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 2026-06-01.

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% 10% 4 8% 5% 5% 5% 4 5% 7% 33%	132957
	Night	15% 10% 4 8% 5% 4 4 4 5% 7% 34%	130221
Transparency	Day	49% 6% 11% 21% 12% 1	136602
	Night	48% 5% 11% 22% 13% 2	134646
Seeing	Day	18% 10% 26% 23% 16% 8%	17287
	Night	18% 8% 26% 31% 15% 2	26957

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	19% 16% 7% 15% 7% 4 4 3 4 6% 16%	5258
	11th to 20th	Day	13% 17% 7% 13% 8% 6% 5% 2 4 4 20%	5280
	21st to end	Day	19% 14% 7% 11% 8% 6% 5% 4 5% 6% 14%	5780
February	1st to 10th	Day	20% 14% 5% 10% 6% 5% 3 3 4 5% 24%	1890
	11th to 20th	Day	17% 12% 6% 13% 7% 4 4 4 5% 5% 24%	2043
	21st to end	Day	17% 14% 5% 9% 5% 4 3 3 5% 6% 30%	1730
March	1st to 10th	Day	17% 13% 6% 12% 5% 5% 4 3 4 5% 27%	4763
	11th to 20th	Day	12% 9% 4 8% 6% 4 4 3 4 6% 40%	5090
	21st to end	Day	20% 9% 4 8% 5% 5% 4 3 5% 6% 32%	5586
April	1st to 10th	Day	15% 12% 3 8% 5% 4 4 4 5% 7% 33%	2380
	11th to 20th	Day	16% 9% 3 7% 4 5% 4 3 4 7% 37%	2639
	21st to end	Day	17% 8% 3 7% 5% 5% 4 4 5% 7% 34%	2730
May	1st to 10th	Day	22% 10% 3 7% 5% 5% 5% 3 5% 7% 29%	5992
	11th to 20th	Day	20% 8% 3 6% 4 4 4 4 5% 8% 33%	5991
	21st to end	Day	16% 6% 3 6% 4 5% 5% 4 6% 9% 34%	6576
June	1st to 10th	Day	12% 7% 2 7% 5% 6% 4 4 6% 9% 37%	2600
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 5% 6% 10% 39%	2668
	21st to end	Day	10% 4 2 6% 6% 5% 6% 6% 7% 10% 38%	2800
July	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 4 6% 9% 42%	6296
	11th to 20th	Day	11% 6% 2 7% 5% 6% 6% 5% 7% 10% 36%	5900
	21st to end	Day	10% 6% 2 6% 5% 5% 6% 5% 7% 10% 37%	6424
August	1st to 10th	Day	9% 6% 2 6% 4 5% 5% 4 7% 10% 41%	4234
	11th to 20th	Day	8% 6% 2 6% 5% 4 5% 5% 8% 10% 41%	4174
	21st to end	Day	7% 3 2 5% 4 5% 5% 5% 6% 10% 48%	4558
September	1st to 10th	Day	12% 6% 2 5% 4 4 3 3 5% 7% 49%	2220
	11th to 20th	Day	8% 5% 2 6% 4 4 4 4 5% 9% 49%	2200
	21st to end	Day	18% 6% 2 7% 4 4 4 4 4 5% 43%	2200
October	1st to 10th	Day	12% 6% 1 6% 5% 4 4 3 6% 6% 46%	2155
	11th to 20th	Day	14% 7% 2 5% 4 4 5% 3 5% 7% 44%	2000
	21st to end	Day	20% 9% 2 9% 4 4 4 4 5% 6% 34%	2120
November	1st to 10th	Day	10% 8% 2 7% 4 4 4 3 4 6% 46%	1852
	11th to 20th	Day	16% 13% 4 7% 5% 4 4 4 5% 6% 33%	1890
	21st to end	Day	19% 14% 5% 8% 5% 3 4 3 4 6% 30%	1890
December	1st to 10th	Day	21% 12% 5% 12% 5% 4 3 3 4 5% 27%	3020
	11th to 20th	Day	23% 16% 6% 9% 4 4 4 3 4 5% 22%	3463
	21st to end	Day	25% 20% 6% 10% 5% 5% 4 3 4 4 14%	4565
January	1st to 10th	Night	16% 15% 7% 16% 6% 5% 5% 4 5% 5% 17%	7635
	11th to 20th	Night	12% 17% 7% 15% 7% 6% 6% 3 4 4 19%	7680
	21st to end	Night	18% 13% 5% 8% 5% 5% 4 3 5% 6% 27%	8401
February	1st to 10th	Night	19% 12% 4 10% 5% 4 3 3 5% 5% 28%	2730
	11th to 20th	Night	18% 11% 3 10% 6% 4 4 3 5% 6% 28%	2462
