Round Top
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Round Top clear sky chart from 2005-11-06 to 2025-07-21.

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 4 4 6% 8% 43%	120364
	Night	13% 7% 2 5% 3 3 3 3 5% 7% 48%	118999
Transparency	Day	37% 6% 11% 20% 17% 9%	124123
	Night	35% 5% 10% 20% 18% 11%	123370
Seeing	Day	12% 7% 20% 27% 24% 9%	15796
	Night	14% 8% 25% 35% 17% 1	25666

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	17% 10% 3 6% 3 3 3 2 4 5% 42%	4077
	11th to 20th	Day	14% 5% 2 5% 4 3 3 3 5% 6% 50%	4080
	21st to end	Day	30% 8% 3 6% 3 2 2 1 3 5% 38%	4511
February	1st to 10th	Day	21% 8% 3 6% 4 3 3 3 3 5% 41%	2000
	11th to 20th	Day	21% 8% 2 6% 4 3 3 3 4 4 41%	1949
	21st to end	Day	18% 8% 3 6% 3 3 3 3 5% 6% 42%	1729
March	1st to 10th	Day	13% 6% 2 7% 4 4 5% 3 6% 7% 43%	4936
	11th to 20th	Day	13% 6% 2 6% 4 4 3 3 4 6% 48%	4980
	21st to end	Day	18% 8% 3 6% 5% 3 4 3 5% 5% 41%	5462
April	1st to 10th	Day	14% 9% 3 8% 5% 5% 5% 3 4 6% 39%	2200
	11th to 20th	Day	14% 8% 3 7% 5% 5% 5% 4 4 5% 40%	2239
	21st to end	Day	12% 8% 3 6% 4 5% 5% 3 5% 6% 43%	2413
May	1st to 10th	Day	17% 7% 3 7% 5% 4 5% 4 6% 8% 33%	5862
	11th to 20th	Day	10% 5% 2 7% 4 5% 4 4 5% 8% 44%	5862
	21st to end	Day	9% 7% 3 7% 5% 5% 6% 5% 8% 10% 35%	6434
June	1st to 10th	Day	5% 5% 2 5% 6% 5% 6% 5% 7% 9% 46%	2600
	11th to 20th	Day	6% 4 1 5% 5% 4 6% 5% 9% 10% 45%	2600
	21st to end	Day	6% 7% 2 6% 6% 6% 7% 6% 9% 10% 35%	2600
July	1st to 10th	Day	8% 4 2 5% 5% 5% 6% 4 6% 10% 44%	4250
	11th to 20th	Day	6% 6% 2 7% 6% 5% 6% 5% 9% 11% 38%	4250
	21st to end	Day	12% 7% 2 7% 6% 5% 6% 5% 8% 11% 32%	4466
August	1st to 10th	Day	1 1 1 3 3 3 4 3 6% 10% 63%	4104
	11th to 20th	Day	3 2 1 4 3 4 4 5% 9% 14% 51%	4023
	21st to end	Day	5% 3 2 6% 6% 6% 7% 6% 8% 10% 43%	3833
September	1st to 10th	Day	6% 6% 2 7% 6% 6% 7% 5% 6% 8% 42%	2090
	11th to 20th	Day	11% 6% 3 7% 5% 5% 5% 4 6% 7% 40%	2090
	21st to end	Day	5% 6% 2 6% 5% 5% 5% 4 6% 7% 49%	2090
October	1st to 10th	Day	5% 3 1 5% 4 5% 4 4 6% 8% 56%	2057
	11th to 20th	Day	12% 5% 2 5% 4 4 3 3 4 6% 53%	1900
	21st to end	Day	13% 5% 1 5% 3 3 4 3 4 5% 56%	2090
November	1st to 10th	Day	12% 7% 2 5% 4 4 5% 3 5% 7% 46%	1958
	11th to 20th	Day	12% 5% 2 5% 4 3 4 3 4 6% 53%	2000
	21st to end	Day	16% 8% 2 6% 4 4 3 3 5% 6% 43%	1810
December	1st to 10th	Day	24% 8% 3 6% 4 3 3 2 4 5% 37%	2940
	11th to 20th	Day	17% 8% 2 7% 4 3 3 2 4 6% 43%	3391
	21st to end	Day	14% 7% 2 6% 3 3 3 2 4 5% 50%	4488
January	1st to 10th	Night	18% 7% 2 5% 2 3 3 2 4 5% 47%	5888
	11th to 20th	Night	13% 5% 2 4 2 3 3 2 3 6% 56%	5900
	21st to end	Night	30% 7% 2 4 3 3 2 2 4 5% 38%	6490
February	1st to 10th	Night	19% 9% 3 7% 3 3 3 2 4 6% 43%	2405
	11th to 20th	Night	21% 8% 3 6% 4 2 2 2 3 6% 44%	2338
	21st to end	Night	22% 7% 3 4 5% 3 3 3 4 5% 41%	1900
