Tarleton Observatory
Clear Sky Chart
History

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

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	9% 5% 2 5% 3 3 4 3 5% 7% 55%	132428
	Night	9% 5% 2 4 3 3 3 3 5% 7% 57%	131078
Transparency	Day	30% 4 9% 23% 24% 10%	136184
	Night	28% 3 8% 22% 25% 14%	135386
Seeing	Day	11% 7% 24% 30% 21% 7%	17124
	Night	11% 7% 30% 36% 15% 1	27080

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	9% 6% 2 5% 3 3 3 2 4 5% 59%	5289
	11th to 20th	Day	7% 4 1 3 2 3 2 2 4 5% 65%	5310
	21st to end	Day	20% 7% 2 5% 3 3 3 3 3 5% 47%	5969
February	1st to 10th	Day	12% 5% 2 5% 3 3 3 3 5% 6% 52%	2100
	11th to 20th	Day	13% 8% 2 6% 4 4 3 2 3 5% 50%	2049
	21st to end	Day	17% 6% 2 4 2 2 2 1 3 5% 58%	1730
March	1st to 10th	Day	11% 6% 2 4 3 4 3 3 5% 6% 54%	4948
	11th to 20th	Day	11% 4 1 3 2 3 3 3 4 6% 58%	5090
	21st to end	Day	8% 4 2 5% 4 3 4 4 6% 8% 53%	5583
April	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 5% 7% 49%	2310
	11th to 20th	Day	13% 6% 2 5% 3 3 4 3 5% 5% 50%	2354
	21st to end	Day	12% 4 1 6% 4 4 3 4 5% 7% 52%	2730
May	1st to 10th	Day	14% 6% 3 6% 4 3 3 3 5% 6% 46%	5992
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 4 6% 48%	5992
	21st to end	Day	11% 6% 3 7% 4 4 5% 4 6% 8% 42%	6577
June	1st to 10th	Day	5% 4 1 5% 4 4 4 4 6% 8% 55%	2601
	11th to 20th	Day	5% 3 1 3 4 4 4 4 6% 9% 57%	2600
	21st to end	Day	5% 4 1 3 4 4 4 4 5% 8% 58%	2600
July	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 4 5% 7% 43%	5900
	11th to 20th	Day	3 2 1 4 3 3 4 3 6% 10% 61%	5900
	21st to end	Day	3 3 1 4 3 4 5% 4 7% 10% 57%	6422
August	1st to 10th	Day	1 1 1 3 3 3 3 3 5% 7% 71%	4234
	11th to 20th	Day	4 2 1 3 2 3 4 3 5% 9% 63%	4008
	21st to end	Day	3 2 1 4 4 4 5% 4 6% 9% 58%	4185
September	1st to 10th	Day	10% 4 2 5% 3 5% 5% 3 4 6% 54%	2200
	11th to 20th	Day	9% 5% 3 6% 4 4 4 4 5% 6% 50%	2200
	21st to end	Day	7% 3 1 4 4 4 4 3 4 7% 60%	2200
October	1st to 10th	Day	5% 3 1 4 2 3 3 3 5% 5% 65%	2200
	11th to 20th	Day	9% 4 1 3 2 2 3 3 4 5% 64%	2035
	21st to end	Day	11% 4 2 5% 2 2 2 2 3 4 64%	2200
November	1st to 10th	Day	11% 5% 2 4 2 2 3 3 4 5% 60%	2042
	11th to 20th	Day	6% 4 1 4 2 3 4 3 6% 5% 63%	1890
	21st to end	Day	12% 6% 2 5% 3 3 3 3 4 5% 54%	1890
December	1st to 10th	Day	15% 7% 2 4 3 3 3 3 4 5% 50%	3030
	11th to 20th	Day	10% 7% 3 7% 4 4 5% 3 5% 4 49%	3481
	21st to end	Day	13% 6% 2 4 3 3 3 2 3 4 57%	4587
January	1st to 10th	Night	11% 5% 2 4 2 3 3 2 4 5% 59%	7634
	11th to 20th	Night	6% 3 1 3 2 2 3 2 4 4 71%	7680
	21st to end	Night	20% 7% 2 5% 3 3 4 3 4 5% 44%	8300
February	1st to 10th	Night	11% 6% 2 5% 3 3 2 3 4 6% 55%	2520
	11th to 20th	Night	13% 7% 2 5% 3 3 3 2 4 6% 52%	2458
	21st to end	Night	14% 6% 2 3 2 2 2 3 4 5% 57%	2076
March	1st to 10th	Night	9% 8% 3 6% 3 3 3 3 4 6% 53%	5128
