Hollow Oak Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hollow Oak Observatory clear sky chart from 2005-11-06 to 2024-01-27.

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% 9% 4 7% 6% 3 5% 5% 5% 7% 34%	75406
	Night	16% 9% 4 6% 6% 3 4 4 5% 7% 36%	74648
Transparency	Day	48% 6% 12% 20% 12% 2	75126
	Night	47% 5% 11% 21% 13% 2	74448
Seeing	Day	18% 9% 25% 23% 17% 8%	14118
	Night	18% 8% 26% 31% 16% 2	23886

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	20% 13% 6% 11% 8% 4 5% 4 4 6% 20%	1706
	11th to 20th	Day	20% 14% 10% 11% 7% 4 4 4 5% 5% 18%	1710
	21st to end	Day	18% 16% 7% 10% 8% 3 5% 4 4 6% 18%	1836
February	1st to 10th	Day	22% 14% 5% 10% 7% 4 4 4 4 5% 22%	1620
	11th to 20th	Day	17% 11% 7% 12% 8% 3 4 5% 5% 6% 23%	1744
	21st to end	Day	19% 14% 5% 8% 5% 3 4 4 4 5% 28%	1480
March	1st to 10th	Day	19% 10% 4 6% 6% 4 5% 5% 5% 5% 31%	1867
	11th to 20th	Day	19% 11% 5% 7% 7% 3 4 4 5% 6% 29%	1980
	21st to end	Day	24% 9% 5% 8% 5% 4 5% 3 5% 6% 28%	2178
April	1st to 10th	Day	14% 11% 4 7% 7% 3 4 4 5% 7% 34%	2039
	11th to 20th	Day	17% 9% 4 6% 5% 4 4 5% 4 7% 36%	2258
	21st to end	Day	19% 8% 3 6% 6% 4 4 6% 5% 7% 31%	2340
May	1st to 10th	Day	19% 9% 4 6% 5% 3 6% 4 5% 7% 33%	2340
	11th to 20th	Day	20% 8% 3 7% 5% 3 5% 5% 6% 8% 31%	2301
	21st to end	Day	9% 5% 3 5% 6% 4 6% 6% 7% 11% 38%	2531
June	1st to 10th	Day	12% 7% 3 6% 6% 4 5% 5% 6% 10% 38%	2340
	11th to 20th	Day	11% 5% 2 6% 5% 4 5% 6% 6% 10% 39%	2400
	21st to end	Day	10% 4 2 5% 6% 4 7% 7% 7% 10% 38%	2520
July	1st to 10th	Day	12% 6% 3 5% 6% 4 5% 6% 6% 9% 40%	2498
	11th to 20th	Day	10% 5% 3 5% 5% 4 6% 5% 6% 9% 43%	2340
	21st to end	Day	9% 4 2 4 6% 4 6% 6% 8% 11% 39%	2547
August	1st to 10th	Day	7% 5% 2 5% 5% 4 6% 6% 7% 10% 42%	2327
	11th to 20th	Day	9% 6% 3 5% 7% 4 6% 6% 8% 11% 37%	2303
	21st to end	Day	8% 3 3 5% 5% 4 5% 6% 6% 10% 44%	2516
September	1st to 10th	Day	13% 6% 2 4 4 3 4 5% 6% 8% 47%	1998
	11th to 20th	Day	10% 5% 2 6% 5% 3 5% 5% 5% 9% 47%	1980
	21st to end	Day	16% 5% 2 6% 4 2 4 4 4 6% 45%	1980
October	1st to 10th	Day	13% 6% 2 6% 6% 3 4 5% 6% 7% 43%	1940
	11th to 20th	Day	15% 7% 2 5% 5% 3 4 4 6% 6% 42%	1800
	21st to end	Day	21% 9% 2 8% 5% 3 5% 4 5% 6% 32%	1908
November	1st to 10th	Day	10% 7% 3 7% 5% 3 4 4 4 6% 47%	1672
	11th to 20th	Day	16% 12% 5% 6% 6% 3 4 4 4 6% 34%	1710
	21st to end	Day	19% 12% 4 7% 6% 3 4 4 4 7% 31%	1710
December	1st to 10th	Day	22% 15% 7% 8% 6% 3 5% 4 5% 6% 20%	1710
	11th to 20th	Day	19% 14% 6% 8% 6% 3 5% 4 5% 6% 24%	2153
	21st to end	Day	25% 22% 6% 9% 6% 3 5% 4 4 4 11%	3124
January	1st to 10th	Night	18% 13% 6% 11% 7% 4 5% 4 3 5% 23%	2459
	11th to 20th	Night	20% 15% 5% 10% 7% 3 5% 3 4 5% 22%	2470
	21st to end	Night	19% 14% 5% 9% 6% 4 5% 4 5% 5% 24%	2664
February	1st to 10th	Night	22% 12% 5% 8% 7% 3 4 3 4 4 26%	2340
	11th to 20th	Night	18% 10% 4 9% 8% 3 5% 4 5% 6% 27%	2102