March	1st to 10th	Night	17% 10% 2 6% 3 3 4 3 5% 7% 40%	4919
	11th to 20th	Night	17% 7% 2 6% 3 3 3 3 4 5% 47%	4980
	21st to end	Night	18% 11% 2 6% 4 3 3 3 4 5% 41%	5467
April	1st to 10th	Night	12% 7% 2 5% 4 3 4 3 6% 7% 46%	2200
	11th to 20th	Night	14% 8% 3 6% 4 4 3 3 4 7% 45%	2110
	21st to end	Night	14% 8% 2 6% 4 3 3 3 4 6% 46%	1981
May	1st to 10th	Night	21% 10% 2 7% 4 5% 4 3 5% 8% 31%	4053
	11th to 20th	Night	10% 6% 3 6% 4 4 4 4 6% 8% 46%	4053
	21st to end	Night	10% 7% 3 7% 5% 5% 5% 4 7% 9% 37%	4443
June	1st to 10th	Night	4 4 2 5% 3 3 4 3 5% 11% 55%	1800
	11th to 20th	Night	4 3 1 4 3 3 4 4 7% 10% 56%	1800
	21st to end	Night	4 4 2 5% 4 5% 6% 5% 9% 10% 47%	1800
July	1st to 10th	Night	5% 5% 1 4 4 4 4 4 7% 11% 51%	2940
	11th to 20th	Night	3 4 2 5% 4 4 5% 4 7% 11% 50%	2940
	21st to end	Night	9% 7% 3 6% 5% 5% 5% 4 6% 11% 39%	3098
August	1st to 10th	Night	1 1 2 2 2 3 3 4 10% 72%	2836
	11th to 20th	Night	2 2 1 3 2 3 5% 4 7% 12% 59%	2798
	21st to end	Night	3 3 1 5% 3 4 5% 5% 9% 12% 52%	3818
September	1st to 10th	Night	5% 5% 2 5% 4 4 5% 4 6% 9% 52%	2090
	11th to 20th	Night	9% 7% 2 7% 4 3 5% 4 6% 9% 46%	2090
	21st to end	Night	7% 4 1 5% 3 4 4 3 6% 9% 54%	2090
October	1st to 10th	Night	4 3 1 4 4 2 3 3 5% 7% 64%	2103
	11th to 20th	Night	10% 5% 1 4 3 3 3 2 5% 6% 59%	2280
	21st to end	Night	11% 6% 2 4 3 2 2 3 3 4 61%	2508
November	1st to 10th	Night	10% 6% 2 5% 4 4 4 2 5% 6% 52%	2340
	11th to 20th	Night	11% 6% 2 5% 3 4 3 3 5% 6% 53%	2400
	21st to end	Night	14% 6% 3 6% 4 3 3 3 5% 7% 47%	2585
December	1st to 10th	Night	23% 10% 3 7% 4 3 3 3 4 6% 36%	4250
	11th to 20th	Night	16% 9% 3 6% 3 3 3 2 4 4 47%	4816
	21st to end	Night	15% 6% 2 4 2 3 2 2 3 5% 55%	6490

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% 4 4 21% 17% 8%	4078
	11th to 20th	Day	36% 3 5% 19% 23% 14%	4080
	21st to end	Day	52% 2 3 9% 19% 15%	4511
February	1st to 10th	Day	47% 4 5% 13% 19% 13%	2000
	11th to 20th	Day	47% 5% 6% 16% 17% 10%	1959
	21st to end	Day	44% 4 8% 15% 16% 12%	1729
March	1st to 10th	Day	40% 5% 8% 17% 16% 15%	4936
	11th to 20th	Day	39% 4 7% 9% 17% 24%	4980
	21st to end	Day	45% 5% 7% 19% 19% 6%	5464
April	1st to 10th	Day	45% 5% 7% 17% 15% 11%	2200
	11th to 20th	Day	46% 5% 7% 14% 19% 9%	2239
	21st to end	Day	41% 5% 7% 20% 19% 7%	2413
May	1st to 10th	Day	46% 7% 11% 21% 11% 4	5900
	11th to 20th	Day	37% 6% 12% 20% 21% 4	5866
	21st to end	Day	39% 7% 13% 27% 12% 2	6447
June	1st to 10th	Day	32% 7% 15% 27% 18% 2	2600
	11th to 20th	Day	29% 9% 15% 25% 18% 4	2600
	21st to end	Day	38% 11% 15% 24% 10% 1	2600
July	1st to 10th	Day	32% 9% 23% 32% 5%	4186
	11th to 20th	Day	35% 10% 18% 25% 12%	4181
	21st to end	Day	43% 10% 19% 22% 5%	4326
August	1st to 10th	Day	15% 6% 14% 41% 24% 1	4090
	11th to 20th	Day	21% 9% 30% 30% 8% 2	4062
	21st to end	Day	32% 12% 18% 24% 13% 2	3832
