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 2025-06-10.

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	11% 7% 3 7% 4 4 4 4 5% 7% 43%	119642
	Night	10% 7% 3 7% 4 4 4 3 5% 7% 46%	118796
Transparency	Day	38% 6% 13% 25% 16% 2	123308
	Night	37% 5% 12% 25% 18% 3	123261
Seeing	Day	15% 8% 31% 29% 14% 3	16583
	Night	14% 8% 33% 33% 11% 1	26530

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	8% 12% 5% 16% 7% 5% 5% 3 6% 7% 27%	4077
	11th to 20th	Day	7% 9% 4 10% 6% 5% 7% 4 5% 6% 37%	4080
	21st to end	Day	7% 11% 5% 14% 7% 5% 5% 3 5% 6% 33%	4488
February	1st to 10th	Day	10% 12% 4 11% 6% 6% 5% 4 5% 7% 31%	1800
	11th to 20th	Day	10% 11% 4 9% 5% 4 4 4 5% 8% 38%	1754
	21st to end	Day	11% 8% 4 9% 4 4 4 4 6% 6% 41%	1645
March	1st to 10th	Day	15% 9% 2 6% 3 3 3 2 3 4 50%	4930
	11th to 20th	Day	11% 7% 2 5% 3 3 4 3 4 8% 49%	4980
	21st to end	Day	19% 12% 4 9% 5% 4 4 3 5% 8% 27%	5462
April	1st to 10th	Day	16% 8% 3 7% 4 4 4 3 4 7% 42%	2285
	11th to 20th	Day	16% 8% 2 6% 4 4 3 4 5% 7% 40%	2449
	21st to end	Day	19% 8% 2 6% 4 4 4 4 5% 7% 36%	2600
May	1st to 10th	Day	13% 7% 3 7% 4 4 5% 3 5% 7% 43%	5862
	11th to 20th	Day	15% 6% 2 6% 3 3 3 3 4 6% 49%	5862
	21st to end	Day	16% 8% 3 7% 4 4 5% 4 6% 7% 36%	6789
June	1st to 10th	Day	9% 5% 2 5% 4 5% 4 4 7% 9% 46%	2787
	11th to 20th	Day	8% 6% 2 5% 5% 5% 4 3 7% 9% 47%	2660
	21st to end	Day	7% 5% 2 6% 4 4 4 4 6% 8% 49%	2660
July	1st to 10th	Day	12% 5% 2 5% 4 4 5% 4 7% 8% 43%	4294
	11th to 20th	Day	6% 4 1 5% 4 4 4 4 6% 7% 56%	4120
	21st to end	Day	4 3 1 5% 4 4 4 4 7% 11% 53%	4465
August	1st to 10th	Day	6% 4 2 5% 5% 5% 5% 5% 7% 9% 46%	4104
	11th to 20th	Day	13% 7% 3 6% 4 4 5% 4 6% 7% 41%	4041
	21st to end	Day	7% 5% 2 6% 5% 4 5% 4 6% 8% 48%	4412
September	1st to 10th	Day	12% 4 1 4 3 4 3 2 6% 6% 55%	2250
	11th to 20th	Day	9% 6% 2 5% 3 4 4 3 5% 7% 53%	2090
	21st to end	Day	10% 5% 2 4 3 4 4 3 4 6% 55%	2090
October	1st to 10th	Day	15% 5% 1 5% 3 2 3 3 5% 7% 52%	1919
	11th to 20th	Day	13% 5% 2 5% 3 3 3 2 4 5% 56%	1900
	21st to end	Day	13% 8% 3 5% 4 4 3 4 4 5% 47%	2090
November	1st to 10th	Day	11% 5% 3 4 4 2 4 3 4 7% 53%	1817
	11th to 20th	Day	10% 8% 2 8% 5% 5% 4 4 4 6% 44%	1680
	21st to end	Day	13% 10% 3 7% 4 3 4 3 6% 7% 42%	1600
December	1st to 10th	Day	5% 6% 2 6% 6% 5% 4 3 5% 6% 51%	2610
	11th to 20th	Day	10% 9% 4 10% 5% 4 5% 3 5% 5% 41%	3008
	21st to end	Day	18% 10% 5% 14% 5% 4 3 4 4 6% 27%	3982
January	1st to 10th	Night	10% 10% 5% 20% 9% 5% 4 3 5% 5% 24%	5888
	11th to 20th	Night	7% 10% 3 8% 6% 5% 7% 4 7% 6% 37%	5900
	21st to end	Night	9% 12% 4 11% 5% 5% 4 3 5% 5% 37%	6490
February	1st to 10th	Night	11% 11% 3 10% 5% 4 4 4 5% 7% 36%	2600
	11th to 20th	Night	10% 10% 3 8% 4 5% 4 3 5% 6% 42%	2533
	21st to end	Night	9% 8% 3 7% 5% 4 3 3 4 6% 49%	2000
March	1st to 10th	Night	15% 7% 2 5% 3 3 3 3 4 6% 51%	4924
