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 2024-01-27.

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% 4 2 4 4 3 4 4 5% 7% 54%	75023
	Night	9% 4 2 3 3 2 3 4 5% 7% 58%	75050
Transparency	Day	30% 5% 10% 23% 23% 10%	74748
	Night	27% 4 9% 22% 24% 13%	74845
Seeing	Day	10% 7% 23% 30% 22% 8%	13970
	Night	11% 7% 30% 36% 15% 1	24034

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	10% 6% 2 4 3 2 3 3 4 5% 58%	1707
	11th to 20th	Day	13% 4 2 3 3 2 2 3 5% 5% 56%	1710
	21st to end	Day	12% 5% 1 3 3 2 3 4 3 6% 57%	1881
February	1st to 10th	Day	13% 4 3 4 4 3 3 4 4 6% 52%	1800
	11th to 20th	Day	14% 7% 2 5% 4 3 4 3 3 5% 50%	1749
	21st to end	Day	19% 5% 2 3 3 1 2 2 3 5% 55%	1480
March	1st to 10th	Day	15% 6% 3 4 4 2 4 4 4 7% 47%	1934
	11th to 20th	Day	13% 6% 2 4 4 3 4 4 5% 6% 50%	1980
	21st to end	Day	11% 4 2 4 4 2 3 4 4 7% 54%	2178
April	1st to 10th	Day	10% 4 2 4 4 2 5% 5% 5% 7% 50%	1980
	11th to 20th	Day	13% 4 2 4 3 2 4 4 5% 6% 52%	2013
	21st to end	Day	11% 3 1 5% 5% 3 3 4 4 6% 54%	2340
May	1st to 10th	Day	10% 4 3 4 5% 3 4 4 5% 6% 51%	2340
	11th to 20th	Day	11% 6% 2 5% 5% 3 5% 5% 5% 7% 46%	2302
	21st to end	Day	11% 5% 2 6% 5% 3 5% 6% 7% 10% 42%	2531
June	1st to 10th	Day	5% 4 2 4 4 3 5% 5% 5% 7% 56%	2340
	11th to 20th	Day	5% 2 1 3 4 3 4 5% 5% 9% 58%	2340
	21st to end	Day	6% 3 2 3 5% 3 5% 4 6% 8% 56%	2340
July	1st to 10th	Day	6% 4 2 5% 6% 5% 6% 6% 6% 9% 44%	2340
	11th to 20th	Day	4 2 1 3 3 3 4 4 5% 7% 64%	2340
	21st to end	Day	3 3 1 3 4 3 6% 5% 7% 11% 55%	2546
August	1st to 10th	Day	2 1 1 3 4 3 4 4 5% 8% 65%	2328
	11th to 20th	Day	7% 3 2 4 5% 3 6% 6% 7% 8% 49%	2213
	21st to end	Day	5% 3 1 4 5% 3 5% 5% 6% 9% 55%	2309
September	1st to 10th	Day	10% 4 2 4 4 3 5% 5% 4 6% 54%	1980
	11th to 20th	Day	10% 5% 3 5% 5% 3 4 5% 5% 6% 48%	1980
	21st to end	Day	8% 3 1 4 4 3 4 4 4 6% 60%	1980
October	1st to 10th	Day	5% 3 1 3 4 3 3 4 5% 6% 62%	1980
	11th to 20th	Day	10% 4 1 2 3 2 3 3 4 5% 63%	1832
	21st to end	Day	12% 4 2 4 3 2 2 2 2 4 63%	1980
November	1st to 10th	Day	11% 4 2 3 3 2 3 3 4 6% 59%	1844
	11th to 20th	Day	6% 4 1 4 3 2 4 4 6% 6% 61%	1710
	21st to end	Day	13% 6% 2 4 3 2 3 3 3 4 55%	1710
December	1st to 10th	Day	12% 7% 2 3 4 2 3 4 4 6% 53%	1710
	11th to 20th	Day	11% 7% 2 5% 4 3 4 4 5% 4 50%	2161
	21st to end	Day	14% 5% 2 4 4 2 3 3 4 4 55%	3135
January	1st to 10th	Night	10% 5% 2 4 3 2 2 3 4 6% 60%	2458
	11th to 20th	Night	13% 4 1 3 3 2 2 3 4 6% 60%	2470
	21st to end	Night	12% 4 2 4 3 2 4 4 5% 6% 55%	2633
February	1st to 10th	Night	12% 5% 3 5% 3 2 2 3 4 6% 55%	2160
	11th to 20th	Night	13% 6% 2 4 4 2 3 3 4 6% 52%	2098
	21st to end	Night	15% 5% 2 3 3 2 3 4 4 5% 55%	1776
March	1st to 10th	Night	13% 6% 3 4 4 2 4 4 5% 7% 48%	2025
	11th to 20th	Night	12% 5% 2 3 4 2 5% 3 5% 6% 52%	1980
