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 2024-10-20.

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	14% 9% 4 8% 5% 5% 5% 4 6% 7% 33%	101410
	Night	15% 10% 4 8% 5% 4 4 4 5% 7% 34%	97193
Transparency	Day	49% 6% 13% 20% 11% 1	101212
	Night	49% 5% 13% 20% 12% 1	97062
Seeing	Day	17% 10% 28% 25% 14% 5%	15034
	Night	18% 10% 28% 30% 13% 2	24290

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% 18% 7% 14% 6% 5% 4 4 4 5% 16%	2535
	11th to 20th	Day	13% 19% 9% 15% 8% 6% 5% 4 5% 5% 11%	2840
	21st to end	Day	26% 20% 8% 11% 5% 3 4 3 4 4 13%	3124
February	1st to 10th	Day	15% 16% 5% 14% 7% 6% 4 4 4 4 21%	1710
	11th to 20th	Day	15% 13% 6% 11% 7% 5% 5% 4 5% 6% 24%	1696
	21st to end	Day	16% 13% 5% 11% 7% 5% 5% 3 5% 5% 24%	1570
March	1st to 10th	Day	20% 12% 5% 10% 5% 5% 4 3 4 5% 27%	3329
	11th to 20th	Day	12% 12% 5% 10% 7% 4 5% 3 5% 7% 31%	3480
	21st to end	Day	17% 12% 4 9% 5% 4 4 4 4 6% 31%	3815
April	1st to 10th	Day	14% 13% 4 8% 5% 5% 5% 3 5% 8% 31%	2143
	11th to 20th	Day	18% 10% 3 7% 4 4 4 4 6% 7% 32%	2442
	21st to end	Day	18% 10% 3 8% 5% 4 4 4 5% 6% 33%	2470
May	1st to 10th	Day	17% 7% 3 7% 5% 5% 4 3 6% 7% 34%	4082
	11th to 20th	Day	16% 9% 3 7% 5% 5% 4 4 5% 9% 33%	4081
	21st to end	Day	10% 5% 2 6% 4 4 4 4 6% 8% 46%	4715
June	1st to 10th	Day	10% 6% 3 7% 6% 5% 5% 4 6% 9% 39%	2660
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 9% 40%	2850
	21st to end	Day	10% 6% 2 6% 5% 4 4 5% 8% 9% 40%	2850
July	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 5% 7% 9% 40%	4637
	11th to 20th	Day	8% 5% 2 5% 4 5% 4 5% 8% 10% 42%	4230
	21st to end	Day	7% 5% 2 7% 6% 6% 5% 5% 7% 9% 41%	4466
August	1st to 10th	Day	13% 5% 2 6% 4 4 5% 5% 8% 9% 38%	4104
	11th to 20th	Day	8% 6% 2 7% 5% 6% 6% 5% 9% 10% 36%	4044
	21st to end	Day	8% 6% 2 7% 5% 5% 5% 4 6% 9% 42%	4503
September	1st to 10th	Day	10% 6% 2 6% 5% 6% 6% 4 6% 8% 42%	2190
	11th to 20th	Day	9% 6% 2 5% 4 4 4 4 5% 10% 46%	2090
	21st to end	Day	17% 7% 2 6% 5% 4 5% 4 6% 7% 36%	2090
October	1st to 10th	Day	13% 7% 3 7% 4 4 4 5% 7% 7% 39%	1962
	11th to 20th	Day	16% 9% 3 8% 5% 5% 5% 4 6% 6% 32%	1890
	21st to end	Day	21% 11% 3 9% 6% 5% 5% 3 5% 6% 26%	1930
November	1st to 10th	Day	12% 8% 3 8% 7% 5% 5% 5% 5% 7% 35%	1672
	11th to 20th	Day	15% 10% 4 9% 5% 5% 5% 4 6% 7% 30%	1614
	21st to end	Day	19% 13% 5% 9% 5% 5% 4 4 6% 6% 23%	1520
