Truchas
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Truchas clear sky chart from 2005-11-06 to 2026-07-31.

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	3 5% 2 5% 4 4 4 4 6% 7% 57%	146420
	Night	3 6% 2 5% 4 3 4 3 6% 7% 57%	144104
Transparency	Day	22% 8% 9% 23% 26% 11%	141431
	Night	24% 8% 9% 22% 26% 11%	140102
Seeing	Day	9% 17% 30% 28% 13% 3	17560
	Night	7% 11% 29% 36% 15% 2	27611

By Month

Percentage Cloud Cover SW (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	2 9% 5% 8% 4 3 3 3 5% 5% 53%	5037
	11th to 20th	Day	2 6% 3 5% 3 2 2 2 4 4 67%	5340
	21st to end	Day	3 8% 3 7% 4 3 3 3 4 4 58%	5844
February	1st to 10th	Day	2 9% 4 7% 3 3 4 4 5% 6% 54%	1924
	11th to 20th	Day	2 9% 4 7% 4 3 4 3 5% 6% 53%	2049
	21st to end	Day	3 9% 3 5% 3 3 3 3 4 7% 56%	1730
March	1st to 10th	Day	2 9% 3 6% 4 3 4 3 5% 6% 56%	4774
	11th to 20th	Day	2 8% 4 8% 4 4 5% 4 7% 7% 47%	5110
	21st to end	Day	2 8% 4 5% 3 3 4 4 5% 7% 54%	5830
April	1st to 10th	Day	4 7% 3 4 4 3 3 4 5% 7% 57%	2520
	11th to 20th	Day	4 8% 2 5% 5% 3 3 3 4 5% 57%	2491
	21st to end	Day	4 7% 2 5% 3 3 4 3 5% 7% 58%	2520
May	1st to 10th	Day	4 5% 2 5% 4 5% 5% 4 7% 7% 52%	7057
	11th to 20th	Day	3 3 1 3 3 3 4 3 5% 7% 65%	7529
	21st to end	Day	2 2 1 3 2 3 4 3 6% 8% 66%	9026
June	1st to 10th	Day	3 2 1 3 3 4 5% 5% 7% 9% 58%	2940
	11th to 20th	Day	2 2 1 2 2 3 3 3 6% 7% 68%	2940
	21st to end	Day	5% 3 1 4 5% 4 5% 4 7% 8% 54%	2940
July	1st to 10th	Day	2 2 1 4 4 4 5% 4 8% 8% 58%	6479
	11th to 20th	Day	1 2 1 3 4 4 7% 7% 11% 13% 47%	6114
	21st to end	Day	3 3 1 6% 6% 7% 7% 7% 9% 11% 40%	6532
August	1st to 10th	Day	3 3 2 5% 5% 5% 7% 7% 10% 10% 44%	5617
	11th to 20th	Day	2 2 1 3 3 4 6% 6% 8% 11% 53%	5366
	21st to end	Day	3 3 2 5% 5% 6% 7% 6% 11% 12% 41%	5984
September	1st to 10th	Day	5% 4 2 5% 4 4 5% 5% 7% 9% 50%	2400
	11th to 20th	Day	4 4 2 4 4 5% 4 5% 6% 6% 56%	2400
	21st to end	Day	3 3 1 3 3 2 4 5% 6% 6% 65%	2186
October	1st to 10th	Day	4 3 1 4 3 3 3 3 5% 6% 68%	3270
	11th to 20th	Day	3 3 2 4 3 4 4 3 5% 7% 62%	3250
	21st to end	Day	4 3 1 3 2 2 2 2 3 4 74%	3548
November	1st to 10th	Day	2 5% 2 3 3 2 3 3 4 6% 68%	1946
	11th to 20th	Day	3 4 1 3 3 2 3 3 5% 5% 68%	1890
