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 2024-11-25.

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% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	101111
	Night	12% 7% 2 6% 4 4 4 4 6% 8% 44%	98330
Transparency	Day	37% 8% 11% 19% 18% 6%	104967
	Night	36% 7% 11% 19% 19% 8%	102600
Seeing	Day	15% 10% 24% 24% 18% 9%	14892
	Night	14% 9% 23% 29% 20% 4	24899

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	16% 9% 3 5% 4 4 3 4 5% 6% 41%	2836
	11th to 20th	Day	12% 7% 2 3 3 3 3 3 5% 7% 53%	2840
	21st to end	Day	22% 12% 2 6% 3 3 2 3 4 5% 40%	3387
February	1st to 10th	Day	15% 8% 3 7% 5% 4 4 3 6% 6% 37%	1900
	11th to 20th	Day	19% 8% 3 6% 3 4 4 3 4 5% 43%	1848
	21st to end	Day	16% 8% 3 8% 4 5% 4 4 5% 5% 39%	1570
March	1st to 10th	Day	21% 11% 3 7% 3 4 4 3 5% 6% 34%	3171
	11th to 20th	Day	12% 7% 2 5% 4 4 4 4 5% 7% 45%	3480
	21st to end	Day	17% 6% 2 6% 4 4 4 4 5% 7% 41%	3815
April	1st to 10th	Day	11% 6% 3 7% 4 4 5% 3 5% 6% 46%	2189
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 5% 7% 47%	2254
	21st to end	Day	7% 5% 2 5% 4 4 4 4 6% 7% 52%	2365
May	1st to 10th	Day	9% 5% 2 7% 5% 5% 5% 4 7% 8% 45%	4082
	11th to 20th	Day	16% 8% 3 6% 4 4 4 4 5% 7% 39%	4081
	21st to end	Day	6% 4 2 7% 4 5% 6% 5% 7% 9% 46%	4476
June	1st to 10th	Day	9% 7% 3 8% 7% 7% 6% 6% 8% 8% 32%	2470
	11th to 20th	Day	6% 4 1 6% 5% 5% 7% 6% 8% 11% 41%	2470
	21st to end	Day	5% 6% 2 8% 6% 6% 7% 6% 8% 10% 35%	2470
July	1st to 10th	Day	10% 7% 3 9% 7% 8% 8% 7% 9% 11% 22%	4120
	11th to 20th	Day	6% 6% 2 7% 8% 7% 8% 6% 8% 11% 32%	4120
	21st to end	Day	8% 8% 3 10% 7% 9% 9% 7% 9% 11% 18%	4466
August	1st to 10th	Day	5% 4 3 7% 4 7% 7% 5% 9% 12% 37%	3984
	11th to 20th	Day	6% 4 2 6% 4 5% 5% 4 7% 8% 48%	3729
	21st to end	Day	7% 4 1 5% 4 5% 5% 5% 7% 10% 47%	4180
September	1st to 10th	Day	11% 7% 3 7% 6% 6% 5% 5% 6% 7% 37%	2120
	11th to 20th	Day	14% 7% 2 6% 4 4 5% 3 6% 8% 41%	2090
	21st to end	Day	12% 6% 2 5% 4 5% 5% 3 5% 6% 48%	2090
October	1st to 10th	Day	11% 5% 2 5% 3 3 3 3 4 7% 53%	2090
	11th to 20th	Day	10% 4 2 4 3 3 3 2 4 7% 58%	2038
	21st to end	Day	11% 5% 2 5% 3 3 3 3 4 5% 55%	2085
November	1st to 10th	Day	10% 7% 3 6% 3 3 3 3 4 5% 52%	1762
	11th to 20th	Day	14% 5% 2 3 3 3 3 3 5% 6% 56%	1800
	21st to end	Day	12% 5% 2 6% 5% 4 4 3 5% 7% 47%	1746
December	1st to 10th	Day	17% 7% 2 5% 4 4 4 4 6% 7% 41%	1710
	11th to 20th	Day	16% 8% 4 7% 4 4 3 3 4 5% 43%	2153
	21st to end	Day	19% 9% 4 6% 4 4 4 3 5% 6% 37%	3124
January	1st to 10th	Night	17% 11% 2 6% 4 4 4 3 5% 6% 39%	4109
	11th to 20th	Night	11% 7% 3 6% 4 4 4 3 5% 6% 47%	4120
	21st to end	Night	21% 13% 3 6% 3 3 3 2 3 4 40%	4266
February	1st to 10th	Night	14% 9% 3 7% 5% 4 4 3 5% 5% 42%	2280
	11th to 20th	Night	19% 8% 3 5% 4 3 3 3 4 4 44%	2218
	21st to end	Night	16% 8% 2 6% 4 4 4 4 5% 7% 40%	1884
March	1st to 10th	Night	22% 10% 3 7% 3 3 3 3 4 5% 35%	3680
