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 2024-01-22.

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% 9% 4 8% 7% 4 6% 5% 5% 7% 30%	74784
	Night	16% 10% 4 8% 6% 4 5% 5% 5% 7% 30%	75147
Transparency	Day	52% 7% 12% 18% 9% 1	74492
	Night	53% 6% 13% 18% 9% 1	74973
Seeing	Day	21% 16% 30% 21% 10% 3	13520
	Night	21% 15% 29% 24% 9% 1	22928

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	13% 15% 8% 12% 9% 4 8% 6% 5% 4 16%	1517
	11th to 20th	Day	16% 17% 7% 13% 11% 5% 5% 4 4 5% 14%	1550
	21st to end	Day	12% 15% 7% 13% 13% 6% 7% 4 4 5% 14%	1791
February	1st to 10th	Day	16% 15% 7% 12% 9% 5% 7% 4 5% 5% 15%	1620
	11th to 20th	Day	13% 14% 9% 13% 10% 5% 7% 5% 4 5% 16%	1573
	21st to end	Day	19% 14% 5% 11% 8% 5% 6% 3 4 6% 19%	1473
March	1st to 10th	Day	14% 12% 7% 12% 8% 4 6% 5% 4 5% 22%	1882
	11th to 20th	Day	14% 14% 6% 9% 8% 4 5% 4 4 6% 27%	1980
	21st to end	Day	20% 11% 4 8% 6% 4 5% 4 5% 6% 28%	2239
April	1st to 10th	Day	16% 11% 4 8% 6% 3 6% 5% 5% 7% 30%	2160
	11th to 20th	Day	19% 8% 3 6% 6% 3 5% 4 4 7% 36%	2134
	21st to end	Day	19% 9% 4 8% 5% 3 4 4 5% 7% 32%	2232
May	1st to 10th	Day	16% 8% 4 6% 8% 4 6% 4 5% 7% 33%	2436
	11th to 20th	Day	15% 7% 3 6% 5% 4 5% 5% 5% 8% 37%	2478
	21st to end	Day	13% 5% 2 6% 6% 4 5% 5% 6% 8% 41%	2725
June	1st to 10th	Day	17% 7% 3 5% 5% 3 4 5% 6% 8% 36%	2520
	11th to 20th	Day	13% 5% 3 5% 6% 3 5% 5% 7% 9% 38%	2520
	21st to end	Day	11% 6% 2 6% 6% 4 6% 5% 7% 8% 38%	2520
July	1st to 10th	Day	9% 5% 3 6% 6% 3 5% 5% 7% 10% 42%	2520
	11th to 20th	Day	9% 5% 3 5% 7% 3 7% 6% 7% 9% 38%	2520
	21st to end	Day	11% 5% 2 6% 7% 4 7% 7% 7% 8% 36%	2704
August	1st to 10th	Day	10% 5% 2 5% 6% 4 7% 7% 8% 9% 37%	2327
	11th to 20th	Day	11% 6% 3 6% 7% 4 8% 7% 7% 8% 32%	2281
	21st to end	Day	13% 6% 3 6% 6% 5% 5% 6% 6% 9% 35%	2376
September	1st to 10th	Day	15% 6% 2 5% 5% 4 5% 6% 5% 8% 39%	2160
	11th to 20th	Day	13% 6% 3 4 5% 4 4 5% 5% 8% 43%	2160
	21st to end	Day	15% 7% 3 5% 7% 4 5% 6% 6% 7% 36%	1899
October	1st to 10th	Day	17% 8% 2 7% 6% 4 5% 5% 6% 6% 34%	1800
	11th to 20th	Day	18% 9% 3 6% 7% 5% 5% 4 6% 7% 29%	1800
	21st to end	Day	24% 10% 4 8% 8% 4 4 4 5% 6% 23%	1945
November	1st to 10th	Day	14% 11% 4 7% 7% 3 6% 5% 5% 6% 32%	1671
	11th to 20th	Day	13% 13% 4 10% 7% 4 6% 4 6% 8% 27%	1553
	21st to end	Day	19% 14% 4 8% 7% 2 6% 5% 6% 7% 22%	1520
December	1st to 10th	Day	18% 16% 8% 10% 7% 4 5% 4 5% 6% 17%	1520
	11th to 20th	Day	19% 14% 7% 12% 7% 4 5% 4 5% 5% 19%	1906
	21st to end	Day	26% 19% 5% 11% 7% 4 4 4 4 5% 12%	2772
January	1st to 10th	Night	15% 16% 6% 12% 8% 4 6% 6% 5% 5% 16%	2649
	11th to 20th	Night	18% 16% 6% 11% 9% 5% 5% 4 5% 6% 15%	2627
	21st to end	Night	12% 13% 7% 11% 10% 6% 7% 5% 5% 6% 17%	2599
February	1st to 10th	Night	17% 14% 7% 12% 7% 3 5% 4 5% 5% 21%	2340
	11th to 20th	Night	15% 12% 6% 12% 8% 5% 6% 5% 5% 6% 21%	2273
