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 2024-11-12.

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	14% 9% 3 8% 5% 4 5% 4 5% 7% 37%	101701
	Night	13% 9% 4 8% 5% 4 4 4 5% 6% 38%	97590
Transparency	Day	45% 5% 13% 25% 11% 1	105258
	Night	46% 4 12% 24% 13% 1	102173
Seeing	Day	17% 10% 30% 26% 14% 4	16062
	Night	16% 9% 31% 31% 11% 1	25926

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	12% 17% 7% 15% 6% 4 5% 2 4 5% 23%	2527
	11th to 20th	Day	6% 11% 5% 15% 8% 10% 10% 7% 5% 5% 18%	2530
	21st to end	Day	10% 18% 9% 14% 5% 4 5% 3 4 6% 22%	3007
February	1st to 10th	Day	9% 11% 4 10% 6% 5% 5% 5% 5% 7% 32%	1710
	11th to 20th	Day	9% 9% 5% 10% 5% 5% 4 4 7% 7% 34%	1664
	21st to end	Day	12% 11% 3 10% 5% 5% 4 4 5% 7% 33%	1529
March	1st to 10th	Day	12% 9% 3 7% 4 4 4 3 4 6% 43%	3231
	11th to 20th	Day	9% 9% 3 9% 5% 5% 4 4 6% 7% 38%	3480
	21st to end	Day	21% 12% 4 11% 6% 5% 4 3 4 6% 26%	3959
April	1st to 10th	Day	17% 12% 4 7% 4 5% 4 4 6% 7% 32%	2280
	11th to 20th	Day	21% 10% 4 8% 4 4 4 4 4 6% 30%	2254
	21st to end	Day	21% 11% 3 7% 5% 4 4 4 6% 6% 29%	2403
May	1st to 10th	Day	18% 6% 2 6% 5% 4 4 3 5% 6% 41%	4394
	11th to 20th	Day	15% 6% 3 6% 4 4 4 4 6% 7% 41%	4399
	21st to end	Day	18% 8% 4 7% 5% 4 4 4 4 7% 36%	4824
June	1st to 10th	Day	12% 5% 2 5% 5% 4 4 4 7% 8% 43%	2660
	11th to 20th	Day	13% 7% 2 8% 6% 5% 5% 5% 7% 8% 35%	2660
	21st to end	Day	12% 4 2 5% 5% 5% 5% 5% 7% 10% 40%	2660
July	1st to 10th	Day	13% 5% 2 5% 4 5% 6% 5% 8% 9% 39%	4440
	11th to 20th	Day	6% 3 1 5% 3 4 4 3 7% 9% 55%	4440
	21st to end	Day	9% 5% 2 5% 4 4 5% 4 6% 8% 47%	4813
August	1st to 10th	Day	13% 6% 2 6% 4 4 5% 4 6% 8% 42%	4234
	11th to 20th	Day	12% 5% 2 5% 4 4 5% 4 6% 8% 44%	4041
	21st to end	Day	13% 7% 2 7% 5% 4 5% 4 6% 8% 40%	4228
September	1st to 10th	Day	10% 6% 2 6% 4 4 4 4 5% 9% 47%	2280
	11th to 20th	Day	13% 6% 2 5% 4 4 4 3 6% 7% 45%	2280
	21st to end	Day	16% 7% 2 6% 4 4 3 4 5% 8% 42%	2005
October	1st to 10th	Day	16% 6% 2 6% 4 3 4 5% 5% 7% 42%	1900
	11th to 20th	Day	16% 11% 3 6% 5% 4 3 4 6% 7% 34%	1900
	21st to end	Day	18% 12% 3 7% 4 4 3 3 5% 6% 36%	1988
November	1st to 10th	Day	14% 12% 4 7% 5% 4 4 3 5% 6% 36%	1712
	11th to 20th	Day	13% 15% 5% 10% 6% 5% 5% 3 5% 7% 25%	1528
	21st to end	Day	15% 10% 5% 6% 6% 4 3 3 6% 7% 35%	1520
December	1st to 10th	Day	12% 14% 6% 12% 8% 6% 6% 4 5% 5% 23%	1520
	11th to 20th	Day	12% 14% 7% 11% 7% 5% 5% 3 4 5% 25%	1918
	21st to end	Day	23% 17% 5% 15% 5% 4 3 2 3 3 18%	2783
January	1st to 10th	Night	11% 14% 8% 13% 5% 4 4 3 4 5% 29%	4428
	11th to 20th	Night	7% 12% 5% 14% 8% 7% 8% 6% 6% 5% 22%	4440
	21st to end	Night	11% 20% 8% 13% 5% 5% 4 3 4 5% 23%	4657
February	1st to 10th	Night	7% 11% 6% 11% 7% 6% 5% 3 4 6% 35%	2470
	11th to 20th	Night	8% 10% 4 9% 5% 5% 5% 4 6% 8% 37%	2403
	21st to end	Night	12% 11% 4 8% 6% 6% 4 4 5% 6% 35%	1923
