WestVale Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the WestVale Observatory clear sky chart from 2005-11-06 to 2024-11-28.

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	14% 6% 2 5% 4 4 4 4 6% 8% 43%	101423
	Night	15% 6% 2 5% 4 4 4 4 5% 7% 44%	98071
Transparency	Day	38% 7% 12% 21% 17% 5%	105139
	Night	38% 6% 11% 19% 19% 6%	102494
Seeing	Day	17% 10% 22% 25% 17% 8%	15272
	Night	17% 7% 23% 30% 20% 3	25342

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	17% 4 1 4 2 2 3 2 4 5% 55%	2836
	11th to 20th	Day	15% 6% 2 5% 3 3 3 2 4 4 52%	2840
	21st to end	Day	25% 4 2 4 2 3 3 3 4 5% 45%	3124
February	1st to 10th	Day	15% 6% 1 4 5% 3 3 4 6% 8% 43%	1710
	11th to 20th	Day	19% 7% 2 4 4 3 3 3 5% 5% 45%	1848
	21st to end	Day	18% 7% 2 6% 4 4 4 3 4 7% 41%	1582
March	1st to 10th	Day	25% 6% 2 5% 3 3 3 3 5% 6% 41%	3435
	11th to 20th	Day	12% 5% 2 5% 3 4 4 3 5% 6% 53%	3480
	21st to end	Day	22% 6% 2 4 4 3 3 3 5% 6% 43%	3815
April	1st to 10th	Day	11% 4 1 5% 4 3 4 4 6% 8% 50%	2090
	11th to 20th	Day	15% 4 1 5% 4 4 3 3 5% 7% 48%	2239
	21st to end	Day	14% 6% 2 5% 5% 4 4 4 6% 6% 47%	2470
May	1st to 10th	Day	12% 7% 2 7% 5% 5% 6% 5% 7% 9% 36%	4082
	11th to 20th	Day	17% 9% 3 6% 4 5% 5% 3 5% 8% 36%	4081
	21st to end	Day	9% 4 2 5% 3 4 5% 5% 7% 10% 47%	4476
June	1st to 10th	Day	12% 6% 2 6% 4 5% 5% 6% 8% 9% 36%	2470
	11th to 20th	Day	7% 5% 2 6% 5% 6% 6% 5% 8% 10% 40%	2470
	21st to end	Day	8% 5% 2 5% 5% 5% 5% 6% 9% 11% 38%	2470
July	1st to 10th	Day	7% 6% 2 7% 5% 7% 7% 6% 9% 10% 35%	4120
	11th to 20th	Day	8% 6% 3 7% 6% 7% 6% 5% 8% 11% 34%	4120
	21st to end	Day	12% 9% 3 10% 7% 7% 7% 6% 8% 10% 23%	4466
August	1st to 10th	Day	17% 7% 3 6% 5% 6% 6% 5% 7% 10% 27%	4104
	11th to 20th	Day	9% 5% 3 6% 5% 5% 6% 5% 8% 11% 36%	4044
	21st to end	Day	6% 4 2 5% 4 4 5% 4 6% 9% 53%	4283
September	1st to 10th	Day	11% 5% 2 7% 5% 5% 5% 4 6% 9% 43%	2090
	11th to 20th	Day	16% 6% 2 6% 4 5% 5% 3 5% 7% 40%	2090
	21st to end	Day	21% 5% 3 5% 4 5% 4 3 5% 6% 40%	2090
October	1st to 10th	Day	15% 4 2 5% 3 2 3 3 4 5% 55%	1997
	11th to 20th	Day	13% 4 2 4 3 2 3 2 3 5% 59%	1900
	21st to end	Day	16% 4 1 4 2 2 3 2 3 4 58%	2090
November	1st to 10th	Day	14% 5% 2 5% 3 3 3 2 4 5% 54%	1951
	11th to 20th	Day	15% 5% 2 4 3 3 3 2 4 6% 54%	1800
	21st to end	Day	15% 5% 2 4 3 3 3 4 5% 6% 51%	1773
December	1st to 10th	Day	19% 7% 2 6% 4 4 4 3 4 5% 42%	1710
	11th to 20th	Day	15% 5% 2 4 3 2 3 3 5% 7% 53%	2153
	21st to end	Day	20% 6% 2 5% 3 4 3 3 5% 6% 44%	3124
January	1st to 10th	Night	16% 5% 2 4 3 3 3 3 4 6% 51%	4109
	11th to 20th	Night	12% 6% 2 6% 4 3 3 3 4 6% 51%	4120
	21st to end	Night	25% 6% 1 4 3 3 3 3 3 5% 43%	4532
February	1st to 10th	Night	16% 5% 2 5% 3 4 4 3 5% 6% 46%	2466
	11th to 20th	Night	19% 6% 2 5% 3 4 3 3 5% 5% 45%	2218
	21st to end	Night	17% 6% 2 5% 4 4 5% 3 5% 6% 43%	1870
