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 2026-08-30.

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% 7% 3 7% 5% 5% 5% 4 6% 8% 38%	143883
	Night	13% 8% 3 7% 4 4 4 3 5% 7% 42%	138434
Transparency	Day	42% 5% 11% 23% 16% 3	147561
	Night	41% 5% 11% 22% 18% 4	142805
Seeing	Day	16% 11% 24% 24% 18% 8%	17608
	Night	17% 8% 25% 29% 18% 3	27201

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	16% 10% 5% 11% 4 5% 4 3 5% 5% 32%	4691
	11th to 20th	Day	14% 7% 3 10% 3 3 3 3 4 4 47%	4952
	21st to end	Day	20% 9% 4 7% 4 4 4 3 5% 6% 35%	6355
February	1st to 10th	Day	17% 11% 4 8% 5% 4 4 3 3 6% 35%	2100
	11th to 20th	Day	20% 10% 4 7% 5% 5% 3 3 4 6% 33%	2048
	21st to end	Day	18% 10% 3 7% 4 4 4 3 5% 5% 38%	1730
March	1st to 10th	Day	15% 12% 4 9% 5% 4 4 3 4 5% 35%	4580
	11th to 20th	Day	12% 8% 2 7% 4 4 4 3 4 5% 47%	4855
	21st to end	Day	11% 8% 3 7% 5% 4 5% 3 5% 6% 43%	5907
April	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 6% 7% 45%	2520
	11th to 20th	Day	13% 7% 3 6% 4 4 4 3 5% 6% 44%	2494
	21st to end	Day	12% 8% 2 6% 6% 4 5% 4 6% 8% 40%	2557
May	1st to 10th	Day	17% 8% 3 6% 4 4 5% 4 7% 10% 31%	7616
	11th to 20th	Day	13% 8% 3 8% 5% 5% 5% 3 5% 8% 36%	7626
	21st to end	Day	18% 10% 4 9% 5% 4 5% 4 5% 7% 30%	8391
June	1st to 10th	Day	9% 6% 2 6% 4 6% 5% 5% 8% 9% 41%	2733
	11th to 20th	Day	5% 4 2 6% 5% 6% 6% 4 7% 11% 44%	2940
	21st to end	Day	4 5% 2 5% 5% 5% 6% 6% 9% 12% 40%	2940
July	1st to 10th	Day	6% 5% 2 7% 6% 6% 7% 5% 8% 11% 35%	8280
	11th to 20th	Day	7% 6% 2 8% 6% 7% 7% 6% 10% 12% 27%	8100
	21st to end	Day	11% 7% 3 6% 5% 5% 6% 5% 8% 12% 31%	8340
August	1st to 10th	Day	4 3 2 5% 4 5% 6% 4 7% 10% 49%	4137
	11th to 20th	Day	6% 4 2 7% 5% 6% 6% 5% 8% 11% 39%	3969
	21st to end	Day	6% 4 2 5% 4 5% 5% 4 6% 8% 53%	4420
September	1st to 10th	Day	10% 6% 2 7% 4 5% 5% 5% 7% 8% 40%	2371
	11th to 20th	Day	12% 6% 2 5% 4 4 5% 4 5% 7% 45%	2200
	21st to end	Day	11% 5% 2 5% 4 4 4 4 6% 7% 48%	2200
October	1st to 10th	Day	11% 4 2 5% 2 3 3 3 5% 6% 56%	2200
	11th to 20th	Day	11% 6% 2 5% 3 3 4 2 4 6% 55%	2155
	21st to end	Day	16% 6% 3 6% 3 3 4 3 4 5% 47%	1999
November	1st to 10th	Day	12% 6% 2 5% 3 2 3 3 4 5% 55%	1852
	11th to 20th	Day	14% 6% 2 6% 3 4 3 3 6% 8% 47%	1890
	21st to end	Day	13% 8% 2 6% 4 4 4 3 4 5% 45%	1890
December	1st to 10th	Day	15% 10% 3 6% 4 4 4 3 4 5% 44%	3020
	11th to 20th	Day	19% 11% 4 9% 4 5% 3 3 4 5% 32%	3463
	21st to end	Day	17% 10% 5% 8% 5% 4 5% 4 6% 7% 28%	4362
January	1st to 10th	Night	16% 9% 3 9% 4 3 4 3 4 6% 39%	8232
	11th to 20th	Night	15% 9% 3 8% 4 3 3 2 3 4 46%	8005
	21st to end	Night	18% 12% 4 8% 4 4 3 2 4 5% 37%	7823
February	1st to 10th	Night	19% 10% 3 8% 5% 4 4 4 4 5% 33%	2520
	11th to 20th	Night	22% 10% 3 8% 4 3 3 2 4 6% 35%	2458
	21st to end	Night	22% 8% 3 7% 4 4 4 4 4 4 36%	2076
March	1st to 10th	Night	19% 10% 2 7% 4 4 4 3 5% 7% 35%	5493
