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 2024-11-25.

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	10% 5% 2 6% 5% 5% 5% 5% 7% 9% 40%	101136
	Night	10% 6% 2 6% 4 4 5% 4 6% 9% 44%	98292
Transparency	Day	36% 10% 15% 17% 15% 7%	104966
	Night	35% 9% 15% 17% 16% 8%	102588
Seeing	Day	11% 8% 23% 26% 21% 11%	15225
	Night	11% 6% 22% 31% 23% 5%	25237

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	15% 6% 3 6% 3 3 4 3 4 6% 47%	2836
	11th to 20th	Day	15% 7% 3 7% 4 3 4 4 6% 7% 41%	2840
	21st to end	Day	13% 6% 2 6% 4 4 4 4 6% 7% 44%	3412
February	1st to 10th	Day	17% 8% 2 5% 4 3 3 3 5% 6% 44%	1900
	11th to 20th	Day	15% 6% 2 5% 3 4 3 3 4 6% 49%	1848
	21st to end	Day	14% 6% 2 4 4 3 4 3 5% 7% 48%	1570
March	1st to 10th	Day	16% 7% 3 7% 4 4 5% 3 7% 8% 37%	3342
	11th to 20th	Day	8% 5% 1 6% 4 4 5% 4 6% 9% 48%	3480
	21st to end	Day	14% 3 1 4 3 3 4 4 5% 7% 50%	3815
April	1st to 10th	Day	8% 4 2 5% 4 5% 5% 5% 7% 7% 47%	2090
	11th to 20th	Day	11% 4 2 6% 3 3 5% 5% 6% 7% 47%	2133
	21st to end	Day	6% 4 2 5% 4 4 4 4 6% 8% 55%	2391
May	1st to 10th	Day	7% 4 2 5% 4 4 5% 4 7% 9% 51%	4082
	11th to 20th	Day	10% 6% 2 7% 5% 5% 5% 5% 7% 9% 38%	4081
	21st to end	Day	4 3 2 5% 4 5% 5% 4 7% 11% 49%	4476
June	1st to 10th	Day	10% 6% 3 7% 7% 6% 6% 7% 8% 11% 31%	2470
	11th to 20th	Day	6% 4 2 6% 6% 6% 8% 7% 10% 13% 31%	2470
	21st to end	Day	8% 5% 2 7% 6% 8% 8% 7% 9% 10% 29%	2470
July	1st to 10th	Day	7% 5% 2 8% 6% 8% 9% 7% 10% 14% 24%	4120
	11th to 20th	Day	4 4 2 7% 6% 6% 8% 7% 10% 14% 32%	4120
	21st to end	Day	5% 5% 2 8% 7% 8% 9% 8% 11% 14% 22%	4466
August	1st to 10th	Day	10% 5% 2 8% 6% 7% 8% 6% 9% 12% 26%	4009
	11th to 20th	Day	8% 4 2 6% 5% 5% 6% 6% 8% 12% 37%	3729
	21st to end	Day	8% 6% 3 9% 7% 7% 7% 6% 8% 11% 28%	4025
September	1st to 10th	Day	10% 9% 3 9% 7% 6% 8% 6% 9% 11% 24%	2090
	11th to 20th	Day	9% 7% 3 8% 7% 7% 7% 6% 8% 9% 28%	2090
	21st to end	Day	10% 6% 2 6% 4 5% 5% 5% 7% 7% 42%	2090
October	1st to 10th	Day	9% 5% 3 7% 5% 5% 4 4 5% 7% 45%	2090
	11th to 20th	Day	6% 4 2 4 3 4 5% 4 4 6% 61%	1994
	21st to end	Day	7% 4 1 4 3 5% 5% 4 5% 7% 54%	2090
November	1st to 10th	Day	9% 4 2 5% 3 3 3 3 6% 7% 54%	1957
	11th to 20th	Day	10% 5% 3 5% 4 3 3 3 4 7% 54%	1827
	21st to end	Day	10% 6% 2 5% 4 3 4 4 6% 6% 51%	1746
December	1st to 10th	Day	12% 6% 2 6% 5% 4 4 4 6% 7% 45%	1710
	11th to 20th	Day	16% 7% 2 6% 4 4 4 3 5% 6% 44%	2153
	21st to end	Day	19% 7% 3 7% 4 3 3 3 4 6% 41%	3124
January	1st to 10th	Night	16% 8% 2 6% 4 3 3 3 5% 7% 42%	4109
	11th to 20th	Night	10% 7% 3 8% 5% 5% 5% 4 5% 8% 41%	4120
	21st to end	Night	17% 8% 3 6% 4 3 4 3 5% 7% 40%	4241
February	1st to 10th	Night	15% 7% 2 5% 4 3 4 3 5% 7% 45%	2280
