Mountville Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mountville Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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	11% 6% 2 6% 4 5% 5% 4 6% 8% 42%	127173
	Night	11% 7% 2 6% 4 4 4 4 6% 8% 44%	124525
Transparency	Day	38% 7% 12% 19% 18% 6%	130985
	Night	36% 7% 11% 19% 19% 8%	128769
Seeing	Day	15% 10% 24% 24% 18% 8%	16382
	Night	14% 9% 23% 29% 20% 4	26419

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% 9% 3 4 3 3 2 3 4 4 49%	4146
	11th to 20th	Day	11% 9% 3 4 3 2 3 3 5% 7% 51%	4114
	21st to end	Day	20% 11% 3 6% 3 3 3 3 4 5% 39%	4855
February	1st to 10th	Day	15% 8% 3 7% 5% 5% 4 3 6% 6% 38%	2000
	11th to 20th	Day	20% 7% 3 6% 3 4 4 3 4 5% 42%	1948
	21st to end	Day	15% 7% 3 8% 4 5% 4 4 5% 5% 40%	1650
March	1st to 10th	Day	19% 9% 3 7% 4 4 4 3 5% 6% 36%	4545
	11th to 20th	Day	10% 7% 2 5% 4 4 4 4 5% 6% 50%	4980
	21st to end	Day	16% 6% 2 6% 4 4 4 3 5% 6% 44%	5465
April	1st to 10th	Day	11% 6% 3 7% 4 4 5% 3 5% 6% 46%	2304
	11th to 20th	Day	12% 6% 2 6% 4 4 3 3 5% 7% 49%	2374
	21st to end	Day	7% 5% 2 5% 5% 4 4 4 6% 8% 51%	2490
May	1st to 10th	Day	11% 6% 2 8% 5% 5% 5% 4 7% 8% 38%	5862
	11th to 20th	Day	15% 8% 2 7% 5% 5% 5% 5% 7% 9% 33%	5861
	21st to end	Day	9% 5% 2 7% 5% 5% 6% 4 6% 9% 43%	6433
June	1st to 10th	Day	8% 7% 3 8% 7% 7% 7% 6% 8% 8% 32%	2600
	11th to 20th	Day	6% 4 1 6% 5% 5% 7% 6% 9% 12% 40%	2600
	21st to end	Day	5% 6% 2 8% 6% 7% 7% 6% 8% 11% 35%	2600
July	1st to 10th	Day	8% 6% 3 8% 6% 7% 7% 6% 9% 12% 29%	5900
	11th to 20th	Day	5% 5% 2 6% 6% 6% 7% 6% 9% 13% 36%	5900
	21st to end	Day	7% 7% 3 9% 7% 8% 8% 7% 10% 12% 22%	6424
August	1st to 10th	Day	6% 5% 3 7% 4 7% 7% 5% 8% 12% 36%	4110
	11th to 20th	Day	7% 4 2 6% 4 5% 5% 4 7% 8% 47%	3849
	21st to end	Day	7% 4 1 5% 4 5% 5% 5% 7% 10% 47%	4312
September	1st to 10th	Day	10% 6% 3 6% 6% 6% 6% 5% 6% 8% 38%	2231
	11th to 20th	Day	13% 7% 2 6% 4 4 5% 3 6% 8% 43%	2200
	21st to end	Day	11% 6% 2 6% 4 5% 5% 3 5% 6% 48%	2200
October	1st to 10th	Day	11% 5% 2 5% 3 3 3 4 4 7% 52%	2200
	11th to 20th	Day	10% 4 2 4 2 3 3 2 4 6% 59%	2145
	21st to end	Day	11% 5% 2 5% 3 3 3 3 4 5% 55%	2195
November	1st to 10th	Day	10% 7% 3 6% 3 3 4 3 4 5% 53%	1852
	11th to 20th	Day	13% 5% 1 3 3 3 3 3 5% 6% 57%	1890
	21st to end	Day	12% 5% 2 6% 4 4 4 3 5% 8% 47%	1890
December	1st to 10th	Day	17% 7% 2 4 3 3 3 4 5% 5% 47%	3020
	11th to 20th	Day	14% 8% 3 6% 4 4 3 3 4 6% 45%	3463
	21st to end	Day	18% 8% 3 5% 4 4 4 3 4 6% 41%	4565
January	1st to 10th	Night	14% 8% 3 5% 4 3 3 3 5% 5% 47%	6019
	11th to 20th	Night	13% 7% 3 7% 4 4 3 3 4 6% 45%	5979
	21st to end	Night	17% 11% 3 5% 3 3 4 3 5% 5% 41%	6098
February	1st to 10th	Night	14% 9% 3 7% 5% 4 3 3 5% 5% 43%	2400
	11th to 20th	Night	19% 8% 3 5% 4 3 3 3 4 4 43%	2338
	21st to end	Night	15% 8% 2 6% 4 4 4 4 5% 6% 42%	1980
March	1st to 10th	Night	20% 9% 3 8% 4 4 4 3 5% 6% 36%	5292
