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 2024-11-10.

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% 5% 2 6% 4 4 5% 4 6% 8% 42%	100963
	Night	15% 6% 2 5% 4 4 4 4 5% 7% 44%	98120
Transparency	Day	38% 6% 12% 23% 17% 4	104613
	Night	38% 5% 11% 23% 18% 5%	102609
Seeing	Day	18% 10% 24% 26% 15% 6%	15088
	Night	17% 7% 24% 34% 16% 2	24924

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% 6% 2 5% 4 3 3 3 5% 5% 48%	2836
	11th to 20th	Day	13% 8% 3 7% 4 5% 4 3 6% 5% 43%	2840
	21st to end	Day	19% 8% 3 7% 4 3 4 3 4 5% 40%	3124
February	1st to 10th	Day	11% 6% 3 6% 5% 4 4 5% 6% 7% 43%	1710
	11th to 20th	Day	18% 6% 2 5% 4 3 4 3 6% 5% 43%	1663
	21st to end	Day	18% 8% 3 6% 4 5% 4 2 4 6% 39%	1651
March	1st to 10th	Day	21% 6% 2 6% 4 3 4 4 5% 7% 38%	3435
	11th to 20th	Day	10% 5% 2 4 4 4 3 4 5% 6% 52%	3480
	21st to end	Day	19% 7% 2 6% 5% 5% 4 4 4 5% 38%	3815
April	1st to 10th	Day	10% 5% 2 5% 4 4 4 3 6% 6% 49%	2228
	11th to 20th	Day	13% 6% 1 5% 3 3 4 4 5% 6% 49%	2254
	21st to end	Day	16% 5% 2 5% 5% 4 5% 3 5% 7% 42%	2320
May	1st to 10th	Day	16% 7% 2 7% 6% 5% 6% 5% 7% 7% 33%	4082
	11th to 20th	Day	21% 6% 2 6% 4 4 5% 4 6% 8% 35%	4081
	21st to end	Day	10% 4 2 5% 3 4 5% 4 7% 10% 46%	4589
June	1st to 10th	Day	10% 4 2 5% 5% 4 5% 5% 7% 10% 44%	2660
	11th to 20th	Day	11% 4 2 6% 4 5% 5% 4 8% 10% 41%	2660
	21st to end	Day	8% 4 2 6% 5% 5% 5% 4 8% 11% 42%	2644
July	1st to 10th	Day	7% 4 2 6% 5% 5% 6% 5% 8% 12% 42%	4120
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 5% 9% 12% 36%	4120
	21st to end	Day	12% 6% 3 7% 7% 6% 6% 6% 7% 9% 30%	4466
August	1st to 10th	Day	15% 6% 2 7% 5% 5% 6% 4 8% 11% 31%	4104
	11th to 20th	Day	8% 4 2 6% 6% 6% 6% 6% 7% 10% 39%	3982
	21st to end	Day	9% 4 2 4 3 4 5% 4 7% 11% 48%	4180
September	1st to 10th	Day	11% 5% 2 5% 4 4 4 3 6% 6% 49%	2280
	11th to 20th	Day	12% 6% 2 5% 4 4 4 4 6% 8% 44%	2174
	21st to end	Day	23% 6% 2 6% 4 3 3 4 5% 6% 37%	2039
October	1st to 10th	Day	14% 5% 2 5% 3 3 3 3 5% 7% 49%	1900
	11th to 20th	Day	15% 4 1 4 3 4 3 3 4 5% 54%	1900
	21st to end	Day	14% 5% 2 5% 3 3 3 3 4 5% 54%	2090
November	1st to 10th	Day	13% 4 1 4 3 4 3 3 4 6% 56%	1947
	11th to 20th	Day	14% 6% 2 4 3 3 4 3 4 5% 51%	1730
	21st to end	Day	14% 4 2 4 4 3 4 3 5% 6% 50%	1520
December	1st to 10th	Day	17% 7% 2 6% 3 4 4 4 6% 7% 40%	1520
	11th to 20th	Day	16% 5% 2 4 3 3 3 2 5% 5% 52%	1910
	21st to end	Day	20% 5% 2 5% 4 4 5% 3 6% 7% 39%	2909
January	1st to 10th	Night	13% 6% 2 5% 4 3 3 3 5% 6% 49%	4109
	11th to 20th	Night	17% 8% 2 4 3 3 3 3 4 5% 49%	4120
	21st to end	Night	21% 7% 2 6% 4 3 3 2 4 5% 43%	4532
February	1st to 10th	Night	13% 7% 2 5% 4 4 4 4 6% 7% 43%	2470
	11th to 20th	Night	21% 7% 2 5% 3 4 3 3 4 5% 44%	2403
	21st to end	Night	14% 7% 2 5% 4 4 4 3 5% 7% 45%	1799
March	1st to 10th	Night	21% 8% 2 6% 4 4 4 3 5% 6% 36%	3418
