Powell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Powell Observatory clear sky chart from 2005-11-06 to 2025-08-05.

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 6% 4 4 4 3 5% 7% 45%	125253
	Night	11% 7% 3 6% 4 3 4 3 5% 6% 48%	123029
Transparency	Day	37% 5% 12% 26% 17% 3	128865
	Night	36% 4 11% 26% 19% 4	127493
Seeing	Day	14% 10% 29% 27% 16% 4	16692
	Night	14% 9% 32% 32% 12% 1	26497

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	16% 9% 3 9% 5% 4 4 3 4 6% 38%	4077
	11th to 20th	Day	7% 8% 4 9% 6% 7% 6% 3 4 5% 41%	4080
	21st to end	Day	12% 14% 4 7% 4 4 4 3 5% 5% 39%	4488
February	1st to 10th	Day	15% 11% 5% 7% 4 4 3 3 4 7% 37%	1800
	11th to 20th	Day	13% 10% 3 7% 5% 4 4 3 5% 5% 41%	1754
	21st to end	Day	11% 9% 3 7% 3 3 4 4 6% 7% 44%	1717
March	1st to 10th	Day	12% 8% 3 6% 4 3 3 2 3 5% 51%	4936
	11th to 20th	Day	9% 6% 2 5% 3 3 3 3 5% 7% 55%	4980
	21st to end	Day	12% 10% 4 9% 5% 5% 4 4 5% 7% 36%	5462
April	1st to 10th	Day	13% 8% 2 7% 5% 3 3 4 5% 7% 44%	2305
	11th to 20th	Day	15% 8% 2 6% 4 5% 4 4 5% 6% 41%	2374
	21st to end	Day	20% 7% 3 8% 5% 4 5% 3 5% 6% 35%	2531
May	1st to 10th	Day	15% 7% 3 7% 5% 4 4 3 5% 6% 42%	5862
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 6% 8% 44%	5862
	21st to end	Day	18% 6% 2 6% 4 4 4 3 5% 7% 40%	6479
June	1st to 10th	Day	8% 4 2 4 4 5% 5% 4 7% 10% 48%	2800
	11th to 20th	Day	7% 5% 2 6% 5% 4 4 5% 6% 9% 47%	2800
	21st to end	Day	6% 3 2 5% 4 4 4 4 7% 9% 51%	2742
July	1st to 10th	Day	8% 5% 2 7% 5% 5% 5% 4 6% 8% 45%	5900
	11th to 20th	Day	7% 4 2 6% 4 4 4 4 7% 10% 47%	5900
	21st to end	Day	6% 5% 2 6% 5% 5% 6% 5% 8% 11% 41%	6422
August	1st to 10th	Day	5% 4 1 5% 4 4 4 4 6% 10% 53%	4157
	11th to 20th	Day	10% 6% 2 6% 5% 4 4 4 6% 7% 46%	4041
	21st to end	Day	5% 4 2 4 3 3 4 4 6% 8% 57%	4257
September	1st to 10th	Day	12% 5% 2 5% 3 3 3 3 4 7% 53%	2280
	11th to 20th	Day	9% 4 2 6% 4 4 5% 4 5% 7% 51%	2103
	21st to end	Day	9% 4 2 5% 3 4 4 4 4 6% 55%	2080
October	1st to 10th	Day	11% 6% 2 5% 2 3 3 3 5% 7% 53%	1900
	11th to 20th	Day	12% 6% 2 4 3 2 2 3 4 5% 57%	1900
	21st to end	Day	15% 7% 3 5% 3 4 3 3 4 5% 49%	2090
November	1st to 10th	Day	9% 7% 2 6% 3 3 4 3 4 6% 55%	1958
	11th to 20th	Day	16% 5% 2 5% 3 2 3 3 4 6% 51%	1758
	21st to end	Day	15% 8% 3 5% 4 3 2 3 4 6% 49%	1600
December	1st to 10th	Day	6% 6% 3 6% 4 3 4 3 5% 6% 55%	2610
	11th to 20th	Day	10% 7% 3 6% 4 4 4 2 4 5% 50%	3008
	21st to end	Day	18% 16% 8% 10% 4 3 3 3 4 5% 26%	4240
January	1st to 10th	Night	13% 9% 4 13% 5% 4 4 3 3 5% 37%	5888
	11th to 20th	Night	9% 8% 3 9% 6% 6% 5% 3 3 5% 43%	5900
	21st to end	Night	14% 9% 3 9% 3 3 4 3 4 5% 42%	6490
February	1st to 10th	Night	11% 10% 3 7% 6% 4 4 4 5% 6% 40%	2600
	11th to 20th	Night	12% 9% 3 7% 4 4 4 3 4 6% 43%	2533
	21st to end	Night	12% 7% 3 6% 4 4 4 3 5% 6% 47%	1929
