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 2025-04-11.

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	12% 5% 2 5% 3 3 4 3 5% 7% 50%	114091
	Night	12% 5% 2 5% 3 3 3 3 4 6% 55%	114404
Transparency	Day	33% 5% 9% 24% 21% 8%	117810
	Night	30% 4 8% 22% 24% 11%	118814
Seeing	Day	12% 7% 23% 28% 23% 7%	15629
	Night	13% 7% 28% 34% 17% 2	25656

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	14% 6% 3 6% 3 3 3 3 4 5% 51%	4077
	11th to 20th	Day	9% 4 2 3 2 2 3 3 4 6% 62%	4080
	21st to end	Day	23% 10% 3 5% 3 2 2 1 2 3 46%	4488
February	1st to 10th	Day	20% 7% 2 7% 3 3 3 3 3 5% 44%	1875
	11th to 20th	Day	19% 8% 2 5% 3 4 3 2 3 5% 45%	1949
	21st to end	Day	20% 7% 2 4 3 2 3 3 3 5% 49%	1714
March	1st to 10th	Day	16% 6% 2 4 3 3 3 3 5% 7% 49%	4936
	11th to 20th	Day	13% 5% 2 5% 3 3 3 3 4 6% 53%	4980
	21st to end	Day	14% 8% 2 5% 4 3 4 3 5% 6% 46%	5462
April	1st to 10th	Day	15% 6% 2 5% 4 3 4 3 6% 6% 46%	2200
	11th to 20th	Day	14% 7% 2 6% 4 4 4 3 5% 7% 44%	2197
	21st to end	Day	12% 5% 2 6% 5% 4 4 4 6% 7% 45%	2359
May	1st to 10th	Day	20% 8% 3 7% 4 4 3 3 5% 6% 37%	4082
	11th to 20th	Day	15% 6% 2 6% 4 4 4 3 4 6% 47%	4082
	21st to end	Day	16% 7% 2 7% 5% 5% 5% 5% 7% 7% 34%	4477
June	1st to 10th	Day	6% 4 1 5% 4 5% 5% 5% 7% 8% 49%	2470
	11th to 20th	Day	5% 4 2 4 4 5% 5% 4 6% 10% 51%	2470
	21st to end	Day	5% 4 1 5% 4 3 4 4 7% 9% 54%	2470
July	1st to 10th	Day	6% 5% 2 5% 5% 5% 5% 4 6% 8% 50%	4120
	11th to 20th	Day	3 2 1 4 3 4 4 4 6% 10% 61%	4120
	21st to end	Day	6% 6% 2 7% 5% 5% 5% 5% 7% 11% 40%	4465
August	1st to 10th	Day	1 1 1 2 2 3 4 3 5% 8% 71%	4104
	11th to 20th	Day	4 2 1 3 3 2 3 4 7% 10% 60%	4041
	21st to end	Day	3 3 1 4 4 4 6% 5% 8% 11% 53%	4107
September	1st to 10th	Day	8% 5% 2 4 4 4 4 4 5% 7% 52%	2090
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 6% 7% 47%	2090
	21st to end	Day	7% 3 2 4 3 3 4 3 5% 8% 59%	2090
October	1st to 10th	Day	5% 4 2 5% 3 3 4 2 5% 6% 63%	2009
	11th to 20th	Day	13% 4 1 3 3 2 3 3 4 5% 60%	1900
	21st to end	Day	14% 5% 1 4 3 2 3 2 3 4 58%	2090
November	1st to 10th	Day	12% 5% 2 5% 3 3 3 2 4 6% 55%	1958
	11th to 20th	Day	8% 4 2 5% 4 4 3 3 4 6% 58%	1920
	21st to end	Day	15% 7% 2 4 3 3 2 2 4 5% 53%	1800
December	1st to 10th	Day	17% 5% 2 5% 3 3 3 2 5% 6% 50%	2940
	11th to 20th	Day	18% 8% 3 6% 4 3 4 3 4 4 44%	3391
	21st to end	Day	17% 8% 3 6% 3 3 3 2 3 4 47%	4488
January	1st to 10th	Night	15% 5% 2 5% 3 3 3 2 3 4 55%	5888
	11th to 20th	Night	9% 3 1 3 2 2 2 2 3 5% 70%	5900
	21st to end	Night	24% 8% 3 4 2 2 2 2 3 4 45%	6490
February	1st to 10th	Night	15% 7% 2 6% 3 3 3 3 4 5% 51%	2540
	11th to 20th	Night	18% 7% 2 5% 3 3 3 2 3 6% 47%	2338
	21st to end	Night	19% 6% 2 4 3 3 3 3 3 5% 50%	1915
March	1st to 10th	Night	18% 7% 2 6% 4 3 3 3 4 6% 45%	4919