	21st to end	Day	3 8% 2 5% 3 3 3 2 5% 6% 60%	1890
December	1st to 10th	Day	3 4 1 3 2 2 1 2 3 7% 73%	2749
	11th to 20th	Day	2 5% 2 5% 4 4 3 3 6% 8% 59%	3106
	21st to end	Day	2 6% 3 5% 3 2 3 2 4 5% 64%	4092
January	1st to 10th	Night	2 8% 4 10% 5% 4 4 3 5% 6% 50%	7927
	11th to 20th	Night	2 8% 3 6% 4 3 3 2 3 5% 62%	7650
	21st to end	Night	2 8% 3 7% 4 3 3 3 5% 6% 56%	8370
February	1st to 10th	Night	2 10% 4 7% 3 2 3 2 4 6% 58%	2693
	11th to 20th	Night	4 12% 3 6% 4 3 4 3 4 6% 52%	2458
	21st to end	Night	4 10% 3 6% 3 3 2 2 4 5% 57%	2076
March	1st to 10th	Night	2 12% 4 6% 3 3 3 3 4 5% 57%	5325
	11th to 20th	Night	3 12% 4 6% 4 4 3 2 4 5% 53%	5110
	21st to end	Night	4 12% 3 5% 3 3 3 3 4 6% 54%	5376
April	1st to 10th	Night	4 7% 2 4 3 3 4 3 5% 6% 58%	2100
	11th to 20th	Night	3 10% 2 6% 3 3 3 3 5% 5% 56%	2077
	21st to end	Night	6% 9% 2 4 3 4 3 3 4 4 57%	2100
May	1st to 10th	Night	7% 6% 2 5% 3 4 4 4 6% 7% 53%	5890
	11th to 20th	Night	4 4 2 4 3 3 4 4 5% 7% 60%	5332
	21st to end	Night	4 3 2 4 3 4 4 4 5% 7% 62%	5181
June	1st to 10th	Night	1 2 1 5% 4 4 5% 4 8% 9% 57%	1680
	11th to 20th	Night	2 2 1 3 3 3 4 4 5% 6% 68%	1680
	21st to end	Night	4 3 2 4 4 4 4 5% 6% 8% 55%	1680
July	1st to 10th	Night	3 3 1 5% 5% 5% 5% 4 6% 7% 57%	3720
	11th to 20th	Night	1 2 1 5% 4 5% 7% 7% 12% 14% 41%	4067
	21st to end	Night	4 4 2 6% 5% 6% 7% 7% 11% 13% 36%	4562
August	1st to 10th	Night	2 3 2 6% 6% 6% 7% 6% 10% 13% 40%	4194
	11th to 20th	Night	2 3 1 5% 4 5% 7% 7% 10% 13% 42%	4483
	21st to end	Night	4 5% 2 8% 6% 6% 7% 6% 10% 11% 34%	4993
September	1st to 10th	Night	5% 3 2 4 4 4 6% 4 6% 9% 53%	2000
	11th to 20th	Night	4 3 1 4 3 3 4 5% 7% 8% 60%	2000
	21st to end	Night	5% 3 1 3 2 3 3 3 5% 6% 65%	2208
October	1st to 10th	Night	3 2 1 2 2 2 2 2 4 5% 75%	3920
	11th to 20th	Night	2 2 1 3 3 3 4 3 5% 8% 66%	3898
	21st to end	Night	3 3 1 2 1 2 2 2 4 6% 74%	4252
November	1st to 10th	Night	2 3 1 3 1 2 3 3 4 6% 72%	2549
	11th to 20th	Night	3 4 1 3 2 2 2 2 5% 5% 71%	2730
	21st to end	Night	4 8% 3 4 3 2 4 3 4 5% 59%	2730
December	1st to 10th	Night	2 6% 2 4 2 2 2 1 3 5% 72%	4644
	11th to 20th	Night	2 5% 2 5% 3 2 3 3 5% 8% 62%	5321
	21st to end	Night	2 10% 3 6% 3 3 3 3 4 5% 58%	7128

