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 2024-01-14.

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	10% 5% 2 5% 5% 4 6% 6% 7% 9% 41%	74860
	Night	10% 5% 2 5% 5% 3 4 5% 6% 8% 46%	74902
Transparency	Day	37% 10% 14% 17% 15% 7%	74583
	Night	34% 9% 15% 18% 16% 8%	74698
Seeing	Day	11% 8% 22% 26% 22% 12%	14024
	Night	11% 6% 22% 32% 24% 5%	24105

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	13% 6% 3 7% 4 3 4 5% 5% 8% 43%	1706
	11th to 20th	Day	10% 4 2 5% 4 2 4 5% 6% 7% 49%	1647
	21st to end	Day	16% 6% 3 6% 5% 3 4 4 4 5% 45%	1944
February	1st to 10th	Day	19% 7% 3 4 4 2 3 4 5% 6% 44%	1800
	11th to 20th	Day	14% 5% 2 4 4 3 3 4 5% 6% 51%	1748
	21st to end	Day	15% 5% 3 4 4 3 4 4 4 8% 47%	1480
March	1st to 10th	Day	8% 4 2 4 4 2 4 4 6% 8% 54%	1924
	11th to 20th	Day	10% 4 2 5% 4 3 5% 5% 6% 7% 49%	1980
	21st to end	Day	10% 4 1 5% 5% 3 4 5% 5% 6% 53%	2178
April	1st to 10th	Day	9% 4 3 4 5% 4 6% 6% 6% 7% 48%	1980
	11th to 20th	Day	12% 4 2 6% 4 3 4 6% 6% 7% 46%	2019
	21st to end	Day	6% 3 2 4 4 2 4 5% 6% 9% 56%	2265
May	1st to 10th	Day	5% 3 1 3 3 2 4 5% 6% 8% 59%	2340
	11th to 20th	Day	7% 5% 2 5% 5% 4 6% 6% 8% 10% 42%	2301
	21st to end	Day	6% 5% 3 6% 6% 5% 7% 6% 6% 10% 42%	2531
June	1st to 10th	Day	10% 6% 3 6% 8% 3 7% 8% 8% 11% 30%	2340
	11th to 20th	Day	6% 3 3 5% 7% 4 7% 9% 10% 13% 32%	2340
	21st to end	Day	9% 4 3 6% 7% 6% 9% 9% 9% 11% 29%	2340
July	1st to 10th	Day	9% 5% 3 7% 6% 4 8% 8% 9% 12% 28%	2340
	11th to 20th	Day	7% 6% 4 7% 8% 6% 8% 9% 10% 11% 24%	2340
	21st to end	Day	7% 5% 3 6% 7% 5% 8% 10% 9% 13% 27%	2547
August	1st to 10th	Day	9% 6% 3 7% 8% 5% 8% 8% 9% 13% 26%	2274
	11th to 20th	Day	12% 6% 3 7% 7% 5% 7% 9% 9% 11% 24%	2125
	21st to end	Day	10% 6% 4 8% 9% 5% 7% 7% 8% 10% 25%	2291
September	1st to 10th	Day	8% 8% 3 8% 8% 5% 8% 8% 9% 11% 25%	1980
	11th to 20th	Day	8% 7% 3 7% 8% 6% 7% 8% 9% 10% 28%	1980
	21st to end	Day	10% 6% 2 5% 5% 4 6% 6% 7% 7% 42%	1980
October	1st to 10th	Day	9% 5% 3 6% 7% 3 4 5% 5% 7% 46%	1980
	11th to 20th	Day	6% 3 2 3 4 3 4 5% 4 7% 59%	1890
	21st to end	Day	8% 4 2 4 4 4 5% 5% 5% 6% 53%	1980
November	1st to 10th	Day	9% 4 2 3 4 2 4 4 6% 7% 55%	1857
	11th to 20th	Day	11% 4 3 4 4 2 3 4 4 7% 55%	1736
	21st to end	Day	10% 6% 2 4 5% 2 4 5% 6% 6% 50%	1710
December	1st to 10th	Day	12% 5% 3 5% 6% 3 4 5% 6% 7% 45%	1710
	11th to 20th	Day	16% 6% 3 5% 4 2 4 4 5% 6% 44%	2153
	21st to end	Day	19% 7% 3 6% 5% 3 4 3 4 6% 41%	3124
January	1st to 10th	Night	15% 7% 3 5% 4 3 3 4 5% 7% 44%	2459
	11th to 20th	Night	10% 5% 3 4 5% 3 4 3 5% 8% 51%	2386
	21st to end	Night	17% 8% 3 5% 5% 3 4 4 5% 6% 41%	2409
February	1st to 10th	Night	16% 6% 3 4 4 2 4 4 5% 7% 45%	2160
	11th to 20th	Night	15% 6% 3 4 4 2 4 4 4 6% 49%	2098
	21st to end	Night	14% 8% 3 6% 4 2 3 3 4 6% 46%	1776
March	1st to 10th	Night	11% 5% 2 3 4 2 4 4 6% 7% 52%	2034
