Welsh Mt. Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Welsh Mt. Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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	13% 5% 2 6% 4 4 5% 4 6% 8% 43%	126440
	Night	14% 6% 2 5% 4 4 4 4 5% 7% 45%	124238
Transparency	Day	38% 5% 12% 23% 17% 5%	130046
	Night	38% 5% 10% 22% 19% 6%	128701
Seeing	Day	19% 11% 25% 25% 15% 6%	16468
	Night	17% 7% 25% 34% 16% 2	26328

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% 5% 2 4 4 3 3 4 5% 5% 51%	4056
	11th to 20th	Day	12% 7% 2 6% 3 4 3 3 4 4 52%	4060
	21st to end	Day	15% 6% 2 6% 4 3 4 3 4 5% 47%	4466
February	1st to 10th	Day	12% 6% 3 6% 5% 4 4 5% 6% 7% 42%	1800
	11th to 20th	Day	19% 6% 2 6% 4 3 4 3 6% 5% 42%	1753
	21st to end	Day	17% 8% 3 6% 4 5% 4 2 5% 6% 40%	1735
March	1st to 10th	Day	17% 5% 2 6% 4 4 5% 4 6% 7% 41%	4935
	11th to 20th	Day	11% 5% 2 5% 4 4 3 4 5% 6% 52%	4980
	21st to end	Day	15% 7% 2 7% 5% 5% 5% 4 5% 5% 41%	5465
April	1st to 10th	Day	11% 6% 2 5% 4 4 4 3 6% 6% 48%	2345
	11th to 20th	Day	12% 6% 2 5% 4 3 4 4 5% 6% 48%	2374
	21st to end	Day	16% 5% 2 6% 5% 4 5% 3 6% 8% 42%	2442
May	1st to 10th	Day	13% 6% 2 6% 6% 6% 6% 5% 7% 10% 32%	5862
	11th to 20th	Day	21% 6% 2 7% 4 5% 5% 4 6% 8% 32%	5861
	21st to end	Day	15% 4 2 6% 3 4 4 4 6% 9% 42%	6588
June	1st to 10th	Day	10% 4 2 5% 5% 4 5% 5% 7% 10% 44%	2800
	11th to 20th	Day	11% 4 2 6% 4 5% 5% 4 8% 10% 41%	2800
	21st to end	Day	7% 4 2 5% 5% 5% 5% 4 8% 12% 43%	2783
July	1st to 10th	Day	7% 4 2 6% 5% 5% 6% 5% 8% 12% 41%	5900
	11th to 20th	Day	8% 5% 2 7% 6% 6% 6% 6% 10% 13% 30%	5900
	21st to end	Day	9% 5% 3 6% 6% 5% 6% 5% 7% 11% 36%	6424
August	1st to 10th	Day	15% 6% 2 7% 5% 5% 6% 4 8% 11% 31%	4234
	11th to 20th	Day	8% 5% 2 6% 6% 6% 6% 6% 8% 10% 39%	4110
	21st to end	Day	8% 4 2 5% 3 4 5% 4 7% 11% 48%	4312
September	1st to 10th	Day	11% 5% 2 5% 4 4 4 3 6% 7% 49%	2400
	11th to 20th	Day	12% 6% 2 5% 4 4 4 4 6% 8% 45%	2288
	21st to end	Day	23% 7% 2 6% 4 3 3 4 5% 6% 36%	2146
October	1st to 10th	Day	14% 5% 2 6% 3 3 3 3 5% 7% 50%	2000
	11th to 20th	Day	15% 4 1 4 3 4 3 3 4 5% 54%	2000
	21st to end	Day	14% 5% 2 5% 3 3 3 3 4 5% 54%	2200
November	1st to 10th	Day	12% 4 1 4 3 4 3 3 4 6% 56%	2057
	11th to 20th	Day	13% 6% 2 4 3 3 3 3 5% 6% 52%	1911
	21st to end	Day	13% 5% 2 4 4 3 4 3 5% 6% 51%	1680
December	1st to 10th	Day	12% 6% 2 6% 3 4 3 3 5% 6% 50%	2616
	11th to 20th	Day	21% 5% 2 4 3 2 3 3 4 6% 47%	2990
	21st to end	Day	21% 6% 2 6% 4 4 4 3 5% 6% 39%	4167
January	1st to 10th	Night	12% 5% 2 4 3 3 3 3 4 5% 55%	5889
	11th to 20th	Night	12% 7% 2 4 3 2 3 3 3 4 56%	5900
	21st to end	Night	15% 6% 2 5% 4 3 3 2 4 4 51%	6490
February	1st to 10th	Night	13% 7% 2 5% 4 4 4 4 6% 7% 43%	2600
	11th to 20th	Night	21% 7% 2 5% 3 4 4 3 4 5% 43%	2533
	21st to end	Night	14% 6% 2 5% 4 4 4 3 6% 7% 46%	1889
March	1st to 10th	Night	17% 6% 2 5% 3 4 4 3 5% 6% 46%	4918
