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 2026-02-01.

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 6% 4 4 4 4 6% 8% 43%	127597
	Night	14% 6% 2 5% 4 4 4 4 5% 7% 45%	124437
Transparency	Day	38% 6% 12% 20% 18% 6%	131269
	Night	38% 6% 11% 19% 20% 7%	128834
Seeing	Day	18% 10% 23% 25% 17% 8%	16817
	Night	17% 7% 24% 30% 19% 3	26935

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	14% 6% 2 4 2 2 2 2 3 4 59%	4146
	11th to 20th	Day	12% 5% 2 4 3 2 2 2 3 4 61%	4150
	21st to end	Day	18% 5% 2 5% 4 3 3 3 4 5% 48%	4565
February	1st to 10th	Day	15% 6% 1 4 5% 3 4 4 6% 8% 44%	1800
	11th to 20th	Day	20% 7% 2 4 4 3 3 3 5% 5% 44%	1948
	21st to end	Day	18% 7% 2 6% 4 4 4 3 4 6% 42%	1663
March	1st to 10th	Day	21% 7% 2 5% 3 3 3 3 5% 6% 42%	4935
	11th to 20th	Day	11% 5% 2 5% 3 3 3 3 5% 6% 54%	4980
	21st to end	Day	18% 6% 2 5% 4 3 3 3 4 5% 47%	5465
April	1st to 10th	Day	12% 4 1 5% 4 3 4 4 6% 8% 50%	2200
	11th to 20th	Day	15% 4 1 5% 4 4 3 3 5% 7% 49%	2359
	21st to end	Day	14% 6% 2 5% 5% 4 4 4 6% 6% 47%	2600
May	1st to 10th	Day	10% 6% 3 7% 5% 5% 7% 5% 8% 10% 34%	5862
	11th to 20th	Day	19% 8% 3 7% 5% 5% 5% 4 6% 8% 30%	5861
	21st to end	Day	13% 5% 2 6% 4 4 5% 4 6% 8% 42%	6433
June	1st to 10th	Day	12% 6% 2 6% 4 5% 5% 6% 9% 9% 36%	2600
	11th to 20th	Day	7% 5% 2 6% 5% 6% 6% 5% 8% 11% 40%	2600
	21st to end	Day	7% 5% 2 5% 5% 5% 5% 6% 9% 11% 40%	2600
July	1st to 10th	Day	7% 6% 2 7% 5% 6% 7% 5% 9% 10% 36%	5900
	11th to 20th	Day	6% 6% 3 8% 6% 7% 7% 5% 9% 12% 31%	5900
	21st to end	Day	10% 7% 3 9% 7% 6% 7% 6% 8% 11% 26%	6424
August	1st to 10th	Day	18% 7% 3 6% 5% 6% 6% 5% 7% 10% 26%	4234
	11th to 20th	Day	10% 6% 3 6% 5% 6% 6% 5% 8% 11% 35%	4174
	21st to end	Day	6% 4 2 5% 4 4 5% 4 6% 9% 53%	4418
September	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 4 6% 9% 43%	2200
	11th to 20th	Day	15% 6% 2 6% 4 5% 5% 3 6% 7% 41%	2200
	21st to end	Day	21% 5% 3 5% 4 5% 4 3 5% 6% 39%	2200
October	1st to 10th	Day	14% 4 2 4 3 2 3 3 4 5% 54%	2102
	11th to 20th	Day	13% 4 2 5% 3 2 3 2 3 5% 59%	2000
	21st to end	Day	16% 4 1 4 2 2 3 2 3 4 57%	2200
November	1st to 10th	Day	13% 5% 2 5% 4 3 3 2 4 5% 55%	2050
	11th to 20th	Day	14% 4 2 4 3 3 3 2 4 6% 55%	1890
	21st to end	Day	15% 5% 2 4 3 3 3 4 4 6% 52%	1890
December	1st to 10th	Day	15% 5% 2 5% 4 3 3 3 4 4 53%	3020
	11th to 20th	Day	18% 6% 2 6% 4 3 4 3 5% 6% 44%	3463
	21st to end	Day	22% 5% 2 4 3 3 3 3 4 7% 46%	4565
January	1st to 10th	Night	13% 5% 1 4 2 2 3 2 3 5% 59%	6019
	11th to 20th	Night	11% 6% 1 4 3 3 3 2 3 5% 59%	6030
	21st to end	Night	18% 6% 2 4 3 3 3 3 4 6% 48%	6633
February	1st to 10th	Night	15% 5% 2 5% 4 4 4 3 5% 6% 46%	2596
	11th to 20th	Night	20% 6% 2 5% 3 4 3 3 5% 5% 44%	2338
	21st to end	Night	16% 6% 2 5% 4 4 4 3 5% 6% 44%	1965
March	1st to 10th	Night	19% 6% 2 5% 3 3 4 2 4 5% 46%	4918
