Treynor
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Treynor clear sky chart from 2005-11-06 to 2026-10-14.

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% 7% 3 7% 4 4 4 4 5% 7% 43%	149281
	Night	10% 7% 3 7% 4 4 4 3 5% 7% 46%	142830
Transparency	Day	38% 6% 13% 25% 15% 2	152905
	Night	36% 5% 13% 26% 17% 3	147274
Seeing	Day	15% 9% 31% 29% 14% 3	18513
	Night	14% 8% 33% 33% 10% 1	28272

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	9% 12% 5% 14% 6% 5% 5% 3 5% 6% 30%	5289
	11th to 20th	Day	6% 8% 4 11% 7% 6% 7% 4 6% 7% 36%	5310
	21st to end	Day	6% 11% 5% 13% 6% 6% 5% 3 5% 6% 35%	5812
February	1st to 10th	Day	9% 12% 4 11% 6% 6% 5% 4 5% 7% 31%	1890
	11th to 20th	Day	10% 11% 4 9% 5% 4 4 4 5% 7% 38%	1844
	21st to end	Day	10% 7% 3 9% 4 4 4 4 6% 6% 42%	1725
March	1st to 10th	Day	15% 9% 2 6% 3 3 3 2 3 4 50%	5040
	11th to 20th	Day	11% 7% 2 5% 3 3 4 3 4 8% 48%	5090
	21st to end	Day	18% 11% 4 9% 5% 4 4 4 5% 8% 27%	5583
April	1st to 10th	Day	17% 8% 3 6% 4 4 4 3 4 6% 41%	2399
	11th to 20th	Day	16% 8% 2 6% 4 4 4 4 5% 6% 40%	2573
	21st to end	Day	19% 8% 2 6% 4 4 4 4 5% 7% 36%	2730
May	1st to 10th	Day	12% 7% 3 6% 4 4 5% 3 5% 7% 44%	7617
	11th to 20th	Day	13% 7% 2 6% 3 4 4 3 4 6% 46%	7627
	21st to end	Day	17% 8% 3 7% 4 4 4 3 5% 7% 36%	8859
June	1st to 10th	Day	9% 5% 2 5% 4 5% 4 4 7% 9% 46%	2940
	11th to 20th	Day	8% 6% 2 5% 5% 5% 4 3 7% 9% 48%	2940
	21st to end	Day	8% 5% 2 6% 5% 4 4 4 6% 8% 48%	2940
July	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 7% 10% 44%	8022
	11th to 20th	Day	5% 4 2 5% 3 4 3 4 5% 7% 57%	7680
	21st to end	Day	8% 5% 2 6% 5% 4 5% 4 7% 10% 44%	8337
August	1st to 10th	Day	7% 4 2 5% 4 5% 5% 4 7% 9% 49%	6014
	11th to 20th	Day	11% 7% 3 7% 5% 4 5% 5% 6% 8% 38%	5948
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 4 6% 9% 46%	6413
September	1st to 10th	Day	12% 4 1 4 3 4 3 3 6% 6% 54%	2486
	11th to 20th	Day	10% 6% 2 5% 3 4 4 3 5% 8% 50%	2310
	21st to end	Day	12% 5% 2 4 3 3 3 3 4 6% 53%	2310
October	1st to 10th	Day	14% 4 1 4 2 2 3 3 5% 7% 54%	2121
	11th to 20th	Day	12% 5% 2 5% 3 3 3 2 4 5% 55%	2030
	21st to end	Day	13% 8% 3 5% 4 4 4 4 4 5% 46%	2200
November	1st to 10th	Day	10% 6% 3 5% 4 3 4 3 4 6% 53%	1910
	11th to 20th	Day	10% 8% 2 8% 5% 5% 4 3 4 6% 45%	1764
	21st to end	Day	14% 9% 3 6% 4 3 4 3 5% 7% 42%	1680
December	1st to 10th	Day	5% 6% 2 7% 6% 5% 4 3 5% 6% 51%	2690
	11th to 20th	Day	10% 9% 4 10% 5% 4 5% 3 5% 5% 41%	3088
	21st to end	Day	18% 10% 5% 14% 5% 4 3 4 4 6% 27%	4070
January	1st to 10th	Night	10% 10% 4 17% 8% 5% 4 3 5% 5% 29%	7634
	11th to 20th	Night	5% 8% 3 8% 6% 5% 7% 4 7% 7% 41%	7680
	21st to end	Night	8% 12% 4 11% 5% 5% 4 3 4 6% 39%	8402
February	1st to 10th	Night	10% 11% 3 10% 5% 4 4 4 5% 7% 37%	2730
	11th to 20th	Night	10% 10% 3 8% 4 5% 4 3 5% 6% 42%	2663
	21st to end	Night	9% 7% 3 7% 5% 4 3 3 4 6% 49%	2097
