Perry Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Perry Observatory 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	16% 7% 2 6% 4 4 4 3 5% 7% 41%	101365
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 40%	98105
Transparency	Day	41% 7% 13% 22% 16% 2	104925
	Night	43% 6% 12% 22% 15% 2	102634
Seeing	Day	18% 8% 25% 30% 15% 3	15157
	Night	20% 8% 29% 28% 13% 2	24218

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	15% 7% 3 7% 4 4 4 3 4 4 45%	2517
	11th to 20th	Day	16% 9% 2 6% 5% 4 5% 3 6% 5% 38%	2520
	21st to end	Day	22% 11% 5% 7% 4 4 3 3 4 5% 33%	2877
February	1st to 10th	Day	17% 7% 2 7% 4 4 3 4 6% 6% 39%	1799
	11th to 20th	Day	16% 5% 3 6% 4 4 4 3 4 6% 46%	1848
	21st to end	Day	19% 8% 3 5% 4 3 4 3 5% 6% 38%	1570
March	1st to 10th	Day	26% 9% 3 6% 3 3 3 2 4 5% 36%	3116
	11th to 20th	Day	9% 5% 2 4 4 4 5% 4 6% 9% 49%	3249
	21st to end	Day	20% 8% 3 6% 3 3 3 3 5% 6% 41%	4166
April	1st to 10th	Day	17% 9% 2 6% 4 4 4 3 5% 6% 39%	2280
	11th to 20th	Day	17% 7% 2 5% 4 3 5% 4 5% 6% 43%	2254
	21st to end	Day	19% 6% 2 5% 4 4 3 3 6% 6% 42%	2371
May	1st to 10th	Day	26% 10% 3 5% 3 3 3 3 5% 8% 30%	4106
	11th to 20th	Day	25% 8% 3 7% 4 4 3 3 4 6% 34%	4206
	21st to end	Day	14% 6% 2 5% 3 4 5% 4 6% 9% 44%	4837
June	1st to 10th	Day	16% 6% 2 5% 4 4 4 4 6% 9% 40%	2660
	11th to 20th	Day	13% 6% 2 5% 4 5% 5% 4 6% 9% 41%	2660
	21st to end	Day	11% 7% 2 5% 4 4 4 4 6% 11% 43%	2660
July	1st to 10th	Day	12% 7% 2 7% 5% 4 5% 4 6% 9% 37%	4440
	11th to 20th	Day	13% 6% 2 5% 4 4 5% 4 6% 9% 41%	4440
	21st to end	Day	15% 8% 3 7% 5% 4 5% 4 7% 7% 35%	4814
August	1st to 10th	Day	18% 6% 2 6% 5% 5% 4 3 6% 9% 35%	4193
	11th to 20th	Day	11% 5% 3 7% 5% 4 4 3 5% 8% 45%	3730
	21st to end	Day	7% 4 2 5% 4 4 5% 4 6% 9% 50%	4180
September	1st to 10th	Day	14% 5% 2 6% 4 4 4 3 5% 7% 47%	2280
	11th to 20th	Day	11% 6% 2 5% 4 3 3 3 4 7% 52%	2142
	21st to end	Day	19% 7% 2 6% 4 5% 4 3 4 6% 40%	2090
October	1st to 10th	Day	21% 5% 2 5% 3 3 3 2 5% 7% 43%	2072
	11th to 20th	Day	14% 5% 2 5% 3 3 4 3 5% 8% 47%	1892
	21st to end	Day	18% 5% 2 6% 4 4 3 3 5% 7% 41%	1881
November	1st to 10th	Day	13% 6% 2 5% 4 4 4 2 3 5% 51%	1761
	11th to 20th	Day	16% 7% 2 5% 3 3 3 2 4 7% 49%	1800
	21st to end	Day	15% 7% 3 4 3 4 4 4 5% 6% 44%	1746
December	1st to 10th	Day	19% 7% 2 6% 4 4 4 3 6% 8% 37%	1530
	11th to 20th	Day	22% 7% 2 4 3 3 2 3 4 5% 45%	1906
	21st to end	Day	18% 10% 4 8% 5% 4 4 3 4 6% 32%	2772
January	1st to 10th	Night	18% 6% 2 4 3 4 3 3 5% 6% 46%	4429
	11th to 20th	Night	18% 9% 3 7% 3 3 4 2 3 4 44%	4440
	21st to end	Night	23% 11% 3 8% 4 4 3 3 4 4 32%	4774
February	1st to 10th	Night	16% 9% 3 6% 5% 3 4 3 4 7% 40%	2380
	11th to 20th	Night	18% 8% 2 5% 4 4 4 4 4 6% 41%	2218
	21st to end	Night	21% 6% 2 4 4 5% 4 3 4 6% 41%	1884
March	1st to 10th	Night	24% 13% 3 6% 3 3 3 2 3 5% 36%	3724
