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 2026-07-01.

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	17% 10% 3 8% 5% 5% 5% 4 5% 7% 33%	138194
	Night	17% 11% 4 8% 5% 4 4 3 5% 6% 32%	134437
Transparency	Day	52% 4 14% 21% 9% 1	141663
	Night	53% 4 12% 20% 10% 1	139061
Seeing	Day	21% 6% 23% 31% 16% 3	18920
	Night	23% 7% 26% 28% 13% 2	26351

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% 17% 5% 11% 5% 5% 4 4 5% 7% 23%	4093
	11th to 20th	Day	14% 15% 7% 13% 5% 4 4 3 5% 7% 24%	4084
	21st to end	Day	4 9% 5% 10% 5% 6% 6% 4 6% 8% 37%	5329
February	1st to 10th	Day	10% 13% 4 9% 6% 5% 4 5% 5% 7% 32%	1890
	11th to 20th	Day	11% 15% 5% 10% 6% 5% 5% 4 4 6% 29%	1843
	21st to end	Day	10% 10% 5% 8% 6% 4 5% 4 6% 7% 35%	1557
March	1st to 10th	Day	16% 14% 5% 10% 5% 5% 4 3 4 6% 28%	4888
	11th to 20th	Day	16% 16% 4 10% 6% 5% 5% 3 5% 6% 24%	5090
	21st to end	Day	22% 15% 4 9% 4 4 4 2 4 5% 27%	5619
April	1st to 10th	Day	20% 12% 4 7% 4 5% 4 4 4 7% 29%	2520
	11th to 20th	Day	17% 10% 3 7% 4 4 4 4 6% 6% 35%	2670
	21st to end	Day	23% 9% 3 6% 4 4 4 3 5% 6% 33%	2730
May	1st to 10th	Day	26% 11% 4 8% 5% 4 4 3 5% 5% 25%	8173
	11th to 20th	Day	17% 9% 3 8% 5% 5% 4 4 5% 7% 32%	8220
	21st to end	Day	22% 8% 3 8% 5% 4 4 4 4 6% 32%	9395
June	1st to 10th	Day	22% 9% 2 8% 5% 4 5% 4 5% 6% 29%	3150
	11th to 20th	Day	18% 8% 3 7% 5% 4 4 5% 5% 8% 34%	3150
	21st to end	Day	24% 9% 3 7% 6% 5% 5% 3 6% 6% 26%	3150
July	1st to 10th	Day	13% 6% 3 7% 5% 5% 5% 5% 8% 9% 34%	6820
	11th to 20th	Day	11% 6% 3 7% 5% 5% 5% 5% 7% 9% 38%	6820
	21st to end	Day	13% 5% 2 6% 4 4 5% 4 5% 8% 44%	7109
August	1st to 10th	Day	13% 7% 2 7% 5% 6% 6% 5% 7% 9% 32%	4367
	11th to 20th	Day	15% 6% 2 7% 5% 4 5% 4 7% 7% 40%	4175
	21st to end	Day	13% 4 2 6% 5% 4 4 3 5% 7% 46%	4594
September	1st to 10th	Day	17% 6% 2 6% 4 4 5% 4 5% 7% 40%	2400
	11th to 20th	Day	16% 7% 3 6% 4 4 4 3 4 5% 45%	2330
	21st to end	Day	17% 7% 3 6% 5% 4 5% 3 4 5% 42%	2135
October	1st to 10th	Day	15% 7% 2 6% 5% 5% 5% 4 6% 5% 41%	2000
	11th to 20th	Day	21% 9% 3 6% 4 4 4 3 5% 7% 34%	2000
	21st to end	Day	22% 9% 2 7% 5% 4 4 3 5% 7% 31%	2053
November	1st to 10th	Day	20% 9% 3 7% 4 5% 5% 3 5% 5% 33%	1645
	11th to 20th	Day	20% 12% 4 7% 6% 5% 5% 4 5% 5% 26%	1680
	21st to end	Day	18% 16% 4 10% 5% 4 3 4 5% 6% 25%	1680
December	1st to 10th	Day	14% 15% 5% 9% 5% 4 4 3 4 4 33%	2594
	11th to 20th	Day	18% 13% 4 9% 5% 4 3 3 4 7% 30%	2688
	21st to end	Day	16% 12% 5% 11% 5% 4 5% 3 4 5% 29%	3553
January	1st to 10th	Night	14% 18% 6% 11% 6% 5% 5% 4 5% 6% 21%	8831
	11th to 20th	Night	15% 13% 6% 13% 5% 4 4 3 4 5% 27%	8831
	21st to end	Night	5% 10% 4 9% 6% 6% 4 4 5% 6% 41%	8860
February	1st to 10th	Night	10% 13% 4 8% 6% 5% 4 3 5% 6% 35%	2730
	11th to 20th	Night	13% 16% 4 10% 5% 5% 4 3 5% 5% 31%	2663
	21st to end	Night	11% 14% 5% 8% 5% 4 4 4 5% 6% 34%	2249