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	28% 9% 5% 29% 24% 4	5040
	11th to 20th	Day	16% 8% 9% 25% 28% 15%	5340
	21st to end	Day	28% 6% 5% 17% 29% 16%	5844
February	1st to 10th	Day	30% 8% 8% 20% 25% 10%	1924
	11th to 20th	Day	31% 6% 6% 24% 25% 8%	2059
	21st to end	Day	28% 7% 5% 17% 29% 15%	1730
March	1st to 10th	Day	24% 9% 3 18% 30% 16%	4774
	11th to 20th	Day	27% 12% 7% 22% 27% 5%	5110
	21st to end	Day	25% 8% 4 37% 21% 4	5830
April	1st to 10th	Day	25% 8% 5% 25% 26% 11%	2520
	11th to 20th	Day	27% 7% 4 20% 31% 11%	2494
	21st to end	Day	24% 8% 5% 24% 31% 7%	2520
May	1st to 10th	Day	23% 11% 8% 23% 29% 7%	7057
	11th to 20th	Day	17% 7% 6% 20% 38% 13%	7533
	21st to end	Day	14% 6% 7% 21% 33% 18%	9027
June	1st to 10th	Day	18% 8% 12% 24% 24% 14%	2940
	11th to 20th	Day	13% 6% 6% 17% 28% 29%	2940
	21st to end	Day	26% 7% 9% 19% 23% 16%	2940
July	1st to 10th	Day	19% 10% 11% 14% 21% 24%	6411
	11th to 20th	Day	19% 12% 25% 31% 8% 6%	6042
	21st to end	Day	30% 13% 21% 23% 11% 2	6348
August	1st to 10th	Day	32% 15% 20% 20% 9% 4	4090
	11th to 20th	Day	19% 10% 18% 27% 18% 9%	3882
	21st to end	Day	28% 14% 20% 23% 13% 2	4311
September	1st to 10th	Day	28% 11% 11% 23% 20% 6%	2400
	11th to 20th	Day	25% 8% 9% 19% 28% 11%	2400
	21st to end	Day	17% 9% 7% 25% 33% 10%	2186
October	1st to 10th	Day	17% 5% 4 18% 36% 19%	3270
	11th to 20th	Day	19% 6% 8% 23% 32% 12%	3250
	21st to end	Day	15% 4 4 20% 41% 15%	3548
November	1st to 10th	Day	17% 6% 7% 25% 35% 11%	1946
	11th to 20th	Day	18% 5% 6% 27% 34% 10%	1890
	21st to end	Day	25% 6% 5% 23% 27% 14%	1890
December	1st to 10th	Day	13% 4 7% 28% 39% 8%	2749
	11th to 20th	Day	23% 5% 5% 25% 35% 7%	3104
	21st to end	Day	21% 5% 8% 26% 30% 11%	4092
January	1st to 10th	Night	32% 7% 7% 26% 22% 6%	7928
	11th to 20th	Night	17% 12% 6% 23% 30% 12%	7650
	21st to end	Night	26% 6% 5% 18% 34% 12%	8370
February	1st to 10th	Night	29% 5% 9% 22% 24% 11%	2693
	11th to 20th	Night	32% 7% 7% 26% 20% 8%	2458
	21st to end	Night	28% 6% 3 19% 31% 12%	2076
March	1st to 10th	Night	25% 9% 3 15% 35% 13%	5325
	11th to 20th	Night	30% 6% 7% 22% 27% 8%	5110
	21st to end	Night	25% 10% 7% 34% 20% 4	5376
April	1st to 10th	Night	25% 6% 6% 26% 27% 10%	2100
	11th to 20th	Night	28% 8% 4 21% 28% 11%	2077
	21st to end	Night	28% 7% 6% 23% 28% 8%	2100
May	1st to 10th	Night	25% 9% 9% 25% 26% 5%	5888
	11th to 20th	Night	20% 6% 8% 21% 29% 15%	5333
	21st to end	Night	19% 8% 8% 17% 27% 22%	5182
June	1st to 10th	Night	21% 7% 12% 22% 22% 15%	1680
	11th to 20th	Night	17% 5% 6% 16% 27% 29%	1680
	21st to end	Night	24% 8% 10% 18% 22% 18%	1680
July	1st to 10th	Night	25% 9% 11% 11% 17% 27%	3680
	11th to 20th	Night	24% 13% 26% 24% 5% 8%	4016
	21st to end	Night	32% 12% 24% 21% 10% 1	4434
August	1st to 10th	Night	34% 14% 21% 16% 10% 5%	3045
	11th to 20th	Night	27% 12% 19% 19% 16% 8%	3243
	21st to end	Night	37% 11% 19% 21% 11% 2	3597
September	1st to 10th	Night	26% 8% 15% 26% 17% 8%	2000
	11th to 20th	Night	20% 7% 14% 18% 27% 14%	2000
	21st to end	Night	19% 6% 9% 22% 34% 10%	2208
October	1st to 10th	Night	14% 4 7% 16% 40% 19%	3920
	11th to 20th	Night	15% 6% 8% 24% 33% 14%	3898
	21st to end	Night	12% 4 5% 25% 37% 18%	4252
November	1st to 10th	Night	13% 5% 6% 26% 38% 11%	2549
	11th to 20th	Night	16% 4 6% 29% 34% 11%	2730
	21st to end	Night	26% 6% 5% 21% 28% 14%	2730
December	1st to 10th	Night	17% 4 5% 28% 35% 10%	4644
	11th to 20th	Night	21% 5% 5% 23% 33% 13%	5322
	21st to end	Night	27% 7% 8% 22% 28% 8%	7128

