Perce
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Perce clear sky chart from 2005-11-06 to 2024-09-29.

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	16% 9% 3 8% 5% 5% 5% 4 5% 7% 33%	101457
	Night	17% 10% 3 8% 5% 4 4 4 5% 6% 33%	96891
Transparency	Day	49% 6% 15% 21% 8% 1	101201
	Night	52% 6% 14% 19% 9% 1	96767
Seeing	Day	21% 6% 24% 31% 15% 3	16611
	Night	23% 7% 27% 28% 13% 2	23973

By Month

Percentage Cloud Cover NE (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	13% 13% 4 10% 6% 6% 5% 4 5% 7% 26%	2207
	11th to 20th	Day	11% 11% 4 9% 4 5% 4 4 6% 9% 32%	2210
	21st to end	Day	7% 7% 4 10% 7% 8% 5% 5% 6% 8% 34%	2860
February	1st to 10th	Day	11% 13% 4 9% 6% 5% 4 5% 5% 7% 32%	1710
	11th to 20th	Day	11% 15% 5% 10% 6% 5% 5% 4 5% 6% 29%	1663
	21st to end	Day	11% 10% 5% 8% 6% 5% 5% 4 6% 7% 34%	1413
March	1st to 10th	Day	13% 14% 4 10% 4 5% 4 4 5% 6% 31%	3323
	11th to 20th	Day	17% 17% 5% 11% 6% 6% 5% 3 5% 5% 21%	3480
	21st to end	Day	23% 14% 3 7% 4 4 4 2 4 5% 30%	3834
April	1st to 10th	Day	20% 12% 4 7% 4 5% 4 4 5% 6% 29%	2280
	11th to 20th	Day	17% 10% 3 7% 4 4 5% 4 6% 6% 36%	2413
	21st to end	Day	24% 10% 3 6% 4 4 4 3 5% 6% 32%	2470
May	1st to 10th	Day	20% 11% 4 8% 5% 4 5% 4 6% 7% 26%	4390
	11th to 20th	Day	14% 8% 3 7% 5% 5% 5% 4 6% 7% 37%	4398
	21st to end	Day	19% 8% 3 8% 6% 5% 5% 4 6% 7% 28%	5018
June	1st to 10th	Day	24% 10% 3 8% 5% 4 5% 4 5% 6% 28%	2850
	11th to 20th	Day	18% 7% 2 6% 4 4 4 5% 5% 8% 36%	2850
	21st to end	Day	25% 9% 3 7% 6% 5% 5% 3 6% 6% 26%	2850
July	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 4 7% 9% 36%	4760
	11th to 20th	Day	11% 7% 3 8% 5% 5% 5% 5% 7% 9% 36%	4760
	21st to end	Day	13% 5% 2 5% 4 4 5% 4 6% 8% 44%	4941
August	1st to 10th	Day	14% 7% 2 8% 5% 6% 6% 5% 7% 9% 31%	4233
	11th to 20th	Day	15% 6% 2 7% 5% 4 5% 4 7% 7% 39%	4045
	21st to end	Day	13% 4 2 5% 5% 4 4 3 5% 7% 46%	4453
September	1st to 10th	Day	18% 6% 2 6% 4 4 5% 4 5% 7% 39%	2280
	11th to 20th	Day	17% 7% 3 6% 4 4 4 3 4 5% 44%	2214
	21st to end	Day	18% 7% 3 6% 4 4 5% 3 4 5% 40%	2010
October	1st to 10th	Day	15% 7% 2 6% 5% 5% 5% 4 6% 5% 41%	1800
	11th to 20th	Day	22% 8% 3 6% 5% 4 4 3 5% 7% 34%	1800
	21st to end	Day	23% 9% 2 7% 5% 5% 4 3 4 7% 30%	1849
November	1st to 10th	Day	19% 9% 3 7% 4 5% 5% 3 5% 5% 34%	1485
	11th to 20th	Day	20% 12% 4 7% 6% 5% 6% 4 5% 6% 26%	1520