March	1st to 10th	Night	13% 9% 3 5% 3 3 3 2 4 6% 49%	4919
	11th to 20th	Night	7% 7% 2 4 2 2 3 4 6% 7% 56%	4980
	21st to end	Night	13% 8% 3 8% 4 4 4 4 5% 7% 40%	5467
April	1st to 10th	Night	12% 6% 2 6% 4 3 3 3 6% 6% 49%	2093
	11th to 20th	Night	13% 8% 3 7% 3 3 2 2 5% 6% 48%	1975
	21st to end	Night	17% 7% 3 6% 4 4 3 3 5% 6% 42%	1865
May	1st to 10th	Night	15% 8% 2 6% 3 3 4 3 4 6% 45%	4053
	11th to 20th	Night	14% 7% 2 5% 3 3 3 3 5% 7% 49%	4053
	21st to end	Night	18% 6% 2 4 3 2 2 3 5% 6% 50%	4380
June	1st to 10th	Night	6% 3 2 3 3 4 4 4 7% 11% 53%	1600
	11th to 20th	Night	5% 4 2 5% 3 4 4 4 7% 11% 52%	1600
	21st to end	Night	4 2 1 4 3 3 3 4 5% 9% 60%	1650
July	1st to 10th	Night	9% 5% 2 5% 4 4 3 3 7% 9% 49%	4078
	11th to 20th	Night	5% 4 2 5% 4 4 4 3 7% 9% 52%	4077
	21st to end	Night	6% 5% 2 5% 4 4 5% 5% 7% 12% 44%	4449
August	1st to 10th	Night	3 2 1 3 3 3 4 4 6% 10% 61%	2872
	11th to 20th	Night	9% 6% 3 6% 4 3 4 3 4 6% 51%	2798
	21st to end	Night	3 3 1 5% 4 4 4 3 5% 9% 59%	3400
September	1st to 10th	Night	10% 4 2 5% 3 3 3 2 5% 5% 58%	1900
	11th to 20th	Night	9% 5% 2 4 3 4 4 4 6% 6% 54%	2074
	21st to end	Night	6% 4 1 5% 3 4 4 4 4 6% 58%	2090
October	1st to 10th	Night	9% 5% 1 4 3 3 4 3 4 5% 59%	2270
	11th to 20th	Night	11% 6% 2 5% 3 2 3 2 3 5% 58%	2280
	21st to end	Night	15% 7% 2 5% 3 2 3 3 4 4 52%	2508
November	1st to 10th	Night	10% 5% 1 4 3 2 2 2 4 5% 60%	2340
	11th to 20th	Night	10% 6% 2 6% 3 3 3 3 5% 6% 53%	2634
	21st to end	Night	14% 8% 2 5% 3 3 3 2 3 5% 53%	2800
December	1st to 10th	Night	7% 6% 2 5% 3 3 4 3 5% 6% 57%	4580
	11th to 20th	Night	13% 7% 4 6% 3 3 4 4 5% 6% 46%	5199
	21st to end	Night	19% 16% 5% 8% 4 4 3 3 3 4 30%	6705

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	49% 4 9% 25% 12% 1	4078
	11th to 20th	Day	47% 3 8% 22% 16% 4	4080
	21st to end	Day	47% 3 10% 18% 17% 4	4488
February	1st to 10th	Day	48% 5% 12% 23% 10% 3	1800
	11th to 20th	Day	45% 4 13% 21% 15% 2	1763
	21st to end	Day	39% 5% 10% 25% 17% 4	1717
March	1st to 10th	Day	37% 4 7% 21% 23% 8%	4936
	11th to 20th	Day	30% 5% 9% 22% 27% 8%	4980
	21st to end	Day	47% 5% 16% 26% 6% 1	5464
April	1st to 10th	Day	40% 6% 11% 23% 17% 2	2305
	11th to 20th	Day	43% 5% 12% 23% 15% 1	2374
	21st to end	Day	50% 6% 11% 22% 10% 1	2531
May	1st to 10th	Day	43% 6% 10% 23% 16% 1	5900
	11th to 20th	Day	37% 6% 12% 23% 18% 3	5866
	21st to end	Day	43% 6% 11% 27% 12% 1	6493
June	1st to 10th	Day	30% 8% 17% 27% 16% 1	2800
	11th to 20th	Day	31% 8% 14% 29% 15% 2	2800
	21st to end	Day	26% 7% 16% 30% 16% 4	2742
July	1st to 10th	Day	37% 4 12% 25% 20% 2	5836
	11th to 20th	Day	33% 4 14% 33% 15% 1	5831
	21st to end	Day	34% 4 17% 35% 10% 1	6240
August	1st to 10th	Day	27% 6% 16% 29% 21% 2	4143
	11th to 20th	Day	36% 5% 18% 25% 15% 1	4080
	21st to end	Day	25% 4 15% 37% 14% 4	4256
