Richard S. Perkin Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Richard S. Perkin Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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	15% 10% 4 9% 6% 5% 5% 4 6% 7% 30%	132274
	Night	15% 11% 4 10% 6% 5% 5% 4 5% 7% 28%	130541
Transparency	Day	51% 7% 12% 19% 10% 1	135829
	Night	53% 6% 13% 18% 9% 1	135072
Seeing	Day	21% 16% 30% 20% 10% 3	16509
	Night	21% 15% 29% 25% 9% 1	25877

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	14% 17% 7% 19% 8% 4 5% 4 5% 4 14%	4661
	11th to 20th	Day	11% 18% 7% 16% 10% 7% 5% 4 4 5% 13%	4749
	21st to end	Day	12% 13% 6% 14% 11% 10% 7% 4 4 5% 14%	5780
February	1st to 10th	Day	15% 16% 5% 13% 9% 6% 6% 4 5% 4 17%	1890
	11th to 20th	Day	12% 16% 7% 16% 9% 6% 6% 3 4 5% 16%	1843
	21st to end	Day	18% 14% 5% 13% 8% 6% 4 2 4 6% 20%	1722
March	1st to 10th	Day	17% 12% 5% 11% 7% 5% 4 3 4 4 27%	4806
	11th to 20th	Day	10% 15% 5% 9% 5% 5% 4 3 5% 7% 32%	5090
	21st to end	Day	15% 12% 4 12% 6% 6% 6% 3 5% 6% 27%	5761
April	1st to 10th	Day	17% 13% 4 8% 6% 5% 5% 3 4 7% 29%	2520
	11th to 20th	Day	18% 10% 3 7% 5% 4 5% 4 4 6% 34%	2494
	21st to end	Day	19% 9% 4 8% 5% 4 4 3 5% 6% 32%	2604
May	1st to 10th	Day	26% 9% 3 7% 4 4 4 4 5% 6% 28%	6287
	11th to 20th	Day	15% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	6402
	21st to end	Day	18% 8% 3 7% 5% 4 5% 4 6% 7% 33%	6949
June	1st to 10th	Day	16% 8% 3 6% 4 5% 4 4 6% 8% 35%	2800
	11th to 20th	Day	12% 6% 2 7% 5% 5% 6% 4 7% 9% 36%	2800
	21st to end	Day	11% 6% 2 7% 6% 6% 5% 4 7% 8% 38%	2800
July	1st to 10th	Day	8% 5% 2 7% 5% 5% 6% 5% 7% 10% 40%	6360
	11th to 20th	Day	8% 5% 2 7% 6% 5% 7% 5% 8% 11% 37%	6360
	21st to end	Day	10% 5% 2 7% 5% 5% 6% 4 6% 8% 43%	6819
August	1st to 10th	Day	16% 6% 2 6% 4 5% 5% 4 6% 8% 37%	4234
	11th to 20th	Day	13% 6% 3 8% 6% 5% 7% 5% 7% 9% 32%	4122
	21st to end	Day	12% 6% 2 7% 5% 5% 4 4 6% 8% 40%	4312
September	1st to 10th	Day	14% 6% 2 6% 4 5% 4 5% 5% 8% 42%	2400
	11th to 20th	Day	12% 6% 3 5% 5% 5% 4 4 6% 8% 44%	2400
	21st to end	Day	15% 9% 3 6% 5% 5% 5% 4 6% 7% 35%	2108
October	1st to 10th	Day	15% 8% 2 8% 4 5% 5% 4 6% 6% 38%	2000
	11th to 20th	Day	18% 9% 3 8% 5% 6% 5% 3 6% 7% 30%	2000
	21st to end	Day	22% 10% 4 9% 7% 5% 4 4 6% 6% 24%	2161
November	1st to 10th	Day	13% 12% 4 9% 6% 5% 6% 3 5% 6% 32%	1851
	11th to 20th	Day	13% 13% 4 11% 6% 5% 5% 3 6% 7% 26%	1715
	21st to end	Day	19% 15% 4 9% 5% 4 5% 4 6% 7% 21%	1680
December	1st to 10th	Day	17% 17% 7% 12% 7% 5% 5% 4 6% 7% 13%	2680
	11th to 20th	Day	18% 14% 6% 14% 6% 5% 4 3 4 5% 20%	3066
	21st to end	Day	20% 17% 4 14% 8% 5% 4 4 5% 6% 13%	4048
January	1st to 10th	Night	14% 16% 7% 19% 7% 5% 5% 4 4 4 15%	8234
	11th to 20th	Night	11% 19% 6% 15% 8% 6% 5% 4 6% 6% 14%	8133
	21st to end	Night	13% 14% 6% 12% 9% 8% 6% 4 5% 6% 19%	8401
February	1st to 10th	Night	15% 15% 5% 13% 7% 6% 4 3 5% 5% 21%	2730
