Oak Beach Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Oak Beach Observatory clear sky chart from 2005-11-06 to 2024-04-25.

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	14% 10% 4 7% 6% 4 5% 5% 5% 7% 33%	83028
	Night	15% 11% 5% 8% 7% 4 5% 4 5% 6% 30%	86094
Transparency	Day	35% 36% 9% 13% 7% 1	82734
	Night	38% 34% 8% 13% 6% 1	85910
Seeing	Day	16% 7% 25% 30% 17% 5%	14593
	Night	19% 8% 28% 29% 14% 2	24281

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	16% 15% 11% 14% 8% 3 5% 4 4 4 16%	2527
	11th to 20th	Day	13% 17% 8% 15% 16% 8% 4 4 4 3 7%	2530
	21st to end	Day	22% 17% 7% 11% 7% 3 4 5% 6% 5% 14%	2967
February	1st to 10th	Day	13% 14% 6% 11% 8% 4 6% 5% 4 7% 22%	1710
	11th to 20th	Day	11% 11% 6% 10% 10% 5% 6% 5% 5% 6% 26%	1744
	21st to end	Day	14% 11% 5% 7% 8% 4 5% 5% 7% 7% 28%	1570
March	1st to 10th	Day	17% 12% 4 7% 7% 3 5% 4 4 6% 31%	3203
	11th to 20th	Day	11% 12% 5% 8% 6% 4 5% 5% 5% 8% 31%	3480
	21st to end	Day	15% 11% 5% 8% 8% 4 5% 5% 5% 6% 29%	3815
April	1st to 10th	Day	16% 10% 5% 7% 6% 3 4 5% 6% 8% 31%	2210
	11th to 20th	Day	17% 7% 3 6% 5% 3 4 4 5% 7% 39%	2441
	21st to end	Day	16% 8% 3 7% 5% 3 5% 4 5% 7% 36%	2394
May	1st to 10th	Day	15% 7% 3 6% 6% 3 4 5% 6% 8% 38%	2340
	11th to 20th	Day	14% 6% 3 6% 6% 4 5% 4 5% 7% 40%	2301
	21st to end	Day	11% 6% 2 4 5% 2 5% 4 6% 8% 46%	2694
June	1st to 10th	Day	11% 6% 3 4 5% 3 4 5% 7% 8% 44%	2700
	11th to 20th	Day	10% 5% 3 4 5% 3 5% 4 7% 9% 45%	2700
	21st to end	Day	11% 5% 2 5% 5% 3 4 6% 5% 9% 45%	2700
July	1st to 10th	Day	7% 4 2 4 3 2 4 4 7% 11% 52%	2607
	11th to 20th	Day	8% 4 2 5% 5% 3 5% 6% 6% 9% 47%	2520
	21st to end	Day	8% 4 2 4 5% 3 5% 6% 7% 10% 46%	2649
August	1st to 10th	Day	9% 5% 3 5% 5% 3 6% 5% 6% 10% 44%	2327
	11th to 20th	Day	9% 4 2 5% 4 4 5% 5% 5% 9% 48%	2303
	21st to end	Day	11% 6% 3 5% 4 2 4 5% 5% 8% 47%	2574
September	1st to 10th	Day	11% 6% 3 6% 5% 3 5% 6% 5% 8% 43%	2062
	11th to 20th	Day	14% 6% 2 4 3 3 3 5% 5% 8% 46%	1980
	21st to end	Day	14% 6% 3 6% 6% 3 6% 6% 6% 8% 36%	1980
October	1st to 10th	Day	16% 9% 3 6% 7% 3 4 4 4 6% 38%	1908
	11th to 20th	Day	22% 12% 3 7% 5% 4 4 5% 4 7% 27%	1800
	21st to end	Day	23% 11% 4 7% 7% 4 5% 5% 5% 7% 22%	1818
November	1st to 10th	Day	13% 13% 4 7% 7% 5% 5% 4 6% 8% 29%	1672
	11th to 20th	Day	15% 15% 6% 9% 7% 4 6% 5% 5% 8% 21%	1656
	21st to end	Day	17% 18% 6% 12% 8% 4 5% 4 3 6% 18%	1520
December	1st to 10th	Day	12% 20% 10% 13% 9% 5% 5% 4 4 4 14%	1506
	11th to 20th	Day	14% 17% 6% 11% 9% 5% 5% 5% 5% 5% 18%	1675
	21st to end	Day	25% 21% 7% 11% 7% 3 5% 4 3 4 9%	2445
January	1st to 10th	Night	16% 15% 8% 13% 8% 3 5% 4 4 5% 20%	4428
	11th to 20th	Night	13% 14% 8% 14% 15% 7% 5% 4 4 4 11%	4440
	21st to end	Night	17% 15% 8% 11% 9% 4 5% 5% 5% 5% 16%	4692
February	1st to 10th	Night	13% 14% 7% 11% 7% 4 5% 5% 6% 6% 22%	2470
	11th to 20th	Night	14% 13% 5% 10% 7% 3 5% 4 5% 6% 27%	2318
