Poetry Ranch Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Poetry Ranch Observatory clear sky chart from 2005-11-06 to 2026-08-15.

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% 5% 2 5% 4 4 4 3 5% 7% 50%	143617
	Night	11% 5% 2 5% 3 3 3 3 5% 7% 54%	138629
Transparency	Day	33% 5% 10% 24% 21% 7%	147295
	Night	30% 4 9% 22% 24% 10%	143018
Seeing	Day	12% 7% 23% 28% 23% 7%	17590
	Night	13% 7% 28% 34% 16% 2	27351

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% 5% 2 6% 3 3 3 3 5% 7% 51%	5289
	11th to 20th	Day	7% 4 2 3 3 3 3 3 4 6% 62%	5310
	21st to end	Day	23% 8% 3 6% 4 3 2 2 3 4 42%	5812
February	1st to 10th	Day	19% 7% 2 7% 3 3 3 3 3 5% 45%	1969
	11th to 20th	Day	18% 8% 3 5% 3 4 3 2 4 6% 44%	2049
	21st to end	Day	19% 7% 2 4 3 2 2 3 3 4 51%	1797
March	1st to 10th	Day	16% 7% 2 5% 3 3 3 3 5% 7% 48%	5046
	11th to 20th	Day	13% 5% 2 5% 3 3 3 3 4 6% 53%	5090
	21st to end	Day	13% 8% 2 5% 4 3 4 3 5% 6% 46%	5583
April	1st to 10th	Day	15% 6% 2 6% 4 3 4 3 6% 6% 45%	2310
	11th to 20th	Day	14% 7% 2 6% 4 4 4 3 5% 7% 44%	2431
	21st to end	Day	14% 5% 2 6% 5% 4 4 4 6% 7% 43%	2607
May	1st to 10th	Day	19% 7% 3 7% 4 4 4 3 4 6% 37%	7617
	11th to 20th	Day	13% 6% 3 8% 5% 4 5% 3 5% 5% 43%	7627
	21st to end	Day	14% 7% 3 7% 5% 5% 5% 4 7% 9% 34%	8392
June	1st to 10th	Day	6% 4 2 6% 5% 5% 6% 5% 7% 8% 47%	2730
	11th to 20th	Day	6% 4 2 5% 4 5% 5% 4 6% 9% 49%	2730
	21st to end	Day	4 4 1 5% 4 3 4 4 6% 9% 55%	2730
July	1st to 10th	Day	5% 4 2 6% 5% 6% 6% 5% 7% 10% 44%	7680
	11th to 20th	Day	3 3 1 4 3 4 5% 4 7% 10% 56%	7680
	21st to end	Day	3 4 1 4 3 3 4 4 6% 9% 59%	8337
August	1st to 10th	Day	1 1 1 2 2 3 4 3 5% 8% 71%	4364
	11th to 20th	Day	4 2 1 3 3 2 3 4 7% 10% 60%	4224
	21st to end	Day	4 3 1 4 4 4 5% 5% 8% 11% 52%	4237
September	1st to 10th	Day	8% 5% 2 5% 4 4 4 4 5% 7% 52%	2200
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 6% 7% 48%	2200
	21st to end	Day	7% 4 2 4 2 3 4 3 5% 8% 59%	2200
October	1st to 10th	Day	5% 4 2 5% 3 3 3 2 4 5% 64%	2114
	11th to 20th	Day	12% 4 1 3 3 2 3 3 3 5% 61%	2000
	21st to end	Day	14% 5% 1 4 3 2 3 2 3 4 58%	2200
November	1st to 10th	Day	12% 5% 2 5% 3 3 3 2 4 5% 57%	2058
	11th to 20th	Day	8% 4 1 5% 4 4 3 2 4 6% 59%	2016
	21st to end	Day	15% 7% 2 4 3 3 3 2 4 5% 52%	1890
December	1st to 10th	Day	17% 5% 2 5% 3 3 3 2 4 6% 50%	3030
	11th to 20th	Day	18% 8% 2 6% 4 3 3 3 4 4 45%	3481
	21st to end	Day	17% 8% 3 6% 3 3 3 2 3 4 47%	4587
January	1st to 10th	Night	12% 5% 2 5% 3 3 3 2 4 5% 56%	7634
	11th to 20th	Night	7% 3 1 3 2 2 2 2 4 5% 71%	7680
	21st to end	Night	24% 7% 2 6% 3 3 3 2 4 5% 42%	8402
February	1st to 10th	Night	15% 7% 2 6% 3 3 3 3 4 5% 51%	2667
	11th to 20th	Night	17% 7% 2 5% 3 3 3 2 3 6% 48%	2458
	21st to end	Night	18% 5% 2 4 3 3 2 3 3 5% 52%	2008
March	1st to 10th	Night	17% 7% 2 6% 4 3 3 3 4 6% 45%	5029
