Frog Hollow Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Frog Hollow Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	13% 6% 2 6% 4 4 4 3 5% 7% 45%	131063
	Night	13% 6% 2 5% 3 3 3 3 5% 6% 50%	130062
Transparency	Day	38% 3 9% 23% 19% 6%	134671
	Night	35% 3 9% 22% 22% 9%	134531
Seeing	Day	15% 9% 23% 26% 20% 6%	16929
	Night	15% 8% 25% 31% 18% 2	26858

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% 9% 3 5% 3 3 3 2 4 6% 46%	5289
	11th to 20th	Day	11% 7% 2 5% 3 3 3 2 4 5% 56%	5310
	21st to end	Day	24% 7% 3 6% 2 2 2 2 3 5% 44%	5812
February	1st to 10th	Day	23% 8% 2 5% 4 3 4 3 4 4 40%	1890
	11th to 20th	Day	20% 9% 3 6% 3 3 4 3 3 5% 41%	1885
	21st to end	Day	23% 7% 3 7% 4 2 3 2 4 4 41%	1815
March	1st to 10th	Day	23% 9% 3 6% 3 2 3 2 4 6% 39%	4936
	11th to 20th	Day	12% 5% 2 6% 4 4 3 3 4 6% 50%	4980
	21st to end	Day	14% 9% 4 8% 5% 3 4 3 4 5% 42%	5462
April	1st to 10th	Day	15% 6% 2 6% 5% 3 4 4 5% 6% 44%	2326
	11th to 20th	Day	16% 6% 2 5% 4 4 4 3 4 5% 45%	2374
	21st to end	Day	12% 7% 3 7% 5% 4 4 3 5% 6% 43%	2400
May	1st to 10th	Day	19% 9% 3 7% 4 3 4 3 4 6% 38%	5484
	11th to 20th	Day	14% 8% 2 8% 6% 5% 5% 4 5% 7% 38%	5862
	21st to end	Day	13% 7% 3 7% 5% 5% 5% 4 5% 8% 37%	6434
June	1st to 10th	Day	7% 5% 3 6% 6% 5% 5% 5% 7% 9% 42%	2600
	11th to 20th	Day	7% 5% 2 6% 6% 6% 6% 5% 7% 8% 43%	2600
	21st to end	Day	4 4 2 5% 5% 4 5% 4 6% 10% 51%	2600
July	1st to 10th	Day	7% 5% 3 7% 6% 6% 7% 5% 9% 12% 33%	5900
	11th to 20th	Day	5% 3 1 4 4 4 4 4 6% 10% 55%	5900
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 9% 40%	6422
August	1st to 10th	Day	2 2 1 3 3 3 4 4 6% 9% 63%	4234
	11th to 20th	Day	5% 4 2 6% 4 5% 5% 5% 7% 9% 48%	3946
	21st to end	Day	5% 4 2 5% 5% 5% 6% 5% 7% 9% 46%	4312
September	1st to 10th	Day	9% 5% 2 6% 4 4 5% 4 6% 7% 48%	2400
	11th to 20th	Day	13% 5% 2 5% 4 5% 5% 4 5% 8% 44%	2282
	21st to end	Day	9% 5% 2 5% 4 4 4 3 4 7% 53%	2141
October	1st to 10th	Day	8% 4 1 5% 3 3 4 2 5% 6% 59%	2000
	11th to 20th	Day	12% 6% 1 4 2 3 3 2 4 5% 59%	2000
	21st to end	Day	17% 6% 2 5% 3 3 3 3 4 4 52%	2200
November	1st to 10th	Day	14% 6% 2 4 2 3 3 3 4 6% 52%	2058
	11th to 20th	Day	11% 6% 2 5% 4 3 3 3 4 5% 53%	2100
	21st to end	Day	20% 7% 2 5% 3 3 3 2 4 4 46%	2011
December	1st to 10th	Day	20% 7% 2 5% 2 2 2 2 3 4 50%	3030
	11th to 20th	Day	20% 9% 3 6% 4 3 3 3 3 4 42%	3481
	21st to end	Day	19% 9% 2 7% 3 3 4 3 4 5% 41%	4587
January	1st to 10th	Night	16% 7% 2 5% 3 3 3 2 4 5% 48%	7634
	11th to 20th	Night	11% 4 1 3 2 2 2 1 3 4 67%	7680
	21st to end	Night	26% 8% 3 5% 3 3 3 2 3 4 41%	8402
February	1st to 10th	Night	19% 8% 2 4 3 3 3 3 4 5% 45%	2730
	11th to 20th	Night	21% 8% 3 5% 4 3 3 2 4 5% 43%	2515
	21st to end	Night	19% 7% 2 5% 4 3 3 2 4 4 47%	1827
March	1st to 10th	Night	19% 8% 3 6% 3 3 3 2 3 5% 44%	4919