	11th to 20th	Night	11% 4 2 4 4 3 4 4 6% 7% 53%	3480
	21st to end	Night	15% 6% 2 5% 3 3 4 3 4 6% 48%	3817
April	1st to 10th	Night	14% 6% 2 6% 4 4 3 3 6% 6% 46%	1952
	11th to 20th	Night	12% 6% 2 5% 3 2 3 3 6% 8% 50%	1875
	21st to end	Night	15% 5% 2 5% 4 4 4 3 6% 7% 45%	1876
May	1st to 10th	Night	19% 7% 3 7% 4 4 4 4 5% 8% 34%	2813
	11th to 20th	Night	23% 7% 2 5% 4 4 4 4 5% 7% 36%	2812
	21st to end	Night	10% 5% 2 5% 5% 4 5% 4 7% 11% 43%	2962
June	1st to 10th	Night	10% 5% 2 4 4 5% 5% 5% 7% 9% 44%	1520
	11th to 20th	Night	10% 6% 2 5% 5% 4 5% 4 5% 10% 44%	1520
	21st to end	Night	8% 4 2 5% 4 5% 4 4 7% 10% 46%	1528
July	1st to 10th	Night	7% 5% 3 5% 5% 6% 6% 5% 8% 10% 40%	2840
	11th to 20th	Night	8% 6% 3 7% 6% 5% 5% 5% 7% 10% 36%	2840
	21st to end	Night	11% 9% 3 9% 6% 6% 6% 6% 8% 8% 29%	3087
August	1st to 10th	Night	17% 6% 2 8% 5% 5% 5% 5% 7% 9% 32%	2825
	11th to 20th	Night	10% 7% 2 8% 5% 5% 6% 5% 7% 9% 36%	2813
	21st to end	Night	11% 6% 2 6% 4 4 5% 4 6% 9% 43%	3476
September	1st to 10th	Night	12% 6% 3 6% 3 4 4 3 6% 7% 47%	1900
	11th to 20th	Night	14% 5% 2 5% 4 5% 5% 3 6% 7% 44%	2002
	21st to end	Night	24% 7% 2 5% 3 3 4 3 5% 6% 38%	2139
October	1st to 10th	Night	18% 4 2 5% 3 4 4 3 5% 6% 46%	2280
	11th to 20th	Night	14% 5% 2 4 3 3 4 3 4 7% 51%	2280
	21st to end	Night	14% 3 2 4 3 4 3 3 4 5% 55%	2508
November	1st to 10th	Night	12% 7% 2 6% 4 3 3 3 4 3 53%	2336
	11th to 20th	Night	13% 5% 2 5% 4 3 3 3 5% 6% 52%	2441
	21st to end	Night	13% 4 2 5% 4 4 3 3 5% 7% 50%	2660
December	1st to 10th	Night	17% 8% 2 6% 4 4 4 4 4 5% 43%	2660
	11th to 20th	Night	16% 7% 2 5% 3 4 3 3 4 6% 48%	3285
	21st to end	Night	17% 6% 2 5% 4 4 5% 4 6% 7% 41%	4742

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	39% 3 6% 31% 15% 6%	2837
	11th to 20th	Day	42% 4 7% 18% 22% 7%	2840
	21st to end	Day	46% 4 7% 23% 16% 3	3124
February	1st to 10th	Day	40% 4 11% 26% 16% 3	1710
	11th to 20th	Day	42% 5% 8% 20% 21% 5%	1672
	21st to end	Day	46% 4 11% 15% 17% 6%	1651
March	1st to 10th	Day	46% 4 10% 19% 14% 7%	3435
	11th to 20th	Day	31% 5% 8% 25% 23% 9%	3480
	21st to end	Day	48% 5% 9% 18% 17% 3	3815
April	1st to 10th	Day	33% 5% 11% 23% 19% 9%	2228
	11th to 20th	Day	35% 4 12% 23% 20% 6%	2254
	21st to end	Day	42% 5% 11% 23% 15% 5%	2320
May	1st to 10th	Day	48% 6% 13% 20% 12% 2	4120
	11th to 20th	Day	47% 7% 12% 22% 11% 1	4085
	21st to end	Day	32% 6% 17% 22% 20% 3	4603
June	1st to 10th	Day	34% 7% 16% 27% 14% 1	2660
	11th to 20th	Day	36% 8% 17% 23% 15% 2	2660
	21st to end	Day	33% 8% 17% 26% 15% 1	2644
July	1st to 10th	Day	33% 10% 23% 20% 10% 5%	4055
	11th to 20th	Day	36% 8% 20% 26% 9% 1	4051
	21st to end	Day	47% 8% 13% 20% 11%	4324
August	1st to 10th	Day	44% 8% 17% 25% 6%	4091
	11th to 20th	Day	37% 8% 15% 28% 11%	4021
	21st to end	Day	30% 6% 19% 28% 15% 1	4180