	11th to 20th	Night	10% 5% 2 6% 4 3 4 3 4 6% 54%	5090
	21st to end	Night	13% 6% 2 5% 4 4 3 3 4 6% 51%	5588
April	1st to 10th	Night	11% 4 2 5% 3 4 4 3 6% 8% 50%	2310
	11th to 20th	Night	13% 6% 2 6% 5% 3 4 3 4 6% 48%	2138
	21st to end	Night	12% 6% 2 4 4 3 3 4 5% 6% 52%	2008
May	1st to 10th	Night	17% 9% 3 7% 4 4 4 3 5% 7% 37%	5266
	11th to 20th	Night	11% 7% 2 6% 4 4 5% 3 4 6% 46%	5265
	21st to end	Night	10% 7% 2 7% 4 4 5% 4 6% 9% 41%	5793
June	1st to 10th	Night	6% 3 2 4 4 3 4 4 7% 9% 55%	1890
	11th to 20th	Night	4 4 1 4 3 3 3 3 5% 8% 62%	1890
	21st to end	Night	3 3 1 4 3 3 4 3 7% 11% 58%	1890
July	1st to 10th	Night	4 4 2 5% 4 5% 5% 4 7% 12% 48%	5308
	11th to 20th	Night	3 3 1 4 3 3 3 3 6% 10% 62%	5307
	21st to end	Night	2 2 1 3 3 3 4 3 5% 8% 66%	5770
August	1st to 10th	Night	1 1 1 2 1 2 2 4 9% 76%	3016
	11th to 20th	Night	3 3 1 2 2 3 2 2 5% 10% 67%	2924
	21st to end	Night	3 2 1 3 3 3 4 3 6% 9% 62%	3643
September	1st to 10th	Night	7% 6% 1 5% 3 3 3 2 5% 8% 56%	2200
	11th to 20th	Night	10% 4 2 5% 4 3 4 3 5% 6% 56%	2200
	21st to end	Night	6% 3 1 4 2 3 4 3 5% 8% 62%	2200
October	1st to 10th	Night	5% 2 1 3 3 2 3 3 5% 6% 67%	2265
	11th to 20th	Night	12% 3 1 3 2 2 3 2 4 5% 64%	2400
	21st to end	Night	15% 4 2 4 2 2 2 1 3 4 61%	2640
November	1st to 10th	Night	10% 5% 2 5% 3 2 2 2 3 5% 61%	2460
	11th to 20th	Night	7% 5% 2 4 4 2 3 3 4 6% 60%	2601
	21st to end	Night	14% 6% 2 4 3 3 3 3 4 5% 53%	2730
December	1st to 10th	Night	16% 6% 2 5% 2 3 2 2 3 5% 53%	4380
	11th to 20th	Night	16% 7% 3 6% 3 3 2 2 3 4 51%	4946
	21st to end	Night	15% 7% 2 6% 3 2 3 2 3 4 53%	6633

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	34% 3 7% 26% 26% 3	5290
	11th to 20th	Day	24% 3 6% 26% 34% 7%	5310
	21st to end	Day	50% 2 3 16% 20% 10%	5812
February	1st to 10th	Day	45% 2 6% 17% 21% 9%	1969
	11th to 20th	Day	44% 2 7% 20% 19% 7%	2059
	21st to end	Day	39% 2 5% 15% 26% 12%	1797
March	1st to 10th	Day	37% 3 6% 16% 22% 15%	5046
	11th to 20th	Day	33% 4 6% 18% 18% 21%	5090
	21st to end	Day	39% 4 10% 22% 20% 5%	5585
April	1st to 10th	Day	39% 5% 9% 19% 18% 9%	2310
	11th to 20th	Day	41% 5% 8% 18% 21% 8%	2431
	21st to end	Day	40% 6% 9% 20% 21% 4	2607
May	1st to 10th	Day	48% 4 9% 21% 14% 3	7655
	11th to 20th	Day	43% 5% 7% 19% 23% 3	7631
	21st to end	Day	45% 8% 13% 18% 13% 3	8405
June	1st to 10th	Day	31% 8% 14% 28% 16% 2	2730
	11th to 20th	Day	30% 7% 14% 27% 18% 4	2730
	21st to end	Day	25% 7% 12% 31% 22% 3	2730
July	1st to 10th	Day	33% 8% 17% 30% 12%	7616
	11th to 20th	Day	22% 7% 17% 34% 18% 1	7611
	21st to end	Day	22% 5% 12% 39% 21% 2	8155
August	1st to 10th	Day	12% 5% 12% 33% 34% 5%	4350
	11th to 20th	Day	18% 5% 17% 41% 17% 2	4263
	21st to end	Day	25% 8% 18% 33% 13% 2	4236
September	1st to 10th	Day	31% 7% 9% 23% 20% 11%	2200