	11th to 20th	Night	9% 6% 2 5% 3 3 3 2 4 6% 56%	4980
	21st to end	Night	14% 10% 3 8% 4 4 5% 4 5% 8% 34%	5467
April	1st to 10th	Night	12% 8% 3 5% 3 4 4 3 5% 7% 46%	2114
	11th to 20th	Night	11% 9% 2 6% 4 4 3 3 5% 6% 46%	1897
	21st to end	Night	17% 7% 3 6% 4 4 3 2 4 8% 42%	1800
May	1st to 10th	Night	14% 6% 2 5% 3 3 3 3 5% 6% 50%	4053
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 5% 7% 52%	4053
	21st to end	Night	16% 8% 2 6% 4 3 4 3 5% 6% 43%	4083
June	1st to 10th	Night	6% 3 2 5% 4 3 4 4 7% 9% 54%	1597
	11th to 20th	Night	7% 4 2 4 3 3 4 4 8% 10% 50%	1520
	21st to end	Night	4 4 1 5% 4 5% 4 3 6% 8% 55%	1520
July	1st to 10th	Night	7% 5% 2 5% 4 4 4 4 5% 9% 51%	2657
	11th to 20th	Night	5% 2 1 3 3 2 3 3 5% 7% 65%	2850
	21st to end	Night	4 2 2 4 3 3 4 4 7% 10% 57%	3098
August	1st to 10th	Night	4 3 1 4 3 4 4 4 6% 10% 58%	2836
	11th to 20th	Night	10% 6% 2 5% 4 4 4 3 5% 8% 50%	2798
	21st to end	Night	7% 4 2 4 3 3 4 4 5% 8% 56%	3250
September	1st to 10th	Night	9% 4 2 4 3 3 3 2 4 7% 59%	1927
	11th to 20th	Night	9% 4 1 4 3 2 3 3 5% 7% 57%	2090
	21st to end	Night	9% 5% 1 4 3 3 3 2 4 6% 60%	2090
October	1st to 10th	Night	12% 5% 2 5% 3 3 3 2 5% 7% 53%	2240
	11th to 20th	Night	13% 7% 1 4 3 3 3 3 4 5% 54%	2280
	21st to end	Night	13% 6% 2 5% 2 3 4 3 4 6% 50%	2508
November	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 5% 6% 55%	2478
	11th to 20th	Night	10% 6% 2 5% 4 3 4 3 4 7% 51%	2718
	21st to end	Night	11% 8% 3 6% 4 3 3 3 5% 6% 49%	2800
December	1st to 10th	Night	5% 7% 3 7% 5% 4 4 4 5% 6% 51%	4580
	11th to 20th	Night	9% 8% 3 7% 5% 4 4 4 5% 7% 43%	5199
	21st to end	Night	18% 13% 5% 11% 4 3 3 3 4 4 30%	6978

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	55% 6% 12% 19% 7% 1	4078
	11th to 20th	Day	47% 5% 11% 20% 15% 2	4080
	21st to end	Day	52% 5% 13% 21% 9%	4488
February	1st to 10th	Day	53% 6% 13% 17% 10% 1	1800
	11th to 20th	Day	46% 3 13% 23% 13% 2	1763
	21st to end	Day	42% 6% 13% 22% 13% 3	1645
March	1st to 10th	Day	39% 4 8% 27% 18% 5%	4930
	11th to 20th	Day	34% 5% 9% 25% 23% 3	4980
	21st to end	Day	54% 8% 19% 14% 4 2	5464
April	1st to 10th	Day	43% 5% 12% 27% 12% 1	2285
	11th to 20th	Day	43% 5% 14% 23% 12% 2	2449
	21st to end	Day	46% 7% 12% 22% 12% 1	2600
May	1st to 10th	Day	38% 8% 11% 26% 15% 1	5900
	11th to 20th	Day	37% 5% 9% 24% 22% 3	5866
	21st to end	Day	44% 7% 12% 21% 14% 1	6803
June	1st to 10th	Day	32% 8% 18% 27% 12% 3	2787
	11th to 20th	Day	32% 7% 14% 30% 16% 1	2660
	21st to end	Day	31% 8% 16% 28% 14% 3	2660
July	1st to 10th	Day	36% 7% 16% 28% 12% 2	4229
	11th to 20th	Day	26% 7% 12% 32% 21% 2	4051
	21st to end	Day	21% 10% 18% 31% 19% 2	4325
August	1st to 10th	Day	30% 8% 16% 28% 17% 1	4090
	11th to 20th	Day	40% 7% 12% 26% 15%	4080
	21st to end	Day	33% 7% 15% 26% 15% 4	4411