September	1st to 10th	Day	35% 6% 13% 18% 23% 4	2280
	11th to 20th	Day	37% 6% 8% 27% 19% 3	2174
	21st to end	Day	48% 5% 9% 19% 18% 2	2039
October	1st to 10th	Day	34% 4 6% 20% 30% 6%	3160
	11th to 20th	Day	24% 3 7% 23% 31% 13%	3140
	21st to end	Day	21% 3 10% 30% 25% 12%	3428
November	1st to 10th	Day	31% 4 9% 24% 28% 5%	1947
	11th to 20th	Day	35% 4 9% 22% 22% 8%	1730
	21st to end	Day	34% 5% 8% 24% 25% 4	1520
December	1st to 10th	Day	43% 4 10% 23% 15% 4	1520
	11th to 20th	Day	35% 3 8% 27% 18% 9%	1906
	21st to end	Day	44% 4 10% 27% 14% 2	2909
January	1st to 10th	Night	36% 5% 6% 25% 23% 5%	4109
	11th to 20th	Night	39% 4 7% 21% 23% 6%	4120
	21st to end	Night	45% 3 6% 25% 16% 6%	4532
February	1st to 10th	Night	39% 5% 10% 24% 18% 3	2470
	11th to 20th	Night	43% 4 8% 19% 19% 8%	2405
	21st to end	Night	40% 4 10% 23% 17% 6%	1799
March	1st to 10th	Night	48% 5% 8% 20% 15% 4	3418
	11th to 20th	Night	30% 5% 6% 22% 28% 9%	3480
	21st to end	Night	38% 5% 9% 23% 19% 5%	3817
April	1st to 10th	Night	37% 5% 11% 17% 22% 7%	1952
	11th to 20th	Night	33% 4 13% 22% 23% 6%	1875
	21st to end	Night	38% 5% 10% 25% 16% 5%	1876
May	1st to 10th	Night	48% 6% 14% 19% 12% 3	2840
	11th to 20th	Night	48% 7% 10% 23% 11% 1	2813
	21st to end	Night	34% 7% 17% 25% 14% 3	2969
June	1st to 10th	Night	34% 8% 12% 29% 16% 1	1520
	11th to 20th	Night	36% 7% 18% 22% 15% 2	1520
	21st to end	Night	32% 7% 16% 23% 18% 3	1528
July	1st to 10th	Night	36% 10% 23% 15% 14% 3	2795
	11th to 20th	Night	41% 8% 19% 23% 8% 1	2786
	21st to end	Night	49% 7% 15% 19% 10%	2988
August	1st to 10th	Night	46% 7% 16% 23% 7% 1	2814
	11th to 20th	Night	42% 6% 15% 25% 11%	2840
	21st to end	Night	36% 7% 16% 31% 9% 2	3476
September	1st to 10th	Night	37% 6% 13% 20% 17% 7%	1900
	11th to 20th	Night	38% 7% 11% 24% 17% 3	2002
	21st to end	Night	47% 5% 8% 20% 16% 4	2139
October	1st to 10th	Night	40% 5% 9% 19% 21% 6%	3800
	11th to 20th	Night	26% 3 7% 22% 30% 13%	3779
	21st to end	Night	20% 3 7% 31% 27% 11%	4120
November	1st to 10th	Night	36% 4 7% 21% 25% 7%	2336
	11th to 20th	Night	35% 3 8% 22% 24% 8%	2441
	21st to end	Night	34% 4 9% 25% 22% 6%	2660
December	1st to 10th	Night	44% 4 8% 24% 17% 4	2660
	11th to 20th	Night	39% 4 8% 21% 21% 7%	3288
	21st to end	Night	40% 6% 10% 25% 16% 2	4742

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% 24% 30% 20% 4	270
	11th to 20th	Day	30% 23% 31% 13% 2	270
	21st to end	Day	26% 22% 34% 13% 5%	297
February	1st to 10th	Day	21% 24% 38% 14% 3	270
	11th to 20th	Day	36% 25% 29% 7% 3	270
	21st to end	Day	32% 19% 27% 19% 3	363
March	1st to 10th	Day	25% 15% 30% 25% 5% 1	460
	11th to 20th	Day	20% 21% 32% 21% 5% 1	460
	21st to end	Day	24% 14% 33% 22% 4 2	506
April	1st to 10th	Day	22% 18% 29% 21% 8% 2	460
	11th to 20th	Day	20% 12% 30% 23% 10% 4	452
	21st to end	Day	22% 12% 30% 23% 11% 2	460