September	1st to 10th	Day	31% 6% 13% 25% 19% 7%	2280
	11th to 20th	Day	31% 7% 11% 25% 24% 2	2103
	21st to end	Day	29% 6% 9% 27% 25% 3	2080
October	1st to 10th	Day	18% 3 6% 23% 41% 8%	3160
	11th to 20th	Day	20% 3 5% 25% 40% 9%	3140
	21st to end	Day	33% 3 9% 26% 26% 4	3428
November	1st to 10th	Day	32% 5% 11% 29% 19% 5%	1958
	11th to 20th	Day	35% 4 9% 26% 22% 4	1758
	21st to end	Day	39% 4 10% 27% 19% 1	1600
December	1st to 10th	Day	31% 3 12% 37% 17% 1	2610
	11th to 20th	Day	37% 3 10% 33% 15% 1	3005
	21st to end	Day	62% 4 10% 17% 6% 1	4240
January	1st to 10th	Night	50% 4 12% 22% 11% 1	5888
	11th to 20th	Night	45% 5% 8% 22% 16% 4	5900
	21st to end	Night	45% 4 10% 19% 17% 6%	6490
February	1st to 10th	Night	45% 5% 12% 22% 14% 3	2600
	11th to 20th	Night	42% 5% 12% 22% 16% 3	2534
	21st to end	Night	38% 5% 9% 23% 19% 7%	1929
March	1st to 10th	Night	38% 3 8% 21% 25% 5%	4919
	11th to 20th	Night	27% 4 7% 22% 32% 8%	4980
	21st to end	Night	42% 6% 15% 24% 10% 2	5467
April	1st to 10th	Night	35% 6% 10% 23% 23% 4	2093
	11th to 20th	Night	38% 4 9% 26% 18% 4	1975
	21st to end	Night	42% 6% 12% 25% 14% 1	1865
May	1st to 10th	Night	40% 6% 13% 24% 16% 1	4080
	11th to 20th	Night	35% 5% 13% 24% 17% 6%	4054
	21st to end	Night	36% 5% 9% 27% 21% 2	4388
June	1st to 10th	Night	24% 9% 16% 30% 17% 3	1600
	11th to 20th	Night	25% 8% 15% 30% 20% 2	1600
	21st to end	Night	20% 5% 14% 32% 24% 4	1650
July	1st to 10th	Night	33% 4 14% 24% 22% 3	4035
	11th to 20th	Night	28% 3 14% 36% 17% 2	4026
	21st to end	Night	32% 4 17% 37% 11% 1	4320
August	1st to 10th	Night	18% 5% 21% 29% 21% 6%	2862
	11th to 20th	Night	34% 4 13% 28% 20% 2	2825
	21st to end	Night	24% 4 13% 38% 17% 4	3400
September	1st to 10th	Night	29% 5% 10% 27% 21% 9%	1900
	11th to 20th	Night	29% 6% 10% 26% 25% 4	2074
	21st to end	Night	27% 5% 9% 25% 29% 5%	2090
October	1st to 10th	Night	17% 3 6% 20% 44% 10%	3790
	11th to 20th	Night	23% 2 5% 25% 31% 14%	3779
	21st to end	Night	33% 3 6% 31% 25% 3	4120
November	1st to 10th	Night	28% 3 12% 27% 23% 6%	2340
	11th to 20th	Night	32% 4 9% 25% 25% 5%	2634
	21st to end	Night	37% 3 8% 29% 20% 3	2800
December	1st to 10th	Night	30% 3 13% 34% 18% 1	4580
	11th to 20th	Night	39% 3 11% 27% 18% 2	5201
	21st to end	Night	59% 4 8% 20% 7% 1	6705

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	22% 14% 37% 22% 5%	300
	11th to 20th	Day	25% 18% 38% 17% 2	300
	21st to end	Day	15% 13% 47% 22% 3	330
February	1st to 10th	Day	25% 17% 39% 16% 3	300
	11th to 20th	Day	20% 20% 41% 15% 3 1	300
	21st to end	Day	16% 24% 45% 12% 2	249
March	1st to 10th	Day	21% 16% 42% 18% 2	300
	11th to 20th	Day	27% 13% 36% 19% 4	300
	21st to end	Day	27% 24% 33% 14% 3	330
April	1st to 10th	Day	22% 19% 36% 16% 5% 1	405
	11th to 20th	Day	22% 14% 38% 19% 6%	492
	21st to end	Day	22% 16% 33% 22% 6% 1	631