	11th to 20th	Night	12% 6% 2 6% 4 4 4 3 6% 6% 45%	3704
	21st to end	Night	22% 9% 3 6% 3 3 3 3 5% 5% 39%	3466
April	1st to 10th	Night	18% 7% 2 6% 5% 4 4 3 4 6% 38%	1900
	11th to 20th	Night	18% 8% 2 6% 4 3 4 3 4 7% 41%	1875
	21st to end	Night	19% 8% 2 5% 4 4 4 3 4 6% 41%	1807
May	1st to 10th	Night	22% 13% 5% 7% 4 4 4 3 5% 7% 27%	2834
	11th to 20th	Night	25% 9% 3 6% 5% 3 4 3 6% 6% 28%	2703
	21st to end	Night	18% 8% 2 6% 3 4 3 4 6% 8% 37%	2738
June	1st to 10th	Night	18% 7% 2 5% 4 4 5% 3 5% 8% 39%	1520
	11th to 20th	Night	15% 6% 2 4 3 4 4 4 7% 9% 41%	1520
	21st to end	Night	11% 6% 3 7% 5% 5% 6% 4 6% 9% 38%	1520
July	1st to 10th	Night	13% 11% 3 8% 5% 4 5% 5% 7% 7% 32%	2520
	11th to 20th	Night	15% 7% 2 6% 4 4 5% 5% 7% 10% 34%	2520
	21st to end	Night	19% 9% 3 7% 4 5% 4 3 6% 7% 33%	2739
August	1st to 10th	Night	22% 7% 3 6% 4 4 5% 5% 6% 8% 30%	2700
	11th to 20th	Night	11% 6% 3 6% 4 5% 5% 5% 7% 8% 39%	3097
	21st to end	Night	8% 4 2 4 4 3 4 4 6% 8% 53%	3476
September	1st to 10th	Night	14% 6% 2 7% 5% 4 5% 3 5% 7% 43%	1900
	11th to 20th	Night	13% 5% 1 4 3 3 3 4 4 6% 52%	2037
	21st to end	Night	18% 6% 2 6% 4 4 4 3 5% 6% 40%	2090
October	1st to 10th	Night	19% 7% 3 7% 4 3 4 2 4 7% 40%	2104
	11th to 20th	Night	15% 6% 2 6% 3 2 4 3 4 6% 50%	2286
	21st to end	Night	20% 6% 3 6% 4 3 3 3 4 6% 41%	2717
November	1st to 10th	Night	12% 6% 3 6% 5% 3 4 3 4 7% 48%	2537
	11th to 20th	Night	18% 7% 2 5% 4 3 3 3 4 5% 47%	2600
	21st to end	Night	15% 5% 2 5% 4 4 4 3 6% 6% 46%	2529
December	1st to 10th	Night	20% 7% 3 6% 4 4 4 3 5% 6% 39%	2645
	11th to 20th	Night	19% 7% 2 5% 4 4 4 3 5% 5% 42%	3288
	21st to end	Night	19% 10% 3 7% 4 4 3 3 4 6% 38%	4884

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	41% 5% 13% 27% 13% 1	2518
	11th to 20th	Day	45% 6% 13% 22% 12% 1	2520
	21st to end	Day	53% 8% 9% 20% 9% 1	2877
February	1st to 10th	Day	45% 6% 14% 21% 13% 1	1799
	11th to 20th	Day	41% 5% 10% 25% 17% 2	1857
	21st to end	Day	46% 5% 11% 20% 14% 4	1570
March	1st to 10th	Day	49% 4 14% 21% 11% 2	3119
	11th to 20th	Day	30% 7% 15% 32% 16% 1	3249
	21st to end	Day	45% 4 7% 19% 20% 5%	4166
April	1st to 10th	Day	43% 5% 11% 20% 18% 4	2280
	11th to 20th	Day	41% 5% 13% 20% 19% 3	2254
	21st to end	Day	42% 6% 10% 20% 18% 4	2371
May	1st to 10th	Day	53% 9% 14% 13% 10% 2	4120
	11th to 20th	Day	50% 6% 14% 18% 9% 2	4210
	21st to end	Day	36% 10% 13% 21% 18% 1	4837
June	1st to 10th	Day	40% 8% 13% 22% 15% 1	2660
	11th to 20th	Day	39% 9% 13% 25% 13% 2	2660
	21st to end	Day	36% 8% 16% 23% 15% 1	2660
July	1st to 10th	Day	47% 9% 11% 17% 16% 1	4370
	11th to 20th	Day	38% 8% 18% 23% 12% 1	4365
	21st to end	Day	45% 8% 13% 18% 14% 2	4661
August	1st to 10th	Day	44% 7% 18% 22% 9%	4179
	11th to 20th	Day	36% 8% 15% 32% 10% 1	3766
	21st to end	Day	28% 8% 15% 29% 19% 1	4180
September	1st to 10th	Day	37% 8% 14% 21% 19% 2	2280
