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 2024-03-23.

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% 7% 3 6% 5% 3 4 4 5% 7% 44%	78831
	Night	11% 7% 3 6% 4 3 4 4 5% 6% 47%	81082
Transparency	Day	39% 7% 12% 24% 15% 2	78554
	Night	37% 6% 11% 25% 18% 3	80879
Seeing	Day	14% 10% 28% 28% 17% 4	14687
	Night	14% 8% 32% 32% 13% 1	24747

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% 9% 4 8% 6% 3 4 4 4 6% 38%	2847
	11th to 20th	Day	10% 10% 4 7% 7% 5% 7% 3 3 4 39%	2850
	21st to end	Day	13% 16% 6% 7% 4 2 4 3 4 6% 35%	3135
February	1st to 10th	Day	15% 10% 5% 6% 4 3 4 4 4 7% 37%	1710
	11th to 20th	Day	12% 8% 4 6% 5% 3 4 4 5% 6% 42%	1664
	21st to end	Day	12% 9% 3 6% 4 2 4 5% 5% 7% 43%	1633
March	1st to 10th	Day	11% 9% 4 7% 5% 3 5% 4 4 5% 41%	2090
	11th to 20th	Day	19% 9% 4 6% 5% 3 4 4 4 6% 38%	2090
	21st to end	Day	20% 10% 4 7% 5% 3 4 3 5% 5% 33%	2200
April	1st to 10th	Day	13% 8% 3 6% 6% 2 3 4 5% 7% 44%	2074
	11th to 20th	Day	15% 8% 2 6% 5% 4 4 5% 4 6% 41%	2134
	21st to end	Day	20% 7% 3 7% 6% 3 5% 4 4 6% 36%	2277
May	1st to 10th	Day	18% 7% 3 6% 6% 3 5% 4 5% 6% 36%	2340
	11th to 20th	Day	16% 7% 2 6% 6% 2 5% 4 5% 7% 40%	2302
	21st to end	Day	14% 7% 3 7% 6% 4 5% 6% 5% 8% 35%	2549
June	1st to 10th	Day	7% 4 2 4 4 3 5% 5% 7% 10% 49%	2520
	11th to 20th	Day	7% 4 2 6% 6% 3 4 6% 6% 10% 47%	2520
	21st to end	Day	7% 3 2 4 5% 3 4 4 6% 10% 52%	2468
July	1st to 10th	Day	9% 5% 3 5% 5% 3 5% 6% 7% 9% 45%	2340
	11th to 20th	Day	3 2 1 4 4 3 4 6% 7% 10% 54%	2340
	21st to end	Day	8% 5% 2 5% 5% 4 5% 6% 6% 10% 44%	2546
August	1st to 10th	Day	7% 5% 2 5% 5% 3 5% 6% 7% 11% 44%	2328
	11th to 20th	Day	8% 5% 2 4 5% 3 5% 5% 7% 8% 49%	2301
	21st to end	Day	7% 4 2 4 4 2 3 5% 7% 8% 52%	2425
September	1st to 10th	Day	13% 5% 2 4 4 3 3 4 5% 7% 52%	2160
	11th to 20th	Day	10% 4 2 5% 4 3 5% 6% 5% 7% 51%	1993
	21st to end	Day	8% 4 2 5% 4 2 4 5% 4 6% 56%	1971
October	1st to 10th	Day	12% 6% 2 4 3 2 4 4 5% 7% 51%	1800
	11th to 20th	Day	13% 6% 2 3 4 2 2 3 4 5% 55%	1800
	21st to end	Day	16% 7% 3 4 4 2 4 4 4 5% 48%	1980
November	1st to 10th	Day	9% 6% 2 4 4 2 3 4 4 6% 56%	1858
	11th to 20th	Day	16% 5% 2 4 3 2 3 3 4 7% 51%	1671
	21st to end	Day	15% 7% 3 5% 4 2 3 3 4 6% 49%	1520
December	1st to 10th	Day	10% 7% 4 6% 5% 2 4 3 5% 7% 47%	1520
	11th to 20th	Day	12% 10% 4 7% 6% 3 4 3 4 5% 43%	1918
	21st to end	Day	17% 12% 8% 11% 6% 3 4 4 5% 5% 27%	2957
January	1st to 10th	Night	9% 9% 4 12% 7% 3 4 4 3 4 42%	4108
	11th to 20th	Night	10% 8% 4 7% 7% 5% 7% 5% 4 5% 38%	4120
	21st to end	Night	16% 12% 4 9% 4 2 3 3 4 5% 36%	4532
February	1st to 10th	Night	11% 10% 4 6% 6% 3 5% 4 5% 6% 40%	2470
	11th to 20th	Night	12% 8% 3 6% 4 3 4 4 5% 6% 44%	2403
	21st to end	Night	13% 7% 3 6% 4 3 4 4 5% 6% 47%	1836
March	1st to 10th	Night	11% 8% 3 5% 4 3 4 3 5% 7% 48%	2090