	11th to 20th	Night	10% 4 2 4 3 3 3 3 5% 7% 57%	5090
	21st to end	Night	9% 5% 2 5% 3 3 3 3 5% 6% 55%	5588
April	1st to 10th	Night	8% 5% 2 4 3 4 4 3 6% 6% 56%	2310
	11th to 20th	Night	11% 5% 2 5% 3 4 4 3 5% 7% 52%	2210
	21st to end	Night	10% 4 2 4 3 2 3 3 6% 6% 56%	1890
May	1st to 10th	Night	15% 7% 2 6% 4 3 3 3 5% 6% 47%	4143
	11th to 20th	Night	8% 6% 2 7% 4 4 4 3 6% 7% 49%	4143
	21st to end	Night	10% 6% 2 5% 4 3 4 3 6% 9% 49%	4542
June	1st to 10th	Night	3 3 1 4 3 3 4 4 6% 9% 60%	1800
	11th to 20th	Night	2 2 1 3 2 3 3 4 5% 9% 65%	1800
	21st to end	Night	3 3 1 3 4 3 3 3 6% 9% 63%	1800
July	1st to 10th	Night	8% 5% 2 5% 4 4 4 4 5% 10% 49%	4078
	11th to 20th	Night	2 2 1 2 2 3 3 3 5% 10% 68%	4077
	21st to end	Night	2 3 1 4 3 3 4 3 6% 10% 61%	4449
August	1st to 10th	Night	1 1 2 1 1 2 2 3 8% 79%	2926
	11th to 20th	Night	3 2 1 2 2 2 3 3 5% 10% 66%	3045
	21st to end	Night	3 2 1 3 3 3 4 4 6% 11% 61%	3717
September	1st to 10th	Night	7% 4 2 5% 3 4 4 4 6% 8% 55%	2200
	11th to 20th	Night	10% 5% 2 5% 3 3 3 3 5% 7% 53%	2200
	21st to end	Night	7% 3 1 3 2 3 3 3 5% 7% 62%	2200
October	1st to 10th	Night	5% 3 1 3 2 2 3 3 4 6% 68%	2200
	11th to 20th	Night	8% 3 1 3 2 2 3 3 4 5% 66%	2344
	21st to end	Night	12% 4 1 3 3 2 2 2 3 4 63%	2640
November	1st to 10th	Night	9% 5% 1 4 3 3 3 2 4 5% 63%	2471
	11th to 20th	Night	5% 4 1 4 3 3 4 3 4 6% 64%	2730
	21st to end	Night	14% 5% 2 4 3 3 3 3 4 6% 55%	2730
December	1st to 10th	Night	14% 6% 2 5% 3 3 3 2 4 5% 54%	4380
	11th to 20th	Night	11% 7% 2 6% 3 3 3 3 4 5% 52%	4946
	21st to end	Night	11% 7% 2 4 2 2 3 2 3 4 59%	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	30% 2 6% 24% 29% 9%	5290
	11th to 20th	Day	24% 2 4 23% 35% 12%	5310
	21st to end	Day	43% 2 3 17% 23% 13%	5969
February	1st to 10th	Day	34% 4 5% 18% 24% 16%	2100
	11th to 20th	Day	39% 2 7% 20% 20% 12%	2059
	21st to end	Day	33% 2 4 15% 27% 18%	1730
March	1st to 10th	Day	32% 3 6% 14% 23% 22%	4948
	11th to 20th	Day	30% 3 10% 13% 18% 27%	5090
	21st to end	Day	31% 4 10% 23% 22% 11%	5585
April	1st to 10th	Day	34% 5% 9% 19% 19% 13%	2310
	11th to 20th	Day	36% 4 8% 19% 20% 12%	2354
	21st to end	Day	35% 4 9% 19% 25% 8%	2730
May	1st to 10th	Day	39% 4 10% 21% 21% 6%	6030
	11th to 20th	Day	38% 4 7% 17% 27% 7%	5996
	21st to end	Day	40% 5% 11% 25% 14% 5%	6590
June	1st to 10th	Day	27% 6% 14% 30% 19% 4	2601
	11th to 20th	Day	23% 5% 13% 30% 21% 7%	2600
	21st to end	Day	24% 5% 14% 26% 24% 7%	2600
July	1st to 10th	Day	40% 6% 13% 31% 11% 1	5836
	11th to 20th	Day	19% 5% 17% 34% 22% 3	5831
	21st to end	Day	21% 6% 15% 33% 23% 1	6240
August	1st to 10th	Day	13% 4 10% 32% 37% 4	4220
	11th to 20th	Day	18% 4 14% 36% 26% 2	4047
	21st to end	Day	22% 7% 17% 33% 19% 3	4184
September	1st to 10th	Day	32% 5% 9% 20% 24% 10%	2200