	11th to 20th	Day	32% 7% 12% 24% 23% 2	2142
	21st to end	Day	46% 6% 9% 18% 19% 2	2090
October	1st to 10th	Day	34% 5% 13% 22% 22% 5%	3449
	11th to 20th	Day	26% 4 12% 23% 32% 2	3120
	21st to end	Day	32% 6% 15% 26% 20% 2	3081
November	1st to 10th	Day	37% 5% 14% 22% 19% 2	1761
	11th to 20th	Day	38% 5% 12% 22% 21% 2	1800
	21st to end	Day	41% 5% 14% 27% 10% 2	1746
December	1st to 10th	Day	46% 7% 14% 18% 13% 1	1530
	11th to 20th	Day	42% 5% 16% 16% 19% 2	1906
	21st to end	Day	50% 6% 13% 21% 9%	2772
January	1st to 10th	Night	39% 4 12% 30% 11% 3	4429
	11th to 20th	Night	46% 5% 12% 23% 12% 2	4440
	21st to end	Night	55% 6% 8% 19% 10% 1	4774
February	1st to 10th	Night	45% 7% 13% 22% 12% 2	2380
	11th to 20th	Night	45% 5% 10% 21% 16% 4	2220
	21st to end	Night	45% 5% 8% 22% 17% 4	1884
March	1st to 10th	Night	53% 3 9% 21% 11% 2	3734
	11th to 20th	Night	37% 6% 13% 26% 15% 3	3705
	21st to end	Night	48% 4 7% 13% 22% 6%	3466
April	1st to 10th	Night	46% 5% 9% 19% 17% 3	1900
	11th to 20th	Night	43% 6% 9% 19% 21% 2	1875
	21st to end	Night	45% 5% 9% 21% 16% 5%	1807
May	1st to 10th	Night	57% 7% 11% 14% 10% 1	2840
	11th to 20th	Night	56% 8% 12% 14% 9% 2	2704
	21st to end	Night	44% 8% 14% 19% 13% 2	2739
June	1st to 10th	Night	46% 6% 9% 21% 16% 2	1520
	11th to 20th	Night	39% 6% 15% 26% 14% 1	1520
	21st to end	Night	41% 7% 13% 26% 12% 1	1520
July	1st to 10th	Night	50% 8% 12% 18% 12% 1	2480
	11th to 20th	Night	41% 8% 16% 24% 10% 1	2472
	21st to end	Night	50% 6% 14% 15% 14% 1	2651
August	1st to 10th	Night	51% 8% 15% 17% 8% 1	2690
	11th to 20th	Night	38% 8% 15% 29% 9%	3127
	21st to end	Night	27% 6% 14% 33% 19% 1	3476
September	1st to 10th	Night	40% 7% 10% 24% 18% 2	1900
	11th to 20th	Night	33% 6% 11% 25% 23% 3	2037
	21st to end	Night	46% 7% 8% 18% 18% 3	2090
October	1st to 10th	Night	40% 5% 11% 23% 18% 2	3507
	11th to 20th	Night	26% 4 9% 26% 30% 5%	3797
	21st to end	Night	32% 5% 13% 26% 21% 2	4467
November	1st to 10th	Night	38% 6% 12% 23% 18% 4	2537
	11th to 20th	Night	41% 5% 10% 22% 19% 4	2600
	21st to end	Night	39% 6% 15% 27% 12% 2	2529
December	1st to 10th	Night	47% 7% 13% 19% 12% 1	2645
	11th to 20th	Night	44% 5% 14% 19% 18% 1	3288
	21st to end	Night	48% 5% 13% 22% 12% 1	4884

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	19% 19% 31% 23% 7% 1	180
	11th to 20th	Day	19% 25% 34% 16% 6%	180
	21st to end	Day	22% 12% 35% 20% 9% 1	229
February	1st to 10th	Day	23% 13% 35% 25% 5%	359
	11th to 20th	Day	21% 17% 39% 21% 3	456
	21st to end	Day	24% 15% 31% 23% 5% 1	382
March	1st to 10th	Day	18% 14% 36% 25% 6%	460
	11th to 20th	Day	15% 13% 40% 25% 6% 1	460
	21st to end	Day	21% 12% 37% 22% 7% 1	506
April	1st to 10th	Day	22% 11% 34% 23% 9% 1	460
	11th to 20th	Day	20% 10% 29% 30% 10% 1	452
	21st to end	Day	24% 9% 28% 30% 8% 2	460
