Perry
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Perry clear sky chart from 2005-11-06 to 2026-01-17.

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	9% 5% 2 6% 5% 5% 5% 5% 7% 9% 42%	127276
	Night	10% 6% 2 6% 4 4 4 4 6% 9% 45%	124410
Transparency	Day	35% 10% 15% 18% 15% 7%	131062
	Night	34% 8% 14% 18% 17% 8%	128680
Seeing	Day	11% 9% 23% 26% 21% 11%	16732
	Night	11% 6% 23% 32% 23% 5%	26775

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	11% 5% 2 5% 3 3 3 3 4 6% 55%	4146
	11th to 20th	Day	18% 6% 3 7% 4 3 4 3 5% 6% 40%	4114
	21st to end	Day	10% 5% 2 5% 4 4 4 4 6% 9% 48%	4880
February	1st to 10th	Day	17% 8% 2 5% 4 3 3 3 5% 6% 44%	2000
	11th to 20th	Day	15% 6% 2 5% 3 4 3 3 4 6% 49%	1948
	21st to end	Day	14% 5% 2 4 3 3 4 3 5% 7% 49%	1650
March	1st to 10th	Day	18% 6% 2 6% 3 3 4 3 6% 8% 40%	4800
	11th to 20th	Day	8% 4 1 5% 3 3 4 4 5% 8% 53%	4980
	21st to end	Day	13% 4 1 5% 3 4 4 3 5% 7% 51%	5465
April	1st to 10th	Day	8% 4 2 5% 4 5% 5% 5% 7% 8% 48%	2200
	11th to 20th	Day	11% 4 2 6% 3 3 5% 5% 6% 7% 49%	2247
	21st to end	Day	6% 4 2 5% 4 4 4 4 6% 8% 55%	2517
May	1st to 10th	Day	8% 5% 2 6% 5% 4 5% 4 7% 9% 44%	5862
	11th to 20th	Day	10% 5% 2 6% 4 4 4 4 6% 10% 45%	5861
	21st to end	Day	3 3 2 5% 4 5% 6% 5% 7% 11% 48%	6433
June	1st to 10th	Day	9% 5% 3 7% 7% 6% 7% 7% 9% 11% 31%	2600
	11th to 20th	Day	5% 4 2 6% 6% 6% 8% 7% 10% 13% 31%	2600
	21st to end	Day	8% 5% 2 7% 6% 8% 8% 7% 9% 10% 30%	2600
July	1st to 10th	Day	6% 6% 3 10% 6% 8% 9% 8% 10% 15% 20%	5900
	11th to 20th	Day	4 3 2 5% 5% 6% 7% 7% 10% 15% 35%	5900
	21st to end	Day	4 4 2 8% 6% 7% 8% 7% 10% 15% 28%	6424
August	1st to 10th	Day	10% 5% 2 8% 6% 7% 8% 6% 9% 12% 26%	4136
	11th to 20th	Day	8% 4 2 6% 5% 5% 6% 6% 8% 13% 37%	3849
	21st to end	Day	8% 6% 3 9% 7% 7% 7% 6% 8% 11% 28%	4152
September	1st to 10th	Day	9% 9% 3 9% 7% 6% 8% 6% 8% 10% 26%	2200
	11th to 20th	Day	9% 7% 3 7% 7% 7% 7% 6% 8% 10% 30%	2200
	21st to end	Day	9% 6% 2 6% 4 5% 5% 5% 7% 7% 42%	2200
October	1st to 10th	Day	9% 5% 3 7% 5% 5% 5% 5% 5% 7% 44%	2200
	11th to 20th	Day	6% 4 1 3 3 4 4 3 4 6% 61%	2099
	21st to end	Day	8% 4 2 4 3 5% 5% 3 5% 7% 54%	2200
November	1st to 10th	Day	9% 4 2 5% 3 4 4 3 5% 7% 55%	2057
	11th to 20th	Day	10% 5% 2 5% 4 2 3 3 4 6% 56%	1918
	21st to end	Day	9% 6% 2 5% 4 4 4 4 6% 7% 50%	1890
December	1st to 10th	Day	8% 5% 2 5% 4 4 4 4 5% 6% 53%	3020
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 5% 6% 47%	3463
	21st to end	Day	17% 7% 2 7% 4 4 4 3 4 6% 43%	4565
January	1st to 10th	Night	11% 6% 2 4 3 3 3 3 5% 6% 55%	6019
	11th to 20th	Night	16% 7% 3 7% 5% 5% 5% 4 5% 7% 36%	5979
	21st to end	Night	16% 7% 2 5% 4 3 3 3 5% 9% 42%	6059
February	1st to 10th	Night	15% 8% 3 6% 4 3 3 3 5% 7% 44%	2400
	11th to 20th	Night	16% 7% 2 5% 3 3 3 3 5% 6% 47%	2338
	21st to end	Night	14% 8% 3 6% 3 3 3 2 4 6% 48%	1980
