Perkins Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Perkins Observatory clear sky chart from 2005-11-06 to 2024-01-27.

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 7% 6% 3 5% 5% 5% 7% 33%	75437
	Night	16% 9% 4 7% 6% 3 5% 4 5% 7% 35%	74611
Transparency	Day	51% 3 11% 22% 12% 2	75156
	Night	50% 3 10% 22% 13% 2	74412
Seeing	Day	20% 7% 25% 24% 16% 8%	14150
	Night	20% 6% 25% 32% 15% 2	23889

By Month

Percentage Cloud Cover SE (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	19% 16% 7% 10% 8% 5% 4 4 4 5% 18%	1706
	11th to 20th	Day	20% 19% 8% 10% 7% 4 4 4 4 4 18%	1710
	21st to end	Day	17% 18% 8% 10% 8% 4 4 3 5% 5% 16%	1836
February	1st to 10th	Day	20% 16% 5% 9% 8% 4 5% 3 4 4 21%	1620
	11th to 20th	Day	16% 12% 6% 13% 9% 3 4 4 5% 5% 22%	1730
	21st to end	Day	18% 14% 5% 8% 6% 4 4 4 4 6% 27%	1480
March	1st to 10th	Day	19% 10% 6% 7% 7% 4 5% 4 4 6% 29%	1870
	11th to 20th	Day	19% 11% 6% 7% 6% 4 4 3 5% 6% 29%	1980
	21st to end	Day	23% 10% 6% 7% 6% 3 4 3 4 6% 28%	2178
April	1st to 10th	Day	15% 12% 4 8% 6% 3 4 4 4 7% 34%	2037
	11th to 20th	Day	18% 8% 3 6% 6% 3 5% 3 5% 7% 35%	2266
	21st to end	Day	19% 8% 4 7% 7% 3 4 5% 5% 7% 32%	2340
May	1st to 10th	Day	18% 9% 4 7% 6% 3 5% 6% 5% 7% 31%	2340
	11th to 20th	Day	19% 8% 3 6% 7% 3 5% 5% 6% 7% 31%	2301
	21st to end	Day	10% 5% 3 6% 6% 4 6% 7% 8% 10% 37%	2531
June	1st to 10th	Day	11% 7% 3 6% 7% 3 5% 5% 6% 9% 37%	2340
	11th to 20th	Day	10% 7% 3 5% 6% 4 5% 6% 7% 9% 38%	2416
	21st to end	Day	11% 4 2 5% 5% 4 6% 7% 8% 10% 38%	2520
July	1st to 10th	Day	10% 7% 3 5% 6% 3 6% 6% 7% 9% 38%	2512
	11th to 20th	Day	9% 5% 3 5% 4 3 5% 6% 7% 10% 42%	2340
	21st to end	Day	9% 4 2 5% 6% 3 6% 7% 7% 11% 40%	2547
August	1st to 10th	Day	8% 5% 2 6% 7% 4 6% 6% 6% 10% 42%	2327
	11th to 20th	Day	9% 5% 3 6% 6% 4 6% 7% 8% 10% 37%	2303
	21st to end	Day	8% 5% 2 5% 5% 4 5% 5% 7% 10% 44%	2529
September	1st to 10th	Day	12% 6% 2 4 5% 3 5% 4 6% 6% 47%	1996
	11th to 20th	Day	9% 5% 2 5% 6% 3 5% 5% 6% 9% 45%	1980
	21st to end	Day	16% 7% 2 4 5% 3 4 5% 4 6% 44%	1980
October	1st to 10th	Day	13% 6% 3 6% 5% 3 4 4 6% 7% 41%	1935
	11th to 20th	Day	16% 7% 3 5% 6% 3 4 4 5% 7% 40%	1800
	21st to end	Day	21% 10% 4 7% 5% 3 5% 5% 5% 6% 31%	1908
November	1st to 10th	Day	10% 9% 3 6% 5% 3 4 3 5% 7% 45%	1672
	11th to 20th	Day	17% 11% 5% 6% 6% 3 4 4 4 6% 35%	1710
	21st to end	Day	19% 13% 5% 8% 5% 3 4 3 4 5% 31%	1710
December	1st to 10th	Day	20% 16% 8% 9% 5% 4 5% 4 4 5% 19%	1710
	11th to 20th	Day	18% 14% 7% 9% 6% 4 5% 5% 4 5% 23%	2153
	21st to end	Day	25% 22% 6% 9% 7% 4 4 4 4 5% 10%	3124
January	1st to 10th	Night	18% 14% 7% 12% 7% 5% 5% 4 3 4 22%	2459
	11th to 20th	Night	22% 16% 5% 10% 7% 3 5% 4 4 5% 20%	2470
	21st to end	Night	18% 16% 6% 9% 7% 4 5% 4 4 4 22%	2664
February	1st to 10th	Night	23% 13% 4 8% 7% 3 4 4 4 4 26%	2340