	21st to end	Night	16% 13% 5% 9% 8% 3 5% 5% 5% 5% 27%	1884
March	1st to 10th	Night	12% 11% 4 6% 5% 3 4 4 4 6% 40%	3649
	11th to 20th	Night	9% 10% 4 8% 7% 3 4 5% 4 7% 39%	3480
	21st to end	Night	14% 10% 5% 9% 7% 4 5% 4 4 5% 31%	3817
April	1st to 10th	Night	18% 11% 5% 6% 5% 3 3 4 4 7% 33%	1986
	11th to 20th	Night	18% 7% 3 5% 5% 3 4 5% 5% 9% 37%	1687
	21st to end	Night	17% 9% 3 6% 6% 4 4 4 5% 6% 35%	1665
May	1st to 10th	Night	12% 8% 4 6% 5% 4 4 4 6% 8% 37%	1620
	11th to 20th	Night	16% 7% 3 6% 5% 4 5% 4 5% 7% 38%	1592
	21st to end	Night	10% 5% 2 5% 4 3 5% 4 6% 7% 49%	1574
June	1st to 10th	Night	13% 6% 3 5% 5% 3 4 5% 5% 9% 42%	1260
	11th to 20th	Night	10% 6% 2 4 5% 3 5% 5% 6% 8% 45%	1260
	21st to end	Night	8% 6% 3 6% 5% 3 5% 5% 6% 8% 45%	1260
July	1st to 10th	Night	6% 4 2 4 4 3 4 4 5% 8% 55%	1349
	11th to 20th	Night	8% 4 2 3 5% 3 5% 6% 6% 10% 48%	1440
	21st to end	Night	8% 5% 2 5% 6% 3 4 5% 6% 9% 48%	1665
August	1st to 10th	Night	8% 4 2 4 5% 3 4 5% 7% 11% 47%	1611
	11th to 20th	Night	9% 4 1 5% 5% 2 4 5% 6% 10% 50%	1598
	21st to end	Night	10% 5% 3 6% 4 3 5% 4 7% 7% 47%	1782
September	1st to 10th	Night	11% 5% 2 7% 5% 3 4 4 5% 8% 45%	1875
	11th to 20th	Night	12% 5% 2 5% 4 2 4 4 6% 7% 49%	1980
	21st to end	Night	15% 7% 3 6% 6% 2 4 4 5% 7% 41%	1980
October	1st to 10th	Night	17% 7% 2 6% 6% 2 5% 4 4 7% 40%	2052
	11th to 20th	Night	22% 11% 4 7% 6% 3 5% 3 4 6% 30%	2160
	21st to end	Night	24% 12% 4 8% 6% 3 4 4 4 6% 23%	2536
November	1st to 10th	Night	15% 12% 4 8% 5% 3 5% 4 4 6% 34%	2406
	11th to 20th	Night	15% 14% 7% 11% 8% 4 5% 5% 5% 5% 21%	2522
	21st to end	Night	21% 17% 6% 10% 6% 4 4 4 4 5% 20%	2660
December	1st to 10th	Night	18% 16% 9% 12% 6% 3 5% 4 5% 5% 16%	2669
	11th to 20th	Night	14% 15% 7% 12% 9% 5% 5% 6% 5% 6% 16%	3522
	21st to end	Night	24% 21% 8% 9% 7% 3 4 4 4 4 11%	5205

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	53% 32% 9% 5% 1	2528
	11th to 20th	Day	64% 28% 4 3 1	2530
	21st to end	Day	56% 27% 11% 4 1	2967
February	1st to 10th	Day	39% 37% 8% 12% 3	1710
	11th to 20th	Day	39% 38% 8% 10% 4 1	1753
	21st to end	Day	38% 37% 9% 12% 3	1570
March	1st to 10th	Day	43% 26% 8% 19% 4	3203
	11th to 20th	Day	40% 26% 10% 16% 6% 2	3480
	21st to end	Day	43% 27% 10% 12% 7% 1	3815
April	1st to 10th	Day	34% 36% 9% 13% 7% 1	2210
	11th to 20th	Day	29% 35% 9% 14% 11% 1	2441
	21st to end	Day	32% 37% 10% 12% 8% 1	2394
May	1st to 10th	Day	28% 38% 8% 15% 10% 1	2340
	11th to 20th	Day	27% 38% 6% 14% 12% 2	2305
	21st to end	Day	24% 39% 10% 14% 11% 2	2694
June	1st to 10th	Day	24% 39% 10% 17% 9% 1	2700
	11th to 20th	Day	20% 38% 9% 19% 14% 1	2700
	21st to end	Day	23% 39% 10% 18% 9% 1	2700
July	1st to 10th	Day	19% 39% 10% 20% 11% 1	2538
	11th to 20th	Day	21% 40% 9% 17% 10% 2	2445
	21st to end	Day	22% 40% 11% 16% 10% 1	2502
August	1st to 10th	Day	25% 39% 10% 18% 7% 1	2314
