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 2025-04-26.

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% 4 5% 4 6% 7% 40%	113970
	Night	13% 8% 3 7% 4 4 4 3 5% 7% 43%	114715
Transparency	Day	41% 6% 10% 23% 17% 3	117692
	Night	40% 5% 9% 23% 19% 4	119109
Seeing	Day	16% 10% 24% 24% 18% 8%	15718
	Night	17% 8% 25% 29% 18% 3	25514

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	14% 11% 6% 13% 4 4 4 3 5% 5% 31%	3617
	11th to 20th	Day	15% 7% 4 12% 3 3 3 3 4 4 41%	3810
	21st to end	Day	19% 9% 5% 6% 3 3 4 3 5% 7% 36%	4905
February	1st to 10th	Day	17% 11% 4 8% 5% 4 4 3 3 6% 35%	2000
	11th to 20th	Day	20% 11% 4 7% 5% 5% 3 3 4 5% 33%	1948
	21st to end	Day	19% 10% 3 7% 4 4 4 3 5% 4 38%	1650
March	1st to 10th	Day	15% 12% 4 9% 5% 4 4 3 4 5% 35%	4480
	11th to 20th	Day	12% 8% 2 7% 4 4 4 3 4 5% 47%	4750
	21st to end	Day	11% 8% 3 7% 5% 4 5% 3 5% 6% 42%	5779
April	1st to 10th	Day	13% 6% 2 5% 4 5% 4 4 6% 7% 45%	2400
	11th to 20th	Day	13% 7% 3 6% 5% 4 4 3 5% 6% 44%	2374
	21st to end	Day	13% 8% 2 7% 6% 5% 5% 4 6% 7% 39%	2373
May	1st to 10th	Day	16% 7% 3 7% 4 4 5% 4 6% 9% 34%	4082
	11th to 20th	Day	16% 9% 2 7% 4 4 5% 3 5% 8% 37%	4081
	21st to end	Day	10% 7% 3 7% 5% 4 5% 5% 6% 9% 40%	4476
June	1st to 10th	Day	9% 6% 2 6% 4 6% 5% 5% 7% 8% 41%	2473
	11th to 20th	Day	5% 4 2 6% 5% 6% 6% 5% 7% 11% 45%	2660
	21st to end	Day	5% 5% 2 5% 5% 6% 6% 6% 8% 11% 40%	2660
July	1st to 10th	Day	6% 5% 2 7% 7% 6% 7% 6% 8% 10% 37%	4440
	11th to 20th	Day	8% 6% 2 7% 6% 6% 6% 5% 8% 12% 35%	4344
	21st to end	Day	16% 10% 4 8% 6% 6% 6% 5% 7% 8% 24%	4466
August	1st to 10th	Day	3 3 2 5% 4 5% 5% 4 7% 10% 50%	3891
	11th to 20th	Day	6% 4 2 7% 6% 7% 6% 5% 8% 11% 38%	3729
	21st to end	Day	6% 4 2 5% 4 5% 5% 4 6% 8% 52%	4180
September	1st to 10th	Day	10% 6% 2 7% 4 5% 5% 5% 7% 8% 40%	2253
	11th to 20th	Day	13% 6% 2 5% 4 4 5% 4 5% 7% 44%	2090
	21st to end	Day	12% 5% 2 4 4 4 4 4 6% 7% 49%	2090
October	1st to 10th	Day	10% 4 2 5% 2 3 3 3 5% 6% 55%	2090
	11th to 20th	Day	11% 6% 2 6% 3 3 4 2 4 6% 53%	2047
	21st to end	Day	16% 6% 3 6% 3 3 4 3 4 5% 48%	1899
November	1st to 10th	Day	13% 5% 2 5% 3 2 3 2 4 5% 55%	1762
	11th to 20th	Day	14% 7% 2 6% 3 4 3 3 6% 7% 46%	1800
	21st to end	Day	14% 8% 3 6% 4 4 4 3 5% 5% 45%	1800
December	1st to 10th	Day	15% 10% 2 5% 4 4 4 3 4 5% 44%	2930
	11th to 20th	Day	19% 11% 4 9% 4 5% 3 3 4 5% 31%	3373
	21st to end	Day	17% 11% 5% 8% 5% 4 5% 4 6% 7% 28%	4268
January	1st to 10th	Night	15% 10% 4 10% 4 3 4 3 4 6% 35%	6348
	11th to 20th	Night	17% 10% 3 8% 4 3 3 2 3 4 43%	6155
	21st to end	Night	18% 11% 4 7% 4 3 3 2 4 5% 39%	6048
February	1st to 10th	Night	20% 10% 3 8% 5% 4 4 4 4 5% 34%	2400
	11th to 20th	Night	22% 10% 3 8% 4 3 3 2 4 6% 35%	2338
	21st to end	Night	22% 8% 3 7% 5% 4 4 4 4 4 35%	1980
March	1st to 10th	Night	19% 10% 3 7% 4 4 4 3 5% 6% 35%	5373