	11th to 20th	Night	13% 5% 2 4 4 3 4 5% 5% 7% 48%	1980
	21st to end	Night	9% 5% 2 4 4 3 4 4 4 7% 54%	2178
April	1st to 10th	Night	11% 4 3 4 4 3 5% 4 5% 6% 49%	1980
	11th to 20th	Night	8% 5% 2 5% 5% 3 5% 5% 4 7% 51%	1902
	21st to end	Night	6% 3 2 3 4 2 3 4 7% 8% 59%	1692
May	1st to 10th	Night	5% 3 2 3 3 2 4 4 5% 10% 59%	1620
	11th to 20th	Night	6% 3 2 5% 5% 4 4 5% 5% 11% 49%	1592
	21st to end	Night	5% 4 2 4 4 3 5% 5% 7% 10% 52%	1744
June	1st to 10th	Night	8% 5% 3 6% 5% 4 5% 6% 8% 12% 38%	1620
	11th to 20th	Night	4 4 2 5% 6% 3 7% 8% 10% 13% 40%	1620
	21st to end	Night	7% 5% 2 5% 5% 5% 5% 6% 7% 12% 40%	1620
July	1st to 10th	Night	6% 5% 2 5% 6% 4 6% 7% 9% 14% 36%	1620
	11th to 20th	Night	5% 4 3 6% 6% 4 7% 8% 9% 14% 33%	1620
	21st to end	Night	3 3 2 6% 6% 4 5% 8% 9% 14% 40%	1771
August	1st to 10th	Night	5% 4 2 5% 5% 4 4 6% 10% 15% 39%	1658
	11th to 20th	Night	6% 5% 2 5% 7% 5% 6% 8% 9% 13% 35%	1776
	21st to end	Night	8% 6% 3 8% 6% 4 6% 9% 7% 11% 32%	2045
September	1st to 10th	Night	8% 5% 2 9% 8% 5% 7% 8% 8% 12% 28%	1980
	11th to 20th	Night	9% 5% 3 6% 7% 5% 7% 8% 8% 10% 32%	1980
	21st to end	Night	8% 5% 2 6% 6% 5% 4 5% 7% 9% 44%	1980
October	1st to 10th	Night	8% 5% 2 6% 5% 3 4 6% 6% 7% 49%	1980
	11th to 20th	Night	7% 4 2 4 4 2 4 4 3 7% 59%	2070
	21st to end	Night	8% 4 2 4 5% 3 4 5% 6% 7% 52%	2376
November	1st to 10th	Night	8% 5% 2 5% 5% 3 4 4 6% 6% 53%	2221
	11th to 20th	Night	11% 5% 2 4 5% 3 4 4 4 6% 52%	2441
	21st to end	Night	10% 7% 3 5% 5% 2 4 4 5% 7% 50%	2470
December	1st to 10th	Night	13% 5% 2 6% 4 3 5% 4 5% 8% 45%	2470
	11th to 20th	Night	14% 6% 2 4 4 2 4 4 5% 6% 47%	3042
	21st to end	Night	18% 7% 3 6% 5% 3 4 4 5% 6% 40%	4532

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	38% 6% 10% 17% 16% 12%	1707
	11th to 20th	Day	32% 5% 10% 20% 22% 10%	1647
	21st to end	Day	41% 6% 7% 15% 21% 10%	1944
February	1st to 10th	Day	42% 5% 10% 13% 18% 12%	1800
	11th to 20th	Day	35% 5% 8% 16% 24% 12%	1758
	21st to end	Day	38% 5% 9% 16% 20% 12%	1480
March	1st to 10th	Day	27% 6% 11% 19% 26% 10%	1924
	11th to 20th	Day	32% 9% 13% 19% 16% 11%	1980
	21st to end	Day	31% 7% 13% 21% 19% 10%	2178
April	1st to 10th	Day	33% 9% 12% 16% 18% 12%	1980
	11th to 20th	Day	35% 7% 10% 20% 19% 9%	2019
	21st to end	Day	25% 7% 12% 23% 23% 9%	2265
May	1st to 10th	Day	22% 6% 13% 24% 24% 10%	2340
	11th to 20th	Day	33% 11% 17% 18% 16% 4	2305
	21st to end	Day	36% 9% 15% 21% 17% 2	2531
June	1st to 10th	Day	43% 14% 19% 16% 7% 1	2340
	11th to 20th	Day	37% 16% 22% 18% 7%	2340
	21st to end	Day	43% 17% 20% 14% 6% 1	2340
July	1st to 10th	Day	42% 19% 25% 13% 2	2275
	11th to 20th	Day	45% 20% 21% 11% 3	2271
	21st to end	Day	41% 17% 25% 14% 2	2405
August	1st to 10th	Day	43% 18% 24% 13% 2	2261
	11th to 20th	Day	47% 19% 22% 9% 2	2125
	21st to end	Day	50% 16% 18% 12% 4 1	2291