	11th to 20th	Night	10% 6% 2 5% 4 3 5% 3 6% 8% 47%	3480
	21st to end	Night	14% 7% 2 7% 4 5% 4 3 5% 7% 43%	3817
April	1st to 10th	Night	10% 6% 2 5% 5% 5% 5% 4 5% 6% 48%	2006
	11th to 20th	Night	12% 5% 2 5% 4 3 3 4 5% 8% 49%	1875
	21st to end	Night	7% 5% 1 4 3 4 5% 3 5% 6% 55%	1811
May	1st to 10th	Night	7% 5% 2 6% 5% 5% 5% 4 8% 11% 43%	2813
	11th to 20th	Night	15% 8% 2 6% 4 4 4 4 6% 7% 40%	2812
	21st to end	Night	5% 4 1 5% 3 4 3 4 7% 13% 50%	3079
June	1st to 10th	Night	6% 6% 3 7% 6% 5% 6% 6% 9% 11% 36%	1710
	11th to 20th	Night	4 5% 2 5% 4 3 5% 5% 9% 11% 48%	1710
	21st to end	Night	5% 5% 2 6% 6% 6% 5% 6% 9% 11% 40%	1710
July	1st to 10th	Night	8% 7% 3 8% 7% 5% 7% 6% 9% 12% 26%	2840
	11th to 20th	Night	6% 5% 2 8% 6% 6% 7% 6% 9% 13% 33%	2840
	21st to end	Night	9% 8% 3 9% 7% 6% 6% 6% 9% 12% 24%	3087
August	1st to 10th	Night	5% 5% 2 6% 5% 6% 7% 5% 10% 13% 37%	2940
	11th to 20th	Night	5% 4 2 6% 4 4 5% 4 7% 11% 49%	3100
	21st to end	Night	6% 4 2 5% 4 4 5% 4 6% 9% 51%	3476
September	1st to 10th	Night	9% 5% 2 6% 4 5% 5% 5% 7% 9% 42%	2039
	11th to 20th	Night	11% 6% 2 7% 5% 4 4 4 6% 7% 43%	2090
	21st to end	Night	9% 5% 2 5% 4 5% 5% 4 6% 7% 47%	2090
October	1st to 10th	Night	9% 5% 1 5% 3 3 4 3 4 6% 57%	2090
	11th to 20th	Night	10% 5% 2 4 4 3 3 2 4 5% 60%	2141
	21st to end	Night	10% 5% 2 6% 4 3 3 2 3 4 59%	2508
November	1st to 10th	Night	10% 6% 2 5% 3 3 3 3 5% 4 55%	2536
	11th to 20th	Night	13% 6% 2 4 3 4 2 3 3 5% 55%	2600
	21st to end	Night	12% 7% 2 5% 4 4 4 3 5% 6% 49%	2529
December	1st to 10th	Night	15% 8% 3 7% 4 4 4 3 4 6% 44%	2470
	11th to 20th	Night	17% 7% 3 5% 3 3 3 3 4 5% 47%	3042
	21st to end	Night	18% 9% 3 8% 5% 5% 4 3 5% 7% 34%	4532

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	41% 6% 7% 16% 23% 7%	2837
	11th to 20th	Day	31% 4 7% 23% 26% 10%	2840
	21st to end	Day	48% 4 5% 13% 21% 9%	3387
February	1st to 10th	Day	46% 5% 8% 16% 20% 6%	1900
	11th to 20th	Day	44% 5% 6% 16% 22% 8%	1858
	21st to end	Day	45% 7% 8% 13% 19% 9%	1570
March	1st to 10th	Day	49% 7% 9% 12% 17% 6%	3171
	11th to 20th	Day	37% 7% 9% 16% 22% 10%	3480
	21st to end	Day	41% 6% 10% 16% 18% 8%	3815
April	1st to 10th	Day	38% 6% 9% 20% 18% 9%	2189
	11th to 20th	Day	35% 6% 8% 18% 24% 10%	2254
	21st to end	Day	29% 7% 11% 22% 27% 4	2365
May	1st to 10th	Day	33% 10% 13% 21% 21% 3	4120
	11th to 20th	Day	41% 9% 13% 22% 13% 1	4085
	21st to end	Day	30% 10% 14% 26% 18% 2	4489
June	1st to 10th	Day	44% 11% 15% 18% 10% 2	2470
	11th to 20th	Day	32% 11% 18% 22% 15% 2	2470
	21st to end	Day	39% 12% 18% 18% 11% 3	2470
July	1st to 10th	Day	48% 14% 22% 12% 3 1	4055
	11th to 20th	Day	41% 14% 19% 21% 5% 1	4051
	21st to end	Day	53% 15% 16% 12% 4	4324
August	1st to 10th	Day	35% 10% 15% 28% 11%	3971
	11th to 20th	Day	31% 9% 14% 29% 16% 1	3765
	21st to end	Day	30% 9% 17% 25% 16% 3	4179
