Wes Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wes 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	11% 6% 2 6% 5% 5% 5% 4 6% 9% 41%	101251
	Night	12% 6% 2 6% 4 4 5% 4 6% 8% 43%	98237
Transparency	Day	37% 8% 13% 18% 17% 7%	105019
	Night	36% 7% 13% 17% 18% 9%	102608
Seeing	Day	12% 7% 20% 24% 23% 15%	15311
	Night	13% 6% 22% 33% 23% 4	25358

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	12% 6% 2 6% 4 4 5% 4 4 7% 45%	2836
	11th to 20th	Day	13% 6% 1 5% 3 3 4 3 5% 8% 50%	2840
	21st to end	Day	13% 8% 3 6% 4 3 4 3 5% 6% 45%	3124
February	1st to 10th	Day	16% 8% 2 6% 5% 4 4 4 5% 8% 39%	1891
	11th to 20th	Day	17% 6% 2 4 4 3 3 3 5% 6% 47%	1848
	21st to end	Day	15% 7% 3 5% 5% 3 4 2 4 7% 44%	1570
March	1st to 10th	Day	19% 9% 3 8% 4 3 3 3 6% 6% 36%	3430
	11th to 20th	Day	8% 4 1 5% 4 4 4 4 5% 8% 52%	3480
	21st to end	Day	17% 6% 2 6% 4 4 4 4 5% 5% 43%	3815
April	1st to 10th	Day	10% 5% 2 6% 4 5% 4 3 5% 7% 48%	2090
	11th to 20th	Day	13% 5% 2 4 5% 4 4 4 5% 7% 48%	2114
	21st to end	Day	6% 4 2 4 3 4 4 4 6% 7% 56%	2470
May	1st to 10th	Day	8% 4 2 5% 5% 4 5% 4 6% 9% 48%	4082
	11th to 20th	Day	16% 6% 2 6% 4 4 4 4 5% 7% 40%	4081
	21st to end	Day	6% 3 1 5% 5% 5% 5% 4 7% 10% 48%	4476
June	1st to 10th	Day	10% 5% 2 7% 6% 6% 6% 5% 8% 11% 34%	2470
	11th to 20th	Day	6% 4 1 5% 5% 6% 6% 7% 8% 11% 42%	2470
	21st to end	Day	6% 5% 2 7% 6% 7% 7% 7% 10% 11% 33%	2470
July	1st to 10th	Day	10% 7% 3 9% 7% 7% 8% 6% 9% 11% 22%	4120
	11th to 20th	Day	4 4 2 7% 7% 7% 8% 7% 10% 14% 30%	4120
	21st to end	Day	6% 6% 3 9% 7% 8% 8% 6% 10% 14% 23%	4466
August	1st to 10th	Day	14% 5% 3 8% 6% 7% 7% 6% 9% 10% 26%	4104
	11th to 20th	Day	7% 4 2 6% 5% 5% 6% 5% 8% 12% 40%	4020
	21st to end	Day	8% 6% 3 8% 6% 5% 6% 5% 8% 11% 34%	3956
September	1st to 10th	Day	10% 7% 3 8% 5% 6% 7% 5% 7% 8% 34%	2090
	11th to 20th	Day	13% 6% 2 8% 7% 4 5% 4 6% 8% 37%	2090
	21st to end	Day	13% 6% 3 6% 4 4 6% 4 5% 7% 41%	2090
October	1st to 10th	Day	12% 5% 3 6% 3 4 4 3 5% 6% 49%	2081
	11th to 20th	Day	11% 3 1 4 3 2 3 3 5% 7% 58%	1900
	21st to end	Day	8% 5% 2 5% 3 3 3 3 4 7% 57%	2090
November	1st to 10th	Day	11% 6% 2 5% 4 4 3 3 5% 5% 52%	1957
	11th to 20th	Day	12% 6% 2 5% 3 3 3 2 4 7% 54%	1850
	21st to end	Day	14% 6% 2 5% 4 3 3 3 4 7% 49%	1773
December	1st to 10th	Day	15% 5% 2 6% 4 3 5% 4 6% 6% 44%	1710
	11th to 20th	Day	16% 6% 2 4 4 3 4 4 4 7% 46%	2153
	21st to end	Day	17% 9% 3 7% 4 4 3 2 4 5% 42%	3124
January	1st to 10th	Night	16% 6% 2 5% 3 4 5% 4 6% 6% 43%	4109
	11th to 20th	Night	8% 6% 2 7% 5% 4 4 4 6% 7% 47%	4120
	21st to end	Night	16% 7% 2 7% 4 4 3 3 5% 6% 42%	4532
February	1st to 10th	Night	13% 7% 3 6% 4 4 4 3 4 7% 44%	2285
	11th to 20th	Night	16% 7% 2 5% 4 3 3 3 4 5% 47%	2218
	21st to end	Night	13% 6% 2 5% 4 4 4 3 5% 7% 48%	1884
March	1st to 10th	Night	21% 10% 3 6% 4 3 3 2 4 6% 38%	3421