December	1st to 10th	Day	16% 15% 5% 10% 6% 5% 5% 3 6% 6% 22%	1520
	11th to 20th	Day	18% 15% 7% 11% 6% 4 5% 4 5% 7% 20%	1910
	21st to end	Day	27% 20% 7% 11% 5% 5% 4 3 4 4 11%	2646
January	1st to 10th	Night	17% 15% 5% 12% 6% 5% 5% 3 5% 6% 21%	4405
	11th to 20th	Night	14% 17% 8% 14% 6% 5% 5% 4 8% 5% 12%	4120
	21st to end	Night	24% 17% 7% 10% 5% 4 4 3 4 4 18%	4532
February	1st to 10th	Night	17% 15% 4 10% 7% 5% 5% 4 6% 5% 23%	2470
	11th to 20th	Night	16% 13% 5% 8% 6% 5% 5% 4 5% 6% 27%	2368
	21st to end	Night	19% 11% 4 9% 6% 5% 5% 5% 5% 6% 26%	1884
March	1st to 10th	Night	18% 13% 4 7% 5% 4 4 3 4 6% 32%	3523
	11th to 20th	Night	10% 8% 3 7% 6% 5% 5% 3 6% 8% 38%	3480
	21st to end	Night	15% 10% 3 7% 5% 4 4 3 5% 4 38%	3817
April	1st to 10th	Night	16% 12% 4 7% 4 5% 3 4 5% 6% 35%	2044
	11th to 20th	Night	17% 7% 3 7% 5% 4 3 3 6% 8% 36%	1696
	21st to end	Night	15% 8% 3 8% 5% 4 5% 4 5% 7% 36%	1710
May	1st to 10th	Night	18% 9% 3 5% 4 4 5% 3 6% 8% 36%	2813
	11th to 20th	Night	11% 9% 4 8% 6% 7% 6% 4 7% 7% 33%	2812
	21st to end	Night	10% 5% 2 5% 4 4 4 4 5% 8% 50%	2836
June	1st to 10th	Night	11% 5% 3 6% 4 4 4 3 6% 10% 44%	1514
	11th to 20th	Night	10% 5% 2 5% 4 4 6% 5% 7% 10% 43%	1330
	21st to end	Night	9% 6% 1 5% 4 3 5% 4 5% 9% 48%	1330
July	1st to 10th	Night	7% 4 1 4 5% 4 4 3 6% 11% 50%	2328
	11th to 20th	Night	8% 5% 2 5% 3 4 3 4 7% 9% 51%	2716
	21st to end	Night	5% 4 2 5% 5% 4 5% 5% 8% 9% 48%	3087
August	1st to 10th	Night	10% 6% 3 5% 4 4 4 4 6% 8% 45%	2825
	11th to 20th	Night	7% 6% 3 6% 4 4 4 3 6% 7% 50%	2785
	21st to end	Night	8% 5% 2 6% 4 5% 5% 4 6% 8% 49%	3124
September	1st to 10th	Night	8% 7% 2 7% 4 4 5% 4 5% 7% 48%	1982
	11th to 20th	Night	9% 5% 2 5% 4 3 3 3 5% 6% 55%	2090
	21st to end	Night	14% 7% 2 8% 5% 4 5% 2 5% 6% 41%	2090
October	1st to 10th	Night	12% 6% 3 6% 4 4 4 3 5% 7% 46%	2218
	11th to 20th	Night	18% 9% 3 8% 4 4 4 3 5% 6% 38%	2275
	21st to end	Night	21% 9% 3 8% 5% 5% 4 4 6% 6% 29%	2424
November	1st to 10th	Night	13% 7% 3 6% 5% 4 4 4 5% 7% 41%	2406
	11th to 20th	Night	16% 10% 3 8% 5% 5% 5% 5% 6% 6% 32%	2565
	21st to end	Night	20% 12% 4 7% 5% 4 4 3 6% 7% 28%	2660
December	1st to 10th	Night	17% 16% 5% 8% 7% 4 5% 4 5% 7% 22%	2660
	11th to 20th	Night	15% 17% 5% 10% 5% 5% 5% 3 5% 5% 25%	3285
	21st to end	Night	26% 19% 7% 11% 5% 5% 4 3 3 4 13%	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% 8% 1	1705
	21st to end	Day	61% 5% 12% 14% 5% 2	1570