March	1st to 10th	Night	17% 9% 3 6% 4 4 4 3 5% 6% 40%	5051
	11th to 20th	Night	9% 4 2 4 3 3 4 3 6% 9% 53%	4980
	21st to end	Night	13% 5% 2 5% 4 4 4 3 4 6% 50%	5467
April	1st to 10th	Night	11% 6% 3 5% 4 4 4 4 5% 6% 49%	2200
	11th to 20th	Night	7% 5% 2 5% 4 4 4 4 4 6% 53%	2116
	21st to end	Night	5% 3 1 4 3 4 4 4 7% 8% 57%	1880
May	1st to 10th	Night	6% 5% 2 5% 3 4 4 3 6% 10% 50%	4033
	11th to 20th	Night	12% 5% 2 5% 4 4 4 3 5% 10% 47%	4032
	21st to end	Night	3 3 1 3 3 3 4 3 6% 12% 57%	4421
June	1st to 10th	Night	7% 6% 2 7% 4 5% 5% 5% 9% 12% 38%	1800
	11th to 20th	Night	4 4 2 5% 6% 4 7% 7% 10% 12% 39%	1800
	21st to end	Night	7% 6% 1 6% 5% 6% 5% 5% 8% 12% 40%	1800
July	1st to 10th	Night	6% 6% 3 10% 7% 8% 8% 7% 10% 13% 22%	4058
	11th to 20th	Night	3 3 1 5% 4 4 5% 5% 9% 16% 44%	4057
	21st to end	Night	4 5% 2 8% 6% 6% 6% 6% 9% 15% 34%	4427
August	1st to 10th	Night	8% 5% 2 7% 5% 6% 5% 5% 10% 13% 33%	3007
	11th to 20th	Night	4 3 2 5% 4 5% 5% 6% 9% 13% 45%	3200
	21st to end	Night	9% 6% 3 10% 5% 6% 6% 7% 7% 11% 30%	3711
September	1st to 10th	Night	9% 6% 3 10% 7% 6% 7% 6% 8% 11% 28%	2200
	11th to 20th	Night	9% 6% 3 7% 6% 6% 6% 6% 8% 10% 33%	2200
	21st to end	Night	9% 5% 2 6% 5% 5% 4 4 7% 9% 45%	2200
October	1st to 10th	Night	8% 6% 2 8% 4 4 4 4 5% 7% 47%	2200
	11th to 20th	Night	7% 4 1 5% 3 3 4 3 3 6% 61%	2300
	21st to end	Night	8% 4 2 5% 4 3 4 4 5% 7% 54%	2640
November	1st to 10th	Night	8% 5% 2 5% 5% 4 4 3 6% 6% 52%	2461
	11th to 20th	Night	11% 5% 1 5% 4 4 4 3 5% 5% 53%	2699
	21st to end	Night	10% 7% 2 6% 4 3 4 3 5% 6% 50%	2730
December	1st to 10th	Night	11% 5% 2 5% 3 4 4 3 4 6% 52%	4380
	11th to 20th	Night	12% 7% 2 5% 4 4 3 3 4 7% 49%	4952
	21st to end	Night	15% 7% 3 7% 4 4 4 3 5% 6% 43%	6633

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	30% 5% 7% 17% 28% 14%	4147
	11th to 20th	Day	43% 5% 7% 12% 20% 13%	4114
	21st to end	Day	32% 7% 12% 19% 20% 10%	4880
February	1st to 10th	Day	40% 5% 10% 14% 19% 12%	2000
	11th to 20th	Day	37% 5% 8% 15% 23% 12%	1958
	21st to end	Day	36% 5% 9% 17% 20% 13%	1650
March	1st to 10th	Day	40% 7% 10% 14% 20% 9%	4800
	11th to 20th	Day	27% 8% 11% 15% 21% 18%	4980
	21st to end	Day	32% 7% 10% 23% 17% 12%	5465
April	1st to 10th	Day	31% 9% 12% 17% 18% 12%	2200
	11th to 20th	Day	32% 7% 10% 19% 21% 11%	2247
	21st to end	Day	25% 7% 13% 24% 21% 9%	2517
May	1st to 10th	Day	32% 11% 18% 22% 14% 4	5897
	11th to 20th	Day	33% 9% 15% 30% 12% 2	5865
	21st to end	Day	26% 9% 20% 26% 17% 2	6447
June	1st to 10th	Day	42% 14% 19% 16% 8% 1	2600
	11th to 20th	Day	37% 16% 22% 17% 7% 1	2600
	21st to end	Day	42% 16% 20% 15% 7% 1	2600
July	1st to 10th	Day	47% 18% 28% 6% 1	5835
	11th to 20th	Day	31% 17% 36% 15% 2	5831
	21st to end	Day	39% 17% 23% 20% 1	6240
August	1st to 10th	Day	44% 14% 23% 14% 5%	4123
	11th to 20th	Day	34% 14% 22% 20% 9%	3885
	21st to end	Day	45% 13% 19% 19% 4	4152