	11th to 20th	Night	10% 5% 2 5% 4 5% 5% 3 5% 7% 51%	3480
	21st to end	Night	17% 5% 2 5% 4 4 4 4 5% 7% 44%	3817
April	1st to 10th	Night	11% 5% 2 5% 4 3 4 4 5% 7% 50%	2090
	11th to 20th	Night	10% 4 2 5% 4 3 4 4 6% 7% 51%	2013
	21st to end	Night	7% 4 1 4 3 5% 3 4 6% 7% 56%	1724
May	1st to 10th	Night	8% 4 2 7% 4 4 4 3 6% 10% 49%	2813
	11th to 20th	Night	11% 6% 2 7% 4 4 4 4 6% 8% 43%	2812
	21st to end	Night	6% 4 2 5% 4 4 4 4 6% 8% 53%	3079
June	1st to 10th	Night	10% 5% 2 7% 5% 6% 6% 6% 8% 12% 34%	1710
	11th to 20th	Night	6% 4 2 5% 5% 5% 6% 6% 7% 11% 42%	1710
	21st to end	Night	6% 4 2 6% 7% 7% 7% 7% 11% 10% 33%	1710
July	1st to 10th	Night	11% 8% 4 9% 6% 7% 7% 5% 8% 10% 25%	2840
	11th to 20th	Night	5% 4 3 8% 7% 6% 7% 6% 10% 15% 29%	2840
	21st to end	Night	8% 6% 3 10% 7% 7% 7% 6% 10% 11% 26%	3087
August	1st to 10th	Night	13% 6% 2 8% 5% 6% 7% 6% 8% 12% 27%	2825
	11th to 20th	Night	6% 5% 2 7% 4 5% 6% 4 8% 11% 41%	2805
	21st to end	Night	7% 5% 2 6% 4 5% 5% 4 7% 11% 43%	3666
September	1st to 10th	Night	12% 6% 2 6% 4 5% 5% 5% 7% 12% 37%	2090
	11th to 20th	Night	10% 6% 3 8% 5% 4 5% 4 6% 8% 40%	2090
	21st to end	Night	12% 6% 2 7% 4 5% 5% 4 6% 8% 41%	2090
October	1st to 10th	Night	11% 6% 2 5% 3 4 4 3 5% 6% 50%	2097
	11th to 20th	Night	12% 5% 2 3 4 3 3 2 4 4 59%	2280
	21st to end	Night	9% 4 1 5% 4 4 4 3 4 6% 56%	2508
November	1st to 10th	Night	12% 6% 2 5% 3 3 4 3 5% 6% 52%	2341
	11th to 20th	Night	13% 6% 2 5% 4 3 4 2 4 6% 51%	2545
	21st to end	Night	13% 6% 2 5% 4 4 4 2 5% 6% 50%	2562
December	1st to 10th	Night	13% 6% 2 6% 3 4 5% 4 5% 7% 45%	2470
	11th to 20th	Night	17% 6% 2 5% 3 3 3 2 4 5% 49%	3042
	21st to end	Night	17% 9% 2 6% 5% 4 4 3 5% 6% 38%	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	38% 5% 7% 17% 22% 11%	2837
	11th to 20th	Day	32% 5% 8% 18% 24% 13%	2840
	21st to end	Day	40% 6% 6% 13% 22% 13%	3124
February	1st to 10th	Day	44% 4 9% 15% 20% 8%	1891
	11th to 20th	Day	38% 4 8% 14% 27% 9%	1858
	21st to end	Day	40% 4 7% 18% 19% 12%	1570
March	1st to 10th	Day	47% 6% 8% 16% 15% 8%	3430
	11th to 20th	Day	29% 6% 12% 17% 21% 15%	3480
	21st to end	Day	42% 5% 8% 20% 17% 8%	3815
April	1st to 10th	Day	34% 6% 10% 20% 18% 12%	2090
	11th to 20th	Day	35% 5% 9% 18% 21% 12%	2114
	21st to end	Day	26% 6% 11% 24% 26% 7%	2470
May	1st to 10th	Day	31% 9% 12% 28% 17% 3	4120
	11th to 20th	Day	42% 7% 11% 23% 15% 2	4085
	21st to end	Day	28% 9% 17% 24% 20% 2	4489
June	1st to 10th	Day	40% 12% 16% 18% 13% 1	2470
	11th to 20th	Day	31% 11% 17% 24% 15% 1	2470
	21st to end	Day	39% 14% 20% 17% 10% 2	2470
July	1st to 10th	Day	48% 15% 21% 11% 3 1	4055
	11th to 20th	Day	38% 17% 27% 15% 3 1	4051
	21st to end	Day	44% 17% 24% 12% 3	4324
August	1st to 10th	Day	48% 12% 22% 16% 2	4091
	11th to 20th	Day	33% 12% 18% 26% 10%	4059
	21st to end	Day	41% 11% 16% 26% 5% 1	3956