March	1st to 10th	Night	23% 8% 3 6% 3 4 4 3 4 6% 38%	3418
	11th to 20th	Night	10% 4 2 4 3 3 4 3 5% 7% 54%	3480
	21st to end	Night	20% 5% 2 4 4 3 4 4 5% 6% 44%	3817
April	1st to 10th	Night	12% 5% 1 4 4 4 5% 4 5% 6% 50%	2090
	11th to 20th	Night	15% 5% 2 5% 4 3 3 3 5% 7% 48%	1904
	21st to end	Night	12% 5% 2 6% 4 4 3 4 3 6% 51%	1710
May	1st to 10th	Night	14% 7% 2 6% 5% 4 4 3 5% 8% 41%	2813
	11th to 20th	Night	18% 7% 2 7% 4 5% 4 4 4 7% 38%	2812
	21st to end	Night	11% 5% 2 5% 4 4 4 4 7% 10% 45%	3079
June	1st to 10th	Night	11% 6% 2 5% 5% 5% 6% 5% 7% 11% 36%	1710
	11th to 20th	Night	11% 6% 2 6% 5% 5% 5% 4 8% 9% 39%	1710
	21st to end	Night	7% 5% 2 6% 6% 5% 6% 6% 9% 10% 38%	1710
July	1st to 10th	Night	9% 6% 3 8% 6% 5% 7% 5% 9% 10% 31%	2840
	11th to 20th	Night	9% 5% 2 8% 5% 7% 6% 5% 8% 12% 35%	2840
	21st to end	Night	12% 10% 4 10% 8% 7% 7% 5% 9% 10% 20%	3087
August	1st to 10th	Night	16% 10% 4 8% 6% 5% 6% 5% 7% 8% 25%	2825
	11th to 20th	Night	11% 8% 3 9% 5% 5% 7% 6% 8% 9% 31%	2785
	21st to end	Night	7% 4 2 6% 4 5% 5% 4 7% 10% 46%	3335
September	1st to 10th	Night	12% 6% 1 5% 5% 4 5% 4 6% 8% 43%	2090
	11th to 20th	Night	17% 6% 2 7% 5% 4 4 4 5% 8% 38%	2090
	21st to end	Night	18% 6% 2 6% 5% 4 4 4 5% 6% 39%	2090
October	1st to 10th	Night	17% 4 2 4 3 3 3 3 4 6% 51%	2181
	11th to 20th	Night	15% 4 2 4 3 3 3 3 4 5% 55%	2280
	21st to end	Night	14% 3 1 4 3 3 2 2 3 4 60%	2508
November	1st to 10th	Night	15% 5% 2 5% 3 3 3 2 4 4 53%	2346
	11th to 20th	Night	13% 5% 2 4 3 3 4 3 4 6% 54%	2600
	21st to end	Night	13% 4 1 4 4 3 3 3 6% 6% 51%	2562
December	1st to 10th	Night	20% 6% 2 6% 4 3 3 4 5% 6% 42%	2470
	11th to 20th	Night	16% 6% 2 4 3 3 3 3 4 5% 52%	3042
	21st to end	Night	20% 5% 2 5% 3 3 4 3 5% 7% 42%	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	33% 3 6% 25% 24% 8%	2837
	11th to 20th	Day	36% 4 6% 16% 32% 6%	2840
	21st to end	Day	42% 3 7% 18% 25% 5%	3124
February	1st to 10th	Day	38% 6% 10% 21% 19% 5%	1710
	11th to 20th	Day	41% 5% 8% 20% 21% 6%	1858
	21st to end	Day	45% 5% 9% 15% 19% 8%	1582
March	1st to 10th	Day	45% 5% 7% 18% 17% 9%	3435
	11th to 20th	Day	31% 6% 8% 19% 24% 12%	3480
	21st to end	Day	43% 4 8% 22% 20% 3	3815
April	1st to 10th	Day	30% 7% 11% 22% 24% 6%	2090
	11th to 20th	Day	35% 6% 10% 20% 22% 8%	2239
	21st to end	Day	36% 6% 12% 24% 17% 4	2470
May	1st to 10th	Day	42% 10% 15% 19% 12% 2	4120
	11th to 20th	Day	45% 8% 12% 20% 13% 1	4085
	21st to end	Day	30% 10% 18% 22% 18% 3	4489
June	1st to 10th	Day	39% 10% 17% 23% 10% 1	2470
	11th to 20th	Day	35% 11% 18% 21% 13% 1	2470
	21st to end	Day	34% 13% 17% 20% 14% 2	2470
July	1st to 10th	Day	41% 11% 19% 18% 9% 2	4055
	11th to 20th	Day	41% 9% 25% 18% 6% 1	4051
	21st to end	Day	53% 11% 15% 16% 5% 1	4324
August	1st to 10th	Day	50% 8% 18% 19% 5%	4091
	11th to 20th	Day	39% 9% 22% 17% 10% 2	4083
	21st to end	Day	27% 7% 15% 35% 14% 2	4283
