	11th to 20th	Day	36% 8% 14% 26% 14% 2	2125
	21st to end	Day	29% 6% 16% 25% 17% 7%	2376
September	1st to 10th	Day	37% 6% 12% 25% 17% 4	2160
	11th to 20th	Day	32% 7% 13% 24% 21% 3	2000
	21st to end	Day	31% 6% 10% 25% 24% 3	1980
October	1st to 10th	Day	34% 5% 11% 25% 22% 3	1980
	11th to 20th	Day	40% 6% 10% 21% 21% 1	1826
	21st to end	Day	48% 4 9% 23% 14% 1	1782
November	1st to 10th	Day	34% 4 14% 27% 16% 5%	1672
	11th to 20th	Day	45% 5% 9% 22% 16% 4	1710
	21st to end	Day	48% 5% 9% 22% 15% 1	1710
December	1st to 10th	Day	56% 5% 10% 18% 10%	1651
	11th to 20th	Day	56% 5% 7% 25% 6% 1	1915
	21st to end	Day	69% 5% 10% 11% 5%	2783
January	1st to 10th	Night	54% 5% 10% 19% 11% 1	2648
	11th to 20th	Night	60% 4 11% 18% 7% 1	2660
	21st to end	Night	55% 5% 12% 19% 9% 1	2831
February	1st to 10th	Night	59% 5% 13% 16% 7% 1	2167
	11th to 20th	Night	52% 5% 11% 19% 11% 2	2099
	21st to end	Night	53% 6% 10% 20% 10% 2	1776
March	1st to 10th	Night	49% 4 9% 23% 13% 2	2160
	11th to 20th	Night	47% 5% 12% 22% 13% 2	2102
	21st to end	Night	52% 6% 12% 21% 8% 2	2018
April	1st to 10th	Night	41% 6% 11% 25% 15% 2	1800
	11th to 20th	Night	41% 6% 10% 20% 18% 5%	1775
	21st to end	Night	48% 7% 9% 20% 13% 4	1715
May	1st to 10th	Night	47% 7% 12% 23% 11% 1	1620
	11th to 20th	Night	42% 8% 13% 20% 16% 1	1593
	21st to end	Night	36% 10% 13% 21% 15% 5%	1609
June	1st to 10th	Night	33% 6% 10% 25% 21% 4	1440
	11th to 20th	Night	30% 7% 12% 24% 21% 5%	1440
	21st to end	Night	29% 9% 16% 22% 19% 5%	1440
July	1st to 10th	Night	29% 11% 17% 21% 18% 5%	1400
	11th to 20th	Night	27% 7% 15% 28% 21% 3	1392
	21st to end	Night	27% 6% 14% 30% 20% 3	1489
August	1st to 10th	Night	30% 8% 15% 26% 18% 3	1613
	11th to 20th	Night	29% 8% 16% 25% 20% 3	1776
	21st to end	Night	25% 6% 14% 25% 22% 7%	1980
September	1st to 10th	Night	33% 7% 11% 25% 18% 6%	1800
	11th to 20th	Night	26% 5% 11% 28% 26% 5%	1941
	21st to end	Night	33% 6% 10% 23% 24% 5%	1980
October	1st to 10th	Night	29% 6% 11% 25% 25% 4	1980
	11th to 20th	Night	37% 4 8% 25% 22% 4	2129
	21st to end	Night	45% 5% 9% 22% 17% 3	2574
November	1st to 10th	Night	35% 4 10% 26% 20% 5%	2406
	11th to 20th	Night	40% 5% 10% 24% 18% 3	2470
	21st to end	Night	50% 4 9% 23% 13% 1	2470
December	1st to 10th	Night	53% 5% 12% 19% 11% 1	2527
	11th to 20th	Night	51% 6% 11% 19% 13% 1	3281
	21st to end	Night	69% 5% 7% 11% 6% 1	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	25% 17% 31% 19% 7%	270
	11th to 20th	Day	26% 16% 38% 16% 4	270
	21st to end	Day	26% 19% 32% 18% 3 1	294
February	1st to 10th	Day	30% 18% 33% 15% 5%	240
	11th to 20th	Day	27% 20% 33% 16% 5%	240
	21st to end	Day	30% 14% 38% 15% 2 1	198
March	1st to 10th	Day	22% 21% 37% 16% 4	240
	11th to 20th	Day	27% 15% 37% 16% 5% 1	298
	21st to end	Day	28% 17% 31% 17% 7% 1	462
April	1st to 10th	Day	18% 15% 35% 23% 9% 1	420