September	1st to 10th	Day	30% 4 13% 29% 19% 5%	2250
	11th to 20th	Day	31% 6% 11% 33% 18% 1	2090
	21st to end	Day	30% 4 10% 32% 23% 1	2090
October	1st to 10th	Day	20% 3 7% 22% 46% 2	3192
	11th to 20th	Day	21% 3 12% 31% 26% 7%	3140
	21st to end	Day	35% 4 9% 27% 21% 3	3428
November	1st to 10th	Day	32% 5% 15% 27% 19% 3	1817
	11th to 20th	Day	40% 5% 13% 24% 17% 1	1680
	21st to end	Day	41% 4 13% 28% 12% 1	1600
December	1st to 10th	Day	33% 5% 19% 29% 14% 1	2610
	11th to 20th	Day	45% 5% 16% 24% 10%	3005
	21st to end	Day	58% 5% 12% 18% 6%	3982
January	1st to 10th	Night	60% 6% 11% 14% 9% 1	5888
	11th to 20th	Night	44% 5% 13% 21% 15% 2	5900
	21st to end	Night	48% 5% 12% 18% 15% 2	6490
February	1st to 10th	Night	47% 6% 13% 21% 12% 2	2600
	11th to 20th	Night	42% 5% 12% 23% 15% 3	2534
	21st to end	Night	37% 4 9% 25% 21% 4	2000
March	1st to 10th	Night	35% 4 8% 29% 22% 3	4924
	11th to 20th	Night	30% 4 7% 30% 22% 7%	4980
	21st to end	Night	46% 8% 19% 20% 5% 1	5467
April	1st to 10th	Night	37% 6% 10% 27% 17% 3	2114
	11th to 20th	Night	38% 5% 11% 26% 17% 3	1897
	21st to end	Night	42% 6% 13% 23% 15% 2	1800
May	1st to 10th	Night	35% 6% 10% 26% 22% 1	4080
	11th to 20th	Night	32% 6% 11% 22% 24% 5%	4054
	21st to end	Night	42% 6% 13% 21% 16% 2	4091
June	1st to 10th	Night	26% 7% 15% 32% 16% 4	1597
	11th to 20th	Night	26% 7% 16% 26% 20% 4	1520
	21st to end	Night	26% 5% 16% 31% 19% 3	1520
July	1st to 10th	Night	30% 6% 16% 30% 16% 2	2614
	11th to 20th	Night	18% 6% 16% 34% 24% 3	2796
	21st to end	Night	19% 9% 17% 34% 20% 1	2999
August	1st to 10th	Night	21% 6% 15% 35% 20% 2	2826
	11th to 20th	Night	33% 6% 14% 26% 19% 2	2825
	21st to end	Night	26% 6% 17% 27% 17% 8%	3250
September	1st to 10th	Night	26% 5% 12% 28% 23% 6%	1927
	11th to 20th	Night	27% 5% 12% 35% 19% 3	2090
	21st to end	Night	27% 4 8% 30% 27% 3	2090
October	1st to 10th	Night	20% 3 7% 18% 49% 3	3734
	11th to 20th	Night	24% 3 7% 28% 29% 8%	3779
	21st to end	Night	35% 4 10% 28% 22% 1	4120
November	1st to 10th	Night	29% 5% 16% 24% 22% 4	2478
	11th to 20th	Night	34% 4 13% 29% 17% 3	2718
	21st to end	Night	37% 4 12% 29% 16% 2	2800
December	1st to 10th	Night	34% 4 13% 36% 13% 1	4580
	11th to 20th	Night	40% 6% 16% 28% 11%	5201
	21st to end	Night	57% 5% 11% 20% 6% 1	6978

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% 24% 7% 2	300
	11th to 20th	Day	19% 15% 37% 22% 5% 1	300
	21st to end	Day	15% 10% 44% 25% 5% 1	330
February	1st to 10th	Day	21% 14% 43% 19% 3	300
	11th to 20th	Day	19% 17% 38% 21% 5% 1	300
	21st to end	Day	16% 15% 43% 22% 5%	249
March	1st to 10th	Day	20% 12% 46% 18% 4 1	300
	11th to 20th	Day	26% 9% 37% 21% 7% 1	300
	21st to end	Day	34% 12% 38% 13% 3 1	330
April	1st to 10th	Day	24% 15% 37% 19% 4 1	385
	11th to 20th	Day	22% 15% 35% 22% 6%	567
	21st to end	Day	23% 13% 35% 20% 8% 1	700