September	1st to 10th	Day	49% 14% 16% 13% 7% 2	2200
	11th to 20th	Day	46% 12% 17% 14% 10% 2	2200
	21st to end	Day	37% 10% 14% 21% 14% 5%	2200
October	1st to 10th	Day	46% 7% 8% 11% 16% 11%	3590
	11th to 20th	Day	15% 3 6% 17% 34% 25%	3391
	21st to end	Day	20% 5% 6% 18% 37% 14%	3538
November	1st to 10th	Day	28% 6% 11% 18% 22% 16%	2057
	11th to 20th	Day	29% 5% 8% 17% 27% 14%	1918
	21st to end	Day	32% 6% 9% 19% 23% 10%	1890
December	1st to 10th	Day	32% 4 9% 16% 25% 13%	3020
	11th to 20th	Day	36% 6% 7% 20% 22% 9%	3460
	21st to end	Day	43% 5% 7% 18% 24% 4	4565
January	1st to 10th	Night	29% 4 7% 15% 30% 15%	6019
	11th to 20th	Night	46% 5% 6% 15% 20% 8%	5979
	21st to end	Night	39% 6% 8% 18% 20% 10%	6059
February	1st to 10th	Night	39% 7% 8% 14% 21% 12%	2400
	11th to 20th	Night	38% 5% 9% 15% 19% 14%	2339
	21st to end	Night	37% 6% 8% 17% 19% 13%	1980
March	1st to 10th	Night	44% 7% 8% 14% 17% 10%	5051
	11th to 20th	Night	25% 7% 10% 21% 20% 18%	4980
	21st to end	Night	34% 5% 9% 20% 19% 12%	5467
April	1st to 10th	Night	34% 8% 11% 18% 17% 12%	2200
	11th to 20th	Night	30% 6% 9% 21% 22% 12%	2116
	21st to end	Night	22% 7% 13% 27% 22% 8%	1880
May	1st to 10th	Night	26% 11% 20% 28% 10% 5%	4060
	11th to 20th	Night	31% 9% 18% 28% 13% 2	4033
	21st to end	Night	18% 8% 22% 35% 16% 1	4429
June	1st to 10th	Night	34% 13% 23% 22% 8%	1800
	11th to 20th	Night	29% 17% 27% 20% 7%	1800
	21st to end	Night	33% 14% 25% 20% 6% 1	1800
July	1st to 10th	Night	50% 17% 25% 7% 1	4015
	11th to 20th	Night	26% 14% 44% 14% 2	4006
	21st to end	Night	36% 13% 31% 18% 1	4297
August	1st to 10th	Night	37% 15% 28% 15% 4	2996
	11th to 20th	Night	26% 14% 30% 23% 6%	3230
	21st to end	Night	45% 13% 23% 15% 4	3711
September	1st to 10th	Night	46% 14% 19% 14% 6% 1	2200
	11th to 20th	Night	41% 15% 17% 17% 9% 1	2200
	21st to end	Night	34% 9% 17% 20% 15% 5%	2200
October	1st to 10th	Night	46% 8% 9% 12% 14% 10%	3590
	11th to 20th	Night	17% 4 5% 19% 32% 22%	3734
	21st to end	Night	20% 5% 5% 19% 37% 15%	4252
November	1st to 10th	Night	31% 5% 9% 21% 19% 15%	2461
	11th to 20th	Night	32% 5% 9% 16% 24% 15%	2699
	21st to end	Night	34% 5% 11% 18% 20% 12%	2730
December	1st to 10th	Night	34% 4 8% 16% 21% 17%	4380
	11th to 20th	Night	36% 7% 8% 19% 21% 10%	4954
	21st to end	Night	41% 5% 8% 18% 22% 5%	6633

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	10% 19% 50% 15% 5% 1	330
	11th to 20th	Day	14% 18% 39% 21% 7% 2	318
	21st to end	Day	18% 23% 42% 15% 2	330
February	1st to 10th	Day	17% 18% 37% 19% 9%	300
	11th to 20th	Day	21% 19% 36% 19% 4	300
	21st to end	Day	14% 19% 42% 19% 6%	249
March	1st to 10th	Day	15% 20% 33% 24% 8%	438
	11th to 20th	Day	14% 18% 33% 23% 11% 1	500
	21st to end	Day	14% 16% 32% 27% 11% 1	550
April	1st to 10th	Day	13% 14% 34% 24% 14% 1	500
	11th to 20th	Day	12% 14% 29% 29% 14% 2	492
	21st to end	Day	5% 8% 31% 32% 19% 6%	500