March	1st to 10th	Night	16% 12% 4 10% 6% 5% 5% 4 5% 6% 27%	5169
	11th to 20th	Night	20% 16% 4 11% 5% 4 5% 3 4 4 22%	5089
	21st to end	Night	24% 13% 4 9% 4 4 4 3 3 6% 25%	5554
April	1st to 10th	Night	23% 11% 4 8% 5% 4 4 2 4 6% 29%	2100
	11th to 20th	Night	21% 9% 2 7% 5% 4 4 3 5% 6% 34%	1897
	21st to end	Night	23% 11% 3 6% 5% 4 4 3 4 5% 34%	1890
May	1st to 10th	Night	23% 11% 3 8% 4 4 4 3 5% 5% 30%	4751
	11th to 20th	Night	20% 7% 2 7% 4 4 4 3 5% 6% 37%	4670
	21st to end	Night	19% 9% 4 9% 4 4 4 3 5% 7% 32%	4854
June	1st to 10th	Night	26% 8% 2 6% 4 4 4 4 5% 7% 30%	1470
	11th to 20th	Night	21% 8% 2 6% 4 4 4 3 5% 7% 37%	1470
	21st to end	Night	24% 8% 2 7% 5% 5% 5% 3 6% 6% 28%	1470
July	1st to 10th	Night	16% 8% 3 7% 4 4 5% 4 6% 9% 33%	3160
	11th to 20th	Night	12% 6% 3 7% 5% 5% 5% 4 7% 8% 38%	3160
	21st to end	Night	11% 6% 2 6% 4 4 5% 4 5% 7% 45%	3720
August	1st to 10th	Night	15% 8% 3 8% 6% 5% 5% 4 5% 6% 34%	2784
	11th to 20th	Night	13% 6% 3 6% 5% 4 4 4 5% 8% 43%	2872
	21st to end	Night	13% 5% 2 4 4 3 4 3 5% 8% 47%	3303
September	1st to 10th	Night	19% 6% 2 6% 4 3 4 3 4 5% 44%	2000
	11th to 20th	Night	17% 6% 2 5% 3 3 3 3 4 6% 47%	2067
	21st to end	Night	17% 7% 2 5% 3 3 3 4 4 6% 44%	2265
October	1st to 10th	Night	17% 7% 2 7% 4 4 4 4 5% 5% 41%	2400
	11th to 20th	Night	20% 10% 3 7% 3 3 4 3 4 6% 38%	2400
	21st to end	Night	22% 10% 3 7% 4 4 4 3 4 6% 34%	2785
November	1st to 10th	Night	21% 10% 2 6% 5% 4 4 3 5% 7% 33%	2873
	11th to 20th	Night	23% 11% 4 7% 4 3 4 2 5% 6% 29%	2940
	21st to end	Night	22% 14% 4 7% 4 5% 3 3 5% 6% 26%	2940
December	1st to 10th	Night	15% 12% 5% 8% 5% 4 4 3 4 5% 35%	4812
	11th to 20th	Night	20% 12% 4 7% 4 3 4 3 4 6% 33%	5741
	21st to end	Night	19% 13% 5% 11% 5% 4 5% 3 5% 7% 23%	7667

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	62% 3 15% 16% 3	4094
	11th to 20th	Day	62% 4 14% 11% 8%	4110
	21st to end	Day	47% 4 12% 21% 16%	5329
February	1st to 10th	Day	51% 5% 12% 19% 10% 2	1890
	11th to 20th	Day	56% 5% 12% 17% 9% 1	1851
	21st to end	Day	48% 5% 9% 24% 11% 2	1557
March	1st to 10th	Day	58% 4 12% 16% 9% 1	4892
	11th to 20th	Day	62% 6% 9% 13% 7% 3	5090
	21st to end	Day	62% 3 7% 20% 8% 1	5619
April	1st to 10th	Day	55% 4 9% 17% 13% 1	2520
	11th to 20th	Day	49% 5% 10% 22% 13% 1	2670
	21st to end	Day	51% 5% 13% 16% 13% 2	2730
May	1st to 10th	Day	60% 5% 10% 18% 7%	8186
	11th to 20th	Day	51% 4 13% 23% 8% 1	8224
	21st to end	Day	53% 5% 10% 21% 11% 1	9396
June	1st to 10th	Day	54% 5% 13% 18% 9%	3150
	11th to 20th	Day	48% 6% 13% 23% 9% 1	3150
	21st to end	Day	57% 6% 11% 18% 7%	3150
July	1st to 10th	Day	46% 4 17% 23% 10%	6745
	11th to 20th	Day	43% 5% 21% 25% 6%	6739
	21st to end	Day	39% 4 22% 26% 9%	6907
August	1st to 10th	Day	48% 5% 20% 22% 5%	4353
	11th to 20th	Day	42% 5% 20% 24% 8% 1	4214
	21st to end	Day	37% 5% 16% 29% 13% 1	4594