	21st to end	Night	8% 4 2 4 4 2 3 4 5% 7% 58%	2178
April	1st to 10th	Night	7% 4 2 4 3 2 4 4 5% 7% 57%	1980
	11th to 20th	Night	11% 4 2 4 4 2 4 4 5% 7% 54%	1893
	21st to end	Night	9% 4 2 3 3 2 2 4 5% 7% 59%	1620
May	1st to 10th	Night	8% 5% 2 4 5% 2 4 5% 5% 8% 53%	1620
	11th to 20th	Night	11% 5% 2 4 4 2 4 4 7% 10% 48%	1593
	21st to end	Night	8% 3 2 3 3 2 4 4 6% 11% 52%	1745
June	1st to 10th	Night	3 2 1 3 4 2 4 5% 5% 9% 62%	1620
	11th to 20th	Night	2 2 1 2 2 2 3 5% 5% 9% 65%	1620
	21st to end	Night	3 2 2 3 4 2 4 5% 6% 9% 62%	1620
July	1st to 10th	Night	6% 2 1 3 4 2 5% 5% 7% 12% 53%	1620
	11th to 20th	Night	2 1 1 2 1 3 4 5% 8% 72%	1620
	21st to end	Night	1 2 1 2 3 2 3 4 7% 13% 60%	1772
August	1st to 10th	Night	1 1 1 2 1 3 2 4 10% 74%	1611
	11th to 20th	Night	5% 4 2 4 4 2 3 5% 5% 8% 57%	1682
	21st to end	Night	4 3 1 3 4 3 4 5% 6% 11% 58%	2044
September	1st to 10th	Night	7% 4 1 4 4 2 4 5% 6% 8% 55%	1980
	11th to 20th	Night	11% 4 2 5% 4 2 3 4 5% 8% 52%	1980
	21st to end	Night	7% 3 1 3 3 2 3 3 5% 8% 62%	1980
October	1st to 10th	Night	5% 3 2 3 2 2 3 3 4 6% 66%	1980
	11th to 20th	Night	10% 3 1 3 3 2 3 4 4 5% 63%	2109
	21st to end	Night	13% 5% 1 3 3 1 2 2 3 4 62%	2376
November	1st to 10th	Night	9% 4 1 3 4 2 3 3 4 5% 63%	2229
	11th to 20th	Night	5% 3 2 4 3 3 4 4 4 6% 63%	2470
	21st to end	Night	14% 4 2 3 3 2 3 4 4 6% 55%	2470
December	1st to 10th	Night	10% 5% 1 3 3 2 3 4 5% 6% 58%	2470
	11th to 20th	Night	10% 5% 2 5% 4 3 3 4 5% 6% 53%	3036
	21st to end	Night	13% 6% 2 3 3 2 3 3 3 5% 57%	4532

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	29% 3 6% 20% 31% 11%	1708
	11th to 20th	Day	31% 3 5% 21% 27% 12%	1710
	21st to end	Day	30% 3 6% 17% 28% 15%	1881
February	1st to 10th	Day	33% 4 6% 19% 23% 16%	1800
	11th to 20th	Day	39% 2 7% 19% 20% 13%	1759
	21st to end	Day	35% 2 5% 15% 24% 19%	1480
March	1st to 10th	Day	39% 3 7% 16% 23% 12%	1934
	11th to 20th	Day	36% 4 10% 15% 23% 12%	1980
	21st to end	Day	32% 5% 9% 21% 22% 11%	2178
April	1st to 10th	Day	33% 5% 10% 20% 18% 13%	1980
	11th to 20th	Day	34% 4 9% 20% 20% 13%	2013
	21st to end	Day	32% 4 8% 20% 27% 9%	2340
May	1st to 10th	Day	34% 5% 9% 23% 20% 9%	2340
	11th to 20th	Day	37% 5% 13% 21% 20% 4	2306
	21st to end	Day	37% 7% 14% 25% 16% 2	2531
June	1st to 10th	Day	27% 6% 14% 30% 19% 5%	2340
	11th to 20th	Day	23% 5% 14% 31% 20% 7%	2340
	21st to end	Day	26% 5% 13% 25% 24% 7%	2340
July	1st to 10th	Day	33% 8% 16% 28% 14% 2	2276
	11th to 20th	Day	19% 5% 13% 34% 24% 6%	2271
	21st to end	Day	20% 8% 17% 35% 19% 1	2405
August	1st to 10th	Day	17% 4 14% 35% 26% 4	2314
	11th to 20th	Day	30% 7% 14% 27% 18% 3	2213
	21st to end	Day	25% 6% 13% 31% 20% 4	2309
September	1st to 10th	Day	31% 5% 10% 21% 24% 9%	1980
