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 2025-09-19.

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 7% 5% 4 5% 4 5% 7% 35%	126141
	Night	16% 9% 3 7% 4 4 4 4 5% 6% 37%	122909
Transparency	Day	46% 6% 11% 21% 14% 2	129648
	Night	44% 6% 12% 21% 14% 2	127489
Seeing	Day	25% 19% 30% 17% 7% 2	16042
	Night	22% 19% 30% 22% 6% 1	24922

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	10% 8% 4 13% 7% 5% 5% 4 5% 5% 33%	3597
	11th to 20th	Day	12% 9% 4 8% 5% 4 5% 3 6% 6% 38%	3570
	21st to end	Day	14% 11% 5% 8% 4 4 4 3 5% 5% 39%	3960
February	1st to 10th	Day	16% 9% 3 7% 5% 6% 6% 5% 6% 6% 31%	1825
	11th to 20th	Day	12% 7% 4 9% 6% 5% 4 4 6% 8% 34%	1948
	21st to end	Day	17% 11% 4 8% 5% 5% 4 4 4 5% 34%	1650
March	1st to 10th	Day	18% 13% 3 10% 5% 4 4 3 4 4 32%	4476
	11th to 20th	Day	15% 9% 3 8% 5% 5% 4 3 5% 6% 37%	4694
	21st to end	Day	22% 10% 3 6% 4 3 4 3 5% 6% 33%	5895
April	1st to 10th	Day	18% 12% 3 7% 5% 4 4 3 5% 5% 34%	2400
	11th to 20th	Day	16% 8% 3 6% 5% 6% 4 4 6% 6% 36%	2374
	21st to end	Day	20% 9% 3 8% 5% 4 4 3 4 6% 34%	2587
May	1st to 10th	Day	32% 10% 3 7% 4 4 5% 3 4 6% 23%	5886
	11th to 20th	Day	19% 11% 4 9% 6% 4 5% 4 4 6% 29%	6166
	21st to end	Day	19% 7% 2 7% 5% 4 5% 4 6% 7% 33%	6949
June	1st to 10th	Day	18% 7% 2 6% 5% 5% 5% 3 6% 9% 33%	2800
	11th to 20th	Day	17% 7% 2 7% 5% 4 5% 3 5% 9% 36%	2800
	21st to end	Day	18% 7% 3 7% 5% 6% 5% 4 6% 8% 33%	2800
July	1st to 10th	Day	14% 7% 3 7% 5% 5% 6% 4 7% 9% 33%	6360
	11th to 20th	Day	10% 6% 2 7% 5% 5% 6% 5% 8% 11% 36%	6360
	21st to end	Day	15% 6% 2 6% 4 4 5% 3 6% 7% 41%	6926
August	1st to 10th	Day	15% 7% 2 8% 5% 5% 5% 4 6% 8% 35%	4400
	11th to 20th	Day	13% 7% 3 8% 5% 5% 5% 4 6% 9% 35%	4053
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 4 7% 9% 43%	4312
September	1st to 10th	Day	12% 6% 2 6% 4 5% 4 3 6% 8% 44%	2400
	11th to 20th	Day	13% 7% 2 5% 3 4 3 3 4 7% 49%	2208
	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% 9% 3 6% 4 3 4 3 4 7% 46%	1761
	11th to 20th	Day	17% 8% 2 5% 3 4 3 3 6% 5% 44%	1800
	21st to end	Day	18% 10% 2 6% 4 4 4 3 5% 6% 37%	1750
December	1st to 10th	Day	13% 10% 4 9% 5% 4 5% 3 5% 7% 36%	2600
	11th to 20th	Day	18% 11% 3 7% 4 4 4 3 3 4 39%	2986
	21st to end	Day	18% 11% 4 8% 5% 3 4 3 5% 6% 34%	3960
January	1st to 10th	Night	12% 7% 4 11% 6% 6% 4 4 5% 6% 35%	6349
	11th to 20th	Night	11% 10% 4 10% 6% 5% 5% 4 6% 7% 33%	6315
	21st to end	Night	14% 9% 4 8% 5% 5% 5% 4 6% 6% 35%	6996
February	1st to 10th	Night	14% 10% 3 8% 5% 5% 4 4 5% 7% 35%	2572
	11th to 20th	Night	11% 9% 3 7% 6% 5% 5% 4 7% 7% 36%	2338