	11th to 20th	Night	14% 13% 6% 13% 8% 6% 5% 4 5% 6% 20%	2663
	21st to end	Night	16% 12% 4 13% 6% 5% 5% 4 5% 6% 23%	2083
March	1st to 10th	Night	15% 9% 5% 10% 7% 6% 5% 4 5% 7% 27%	5251
	11th to 20th	Night	14% 14% 4 7% 4 4 4 3 6% 7% 32%	5089
	21st to end	Night	18% 10% 4 11% 5% 5% 4 4 5% 6% 28%	5412
April	1st to 10th	Night	18% 10% 3 8% 7% 5% 4 5% 5% 6% 29%	2100
	11th to 20th	Night	19% 9% 2 7% 5% 5% 5% 3 4 5% 35%	2075
	21st to end	Night	16% 8% 2 7% 5% 5% 4 4 6% 7% 35%	2032
May	1st to 10th	Night	29% 11% 3 7% 4 4 4 3 4 5% 25%	3872
	11th to 20th	Night	15% 8% 3 7% 5% 5% 5% 4 6% 7% 36%	3623
	21st to end	Night	17% 7% 2 7% 5% 5% 5% 4 5% 7% 36%	3926
June	1st to 10th	Night	16% 7% 3 7% 3 4 4 4 5% 9% 38%	1600
	11th to 20th	Night	10% 7% 3 7% 5% 4 5% 4 6% 8% 42%	1600
	21st to end	Night	10% 5% 2 7% 5% 5% 5% 4 6% 8% 43%	1600
July	1st to 10th	Night	8% 6% 3 7% 5% 5% 5% 4 7% 11% 40%	3600
	11th to 20th	Night	8% 5% 2 6% 4 5% 5% 4 6% 8% 45%	3600
	21st to end	Night	10% 5% 2 6% 4 4 4 4 6% 8% 47%	4029
August	1st to 10th	Night	14% 7% 2 5% 4 4 4 4 7% 8% 40%	2915
	11th to 20th	Night	15% 6% 2 7% 5% 4 4 4 4 7% 40%	2893
	21st to end	Night	8% 4 2 6% 5% 4 5% 4 6% 9% 47%	3586
September	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 5% 7% 45%	2000
	11th to 20th	Night	13% 5% 2 4 4 4 4 3 5% 7% 48%	2000
	21st to end	Night	16% 7% 2 6% 5% 4 5% 4 6% 7% 38%	2289
October	1st to 10th	Night	18% 7% 3 8% 5% 5% 4 4 5% 6% 37%	2400
	11th to 20th	Night	18% 9% 3 7% 4 5% 4 4 5% 7% 34%	2400
	21st to end	Night	23% 11% 3 8% 5% 4 4 3 5% 6% 28%	2677
November	1st to 10th	Night	16% 11% 3 8% 5% 4 4 3 5% 6% 34%	2667
	11th to 20th	Night	16% 11% 3 8% 6% 5% 5% 4 6% 7% 29%	2903
	21st to end	Night	16% 15% 4 8% 5% 6% 5% 4 6% 7% 24%	2940
December	1st to 10th	Night	16% 16% 6% 13% 6% 6% 6% 4 6% 6% 15%	4720
	11th to 20th	Night	19% 16% 5% 11% 6% 4 5% 3 5% 5% 21%	5348
	21st to end	Night	24% 16% 5% 11% 6% 5% 5% 4 5% 5% 14%	7150

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	70% 6% 12% 10% 3	4662
	11th to 20th	Day	70% 5% 9% 9% 6% 1	4779
	21st to end	Day	70% 4 11% 11% 4	5780
February	1st to 10th	Day	69% 6% 10% 11% 4	1890
	11th to 20th	Day	71% 5% 8% 13% 3	1852
	21st to end	Day	65% 5% 10% 15% 5% 1	1722
March	1st to 10th	Day	59% 4 12% 17% 7%	4809
	11th to 20th	Day	51% 5% 18% 16% 9% 1	5090
	21st to end	Day	57% 6% 8% 19% 9% 1	5761
April	1st to 10th	Day	56% 5% 10% 18% 9% 1	2520
	11th to 20th	Day	51% 7% 10% 18% 13% 1	2494
	21st to end	Day	51% 6% 9% 19% 13% 2	2604
May	1st to 10th	Day	57% 5% 11% 15% 10% 2	6296
	11th to 20th	Day	49% 8% 11% 20% 11% 1	6406
	21st to end	Day	49% 7% 12% 20% 11% 2	6949
June	1st to 10th	Day	45% 7% 11% 20% 16% 1	2800
	11th to 20th	Day	41% 9% 14% 23% 12% 1	2800
	21st to end	Day	39% 10% 15% 22% 14% 1	2800
July	1st to 10th	Day	37% 7% 14% 27% 14% 1	6290
	11th to 20th	Day	36% 9% 17% 23% 14% 1	6285
	21st to end	Day	37% 7% 14% 27% 15%	6623
