Oak Tree Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Oak Tree Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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% 5% 4 5% 4 6% 8% 42%	131565
	Night	14% 6% 2 5% 4 3 4 3 5% 7% 46%	130548
Transparency	Day	39% 6% 12% 20% 17% 8%	135254
	Night	36% 5% 11% 20% 18% 10%	134923
Seeing	Day	12% 6% 18% 24% 26% 14%	16618
	Night	15% 5% 22% 34% 22% 2	26617

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	18% 10% 3 6% 3 3 3 3 3 5% 44%	5289
	11th to 20th	Day	15% 6% 2 6% 4 3 3 2 3 4 52%	5310
	21st to end	Day	28% 8% 3 7% 4 3 3 2 4 6% 33%	5812
February	1st to 10th	Day	22% 8% 3 6% 4 4 3 3 4 6% 39%	2048
	11th to 20th	Day	21% 9% 3 7% 4 4 3 3 4 5% 38%	2049
	21st to end	Day	19% 7% 3 6% 4 3 3 2 4 6% 43%	1871
March	1st to 10th	Day	14% 6% 2 7% 5% 5% 4 4 6% 8% 39%	5046
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 4 6% 47%	5090
	21st to end	Day	18% 7% 2 6% 4 4 4 3 5% 5% 41%	5583
April	1st to 10th	Day	14% 8% 3 8% 5% 5% 5% 4 4 6% 38%	2211
	11th to 20th	Day	17% 7% 2 6% 5% 4 4 4 5% 6% 40%	2286
	21st to end	Day	12% 7% 3 7% 5% 5% 4 4 5% 7% 43%	2400
May	1st to 10th	Day	18% 7% 3 8% 5% 5% 5% 4 5% 8% 32%	5696
	11th to 20th	Day	11% 5% 2 7% 4 5% 5% 4 6% 8% 43%	5862
	21st to end	Day	10% 6% 2 7% 5% 5% 6% 5% 8% 10% 35%	6434
June	1st to 10th	Day	5% 4 2 6% 5% 5% 6% 5% 8% 11% 44%	2600
	11th to 20th	Day	6% 4 2 5% 5% 5% 6% 6% 8% 10% 43%	2600
	21st to end	Day	5% 6% 2 7% 8% 8% 6% 7% 9% 10% 31%	2600
July	1st to 10th	Day	8% 5% 2 7% 6% 6% 6% 6% 8% 11% 34%	5900
	11th to 20th	Day	6% 6% 3 7% 6% 6% 7% 6% 9% 13% 32%	5900
	21st to end	Day	10% 6% 3 7% 5% 5% 6% 5% 7% 11% 35%	6422
August	1st to 10th	Day	2 2 1 3 3 4 4 3 6% 10% 61%	4234
	11th to 20th	Day	3 2 1 4 4 5% 7% 6% 9% 12% 46%	4082
	21st to end	Day	5% 4 2 7% 6% 6% 7% 6% 8% 10% 40%	4090
September	1st to 10th	Day	6% 5% 3 7% 5% 6% 6% 5% 7% 10% 40%	2200
	11th to 20th	Day	10% 6% 3 7% 6% 6% 5% 5% 5% 9% 40%	2200
	21st to end	Day	7% 5% 2 6% 4 4 6% 5% 6% 7% 47%	2200
October	1st to 10th	Day	6% 3 2 4 4 3 5% 4 5% 7% 58%	2120
	11th to 20th	Day	10% 5% 2 6% 4 4 4 4 4 5% 54%	2000
	21st to end	Day	12% 5% 2 6% 3 3 3 3 4 5% 54%	2200
November	1st to 10th	Day	10% 6% 2 6% 4 4 4 4 5% 8% 47%	2058
	11th to 20th	Day	11% 6% 2 5% 3 4 3 3 6% 7% 50%	2100
	21st to end	Day	16% 7% 2 7% 5% 5% 4 3 4 6% 42%	1974
December	1st to 10th	Day	25% 7% 2 6% 3 3 3 3 4 6% 37%	3030
	11th to 20th	Day	15% 7% 3 7% 4 4 4 3 5% 6% 43%	3481
	21st to end	Day	15% 7% 2 6% 4 3 4 3 4 6% 47%	4587
January	1st to 10th	Night	19% 6% 2 5% 3 3 3 2 4 5% 48%	7634
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 4 5% 54%	7680
	21st to end	Night	29% 8% 2 6% 3 2 3 2 4 5% 35%	8402
February	1st to 10th	Night	20% 9% 3 6% 3 3 3 3 4 6% 41%	2588
	11th to 20th	Night	23% 7% 2 4 3 3 2 3 3 5% 44%	2458
	21st to end	Night	22% 7% 3 5% 3 4 3 3 4 5% 42%	1933