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 6% 6% 49%	5210
	21st to end	Night	13% 8% 3 8% 4 4 4 3 5% 6% 41%	5192
April	1st to 10th	Night	11% 6% 2 7% 4 4 4 4 5% 7% 46%	2000
	11th to 20th	Night	14% 7% 2 4 3 4 4 3 6% 7% 47%	1975
	21st to end	Night	13% 7% 3 5% 5% 4 4 4 6% 8% 43%	1922
May	1st to 10th	Night	12% 7% 3 8% 5% 4 4 4 5% 7% 41%	2813
	11th to 20th	Night	12% 7% 3 7% 5% 4 4 3 6% 7% 42%	2812
	21st to end	Night	6% 6% 2 7% 5% 5% 6% 4 6% 11% 42%	3079
June	1st to 10th	Night	8% 6% 2 6% 5% 4 4 5% 7% 8% 46%	1701
	11th to 20th	Night	4 3 1 4 3 4 4 4 7% 10% 55%	1520
	21st to end	Night	4 3 1 5% 4 5% 4 5% 8% 12% 49%	1520
July	1st to 10th	Night	6% 4 2 6% 5% 6% 5% 5% 9% 12% 41%	2530
	11th to 20th	Night	6% 6% 2 5% 4 4 5% 4 7% 12% 46%	2618
	21st to end	Night	11% 9% 4 9% 6% 6% 7% 5% 7% 8% 27%	3087
August	1st to 10th	Night	3 3 2 4 4 3 4 3 6% 8% 61%	3034
	11th to 20th	Night	4 4 1 6% 4 5% 5% 5% 8% 12% 47%	3100
	21st to end	Night	4 3 1 4 3 4 4 4 6% 7% 58%	3476
September	1st to 10th	Night	11% 6% 2 6% 4 4 4 4 5% 7% 48%	1906
	11th to 20th	Night	10% 6% 2 6% 4 4 4 4 6% 7% 48%	2090
	21st to end	Night	11% 4 2 4 4 4 3 3 5% 7% 52%	2090
October	1st to 10th	Night	9% 4 2 4 3 3 2 3 4 5% 62%	2090
	11th to 20th	Night	14% 5% 2 5% 3 3 3 2 4 6% 51%	2128
	21st to end	Night	14% 6% 2 5% 4 3 3 3 4 6% 49%	2698
November	1st to 10th	Night	12% 6% 2 5% 4 3 4 2 5% 5% 53%	2536
	11th to 20th	Night	13% 7% 2 4 3 3 3 3 6% 7% 48%	2600
	21st to end	Night	16% 8% 2 5% 3 3 3 3 5% 5% 46%	2600
December	1st to 10th	Night	17% 11% 3 8% 4 3 3 2 4 4 41%	4250
	11th to 20th	Night	22% 10% 3 6% 3 3 3 3 3 4 41%	4822
	21st to end	Night	17% 11% 5% 9% 5% 5% 5% 4 5% 5% 28%	6674

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	55% 4 11% 18% 11% 2	3618
	11th to 20th	Day	47% 3 6% 20% 23% 1	3810
	21st to end	Day	47% 3 7% 22% 16% 5%	4905
February	1st to 10th	Day	53% 4 8% 21% 13% 3	2000
	11th to 20th	Day	55% 4 7% 19% 13% 3	1958
	21st to end	Day	49% 4 9% 17% 16% 6%	1650
March	1st to 10th	Day	52% 5% 4 15% 16% 9%	4480
	11th to 20th	Day	40% 4 7% 21% 20% 8%	4750
	21st to end	Day	42% 5% 8% 28% 16% 1	5781
April	1st to 10th	Day	38% 5% 12% 22% 20% 3	2400
	11th to 20th	Day	40% 5% 7% 22% 21% 6%	2374
	21st to end	Day	43% 6% 9% 25% 15% 2	2373
May	1st to 10th	Day	44% 8% 12% 22% 13% 1	4120
	11th to 20th	Day	44% 7% 12% 25% 13%	4085
	21st to end	Day	39% 9% 10% 22% 19% 1	4489
June	1st to 10th	Day	37% 9% 12% 22% 17% 3	2473
	11th to 20th	Day	30% 9% 16% 24% 19% 3	2660
	21st to end	Day	32% 10% 16% 22% 16% 4	2660
July	1st to 10th	Day	38% 8% 17% 27% 8% 2	4371
	11th to 20th	Day	39% 8% 15% 27% 9% 2	4269
	21st to end	Day	55% 9% 12% 17% 7% 1	4324
August	1st to 10th	Day	26% 7% 13% 34% 18% 1	3878
	11th to 20th	Day	36% 7% 13% 26% 16% 2	3765
	21st to end	Day	29% 6% 9% 31% 23% 2	4179
September	1st to 10th	Day	38% 10% 11% 20% 17% 4	2253