	21st to end	Day	17% 16% 4 10% 5% 3 3 4 5% 7% 25%	1520
December	1st to 10th	Day	21% 13% 4 9% 5% 4 4 4 5% 5% 26%	1469
	11th to 20th	Day	14% 15% 5% 11% 6% 6% 4 4 5% 7% 23%	1668
	21st to end	Day	16% 12% 5% 11% 5% 5% 5% 3 5% 6% 28%	2431
January	1st to 10th	Night	10% 12% 5% 11% 8% 6% 6% 6% 7% 8% 22%	4748
	11th to 20th	Night	17% 11% 3 9% 6% 5% 5% 4 5% 5% 30%	4760
	21st to end	Night	8% 10% 4 8% 6% 6% 4 4 5% 7% 38%	4797
February	1st to 10th	Night	10% 14% 5% 7% 6% 4 4 3 5% 6% 35%	2470
	11th to 20th	Night	13% 15% 4 10% 5% 5% 4 3 5% 6% 31%	2403
	21st to end	Night	11% 13% 5% 7% 5% 4 4 4 5% 6% 33%	2041
March	1st to 10th	Night	11% 10% 4 9% 7% 5% 6% 4 6% 7% 31%	3514
	11th to 20th	Night	21% 18% 5% 10% 5% 5% 5% 3 4 4 20%	3479
	21st to end	Night	26% 12% 4 7% 4 3 4 3 3 7% 27%	3797
April	1st to 10th	Night	24% 11% 3 8% 5% 4 3 2 4 6% 29%	1900
	11th to 20th	Night	20% 9% 3 7% 5% 4 4 3 5% 5% 35%	1715
	21st to end	Night	23% 11% 3 6% 5% 3 4 3 4 5% 33%	1710
May	1st to 10th	Night	22% 11% 4 9% 5% 5% 4 3 5% 5% 27%	2558
	11th to 20th	Night	14% 7% 2 7% 4 4 5% 3 5% 7% 41%	2506
	21st to end	Night	14% 11% 5% 10% 5% 5% 5% 4 6% 7% 28%	2596
June	1st to 10th	Night	26% 9% 2 6% 5% 3 4 4 5% 7% 30%	1330
	11th to 20th	Night	19% 7% 2 6% 4 4 4 3 6% 7% 38%	1330
	21st to end	Night	24% 8% 2 6% 6% 5% 6% 3 6% 6% 28%	1330
July	1st to 10th	Night	15% 7% 3 7% 5% 4 4 4 6% 8% 39%	2210
	11th to 20th	Night	14% 7% 3 6% 5% 5% 5% 4 6% 7% 38%	2210
	21st to end	Night	12% 6% 2 5% 4 4 4 4 5% 7% 46%	2592
August	1st to 10th	Night	15% 8% 3 8% 6% 5% 6% 4 5% 6% 33%	2698
	11th to 20th	Night	14% 6% 3 6% 5% 4 5% 4 5% 7% 42%	2782
	21st to end	Night	14% 5% 2 4 4 3 4 3 5% 8% 48%	3202
September	1st to 10th	Night	19% 6% 2 6% 4 3 4 3 5% 5% 43%	1900
	11th to 20th	Night	18% 6% 2 5% 3 3 3 3 4 6% 47%	1963
	21st to end	Night	17% 7% 3 5% 3 3 3 4 4 6% 43%	2138
October	1st to 10th	Night	18% 7% 2 7% 4 4 4 4 4 5% 40%	2160
	11th to 20th	Night	21% 10% 3 7% 3 3 4 3 4 6% 36%	2160
	21st to end	Night	23% 10% 3 7% 4 4 3 3 4 6% 34%	2505
November	1st to 10th	Night	22% 10% 2 6% 5% 4 4 3 4 7% 34%	2593
	11th to 20th	Night	22% 11% 4 8% 4 3 4 3 5% 7% 30%	2660
	21st to end	Night	21% 14% 4 8% 5% 5% 3 3 5% 6% 26%	2660
December	1st to 10th	Night	21% 13% 4 8% 6% 4 4 3 5% 6% 26%	2707
	11th to 20th	Night	23% 15% 4 8% 5% 4 4 3 4 6% 23%	3531
	21st to end	Night	19% 12% 4 11% 5% 5% 5% 3 5% 7% 24%	5236