	21st to end	Night	21% 12% 6% 9% 6% 2 4 3 5% 5% 27%	1776
March	1st to 10th	Night	17% 9% 4 7% 6% 3 5% 5% 4 6% 34%	2093
	11th to 20th	Night	16% 10% 4 5% 6% 3 4 5% 5% 7% 36%	1980
	21st to end	Night	21% 9% 4 6% 5% 3 4 4 5% 6% 34%	2178
April	1st to 10th	Night	14% 9% 4 6% 5% 4 4 4 5% 7% 38%	1936
	11th to 20th	Night	14% 7% 3 6% 5% 3 5% 5% 6% 7% 39%	1651
	21st to end	Night	16% 7% 3 6% 6% 3 4 5% 5% 8% 37%	1620
May	1st to 10th	Night	19% 7% 3 6% 6% 3 6% 5% 5% 7% 33%	1620
	11th to 20th	Night	18% 7% 2 6% 5% 3 5% 4 5% 8% 38%	1592
	21st to end	Night	8% 5% 2 4 5% 3 6% 6% 7% 12% 43%	1744
June	1st to 10th	Night	8% 5% 3 4 5% 4 5% 5% 5% 8% 48%	1620
	11th to 20th	Night	10% 4 2 4 5% 2 4 8% 7% 10% 45%	1552
	21st to end	Night	9% 4 2 4 4 4 6% 6% 7% 10% 45%	1440
July	1st to 10th	Night	9% 5% 2 5% 6% 3 5% 5% 5% 8% 47%	1458
	11th to 20th	Night	6% 3 3 5% 5% 3 6% 6% 7% 9% 48%	1620
	21st to end	Night	9% 5% 3 5% 6% 3 5% 5% 5% 7% 48%	1771
August	1st to 10th	Night	6% 4 2 4 6% 4 5% 7% 7% 10% 45%	1611
	11th to 20th	Night	7% 5% 3 6% 6% 3 4 4 5% 10% 47%	1598
	21st to end	Night	6% 3 2 4 5% 3 5% 5% 7% 8% 53%	1836
September	1st to 10th	Night	13% 5% 2 4 5% 2 4 4 4 7% 51%	1944
	11th to 20th	Night	9% 4 2 4 4 2 4 4 5% 7% 55%	1980
	21st to end	Night	14% 5% 2 4 5% 2 4 3 5% 6% 49%	1980
October	1st to 10th	Night	10% 5% 3 5% 6% 2 4 4 5% 7% 50%	2020
	11th to 20th	Night	17% 7% 3 5% 5% 2 4 4 4 7% 42%	2160
	21st to end	Night	20% 9% 3 6% 5% 2 4 4 4 5% 38%	2443
November	1st to 10th	Night	14% 6% 2 5% 5% 3 4 4 4 6% 47%	2406
	11th to 20th	Night	17% 9% 4 6% 5% 3 4 5% 4 6% 38%	2470
	21st to end	Night	19% 10% 5% 7% 6% 3 4 5% 5% 6% 30%	2470
December	1st to 10th	Night	22% 14% 6% 8% 6% 3 4 4 5% 5% 25%	2470
	11th to 20th	Night	18% 13% 6% 8% 6% 3 4 4 4 7% 27%	3042
	21st to end	Night	26% 19% 5% 9% 6% 3 3 4 4 5% 16%	4532

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	67% 5% 8% 14% 6%	1707
	11th to 20th	Day	69% 5% 12% 12% 3	1710
	21st to end	Day	67% 4 11% 13% 4 1	1836
February	1st to 10th	Day	65% 4 11% 13% 7%	1620
	11th to 20th	Day	62% 6% 9% 16% 7%	1754
	21st to end	Day	59% 5% 11% 17% 7% 2	1480
March	1st to 10th	Day	53% 6% 12% 22% 6% 1	1867
	11th to 20th	Day	56% 4 10% 20% 9% 1	1980
	21st to end	Day	58% 5% 9% 21% 6% 1	2178
April	1st to 10th	Day	49% 6% 12% 19% 13% 1	2039
	11th to 20th	Day	48% 5% 12% 19% 13% 3	2258
	21st to end	Day	51% 6% 11% 20% 11% 2	2340
May	1st to 10th	Day	51% 6% 13% 20% 9% 2	2340
	11th to 20th	Day	51% 8% 12% 17% 11% 1	2305
	21st to end	Day	39% 10% 15% 24% 12% 1	2531
June	1st to 10th	Day	42% 7% 10% 25% 14% 2	2340
	11th to 20th	Day	39% 8% 15% 21% 15% 2	2400
	21st to end	Day	38% 8% 16% 21% 14% 3	2520
July	1st to 10th	Day	40% 8% 12% 21% 15% 3	2430
	11th to 20th	Day	36% 7% 14% 25% 16% 1	2271
	21st to end	Day	35% 9% 15% 26% 12% 3	2405
August	1st to 10th	Day	34% 9% 14% 27% 14% 2	2314
