Vanderbilt Planetarium & Observatory
Clear Sky Chart
History

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

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	16% 7% 2 6% 4 4 4 4 5% 7% 41%	101262
	Night	17% 8% 2 6% 4 4 4 3 5% 7% 40%	97812
Transparency	Day	41% 5% 12% 23% 16% 2	104820
	Night	43% 4 11% 23% 16% 2	102343
Seeing	Day	18% 10% 28% 28% 13% 3	15051
	Night	20% 10% 31% 28% 10% 1	24129

By Month

Percentage Cloud Cover NE (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% 6% 2 7% 3 4 3 2 5% 6% 46%	2517
	11th to 20th	Day	16% 8% 3 6% 4 5% 5% 4 5% 6% 37%	2520
	21st to end	Day	22% 10% 3 8% 4 4 3 3 5% 6% 32%	2935
February	1st to 10th	Day	18% 7% 2 6% 4 4 4 3 5% 7% 40%	1761
	11th to 20th	Day	14% 7% 1 6% 4 4 4 2 5% 6% 46%	1848
	21st to end	Day	19% 9% 2 6% 4 4 4 3 4 6% 40%	1570
March	1st to 10th	Day	26% 10% 2 5% 3 3 2 3 4 5% 37%	3116
	11th to 20th	Day	9% 5% 1 4 4 4 5% 4 7% 9% 48%	3274
	21st to end	Day	20% 7% 2 5% 4 3 3 3 5% 5% 43%	4166
April	1st to 10th	Day	18% 8% 2 5% 4 5% 3 3 5% 8% 40%	2280
	11th to 20th	Day	16% 7% 2 5% 5% 4 3 3 4 7% 44%	2254
	21st to end	Day	19% 6% 2 5% 3 4 4 4 5% 6% 43%	2343
May	1st to 10th	Day	25% 10% 2 5% 3 3 4 3 5% 7% 33%	4106
	11th to 20th	Day	23% 7% 2 6% 4 4 4 3 5% 7% 35%	4288
	21st to end	Day	13% 5% 2 5% 4 3 4 3 6% 8% 47%	4837
June	1st to 10th	Day	16% 5% 2 5% 4 4 3 4 6% 9% 41%	2660
	11th to 20th	Day	13% 6% 2 5% 4 5% 5% 4 7% 8% 41%	2660
	21st to end	Day	10% 6% 2 5% 5% 4 4 4 5% 9% 45%	2660
July	1st to 10th	Day	8% 6% 2 7% 5% 6% 5% 5% 8% 10% 37%	4440
	11th to 20th	Day	10% 6% 2 6% 4 4 5% 5% 7% 10% 41%	4440
	21st to end	Day	14% 8% 3 7% 5% 5% 5% 4 6% 7% 36%	4814
August	1st to 10th	Day	17% 7% 2 7% 4 5% 5% 4 7% 9% 35%	4192
	11th to 20th	Day	11% 6% 2 6% 3 4 4 4 6% 9% 45%	3730
	21st to end	Day	8% 5% 2 5% 4 4 4 4 6% 10% 48%	4180
September	1st to 10th	Day	13% 5% 2 5% 3 4 4 3 5% 7% 47%	2280
	11th to 20th	Day	11% 6% 2 4 3 3 4 3 5% 7% 52%	2166
	21st to end	Day	18% 6% 2 6% 5% 4 4 4 4 6% 41%	2090
October	1st to 10th	Day	21% 6% 2 5% 3 2 4 3 5% 7% 43%	2033
	11th to 20th	Day	14% 5% 2 5% 4 3 4 4 5% 6% 48%	1900
	21st to end	Day	20% 5% 2 5% 3 4 4 3 5% 7% 42%	1900
November	1st to 10th	Day	14% 5% 2 5% 4 4 3 3 4 5% 50%	1717
	11th to 20th	Day	17% 7% 2 5% 3 3 3 3 4 6% 47%	1710
