Vanderbilt Dyer Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Vanderbilt Dyer Observatory clear sky chart from 2005-11-06 to 2024-02-25.

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	13% 7% 3 6% 5% 3 5% 5% 6% 8% 39%	78909
	Night	14% 7% 3 5% 5% 3 4 4 5% 7% 43%	80396
Transparency	Day	42% 7% 11% 21% 16% 3	78622
	Night	41% 6% 10% 22% 18% 4	80203
Seeing	Day	16% 10% 23% 24% 19% 9%	14145
	Night	16% 8% 25% 29% 19% 3	23991

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	13% 11% 5% 9% 6% 4 6% 5% 5% 6% 29%	2527
	11th to 20th	Day	20% 8% 4 9% 6% 3 4 5% 4 5% 33%	2655
	21st to end	Day	22% 12% 6% 6% 4 2 4 3 4 6% 30%	3423
February	1st to 10th	Day	17% 11% 4 6% 6% 3 4 4 3 6% 35%	1900
	11th to 20th	Day	20% 10% 4 6% 6% 3 4 3 4 6% 34%	1848
	21st to end	Day	20% 9% 3 6% 5% 2 4 3 4 5% 37%	1520
March	1st to 10th	Day	14% 10% 4 5% 6% 4 4 4 4 6% 39%	1800
	11th to 20th	Day	18% 9% 3 7% 6% 3 4 4 6% 5% 34%	1890
	21st to end	Day	15% 8% 4 7% 7% 3 6% 4 6% 6% 33%	2300
April	1st to 10th	Day	12% 5% 3 5% 5% 3 4 4 6% 7% 46%	2160
	11th to 20th	Day	14% 7% 3 5% 5% 3 5% 4 5% 6% 44%	2134
	21st to end	Day	13% 7% 3 5% 6% 3 5% 5% 5% 7% 40%	2191
May	1st to 10th	Day	15% 6% 3 5% 4 3 5% 5% 5% 9% 41%	2340
	11th to 20th	Day	14% 8% 3 6% 5% 4 5% 4 5% 7% 39%	2301
	21st to end	Day	8% 5% 3 6% 6% 4 6% 7% 7% 10% 38%	2531
June	1st to 10th	Day	9% 6% 2 5% 5% 4 6% 6% 7% 9% 42%	2342
	11th to 20th	Day	5% 3 2 5% 6% 4 6% 6% 7% 12% 45%	2520
	21st to end	Day	5% 4 2 5% 6% 4 7% 7% 8% 11% 41%	2520
July	1st to 10th	Day	7% 6% 2 6% 8% 5% 8% 7% 8% 10% 33%	2520
	11th to 20th	Day	7% 5% 2 6% 7% 5% 7% 6% 8% 11% 36%	2466
	21st to end	Day	7% 5% 3 5% 6% 4 8% 7% 9% 12% 35%	2547
August	1st to 10th	Day	6% 5% 3 7% 7% 4 7% 8% 8% 10% 35%	2212
	11th to 20th	Day	9% 6% 2 7% 7% 5% 6% 6% 7% 10% 35%	2125
	21st to end	Day	10% 5% 2 6% 6% 4 7% 6% 7% 9% 38%	2376
September	1st to 10th	Day	11% 5% 2 6% 5% 4 6% 7% 7% 8% 39%	2135
	11th to 20th	Day	13% 6% 2 5% 5% 4 5% 5% 5% 7% 45%	1980
	21st to end	Day	11% 5% 2 3 4 3 3 5% 6% 7% 51%	1980
October	1st to 10th	Day	11% 4 2 5% 3 3 3 4 5% 6% 55%	1980
	11th to 20th	Day	12% 6% 2 5% 4 2 4 3 4 6% 52%	1940
	21st to end	Day	17% 6% 2 5% 4 3 4 4 4 5% 46%	1800
November	1st to 10th	Day	12% 4 2 4 3 2 2 3 5% 6% 58%	1672
	11th to 20th	Day	14% 5% 3 5% 3 2 3 4 6% 7% 47%	1710
	21st to end	Day	14% 7% 3 5% 4 3 5% 4 4 6% 45%	1710
December	1st to 10th	Day	22% 11% 3 4 4 2 4 4 4 6% 35%	1710
	11th to 20th	Day	16% 9% 2 6% 5% 4 4 5% 5% 6% 37%	2153
	21st to end	Day	16% 11% 4 7% 7% 4 5% 5% 5% 7% 29%	2991
January	1st to 10th	Night	18% 9% 4 6% 5% 3 4 4 5% 6% 37%	4428
	11th to 20th	Night	18% 10% 4 8% 6% 3 3 3 3 4 37%	4305
	21st to end	Night	21% 13% 4 6% 4 3 3 3 5% 5% 34%	4230
February	1st to 10th	Night	20% 9% 4 7% 6% 3 4 4 4 5% 34%	2280
	11th to 20th	Night	22% 9% 4 7% 4 2 4 3 4 6% 36%	2218
	21st to end	Night	23% 8% 4 6% 5% 3 4 4 5% 4 33%	1831