	11th to 20th	Night	16% 6% 2 5% 4 3 3 3 5% 6% 48%	2218
	21st to end	Night	13% 8% 3 6% 4 3 3 2 4 5% 48%	1884
March	1st to 10th	Night	17% 10% 3 7% 5% 4 5% 3 5% 6% 35%	3509
	11th to 20th	Night	11% 5% 2 5% 4 4 5% 4 7% 8% 45%	3480
	21st to end	Night	15% 5% 2 4 4 4 3 3 4 7% 47%	3817
April	1st to 10th	Night	11% 5% 2 5% 4 4 5% 4 5% 6% 48%	2090
	11th to 20th	Night	8% 5% 2 6% 4 4 4 4 4 6% 52%	2009
	21st to end	Night	5% 3 1 3 3 3 3 3 6% 8% 58%	1786
May	1st to 10th	Night	6% 4 2 4 3 4 4 3 7% 10% 53%	2813
	11th to 20th	Night	11% 5% 2 7% 5% 4 4 3 5% 9% 44%	2812
	21st to end	Night	3 3 1 3 3 3 4 3 7% 11% 59%	3079
June	1st to 10th	Night	7% 6% 2 7% 4 5% 5% 5% 8% 12% 38%	1710
	11th to 20th	Night	4 4 2 5% 6% 5% 7% 7% 10% 12% 39%	1710
	21st to end	Night	7% 6% 1 6% 4 6% 5% 5% 8% 12% 40%	1710
July	1st to 10th	Night	6% 6% 3 9% 7% 8% 7% 7% 10% 12% 25%	2840
	11th to 20th	Night	3 3 2 5% 4 4 5% 5% 8% 14% 45%	2840
	21st to end	Night	4 5% 2 8% 6% 6% 6% 7% 9% 14% 32%	3087
August	1st to 10th	Night	7% 4 2 6% 5% 6% 5% 5% 10% 13% 34%	2914
	11th to 20th	Night	4 3 2 5% 4 5% 5% 6% 9% 13% 45%	3100
	21st to end	Night	9% 6% 3 9% 5% 6% 6% 7% 7% 11% 30%	3597
September	1st to 10th	Night	9% 7% 3 10% 7% 6% 7% 6% 8% 11% 27%	2090
	11th to 20th	Night	9% 6% 3 7% 6% 6% 7% 6% 8% 10% 31%	2090
	21st to end	Night	9% 5% 2 6% 5% 5% 4 4 7% 9% 45%	2090
October	1st to 10th	Night	9% 6% 2 8% 4 4 4 4 5% 7% 48%	2090
	11th to 20th	Night	7% 4 1 5% 3 3 4 3 3 7% 61%	2185
	21st to end	Night	8% 4 2 5% 4 3 4 4 5% 7% 54%	2508
November	1st to 10th	Night	8% 5% 2 6% 5% 4 4 3 6% 6% 51%	2341
	11th to 20th	Night	11% 5% 2 5% 5% 4 4 3 5% 5% 51%	2570
	21st to end	Night	10% 7% 2 6% 4 3 4 3 5% 7% 51%	2529
December	1st to 10th	Night	12% 6% 2 7% 4 4 5% 3 6% 8% 45%	2470
	11th to 20th	Night	14% 7% 2 5% 4 4 3 3 5% 6% 47%	3042
	21st to end	Night	18% 7% 3 7% 4 4 4 3 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	37% 6% 7% 17% 19% 14%	2837
	11th to 20th	Day	39% 7% 9% 13% 16% 16%	2840
	21st to end	Day	36% 9% 10% 13% 21% 10%	3412
February	1st to 10th	Day	41% 5% 10% 14% 18% 12%	1900
	11th to 20th	Day	36% 5% 7% 15% 23% 12%	1858
	21st to end	Day	36% 5% 10% 16% 21% 12%	1570
March	1st to 10th	Day	41% 9% 12% 17% 15% 6%	3342
	11th to 20th	Day	30% 10% 15% 15% 21% 10%	3480
	21st to end	Day	31% 8% 12% 25% 14% 10%	3815
April	1st to 10th	Day	32% 9% 13% 16% 18% 12%	2090
	11th to 20th	Day	34% 7% 10% 20% 20% 10%	2133
	21st to end	Day	25% 7% 13% 23% 22% 10%	2391
May	1st to 10th	Day	23% 13% 18% 23% 17% 6%	4120
	11th to 20th	Day	37% 11% 14% 21% 14% 2	4085
	21st to end	Day	23% 10% 21% 26% 18% 1	4489
June	1st to 10th	Day	42% 14% 19% 17% 8% 1	2470
	11th to 20th	Day	37% 16% 21% 18% 7% 1	2470
	21st to end	Day	42% 16% 20% 14% 7% 1	2470