March	1st to 10th	Night	15% 7% 2 5% 3 3 3 3 4 6% 50%	5034
	11th to 20th	Night	9% 6% 2 5% 3 3 3 2 4 6% 56%	5090
	21st to end	Night	14% 10% 3 8% 4 4 5% 4 5% 8% 35%	5588
April	1st to 10th	Night	13% 8% 3 6% 3 4 4 3 5% 7% 45%	2220
	11th to 20th	Night	11% 9% 2 6% 4 4 3 3 5% 6% 47%	1993
	21st to end	Night	16% 8% 3 6% 3 4 3 2 4 7% 42%	1890
May	1st to 10th	Night	12% 6% 2 5% 4 4 4 3 5% 6% 50%	5266
	11th to 20th	Night	13% 6% 2 5% 4 3 4 3 5% 7% 50%	5265
	21st to end	Night	17% 8% 3 6% 4 3 3 3 5% 6% 41%	5321
June	1st to 10th	Night	7% 4 2 5% 4 4 4 4 7% 9% 52%	1680
	11th to 20th	Night	6% 4 2 4 3 3 4 4 7% 10% 51%	1680
	21st to end	Night	5% 4 1 5% 3 5% 4 3 6% 8% 54%	1680
July	1st to 10th	Night	6% 5% 2 6% 4 4 5% 4 6% 9% 49%	4947
	11th to 20th	Night	5% 3 1 4 3 3 4 3 6% 7% 61%	5307
	21st to end	Night	7% 5% 2 5% 4 4 5% 5% 7% 10% 45%	5770
August	1st to 10th	Night	5% 3 1 5% 3 4 4 4 6% 8% 57%	4156
	11th to 20th	Night	9% 6% 2 6% 4 4 5% 4 6% 9% 45%	4118
	21st to end	Night	6% 4 2 4 3 3 4 4 6% 9% 55%	4726
September	1st to 10th	Night	9% 4 2 4 3 3 3 2 5% 8% 58%	2130
	11th to 20th	Night	10% 5% 1 4 3 3 4 3 5% 7% 55%	2310
	21st to end	Night	11% 5% 1 4 3 3 3 2 4 6% 57%	2310
October	1st to 10th	Night	12% 5% 1 4 2 3 3 2 4 7% 54%	2476
	11th to 20th	Night	12% 7% 2 4 3 3 3 3 4 5% 54%	2437
	21st to end	Night	14% 7% 2 5% 3 3 4 3 4 5% 50%	2640
November	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 5% 7% 55%	2605
	11th to 20th	Night	10% 6% 2 6% 4 3 4 3 4 7% 51%	2854
	21st to end	Night	11% 8% 4 6% 4 3 3 3 5% 6% 48%	2940
December	1st to 10th	Night	5% 7% 3 7% 5% 4 4 4 5% 6% 50%	4720
	11th to 20th	Night	9% 8% 3 8% 5% 4 4 4 5% 7% 43%	5339
	21st to end	Night	17% 13% 5% 11% 4 4 3 3 4 5% 31%	7132

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	53% 6% 13% 18% 10% 1	5290
	11th to 20th	Day	46% 5% 11% 21% 15% 3	5310
	21st to end	Day	50% 4 14% 22% 9%	5812
February	1st to 10th	Day	52% 6% 14% 18% 10% 1	1890
	11th to 20th	Day	46% 3 12% 24% 12% 3	1853
	21st to end	Day	41% 6% 13% 23% 14% 3	1725
March	1st to 10th	Day	40% 4 8% 26% 18% 5%	5040
	11th to 20th	Day	34% 5% 10% 25% 23% 3	5090
	21st to end	Day	54% 8% 19% 14% 4 2	5585
April	1st to 10th	Day	44% 5% 12% 26% 11% 1	2399
	11th to 20th	Day	43% 5% 14% 23% 12% 2	2573
	21st to end	Day	46% 7% 12% 22% 12% 1	2730
May	1st to 10th	Day	37% 7% 10% 28% 16% 1	7655
	11th to 20th	Day	38% 6% 9% 24% 21% 3	7631
	21st to end	Day	45% 7% 12% 22% 14% 1	8873
June	1st to 10th	Day	33% 8% 17% 27% 12% 3	2940
	11th to 20th	Day	31% 7% 14% 29% 17% 1	2940
	21st to end	Day	33% 8% 15% 27% 14% 3	2940
July	1st to 10th	Day	34% 7% 17% 29% 12% 1	7957
	11th to 20th	Day	25% 6% 14% 30% 21% 4	7611
	21st to end	Day	32% 9% 19% 29% 11% 1	8155
August	1st to 10th	Day	30% 7% 15% 30% 18% 1	6000
	11th to 20th	Day	41% 8% 15% 25% 10%	5987
	21st to end	Day	34% 8% 15% 27% 13% 3	6412
