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 2024-01-12.

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% 8% 3 6% 6% 3 5% 5% 5% 7% 37%	74787
	Night	16% 8% 3 6% 5% 3 4 5% 5% 6% 39%	74923
Transparency	Day	45% 7% 11% 20% 14% 2	74492
	Night	44% 7% 12% 21% 14% 2	74752
Seeing	Day	24% 20% 30% 18% 7% 2	13811
	Night	22% 19% 31% 22% 6% 1	22866

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% 10% 5% 6% 6% 3 5% 5% 6% 7% 36%	1517
	11th to 20th	Day	16% 8% 4 6% 6% 4 6% 4 6% 8% 32%	1454
	21st to end	Day	11% 8% 3 6% 6% 4 5% 5% 7% 6% 39%	1584
February	1st to 10th	Day	17% 8% 4 6% 6% 5% 7% 6% 6% 7% 28%	1643
	11th to 20th	Day	13% 7% 4 8% 7% 3 4 5% 5% 8% 35%	1748
	21st to end	Day	18% 10% 5% 7% 6% 3 5% 5% 4 4 33%	1480
March	1st to 10th	Day	14% 9% 3 7% 7% 4 4 5% 5% 6% 36%	1797
	11th to 20th	Day	16% 9% 4 6% 5% 4 5% 4 5% 6% 35%	1862
	21st to end	Day	17% 9% 4 6% 7% 2 4 3 4 5% 38%	2343
April	1st to 10th	Day	17% 10% 4 7% 6% 3 4 4 5% 5% 36%	2160
	11th to 20th	Day	16% 7% 3 5% 6% 4 5% 5% 5% 7% 37%	2134
	21st to end	Day	21% 8% 3 7% 6% 3 5% 4 4 6% 32%	2327
May	1st to 10th	Day	23% 9% 3 7% 5% 3 5% 4 4 6% 31%	2340
	11th to 20th	Day	18% 8% 2 6% 5% 3 3 4 4 6% 39%	2410
	21st to end	Day	16% 6% 3 5% 6% 3 4 4 5% 8% 41%	2725
June	1st to 10th	Day	19% 7% 3 5% 7% 3 5% 4 6% 8% 33%	2520
	11th to 20th	Day	17% 6% 3 6% 6% 3 5% 4 4 9% 35%	2520
	21st to end	Day	19% 7% 3 6% 6% 4 5% 5% 6% 8% 32%	2520
July	1st to 10th	Day	15% 6% 3 6% 5% 3 6% 5% 6% 7% 39%	2520
	11th to 20th	Day	13% 6% 3 5% 6% 3 5% 6% 6% 9% 37%	2520
	21st to end	Day	14% 5% 3 6% 6% 3 7% 5% 7% 8% 37%	2744
August	1st to 10th	Day	11% 6% 2 5% 6% 3 6% 6% 6% 9% 38%	2417
	11th to 20th	Day	13% 5% 2 6% 6% 4 7% 5% 6% 9% 36%	2242
	21st to end	Day	12% 5% 3 6% 5% 3 5% 5% 6% 8% 43%	2376
September	1st to 10th	Day	13% 6% 2 6% 5% 3 4 4 5% 8% 42%	2160
	11th to 20th	Day	14% 7% 2 4 4 3 4 3 4 7% 48%	2008
	21st to end	Day	17% 6% 3 5% 4 3 4 4 5% 6% 44%	1980
October	1st to 10th	Day	23% 8% 3 6% 4 3 4 4 3 5% 37%	1980
	11th to 20th	Day	14% 7% 3 6% 4 3 5% 5% 5% 6% 42%	1732
	21st to end	Day	23% 8% 3 5% 6% 3 4 4 4 7% 33%	1782
November	1st to 10th	Day	12% 9% 3 6% 4 2 4 4 4 7% 46%	1671
	11th to 20th	Day	18% 7% 2 5% 4 3 4 4 5% 6% 42%	1710
	21st to end	Day	18% 8% 3 5% 5% 3 4 4 5% 6% 38%	1663
December	1st to 10th	Day	17% 9% 4 8% 6% 3 5% 4 5% 6% 32%	1520
	11th to 20th	Day	21% 9% 3 5% 4 3 4 4 3 4 38%	1906
	21st to end	Day	20% 12% 4 6% 6% 3 5% 5% 5% 6% 29%	2772
January	1st to 10th	Night	13% 9% 3 6% 5% 3 4 5% 6% 7% 39%	2649
	11th to 20th	Night	16% 11% 4 6% 6% 3 6% 5% 6% 6% 32%	2546
	21st to end	Night	10% 7% 5% 8% 6% 4 5% 5% 5% 6% 38%	2772