	11th to 20th	Night	10% 6% 2 6% 4 3 4 3 5% 7% 50%	4980
	21st to end	Night	12% 6% 2 7% 4 4 4 3 5% 6% 46%	5467
April	1st to 10th	Night	11% 6% 2 5% 4 5% 5% 4 5% 6% 48%	2111
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 5% 8% 50%	1975
	21st to end	Night	6% 5% 2 5% 3 4 6% 3 5% 6% 53%	1906
May	1st to 10th	Night	10% 6% 2 7% 5% 5% 5% 4 7% 11% 38%	4033
	11th to 20th	Night	16% 8% 2 6% 5% 5% 5% 4 7% 8% 33%	4032
	21st to end	Night	8% 5% 2 5% 4 4 4 4 7% 12% 45%	4421
June	1st to 10th	Night	6% 6% 3 7% 6% 5% 6% 6% 9% 11% 37%	1800
	11th to 20th	Night	4 5% 2 6% 4 3 5% 5% 9% 11% 47%	1800
	21st to end	Night	5% 5% 2 6% 6% 6% 5% 6% 9% 11% 40%	1800
July	1st to 10th	Night	8% 7% 3 8% 6% 6% 7% 6% 8% 11% 29%	4058
	11th to 20th	Night	5% 5% 2 6% 5% 5% 6% 6% 8% 14% 38%	4057
	21st to end	Night	7% 7% 3 8% 7% 6% 7% 6% 9% 13% 28%	4427
August	1st to 10th	Night	6% 5% 2 7% 5% 6% 7% 5% 10% 12% 36%	3034
	11th to 20th	Night	5% 4 2 6% 4 4 5% 4 7% 11% 48%	3200
	21st to end	Night	7% 4 2 5% 4 4 5% 4 6% 9% 50%	3586
September	1st to 10th	Night	8% 5% 2 5% 4 5% 5% 5% 7% 9% 43%	2147
	11th to 20th	Night	10% 6% 2 7% 5% 4 4 4 6% 7% 44%	2200
	21st to end	Night	9% 5% 2 5% 4 5% 5% 4 6% 7% 47%	2200
October	1st to 10th	Night	9% 5% 2 5% 3 3 4 3 5% 6% 56%	2200
	11th to 20th	Night	9% 5% 2 4 4 3 3 2 4 5% 61%	2254
	21st to end	Night	11% 5% 2 6% 4 3 3 2 3 4 59%	2640
November	1st to 10th	Night	10% 6% 2 5% 3 3 3 3 5% 4 56%	2666
	11th to 20th	Night	13% 5% 2 4 3 4 2 3 3 5% 56%	2730
	21st to end	Night	12% 7% 2 5% 4 4 4 3 5% 6% 49%	2730
December	1st to 10th	Night	14% 8% 2 6% 3 3 3 2 4 6% 51%	4380
	11th to 20th	Night	15% 6% 2 5% 3 3 4 3 5% 6% 48%	4952
	21st to end	Night	17% 8% 2 6% 4 4 3 3 4 6% 42%	6633

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	37% 4 5% 20% 26% 7%	4147
	11th to 20th	Day	33% 3 5% 19% 31% 9%	4114
	21st to end	Day	48% 3 7% 12% 20% 10%	4855
February	1st to 10th	Day	45% 5% 8% 16% 20% 6%	2000
	11th to 20th	Day	45% 4 6% 16% 22% 7%	1958
	21st to end	Day	43% 7% 8% 13% 20% 9%	1650
March	1st to 10th	Day	47% 6% 7% 11% 22% 7%	4545
	11th to 20th	Day	33% 6% 7% 13% 21% 19%	4980
	21st to end	Day	41% 5% 8% 18% 20% 9%	5465
April	1st to 10th	Day	38% 6% 10% 20% 18% 9%	2304
	11th to 20th	Day	34% 5% 8% 18% 25% 10%	2374
	21st to end	Day	30% 7% 12% 21% 26% 4	2490
May	1st to 10th	Day	40% 8% 14% 19% 17% 2	5897
	11th to 20th	Day	44% 8% 18% 19% 10% 1	5865
	21st to end	Day	35% 9% 13% 22% 19% 1	6447
June	1st to 10th	Day	44% 11% 15% 18% 11% 2	2600
	11th to 20th	Day	32% 11% 19% 22% 15% 2	2600
	21st to end	Day	39% 12% 18% 19% 10% 3	2600
July	1st to 10th	Day	42% 13% 21% 19% 4 1	5835
	11th to 20th	Day	35% 13% 30% 19% 3	5831
	21st to end	Day	48% 14% 19% 17% 3	6240
August	1st to 10th	Day	37% 9% 15% 28% 11%	4097
	11th to 20th	Day	32% 9% 14% 29% 15% 1	3885
	21st to end	Day	30% 9% 17% 24% 16% 3	4311
