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 2024-11-04.

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%	100948
	Night	11% 7% 3 6% 4 4 4 3 5% 6% 48%	98123
Transparency	Day	37% 6% 12% 26% 17% 2	104599
	Night	35% 5% 11% 26% 19% 4	102612
Seeing	Day	14% 10% 28% 28% 17% 4	15645
	Night	14% 8% 32% 32% 13% 1	25462

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 9% 5% 4 4 3 4 6% 38%	2847
	11th to 20th	Day	10% 10% 4 8% 6% 6% 6% 2 4 4 39%	2850
	21st to end	Day	13% 17% 6% 8% 3 3 4 2 4 5% 35%	3135
February	1st to 10th	Day	15% 11% 5% 7% 4 4 3 3 4 7% 37%	1710
	11th to 20th	Day	12% 10% 3 7% 5% 4 4 3 5% 5% 42%	1664
	21st to end	Day	11% 9% 3 7% 3 3 4 4 6% 7% 43%	1633
March	1st to 10th	Day	13% 10% 3 6% 4 4 3 2 3 5% 48%	3436
	11th to 20th	Day	11% 7% 2 5% 3 3 3 3 5% 7% 52%	3480
	21st to end	Day	15% 12% 4 7% 4 4 3 3 5% 6% 37%	3814
April	1st to 10th	Day	13% 8% 2 6% 5% 3 3 4 5% 7% 44%	2190
	11th to 20th	Day	15% 8% 2 6% 4 5% 4 4 4 6% 41%	2254
	21st to end	Day	20% 7% 3 8% 5% 4 5% 3 4 6% 36%	2404
May	1st to 10th	Day	15% 9% 3 7% 5% 4 4 3 5% 6% 37%	4082
	11th to 20th	Day	15% 7% 2 7% 5% 4 4 3 6% 7% 39%	4082
	21st to end	Day	12% 6% 2 6% 5% 4 5% 3 6% 8% 43%	4508
June	1st to 10th	Day	7% 4 1 4 4 5% 5% 4 7% 10% 48%	2660
	11th to 20th	Day	7% 5% 2 6% 5% 4 4 5% 6% 10% 47%	2660
	21st to end	Day	7% 3 2 5% 4 4 4 4 7% 9% 51%	2605
July	1st to 10th	Day	11% 7% 3 7% 5% 5% 5% 4 6% 8% 42%	4120
	11th to 20th	Day	4 3 2 5% 4 3 5% 4 7% 11% 53%	4120
	21st to end	Day	6% 4 2 5% 4 6% 6% 5% 8% 11% 43%	4465
August	1st to 10th	Day	5% 4 1 5% 4 4 4 4 6% 9% 53%	4104
	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%	1888
	11th to 20th	Day	16% 6% 2 5% 3 2 3 3 4 7% 51%	1671
	21st to end	Day	15% 8% 3 5% 4 3 2 3 4 5% 48%	1520
December	1st to 10th	Day	10% 8% 4 6% 4 3 3 3 6% 7% 46%	1520
	11th to 20th	Day	11% 10% 4 7% 5% 4 4 2 5% 5% 43%	1918
	21st to end	Day	16% 12% 8% 12% 5% 4 4 3 5% 5% 27%	2957
January	1st to 10th	Night	9% 9% 3 13% 5% 4 4 3 3 5% 41%	4108
	11th to 20th	Night	10% 9% 4 8% 6% 7% 6% 3 4 5% 38%	4120
	21st to end	Night	16% 12% 4 10% 3 3 4 3 4 5% 36%	4532
February	1st to 10th	Night	11% 11% 3 7% 6% 4 4 4 5% 6% 40%	2470
	11th to 20th	Night	12% 9% 3 7% 4 4 3 4 5% 6% 44%	2403
	21st to end	Night	13% 8% 3 6% 4 4 3 3 5% 6% 46%	1836
March	1st to 10th	Night	13% 8% 3 5% 3 3 3 3 5% 6% 49%	3419
	11th to 20th	Night	10% 7% 2 4 2 2 3 3 6% 7% 55%	3480
	21st to end	Night	14% 10% 4 8% 5% 4 4 3 5% 6% 38%	3817
April	1st to 10th	Night	11% 6% 2 6% 4 3 4 2 6% 6% 50%	1989
	11th to 20th	Night	13% 8% 3 7% 3 3 2 2 5% 6% 48%	1875
	21st to end	Night	17% 6% 3 6% 4 4 3 3 5% 5% 44%	1772
May	1st to 10th	Night	15% 8% 3 6% 4 4 5% 3 5% 6% 41%	2823
	11th to 20th	Night	14% 8% 2 6% 4 3 3 3 6% 8% 43%	2823
	21st to end	Night	12% 5% 1 4 4 2 3 3 6% 6% 53%	3041
June	1st to 10th	Night	5% 3 2 3 3 4 4 4 7% 11% 54%	1520
	11th to 20th	Night	5% 4 1 5% 4 4 4 4 7% 10% 52%	1520
	21st to end	Night	5% 2 1 4 3 3 3 4 5% 9% 60%	1567
July	1st to 10th	Night	11% 6% 2 5% 4 4 3 3 7% 7% 46%	2850
	11th to 20th	Night	3 2 2 4 3 4 4 3 7% 10% 59%	2850
	21st to end	Night	5% 4 2 5% 4 4 5% 5% 7% 11% 46%	3098
August	1st to 10th	Night	3 2 1 3 3 3 4 4 6% 10% 61%	2836
	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 3 2 2 4 5% 60%	2263
	11th to 20th	Night	10% 6% 2 6% 3 3 3 3 5% 6% 53%	2501
	21st to end	Night	15% 8% 2 5% 3 2 3 2 3 5% 52%	2660
December	1st to 10th	Night	11% 8% 3 5% 4 3 4 4 5% 6% 46%	2660
	11th to 20th	Night	12% 7% 3 7% 4 3 4 3 5% 6% 45%	3279
