Powerhouse Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Powerhouse Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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	13% 9% 3 8% 5% 4 4 4 5% 7% 38%	127812
	Night	12% 9% 4 9% 5% 4 4 4 5% 6% 38%	123819
Transparency	Day	45% 4 12% 25% 12% 1	131324
	Night	46% 4 12% 23% 13% 1	128380
Seeing	Day	17% 11% 30% 25% 14% 4	17611
	Night	16% 9% 32% 31% 11% 1	27549

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	8% 13% 6% 18% 7% 5% 4 2 4 5% 28%	3625
	11th to 20th	Day	6% 12% 6% 13% 6% 8% 9% 9% 7% 5% 20%	3620
	21st to end	Day	8% 15% 8% 13% 5% 3 4 3 5% 6% 30%	4304
February	1st to 10th	Day	9% 12% 4 10% 7% 5% 5% 5% 5% 7% 31%	1800
	11th to 20th	Day	9% 9% 5% 10% 6% 5% 4 4 7% 7% 35%	1754
	21st to end	Day	12% 11% 3 10% 6% 5% 5% 4 5% 7% 33%	1607
March	1st to 10th	Day	13% 8% 3 6% 3 3 4 3 5% 8% 45%	4633
	11th to 20th	Day	9% 9% 3 8% 5% 5% 4 4 6% 7% 39%	4980
	21st to end	Day	22% 10% 3 10% 5% 5% 4 3 4 6% 28%	5663
April	1st to 10th	Day	17% 12% 4 7% 4 5% 4 4 6% 7% 32%	2400
	11th to 20th	Day	21% 10% 4 8% 5% 4 4 4 4 6% 30%	2374
	21st to end	Day	21% 11% 3 7% 5% 4 4 4 6% 6% 29%	2530
May	1st to 10th	Day	15% 6% 2 5% 4 3 3 3 4 5% 50%	6314
	11th to 20th	Day	16% 8% 3 6% 4 4 4 4 5% 6% 41%	6319
	21st to end	Day	17% 9% 3 7% 4 4 5% 3 4 7% 37%	6935
June	1st to 10th	Day	12% 5% 2 5% 5% 4 4 4 7% 8% 43%	2800
	11th to 20th	Day	14% 7% 2 8% 5% 5% 5% 5% 7% 8% 34%	2800
	21st to end	Day	12% 5% 2 5% 5% 5% 5% 5% 7% 10% 39%	2800
July	1st to 10th	Day	11% 4 2 5% 3 4 5% 4 7% 9% 47%	6360
	11th to 20th	Day	9% 4 2 5% 3 3 4 3 6% 8% 53%	6360
	21st to end	Day	10% 5% 2 6% 4 4 4 4 6% 9% 46%	6924
August	1st to 10th	Day	13% 6% 2 6% 4 5% 4 4 6% 8% 42%	4368
	11th to 20th	Day	12% 5% 2 5% 4 4 5% 4 6% 8% 43%	4171
	21st to end	Day	13% 7% 2 7% 4 4 5% 4 6% 8% 40%	4362
September	1st to 10th	Day	10% 6% 2 6% 4 4 4 4 5% 9% 46%	2400
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 6% 8% 44%	2400
	21st to end	Day	15% 7% 2 6% 4 4 3 4 5% 8% 44%	2110
October	1st to 10th	Day	16% 6% 2 6% 4 3 4 5% 5% 7% 43%	2000
	11th to 20th	Day	17% 11% 4 6% 5% 4 3 4 6% 7% 35%	2000
	21st to end	Day	18% 12% 3 7% 4 4 3 3 5% 5% 36%	2092
November	1st to 10th	Day	13% 12% 4 7% 5% 4 4 3 5% 6% 36%	1799
	11th to 20th	Day	12% 15% 5% 9% 6% 5% 5% 3 5% 7% 27%	1680
	21st to end	Day	15% 11% 5% 7% 6% 4 3 3 6% 7% 33%	1680
December	1st to 10th	Day	8% 12% 6% 15% 13% 6% 6% 3 4 5% 22%	2690
	11th to 20th	Day	12% 17% 7% 10% 6% 4 4 2 3 5% 29%	3088
	21st to end	Day	21% 20% 7% 17% 6% 4 3 2 3 3 14%	4070
January	1st to 10th	Night	8% 11% 6% 14% 9% 4 4 3 4 5% 32%	6363
	11th to 20th	Night	7% 12% 5% 12% 7% 6% 9% 7% 7% 5% 23%	6360
	21st to end	Night	8% 17% 7% 12% 5% 4 4 3 5% 6% 30%	6657
February	1st to 10th	Night	7% 11% 6% 11% 7% 6% 5% 3 5% 6% 34%	2600
	11th to 20th	Night	8% 10% 4 10% 5% 5% 5% 4 6% 8% 37%	2533