September	1st to 10th	Day	42% 10% 13% 16% 14% 4	2231
	11th to 20th	Day	38% 8% 12% 19% 19% 3	2200
	21st to end	Day	36% 6% 10% 20% 21% 6%	2200
October	1st to 10th	Day	27% 4 6% 13% 31% 18%	3590
	11th to 20th	Day	17% 3 5% 17% 28% 32%	3476
	21st to end	Day	23% 3 5% 18% 39% 12%	3521
November	1st to 10th	Day	34% 4 7% 15% 27% 13%	1852
	11th to 20th	Day	29% 4 8% 18% 26% 15%	1890
	21st to end	Day	36% 6% 8% 17% 25% 9%	1890
December	1st to 10th	Day	39% 5% 5% 18% 25% 8%	3020
	11th to 20th	Day	42% 3 8% 18% 21% 8%	3460
	21st to end	Day	43% 5% 7% 21% 21% 3	4565
January	1st to 10th	Night	38% 4 5% 20% 26% 6%	6019
	11th to 20th	Night	40% 4 5% 18% 25% 9%	5979
	21st to end	Night	45% 3 6% 13% 27% 6%	6098
February	1st to 10th	Night	42% 4 6% 18% 19% 10%	2400
	11th to 20th	Night	44% 4 5% 15% 22% 9%	2339
	21st to end	Night	40% 6% 8% 15% 18% 13%	1980
March	1st to 10th	Night	48% 5% 7% 14% 16% 10%	5292
	11th to 20th	Night	33% 6% 8% 13% 21% 19%	4980
	21st to end	Night	38% 5% 6% 19% 21% 10%	5467
April	1st to 10th	Night	36% 6% 10% 18% 20% 11%	2111
	11th to 20th	Night	32% 5% 7% 22% 24% 10%	1975
	21st to end	Night	29% 7% 12% 23% 22% 7%	1906
May	1st to 10th	Night	35% 11% 18% 20% 14% 2	4060
	11th to 20th	Night	44% 9% 17% 21% 8% 1	4033
	21st to end	Night	29% 7% 18% 25% 18% 2	4429
June	1st to 10th	Night	35% 12% 19% 18% 15% 1	1800
	11th to 20th	Night	27% 10% 22% 27% 12% 2	1800
	21st to end	Night	32% 12% 20% 21% 12% 3	1800
July	1st to 10th	Night	44% 13% 21% 17% 5% 1	4015
	11th to 20th	Night	34% 13% 31% 17% 4 1	4006
	21st to end	Night	44% 13% 23% 16% 3	4297
August	1st to 10th	Night	35% 11% 17% 27% 10% 1	3023
	11th to 20th	Night	29% 9% 16% 31% 13% 2	3230
	21st to end	Night	29% 8% 16% 25% 19% 3	3586
September	1st to 10th	Night	34% 10% 16% 21% 15% 5%	2147
	11th to 20th	Night	37% 9% 11% 23% 17% 4	2200
	21st to end	Night	34% 8% 10% 20% 20% 8%	2200
October	1st to 10th	Night	31% 5% 8% 14% 27% 16%	3590
	11th to 20th	Night	16% 3 4 15% 29% 32%	3662
	21st to end	Night	21% 3 4 20% 34% 18%	4252
November	1st to 10th	Night	31% 5% 9% 15% 27% 14%	2666
	11th to 20th	Night	32% 4 5% 17% 26% 15%	2730
	21st to end	Night	36% 4 8% 17% 25% 10%	2730
December	1st to 10th	Night	37% 3 6% 19% 26% 8%	4380
	11th to 20th	Night	38% 4 11% 16% 24% 7%	4954
	21st to end	Night	42% 5% 7% 22% 22% 2	6633

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	28% 20% 40% 11% 1	330
	11th to 20th	Day	25% 22% 34% 13% 5% 1	318
	21st to end	Day	26% 26% 35% 9% 3 1	330
February	1st to 10th	Day	25% 19% 38% 16% 2	300
	11th to 20th	Day	36% 21% 32% 10% 1	300
	21st to end	Day	26% 22% 41% 10% 2	249
March	1st to 10th	Day	22% 25% 30% 18% 3 1	326
	11th to 20th	Day	22% 18% 36% 16% 7%	500
	21st to end	Day	19% 20% 35% 20% 5% 1	550
April	1st to 10th	Day	22% 16% 33% 19% 9% 1	500
	11th to 20th	Day	15% 17% 34% 22% 10% 2	492
	21st to end	Day	13% 11% 31% 28% 14% 2	500
