McKethan Park Observatory (VMI)
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the McKethan Park Observatory (VMI) clear sky chart from 2005-11-06 to 2026-05-17.

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% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	131515
	Night	13% 6% 2 6% 4 4 5% 4 5% 7% 43%	131280
Transparency	Day	38% 8% 13% 21% 15% 4	135120
	Night	37% 7% 12% 22% 17% 4	135745
Seeing	Day	20% 14% 25% 23% 12% 5%	17012
	Night	17% 12% 24% 30% 15% 2	27107

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	15% 7% 2 6% 4 4 4 3 4 6% 45%	5258
	11th to 20th	Day	10% 7% 3 7% 4 4 4 3 3 4 49%	5280
	21st to end	Day	15% 8% 3 6% 4 4 3 2 4 5% 47%	5780
February	1st to 10th	Day	12% 8% 3 7% 5% 5% 6% 4 5% 6% 41%	1890
	11th to 20th	Day	19% 6% 3 7% 4 4 4 3 4 6% 39%	1843
	21st to end	Day	18% 7% 2 6% 5% 4 5% 4 4 5% 39%	1828
March	1st to 10th	Day	14% 9% 3 8% 4 4 4 4 5% 8% 37%	5045
	11th to 20th	Day	10% 4 2 6% 4 4 4 4 5% 8% 49%	5090
	21st to end	Day	14% 7% 3 8% 5% 5% 4 3 5% 6% 40%	5586
April	1st to 10th	Day	10% 6% 3 5% 5% 4 4 3 6% 7% 46%	2428
	11th to 20th	Day	12% 6% 2 5% 5% 4 5% 4 5% 8% 45%	2494
	21st to end	Day	14% 7% 2 7% 5% 4 4 3 6% 6% 41%	2620
May	1st to 10th	Day	13% 6% 2 7% 6% 5% 7% 5% 7% 10% 32%	5992
	11th to 20th	Day	19% 8% 3 7% 5% 5% 5% 4 6% 8% 30%	5939
	21st to end	Day	14% 6% 2 6% 5% 5% 5% 4 7% 9% 38%	6433
June	1st to 10th	Day	10% 5% 2 5% 5% 5% 5% 5% 7% 9% 42%	2750
	11th to 20th	Day	10% 5% 2 6% 4 6% 6% 5% 7% 11% 37%	2800
	21st to end	Day	7% 5% 1 5% 6% 5% 5% 5% 8% 10% 40%	2660
July	1st to 10th	Day	6% 4 2 7% 6% 5% 6% 5% 8% 10% 40%	5900
	11th to 20th	Day	7% 6% 2 8% 6% 7% 7% 6% 10% 13% 29%	5900
	21st to end	Day	9% 6% 3 8% 6% 6% 7% 5% 8% 11% 30%	6424
August	1st to 10th	Day	13% 7% 3 8% 5% 6% 6% 5% 7% 9% 30%	4234
	11th to 20th	Day	9% 4 2 7% 6% 6% 7% 6% 9% 10% 35%	4174
	21st to end	Day	6% 5% 2 6% 4 5% 5% 4 7% 10% 46%	4312
September	1st to 10th	Day	11% 6% 2 6% 4 4 4 4 5% 8% 46%	2400
	11th to 20th	Day	12% 7% 2 7% 4 5% 5% 3 6% 7% 42%	2231
	21st to end	Day	19% 7% 3 7% 5% 4 5% 4 5% 6% 34%	2177
October	1st to 10th	Day	14% 5% 2 6% 4 4 4 3 5% 7% 47%	2000
	11th to 20th	Day	14% 5% 2 5% 3 3 4 3 5% 6% 50%	2000
	21st to end	Day	14% 6% 2 5% 4 3 4 2 5% 6% 49%	2200
November	1st to 10th	Day	11% 5% 2 5% 4 3 4 3 5% 6% 54%	2057
	11th to 20th	Day	13% 5% 2 5% 4 3 4 4 6% 7% 49%	1936
	21st to end	Day	13% 6% 3 6% 4 3 4 3 4 7% 47%	1680
December	1st to 10th	Day	12% 6% 3 7% 4 4 4 3 6% 6% 45%	2680
	11th to 20th	Day	18% 6% 2 6% 3 4 3 3 4 6% 45%	3070
	21st to end	Day	18% 6% 2 8% 5% 6% 5% 5% 6% 7% 32%	4424
January	1st to 10th	Night	12% 7% 2 6% 4 4 4 4 5% 5% 46%	7635
	11th to 20th	Night	12% 8% 2 5% 3 3 3 3 4 5% 53%	7680
	21st to end	Night	15% 6% 2 6% 4 3 3 3 4 5% 49%	8401
February	1st to 10th	Night	12% 8% 2 6% 5% 4 4 4 6% 7% 42%	2730
	11th to 20th	Night	19% 8% 2 6% 5% 3 4 3 5% 6% 40%	2663