	11th to 20th	Night	10% 4 2 4 3 3 4 3 5% 7% 56%	4980
	21st to end	Night	16% 4 2 4 4 4 4 4 5% 7% 47%	5467
April	1st to 10th	Night	12% 6% 1 4 4 4 5% 4 5% 6% 49%	2200
	11th to 20th	Night	14% 4 2 5% 4 3 3 3 5% 7% 49%	2005
	21st to end	Night	12% 5% 2 7% 4 4 3 4 3 6% 50%	1800
May	1st to 10th	Night	13% 7% 3 6% 5% 4 5% 4 6% 10% 38%	4033
	11th to 20th	Night	20% 7% 2 7% 5% 5% 4 4 5% 8% 32%	4032
	21st to end	Night	15% 5% 2 6% 4 4 4 3 6% 8% 42%	4421
June	1st to 10th	Night	10% 6% 2 6% 6% 5% 6% 5% 7% 11% 37%	1800
	11th to 20th	Night	12% 6% 2 6% 5% 5% 5% 4 8% 9% 38%	1800
	21st to end	Night	6% 4 2 6% 6% 5% 6% 6% 9% 10% 39%	1800
July	1st to 10th	Night	9% 7% 3 8% 6% 6% 7% 5% 8% 10% 30%	4058
	11th to 20th	Night	9% 6% 2 9% 6% 7% 7% 6% 9% 11% 28%	4057
	21st to end	Night	10% 8% 4 10% 7% 6% 7% 5% 9% 11% 23%	4427
August	1st to 10th	Night	17% 10% 4 8% 5% 5% 6% 5% 7% 8% 25%	2915
	11th to 20th	Night	12% 8% 3 9% 5% 5% 7% 6% 7% 9% 30%	2875
	21st to end	Night	7% 4 2 6% 4 5% 5% 4 7% 10% 45%	3441
September	1st to 10th	Night	12% 5% 1 5% 5% 4 5% 4 6% 8% 44%	2200
	11th to 20th	Night	17% 5% 2 7% 5% 4 5% 4 5% 8% 38%	2200
	21st to end	Night	19% 6% 2 6% 5% 5% 4 4 5% 6% 39%	2200
October	1st to 10th	Night	16% 4 2 4 3 3 3 3 4 6% 51%	2296
	11th to 20th	Night	14% 4 2 4 3 3 3 3 4 5% 55%	2400
	21st to end	Night	15% 3 1 4 3 3 2 2 3 4 59%	2640
November	1st to 10th	Night	15% 5% 2 5% 3 3 3 2 4 4 54%	2466
	11th to 20th	Night	13% 5% 2 4 3 3 4 3 4 6% 55%	2730
	21st to end	Night	13% 4 2 4 3 3 3 3 6% 6% 52%	2730
December	1st to 10th	Night	15% 6% 2 4 3 3 3 3 4 5% 53%	4380
	11th to 20th	Night	21% 8% 2 5% 3 3 3 3 4 4 43%	4952
	21st to end	Night	20% 5% 2 4 3 3 3 3 4 6% 48%	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	32% 2 5% 19% 33% 8%	4147
	11th to 20th	Day	30% 3 4 15% 37% 11%	4150
	21st to end	Day	40% 3 5% 21% 24% 8%	4565
February	1st to 10th	Day	38% 6% 11% 22% 19% 5%	1800
	11th to 20th	Day	42% 4 8% 19% 20% 6%	1958
	21st to end	Day	44% 5% 8% 14% 21% 7%	1663
March	1st to 10th	Day	44% 4 5% 15% 19% 13%	4935
	11th to 20th	Day	31% 4 7% 16% 23% 19%	4980
	21st to end	Day	41% 3 7% 24% 19% 6%	5465
April	1st to 10th	Day	31% 6% 11% 22% 23% 6%	2200
	11th to 20th	Day	34% 5% 10% 21% 22% 8%	2359
	21st to end	Day	37% 6% 12% 24% 17% 4	2600
May	1st to 10th	Day	43% 8% 17% 20% 11% 1	5897
	11th to 20th	Day	50% 7% 14% 19% 10% 1	5865
	21st to end	Day	38% 7% 14% 20% 17% 4	6447
June	1st to 10th	Day	39% 10% 17% 23% 10% 1	2600
	11th to 20th	Day	35% 10% 19% 20% 13% 1	2600
	21st to end	Day	33% 12% 18% 21% 14% 2	2600
July	1st to 10th	Day	40% 9% 17% 22% 10% 1	5835
	11th to 20th	Day	43% 9% 29% 15% 4	5831
	21st to end	Day	47% 10% 17% 20% 6% 1	6240
August	1st to 10th	Day	51% 7% 18% 18% 5%	4221
	11th to 20th	Day	40% 9% 22% 17% 10% 2	4213
	21st to end	Day	28% 7% 15% 35% 14% 2	4418
September	1st to 10th	Day	36% 10% 12% 21% 18% 3	2200