	11th to 20th	Night	12% 5% 2 5% 4 4 4 3 6% 6% 49%	5325
	21st to end	Night	13% 7% 3 7% 4 4 4 3 5% 7% 42%	5307
April	1st to 10th	Night	11% 6% 2 7% 4 4 4 4 5% 7% 46%	2100
	11th to 20th	Night	14% 7% 2 4 3 4 4 3 6% 8% 48%	2075
	21st to end	Night	12% 7% 3 5% 5% 4 4 4 6% 7% 43%	2062
May	1st to 10th	Night	14% 9% 4 8% 5% 5% 4 4 6% 6% 36%	5237
	11th to 20th	Night	12% 8% 3 8% 6% 5% 5% 4 6% 7% 37%	5234
	21st to end	Night	16% 9% 3 9% 5% 5% 5% 4 6% 9% 30%	5761
June	1st to 10th	Night	8% 6% 2 6% 5% 4 4 4 7% 9% 46%	1880
	11th to 20th	Night	5% 3 2 5% 4 4 4 5% 6% 10% 53%	1680
	21st to end	Night	4 3 1 5% 4 5% 4 5% 7% 13% 49%	1680
July	1st to 10th	Night	6% 5% 2 6% 4 5% 5% 4 9% 11% 41%	4708
	11th to 20th	Night	6% 6% 2 6% 5% 6% 7% 5% 9% 14% 33%	4873
	21st to end	Night	8% 7% 3 8% 6% 5% 6% 5% 8% 12% 32%	5736
August	1st to 10th	Night	4 3 2 4 4 3 4 3 6% 8% 59%	3227
	11th to 20th	Night	4 4 1 5% 4 4 5% 5% 8% 12% 48%	3300
	21st to end	Night	4 3 1 4 3 4 4 4 6% 7% 58%	3676
September	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 5% 7% 49%	2007
	11th to 20th	Night	10% 6% 2 6% 4 4 4 4 6% 7% 48%	2200
	21st to end	Night	11% 5% 2 4 4 4 3 3 5% 7% 52%	2200
October	1st to 10th	Night	9% 4 2 3 3 2 2 3 4 5% 63%	2200
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 4 6% 52%	2240
	21st to end	Night	15% 6% 2 5% 4 3 3 3 4 6% 49%	2840
November	1st to 10th	Night	12% 6% 2 5% 4 3 4 2 4 5% 53%	2666
	11th to 20th	Night	12% 7% 2 4 3 3 4 3 6% 7% 49%	2730
	21st to end	Night	16% 8% 2 5% 3 3 3 3 5% 5% 45%	2730
December	1st to 10th	Night	17% 10% 3 8% 4 3 3 2 4 4 40%	4380
	11th to 20th	Night	21% 9% 3 6% 3 3 3 3 3 4 41%	4952
	21st to end	Night	18% 11% 5% 9% 5% 5% 5% 4 5% 5% 28%	6821

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% 3 11% 21% 9% 1	4692
	11th to 20th	Day	42% 3 5% 22% 28% 1	4952
	21st to end	Day	51% 2 7% 22% 13% 4	6355
February	1st to 10th	Day	52% 3 8% 21% 13% 2	2100
	11th to 20th	Day	54% 4 7% 19% 13% 3	2058
	21st to end	Day	49% 3 8% 17% 16% 6%	1730
March	1st to 10th	Day	52% 4 5% 14% 16% 9%	4580
	11th to 20th	Day	40% 4 7% 21% 20% 7%	4855
	21st to end	Day	42% 5% 9% 28% 16% 1	5909
April	1st to 10th	Day	37% 5% 12% 23% 19% 3	2520
	11th to 20th	Day	40% 5% 7% 22% 20% 5%	2494
	21st to end	Day	42% 5% 10% 25% 15% 2	2557
May	1st to 10th	Day	47% 5% 14% 22% 12% 1	7651
	11th to 20th	Day	46% 5% 12% 22% 14% 1	7630
	21st to end	Day	54% 6% 8% 17% 13% 2	8405
June	1st to 10th	Day	38% 8% 12% 22% 17% 2	2733
	11th to 20th	Day	31% 8% 16% 24% 18% 3	2940
	21st to end	Day	32% 10% 16% 23% 16% 4	2940
July	1st to 10th	Day	40% 7% 17% 26% 9% 1	8211
	11th to 20th	Day	44% 9% 20% 21% 5% 1	8025
	21st to end	Day	44% 6% 21% 21% 7% 1	8156
August	1st to 10th	Day	28% 7% 13% 33% 18% 1	4124
	11th to 20th	Day	36% 7% 13% 27% 15% 2	4005
	21st to end	Day	29% 6% 9% 31% 23% 2	4419