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	12% 20% 40% 20% 7% 1	330
	11th to 20th	Day	16% 20% 30% 27% 5% 2	330
	21st to end	Day	5% 25% 39% 22% 9% 1	363
February	1st to 10th	Day	9% 27% 37% 22% 5% 1	330
	11th to 20th	Day	14% 28% 38% 16% 3	330
	21st to end	Day	18% 30% 31% 18% 2	273
March	1st to 10th	Day	14% 26% 33% 23% 5%	400
	11th to 20th	Day	15% 29% 30% 19% 6% 1	540
	21st to end	Day	17% 36% 27% 16% 4	594
April	1st to 10th	Day	15% 29% 36% 16% 4 1	540
	11th to 20th	Day	18% 35% 33% 10% 4	532
	21st to end	Day	15% 35% 34% 14% 2	540
May	1st to 10th	Day	10% 30% 40% 15% 3 1	543
	11th to 20th	Day	9% 21% 35% 24% 9% 1	670
	21st to end	Day	7% 21% 33% 28% 10% 1	844
June	1st to 10th	Day	3 10% 32% 38% 16% 2	780
	11th to 20th	Day	3 12% 37% 31% 14% 2	780
	21st to end	Day	7% 10% 28% 34% 18% 4	780
July	1st to 10th	Day	4 4 23% 40% 25% 4	771
	11th to 20th	Day	4 2 11% 43% 31% 9%	613
	21st to end	Day	4 3 11% 37% 37% 7%	616
August	1st to 10th	Day	4 5% 20% 39% 26% 6%	527
	11th to 20th	Day	3 6% 18% 44% 25% 5%	517
	21st to end	Day	5% 4 20% 42% 22% 6%	583
September	1st to 10th	Day	5% 5% 23% 40% 22% 4	530
	11th to 20th	Day	5% 8% 34% 36% 13% 4	530
	21st to end	Day	2 13% 29% 34% 16% 5%	338
October	1st to 10th	Day	6% 22% 23% 29% 14% 6%	330
	11th to 20th	Day	8% 17% 33% 27% 11% 4	330
	21st to end	Day	13% 13% 31% 29% 12% 2	363
November	1st to 10th	Day	8% 18% 35% 29% 8% 2	330
	11th to 20th	Day	8% 15% 34% 31% 12% 1	330
	21st to end	Day	9% 24% 35% 21% 11% 1	330
December	1st to 10th	Day	11% 18% 36% 25% 9% 1	330
	11th to 20th	Day	8% 19% 38% 28% 5% 1	330
	21st to end	Day	15% 25% 37% 19% 3 1	363
January	1st to 10th	Night	7% 11% 32% 37% 11% 2	958
	11th to 20th	Night	10% 10% 30% 36% 11% 2	840
	21st to end	Night	6% 14% 32% 35% 11% 2	924
February	1st to 10th	Night	8% 15% 36% 32% 8% 1	840
	11th to 20th	Night	11% 17% 38% 28% 7% 1	824
	21st to end	Night	13% 16% 41% 27% 3	692
March	1st to 10th	Night	8% 21% 36% 28% 6% 1	840
	11th to 20th	Night	10% 15% 31% 30% 13% 1	840
	21st to end	Night	11% 19% 34% 31% 5%	819
April	1st to 10th	Night	10% 17% 43% 26% 4	630
	11th to 20th	Night	11% 21% 35% 26% 6%	623
	21st to end	Night	11% 19% 37% 26% 7%	630
May	1st to 10th	Night	7% 16% 36% 32% 9%	630
	11th to 20th	Night	9% 13% 31% 32% 13% 1	621
	21st to end	Night	6% 13% 34% 35% 11%	683
June	1st to 10th	Night	2 6% 32% 41% 18%	630
	11th to 20th	Night	2 8% 34% 41% 14% 1	630
	21st to end	Night	3 5% 27% 41% 22% 2	630
July	1st to 10th	Night	4 2 13% 47% 31% 3	615
	11th to 20th	Night	2 1 9% 44% 38% 5%	612
	21st to end	Night	4 3 9% 34% 41% 9%	687
August	1st to 10th	Night	3 2 9% 38% 38% 10%	597
	11th to 20th	Night	4 3 12% 47% 30% 3	594
	21st to end	Night	3 3 11% 42% 35% 6%	660
September	1st to 10th	Night	5% 2 18% 44% 28% 3	600
	11th to 20th	Night	4 3 21% 45% 25% 3	600
	21st to end	Night	5% 8% 24% 42% 20% 1	618
October	1st to 10th	Night	5% 10% 26% 43% 15% 2	800
	11th to 20th	Night	6% 10% 29% 37% 16% 3	800
	21st to end	Night	7% 10% 28% 34% 17% 4	880
November	1st to 10th	Night	5% 11% 28% 39% 15% 2	909
	11th to 20th	Night	4 11% 29% 40% 13% 3	1050
	21st to end	Night	8% 15% 28% 31% 15% 2	1050
December	1st to 10th	Night	9% 12% 32% 36% 9% 1	1050
	11th to 20th	Night	10% 13% 31% 35% 9% 1	1050
	21st to end	Night	10% 15% 35% 32% 7% 1	1155

Page generated at 2026-07-28 01:25:24. Version ac07

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