	11th to 20th	Night	10% 5% 2 6% 4 3 4 3 4 6% 53%	4980
	21st to end	Night	13% 6% 2 5% 4 4 3 3 4 5% 50%	5467
April	1st to 10th	Night	11% 4 2 5% 3 4 4 3 6% 7% 51%	2200
	11th to 20th	Night	14% 5% 2 6% 5% 3 4 3 4 6% 49%	1938
	21st to end	Night	11% 5% 2 4 3 3 3 4 5% 6% 54%	1817
May	1st to 10th	Night	19% 8% 2 6% 4 4 4 3 5% 8% 37%	2823
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 4 6% 49%	2823
	21st to end	Night	10% 6% 2 6% 4 4 5% 4 5% 8% 46%	3090
June	1st to 10th	Night	5% 3 1 4 3 3 3 4 7% 9% 57%	1710
	11th to 20th	Night	4 3 1 3 3 3 3 3 5% 9% 63%	1710
	21st to end	Night	3 3 1 4 3 3 4 3 7% 11% 58%	1710
July	1st to 10th	Night	5% 3 2 4 3 3 4 3 6% 12% 57%	2850
	11th to 20th	Night	2 2 1 3 2 2 2 3 6% 9% 67%	2850
	21st to end	Night	4 4 2 5% 4 4 5% 5% 7% 9% 51%	3098
August	1st to 10th	Night	1 1 1 2 1 2 2 4 10% 76%	2836
	11th to 20th	Night	3 3 1 2 2 3 2 3 5% 10% 66%	2798
	21st to end	Night	3 2 1 3 3 3 4 3 6% 9% 62%	3531
September	1st to 10th	Night	7% 6% 1 5% 3 3 3 2 6% 8% 56%	2090
	11th to 20th	Night	10% 4 2 5% 4 3 4 3 4 6% 55%	2090
	21st to end	Night	6% 3 1 4 2 3 3 3 4 8% 63%	2090
October	1st to 10th	Night	6% 2 1 3 3 2 3 3 5% 6% 66%	2152
	11th to 20th	Night	12% 3 1 3 2 2 3 2 4 5% 62%	2280
	21st to end	Night	16% 4 2 4 2 2 2 1 3 3 61%	2508
November	1st to 10th	Night	11% 5% 2 5% 3 2 3 2 3 5% 60%	2340
	11th to 20th	Night	7% 6% 2 4 4 3 3 3 4 6% 59%	2477
	21st to end	Night	14% 6% 2 4 3 3 3 3 4 5% 53%	2600
December	1st to 10th	Night	16% 6% 2 5% 2 3 2 2 3 5% 53%	4250
	11th to 20th	Night	16% 7% 3 6% 3 3 2 2 3 4 51%	4816
	21st to end	Night	15% 7% 2 6% 3 2 3 2 3 4 52%	6490

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	37% 3 6% 28% 23% 3	4078
	11th to 20th	Day	25% 3 7% 23% 35% 7%	4080
	21st to end	Day	48% 2 3 14% 21% 12%	4488
February	1st to 10th	Day	46% 2 6% 16% 21% 9%	1875
	11th to 20th	Day	44% 2 7% 20% 19% 8%	1959
	21st to end	Day	41% 3 4 15% 25% 12%	1714
March	1st to 10th	Day	37% 3 6% 16% 22% 16%	4936
	11th to 20th	Day	34% 4 6% 17% 17% 21%	4980
	21st to end	Day	39% 4 10% 22% 20% 5%	5464
April	1st to 10th	Day	39% 5% 9% 19% 19% 9%	2200
	11th to 20th	Day	40% 5% 8% 18% 21% 8%	2197
	21st to end	Day	37% 6% 9% 21% 23% 4	2359
May	1st to 10th	Day	49% 5% 8% 21% 13% 4	4120
	11th to 20th	Day	39% 5% 8% 25% 22% 1	4086
	21st to end	Day	45% 9% 12% 18% 11% 4	4489
June	1st to 10th	Day	30% 8% 14% 29% 16% 2	2470
	11th to 20th	Day	28% 7% 14% 27% 19% 5%	2470
	21st to end	Day	26% 7% 11% 30% 23% 3	2470
July	1st to 10th	Day	30% 8% 17% 34% 10%	4056
	11th to 20th	Day	18% 6% 13% 41% 21% 2	4051
	21st to end	Day	35% 8% 14% 32% 11%	4325
August	1st to 10th	Day	12% 5% 12% 32% 35% 4	4090
	11th to 20th	Day	19% 5% 16% 41% 17% 2	4080
	21st to end	Day	25% 9% 18% 34% 13% 2	4106
September	1st to 10th	Day	31% 7% 9% 23% 19% 11%	2090
