Big Bend Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Big Bend Observatory clear sky chart from 2005-11-06 to 2024-02-20.

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	15% 9% 4 7% 6% 4 5% 5% 6% 7% 31%	78628
	Night	16% 10% 4 7% 6% 3 5% 5% 5% 7% 33%	80533
Transparency	Day	50% 6% 13% 19% 10% 1	78339
	Night	51% 6% 12% 19% 11% 1	80342
Seeing	Day	17% 10% 28% 25% 14% 5%	14042
	Night	18% 10% 28% 30% 13% 2	23505

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% 17% 7% 12% 7% 4 5% 5% 4 6% 16%	2535
	11th to 20th	Day	13% 18% 9% 14% 9% 5% 5% 5% 5% 5% 11%	2840
	21st to end	Day	26% 19% 8% 10% 6% 3 4 3 3 4 13%	3124
February	1st to 10th	Day	15% 15% 5% 12% 9% 4 5% 5% 4 4 21%	1710
	11th to 20th	Day	15% 11% 7% 10% 8% 4 5% 5% 5% 6% 23%	1686
	21st to end	Day	17% 12% 6% 11% 8% 4 6% 4 5% 6% 24%	1480
March	1st to 10th	Day	17% 10% 5% 9% 7% 5% 5% 5% 6% 6% 28%	1911
	11th to 20th	Day	14% 11% 5% 8% 6% 3 4 5% 5% 6% 31%	1980
	21st to end	Day	19% 12% 5% 8% 6% 3 4 4 4 6% 28%	2178
April	1st to 10th	Day	15% 11% 5% 7% 6% 3 5% 4 5% 8% 30%	2029
	11th to 20th	Day	18% 10% 4 6% 5% 3 4 5% 6% 7% 32%	2312
	21st to end	Day	19% 9% 4 7% 5% 3 4 5% 5% 6% 32%	2340
May	1st to 10th	Day	17% 8% 3 7% 6% 4 5% 4 6% 7% 33%	2340
	11th to 20th	Day	16% 7% 3 7% 6% 3 5% 5% 5% 8% 36%	2301
	21st to end	Day	10% 5% 2 6% 5% 3 4 5% 7% 9% 42%	2659
June	1st to 10th	Day	11% 6% 3 6% 6% 4 5% 5% 6% 9% 40%	2520
	11th to 20th	Day	9% 6% 2 5% 6% 3 5% 6% 7% 9% 40%	2700
	21st to end	Day	10% 5% 2 5% 5% 3 5% 6% 8% 9% 41%	2700
July	1st to 10th	Day	9% 4 2 4 5% 3 6% 6% 6% 9% 45%	2633
	11th to 20th	Day	9% 6% 3 5% 5% 3 5% 6% 7% 10% 41%	2408
	21st to end	Day	7% 5% 2 6% 6% 4 5% 7% 8% 10% 39%	2547
August	1st to 10th	Day	7% 4 2 6% 6% 3 6% 7% 8% 10% 40%	2327
	11th to 20th	Day	8% 5% 3 6% 6% 5% 6% 7% 8% 10% 37%	2303
	21st to end	Day	10% 7% 2 6% 5% 4 5% 5% 6% 8% 42%	2559
September	1st to 10th	Day	10% 6% 2 5% 6% 5% 6% 6% 6% 8% 40%	2075
	11th to 20th	Day	9% 5% 2 4 5% 3 4 5% 5% 11% 45%	1980
	21st to end	Day	17% 7% 2 5% 6% 3 4 5% 6% 8% 38%	1980
October	1st to 10th	Day	14% 7% 3 6% 4 3 5% 5% 6% 7% 39%	1859
	11th to 20th	Day	17% 9% 3 7% 6% 4 5% 5% 6% 6% 31%	1800
	21st to end	Day	22% 10% 3 7% 7% 3 5% 5% 5% 6% 26%	1930
November	1st to 10th	Day	13% 7% 4 7% 7% 3 6% 6% 5% 7% 35%	1672
	11th to 20th	Day	15% 9% 4 9% 6% 4 5% 6% 6% 7% 30%	1614
	21st to end	Day	19% 12% 5% 8% 6% 4 5% 5% 5% 7% 23%	1520
December	1st to 10th	Day	17% 14% 6% 9% 7% 4 5% 3 6% 7% 22%	1520
	11th to 20th	Day	18% 14% 7% 10% 7% 3 4 6% 5% 7% 20%	1910
	21st to end	Day	27% 20% 7% 10% 6% 4 4 4 4 4 11%	2646
January	1st to 10th	Night	17% 14% 6% 11% 7% 4 5% 4 5% 6% 22%	4405
	11th to 20th	Night	15% 17% 8% 12% 8% 4 5% 6% 8% 5% 12%	4120
	21st to end	Night	25% 16% 8% 9% 6% 3 4 4 4 4 18%	4532
February	1st to 10th	Night	17% 14% 5% 9% 8% 3 5% 5% 5% 5% 23%	2470
