Tall Timbers
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tall Timbers clear sky chart from 2005-11-06 to 2026-04-17.

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 5% 4 4 4 4 6% 8% 44%	131480
	Night	13% 6% 2 6% 4 4 4 3 5% 7% 44%	130663
Transparency	Day	24% 38% 8% 15% 12% 3	135070
	Night	25% 37% 7% 16% 12% 3	135143
Seeing	Day	15% 5% 17% 29% 24% 10%	16936
	Night	16% 7% 24% 32% 17% 3	26560

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	13% 5% 2 5% 3 3 4 4 5% 6% 49%	4781
	11th to 20th	Day	13% 8% 2 5% 3 3 3 3 4 4 52%	5280
	21st to end	Day	11% 8% 3 6% 4 4 4 3 5% 6% 46%	5780
February	1st to 10th	Day	13% 7% 2 6% 5% 5% 5% 4 6% 8% 39%	1890
	11th to 20th	Day	19% 6% 3 5% 3 4 4 3 5% 6% 43%	1962
	21st to end	Day	16% 8% 2 6% 5% 4 5% 3 5% 6% 40%	1730
March	1st to 10th	Day	18% 6% 2 5% 3 3 4 4 6% 9% 40%	4861
	11th to 20th	Day	11% 7% 2 4 3 3 3 3 5% 7% 52%	5090
	21st to end	Day	16% 7% 2 5% 4 4 4 4 5% 6% 43%	5758
April	1st to 10th	Day	11% 6% 2 4 4 3 4 4 6% 8% 48%	2520
	11th to 20th	Day	11% 6% 2 5% 4 4 4 4 5% 6% 50%	2446
	21st to end	Day	14% 6% 2 5% 4 3 4 4 5% 8% 45%	2400
May	1st to 10th	Day	17% 7% 3 7% 4 5% 5% 4 7% 10% 33%	5558
	11th to 20th	Day	17% 7% 2 6% 4 4 5% 4 6% 10% 35%	6194
	21st to end	Day	14% 5% 2 6% 4 4 4 4 7% 9% 41%	6934
June	1st to 10th	Day	10% 5% 2 5% 4 4 5% 4 6% 10% 45%	2800
	11th to 20th	Day	10% 5% 1 5% 4 4 5% 5% 8% 10% 45%	2800
	21st to end	Day	8% 4 2 4 5% 4 6% 5% 8% 10% 46%	2800
July	1st to 10th	Day	7% 4 2 5% 4 5% 5% 5% 8% 11% 44%	6360
	11th to 20th	Day	9% 6% 2 7% 6% 6% 6% 5% 9% 13% 31%	6184
	21st to end	Day	7% 4 2 6% 5% 5% 5% 5% 8% 10% 42%	6424
August	1st to 10th	Day	13% 8% 3 7% 4 5% 5% 5% 8% 11% 33%	4234
	11th to 20th	Day	9% 6% 2 6% 5% 6% 5% 4 6% 9% 41%	3890
	21st to end	Day	9% 3 2 4 3 3 4 3 6% 9% 53%	4312
September	1st to 10th	Day	12% 5% 2 5% 4 5% 4 4 6% 8% 46%	2400
	11th to 20th	Day	13% 6% 3 6% 4 4 4 3 5% 8% 46%	2400
	21st to end	Day	18% 8% 3 6% 3 3 3 3 6% 7% 38%	2130
October	1st to 10th	Day	15% 5% 2 6% 4 3 5% 3 5% 6% 48%	2000
	11th to 20th	Day	17% 4 2 4 3 3 3 3 4 6% 52%	2000
	21st to end	Day	12% 6% 2 5% 3 3 3 3 5% 7% 51%	2200
November	1st to 10th	Day	13% 5% 2 4 3 3 3 2 5% 6% 54%	1926
	11th to 20th	Day	13% 5% 2 5% 3 4 4 3 5% 6% 49%	1890
	21st to end	Day	14% 6% 2 6% 3 3 3 3 4 5% 51%	1748
December	1st to 10th	Day	10% 6% 3 5% 4 3 4 3 6% 6% 51%	2680
	11th to 20th	Day	17% 9% 2 6% 3 3 3 2 3 5% 48%	3070
	21st to end	Day	13% 6% 2 6% 4 3 4 3 5% 8% 46%	4048
January	1st to 10th	Night	11% 7% 2 5% 4 4 4 4 5% 7% 46%	8095
	11th to 20th	Night	14% 6% 2 6% 3 3 3 2 3 5% 53%	7680
	21st to end	Night	14% 7% 2 5% 3 4 4 3 5% 5% 47%	8401
February	1st to 10th	Night	16% 7% 3 5% 3 4 4 4 5% 6% 43%	2730
	11th to 20th	Night	19% 6% 2 5% 4 3 4 3 4 6% 42%	2541
	21st to end	Night	16% 8% 2 5% 4 3 3 3 5% 6% 45%	2076
March	1st to 10th	Night	16% 6% 3 5% 4 3 3 3 4 6% 47%	5194