March	1st to 10th	Night	19% 10% 2 6% 3 3 4 3 5% 6% 38%	5029
	11th to 20th	Night	19% 7% 2 6% 3 3 3 2 4 5% 46%	5090
	21st to end	Night	18% 9% 3 6% 4 3 3 3 4 5% 42%	5588
April	1st to 10th	Night	14% 7% 3 5% 4 4 4 4 6% 7% 43%	2211
	11th to 20th	Night	15% 7% 3 6% 3 4 3 3 4 7% 45%	2060
	21st to end	Night	15% 7% 2 6% 4 3 4 3 4 7% 44%	2000
May	1st to 10th	Night	22% 9% 3 7% 5% 5% 4 3 6% 7% 30%	4194
	11th to 20th	Night	10% 7% 2 6% 4 4 5% 4 7% 9% 43%	4053
	21st to end	Night	11% 8% 3 8% 5% 5% 5% 4 7% 10% 33%	4443
June	1st to 10th	Night	6% 3 1 4 2 3 4 3 8% 10% 56%	1800
	11th to 20th	Night	4 3 1 4 4 4 3 4 6% 12% 55%	1800
	21st to end	Night	5% 4 2 4 6% 4 4 5% 9% 11% 46%	1800
July	1st to 10th	Night	6% 5% 2 6% 4 5% 5% 5% 9% 12% 41%	4078
	11th to 20th	Night	4 4 2 6% 4 4 5% 4 6% 11% 50%	4077
	21st to end	Night	7% 5% 2 6% 5% 4 5% 4 7% 10% 45%	4449
August	1st to 10th	Night	1 1 1 2 2 2 2 2 5% 10% 71%	2926
	11th to 20th	Night	2 2 1 3 3 3 4 4 7% 12% 60%	2972
	21st to end	Night	4 2 1 5% 4 5% 5% 5% 8% 12% 49%	3803
September	1st to 10th	Night	5% 5% 2 5% 4 5% 5% 5% 8% 9% 48%	2200
	11th to 20th	Night	10% 5% 3 6% 4 5% 4 4 5% 8% 46%	2200
	21st to end	Night	7% 4 2 4 3 4 3 3 6% 8% 56%	2200
October	1st to 10th	Night	4 3 1 3 4 2 3 3 5% 7% 64%	2260
	11th to 20th	Night	10% 4 1 4 3 3 3 2 3 6% 61%	2400
	21st to end	Night	10% 5% 2 4 2 2 3 2 3 5% 61%	2640
November	1st to 10th	Night	10% 5% 2 5% 4 3 3 3 4 6% 54%	2460
	11th to 20th	Night	12% 5% 2 4 4 3 3 3 4 5% 53%	2520
	21st to end	Night	17% 6% 3 5% 4 3 4 3 4 6% 46%	2641
December	1st to 10th	Night	22% 11% 3 8% 3 3 3 2 4 6% 35%	4380
	11th to 20th	Night	16% 9% 3 6% 3 3 3 2 4 5% 45%	4946
	21st to end	Night	15% 7% 2 5% 3 3 3 3 4 5% 50%	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	45% 3 7% 21% 18% 6%	5290
	11th to 20th	Day	38% 2 4 17% 23% 15%	5310
	21st to end	Day	54% 2 4 12% 17% 12%	5812
February	1st to 10th	Day	47% 3 6% 13% 17% 13%	2048
	11th to 20th	Day	49% 4 7% 14% 17% 9%	2059
	21st to end	Day	44% 3 8% 16% 17% 11%	1871
March	1st to 10th	Day	43% 5% 8% 16% 16% 12%	5046
	11th to 20th	Day	38% 5% 6% 10% 19% 21%	5090
	21st to end	Day	44% 4 7% 19% 19% 6%	5585
April	1st to 10th	Day	47% 5% 7% 16% 15% 10%	2211
	11th to 20th	Day	44% 5% 8% 14% 18% 9%	2286
	21st to end	Day	41% 5% 8% 20% 19% 6%	2400
May	1st to 10th	Day	48% 7% 10% 21% 12% 3	5732
	11th to 20th	Day	38% 6% 12% 21% 21% 2	5866
	21st to end	Day	40% 7% 12% 28% 12% 1	6447
June	1st to 10th	Day	31% 8% 17% 26% 16% 2	2600
	11th to 20th	Day	31% 11% 15% 24% 17% 2	2600
	21st to end	Day	42% 10% 18% 20% 9%	2600
July	1st to 10th	Day	41% 7% 29% 20% 3	5836
	11th to 20th	Day	39% 8% 25% 22% 6%	5831
	21st to end	Day	41% 8% 18% 25% 9%	6240
August	1st to 10th	Day	19% 6% 14% 40% 21%	4220
	11th to 20th	Day	25% 8% 32% 27% 7% 1	4121
	21st to end	Day	36% 10% 18% 22% 12% 1	4089