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	55% 5% 18% 19% 4 1	2208
	11th to 20th	Day	48% 7% 16% 18% 9%	2210
	21st to end	Day	46% 8% 13% 23% 9% 1	2860
February	1st to 10th	Day	51% 6% 12% 20% 10% 2	1710
	11th to 20th	Day	56% 6% 12% 18% 9% 1	1671
	21st to end	Day	48% 6% 9% 24% 11% 3	1413
March	1st to 10th	Day	52% 5% 15% 20% 6% 1	3327
	11th to 20th	Day	64% 7% 8% 13% 6% 1	3480
	21st to end	Day	57% 4 9% 20% 8% 1	3834
April	1st to 10th	Day	54% 5% 9% 18% 13% 1	2280
	11th to 20th	Day	48% 6% 10% 22% 13% 1	2413
	21st to end	Day	51% 6% 13% 15% 13% 2	2470
May	1st to 10th	Day	54% 8% 11% 19% 7% 1	4405
	11th to 20th	Day	44% 8% 14% 25% 8% 1	4402
	21st to end	Day	52% 9% 11% 20% 8% 1	5018
June	1st to 10th	Day	56% 6% 13% 17% 8%	2850
	11th to 20th	Day	46% 6% 13% 25% 9% 1	2850
	21st to end	Day	57% 6% 11% 18% 7%	2850
July	1st to 10th	Day	44% 5% 16% 23% 11%	4685
	11th to 20th	Day	43% 6% 19% 25% 7%	4679
	21st to end	Day	38% 6% 23% 23% 9%	4784
August	1st to 10th	Day	49% 5% 20% 21% 5%	4219
	11th to 20th	Day	43% 5% 20% 23% 8% 1	4084
	21st to end	Day	37% 5% 16% 29% 13% 1	4453
September	1st to 10th	Day	43% 7% 15% 24% 10%	2280
	11th to 20th	Day	42% 6% 14% 27% 11% 1	2214
	21st to end	Day	45% 6% 13% 24% 11% 1	2010
October	1st to 10th	Day	43% 6% 15% 26% 9% 2	1800
	11th to 20th	Day	50% 5% 18% 20% 7% 1	1800
	21st to end	Day	55% 5% 14% 17% 9% 1	1849
November	1st to 10th	Day	52% 5% 15% 16% 9% 2	1485
	11th to 20th	Day	57% 7% 15% 14% 7% 1	1520
	21st to end	Day	61% 5% 14% 14% 5%	1520
December	1st to 10th	Day	61% 7% 14% 13% 5%	1469
	11th to 20th	Day	62% 5% 15% 13% 5%	1668
	21st to end	Day	57% 6% 19% 12% 5% 2	2431
January	1st to 10th	Night	56% 8% 20% 13% 3	4748
	11th to 20th	Night	54% 6% 14% 15% 10%	4760
	21st to end	Night	45% 8% 18% 17% 10% 1	4797
February	1st to 10th	Night	50% 4 11% 19% 13% 3	2470
	11th to 20th	Night	55% 5% 11% 18% 10% 1	2406
	21st to end	Night	50% 6% 9% 21% 13% 1	2041
March	1st to 10th	Night	50% 8% 11% 21% 9% 1	3523
	11th to 20th	Night	66% 6% 7% 14% 6% 1	3480
	21st to end	Night	58% 4 8% 18% 11% 2	3797
April	1st to 10th	Night	57% 4 10% 14% 13% 1	1900
	11th to 20th	Night	50% 5% 8% 23% 12% 1	1715
	21st to end	Night	53% 5% 9% 16% 16% 2	1710
May	1st to 10th	Night	58% 6% 9% 17% 8% 1	2563
	11th to 20th	Night	40% 9% 13% 28% 9% 1	2507
	21st to end	Night	53% 8% 10% 20% 8% 1	2597
June	1st to 10th	Night	54% 6% 11% 20% 9%	1330
	11th to 20th	Night	45% 7% 14% 19% 13% 1	1330
	21st to end	Night	56% 6% 12% 18% 7% 1	1330
July	1st to 10th	Night	44% 5% 17% 20% 14%	2175
	11th to 20th	Night	45% 4 18% 28% 5%	2168
	21st to end	Night	37% 8% 22% 23% 10%	2509
August	1st to 10th	Night	52% 6% 15% 20% 7%	2687
	11th to 20th	Night	42% 5% 19% 26% 7% 1	2809
	21st to end	Night	35% 4 18% 30% 12%	3201
September	1st to 10th	Night	44% 6% 13% 28% 9% 1	1900
	11th to 20th	Night	40% 5% 15% 28% 12% 1	1963
	21st to end	Night	42% 5% 13% 25% 14% 1	2138
October	1st to 10th	Night	46% 6% 12% 23% 12% 2	2160
	11th to 20th	Night	50% 4 15% 23% 7%	2160
	21st to end	Night	52% 5% 14% 19% 8% 1	2505
November	1st to 10th	Night	52% 6% 14% 18% 9% 1	2593
	11th to 20th	Night	55% 6% 15% 18% 6% 1	2660
	21st to end	Night	61% 6% 16% 12% 5%	2660
December	1st to 10th	Night	61% 5% 15% 15% 4	2707
	11th to 20th	Night	65% 4 13% 13% 5% 1	3532
	21st to end	Night	59% 7% 19% 13% 3	5236