	11th to 20th	Night	12% 6% 3 5% 3 3 4 4 5% 7% 50%	5090
	21st to end	Night	14% 8% 2 6% 4 4 3 3 4 6% 46%	5414
April	1st to 10th	Night	15% 8% 2 6% 4 4 4 3 5% 7% 43%	2100
	11th to 20th	Night	11% 6% 2 5% 4 4 4 3 5% 8% 48%	2036
	21st to end	Night	17% 5% 2 5% 4 4 4 4 5% 7% 45%	2000
May	1st to 10th	Night	16% 8% 3 8% 5% 5% 5% 4 6% 8% 33%	4366
	11th to 20th	Night	20% 7% 2 7% 4 4 5% 4 5% 7% 33%	3695
	21st to end	Night	16% 7% 3 7% 3 5% 4 4 6% 8% 38%	3920
June	1st to 10th	Night	11% 5% 1 5% 5% 5% 5% 5% 6% 11% 40%	1600
	11th to 20th	Night	12% 6% 2 6% 4 5% 5% 4 7% 9% 40%	1600
	21st to end	Night	8% 4 2 5% 6% 5% 5% 4 8% 9% 44%	1600
July	1st to 10th	Night	9% 4 2 7% 5% 5% 5% 5% 8% 10% 40%	3598
	11th to 20th	Night	11% 6% 3 7% 6% 5% 5% 4 9% 12% 31%	3769
	21st to end	Night	8% 7% 3 7% 5% 5% 6% 5% 8% 9% 37%	4427
August	1st to 10th	Night	15% 8% 3 7% 5% 4 5% 3 7% 10% 33%	2915
	11th to 20th	Night	10% 7% 3 8% 5% 5% 6% 4 7% 9% 37%	3124
	21st to end	Night	9% 5% 2 5% 4 4 4 4 5% 8% 52%	3586
September	1st to 10th	Night	13% 5% 3 6% 4 5% 4 3 5% 7% 47%	2000
	11th to 20th	Night	11% 5% 2 5% 3 4 4 4 7% 8% 47%	2000
	21st to end	Night	21% 8% 3 6% 4 4 4 4 5% 6% 36%	2265
October	1st to 10th	Night	16% 6% 2 6% 3 4 4 3 4 6% 46%	2400
	11th to 20th	Night	17% 5% 2 4 4 3 4 3 4 5% 50%	2400
	21st to end	Night	15% 5% 2 5% 3 3 3 3 5% 6% 50%	2640
November	1st to 10th	Night	12% 6% 3 5% 3 3 3 3 4 6% 52%	2589
	11th to 20th	Night	15% 7% 2 5% 4 3 4 2 5% 5% 48%	2730
	21st to end	Night	12% 5% 2 5% 3 4 4 3 5% 7% 51%	2867
December	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 5% 7% 54%	4720
	11th to 20th	Night	19% 7% 3 6% 4 4 4 3 5% 7% 39%	5345
	21st to end	Night	12% 8% 2 6% 4 4 4 4 6% 8% 44%	7150

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	28% 21% 6% 21% 18% 5%	4782
	11th to 20th	Day	28% 21% 4 15% 25% 7%	5280
	21st to end	Day	31% 24% 7% 25% 12% 2	5780
February	1st to 10th	Day	17% 60% 6% 10% 6% 1	1890
	11th to 20th	Day	20% 58% 3 8% 8% 2	1971
	21st to end	Day	19% 59% 5% 8% 7% 2	1730
March	1st to 10th	Day	32% 30% 7% 12% 13% 6%	4861
	11th to 20th	Day	21% 29% 7% 19% 18% 6%	5090
	21st to end	Day	30% 29% 8% 13% 14% 5%	5758
April	1st to 10th	Day	15% 60% 5% 10% 9% 1	2520
	11th to 20th	Day	14% 61% 5% 11% 8% 1	2446
	21st to end	Day	14% 63% 5% 11% 6% 2	2400
May	1st to 10th	Day	34% 33% 12% 14% 7% 1	5590
	11th to 20th	Day	33% 33% 9% 18% 6%	6198
	21st to end	Day	28% 32% 10% 17% 12% 1	6949
June	1st to 10th	Day	15% 63% 5% 10% 6% 1	2800
	11th to 20th	Day	11% 64% 7% 10% 9%	2800
	21st to end	Day	11% 62% 7% 12% 8% 1	2800
July	1st to 10th	Day	24% 31% 18% 15% 10% 3	6290
	11th to 20th	Day	34% 32% 17% 14% 3	6109
	21st to end	Day	25% 29% 12% 17% 18%	6240
August	1st to 10th	Day	31% 38% 11% 16% 3	4221
	11th to 20th	Day	22% 39% 10% 21% 8%	3927
	21st to end	Day	15% 37% 11% 20% 16% 1	4312