	11th to 20th	Night	16% 12% 6% 8% 6% 3 5% 5% 5% 6% 28%	2363
	21st to end	Night	20% 10% 4 8% 7% 3 5% 5% 5% 6% 26%	1776
March	1st to 10th	Night	16% 9% 4 6% 7% 4 5% 5% 5% 8% 31%	2048
	11th to 20th	Night	15% 8% 3 7% 6% 4 4 4 5% 7% 37%	1980
	21st to end	Night	18% 10% 4 7% 6% 3 5% 4 5% 5% 34%	2178
April	1st to 10th	Night	16% 10% 4 6% 5% 4 4 5% 5% 6% 36%	1937
	11th to 20th	Night	17% 6% 4 6% 6% 3 4 4 5% 9% 37%	1606
	21st to end	Night	16% 8% 3 7% 6% 4 4 5% 5% 7% 36%	1620
May	1st to 10th	Night	16% 8% 4 5% 5% 4 5% 5% 7% 8% 33%	1620
	11th to 20th	Night	13% 6% 3 6% 6% 4 6% 4 6% 8% 38%	1592
	21st to end	Night	9% 5% 2 4 4 4 4 5% 5% 8% 49%	1612
June	1st to 10th	Night	11% 4 3 5% 4 3 4 4 6% 10% 46%	1435
	11th to 20th	Night	10% 5% 2 4 5% 3 6% 5% 7% 10% 44%	1260
	21st to end	Night	9% 5% 2 4 5% 3 5% 5% 5% 9% 49%	1260
July	1st to 10th	Night	6% 4 2 4 6% 3 4 3 4 8% 56%	1322
	11th to 20th	Night	8% 4 2 4 4 2 4 5% 6% 10% 50%	1548
	21st to end	Night	7% 4 2 5% 5% 3 5% 6% 7% 9% 48%	1771
August	1st to 10th	Night	7% 3 2 4 6% 3 5% 6% 7% 9% 49%	1611
	11th to 20th	Night	6% 4 3 5% 5% 4 4 6% 7% 8% 50%	1598
	21st to end	Night	9% 4 2 4 5% 3 5% 4 7% 8% 50%	1782
September	1st to 10th	Night	8% 7% 2 6% 6% 3 5% 5% 5% 7% 47%	1877
	11th to 20th	Night	9% 5% 2 4 5% 2 4 3 5% 6% 54%	1980
	21st to end	Night	14% 7% 2 6% 5% 3 4 4 5% 7% 43%	1980
October	1st to 10th	Night	12% 6% 3 6% 5% 3 4 4 5% 7% 44%	2101
	11th to 20th	Night	17% 9% 3 7% 5% 3 5% 4 5% 6% 37%	2160
	21st to end	Night	21% 9% 4 7% 6% 4 4 5% 5% 6% 29%	2424
November	1st to 10th	Night	14% 6% 3 5% 5% 3 4 5% 6% 7% 41%	2406
	11th to 20th	Night	16% 9% 4 7% 6% 4 5% 6% 6% 6% 33%	2565
	21st to end	Night	21% 11% 4 7% 6% 3 4 4 5% 7% 29%	2660
December	1st to 10th	Night	18% 15% 6% 8% 7% 3 4 5% 5% 7% 22%	2660
	11th to 20th	Night	16% 16% 5% 9% 6% 4 5% 5% 5% 5% 25%	3285
	21st to end	Night	26% 18% 7% 10% 6% 4 5% 4 3 4 14%	4989

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	69% 7% 11% 10% 3	2536
	11th to 20th	Day	76% 5% 11% 6% 3	2840
	21st to end	Day	77% 3 10% 7% 3	3124
February	1st to 10th	Day	65% 6% 11% 13% 5% 1	1710
	11th to 20th	Day	61% 5% 11% 15% 7% 1	1695
	21st to end	Day	62% 5% 11% 15% 6% 1	1480
March	1st to 10th	Day	57% 5% 13% 17% 8% 1	1911
	11th to 20th	Day	54% 6% 11% 18% 11% 1	1980
	21st to end	Day	57% 5% 10% 18% 9% 2	2178
April	1st to 10th	Day	52% 7% 12% 19% 10%	2029
	11th to 20th	Day	50% 6% 11% 19% 13% 1	2312
	21st to end	Day	52% 4 13% 17% 11% 2	2340
May	1st to 10th	Day	49% 6% 10% 22% 10% 2	2340
	11th to 20th	Day	47% 6% 13% 18% 13% 2	2305
	21st to end	Day	37% 7% 15% 23% 15% 2	2659
June	1st to 10th	Day	39% 6% 14% 26% 13% 2	2520
	11th to 20th	Day	37% 8% 15% 22% 16% 2	2700
	21st to end	Day	35% 8% 14% 25% 16% 1	2700
July	1st to 10th	Day	34% 7% 11% 27% 18% 3	2562
	11th to 20th	Day	35% 7% 15% 25% 16% 2	2335
	21st to end	Day	34% 10% 16% 23% 15% 2	2405
August	1st to 10th	Day	34% 9% 18% 26% 12% 1	2314
