Perkin Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Perkin Observatory clear sky chart from 2005-11-06 to 2024-11-05.

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% 9% 3 7% 5% 4 5% 4 5% 7% 36%	101307
	Night	16% 9% 3 7% 4 4 4 4 5% 6% 37%	97746
Transparency	Day	45% 7% 12% 21% 13% 2	104832
	Night	44% 7% 12% 22% 13% 2	102310
Seeing	Day	24% 20% 30% 18% 7% 2	14932
	Night	22% 19% 30% 22% 6% 1	23856

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	12% 9% 4 7% 4 4 4 4 5% 6% 40%	2517
	11th to 20th	Day	15% 9% 4 9% 5% 5% 5% 3 5% 6% 33%	2520
	21st to end	Day	18% 14% 5% 8% 4 4 4 4 5% 5% 30%	2772
February	1st to 10th	Day	16% 9% 4 7% 5% 6% 6% 5% 6% 6% 31%	1733
	11th to 20th	Day	12% 8% 4 9% 6% 5% 4 4 6% 8% 35%	1848
	21st to end	Day	17% 11% 4 8% 5% 5% 4 4 4 4 34%	1570
March	1st to 10th	Day	20% 16% 3 9% 6% 4 3 3 4 4 28%	3116
	11th to 20th	Day	12% 10% 3 7% 5% 6% 5% 4 6% 7% 36%	3278
	21st to end	Day	19% 10% 4 7% 5% 4 4 3 5% 6% 34%	4119
April	1st to 10th	Day	17% 12% 3 7% 5% 4 4 3 5% 5% 35%	2280
	11th to 20th	Day	16% 8% 2 6% 5% 6% 4 5% 6% 6% 36%	2254
	21st to end	Day	21% 9% 3 8% 5% 4 4 3 4 6% 33%	2457
May	1st to 10th	Day	23% 11% 3 8% 5% 4 5% 3 4 6% 27%	4106
	11th to 20th	Day	19% 11% 3 8% 5% 4 4 3 4 6% 32%	4288
	21st to end	Day	14% 6% 2 6% 4 4 4 4 6% 8% 42%	4837
June	1st to 10th	Day	18% 7% 2 6% 5% 5% 5% 4 6% 8% 33%	2660
	11th to 20th	Day	16% 7% 2 7% 6% 4 5% 4 5% 9% 35%	2660
	21st to end	Day	18% 7% 3 7% 5% 6% 5% 4 6% 8% 32%	2660
July	1st to 10th	Day	12% 6% 2 7% 5% 5% 6% 5% 7% 9% 36%	4440
	11th to 20th	Day	11% 6% 2 6% 5% 5% 6% 5% 7% 11% 36%	4440
	21st to end	Day	18% 7% 3 7% 5% 5% 5% 3 6% 7% 34%	4814
August	1st to 10th	Day	15% 7% 2 8% 5% 5% 5% 4 6% 8% 35%	4265
	11th to 20th	Day	14% 7% 3 8% 5% 5% 5% 4 6% 9% 34%	3927
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 4 7% 9% 42%	4180
September	1st to 10th	Day	13% 6% 2 6% 4 5% 4 3 6% 8% 43%	2280
	11th to 20th	Day	13% 7% 2 5% 3 4 3 3 4 7% 49%	2119
	21st to end	Day	16% 6% 3 5% 4 4 4 3 5% 6% 42%	2090
October	1st to 10th	Day	22% 8% 3 6% 4 5% 4 3 3 5% 37%	2090
	11th to 20th	Day	14% 7% 3 6% 4 4 5% 4 5% 6% 43%	1827
	21st to end	Day	22% 9% 3 6% 5% 5% 4 3 4 7% 35%	1881
November	1st to 10th	Day	11% 10% 3 6% 4 3 4 3 4 7% 45%	1708
	11th to 20th	Day	18% 8% 2 5% 3 4 4 4 5% 5% 42%	1710
	21st to end	Day	18% 10% 2 6% 5% 4 4 3 5% 6% 38%	1663
December	1st to 10th	Day	17% 10% 4 8% 5% 4 5% 3 5% 6% 32%	1520
	11th to 20th	Day	21% 10% 3 5% 3 4 4 3 4 4 38%	1906
	21st to end	Day	20% 12% 3 7% 5% 4 5% 4 5% 6% 28%	2772
January	1st to 10th	Night	16% 8% 3 7% 4 4 3 3 5% 6% 40%	4429
	11th to 20th	Night	13% 9% 3 10% 6% 5% 5% 4 6% 7% 31%	4440
	21st to end	Night	20% 12% 5% 9% 5% 5% 5% 3 5% 4 27%	4884
February	1st to 10th	Night	14% 10% 3 8% 4 5% 4 4 5% 7% 35%	2444
	11th to 20th	Night	11% 9% 3 7% 5% 5% 5% 4 7% 6% 37%	2218