	11th to 20th	Night	10% 3 1 4 3 3 4 3 5% 7% 55%	4980
	21st to end	Night	13% 6% 2 6% 4 4 5% 4 5% 6% 45%	5467
April	1st to 10th	Night	14% 6% 2 6% 4 4 3 3 6% 7% 45%	2055
	11th to 20th	Night	11% 6% 2 5% 3 3 3 3 6% 8% 49%	1975
	21st to end	Night	15% 5% 2 5% 4 4 4 3 6% 7% 45%	1974
May	1st to 10th	Night	17% 8% 3 7% 5% 5% 5% 4 5% 9% 34%	4033
	11th to 20th	Night	22% 7% 2 7% 4 4 4 4 6% 7% 32%	4032
	21st to end	Night	14% 6% 2 6% 5% 4 5% 4 7% 9% 39%	4248
June	1st to 10th	Night	10% 5% 2 4 5% 5% 5% 5% 7% 9% 44%	1600
	11th to 20th	Night	10% 6% 2 5% 5% 4 5% 4 5% 9% 44%	1600
	21st to end	Night	8% 4 2 5% 5% 5% 4 4 7% 10% 46%	1609
July	1st to 10th	Night	10% 6% 3 6% 5% 6% 6% 5% 7% 9% 37%	4058
	11th to 20th	Night	12% 8% 4 9% 7% 6% 6% 5% 7% 10% 27%	4057
	21st to end	Night	8% 7% 3 9% 6% 6% 6% 6% 9% 9% 31%	4427
August	1st to 10th	Night	17% 6% 2 8% 5% 4 5% 5% 7% 9% 32%	2915
	11th to 20th	Night	10% 7% 2 8% 6% 5% 6% 5% 7% 9% 36%	2904
	21st to end	Night	10% 6% 2 6% 5% 4 5% 4 6% 9% 43%	3586
September	1st to 10th	Night	12% 6% 3 5% 3 4 4 3 5% 6% 48%	2000
	11th to 20th	Night	14% 5% 2 5% 4 4 5% 3 6% 7% 44%	2108
	21st to end	Night	24% 7% 2 5% 4 3 4 3 5% 6% 37%	2252
October	1st to 10th	Night	18% 4 2 4 3 4 4 3 5% 6% 48%	2400
	11th to 20th	Night	14% 5% 2 4 3 3 4 3 4 6% 51%	2400
	21st to end	Night	14% 4 2 4 3 4 3 3 4 5% 55%	2640
November	1st to 10th	Night	12% 7% 2 6% 4 3 3 3 4 3 53%	2461
	11th to 20th	Night	13% 5% 2 5% 4 3 3 3 5% 6% 51%	2700
	21st to end	Night	13% 4 2 5% 4 4 3 3 5% 7% 50%	2940
December	1st to 10th	Night	10% 6% 2 5% 3 4 4 3 5% 6% 54%	4609
	11th to 20th	Night	19% 8% 3 5% 3 4 3 3 4 5% 43%	5205
	21st to end	Night	17% 6% 2 5% 3 4 4 4 6% 7% 43%	6784

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	36% 2 5% 25% 20% 11%	4057
	11th to 20th	Day	36% 3 5% 18% 24% 14%	4060
	21st to end	Day	40% 3 6% 26% 20% 5%	4466
February	1st to 10th	Day	41% 4 11% 26% 15% 3	1800
	11th to 20th	Day	43% 4 8% 20% 20% 5%	1762
	21st to end	Day	45% 4 10% 17% 17% 6%	1735
March	1st to 10th	Day	41% 4 9% 19% 17% 10%	4935
	11th to 20th	Day	32% 4 8% 25% 22% 10%	4980
	21st to end	Day	44% 5% 9% 18% 17% 7%	5465
April	1st to 10th	Day	35% 5% 11% 23% 18% 9%	2345
	11th to 20th	Day	35% 4 12% 23% 20% 6%	2374
	21st to end	Day	41% 5% 11% 24% 15% 4	2442
May	1st to 10th	Day	45% 6% 15% 21% 11% 1	5897
	11th to 20th	Day	49% 7% 12% 19% 12% 1	5865
	21st to end	Day	38% 5% 15% 20% 19% 3	6603
June	1st to 10th	Day	33% 7% 16% 27% 15% 1	2800
	11th to 20th	Day	36% 8% 17% 22% 15% 2	2800
	21st to end	Day	32% 8% 17% 27% 15% 1	2783
July	1st to 10th	Day	34% 9% 22% 20% 12% 4	5835
	11th to 20th	Day	40% 11% 21% 21% 7% 1	5831
	21st to end	Day	39% 8% 15% 24% 13%	6240
August	1st to 10th	Day	44% 7% 17% 25% 6%	4221
	11th to 20th	Day	37% 8% 16% 28% 11%	4149
	21st to end	Day	30% 6% 19% 28% 15% 1	4312