	21st to end	Night	19% 13% 5% 11% 5% 3 4 3 5% 5% 28%	2076
March	1st to 10th	Night	19% 12% 4 7% 4 4 3 3 4 5% 34%	5310
	11th to 20th	Night	13% 7% 2 5% 4 4 5% 4 6% 8% 41%	5090
	21st to end	Night	20% 9% 3 7% 4 4 4 3 5% 6% 36%	5588
April	1st to 10th	Night	16% 10% 3 7% 5% 4 4 3 5% 7% 36%	2258
	11th to 20th	Night	14% 8% 2 7% 5% 5% 5% 4 6% 7% 38%	1929
	21st to end	Night	15% 8% 3 7% 6% 4 4 4 4 7% 38%	1890
May	1st to 10th	Night	26% 8% 3 7% 5% 4 4 4 5% 6% 29%	4123
	11th to 20th	Night	18% 7% 2 7% 4 4 3 3 6% 10% 37%	4122
	21st to end	Night	14% 6% 2 6% 5% 5% 5% 4 6% 9% 39%	4520
June	1st to 10th	Night	9% 6% 2 5% 5% 5% 5% 3 5% 8% 48%	1800
	11th to 20th	Night	10% 5% 2 6% 4 3 5% 6% 7% 10% 44%	1724
	21st to end	Night	8% 4 2 5% 4 5% 5% 5% 7% 10% 45%	1600
July	1st to 10th	Night	11% 6% 2 6% 4 4 5% 4 6% 7% 45%	3662
	11th to 20th	Night	9% 5% 2 6% 5% 6% 5% 5% 7% 9% 42%	4057
	21st to end	Night	9% 5% 3 7% 5% 5% 5% 4 6% 9% 42%	4427
August	1st to 10th	Night	11% 6% 2 6% 5% 4 5% 5% 6% 9% 40%	2915
	11th to 20th	Night	9% 7% 3 7% 5% 4 4 4 5% 8% 44%	2875
	21st to end	Night	4 3 2 6% 5% 5% 5% 4 8% 8% 52%	3336
September	1st to 10th	Night	12% 5% 2 4 4 3 3 3 4 6% 54%	2160
	11th to 20th	Night	8% 4 2 5% 3 3 4 4 5% 7% 56%	2200
	21st to end	Night	16% 6% 2 5% 4 4 3 3 5% 6% 47%	2200
October	1st to 10th	Night	9% 5% 2 5% 5% 3 4 3 6% 6% 52%	2245
	11th to 20th	Night	16% 7% 2 6% 4 3 4 3 4 7% 45%	2400
	21st to end	Night	19% 8% 2 7% 4 3 4 3 4 6% 39%	2715
November	1st to 10th	Night	13% 7% 2 5% 4 4 4 3 4 6% 48%	2666
	11th to 20th	Night	16% 10% 3 7% 4 4 4 4 5% 6% 38%	2730
	21st to end	Night	19% 12% 5% 8% 5% 4 4 4 5% 6% 29%	2730
December	1st to 10th	Night	18% 11% 5% 10% 6% 5% 4 3 5% 5% 28%	4380
	11th to 20th	Night	19% 17% 7% 11% 5% 4 4 3 4 5% 23%	4952
	21st to end	Night	27% 19% 5% 10% 5% 4 3 3 4 5% 16%	6633

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	70% 4 10% 12% 3	5259
	11th to 20th	Day	69% 3 8% 10% 10% 1	5280
	21st to end	Day	71% 4 10% 11% 5%	5780
February	1st to 10th	Day	62% 4 10% 15% 8%	1890
	11th to 20th	Day	61% 5% 8% 17% 8%	2053
	21st to end	Day	57% 4 13% 18% 7% 2	1730
March	1st to 10th	Day	59% 5% 8% 21% 7% 1	4763
	11th to 20th	Day	45% 5% 9% 29% 12% 1	5090
	21st to end	Day	53% 5% 10% 21% 9% 2	5586
April	1st to 10th	Day	50% 5% 12% 18% 13% 1	2380
	11th to 20th	Day	48% 5% 12% 20% 13% 2	2639
	21st to end	Day	48% 6% 11% 22% 11% 2	2730
May	1st to 10th	Day	55% 6% 10% 17% 11% 1	6027
	11th to 20th	Day	49% 6% 11% 23% 11%	5995
	21st to end	Day	44% 8% 12% 23% 11% 3	6590
June	1st to 10th	Day	42% 6% 10% 25% 15% 2	2600
	11th to 20th	Day	39% 7% 15% 22% 15% 1	2668
	21st to end	Day	37% 8% 16% 21% 15% 3	2800
July	1st to 10th	Day	38% 7% 10% 27% 16% 2	6228
	11th to 20th	Day	41% 8% 13% 23% 15%	5831
	21st to end	Day	40% 7% 12% 21% 17% 3	6240
August	1st to 10th	Day	35% 8% 16% 27% 13% 1	4221
	11th to 20th	Day	35% 7% 11% 32% 15% 1	4213