	11th to 20th	Night	18% 11% 4 8% 8% 3 6% 5% 5% 6% 26%	2112
	21st to end	Night	20% 13% 6% 10% 5% 4 4 3 4 5% 26%	1776
March	1st to 10th	Night	17% 10% 4 7% 7% 4 5% 4 4 5% 33%	2089
	11th to 20th	Night	16% 10% 4 6% 5% 3 4 4 5% 6% 36%	1980
	21st to end	Night	20% 9% 5% 7% 5% 3 4 5% 5% 6% 33%	2178
April	1st to 10th	Night	15% 8% 4 6% 6% 3 5% 4 5% 8% 36%	1939
	11th to 20th	Night	14% 7% 3 6% 5% 3 5% 6% 5% 7% 37%	1643
	21st to end	Night	16% 8% 3 7% 5% 4 3 5% 5% 8% 37%	1620
May	1st to 10th	Night	19% 8% 3 6% 6% 3 6% 4 6% 7% 32%	1620
	11th to 20th	Night	18% 8% 3 6% 5% 3 4 5% 4 7% 37%	1592
	21st to end	Night	7% 5% 2 4 6% 3 7% 6% 8% 11% 42%	1744
June	1st to 10th	Night	8% 5% 3 5% 5% 2 6% 5% 6% 8% 47%	1620
	11th to 20th	Night	9% 4 2 5% 4 3 6% 5% 8% 9% 44%	1536
	21st to end	Night	9% 4 2 5% 4 3 5% 6% 7% 10% 45%	1440
July	1st to 10th	Night	9% 5% 2 6% 6% 4 4 5% 5% 8% 47%	1444
	11th to 20th	Night	6% 4 2 5% 5% 3 4 5% 8% 9% 49%	1620
	21st to end	Night	8% 5% 2 5% 6% 4 4 6% 5% 9% 45%	1771
August	1st to 10th	Night	5% 4 3 5% 6% 4 5% 7% 8% 10% 43%	1611
	11th to 20th	Night	7% 6% 3 6% 5% 4 5% 5% 5% 8% 46%	1598
	21st to end	Night	5% 3 2 4 4 3 6% 6% 7% 9% 50%	1824
September	1st to 10th	Night	13% 5% 2 4 4 2 5% 3 4 7% 50%	1944
	11th to 20th	Night	9% 4 2 4 4 3 4 3 5% 7% 55%	1980
	21st to end	Night	13% 5% 2 5% 4 3 4 5% 4 6% 47%	1980
October	1st to 10th	Night	10% 5% 2 5% 5% 2 4 4 4 7% 50%	2024
	11th to 20th	Night	18% 7% 3 5% 5% 3 5% 5% 5% 6% 40%	2160
	21st to end	Night	20% 8% 4 7% 5% 3 4 4 4 5% 37%	2443
November	1st to 10th	Night	14% 6% 3 5% 4 2 4 4 5% 6% 46%	2406
	11th to 20th	Night	17% 10% 4 6% 6% 4 4 4 5% 6% 36%	2470
	21st to end	Night	20% 10% 4 8% 5% 4 5% 5% 5% 6% 29%	2470
December	1st to 10th	Night	21% 14% 6% 9% 6% 3 4 4 4 6% 23%	2470
	11th to 20th	Night	18% 14% 6% 9% 6% 3 4 5% 4 6% 25%	3042
	21st to end	Night	25% 20% 7% 9% 6% 3 4 4 3 5% 15%	4532

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% 3 7% 14% 7%	1707
	11th to 20th	Day	71% 2 12% 10% 4	1710
	21st to end	Day	71% 1 9% 14% 5% 1	1836
February	1st to 10th	Day	68% 2 9% 13% 7%	1620
	11th to 20th	Day	65% 3 8% 16% 7% 1	1740
	21st to end	Day	60% 2 10% 18% 7% 2	1480
March	1st to 10th	Day	58% 1 11% 23% 6% 1	1870
	11th to 20th	Day	57% 2 10% 20% 9% 1	1980
	21st to end	Day	60% 2 9% 22% 7% 1	2178
April	1st to 10th	Day	52% 2 12% 20% 13% 1	2037
	11th to 20th	Day	50% 3 12% 19% 14% 2	2266
	21st to end	Day	53% 3 9% 22% 11% 2	2340
May	1st to 10th	Day	53% 2 13% 20% 9% 2	2340
	11th to 20th	Day	53% 4 13% 18% 12% 1	2305
	21st to end	Day	40% 5% 15% 26% 12% 1	2531
June	1st to 10th	Day	44% 3 11% 27% 14% 2	2340
	11th to 20th	Day	40% 4 15% 24% 16% 2	2416
	21st to end	Day	39% 5% 15% 22% 16% 3	2520
July	1st to 10th	Day	42% 5% 12% 23% 16% 3	2443
	11th to 20th	Day	36% 4 13% 28% 18% 1	2271
	21st to end	Day	36% 4 14% 30% 13% 3	2405
August	1st to 10th	Day	39% 3 14% 28% 15% 1	2314