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 5% 7% 51%	4980
	21st to end	Night	15% 7% 3 6% 4 3 3 3 5% 7% 44%	5467
April	1st to 10th	Night	14% 6% 2 5% 4 3 3 2 5% 5% 50%	2072
	11th to 20th	Night	12% 5% 2 5% 4 3 4 3 5% 7% 50%	1975
	21st to end	Night	12% 6% 2 5% 4 3 3 3 5% 6% 51%	2000
May	1st to 10th	Night	19% 7% 3 6% 4 4 4 3 5% 7% 37%	4411
	11th to 20th	Night	18% 8% 2 6% 5% 4 4 4 4 7% 37%	4053
	21st to end	Night	15% 7% 3 6% 4 4 4 3 5% 8% 41%	4443
June	1st to 10th	Night	4 4 2 5% 4 4 4 5% 6% 11% 51%	1800
	11th to 20th	Night	4 4 2 5% 3 4 4 4 6% 7% 57%	1800
	21st to end	Night	4 3 1 3 3 3 4 4 6% 9% 60%	1800
July	1st to 10th	Night	7% 4 2 8% 5% 4 6% 5% 8% 11% 40%	4078
	11th to 20th	Night	4 3 1 4 3 3 4 3 7% 13% 55%	4077
	21st to end	Night	8% 5% 2 7% 4 4 4 4 6% 9% 47%	4449
August	1st to 10th	Night	1 2 1 2 2 2 3 3 5% 8% 72%	2926
	11th to 20th	Night	5% 2 1 4 3 3 5% 4 7% 12% 53%	3105
	21st to end	Night	4 4 1 5% 3 4 5% 4 8% 9% 52%	3586
September	1st to 10th	Night	6% 4 2 5% 3 4 5% 3 5% 7% 55%	2000
	11th to 20th	Night	9% 5% 2 4 4 4 4 4 6% 8% 50%	2113
	21st to end	Night	7% 4 2 4 3 2 4 3 5% 7% 60%	2237
October	1st to 10th	Night	7% 4 1 3 3 3 3 3 5% 6% 64%	2400
	11th to 20th	Night	11% 4 2 3 2 2 3 2 5% 5% 61%	2400
	21st to end	Night	15% 6% 2 5% 3 3 3 2 3 4 55%	2640
November	1st to 10th	Night	12% 5% 1 5% 4 4 3 2 4 5% 55%	2460
	11th to 20th	Night	10% 5% 2 5% 3 3 3 3 3 5% 57%	2520
	21st to end	Night	16% 6% 3 5% 3 3 3 3 3 4 51%	2604
December	1st to 10th	Night	16% 7% 2 6% 2 3 3 3 4 5% 49%	4380
	11th to 20th	Night	19% 8% 3 6% 4 4 3 3 3 4 44%	4946
	21st to end	Night	18% 9% 3 6% 3 3 2 2 3 4 46%	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	43% 1 8% 30% 16% 3	5290
	11th to 20th	Day	33% 1 7% 24% 30% 5%	5310
	21st to end	Day	46% 1 3 15% 23% 11%	5812
February	1st to 10th	Day	49% 2 5% 19% 19% 5%	1890
	11th to 20th	Day	48% 2 5% 20% 18% 6%	1894
	21st to end	Day	49% 2 5% 13% 21% 9%	1815
March	1st to 10th	Day	49% 2 4 14% 19% 12%	4936
	11th to 20th	Day	37% 1 6% 16% 22% 18%	4980
	21st to end	Day	46% 2 8% 21% 20% 3	5464
April	1st to 10th	Day	42% 3 8% 22% 19% 7%	2326
	11th to 20th	Day	42% 3 7% 21% 20% 7%	2374
	21st to end	Day	41% 3 10% 22% 20% 4	2400
May	1st to 10th	Day	49% 3 8% 27% 11% 2	5519
	11th to 20th	Day	47% 4 9% 20% 19% 2	5866
	21st to end	Day	46% 4 12% 22% 12% 4	6447
June	1st to 10th	Day	37% 5% 13% 27% 16% 2	2600
	11th to 20th	Day	36% 6% 12% 27% 17% 2	2600
	21st to end	Day	28% 5% 13% 33% 19% 2	2600
July	1st to 10th	Day	38% 8% 19% 26% 7% 1	5836
	11th to 20th	Day	24% 6% 17% 35% 17% 1	5831
	21st to end	Day	38% 6% 14% 29% 12% 1	6240
August	1st to 10th	Day	18% 4 15% 30% 27% 6%	4220
	11th to 20th	Day	31% 5% 19% 33% 10% 1	3984
	21st to end	Day	32% 7% 14% 27% 18% 2	4311
September	1st to 10th	Day	35% 6% 11% 19% 20% 9%	2400