August	1st to 10th	Day	43% 8% 14% 20% 14% 1	4221
	11th to 20th	Day	44% 9% 15% 25% 6%	4161
	21st to end	Day	41% 6% 11% 33% 8% 1	4312
September	1st to 10th	Day	40% 7% 11% 29% 12% 1	2400
	11th to 20th	Day	37% 6% 13% 28% 16% 1	2400
	21st to end	Day	47% 8% 11% 21% 12% 1	2108
October	1st to 10th	Day	37% 7% 14% 27% 14% 1	3260
	11th to 20th	Day	45% 6% 14% 21% 13%	3240
	21st to end	Day	45% 7% 15% 25% 8% 1	3475
November	1st to 10th	Day	51% 8% 12% 18% 10% 2	1851
	11th to 20th	Day	54% 7% 15% 16% 7% 1	1715
	21st to end	Day	61% 6% 17% 13% 2	1680
December	1st to 10th	Day	69% 6% 14% 10% 2	2680
	11th to 20th	Day	65% 6% 13% 10% 6%	3066
	21st to end	Day	72% 5% 13% 7% 2	4048
January	1st to 10th	Night	70% 5% 11% 10% 4	8234
	11th to 20th	Night	68% 6% 10% 10% 5% 1	8178
	21st to end	Night	64% 5% 9% 12% 9% 1	8401
February	1st to 10th	Night	63% 6% 12% 13% 6%	2730
	11th to 20th	Night	64% 5% 12% 14% 4 1	2665
	21st to end	Night	60% 5% 14% 13% 7% 1	2083
March	1st to 10th	Night	54% 6% 15% 17% 8%	5261
	11th to 20th	Night	51% 5% 18% 17% 6% 3	5090
	21st to end	Night	55% 5% 9% 19% 10% 1	5412
April	1st to 10th	Night	54% 6% 12% 18% 9% 1	2100
	11th to 20th	Night	51% 6% 11% 17% 14% 1	2075
	21st to end	Night	46% 5% 12% 21% 13% 3	2032
May	1st to 10th	Night	60% 7% 9% 14% 9% 3	3879
	11th to 20th	Night	45% 8% 15% 20% 12% 1	3624
	21st to end	Night	46% 7% 14% 18% 13% 2	3927
June	1st to 10th	Night	43% 7% 12% 22% 15% 1	1600
	11th to 20th	Night	38% 7% 17% 22% 14% 2	1600
	21st to end	Night	37% 8% 13% 26% 15% 1	1600
July	1st to 10th	Night	38% 7% 16% 24% 13% 2	3560
	11th to 20th	Night	33% 8% 15% 28% 15% 1	3552
	21st to end	Night	34% 7% 13% 27% 19% 1	3910
August	1st to 10th	Night	40% 7% 15% 26% 11% 1	2904
	11th to 20th	Night	42% 7% 14% 28% 9%	2920
	21st to end	Night	32% 8% 14% 33% 11% 1	3586
September	1st to 10th	Night	37% 7% 15% 26% 13% 2	2000
	11th to 20th	Night	35% 6% 13% 29% 16% 1	2000
	21st to end	Night	44% 8% 13% 22% 12% 1	2289
October	1st to 10th	Night	45% 6% 12% 25% 12%	3920
	11th to 20th	Night	44% 6% 13% 22% 13% 2	3899
	21st to end	Night	44% 5% 15% 26% 9% 1	4313
November	1st to 10th	Night	49% 6% 12% 21% 9% 2	2667
	11th to 20th	Night	52% 6% 15% 17% 7% 2	2903
	21st to end	Night	58% 7% 17% 15% 4	2940
December	1st to 10th	Night	67% 8% 14% 10% 1	4720
	11th to 20th	Night	63% 5% 11% 14% 7%	5348
	21st to end	Night	70% 7% 10% 10% 3	7150

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	28% 25% 34% 10% 4	220
	11th to 20th	Day	28% 30% 31% 9% 1 1	239
	21st to end	Day	21% 28% 38% 10% 2	363
February	1st to 10th	Day	27% 28% 34% 8% 2	330
	11th to 20th	Day	31% 29% 33% 7%	330
	21st to end	Day	32% 20% 35% 12% 1	438
March	1st to 10th	Day	25% 24% 31% 16% 3 1	540
	11th to 20th	Day	28% 25% 29% 14% 4	540
	21st to end	Day	30% 22% 30% 13% 4 1	594
April	1st to 10th	Day	29% 21% 32% 13% 4 1	540
	11th to 20th	Day	26% 23% 28% 14% 7% 2	532