	21st to end	Day	14% 7% 3 5% 4 4 4 3 5% 7% 45%	1677
December	1st to 10th	Day	17% 7% 2 7% 4 4 5% 4 7% 8% 36%	1520
	11th to 20th	Day	21% 7% 2 4 2 2 3 3 4 6% 45%	1906
	21st to end	Day	18% 10% 3 8% 5% 4 4 3 5% 6% 33%	2772
January	1st to 10th	Night	18% 7% 2 5% 3 4 3 2 4 7% 45%	4429
	11th to 20th	Night	17% 9% 3 7% 3 3 3 3 4 5% 43%	4440
	21st to end	Night	24% 11% 4 7% 4 3 3 3 4 5% 32%	4720
February	1st to 10th	Night	16% 9% 3 5% 4 4 3 3 5% 5% 41%	2418
	11th to 20th	Night	18% 8% 2 5% 4 4 4 4 5% 6% 40%	2218
	21st to end	Night	20% 6% 2 5% 4 4 3 3 6% 5% 42%	1884
March	1st to 10th	Night	23% 11% 2 6% 4 3 3 3 4 5% 36%	3724
	11th to 20th	Night	13% 5% 2 6% 5% 5% 4 3 6% 6% 45%	3679
	21st to end	Night	22% 8% 2 5% 3 3 3 3 5% 5% 40%	3466
April	1st to 10th	Night	19% 7% 2 5% 4 4 5% 4 5% 6% 38%	1900
	11th to 20th	Night	19% 7% 3 5% 4 3 4 3 4 6% 41%	1875
	21st to end	Night	19% 7% 3 6% 4 3 3 3 4 5% 42%	1834
May	1st to 10th	Night	24% 10% 3 7% 5% 4 3 3 4 8% 29%	2834
	11th to 20th	Night	24% 9% 3 7% 5% 4 4 4 4 6% 31%	2633
	21st to end	Night	16% 8% 3 5% 4 3 4 3 5% 7% 43%	2738
June	1st to 10th	Night	16% 7% 2 6% 5% 5% 3 4 4 8% 40%	1520
	11th to 20th	Night	14% 6% 2 6% 3 4 3 4 7% 9% 40%	1520
	21st to end	Night	9% 6% 2 7% 4 4 6% 4 7% 9% 41%	1520
July	1st to 10th	Night	10% 9% 3 8% 5% 5% 6% 5% 7% 9% 34%	2520
	11th to 20th	Night	11% 8% 3 8% 4 4 6% 4 7% 8% 37%	2520
	21st to end	Night	16% 9% 3 7% 5% 4 4 4 7% 9% 32%	2739
August	1st to 10th	Night	21% 7% 2 6% 5% 5% 5% 4 6% 7% 31%	2703
	11th to 20th	Night	13% 7% 3 7% 4 5% 4 4 7% 8% 38%	3097
	21st to end	Night	8% 5% 1 5% 4 4 4 3 6% 8% 52%	3476
September	1st to 10th	Night	13% 5% 2 6% 5% 4 5% 4 6% 6% 45%	1900
	11th to 20th	Night	13% 5% 2 5% 3 3 3 2 5% 6% 51%	2012
	21st to end	Night	18% 5% 2 6% 5% 3 4 3 4 7% 42%	2090
October	1st to 10th	Night	19% 8% 3 7% 4 3 3 3 4 6% 40%	2142
	11th to 20th	Night	15% 5% 2 5% 3 3 3 3 5% 6% 50%	2280
	21st to end	Night	21% 6% 2 6% 4 4 3 3 4 6% 41%	2698
November	1st to 10th	Night	13% 5% 2 6% 4 4 4 3 4 6% 49%	2483
	11th to 20th	Night	18% 7% 2 4 4 2 3 3 4 5% 47%	2470
	21st to end	Night	15% 6% 2 5% 4 4 3 3 6% 6% 47%	2498
December	1st to 10th	Night	21% 7% 3 5% 4 4 4 3 5% 7% 38%	2660
	11th to 20th	Night	19% 7% 2 5% 4 4 3 3 5% 6% 41%	3288
	21st to end	Night	20% 9% 3 6% 4 4 4 3 5% 6% 37%	4884