	21st to end	Night	14% 8% 3 6% 4 4 5% 4 5% 7% 40%	1974
March	1st to 10th	Night	16% 6% 2 6% 4 4 4 4 5% 5% 43%	5028
	11th to 20th	Night	10% 4 2 4 4 4 4 4 6% 8% 52%	5090
	21st to end	Night	13% 7% 2 6% 5% 4 4 3 5% 7% 43%	5588
April	1st to 10th	Night	13% 6% 3 5% 4 4 4 4 6% 8% 43%	2190
	11th to 20th	Night	11% 6% 2 5% 4 4 4 5% 5% 8% 47%	2075
	21st to end	Night	12% 5% 2 7% 4 4 5% 4 6% 8% 43%	2016
May	1st to 10th	Night	14% 8% 3 7% 5% 4 5% 4 5% 9% 35%	4123
	11th to 20th	Night	20% 9% 3 8% 5% 5% 5% 4 5% 7% 30%	4086
	21st to end	Night	15% 6% 2 6% 4 4 5% 4 7% 9% 37%	4421
June	1st to 10th	Night	7% 6% 2 6% 5% 5% 5% 5% 7% 11% 42%	1640
	11th to 20th	Night	10% 6% 2 7% 4 4 4 5% 6% 8% 45%	1600
	21st to end	Night	8% 4 2 5% 5% 4 5% 4 7% 10% 46%	1729
July	1st to 10th	Night	7% 5% 2 8% 5% 5% 5% 5% 7% 10% 40%	4058
	11th to 20th	Night	10% 6% 3 9% 6% 6% 7% 5% 8% 11% 29%	4057
	21st to end	Night	9% 7% 3 9% 7% 7% 6% 5% 7% 10% 29%	4427
August	1st to 10th	Night	13% 7% 3 8% 5% 5% 5% 4 7% 9% 33%	2915
	11th to 20th	Night	10% 6% 3 8% 5% 6% 6% 5% 7% 9% 36%	2875
	21st to end	Night	8% 6% 2 6% 6% 4 4 4 7% 9% 44%	3553
September	1st to 10th	Night	9% 6% 2 7% 3 4 4 4 5% 7% 49%	2000
	11th to 20th	Night	11% 7% 3 6% 4 4 5% 4 7% 8% 42%	2165
	21st to end	Night	19% 8% 3 8% 4 4 4 3 5% 6% 36%	2222
October	1st to 10th	Night	13% 6% 2 6% 3 4 4 3 5% 6% 48%	2400
	11th to 20th	Night	13% 6% 2 5% 4 4 4 3 5% 7% 49%	2400
	21st to end	Night	13% 5% 2 4 3 3 4 3 5% 6% 52%	2640
November	1st to 10th	Night	11% 8% 2 6% 4 3 3 3 4 4 53%	2461
	11th to 20th	Night	13% 5% 2 6% 4 3 4 3 5% 7% 49%	2677
	21st to end	Night	12% 6% 2 5% 4 4 5% 3 5% 6% 48%	2940
December	1st to 10th	Night	12% 7% 2 6% 4 3 4 3 5% 5% 49%	4720
	11th to 20th	Night	17% 7% 3 6% 4 4 5% 4 5% 6% 39%	5345
	21st to end	Night	15% 6% 3 7% 4 4 5% 4 6% 8% 38%	6756

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	38% 5% 11% 21% 19% 6%	5259
	11th to 20th	Day	35% 5% 8% 18% 23% 11%	5280
	21st to end	Day	41% 4 6% 26% 18% 5%	5780
February	1st to 10th	Day	42% 7% 10% 21% 16% 3	1890
	11th to 20th	Day	45% 5% 10% 20% 17% 4	1852
	21st to end	Day	46% 6% 9% 18% 15% 5%	1828
March	1st to 10th	Day	42% 6% 12% 17% 15% 8%	5045
	11th to 20th	Day	32% 7% 9% 24% 21% 8%	5090
	21st to end	Day	44% 7% 9% 18% 16% 5%	5586
April	1st to 10th	Day	36% 7% 12% 22% 17% 6%	2428
	11th to 20th	Day	36% 7% 12% 23% 17% 4	2494
	21st to end	Day	42% 8% 11% 23% 13% 4	2620
May	1st to 10th	Day	43% 11% 15% 20% 10% 2	6027
	11th to 20th	Day	49% 9% 13% 18% 10% 1	5943
	21st to end	Day	39% 11% 15% 19% 15% 3	6447
June	1st to 10th	Day	36% 10% 16% 24% 14% 1	2750
	11th to 20th	Day	38% 12% 16% 20% 14% 1	2800
	21st to end	Day	34% 13% 15% 24% 12% 2	2660
July	1st to 10th	Day	35% 13% 19% 19% 10% 4	5835
	11th to 20th	Day	39% 16% 21% 17% 6% 1	5831
	21st to end	Day	42% 13% 16% 21% 8%	6240
August	1st to 10th	Day	44% 10% 20% 20% 6%	4221
	11th to 20th	Day	37% 12% 16% 26% 9% 1	4213