	21st to end	Night	15% 10% 3 8% 5% 4 4 4 4 6% 37%	1884
March	1st to 10th	Night	23% 12% 3 6% 4 4 4 3 5% 5% 31%	3724
	11th to 20th	Night	11% 9% 3 7% 6% 5% 6% 5% 6% 6% 36%	3679
	21st to end	Night	22% 8% 3 7% 4 3 5% 4 6% 6% 32%	3511
April	1st to 10th	Night	18% 10% 3 8% 5% 3 5% 3 6% 6% 35%	1900
	11th to 20th	Night	18% 8% 2 6% 5% 3 5% 3 5% 6% 39%	1875
	21st to end	Night	20% 9% 4 6% 4 4 3 4 5% 5% 37%	1721
May	1st to 10th	Night	23% 11% 4 7% 4 5% 4 4 6% 6% 27%	2834
	11th to 20th	Night	16% 10% 4 10% 7% 5% 5% 4 6% 7% 27%	2633
	21st to end	Night	15% 7% 2 6% 5% 3 4 3 6% 8% 41%	2738
June	1st to 10th	Night	17% 9% 3 7% 5% 4 3 3 5% 7% 38%	1520
	11th to 20th	Night	18% 8% 2 6% 4 4 4 4 5% 7% 38%	1520
	21st to end	Night	16% 8% 3 5% 4 4 4 5% 6% 8% 39%	1520
July	1st to 10th	Night	10% 7% 3 8% 5% 4 6% 4 7% 8% 38%	2520
	11th to 20th	Night	14% 7% 2 6% 4 4 4 4 6% 8% 41%	2520
	21st to end	Night	17% 7% 3 7% 3 4 5% 4 4 6% 41%	2739
August	1st to 10th	Night	21% 8% 3 6% 4 4 4 3 5% 6% 35%	2634
	11th to 20th	Night	13% 9% 3 9% 5% 4 4 3 5% 6% 38%	2896
	21st to end	Night	9% 4 2 4 3 3 3 3 5% 8% 57%	3476
September	1st to 10th	Night	13% 5% 2 5% 3 3 4 5% 4 6% 49%	1900
	11th to 20th	Night	13% 6% 3 5% 2 3 3 3 4 6% 52%	2059
	21st to end	Night	13% 7% 2 7% 3 4 4 3 4 7% 46%	2090
October	1st to 10th	Night	20% 7% 3 8% 4 4 4 3 3 5% 40%	2090
	11th to 20th	Night	15% 7% 3 5% 3 3 4 5% 5% 6% 43%	2347
	21st to end	Night	21% 9% 3 6% 5% 4 4 3 4 6% 35%	2717
November	1st to 10th	Night	14% 7% 2 7% 4 3 4 3 5% 5% 46%	2470
	11th to 20th	Night	16% 6% 3 5% 5% 4 4 4 4 5% 42%	2470
	21st to end	Night	14% 9% 3 6% 4 4 5% 4 5% 6% 41%	2512
December	1st to 10th	Night	17% 11% 4 7% 4 4 4 3 5% 7% 33%	2660
	11th to 20th	Night	19% 9% 4 6% 4 4 5% 4 5% 5% 36%	3288
	21st to end	Night	19% 13% 4 9% 5% 4 4 3 5% 5% 29%	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	43% 6% 10% 31% 9% 1	2518
	11th to 20th	Day	48% 8% 12% 22% 9% 2	2520
	21st to end	Day	54% 8% 8% 20% 9% 2	2772
February	1st to 10th	Day	50% 8% 12% 17% 11% 2	1733
	11th to 20th	Day	47% 8% 10% 19% 14% 3	1857
	21st to end	Day	51% 6% 9% 18% 13% 2	1570
March	1st to 10th	Day	59% 5% 11% 12% 12% 2	3119
	11th to 20th	Day	40% 12% 14% 19% 13% 3	3278
	21st to end	Day	49% 4 8% 16% 18% 5%	4119
April	1st to 10th	Day	50% 5% 9% 19% 14% 3	2280
	11th to 20th	Day	45% 8% 10% 19% 16% 2	2254
	21st to end	Day	52% 6% 7% 17% 15% 3	2457
May	1st to 10th	Day	57% 8% 10% 15% 9% 1	4120
	11th to 20th	Day	52% 7% 14% 16% 9% 1	4292
	21st to end	Day	37% 8% 13% 25% 16% 1	4837
June	1st to 10th	Day	47% 7% 13% 21% 11% 1	2660
	11th to 20th	Day	45% 8% 12% 19% 14% 2	2660
	21st to end	Day	49% 8% 12% 19% 11% 1	2660
July	1st to 10th	Day	42% 7% 13% 20% 17% 1	4370
	11th to 20th	Day	40% 7% 16% 24% 12% 1	4365
	21st to end	Day	47% 6% 13% 19% 14% 1	4661
August	1st to 10th	Day	46% 7% 15% 21% 10%	4251
	11th to 20th	Day	45% 6% 16% 25% 7% 1	3966