March	1st to 10th	Night	16% 7% 3 6% 5% 3 5% 4 5% 7% 41%	2160
	11th to 20th	Night	18% 8% 3 6% 6% 4 4 4 4 6% 37%	2070
	21st to end	Night	14% 7% 3 7% 5% 3 5% 4 6% 7% 39%	2070
April	1st to 10th	Night	10% 5% 2 6% 5% 3 4 5% 5% 7% 48%	1800
	11th to 20th	Night	15% 6% 2 3 4 2 4 3 5% 7% 48%	1775
	21st to end	Night	13% 6% 3 4 5% 3 4 5% 6% 8% 43%	1768
May	1st to 10th	Night	10% 5% 2 6% 5% 3 5% 4 5% 7% 48%	1620
	11th to 20th	Night	11% 5% 3 5% 5% 3 4 4 7% 8% 45%	1592
	21st to end	Night	6% 5% 2 6% 6% 4 7% 5% 7% 12% 42%	1744
June	1st to 10th	Night	8% 6% 2 5% 5% 3 4 6% 6% 9% 47%	1612
	11th to 20th	Night	4 3 1 3 4 3 5% 5% 7% 11% 55%	1440
	21st to end	Night	4 3 1 5% 4 4 5% 6% 7% 13% 48%	1440
July	1st to 10th	Night	8% 5% 2 5% 6% 4 5% 6% 9% 11% 40%	1440
	11th to 20th	Night	6% 5% 2 4 5% 3 5% 5% 7% 13% 45%	1489
	21st to end	Night	6% 4 2 5% 5% 4 6% 6% 8% 12% 41%	1771
August	1st to 10th	Night	6% 4 2 6% 5% 3 5% 5% 7% 10% 47%	1722
	11th to 20th	Night	6% 4 2 6% 6% 3 5% 5% 7% 11% 45%	1776
	21st to end	Night	8% 4 2 5% 5% 3 6% 6% 7% 8% 47%	1980
September	1st to 10th	Night	11% 5% 2 5% 5% 3 3 5% 5% 8% 48%	1805
	11th to 20th	Night	10% 5% 2 5% 5% 3 5% 5% 6% 6% 49%	1980
	21st to end	Night	10% 4 2 3 4 3 4 4 5% 7% 54%	1980
October	1st to 10th	Night	10% 4 2 3 3 2 3 4 4 5% 61%	1980
	11th to 20th	Night	15% 5% 2 5% 5% 3 4 3 4 6% 49%	2016
	21st to end	Night	15% 6% 2 4 5% 2 3 4 4 6% 48%	2556
November	1st to 10th	Night	11% 5% 2 4 4 2 4 3 4 5% 55%	2406
	11th to 20th	Night	13% 5% 3 4 4 2 4 3 6% 8% 50%	2470
	21st to end	Night	16% 7% 2 5% 4 3 4 4 5% 5% 46%	2470
December	1st to 10th	Night	20% 10% 4 6% 5% 2 4 4 5% 5% 36%	2470
	11th to 20th	Night	15% 9% 3 6% 4 2 4 4 4 4 45%	3042
	21st to end	Night	17% 10% 4 8% 7% 4 5% 5% 5% 6% 29%	4660

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	54% 5% 9% 20% 12% 2	2528
	11th to 20th	Day	53% 4 8% 17% 17% 1	2655
	21st to end	Day	55% 3 8% 22% 11% 1	3423
February	1st to 10th	Day	52% 4 8% 20% 14% 3	1900
	11th to 20th	Day	53% 4 6% 19% 14% 4	1858
	21st to end	Day	51% 4 9% 16% 15% 6%	1520
March	1st to 10th	Day	46% 6% 7% 18% 18% 6%	1800
	11th to 20th	Day	51% 5% 10% 18% 14% 2	1890
	21st to end	Day	50% 7% 10% 19% 13% 1	2300
April	1st to 10th	Day	37% 6% 11% 23% 21% 3	2160
	11th to 20th	Day	41% 5% 7% 21% 21% 5%	2134
	21st to end	Day	43% 6% 9% 25% 16% 2	2191
May	1st to 10th	Day	40% 7% 11% 25% 15% 2	2340
	11th to 20th	Day	43% 7% 12% 23% 15% 1	2305
	21st to end	Day	38% 10% 14% 24% 12% 2	2531
June	1st to 10th	Day	37% 9% 12% 23% 17% 2	2342
	11th to 20th	Day	29% 9% 17% 24% 18% 3	2520
	21st to end	Day	33% 10% 16% 22% 15% 5%	2520
July	1st to 10th	Day	40% 13% 15% 18% 12% 3	2451
	11th to 20th	Day	38% 12% 17% 19% 11% 4	2391
	21st to end	Day	36% 13% 17% 23% 10% 2	2405
August	1st to 10th	Day	38% 11% 16% 23% 11% 1	2199
	11th to 20th	Day	41% 9% 12% 21% 15% 2	2125