	11th to 20th	Day	39% 4 12% 31% 12% 2	2303
	21st to end	Day	36% 4 13% 25% 17% 5%	2529
September	1st to 10th	Day	37% 4 10% 25% 20% 4	1996
	11th to 20th	Day	37% 3 11% 27% 19% 2	1980
	21st to end	Day	43% 2 9% 24% 22% 1	1980
October	1st to 10th	Day	42% 4 9% 26% 19% 1	1935
	11th to 20th	Day	44% 3 9% 25% 17% 2	1800
	21st to end	Day	55% 2 10% 21% 10% 1	1908
November	1st to 10th	Day	41% 2 10% 26% 18% 3	1672
	11th to 20th	Day	52% 2 9% 22% 12% 2	1710
	21st to end	Day	57% 2 10% 20% 10% 1	1710
December	1st to 10th	Day	68% 1 9% 17% 4	1710
	11th to 20th	Day	64% 2 8% 19% 6%	2150
	21st to end	Day	80% 1 8% 8% 4	3124
January	1st to 10th	Night	67% 2 6% 17% 8%	2459
	11th to 20th	Night	69% 2 10% 14% 6% 1	2470
	21st to end	Night	66% 2 8% 17% 6% 1	2664
February	1st to 10th	Night	63% 2 10% 17% 7% 1	2340
	11th to 20th	Night	59% 4 10% 17% 10% 1	2113
	21st to end	Night	63% 2 9% 16% 9% 2	1776
March	1st to 10th	Night	54% 2 9% 22% 11% 1	2089
	11th to 20th	Night	49% 3 9% 26% 11% 3	1980
	21st to end	Night	53% 3 7% 24% 12% 1	2178
April	1st to 10th	Night	46% 3 12% 22% 14% 2	1939
	11th to 20th	Night	45% 3 11% 18% 19% 4	1643
	21st to end	Night	47% 3 12% 21% 15% 2	1620
May	1st to 10th	Night	51% 3 12% 21% 11% 3	1620
	11th to 20th	Night	48% 4 10% 21% 14% 3	1593
	21st to end	Night	35% 4 15% 29% 13% 3	1745
June	1st to 10th	Night	35% 4 13% 25% 20% 2	1620
	11th to 20th	Night	35% 5% 13% 27% 16% 3	1536
	21st to end	Night	34% 5% 15% 26% 17% 3	1440
July	1st to 10th	Night	37% 4 13% 22% 20% 5%	1404
	11th to 20th	Night	31% 5% 12% 31% 19% 2	1566
	21st to end	Night	35% 4 12% 29% 16% 4	1672
August	1st to 10th	Night	34% 5% 15% 29% 18% 1	1600
	11th to 20th	Night	36% 5% 13% 26% 19% 1	1598
	21st to end	Night	30% 4 14% 27% 21% 4	1824
September	1st to 10th	Night	36% 2 12% 26% 19% 6%	1944
	11th to 20th	Night	31% 2 9% 30% 25% 3	1980
	21st to end	Night	38% 3 9% 22% 26% 2	1980
October	1st to 10th	Night	36% 3 10% 28% 21% 2	2024
	11th to 20th	Night	45% 3 7% 27% 17% 1	2160
	21st to end	Night	51% 2 7% 23% 14% 3	2443
November	1st to 10th	Night	40% 2 11% 24% 20% 3	2406
	11th to 20th	Night	49% 3 7% 24% 15% 2	2470
	21st to end	Night	56% 3 11% 22% 8% 1	2470
December	1st to 10th	Night	64% 2 9% 19% 6%	2470
	11th to 20th	Night	61% 2 10% 21% 6%	3044
	21st to end	Night	75% 2 8% 12% 3	4532

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	29% 12% 35% 16% 6% 2	270
	11th to 20th	Day	30% 13% 36% 16% 4 1	270
	21st to end	Day	32% 13% 33% 17% 5%	282
February	1st to 10th	Day	35% 15% 31% 17% 2	240
	11th to 20th	Day	26% 16% 41% 14% 3	240
	21st to end	Day	26% 14% 44% 15% 1	198
March	1st to 10th	Day	21% 18% 34% 22% 5%	310
	11th to 20th	Day	27% 11% 35% 19% 7% 1	420
	21st to end	Day	31% 9% 32% 20% 6% 1	462
April	1st to 10th	Day	24% 9% 31% 22% 11% 2	420
	11th to 20th	Day	22% 12% 28% 20% 11% 6%	412
