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 2024-10-25.

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	12% 7% 3 7% 5% 4 5% 4 6% 8% 40%	101083
	Night	13% 7% 2 6% 4 4 4 3 5% 7% 43%	97716
Transparency	Day	41% 6% 10% 23% 16% 3	100882
	Night	40% 5% 10% 23% 18% 4	97587
Seeing	Day	16% 10% 23% 24% 18% 8%	15130
	Night	16% 8% 24% 29% 19% 3	24796

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% 12% 5% 10% 5% 5% 6% 4 6% 5% 29%	2527
	11th to 20th	Day	20% 8% 3 11% 4 4 4 4 4 5% 33%	2655
	21st to end	Day	22% 12% 6% 7% 3 3 4 3 5% 5% 30%	3423
February	1st to 10th	Day	17% 11% 3 7% 5% 4 4 3 3 6% 35%	1900
	11th to 20th	Day	20% 11% 4 7% 5% 4 3 3 4 6% 34%	1848
	21st to end	Day	19% 10% 3 7% 4 4 4 3 5% 5% 36%	1570
March	1st to 10th	Day	19% 13% 4 7% 5% 4 4 3 4 5% 31%	3120
	11th to 20th	Day	12% 7% 2 7% 4 4 4 3 5% 6% 47%	3320
	21st to end	Day	13% 8% 2 7% 5% 4 5% 3 6% 6% 41%	4033
April	1st to 10th	Day	13% 5% 2 6% 4 5% 4 4 6% 7% 45%	2280
	11th to 20th	Day	14% 8% 3 6% 5% 3 4 3 5% 6% 43%	2254
	21st to end	Day	13% 8% 2 6% 5% 5% 5% 4 5% 7% 40%	2313
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	17% 6% 2 6% 3 4 4 3 4 5% 47%	1842
November	1st to 10th	Day	12% 5% 2 4 3 2 3 2 5% 6% 57%	1672
	11th to 20th	Day	14% 6% 2 6% 3 4 3 3 6% 7% 47%	1710
	21st to end	Day	14% 8% 2 5% 4 4 4 3 5% 5% 45%	1710
December	1st to 10th	Day	22% 12% 3 5% 4 3 3 3 5% 6% 35%	1710
	11th to 20th	Day	16% 10% 2 7% 4 5% 4 4 5% 6% 37%	2153
	21st to end	Day	16% 11% 4 8% 6% 5% 5% 4 6% 7% 29%	2991
January	1st to 10th	Night	18% 10% 3 7% 4 3 4 3 5% 6% 36%	4428
	11th to 20th	Night	18% 11% 3 9% 5% 4 3 3 3 4 37%	4305
	21st to end	Night	21% 13% 4 6% 3 3 3 3 5% 5% 34%	4230
February	1st to 10th	Night	20% 10% 3 8% 5% 4 4 4 4 5% 34%	2280
	11th to 20th	Night	21% 10% 3 8% 4 3 3 2 4 5% 36%	2218
	21st to end	Night	23% 8% 3 7% 5% 4 4 4 5% 4 33%	1884
March	1st to 10th	Night	23% 10% 2 7% 4 4 4 3 4 5% 33%	3733
	11th to 20th	Night	13% 6% 2 5% 4 4 4 3 4 6% 48%	3640
	21st to end	Night	13% 8% 3 8% 4 4 4 3 5% 6% 41%	3626
April	1st to 10th	Night	10% 6% 2 7% 4 4 4 4 5% 7% 47%	1900
	11th to 20th	Night	15% 7% 2 4 3 3 4 2 5% 7% 47%	1875
	21st to end	Night	13% 7% 3 5% 4 4 4 4 6% 8% 44%	1866
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	15% 6% 2 5% 4 3 3 3 4 6% 49%	2619
November	1st to 10th	Night	11% 6% 2 5% 4 3 4 2 5% 5% 55%	2406
	11th to 20th	Night	12% 6% 2 4 3 3 3 3 6% 7% 49%	2470
	21st to end	Night	16% 7% 2 6% 3 4 3 3 5% 5% 46%	2470
December	1st to 10th	Night	20% 11% 3 7% 4 3 4 3 5% 5% 35%	2470
	11th to 20th	Night	15% 10% 3 6% 4 3 3 3 4 4 45%	3042
	21st to end	Night	16% 10% 4 10% 6% 5% 5% 4 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%	1570
March	1st to 10th	Day	55% 6% 6% 14% 12% 7%	3120
	11th to 20th	Day	39% 5% 9% 24% 18% 6%	3320
	21st to end	Day	42% 6% 9% 26% 16% 1	4033
April	1st to 10th	Day	38% 6% 11% 22% 20% 3	2280
	11th to 20th	Day	41% 5% 7% 21% 21% 6%	2254
	21st to end	Day	42% 6% 9% 26% 15% 2	2313
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	30% 5% 8% 23% 28% 6%	2090
	11th to 20th	Day	33% 4 7% 22% 26% 9%	2047
	21st to end	Day	40% 4 6% 20% 24% 5%	1842
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 8% 14% 15% 5%	1884
March	1st to 10th	Night	53% 5% 8% 12% 16% 6%	3733
	11th to 20th	Night	38% 5% 9% 22% 16% 10%	3640
	21st to end	Night	43% 6% 8% 23% 19% 1	3626
April	1st to 10th	Night	36% 5% 11% 24% 22% 3	1900
	11th to 20th	Night	37% 4 9% 20% 22% 9%	1875
	21st to end	Night	38% 5% 9% 28% 17% 2	1866
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	25% 5% 8% 23% 30% 9%	2090
	11th to 20th	Night	35% 4 8% 21% 24% 8%	2128
	21st to end	Night	36% 4 6% 22% 26% 6%	2619
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% 20% 30% 12% 1	225
March	1st to 10th	Day	25% 22% 30% 19% 3 1	270
	11th to 20th	Day	22% 23% 31% 17% 7% 1	365
	21st to end	Day	21% 20% 34% 17% 8% 1	506
April	1st to 10th	Day	20% 18% 28% 24% 9% 2	460
	11th to 20th	Day	22% 12% 30% 24% 9% 2	452
	21st to end	Day	18% 17% 31% 19% 12% 3	460
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	23% 15% 24% 28% 8% 2	311
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	29% 18% 32% 20% 2	628
March	1st to 10th	Night	22% 13% 32% 27% 6%	760
	11th to 20th	Night	20% 12% 34% 26% 7% 1	760
	21st to end	Night	19% 14% 31% 27% 9%	722
April	1st to 10th	Night	12% 14% 33% 32% 9%	570
	11th to 20th	Night	18% 12% 33% 27% 9% 1	563
	21st to end	Night	16% 11% 29% 32% 10% 1	570
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% 27% 28% 17% 1	812
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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