September	1st to 10th	Day	49% 14% 16% 13% 6% 2	1980
	11th to 20th	Day	47% 13% 18% 11% 9% 2	1980
	21st to end	Day	37% 10% 14% 19% 13% 6%	1980
October	1st to 10th	Day	36% 9% 11% 16% 17% 11%	1980
	11th to 20th	Day	25% 6% 10% 22% 28% 10%	1890
	21st to end	Day	29% 7% 10% 14% 23% 17%	1980
November	1st to 10th	Day	27% 6% 11% 17% 22% 16%	1857
	11th to 20th	Day	31% 4 8% 17% 27% 13%	1736
	21st to end	Day	33% 6% 9% 19% 24% 8%	1710
December	1st to 10th	Day	38% 6% 12% 19% 18% 7%	1710
	11th to 20th	Day	40% 6% 8% 16% 20% 10%	2150
	21st to end	Day	45% 4 6% 20% 19% 6%	3124
January	1st to 10th	Night	40% 6% 10% 15% 18% 12%	2459
	11th to 20th	Night	32% 5% 8% 16% 26% 12%	2386
	21st to end	Night	44% 5% 7% 12% 21% 11%	2409
February	1st to 10th	Night	38% 7% 9% 14% 20% 12%	2160
	11th to 20th	Night	37% 5% 8% 16% 19% 15%	2099
	21st to end	Night	39% 6% 9% 16% 17% 13%	1776
March	1st to 10th	Night	28% 8% 10% 20% 21% 12%	2034
	11th to 20th	Night	34% 7% 12% 20% 16% 10%	1980
	21st to end	Night	30% 6% 12% 22% 19% 11%	2178
April	1st to 10th	Night	34% 8% 11% 17% 18% 12%	1980
	11th to 20th	Night	32% 7% 10% 21% 22% 8%	1902
	21st to end	Night	22% 7% 13% 28% 23% 8%	1692
May	1st to 10th	Night	21% 7% 13% 25% 22% 11%	1620
	11th to 20th	Night	27% 11% 20% 21% 16% 4	1593
	21st to end	Night	24% 10% 19% 23% 20% 3	1745
June	1st to 10th	Night	35% 13% 23% 22% 7%	1620
	11th to 20th	Night	29% 17% 26% 21% 7%	1620
	21st to end	Night	34% 14% 25% 20% 6% 1	1620
July	1st to 10th	Night	35% 18% 27% 17% 3	1575
	11th to 20th	Night	37% 19% 27% 13% 4	1566
	21st to end	Night	30% 17% 33% 16% 3	1672
August	1st to 10th	Night	29% 20% 30% 17% 3	1647
	11th to 20th	Night	35% 20% 30% 13% 2	1776
	21st to end	Night	42% 16% 21% 16% 3 1	2045
September	1st to 10th	Night	44% 15% 20% 14% 5% 2	1980
	11th to 20th	Night	41% 16% 18% 16% 8% 1	1980
	21st to end	Night	35% 9% 18% 18% 14% 6%	1980
October	1st to 10th	Night	33% 8% 14% 15% 17% 13%	1980
	11th to 20th	Night	26% 6% 9% 24% 28% 8%	2070
	21st to end	Night	30% 7% 8% 16% 23% 16%	2376
November	1st to 10th	Night	31% 5% 9% 20% 19% 15%	2221
	11th to 20th	Night	33% 5% 9% 15% 23% 14%	2441
	21st to end	Night	34% 5% 11% 18% 20% 11%	2470
December	1st to 10th	Night	37% 7% 12% 20% 17% 7%	2470
	11th to 20th	Night	37% 7% 8% 15% 23% 10%	3044
	21st to end	Night	43% 6% 9% 18% 20% 5%	4532

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% 49% 15% 6% 1	270
	11th to 20th	Day	12% 17% 40% 20% 8% 2	249
	21st to end	Day	22% 20% 42% 14% 2	264
February	1st to 10th	Day	20% 20% 36% 16% 8%	240
	11th to 20th	Day	19% 18% 37% 20% 5%	240
	21st to end	Day	14% 19% 41% 20% 6%	198
March	1st to 10th	Day	11% 19% 34% 26% 10%	364
	11th to 20th	Day	15% 18% 32% 21% 13% 1	420
	21st to end	Day	12% 15% 32% 27% 12% 2	462
April	1st to 10th	Day	14% 13% 33% 24% 15% 1	420
	11th to 20th	Day	13% 15% 27% 29% 14% 3	412
