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 2025-02-25.

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% 10% 3 8% 5% 5% 5% 4 5% 7% 33%	108398
	Night	17% 11% 4 8% 5% 4 4 3 5% 6% 33%	110439
Transparency	Day	50% 5% 14% 21% 9% 1	111913
	Night	52% 5% 13% 20% 9% 1	115092
Seeing	Day	21% 6% 24% 31% 15% 3	17010
	Night	24% 7% 27% 28% 13% 2	24657

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	14% 17% 6% 11% 5% 6% 4 4 5% 7% 21%	3157
	11th to 20th	Day	10% 14% 7% 14% 4 4 4 3 5% 7% 25%	3134
	21st to end	Day	5% 6% 3 9% 6% 7% 5% 4 6% 9% 40%	4108
February	1st to 10th	Day	10% 12% 4 9% 6% 5% 4 5% 5% 7% 33%	1800
	11th to 20th	Day	11% 15% 5% 9% 6% 5% 5% 4 4 6% 29%	1753
	21st to end	Day	10% 10% 5% 8% 5% 5% 5% 4 6% 7% 35%	1449
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% 4 4 5% 41%	2028
October	1st to 10th	Day	15% 7% 2 6% 5% 5% 5% 4 6% 5% 40%	1900
	11th to 20th	Day	21% 8% 3 6% 5% 4 4 3 5% 7% 35%	1900
	21st to end	Day	22% 9% 2 7% 5% 4 4 3 5% 7% 31%	1951
November	1st to 10th	Day	19% 9% 3 7% 4 5% 5% 3 5% 5% 33%	1565
	11th to 20th	Day	21% 12% 4 7% 6% 5% 6% 4 5% 6% 26%	1600
	21st to end	Day	19% 16% 4 10% 5% 3 3 4 5% 6% 25%	1600
December	1st to 10th	Day	15% 15% 5% 9% 5% 4 4 3 4 4 33%	2517
	11th to 20th	Day	18% 13% 4 9% 5% 4 3 3 4 7% 29%	2618
	21st to end	Day	17% 13% 5% 11% 5% 4 5% 3 4 5% 29%	3476
January	1st to 10th	Night	14% 17% 6% 12% 6% 5% 5% 4 5% 6% 18%	6808
	11th to 20th	Night	14% 13% 6% 15% 5% 4 4 3 4 5% 25%	6771
	21st to end	Night	6% 9% 3 8% 6% 6% 4 4 5% 6% 44%	6853
February	1st to 10th	Night	10% 14% 4 7% 6% 4 4 3 5% 7% 35%	2600
	11th to 20th	Night	13% 15% 4 10% 5% 5% 4 3 5% 5% 31%	2533
	21st to end	Night	11% 13% 5% 7% 5% 4 4 4 5% 6% 34%	2100
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% 4 3 3 4 4 6% 43%	2152
October	1st to 10th	Night	18% 7% 2 7% 4 4 4 4 5% 5% 40%	2280
	11th to 20th	Night	21% 9% 3 7% 3 3 4 3 4 6% 37%	2280
	21st to end	Night	22% 9% 3 7% 4 4 4 3 4 7% 34%	2645
November	1st to 10th	Night	22% 10% 2 5% 5% 4 4 3 5% 7% 34%	2733
	11th to 20th	Night	23% 11% 4 8% 4 3 4 3 5% 6% 30%	2800
	21st to end	Night	23% 14% 4 8% 4 5% 3 3 5% 6% 25%	2800
December	1st to 10th	Night	16% 12% 5% 8% 5% 3 4 3 4 5% 35%	4669
	11th to 20th	Night	20% 12% 4 7% 4 3 4 3 4 6% 33%	5591
	21st to end	Night	19% 13% 5% 11% 5% 4 5% 3 5% 7% 24%	7502