	11th to 20th	Day	38% 5% 9% 22% 20% 6%	2282
	21st to end	Day	31% 3 10% 22% 25% 9%	2141
October	1st to 10th	Day	16% 2 6% 13% 27% 36%	3260
	11th to 20th	Day	20% 1 3 18% 34% 23%	3240
	21st to end	Day	30% 2 5% 23% 31% 7%	3538
November	1st to 10th	Day	35% 2 6% 22% 26% 9%	2058
	11th to 20th	Day	34% 2 6% 19% 27% 12%	2100
	21st to end	Day	43% 2 6% 18% 25% 7%	2011
December	1st to 10th	Day	40% 2 6% 29% 18% 4	3030
	11th to 20th	Day	48% 2 3 18% 21% 8%	3479
	21st to end	Day	47% 2 4 22% 21% 4	4587
January	1st to 10th	Night	40% 2 7% 26% 23% 3	7635
	11th to 20th	Night	25% 1 5% 20% 36% 12%	7680
	21st to end	Night	50% 1 5% 14% 20% 10%	8402
February	1st to 10th	Night	43% 2 6% 20% 21% 8%	2730
	11th to 20th	Night	46% 2 7% 19% 19% 7%	2516
	21st to end	Night	42% 2 6% 15% 23% 11%	1827
March	1st to 10th	Night	45% 1 5% 15% 20% 14%	4919
	11th to 20th	Night	34% 1 7% 19% 19% 19%	4980
	21st to end	Night	42% 2 8% 21% 23% 5%	5467
April	1st to 10th	Night	38% 2 9% 22% 17% 12%	2072
	11th to 20th	Night	36% 3 7% 21% 22% 10%	1975
	21st to end	Night	36% 2 10% 25% 24% 4	2000
May	1st to 10th	Night	48% 3 10% 22% 13% 3	4441
	11th to 20th	Night	48% 3 10% 22% 16% 1	4054
	21st to end	Night	43% 4 14% 20% 14% 6%	4452
June	1st to 10th	Night	27% 5% 15% 31% 18% 3	1800
	11th to 20th	Night	25% 4 11% 32% 23% 4	1800
	21st to end	Night	21% 5% 14% 31% 25% 4	1800
July	1st to 10th	Night	35% 7% 20% 27% 10% 1	4035
	11th to 20th	Night	21% 5% 19% 35% 18% 2	4026
	21st to end	Night	34% 5% 14% 29% 15% 1	4320
August	1st to 10th	Night	12% 2 17% 29% 34% 6%	2916
	11th to 20th	Night	23% 5% 20% 32% 18% 3	3133
	21st to end	Night	26% 6% 14% 24% 28% 2	3586
September	1st to 10th	Night	28% 6% 12% 21% 23% 10%	2000
	11th to 20th	Night	32% 4 10% 24% 22% 8%	2113
	21st to end	Night	26% 4 9% 23% 26% 13%	2237
October	1st to 10th	Night	15% 1 7% 14% 25% 38%	3920
	11th to 20th	Night	17% 2 3 16% 35% 28%	3899
	21st to end	Night	27% 2 5% 20% 31% 15%	4252
November	1st to 10th	Night	32% 3 7% 20% 26% 12%	2460
	11th to 20th	Night	31% 2 7% 20% 27% 13%	2520
	21st to end	Night	38% 2 5% 20% 26% 8%	2604
December	1st to 10th	Night	40% 2 6% 26% 21% 5%	4380
	11th to 20th	Night	46% 2 5% 17% 20% 9%	4947
	21st to end	Night	44% 2 7% 20% 24% 4	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	28% 13% 37% 18% 4	330
	11th to 20th	Day	21% 21% 39% 16% 2 1	330
	21st to end	Day	24% 24% 36% 12% 4	363
February	1st to 10th	Day	27% 16% 37% 16% 4	330
	11th to 20th	Day	34% 21% 31% 12% 1	315
	21st to end	Day	35% 25% 29% 8% 3	249
March	1st to 10th	Day	29% 19% 32% 16% 4	300
	11th to 20th	Day	27% 23% 34% 13% 3	300
	21st to end	Day	23% 21% 38% 16% 2	330
April	1st to 10th	Day	25% 16% 35% 17% 7%	426
	11th to 20th	Day	24% 16% 32% 20% 7% 1	492
	21st to end	Day	16% 17% 32% 24% 9% 2	500
May	1st to 10th	Day	19% 13% 33% 26% 8% 1	530