	21st to end	Day	31% 10% 21% 26% 11% 1	4312
September	1st to 10th	Day	36% 9% 13% 18% 21% 4	2400
	11th to 20th	Day	39% 7% 10% 23% 17% 3	2231
	21st to end	Day	49% 7% 10% 16% 16% 1	2177
October	1st to 10th	Day	31% 6% 9% 19% 28% 7%	3260
	11th to 20th	Day	24% 4 10% 24% 24% 13%	3240
	21st to end	Day	22% 4 11% 30% 23% 10%	3538
November	1st to 10th	Day	31% 5% 10% 23% 26% 5%	2057
	11th to 20th	Day	34% 5% 10% 21% 22% 7%	1936
	21st to end	Day	37% 5% 12% 23% 21% 3	1680
December	1st to 10th	Day	38% 7% 11% 19% 20% 6%	2680
	11th to 20th	Day	41% 5% 11% 20% 17% 5%	3066
	21st to end	Day	46% 7% 13% 21% 12% 1	4424
January	1st to 10th	Night	35% 6% 11% 23% 19% 6%	7636
	11th to 20th	Night	33% 4 8% 21% 25% 9%	7680
	21st to end	Night	37% 4 8% 26% 20% 5%	8401
February	1st to 10th	Night	40% 6% 12% 21% 18% 2	2730
	11th to 20th	Night	44% 5% 9% 19% 17% 5%	2665
	21st to end	Night	42% 7% 11% 21% 14% 5%	1974
March	1st to 10th	Night	39% 7% 9% 20% 18% 7%	5028
	11th to 20th	Night	28% 6% 9% 23% 28% 7%	5090
	21st to end	Night	40% 6% 12% 21% 17% 5%	5588
April	1st to 10th	Night	37% 7% 14% 19% 19% 4	2190
	11th to 20th	Night	34% 7% 14% 22% 19% 5%	2075
	21st to end	Night	37% 7% 12% 25% 15% 2	2016
May	1st to 10th	Night	43% 11% 15% 18% 11% 2	4150
	11th to 20th	Night	51% 10% 11% 18% 10% 1	4087
	21st to end	Night	39% 12% 16% 22% 8% 2	4429
June	1st to 10th	Night	35% 11% 12% 27% 14% 1	1640
	11th to 20th	Night	36% 10% 17% 23% 13% 2	1600
	21st to end	Night	30% 12% 17% 23% 16% 2	1729
July	1st to 10th	Night	36% 13% 19% 17% 13% 2	4015
	11th to 20th	Night	44% 15% 19% 16% 5%	4006
	21st to end	Night	44% 14% 16% 18% 9%	4297
August	1st to 10th	Night	43% 11% 20% 20% 5%	2904
	11th to 20th	Night	39% 11% 15% 23% 10% 1	2902
	21st to end	Night	32% 9% 20% 30% 8% 2	3553
September	1st to 10th	Night	34% 9% 15% 18% 19% 5%	2000
	11th to 20th	Night	37% 8% 13% 22% 17% 2	2165
	21st to end	Night	49% 7% 10% 17% 15% 3	2222
October	1st to 10th	Night	35% 6% 12% 20% 22% 6%	3920
	11th to 20th	Night	25% 6% 11% 20% 26% 12%	3899
	21st to end	Night	20% 4 11% 29% 26% 11%	4252
November	1st to 10th	Night	36% 5% 8% 21% 24% 6%	2461
	11th to 20th	Night	35% 5% 10% 21% 23% 6%	2677
	21st to end	Night	35% 5% 10% 24% 20% 5%	2940
December	1st to 10th	Night	34% 6% 10% 27% 21% 2	4720
	11th to 20th	Night	43% 5% 12% 19% 16% 4	5348
	21st to end	Night	39% 7% 15% 22% 15% 1	6756

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	37% 25% 24% 12% 2 1	330
	11th to 20th	Day	38% 26% 24% 11%	330
	21st to end	Day	34% 27% 26% 11% 1	363
February	1st to 10th	Day	26% 34% 26% 12% 2	330
	11th to 20th	Day	46% 21% 27% 5% 1	330
	21st to end	Day	33% 25% 26% 15% 2	387
March	1st to 10th	Day	30% 19% 29% 19% 4	540
	11th to 20th	Day	28% 21% 29% 17% 4 1	540
	21st to end	Day	25% 22% 30% 16% 6% 1	594
April	1st to 10th	Day	23% 26% 25% 17% 9%	540
	11th to 20th	Day	21% 19% 28% 20% 9% 3	532
	21st to end	Day	24% 17% 27% 19% 11% 2	540