	21st to end	Day	35% 7% 12% 29% 15% 1	4180
September	1st to 10th	Day	38% 6% 12% 25% 17% 1	2280
	11th to 20th	Day	36% 5% 9% 25% 23% 2	2119
	21st to end	Day	41% 5% 10% 23% 18% 3	2090
October	1st to 10th	Day	47% 5% 6% 23% 16% 2	3480
	11th to 20th	Day	35% 5% 8% 27% 20% 5%	3004
	21st to end	Day	37% 4 9% 30% 16% 4	3081
November	1st to 10th	Day	40% 5% 13% 24% 15% 2	1708
	11th to 20th	Day	42% 6% 11% 21% 17% 3	1710
	21st to end	Day	45% 6% 17% 20% 10% 2	1663
December	1st to 10th	Day	51% 7% 13% 14% 13% 1	1520
	11th to 20th	Day	47% 6% 14% 17% 15% 1	1906
	21st to end	Day	53% 8% 10% 17% 10% 1	2772
January	1st to 10th	Night	41% 8% 12% 25% 11% 2	4429
	11th to 20th	Night	47% 10% 13% 18% 10% 3	4440
	21st to end	Night	57% 7% 10% 15% 10% 1	4884
February	1st to 10th	Night	46% 8% 13% 19% 11% 2	2444
	11th to 20th	Night	43% 8% 13% 20% 15% 2	2220
	21st to end	Night	46% 6% 11% 23% 11% 4	1884
March	1st to 10th	Night	54% 6% 9% 19% 10% 2	3734
	11th to 20th	Night	44% 10% 12% 20% 12% 2	3680
	21st to end	Night	48% 5% 12% 11% 20% 5%	3511
April	1st to 10th	Night	47% 6% 11% 18% 15% 2	1900
	11th to 20th	Night	45% 6% 11% 19% 17% 2	1875
	21st to end	Night	47% 6% 11% 17% 15% 4	1721
May	1st to 10th	Night	54% 9% 11% 17% 7% 1	2840
	11th to 20th	Night	53% 10% 14% 14% 8% 2	2634
	21st to end	Night	40% 9% 14% 22% 13% 2	2739
June	1st to 10th	Night	46% 7% 12% 24% 11%	1520
	11th to 20th	Night	46% 7% 13% 20% 13% 2	1520
	21st to end	Night	43% 8% 14% 21% 13% 1	1520
July	1st to 10th	Night	41% 8% 14% 23% 14% 1	2480
	11th to 20th	Night	40% 7% 14% 24% 15%	2472
	21st to end	Night	44% 5% 12% 23% 15% 1	2651
August	1st to 10th	Night	50% 7% 13% 20% 10%	2624
	11th to 20th	Night	46% 6% 16% 23% 9%	2923
	21st to end	Night	26% 4 16% 34% 18% 1	3476
September	1st to 10th	Night	35% 7% 12% 27% 19% 1	1900
	11th to 20th	Night	34% 6% 10% 27% 22% 2	2059
	21st to end	Night	39% 6% 9% 27% 17% 2	2090
October	1st to 10th	Night	41% 5% 15% 24% 14% 2	3480
	11th to 20th	Night	31% 5% 11% 27% 23% 4	3909
	21st to end	Night	38% 4 10% 32% 13% 2	4467
November	1st to 10th	Night	40% 6% 12% 22% 18% 3	2470
	11th to 20th	Night	42% 5% 11% 21% 17% 3	2470
	21st to end	Night	41% 8% 15% 23% 11% 1	2512
December	1st to 10th	Night	49% 7% 12% 20% 11% 1	2660
	11th to 20th	Night	46% 7% 15% 15% 16% 2	3288
	21st to end	Night	52% 9% 8% 21% 9%	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	31% 25% 26% 17% 2	180
	11th to 20th	Day	33% 24% 32% 8% 2 1	180
	21st to end	Day	23% 31% 32% 10% 4	198
February	1st to 10th	Day	33% 31% 26% 8% 2	318
	11th to 20th	Day	34% 28% 27% 10% 1	456
	21st to end	Day	31% 27% 28% 11% 2	382
March	1st to 10th	Day	31% 21% 35% 11% 3	460
	11th to 20th	Day	29% 30% 27% 9% 4 1	460
	21st to end	Day	31% 26% 29% 8% 4 2	506
April	1st to 10th	Day	33% 24% 25% 10% 6% 1	460
	11th to 20th	Day	28% 28% 27% 11% 5% 1	452
	21st to end	Day	32% 19% 32% 12% 5% 1	460