	21st to end	Day	39% 10% 13% 21% 14% 3	2376
September	1st to 10th	Day	38% 10% 11% 21% 16% 3	2135
	11th to 20th	Day	38% 6% 10% 23% 20% 3	1980
	21st to end	Day	32% 5% 9% 26% 22% 5%	1980
October	1st to 10th	Day	31% 5% 8% 24% 27% 5%	1980
	11th to 20th	Day	34% 4 7% 21% 25% 9%	1940
	21st to end	Day	41% 4 7% 20% 24% 5%	1800
November	1st to 10th	Day	30% 4 8% 24% 29% 5%	1672
	11th to 20th	Day	36% 4 7% 23% 24% 6%	1710
	21st to end	Day	40% 3 7% 25% 19% 5%	1710
December	1st to 10th	Day	51% 4 6% 21% 16% 2	1710
	11th to 20th	Day	46% 5% 11% 20% 17% 1	2150
	21st to end	Day	52% 5% 12% 20% 9% 2	2991
January	1st to 10th	Night	47% 4 9% 23% 16% 1	4428
	11th to 20th	Night	53% 4 5% 18% 16% 5%	4305
	21st to end	Night	53% 3 7% 21% 14% 2	4230
February	1st to 10th	Night	53% 5% 8% 17% 16% 1	2280
	11th to 20th	Night	52% 4 5% 19% 16% 4	2219
	21st to end	Night	53% 4 9% 15% 14% 5%	1831
March	1st to 10th	Night	44% 6% 8% 14% 20% 8%	2160
	11th to 20th	Night	48% 5% 8% 20% 16% 2	2070
	21st to end	Night	44% 6% 10% 24% 14% 2	2070
April	1st to 10th	Night	35% 5% 11% 24% 22% 3	1800
	11th to 20th	Night	36% 4 9% 20% 22% 9%	1775
	21st to end	Night	38% 5% 10% 27% 18% 2	1768
May	1st to 10th	Night	37% 6% 11% 25% 19% 2	1620
	11th to 20th	Night	35% 9% 17% 21% 17% 1	1593
	21st to end	Night	33% 10% 15% 25% 15% 2	1745
June	1st to 10th	Night	32% 8% 14% 25% 17% 3	1612
	11th to 20th	Night	23% 7% 18% 31% 18% 3	1440
	21st to end	Night	25% 9% 17% 30% 15% 4	1440
July	1st to 10th	Night	33% 11% 19% 21% 14% 4	1400
	11th to 20th	Night	30% 8% 19% 24% 14% 5%	1441
	21st to end	Night	32% 12% 16% 26% 14% 2	1672
August	1st to 10th	Night	31% 9% 15% 29% 15% 1	1711
	11th to 20th	Night	31% 10% 18% 23% 16% 2	1776
	21st to end	Night	32% 9% 14% 26% 14% 5%	1980
September	1st to 10th	Night	34% 7% 12% 25% 18% 4	1805
	11th to 20th	Night	34% 6% 10% 21% 26% 3	1980
	21st to end	Night	29% 5% 10% 24% 25% 7%	1980
October	1st to 10th	Night	26% 5% 8% 24% 29% 8%	1980
	11th to 20th	Night	37% 4 9% 21% 22% 8%	2016
	21st to end	Night	37% 4 6% 22% 25% 5%	2556
November	1st to 10th	Night	33% 4 7% 22% 26% 8%	2406
	11th to 20th	Night	33% 4 8% 21% 27% 7%	2470
	21st to end	Night	40% 3 8% 23% 21% 5%	2470
December	1st to 10th	Night	50% 5% 8% 18% 16% 3	2470
	11th to 20th	Night	42% 3 7% 22% 22% 4	3044
	21st to end	Night	55% 5% 9% 18% 12% 2	4660

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	23% 19% 36% 15% 7% 1	270
	11th to 20th	Day	31% 22% 27% 14% 4 1	270
	21st to end	Day	22% 22% 38% 14% 5%	297
February	1st to 10th	Day	28% 18% 32% 17% 4 1	270
	11th to 20th	Day	37% 17% 31% 11% 3 1	270
	21st to end	Day	37% 19% 30% 12% 1	210
March	1st to 10th	Day	24% 22% 32% 20% 3	240
	11th to 20th	Day	23% 23% 30% 16% 7% 1	330
	21st to end	Day	22% 19% 32% 18% 8% 1	462
April	1st to 10th	Day	17% 18% 28% 25% 10% 2	420
	11th to 20th	Day	21% 12% 31% 25% 10% 2	412