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	64% 3 16% 13% 3	3158
	11th to 20th	Day	60% 5% 15% 13% 7%	3160
	21st to end	Day	41% 5% 14% 23% 17% 1	4108
February	1st to 10th	Day	50% 6% 12% 20% 11% 2	1800
	11th to 20th	Day	55% 5% 12% 17% 9% 1	1761
	21st to end	Day	48% 6% 8% 25% 11% 3	1449
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% 25% 11% 1	2028
October	1st to 10th	Day	48% 3 14% 27% 7% 1	3160
	11th to 20th	Day	51% 3 14% 24% 7% 1	3140
	21st to end	Day	49% 3 16% 20% 11%	3196
November	1st to 10th	Day	53% 5% 15% 16% 9% 2	1565
	11th to 20th	Day	57% 6% 15% 14% 7% 1	1600
	21st to end	Day	62% 4 14% 15% 5%	1600
December	1st to 10th	Day	57% 4 10% 21% 8%	2517
	11th to 20th	Day	57% 3 14% 15% 10% 1	2618
	21st to end	Day	59% 4 16% 17% 4 1	3476
January	1st to 10th	Night	66% 5% 15% 11% 2	6808
	11th to 20th	Night	62% 5% 13% 13% 7%	6820
	21st to end	Night	42% 5% 14% 19% 19% 1	6853
February	1st to 10th	Night	50% 4 11% 20% 14% 3	2600
	11th to 20th	Night	55% 4 11% 18% 10% 1	2536
	21st to end	Night	49% 6% 9% 22% 12% 1	2100
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% 12% 26% 14% 1	2152
October	1st to 10th	Night	50% 3 12% 26% 8% 1	3800
	11th to 20th	Night	44% 3 11% 30% 12%	3779
	21st to end	Night	52% 4 13% 21% 10% 1	4354
November	1st to 10th	Night	52% 6% 14% 19% 9% 1	2733
	11th to 20th	Night	55% 5% 14% 18% 6% 1	2800
	21st to end	Night	62% 6% 15% 12% 5%	2800
December	1st to 10th	Night	55% 3 12% 23% 8%	4669
	11th to 20th	Night	57% 3 11% 18% 9% 1	5592
	21st to end	Night	63% 5% 16% 14% 2	7502

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	31% 9% 24% 28% 8% 3	200
	11th to 20th	Day	26% 11% 23% 23% 14% 4	200
	21st to end	Day	14% 7% 32% 32% 13% 2	372
February	1st to 10th	Day	17% 10% 34% 27% 9% 3	400
	11th to 20th	Day	18% 6% 33% 31% 12% 2	396
	21st to end	Day	15% 5% 28% 34% 15% 3	319
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	457
October	1st to 10th	Day	17% 7% 25% 32% 16% 3	390
	11th to 20th	Day	27% 5% 29% 23% 13% 3	390
	21st to end	Day	27% 7% 32% 24% 8% 1	429
November	1st to 10th	Day	24% 8% 31% 25% 10% 2	395
	11th to 20th	Day	29% 11% 26% 26% 7% 3	400
	21st to end	Day	28% 14% 29% 25% 5%	400
December	1st to 10th	Day	25% 7% 29% 29% 8% 1	346
	11th to 20th	Day	21% 14% 33% 26% 6% 2	200
	21st to end	Day	23% 11% 26% 28% 11% 1	220
January	1st to 10th	Night	26% 8% 35% 22% 8% 1	900
	11th to 20th	Night	24% 11% 32% 22% 10% 1	900
	21st to end	Night	16% 8% 36% 28% 11% 1	925
February	1st to 10th	Night	18% 8% 36% 28% 10% 2	800
	11th to 20th	Night	22% 7% 34% 28% 8% 1	783
	21st to end	Night	17% 7% 28% 32% 12% 3	647
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	760
October	1st to 10th	Night	22% 5% 28% 30% 13% 2	760
	11th to 20th	Night	26% 7% 28% 26% 12% 2	760
	21st to end	Night	28% 7% 30% 25% 9%	871
November	1st to 10th	Night	27% 11% 30% 23% 8% 1	880
	11th to 20th	Night	28% 12% 30% 20% 7% 2	900
	21st to end	Night	29% 11% 32% 22% 5%	900
December	1st to 10th	Night	27% 11% 34% 21% 6% 1	900
	11th to 20th	Night	27% 17% 35% 17% 4 1	900
	21st to end	Night	28% 12% 29% 22% 7% 2	990

Page generated at 2025-02-22 06:51:33. Version ac07

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