September	1st to 10th	Day	45% 11% 16% 14% 11% 4	2090
	11th to 20th	Day	46% 8% 12% 15% 15% 5%	2090
	21st to end	Day	41% 7% 10% 19% 18% 6%	2090
October	1st to 10th	Day	39% 4 8% 12% 22% 15%	3458
	11th to 20th	Day	16% 3 5% 14% 26% 35%	3140
	21st to end	Day	22% 3 5% 18% 36% 16%	3428
November	1st to 10th	Day	33% 5% 8% 16% 23% 15%	1957
	11th to 20th	Day	32% 4 8% 18% 24% 15%	1850
	21st to end	Day	36% 4 9% 19% 22% 11%	1773
December	1st to 10th	Day	39% 7% 11% 17% 17% 8%	1710
	11th to 20th	Day	39% 4 7% 16% 27% 7%	2150
	21st to end	Day	46% 4 6% 15% 23% 5%	3124
January	1st to 10th	Night	38% 6% 8% 14% 21% 14%	4109
	11th to 20th	Night	35% 5% 7% 17% 26% 11%	4120
	21st to end	Night	42% 6% 6% 11% 24% 11%	4532
February	1st to 10th	Night	39% 5% 8% 17% 20% 11%	2285
	11th to 20th	Night	40% 4 7% 15% 21% 13%	2219
	21st to end	Night	36% 6% 9% 18% 17% 15%	1884
March	1st to 10th	Night	48% 5% 11% 14% 14% 8%	3421
	11th to 20th	Night	32% 5% 10% 15% 23% 14%	3480
	21st to end	Night	39% 5% 9% 16% 20% 10%	3817
April	1st to 10th	Night	32% 7% 11% 18% 19% 13%	2090
	11th to 20th	Night	29% 7% 8% 22% 21% 12%	2013
	21st to end	Night	25% 7% 12% 26% 22% 8%	1724
May	1st to 10th	Night	29% 8% 16% 25% 17% 5%	2840
	11th to 20th	Night	36% 9% 14% 23% 15% 2	2813
	21st to end	Night	26% 9% 15% 29% 19% 3	3087
June	1st to 10th	Night	38% 13% 19% 19% 10% 1	1710
	11th to 20th	Night	32% 11% 21% 25% 11%	1710
	21st to end	Night	36% 15% 21% 16% 10% 1	1710
July	1st to 10th	Night	49% 12% 21% 13% 4 1	2795
	11th to 20th	Night	39% 15% 29% 13% 3	2786
	21st to end	Night	46% 19% 19% 12% 3 1	2988
August	1st to 10th	Night	45% 13% 28% 12% 2	2814
	11th to 20th	Night	33% 10% 23% 26% 8% 1	2832
	21st to end	Night	31% 11% 22% 30% 5% 1	3666
September	1st to 10th	Night	39% 11% 20% 15% 12% 3	2090
	11th to 20th	Night	39% 12% 17% 15% 14% 3	2090
	21st to end	Night	39% 8% 15% 15% 19% 4	2090
October	1st to 10th	Night	37% 5% 12% 13% 22% 12%	3500
	11th to 20th	Night	19% 2 4 12% 27% 36%	3779
	21st to end	Night	23% 3 5% 16% 33% 20%	4120
November	1st to 10th	Night	34% 4 10% 13% 24% 15%	2341
	11th to 20th	Night	36% 4 6% 15% 24% 14%	2545
	21st to end	Night	35% 5% 8% 20% 20% 13%	2562
December	1st to 10th	Night	38% 6% 13% 16% 21% 6%	2470
	11th to 20th	Night	39% 4 8% 15% 26% 9%	3044
	21st to end	Night	46% 5% 7% 17% 21% 4	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	10% 16% 50% 19% 5% 1	270
	11th to 20th	Day	15% 14% 33% 24% 11% 3	270
	21st to end	Day	23% 19% 40% 15% 3 1	297
February	1st to 10th	Day	18% 18% 34% 22% 7%	270
	11th to 20th	Day	24% 12% 32% 27% 4	270
	21st to end	Day	16% 18% 39% 22% 3 1	225
March	1st to 10th	Day	13% 15% 29% 29% 13% 1	455
	11th to 20th	Day	13% 10% 37% 29% 10% 1	460
	21st to end	Day	15% 11% 33% 29% 9% 2	506
April	1st to 10th	Day	15% 14% 28% 26% 15% 3	460
	11th to 20th	Day	13% 8% 29% 26% 18% 7%	452