	11th to 20th	Day	16% 9% 31% 28% 14% 3	725
	21st to end	Day	11% 7% 27% 34% 17% 4	789
June	1st to 10th	Day	10% 5% 18% 32% 27% 8%	730
	11th to 20th	Day	7% 2 18% 37% 28% 7%	730
	21st to end	Day	4 2 18% 34% 33% 9%	730
July	1st to 10th	Day	6% 2 12% 33% 37% 9%	721
	11th to 20th	Day	5% 2 8% 32% 39% 13%	718
	21st to end	Day	8% 1 8% 32% 38% 14%	794
August	1st to 10th	Day	5% 1 10% 33% 41% 11%	726
	11th to 20th	Day	10% 2 6% 32% 38% 12%	580
	21st to end	Day	11% 2 7% 23% 40% 17%	583
September	1st to 10th	Day	8% 3 10% 29% 37% 13%	530
	11th to 20th	Day	10% 3 13% 25% 33% 16%	412
	21st to end	Day	14% 5% 14% 35% 23% 9%	330
October	1st to 10th	Day	6% 2 27% 39% 22% 3	330
	11th to 20th	Day	10% 9% 35% 28% 15% 3	330
	21st to end	Day	25% 9% 29% 25% 10% 1	363
November	1st to 10th	Day	19% 12% 29% 25% 12% 2	330
	11th to 20th	Day	16% 12% 37% 26% 8% 1	330
	21st to end	Day	23% 15% 32% 18% 10% 2	330
December	1st to 10th	Day	23% 17% 35% 18% 5% 2	330
	11th to 20th	Day	26% 15% 36% 16% 6% 2	330
	21st to end	Day	23% 20% 37% 16% 3 1	363
January	1st to 10th	Night	24% 12% 31% 28% 5%	1050
	11th to 20th	Night	22% 15% 33% 25% 5%	1050
	21st to end	Night	22% 13% 34% 27% 4	1155
February	1st to 10th	Night	24% 15% 33% 24% 4	1050
	11th to 20th	Night	26% 16% 35% 20% 3	921
	21st to end	Night	22% 18% 34% 23% 4	507
March	1st to 10th	Night	22% 13% 34% 26% 6%	600
	11th to 20th	Night	19% 15% 34% 27% 4 1	600
	21st to end	Night	19% 12% 38% 26% 5%	660
April	1st to 10th	Night	19% 13% 35% 26% 7%	600
	11th to 20th	Night	13% 14% 36% 26% 10% 1	593
	21st to end	Night	13% 12% 35% 29% 11%	600
May	1st to 10th	Night	12% 8% 33% 37% 10% 1	600
	11th to 20th	Night	14% 7% 31% 31% 16% 2	591
	21st to end	Night	10% 4 25% 36% 21% 3	650
June	1st to 10th	Night	6% 3 13% 42% 34% 3	600
	11th to 20th	Night	5% 1 13% 47% 32% 2	600
	21st to end	Night	4 2 10% 39% 39% 7%	600
July	1st to 10th	Night	6% 1 9% 34% 44% 7%	585
	11th to 20th	Night	5% 1 5% 35% 44% 9%	582
	21st to end	Night	5% 1 6% 38% 42% 9%	657
August	1st to 10th	Night	3 1 7% 38% 44% 7%	597
	11th to 20th	Night	7% 9% 38% 38% 8%	594
	21st to end	Night	7% 1 7% 34% 44% 7%	660
September	1st to 10th	Night	7% 1 11% 37% 40% 5%	600
	11th to 20th	Night	12% 1 13% 30% 39% 5%	600
	21st to end	Night	8% 3 13% 38% 34% 4	637
October	1st to 10th	Night	8% 3 13% 40% 33% 3	800
	11th to 20th	Night	13% 5% 24% 37% 20% 1	800
	21st to end	Night	18% 7% 27% 30% 16% 2	880
November	1st to 10th	Night	13% 7% 27% 36% 16% 1	820
	11th to 20th	Night	12% 10% 30% 37% 11%	840
	21st to end	Night	19% 13% 29% 28% 11%	924
December	1st to 10th	Night	21% 9% 33% 30% 7%	1050
	11th to 20th	Night	24% 11% 29% 29% 7%	1050
	21st to end	Night	23% 13% 31% 27% 6%	1155

Page generated at 2026-02-13 00:42:13. Version ac07

Copyright © 2026 Attilla Danko.