	11th to 20th	Day	34% 5% 10% 22% 21% 8%	2200
	21st to end	Day	25% 4 9% 20% 28% 14%	2200
October	1st to 10th	Day	13% 2 6% 15% 34% 30%	3590
	11th to 20th	Day	16% 3 7% 20% 33% 21%	3288
	21st to end	Day	21% 2 4 24% 34% 15%	3538
November	1st to 10th	Day	28% 4 6% 21% 27% 13%	2042
	11th to 20th	Day	24% 3 8% 22% 28% 16%	1890
	21st to end	Day	35% 3 4 21% 24% 13%	1890
December	1st to 10th	Day	38% 3 5% 21% 25% 9%	3030
	11th to 20th	Day	39% 3 6% 12% 28% 13%	3479
	21st to end	Day	33% 2 4 20% 26% 15%	4587
January	1st to 10th	Night	30% 2 5% 20% 33% 10%	7635
	11th to 20th	Night	20% 1 3 20% 35% 21%	7680
	21st to end	Night	44% 2 4 13% 23% 13%	8300
February	1st to 10th	Night	32% 3 7% 15% 25% 18%	2520
	11th to 20th	Night	35% 3 6% 16% 26% 13%	2458
	21st to end	Night	31% 3 5% 15% 24% 21%	2076
March	1st to 10th	Night	35% 3 5% 16% 22% 20%	5128
	11th to 20th	Night	28% 3 9% 17% 16% 27%	5090
	21st to end	Night	31% 4 8% 19% 28% 12%	5588
April	1st to 10th	Night	29% 4 8% 23% 19% 16%	2310
	11th to 20th	Night	33% 4 11% 18% 21% 14%	2210
	21st to end	Night	29% 4 12% 20% 27% 8%	1890
May	1st to 10th	Night	40% 5% 7% 25% 16% 8%	4170
	11th to 20th	Night	36% 4 11% 18% 21% 11%	4144
	21st to end	Night	33% 5% 14% 28% 12% 8%	4551
June	1st to 10th	Night	21% 5% 16% 35% 19% 5%	1800
	11th to 20th	Night	16% 5% 16% 32% 24% 8%	1800
	21st to end	Night	19% 6% 11% 30% 27% 7%	1800
July	1st to 10th	Night	32% 5% 14% 36% 12% 2	4035
	11th to 20th	Night	13% 4 16% 35% 28% 4	4026
	21st to end	Night	20% 6% 15% 33% 25% 2	4320
August	1st to 10th	Night	7% 2 10% 34% 40% 7%	2916
	11th to 20th	Night	15% 3 15% 40% 23% 4	3072
	21st to end	Night	17% 6% 21% 34% 19% 3	3717
September	1st to 10th	Night	28% 5% 9% 23% 24% 11%	2200
	11th to 20th	Night	30% 5% 12% 21% 23% 9%	2200
	21st to end	Night	22% 4 7% 24% 25% 18%	2200
October	1st to 10th	Night	12% 2 6% 16% 28% 37%	3590
	11th to 20th	Night	15% 2 8% 18% 32% 25%	3817
	21st to end	Night	21% 2 4 22% 37% 15%	4252
November	1st to 10th	Night	27% 3 6% 21% 27% 17%	2471
	11th to 20th	Night	23% 3 7% 23% 27% 18%	2730
	21st to end	Night	32% 3 6% 18% 26% 15%	2730
December	1st to 10th	Night	34% 3 5% 21% 26% 12%	4380
	11th to 20th	Night	36% 3 5% 12% 28% 16%	4947
	21st to end	Night	30% 3 5% 19% 25% 17%	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	15% 15% 39% 25% 5%	330
	11th to 20th	Day	14% 15% 36% 26% 7% 2	330
	21st to end	Day	18% 15% 38% 21% 7% 1	363
February	1st to 10th	Day	17% 16% 38% 25% 5%	330
	11th to 20th	Day	21% 16% 41% 18% 3	330
	21st to end	Day	23% 14% 40% 20% 3	273
March	1st to 10th	Day	16% 15% 40% 25% 3	330
	11th to 20th	Day	17% 16% 37% 24% 4 2	330
	21st to end	Day	10% 15% 41% 27% 6%	363
April	1st to 10th	Day	16% 17% 38% 22% 7%	330
	11th to 20th	Day	18% 15% 40% 22% 4	392
	21st to end	Day	15% 14% 39% 24% 7% 1	750