September	1st to 10th	Day	30% 5% 14% 28% 18% 4	2486
	11th to 20th	Day	33% 6% 12% 32% 17% 1	2310
	21st to end	Day	32% 4 10% 30% 23% 1	2310
October	1st to 10th	Day	19% 3 7% 23% 45% 2	3394
	11th to 20th	Day	21% 3 12% 31% 26% 7%	3270
	21st to end	Day	36% 4 9% 27% 21% 3	3538
November	1st to 10th	Day	32% 4 14% 27% 19% 2	1910
	11th to 20th	Day	39% 5% 14% 25% 17% 1	1764
	21st to end	Day	42% 4 13% 28% 12% 1	1680
December	1st to 10th	Day	34% 5% 18% 29% 14% 1	2690
	11th to 20th	Day	45% 5% 16% 24% 10%	3085
	21st to end	Day	58% 5% 12% 19% 6%	4070
January	1st to 10th	Night	55% 6% 13% 15% 10% 1	7635
	11th to 20th	Night	39% 5% 13% 24% 17% 2	7680
	21st to end	Night	47% 5% 14% 18% 15% 2	8402
February	1st to 10th	Night	46% 6% 13% 22% 11% 2	2730
	11th to 20th	Night	42% 5% 12% 23% 15% 3	2664
	21st to end	Night	37% 4 10% 25% 21% 4	2097
March	1st to 10th	Night	35% 4 8% 29% 22% 3	5034
	11th to 20th	Night	30% 4 7% 30% 22% 7%	5090
	21st to end	Night	45% 8% 19% 21% 6% 1	5588
April	1st to 10th	Night	38% 6% 11% 27% 17% 2	2220
	11th to 20th	Night	37% 5% 11% 26% 17% 3	1993
	21st to end	Night	42% 5% 13% 23% 15% 2	1890
May	1st to 10th	Night	34% 5% 9% 25% 24% 3	5293
	11th to 20th	Night	34% 6% 10% 22% 23% 5%	5266
	21st to end	Night	44% 6% 12% 22% 15% 2	5332
June	1st to 10th	Night	27% 7% 16% 32% 15% 3	1680
	11th to 20th	Night	26% 7% 15% 26% 22% 4	1680
	21st to end	Night	27% 5% 16% 29% 19% 3	1680
July	1st to 10th	Night	30% 6% 15% 31% 15% 2	4906
	11th to 20th	Night	21% 6% 15% 31% 22% 4	5256
	21st to end	Night	30% 10% 19% 28% 13% 1	5641
August	1st to 10th	Night	23% 6% 14% 34% 20% 2	4146
	11th to 20th	Night	35% 8% 19% 24% 13% 1	4145
	21st to end	Night	25% 6% 17% 32% 14% 6%	4726
September	1st to 10th	Night	26% 5% 13% 28% 23% 6%	2130
	11th to 20th	Night	29% 5% 13% 33% 18% 2	2310
	21st to end	Night	30% 4 9% 28% 26% 3	2310
October	1st to 10th	Night	20% 3 7% 19% 48% 3	3970
	11th to 20th	Night	24% 3 8% 28% 29% 8%	3936
	21st to end	Night	35% 4 10% 28% 22% 1	4252
November	1st to 10th	Night	29% 5% 16% 25% 22% 4	2605
	11th to 20th	Night	34% 4 13% 30% 17% 2	2854
	21st to end	Night	38% 4 12% 28% 16% 2	2940
December	1st to 10th	Night	34% 5% 12% 35% 13% 1	4720
	11th to 20th	Night	40% 6% 15% 28% 11%	5341
	21st to end	Night	57% 5% 11% 21% 7% 1	7132

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% 11% 36% 25% 6% 2	330
	11th to 20th	Day	18% 16% 38% 21% 5% 1	330
	21st to end	Day	14% 11% 44% 25% 6% 1	363
February	1st to 10th	Day	20% 15% 44% 18% 3	330
	11th to 20th	Day	19% 17% 38% 20% 5% 1	330
	21st to end	Day	14% 16% 43% 21% 5%	273
March	1st to 10th	Day	20% 12% 45% 18% 3 1	330
	11th to 20th	Day	27% 10% 35% 21% 7% 1	330
	21st to end	Day	32% 13% 39% 12% 3 1	363
April	1st to 10th	Day	25% 16% 37% 18% 4	419
	11th to 20th	Day	22% 16% 35% 22% 6%	611
	21st to end	Day	23% 13% 36% 20% 7% 1	750