	21st to end	Day	19% 16% 30% 19% 13% 3	420
May	1st to 10th	Day	19% 13% 26% 25% 12% 5%	423
	11th to 20th	Day	16% 7% 28% 25% 16% 8%	447
	21st to end	Day	11% 4 20% 29% 22% 14%	485
June	1st to 10th	Day	11% 6% 17% 24% 24% 19%	452
	11th to 20th	Day	5% 4 17% 29% 29% 15%	630
	21st to end	Day	6% 4 20% 36% 21% 14%	630
July	1st to 10th	Day	10% 2 14% 29% 28% 18%	621
	11th to 20th	Day	8% 4 11% 26% 33% 18%	564
	21st to end	Day	8% 2 14% 28% 28% 22%	488
August	1st to 10th	Day	10% 3 12% 25% 33% 17%	446
	11th to 20th	Day	11% 3 13% 30% 35% 8%	439
	21st to end	Day	13% 1 11% 22% 35% 18%	495
September	1st to 10th	Day	9% 3 14% 27% 33% 14%	450
	11th to 20th	Day	13% 3 10% 24% 31% 18%	450
	21st to end	Day	15% 4 18% 27% 26% 10%	450
October	1st to 10th	Day	14% 3 22% 27% 26% 9%	450
	11th to 20th	Day	14% 15% 26% 29% 12% 4	410
	21st to end	Day	24% 15% 25% 26% 8% 2	297
November	1st to 10th	Day	17% 9% 30% 27% 14% 3	270
	11th to 20th	Day	14% 15% 41% 22% 8%	270
	21st to end	Day	19% 15% 33% 19% 11% 3	270
December	1st to 10th	Day	29% 18% 27% 19% 7%	270
	11th to 20th	Day	18% 20% 33% 19% 10% 1	270
	21st to end	Day	26% 20% 32% 19% 4	297
January	1st to 10th	Night	22% 15% 32% 26% 5%	950
	11th to 20th	Night	26% 16% 31% 22% 5%	867
	21st to end	Night	20% 13% 39% 25% 3	836
February	1st to 10th	Night	25% 12% 34% 25% 4	760
	11th to 20th	Night	29% 13% 33% 23% 2	743
	21st to end	Night	28% 17% 31% 21% 2	611
March	1st to 10th	Night	21% 14% 33% 28% 6%	720
	11th to 20th	Night	21% 12% 34% 26% 7% 1	720
	21st to end	Night	19% 13% 30% 28% 9%	684
April	1st to 10th	Night	12% 12% 34% 33% 9%	540
	11th to 20th	Night	18% 11% 32% 29% 9% 1	533
	21st to end	Night	16% 11% 28% 33% 11% 1	540
May	1st to 10th	Night	13% 6% 32% 30% 17% 2	540
	11th to 20th	Night	11% 5% 28% 31% 20% 4	531
	21st to end	Night	7% 2 18% 37% 30% 5%	584
June	1st to 10th	Night	11% 4 14% 35% 31% 5%	540
	11th to 20th	Night	5% 2 12% 40% 36% 5%	540
	21st to end	Night	5% 1 14% 38% 35% 6%	540
July	1st to 10th	Night	9% 2 8% 32% 38% 11%	525
	11th to 20th	Night	9% 2 10% 28% 42% 10%	522
	21st to end	Night	7% 2 8% 31% 45% 8%	591
August	1st to 10th	Night	7% 1 7% 30% 48% 7%	537
	11th to 20th	Night	7% 1 10% 40% 36% 6%	534
	21st to end	Night	9% 1 8% 30% 44% 8%	594
September	1st to 10th	Night	11% 2 12% 28% 43% 5%	545
	11th to 20th	Night	12% 2 13% 30% 37% 6%	720
	21st to end	Night	12% 2 18% 30% 34% 4	720
October	1st to 10th	Night	11% 5% 19% 29% 30% 6%	720
	11th to 20th	Night	18% 8% 23% 31% 18% 2	720
	21st to end	Night	18% 8% 28% 28% 17% 2	792
November	1st to 10th	Night	14% 6% 24% 36% 18% 3	740
	11th to 20th	Night	14% 10% 33% 30% 13% 1	760
	21st to end	Night	19% 10% 27% 32% 11% 1	760
December	1st to 10th	Night	27% 11% 28% 26% 7% 1	760
	11th to 20th	Night	21% 8% 32% 30% 8% 1	760
	21st to end	Night	26% 12% 31% 26% 5%	912

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