July	1st to 10th	Day	43% 18% 31% 8% 1	4055
	11th to 20th	Day	35% 20% 33% 11% 2	4051
	21st to end	Day	43% 19% 26% 9% 1	4324
August	1st to 10th	Day	44% 15% 22% 14% 5%	3996
	11th to 20th	Day	34% 14% 22% 21% 9%	3765
	21st to end	Day	45% 13% 19% 19% 4	4025
September	1st to 10th	Day	50% 14% 16% 12% 6% 2	2090
	11th to 20th	Day	48% 13% 17% 11% 9% 2	2090
	21st to end	Day	37% 10% 14% 20% 14% 6%	2090
October	1st to 10th	Day	46% 7% 8% 11% 16% 12%	3480
	11th to 20th	Day	15% 3 6% 18% 33% 25%	3286
	21st to end	Day	19% 5% 6% 18% 38% 14%	3428
November	1st to 10th	Day	28% 6% 11% 18% 22% 15%	1957
	11th to 20th	Day	31% 5% 8% 16% 26% 14%	1827
	21st to end	Day	33% 6% 9% 19% 24% 10%	1746
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% 5% 8% 13% 18% 16%	4109
	11th to 20th	Night	39% 6% 8% 14% 22% 11%	4120
	21st to end	Night	43% 8% 8% 12% 20% 10%	4241
February	1st to 10th	Night	38% 7% 9% 14% 20% 12%	2280
	11th to 20th	Night	38% 5% 8% 15% 19% 15%	2219
	21st to end	Night	38% 6% 9% 17% 18% 13%	1884
March	1st to 10th	Night	47% 10% 10% 14% 13% 7%	3509
	11th to 20th	Night	31% 9% 13% 21% 17% 9%	3480
	21st to end	Night	36% 6% 10% 20% 18% 10%	3817
April	1st to 10th	Night	34% 8% 11% 17% 17% 12%	2090
	11th to 20th	Night	32% 7% 10% 21% 21% 10%	2009
	21st to end	Night	22% 7% 13% 27% 23% 8%	1786
May	1st to 10th	Night	20% 14% 20% 25% 14% 7%	2840
	11th to 20th	Night	32% 12% 19% 23% 11% 2	2813
	21st to end	Night	16% 9% 22% 34% 17% 2	3087
June	1st to 10th	Night	34% 13% 23% 22% 7%	1710
	11th to 20th	Night	29% 17% 25% 21% 8%	1710
	21st to end	Night	33% 14% 25% 20% 6% 1	1710
July	1st to 10th	Night	48% 16% 25% 10% 2	2795
	11th to 20th	Night	26% 15% 44% 13% 2	2786
	21st to end	Night	38% 17% 34% 10% 2	2988
August	1st to 10th	Night	36% 16% 28% 16% 4	2903
	11th to 20th	Night	26% 15% 30% 23% 6%	3130
	21st to end	Night	45% 13% 23% 14% 4	3597
September	1st to 10th	Night	46% 15% 19% 13% 5% 2	2090
	11th to 20th	Night	43% 15% 17% 16% 8% 1	2090
	21st to end	Night	34% 9% 17% 19% 15% 6%	2090
October	1st to 10th	Night	46% 8% 9% 12% 14% 11%	3480
	11th to 20th	Night	17% 4 5% 20% 32% 22%	3619
	21st to end	Night	19% 5% 5% 19% 37% 14%	4120
November	1st to 10th	Night	32% 5% 9% 20% 18% 14%	2341
	11th to 20th	Night	33% 5% 9% 15% 23% 14%	2570
	21st to end	Night	33% 5% 11% 18% 20% 12%	2529
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% 18% 40% 21% 8% 2	270
	21st to end	Day	20% 22% 42% 14% 2	297
February	1st to 10th	Day	19% 20% 38% 16% 7%	270
	11th to 20th	Day	21% 19% 36% 20% 5%	270
	21st to end	Day	14% 20% 41% 20% 6%	225
March	1st to 10th	Day	12% 18% 34% 25% 9%	401
	11th to 20th	Day	14% 18% 33% 23% 12% 1	460
	21st to end	Day	13% 16% 31% 26% 12% 1	506