September	1st to 10th	Day	34% 6% 14% 18% 23% 4	2400
	11th to 20th	Day	37% 6% 8% 27% 19% 3	2288
	21st to end	Day	48% 5% 9% 19% 17% 2	2146
October	1st to 10th	Day	33% 4 6% 20% 31% 5%	3260
	11th to 20th	Day	24% 3 7% 23% 30% 13%	3240
	21st to end	Day	21% 3 10% 29% 25% 12%	3538
November	1st to 10th	Day	30% 4 9% 24% 27% 5%	2057
	11th to 20th	Day	34% 3 9% 24% 22% 8%	1911
	21st to end	Day	34% 5% 8% 25% 25% 4	1680
December	1st to 10th	Day	34% 4 9% 25% 19% 9%	2616
	11th to 20th	Day	39% 3 14% 22% 17% 6%	2986
	21st to end	Day	46% 3 10% 24% 16% 1	4167
January	1st to 10th	Night	31% 4 5% 22% 25% 13%	5889
	11th to 20th	Night	32% 3 5% 21% 28% 11%	5900
	21st to end	Night	38% 2 6% 27% 20% 7%	6490
February	1st to 10th	Night	39% 5% 11% 24% 18% 3	2600
	11th to 20th	Night	44% 3 8% 19% 19% 7%	2535
	21st to end	Night	38% 4 10% 23% 18% 7%	1889
March	1st to 10th	Night	38% 4 6% 22% 20% 10%	4918
	11th to 20th	Night	28% 4 8% 20% 30% 9%	4980
	21st to end	Night	39% 4 9% 22% 20% 6%	5467
April	1st to 10th	Night	38% 5% 12% 17% 21% 7%	2055
	11th to 20th	Night	34% 4 13% 22% 23% 5%	1975
	21st to end	Night	38% 6% 10% 25% 17% 5%	1974
May	1st to 10th	Night	47% 7% 15% 18% 10% 3	4060
	11th to 20th	Night	50% 6% 11% 21% 11% 1	4033
	21st to end	Night	40% 6% 16% 25% 11% 2	4255
June	1st to 10th	Night	34% 8% 12% 28% 16% 1	1600
	11th to 20th	Night	37% 6% 18% 21% 16% 2	1600
	21st to end	Night	32% 7% 16% 23% 18% 3	1609
July	1st to 10th	Night	40% 10% 19% 16% 13% 2	4015
	11th to 20th	Night	51% 9% 17% 17% 5% 1	4006
	21st to end	Night	44% 8% 16% 21% 11%	4297
August	1st to 10th	Night	46% 7% 15% 24% 7% 1	2904
	11th to 20th	Night	42% 7% 15% 24% 11%	2931
	21st to end	Night	36% 7% 16% 31% 9% 2	3586
September	1st to 10th	Night	36% 6% 13% 21% 18% 6%	2000
	11th to 20th	Night	38% 7% 11% 24% 17% 3	2108
	21st to end	Night	48% 5% 9% 19% 16% 3	2252
October	1st to 10th	Night	40% 4 9% 19% 22% 6%	3920
	11th to 20th	Night	26% 3 7% 22% 30% 12%	3899
	21st to end	Night	21% 3 7% 30% 27% 11%	4252
November	1st to 10th	Night	36% 4 6% 21% 26% 7%	2461
	11th to 20th	Night	35% 3 8% 22% 24% 8%	2700
	21st to end	Night	34% 4 9% 24% 23% 6%	2940
December	1st to 10th	Night	31% 4 8% 28% 24% 5%	4609
	11th to 20th	Night	44% 4 10% 19% 16% 7%	5208
	21st to end	Night	39% 5% 11% 27% 16% 2	6784

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	21% 26% 30% 18% 4	300
	11th to 20th	Day	30% 23% 32% 13% 2	300
	21st to end	Day	25% 25% 32% 12% 5%	330
February	1st to 10th	Day	21% 24% 39% 13% 3	300
	11th to 20th	Day	39% 24% 27% 7% 3	300
	21st to end	Day	29% 20% 28% 19% 4	394
March	1st to 10th	Day	25% 16% 31% 24% 4 1	500
	11th to 20th	Day	20% 20% 32% 22% 5% 1	500
	21st to end	Day	25% 14% 34% 22% 4 2	550
April	1st to 10th	Day	23% 18% 28% 21% 8% 1	500
	11th to 20th	Day	21% 12% 30% 23% 10% 4	492
	21st to end	Day	21% 12% 30% 23% 11% 2	500