	21st to end	Night	19% 15% 6% 9% 4 4 3 3 3 4 30%	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	40% 5% 8% 21% 22% 3	3436
	11th to 20th	Day	32% 7% 12% 27% 19% 3	3480
	21st to end	Day	47% 7% 16% 23% 6% 1	3814
April	1st to 10th	Day	39% 6% 12% 23% 17% 2	2190
	11th to 20th	Day	43% 6% 12% 23% 16% 1	2254
	21st to end	Day	49% 6% 11% 23% 11% 1	2404
May	1st to 10th	Day	46% 8% 10% 21% 14% 1	4120
	11th to 20th	Day	41% 9% 14% 22% 14%	4086
	21st to end	Day	37% 9% 13% 28% 12% 1	4521
June	1st to 10th	Day	29% 9% 18% 28% 16% 1	2660
	11th to 20th	Day	31% 9% 15% 29% 15% 2	2660
	21st to end	Day	26% 8% 16% 30% 16% 4	2605
July	1st to 10th	Day	41% 6% 12% 21% 18% 3	4056
	11th to 20th	Day	26% 6% 13% 37% 17% 2	4051
	21st to end	Day	31% 5% 19% 32% 11% 1	4325
August	1st to 10th	Day	27% 6% 15% 29% 21% 2	4090
	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	31% 5% 12% 29% 19% 5%	1888
	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	36% 5% 11% 23% 21% 4	3419
	11th to 20th	Night	28% 6% 10% 26% 26% 4	3480
	21st to end	Night	45% 8% 15% 22% 9% 1	3817
April	1st to 10th	Night	34% 6% 10% 23% 23% 4	1989
	11th to 20th	Night	38% 4 9% 26% 18% 4	1875
	21st to end	Night	41% 6% 11% 26% 14% 2	1772
May	1st to 10th	Night	42% 8% 14% 19% 15% 2	2850
	11th to 20th	Night	38% 7% 15% 27% 12% 1	2824
	21st to end	Night	30% 7% 11% 28% 22% 1	3049
June	1st to 10th	Night	23% 10% 17% 31% 17% 3	1520
	11th to 20th	Night	24% 8% 16% 31% 19% 2	1520
	21st to end	Night	20% 6% 14% 32% 24% 4	1567
July	1st to 10th	Night	36% 6% 13% 24% 18% 4	2805
	11th to 20th	Night	21% 4 13% 39% 21% 3	2796
	21st to end	Night	30% 5% 19% 31% 14% 1	2999
August	1st to 10th	Night	18% 5% 21% 29% 21% 6%	2826
	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	27% 4 12% 27% 24% 6%	2263
	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% 13% 37% 19% 4	270
	21st to end	Day	28% 24% 30% 15% 3	297
April	1st to 10th	Day	19% 20% 37% 17% 6% 1	370
	11th to 20th	Day	21% 14% 38% 20% 6%	452
	21st to end	Day	21% 16% 32% 22% 7% 1	584
May	1st to 10th	Day	18% 13% 37% 24% 8% 1	653
	11th to 20th	Day	18% 10% 30% 27% 12% 3	675
	21st to end	Day	15% 10% 29% 25% 14% 6%	734
June	1st to 10th	Day	7% 3 21% 36% 25% 9%	680
	11th to 20th	Day	8% 3 28% 38% 20% 4	680
	21st to end	Day	5% 7% 34% 33% 19% 2	680
July	1st to 10th	Day	11% 4 15% 28% 30% 11%	671
	11th to 20th	Day	4 3 18% 39% 29% 8%	668
	21st to end	Day	7% 3 15% 33% 33% 8%	739
August	1st to 10th	Day	7% 3 15% 39% 32% 5%	676
	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% 11% 36% 27% 8% 3	279
	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% 26% 6% 1	570
	21st to end	Night	24% 12% 37% 23% 4	627
April	1st to 10th	Night	13% 14% 39% 29% 5%	570
	11th to 20th	Night	18% 13% 34% 28% 7%	563
	21st to end	Night	21% 10% 36% 25% 7%	570
May	1st to 10th	Night	17% 7% 35% 31% 9%	570
	11th to 20th	Night	16% 6% 30% 34% 15% 1	561
	21st to end	Night	13% 4 28% 35% 17% 2	617
June	1st to 10th	Night	7% 3 18% 45% 25% 2	570
	11th to 20th	Night	6% 2 26% 44% 21% 1	570
	21st to end	Night	5% 2 28% 49% 15% 1	570
July	1st to 10th	Night	10% 1 12% 40% 33% 4	555
	11th to 20th	Night	3 3 13% 46% 33% 3	552
	21st to end	Night	7% 2 13% 42% 34% 2	624
August	1st to 10th	Night	6% 2 12% 47% 32% 1	567
	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	12% 9% 36% 32% 11% 1	755
	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 2024-11-01 21:47:45. Version ac07

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