	11th to 20th	Night	16% 8% 3 5% 4 2 4 4 5% 6% 43%	2090
	21st to end	Night	19% 10% 3 7% 5% 3 4 4 4 6% 35%	2200
April	1st to 10th	Night	11% 6% 3 5% 4 2 3 4 6% 6% 51%	1885
	11th to 20th	Night	13% 8% 3 6% 5% 2 3 2 4 6% 47%	1775
	21st to end	Night	17% 6% 3 5% 5% 2 4 3 5% 5% 44%	1679
May	1st to 10th	Night	16% 6% 3 6% 4 2 5% 5% 5% 6% 41%	1620
	11th to 20th	Night	14% 6% 2 4 4 2 4 4 5% 7% 49%	1593
	21st to end	Night	11% 5% 2 5% 5% 2 4 5% 7% 9% 44%	1723
June	1st to 10th	Night	5% 3 1 2 4 3 5% 5% 6% 12% 55%	1440
	11th to 20th	Night	5% 4 2 4 4 3 5% 5% 6% 10% 53%	1440
	21st to end	Night	5% 2 1 3 4 2 3 5% 5% 9% 62%	1484
July	1st to 10th	Night	8% 3 1 3 4 3 4 5% 9% 9% 51%	1620
	11th to 20th	Night	3 2 2 3 4 3 4 4 7% 10% 59%	1620
	21st to end	Night	6% 4 2 4 4 2 5% 6% 6% 10% 49%	1772
August	1st to 10th	Night	5% 2 2 3 5% 3 5% 6% 6% 10% 54%	1611
	11th to 20th	Night	6% 5% 2 3 4 2 4 5% 4 8% 57%	1600
	21st to end	Night	5% 3 2 4 4 2 4 4 5% 9% 59%	1930
September	1st to 10th	Night	11% 4 2 4 4 2 3 3 5% 6% 57%	1800
	11th to 20th	Night	9% 5% 2 4 3 3 4 5% 5% 6% 54%	1964
	21st to end	Night	6% 3 2 4 4 3 4 5% 5% 6% 59%	1980
October	1st to 10th	Night	10% 5% 2 4 3 3 4 4 4 6% 57%	2150
	11th to 20th	Night	12% 5% 2 4 4 2 3 3 3 5% 56%	2160
	21st to end	Night	15% 7% 3 4 4 2 3 3 3 4 52%	2376
November	1st to 10th	Night	9% 5% 2 4 3 2 3 3 4 5% 61%	2220
	11th to 20th	Night	10% 5% 2 5% 3 2 4 4 5% 6% 53%	2501
	21st to end	Night	15% 7% 2 4 3 2 3 3 3 5% 52%	2660
December	1st to 10th	Night	11% 7% 3 5% 4 2 4 5% 5% 7% 47%	2660
	11th to 20th	Night	12% 7% 3 6% 5% 2 4 4 5% 6% 45%	3279
	21st to end	Night	19% 15% 6% 7% 5% 3 3 3 3 4 31%	4691

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	47% 6% 9% 25% 13% 1	2848
	11th to 20th	Day	49% 5% 11% 23% 12%	2850
	21st to end	Day	51% 4 11% 17% 15% 1	3135
February	1st to 10th	Day	47% 5% 12% 22% 10% 3	1710
	11th to 20th	Day	44% 4 13% 22% 15% 2	1673
	21st to end	Day	41% 5% 11% 23% 16% 4	1633
March	1st to 10th	Day	44% 6% 9% 21% 16% 4	2090
	11th to 20th	Day	48% 5% 10% 22% 12% 2	2090
	21st to end	Day	53% 6% 14% 18% 8% 1	2200
April	1st to 10th	Day	40% 6% 12% 23% 16% 2	2074
	11th to 20th	Day	44% 6% 12% 23% 14% 1	2134
	21st to end	Day	49% 6% 11% 23% 11% 1	2277
May	1st to 10th	Day	50% 7% 10% 20% 13% 1	2340
	11th to 20th	Day	43% 8% 14% 22% 12%	2306
	21st to end	Day	47% 9% 14% 20% 9% 1	2549
June	1st to 10th	Day	29% 8% 19% 29% 15% 1	2520
	11th to 20th	Day	31% 9% 14% 28% 15% 2	2520
	21st to end	Day	26% 8% 15% 31% 16% 3	2468
July	1st to 10th	Day	34% 9% 14% 24% 16% 3	2276
	11th to 20th	Day	23% 10% 17% 29% 18% 4	2271
	21st to end	Day	32% 8% 15% 31% 13% 1	2405
August	1st to 10th	Day	31% 10% 18% 27% 13% 1	2314
	11th to 20th	Day	30% 8% 14% 30% 16% 2	2301
	21st to end	Day	28% 7% 13% 26% 19% 6%	2425
September	1st to 10th	Day	32% 6% 13% 26% 16% 6%	2160