	11th to 20th	Day	38% 9% 16% 25% 11% 1	2303
	21st to end	Day	38% 7% 13% 25% 15% 2	2559
September	1st to 10th	Day	40% 8% 15% 23% 13% 1	2075
	11th to 20th	Day	34% 6% 14% 32% 14%	1980
	21st to end	Day	43% 7% 14% 25% 11% 1	1980
October	1st to 10th	Day	43% 5% 14% 24% 14% 1	1859
	11th to 20th	Day	51% 6% 16% 19% 8% 1	1800
	21st to end	Day	58% 5% 14% 16% 7%	1930
November	1st to 10th	Day	47% 7% 13% 23% 9% 2	1672
	11th to 20th	Day	51% 6% 18% 15% 9% 1	1614
	21st to end	Day	61% 6% 15% 15% 3	1520
December	1st to 10th	Day	61% 6% 15% 14% 3	1520
	11th to 20th	Day	64% 7% 14% 12% 3	1906
	21st to end	Day	79% 4 11% 5% 1	2646
January	1st to 10th	Night	64% 5% 13% 13% 4	4405
	11th to 20th	Night	69% 7% 13% 7% 3	4120
	21st to end	Night	70% 5% 10% 12% 2	4532
February	1st to 10th	Night	62% 6% 11% 15% 6% 1	2470
	11th to 20th	Night	57% 5% 11% 17% 9% 1	2365
	21st to end	Night	58% 4 12% 18% 7% 1	1776
March	1st to 10th	Night	52% 5% 12% 22% 9% 1	2048
	11th to 20th	Night	48% 4 11% 22% 13% 2	1980
	21st to end	Night	52% 5% 8% 22% 11% 2	2178
April	1st to 10th	Night	49% 5% 10% 20% 14% 2	1937
	11th to 20th	Night	45% 5% 11% 21% 17% 2	1606
	21st to end	Night	46% 5% 11% 19% 16% 3	1620
May	1st to 10th	Night	47% 7% 10% 19% 12% 5%	1620
	11th to 20th	Night	44% 6% 9% 21% 16% 3	1593
	21st to end	Night	33% 7% 13% 26% 18% 3	1613
June	1st to 10th	Night	34% 5% 13% 28% 17% 3	1435
	11th to 20th	Night	34% 6% 14% 23% 20% 2	1260
	21st to end	Night	32% 6% 13% 27% 19% 2	1260
July	1st to 10th	Night	29% 4 9% 30% 22% 5%	1284
	11th to 20th	Night	31% 5% 13% 30% 18% 2	1498
	21st to end	Night	30% 9% 16% 23% 19% 3	1672
August	1st to 10th	Night	30% 7% 16% 31% 15% 1	1600
	11th to 20th	Night	30% 8% 15% 31% 14% 2	1598
	21st to end	Night	30% 7% 12% 31% 18% 2	1782
September	1st to 10th	Night	36% 7% 13% 26% 15% 3	1877
	11th to 20th	Night	31% 5% 13% 31% 19% 1	1980
	21st to end	Night	42% 4 12% 27% 13% 1	1980
October	1st to 10th	Night	38% 7% 12% 24% 18% 1	2101
	11th to 20th	Night	48% 4 13% 21% 13% 1	2160
	21st to end	Night	55% 5% 12% 17% 10% 1	2424
November	1st to 10th	Night	41% 6% 13% 26% 14% 1	2406
	11th to 20th	Night	50% 5% 14% 20% 9% 1	2565
	21st to end	Night	56% 5% 17% 17% 5% 1	2660
December	1st to 10th	Night	62% 5% 15% 14% 4	2660
	11th to 20th	Night	60% 5% 14% 15% 5%	3288
	21st to end	Night	75% 5% 11% 7% 2	4989

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	22% 20% 33% 17% 7% 1	266
	11th to 20th	Day	25% 15% 41% 13% 4 2	270
	21st to end	Day	28% 16% 29% 23% 4 1	297
February	1st to 10th	Day	19% 21% 41% 16% 4	270
	11th to 20th	Day	23% 20% 39% 14% 3	267
	21st to end	Day	27% 21% 38% 13% 1	198
March	1st to 10th	Day	22% 17% 36% 19% 5% 1	351
	11th to 20th	Day	21% 13% 34% 25% 7%	420
	21st to end	Day	27% 14% 31% 22% 6%	462
April	1st to 10th	Day	18% 13% 36% 24% 8% 1	420
	11th to 20th	Day	26% 13% 27% 20% 9% 4	412