	11th to 20th	Day	35% 8% 11% 18% 22% 7%	2090
	21st to end	Day	26% 4 9% 22% 27% 11%	2090
October	1st to 10th	Day	14% 3 6% 14% 32% 31%	3334
	11th to 20th	Day	18% 2 4 22% 32% 22%	3140
	21st to end	Day	24% 3 4 31% 29% 9%	3428
November	1st to 10th	Day	33% 4 7% 23% 24% 10%	1958
	11th to 20th	Day	29% 3 8% 19% 29% 12%	1920
	21st to end	Day	37% 2 6% 22% 24% 10%	1800
December	1st to 10th	Day	37% 3 6% 27% 23% 5%	2940
	11th to 20th	Day	44% 3 4 13% 25% 10%	3389
	21st to end	Day	42% 3 5% 23% 21% 7%	4488
January	1st to 10th	Night	34% 3 6% 26% 26% 6%	5888
	11th to 20th	Night	21% 2 6% 19% 36% 16%	5900
	21st to end	Night	46% 2 4 13% 21% 14%	6490
February	1st to 10th	Night	37% 4 7% 18% 22% 12%	2540
	11th to 20th	Night	41% 3 7% 16% 24% 9%	2338
	21st to end	Night	38% 4 7% 15% 21% 16%	1915
March	1st to 10th	Night	43% 3 5% 11% 21% 17%	4919
	11th to 20th	Night	32% 4 8% 15% 19% 22%	4980
	21st to end	Night	37% 4 7% 19% 27% 6%	5467
April	1st to 10th	Night	32% 5% 10% 20% 19% 14%	2200
	11th to 20th	Night	35% 5% 9% 20% 18% 13%	1938
	21st to end	Night	30% 5% 11% 21% 28% 5%	1817
May	1st to 10th	Night	46% 7% 10% 19% 13% 6%	2850
	11th to 20th	Night	36% 6% 13% 19% 24% 2	2824
	21st to end	Night	35% 7% 15% 25% 13% 6%	3099
June	1st to 10th	Night	22% 8% 15% 30% 20% 5%	1710
	11th to 20th	Night	19% 5% 13% 35% 20% 7%	1710
	21st to end	Night	21% 7% 12% 30% 25% 5%	1710
July	1st to 10th	Night	22% 6% 19% 39% 13% 1	2805
	11th to 20th	Night	14% 4 12% 44% 23% 3	2796
	21st to end	Night	27% 7% 17% 35% 13% 1	2999
August	1st to 10th	Night	7% 3 10% 33% 40% 6%	2826
	11th to 20th	Night	15% 4 13% 43% 20% 4	2825
	21st to end	Night	17% 7% 20% 33% 21% 2	3531
September	1st to 10th	Night	26% 5% 11% 24% 22% 12%	2090
	11th to 20th	Night	30% 4 12% 22% 23% 10%	2090
	21st to end	Night	22% 4 8% 22% 28% 16%	2090
October	1st to 10th	Night	11% 3 6% 15% 27% 39%	3594
	11th to 20th	Night	17% 2 4 19% 31% 27%	3779
	21st to end	Night	24% 3 3 25% 29% 16%	4120
November	1st to 10th	Night	30% 3 7% 21% 26% 14%	2340
	11th to 20th	Night	28% 3 6% 20% 28% 16%	2477
	21st to end	Night	35% 4 7% 16% 27% 11%	2600
December	1st to 10th	Night	37% 2 5% 19% 27% 9%	4250
	11th to 20th	Night	41% 2 6% 14% 24% 13%	4817
	21st to end	Night	37% 3 5% 19% 26% 9%	6490

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% 13% 36% 23% 6%	300
	11th to 20th	Day	18% 18% 39% 19% 5% 2	300
	21st to end	Day	23% 15% 33% 22% 7% 1	330
February	1st to 10th	Day	23% 14% 38% 19% 6%	300
	11th to 20th	Day	23% 19% 40% 13% 4	300
	21st to end	Day	29% 19% 37% 13% 2	249
March	1st to 10th	Day	23% 16% 35% 23% 4	300
	11th to 20th	Day	20% 19% 40% 17% 3 1	300
	21st to end	Day	18% 16% 40% 19% 6% 1	330
April	1st to 10th	Day	20% 16% 37% 21% 6% 1	300
	11th to 20th	Day	22% 12% 35% 23% 8% 1	395
	21st to end	Day	16% 15% 35% 24% 9% 1	539