	21st to end	Night	14% 10% 3 8% 5% 4 4 4 5% 6% 37%	1980
March	1st to 10th	Night	19% 10% 3 6% 4 4 5% 3 6% 6% 33%	5364
	11th to 20th	Night	15% 9% 3 7% 6% 5% 5% 4 5% 6% 36%	5263
	21st to end	Night	23% 9% 2 7% 4 3 5% 4 6% 6% 32%	5021
April	1st to 10th	Night	18% 10% 3 8% 5% 3 5% 3 6% 6% 34%	2000
	11th to 20th	Night	18% 9% 2 6% 5% 3 5% 3 5% 6% 38%	1975
	21st to end	Night	19% 9% 4 6% 4 3 3 4 4 5% 38%	1811
May	1st to 10th	Night	32% 10% 3 6% 4 4 4 3 6% 6% 23%	4054
	11th to 20th	Night	15% 11% 4 10% 6% 5% 5% 3 5% 7% 28%	3769
	21st to end	Night	20% 9% 3 8% 6% 4 4 3 5% 7% 31%	3926
June	1st to 10th	Night	18% 9% 3 7% 4 4 3 3 5% 7% 38%	1600
	11th to 20th	Night	18% 8% 2 6% 5% 4 4 4 5% 7% 37%	1600
	21st to end	Night	16% 8% 3 5% 3 4 4 5% 6% 8% 39%	1600
July	1st to 10th	Night	12% 8% 2 7% 4 4 5% 4 6% 8% 40%	3600
	11th to 20th	Night	13% 8% 3 7% 5% 4 6% 4 6% 8% 38%	3600
	21st to end	Night	12% 6% 2 6% 3 4 4 3 4 6% 49%	3926
August	1st to 10th	Night	21% 8% 3 6% 4 4 4 3 5% 6% 36%	2718
	11th to 20th	Night	13% 9% 3 9% 5% 4 4 3 5% 6% 39%	2989
	21st to end	Night	9% 4 2 4 3 3 3 3 5% 8% 57%	3586
September	1st to 10th	Night	13% 5% 2 5% 3 3 4 4 4 6% 50%	2000
	11th to 20th	Night	12% 5% 3 5% 3 3 3 3 4 6% 53%	2147
	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%	2537
	11th to 20th	Night	16% 6% 3 5% 5% 4 4 4 5% 5% 43%	2600
	21st to end	Night	14% 9% 3 6% 4 4 5% 4 5% 6% 40%	2645
December	1st to 10th	Night	15% 11% 4 9% 5% 4 4 3 5% 7% 33%	4580
	11th to 20th	Night	22% 9% 4 6% 3 3 4 3 4 4 37%	5208
	21st to end	Night	17% 12% 4 8% 4 4 3 3 5% 5% 34%	6996

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	49% 5% 8% 25% 12% 1	3598
	11th to 20th	Day	43% 6% 11% 20% 16% 3	3600
	21st to end	Day	46% 6% 7% 21% 18% 3	3960
February	1st to 10th	Day	50% 7% 12% 17% 12% 2	1825
	11th to 20th	Day	46% 7% 10% 19% 14% 3	1957
	21st to end	Day	50% 6% 9% 19% 13% 2	1650
March	1st to 10th	Day	54% 4 10% 13% 11% 8%	4479
	11th to 20th	Day	44% 9% 14% 21% 11% 2	4694
	21st to end	Day	51% 3 8% 15% 18% 5%	5895
April	1st to 10th	Day	51% 5% 9% 19% 14% 3	2400
	11th to 20th	Day	45% 7% 11% 18% 17% 2	2374
	21st to end	Day	51% 5% 7% 18% 15% 3	2587
May	1st to 10th	Day	62% 7% 10% 13% 8% 1	5897
	11th to 20th	Day	55% 6% 14% 16% 9% 1	6170
	21st to end	Day	48% 7% 12% 21% 12% 1	6949
June	1st to 10th	Day	47% 7% 12% 21% 11% 1	2800
	11th to 20th	Day	45% 7% 12% 19% 14% 2	2800
	21st to end	Day	49% 7% 12% 19% 12% 1	2800
July	1st to 10th	Day	46% 6% 13% 19% 15% 1	6290
	11th to 20th	Day	40% 7% 17% 23% 13% 1	6285
	21st to end	Day	41% 5% 12% 26% 15% 1	6728