	21st to end	Day	8% 7% 21% 33% 21% 10%	460
May	1st to 10th	Day	5% 10% 24% 27% 23% 12%	463
	11th to 20th	Day	12% 2 15% 26% 27% 18%	487
	21st to end	Day	9% 2 13% 21% 30% 24%	529
June	1st to 10th	Day	10% 2 12% 24% 29% 24%	490
	11th to 20th	Day	7% 3 11% 23% 25% 30%	490
	21st to end	Day	7% 2 9% 24% 31% 27%	490
July	1st to 10th	Day	5% 2 4 16% 34% 38%	485
	11th to 20th	Day	6% 1 3 13% 31% 46%	484
	21st to end	Day	5% 2 6% 16% 39% 31%	532
August	1st to 10th	Day	11% 2 8% 17% 33% 29%	486
	11th to 20th	Day	9% 2 4 13% 33% 39%	479
	21st to end	Day	12% 2 10% 20% 29% 27%	539
September	1st to 10th	Day	12% 3 9% 22% 31% 23%	490
	11th to 20th	Day	15% 2 12% 27% 31% 13%	490
	21st to end	Day	14% 3 11% 25% 33% 15%	490
October	1st to 10th	Day	12% 5% 23% 28% 24% 9%	490
	11th to 20th	Day	9% 5% 21% 32% 22% 10%	490
	21st to end	Day	10% 5% 21% 32% 26% 5%	539
November	1st to 10th	Day	18% 6% 23% 25% 22% 5%	495
	11th to 20th	Day	14% 9% 35% 26% 13% 2	350
	21st to end	Day	10% 15% 24% 32% 17% 1	291
December	1st to 10th	Day	18% 14% 33% 20% 13% 2	270
	11th to 20th	Day	19% 13% 33% 22% 11% 1	270
	21st to end	Day	24% 8% 33% 20% 13% 1	297
January	1st to 10th	Night	16% 11% 36% 30% 6% 1	760
	11th to 20th	Night	13% 10% 32% 33% 11%	760
	21st to end	Night	19% 10% 39% 26% 6%	836
February	1st to 10th	Night	16% 9% 34% 34% 7%	760
	11th to 20th	Night	20% 10% 34% 32% 4	743
	21st to end	Night	14% 13% 35% 35% 3	628
March	1st to 10th	Night	12% 11% 31% 40% 6%	760
	11th to 20th	Night	13% 10% 33% 33% 11% 1	760
	21st to end	Night	13% 10% 35% 31% 10%	836
April	1st to 10th	Night	12% 8% 34% 33% 12% 1	760
	11th to 20th	Night	12% 9% 30% 34% 15% 1	751
	21st to end	Night	8% 6% 26% 43% 18%	584
May	1st to 10th	Night	8% 5% 26% 41% 20% 1	570
	11th to 20th	Night	8% 4 20% 41% 24% 3	561
	21st to end	Night	8% 1 11% 41% 35% 5%	617
June	1st to 10th	Night	11% 2 10% 38% 35% 5%	570
	11th to 20th	Night	8% 1 11% 34% 36% 10%	570
	21st to end	Night	6% 2 6% 38% 37% 11%	570
July	1st to 10th	Night	8% 1 5% 27% 46% 12%	555
	11th to 20th	Night	9% 1 5% 20% 52% 13%	552
	21st to end	Night	6% 1 5% 29% 54% 7%	624
August	1st to 10th	Night	8% 1 4 27% 48% 12%	567
	11th to 20th	Night	10% 1 4 23% 51% 11%	564
	21st to end	Night	11% 2 8% 25% 42% 12%	739
September	1st to 10th	Night	12% 2 7% 28% 41% 10%	760
	11th to 20th	Night	11% 3 10% 33% 39% 4	760
	21st to end	Night	14% 2 12% 31% 35% 5%	760
October	1st to 10th	Night	13% 3 17% 36% 26% 5%	760
	11th to 20th	Night	14% 4 19% 34% 26% 3	760
	21st to end	Night	10% 4 21% 38% 25% 2	836
November	1st to 10th	Night	13% 6% 20% 36% 22% 2	780
	11th to 20th	Night	15% 8% 27% 34% 15% 2	800
	21st to end	Night	15% 7% 27% 34% 16% 1	789
December	1st to 10th	Night	16% 7% 29% 34% 13% 1	760
	11th to 20th	Night	18% 7% 27% 33% 13% 1	760
	21st to end	Night	21% 9% 27% 33% 10%	836

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