	21st to end	Day	23% 12% 30% 24% 8% 2	420
May	1st to 10th	Day	22% 13% 27% 25% 9% 3	423
	11th to 20th	Day	17% 8% 29% 26% 15% 4	447
	21st to end	Day	13% 5% 24% 30% 21% 6%	485
June	1st to 10th	Day	12% 7% 26% 35% 16% 4	450
	11th to 20th	Day	12% 7% 27% 35% 16% 3	630
	21st to end	Day	12% 5% 28% 37% 14% 4	630
July	1st to 10th	Day	10% 2 21% 31% 26% 11%	621
	11th to 20th	Day	12% 5% 23% 30% 20% 10%	508
	21st to end	Day	9% 3 22% 31% 24% 11%	488
August	1st to 10th	Day	9% 4 17% 30% 24% 16%	446
	11th to 20th	Day	9% 5% 18% 32% 20% 15%	439
	21st to end	Day	12% 3 21% 27% 26% 11%	495
September	1st to 10th	Day	12% 3 22% 29% 23% 11%	450
	11th to 20th	Day	9% 3 20% 25% 31% 12%	450
	21st to end	Day	18% 8% 23% 27% 16% 9%	450
October	1st to 10th	Day	18% 8% 26% 24% 17% 6%	450
	11th to 20th	Day	17% 16% 33% 22% 9% 2	450
	21st to end	Day	25% 14% 36% 19% 5% 1	445
November	1st to 10th	Day	16% 18% 36% 20% 7% 3	270
	11th to 20th	Day	21% 20% 32% 18% 8% 2	224
	21st to end	Day	21% 13% 42% 19% 4	180
December	1st to 10th	Day	24% 18% 32% 17% 8% 1	180
	11th to 20th	Day	29% 17% 34% 17% 3	180
	21st to end	Day	29% 19% 31% 17% 4	198
January	1st to 10th	Night	24% 16% 35% 17% 6% 1	922
	11th to 20th	Night	29% 14% 31% 23% 4	760
	21st to end	Night	21% 12% 34% 27% 6%	836
February	1st to 10th	Night	25% 13% 35% 24% 4	760
	11th to 20th	Night	22% 15% 32% 25% 6%	742
	21st to end	Night	24% 16% 37% 21% 3	592
March	1st to 10th	Night	20% 12% 33% 29% 6%	720
	11th to 20th	Night	18% 12% 36% 25% 8% 1	720
	21st to end	Night	21% 13% 30% 27% 8% 1	792
April	1st to 10th	Night	20% 14% 32% 29% 5% 1	720
	11th to 20th	Night	20% 10% 32% 26% 11% 1	542
	21st to end	Night	20% 8% 33% 30% 9%	540
May	1st to 10th	Night	18% 6% 28% 34% 13% 1	540
	11th to 20th	Night	15% 5% 25% 39% 14% 2	531
	21st to end	Night	11% 3 22% 41% 22% 1	452
June	1st to 10th	Night	12% 5% 27% 40% 16%	360
	11th to 20th	Night	13% 3 22% 46% 16%	360
	21st to end	Night	10% 4 24% 43% 18% 1	360
July	1st to 10th	Night	7% 1 15% 44% 31% 2	409
	11th to 20th	Night	10% 2 17% 39% 28% 3	522
	21st to end	Night	8% 3 16% 43% 27% 3	591
August	1st to 10th	Night	8% 2 12% 41% 31% 5%	537
	11th to 20th	Night	6% 2 14% 38% 34% 5%	534
	21st to end	Night	11% 2 17% 38% 29% 3	594
September	1st to 10th	Night	11% 3 20% 34% 26% 6%	715
	11th to 20th	Night	11% 4 16% 35% 28% 6%	720
	21st to end	Night	16% 6% 20% 33% 18% 7%	720
October	1st to 10th	Night	16% 6% 24% 34% 17% 3	720
	11th to 20th	Night	21% 11% 28% 27% 12% 1	720
	21st to end	Night	25% 12% 30% 27% 5% 1	792
November	1st to 10th	Night	16% 12% 29% 28% 14% 1	740
	11th to 20th	Night	19% 14% 36% 22% 7% 1	760
	21st to end	Night	27% 12% 30% 23% 8%	760
December	1st to 10th	Night	25% 15% 34% 19% 6% 1	760
	11th to 20th	Night	23% 15% 37% 21% 4	760
	21st to end	Night	29% 12% 33% 22% 3	902

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