March	1st to 10th	Day	60% 4 9% 22% 5%	3329
	11th to 20th	Day	53% 5% 17% 17% 7% 1	3480
	21st to end	Day	54% 5% 12% 17% 11% 1	3815
April	1st to 10th	Day	51% 6% 12% 19% 10% 1	2143
	11th to 20th	Day	49% 6% 11% 20% 13% 1	2442
	21st to end	Day	52% 4 13% 17% 11% 2	2470
May	1st to 10th	Day	47% 6% 11% 22% 11% 2	4120
	11th to 20th	Day	48% 5% 17% 21% 8% 1	4085
	21st to end	Day	36% 6% 13% 20% 19% 7%	4729
June	1st to 10th	Day	40% 6% 13% 26% 12% 2	2660
	11th to 20th	Day	37% 8% 15% 22% 17% 2	2850
	21st to end	Day	36% 8% 14% 25% 16% 1	2850
July	1st to 10th	Day	38% 7% 12% 25% 15% 3	4566
	11th to 20th	Day	32% 7% 15% 29% 15% 1	4157
	21st to end	Day	36% 8% 16% 19% 20% 2	4324
August	1st to 10th	Day	39% 7% 16% 23% 15% 1	4091
	11th to 20th	Day	38% 8% 14% 31% 8% 1	4083
	21st to end	Day	37% 6% 13% 31% 13% 1	4503
September	1st to 10th	Day	39% 8% 15% 24% 13% 1	2190
	11th to 20th	Day	32% 6% 14% 32% 15%	2090
	21st to end	Day	45% 7% 13% 24% 11% 1	2090
October	1st to 10th	Day	41% 5% 14% 24% 14% 1	1962
	11th to 20th	Day	50% 6% 15% 20% 8% 1	1890
	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% 18% 9% 1	2370
	21st to end	Night	57% 4 12% 18% 7% 1	1884
March	1st to 10th	Night	54% 4 14% 22% 6%	3523
	11th to 20th	Night	43% 5% 16% 21% 14% 1	3480
	21st to end	Night	48% 4 11% 21% 15% 1	3817
April	1st to 10th	Night	49% 5% 11% 20% 13% 2	2044
	11th to 20th	Night	45% 5% 11% 21% 16% 2	1696
	21st to end	Night	46% 5% 11% 19% 16% 3	1710
May	1st to 10th	Night	46% 5% 10% 18% 15% 5%	2840
	11th to 20th	Night	46% 7% 11% 23% 11% 2	2813
	21st to end	Night	33% 6% 11% 23% 23% 5%	2843
June	1st to 10th	Night	35% 5% 12% 27% 17% 3	1514
	11th to 20th	Night	34% 6% 14% 23% 20% 2	1330
	21st to end	Night	33% 6% 13% 27% 19% 2	1330
July	1st to 10th	Night	29% 4 12% 30% 18% 6%	2290
	11th to 20th	Night	29% 4 15% 31% 20% 1	2666
	21st to end	Night	29% 8% 17% 21% 21% 3	2988
August	1st to 10th	Night	36% 5% 15% 25% 17% 2	2814
	11th to 20th	Night	33% 6% 14% 31% 14% 1	2812
	21st to end	Night	32% 5% 11% 33% 17% 1	3124
September	1st to 10th	Night	35% 7% 13% 27% 16% 3	1982
	11th to 20th	Night	29% 5% 13% 31% 20% 1	2090
	21st to end	Night	44% 4 12% 26% 13% 1	2090
October	1st to 10th	Night	37% 7% 12% 24% 19% 1	2218
	11th to 20th	Night	48% 4 13% 21% 14%	2275
	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% 15% 3	270
	21st to end	Day	26% 20% 40% 13% 1	225