September	1st to 10th	Day	36% 11% 12% 21% 17% 3	2090
	11th to 20th	Day	41% 8% 12% 18% 18% 3	2090
	21st to end	Day	45% 6% 9% 19% 16% 4	2090
October	1st to 10th	Day	29% 3 7% 21% 30% 10%	3322
	11th to 20th	Day	20% 3 6% 19% 26% 25%	3140
	21st to end	Day	29% 3 5% 24% 22% 17%	3428
November	1st to 10th	Day	33% 5% 7% 19% 28% 7%	1951
	11th to 20th	Day	33% 3 8% 23% 26% 8%	1800
	21st to end	Day	34% 4 8% 24% 25% 6%	1773
December	1st to 10th	Day	45% 5% 8% 20% 17% 5%	1710
	11th to 20th	Day	33% 4 8% 21% 23% 11%	2150
	21st to end	Day	41% 4 9% 23% 20% 3	3124
January	1st to 10th	Night	35% 4 9% 18% 28% 7%	4109
	11th to 20th	Night	36% 4 8% 16% 29% 7%	4120
	21st to end	Night	44% 4 6% 17% 23% 6%	4532
February	1st to 10th	Night	38% 5% 11% 19% 21% 5%	2466
	11th to 20th	Night	43% 4 8% 17% 22% 7%	2219
	21st to end	Night	42% 5% 9% 16% 21% 7%	1870
March	1st to 10th	Night	47% 7% 8% 15% 17% 7%	3418
	11th to 20th	Night	29% 5% 8% 20% 22% 15%	3480
	21st to end	Night	41% 6% 7% 21% 17% 7%	3817
April	1st to 10th	Night	34% 6% 11% 20% 23% 5%	2090
	11th to 20th	Night	34% 5% 9% 22% 22% 8%	1904
	21st to end	Night	36% 6% 11% 23% 21% 4	1710
May	1st to 10th	Night	42% 8% 14% 24% 12% 2	2840
	11th to 20th	Night	44% 9% 12% 20% 14% 1	2813
	21st to end	Night	31% 9% 16% 28% 15% 1	3087
June	1st to 10th	Night	39% 11% 17% 21% 11% 1	1710
	11th to 20th	Night	39% 10% 19% 21% 10% 1	1710
	21st to end	Night	36% 12% 16% 18% 13% 4	1710
July	1st to 10th	Night	44% 11% 18% 17% 9% 2	2795
	11th to 20th	Night	40% 10% 24% 20% 3 1	2786
	21st to end	Night	55% 12% 15% 13% 5%	2988
August	1st to 10th	Night	53% 8% 20% 14% 5%	2814
	11th to 20th	Night	47% 8% 21% 13% 9% 1	2812
	21st to end	Night	30% 10% 14% 30% 14% 2	3335
September	1st to 10th	Night	37% 10% 14% 18% 15% 4	2090
	11th to 20th	Night	43% 10% 12% 15% 18% 2	2090
	21st to end	Night	44% 7% 12% 18% 16% 4	2090
October	1st to 10th	Night	36% 5% 7% 18% 25% 9%	3636
	11th to 20th	Night	22% 4 6% 21% 27% 21%	3779
	21st to end	Night	28% 3 5% 22% 27% 15%	4120
November	1st to 10th	Night	36% 4 7% 19% 24% 10%	2346
	11th to 20th	Night	33% 4 7% 19% 26% 11%	2600
	21st to end	Night	33% 5% 7% 22% 26% 6%	2562
December	1st to 10th	Night	43% 5% 10% 18% 18% 6%	2470
	11th to 20th	Night	35% 4 8% 16% 26% 9%	3044
	21st to end	Night	41% 6% 9% 20% 21% 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	24% 21% 32% 20% 3	270
	11th to 20th	Day	27% 19% 32% 19% 3	270
	21st to end	Day	21% 23% 37% 15% 3	297
February	1st to 10th	Day	23% 20% 36% 16% 5%	270
	11th to 20th	Day	35% 21% 32% 11% 1	270
	21st to end	Day	30% 27% 26% 14% 2	237
March	1st to 10th	Day	21% 17% 30% 25% 6% 1	460
	11th to 20th	Day	21% 20% 34% 21% 4 1	460
	21st to end	Day	23% 18% 32% 21% 5%	506
April	1st to 10th	Day	19% 18% 32% 20% 10% 1	460
	11th to 20th	Day	17% 17% 25% 23% 14% 2	452
	21st to end	Day	18% 12% 32% 22% 12% 5%	460