	11th to 20th	Day	38% 6% 10% 23% 20% 3	2090
	21st to end	Day	34% 5% 10% 25% 21% 5%	2090
October	1st to 10th	Day	21% 4 5% 18% 43% 9%	3480
	11th to 20th	Day	24% 3 6% 25% 29% 13%	3378
	21st to end	Day	31% 3 6% 24% 29% 8%	3099
November	1st to 10th	Day	32% 4 8% 23% 27% 5%	1762
	11th to 20th	Day	37% 4 7% 23% 24% 6%	1800
	21st to end	Day	40% 3 7% 26% 19% 5%	1800
December	1st to 10th	Day	43% 3 5% 28% 21% 1	2930
	11th to 20th	Day	55% 3 8% 19% 13% 2	3370
	21st to end	Day	54% 4 11% 23% 7% 1	4268
January	1st to 10th	Night	50% 3 10% 21% 15% 1	6348
	11th to 20th	Night	48% 3 4 19% 21% 4	6155
	21st to end	Night	49% 3 5% 21% 17% 5%	6048
February	1st to 10th	Night	53% 4 9% 17% 16% 1	2400
	11th to 20th	Night	53% 4 5% 18% 16% 4	2339
	21st to end	Night	51% 4 8% 16% 16% 5%	1980
March	1st to 10th	Night	50% 4 6% 15% 19% 7%	5373
	11th to 20th	Night	35% 4 7% 20% 20% 13%	5210
	21st to end	Night	43% 5% 8% 26% 17% 2	5192
April	1st to 10th	Night	38% 5% 10% 23% 21% 3	2000
	11th to 20th	Night	36% 4 8% 21% 22% 8%	1975
	21st to end	Night	39% 5% 9% 27% 17% 2	1922
May	1st to 10th	Night	42% 8% 12% 23% 13% 1	2840
	11th to 20th	Night	39% 8% 14% 20% 17%	2813
	21st to end	Night	33% 10% 13% 25% 17% 1	3087
June	1st to 10th	Night	33% 8% 14% 25% 17% 3	1701
	11th to 20th	Night	23% 7% 17% 31% 19% 3	1520
	21st to end	Night	26% 9% 17% 30% 15% 4	1520
July	1st to 10th	Night	33% 8% 21% 28% 9% 2	2490
	11th to 20th	Night	31% 6% 17% 35% 9% 3	2570
	21st to end	Night	53% 9% 12% 17% 9% 1	2988
August	1st to 10th	Night	21% 6% 10% 38% 23% 1	3023
	11th to 20th	Night	27% 7% 19% 31% 15% 1	3130
	21st to end	Night	24% 6% 11% 34% 23% 3	3476
September	1st to 10th	Night	34% 6% 11% 24% 19% 5%	1906
	11th to 20th	Night	35% 6% 10% 21% 26% 3	2090
	21st to end	Night	30% 5% 11% 23% 24% 7%	2090
October	1st to 10th	Night	20% 4 5% 20% 41% 11%	3480
	11th to 20th	Night	22% 3 6% 22% 32% 16%	3523
	21st to end	Night	25% 3 7% 25% 30% 11%	4436
November	1st to 10th	Night	35% 4 7% 22% 25% 7%	2536
	11th to 20th	Night	34% 4 7% 21% 27% 7%	2600
	21st to end	Night	40% 3 8% 23% 21% 5%	2600
December	1st to 10th	Night	48% 3 6% 17% 24% 2	4250
	11th to 20th	Night	48% 2 5% 23% 19% 2	4824
	21st to end	Night	57% 4 8% 21% 9% 1	6674

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% 36% 15% 6% 1	300
	11th to 20th	Day	30% 22% 30% 14% 3 1	300
	21st to end	Day	21% 22% 39% 14% 4	330
February	1st to 10th	Day	26% 20% 33% 17% 3 1	300
	11th to 20th	Day	37% 19% 31% 10% 2 1	300
	21st to end	Day	34% 20% 32% 13% 2	249
March	1st to 10th	Day	25% 24% 29% 19% 3 1	300
	11th to 20th	Day	22% 23% 32% 17% 6% 1	400
	21st to end	Day	21% 21% 34% 16% 7% 1	550
April	1st to 10th	Day	21% 18% 29% 23% 8% 1	500
	11th to 20th	Day	22% 13% 31% 24% 9% 2	492
	21st to end	Day	18% 17% 31% 20% 12% 3	480
May	1st to 10th	Day	19% 14% 27% 24% 13% 5%	463