March	1st to 10th	Day	22% 17% 36% 19% 6% 1	388
	11th to 20th	Day	21% 13% 35% 24% 7%	460
	21st to end	Day	26% 14% 31% 23% 5%	506
April	1st to 10th	Day	18% 14% 36% 24% 7% 1	460
	11th to 20th	Day	25% 15% 27% 20% 8% 4	452
	21st to end	Day	23% 12% 31% 24% 8% 2	460
May	1st to 10th	Day	21% 14% 27% 27% 9% 3	463
	11th to 20th	Day	17% 9% 29% 25% 16% 4	487
	21st to end	Day	13% 5% 25% 30% 22% 5%	529
June	1st to 10th	Day	12% 8% 27% 34% 16% 4	490
	11th to 20th	Day	12% 7% 28% 36% 15% 3	680
	21st to end	Day	12% 6% 29% 35% 14% 4	680
July	1st to 10th	Day	10% 3 23% 30% 24% 10%	671
	11th to 20th	Day	12% 6% 24% 29% 19% 11%	551
	21st to end	Day	9% 3 21% 31% 25% 11%	532
August	1st to 10th	Day	10% 4 18% 29% 23% 15%	486
	11th to 20th	Day	9% 6% 19% 31% 20% 14%	479
	21st to end	Day	11% 4 21% 28% 26% 11%	539
September	1st to 10th	Day	11% 4 22% 29% 23% 11%	490
	11th to 20th	Day	8% 2 19% 26% 32% 12%	490
	21st to end	Day	19% 7% 24% 27% 15% 8%	490
October	1st to 10th	Day	18% 9% 26% 25% 18% 6%	490
	11th to 20th	Day	17% 15% 33% 21% 10% 3	486
	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%	743
	21st to end	Night	23% 16% 37% 21% 3	628
March	1st to 10th	Night	20% 12% 33% 29% 6%	760
	11th to 20th	Night	18% 13% 37% 25% 8% 1	760
	21st to end	Night	21% 13% 30% 27% 8% 1	836
April	1st to 10th	Night	20% 15% 32% 28% 5% 1	760
	11th to 20th	Night	20% 11% 32% 26% 10% 1	572
	21st to end	Night	20% 8% 33% 30% 9%	570
May	1st to 10th	Night	18% 6% 29% 33% 13% 1	570
	11th to 20th	Night	14% 6% 25% 39% 15% 1	561
	21st to end	Night	11% 3 22% 41% 21% 1	477
June	1st to 10th	Night	13% 5% 28% 39% 16%	380
	11th to 20th	Night	12% 4 22% 46% 16%	380
	21st to end	Night	11% 5% 24% 42% 17% 1	380
July	1st to 10th	Night	7% 1 15% 45% 30% 2	433
	11th to 20th	Night	10% 2 18% 38% 28% 3	552
	21st to end	Night	7% 3 16% 44% 28% 2	624
August	1st to 10th	Night	9% 2 14% 41% 30% 5%	567
	11th to 20th	Night	6% 2 15% 38% 34% 5%	564
	21st to end	Night	11% 2 17% 38% 30% 3	627
September	1st to 10th	Night	10% 4 21% 34% 26% 5%	755
	11th to 20th	Night	10% 4 15% 36% 29% 5%	760
	21st to end	Night	17% 6% 19% 34% 17% 6%	760
October	1st to 10th	Night	16% 6% 25% 33% 18% 3	760
	11th to 20th	Night	21% 10% 28% 26% 12% 2	759
	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

Page generated at 2024-10-17 13:09:41. Version ac07

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