September	1st to 10th	Day	43% 11% 13% 15% 14% 5%	2120
	11th to 20th	Day	40% 9% 12% 18% 18% 4	2090
	21st to end	Day	36% 6% 9% 21% 21% 7%	2090
October	1st to 10th	Day	26% 4 6% 13% 31% 18%	3480
	11th to 20th	Day	17% 3 5% 17% 27% 32%	3369
	21st to end	Day	22% 3 5% 18% 40% 12%	3411
November	1st to 10th	Day	35% 4 7% 15% 26% 13%	1762
	11th to 20th	Day	30% 4 7% 17% 25% 16%	1800
	21st to end	Day	36% 6% 8% 16% 24% 9%	1746
December	1st to 10th	Day	43% 7% 8% 17% 19% 7%	1710
	11th to 20th	Day	44% 4 5% 15% 24% 8%	2150
	21st to end	Day	45% 6% 9% 16% 20% 3	3124
January	1st to 10th	Night	43% 6% 6% 14% 23% 8%	4109
	11th to 20th	Night	35% 5% 7% 22% 22% 9%	4120
	21st to end	Night	49% 4 4 11% 25% 7%	4266
February	1st to 10th	Night	43% 4 6% 18% 19% 10%	2280
	11th to 20th	Night	44% 5% 5% 15% 22% 10%	2219
	21st to end	Night	42% 7% 9% 15% 17% 12%	1884
March	1st to 10th	Night	49% 7% 9% 15% 13% 7%	3680
	11th to 20th	Night	33% 8% 10% 14% 24% 11%	3480
	21st to end	Night	40% 6% 8% 18% 21% 9%	3817
April	1st to 10th	Night	36% 6% 10% 18% 20% 10%	2006
	11th to 20th	Night	33% 5% 7% 22% 23% 10%	1875
	21st to end	Night	27% 7% 12% 23% 23% 7%	1811
May	1st to 10th	Night	28% 13% 17% 21% 19% 3	2840
	11th to 20th	Night	40% 9% 15% 23% 12% 2	2813
	21st to end	Night	23% 9% 18% 28% 19% 3	3087
June	1st to 10th	Night	36% 13% 19% 17% 15% 1	1710
	11th to 20th	Night	26% 9% 22% 27% 13% 2	1710
	21st to end	Night	33% 11% 21% 20% 12% 3	1710
July	1st to 10th	Night	45% 15% 22% 12% 5% 1	2795
	11th to 20th	Night	38% 14% 25% 17% 6% 1	2786
	21st to end	Night	48% 15% 18% 14% 4 1	2988
August	1st to 10th	Night	33% 11% 17% 28% 11% 1	2929
	11th to 20th	Night	28% 9% 16% 31% 14% 2	3130
	21st to end	Night	29% 8% 16% 25% 19% 3	3476
September	1st to 10th	Night	35% 10% 15% 20% 15% 5%	2039
	11th to 20th	Night	38% 9% 11% 22% 16% 5%	2090
	21st to end	Night	33% 8% 10% 20% 20% 8%	2090
October	1st to 10th	Night	30% 5% 8% 14% 27% 16%	3480
	11th to 20th	Night	17% 3 4 15% 29% 32%	3549
	21st to end	Night	21% 3 4 21% 34% 18%	4120
November	1st to 10th	Night	32% 5% 9% 15% 26% 14%	2536
	11th to 20th	Night	34% 4 5% 17% 26% 15%	2600
	21st to end	Night	36% 4 8% 16% 26% 10%	2529
December	1st to 10th	Night	41% 5% 10% 17% 21% 6%	2470
	11th to 20th	Night	40% 4 7% 13% 28% 8%	3044
	21st to end	Night	46% 7% 8% 18% 17% 3	4532

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	26% 20% 40% 11% 1	270
	11th to 20th	Day	26% 20% 35% 13% 5% 1	270
	21st to end	Day	26% 26% 36% 9% 2 1	297
February	1st to 10th	Day	26% 20% 36% 15% 2	270
	11th to 20th	Day	33% 21% 34% 10% 1	270
	21st to end	Day	28% 23% 41% 8%	225
March	1st to 10th	Day	22% 23% 31% 20% 3 1	295
	11th to 20th	Day	22% 18% 36% 17% 8%	460
	21st to end	Day	19% 20% 33% 21% 6% 1	506
April	1st to 10th	Day	22% 16% 33% 19% 10% 1	460
	11th to 20th	Day	16% 17% 34% 20% 11% 2	452
