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 2025-02-06.

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	15% 10% 4 8% 5% 5% 4 4 5% 7% 33%	109522
	Night	16% 10% 4 8% 5% 4 4 3 5% 6% 35%	108750
Transparency	Day	48% 6% 11% 21% 12% 1	113211
	Night	48% 5% 11% 22% 13% 2	113200
Seeing	Day	18% 9% 26% 23% 16% 8%	15503
	Night	18% 8% 26% 31% 15% 2	25239

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% 18% 7% 17% 7% 4 4 3 4 5% 16%	4056
	11th to 20th	Day	15% 19% 7% 13% 7% 6% 5% 2 4 4 19%	4060
	21st to end	Day	21% 16% 8% 10% 9% 6% 4 3 3 4 15%	4466
February	1st to 10th	Day	21% 14% 4 11% 6% 5% 4 3 4 5% 24%	1755
	11th to 20th	Day	16% 12% 5% 13% 7% 4 4 4 5% 6% 24%	1843
	21st to end	Day	19% 15% 5% 8% 5% 4 3 3 5% 5% 29%	1570
March	1st to 10th	Day	21% 14% 5% 10% 5% 5% 4 3 4 5% 23%	3243
	11th to 20th	Day	14% 10% 3 7% 6% 4 4 3 5% 6% 38%	3480
	21st to end	Day	21% 8% 3 8% 5% 5% 4 2 5% 6% 33%	3815
April	1st to 10th	Day	14% 12% 4 8% 5% 4 4 4 5% 7% 33%	2153
	11th to 20th	Day	17% 9% 3 7% 4 5% 4 3 4 6% 37%	2385
	21st to end	Day	18% 8% 3 7% 5% 5% 4 4 5% 7% 32%	2470
May	1st to 10th	Day	22% 10% 3 7% 5% 5% 4 3 5% 7% 30%	4082
	11th to 20th	Day	21% 8% 3 6% 4 4 4 4 5% 7% 34%	4081
	21st to end	Day	11% 5% 3 7% 5% 6% 6% 5% 7% 10% 35%	4476
June	1st to 10th	Day	11% 8% 2 7% 5% 6% 4 4 6% 10% 37%	2470
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 5% 6% 10% 40%	2534
	21st to end	Day	10% 5% 2 6% 6% 5% 6% 6% 7% 10% 38%	2660
July	1st to 10th	Day	11% 6% 2 7% 5% 6% 5% 5% 6% 9% 38%	4390
	11th to 20th	Day	8% 5% 2 5% 4 5% 5% 4 7% 11% 45%	4120
	21st to end	Day	10% 5% 2 6% 5% 5% 6% 5% 8% 10% 36%	4466
August	1st to 10th	Day	10% 6% 2 6% 4 5% 5% 4 7% 10% 40%	4104
	11th to 20th	Day	8% 6% 2 6% 5% 4 5% 5% 8% 10% 41%	4044
	21st to end	Day	7% 3 2 5% 4 5% 5% 5% 6% 10% 48%	4418
September	1st to 10th	Day	12% 6% 2 5% 4 4 3 4 5% 7% 49%	2109
	11th to 20th	Day	9% 5% 2 6% 4 4 4 4 5% 9% 48%	2090
	21st to end	Day	18% 6% 2 7% 4 3 4 4 4 6% 43%	2090
October	1st to 10th	Day	12% 6% 1 6% 5% 4 4 4 6% 7% 45%	2047
	11th to 20th	Day	14% 7% 2 5% 4 4 5% 3 5% 6% 43%	1900
	21st to end	Day	20% 9% 2 9% 4 4 4 4 5% 6% 33%	2014
November	1st to 10th	Day	10% 8% 2 8% 4 4 4 3 4 6% 47%	1762
	11th to 20th	Day	16% 13% 4 7% 5% 4 4 4 5% 6% 33%	1800
	21st to end	Day	19% 14% 4 8% 5% 3 4 3 4 6% 30%	1800
December	1st to 10th	Day	22% 11% 5% 11% 5% 4 3 3 4 5% 27%	2930
	11th to 20th	Day	23% 16% 6% 9% 4 4 4 3 4 5% 22%	3373
	21st to end	Day	25% 20% 6% 10% 5% 5% 4 3 4 4 14%	4466
January	1st to 10th	Night	16% 16% 7% 16% 5% 5% 3 3 5% 5% 18%	5889
	11th to 20th	Night	14% 18% 7% 15% 7% 6% 6% 3 4 5% 17%	5900
	21st to end	Night	22% 14% 6% 8% 5% 4 4 2 4 5% 25%	6490
February	1st to 10th	Night	21% 13% 4 10% 5% 4 3 3 5% 5% 27%	2536
	11th to 20th	Night	17% 12% 3 10% 6% 5% 4 3 5% 6% 28%	2222
	21st to end	Night	20% 13% 5% 10% 6% 3 4 3 5% 5% 27%	1884