May	1st to 10th	Day	13% 12% 34% 31% 8% 1	653
	11th to 20th	Day	16% 6% 29% 31% 15% 3	675
	21st to end	Day	16% 5% 27% 32% 17% 3	734
June	1st to 10th	Day	7% 2 15% 39% 29% 9%	680
	11th to 20th	Day	6% 1 18% 41% 28% 6%	680
	21st to end	Day	4 1 13% 37% 34% 11%	680
July	1st to 10th	Day	7% 1 10% 30% 39% 13%	671
	11th to 20th	Day	2 7% 35% 42% 14%	668
	21st to end	Day	4 1 5% 31% 45% 14%	739
August	1st to 10th	Day	2 1 6% 30% 47% 14%	676
	11th to 20th	Day	8% 1 6% 31% 41% 13%	665
	21st to end	Day	6% 1 6% 24% 42% 22%	505
September	1st to 10th	Day	8% 5% 20% 43% 24%	300
	11th to 20th	Day	5% 2 7% 25% 39% 21%	300
	21st to end	Day	9% 3 11% 26% 35% 16%	300
October	1st to 10th	Day	2 1 17% 32% 37% 10%	300
	11th to 20th	Day	11% 6% 27% 32% 20% 4	300
	21st to end	Day	24% 9% 21% 25% 18% 3	330
November	1st to 10th	Day	18% 6% 34% 23% 15% 4	300
	11th to 20th	Day	12% 8% 32% 31% 15% 2	300
	21st to end	Day	17% 16% 31% 27% 8% 2	300
December	1st to 10th	Day	19% 10% 33% 26% 10% 2	300
	11th to 20th	Day	21% 13% 38% 22% 6%	300
	21st to end	Day	20% 15% 38% 20% 6% 1	330
January	1st to 10th	Night	16% 10% 35% 32% 7%	1000
	11th to 20th	Night	17% 14% 34% 30% 5% 1	1000
	21st to end	Night	18% 10% 37% 31% 6%	1100
February	1st to 10th	Night	19% 12% 37% 27% 5%	940
	11th to 20th	Night	22% 13% 38% 24% 2	784
	21st to end	Night	23% 15% 39% 20% 3	595
March	1st to 10th	Night	21% 12% 34% 29% 5%	600
	11th to 20th	Night	16% 12% 37% 29% 6%	600
	21st to end	Night	14% 13% 38% 29% 5%	660
April	1st to 10th	Night	12% 12% 43% 27% 5%	600
	11th to 20th	Night	17% 11% 38% 26% 8% 1	565
	21st to end	Night	14% 8% 40% 28% 9% 1	570
May	1st to 10th	Night	10% 9% 39% 34% 7% 1	570
	11th to 20th	Night	10% 5% 34% 35% 16% 1	561
	21st to end	Night	8% 5% 30% 39% 16% 2	617
June	1st to 10th	Night	6% 2 18% 42% 29% 3	570
	11th to 20th	Night	4 2 21% 53% 18% 1	570
	21st to end	Night	5% 1 15% 42% 33% 5%	570
July	1st to 10th	Night	4 1 11% 42% 37% 5%	555
	11th to 20th	Night	3 1 8% 44% 41% 4	552
	21st to end	Night	2 6% 45% 42% 5%	624
August	1st to 10th	Night	2 1 5% 43% 45% 4	567
	11th to 20th	Night	7% 7% 38% 43% 5%	564
	21st to end	Night	4 4 39% 45% 7%	627
September	1st to 10th	Night	9% 9% 35% 42% 4	570
	11th to 20th	Night	12% 1 8% 38% 38% 3	570
	21st to end	Night	7% 1 12% 42% 33% 5%	570
October	1st to 10th	Night	7% 1 18% 43% 28% 3	632
	11th to 20th	Night	14% 4 24% 39% 18% 1	760
	21st to end	Night	17% 8% 27% 31% 16% 1	836
November	1st to 10th	Night	13% 4 29% 38% 16% 1	780
	11th to 20th	Night	9% 10% 31% 37% 11% 1	877
	21st to end	Night	18% 10% 34% 28% 10% 1	1000
December	1st to 10th	Night	17% 9% 34% 33% 7%	1000
	11th to 20th	Night	20% 9% 33% 33% 6%	1000
	21st to end	Night	19% 9% 35% 30% 6%	1100

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