	21st to end	Night	18% 10% 5% 12% 7% 4 5% 6% 5% 6% 23%	1782
March	1st to 10th	Night	14% 13% 6% 8% 8% 4 6% 6% 5% 6% 24%	2062
	11th to 20th	Night	17% 11% 6% 8% 6% 3 4 4 5% 5% 30%	1979
	21st to end	Night	18% 10% 4 8% 7% 3 5% 4 4 6% 32%	2116
April	1st to 10th	Night	18% 9% 4 7% 7% 4 4 5% 4 7% 31%	1800
	11th to 20th	Night	19% 7% 3 6% 6% 4 5% 5% 4 6% 35%	1775
	21st to end	Night	17% 8% 2 7% 6% 3 5% 4 6% 7% 35%	1742
May	1st to 10th	Night	19% 9% 3 6% 5% 4 5% 5% 5% 7% 32%	1522
	11th to 20th	Night	12% 5% 2 5% 6% 4 5% 6% 6% 9% 40%	1415
	21st to end	Night	9% 5% 3 5% 6% 3 5% 4 6% 8% 46%	1550
June	1st to 10th	Night	15% 5% 3 6% 4 4 4 6% 5% 9% 38%	1440
	11th to 20th	Night	10% 6% 3 6% 5% 3 4 5% 6% 8% 43%	1440
	21st to end	Night	10% 5% 2 5% 6% 5% 5% 5% 6% 8% 43%	1440
July	1st to 10th	Night	6% 4 2 6% 5% 4 6% 5% 6% 10% 46%	1440
	11th to 20th	Night	9% 5% 3 4 5% 3 5% 4 6% 8% 48%	1440
	21st to end	Night	12% 5% 3 4 5% 2 5% 5% 6% 10% 42%	1610
August	1st to 10th	Night	9% 5% 2 5% 5% 2 5% 5% 7% 8% 47%	1611
	11th to 20th	Night	11% 5% 2 7% 6% 3 5% 5% 5% 7% 43%	1602
	21st to end	Night	10% 4 2 6% 4 3 4 4 7% 9% 47%	1980
September	1st to 10th	Night	14% 6% 2 5% 5% 3 4 5% 6% 8% 43%	1800
	11th to 20th	Night	14% 5% 3 4 4 3 4 4 5% 8% 46%	1800
	21st to end	Night	15% 5% 2 5% 6% 3 5% 5% 6% 7% 40%	2059
October	1st to 10th	Night	18% 8% 3 7% 6% 4 4 5% 5% 6% 35%	2160
	11th to 20th	Night	19% 9% 3 6% 5% 3 5% 5% 5% 7% 33%	2160
	21st to end	Night	25% 10% 3 7% 7% 3 4 5% 5% 6% 25%	2409
November	1st to 10th	Night	16% 11% 4 7% 6% 3 4 4 5% 7% 34%	2407
	11th to 20th	Night	17% 10% 4 7% 6% 3 6% 5% 6% 7% 31%	2626
	21st to end	Night	16% 14% 5% 7% 6% 4 5% 5% 6% 7% 25%	2660
December	1st to 10th	Night	19% 15% 6% 8% 6% 4 6% 5% 6% 6% 19%	2660
	11th to 20th	Night	19% 15% 6% 10% 7% 3 5% 4 4 5% 21%	3288
	21st to end	Night	30% 14% 5% 8% 6% 4 5% 5% 5% 5% 13%	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	69% 6% 11% 11% 3	1518
	11th to 20th	Day	73% 5% 11% 8% 3	1550
	21st to end	Day	72% 5% 12% 8% 3	1791
February	1st to 10th	Day	70% 6% 10% 9% 4	1620
	11th to 20th	Day	71% 5% 8% 13% 3	1582
	21st to end	Day	67% 4 10% 13% 5% 1	1473
March	1st to 10th	Day	63% 6% 13% 14% 4 1	1885
	11th to 20th	Day	59% 6% 10% 15% 9% 1	1980
	21st to end	Day	58% 5% 8% 17% 10% 2	2239
April	1st to 10th	Day	54% 6% 10% 19% 10% 1	2160
	11th to 20th	Day	49% 7% 10% 19% 14% 2	2134
	21st to end	Day	52% 6% 8% 18% 14% 2	2232
May	1st to 10th	Day	51% 6% 11% 19% 11% 2	2436
	11th to 20th	Day	44% 8% 12% 21% 13% 2	2482
	21st to end	Day	41% 8% 15% 22% 12% 1	2725
June	1st to 10th	Day	45% 7% 11% 20% 16% 1	2520
	11th to 20th	Day	40% 9% 14% 24% 12% 1	2520
	21st to end	Day	39% 10% 15% 22% 13% 1	2520
July	1st to 10th	Day	37% 8% 13% 27% 13% 2	2450
	11th to 20th	Day	38% 10% 16% 23% 12% 1	2445
	21st to end	Day	42% 10% 14% 22% 11% 1	2553
