Forest Acres Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Forest Acres Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	12% 6% 2 6% 4 4 5% 4 6% 8% 42%	127567
	Night	13% 6% 2 6% 4 4 4 4 6% 7% 44%	124450
Transparency	Day	38% 7% 12% 19% 18% 6%	131226
	Night	38% 6% 11% 18% 20% 8%	128860
Seeing	Day	15% 8% 23% 24% 19% 9%	16980
	Night	15% 7% 23% 32% 20% 3	27101

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	15% 5% 1 4 3 3 2 2 5% 6% 54%	4146
	11th to 20th	Day	12% 6% 2 4 3 3 2 2 3 4 59%	4150
	21st to end	Day	15% 7% 3 7% 4 4 3 3 4 5% 45%	4565
February	1st to 10th	Day	15% 6% 2 7% 4 4 4 3 6% 8% 41%	1810
	11th to 20th	Day	20% 7% 2 5% 4 3 3 3 4 5% 44%	1948
	21st to end	Day	18% 5% 3 6% 3 5% 3 3 4 5% 44%	1717
March	1st to 10th	Day	21% 9% 3 6% 4 3 3 3 4 5% 40%	4935
	11th to 20th	Day	8% 4 2 5% 4 4 4 3 5% 6% 55%	4980
	21st to end	Day	14% 7% 2 6% 4 4 4 3 4 5% 47%	5465
April	1st to 10th	Day	10% 4 2 5% 4 4 4 4 6% 7% 52%	2200
	11th to 20th	Day	10% 5% 2 6% 3 3 4 4 6% 8% 50%	2350
	21st to end	Day	8% 6% 2 4 3 4 4 4 6% 9% 50%	2511
May	1st to 10th	Day	10% 7% 2 6% 5% 5% 5% 4 7% 10% 39%	5862
	11th to 20th	Day	17% 7% 3 7% 5% 5% 5% 4 7% 9% 32%	5861
	21st to end	Day	10% 6% 2 7% 4 5% 4 4 6% 8% 44%	6433
June	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 6% 7% 10% 37%	2600
	11th to 20th	Day	7% 5% 2 4 5% 6% 5% 5% 8% 11% 43%	2600
	21st to end	Day	5% 5% 2 5% 7% 6% 6% 6% 9% 12% 39%	2600
July	1st to 10th	Day	11% 7% 3 8% 6% 6% 7% 6% 9% 11% 25%	5900
	11th to 20th	Day	4 4 2 6% 5% 6% 8% 6% 11% 15% 31%	5900
	21st to end	Day	7% 7% 2 8% 6% 6% 7% 6% 10% 13% 27%	6424
August	1st to 10th	Day	22% 7% 3 7% 5% 6% 6% 5% 7% 9% 23%	4234
	11th to 20th	Day	10% 6% 2 7% 5% 5% 6% 6% 7% 11% 34%	4174
	21st to end	Day	7% 4 2 5% 4 5% 5% 5% 8% 11% 46%	4330
September	1st to 10th	Day	7% 4 2 6% 6% 5% 6% 4 7% 9% 43%	2200
	11th to 20th	Day	12% 6% 3 6% 5% 5% 6% 3 6% 8% 40%	2200
	21st to end	Day	17% 6% 2 6% 4 3 5% 4 4 7% 42%	2200
October	1st to 10th	Day	12% 6% 2 5% 4 4 4 3 5% 6% 48%	2085
	11th to 20th	Day	11% 4 2 4 4 3 3 2 4 5% 59%	2000
	21st to end	Day	12% 4 2 4 3 4 3 3 4 4 58%	2200
November	1st to 10th	Day	12% 6% 2 4 4 4 3 3 4 5% 54%	2057
	11th to 20th	Day	14% 5% 2 4 3 2 2 3 4 6% 54%	1992
	21st to end	Day	13% 6% 2 5% 4 3 3 3 5% 6% 49%	1890
December	1st to 10th	Day	12% 5% 2 6% 4 4 4 3 4 6% 50%	3020
	11th to 20th	Day	14% 6% 2 6% 4 3 4 3 4 6% 47%	3463
	21st to end	Day	20% 7% 2 5% 3 2 3 2 5% 6% 45%	4565
January	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 4 5% 56%	6019
	11th to 20th	Night	11% 6% 2 5% 4 4 3 3 5% 6% 50%	6030
	21st to end	Night	15% 8% 2 6% 4 4 3 3 4 5% 46%	6633
February	1st to 10th	Night	15% 6% 2 6% 3 4 4 3 5% 6% 45%	2587
	11th to 20th	Night	19% 7% 3 6% 3 4 3 3 4 5% 44%	2338
	21st to end	Night	14% 6% 2 6% 4 4 3 3 5% 6% 47%	1911
March	1st to 10th	Night	19% 8% 2 6% 3 4 4 3 4 6% 40%	4918