February	1st to 10th	Night	15% 9% 4 8% 4 4 5% 5% 5% 7% 34%	2314
	11th to 20th	Night	11% 9% 3 6% 6% 4 6% 5% 6% 7% 37%	2098
	21st to end	Night	15% 10% 3 7% 6% 3 5% 5% 4 7% 36%	1776
March	1st to 10th	Night	14% 9% 3 6% 6% 3 4 4 6% 6% 39%	2150
	11th to 20th	Night	15% 8% 3 5% 7% 3 6% 5% 5% 6% 35%	2095
	21st to end	Night	17% 7% 3 6% 5% 3 5% 4 6% 6% 40%	2011
April	1st to 10th	Night	17% 9% 3 6% 6% 2 5% 4 6% 6% 36%	1800
	11th to 20th	Night	18% 7% 2 6% 5% 3 5% 4 5% 6% 39%	1775
	21st to end	Night	20% 8% 4 6% 5% 3 3 4 5% 5% 36%	1631
May	1st to 10th	Night	21% 10% 5% 6% 5% 3 4 5% 5% 5% 31%	1620
	11th to 20th	Night	18% 8% 3 6% 5% 3 4 5% 5% 7% 37%	1497
	21st to end	Night	15% 7% 3 7% 7% 3 3 4 5% 8% 38%	1550
June	1st to 10th	Night	18% 8% 3 6% 5% 3 4 4 5% 7% 37%	1440
	11th to 20th	Night	20% 7% 3 5% 5% 3 4 5% 5% 7% 37%	1440
	21st to end	Night	16% 6% 3 5% 4 3 4 6% 6% 8% 40%	1440
July	1st to 10th	Night	13% 5% 3 6% 5% 3 5% 5% 6% 7% 43%	1440
	11th to 20th	Night	14% 5% 2 5% 5% 3 5% 6% 7% 10% 39%	1440
	21st to end	Night	13% 4 2 4 3 3 6% 5% 5% 8% 46%	1574
August	1st to 10th	Night	9% 6% 3 5% 5% 3 4 5% 6% 8% 46%	1504
	11th to 20th	Night	13% 7% 3 6% 6% 3 4 5% 6% 8% 38%	1654
	21st to end	Night	15% 5% 2 4 4 3 4 4 5% 8% 48%	1980
September	1st to 10th	Night	14% 5% 2 5% 3 2 3 6% 5% 6% 49%	1800
	11th to 20th	Night	14% 6% 3 4 3 2 3 4 4 6% 52%	1950
	21st to end	Night	13% 7% 3 6% 4 3 4 4 4 7% 47%	1980
October	1st to 10th	Night	21% 7% 3 6% 5% 2 4 4 3 5% 40%	1980
	11th to 20th	Night	15% 7% 3 5% 4 3 4 6% 5% 6% 42%	2222
	21st to end	Night	21% 9% 3 6% 5% 3 5% 4 4 6% 34%	2574
November	1st to 10th	Night	15% 7% 3 6% 5% 2 4 4 5% 5% 45%	2407
	11th to 20th	Night	17% 6% 3 5% 5% 3 5% 4 4 5% 43%	2470
	21st to end	Night	15% 7% 3 6% 4 3 5% 5% 5% 6% 41%	2512
December	1st to 10th	Night	18% 10% 4 6% 5% 3 4 4 5% 7% 33%	2660
	11th to 20th	Night	19% 9% 4 5% 5% 3 5% 5% 5% 5% 37%	3288
	21st to end	Night	19% 12% 4 8% 6% 3 3 4 5% 6% 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	45% 6% 10% 25% 12% 2	1518
	11th to 20th	Day	48% 8% 14% 17% 11% 3	1454
	21st to end	Day	41% 7% 12% 22% 14% 3	1584
February	1st to 10th	Day	52% 8% 12% 15% 11% 2	1643
	11th to 20th	Day	47% 8% 10% 19% 14% 3	1757
	21st to end	Day	52% 6% 9% 18% 13% 3	1480
March	1st to 10th	Day	47% 5% 13% 18% 13% 4	1800
	11th to 20th	Day	47% 7% 11% 16% 15% 4	1862
	21st to end	Day	48% 5% 8% 17% 16% 5%	2343
April	1st to 10th	Day	49% 5% 9% 20% 14% 3	2160
	11th to 20th	Day	44% 7% 11% 19% 17% 2	2134
	21st to end	Day	53% 6% 8% 17% 14% 3	2327
May	1st to 10th	Day	54% 6% 8% 18% 12% 2	2340
	11th to 20th	Day	46% 5% 10% 20% 16% 3	2414