September	1st to 10th	Day	36% 9% 14% 20% 15% 6%	2090
	11th to 20th	Day	42% 8% 11% 17% 18% 4	2090
	21st to end	Day	33% 7% 9% 20% 17% 13%	2090
October	1st to 10th	Day	16% 3 7% 13% 21% 39%	3421
	11th to 20th	Day	21% 3 8% 16% 34% 17%	3140
	21st to end	Day	27% 4 5% 20% 28% 15%	3428
November	1st to 10th	Day	36% 6% 9% 17% 20% 11%	1958
	11th to 20th	Day	33% 5% 10% 16% 22% 15%	2000
	21st to end	Day	43% 4 8% 16% 17% 12%	1810
December	1st to 10th	Day	50% 3 6% 16% 18% 7%	2940
	11th to 20th	Day	42% 5% 8% 12% 23% 11%	3389
	21st to end	Day	37% 4 5% 16% 26% 13%	4488
January	1st to 10th	Night	40% 3 5% 20% 20% 11%	5888
	11th to 20th	Night	32% 3 4 19% 27% 14%	5900
	21st to end	Night	51% 3 3 12% 18% 14%	6490
February	1st to 10th	Night	45% 4 6% 11% 19% 15%	2405
	11th to 20th	Night	44% 4 6% 16% 17% 13%	2338
	21st to end	Night	47% 4 6% 14% 16% 14%	1900
March	1st to 10th	Night	44% 5% 8% 15% 18% 12%	4919
	11th to 20th	Night	41% 4 6% 14% 13% 22%	4980
	21st to end	Night	46% 4 7% 15% 18% 9%	5467
April	1st to 10th	Night	38% 6% 9% 19% 17% 12%	2200
	11th to 20th	Night	40% 4 10% 15% 16% 14%	2110
	21st to end	Night	40% 5% 9% 20% 18% 8%	1981
May	1st to 10th	Night	51% 7% 8% 21% 9% 4	4080
	11th to 20th	Night	35% 6% 12% 23% 17% 6%	4054
	21st to end	Night	41% 7% 14% 29% 9% 1	4452
June	1st to 10th	Night	24% 5% 18% 32% 19% 2	1800
	11th to 20th	Night	22% 7% 17% 32% 18% 4	1800
	21st to end	Night	29% 9% 20% 28% 13% 1	1800
July	1st to 10th	Night	27% 8% 28% 31% 5%	2895
	11th to 20th	Night	27% 9% 22% 31% 12%	2886
	21st to end	Night	39% 9% 23% 23% 6%	2999
August	1st to 10th	Night	9% 5% 18% 45% 24% 1	2826
	11th to 20th	Night	16% 8% 33% 34% 9% 1	2825
	21st to end	Night	24% 12% 23% 28% 12% 1	3818
September	1st to 10th	Night	28% 9% 19% 23% 15% 7%	2090
	11th to 20th	Night	35% 7% 15% 22% 14% 6%	2090
	21st to end	Night	28% 6% 15% 20% 18% 13%	2090
October	1st to 10th	Night	13% 4 8% 16% 17% 41%	3506
	11th to 20th	Night	17% 3 7% 20% 32% 21%	3779
	21st to end	Night	22% 3 5% 23% 29% 18%	4120
November	1st to 10th	Night	34% 6% 9% 19% 19% 12%	2340
	11th to 20th	Night	33% 4 10% 18% 20% 15%	2400
	21st to end	Night	38% 4 9% 18% 18% 13%	2585
December	1st to 10th	Night	51% 4 5% 15% 17% 8%	4250
	11th to 20th	Night	42% 5% 8% 9% 21% 16%	4817
	21st to end	Night	34% 4 4 16% 29% 12%	6490

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	24% 11% 34% 24% 5% 1	300
	11th to 20th	Day	23% 15% 27% 27% 6% 2	300
	21st to end	Day	25% 15% 34% 21% 5% 1	330
February	1st to 10th	Day	30% 12% 29% 20% 9%	300
	11th to 20th	Day	29% 18% 30% 16% 7%	300
	21st to end	Day	28% 14% 39% 15% 4	249
March	1st to 10th	Day	16% 20% 37% 20% 7%	300
	11th to 20th	Day	17% 19% 40% 18% 5%	300
	21st to end	Day	20% 14% 35% 23% 8% 1	330
April	1st to 10th	Day	19% 18% 36% 20% 6% 1	300
	11th to 20th	Day	19% 18% 29% 24% 8% 1	357
	21st to end	Day	16% 13% 34% 26% 10% 1	513