	21st to end	Day	30% 7% 14% 31% 15% 3	4557
September	1st to 10th	Day	33% 7% 11% 24% 22% 4	2220
	11th to 20th	Day	32% 5% 12% 27% 22% 3	2200
	21st to end	Day	42% 5% 9% 22% 22% 1	2200
October	1st to 10th	Day	25% 4 10% 34% 26% 1	3506
	11th to 20th	Day	33% 4 10% 27% 23% 2	3240
	21st to end	Day	40% 4 11% 26% 19% 1	3409
November	1st to 10th	Day	39% 5% 10% 27% 17% 2	1852
	11th to 20th	Day	51% 4 10% 21% 12% 2	1890
	21st to end	Day	56% 3 10% 20% 9% 1	1890
December	1st to 10th	Day	61% 3 13% 18% 4	3020
	11th to 20th	Day	65% 3 11% 15% 5%	3460
	21st to end	Day	73% 4 8% 10% 5%	4565
January	1st to 10th	Night	68% 4 10% 14% 4	7636
	11th to 20th	Night	68% 4 7% 12% 8% 1	7680
	21st to end	Night	58% 3 13% 18% 7% 1	8401
February	1st to 10th	Night	59% 4 10% 18% 8% 1	2730
	11th to 20th	Night	57% 4 11% 17% 10% 1	2463
	21st to end	Night	59% 4 10% 17% 8% 2	2076
March	1st to 10th	Night	52% 4 7% 25% 11% 2	5310
	11th to 20th	Night	39% 4 12% 33% 11% 1	5090
	21st to end	Night	49% 4 8% 24% 13% 1	5588
April	1st to 10th	Night	47% 5% 11% 22% 13% 2	2258
	11th to 20th	Night	43% 6% 13% 17% 18% 3	1929
	21st to end	Night	45% 6% 10% 22% 14% 2	1890
May	1st to 10th	Night	54% 7% 10% 17% 9% 3	4150
	11th to 20th	Night	43% 7% 11% 24% 14% 1	4123
	21st to end	Night	40% 7% 13% 27% 11% 2	4528
June	1st to 10th	Night	34% 8% 12% 23% 21% 2	1800
	11th to 20th	Night	32% 9% 13% 26% 16% 3	1724
	21st to end	Night	31% 9% 15% 24% 18% 3	1600
July	1st to 10th	Night	38% 6% 10% 25% 18% 4	3623
	11th to 20th	Night	37% 7% 14% 27% 14% 1	4006
	21st to end	Night	38% 7% 13% 20% 18% 3	4297
August	1st to 10th	Night	39% 7% 14% 26% 13% 1	2904
	11th to 20th	Night	37% 7% 9% 27% 19%	2902
	21st to end	Night	28% 7% 14% 29% 19% 2	3336
September	1st to 10th	Night	32% 5% 12% 25% 21% 6%	2160
	11th to 20th	Night	26% 6% 9% 28% 27% 3	2200
	21st to end	Night	38% 5% 10% 20% 26% 1	2200
October	1st to 10th	Night	25% 4 7% 29% 33% 2	3674
	11th to 20th	Night	30% 3 10% 29% 26% 3	3899
	21st to end	Night	34% 4 7% 26% 25% 4	4376
November	1st to 10th	Night	37% 5% 11% 25% 20% 3	2666
	11th to 20th	Night	47% 4 9% 23% 14% 3	2730
	21st to end	Night	55% 5% 11% 20% 8% 1	2730
December	1st to 10th	Night	58% 4 13% 19% 6%	4380
	11th to 20th	Night	64% 3 12% 15% 6%	4954
	21st to end	Night	71% 4 10% 12% 3	6633

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	26% 17% 32% 17% 6% 1	330
	11th to 20th	Day	27% 21% 37% 10% 4	330
	21st to end	Day	29% 17% 34% 15% 5%	363
February	1st to 10th	Day	28% 18% 32% 17% 4	330
	11th to 20th	Day	26% 19% 36% 14% 4 1	330
	21st to end	Day	25% 20% 39% 15% 1	273
March	1st to 10th	Day	23% 20% 32% 20% 5%	409
	11th to 20th	Day	25% 14% 34% 19% 6% 1	540
	21st to end	Day	28% 14% 32% 20% 5% 1	594
April	1st to 10th	Day	21% 14% 35% 19% 9% 2	540
	11th to 20th	Day	22% 15% 28% 22% 9% 4	532
	21st to end	Day	19% 13% 32% 22% 12% 2	540