	11th to 20th	Day	31% 7% 11% 24% 24% 2	1993
	21st to end	Day	28% 7% 10% 28% 24% 3	1971
October	1st to 10th	Day	32% 6% 11% 28% 21% 3	1800
	11th to 20th	Day	32% 4 8% 26% 24% 5%	1800
	21st to end	Day	39% 5% 10% 22% 19% 5%	1980
November	1st to 10th	Day	31% 5% 12% 29% 19% 5%	1858
	11th to 20th	Day	35% 4 10% 26% 22% 3	1671
	21st to end	Day	39% 5% 9% 26% 20% 2	1520
December	1st to 10th	Day	38% 5% 13% 30% 13% 1	1520
	11th to 20th	Day	44% 5% 11% 25% 12% 2	1915
	21st to end	Day	58% 6% 10% 17% 8% 2	2957
January	1st to 10th	Night	45% 6% 14% 22% 12% 1	4108
	11th to 20th	Night	48% 7% 11% 21% 12% 1	4120
	21st to end	Night	51% 5% 12% 20% 11% 2	4532
February	1st to 10th	Night	45% 6% 11% 22% 14% 3	2470
	11th to 20th	Night	40% 6% 12% 23% 17% 3	2404
	21st to end	Night	39% 5% 9% 23% 18% 6%	1836
March	1st to 10th	Night	37% 4 11% 23% 20% 5%	2090
	11th to 20th	Night	41% 6% 10% 23% 16% 4	2090
	21st to end	Night	51% 6% 11% 21% 10% 2	2200
April	1st to 10th	Night	34% 6% 10% 24% 22% 4	1885
	11th to 20th	Night	39% 4 9% 27% 17% 3	1775
	21st to end	Night	41% 6% 11% 26% 15% 1	1679
May	1st to 10th	Night	43% 6% 12% 22% 15% 1	1620
	11th to 20th	Night	36% 6% 16% 25% 16% 2	1594
	21st to end	Night	36% 7% 16% 27% 11% 2	1724
June	1st to 10th	Night	22% 9% 18% 32% 16% 3	1440
	11th to 20th	Night	25% 9% 16% 29% 19% 2	1440
	21st to end	Night	19% 6% 14% 33% 24% 4	1484
July	1st to 10th	Night	26% 9% 16% 30% 16% 4	1575
	11th to 20th	Night	20% 6% 15% 32% 23% 4	1566
	21st to end	Night	26% 9% 13% 30% 20% 2	1673
August	1st to 10th	Night	23% 9% 17% 33% 15% 3	1601
	11th to 20th	Night	26% 6% 13% 30% 24% 2	1600
	21st to end	Night	23% 7% 12% 26% 25% 7%	1930
September	1st to 10th	Night	30% 5% 10% 28% 20% 8%	1800
	11th to 20th	Night	29% 6% 10% 25% 25% 5%	1964
	21st to end	Night	26% 5% 10% 25% 29% 5%	1980
October	1st to 10th	Night	29% 5% 10% 25% 25% 6%	2150
	11th to 20th	Night	32% 4 6% 28% 22% 7%	2160
	21st to end	Night	37% 4 8% 23% 24% 4	2376
November	1st to 10th	Night	27% 4 12% 27% 24% 6%	2220
	11th to 20th	Night	32% 5% 9% 25% 25% 4	2501
	21st to end	Night	37% 3 8% 28% 20% 3	2660
December	1st to 10th	Night	38% 6% 13% 26% 17% 1	2660
	11th to 20th	Night	38% 5% 10% 28% 16% 2	3281
	21st to end	Night	57% 6% 7% 19% 9% 2	4691

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% 15% 34% 24% 5%	270
	11th to 20th	Day	26% 18% 37% 18% 2	270
	21st to end	Day	15% 13% 46% 22% 3	297
February	1st to 10th	Day	24% 17% 39% 17% 3	270
	11th to 20th	Day	17% 19% 43% 16% 4 1	270
	21st to end	Day	17% 26% 42% 13% 2	225
March	1st to 10th	Day	20% 17% 44% 18% 2	270
	11th to 20th	Day	27% 12% 38% 19% 4	270
	21st to end	Day	29% 23% 30% 15% 3	270
April	1st to 10th	Day	18% 20% 37% 18% 6% 1	334
	11th to 20th	Day	21% 14% 38% 21% 7%	412
	21st to end	Day	21% 16% 33% 23% 7% 1	537