	11th to 20th	Day	38% 8% 14% 29% 10% 1	2303
	21st to end	Day	33% 9% 14% 23% 17% 5%	2516
September	1st to 10th	Day	34% 7% 11% 23% 21% 3	1998
	11th to 20th	Day	34% 6% 12% 26% 20% 2	1980
	21st to end	Day	40% 5% 9% 23% 22% 1	1980
October	1st to 10th	Day	39% 7% 11% 24% 19% 1	1940
	11th to 20th	Day	41% 5% 12% 23% 18% 2	1800
	21st to end	Day	52% 5% 12% 20% 10% 1	1908
November	1st to 10th	Day	39% 5% 10% 26% 18% 3	1672
	11th to 20th	Day	50% 4 10% 21% 13% 2	1710
	21st to end	Day	54% 3 11% 20% 10% 1	1710
December	1st to 10th	Day	64% 5% 11% 17% 3	1710
	11th to 20th	Day	61% 4 11% 18% 6%	2150
	21st to end	Day	76% 5% 8% 7% 4	3124
January	1st to 10th	Night	63% 4 8% 17% 8%	2459
	11th to 20th	Night	65% 4 12% 13% 5% 1	2470
	21st to end	Night	62% 4 10% 17% 6% 1	2664
February	1st to 10th	Night	62% 4 10% 17% 7% 1	2340
	11th to 20th	Night	57% 4 11% 17% 10% 1	2103
	21st to end	Night	60% 4 9% 17% 8% 2	1776
March	1st to 10th	Night	50% 5% 10% 23% 11% 1	2093
	11th to 20th	Night	48% 4 11% 24% 10% 3	1980
	21st to end	Night	50% 5% 9% 23% 12% 1	2178
April	1st to 10th	Night	44% 5% 12% 23% 14% 2	1936
	11th to 20th	Night	42% 5% 13% 17% 19% 4	1651
	21st to end	Night	45% 7% 11% 21% 14% 3	1620
May	1st to 10th	Night	48% 8% 12% 19% 10% 3	1620
	11th to 20th	Night	45% 7% 12% 21% 13% 2	1593
	21st to end	Night	33% 8% 17% 27% 13% 3	1745
June	1st to 10th	Night	33% 8% 13% 23% 21% 2	1620
	11th to 20th	Night	31% 10% 14% 26% 16% 3	1552
	21st to end	Night	31% 9% 15% 23% 18% 3	1440
July	1st to 10th	Night	36% 7% 11% 22% 20% 4	1417
	11th to 20th	Night	30% 7% 14% 28% 19% 2	1566
	21st to end	Night	34% 8% 12% 26% 16% 3	1672
August	1st to 10th	Night	32% 9% 15% 27% 16% 1	1600
	11th to 20th	Night	33% 8% 14% 26% 18% 1	1598
	21st to end	Night	27% 7% 14% 26% 20% 4	1836
September	1st to 10th	Night	34% 5% 12% 24% 19% 6%	1944
	11th to 20th	Night	27% 6% 10% 28% 26% 3	1980
	21st to end	Night	35% 5% 10% 21% 27% 1	1980
October	1st to 10th	Night	34% 6% 12% 26% 21% 2	2020
	11th to 20th	Night	42% 4 9% 26% 18% 2	2160
	21st to end	Night	49% 4 8% 23% 14% 2	2443
November	1st to 10th	Night	38% 5% 11% 23% 20% 3	2406
	11th to 20th	Night	47% 4 9% 22% 15% 3	2470
	21st to end	Night	53% 5% 11% 21% 9% 1	2470
December	1st to 10th	Night	62% 4 10% 17% 6%	2470
	11th to 20th	Night	57% 4 12% 20% 6%	3044
	21st to end	Night	71% 4 10% 12% 3	4532

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	24% 16% 33% 19% 7% 1	270
	11th to 20th	Day	25% 19% 39% 12% 5%	270
	21st to end	Day	29% 14% 35% 16% 6%	282
February	1st to 10th	Day	33% 16% 33% 15% 3	240
	11th to 20th	Day	24% 20% 39% 14% 3	240
	21st to end	Day	24% 19% 39% 16% 1	198
March	1st to 10th	Day	21% 19% 34% 21% 5%	307
	11th to 20th	Day	22% 13% 35% 21% 8% 1	420
	21st to end	Day	29% 11% 33% 21% 6% 1	462
April	1st to 10th	Day	20% 10% 34% 22% 11% 3	420
	11th to 20th	Day	21% 13% 27% 24% 10% 6%	412