	11th to 20th	Day	41% 8% 12% 19% 18% 3	2200
	21st to end	Day	46% 6% 11% 18% 15% 4	2200
October	1st to 10th	Day	29% 3 8% 21% 30% 10%	3427
	11th to 20th	Day	21% 3 6% 19% 27% 24%	3240
	21st to end	Day	29% 3 5% 24% 22% 16%	3538
November	1st to 10th	Day	33% 5% 8% 19% 28% 8%	2050
	11th to 20th	Day	32% 3 8% 24% 26% 8%	1890
	21st to end	Day	34% 3 8% 23% 25% 6%	1890
December	1st to 10th	Day	36% 4 6% 19% 26% 9%	3020
	11th to 20th	Day	43% 4 10% 16% 20% 7%	3460
	21st to end	Day	40% 3 10% 27% 17% 2	4565
January	1st to 10th	Night	30% 3 7% 16% 34% 10%	6019
	11th to 20th	Night	31% 3 5% 17% 35% 9%	6030
	21st to end	Night	39% 3 6% 19% 24% 9%	6633
February	1st to 10th	Night	39% 5% 12% 20% 20% 5%	2596
	11th to 20th	Night	43% 3 8% 17% 22% 6%	2339
	21st to end	Night	40% 5% 9% 16% 23% 7%	1965
March	1st to 10th	Night	41% 5% 6% 14% 26% 9%	4918
	11th to 20th	Night	28% 4 7% 17% 26% 19%	4980
	21st to end	Night	38% 4 7% 25% 19% 7%	5467
April	1st to 10th	Night	35% 6% 11% 20% 23% 5%	2200
	11th to 20th	Night	34% 5% 9% 23% 22% 8%	2005
	21st to end	Night	36% 5% 12% 22% 21% 4	1800
May	1st to 10th	Night	43% 6% 15% 23% 12% 1	4060
	11th to 20th	Night	49% 8% 13% 18% 11%	4033
	21st to end	Night	39% 6% 13% 25% 14% 2	4429
June	1st to 10th	Night	39% 11% 17% 21% 12% 1	1800
	11th to 20th	Night	40% 10% 19% 20% 10% 1	1800
	21st to end	Night	35% 12% 17% 19% 14% 4	1800
July	1st to 10th	Night	45% 10% 17% 18% 8% 1	4015
	11th to 20th	Night	47% 10% 25% 14% 2 1	4006
	21st to end	Night	52% 10% 17% 16% 5%	4297
August	1st to 10th	Night	54% 8% 19% 14% 4	2904
	11th to 20th	Night	48% 8% 21% 14% 9% 1	2902
	21st to end	Night	31% 10% 14% 29% 14% 2	3441
September	1st to 10th	Night	37% 10% 15% 18% 16% 4	2200
	11th to 20th	Night	43% 10% 13% 16% 18% 1	2200
	21st to end	Night	45% 7% 13% 17% 15% 4	2200
October	1st to 10th	Night	35% 5% 7% 19% 25% 9%	3751
	11th to 20th	Night	22% 4 5% 21% 27% 21%	3899
	21st to end	Night	29% 3 5% 22% 27% 15%	4252
November	1st to 10th	Night	36% 4 7% 19% 25% 10%	2466
	11th to 20th	Night	32% 4 7% 19% 26% 11%	2730
	21st to end	Night	33% 5% 7% 23% 26% 6%	2730
December	1st to 10th	Night	35% 3 7% 21% 26% 8%	4380
	11th to 20th	Night	46% 4 8% 15% 19% 9%	4954
	21st to end	Night	38% 4 12% 22% 21% 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	25% 22% 33% 17% 4	330
	11th to 20th	Day	27% 22% 31% 18% 3	330
	21st to end	Day	22% 24% 37% 15% 3	363
February	1st to 10th	Day	21% 20% 39% 16% 5%	300
	11th to 20th	Day	37% 21% 31% 10% 1	300
	21st to end	Day	28% 26% 27% 17% 2	262
March	1st to 10th	Day	22% 16% 31% 25% 6% 1	500
	11th to 20th	Day	22% 19% 34% 21% 4 1	500
	21st to end	Day	23% 19% 33% 20% 5%	550
April	1st to 10th	Day	21% 18% 31% 20% 9% 1	500
	11th to 20th	Day	17% 17% 28% 23% 13% 2	492
	21st to end	Day	17% 11% 32% 23% 12% 4	500
May	1st to 10th	Day	13% 16% 31% 25% 11% 4	503