August	1st to 10th	Day	39% 10% 16% 22% 12% 1	2314
	11th to 20th	Day	44% 10% 14% 23% 8%	2281
	21st to end	Day	44% 8% 12% 24% 11% 1	2376
September	1st to 10th	Day	43% 8% 11% 26% 12% 1	2160
	11th to 20th	Day	39% 6% 13% 26% 16% 1	2160
	21st to end	Day	46% 8% 11% 22% 12% 1	1899
October	1st to 10th	Day	48% 7% 14% 19% 12% 1	1800
	11th to 20th	Day	52% 7% 14% 19% 7% 1	1800
	21st to end	Day	60% 7% 12% 14% 5% 2	1945
November	1st to 10th	Day	51% 9% 12% 17% 10% 2	1671
	11th to 20th	Day	54% 7% 15% 16% 6% 1	1553
	21st to end	Day	61% 6% 17% 14% 3	1520
December	1st to 10th	Day	68% 6% 13% 11% 2	1520
	11th to 20th	Day	68% 6% 15% 8% 4	1906
	21st to end	Day	76% 5% 10% 8% 1	2772
January	1st to 10th	Night	69% 6% 12% 9% 4	2649
	11th to 20th	Night	70% 7% 11% 9% 3	2627
	21st to end	Night	67% 5% 12% 11% 4 1	2599
February	1st to 10th	Night	65% 6% 12% 12% 6%	2340
	11th to 20th	Night	64% 5% 12% 14% 4 1	2275
	21st to end	Night	60% 5% 15% 14% 7% 1	1782
March	1st to 10th	Night	60% 5% 13% 17% 5%	2072
	11th to 20th	Night	57% 5% 11% 19% 8% 1	1980
	21st to end	Night	54% 4 10% 17% 12% 2	2116
April	1st to 10th	Night	52% 6% 13% 19% 9% 1	1800
	11th to 20th	Night	50% 6% 11% 17% 15% 1	1775
	21st to end	Night	48% 5% 12% 20% 13% 3	1742
May	1st to 10th	Night	50% 8% 9% 17% 13% 2	1522
	11th to 20th	Night	39% 9% 12% 24% 15% 2	1416
	21st to end	Night	36% 7% 18% 22% 15% 3	1551
June	1st to 10th	Night	43% 7% 13% 22% 15% 1	1440
	11th to 20th	Night	38% 7% 17% 22% 14% 3	1440
	21st to end	Night	36% 8% 13% 27% 15% 1	1440
July	1st to 10th	Night	33% 9% 15% 23% 17% 4	1400
	11th to 20th	Night	32% 8% 13% 29% 17% 1	1392
	21st to end	Night	37% 8% 15% 24% 15% 2	1520
August	1st to 10th	Night	34% 8% 15% 31% 12% 1	1600
	11th to 20th	Night	38% 8% 16% 30% 8%	1602
	21st to end	Night	32% 9% 16% 28% 13% 2	1980
September	1st to 10th	Night	38% 7% 16% 24% 12% 2	1800
	11th to 20th	Night	37% 6% 13% 27% 16% 1	1800
	21st to end	Night	41% 8% 14% 23% 13% 1	2059
October	1st to 10th	Night	48% 7% 14% 18% 12%	2160
	11th to 20th	Night	48% 6% 16% 19% 9% 1	2160
	21st to end	Night	60% 4 14% 13% 6% 1	2409
November	1st to 10th	Night	49% 7% 13% 20% 9% 2	2407
	11th to 20th	Night	52% 6% 15% 18% 7% 2	2626
	21st to end	Night	58% 7% 17% 15% 4	2660
December	1st to 10th	Night	64% 8% 15% 11% 2	2660
	11th to 20th	Night	65% 5% 12% 14% 3	3288
	21st to end	Night	73% 6% 11% 9% 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	27% 22% 34% 12% 4	180
	11th to 20th	Day	32% 28% 29% 10% 1 1	195
	21st to end	Day	20% 28% 37% 11% 3	267
February	1st to 10th	Day	31% 30% 33% 7%	240
	11th to 20th	Day	31% 29% 37% 4	240
	21st to end	Day	34% 21% 31% 13% 1	339
March	1st to 10th	Day	24% 24% 32% 17% 2	420
	11th to 20th	Day	25% 25% 30% 15% 5%	420
	21st to end	Day	31% 20% 29% 14% 5% 1	462
April	1st to 10th	Day	27% 20% 32% 15% 5% 1	420
	11th to 20th	Day	27% 22% 25% 15% 8% 3	412