May	1st to 10th	Day	17% 13% 36% 25% 8% 1	703
	11th to 20th	Day	19% 11% 30% 26% 12% 3	725
	21st to end	Day	16% 10% 30% 25% 13% 5%	789
June	1st to 10th	Day	8% 4 21% 35% 24% 8%	730
	11th to 20th	Day	9% 3 28% 37% 20% 4	730
	21st to end	Day	5% 7% 34% 34% 18% 2	730
July	1st to 10th	Day	10% 4 16% 29% 30% 11%	721
	11th to 20th	Day	5% 3 18% 39% 28% 7%	718
	21st to end	Day	7% 3 16% 33% 33% 8%	794
August	1st to 10th	Day	7% 3 15% 40% 31% 5%	697
	11th to 20th	Day	8% 3 16% 35% 32% 6%	665
	21st to end	Day	9% 1 13% 30% 37% 10%	590
September	1st to 10th	Day	12% 2 15% 34% 28% 8%	490
	11th to 20th	Day	6% 2 19% 39% 26% 7%	313
	21st to end	Day	11% 6% 27% 35% 15% 6%	300
October	1st to 10th	Day	12% 6% 31% 27% 15% 8%	300
	11th to 20th	Day	12% 14% 43% 23% 8% 1	300
	21st to end	Day	23% 16% 34% 21% 5% 1	330
November	1st to 10th	Day	15% 12% 37% 26% 7% 3	300
	11th to 20th	Day	19% 15% 33% 19% 12% 1	300
	21st to end	Day	22% 15% 35% 21% 7%	300
December	1st to 10th	Day	10% 18% 40% 23% 6% 3	300
	11th to 20th	Day	20% 17% 36% 20% 6% 1	300
	21st to end	Day	22% 24% 35% 18% 1	330
January	1st to 10th	Night	14% 13% 40% 27% 5%	1000
	11th to 20th	Night	15% 13% 41% 26% 5% 1	1000
	21st to end	Night	13% 13% 41% 29% 4	1100
February	1st to 10th	Night	18% 13% 41% 25% 3	1000
	11th to 20th	Night	19% 14% 39% 26% 2	979
	21st to end	Night	17% 14% 45% 20% 4	609
March	1st to 10th	Night	16% 13% 43% 24% 5%	600
	11th to 20th	Night	20% 11% 36% 26% 6% 1	600
	21st to end	Night	23% 12% 38% 22% 3	660
April	1st to 10th	Night	15% 14% 39% 28% 5%	600
	11th to 20th	Night	19% 14% 34% 28% 6%	593
	21st to end	Night	21% 11% 36% 25% 7%	600
May	1st to 10th	Night	17% 7% 35% 32% 9%	600
	11th to 20th	Night	15% 7% 29% 33% 15% 1	591
	21st to end	Night	15% 4 28% 34% 16% 2	650
June	1st to 10th	Night	7% 3 18% 44% 25% 2	600
	11th to 20th	Night	7% 2 26% 44% 21% 1	600
	21st to end	Night	5% 2 29% 49% 14% 1	600
July	1st to 10th	Night	9% 1 12% 40% 34% 3	585
	11th to 20th	Night	4 2 13% 46% 32% 3	582
	21st to end	Night	7% 2 13% 42% 34% 2	657
August	1st to 10th	Night	6% 2 13% 47% 31% 1	579
	11th to 20th	Night	9% 2 11% 46% 29% 3	564
	21st to end	Night	5% 2 12% 39% 40% 3	627
September	1st to 10th	Night	12% 2 17% 41% 24% 4	570
	11th to 20th	Night	10% 2 20% 35% 28% 5%	570
	21st to end	Night	9% 4 24% 39% 21% 4	570
October	1st to 10th	Night	10% 5% 27% 36% 19% 2	750
	11th to 20th	Night	14% 7% 39% 30% 9%	760
	21st to end	Night	19% 13% 32% 27% 9%	836
November	1st to 10th	Night	13% 9% 36% 31% 11% 1	780
	11th to 20th	Night	14% 12% 37% 28% 9%	985
	21st to end	Night	21% 10% 35% 28% 6%	1000
December	1st to 10th	Night	16% 11% 37% 30% 5% 1	1000
	11th to 20th	Night	20% 9% 38% 29% 4	1000
	21st to end	Night	19% 14% 37% 27% 3	1100

Page generated at 2025-08-02 00:32:25. Version ac07

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