	21st to end	Day	13% 12% 31% 28% 13% 2	460
May	1st to 10th	Day	11% 13% 31% 26% 14% 6%	463
	11th to 20th	Day	11% 7% 25% 30% 18% 9%	487
	21st to end	Day	10% 5% 20% 33% 20% 12%	529
June	1st to 10th	Day	12% 3 19% 31% 24% 11%	490
	11th to 20th	Day	6% 4 18% 26% 28% 19%	490
	21st to end	Day	8% 4 16% 30% 24% 17%	490
July	1st to 10th	Day	10% 1 11% 24% 30% 23%	485
	11th to 20th	Day	7% 2 8% 24% 35% 26%	484
	21st to end	Day	8% 1 9% 27% 35% 20%	532
August	1st to 10th	Day	9% 2 13% 24% 31% 21%	486
	11th to 20th	Day	8% 1 10% 29% 34% 18%	479
	21st to end	Day	13% 3 8% 29% 29% 17%	539
September	1st to 10th	Day	9% 5% 14% 28% 30% 14%	490
	11th to 20th	Day	17% 3 14% 29% 28% 9%	490
	21st to end	Day	14% 3 16% 30% 28% 9%	490
October	1st to 10th	Day	14% 5% 27% 28% 18% 8%	490
	11th to 20th	Day	11% 9% 23% 32% 20% 5%	490
	21st to end	Day	16% 9% 27% 27% 18% 4	534
November	1st to 10th	Day	20% 11% 26% 29% 12% 2	300
	11th to 20th	Day	19% 15% 38% 21% 6% 1	300
	21st to end	Day	14% 15% 39% 22% 9% 1	282
December	1st to 10th	Day	24% 19% 32% 21% 4	270
	11th to 20th	Day	25% 19% 31% 19% 5%	270
	21st to end	Day	28% 15% 36% 15% 5% 1	297
January	1st to 10th	Night	21% 19% 32% 23% 5%	760
	11th to 20th	Night	20% 16% 31% 26% 7%	760
	21st to end	Night	21% 18% 35% 23% 4	836
February	1st to 10th	Night	19% 14% 33% 28% 7%	760
	11th to 20th	Night	23% 17% 32% 25% 3	743
	21st to end	Night	22% 16% 34% 25% 3	628
March	1st to 10th	Night	17% 16% 32% 29% 6%	760
	11th to 20th	Night	15% 17% 34% 23% 12%	760
	21st to end	Night	16% 17% 29% 29% 10%	836
April	1st to 10th	Night	13% 14% 33% 28% 12%	676
	11th to 20th	Night	15% 13% 30% 27% 13% 2	563
	21st to end	Night	9% 8% 27% 38% 18% 1	570
May	1st to 10th	Night	9% 9% 25% 31% 24% 2	570
	11th to 20th	Night	13% 2 22% 34% 23% 5%	561
	21st to end	Night	10% 2 17% 34% 32% 5%	617
June	1st to 10th	Night	8% 1 14% 41% 31% 5%	570
	11th to 20th	Night	4 3 15% 29% 33% 16%	570
	21st to end	Night	7% 2 7% 35% 40% 9%	570
July	1st to 10th	Night	8% 1 7% 25% 45% 13%	555
	11th to 20th	Night	7% 2 5% 23% 44% 20%	552
	21st to end	Night	6% 1 7% 26% 46% 13%	624
August	1st to 10th	Night	6% 1 8% 29% 40% 17%	567
	11th to 20th	Night	9% 1 7% 29% 44% 10%	564
	21st to end	Night	12% 2 10% 29% 35% 13%	627
September	1st to 10th	Night	10% 3 11% 26% 40% 10%	709
	11th to 20th	Night	13% 3 13% 31% 34% 5%	760
	21st to end	Night	12% 3 15% 34% 33% 4	760
October	1st to 10th	Night	12% 3 22% 35% 23% 6%	760
	11th to 20th	Night	13% 7% 23% 32% 22% 3	760
	21st to end	Night	13% 7% 24% 35% 18% 3	836
November	1st to 10th	Night	15% 7% 23% 34% 20% 1	780
	11th to 20th	Night	17% 12% 28% 30% 13% 1	800
	21st to end	Night	16% 12% 28% 30% 13% 1	779
December	1st to 10th	Night	18% 14% 30% 28% 9% 1	760
	11th to 20th	Night	22% 10% 32% 27% 9% 1	760
	21st to end	Night	25% 15% 26% 28% 6%	836

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