September	1st to 10th	Day	38% 11% 14% 17% 15% 6%	2200
	11th to 20th	Day	40% 8% 11% 19% 17% 3	2200
	21st to end	Day	32% 7% 12% 18% 17% 13%	2200
October	1st to 10th	Day	17% 4 6% 15% 20% 39%	3458
	11th to 20th	Day	22% 3 7% 15% 35% 17%	3240
	21st to end	Day	27% 4 4 18% 33% 14%	3538
November	1st to 10th	Day	35% 6% 9% 17% 23% 10%	2058
	11th to 20th	Day	35% 4 10% 16% 22% 14%	2100
	21st to end	Day	44% 5% 7% 16% 18% 10%	1974
December	1st to 10th	Day	49% 3 5% 16% 21% 5%	3030
	11th to 20th	Day	42% 5% 7% 13% 23% 10%	3479
	21st to end	Day	40% 4 5% 16% 24% 11%	4587
January	1st to 10th	Night	39% 3 9% 19% 19% 10%	7635
	11th to 20th	Night	35% 2 4 19% 25% 15%	7680
	21st to end	Night	54% 2 3 13% 17% 10%	8402
February	1st to 10th	Night	47% 3 7% 11% 18% 15%	2588
	11th to 20th	Night	45% 3 7% 17% 17% 12%	2458
	21st to end	Night	46% 4 7% 13% 17% 14%	1933
March	1st to 10th	Night	47% 4 7% 15% 18% 10%	5029
	11th to 20th	Night	42% 4 5% 14% 16% 19%	5090
	21st to end	Night	45% 4 8% 15% 20% 8%	5588
April	1st to 10th	Night	40% 7% 9% 18% 15% 10%	2211
	11th to 20th	Night	40% 5% 10% 17% 15% 14%	2060
	21st to end	Night	40% 5% 9% 21% 18% 7%	2000
May	1st to 10th	Night	53% 6% 9% 20% 9% 4	4223
	11th to 20th	Night	36% 6% 14% 23% 17% 5%	4054
	21st to end	Night	43% 8% 15% 27% 7% 1	4452
June	1st to 10th	Night	22% 7% 18% 33% 18% 1	1800
	11th to 20th	Night	22% 8% 18% 30% 18% 4	1800
	21st to end	Night	30% 9% 22% 26% 13% 1	1800
July	1st to 10th	Night	34% 7% 34% 22% 3	4035
	11th to 20th	Night	28% 7% 27% 31% 7%	4026
	21st to end	Night	33% 9% 20% 28% 9%	4320
August	1st to 10th	Night	11% 4 18% 48% 19%	2916
	11th to 20th	Night	16% 6% 35% 35% 7% 1	2999
	21st to end	Night	26% 13% 22% 27% 10% 1	3803
September	1st to 10th	Night	29% 10% 21% 21% 14% 6%	2200
	11th to 20th	Night	35% 7% 15% 22% 16% 5%	2200
	21st to end	Night	25% 6% 15% 23% 18% 12%	2200
October	1st to 10th	Night	14% 4 7% 18% 17% 39%	3689
	11th to 20th	Night	16% 3 6% 23% 31% 21%	3899
	21st to end	Night	21% 2 5% 21% 34% 17%	4252
November	1st to 10th	Night	32% 5% 10% 20% 23% 11%	2460
	11th to 20th	Night	33% 4 10% 18% 21% 14%	2520
	21st to end	Night	41% 4 8% 17% 18% 12%	2641
December	1st to 10th	Night	52% 4 5% 15% 18% 7%	4380
	11th to 20th	Night	43% 3 9% 9% 20% 15%	4947
	21st to end	Night	37% 4 5% 15% 29% 10%	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	23% 9% 35% 25% 7% 1	330
	11th to 20th	Day	22% 10% 28% 29% 9% 1	330
	21st to end	Day	26% 15% 29% 21% 10% 1	363
February	1st to 10th	Day	25% 14% 25% 26% 9% 1	330
	11th to 20th	Day	25% 16% 32% 21% 6% 1	330
	21st to end	Day	26% 12% 38% 16% 8%	273
March	1st to 10th	Day	20% 13% 34% 24% 8% 1	330
	11th to 20th	Day	14% 17% 39% 22% 6% 2	330
	21st to end	Day	19% 10% 29% 27% 12% 2	363
April	1st to 10th	Day	15% 16% 36% 23% 10%	303
	11th to 20th	Day	24% 10% 29% 26% 10% 1	404