May	1st to 10th	Day	13% 13% 37% 30% 7%	753
	11th to 20th	Day	15% 10% 29% 29% 14% 3	775
	21st to end	Day	11% 6% 30% 35% 15% 3	844
June	1st to 10th	Day	5% 2 20% 41% 25% 7%	731
	11th to 20th	Day	5% 1 19% 47% 22% 5%	730
	21st to end	Day	4 1 12% 38% 33% 11%	730
July	1st to 10th	Day	8% 2 8% 34% 36% 13%	721
	11th to 20th	Day	3 6% 35% 42% 14%	718
	21st to end	Day	2 1 7% 30% 45% 16%	794
August	1st to 10th	Day	2 4 34% 44% 17%	726
	11th to 20th	Day	7% 1 7% 31% 41% 14%	617
	21st to end	Day	6% 6% 28% 36% 25%	508
September	1st to 10th	Day	9% 2 8% 22% 38% 22%	330
	11th to 20th	Day	5% 1 5% 24% 46% 19%	330
	21st to end	Day	11% 1 7% 28% 37% 16%	330
October	1st to 10th	Day	3 2 11% 35% 37% 12%	330
	11th to 20th	Day	10% 5% 27% 32% 22% 4	330
	21st to end	Day	18% 10% 26% 26% 17% 2	363
November	1st to 10th	Day	19% 6% 28% 28% 15% 4	330
	11th to 20th	Day	10% 11% 30% 33% 13% 3	330
	21st to end	Day	18% 14% 37% 22% 8% 2	330
December	1st to 10th	Day	16% 11% 35% 25% 10% 2	330
	11th to 20th	Day	16% 10% 40% 25% 8% 1	330
	21st to end	Day	17% 11% 33% 29% 8% 1	363
January	1st to 10th	Night	12% 10% 40% 31% 7%	1050
	11th to 20th	Night	14% 11% 35% 33% 6%	1050
	21st to end	Night	16% 14% 35% 29% 6%	1118
February	1st to 10th	Night	13% 10% 43% 30% 4	840
	11th to 20th	Night	16% 14% 41% 27% 3	824
	21st to end	Night	17% 13% 45% 22% 2	692
March	1st to 10th	Night	17% 14% 39% 26% 4	681
	11th to 20th	Night	15% 11% 42% 28% 5%	630
	21st to end	Night	9% 13% 45% 31% 3	693
April	1st to 10th	Night	12% 11% 44% 29% 4	630
	11th to 20th	Night	13% 11% 44% 26% 5%	623
	21st to end	Night	10% 11% 46% 26% 7%	630
May	1st to 10th	Night	11% 11% 42% 33% 4	630
	11th to 20th	Night	11% 7% 37% 32% 13% 1	621
	21st to end	Night	8% 5% 36% 37% 14% 1	683
June	1st to 10th	Night	4 2 23% 49% 20% 2	600
	11th to 20th	Night	4 2 28% 52% 15%	600
	21st to end	Night	3 2 16% 50% 26% 3	600
July	1st to 10th	Night	7% 1 10% 48% 31% 4	585
	11th to 20th	Night	3 11% 52% 32% 2	582
	21st to end	Night	2 1 7% 52% 37% 2	657
August	1st to 10th	Night	1 5% 50% 40% 4	597
	11th to 20th	Night	6% 1 8% 46% 37% 3	594
	21st to end	Night	5% 4 43% 42% 6%	660
September	1st to 10th	Night	8% 2 8% 43% 36% 4	600
	11th to 20th	Night	11% 1 9% 43% 34% 3	600
	21st to end	Night	9% 1 14% 40% 34% 2	600
October	1st to 10th	Night	6% 2 17% 45% 28% 2	600
	11th to 20th	Night	10% 4 28% 41% 16% 1	744
	21st to end	Night	14% 9% 29% 33% 13% 1	880
November	1st to 10th	Night	12% 4 29% 40% 14% 1	831
	11th to 20th	Night	6% 8% 36% 36% 13% 1	1050
	21st to end	Night	15% 9% 38% 27% 10% 1	1050
December	1st to 10th	Night	13% 8% 36% 36% 6%	1050
	11th to 20th	Night	16% 9% 34% 33% 7%	1050
	21st to end	Night	15% 10% 37% 30% 8%	1155

Page generated at 2026-05-29 00:53:32. Version ac07

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