	11th to 20th	Day	18% 38% 11% 20% 11% 2	2303
	21st to end	Day	23% 38% 9% 20% 9% 1	2574
September	1st to 10th	Day	25% 39% 8% 19% 8% 1	2062
	11th to 20th	Day	24% 39% 11% 20% 6%	1980
	21st to end	Day	31% 38% 10% 15% 6%	1980
October	1st to 10th	Day	30% 38% 7% 17% 7% 1	1908
	11th to 20th	Day	39% 38% 8% 11% 4	1800
	21st to end	Day	39% 40% 9% 8% 3	1818
November	1st to 10th	Day	32% 44% 7% 12% 5%	1672
	11th to 20th	Day	36% 42% 9% 8% 5%	1656
	21st to end	Day	43% 41% 7% 8% 1	1520
December	1st to 10th	Day	47% 42% 7% 4 1	1506
	11th to 20th	Day	45% 35% 9% 9% 1	1671
	21st to end	Day	67% 26% 4 2	2445
January	1st to 10th	Night	50% 28% 11% 10% 1	4428
	11th to 20th	Night	61% 28% 6% 5% 1	4440
	21st to end	Night	57% 27% 7% 7% 2	4692
February	1st to 10th	Night	41% 36% 8% 12% 4	2470
	11th to 20th	Night	40% 37% 8% 10% 4 1	2320
	21st to end	Night	41% 35% 8% 12% 4	1884
March	1st to 10th	Night	35% 24% 12% 23% 5%	3649
	11th to 20th	Night	33% 25% 14% 18% 9% 1	3480
	21st to end	Night	40% 26% 11% 18% 5% 1	3817
April	1st to 10th	Night	34% 34% 6% 15% 9% 1	1986
	11th to 20th	Night	30% 34% 7% 15% 12% 1	1687
	21st to end	Night	32% 36% 6% 12% 12% 2	1665
May	1st to 10th	Night	30% 38% 6% 14% 11% 1	1620
	11th to 20th	Night	30% 37% 5% 13% 12% 4	1593
	21st to end	Night	21% 39% 9% 16% 12% 3	1575
June	1st to 10th	Night	28% 38% 7% 15% 10% 1	1260
	11th to 20th	Night	22% 39% 8% 14% 16% 1	1260
	21st to end	Night	26% 38% 10% 14% 11% 2	1260
July	1st to 10th	Night	20% 37% 8% 18% 16% 1	1310
	11th to 20th	Night	20% 41% 10% 17% 11% 1	1392
	21st to end	Night	22% 41% 11% 15% 9% 2	1572
August	1st to 10th	Night	21% 40% 11% 21% 7%	1600
	11th to 20th	Night	17% 36% 11% 20% 13% 2	1598
	21st to end	Night	22% 38% 9% 20% 10% 2	1782
September	1st to 10th	Night	25% 37% 7% 19% 11% 1	1875
	11th to 20th	Night	23% 38% 8% 24% 7%	1980
	21st to end	Night	30% 38% 9% 17% 7% 1	1980
October	1st to 10th	Night	29% 38% 8% 16% 9%	2052
	11th to 20th	Night	40% 38% 6% 11% 5%	2160
	21st to end	Night	42% 38% 8% 8% 4	2536
November	1st to 10th	Night	27% 43% 8% 16% 6% 1	2406
	11th to 20th	Night	39% 41% 8% 9% 3	2522
	21st to end	Night	41% 42% 8% 9%	2660
December	1st to 10th	Night	46% 41% 6% 6% 1	2669
	11th to 20th	Night	45% 36% 11% 6% 1	3525
	21st to end	Night	62% 29% 6% 3	5205

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	16% 14% 39% 21% 8% 1	270
	11th to 20th	Day	20% 11% 36% 23% 9%	270
	21st to end	Day	24% 6% 32% 30% 7% 1	297
February	1st to 10th	Day	20% 10% 42% 23% 4 1	270
	11th to 20th	Day	15% 14% 40% 26% 5%	270
	21st to end	Day	17% 16% 41% 23% 3	225
March	1st to 10th	Day	18% 14% 34% 27% 7% 1	314
	11th to 20th	Day	15% 13% 30% 31% 9% 2	460
	21st to end	Day	20% 11% 29% 32% 8% 1	506
April	1st to 10th	Day	19% 9% 32% 30% 9% 1	460
	11th to 20th	Day	22% 11% 23% 28% 12% 5%	452
	21st to end	Day	19% 6% 30% 30% 12% 2	437