March	1st to 10th	Night	20% 13% 4 6% 4 4 3 4 4 6% 32%	3610
	11th to 20th	Night	13% 8% 3 5% 5% 4 4 4 5% 8% 41%	3480
	21st to end	Night	20% 8% 3 6% 4 3 4 3 5% 5% 40%	3817
April	1st to 10th	Night	15% 10% 3 7% 5% 4 4 3 6% 7% 36%	2043
	11th to 20th	Night	15% 8% 2 6% 4 5% 5% 4 6% 7% 38%	1743
	21st to end	Night	15% 8% 3 7% 5% 4 4 4 5% 7% 37%	1710
May	1st to 10th	Night	26% 9% 3 6% 5% 4 4 4 5% 6% 27%	2813
	11th to 20th	Night	19% 7% 2 6% 4 4 3 3 5% 8% 40%	2812
	21st to end	Night	10% 5% 1 5% 5% 4 5% 4 6% 10% 45%	3079
June	1st to 10th	Night	8% 6% 2 5% 5% 5% 5% 3 6% 8% 47%	1710
	11th to 20th	Night	9% 4 1 5% 5% 3 5% 6% 7% 10% 45%	1638
	21st to end	Night	8% 4 1 5% 4 5% 4 5% 7% 10% 45%	1520
July	1st to 10th	Night	12% 6% 2 6% 4 4 4 4 5% 7% 46%	2561
	11th to 20th	Night	7% 3 2 5% 5% 5% 5% 4 7% 8% 50%	2840
	21st to end	Night	10% 5% 3 7% 5% 5% 5% 4 7% 8% 42%	3087
August	1st to 10th	Night	11% 6% 2 6% 6% 4 5% 5% 7% 9% 39%	2825
	11th to 20th	Night	9% 7% 3 7% 5% 4 4 4 5% 8% 44%	2785
	21st to end	Night	4 3 2 6% 5% 5% 5% 4 8% 8% 52%	3234
September	1st to 10th	Night	12% 5% 2 4 4 3 3 3 4 7% 53%	2052
	11th to 20th	Night	8% 4 2 5% 3 3 4 4 6% 7% 55%	2090
	21st to end	Night	16% 6% 2 5% 4 4 3 3 5% 6% 47%	2090
October	1st to 10th	Night	9% 5% 3 5% 5% 3 4 3 6% 7% 51%	2133
	11th to 20th	Night	16% 7% 2 6% 4 3 4 3 4 7% 44%	2280
	21st to end	Night	19% 9% 2 7% 4 3 4 3 4 6% 39%	2579
November	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 4 6% 47%	2536
	11th to 20th	Night	17% 10% 3 7% 4 4 4 4 5% 6% 37%	2600
	21st to end	Night	19% 11% 4 8% 5% 4 4 4 5% 6% 29%	2600
December	1st to 10th	Night	18% 11% 5% 10% 6% 5% 4 3 5% 5% 28%	4250
	11th to 20th	Night	19% 17% 7% 11% 5% 4 4 3 4 5% 23%	4822
	21st to end	Night	27% 19% 5% 9% 5% 4 3 3 4 5% 16%	6490

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	71% 4 7% 13% 4	4057
	11th to 20th	Day	70% 3 8% 10% 8% 1	4060
	21st to end	Day	74% 4 9% 8% 5%	4466
February	1st to 10th	Day	62% 4 11% 14% 8%	1755
	11th to 20th	Day	60% 5% 8% 17% 9%	1853
	21st to end	Day	58% 5% 11% 17% 7% 2	1570
March	1st to 10th	Day	62% 6% 9% 19% 4 1	3243
	11th to 20th	Day	46% 5% 9% 27% 12% 1	3480
	21st to end	Day	51% 6% 11% 23% 9%	3815
April	1st to 10th	Day	50% 6% 12% 18% 13% 1	2153
	11th to 20th	Day	48% 5% 12% 20% 13% 3	2385
	21st to end	Day	49% 6% 11% 20% 11% 2	2470
May	1st to 10th	Day	53% 6% 11% 22% 7% 1	4120
	11th to 20th	Day	48% 7% 12% 22% 11% 1	4085
	21st to end	Day	39% 10% 14% 21% 12% 4	4489
June	1st to 10th	Day	42% 7% 10% 26% 14% 2	2470
	11th to 20th	Day	38% 7% 15% 22% 16% 1	2534
	21st to end	Day	37% 8% 16% 21% 15% 3	2660
July	1st to 10th	Day	41% 7% 11% 22% 16% 3	4322
	11th to 20th	Day	32% 7% 13% 28% 20%	4051
	21st to end	Day	39% 8% 13% 19% 17% 4	4324
August	1st to 10th	Day	36% 8% 16% 27% 13% 1	4091
	11th to 20th	Day	35% 7% 11% 32% 15% 1	4083