	21st to end	Day	21% 13% 32% 21% 10% 2	420
May	1st to 10th	Day	25% 11% 29% 18% 12% 5%	423
	11th to 20th	Day	21% 6% 25% 25% 16% 6%	447
	21st to end	Day	13% 6% 23% 25% 23% 11%	485
June	1st to 10th	Day	12% 5% 21% 30% 22% 10%	450
	11th to 20th	Day	13% 5% 21% 27% 25% 9%	510
	21st to end	Day	11% 5% 24% 30% 20% 8%	630
July	1st to 10th	Day	14% 2 14% 26% 23% 21%	600
	11th to 20th	Day	11% 2 15% 25% 29% 18%	444
	21st to end	Day	9% 2 14% 26% 29% 19%	488
August	1st to 10th	Day	10% 3 12% 23% 33% 20%	446
	11th to 20th	Day	12% 3 17% 27% 25% 15%	439
	21st to end	Day	7% 3 14% 28% 28% 21%	495
September	1st to 10th	Day	13% 4 16% 22% 27% 18%	450
	11th to 20th	Day	10% 3 12% 20% 35% 20%	450
	21st to end	Day	17% 10% 15% 25% 21% 12%	450
October	1st to 10th	Day	11% 4 24% 31% 20% 10%	450
	11th to 20th	Day	13% 12% 32% 26% 15% 2	450
	21st to end	Day	23% 13% 31% 24% 9%	423
November	1st to 10th	Day	14% 10% 34% 24% 13% 4	270
	11th to 20th	Day	21% 16% 27% 23% 11% 3	270
	21st to end	Day	26% 9% 37% 22% 6% 1	270
December	1st to 10th	Day	26% 16% 31% 19% 6% 2	270
	11th to 20th	Day	23% 15% 38% 19% 4	270
	21st to end	Day	34% 14% 27% 19% 6%	297
January	1st to 10th	Night	22% 13% 37% 23% 5%	760
	11th to 20th	Night	29% 12% 32% 23% 4	760
	21st to end	Night	26% 12% 32% 26% 4	820
February	1st to 10th	Night	30% 9% 30% 27% 3	720
	11th to 20th	Night	23% 13% 36% 26% 2	703
	21st to end	Night	27% 11% 36% 23% 3	592
March	1st to 10th	Night	22% 11% 33% 27% 7%	720
	11th to 20th	Night	20% 12% 33% 28% 7%	720
	21st to end	Night	24% 9% 31% 27% 8%	792
April	1st to 10th	Night	18% 11% 34% 29% 7% 1	676
	11th to 20th	Night	17% 8% 33% 29% 13% 1	533
	21st to end	Night	17% 6% 37% 31% 9% 1	540
May	1st to 10th	Night	20% 8% 31% 29% 12%	540
	11th to 20th	Night	21% 4 24% 34% 16% 1	531
	21st to end	Night	9% 2 19% 38% 29% 3	584
June	1st to 10th	Night	8% 3 23% 43% 22% 1	540
	11th to 20th	Night	10% 4 17% 41% 26% 2	540
	21st to end	Night	9% 3 18% 44% 24% 2	540
July	1st to 10th	Night	10% 2 12% 37% 31% 9%	525
	11th to 20th	Night	6% 3 13% 37% 36% 5%	522
	21st to end	Night	9% 2 11% 39% 37% 2	591
August	1st to 10th	Night	8% 3 8% 43% 35% 4	537
	11th to 20th	Night	9% 3 11% 39% 34% 4	534
	21st to end	Night	7% 2 12% 35% 39% 6%	594
September	1st to 10th	Night	14% 2 15% 32% 32% 5%	684
	11th to 20th	Night	9% 3 12% 34% 35% 6%	720
	21st to end	Night	16% 5% 18% 32% 26% 3	720
October	1st to 10th	Night	11% 5% 23% 36% 21% 4	720
	11th to 20th	Night	19% 9% 28% 31% 13% 1	720
	21st to end	Night	23% 8% 30% 29% 9% 1	792
November	1st to 10th	Night	17% 7% 26% 32% 17% 1	740
	11th to 20th	Night	20% 13% 29% 27% 10%	760
	21st to end	Night	23% 10% 28% 31% 8% 1	760
December	1st to 10th	Night	29% 9% 31% 26% 6% 1	760
	11th to 20th	Night	24% 8% 32% 28% 6% 1	760
	21st to end	Night	29% 12% 29% 26% 4	836

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