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	40% 4 9% 32% 13% 2	2518
	11th to 20th	Day	46% 5% 10% 25% 12% 1	2520
	21st to end	Day	54% 6% 8% 21% 9% 1	2935
February	1st to 10th	Day	45% 5% 12% 23% 14% 1	1761
	11th to 20th	Day	41% 4 9% 26% 17% 2	1857
	21st to end	Day	48% 5% 10% 21% 14% 3	1570
March	1st to 10th	Day	50% 6% 11% 21% 11% 2	3119
	11th to 20th	Day	31% 8% 13% 32% 15% 1	3274
	21st to end	Day	44% 4 8% 18% 21% 6%	4166
April	1st to 10th	Day	45% 5% 8% 21% 16% 4	2280
	11th to 20th	Day	42% 6% 11% 22% 17% 3	2254
	21st to end	Day	42% 6% 10% 21% 17% 4	2343
May	1st to 10th	Day	52% 5% 15% 15% 11% 1	4120
	11th to 20th	Day	50% 7% 14% 19% 9% 2	4292
	21st to end	Day	35% 7% 13% 26% 18% 2	4837
June	1st to 10th	Day	40% 7% 13% 23% 15% 2	2660
	11th to 20th	Day	41% 7% 13% 25% 13% 2	2660
	21st to end	Day	37% 7% 16% 24% 15% 1	2660
July	1st to 10th	Day	42% 8% 13% 20% 16% 1	4370
	11th to 20th	Day	40% 5% 18% 23% 13% 1	4365
	21st to end	Day	47% 6% 12% 18% 15% 2	4661
August	1st to 10th	Day	47% 5% 16% 22% 9%	4178
	11th to 20th	Day	38% 5% 13% 34% 10% 1	3766
	21st to end	Day	33% 4 13% 28% 20% 2	4180
September	1st to 10th	Day	38% 5% 13% 21% 20% 2	2280
	11th to 20th	Day	34% 4 11% 25% 24% 2	2166
	21st to end	Day	44% 5% 10% 19% 19% 3	2090
October	1st to 10th	Day	38% 4 10% 20% 24% 5%	3384
	11th to 20th	Day	26% 3 10% 23% 33% 5%	3140
	21st to end	Day	30% 3 14% 26% 24% 3	3112
November	1st to 10th	Day	36% 4 14% 23% 21% 2	1717
	11th to 20th	Day	39% 3 10% 23% 21% 3	1710
	21st to end	Day	39% 4 14% 28% 12% 2	1677
December	1st to 10th	Day	45% 5% 16% 19% 14% 1	1520
	11th to 20th	Day	42% 4 14% 22% 16% 2	1906
	21st to end	Day	50% 6% 11% 23% 9%	2772
January	1st to 10th	Night	40% 2 11% 30% 12% 3	4429
	11th to 20th	Night	45% 4 12% 24% 13% 2	4440
	21st to end	Night	54% 3 8% 22% 12% 1	4720
February	1st to 10th	Night	44% 4 13% 24% 13% 2	2418
	11th to 20th	Night	44% 5% 9% 20% 17% 4	2220
	21st to end	Night	44% 4 7% 24% 17% 4	1884
March	1st to 10th	Night	53% 3 9% 21% 11% 3	3734
	11th to 20th	Night	39% 4 12% 26% 15% 3	3680
	21st to end	Night	47% 3 6% 15% 22% 6%	3466
April	1st to 10th	Night	47% 4 9% 19% 17% 3	1900
	11th to 20th	Night	44% 5% 10% 19% 21% 1	1875
	21st to end	Night	45% 4 9% 21% 17% 5%	1834
May	1st to 10th	Night	56% 5% 12% 16% 10% 1	2840
	11th to 20th	Night	55% 6% 14% 15% 9% 1	2634
	21st to end	Night	42% 5% 13% 23% 14% 2	2739
June	1st to 10th	Night	43% 5% 10% 21% 19% 1	1520
	11th to 20th	Night	39% 7% 15% 27% 12% 1	1520
	21st to end	Night	38% 8% 13% 26% 14% 1	1520
July	1st to 10th	Night	47% 7% 14% 19% 13% 1	2480
	11th to 20th	Night	45% 5% 14% 25% 10% 1	2472
	21st to end	Night	49% 5% 15% 16% 15% 2	2651
August	1st to 10th	Night	50% 6% 17% 17% 9% 1	2693
	11th to 20th	Night	42% 5% 14% 30% 9% 1	3127
	21st to end	Night	30% 5% 11% 34% 18% 1	3476
September	1st to 10th	Night	39% 6% 10% 25% 19% 1	1900
	11th to 20th	Night	35% 4 11% 25% 23% 3	2012
	21st to end	Night	43% 5% 8% 21% 20% 3	2090
October	1st to 10th	Night	42% 3 10% 24% 19% 2	3571
	11th to 20th	Night	25% 3 10% 28% 31% 4	3779
	21st to end	Night	32% 3 13% 29% 21% 3	4436
November	1st to 10th	Night	36% 4 11% 24% 20% 4	2483
	11th to 20th	Night	40% 3 9% 25% 19% 4	2470
	21st to end	Night	37% 4 13% 30% 14% 2	2498
December	1st to 10th	Night	46% 5% 13% 22% 14% 1	2660
	11th to 20th	Night	43% 4 12% 20% 18% 3	3288
	21st to end	Night	48% 5% 10% 25% 12% 1	4884