September	1st to 10th	Day	38% 10% 11% 20% 17% 4	2371
	11th to 20th	Day	38% 6% 9% 23% 21% 3	2200
	21st to end	Day	35% 5% 10% 24% 22% 5%	2200
October	1st to 10th	Day	22% 4 5% 19% 43% 8%	3590
	11th to 20th	Day	24% 2 6% 25% 29% 13%	3486
	21st to end	Day	31% 3 6% 24% 29% 7%	3199
November	1st to 10th	Day	33% 4 8% 24% 27% 5%	1852
	11th to 20th	Day	36% 3 8% 24% 23% 6%	1890
	21st to end	Day	41% 3 7% 26% 19% 5%	1890
December	1st to 10th	Day	44% 3 5% 27% 20% 1	3020
	11th to 20th	Day	54% 3 8% 18% 13% 2	3460
	21st to end	Day	54% 4 11% 23% 7% 1	4362
January	1st to 10th	Night	48% 3 14% 22% 13% 1	8233
	11th to 20th	Night	45% 3 4 20% 25% 4	8005
	21st to end	Night	52% 2 7% 18% 15% 5%	7823
February	1st to 10th	Night	53% 4 9% 17% 15% 1	2520
	11th to 20th	Night	53% 3 5% 19% 15% 4	2459
	21st to end	Night	51% 4 8% 16% 17% 5%	2076
March	1st to 10th	Night	50% 4 6% 15% 18% 6%	5493
	11th to 20th	Night	36% 4 7% 21% 19% 13%	5325
	21st to end	Night	43% 4 8% 26% 17% 2	5307
April	1st to 10th	Night	37% 5% 11% 23% 21% 3	2100
	11th to 20th	Night	36% 4 9% 21% 22% 8%	2075
	21st to end	Night	39% 5% 10% 27% 17% 2	2062
May	1st to 10th	Night	48% 5% 12% 22% 12% 1	5264
	11th to 20th	Night	45% 6% 14% 17% 16% 1	5235
	21st to end	Night	51% 7% 12% 19% 11% 1	5771
June	1st to 10th	Night	33% 7% 14% 25% 17% 3	1880
	11th to 20th	Night	25% 6% 17% 30% 18% 3	1680
	21st to end	Night	25% 9% 17% 30% 15% 4	1680
July	1st to 10th	Night	34% 7% 21% 26% 10% 1	4670
	11th to 20th	Night	39% 8% 24% 22% 5% 1	4828
	21st to end	Night	43% 6% 24% 19% 7% 1	5606
August	1st to 10th	Night	23% 6% 11% 37% 22% 1	3216
	11th to 20th	Night	26% 7% 19% 32% 15% 1	3330
	21st to end	Night	24% 6% 11% 34% 23% 3	3676
September	1st to 10th	Night	34% 6% 12% 24% 19% 5%	2007
	11th to 20th	Night	35% 6% 10% 22% 26% 2	2200
	21st to end	Night	31% 5% 11% 22% 24% 7%	2200
October	1st to 10th	Night	20% 4 5% 20% 40% 11%	3590
	11th to 20th	Night	22% 2 5% 23% 32% 16%	3635
	21st to end	Night	26% 3 7% 24% 30% 11%	4578
November	1st to 10th	Night	35% 4 7% 22% 24% 7%	2666
	11th to 20th	Night	34% 4 7% 22% 26% 7%	2730
	21st to end	Night	41% 3 8% 23% 20% 5%	2730
December	1st to 10th	Night	49% 3 6% 17% 24% 2	4380
	11th to 20th	Night	48% 2 6% 23% 20% 2	4954
	21st to end	Night	57% 4 8% 21% 9% 1	6821

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% 20% 35% 16% 6% 1	330
	11th to 20th	Day	29% 22% 30% 14% 3 1	330
	21st to end	Day	23% 22% 39% 13% 4	363
February	1st to 10th	Day	26% 19% 35% 16% 3 1	330
	11th to 20th	Day	37% 21% 30% 10% 2 1	330
	21st to end	Day	34% 20% 32% 13% 2	273
March	1st to 10th	Day	24% 23% 32% 18% 3 1	330
	11th to 20th	Day	22% 24% 33% 15% 6%	435
	21st to end	Day	21% 20% 37% 15% 7% 1	594
April	1st to 10th	Day	19% 18% 30% 24% 8% 1	540
	11th to 20th	Day	22% 13% 32% 24% 9% 2	532
	21st to end	Day	18% 17% 31% 21% 11% 3	540
May	1st to 10th	Day	20% 13% 29% 22% 11% 4	543