	21st to end	Day	30% 7% 14% 31% 15% 3	4417
September	1st to 10th	Day	33% 7% 11% 23% 22% 4	2109
	11th to 20th	Day	33% 5% 12% 26% 21% 3	2090
	21st to end	Day	42% 4 9% 22% 21% 1	2090
October	1st to 10th	Day	25% 4 10% 33% 26% 1	3398
	11th to 20th	Day	34% 4 10% 27% 23% 2	3140
	21st to end	Day	39% 4 11% 26% 19% 1	3303
November	1st to 10th	Day	39% 5% 9% 27% 18% 3	1762
	11th to 20th	Day	51% 4 10% 20% 12% 2	1800
	21st to end	Day	56% 3 10% 20% 10% 1	1800
December	1st to 10th	Day	60% 3 14% 18% 4	2930
	11th to 20th	Day	65% 3 11% 15% 5%	3370
	21st to end	Day	73% 4 8% 10% 4	4466
January	1st to 10th	Night	67% 4 8% 15% 5%	5889
	11th to 20th	Night	69% 4 9% 10% 7% 1	5900
	21st to end	Night	61% 3 12% 16% 7% 2	6490
February	1st to 10th	Night	60% 4 10% 17% 8% 1	2536
	11th to 20th	Night	56% 4 11% 17% 11% 1	2223
	21st to end	Night	60% 4 10% 16% 8% 2	1884
March	1st to 10th	Night	53% 5% 9% 23% 10% 1	3610
	11th to 20th	Night	40% 5% 12% 31% 11% 2	3480
	21st to end	Night	45% 5% 8% 29% 11% 1	3817
April	1st to 10th	Night	45% 5% 12% 22% 14% 2	2043
	11th to 20th	Night	43% 5% 13% 17% 19% 4	1743
	21st to end	Night	45% 7% 11% 21% 14% 2	1710
May	1st to 10th	Night	55% 7% 9% 19% 8% 1	2840
	11th to 20th	Night	42% 8% 12% 27% 10% 1	2813
	21st to end	Night	32% 8% 15% 29% 14% 3	3087
June	1st to 10th	Night	33% 8% 12% 23% 21% 2	1710
	11th to 20th	Night	31% 9% 14% 27% 16% 3	1638
	21st to end	Night	32% 9% 15% 23% 18% 3	1520
July	1st to 10th	Night	37% 5% 10% 24% 17% 5%	2520
	11th to 20th	Night	30% 6% 13% 32% 18% 1	2786
	21st to end	Night	39% 7% 13% 21% 16% 5%	2988
August	1st to 10th	Night	40% 7% 15% 25% 12% 1	2814
	11th to 20th	Night	38% 7% 9% 26% 19%	2812
	21st to end	Night	28% 7% 15% 29% 19% 3	3234
September	1st to 10th	Night	33% 5% 12% 24% 20% 6%	2052
	11th to 20th	Night	27% 6% 10% 28% 26% 4	2090
	21st to end	Night	37% 5% 10% 21% 25% 1	2090
October	1st to 10th	Night	25% 4 8% 29% 32% 2	3562
	11th to 20th	Night	30% 3 10% 28% 26% 3	3779
	21st to end	Night	34% 4 7% 26% 25% 4	4240
November	1st to 10th	Night	38% 5% 11% 24% 20% 3	2536
	11th to 20th	Night	47% 4 9% 22% 15% 3	2600
	21st to end	Night	55% 5% 11% 20% 8% 1	2600
December	1st to 10th	Night	58% 4 13% 20% 6%	4250
	11th to 20th	Night	64% 3 13% 15% 6%	4824
	21st to end	Night	71% 4 10% 12% 3	6490

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% 33% 18% 6% 1	300
	11th to 20th	Day	25% 21% 38% 11% 4	300
	21st to end	Day	28% 16% 35% 15% 6%	330
February	1st to 10th	Day	30% 16% 34% 16% 4	285
	11th to 20th	Day	23% 19% 39% 14% 4 1	270
	21st to end	Day	25% 18% 38% 17% 2	225
March	1st to 10th	Day	22% 18% 32% 22% 5%	341
	11th to 20th	Day	22% 14% 34% 21% 7% 1	460
	21st to end	Day	28% 12% 32% 22% 6% 1	506
April	1st to 10th	Day	21% 12% 35% 21% 10% 2	460
	11th to 20th	Day	21% 14% 27% 24% 9% 5%	452
	21st to end	Day	20% 12% 33% 22% 11% 2	460