May	1st to 10th	Day	21% 10% 31% 27% 10% 2	703
	11th to 20th	Day	20% 8% 29% 27% 13% 3	725
	21st to end	Day	19% 7% 28% 29% 15% 2	789
June	1st to 10th	Day	8% 5% 22% 36% 23% 6%	726
	11th to 20th	Day	8% 5% 30% 36% 17% 3	680
	21st to end	Day	9% 6% 32% 34% 16% 3	680
July	1st to 10th	Day	11% 2 17% 32% 29% 9%	671
	11th to 20th	Day	6% 1 25% 37% 24% 7%	668
	21st to end	Day	5% 1 18% 43% 27% 5%	739
August	1st to 10th	Day	8% 3 20% 40% 24% 5%	676
	11th to 20th	Day	11% 2 19% 37% 27% 3	665
	21st to end	Day	10% 3 17% 42% 21% 5%	680
September	1st to 10th	Day	13% 2 27% 35% 19% 4	460
	11th to 20th	Day	8% 4 32% 32% 18% 6%	300
	21st to end	Day	14% 7% 33% 28% 16% 1	300
October	1st to 10th	Day	18% 6% 29% 26% 16% 5%	300
	11th to 20th	Day	13% 15% 41% 23% 8% 1	300
	21st to end	Day	20% 14% 38% 17% 9% 2	330
November	1st to 10th	Day	18% 12% 37% 25% 6% 2	300
	11th to 20th	Day	16% 16% 40% 21% 6% 1	300
	21st to end	Day	21% 12% 38% 21% 7% 1	300
December	1st to 10th	Day	10% 14% 38% 30% 7% 1	300
	11th to 20th	Day	15% 15% 37% 23% 8% 1	300
	21st to end	Day	22% 12% 43% 17% 3 1	330
January	1st to 10th	Night	13% 12% 37% 30% 8% 1	1000
	11th to 20th	Night	15% 13% 37% 29% 6%	1000
	21st to end	Night	17% 9% 38% 31% 6%	1100
February	1st to 10th	Night	17% 12% 38% 29% 4	1000
	11th to 20th	Night	15% 12% 40% 28% 4	979
	21st to end	Night	14% 13% 43% 25% 5% 1	680
March	1st to 10th	Night	17% 13% 36% 28% 6%	605
	11th to 20th	Night	20% 8% 38% 28% 7%	600
	21st to end	Night	22% 11% 40% 22% 5%	660
April	1st to 10th	Night	17% 13% 42% 23% 5%	600
	11th to 20th	Night	17% 13% 38% 27% 4	593
	21st to end	Night	20% 9% 38% 26% 8% 1	600
May	1st to 10th	Night	18% 8% 30% 36% 9% 1	600
	11th to 20th	Night	17% 7% 30% 33% 12% 1	591
	21st to end	Night	18% 4 29% 35% 14%	650
June	1st to 10th	Night	8% 3 25% 45% 19% 1	599
	11th to 20th	Night	8% 3 31% 44% 14% 1	570
	21st to end	Night	6% 3 32% 43% 15%	570
July	1st to 10th	Night	7% 1 17% 45% 28% 1	555
	11th to 20th	Night	4 1 20% 51% 23% 2	552
	21st to end	Night	5% 1 15% 52% 25% 1	624
August	1st to 10th	Night	6% 1 20% 47% 23% 3	567
	11th to 20th	Night	8% 3 15% 48% 25% 1	564
	21st to end	Night	9% 2 21% 43% 22% 3	627
September	1st to 10th	Night	12% 2 25% 39% 21% 2	570
	11th to 20th	Night	11% 2 29% 39% 17% 2	570
	21st to end	Night	13% 4 31% 36% 15% 1	570
October	1st to 10th	Night	15% 6% 32% 32% 12% 2	720
	11th to 20th	Night	17% 8% 36% 30% 9%	760
	21st to end	Night	19% 9% 33% 30% 8%	836
November	1st to 10th	Night	14% 10% 33% 35% 8%	918
	11th to 20th	Night	14% 11% 41% 28% 6%	1000
	21st to end	Night	17% 10% 36% 28% 9%	1000
December	1st to 10th	Night	15% 11% 37% 32% 6%	1000
	11th to 20th	Night	17% 8% 37% 31% 7%	1000
	21st to end	Night	21% 13% 34% 28% 5%	1100

Page generated at 2025-06-07 09:28:16. Version ac07

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