	11th to 20th	Day	19% 6% 23% 28% 17% 8%	527
	21st to end	Day	18% 6% 17% 27% 21% 11%	573
June	1st to 10th	Day	11% 6% 19% 32% 21% 11%	530
	11th to 20th	Day	10% 2 20% 28% 26% 14%	530
	21st to end	Day	9% 3 14% 34% 26% 14%	530
July	1st to 10th	Day	11% 1 11% 24% 31% 22%	525
	11th to 20th	Day	6% 2 11% 28% 29% 23%	524
	21st to end	Day	9% 3 9% 28% 30% 21%	576
August	1st to 10th	Day	18% 3 14% 28% 20% 17%	526
	11th to 20th	Day	13% 5% 11% 28% 28% 16%	519
	21st to end	Day	12% 3 12% 22% 31% 19%	583
September	1st to 10th	Day	13% 2 13% 28% 27% 17%	530
	11th to 20th	Day	18% 3 12% 23% 31% 13%	530
	21st to end	Day	24% 6% 18% 25% 22% 5%	530
October	1st to 10th	Day	17% 5% 18% 29% 22% 8%	530
	11th to 20th	Day	15% 10% 23% 28% 19% 5%	530
	21st to end	Day	20% 9% 20% 33% 16% 3	583
November	1st to 10th	Day	21% 9% 27% 27% 12% 3	528
	11th to 20th	Day	17% 17% 35% 22% 8% 1	330
	21st to end	Day	14% 18% 35% 22% 10% 2	330
December	1st to 10th	Day	27% 15% 34% 19% 5%	330
	11th to 20th	Day	20% 14% 33% 26% 7%	330
	21st to end	Day	26% 17% 31% 19% 6% 1	363
January	1st to 10th	Night	19% 15% 34% 26% 7%	840
	11th to 20th	Night	20% 13% 34% 29% 5%	840
	21st to end	Night	19% 10% 38% 29% 3	924
February	1st to 10th	Night	18% 12% 35% 32% 3	801
	11th to 20th	Night	23% 13% 38% 23% 2	783
	21st to end	Night	19% 16% 37% 25% 3	660
March	1st to 10th	Night	18% 12% 32% 34% 5%	800
	11th to 20th	Night	18% 13% 34% 27% 8% 1	800
	21st to end	Night	19% 11% 35% 28% 7%	880
April	1st to 10th	Night	16% 13% 34% 28% 9%	800
	11th to 20th	Night	16% 12% 28% 30% 13%	791
	21st to end	Night	14% 7% 28% 35% 16%	600
May	1st to 10th	Night	14% 6% 28% 34% 17% 1	600
	11th to 20th	Night	18% 4 20% 32% 24% 2	591
	21st to end	Night	17% 2 14% 30% 32% 5%	650
June	1st to 10th	Night	12% 2 15% 34% 31% 6%	600
	11th to 20th	Night	12% 4 13% 29% 34% 9%	600
	21st to end	Night	7% 2 11% 31% 43% 6%	600
July	1st to 10th	Night	11% 2 6% 25% 47% 8%	585
	11th to 20th	Night	10% 2 5% 26% 45% 11%	582
	21st to end	Night	8% 2 7% 30% 44% 9%	657
August	1st to 10th	Night	17% 3 7% 28% 38% 7%	597
	11th to 20th	Night	15% 3 6% 29% 37% 9%	594
	21st to end	Night	13% 2 10% 22% 45% 8%	776
September	1st to 10th	Night	13% 3 10% 29% 37% 9%	800
	11th to 20th	Night	18% 3 11% 24% 37% 7%	800
	21st to end	Night	22% 4 15% 30% 28% 2	800
October	1st to 10th	Night	17% 4 18% 28% 27% 6%	800
	11th to 20th	Night	18% 6% 23% 31% 20% 2	800
	21st to end	Night	16% 6% 22% 35% 17% 3	880
November	1st to 10th	Night	16% 6% 24% 35% 17% 2	820
	11th to 20th	Night	15% 8% 29% 36% 11% 1	840
	21st to end	Night	15% 9% 28% 34% 14% 1	840
December	1st to 10th	Night	21% 10% 27% 34% 8%	840
	11th to 20th	Night	20% 7% 32% 32% 8% 1	840
	21st to end	Night	23% 8% 29% 32% 7%	924

Page generated at 2026-01-29 01:21:42. Version ac07

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