March	1st to 10th	Night	9% 7% 2 7% 4 4 4 3 6% 8% 45%	3651
	11th to 20th	Night	8% 8% 3 10% 5% 5% 5% 4 5% 6% 40%	3480
	21st to end	Night	24% 11% 4 11% 5% 4 3 3 3 4 29%	3672
April	1st to 10th	Night	18% 10% 3 7% 4 4 4 3 4 6% 35%	1900
	11th to 20th	Night	23% 9% 3 6% 4 3 4 3 5% 6% 35%	1875
	21st to end	Night	19% 9% 3 7% 5% 3 3 4 5% 7% 35%	1774
May	1st to 10th	Night	16% 8% 2 7% 5% 4 4 4 6% 6% 40%	2509
	11th to 20th	Night	16% 5% 2 5% 3 3 3 3 5% 5% 51%	2506
	21st to end	Night	13% 7% 2 7% 4 4 4 4 6% 7% 41%	2743
June	1st to 10th	Night	11% 5% 2 4 4 4 4 4 7% 9% 45%	1520
	11th to 20th	Night	10% 6% 2 6% 5% 4 4 4 6% 7% 47%	1520
	21st to end	Night	8% 5% 2 5% 4 4 5% 5% 7% 8% 47%	1520
July	1st to 10th	Night	12% 4 1 4 3 4 5% 4 6% 8% 48%	2530
	11th to 20th	Night	6% 3 1 4 3 3 4 4 6% 9% 57%	2530
	21st to end	Night	5% 5% 2 5% 4 3 4 4 5% 7% 57%	2750
August	1st to 10th	Night	12% 5% 2 6% 4 4 4 3 5% 7% 47%	2703
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 6% 8% 48%	2798
	21st to end	Night	13% 6% 3 7% 5% 4 4 3 4 6% 44%	3427
September	1st to 10th	Night	9% 4 2 5% 3 4 4 3 5% 8% 53%	1900
	11th to 20th	Night	10% 4 2 4 4 3 3 4 6% 7% 52%	1900
	21st to end	Night	15% 5% 2 5% 3 4 4 3 5% 8% 48%	2153
October	1st to 10th	Night	15% 5% 2 6% 4 4 3 3 4 6% 49%	2280
	11th to 20th	Night	16% 8% 3 6% 5% 4 4 3 5% 6% 40%	2280
	21st to end	Night	17% 10% 4 6% 4 4 4 3 5% 5% 36%	2608
November	1st to 10th	Night	14% 10% 3 7% 4 3 4 3 5% 7% 40%	2582
	11th to 20th	Night	13% 10% 4 10% 5% 4 4 3 5% 7% 34%	2675
	21st to end	Night	14% 10% 4 10% 5% 4 4 3 6% 6% 34%	2660
December	1st to 10th	Night	10% 13% 5% 11% 7% 5% 4 3 5% 6% 29%	2660
	11th to 20th	Night	13% 15% 5% 11% 6% 5% 5% 4 5% 5% 26%	3279
	21st to end	Night	24% 17% 5% 10% 5% 3 3 3 4 4 21%	4884

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	63% 4 10% 18% 5%	2528
	11th to 20th	Day	67% 4 9% 14% 6%	2530
	21st to end	Day	63% 4 13% 15% 5%	3007
February	1st to 10th	Day	51% 4 13% 21% 10% 1	1710
	11th to 20th	Day	48% 4 13% 25% 10% 1	1673
	21st to end	Day	51% 4 11% 22% 10% 2	1529
March	1st to 10th	Day	42% 4 16% 22% 15% 2	3231
	11th to 20th	Day	44% 4 11% 29% 11% 1	3480
	21st to end	Day	62% 3 9% 20% 5% 1	3959
April	1st to 10th	Day	51% 5% 11% 20% 13% 1	2280
	11th to 20th	Day	55% 4 8% 19% 12% 1	2254
	21st to end	Day	54% 4 13% 18% 9% 1	2403
May	1st to 10th	Day	44% 4 9% 27% 13% 2	4435
	11th to 20th	Day	41% 4 14% 26% 13% 1	4403
	21st to end	Day	49% 5% 11% 23% 11% 1	4837
June	1st to 10th	Day	37% 5% 15% 30% 12% 1	2660
	11th to 20th	Day	44% 6% 13% 23% 13% 1	2660
	21st to end	Day	38% 6% 17% 28% 10% 1	2660
July	1st to 10th	Day	40% 7% 17% 25% 10% 1	4371
	11th to 20th	Day	25% 5% 16% 35% 18% 1	4365
	21st to end	Day	32% 7% 14% 29% 17% 1	4662
August	1st to 10th	Day	39% 5% 14% 27% 14% 1	4219
	11th to 20th	Day	36% 6% 13% 35% 9%	4080