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% 18% 34% 20% 4	180
	11th to 20th	Day	17% 28% 36% 16% 4	180
	21st to end	Day	19% 14% 42% 18% 5% 2	245
February	1st to 10th	Day	24% 12% 44% 18% 2	321
	11th to 20th	Day	19% 20% 40% 19% 2	456
	21st to end	Day	24% 19% 32% 20% 6%	382
March	1st to 10th	Day	16% 18% 37% 25% 4	460
	11th to 20th	Day	17% 14% 39% 25% 4 1	460
	21st to end	Day	23% 15% 36% 19% 6% 1	506
April	1st to 10th	Day	22% 16% 37% 17% 8% 1	460
	11th to 20th	Day	20% 13% 33% 24% 9%	452
	21st to end	Day	25% 11% 32% 25% 6% 1	460
May	1st to 10th	Day	28% 11% 30% 23% 8% 1	463
	11th to 20th	Day	20% 8% 29% 33% 10% 1	487
	21st to end	Day	16% 4 21% 35% 21% 3	529
June	1st to 10th	Day	18% 5% 22% 34% 17% 2	490
	11th to 20th	Day	14% 8% 27% 36% 13% 2	490
	21st to end	Day	13% 5% 28% 33% 19% 3	490
July	1st to 10th	Day	13% 3 19% 37% 24% 4	485
	11th to 20th	Day	10% 4 20% 38% 24% 5%	484
	21st to end	Day	13% 5% 20% 41% 18% 3	532
August	1st to 10th	Day	14% 3 17% 37% 22% 6%	486
	11th to 20th	Day	14% 4 18% 36% 24% 4	479
	21st to end	Day	12% 4 21% 37% 19% 7%	539
September	1st to 10th	Day	18% 6% 21% 28% 21% 6%	490
	11th to 20th	Day	15% 3 21% 31% 21% 9%	490
	21st to end	Day	24% 4 27% 27% 14% 4	490
October	1st to 10th	Day	24% 8% 23% 24% 16% 6%	490
	11th to 20th	Day	14% 12% 30% 28% 13% 2	490
	21st to end	Day	21% 12% 31% 24% 10% 2	439
November	1st to 10th	Day	15% 12% 26% 33% 11% 2	381
	11th to 20th	Day	19% 19% 31% 24% 7% 1	370
	21st to end	Day	18% 16% 32% 26% 8% 1	337
December	1st to 10th	Day	19% 18% 37% 19% 7% 1	180
	11th to 20th	Day	29% 22% 27% 17% 5%	180
	21st to end	Day	23% 18% 38% 14% 7% 1	198
January	1st to 10th	Night	20% 18% 39% 19% 4	760
	11th to 20th	Night	25% 16% 32% 24% 2	760
	21st to end	Night	19% 16% 41% 21% 2	836
February	1st to 10th	Night	20% 14% 39% 24% 3	760
	11th to 20th	Night	23% 17% 40% 18% 2	743
	21st to end	Night	24% 17% 35% 19% 5%	628
March	1st to 10th	Night	21% 16% 36% 22% 5%	760
	11th to 20th	Night	20% 13% 39% 22% 5% 1	760
	21st to end	Night	22% 14% 38% 21% 5%	836
April	1st to 10th	Night	22% 12% 37% 24% 5%	760
	11th to 20th	Night	21% 12% 33% 28% 6%	751
	21st to end	Night	21% 9% 35% 29% 6% 1	694
May	1st to 10th	Night	24% 8% 32% 28% 8% 1	570
	11th to 20th	Night	22% 9% 34% 29% 6%	460
	21st to end	Night	21% 5% 18% 37% 17% 2	411
June	1st to 10th	Night	20% 5% 25% 34% 16%	380
	11th to 20th	Night	18% 4 19% 43% 16% 1	380
	21st to end	Night	11% 4 23% 48% 13% 1	380
July	1st to 10th	Night	12% 3 23% 42% 18% 2	370
	11th to 20th	Night	13% 5% 17% 44% 19% 2	368
	21st to end	Night	15% 2 19% 46% 17% 1	416
August	1st to 10th	Night	11% 2 21% 45% 20% 1	484
	11th to 20th	Night	16% 3 17% 37% 25% 2	750
	21st to end	Night	13% 4 17% 38% 24% 4	836
September	1st to 10th	Night	16% 3 24% 36% 18% 4	760
	11th to 20th	Night	17% 5% 19% 34% 21% 6%	760
	21st to end	Night	21% 3 24% 33% 17% 2	760
October	1st to 10th	Night	23% 7% 25% 27% 16% 3	760
	11th to 20th	Night	17% 10% 30% 30% 12% 1	760
	21st to end	Night	23% 10% 33% 25% 8%	836
November	1st to 10th	Night	14% 11% 34% 28% 11% 2	764
	11th to 20th	Night	21% 14% 33% 23% 8%	760
	21st to end	Night	17% 15% 34% 25% 7% 1	760
December	1st to 10th	Night	25% 16% 33% 19% 5% 1	760
	11th to 20th	Night	22% 15% 36% 22% 4	760
	21st to end	Night	23% 14% 39% 21% 3	836

Page generated at 2024-11-03 13:19:01. Version ac07

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