May	1st to 10th	Day	20% 10% 33% 26% 9% 1	753
	11th to 20th	Day	20% 9% 29% 27% 12% 3	775
	21st to end	Day	19% 7% 27% 29% 16% 2	844
June	1st to 10th	Day	8% 5% 22% 36% 23% 6%	780
	11th to 20th	Day	8% 6% 30% 35% 18% 3	780
	21st to end	Day	10% 7% 32% 34% 15% 3	780
July	1st to 10th	Day	11% 2 18% 32% 28% 10%	771
	11th to 20th	Day	6% 2 24% 36% 25% 8%	768
	21st to end	Day	6% 2 19% 42% 26% 5%	849
August	1st to 10th	Day	8% 3 22% 39% 23% 5%	776
	11th to 20th	Day	11% 2 20% 37% 26% 4	765
	21st to end	Day	10% 4 18% 43% 20% 5%	782
September	1st to 10th	Day	12% 3 27% 36% 18% 4	536
	11th to 20th	Day	11% 5% 32% 31% 16% 6%	360
	21st to end	Day	17% 7% 30% 28% 15% 3	360
October	1st to 10th	Day	17% 5% 28% 29% 15% 5%	360
	11th to 20th	Day	12% 15% 42% 23% 7% 1	339
	21st to end	Day	20% 14% 39% 17% 9% 1	363
November	1st to 10th	Day	17% 12% 40% 25% 5% 2	330
	11th to 20th	Day	15% 15% 41% 22% 5% 1	330
	21st to end	Day	23% 13% 37% 20% 7% 1	330
December	1st to 10th	Day	10% 14% 38% 28% 7% 2	330
	11th to 20th	Day	16% 15% 38% 22% 8% 1	330
	21st to end	Day	21% 12% 44% 18% 3 1	363
January	1st to 10th	Night	13% 11% 37% 30% 8% 1	1050
	11th to 20th	Night	14% 14% 37% 29% 6%	1050
	21st to end	Night	16% 9% 38% 30% 6%	1155
February	1st to 10th	Night	16% 12% 39% 29% 4	1050
	11th to 20th	Night	15% 12% 40% 28% 4	1029
	21st to end	Night	14% 13% 43% 25% 5% 1	713
March	1st to 10th	Night	17% 13% 36% 27% 6%	635
	11th to 20th	Night	19% 8% 39% 27% 6%	630
	21st to end	Night	21% 11% 41% 22% 5%	693
April	1st to 10th	Night	18% 13% 41% 23% 5%	630
	11th to 20th	Night	16% 13% 39% 27% 4	623
	21st to end	Night	20% 9% 38% 26% 7% 1	630
May	1st to 10th	Night	17% 8% 31% 36% 8%	630
	11th to 20th	Night	17% 8% 30% 33% 12%	621
	21st to end	Night	18% 4 29% 35% 13%	683
June	1st to 10th	Night	8% 4 25% 45% 18%	630
	11th to 20th	Night	7% 4 31% 44% 13%	630
	21st to end	Night	8% 3 32% 43% 15%	630
July	1st to 10th	Night	7% 1 17% 46% 28% 1	615
	11th to 20th	Night	4 1 18% 51% 25% 2	612
	21st to end	Night	6% 1 16% 52% 24% 1	690
August	1st to 10th	Night	7% 1 21% 47% 22% 3	627
	11th to 20th	Night	8% 3 14% 49% 24% 1	624
	21st to end	Night	8% 2 21% 45% 22% 2	693
September	1st to 10th	Night	12% 2 25% 40% 19% 2	630
	11th to 20th	Night	11% 3 28% 40% 16% 2	630
	21st to end	Night	14% 4 30% 36% 16% 1	630
October	1st to 10th	Night	15% 6% 32% 33% 12% 2	796
	11th to 20th	Night	16% 9% 38% 29% 8%	813
	21st to end	Night	20% 10% 33% 29% 8%	880
November	1st to 10th	Night	13% 10% 34% 34% 8%	965
	11th to 20th	Night	13% 11% 42% 28% 5%	1050
	21st to end	Night	18% 10% 36% 27% 9%	1050
December	1st to 10th	Night	14% 10% 37% 32% 5%	1050
	11th to 20th	Night	17% 9% 37% 30% 7%	1050
	21st to end	Night	20% 13% 35% 28% 5%	1155

Page generated at 2026-10-11 04:06:30. Version ac07

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