May	1st to 10th	Day	22% 21% 28% 18% 8% 3	543
	11th to 20th	Day	22% 15% 28% 20% 12% 3	551
	21st to end	Day	19% 10% 25% 23% 15% 7%	573
June	1st to 10th	Day	12% 10% 30% 27% 15% 6%	530
	11th to 20th	Day	12% 8% 25% 31% 17% 7%	530
	21st to end	Day	9% 8% 27% 32% 16% 7%	530
July	1st to 10th	Day	10% 4 20% 31% 24% 10%	525
	11th to 20th	Day	6% 3 21% 35% 23% 12%	524
	21st to end	Day	11% 5% 20% 32% 23% 10%	576
August	1st to 10th	Day	17% 6% 21% 24% 22% 10%	526
	11th to 20th	Day	11% 4 23% 29% 22% 10%	519
	21st to end	Day	9% 5% 17% 32% 22% 14%	583
September	1st to 10th	Day	11% 5% 20% 30% 22% 12%	530
	11th to 20th	Day	14% 5% 18% 31% 23% 9%	530
	21st to end	Day	25% 13% 21% 25% 13% 4	530
October	1st to 10th	Day	17% 9% 21% 30% 18% 5%	530
	11th to 20th	Day	17% 15% 24% 28% 14% 2	530
	21st to end	Day	20% 16% 27% 26% 8% 2	583
November	1st to 10th	Day	18% 12% 31% 26% 11% 2	535
	11th to 20th	Day	25% 19% 28% 21% 5% 1	388
	21st to end	Day	26% 20% 28% 20% 5% 1	220
December	1st to 10th	Day	28% 23% 30% 16% 3	220
	11th to 20th	Day	31% 17% 27% 18% 6% 1	220
	21st to end	Day	30% 19% 29% 17% 5%	330
January	1st to 10th	Night	23% 21% 30% 23% 4	840
	11th to 20th	Night	22% 21% 29% 25% 3	840
	21st to end	Night	19% 18% 38% 23% 2	924
February	1st to 10th	Night	19% 18% 33% 29% 2	840
	11th to 20th	Night	26% 23% 31% 18% 1	823
	21st to end	Night	22% 20% 34% 23% 2	692
March	1st to 10th	Night	17% 16% 34% 28% 4	840
	11th to 20th	Night	25% 17% 29% 23% 7%	840
	21st to end	Night	19% 15% 35% 26% 6%	924
April	1st to 10th	Night	19% 19% 30% 27% 5%	840
	11th to 20th	Night	15% 18% 27% 30% 10%	831
	21st to end	Night	18% 10% 29% 34% 9%	756
May	1st to 10th	Night	17% 10% 28% 31% 14% 1	630
	11th to 20th	Night	18% 7% 26% 34% 14% 1	609
	21st to end	Night	16% 5% 14% 30% 31% 4	650
June	1st to 10th	Night	8% 5% 14% 44% 27% 2	440
	11th to 20th	Night	12% 4 16% 38% 29% 2	400
	21st to end	Night	8% 3 14% 43% 27% 4	529
July	1st to 10th	Night	11% 3 11% 36% 34% 6%	585
	11th to 20th	Night	6% 1 10% 33% 44% 5%	582
	21st to end	Night	6% 3 11% 39% 37% 4	657
August	1st to 10th	Night	14% 2 10% 36% 34% 4	597
	11th to 20th	Night	14% 3 12% 33% 36% 4	594
	21st to end	Night	11% 4 12% 34% 34% 6%	860
September	1st to 10th	Night	13% 3 11% 34% 33% 6%	800
	11th to 20th	Night	15% 4 13% 35% 29% 5%	800
	21st to end	Night	22% 8% 19% 29% 20% 2	800
October	1st to 10th	Night	15% 9% 19% 28% 25% 5%	800
	11th to 20th	Night	15% 11% 25% 31% 17% 1	800
	21st to end	Night	17% 9% 26% 34% 12% 1	880
November	1st to 10th	Night	17% 10% 26% 32% 13% 2	820
	11th to 20th	Night	18% 14% 25% 33% 9% 1	840
	21st to end	Night	16% 15% 28% 31% 8% 1	840
December	1st to 10th	Night	22% 15% 32% 24% 6%	840
	11th to 20th	Night	19% 13% 30% 33% 5%	840
	21st to end	Night	21% 14% 30% 32% 3	924

Page generated at 2026-05-14 01:59:00. Version ac07

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