May	1st to 10th	Day	28% 8% 29% 26% 9% 2	463
	11th to 20th	Day	22% 8% 23% 34% 12% 1	487
	21st to end	Day	19% 2 17% 33% 24% 5%	529
June	1st to 10th	Day	16% 4 20% 37% 20% 3	490
	11th to 20th	Day	14% 4 21% 39% 19% 3	490
	21st to end	Day	14% 4 18% 39% 21% 4	490
July	1st to 10th	Day	15% 4 12% 38% 25% 7%	485
	11th to 20th	Day	11% 3 11% 40% 27% 8%	484
	21st to end	Day	15% 2 15% 40% 25% 3	532
August	1st to 10th	Day	12% 3 14% 37% 29% 5%	486
	11th to 20th	Day	13% 3 10% 38% 28% 8%	479
	21st to end	Day	11% 4 16% 35% 25% 9%	539
September	1st to 10th	Day	19% 4 20% 26% 22% 9%	490
	11th to 20th	Day	13% 3 19% 31% 22% 12%	490
	21st to end	Day	23% 5% 24% 30% 16% 3	490
October	1st to 10th	Day	24% 6% 19% 24% 19% 7%	490
	11th to 20th	Day	14% 7% 30% 30% 15% 4	485
	21st to end	Day	22% 10% 31% 23% 13% 2	429
November	1st to 10th	Day	16% 9% 28% 31% 14% 3	395
	11th to 20th	Day	19% 16% 30% 26% 10%	400
	21st to end	Day	17% 14% 32% 25% 10% 1	382
December	1st to 10th	Day	19% 15% 34% 25% 7% 1	190
	11th to 20th	Day	23% 23% 29% 20% 4	180
	21st to end	Day	25% 12% 38% 15% 8% 2	198
January	1st to 10th	Night	20% 14% 41% 20% 5%	760
	11th to 20th	Night	25% 16% 32% 22% 4	760
	21st to end	Night	20% 13% 42% 21% 4	836
February	1st to 10th	Night	20% 13% 39% 26% 3	760
	11th to 20th	Night	24% 14% 42% 18% 2	743
	21st to end	Night	23% 13% 38% 21% 6%	628
March	1st to 10th	Night	20% 13% 36% 24% 6%	760
	11th to 20th	Night	19% 12% 39% 23% 6% 1	760
	21st to end	Night	20% 9% 38% 24% 7% 1	836
April	1st to 10th	Night	23% 8% 34% 28% 7%	760
	11th to 20th	Night	21% 7% 30% 32% 10%	751
	21st to end	Night	23% 4 31% 33% 9% 1	667
May	1st to 10th	Night	24% 6% 25% 31% 13% 2	570
	11th to 20th	Night	23% 4 29% 34% 11%	504
	21st to end	Night	21% 3 12% 38% 24% 2	411
June	1st to 10th	Night	21% 3 19% 33% 23% 2	380
	11th to 20th	Night	17% 2 15% 43% 21% 2	380
	21st to end	Night	14% 3 18% 41% 23% 1	380
July	1st to 10th	Night	13% 5% 13% 41% 25% 3	370
	11th to 20th	Night	13% 1 14% 40% 27% 4	368
	21st to end	Night	13% 3 15% 44% 22% 1	416
August	1st to 10th	Night	10% 2 14% 45% 28% 2	481
	11th to 20th	Night	13% 2 14% 37% 29% 4	750
	21st to end	Night	13% 2 11% 38% 28% 7%	836
September	1st to 10th	Night	16% 2 21% 33% 23% 5%	760
	11th to 20th	Night	17% 3 17% 31% 24% 9%	760
	21st to end	Night	23% 4 22% 30% 19% 3	760
October	1st to 10th	Night	23% 6% 25% 22% 20% 4	760
	11th to 20th	Night	19% 8% 28% 28% 16% 2	760
	21st to end	Night	24% 9% 30% 26% 11% 1	836
November	1st to 10th	Night	16% 8% 33% 27% 14% 2	780
	11th to 20th	Night	22% 12% 32% 24% 10% 1	800
	21st to end	Night	17% 15% 34% 25% 8% 1	779
December	1st to 10th	Night	24% 14% 32% 22% 6% 1	760
	11th to 20th	Night	23% 15% 35% 23% 4	760
	21st to end	Night	23% 13% 37% 23% 4	836

Page generated at 2024-11-22 08:02:27. Version ac07

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