	11th to 20th	Day	16% 9% 29% 25% 15% 7%	567
	21st to end	Day	13% 7% 22% 27% 19% 12%	617
June	1st to 10th	Day	11% 6% 21% 25% 21% 16%	573
	11th to 20th	Day	7% 5% 20% 28% 27% 13%	780
	21st to end	Day	6% 4 19% 35% 23% 14%	780
July	1st to 10th	Day	9% 2 14% 31% 28% 17%	771
	11th to 20th	Day	8% 4 11% 28% 32% 17%	705
	21st to end	Day	10% 2 14% 28% 28% 18%	620
August	1st to 10th	Day	10% 2 12% 27% 32% 16%	566
	11th to 20th	Day	9% 3 16% 31% 33% 8%	559
	21st to end	Day	11% 2 12% 24% 34% 17%	619
September	1st to 10th	Day	10% 3 14% 28% 32% 13%	530
	11th to 20th	Day	13% 3 10% 25% 31% 17%	530
	21st to end	Day	16% 5% 19% 27% 25% 8%	530
October	1st to 10th	Day	13% 3 20% 29% 26% 9%	530
	11th to 20th	Day	13% 13% 26% 30% 14% 4	485
	21st to end	Day	22% 14% 29% 26% 7% 2	363
November	1st to 10th	Day	19% 11% 30% 26% 12% 2	330
	11th to 20th	Day	15% 17% 39% 22% 7%	330
	21st to end	Day	20% 16% 33% 18% 10% 2	330
December	1st to 10th	Day	27% 18% 31% 18% 6%	330
	11th to 20th	Day	21% 18% 35% 18% 8% 1	330
	21st to end	Day	26% 20% 31% 19% 4	363
January	1st to 10th	Night	22% 14% 33% 26% 5%	1050
	11th to 20th	Night	25% 16% 32% 22% 5%	957
	21st to end	Night	20% 13% 38% 25% 3	924
February	1st to 10th	Night	24% 12% 35% 25% 4	840
	11th to 20th	Night	30% 14% 32% 22% 2	823
	21st to end	Night	28% 17% 32% 21% 2	692
March	1st to 10th	Night	21% 14% 33% 26% 6%	840
	11th to 20th	Night	20% 12% 35% 26% 6% 1	840
	21st to end	Night	18% 14% 33% 26% 8%	798
April	1st to 10th	Night	14% 13% 33% 31% 9%	630
	11th to 20th	Night	17% 12% 34% 28% 8% 1	623
	21st to end	Night	16% 11% 29% 32% 11% 1	630
May	1st to 10th	Night	14% 7% 33% 29% 15% 1	630
	11th to 20th	Night	12% 5% 29% 33% 18% 3	621
	21st to end	Night	10% 3 19% 37% 26% 5%	683
June	1st to 10th	Night	10% 4 16% 37% 29% 4	630
	11th to 20th	Night	5% 2 14% 39% 35% 5%	630
	21st to end	Night	5% 1 13% 38% 36% 7%	630
July	1st to 10th	Night	9% 2 9% 32% 38% 10%	615
	11th to 20th	Night	9% 2 9% 28% 43% 9%	612
	21st to end	Night	8% 2 9% 32% 42% 7%	690
August	1st to 10th	Night	7% 1 9% 30% 46% 6%	627
	11th to 20th	Night	6% 11% 40% 36% 7%	624
	21st to end	Night	8% 1 9% 32% 42% 8%	687
September	1st to 10th	Night	10% 1 13% 30% 41% 5%	607
	11th to 20th	Night	12% 1 12% 30% 39% 6%	800
	21st to end	Night	13% 3 18% 30% 33% 4	800
October	1st to 10th	Night	11% 5% 18% 31% 30% 6%	800
	11th to 20th	Night	16% 8% 22% 32% 20% 1	800
	21st to end	Night	18% 8% 29% 28% 17% 1	880
November	1st to 10th	Night	15% 6% 23% 35% 17% 3	820
	11th to 20th	Night	15% 10% 33% 30% 12% 1	840
	21st to end	Night	21% 10% 28% 31% 11% 1	840
December	1st to 10th	Night	27% 12% 28% 26% 7%	840
	11th to 20th	Night	22% 8% 33% 29% 7% 1	840
	21st to end	Night	26% 12% 31% 26% 5%	1008

Page generated at 2026-08-27 03:07:11. Version ac07

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