	11th to 20th	Day	34% 8% 11% 20% 22% 7%	2200
	21st to end	Day	25% 4 9% 23% 27% 11%	2200
October	1st to 10th	Day	14% 3 6% 14% 33% 31%	3439
	11th to 20th	Day	18% 2 4 23% 32% 22%	3240
	21st to end	Day	24% 3 4 30% 29% 9%	3538
November	1st to 10th	Day	32% 4 6% 23% 26% 9%	2058
	11th to 20th	Day	28% 3 8% 20% 28% 12%	2016
	21st to end	Day	37% 2 6% 22% 24% 9%	1890
December	1st to 10th	Day	37% 3 6% 26% 23% 5%	3030
	11th to 20th	Day	44% 3 4 13% 26% 10%	3479
	21st to end	Day	42% 3 5% 23% 21% 7%	4587
January	1st to 10th	Night	32% 3 7% 23% 30% 6%	7635
	11th to 20th	Night	19% 2 6% 19% 36% 17%	7680
	21st to end	Night	47% 2 4 15% 20% 11%	8402
February	1st to 10th	Night	37% 4 6% 18% 23% 12%	2667
	11th to 20th	Night	40% 3 7% 17% 24% 9%	2458
	21st to end	Night	36% 4 7% 15% 22% 15%	2008
March	1st to 10th	Night	43% 3 5% 11% 21% 17%	5029
	11th to 20th	Night	32% 4 7% 15% 19% 22%	5090
	21st to end	Night	36% 4 7% 19% 27% 6%	5588
April	1st to 10th	Night	32% 5% 11% 21% 18% 13%	2310
	11th to 20th	Night	36% 5% 9% 19% 19% 12%	2138
	21st to end	Night	32% 5% 11% 21% 26% 5%	2008
May	1st to 10th	Night	47% 6% 10% 20% 14% 4	5293
	11th to 20th	Night	39% 6% 11% 17% 24% 3	5266
	21st to end	Night	38% 7% 17% 23% 9% 7%	5805
June	1st to 10th	Night	24% 8% 15% 29% 20% 4	1890
	11th to 20th	Night	21% 5% 13% 33% 20% 7%	1890
	21st to end	Night	20% 7% 14% 31% 24% 5%	1890
July	1st to 10th	Night	28% 7% 18% 34% 13% 1	5265
	11th to 20th	Night	19% 5% 18% 37% 19% 3	5256
	21st to end	Night	17% 5% 13% 37% 24% 3	5641
August	1st to 10th	Night	7% 3 10% 34% 39% 7%	3006
	11th to 20th	Night	15% 4 13% 44% 20% 4	2951
	21st to end	Night	18% 7% 20% 32% 21% 2	3643
September	1st to 10th	Night	27% 5% 11% 24% 23% 11%	2200
	11th to 20th	Night	29% 4 11% 23% 22% 9%	2200
	21st to end	Night	22% 4 9% 22% 28% 15%	2200
October	1st to 10th	Night	11% 3 6% 15% 28% 38%	3707
	11th to 20th	Night	17% 2 4 19% 31% 27%	3899
	21st to end	Night	24% 3 4 25% 29% 16%	4252
November	1st to 10th	Night	28% 3 7% 21% 27% 14%	2460
	11th to 20th	Night	27% 3 7% 21% 27% 15%	2601
	21st to end	Night	35% 4 7% 16% 27% 11%	2730
December	1st to 10th	Night	37% 3 5% 19% 27% 9%	4380
	11th to 20th	Night	41% 2 6% 14% 25% 13%	4947
	21st to end	Night	37% 3 5% 19% 26% 10%	6633

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	20% 13% 37% 24% 6%	330
	11th to 20th	Day	17% 18% 39% 18% 6% 2	330
	21st to end	Day	23% 14% 33% 23% 7% 1	363
February	1st to 10th	Day	22% 14% 39% 20% 5%	330
	11th to 20th	Day	22% 19% 41% 14% 4	330
	21st to end	Day	27% 19% 38% 13% 3	273
March	1st to 10th	Day	22% 17% 35% 22% 4	330
	11th to 20th	Day	19% 19% 41% 17% 3 1	330
	21st to end	Day	17% 15% 39% 22% 6% 1	363
April	1st to 10th	Day	19% 16% 37% 22% 6% 1	330
	11th to 20th	Day	20% 14% 36% 23% 6%	469
	21st to end	Day	19% 15% 34% 23% 8% 1	627