	21st to end	Day	41% 6% 15% 26% 12% 1	4227
September	1st to 10th	Day	34% 6% 14% 32% 14%	2280
	11th to 20th	Day	38% 5% 18% 32% 8% 1	2280
	21st to end	Day	40% 4 13% 31% 10% 1	2005
October	1st to 10th	Day	30% 2 9% 38% 20%	3160
	11th to 20th	Day	41% 4 10% 27% 17% 1	3140
	21st to end	Day	44% 3 17% 26% 10%	3252
November	1st to 10th	Day	49% 5% 11% 21% 13% 2	1712
	11th to 20th	Day	58% 4 13% 17% 7% 1	1528
	21st to end	Day	48% 5% 16% 23% 7% 2	1520
December	1st to 10th	Day	63% 5% 13% 16% 4	1520
	11th to 20th	Day	61% 3 15% 15% 5% 1	1915
	21st to end	Day	72% 3 7% 12% 6%	2783
January	1st to 10th	Night	58% 4 11% 19% 7% 1	4428
	11th to 20th	Night	62% 5% 12% 14% 7%	4440
	21st to end	Night	64% 3 8% 18% 6% 1	4657
February	1st to 10th	Night	51% 4 11% 22% 10% 2	2470
	11th to 20th	Night	45% 5% 13% 26% 10% 2	2404
	21st to end	Night	49% 4 12% 22% 11% 2	1923
March	1st to 10th	Night	37% 5% 14% 27% 15% 2	3651
	11th to 20th	Night	43% 5% 9% 27% 14% 1	3480
	21st to end	Night	62% 3 7% 19% 9% 1	3672
April	1st to 10th	Night	50% 4 7% 20% 17% 1	1900
	11th to 20th	Night	50% 4 8% 20% 15% 2	1875
	21st to end	Night	47% 5% 10% 22% 15% 1	1774
May	1st to 10th	Night	45% 5% 13% 18% 19% 1	2533
	11th to 20th	Night	35% 3 8% 30% 20% 4	2507
	21st to end	Night	41% 5% 14% 23% 15% 1	2751
June	1st to 10th	Night	34% 6% 14% 29% 16% 1	1520
	11th to 20th	Night	36% 5% 14% 25% 19% 1	1520
	21st to end	Night	31% 7% 15% 32% 13% 2	1520
July	1st to 10th	Night	33% 6% 20% 26% 14% 1	2490
	11th to 20th	Night	23% 5% 13% 37% 20% 2	2482
	21st to end	Night	26% 5% 15% 31% 21% 2	2662
August	1st to 10th	Night	37% 5% 11% 30% 14% 3	2694
	11th to 20th	Night	34% 5% 14% 32% 13% 2	2825
	21st to end	Night	41% 5% 14% 26% 12% 2	3427
September	1st to 10th	Night	31% 4 14% 35% 15% 1	1900
	11th to 20th	Night	31% 4 16% 34% 15%	1900
	21st to end	Night	35% 4 14% 33% 13% 1	2153
October	1st to 10th	Night	27% 3 7% 35% 25% 4	3800
	11th to 20th	Night	36% 4 12% 24% 22% 1	3779
	21st to end	Night	48% 3 11% 24% 13%	4294
November	1st to 10th	Night	44% 4 12% 25% 14% 1	2582
	11th to 20th	Night	51% 4 14% 18% 12% 1	2675
	21st to end	Night	49% 4 16% 23% 6% 1	2660
December	1st to 10th	Night	55% 4 14% 21% 6%	2660
	11th to 20th	Night	59% 4 12% 18% 6% 1	3281
	21st to end	Night	68% 3 8% 15% 6%	4884

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	17% 11% 34% 25% 10% 3	270
	11th to 20th	Day	18% 12% 37% 26% 6% 1	270
	21st to end	Day	18% 9% 38% 23% 11% 1	297
February	1st to 10th	Day	14% 14% 44% 19% 8% 1	270
	11th to 20th	Day	15% 20% 39% 18% 8% 1	270
	21st to end	Day	16% 12% 48% 22% 2	225
March	1st to 10th	Day	19% 11% 40% 20% 8% 2	270
	11th to 20th	Day	20% 11% 34% 23% 9% 2	270
	21st to end	Day	24% 14% 30% 21% 8% 3	378
April	1st to 10th	Day	23% 17% 31% 23% 6%	460
	11th to 20th	Day	29% 12% 26% 24% 8% 1	452
	21st to end	Day	26% 14% 28% 20% 10% 2	583