	11th to 20th	Night	11% 3 1 4 3 3 3 3 6% 7% 55%	4980
	21st to end	Night	14% 4 2 5% 4 4 4 3 5% 7% 49%	5467
April	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 5% 7% 50%	2200
	11th to 20th	Night	11% 5% 2 5% 3 4 3 3 5% 6% 54%	1996
	21st to end	Night	9% 4 2 5% 3 4 4 3 5% 8% 53%	1904
May	1st to 10th	Night	9% 7% 2 7% 5% 4 5% 4 7% 10% 40%	4033
	11th to 20th	Night	18% 6% 2 6% 4 5% 5% 4 6% 10% 35%	4032
	21st to end	Night	11% 5% 2 6% 4 5% 5% 4 7% 9% 43%	4421
June	1st to 10th	Night	11% 6% 2 7% 5% 4 6% 6% 9% 9% 36%	1800
	11th to 20th	Night	8% 5% 2 6% 6% 6% 6% 4 8% 10% 39%	1800
	21st to end	Night	6% 5% 2 6% 6% 6% 7% 6% 10% 13% 34%	1800
July	1st to 10th	Night	12% 8% 3 9% 6% 6% 7% 6% 8% 10% 25%	4058
	11th to 20th	Night	6% 6% 2 8% 6% 6% 7% 7% 11% 15% 26%	4057
	21st to end	Night	10% 8% 4 11% 7% 7% 6% 6% 8% 10% 22%	4427
August	1st to 10th	Night	22% 8% 3 7% 5% 6% 6% 5% 8% 9% 22%	2915
	11th to 20th	Night	10% 7% 3 7% 5% 6% 6% 5% 7% 9% 35%	2875
	21st to end	Night	6% 4 2 5% 4 3 3 4 6% 12% 50%	3518
September	1st to 10th	Night	8% 6% 2 7% 5% 5% 5% 4 8% 9% 40%	2197
	11th to 20th	Night	11% 7% 3 8% 5% 5% 5% 4 6% 6% 41%	2200
	21st to end	Night	16% 6% 2 6% 3 4 5% 4 5% 6% 41%	2200
October	1st to 10th	Night	13% 6% 1 6% 3 3 4 3 4 6% 52%	2313
	11th to 20th	Night	13% 5% 1 4 3 3 3 3 5% 5% 55%	2400
	21st to end	Night	11% 4 1 5% 3 2 4 3 5% 5% 56%	2640
November	1st to 10th	Night	13% 6% 2 4 3 3 3 3 4 6% 54%	2461
	11th to 20th	Night	13% 6% 2 4 3 3 3 3 4 6% 53%	2625
	21st to end	Night	13% 5% 2 5% 3 4 4 3 6% 7% 49%	2730
December	1st to 10th	Night	14% 5% 2 5% 4 3 3 2 4 5% 53%	4380
	11th to 20th	Night	17% 7% 2 6% 4 3 3 3 4 6% 45%	4952
	21st to end	Night	18% 7% 2 5% 3 3 4 3 4 6% 43%	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	32% 3 6% 22% 30% 7%	4147
	11th to 20th	Day	31% 3 4 15% 37% 10%	4150
	21st to end	Day	43% 3 5% 16% 22% 11%	4565
February	1st to 10th	Day	41% 4 10% 20% 18% 6%	1810
	11th to 20th	Day	44% 4 5% 17% 25% 6%	1958
	21st to end	Day	42% 5% 7% 15% 22% 9%	1717
March	1st to 10th	Day	48% 4 5% 13% 21% 10%	4935
	11th to 20th	Day	30% 5% 8% 14% 26% 18%	4980
	21st to end	Day	41% 4 6% 18% 22% 8%	5465
April	1st to 10th	Day	31% 6% 10% 21% 25% 7%	2200
	11th to 20th	Day	32% 6% 10% 20% 22% 10%	2350
	21st to end	Day	30% 7% 11% 26% 22% 4	2511
May	1st to 10th	Day	38% 7% 17% 25% 12% 2	5897
	11th to 20th	Day	48% 7% 16% 18% 11% 1	5865
	21st to end	Day	36% 7% 14% 19% 21% 3	6447
June	1st to 10th	Day	39% 10% 16% 23% 11% 2	2600
	11th to 20th	Day	32% 10% 18% 25% 13% 2	2600
	21st to end	Day	34% 11% 18% 22% 13% 2	2600
July	1st to 10th	Day	46% 11% 17% 19% 6% 1	5835
	11th to 20th	Day	36% 15% 32% 14% 3 1	5831
	21st to end	Day	43% 12% 17% 21% 6%	6240
August	1st to 10th	Day	55% 9% 18% 13% 4 1	4221
	11th to 20th	Day	40% 10% 16% 20% 13%	4213
	21st to end	Day	30% 9% 14% 31% 15% 2	4330