	11th to 20th	Day	36% 6% 11% 21% 19% 8%	1980
	21st to end	Day	25% 5% 9% 19% 29% 13%	1980
October	1st to 10th	Day	24% 4 10% 24% 22% 16%	1980
	11th to 20th	Day	24% 3 7% 17% 28% 20%	1832
	21st to end	Day	28% 2 4 19% 26% 21%	1980
November	1st to 10th	Day	28% 4 6% 23% 25% 13%	1844
	11th to 20th	Day	25% 3 8% 22% 28% 14%	1710
	21st to end	Day	35% 3 4 20% 23% 14%	1710
December	1st to 10th	Day	33% 4 6% 22% 26% 9%	1710
	11th to 20th	Day	38% 3 6% 15% 25% 13%	2159
	21st to end	Day	34% 3 4 19% 25% 14%	3135
January	1st to 10th	Night	28% 2 6% 19% 29% 17%	2458
	11th to 20th	Night	28% 2 6% 21% 28% 15%	2470
	21st to end	Night	31% 4 4 16% 26% 18%	2633
February	1st to 10th	Night	33% 3 7% 15% 23% 18%	2160
	11th to 20th	Night	35% 3 6% 15% 26% 15%	2098
	21st to end	Night	33% 4 6% 15% 21% 22%	1776
March	1st to 10th	Night	36% 4 7% 17% 21% 14%	2025
	11th to 20th	Night	34% 4 10% 16% 22% 13%	1980
	21st to end	Night	26% 5% 12% 21% 26% 10%	2178
April	1st to 10th	Night	27% 4 9% 24% 20% 16%	1980
	11th to 20th	Night	31% 4 11% 18% 21% 16%	1893
	21st to end	Night	26% 4 12% 20% 29% 8%	1620
May	1st to 10th	Night	31% 6% 10% 22% 21% 10%	1620
	11th to 20th	Night	32% 6% 16% 23% 19% 5%	1594
	21st to end	Night	27% 6% 17% 27% 19% 3	1746
June	1st to 10th	Night	20% 5% 15% 36% 18% 6%	1620
	11th to 20th	Night	16% 5% 15% 33% 24% 7%	1620
	21st to end	Night	20% 7% 11% 28% 27% 7%	1620
July	1st to 10th	Night	23% 7% 15% 36% 16% 3	1575
	11th to 20th	Night	9% 5% 13% 36% 30% 7%	1566
	21st to end	Night	14% 6% 18% 37% 23% 2	1673
August	1st to 10th	Night	8% 3 14% 38% 30% 6%	1601
	11th to 20th	Night	26% 5% 12% 31% 21% 6%	1682
	21st to end	Night	21% 5% 15% 32% 22% 5%	2044
September	1st to 10th	Night	26% 5% 10% 24% 24% 10%	1980
	11th to 20th	Night	31% 5% 13% 20% 22% 9%	1980
	21st to end	Night	22% 4 7% 24% 26% 17%	1980
October	1st to 10th	Night	20% 3 10% 27% 20% 19%	1980
	11th to 20th	Night	25% 3 8% 16% 24% 24%	2109
	21st to end	Night	28% 2 5% 17% 26% 21%	2376
November	1st to 10th	Night	27% 3 6% 21% 26% 16%	2229
	11th to 20th	Night	24% 3 6% 23% 26% 17%	2470
	21st to end	Night	32% 3 6% 17% 26% 15%	2470
December	1st to 10th	Night	28% 4 8% 22% 25% 13%	2470
	11th to 20th	Night	34% 3 5% 14% 28% 16%	3037
	21st to end	Night	32% 3 5% 17% 25% 19%	4532

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	14% 16% 38% 26% 5%	270
	11th to 20th	Day	15% 14% 35% 26% 7% 2	270
	21st to end	Day	16% 14% 39% 22% 7% 1	282
February	1st to 10th	Day	18% 15% 39% 23% 5%	240
	11th to 20th	Day	24% 14% 40% 19% 3	240
	21st to end	Day	27% 15% 39% 17% 2 1	198
March	1st to 10th	Day	16% 15% 41% 23% 4	240
	11th to 20th	Day	18% 16% 34% 25% 5% 2	240
	21st to end	Day	11% 17% 42% 24% 6%	264
April	1st to 10th	Day	15% 16% 39% 23% 6%	240
	11th to 20th	Day	17% 14% 39% 24% 6%	291