May	1st to 10th	Day	33% 22% 26% 12% 5% 1	463
	11th to 20th	Day	21% 21% 37% 14% 6% 1	487
	21st to end	Day	20% 17% 31% 26% 6% 1	529
June	1st to 10th	Day	20% 17% 35% 18% 9% 1	490
	11th to 20th	Day	17% 19% 34% 22% 8% 1	490
	21st to end	Day	19% 19% 35% 17% 9% 2	490
July	1st to 10th	Day	17% 10% 32% 27% 12% 2	485
	11th to 20th	Day	16% 9% 34% 29% 10% 2	484
	21st to end	Day	16% 12% 36% 26% 8% 2	532
August	1st to 10th	Day	14% 12% 30% 30% 12% 3	486
	11th to 20th	Day	15% 11% 31% 26% 14% 3	479
	21st to end	Day	14% 12% 32% 26% 12% 4	539
September	1st to 10th	Day	18% 14% 26% 24% 13% 5%	490
	11th to 20th	Day	18% 15% 25% 20% 13% 8%	490
	21st to end	Day	18% 16% 32% 19% 10% 5%	490
October	1st to 10th	Day	28% 16% 23% 20% 10% 3	490
	11th to 20th	Day	22% 19% 29% 22% 6% 2	448
	21st to end	Day	32% 17% 27% 19% 4 1	429
November	1st to 10th	Day	23% 21% 27% 17% 10% 1	378
	11th to 20th	Day	31% 24% 24% 17% 4 1	370
	21st to end	Day	32% 20% 30% 14% 4 1	323
December	1st to 10th	Day	23% 36% 26% 11% 4	180
	11th to 20th	Day	38% 24% 28% 8% 1	180
	21st to end	Day	32% 25% 30% 9% 4	198
January	1st to 10th	Night	24% 26% 34% 15% 2	760
	11th to 20th	Night	28% 25% 30% 14% 2	760
	21st to end	Night	20% 26% 34% 17% 3	836
February	1st to 10th	Night	25% 25% 33% 15% 1	760
	11th to 20th	Night	22% 27% 37% 13% 1	743
	21st to end	Night	28% 27% 29% 15% 1	628
March	1st to 10th	Night	27% 22% 32% 17% 2	760
	11th to 20th	Night	25% 25% 31% 16% 3	760
	21st to end	Night	26% 22% 31% 16% 5%	836
April	1st to 10th	Night	26% 20% 31% 20% 3	760
	11th to 20th	Night	23% 24% 29% 18% 5%	751
	21st to end	Night	27% 19% 31% 19% 4	581
May	1st to 10th	Night	28% 16% 32% 20% 4	570
	11th to 20th	Night	22% 16% 35% 23% 5%	460
	21st to end	Night	19% 9% 35% 31% 7%	411
June	1st to 10th	Night	19% 11% 32% 29% 8%	380
	11th to 20th	Night	20% 12% 32% 27% 7% 1	380
	21st to end	Night	17% 9% 31% 36% 6%	380
July	1st to 10th	Night	15% 6% 34% 36% 9%	370
	11th to 20th	Night	16% 3 29% 37% 13% 2	368
	21st to end	Night	12% 5% 34% 37% 12%	416
August	1st to 10th	Night	14% 7% 27% 37% 15%	454
	11th to 20th	Night	15% 10% 28% 33% 13% 2	624
	21st to end	Night	16% 6% 25% 35% 16% 2	836
September	1st to 10th	Night	16% 9% 30% 29% 14% 1	760
	11th to 20th	Night	16% 10% 28% 28% 16% 2	760
	21st to end	Night	16% 13% 28% 28% 13% 2	760
October	1st to 10th	Night	24% 14% 25% 25% 10% 2	760
	11th to 20th	Night	21% 15% 31% 22% 10% 1	760
	21st to end	Night	27% 22% 25% 20% 5% 1	836
November	1st to 10th	Night	20% 21% 30% 21% 8% 1	760
	11th to 20th	Night	22% 26% 26% 19% 6% 1	760
	21st to end	Night	24% 23% 32% 16% 4 1	760
December	1st to 10th	Night	26% 25% 31% 15% 3	760
	11th to 20th	Night	26% 24% 33% 14% 3	760
	21st to end	Night	28% 25% 31% 13% 2	836

Page generated at 2024-11-03 02:11:29. Version ac07

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