	21st to end	Day	27% 19% 29% 19% 6% 1	540
May	1st to 10th	Day	24% 18% 32% 16% 7% 3	543
	11th to 20th	Day	18% 14% 32% 24% 10% 2	551
	21st to end	Day	16% 12% 31% 23% 14% 3	573
June	1st to 10th	Day	18% 13% 31% 25% 11% 2	530
	11th to 20th	Day	14% 11% 32% 30% 11% 3	530
	21st to end	Day	13% 12% 35% 28% 9% 2	530
July	1st to 10th	Day	10% 7% 27% 33% 18% 5%	525
	11th to 20th	Day	9% 7% 26% 31% 23% 4	524
	21st to end	Day	10% 6% 34% 32% 14% 4	576
August	1st to 10th	Day	10% 7% 24% 30% 20% 8%	526
	11th to 20th	Day	14% 6% 28% 27% 16% 9%	519
	21st to end	Day	12% 9% 26% 28% 16% 9%	583
September	1st to 10th	Day	17% 11% 23% 22% 16% 11%	530
	11th to 20th	Day	10% 8% 22% 24% 22% 14%	530
	21st to end	Day	18% 14% 26% 22% 15% 5%	530
October	1st to 10th	Day	18% 12% 29% 20% 16% 6%	530
	11th to 20th	Day	23% 16% 32% 20% 7% 2	530
	21st to end	Day	28% 19% 30% 19% 4	561
November	1st to 10th	Day	24% 20% 29% 19% 8% 1	425
	11th to 20th	Day	25% 25% 29% 17% 3 1	255
	21st to end	Day	28% 30% 30% 11% 1	220
December	1st to 10th	Day	28% 19% 37% 11% 4 1	220
	11th to 20th	Day	30% 25% 31% 12% 1	220
	21st to end	Day	30% 21% 36% 11% 2	242
January	1st to 10th	Night	25% 24% 34% 15% 2	840
	11th to 20th	Night	26% 23% 31% 18% 2	840
	21st to end	Night	20% 24% 35% 19% 3	924
February	1st to 10th	Night	26% 21% 32% 19% 2	840
	11th to 20th	Night	24% 23% 36% 15% 2	823
	21st to end	Night	26% 22% 33% 17% 3	692
March	1st to 10th	Night	24% 19% 35% 20% 3	840
	11th to 20th	Night	25% 21% 28% 23% 3	840
	21st to end	Night	26% 17% 32% 19% 5%	924
April	1st to 10th	Night	24% 18% 34% 21% 3	840
	11th to 20th	Night	24% 16% 30% 23% 6% 1	831
	21st to end	Night	19% 13% 31% 29% 8%	772
May	1st to 10th	Night	24% 8% 27% 30% 10% 1	515
	11th to 20th	Night	14% 9% 32% 35% 9% 1	406
	21st to end	Night	12% 7% 30% 32% 18%	433
June	1st to 10th	Night	16% 8% 28% 33% 16% 1	400
	11th to 20th	Night	12% 9% 25% 40% 15%	400
	21st to end	Night	11% 7% 27% 44% 10% 1	400
July	1st to 10th	Night	8% 4 20% 46% 20% 2	390
	11th to 20th	Night	11% 4 19% 43% 23% 1	388
	21st to end	Night	11% 4 23% 42% 17% 2	479
August	1st to 10th	Night	10% 4 24% 35% 23% 4	597
	11th to 20th	Night	12% 6% 18% 34% 27% 3	599
	21st to end	Night	11% 6% 22% 37% 22% 1	880
September	1st to 10th	Night	15% 7% 23% 32% 19% 3	800
	11th to 20th	Night	13% 7% 19% 30% 26% 5%	800
	21st to end	Night	16% 11% 24% 27% 19% 3	800
October	1st to 10th	Night	22% 10% 28% 21% 19% 2	800
	11th to 20th	Night	22% 15% 30% 24% 7% 1	800
	21st to end	Night	30% 13% 32% 21% 4	880
November	1st to 10th	Night	22% 16% 31% 24% 7%	820
	11th to 20th	Night	23% 21% 31% 19% 5% 2	840
	21st to end	Night	23% 22% 30% 20% 5%	840
December	1st to 10th	Night	26% 24% 32% 15% 3	840
	11th to 20th	Night	26% 24% 30% 18% 3	840
	21st to end	Night	31% 21% 31% 15% 2	924

Page generated at 2026-05-14 01:58:55. Version ac07

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