	21st to end	Night	11% 11% 4 8% 6% 6% 5% 4 5% 6% 35%	2021
March	1st to 10th	Night	10% 7% 2 7% 4 4 4 3 5% 8% 47%	5263
	11th to 20th	Night	9% 8% 3 10% 5% 5% 6% 4 6% 9% 37%	4980
	21st to end	Night	23% 11% 4 9% 4 3 4 3 4 5% 29%	5252
April	1st to 10th	Night	18% 10% 3 7% 4 5% 4 3 4 6% 36%	2000
	11th to 20th	Night	22% 9% 3 6% 4 3 4 3 5% 6% 35%	1975
	21st to end	Night	19% 9% 3 7% 4 3 3 4 5% 7% 35%	1867
May	1st to 10th	Night	13% 7% 2 6% 4 4 3 3 5% 5% 49%	3599
	11th to 20th	Night	14% 6% 2 5% 3 3 3 2 4 5% 52%	3596
	21st to end	Night	15% 8% 3 7% 4 4 4 3 6% 7% 39%	3942
June	1st to 10th	Night	11% 5% 2 5% 4 4 4 4 7% 8% 45%	1600
	11th to 20th	Night	11% 6% 2 6% 5% 5% 4 4 6% 7% 45%	1600
	21st to end	Night	8% 5% 2 5% 4 4 5% 5% 7% 9% 46%	1600
July	1st to 10th	Night	9% 3 2 5% 3 4 4 4 6% 9% 50%	3618
	11th to 20th	Night	7% 3 2 5% 3 3 4 4 6% 9% 54%	3617
	21st to end	Night	7% 6% 2 6% 4 3 5% 4 6% 7% 50%	3947
August	1st to 10th	Night	12% 5% 2 6% 4 4 4 3 5% 7% 47%	2789
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 6% 8% 47%	2888
	21st to end	Night	13% 6% 3 7% 4 4 4 3 4 6% 44%	3535
September	1st to 10th	Night	9% 4 3 5% 3 4 4 3 5% 7% 53%	2000
	11th to 20th	Night	10% 5% 2 4 4 3 4 4 6% 8% 51%	2000
	21st to end	Night	14% 5% 2 5% 3 4 3 3 5% 8% 49%	2267
October	1st to 10th	Night	14% 5% 2 5% 4 4 3 3 4 7% 49%	2400
	11th to 20th	Night	17% 8% 3 6% 5% 4 4 3 5% 6% 40%	2400
	21st to end	Night	17% 10% 4 6% 4 4 4 3 5% 5% 37%	2746
November	1st to 10th	Night	14% 10% 3 7% 4 3 4 3 5% 7% 40%	2715
	11th to 20th	Night	13% 10% 4 10% 5% 4 4 3 5% 7% 35%	2940
	21st to end	Night	14% 10% 4 10% 5% 4 4 3 6% 6% 33%	2940
December	1st to 10th	Night	7% 10% 5% 15% 8% 7% 5% 4 6% 7% 28%	4720
	11th to 20th	Night	12% 17% 7% 13% 5% 5% 4 4 4 5% 24%	5339
	21st to end	Night	23% 16% 5% 14% 6% 4 3 3 3 4 18%	7150

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	60% 3 8% 18% 10%	3626
	11th to 20th	Day	61% 4 7% 17% 11% 1	3620
	21st to end	Day	56% 4 15% 18% 8%	4304
February	1st to 10th	Day	52% 4 13% 20% 10% 1	1800
	11th to 20th	Day	47% 4 12% 25% 10% 1	1763
	21st to end	Day	50% 4 12% 23% 10% 1	1607
March	1st to 10th	Day	38% 4 17% 24% 13% 4	4633
	11th to 20th	Day	43% 4 13% 27% 11% 2	4980
	21st to end	Day	59% 3 9% 18% 10% 1	5665
April	1st to 10th	Day	51% 4 11% 20% 14% 1	2400
	11th to 20th	Day	56% 4 9% 18% 12% 1	2374
	21st to end	Day	54% 4 13% 19% 9% 1	2530
May	1st to 10th	Day	37% 4 10% 30% 17% 2	6352
	11th to 20th	Day	43% 4 12% 23% 14% 4	6323
	21st to end	Day	48% 4 9% 27% 12% 1	6949
June	1st to 10th	Day	36% 5% 15% 30% 12% 1	2800
	11th to 20th	Day	45% 6% 14% 22% 12% 1	2800
	21st to end	Day	39% 6% 17% 27% 10% 1	2800
July	1st to 10th	Day	33% 6% 16% 35% 9%	6291
	11th to 20th	Day	28% 4 14% 37% 15% 1	6285
	21st to end	Day	34% 6% 14% 29% 16% 1	6728
August	1st to 10th	Day	39% 5% 14% 27% 14% 1	4353