	21st to end	Day	14% 9% 32% 30% 13% 2	500
May	1st to 10th	Day	14% 7% 34% 30% 15% 1	626
	11th to 20th	Day	14% 5% 23% 33% 22% 3	725
	21st to end	Day	8% 3 25% 37% 23% 4	789
June	1st to 10th	Day	6% 2 8% 32% 38% 14%	730
	11th to 20th	Day	6% 2 9% 31% 37% 15%	730
	21st to end	Day	8% 2 7% 23% 36% 24%	730
July	1st to 10th	Day	9% 2 5% 20% 39% 26%	721
	11th to 20th	Day	4 1 4 16% 46% 30%	718
	21st to end	Day	6% 2 4 17% 44% 28%	794
August	1st to 10th	Day	4 2 14% 46% 34%	726
	11th to 20th	Day	6% 2 14% 41% 37%	665
	21st to end	Day	9% 1 2 12% 34% 43%	452
September	1st to 10th	Day	8% 2 3 17% 42% 29%	330
	11th to 20th	Day	8% 2 4 18% 35% 33%	330
	21st to end	Day	8% 1 8% 20% 41% 22%	330
October	1st to 10th	Day	6% 1 7% 33% 40% 14%	330
	11th to 20th	Day	10% 2 15% 31% 28% 14%	330
	21st to end	Day	18% 6% 22% 20% 25% 9%	363
November	1st to 10th	Day	13% 3 18% 36% 22% 7%	330
	11th to 20th	Day	14% 5% 23% 33% 22% 4	330
	21st to end	Day	19% 13% 28% 26% 12% 3	330
December	1st to 10th	Day	21% 5% 32% 24% 13% 5%	330
	11th to 20th	Day	22% 12% 25% 27% 11% 2	330
	21st to end	Day	18% 6% 34% 29% 11% 3	363
January	1st to 10th	Night	23% 8% 30% 30% 9%	1050
	11th to 20th	Night	19% 12% 29% 32% 7%	1050
	21st to end	Night	23% 9% 30% 32% 7%	1155
February	1st to 10th	Night	25% 9% 30% 30% 5%	908
	11th to 20th	Night	26% 8% 29% 32% 5%	824
	21st to end	Night	27% 11% 36% 24% 3	549
March	1st to 10th	Night	24% 9% 35% 28% 5%	630
	11th to 20th	Night	20% 8% 33% 31% 7%	630
	21st to end	Night	17% 7% 35% 33% 7% 1	693
April	1st to 10th	Night	16% 8% 36% 35% 4	603
	11th to 20th	Night	18% 8% 32% 32% 9%	593
	21st to end	Night	16% 7% 36% 31% 9%	600
May	1st to 10th	Night	15% 8% 33% 36% 8%	600
	11th to 20th	Night	13% 4 29% 39% 13% 2	591
	21st to end	Night	9% 2 26% 41% 20% 2	650
June	1st to 10th	Night	6% 1 10% 46% 33% 3	600
	11th to 20th	Night	4 1 11% 49% 33% 1	600
	21st to end	Night	6% 1 7% 37% 42% 8%	600
July	1st to 10th	Night	5% 1 4 35% 47% 8%	585
	11th to 20th	Night	5% 4 32% 53% 7%	582
	21st to end	Night	2 1 4 35% 51% 7%	657
August	1st to 10th	Night	2 1 1 33% 57% 6%	597
	11th to 20th	Night	4 1 2 30% 56% 7%	594
	21st to end	Night	5% 3 25% 55% 10%	660
September	1st to 10th	Night	6% 1 5% 29% 51% 9%	600
	11th to 20th	Night	10% 2 4 31% 48% 5%	600
	21st to end	Night	9% 1 8% 33% 43% 7%	600
October	1st to 10th	Night	4 2 9% 39% 41% 5%	660
	11th to 20th	Night	11% 2 18% 42% 25% 2	800
	21st to end	Night	12% 6% 22% 36% 20% 3	880
November	1st to 10th	Night	11% 4 19% 40% 24% 1	820
	11th to 20th	Night	14% 6% 21% 39% 19% 1	840
	21st to end	Night	19% 7% 27% 33% 13% 1	961
December	1st to 10th	Night	22% 6% 25% 32% 14%	1050
	11th to 20th	Night	23% 8% 26% 32% 9% 1	1050
	21st to end	Night	21% 8% 28% 33% 9% 1	1155

Page generated at 2026-03-30 01:18:42. Version ac07

Copyright © 2026 Attilla Danko.