May	1st to 10th	Day	21% 14% 32% 21% 9% 3	503
	11th to 20th	Day	23% 9% 25% 25% 12% 5%	527
	21st to end	Day	19% 7% 21% 28% 18% 6%	573
June	1st to 10th	Day	10% 6% 27% 27% 22% 8%	530
	11th to 20th	Day	13% 7% 21% 32% 17% 11%	530
	21st to end	Day	9% 4 24% 32% 23% 8%	530
July	1st to 10th	Day	11% 2 17% 29% 28% 13%	525
	11th to 20th	Day	6% 2 16% 32% 28% 14%	524
	21st to end	Day	11% 3 18% 30% 24% 14%	576
August	1st to 10th	Day	16% 5% 17% 26% 25% 12%	526
	11th to 20th	Day	11% 2 19% 29% 25% 14%	519
	21st to end	Day	10% 2 14% 31% 26% 17%	583
September	1st to 10th	Day	14% 3 16% 27% 23% 16%	530
	11th to 20th	Day	17% 5% 11% 27% 29% 10%	530
	21st to end	Day	31% 6% 18% 25% 16% 4	530
October	1st to 10th	Day	19% 5% 19% 32% 16% 9%	530
	11th to 20th	Day	15% 12% 25% 28% 17% 3	530
	21st to end	Day	17% 10% 28% 30% 11% 3	583
November	1st to 10th	Day	17% 10% 29% 29% 13% 3	535
	11th to 20th	Day	19% 12% 33% 22% 13% 2	399
	21st to end	Day	17% 19% 31% 22% 8% 2	220
December	1st to 10th	Day	21% 16% 37% 21% 4 1	204
	11th to 20th	Day	24% 15% 33% 21% 7% 1	200
	21st to end	Day	27% 15% 28% 22% 7% 1	265
January	1st to 10th	Night	18% 11% 38% 29% 4	800
	11th to 20th	Night	21% 12% 34% 30% 3	800
	21st to end	Night	17% 11% 40% 29% 4	880
February	1st to 10th	Night	17% 11% 40% 30% 3	800
	11th to 20th	Night	25% 13% 37% 23% 1	783
	21st to end	Night	19% 15% 35% 28% 3	660
March	1st to 10th	Night	17% 13% 34% 31% 5%	800
	11th to 20th	Night	22% 11% 32% 29% 7% 1	800
	21st to end	Night	19% 11% 33% 31% 6%	880
April	1st to 10th	Night	17% 15% 37% 26% 6%	800
	11th to 20th	Night	15% 10% 31% 32% 10% 1	791
	21st to end	Night	17% 6% 30% 37% 9% 1	774
May	1st to 10th	Night	19% 6% 25% 37% 13% 1	600
	11th to 20th	Night	21% 4 20% 35% 18% 2	591
	21st to end	Night	16% 2 13% 37% 29% 3	580
June	1st to 10th	Night	9% 3 13% 46% 27% 2	400
	11th to 20th	Night	12% 2 14% 41% 29% 2	400
	21st to end	Night	9% 2 14% 48% 26% 2	409
July	1st to 10th	Night	12% 2 10% 34% 36% 6%	585
	11th to 20th	Night	5% 2 9% 34% 43% 8%	582
	21st to end	Night	9% 2 10% 39% 37% 4	657
August	1st to 10th	Night	15% 2 7% 37% 33% 5%	597
	11th to 20th	Night	16% 1 8% 33% 38% 4	610
	21st to end	Night	13% 2 10% 36% 34% 5%	880
September	1st to 10th	Night	15% 2 11% 35% 31% 7%	800
	11th to 20th	Night	15% 3 12% 33% 32% 6%	800
	21st to end	Night	27% 3 16% 32% 21% 2	800
October	1st to 10th	Night	20% 4 16% 31% 24% 6%	800
	11th to 20th	Night	17% 6% 24% 34% 18% 2	800
	21st to end	Night	16% 6% 24% 38% 15% 1	880
November	1st to 10th	Night	15% 6% 25% 39% 12% 2	820
	11th to 20th	Night	16% 9% 29% 35% 10% 1	840
	21st to end	Night	15% 8% 32% 37% 8% 1	840
December	1st to 10th	Night	19% 10% 33% 31% 6%	809
	11th to 20th	Night	17% 8% 31% 38% 6%	800
	21st to end	Night	20% 9% 29% 39% 3	880

Page generated at 2025-11-30 01:00:56. Version ac07

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