	21st to end	Day	24% 9% 33% 20% 11% 2	420
May	1st to 10th	Day	25% 7% 29% 24% 11% 4	423
	11th to 20th	Day	24% 4 27% 23% 16% 6%	447
	21st to end	Day	14% 6% 19% 26% 23% 12%	485
June	1st to 10th	Day	13% 3 19% 35% 20% 9%	450
	11th to 20th	Day	14% 4 22% 28% 24% 9%	526
	21st to end	Day	13% 5% 23% 29% 22% 8%	630
July	1st to 10th	Day	14% 2 11% 27% 26% 20%	613
	11th to 20th	Day	14% 2 16% 24% 24% 20%	444
	21st to end	Day	11% 1 14% 27% 28% 19%	488
August	1st to 10th	Day	12% 3 9% 24% 30% 21%	446
	11th to 20th	Day	13% 3 14% 28% 24% 19%	439
	21st to end	Day	11% 3 14% 24% 29% 20%	495
September	1st to 10th	Day	14% 3 14% 25% 27% 17%	450
	11th to 20th	Day	12% 2 11% 24% 31% 21%	450
	21st to end	Day	19% 8% 17% 24% 22% 10%	450
October	1st to 10th	Day	13% 5% 21% 30% 19% 12%	450
	11th to 20th	Day	17% 10% 30% 26% 14% 3	450
	21st to end	Day	27% 10% 31% 23% 8% 1	423
November	1st to 10th	Day	15% 13% 31% 25% 12% 4	270
	11th to 20th	Day	24% 13% 29% 21% 10% 2	270
	21st to end	Day	26% 13% 35% 22% 4	270
December	1st to 10th	Day	24% 15% 35% 19% 5% 2	270
	11th to 20th	Day	24% 11% 42% 17% 5%	270
	21st to end	Day	36% 11% 26% 23% 3	297
January	1st to 10th	Night	25% 12% 35% 23% 4 1	760
	11th to 20th	Night	30% 11% 31% 23% 4 1	760
	21st to end	Night	28% 9% 32% 28% 4	820
February	1st to 10th	Night	31% 7% 31% 27% 4	720
	11th to 20th	Night	26% 9% 35% 28% 2	703
	21st to end	Night	29% 9% 37% 23% 3	592
March	1st to 10th	Night	23% 11% 32% 27% 7%	720
	11th to 20th	Night	23% 9% 31% 28% 8% 1	720
	21st to end	Night	24% 8% 31% 28% 9% 1	792
April	1st to 10th	Night	20% 10% 32% 31% 6% 1	679
	11th to 20th	Night	18% 8% 32% 28% 13% 1	533
	21st to end	Night	20% 5% 35% 31% 9% 1	540
May	1st to 10th	Night	24% 3 27% 34% 11% 1	540
	11th to 20th	Night	21% 4 24% 34% 16% 1	531
	21st to end	Night	9% 4 19% 38% 28% 3	584
June	1st to 10th	Night	9% 2 22% 47% 19% 2	540
	11th to 20th	Night	11% 2 18% 44% 24% 2	540
	21st to end	Night	11% 1 16% 43% 27% 2	540
July	1st to 10th	Night	12% 2 8% 38% 31% 9%	525
	11th to 20th	Night	10% 2 10% 39% 35% 4	522
	21st to end	Night	11% 2 8% 40% 38% 2	591
August	1st to 10th	Night	8% 2 9% 43% 35% 4	537
	11th to 20th	Night	10% 2 8% 41% 34% 5%	534
	21st to end	Night	7% 1 10% 40% 37% 6%	594
September	1st to 10th	Night	15% 4 10% 34% 32% 6%	684
	11th to 20th	Night	11% 3 10% 35% 36% 5%	720
	21st to end	Night	15% 5% 18% 34% 27% 3	720
October	1st to 10th	Night	13% 3 22% 35% 23% 4	720
	11th to 20th	Night	21% 4 29% 32% 13% 1	720
	21st to end	Night	25% 7% 28% 31% 9% 1	792
November	1st to 10th	Night	18% 6% 26% 33% 16% 1	740
	11th to 20th	Night	22% 11% 30% 27% 9% 1	760
	21st to end	Night	24% 7% 29% 31% 8% 1	760
December	1st to 10th	Night	29% 7% 31% 27% 5% 1	760
	11th to 20th	Night	26% 9% 31% 28% 6% 1	760
	21st to end	Night	32% 9% 28% 26% 4	836

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