September	1st to 10th	Day	15% 59% 8% 11% 6% 1	2400
	11th to 20th	Day	18% 58% 4 11% 8% 2	2400
	21st to end	Day	25% 57% 5% 10% 2 1	2130
October	1st to 10th	Day	21% 37% 5% 13% 19% 4	3260
	11th to 20th	Day	13% 36% 3 15% 24% 9%	3240
	21st to end	Day	9% 35% 6% 21% 20% 10%	3538
November	1st to 10th	Day	17% 53% 5% 12% 9% 3	1926
	11th to 20th	Day	16% 54% 4 10% 13% 3	1890
	21st to end	Day	17% 54% 3 15% 10% 1	1748
December	1st to 10th	Day	22% 36% 6% 19% 14% 3	2680
	11th to 20th	Day	29% 31% 9% 17% 11% 3	3066
	21st to end	Day	26% 29% 12% 20% 12% 1	4048
January	1st to 10th	Night	30% 21% 9% 18% 18% 4	8096
	11th to 20th	Night	27% 21% 3 21% 21% 7%	7680
	21st to end	Night	31% 23% 5% 24% 15% 3	8401
February	1st to 10th	Night	18% 60% 5% 10% 6% 1	2730
	11th to 20th	Night	20% 57% 3 9% 9% 2	2543
	21st to end	Night	17% 59% 4 11% 7% 3	2076
March	1st to 10th	Night	30% 30% 5% 15% 14% 7%	5194
	11th to 20th	Night	22% 29% 7% 18% 18% 7%	5090
	21st to end	Night	29% 29% 5% 17% 14% 5%	5414
April	1st to 10th	Night	18% 59% 4 9% 9% 1	2100
	11th to 20th	Night	15% 61% 5% 11% 8% 1	2036
	21st to end	Night	16% 63% 5% 8% 7% 1	2000
May	1st to 10th	Night	37% 32% 12% 14% 5% 1	4396
	11th to 20th	Night	37% 32% 8% 16% 6%	3696
	21st to end	Night	32% 32% 9% 17% 10% 1	3927
June	1st to 10th	Night	16% 63% 5% 8% 7% 1	1600
	11th to 20th	Night	14% 63% 6% 9% 7% 1	1600
	21st to end	Night	12% 63% 5% 11% 8% 2	1600
July	1st to 10th	Night	30% 32% 14% 12% 9% 2	3560
	11th to 20th	Night	35% 31% 20% 11% 2	3724
	21st to end	Night	31% 30% 10% 16% 13%	4297
August	1st to 10th	Night	33% 38% 10% 15% 4	2904
	11th to 20th	Night	25% 39% 11% 16% 8%	3153
	21st to end	Night	16% 37% 5% 19% 22%	3586
September	1st to 10th	Night	15% 59% 8% 12% 5% 2	2000
	11th to 20th	Night	17% 57% 4 12% 8% 2	2000
	21st to end	Night	26% 58% 5% 8% 3	2265
October	1st to 10th	Night	24% 37% 7% 13% 14% 5%	3920
	11th to 20th	Night	12% 36% 5% 15% 23% 8%	3899
	21st to end	Night	9% 35% 5% 23% 19% 9%	4252
November	1st to 10th	Night	18% 54% 4 12% 11% 2	2589
	11th to 20th	Night	18% 54% 3 9% 11% 5%	2730
	21st to end	Night	14% 54% 6% 12% 12% 2	2867
December	1st to 10th	Night	19% 35% 7% 20% 16% 2	4720
	11th to 20th	Night	33% 32% 7% 15% 9% 4	5348
	21st to end	Night	28% 28% 10% 22% 11% 1	7150

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	20% 7% 25% 37% 11% 1	242
	11th to 20th	Day	17% 10% 33% 31% 9%	330
	21st to end	Day	17% 7% 26% 36% 13% 1	363
February	1st to 10th	Day	16% 8% 23% 39% 12% 2	330
	11th to 20th	Day	24% 9% 29% 31% 7%	450
	21st to end	Day	21% 8% 27% 35% 9% 1	446
March	1st to 10th	Day	17% 7% 28% 33% 13% 2	540
	11th to 20th	Day	17% 7% 26% 37% 11% 2	540
	21st to end	Day	16% 7% 26% 36% 12% 2	594
April	1st to 10th	Day	15% 7% 26% 33% 17% 3	540
	11th to 20th	Day	10% 5% 22% 37% 21% 6%	516
	21st to end	Day	15% 5% 22% 31% 23% 4	500