May	1st to 10th	Day	24% 11% 29% 20% 12% 5%	463
	11th to 20th	Day	21% 6% 24% 25% 16% 8%	487
	21st to end	Day	12% 6% 24% 25% 22% 10%	529
June	1st to 10th	Day	12% 7% 22% 29% 21% 10%	490
	11th to 20th	Day	12% 5% 21% 28% 26% 9%	554
	21st to end	Day	11% 5% 26% 30% 20% 8%	680
July	1st to 10th	Day	14% 2 15% 26% 23% 20%	648
	11th to 20th	Day	11% 3 17% 24% 29% 16%	484
	21st to end	Day	10% 2 14% 28% 28% 18%	532
August	1st to 10th	Day	10% 3 13% 24% 31% 19%	486
	11th to 20th	Day	11% 3 19% 28% 24% 14%	479
	21st to end	Day	6% 3 13% 28% 29% 20%	539
September	1st to 10th	Day	12% 4 15% 24% 26% 18%	490
	11th to 20th	Day	10% 3 12% 21% 35% 20%	490
	21st to end	Day	20% 10% 16% 24% 20% 11%	490
October	1st to 10th	Day	10% 4 24% 32% 20% 10%	490
	11th to 20th	Day	12% 12% 32% 25% 15% 4	490
	21st to end	Day	21% 12% 32% 24% 10% 1	463
November	1st to 10th	Day	15% 11% 35% 23% 12% 4	300
	11th to 20th	Day	22% 15% 29% 22% 10% 2	300
	21st to end	Day	25% 12% 36% 21% 5% 1	300
December	1st to 10th	Day	26% 16% 32% 18% 6% 2	300
	11th to 20th	Day	24% 16% 37% 18% 4	300
	21st to end	Day	35% 14% 27% 18% 5%	330
January	1st to 10th	Night	22% 14% 37% 22% 5%	800
	11th to 20th	Night	28% 14% 32% 22% 4	800
	21st to end	Night	26% 13% 32% 26% 3	880
February	1st to 10th	Night	29% 10% 31% 28% 3	781
	11th to 20th	Night	22% 14% 36% 26% 2	743
	21st to end	Night	26% 12% 35% 24% 3	628
March	1st to 10th	Night	22% 11% 32% 28% 7%	760
	11th to 20th	Night	19% 13% 33% 27% 7%	760
	21st to end	Night	24% 10% 31% 27% 8%	836
April	1st to 10th	Night	19% 12% 34% 29% 7% 1	713
	11th to 20th	Night	16% 9% 33% 28% 12% 1	563
	21st to end	Night	17% 6% 37% 32% 8% 1	570
May	1st to 10th	Night	21% 8% 30% 29% 12%	570
	11th to 20th	Night	21% 3 24% 34% 16% 1	561
	21st to end	Night	9% 3 19% 39% 28% 3	617
June	1st to 10th	Night	8% 4 24% 42% 22% 1	570
	11th to 20th	Night	9% 4 16% 43% 26% 2	570
	21st to end	Night	9% 4 19% 43% 24% 2	570
July	1st to 10th	Night	10% 2 13% 38% 29% 8%	555
	11th to 20th	Night	5% 3 14% 38% 35% 5%	552
	21st to end	Night	10% 2 11% 39% 36% 2	624
August	1st to 10th	Night	8% 3 8% 42% 35% 4	567
	11th to 20th	Night	9% 3 11% 41% 32% 4	564
	21st to end	Night	7% 2 11% 35% 40% 6%	627
September	1st to 10th	Night	13% 2 15% 34% 31% 5%	722
	11th to 20th	Night	9% 3 12% 35% 35% 6%	760
	21st to end	Night	18% 5% 18% 32% 25% 3	760
October	1st to 10th	Night	10% 5% 23% 36% 21% 4	760
	11th to 20th	Night	18% 9% 27% 31% 14% 1	760
	21st to end	Night	22% 8% 30% 29% 10% 1	836
November	1st to 10th	Night	17% 7% 27% 32% 16% 1	780
	11th to 20th	Night	21% 13% 29% 27% 10%	800
	21st to end	Night	24% 10% 27% 31% 7% 1	800
December	1st to 10th	Night	28% 10% 31% 26% 6% 1	800
	11th to 20th	Night	25% 9% 32% 28% 6% 1	800
	21st to end	Night	30% 12% 28% 26% 4	880

Page generated at 2025-02-03 03:05:55. Version ac07

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