	11th to 20th	Day	17% 7% 26% 25% 16% 8%	487
	21st to end	Day	10% 6% 21% 28% 21% 13%	529
June	1st to 10th	Day	11% 7% 19% 23% 23% 17%	493
	11th to 20th	Day	6% 4 17% 30% 29% 14%	680
	21st to end	Day	6% 4 19% 36% 21% 14%	680
July	1st to 10th	Day	9% 2 15% 30% 27% 17%	671
	11th to 20th	Day	8% 4 11% 27% 32% 18%	611
	21st to end	Day	10% 2 14% 28% 27% 20%	532
August	1st to 10th	Day	9% 2 12% 27% 33% 17%	486
	11th to 20th	Day	10% 3 17% 29% 33% 8%	479
	21st to end	Day	12% 1 10% 22% 35% 19%	539
September	1st to 10th	Day	10% 3 14% 27% 32% 13%	490
	11th to 20th	Day	14% 3 11% 24% 31% 17%	490
	21st to end	Day	16% 5% 19% 27% 25% 9%	490
October	1st to 10th	Day	13% 3 20% 29% 26% 9%	490
	11th to 20th	Day	13% 14% 27% 28% 14% 4	447
	21st to end	Day	22% 14% 27% 28% 7% 2	330
November	1st to 10th	Day	18% 10% 30% 27% 13% 3	300
	11th to 20th	Day	16% 16% 39% 22% 7%	300
	21st to end	Day	20% 15% 33% 18% 11% 3	300
December	1st to 10th	Day	28% 18% 29% 19% 6%	300
	11th to 20th	Day	21% 19% 32% 18% 9% 1	300
	21st to end	Day	26% 18% 31% 20% 4	330
January	1st to 10th	Night	21% 14% 33% 26% 5%	1000
	11th to 20th	Night	26% 16% 31% 22% 5%	912
	21st to end	Night	20% 13% 39% 26% 3	880
February	1st to 10th	Night	25% 12% 35% 25% 4	800
	11th to 20th	Night	30% 14% 32% 22% 2	783
	21st to end	Night	27% 17% 32% 21% 2	660
March	1st to 10th	Night	22% 14% 32% 26% 6%	800
	11th to 20th	Night	20% 12% 34% 26% 6% 1	800
	21st to end	Night	19% 14% 32% 27% 8%	760
April	1st to 10th	Night	14% 14% 33% 30% 9%	600
	11th to 20th	Night	17% 12% 33% 28% 8% 1	593
	21st to end	Night	17% 11% 28% 32% 11% 1	587
May	1st to 10th	Night	14% 7% 31% 29% 17% 2	570
	11th to 20th	Night	11% 5% 28% 32% 20% 4	561
	21st to end	Night	7% 3 18% 38% 29% 5%	617
June	1st to 10th	Night	11% 4 15% 35% 30% 4	570
	11th to 20th	Night	5% 2 12% 39% 36% 5%	570
	21st to end	Night	5% 1 14% 39% 35% 6%	570
July	1st to 10th	Night	9% 2 9% 32% 37% 11%	555
	11th to 20th	Night	9% 2 10% 29% 42% 10%	552
	21st to end	Night	8% 2 9% 31% 43% 7%	624
August	1st to 10th	Night	7% 1 8% 30% 47% 7%	567
	11th to 20th	Night	7% 1 11% 41% 35% 6%	564
	21st to end	Night	8% 1 8% 30% 44% 9%	627
September	1st to 10th	Night	11% 2 13% 29% 41% 5%	576
	11th to 20th	Night	13% 1 13% 30% 37% 6%	760
	21st to end	Night	13% 2 17% 30% 33% 4	760
October	1st to 10th	Night	11% 5% 18% 30% 30% 6%	760
	11th to 20th	Night	17% 8% 23% 31% 19% 1	760
	21st to end	Night	17% 8% 28% 28% 18% 1	836
November	1st to 10th	Night	15% 6% 23% 35% 18% 3	780
	11th to 20th	Night	15% 10% 33% 29% 13% 1	800
	21st to end	Night	20% 10% 27% 32% 11% 1	800
December	1st to 10th	Night	27% 12% 28% 26% 7% 1	800
	11th to 20th	Night	22% 8% 32% 30% 7% 1	800
	21st to end	Night	26% 12% 31% 26% 5%	960

Page generated at 2025-04-23 08:25:06. Version ac07

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