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	28% 7% 25% 29% 7% 3	180
	11th to 20th	Day	27% 12% 24% 24% 11% 3	180
	21st to end	Day	15% 7% 32% 30% 14% 2	342
February	1st to 10th	Day	18% 11% 33% 26% 9% 3	370
	11th to 20th	Day	17% 6% 33% 31% 11% 2	366
	21st to end	Day	15% 6% 29% 34% 14% 2	307
March	1st to 10th	Day	19% 6% 29% 35% 10% 2	440
	11th to 20th	Day	17% 9% 25% 28% 17% 4	460
	21st to end	Day	22% 8% 29% 28% 11% 1	506
April	1st to 10th	Day	25% 8% 28% 28% 10%	460
	11th to 20th	Day	22% 6% 30% 28% 12% 3	452
	21st to end	Day	26% 8% 19% 28% 17% 3	460
May	1st to 10th	Day	26% 6% 19% 30% 15% 4	631
	11th to 20th	Day	22% 4 20% 30% 19% 5%	674
	21st to end	Day	22% 6% 26% 34% 11% 1	735
June	1st to 10th	Day	29% 5% 22% 26% 16% 2	680
	11th to 20th	Day	21% 4 20% 32% 19% 4	680
	21st to end	Day	25% 3 19% 34% 17% 2	680
July	1st to 10th	Day	19% 2 19% 38% 19% 3	670
	11th to 20th	Day	15% 3 18% 40% 21% 3	668
	21st to end	Day	20% 3 14% 36% 21% 6%	740
August	1st to 10th	Day	14% 2 15% 35% 30% 3	562
	11th to 20th	Day	15% 3 13% 38% 26% 5%	479
	21st to end	Day	14% 3 19% 36% 22% 4	539
September	1st to 10th	Day	19% 4 24% 34% 17% 2	490
	11th to 20th	Day	22% 6% 22% 30% 17% 4	490
	21st to end	Day	21% 7% 25% 32% 12% 4	452
October	1st to 10th	Day	16% 8% 27% 32% 16% 2	360
	11th to 20th	Day	28% 5% 29% 23% 13% 3	360
	21st to end	Day	28% 8% 31% 24% 8% 2	396
November	1st to 10th	Day	24% 8% 31% 25% 11% 2	365
	11th to 20th	Day	27% 11% 26% 26% 7% 3	370
	21st to end	Day	25% 15% 30% 25% 5%	370
December	1st to 10th	Day	26% 7% 29% 30% 7% 1	319
	11th to 20th	Day	20% 14% 34% 26% 6% 1	180
	21st to end	Day	23% 13% 28% 27% 8% 1	198
January	1st to 10th	Night	25% 8% 37% 22% 8% 1	850
	11th to 20th	Night	24% 12% 33% 23% 8% 1	850
	21st to end	Night	17% 8% 35% 28% 11% 1	877
February	1st to 10th	Night	18% 8% 35% 27% 10% 2	760
	11th to 20th	Night	21% 7% 34% 28% 8% 1	743
	21st to end	Night	18% 7% 29% 32% 11% 3	628
March	1st to 10th	Night	19% 7% 33% 30% 10% 1	760
	11th to 20th	Night	21% 9% 30% 29% 10% 2	760
	21st to end	Night	24% 9% 28% 28% 9% 1	829
April	1st to 10th	Night	29% 5% 28% 26% 11% 1	570
	11th to 20th	Night	24% 7% 26% 26% 15% 2	564
	21st to end	Night	28% 6% 20% 29% 13% 3	570
May	1st to 10th	Night	28% 4 18% 29% 20% 2	570
	11th to 20th	Night	23% 3 18% 33% 17% 6%	562
	21st to end	Night	23% 4 23% 36% 12% 2	528
June	1st to 10th	Night	31% 3 17% 28% 19% 2	380
	11th to 20th	Night	23% 3 14% 35% 22% 3	380
	21st to end	Night	26% 3 12% 32% 23% 3	380
July	1st to 10th	Night	18% 2 13% 37% 26% 4	370
	11th to 20th	Night	18% 2 14% 39% 25% 2	368
	21st to end	Night	20% 2 13% 36% 26% 2	530
August	1st to 10th	Night	19% 2 10% 37% 29% 4	568
	11th to 20th	Night	20% 1 13% 30% 32% 4	562
	21st to end	Night	16% 4 16% 31% 26% 6%	627
September	1st to 10th	Night	22% 5% 24% 30% 16% 3	570
	11th to 20th	Night	20% 6% 20% 34% 17% 2	633
	21st to end	Night	22% 6% 25% 31% 14% 2	756
October	1st to 10th	Night	22% 5% 29% 30% 12% 2	720
	11th to 20th	Night	26% 7% 28% 25% 12% 2	720
	21st to end	Night	28% 7% 30% 24% 9%	823
November	1st to 10th	Night	27% 11% 30% 23% 8% 1	830
	11th to 20th	Night	27% 12% 31% 20% 8% 2	850
	21st to end	Night	28% 12% 32% 22% 6%	850
December	1st to 10th	Night	27% 12% 34% 21% 6% 1	850
	11th to 20th	Night	27% 17% 35% 16% 4 1	850
	21st to end	Night	28% 13% 30% 21% 6% 1	935

Page generated at 2024-09-26 09:52:25. Version ac07

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