May	1st to 10th	Day	16% 6% 21% 30% 22% 5%	503
	11th to 20th	Day	20% 3 15% 30% 24% 9%	527
	21st to end	Day	19% 3 10% 23% 29% 16%	573
June	1st to 10th	Day	11% 3 10% 30% 28% 18%	530
	11th to 20th	Day	12% 1 9% 30% 35% 14%	530
	21st to end	Day	9% 2 10% 25% 36% 17%	530
July	1st to 10th	Day	10% 3 6% 24% 35% 22%	525
	11th to 20th	Day	4 2 4 22% 43% 25%	524
	21st to end	Day	9% 2 6% 24% 41% 18%	576
August	1st to 10th	Day	13% 2 6% 24% 37% 18%	526
	11th to 20th	Day	10% 2 9% 22% 38% 19%	519
	21st to end	Day	10% 3 7% 26% 33% 22%	583
September	1st to 10th	Day	14% 3 10% 23% 29% 22%	530
	11th to 20th	Day	14% 3 8% 22% 33% 19%	530
	21st to end	Day	25% 4 15% 24% 24% 9%	530
October	1st to 10th	Day	18% 4 11% 28% 23% 16%	530
	11th to 20th	Day	18% 5% 17% 28% 24% 9%	530
	21st to end	Day	13% 7% 23% 29% 21% 7%	583
November	1st to 10th	Day	16% 4 24% 28% 21% 7%	466
	11th to 20th	Day	14% 9% 28% 32% 15% 2	430
	21st to end	Day	13% 9% 27% 27% 19% 5%	288
December	1st to 10th	Day	16% 6% 25% 37% 14% 1	220
	11th to 20th	Day	18% 7% 26% 31% 18%	220
	21st to end	Day	16% 5% 26% 36% 15% 2	242
January	1st to 10th	Night	18% 15% 35% 27% 5%	840
	11th to 20th	Night	18% 12% 33% 30% 8%	840
	21st to end	Night	18% 13% 35% 31% 3	924
February	1st to 10th	Night	19% 8% 36% 32% 5%	840
	11th to 20th	Night	22% 13% 37% 25% 3	823
	21st to end	Night	18% 10% 34% 33% 4 1	692
March	1st to 10th	Night	16% 8% 35% 34% 6%	840
	11th to 20th	Night	18% 9% 32% 31% 9% 1	840
	21st to end	Night	17% 7% 33% 35% 7% 1	924
April	1st to 10th	Night	19% 7% 34% 31% 9%	840
	11th to 20th	Night	14% 5% 27% 39% 14% 1	816
	21st to end	Night	19% 6% 24% 38% 13% 1	800
May	1st to 10th	Night	17% 4 26% 36% 15% 1	754
	11th to 20th	Night	18% 6% 21% 37% 17% 1	449
	21st to end	Night	15% 2 12% 30% 37% 4	433
June	1st to 10th	Night	11% 3 14% 38% 31% 4	400
	11th to 20th	Night	14% 2 10% 42% 28% 5%	400
	21st to end	Night	8% 2 12% 41% 33% 5%	400
July	1st to 10th	Night	9% 2 9% 32% 41% 8%	390
	11th to 20th	Night	7% 6% 33% 43% 9%	456
	21st to end	Night	7% 2 11% 35% 39% 6%	657
August	1st to 10th	Night	11% 2 8% 40% 32% 6%	597
	11th to 20th	Night	13% 3 10% 33% 35% 6%	741
	21st to end	Night	13% 2 9% 32% 35% 9%	880
September	1st to 10th	Night	16% 2 10% 31% 34% 8%	800
	11th to 20th	Night	13% 3 10% 30% 37% 8%	800
	21st to end	Night	24% 5% 14% 30% 25% 2	800
October	1st to 10th	Night	19% 5% 15% 28% 25% 8%	800
	11th to 20th	Night	20% 8% 24% 29% 18% 2	800
	21st to end	Night	18% 9% 25% 29% 16% 3	880
November	1st to 10th	Night	15% 8% 24% 32% 17% 3	820
	11th to 20th	Night	18% 10% 30% 29% 12% 1	840
	21st to end	Night	14% 11% 29% 34% 11% 2	840
December	1st to 10th	Night	19% 10% 33% 27% 10% 1	840
	11th to 20th	Night	17% 9% 38% 27% 8%	840
	21st to end	Night	17% 10% 32% 33% 9%	924

Page generated at 2026-04-14 02:44:29. Version ac07

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