August	1st to 10th	Day	45% 7% 15% 21% 11%	4386
	11th to 20th	Day	44% 6% 16% 25% 8% 1	4092
	21st to end	Day	35% 7% 12% 29% 15% 2	4312
September	1st to 10th	Day	38% 6% 12% 26% 18% 1	2400
	11th to 20th	Day	36% 5% 9% 25% 24% 2	2208
	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	39% 5% 13% 25% 15% 2	1761
	11th to 20th	Day	41% 6% 11% 20% 20% 3	1800
	21st to end	Day	45% 6% 16% 21% 10% 2	1750
December	1st to 10th	Day	46% 6% 12% 27% 9% 1	2600
	11th to 20th	Day	47% 4 12% 17% 18% 2	2986
	21st to end	Day	50% 6% 8% 18% 13% 6%	3960
January	1st to 10th	Night	44% 6% 11% 25% 12% 2	6349
	11th to 20th	Night	47% 8% 13% 17% 13% 3	6360
	21st to end	Night	47% 6% 10% 16% 17% 4	6996
February	1st to 10th	Night	46% 8% 12% 20% 11% 2	2572
	11th to 20th	Night	43% 7% 13% 20% 14% 3	2340
	21st to end	Night	46% 6% 10% 22% 11% 4	1980
March	1st to 10th	Night	48% 5% 10% 16% 16% 5%	5374
	11th to 20th	Night	46% 8% 15% 18% 10% 2	5264
	21st to end	Night	50% 4 9% 12% 21% 4	5021
April	1st to 10th	Night	48% 6% 10% 18% 15% 2	2000
	11th to 20th	Night	46% 6% 11% 19% 17% 2	1975
	21st to end	Night	47% 6% 10% 17% 16% 4	1811
May	1st to 10th	Night	59% 7% 12% 13% 7% 1	4060
	11th to 20th	Night	53% 8% 13% 16% 9% 1	3770
	21st to end	Night	52% 7% 12% 18% 10% 1	3927
June	1st to 10th	Night	46% 7% 12% 24% 11% 1	1600
	11th to 20th	Night	46% 8% 13% 20% 12% 2	1600
	21st to end	Night	42% 8% 14% 22% 13% 1	1600
July	1st to 10th	Night	41% 6% 14% 26% 13%	3560
	11th to 20th	Night	41% 7% 17% 20% 15% 1	3552
	21st to end	Night	36% 4 14% 31% 14% 1	3809
August	1st to 10th	Night	49% 7% 13% 20% 11% 1	2708
	11th to 20th	Night	45% 6% 16% 23% 10%	3016
	21st to end	Night	26% 4 16% 35% 17% 1	3586
September	1st to 10th	Night	35% 7% 12% 27% 19% 1	2000
	11th to 20th	Night	33% 5% 10% 27% 22% 2	2147
	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	39% 6% 12% 22% 19% 3	2537
	11th to 20th	Night	42% 5% 11% 21% 18% 3	2600
	21st to end	Night	41% 7% 15% 24% 11% 1	2645
December	1st to 10th	Night	48% 6% 10% 27% 9%	4580
	11th to 20th	Night	48% 5% 14% 12% 20% 1	5208
	21st to end	Night	48% 7% 7% 19% 15% 4	6996

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	34% 25% 26% 15% 2	200
	11th to 20th	Day	34% 24% 33% 8% 2 1	200
	21st to end	Day	27% 30% 30% 10% 3	220
February	1st to 10th	Day	35% 31% 25% 8% 1	350
	11th to 20th	Day	35% 28% 27% 10% 1	496
	21st to end	Day	30% 29% 28% 10% 2	414
March	1st to 10th	Day	33% 22% 33% 10% 3	500
	11th to 20th	Day	29% 30% 28% 9% 4 1	500
	21st to end	Day	32% 25% 29% 8% 4 2	550
April	1st to 10th	Day	35% 24% 25% 9% 6% 1	500
	11th to 20th	Day	30% 28% 26% 10% 5% 1	492