May	1st to 10th	Day	19% 13% 36% 24% 9% 1	603
	11th to 20th	Day	19% 10% 31% 25% 11% 3	625
	21st to end	Day	16% 9% 29% 25% 15% 6%	679
June	1st to 10th	Day	7% 3 20% 36% 26% 9%	630
	11th to 20th	Day	8% 3 27% 37% 22% 4	630
	21st to end	Day	5% 7% 33% 33% 20% 2	630
July	1st to 10th	Day	10% 4 13% 29% 32% 12%	621
	11th to 20th	Day	4 3 16% 39% 31% 8%	618
	21st to end	Day	8% 3 14% 34% 33% 8%	684
August	1st to 10th	Day	7% 3 13% 39% 33% 5%	626
	11th to 20th	Day	7% 3 15% 35% 34% 6%	615
	21st to end	Day	9% 1 13% 29% 37% 10%	544
September	1st to 10th	Day	13% 2 15% 34% 27% 8%	450
	11th to 20th	Day	6% 2 17% 40% 26% 8%	283
	21st to end	Day	9% 7% 26% 37% 16% 6%	270
October	1st to 10th	Day	14% 7% 31% 27% 14% 8%	270
	11th to 20th	Day	12% 15% 44% 21% 7% 1	270
	21st to end	Day	25% 15% 33% 21% 6% 1	297
November	1st to 10th	Day	14% 11% 36% 28% 8% 3	270
	11th to 20th	Day	19% 14% 34% 19% 13% 1	270
	21st to end	Day	21% 13% 36% 22% 7%	270
December	1st to 10th	Day	10% 16% 40% 23% 6% 3	270
	11th to 20th	Day	21% 16% 34% 22% 7% 1	270
	21st to end	Day	19% 25% 36% 19% 1	297
January	1st to 10th	Night	14% 12% 41% 27% 6%	950
	11th to 20th	Night	15% 13% 41% 26% 5% 1	950
	21st to end	Night	13% 13% 40% 30% 4	1045
February	1st to 10th	Night	17% 13% 41% 25% 4	950
	11th to 20th	Night	18% 13% 40% 27% 3	929
	21st to end	Night	18% 15% 44% 19% 4 1	580
March	1st to 10th	Night	15% 12% 44% 25% 5%	570
	11th to 20th	Night	20% 11% 36% 27% 6% 1	570
	21st to end	Night	24% 12% 38% 23% 4 1	600
April	1st to 10th	Night	13% 13% 39% 30% 5%	540
	11th to 20th	Night	19% 12% 34% 29% 7%	533
	21st to end	Night	21% 10% 36% 26% 7%	540
May	1st to 10th	Night	17% 7% 34% 32% 9%	540
	11th to 20th	Night	16% 5% 30% 34% 14% 1	531
	21st to end	Night	13% 4 28% 34% 18% 2	584
June	1st to 10th	Night	6% 2 17% 45% 27% 3	540
	11th to 20th	Night	6% 2 26% 44% 22% 1	540
	21st to end	Night	5% 2 27% 50% 15% 1	540
July	1st to 10th	Night	8% 1 11% 41% 35% 4	525
	11th to 20th	Night	3 3 10% 47% 34% 3	522
	21st to end	Night	7% 2 12% 43% 35% 2	591
August	1st to 10th	Night	6% 2 11% 48% 32% 1	537
	11th to 20th	Night	8% 2 10% 46% 30% 3	534
	21st to end	Night	5% 2 12% 38% 41% 3	594
September	1st to 10th	Night	12% 2 17% 41% 24% 3	540
	11th to 20th	Night	10% 2 20% 35% 28% 5%	540
	21st to end	Night	8% 4 24% 40% 21% 4	540
October	1st to 10th	Night	11% 5% 28% 36% 18% 2	710
	11th to 20th	Night	15% 8% 40% 29% 8%	720
	21st to end	Night	20% 12% 32% 27% 9%	792
November	1st to 10th	Night	11% 9% 36% 32% 11% 1	740
	11th to 20th	Night	14% 11% 38% 28% 9%	935
	21st to end	Night	21% 10% 35% 29% 6%	950
December	1st to 10th	Night	16% 11% 37% 29% 5% 1	950
	11th to 20th	Night	19% 8% 38% 30% 4	950
	21st to end	Night	18% 15% 38% 26% 3	1045

Page generated at 2024-03-19 22:31:02. Version ac07

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