	21st to end	Day	6% 7% 33% 30% 18% 5%	420
May	1st to 10th	Day	6% 7% 27% 32% 19% 8%	423
	11th to 20th	Day	7% 4 20% 34% 23% 12%	447
	21st to end	Day	9% 3 13% 30% 28% 17%	485
June	1st to 10th	Day	12% 3 13% 28% 29% 15%	450
	11th to 20th	Day	7% 3 14% 23% 30% 23%	450
	21st to end	Day	7% 2 8% 27% 30% 26%	450
July	1st to 10th	Day	8% 2 7% 21% 29% 34%	445
	11th to 20th	Day	6% 1 2 24% 34% 32%	444
	21st to end	Day	6% 2 7% 23% 37% 26%	488
August	1st to 10th	Day	13% 2 6% 18% 33% 28%	446
	11th to 20th	Day	11% 2 3 17% 39% 29%	439
	21st to end	Day	9% 1 9% 27% 32% 22%	495
September	1st to 10th	Day	11% 4 11% 22% 31% 22%	450
	11th to 20th	Day	10% 3 13% 31% 29% 14%	450
	21st to end	Day	11% 2 14% 24% 35% 14%	450
October	1st to 10th	Day	10% 6% 24% 33% 21% 5%	450
	11th to 20th	Day	5% 6% 19% 34% 26% 10%	450
	21st to end	Day	9% 6% 27% 36% 18% 3	495
November	1st to 10th	Day	13% 6% 28% 33% 17% 4	455
	11th to 20th	Day	14% 9% 40% 27% 8% 2	296
	21st to end	Day	9% 14% 38% 30% 9%	270
December	1st to 10th	Day	13% 15% 33% 26% 11% 3	270
	11th to 20th	Day	18% 14% 37% 24% 6%	270
	21st to end	Day	14% 13% 39% 22% 11% 1	297
January	1st to 10th	Night	18% 13% 36% 25% 8%	760
	11th to 20th	Night	12% 11% 30% 34% 13% 1	735
	21st to end	Night	19% 11% 37% 26% 6%	792
February	1st to 10th	Night	17% 11% 31% 31% 10%	720
	11th to 20th	Night	18% 10% 31% 32% 8%	703
	21st to end	Night	16% 11% 31% 36% 6%	592
March	1st to 10th	Night	12% 11% 32% 36% 9%	720
	11th to 20th	Night	14% 9% 30% 32% 14% 1	720
	21st to end	Night	11% 10% 33% 34% 12%	792
April	1st to 10th	Night	12% 9% 27% 37% 14%	720
	11th to 20th	Night	9% 9% 31% 34% 15% 1	660
	21st to end	Night	6% 4 27% 40% 21% 1	540
May	1st to 10th	Night	5% 4 24% 42% 22% 3	540
	11th to 20th	Night	7% 3 20% 37% 31% 3	531
	21st to end	Night	5% 2 12% 37% 40% 5%	584
June	1st to 10th	Night	9% 2 10% 35% 36% 7%	540
	11th to 20th	Night	5% 2 7% 34% 36% 17%	540
	21st to end	Night	8% 1 6% 26% 44% 14%	540
July	1st to 10th	Night	5% 1 5% 20% 45% 24%	525
	11th to 20th	Night	6% 2 3 17% 46% 26%	522
	21st to end	Night	4 1 3 18% 54% 20%	591
August	1st to 10th	Night	7% 1 3 18% 51% 19%	537
	11th to 20th	Night	6% 1 5% 15% 51% 23%	534
	21st to end	Night	10% 1 8% 26% 38% 17%	659
September	1st to 10th	Night	10% 2 8% 26% 40% 15%	720
	11th to 20th	Night	10% 3 12% 32% 35% 8%	720
	21st to end	Night	11% 2 14% 32% 36% 6%	720
October	1st to 10th	Night	9% 5% 23% 36% 25% 2	720
	11th to 20th	Night	6% 3 23% 38% 27% 2	720
	21st to end	Night	10% 4 25% 39% 21% 1	792
November	1st to 10th	Night	9% 7% 29% 35% 18% 2	740
	11th to 20th	Night	13% 8% 29% 34% 15% 1	760
	21st to end	Night	11% 10% 29% 37% 12%	760
December	1st to 10th	Night	15% 8% 27% 33% 16% 1	760
	11th to 20th	Night	15% 8% 29% 32% 15% 1	760
	21st to end	Night	17% 10% 28% 32% 13%	836

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