	21st to end	Day	42% 7% 13% 23% 13% 2	2725
June	1st to 10th	Day	48% 7% 13% 21% 10% 1	2520
	11th to 20th	Day	45% 8% 11% 19% 15% 2	2520
	21st to end	Day	49% 8% 12% 19% 11% 1	2520
July	1st to 10th	Day	41% 9% 11% 22% 16% 1	2450
	11th to 20th	Day	42% 9% 14% 24% 11% 1	2445
	21st to end	Day	41% 9% 14% 22% 13% 2	2591
August	1st to 10th	Day	39% 10% 15% 23% 12% 1	2403
	11th to 20th	Day	42% 8% 14% 23% 11% 2	2242
	21st to end	Day	38% 9% 12% 24% 15% 2	2376
September	1st to 10th	Day	40% 6% 12% 24% 16% 1	2160
	11th to 20th	Day	36% 6% 9% 24% 24% 2	2008
	21st to end	Day	39% 6% 10% 24% 18% 3	1980
October	1st to 10th	Day	51% 5% 7% 21% 12% 4	1980
	11th to 20th	Day	40% 7% 12% 28% 11% 2	1732
	21st to end	Day	51% 6% 10% 18% 11% 3	1782
November	1st to 10th	Day	40% 5% 13% 24% 15% 3	1671
	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	43% 7% 11% 21% 15% 3	2649
	11th to 20th	Night	49% 9% 12% 19% 8% 3	2546
	21st to end	Night	45% 7% 11% 18% 17% 2	2772
February	1st to 10th	Night	48% 8% 12% 18% 11% 2	2314
	11th to 20th	Night	44% 8% 12% 20% 14% 3	2100
	21st to end	Night	46% 7% 11% 22% 11% 3	1776
March	1st to 10th	Night	44% 7% 10% 23% 14% 3	2160
	11th to 20th	Night	47% 8% 11% 16% 15% 4	2096
	21st to end	Night	44% 5% 11% 16% 19% 5%	2011
April	1st to 10th	Night	47% 6% 10% 19% 15% 2	1800
	11th to 20th	Night	44% 6% 11% 20% 17% 2	1775
	21st to end	Night	48% 6% 11% 18% 13% 4	1631
May	1st to 10th	Night	53% 7% 8% 18% 12% 2	1620
	11th to 20th	Night	45% 7% 13% 17% 15% 3	1498
	21st to end	Night	45% 8% 11% 23% 11% 1	1551
June	1st to 10th	Night	47% 7% 12% 23% 11%	1440
	11th to 20th	Night	47% 7% 12% 19% 13% 2	1440
	21st to end	Night	41% 8% 15% 22% 13% 1	1440
July	1st to 10th	Night	39% 10% 13% 23% 14% 1	1400
	11th to 20th	Night	38% 10% 16% 26% 11%	1392
	21st to end	Night	34% 8% 16% 27% 12% 2	1486
August	1st to 10th	Night	34% 9% 16% 26% 14% 1	1494
	11th to 20th	Night	42% 8% 15% 22% 11% 1	1654
	21st to end	Night	36% 6% 12% 28% 17% 1	1980
September	1st to 10th	Night	36% 7% 12% 26% 18% 1	1800
	11th to 20th	Night	34% 6% 10% 26% 22% 2	1950
	21st to end	Night	38% 6% 9% 27% 17% 2	1980
October	1st to 10th	Night	46% 6% 11% 22% 11% 3	1980
	11th to 20th	Night	39% 8% 13% 27% 12% 1	2222
	21st to end	Night	51% 7% 11% 20% 10% 2	2574
November	1st to 10th	Night	40% 6% 12% 21% 18% 3	2407
	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% 26% 26% 17% 2	180
	11th to 20th	Day	32% 25% 31% 9% 2 1	164
	21st to end	Day	23% 32% 32% 9% 4	176
February	1st to 10th	Day	34% 32% 26% 7% 1	288
	11th to 20th	Day	32% 29% 27% 11% 1	416
	21st to end	Day	31% 27% 29% 10% 2	346
March	1st to 10th	Day	30% 22% 35% 10% 3	420