May	1st to 10th	Day	12% 13% 31% 26% 13% 5%	503
	11th to 20th	Day	12% 8% 26% 29% 17% 8%	527
	21st to end	Day	11% 6% 22% 32% 18% 11%	573
June	1st to 10th	Day	12% 3 19% 30% 24% 11%	530
	11th to 20th	Day	6% 3 18% 28% 28% 17%	530
	21st to end	Day	7% 4 15% 29% 26% 18%	530
July	1st to 10th	Day	9% 2 11% 25% 31% 22%	525
	11th to 20th	Day	6% 2 7% 23% 36% 25%	524
	21st to end	Day	8% 1 8% 29% 35% 20%	576
August	1st to 10th	Day	11% 2 12% 25% 30% 19%	526
	11th to 20th	Day	8% 1 9% 28% 34% 19%	519
	21st to end	Day	13% 4 10% 30% 27% 16%	583
September	1st to 10th	Day	8% 5% 14% 30% 29% 14%	530
	11th to 20th	Day	15% 3 14% 29% 29% 9%	530
	21st to end	Day	14% 3 15% 31% 28% 8%	530
October	1st to 10th	Day	14% 5% 28% 29% 17% 7%	530
	11th to 20th	Day	11% 9% 22% 33% 20% 5%	530
	21st to end	Day	17% 10% 27% 26% 17% 4	578
November	1st to 10th	Day	19% 11% 27% 29% 12% 2	330
	11th to 20th	Day	17% 15% 39% 22% 6% 2	330
	21st to end	Day	15% 18% 38% 21% 7% 1	330
December	1st to 10th	Day	27% 19% 33% 18% 3	330
	11th to 20th	Day	24% 18% 33% 18% 6% 1	330
	21st to end	Day	27% 15% 36% 18% 4 1	363
January	1st to 10th	Night	21% 19% 32% 23% 5%	840
	11th to 20th	Night	20% 16% 32% 25% 7%	825
	21st to end	Night	20% 18% 35% 24% 4	880
February	1st to 10th	Night	19% 14% 33% 28% 7%	800
	11th to 20th	Night	23% 18% 31% 25% 3	783
	21st to end	Night	21% 17% 34% 25% 3	660
March	1st to 10th	Night	17% 17% 32% 28% 6%	800
	11th to 20th	Night	15% 17% 34% 23% 11%	800
	21st to end	Night	16% 16% 30% 29% 9%	880
April	1st to 10th	Night	13% 14% 33% 27% 12%	711
	11th to 20th	Night	15% 13% 30% 27% 12% 2	593
	21st to end	Night	9% 8% 27% 39% 18% 1	600
May	1st to 10th	Night	10% 9% 25% 32% 23% 2	600
	11th to 20th	Night	13% 3 22% 34% 22% 5%	591
	21st to end	Night	10% 2 18% 34% 31% 4	650
June	1st to 10th	Night	9% 2 14% 40% 31% 5%	600
	11th to 20th	Night	5% 3 15% 30% 32% 15%	600
	21st to end	Night	7% 2 6% 35% 41% 9%	600
July	1st to 10th	Night	8% 1 7% 25% 45% 13%	585
	11th to 20th	Night	7% 2 4 23% 45% 19%	582
	21st to end	Night	6% 1 7% 26% 46% 13%	657
August	1st to 10th	Night	8% 1 8% 29% 39% 16%	597
	11th to 20th	Night	9% 1 7% 28% 43% 11%	594
	21st to end	Night	12% 2 10% 29% 34% 13%	660
September	1st to 10th	Night	9% 3 12% 28% 39% 10%	747
	11th to 20th	Night	12% 3 13% 32% 35% 5%	800
	21st to end	Night	12% 3 16% 34% 32% 4	800
October	1st to 10th	Night	12% 3 23% 35% 22% 6%	800
	11th to 20th	Night	13% 7% 23% 32% 23% 3	800
	21st to end	Night	14% 8% 24% 34% 17% 3	880
November	1st to 10th	Night	15% 7% 23% 35% 19% 1	820
	11th to 20th	Night	16% 11% 29% 30% 13% 1	840
	21st to end	Night	16% 12% 29% 30% 12% 1	840
December	1st to 10th	Night	19% 14% 30% 28% 8% 1	840
	11th to 20th	Night	21% 11% 31% 28% 9% 1	840
	21st to end	Night	24% 15% 27% 28% 6%	924

Page generated at 2026-01-14 01:14:31. Version ac07

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