September	1st to 10th	Day	42% 7% 15% 26% 10%	2400
	11th to 20th	Day	42% 6% 15% 27% 10% 1	2330
	21st to end	Day	44% 5% 13% 26% 12% 1	2135
October	1st to 10th	Day	47% 3 14% 27% 7% 1	3260
	11th to 20th	Day	52% 3 14% 24% 7% 1	3240
	21st to end	Day	49% 3 16% 20% 11%	3298
November	1st to 10th	Day	54% 5% 15% 17% 9% 1	1645
	11th to 20th	Day	58% 6% 14% 14% 7% 1	1680
	21st to end	Day	62% 4 14% 15% 6%	1680
December	1st to 10th	Day	57% 4 10% 21% 9%	2594
	11th to 20th	Day	57% 3 14% 15% 10% 1	2688
	21st to end	Day	59% 4 15% 16% 4 1	3553
January	1st to 10th	Night	64% 4 14% 15% 3	8831
	11th to 20th	Night	62% 4 12% 14% 8%	8880
	21st to end	Night	45% 4 14% 19% 18% 1	8860
February	1st to 10th	Night	51% 4 11% 19% 13% 3	2730
	11th to 20th	Night	55% 4 11% 19% 10% 1	2666
	21st to end	Night	49% 6% 9% 21% 13% 1	2249
March	1st to 10th	Night	57% 6% 8% 17% 11% 1	5178
	11th to 20th	Night	66% 4 8% 13% 8% 1	5090
	21st to end	Night	63% 3 6% 16% 10% 2	5554
April	1st to 10th	Night	57% 4 9% 14% 14% 2	2100
	11th to 20th	Night	51% 5% 8% 23% 12% 1	1897
	21st to end	Night	53% 5% 9% 16% 16% 2	1890
May	1st to 10th	Night	57% 4 9% 19% 9% 2	4756
	11th to 20th	Night	48% 5% 9% 25% 11% 1	4671
	21st to end	Night	53% 5% 9% 19% 14% 1	4855
June	1st to 10th	Night	54% 5% 11% 20% 10%	1470
	11th to 20th	Night	47% 6% 14% 19% 13% 1	1470
	21st to end	Night	56% 5% 12% 17% 8% 1	1470
July	1st to 10th	Night	48% 4 15% 22% 11%	3125
	11th to 20th	Night	43% 4 20% 29% 4	3118
	21st to end	Night	40% 6% 21% 24% 10%	3608
August	1st to 10th	Night	51% 6% 15% 21% 7%	2773
	11th to 20th	Night	41% 5% 19% 27% 8% 1	2899
	21st to end	Night	36% 4 18% 30% 12%	3302
September	1st to 10th	Night	43% 5% 14% 29% 9% 1	2000
	11th to 20th	Night	40% 5% 16% 27% 12% 1	2067
	21st to end	Night	41% 5% 12% 26% 15% 1	2265
October	1st to 10th	Night	49% 3 12% 26% 8% 1	3920
	11th to 20th	Night	44% 2 11% 30% 12%	3899
	21st to end	Night	52% 4 13% 21% 9% 1	4494
November	1st to 10th	Night	52% 5% 14% 19% 9% 1	2873
	11th to 20th	Night	56% 5% 14% 18% 6% 1	2940
	21st to end	Night	61% 5% 15% 13% 5%	2940
December	1st to 10th	Night	55% 3 11% 22% 8%	4812
	11th to 20th	Night	57% 3 11% 18% 9% 1	5742
	21st to end	Night	63% 5% 16% 14% 2	7667

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	30% 8% 22% 27% 10% 2	220
	11th to 20th	Day	27% 11% 24% 22% 13% 4	220
	21st to end	Day	13% 6% 33% 32% 13% 2	402
February	1st to 10th	Day	16% 9% 33% 28% 10% 4	430
	11th to 20th	Day	18% 5% 31% 31% 12% 3	426
	21st to end	Day	14% 6% 29% 34% 14% 3	355
March	1st to 10th	Day	19% 6% 30% 33% 9% 2	516
	11th to 20th	Day	19% 8% 28% 27% 15% 4	540
	21st to end	Day	22% 7% 30% 29% 11% 2	594
April	1st to 10th	Day	25% 7% 28% 30% 9%	540
	11th to 20th	Day	23% 6% 27% 31% 11% 2	532
	21st to end	Day	26% 7% 18% 27% 19% 3	540
May	1st to 10th	Day	28% 6% 19% 31% 14% 4	729