	21st to end	Day	30% 18% 30% 16% 6% 1	420
May	1st to 10th	Day	22% 20% 30% 16% 9% 3	423
	11th to 20th	Day	18% 14% 32% 24% 9% 2	447
	21st to end	Day	15% 12% 31% 23% 15% 3	485
June	1st to 10th	Day	18% 13% 30% 27% 10% 2	450
	11th to 20th	Day	14% 11% 31% 30% 11% 3	450
	21st to end	Day	13% 13% 34% 28% 9% 3	450
July	1st to 10th	Day	10% 7% 27% 32% 19% 5%	445
	11th to 20th	Day	10% 6% 26% 32% 22% 4	444
	21st to end	Day	11% 7% 34% 31% 13% 4	488
August	1st to 10th	Day	10% 6% 27% 31% 18% 8%	446
	11th to 20th	Day	14% 5% 28% 27% 16% 10%	439
	21st to end	Day	12% 10% 26% 27% 16% 9%	495
September	1st to 10th	Day	18% 10% 23% 23% 16% 9%	450
	11th to 20th	Day	12% 9% 25% 26% 18% 10%	450
	21st to end	Day	16% 14% 27% 22% 15% 5%	450
October	1st to 10th	Day	20% 13% 29% 18% 15% 5%	450
	11th to 20th	Day	23% 16% 33% 20% 7% 1	450
	21st to end	Day	31% 20% 29% 17% 4	477
November	1st to 10th	Day	24% 18% 29% 20% 9% 1	365
	11th to 20th	Day	26% 25% 29% 17% 2	213
	21st to end	Day	28% 28% 30% 12% 1	180
December	1st to 10th	Day	28% 19% 37% 10% 4 1	180
	11th to 20th	Day	31% 25% 29% 14% 1	180
	21st to end	Day	31% 21% 37% 10% 1	198
January	1st to 10th	Night	26% 23% 33% 15% 3	760
	11th to 20th	Night	26% 22% 31% 18% 2 1	760
	21st to end	Night	19% 22% 37% 19% 3	800
February	1st to 10th	Night	27% 22% 33% 17% 1	720
	11th to 20th	Night	23% 23% 38% 14% 2	703
	21st to end	Night	26% 23% 32% 16% 3	592
March	1st to 10th	Night	24% 20% 33% 20% 3	720
	11th to 20th	Night	25% 21% 28% 23% 3	720
	21st to end	Night	25% 16% 32% 21% 6% 1	792
April	1st to 10th	Night	23% 17% 34% 22% 4	720
	11th to 20th	Night	23% 16% 31% 23% 7% 1	711
	21st to end	Night	20% 13% 30% 28% 8%	662
May	1st to 10th	Night	23% 8% 27% 30% 11% 1	442
	11th to 20th	Night	14% 9% 33% 34% 9% 1	354
	21st to end	Night	12% 7% 31% 32% 19%	389
June	1st to 10th	Night	16% 7% 27% 33% 16% 1	360
	11th to 20th	Night	12% 9% 25% 38% 16%	360
	21st to end	Night	11% 7% 27% 44% 9% 1	360
July	1st to 10th	Night	8% 4 19% 46% 21% 2	350
	11th to 20th	Night	12% 3 20% 41% 22% 1	348
	21st to end	Night	12% 4 25% 40% 17% 3	430
August	1st to 10th	Night	10% 4 26% 34% 22% 4	537
	11th to 20th	Night	12% 5% 19% 35% 26% 3	538
	21st to end	Night	11% 6% 22% 36% 23% 1	792
September	1st to 10th	Night	16% 7% 23% 32% 19% 3	720
	11th to 20th	Night	14% 8% 21% 30% 23% 5%	720
	21st to end	Night	15% 11% 25% 27% 19% 3	720
October	1st to 10th	Night	22% 11% 28% 19% 18% 2	720
	11th to 20th	Night	22% 16% 31% 24% 7%	720
	21st to end	Night	32% 13% 32% 20% 3	792
November	1st to 10th	Night	22% 16% 31% 24% 7%	740
	11th to 20th	Night	23% 20% 32% 18% 5% 2	760
	21st to end	Night	23% 21% 29% 21% 5%	760
December	1st to 10th	Night	27% 24% 31% 15% 3	760
	11th to 20th	Night	26% 23% 31% 18% 3	760
	21st to end	Night	31% 21% 31% 15% 1	836

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