April	1st to 10th	Day	13% 14% 34% 23% 14% 1	460
	11th to 20th	Day	12% 15% 28% 29% 13% 2	452
	21st to end	Day	5% 8% 32% 32% 17% 5%	460
May	1st to 10th	Day	6% 8% 27% 32% 21% 7%	463
	11th to 20th	Day	7% 5% 22% 33% 21% 11%	487
	21st to end	Day	8% 3 15% 31% 27% 16%	529
June	1st to 10th	Day	12% 2 12% 30% 29% 14%	490
	11th to 20th	Day	7% 4 16% 23% 29% 21%	490
	21st to end	Day	7% 2 8% 29% 30% 25%	490
July	1st to 10th	Day	7% 2 7% 21% 28% 34%	485
	11th to 20th	Day	5% 1 2 23% 35% 33%	484
	21st to end	Day	6% 2 7% 22% 36% 27%	532
August	1st to 10th	Day	13% 3 7% 18% 31% 28%	486
	11th to 20th	Day	10% 2 3 18% 39% 29%	479
	21st to end	Day	9% 1 9% 27% 32% 22%	539
September	1st to 10th	Day	14% 4 10% 21% 31% 21%	490
	11th to 20th	Day	12% 3 13% 31% 28% 13%	490
	21st to end	Day	11% 3 14% 23% 34% 14%	490
October	1st to 10th	Day	11% 7% 24% 32% 21% 5%	490
	11th to 20th	Day	5% 7% 22% 33% 24% 9%	490
	21st to end	Day	9% 6% 27% 37% 19% 3	539
November	1st to 10th	Day	12% 6% 27% 34% 17% 3	495
	11th to 20th	Day	13% 9% 41% 26% 9% 2	327
	21st to end	Day	8% 15% 38% 29% 9% 1	282
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	760
	21st to end	Night	19% 13% 37% 25% 6%	836
February	1st to 10th	Night	16% 11% 31% 32% 9%	760
	11th to 20th	Night	19% 10% 31% 31% 7%	743
	21st to end	Night	15% 12% 32% 35% 6%	628
March	1st to 10th	Night	13% 11% 32% 35% 9%	760
	11th to 20th	Night	13% 10% 30% 33% 14% 1	760
	21st to end	Night	11% 11% 33% 33% 12%	836
April	1st to 10th	Night	11% 10% 28% 37% 14%	760
	11th to 20th	Night	9% 9% 31% 34% 15% 1	697
	21st to end	Night	6% 4 28% 40% 22% 1	570
May	1st to 10th	Night	5% 4 24% 41% 23% 4	570
	11th to 20th	Night	7% 3 21% 37% 29% 3	561
	21st to end	Night	5% 1 12% 39% 38% 4	617
June	1st to 10th	Night	9% 2 11% 35% 36% 7%	570
	11th to 20th	Night	5% 2 8% 34% 35% 16%	570
	21st to end	Night	8% 1 6% 26% 44% 14%	570
July	1st to 10th	Night	6% 1 6% 19% 45% 24%	555
	11th to 20th	Night	6% 1 3 16% 46% 27%	552
	21st to end	Night	4 1 3 18% 53% 20%	624
August	1st to 10th	Night	8% 1 4 18% 50% 19%	567
	11th to 20th	Night	6% 1 5% 15% 51% 23%	564
	21st to end	Night	10% 1 9% 26% 38% 16%	696
September	1st to 10th	Night	11% 2 8% 26% 38% 15%	760
	11th to 20th	Night	11% 4 12% 31% 35% 8%	760
	21st to end	Night	11% 2 14% 31% 36% 6%	760
October	1st to 10th	Night	11% 5% 23% 35% 24% 2	760
	11th to 20th	Night	6% 3 26% 37% 25% 2	760
	21st to end	Night	10% 3 26% 39% 21% 1	836
November	1st to 10th	Night	9% 7% 29% 36% 17% 2	780
	11th to 20th	Night	14% 8% 28% 33% 16% 1	800
	21st to end	Night	11% 10% 30% 36% 13% 1	779
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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