May	1st to 10th	Day	14% 12% 34% 31% 8% 1	753
	11th to 20th	Day	16% 7% 30% 29% 15% 3	775
	21st to end	Day	14% 5% 26% 34% 18% 4	844
June	1st to 10th	Day	7% 2 17% 39% 27% 8%	780
	11th to 20th	Day	8% 2 18% 40% 27% 6%	780
	21st to end	Day	4 1 14% 38% 33% 10%	780
July	1st to 10th	Day	6% 1 10% 30% 40% 12%	771
	11th to 20th	Day	2 1 7% 37% 41% 13%	768
	21st to end	Day	4 1 5% 31% 44% 15%	849
August	1st to 10th	Day	2 1 6% 30% 47% 14%	776
	11th to 20th	Day	7% 1 6% 32% 41% 13%	736
	21st to end	Day	6% 1 6% 25% 42% 20%	547
September	1st to 10th	Day	8% 6% 22% 42% 22%	330
	11th to 20th	Day	5% 2 6% 24% 40% 23%	330
	21st to end	Day	8% 3 12% 26% 33% 17%	330
October	1st to 10th	Day	2 1 16% 32% 37% 12%	330
	11th to 20th	Day	10% 6% 26% 31% 22% 5%	330
	21st to end	Day	24% 9% 21% 25% 18% 3	363
November	1st to 10th	Day	16% 7% 33% 24% 17% 3	330
	11th to 20th	Day	12% 8% 32% 31% 15% 2	330
	21st to end	Day	16% 15% 32% 27% 8% 2	330
December	1st to 10th	Day	19% 11% 34% 25% 10% 2	330
	11th to 20th	Day	21% 13% 36% 23% 6% 1	330
	21st to end	Day	19% 15% 36% 23% 7% 1	363
January	1st to 10th	Night	16% 10% 36% 32% 6%	1050
	11th to 20th	Night	16% 14% 35% 30% 4	1050
	21st to end	Night	18% 10% 36% 30% 5%	1155
February	1st to 10th	Night	18% 12% 38% 28% 5%	987
	11th to 20th	Night	22% 13% 38% 24% 3	824
	21st to end	Night	22% 15% 39% 21% 3	624
March	1st to 10th	Night	20% 11% 35% 28% 5%	630
	11th to 20th	Night	16% 12% 37% 29% 5%	630
	21st to end	Night	13% 13% 39% 29% 6%	693
April	1st to 10th	Night	12% 12% 43% 28% 5%	630
	11th to 20th	Night	17% 12% 38% 26% 7%	623
	21st to end	Night	14% 9% 39% 29% 9% 1	630
May	1st to 10th	Night	11% 9% 38% 34% 7% 1	630
	11th to 20th	Night	10% 7% 33% 33% 16% 1	621
	21st to end	Night	8% 5% 30% 40% 16% 2	683
June	1st to 10th	Night	7% 3 18% 42% 27% 3	630
	11th to 20th	Night	5% 2 22% 53% 17% 1	630
	21st to end	Night	4 1 15% 44% 31% 5%	630
July	1st to 10th	Night	5% 1 10% 43% 37% 4	615
	11th to 20th	Night	3 1 8% 45% 40% 3	612
	21st to end	Night	2 6% 45% 42% 5%	690
August	1st to 10th	Night	2 5% 43% 45% 4	627
	11th to 20th	Night	6% 6% 39% 42% 6%	606
	21st to end	Night	4 1 5% 39% 44% 7%	660
September	1st to 10th	Night	9% 1 10% 36% 40% 4	600
	11th to 20th	Night	12% 1 8% 38% 39% 3	600
	21st to end	Night	7% 1 12% 42% 33% 5%	600
October	1st to 10th	Night	6% 1 18% 43% 28% 3	665
	11th to 20th	Night	13% 4 24% 39% 19% 1	800
	21st to end	Night	17% 8% 27% 31% 16% 1	880
November	1st to 10th	Night	12% 4 29% 38% 15% 1	820
	11th to 20th	Night	9% 10% 31% 37% 11% 1	921
	21st to end	Night	18% 10% 34% 28% 9% 1	1050
December	1st to 10th	Night	16% 9% 34% 33% 7%	1050
	11th to 20th	Night	20% 9% 33% 33% 6%	1050
	21st to end	Night	19% 9% 35% 31% 6%	1155

Page generated at 2026-08-12 02:37:20. Version ac07

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