	11th to 20th	Day	36% 6% 13% 34% 9%	4210
	21st to end	Day	41% 5% 14% 27% 12% 1	4361
September	1st to 10th	Day	35% 6% 15% 32% 13%	2400
	11th to 20th	Day	38% 5% 18% 31% 7% 1	2400
	21st to end	Day	40% 4 13% 32% 11% 1	2110
October	1st to 10th	Day	30% 2 9% 38% 20%	3260
	11th to 20th	Day	41% 3 10% 26% 18% 2	3240
	21st to end	Day	44% 3 16% 27% 10%	3356
November	1st to 10th	Day	49% 5% 11% 20% 13% 2	1799
	11th to 20th	Day	56% 4 13% 19% 7% 1	1680
	21st to end	Day	51% 5% 15% 22% 6% 1	1680
December	1st to 10th	Day	65% 4 9% 17% 4 1	2690
	11th to 20th	Day	60% 2 10% 19% 9% 1	3085
	21st to end	Day	77% 3 6% 9% 5%	4070
January	1st to 10th	Night	55% 3 10% 19% 12% 1	6363
	11th to 20th	Night	61% 4 10% 14% 12%	6360
	21st to end	Night	56% 3 11% 21% 9% 1	6657
February	1st to 10th	Night	51% 4 11% 21% 10% 2	2600
	11th to 20th	Night	45% 5% 13% 26% 10% 2	2534
	21st to end	Night	49% 4 13% 22% 11% 2	2021
March	1st to 10th	Night	37% 4 15% 23% 16% 5%	5263
	11th to 20th	Night	43% 4 15% 23% 13% 1	4980
	21st to end	Night	59% 3 10% 15% 13% 1	5252
April	1st to 10th	Night	50% 4 7% 20% 17% 1	2000
	11th to 20th	Night	50% 4 9% 20% 15% 2	1975
	21st to end	Night	47% 5% 10% 22% 15% 1	1867
May	1st to 10th	Night	38% 4 10% 27% 21% 1	3623
	11th to 20th	Night	35% 3 11% 24% 21% 6%	3597
	21st to end	Night	44% 4 12% 26% 13% 1	3950
June	1st to 10th	Night	35% 6% 14% 29% 16% 1	1600
	11th to 20th	Night	37% 5% 13% 25% 18% 1	1600
	21st to end	Night	32% 6% 15% 31% 14% 2	1600
July	1st to 10th	Night	30% 6% 18% 33% 12% 1	3580
	11th to 20th	Night	26% 5% 15% 36% 17% 1	3572
	21st to end	Night	33% 5% 15% 27% 19% 1	3832
August	1st to 10th	Night	38% 4 11% 29% 15% 2	2780
	11th to 20th	Night	35% 5% 14% 32% 12% 2	2915
	21st to end	Night	40% 5% 14% 27% 12% 2	3535
September	1st to 10th	Night	31% 4 13% 35% 15% 1	2000
	11th to 20th	Night	32% 4 17% 33% 14%	2000
	21st to end	Night	34% 4 13% 34% 14% 1	2267
October	1st to 10th	Night	27% 3 7% 35% 25% 4	3920
	11th to 20th	Night	36% 4 12% 24% 22% 1	3899
	21st to end	Night	48% 3 10% 25% 13%	4432
November	1st to 10th	Night	44% 4 12% 26% 13% 1	2715
	11th to 20th	Night	50% 4 14% 19% 12% 1	2940
	21st to end	Night	51% 4 15% 22% 6% 1	2940
December	1st to 10th	Night	56% 5% 16% 19% 4 1	4720
	11th to 20th	Night	63% 3 10% 16% 8%	5341
	21st to end	Night	71% 2 7% 12% 7%	7150

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	15% 12% 35% 24% 11% 3	303
	11th to 20th	Day	17% 12% 38% 26% 6% 1	300
	21st to end	Day	18% 10% 38% 22% 10% 1	330
February	1st to 10th	Day	15% 14% 45% 18% 7% 1	300
	11th to 20th	Day	13% 20% 39% 19% 8% 1	300
	21st to end	Day	16% 14% 47% 21% 2	249
March	1st to 10th	Day	19% 12% 40% 20% 7% 2	300
	11th to 20th	Day	21% 13% 35% 21% 8% 2	300
	21st to end	Day	25% 15% 30% 20% 7% 3	415
April	1st to 10th	Day	23% 19% 30% 23% 6%	500
	11th to 20th	Day	29% 13% 26% 23% 7% 1	492