	21st to end	Day	31% 19% 31% 12% 5% 1	500
May	1st to 10th	Day	36% 21% 25% 12% 5% 1	503
	11th to 20th	Day	21% 22% 37% 13% 6% 2	527
	21st to end	Day	21% 16% 31% 25% 5% 1	573
June	1st to 10th	Day	21% 16% 35% 18% 8% 1	530
	11th to 20th	Day	18% 18% 34% 21% 7% 1	530
	21st to end	Day	19% 18% 35% 17% 8% 2	530
July	1st to 10th	Day	17% 11% 33% 27% 11% 2	525
	11th to 20th	Day	15% 9% 34% 30% 10% 2	524
	21st to end	Day	16% 13% 36% 26% 8% 2	576
August	1st to 10th	Day	13% 11% 30% 29% 13% 3	526
	11th to 20th	Day	14% 10% 30% 26% 15% 4	519
	21st to end	Day	13% 12% 33% 26% 11% 4	583
September	1st to 10th	Day	17% 14% 27% 24% 13% 5%	530
	11th to 20th	Day	17% 15% 25% 22% 13% 8%	522
	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% 23% 26% 17% 10% 1	395
	11th to 20th	Day	31% 24% 24% 17% 4 1	400
	21st to end	Day	34% 21% 28% 13% 3 1	350
December	1st to 10th	Day	23% 35% 27% 11% 4	200
	11th to 20th	Day	39% 24% 28% 8% 2	200
	21st to end	Day	30% 24% 31% 11% 4	220
January	1st to 10th	Night	24% 27% 33% 14% 2	800
	11th to 20th	Night	28% 25% 31% 14% 3	800
	21st to end	Night	20% 26% 34% 17% 3	880
February	1st to 10th	Night	26% 25% 33% 15% 1	800
	11th to 20th	Night	24% 26% 36% 13% 1	783
	21st to end	Night	28% 27% 30% 14% 1	660
March	1st to 10th	Night	28% 23% 31% 16% 2	800
	11th to 20th	Night	25% 25% 30% 17% 3	800
	21st to end	Night	27% 22% 31% 16% 5%	880
April	1st to 10th	Night	28% 20% 31% 19% 3	800
	11th to 20th	Night	24% 24% 29% 18% 5%	791
	21st to end	Night	27% 20% 31% 19% 3	611
May	1st to 10th	Night	29% 15% 32% 20% 4	600
	11th to 20th	Night	22% 15% 35% 23% 4 1	484
	21st to end	Night	20% 9% 34% 30% 6%	433
June	1st to 10th	Night	20% 12% 33% 28% 8%	400
	11th to 20th	Night	20% 12% 33% 28% 7% 1	400
	21st to end	Night	17% 9% 31% 36% 7%	400
July	1st to 10th	Night	15% 6% 34% 36% 8%	390
	11th to 20th	Night	16% 4 29% 36% 13% 2	388
	21st to end	Night	12% 5% 36% 36% 11%	438
August	1st to 10th	Night	14% 6% 27% 37% 15%	478
	11th to 20th	Night	15% 9% 28% 34% 13% 2	657
	21st to end	Night	16% 6% 26% 34% 15% 1	880
September	1st to 10th	Night	16% 9% 29% 30% 14% 1	800
	11th to 20th	Night	15% 9% 27% 28% 17% 2	793
	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% 22% 30% 21% 7% 1	780
	11th to 20th	Night	22% 26% 26% 18% 6% 1	800
	21st to end	Night	25% 23% 32% 16% 4 1	800
December	1st to 10th	Night	26% 24% 31% 15% 4	800
	11th to 20th	Night	27% 23% 32% 14% 3	800
	21st to end	Night	27% 25% 31% 14% 3	880

Page generated at 2025-09-16 01:49:14. Version ac07

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