May	1st to 10th	Day	13% 15% 31% 25% 11% 5%	463
	11th to 20th	Day	18% 6% 22% 29% 18% 8%	487
	21st to end	Day	16% 5% 16% 28% 23% 12%	529
June	1st to 10th	Day	12% 5% 18% 33% 21% 11%	490
	11th to 20th	Day	11% 2 19% 26% 26% 15%	490
	21st to end	Day	10% 3 14% 34% 26% 13%	490
July	1st to 10th	Day	12% 1 10% 24% 31% 23%	485
	11th to 20th	Day	7% 2 12% 28% 29% 22%	484
	21st to end	Day	9% 3 10% 30% 28% 21%	532
August	1st to 10th	Day	16% 3 14% 29% 20% 19%	486
	11th to 20th	Day	12% 5% 11% 28% 28% 16%	479
	21st to end	Day	12% 2 10% 22% 32% 21%	539
September	1st to 10th	Day	14% 2 12% 26% 28% 18%	490
	11th to 20th	Day	20% 3 12% 22% 30% 13%	490
	21st to end	Day	24% 6% 18% 26% 22% 5%	490
October	1st to 10th	Day	19% 5% 18% 28% 22% 9%	490
	11th to 20th	Day	15% 10% 23% 27% 20% 5%	490
	21st to end	Day	19% 8% 19% 34% 17% 4	539
November	1st to 10th	Day	23% 9% 26% 27% 13% 3	489
	11th to 20th	Day	18% 15% 34% 23% 8% 1	300
	21st to end	Day	13% 15% 35% 24% 11% 2	291
December	1st to 10th	Day	27% 15% 30% 21% 6%	270
	11th to 20th	Day	19% 14% 32% 27% 8%	270
	21st to end	Day	26% 17% 33% 18% 5% 1	297
January	1st to 10th	Night	19% 13% 34% 26% 7%	760
	11th to 20th	Night	20% 13% 33% 30% 5%	760
	21st to end	Night	20% 11% 36% 30% 4	836
February	1st to 10th	Night	18% 11% 36% 32% 3	760
	11th to 20th	Night	23% 14% 39% 23% 2	743
	21st to end	Night	20% 16% 36% 25% 3	628
March	1st to 10th	Night	18% 11% 31% 34% 5%	760
	11th to 20th	Night	19% 12% 34% 26% 8% 1	760
	21st to end	Night	19% 10% 34% 29% 7%	836
April	1st to 10th	Night	15% 13% 34% 28% 9%	760
	11th to 20th	Night	17% 12% 27% 30% 14%	751
	21st to end	Night	14% 7% 27% 35% 16%	570
May	1st to 10th	Night	14% 5% 28% 34% 18% 1	570
	11th to 20th	Night	18% 4 20% 32% 25% 2	561
	21st to end	Night	16% 1 13% 30% 34% 5%	617
June	1st to 10th	Night	12% 2 14% 34% 32% 6%	570
	11th to 20th	Night	11% 4 12% 29% 34% 10%	570
	21st to end	Night	8% 2 11% 32% 42% 6%	570
July	1st to 10th	Night	11% 2 7% 24% 49% 8%	555
	11th to 20th	Night	11% 2 6% 26% 45% 11%	552
	21st to end	Night	8% 2 7% 30% 44% 9%	624
August	1st to 10th	Night	15% 3 7% 29% 40% 7%	567
	11th to 20th	Night	15% 2 6% 30% 37% 9%	564
	21st to end	Night	13% 2 10% 21% 46% 9%	737
September	1st to 10th	Night	13% 3 10% 27% 38% 10%	760
	11th to 20th	Night	19% 3 11% 23% 36% 7%	760
	21st to end	Night	22% 4 16% 29% 28% 2	760
October	1st to 10th	Night	18% 4 19% 27% 27% 6%	760
	11th to 20th	Night	18% 6% 22% 31% 21% 2	760
	21st to end	Night	16% 6% 21% 36% 18% 3	836
November	1st to 10th	Night	17% 6% 23% 35% 17% 2	780
	11th to 20th	Night	16% 7% 28% 37% 12% 1	800
	21st to end	Night	15% 9% 27% 35% 14% 1	789
December	1st to 10th	Night	21% 9% 26% 35% 9%	760
	11th to 20th	Night	20% 7% 32% 33% 8% 1	760
	21st to end	Night	24% 8% 29% 32% 7%	836

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