	11th to 20th	Day	19% 14% 29% 23% 12% 2	412
	21st to end	Day	24% 15% 29% 20% 9% 2	420
May	1st to 10th	Day	23% 11% 28% 26% 9% 3	423
	11th to 20th	Day	19% 7% 29% 25% 16% 4	447
	21st to end	Day	11% 4 28% 27% 19% 12%	617
June	1st to 10th	Day	12% 4 20% 33% 24% 6%	630
	11th to 20th	Day	11% 4 22% 33% 23% 7%	630
	21st to end	Day	12% 4 27% 28% 21% 7%	630
July	1st to 10th	Day	11% 2 14% 28% 24% 21%	621
	11th to 20th	Day	7% 3 17% 32% 29% 13%	618
	21st to end	Day	11% 3 14% 28% 32% 12%	680
August	1st to 10th	Day	9% 2 12% 26% 32% 17%	446
	11th to 20th	Day	14% 3 13% 31% 26% 13%	439
	21st to end	Day	5% 2 13% 30% 33% 18%	495
September	1st to 10th	Day	11% 4 16% 26% 32% 11%	450
	11th to 20th	Day	8% 2 12% 30% 31% 17%	450
	21st to end	Day	13% 6% 24% 23% 21% 14%	450
October	1st to 10th	Day	11% 5% 26% 33% 17% 8%	450
	11th to 20th	Day	16% 14% 34% 23% 13% 1	309
	21st to end	Day	25% 19% 31% 20% 6%	297
November	1st to 10th	Day	12% 14% 32% 25% 14% 3	270
	11th to 20th	Day	21% 17% 29% 20% 10% 3	270
	21st to end	Day	24% 12% 34% 22% 7%	270
December	1st to 10th	Day	18% 21% 35% 17% 8% 2	270
	11th to 20th	Day	24% 11% 38% 19% 6% 1	270
	21st to end	Day	34% 14% 33% 14% 4	297
January	1st to 10th	Night	22% 14% 32% 27% 5%	950
	11th to 20th	Night	26% 12% 37% 21% 4	950
	21st to end	Night	23% 12% 35% 27% 3	1043
February	1st to 10th	Night	26% 11% 35% 25% 2	727
	11th to 20th	Night	23% 14% 36% 25% 3	703
	21st to end	Night	27% 11% 36% 23% 2	592
March	1st to 10th	Night	23% 14% 35% 26% 3	720
	11th to 20th	Night	23% 10% 34% 27% 6% 1	720
	21st to end	Night	25% 7% 30% 31% 6%	632
April	1st to 10th	Night	15% 12% 32% 35% 6%	540
	11th to 20th	Night	20% 11% 27% 30% 12%	533
	21st to end	Night	21% 8% 34% 30% 7% 1	540
May	1st to 10th	Night	18% 7% 32% 31% 11% 1	540
	11th to 20th	Night	15% 5% 26% 35% 17% 2	531
	21st to end	Night	12% 3 21% 38% 25% 2	584
June	1st to 10th	Night	11% 3 17% 46% 22% 1	540
	11th to 20th	Night	10% 3 18% 45% 24% 1	540
	21st to end	Night	8% 3 21% 41% 27% 1	540
July	1st to 10th	Night	10% 2 9% 35% 37% 7%	525
	11th to 20th	Night	6% 3 14% 38% 35% 5%	522
	21st to end	Night	6% 2 10% 45% 34% 4	591
August	1st to 10th	Night	8% 1 9% 43% 36% 3	537
	11th to 20th	Night	8% 1 12% 40% 36% 4	534
	21st to end	Night	8% 1 12% 32% 40% 6%	594
September	1st to 10th	Night	12% 2 13% 37% 31% 5%	540
	11th to 20th	Night	8% 2 16% 35% 34% 5%	681
	21st to end	Night	14% 5% 18% 34% 26% 4	720
October	1st to 10th	Night	10% 4 22% 38% 22% 4	720
	11th to 20th	Night	20% 9% 26% 29% 14% 1	720
	21st to end	Night	22% 10% 33% 28% 7%	792
November	1st to 10th	Night	15% 6% 33% 33% 12% 1	740
	11th to 20th	Night	17% 11% 32% 30% 9% 1	760
	21st to end	Night	24% 11% 31% 27% 8%	760
December	1st to 10th	Night	23% 11% 36% 24% 5%	817
	11th to 20th	Night	25% 9% 32% 27% 6% 1	950
	21st to end	Night	30% 13% 31% 22% 4	1045

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