September	1st to 10th	Day	35% 9% 13% 19% 18% 5%	2200
	11th to 20th	Day	41% 8% 11% 17% 20% 3	2200
	21st to end	Day	41% 6% 9% 21% 16% 7%	2200
October	1st to 10th	Day	35% 5% 6% 18% 27% 9%	3397
	11th to 20th	Day	18% 3 5% 17% 28% 29%	3240
	21st to end	Day	26% 3 4 21% 31% 15%	3538
November	1st to 10th	Day	33% 4 7% 18% 27% 12%	2057
	11th to 20th	Day	32% 4 7% 20% 26% 11%	1992
	21st to end	Day	36% 4 8% 20% 23% 9%	1890
December	1st to 10th	Day	36% 4 7% 17% 26% 9%	3020
	11th to 20th	Day	39% 4 9% 21% 20% 7%	3460
	21st to end	Day	41% 3 8% 22% 24% 2	4565
January	1st to 10th	Night	31% 3 6% 19% 35% 7%	6019
	11th to 20th	Night	35% 4 6% 16% 32% 8%	6030
	21st to end	Night	41% 4 4 14% 28% 9%	6633
February	1st to 10th	Night	39% 4 8% 20% 20% 8%	2587
	11th to 20th	Night	44% 4 6% 18% 21% 8%	2339
	21st to end	Night	38% 5% 8% 18% 20% 12%	1911
March	1st to 10th	Night	45% 4 7% 11% 24% 8%	4918
	11th to 20th	Night	28% 4 7% 12% 29% 19%	4980
	21st to end	Night	37% 4 7% 20% 21% 11%	5467
April	1st to 10th	Night	33% 6% 11% 19% 22% 9%	2200
	11th to 20th	Night	31% 5% 9% 21% 22% 12%	1996
	21st to end	Night	30% 6% 11% 25% 23% 5%	1904
May	1st to 10th	Night	37% 8% 14% 25% 13% 2	4060
	11th to 20th	Night	45% 7% 20% 16% 12%	4033
	21st to end	Night	35% 8% 12% 23% 19% 3	4429
June	1st to 10th	Night	39% 11% 17% 20% 12% 2	1800
	11th to 20th	Night	37% 10% 18% 23% 10% 2	1800
	21st to end	Night	36% 13% 19% 19% 11% 2	1800
July	1st to 10th	Night	50% 11% 19% 13% 5% 1	4015
	11th to 20th	Night	41% 16% 29% 11% 3 1	4006
	21st to end	Night	55% 10% 16% 14% 5%	4297
August	1st to 10th	Night	56% 10% 17% 13% 3 1	2904
	11th to 20th	Night	43% 9% 22% 20% 7% 1	2902
	21st to end	Night	27% 8% 15% 32% 16% 2	3518
September	1st to 10th	Night	37% 9% 15% 20% 14% 5%	2197
	11th to 20th	Night	42% 8% 11% 17% 18% 3	2200
	21st to end	Night	41% 7% 13% 18% 16% 5%	2200
October	1st to 10th	Night	34% 5% 8% 18% 29% 7%	3781
	11th to 20th	Night	21% 2 5% 15% 25% 31%	3899
	21st to end	Night	25% 4 5% 19% 28% 19%	4252
November	1st to 10th	Night	32% 4 8% 15% 26% 14%	2461
	11th to 20th	Night	33% 4 6% 17% 26% 13%	2625
	21st to end	Night	35% 4 8% 19% 24% 10%	2730
December	1st to 10th	Night	35% 4 6% 16% 30% 9%	4380
	11th to 20th	Night	41% 3 10% 18% 19% 9%	4954
	21st to end	Night	43% 4 6% 23% 21% 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	17% 20% 44% 16% 4	330
	11th to 20th	Day	21% 19% 38% 16% 5%	330
	21st to end	Day	22% 24% 36% 14% 3	363
February	1st to 10th	Day	17% 21% 35% 21% 5%	300
	11th to 20th	Day	35% 15% 35% 13% 2	300
	21st to end	Day	23% 15% 37% 18% 6%	316
March	1st to 10th	Day	21% 17% 28% 26% 9%	500
	11th to 20th	Day	17% 15% 41% 20% 7%	500
	21st to end	Day	19% 16% 33% 22% 9% 1	550
April	1st to 10th	Day	16% 16% 32% 24% 11% 1	500
	11th to 20th	Day	13% 14% 29% 25% 16% 3	492
	21st to end	Day	12% 8% 27% 28% 18% 7%	500