May	1st to 10th	Day	22% 13% 31% 21% 10% 3	463
	11th to 20th	Day	23% 9% 24% 26% 13% 6%	487
	21st to end	Day	17% 5% 21% 30% 19% 7%	529
June	1st to 10th	Day	11% 6% 25% 27% 22% 8%	490
	11th to 20th	Day	12% 7% 21% 32% 17% 12%	490
	21st to end	Day	10% 4 26% 32% 22% 7%	490
July	1st to 10th	Day	11% 2 17% 27% 28% 14%	485
	11th to 20th	Day	7% 2 16% 33% 29% 13%	484
	21st to end	Day	12% 3 19% 30% 23% 14%	532
August	1st to 10th	Day	14% 5% 17% 27% 26% 12%	486
	11th to 20th	Day	10% 2 20% 29% 25% 15%	479
	21st to end	Day	10% 2 13% 29% 27% 18%	539
September	1st to 10th	Day	14% 3 14% 26% 24% 17%	490
	11th to 20th	Day	17% 6% 11% 28% 29% 10%	490
	21st to end	Day	31% 6% 17% 25% 16% 4	490
October	1st to 10th	Day	20% 5% 19% 31% 17% 7%	490
	11th to 20th	Day	15% 11% 24% 29% 18% 3	490
	21st to end	Day	16% 10% 28% 31% 12% 3	539
November	1st to 10th	Day	18% 9% 26% 31% 14% 3	491
	11th to 20th	Day	20% 12% 29% 23% 15% 2	338
	21st to end	Day	16% 15% 32% 26% 8% 2	180
December	1st to 10th	Day	20% 17% 36% 22% 3 1	180
	11th to 20th	Day	23% 15% 33% 21% 8% 1	180
	21st to end	Day	27% 14% 30% 21% 7% 1	238
January	1st to 10th	Night	18% 11% 37% 31% 4	760
	11th to 20th	Night	22% 11% 33% 31% 3	760
	21st to end	Night	17% 10% 39% 29% 4	836
February	1st to 10th	Night	17% 10% 39% 31% 3	760
	11th to 20th	Night	25% 13% 36% 24% 1	743
	21st to end	Night	20% 15% 34% 28% 3	628
March	1st to 10th	Night	17% 13% 33% 32% 5%	760
	11th to 20th	Night	22% 11% 32% 28% 7% 1	760
	21st to end	Night	19% 11% 31% 32% 6%	836
April	1st to 10th	Night	16% 14% 37% 26% 7%	760
	11th to 20th	Night	15% 9% 30% 34% 11% 1	751
	21st to end	Night	18% 6% 30% 37% 9% 1	736
May	1st to 10th	Night	19% 6% 25% 37% 13% 1	570
	11th to 20th	Night	21% 3 20% 35% 19% 2	561
	21st to end	Night	15% 1 12% 38% 30% 3	551
June	1st to 10th	Night	8% 3 13% 46% 28% 2	380
	11th to 20th	Night	11% 2 14% 41% 30% 2	380
	21st to end	Night	9% 2 14% 49% 24% 2	388
July	1st to 10th	Night	11% 1 10% 34% 37% 6%	555
	11th to 20th	Night	5% 1 9% 34% 44% 7%	552
	21st to end	Night	9% 2 10% 40% 36% 4	624
August	1st to 10th	Night	14% 2 7% 37% 34% 5%	567
	11th to 20th	Night	15% 1 8% 34% 38% 4	579
	21st to end	Night	13% 2 10% 36% 35% 5%	836
September	1st to 10th	Night	15% 2 11% 34% 32% 7%	760
	11th to 20th	Night	15% 3 12% 32% 32% 6%	760
	21st to end	Night	27% 3 16% 31% 22% 2	760
October	1st to 10th	Night	20% 4 16% 31% 23% 5%	760
	11th to 20th	Night	17% 6% 23% 34% 18% 2	760
	21st to end	Night	16% 5% 24% 39% 15% 1	836
November	1st to 10th	Night	16% 6% 25% 39% 12% 3	779
	11th to 20th	Night	16% 8% 29% 35% 11% 1	760
	21st to end	Night	15% 7% 30% 38% 9% 1	760
December	1st to 10th	Night	20% 10% 31% 32% 7% 1	760
	11th to 20th	Night	16% 8% 31% 39% 6%	760
	21st to end	Night	20% 8% 29% 39% 3	836

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