	21st to end	Day	14% 14% 40% 24% 8% 1	600
May	1st to 10th	Day	11% 11% 37% 33% 8%	603
	11th to 20th	Day	15% 8% 28% 31% 16% 4	625
	21st to end	Day	11% 5% 30% 35% 16% 3	679
June	1st to 10th	Day	4 2 17% 41% 28% 7%	630
	11th to 20th	Day	4 1 19% 47% 23% 6%	630
	21st to end	Day	5% 1 12% 38% 32% 12%	630
July	1st to 10th	Day	7% 1 6% 34% 37% 14%	621
	11th to 20th	Day	2 5% 33% 44% 16%	618
	21st to end	Day	2 1 5% 28% 48% 17%	684
August	1st to 10th	Day	2 4 33% 43% 17%	626
	11th to 20th	Day	8% 1 7% 30% 40% 14%	527
	21st to end	Day	6% 6% 26% 35% 28%	428
September	1st to 10th	Day	8% 1 4 18% 42% 27%	270
	11th to 20th	Day	5% 1 4 22% 46% 23%	270
	21st to end	Day	11% 1 5% 27% 36% 19%	270
October	1st to 10th	Day	4 2 13% 37% 35% 9%	270
	11th to 20th	Day	12% 5% 28% 30% 21% 4	270
	21st to end	Day	20% 11% 27% 22% 18% 3	297
November	1st to 10th	Day	17% 5% 31% 28% 14% 5%	270
	11th to 20th	Day	10% 11% 29% 33% 13% 3	270
	21st to end	Day	20% 13% 35% 23% 7% 1	270
December	1st to 10th	Day	16% 10% 36% 26% 11% 3	270
	11th to 20th	Day	16% 10% 41% 24% 8% 1	270
	21st to end	Day	19% 11% 35% 26% 7% 2	297
January	1st to 10th	Night	12% 9% 40% 31% 7%	950
	11th to 20th	Night	15% 11% 34% 33% 7%	950
	21st to end	Night	14% 13% 35% 30% 7%	993
February	1st to 10th	Night	14% 10% 43% 28% 5%	720
	11th to 20th	Night	16% 12% 42% 28% 3	704
	21st to end	Night	18% 13% 45% 21% 2	592
March	1st to 10th	Night	17% 13% 38% 27% 5%	585
	11th to 20th	Night	15% 9% 41% 29% 6%	540
	21st to end	Night	8% 14% 45% 29% 3	594
April	1st to 10th	Night	10% 11% 46% 29% 4	540
	11th to 20th	Night	12% 10% 44% 28% 5%	533
	21st to end	Night	8% 10% 48% 26% 7%	540
May	1st to 10th	Night	10% 10% 43% 33% 4	540
	11th to 20th	Night	11% 5% 38% 31% 14% 1	531
	21st to end	Night	8% 4 36% 37% 15% 1	584
June	1st to 10th	Night	3 2 20% 51% 21% 2	540
	11th to 20th	Night	3 2 28% 52% 16%	540
	21st to end	Night	4 1 17% 48% 27% 3	540
July	1st to 10th	Night	5% 1 10% 46% 33% 4	525
	11th to 20th	Night	3 10% 51% 34% 2	522
	21st to end	Night	1 1 5% 52% 39% 2	591
August	1st to 10th	Night	1 6% 51% 38% 4	537
	11th to 20th	Night	6% 1 7% 44% 39% 3	534
	21st to end	Night	5% 1 4 42% 43% 6%	594
September	1st to 10th	Night	8% 1 8% 43% 37% 4	540
	11th to 20th	Night	12% 1 9% 41% 35% 3	540
	21st to end	Night	9% 1 14% 40% 34% 3	540
October	1st to 10th	Night	6% 2 18% 45% 26% 2	540
	11th to 20th	Night	12% 4 27% 41% 15% 1	669
	21st to end	Night	15% 9% 28% 33% 14% 2	792
November	1st to 10th	Night	11% 4 29% 40% 15% 1	749
	11th to 20th	Night	6% 8% 35% 36% 14% 1	950
	21st to end	Night	15% 9% 38% 27% 10% 1	950
December	1st to 10th	Night	12% 9% 35% 37% 7%	950
	11th to 20th	Night	16% 9% 34% 33% 7%	950
	21st to end	Night	15% 10% 38% 29% 8%	1045

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