	21st to end	Day	26% 15% 29% 20% 9% 2	630
May	1st to 10th	Day	20% 14% 28% 27% 10% 2	703
	11th to 20th	Day	22% 12% 26% 24% 13% 4	725
	21st to end	Day	19% 11% 26% 23% 18% 3	789
June	1st to 10th	Day	11% 9% 30% 30% 17% 3	730
	11th to 20th	Day	15% 10% 35% 27% 12% 3	730
	21st to end	Day	15% 7% 32% 27% 14% 6%	730
July	1st to 10th	Day	9% 4 20% 31% 25% 11%	721
	11th to 20th	Day	10% 7% 25% 30% 20% 8%	718
	21st to end	Day	7% 4 27% 32% 21% 8%	794
August	1st to 10th	Day	11% 5% 22% 33% 21% 8%	726
	11th to 20th	Day	14% 4 19% 29% 27% 7%	715
	21st to end	Day	15% 6% 23% 29% 23% 5%	620
September	1st to 10th	Day	13% 8% 25% 31% 18% 5%	530
	11th to 20th	Day	12% 5% 28% 34% 19% 2	530
	21st to end	Day	21% 11% 24% 23% 16% 5%	395
October	1st to 10th	Day	17% 12% 30% 23% 14% 3	330
	11th to 20th	Day	21% 21% 31% 20% 6%	330
	21st to end	Day	24% 16% 38% 14% 7% 1	363
November	1st to 10th	Day	21% 14% 36% 21% 8% 1	330
	11th to 20th	Day	22% 17% 36% 19% 6% 1	330
	21st to end	Day	21% 18% 36% 19% 6%	330
December	1st to 10th	Day	16% 15% 38% 22% 8% 1	330
	11th to 20th	Day	22% 17% 33% 22% 6%	330
	21st to end	Day	28% 12% 40% 16% 2 1	363
January	1st to 10th	Night	14% 13% 34% 30% 8% 1	1006
	11th to 20th	Night	17% 11% 37% 28% 6% 1	1000
	21st to end	Night	16% 11% 35% 31% 7%	1100
February	1st to 10th	Night	13% 12% 39% 28% 7% 1	1000
	11th to 20th	Night	13% 14% 40% 26% 7% 1	979
	21st to end	Night	17% 12% 33% 30% 7%	701
March	1st to 10th	Night	16% 11% 37% 27% 7% 1	755
	11th to 20th	Night	16% 12% 37% 28% 7% 1	600
	21st to end	Night	20% 10% 32% 26% 9% 2	660
April	1st to 10th	Night	22% 11% 35% 26% 6% 1	600
	11th to 20th	Night	28% 9% 29% 27% 7%	593
	21st to end	Night	21% 9% 33% 28% 8% 1	600
May	1st to 10th	Night	19% 9% 32% 32% 6% 1	600
	11th to 20th	Night	19% 8% 29% 32% 11% 1	591
	21st to end	Night	15% 7% 25% 38% 14% 1	650
June	1st to 10th	Night	13% 5% 28% 42% 12% 1	600
	11th to 20th	Night	13% 5% 32% 41% 9% 1	600
	21st to end	Night	10% 4 26% 45% 16% 1	600
July	1st to 10th	Night	7% 4 20% 42% 25% 3	585
	11th to 20th	Night	10% 4 24% 39% 21% 2	582
	21st to end	Night	5% 2 22% 47% 22% 2	657
August	1st to 10th	Night	9% 3 17% 46% 25% 1	597
	11th to 20th	Night	14% 3 18% 35% 26% 3	594
	21st to end	Night	12% 3 24% 34% 25% 2	660
September	1st to 10th	Night	10% 4 28% 35% 20% 3	600
	11th to 20th	Night	11% 5% 27% 37% 18% 2	600
	21st to end	Night	14% 9% 29% 30% 17% 1	735
October	1st to 10th	Night	17% 8% 29% 31% 14% 1	800
	11th to 20th	Night	19% 13% 32% 29% 7%	800
	21st to end	Night	22% 10% 30% 29% 8%	880
November	1st to 10th	Night	18% 10% 35% 28% 8% 1	869
	11th to 20th	Night	18% 13% 35% 26% 8%	1050
	21st to end	Night	20% 15% 34% 26% 5%	1050
December	1st to 10th	Night	17% 10% 37% 28% 7% 1	1050
	11th to 20th	Night	21% 11% 36% 23% 7% 2	1050
	21st to end	Night	27% 9% 34% 24% 5%	1155

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