	11th to 20th	Day	23% 3 20% 30% 19% 5%	774
	21st to end	Day	23% 5% 24% 33% 13% 2	845
June	1st to 10th	Day	26% 5% 20% 29% 17% 2	780
	11th to 20th	Day	22% 4 20% 31% 20% 4	780
	21st to end	Day	25% 3 18% 34% 17% 2	780
July	1st to 10th	Day	18% 2 18% 39% 21% 3	720
	11th to 20th	Day	15% 3 18% 41% 21% 3	718
	21st to end	Day	20% 3 14% 37% 21% 5%	795
August	1st to 10th	Day	13% 2 15% 35% 31% 4	606
	11th to 20th	Day	15% 3 13% 37% 26% 6%	519
	21st to end	Day	14% 3 19% 37% 23% 5%	583
September	1st to 10th	Day	19% 3 23% 34% 18% 3	530
	11th to 20th	Day	21% 6% 21% 30% 18% 4	530
	21st to end	Day	19% 6% 25% 34% 12% 4	494
October	1st to 10th	Day	18% 7% 24% 33% 16% 2	420
	11th to 20th	Day	27% 5% 28% 22% 15% 4	420
	21st to end	Day	27% 7% 31% 24% 10% 2	462
November	1st to 10th	Day	25% 8% 30% 24% 11% 1	425
	11th to 20th	Day	28% 10% 25% 26% 8% 3	430
	21st to end	Day	27% 13% 28% 25% 5% 1	430
December	1st to 10th	Day	24% 8% 31% 29% 8% 1	373
	11th to 20th	Day	20% 12% 35% 25% 6% 1	220
	21st to end	Day	22% 12% 26% 27% 11% 2	242
January	1st to 10th	Night	26% 8% 35% 22% 8% 1	950
	11th to 20th	Night	24% 11% 32% 23% 9% 1	950
	21st to end	Night	16% 8% 36% 29% 10% 1	973
February	1st to 10th	Night	17% 7% 35% 29% 10% 2	840
	11th to 20th	Night	22% 7% 33% 28% 9% 1	823
	21st to end	Night	17% 6% 29% 33% 12% 3	692
March	1st to 10th	Night	20% 8% 32% 29% 10% 1	840
	11th to 20th	Night	22% 8% 30% 29% 9% 2	840
	21st to end	Night	23% 8% 29% 29% 9% 2	916
April	1st to 10th	Night	28% 5% 28% 27% 10% 1	630
	11th to 20th	Night	26% 6% 25% 28% 14% 2	624
	21st to end	Night	28% 6% 20% 29% 14% 3	630
May	1st to 10th	Night	28% 4 17% 30% 19% 2	630
	11th to 20th	Night	23% 3 17% 33% 18% 6%	622
	21st to end	Night	24% 4 22% 35% 13% 2	584
June	1st to 10th	Night	29% 3 17% 29% 20% 2	420
	11th to 20th	Night	25% 2 13% 34% 22% 4	420
	21st to end	Night	26% 3 12% 32% 22% 5%	420
July	1st to 10th	Night	18% 2 13% 38% 26% 4	390
	11th to 20th	Night	18% 2 14% 39% 25% 2	388
	21st to end	Night	20% 2 13% 38% 25% 2	558
August	1st to 10th	Night	18% 2 10% 36% 29% 5%	598
	11th to 20th	Night	19% 1 13% 30% 33% 5%	592
	21st to end	Night	16% 3 16% 31% 27% 6%	660
September	1st to 10th	Night	22% 5% 24% 29% 17% 3	600
	11th to 20th	Night	20% 6% 20% 34% 18% 2	667
	21st to end	Night	21% 6% 25% 32% 15% 2	800
October	1st to 10th	Night	21% 5% 28% 31% 13% 2	800
	11th to 20th	Night	25% 7% 27% 26% 13% 2	800
	21st to end	Night	28% 7% 30% 25% 11%	919
November	1st to 10th	Night	26% 11% 30% 24% 8% 1	930
	11th to 20th	Night	29% 11% 30% 20% 7% 2	950
	21st to end	Night	28% 11% 32% 23% 6% 1	950
December	1st to 10th	Night	26% 11% 35% 21% 6% 1	950
	11th to 20th	Night	26% 17% 35% 17% 4 1	950
	21st to end	Night	28% 13% 29% 22% 7% 2	1045

Page generated at 2026-06-28 02:10:43. Version ac07

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