May	1st to 10th	Day	10% 11% 31% 24% 18% 6%	503
	11th to 20th	Day	18% 6% 20% 26% 20% 10%	527
	21st to end	Day	14% 6% 16% 24% 23% 17%	573
June	1st to 10th	Day	11% 3 15% 29% 30% 12%	530
	11th to 20th	Day	10% 4 16% 27% 27% 17%	530
	21st to end	Day	8% 2 13% 30% 30% 17%	530
July	1st to 10th	Day	10% 2 8% 23% 31% 27%	525
	11th to 20th	Day	9% 2 8% 21% 37% 23%	524
	21st to end	Day	6% 2 9% 29% 33% 21%	576
August	1st to 10th	Day	17% 4 12% 24% 25% 18%	526
	11th to 20th	Day	15% 3 10% 23% 29% 21%	519
	21st to end	Day	13% 2 12% 23% 31% 20%	583
September	1st to 10th	Day	10% 2 14% 27% 31% 16%	530
	11th to 20th	Day	15% 3 15% 23% 31% 14%	530
	21st to end	Day	20% 2 16% 26% 26% 10%	530
October	1st to 10th	Day	16% 4 20% 31% 21% 8%	530
	11th to 20th	Day	11% 6% 23% 29% 22% 9%	530
	21st to end	Day	15% 6% 26% 29% 18% 6%	583
November	1st to 10th	Day	16% 6% 29% 25% 19% 5%	535
	11th to 20th	Day	19% 12% 30% 27% 10% 2	432
	21st to end	Day	14% 13% 34% 25% 12% 1	330
December	1st to 10th	Day	25% 15% 36% 18% 5% 1	330
	11th to 20th	Day	21% 12% 36% 22% 9% 1	330
	21st to end	Day	25% 14% 32% 20% 6% 2	363
January	1st to 10th	Night	21% 12% 31% 29% 8%	840
	11th to 20th	Night	18% 11% 34% 31% 7%	840
	21st to end	Night	20% 11% 35% 31% 3	924
February	1st to 10th	Night	19% 12% 30% 35% 4	801
	11th to 20th	Night	23% 13% 33% 29% 3	783
	21st to end	Night	18% 13% 33% 30% 6%	660
March	1st to 10th	Night	17% 12% 31% 35% 5%	800
	11th to 20th	Night	17% 9% 36% 30% 7%	800
	21st to end	Night	16% 11% 35% 29% 9%	880
April	1st to 10th	Night	15% 13% 32% 30% 10%	800
	11th to 20th	Night	13% 11% 28% 33% 14%	791
	21st to end	Night	12% 6% 27% 41% 14%	704
May	1st to 10th	Night	11% 6% 27% 39% 16% 1	600
	11th to 20th	Night	13% 3 25% 37% 19% 3	591
	21st to end	Night	10% 2 14% 37% 32% 4	650
June	1st to 10th	Night	13% 3 13% 39% 28% 5%	600
	11th to 20th	Night	9% 2 13% 34% 32% 10%	600
	21st to end	Night	8% 2 11% 37% 35% 7%	600
July	1st to 10th	Night	10% 2 9% 32% 39% 8%	585
	11th to 20th	Night	10% 1 4 31% 46% 8%	582
	21st to end	Night	9% 2 7% 31% 45% 6%	657
August	1st to 10th	Night	15% 1 9% 26% 41% 8%	597
	11th to 20th	Night	13% 2 7% 30% 40% 8%	594
	21st to end	Night	12% 1 12% 26% 40% 8%	838
September	1st to 10th	Night	10% 2 13% 28% 41% 7%	800
	11th to 20th	Night	15% 2 13% 28% 38% 5%	800
	21st to end	Night	20% 3 13% 31% 30% 4	800
October	1st to 10th	Night	14% 3 19% 30% 29% 5%	800
	11th to 20th	Night	15% 5% 21% 32% 25% 3	800
	21st to end	Night	13% 6% 23% 33% 23% 2	880
November	1st to 10th	Night	15% 6% 24% 34% 18% 3	820
	11th to 20th	Night	16% 9% 28% 33% 14% 1	840
	21st to end	Night	14% 8% 26% 34% 17% 1	840
December	1st to 10th	Night	20% 10% 26% 35% 9% 1	840
	11th to 20th	Night	19% 8% 25% 36% 11% 1	840
	21st to end	Night	23% 11% 24% 33% 9%	924

Page generated at 2026-01-29 01:21:45. Version ac07

Copyright © 2026 Attilla Danko.