May	1st to 10th	Day	6% 8% 28% 32% 20% 6%	503
	11th to 20th	Day	7% 6% 24% 32% 21% 10%	527
	21st to end	Day	8% 4 16% 30% 26% 16%	573
June	1st to 10th	Day	11% 3 14% 31% 27% 14%	530
	11th to 20th	Day	6% 4 16% 23% 30% 21%	530
	21st to end	Day	7% 2 8% 29% 29% 25%	530
July	1st to 10th	Day	7% 3 7% 21% 29% 33%	525
	11th to 20th	Day	5% 1 2 23% 36% 32%	524
	21st to end	Day	5% 2 6% 23% 37% 26%	576
August	1st to 10th	Day	13% 3 6% 18% 31% 28%	526
	11th to 20th	Day	10% 2 3 18% 39% 28%	519
	21st to end	Day	9% 1 10% 27% 32% 21%	583
September	1st to 10th	Day	13% 4 13% 22% 30% 20%	530
	11th to 20th	Day	11% 3 14% 32% 28% 13%	530
	21st to end	Day	10% 3 15% 24% 34% 14%	530
October	1st to 10th	Day	11% 7% 25% 33% 20% 4	530
	11th to 20th	Day	5% 7% 22% 32% 25% 9%	530
	21st to end	Day	10% 7% 27% 37% 17% 2	583
November	1st to 10th	Day	12% 6% 27% 33% 18% 3	535
	11th to 20th	Day	12% 9% 39% 27% 9% 3	358
	21st to end	Day	8% 17% 38% 28% 8% 1	330
December	1st to 10th	Day	14% 17% 34% 23% 9% 3	330
	11th to 20th	Day	16% 14% 38% 25% 7%	330
	21st to end	Day	14% 14% 38% 22% 11% 1	363
January	1st to 10th	Night	17% 13% 36% 26% 8%	840
	11th to 20th	Night	12% 11% 31% 34% 12% 1	825
	21st to end	Night	19% 12% 37% 26% 6%	880
February	1st to 10th	Night	16% 11% 31% 33% 10%	800
	11th to 20th	Night	19% 11% 32% 31% 7%	783
	21st to end	Night	15% 12% 32% 35% 5%	660
March	1st to 10th	Night	14% 11% 33% 34% 8%	800
	11th to 20th	Night	13% 10% 31% 32% 13% 1	800
	21st to end	Night	12% 11% 33% 34% 11%	880
April	1st to 10th	Night	12% 10% 28% 36% 14%	800
	11th to 20th	Night	9% 9% 31% 35% 15% 1	734
	21st to end	Night	6% 4 27% 40% 23% 1	600
May	1st to 10th	Night	6% 4 23% 41% 23% 3	600
	11th to 20th	Night	8% 3 21% 38% 28% 3	591
	21st to end	Night	5% 1 12% 38% 39% 4	650
June	1st to 10th	Night	9% 2 12% 36% 35% 7%	600
	11th to 20th	Night	5% 2 8% 34% 36% 17%	600
	21st to end	Night	8% 2 6% 25% 46% 14%	600
July	1st to 10th	Night	6% 1 6% 19% 44% 23%	585
	11th to 20th	Night	6% 2 3 16% 47% 26%	582
	21st to end	Night	4 1 3 18% 54% 20%	657
August	1st to 10th	Night	9% 1 4 18% 49% 18%	597
	11th to 20th	Night	5% 1 5% 15% 52% 22%	594
	21st to end	Night	10% 1 9% 26% 38% 16%	733
September	1st to 10th	Night	11% 2 10% 27% 37% 14%	800
	11th to 20th	Night	10% 4 12% 33% 34% 7%	800
	21st to end	Night	11% 3 14% 31% 36% 6%	800
October	1st to 10th	Night	10% 5% 24% 35% 23% 2	800
	11th to 20th	Night	7% 3 25% 38% 26% 2	800
	21st to end	Night	10% 4 26% 39% 20% 1	880
November	1st to 10th	Night	9% 7% 28% 37% 18% 2	820
	11th to 20th	Night	13% 8% 27% 34% 16% 1	840
	21st to end	Night	11% 10% 29% 36% 13%	840
December	1st to 10th	Night	15% 8% 28% 33% 15% 1	840
	11th to 20th	Night	15% 8% 29% 33% 14% 1	840
	21st to end	Night	16% 10% 28% 32% 13%	924

Page generated at 2026-01-14 01:14:28. Version ac07

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