	11th to 20th	Day	28% 32% 27% 9% 3 1	420
	21st to end	Day	30% 25% 29% 9% 5% 2	462
April	1st to 10th	Day	33% 23% 25% 10% 6% 1	420
	11th to 20th	Day	26% 28% 28% 11% 6% 1	412
	21st to end	Day	33% 18% 32% 11% 5% 1	420
May	1st to 10th	Day	35% 22% 25% 11% 6% 1	423
	11th to 20th	Day	21% 22% 37% 14% 5% 2	447
	21st to end	Day	20% 15% 32% 25% 6% 1	485
June	1st to 10th	Day	20% 16% 36% 18% 9% 1	450
	11th to 20th	Day	18% 19% 34% 21% 7% 1	450
	21st to end	Day	19% 17% 35% 18% 9% 2	450
July	1st to 10th	Day	18% 10% 29% 28% 13% 2	445
	11th to 20th	Day	17% 9% 32% 30% 11% 2	444
	21st to end	Day	15% 12% 35% 27% 9% 2	488
August	1st to 10th	Day	13% 12% 29% 30% 13% 3	446
	11th to 20th	Day	14% 11% 31% 26% 15% 3	439
	21st to end	Day	14% 13% 31% 26% 12% 4	495
September	1st to 10th	Day	18% 14% 25% 25% 14% 5%	450
	11th to 20th	Day	19% 16% 26% 19% 12% 7%	450
	21st to end	Day	18% 17% 32% 19% 9% 5%	450
October	1st to 10th	Day	29% 16% 21% 20% 10% 3	450
	11th to 20th	Day	20% 19% 31% 22% 6% 2	413
	21st to end	Day	33% 17% 26% 19% 4 1	396
November	1st to 10th	Day	23% 22% 26% 18% 10% 1	365
	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	27% 26% 30% 15% 2	728
	21st to end	Night	20% 26% 34% 17% 3	792
February	1st to 10th	Night	27% 26% 33% 14% 1	720
	11th to 20th	Night	22% 27% 37% 13% 1	703
	21st to end	Night	29% 27% 29% 14% 1	592
March	1st to 10th	Night	26% 23% 32% 17% 2	720
	11th to 20th	Night	25% 25% 31% 16% 3	720
	21st to end	Night	25% 23% 30% 17% 5%	792
April	1st to 10th	Night	26% 20% 31% 20% 3	720
	11th to 20th	Night	22% 24% 29% 19% 6%	711
	21st to end	Night	28% 18% 32% 19% 3	551
May	1st to 10th	Night	29% 15% 32% 20% 4	540
	11th to 20th	Night	22% 16% 35% 22% 5%	436
	21st to end	Night	19% 9% 34% 31% 7% 1	389
June	1st to 10th	Night	20% 11% 32% 29% 9%	360
	11th to 20th	Night	21% 12% 32% 28% 8% 1	360
	21st to end	Night	16% 9% 32% 37% 6%	360
July	1st to 10th	Night	15% 6% 33% 37% 9%	350
	11th to 20th	Night	16% 3 29% 38% 13% 2	348
	21st to end	Night	11% 4 35% 38% 12% 1	394
August	1st to 10th	Night	12% 7% 28% 37% 16%	430
	11th to 20th	Night	15% 9% 28% 33% 13% 2	590
	21st to end	Night	17% 7% 25% 34% 15% 2	792
September	1st to 10th	Night	17% 8% 30% 30% 14% 1	720
	11th to 20th	Night	16% 10% 29% 27% 15% 2	720
	21st to end	Night	16% 13% 29% 27% 13% 2	720
October	1st to 10th	Night	25% 14% 25% 24% 10% 2	720
	11th to 20th	Night	21% 15% 32% 21% 9% 1	720
	21st to end	Night	28% 21% 25% 20% 5% 1	792
November	1st to 10th	Night	20% 21% 29% 21% 7% 1	740
	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 2024-01-09 16:24:27. Version ac07

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