May	1st to 10th	Day	14% 11% 39% 26% 11%	703
	11th to 20th	Day	15% 7% 28% 33% 14% 3	725
	21st to end	Day	11% 6% 27% 35% 19% 2	789
June	1st to 10th	Day	5% 2 15% 39% 32% 7%	730
	11th to 20th	Day	7% 2 14% 39% 32% 7%	730
	21st to end	Day	7% 1 11% 31% 36% 13%	730
July	1st to 10th	Day	8% 2 7% 31% 34% 17%	721
	11th to 20th	Day	3 2 3 31% 45% 16%	718
	21st to end	Day	5% 1 4 28% 46% 16%	740
August	1st to 10th	Day	3 2 25% 49% 21%	676
	11th to 20th	Day	6% 3 22% 49% 20%	660
	21st to end	Day	6% 2 4 21% 27% 41%	335
September	1st to 10th	Day	5% 1 6% 25% 35% 27%	300
	11th to 20th	Day	6% 2 9% 19% 41% 24%	300
	21st to end	Day	8% 2 7% 21% 39% 22%	300
October	1st to 10th	Day	4 2 9% 33% 38% 14%	300
	11th to 20th	Day	10% 4 15% 37% 26% 9%	300
	21st to end	Day	18% 9% 21% 24% 20% 7%	330
November	1st to 10th	Day	16% 7% 21% 33% 18% 5%	300
	11th to 20th	Day	17% 7% 22% 31% 19% 4	300
	21st to end	Day	20% 16% 31% 19% 11% 3	300
December	1st to 10th	Day	21% 7% 29% 26% 13% 4	300
	11th to 20th	Day	21% 16% 31% 23% 8% 2	300
	21st to end	Day	19% 10% 37% 25% 7% 2	330
January	1st to 10th	Night	20% 10% 32% 30% 8%	1000
	11th to 20th	Night	21% 13% 29% 29% 7%	1000
	21st to end	Night	21% 12% 31% 30% 6%	1100
February	1st to 10th	Night	25% 13% 32% 26% 4	805
	11th to 20th	Night	26% 12% 33% 25% 4	784
	21st to end	Night	27% 13% 38% 19% 3	580
March	1st to 10th	Night	21% 13% 38% 25% 3	600
	11th to 20th	Night	18% 12% 38% 27% 5%	600
	21st to end	Night	16% 10% 42% 27% 4	660
April	1st to 10th	Night	14% 13% 42% 30% 3	600
	11th to 20th	Night	17% 12% 39% 25% 6%	593
	21st to end	Night	15% 12% 38% 28% 7% 1	600
May	1st to 10th	Night	15% 13% 40% 28% 4	600
	11th to 20th	Night	11% 8% 31% 37% 12% 1	591
	21st to end	Night	8% 3 33% 40% 15%	650
June	1st to 10th	Night	4 3 17% 51% 24% 1	600
	11th to 20th	Night	4 2 24% 51% 19% 1	600
	21st to end	Night	5% 1 11% 48% 32% 3	600
July	1st to 10th	Night	8% 1 8% 45% 33% 4	585
	11th to 20th	Night	3 2 7% 48% 39% 2	582
	21st to end	Night	3 1 7% 50% 35% 4	624
August	1st to 10th	Night	1 5% 52% 39% 2	567
	11th to 20th	Night	5% 4 49% 36% 5%	564
	21st to end	Night	5% 1 4 36% 49% 6%	627
September	1st to 10th	Night	6% 1 8% 34% 45% 6%	570
	11th to 20th	Night	10% 1 8% 36% 41% 3	570
	21st to end	Night	9% 2 13% 33% 39% 4	570
October	1st to 10th	Night	4 3 11% 44% 34% 5%	583
	11th to 20th	Night	11% 4 21% 43% 19% 2	760
	21st to end	Night	12% 8% 25% 33% 19% 2	836
November	1st to 10th	Night	11% 6% 23% 39% 19% 1	780
	11th to 20th	Night	13% 8% 24% 39% 15% 1	800
	21st to end	Night	17% 10% 30% 29% 14% 1	985
December	1st to 10th	Night	21% 10% 26% 31% 12% 1	1000
	11th to 20th	Night	22% 14% 26% 31% 7% 1	1000
	21st to end	Night	20% 10% 30% 31% 8% 1	1100

Page generated at 2025-07-18 00:09:59. Version ac07

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