May	1st to 10th	Day	24% 14% 29% 19% 11% 4	543
	11th to 20th	Day	21% 8% 25% 24% 15% 7%	567
	21st to end	Day	16% 7% 25% 23% 20% 9%	617
June	1st to 10th	Day	12% 7% 22% 29% 20% 9%	530
	11th to 20th	Day	13% 6% 21% 27% 25% 8%	598
	21st to end	Day	11% 5% 25% 30% 20% 8%	730
July	1st to 10th	Day	13% 2 15% 27% 23% 19%	697
	11th to 20th	Day	11% 3 17% 24% 28% 16%	524
	21st to end	Day	10% 3 14% 28% 27% 18%	576
August	1st to 10th	Day	9% 3 13% 25% 31% 18%	526
	11th to 20th	Day	11% 3 18% 29% 25% 14%	519
	21st to end	Day	7% 3 14% 28% 29% 19%	583
September	1st to 10th	Day	12% 4 15% 24% 26% 19%	530
	11th to 20th	Day	9% 2 11% 21% 36% 21%	530
	21st to end	Day	19% 9% 15% 23% 21% 12%	530
October	1st to 10th	Day	10% 4 23% 31% 22% 10%	530
	11th to 20th	Day	12% 12% 32% 25% 15% 4	530
	21st to end	Day	22% 12% 33% 24% 9% 1	503
November	1st to 10th	Day	16% 11% 35% 23% 11% 4	330
	11th to 20th	Day	23% 15% 30% 21% 9% 2	330
	21st to end	Day	26% 13% 36% 19% 5% 1	330
December	1st to 10th	Day	25% 16% 34% 18% 5% 2	330
	11th to 20th	Day	24% 18% 36% 17% 4	330
	21st to end	Day	35% 14% 28% 18% 5%	363
January	1st to 10th	Night	23% 14% 36% 23% 5%	840
	11th to 20th	Night	28% 14% 33% 21% 4	840
	21st to end	Night	25% 13% 33% 25% 3	924
February	1st to 10th	Night	27% 11% 30% 28% 3	840
	11th to 20th	Night	23% 14% 36% 25% 2	823
	21st to end	Night	25% 13% 36% 24% 3	692
March	1st to 10th	Night	23% 12% 33% 26% 6%	840
	11th to 20th	Night	20% 14% 33% 26% 6%	840
	21st to end	Night	23% 12% 31% 27% 7%	924
April	1st to 10th	Night	20% 13% 32% 28% 6% 1	788
	11th to 20th	Night	16% 11% 34% 27% 11% 1	623
	21st to end	Night	17% 6% 36% 31% 9%	630
May	1st to 10th	Night	20% 9% 30% 29% 11%	630
	11th to 20th	Night	20% 4 26% 33% 15% 1	621
	21st to end	Night	11% 3 19% 40% 25% 2	683
June	1st to 10th	Night	9% 3 24% 41% 21% 1	600
	11th to 20th	Night	10% 5% 17% 43% 25% 2	600
	21st to end	Night	9% 3 19% 42% 24% 2	600
July	1st to 10th	Night	10% 2 12% 38% 30% 8%	585
	11th to 20th	Night	6% 3 14% 37% 36% 4	582
	21st to end	Night	10% 2 11% 39% 36% 2	657
August	1st to 10th	Night	7% 3 9% 43% 35% 3	597
	11th to 20th	Night	9% 3 11% 40% 34% 4	594
	21st to end	Night	6% 2 12% 35% 39% 6%	660
September	1st to 10th	Night	13% 2 15% 34% 31% 5%	760
	11th to 20th	Night	9% 3 11% 35% 36% 6%	800
	21st to end	Night	18% 5% 18% 32% 25% 2	800
October	1st to 10th	Night	10% 5% 22% 36% 23% 4	800
	11th to 20th	Night	18% 9% 27% 32% 14% 1	800
	21st to end	Night	22% 9% 30% 29% 10% 1	880
November	1st to 10th	Night	17% 7% 28% 31% 15% 1	820
	11th to 20th	Night	20% 14% 29% 27% 9%	840
	21st to end	Night	25% 11% 27% 31% 7%	840
December	1st to 10th	Night	28% 10% 31% 26% 5%	840
	11th to 20th	Night	25% 10% 32% 28% 5%	840
	21st to end	Night	29% 13% 28% 25% 4	924

Page generated at 2026-05-29 01:53:30. Version ac07

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