May	1st to 10th	Day	16% 7% 29% 31% 13% 3	423
	11th to 20th	Day	15% 4 24% 30% 21% 6%	447
	21st to end	Day	12% 3 19% 32% 27% 8%	552
June	1st to 10th	Day	12% 3 19% 38% 23% 4	630
	11th to 20th	Day	12% 2 20% 38% 21% 6%	630
	21st to end	Day	11% 3 19% 40% 22% 5%	630
July	1st to 10th	Day	8% 2 12% 35% 33% 10%	621
	11th to 20th	Day	11% 3 15% 39% 24% 8%	618
	21st to end	Day	8% 2 15% 42% 27% 7%	590
August	1st to 10th	Day	11% 2 13% 33% 29% 12%	446
	11th to 20th	Day	10% 2 10% 31% 35% 12%	439
	21st to end	Day	14% 4 17% 28% 29% 7%	495
September	1st to 10th	Day	12% 4 21% 30% 23% 10%	450
	11th to 20th	Day	13% 4 17% 26% 28% 12%	450
	21st to end	Day	17% 6% 23% 20% 24% 10%	450
October	1st to 10th	Day	20% 8% 24% 24% 18% 6%	450
	11th to 20th	Day	25% 12% 29% 25% 7% 1	450
	21st to end	Day	30% 12% 27% 25% 5% 1	333
November	1st to 10th	Day	18% 9% 34% 25% 12% 1	270
	11th to 20th	Day	19% 17% 35% 22% 5% 2	244
	21st to end	Day	24% 16% 37% 21% 2	180
December	1st to 10th	Day	21% 17% 36% 24% 3	180
	11th to 20th	Day	25% 21% 32% 18% 3 1	180
	21st to end	Day	27% 16% 37% 17% 2	204
January	1st to 10th	Night	21% 12% 36% 23% 6% 1	950
	11th to 20th	Night	22% 13% 35% 23% 7% 1	950
	21st to end	Night	18% 7% 38% 27% 9%	931
February	1st to 10th	Night	20% 9% 39% 27% 5% 1	760
	11th to 20th	Night	21% 10% 36% 28% 6%	743
	21st to end	Night	21% 11% 38% 26% 4	628
March	1st to 10th	Night	16% 8% 36% 29% 10%	760
	11th to 20th	Night	16% 9% 37% 29% 8% 1	760
	21st to end	Night	21% 9% 29% 29% 11% 1	836
April	1st to 10th	Night	21% 9% 31% 29% 9%	656
	11th to 20th	Night	21% 6% 29% 29% 12% 3	563
	21st to end	Night	21% 5% 24% 35% 14% 1	555
May	1st to 10th	Night	15% 4 24% 41% 14% 3	540
	11th to 20th	Night	18% 4 20% 35% 22% 2	531
	21st to end	Night	14% 2 17% 35% 29% 3	486
June	1st to 10th	Night	14% 2 19% 39% 23% 2	360
	11th to 20th	Night	11% 3 19% 39% 25% 2	360
	21st to end	Night	10% 3 20% 38% 25% 4	360
July	1st to 10th	Night	7% 2 13% 44% 31% 3	435
	11th to 20th	Night	9% 3 15% 38% 28% 8%	522
	21st to end	Night	8% 2 14% 46% 27% 3	591
August	1st to 10th	Night	11% 1 12% 38% 33% 5%	537
	11th to 20th	Night	10% 2 15% 30% 35% 8%	534
	21st to end	Night	13% 3 18% 31% 31% 5%	594
September	1st to 10th	Night	12% 4 19% 34% 23% 8%	643
	11th to 20th	Night	14% 2 18% 31% 27% 8%	720
	21st to end	Night	17% 7% 22% 27% 20% 8%	720
October	1st to 10th	Night	20% 6% 27% 27% 18% 3	720
	11th to 20th	Night	29% 12% 25% 25% 9%	720
	21st to end	Night	31% 11% 26% 25% 5% 1	792
November	1st to 10th	Night	23% 12% 28% 25% 11% 1	740
	11th to 20th	Night	23% 13% 35% 20% 8%	760
	21st to end	Night	30% 13% 32% 20% 4	760
December	1st to 10th	Night	28% 15% 34% 17% 6%	769
	11th to 20th	Night	24% 15% 37% 20% 3 1	950
	21st to end	Night	28% 13% 36% 20% 2	1045

Page generated at 2024-04-22 09:25:43. Version ac07

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