May	1st to 10th	Day	21% 14% 29% 26% 8% 2	653
	11th to 20th	Day	21% 12% 26% 24% 12% 4	675
	21st to end	Day	19% 10% 27% 22% 19% 4	734
June	1st to 10th	Day	11% 9% 29% 30% 18% 4	680
	11th to 20th	Day	13% 10% 35% 27% 12% 3	680
	21st to end	Day	14% 6% 31% 28% 15% 7%	680
July	1st to 10th	Day	9% 4 21% 29% 25% 11%	671
	11th to 20th	Day	10% 6% 25% 29% 21% 9%	668
	21st to end	Day	8% 4 26% 33% 21% 8%	739
August	1st to 10th	Day	11% 5% 22% 34% 21% 7%	676
	11th to 20th	Day	14% 4 19% 29% 27% 7%	665
	21st to end	Day	15% 6% 24% 29% 22% 5%	574
September	1st to 10th	Day	12% 8% 25% 32% 18% 4	490
	11th to 20th	Day	11% 6% 27% 34% 19% 2	490
	21st to end	Day	23% 11% 25% 24% 13% 5%	362
October	1st to 10th	Day	18% 12% 28% 25% 14% 3	300
	11th to 20th	Day	19% 21% 32% 22% 6%	300
	21st to end	Day	25% 15% 39% 14% 6% 1	330
November	1st to 10th	Day	22% 11% 36% 22% 8% 1	300
	11th to 20th	Day	22% 16% 38% 19% 5% 1	273
	21st to end	Day	20% 16% 38% 19% 7%	270
December	1st to 10th	Day	16% 14% 36% 23% 10% 1	270
	11th to 20th	Day	21% 14% 35% 23% 6%	270
	21st to end	Day	29% 10% 41% 17% 2 1	297
January	1st to 10th	Night	14% 12% 35% 30% 8% 1	950
	11th to 20th	Night	17% 11% 37% 28% 6% 1	950
	21st to end	Night	16% 10% 35% 32% 7%	1045
February	1st to 10th	Night	12% 12% 40% 29% 7% 1	950
	11th to 20th	Night	13% 13% 40% 26% 7% 1	929
	21st to end	Night	18% 12% 33% 30% 7%	667
March	1st to 10th	Night	16% 11% 37% 27% 7% 1	717
	11th to 20th	Night	16% 11% 37% 29% 7% 1	570
	21st to end	Night	21% 10% 32% 26% 10% 3	627
April	1st to 10th	Night	22% 11% 35% 26% 5% 1	570
	11th to 20th	Night	28% 8% 30% 27% 6%	563
	21st to end	Night	20% 9% 33% 28% 8% 1	570
May	1st to 10th	Night	19% 10% 32% 32% 6% 1	570
	11th to 20th	Night	19% 8% 29% 31% 11% 1	561
	21st to end	Night	14% 7% 25% 38% 15% 1	617
June	1st to 10th	Night	12% 5% 28% 42% 12% 1	570
	11th to 20th	Night	12% 5% 32% 41% 9% 1	570
	21st to end	Night	9% 3 25% 45% 16% 1	570
July	1st to 10th	Night	7% 4 20% 41% 25% 3	555
	11th to 20th	Night	10% 4 23% 39% 22% 2	552
	21st to end	Night	5% 2 21% 47% 23% 2	624
August	1st to 10th	Night	9% 3 17% 47% 23% 1	567
	11th to 20th	Night	14% 3 18% 35% 27% 2	564
	21st to end	Night	13% 3 24% 34% 24% 2	627
September	1st to 10th	Night	9% 4 28% 36% 19% 4	570
	11th to 20th	Night	11% 5% 27% 37% 18% 2	570
	21st to end	Night	14% 9% 30% 30% 16% 1	698
October	1st to 10th	Night	18% 8% 28% 31% 14% 1	760
	11th to 20th	Night	19% 13% 31% 29% 8%	760
	21st to end	Night	22% 11% 30% 29% 8%	836
November	1st to 10th	Night	19% 9% 35% 28% 8% 1	826
	11th to 20th	Night	18% 12% 36% 25% 8%	956
	21st to end	Night	20% 14% 33% 27% 6%	950
December	1st to 10th	Night	17% 10% 36% 28% 7% 1	950
	11th to 20th	Night	21% 10% 36% 24% 8% 2	950
	21st to end	Night	27% 9% 35% 24% 5%	1045

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