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 2026-06-01.

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% 5% 5% 5% 4 6% 7% 32%	133015
	Night	15% 11% 4 9% 5% 5% 5% 3 5% 7% 33%	130158
Transparency	Day	51% 3 11% 22% 12% 1	136659
	Night	50% 3 10% 22% 13% 2	134584
Seeing	Day	19% 9% 25% 23% 16% 8%	17322
	Night	20% 7% 25% 32% 15% 2	26961

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	17% 18% 7% 15% 7% 5% 4 3 4 5% 14%	5258
	11th to 20th	Day	13% 20% 6% 13% 8% 6% 6% 3 3 3 20%	5280
	21st to end	Day	19% 15% 7% 11% 8% 7% 4 4 5% 5% 14%	5780
February	1st to 10th	Day	18% 17% 4 11% 6% 5% 4 3 4 5% 23%	1890
	11th to 20th	Day	16% 13% 6% 14% 7% 5% 4 3 5% 5% 23%	2027
	21st to end	Day	16% 15% 4 9% 5% 4 4 3 4 6% 28%	1730
March	1st to 10th	Day	17% 13% 7% 12% 5% 4 4 3 5% 5% 26%	4766
	11th to 20th	Day	12% 9% 4 8% 5% 5% 5% 3 5% 6% 39%	5090
	21st to end	Day	19% 10% 4 9% 5% 5% 5% 3 5% 6% 29%	5586
April	1st to 10th	Day	15% 13% 3 9% 6% 5% 4 3 4 7% 32%	2377
	11th to 20th	Day	16% 9% 3 8% 5% 5% 4 3 5% 6% 36%	2649
	21st to end	Day	17% 8% 3 8% 6% 4 5% 4 5% 6% 34%	2730
May	1st to 10th	Day	21% 9% 4 8% 5% 5% 5% 4 5% 6% 28%	5992
	11th to 20th	Day	18% 9% 3 7% 5% 4 5% 4 6% 8% 33%	5991
	21st to end	Day	16% 7% 3 7% 5% 4 5% 5% 7% 9% 33%	6576
June	1st to 10th	Day	11% 8% 2 7% 6% 5% 5% 4 6% 9% 37%	2600
	11th to 20th	Day	10% 7% 3 6% 5% 5% 5% 4 8% 9% 38%	2686
	21st to end	Day	10% 5% 2 6% 5% 5% 5% 6% 8% 10% 38%	2800
July	1st to 10th	Day	8% 6% 3 7% 5% 5% 5% 4 7% 8% 41%	6324
	11th to 20th	Day	9% 6% 3 7% 5% 5% 6% 5% 7% 11% 35%	5900
	21st to end	Day	9% 7% 2 7% 5% 5% 5% 5% 8% 10% 37%	6424
August	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 4 7% 10% 39%	4234
	11th to 20th	Day	8% 6% 2 6% 5% 5% 6% 5% 8% 11% 39%	4174
	21st to end	Day	6% 5% 2 6% 4 5% 5% 4 7% 10% 45%	4585
September	1st to 10th	Day	11% 6% 2 5% 4 4 4 3 6% 6% 49%	2217
	11th to 20th	Day	9% 5% 2 6% 5% 4 4 4 6% 9% 47%	2200
	21st to end	Day	17% 8% 2 6% 4 4 4 3 4 6% 42%	2200
October	1st to 10th	Day	13% 6% 3 6% 4 4 4 4 6% 7% 45%	2149
	11th to 20th	Day	15% 7% 2 6% 4 4 3 4 5% 7% 43%	2000
	21st to end	Day	20% 10% 3 7% 4 4 5% 4 5% 6% 32%	2120
November	1st to 10th	Day	11% 9% 3 6% 4 5% 3 3 5% 6% 44%	1852
	11th to 20th	Day	17% 13% 4 6% 5% 4 4 4 4 6% 34%	1890
	21st to end	Day	19% 14% 6% 9% 4 4 4 2 4 5% 29%	1890
December	1st to 10th	Day	19% 13% 6% 15% 4 5% 4 3 4 4 24%	3020
	11th to 20th	Day	21% 17% 7% 10% 4 4 4 3 4 5% 21%	3463
	21st to end	Day	25% 21% 6% 11% 6% 5% 4 3 4 4 12%	4565
January	1st to 10th	Night	16% 16% 7% 17% 6% 5% 5% 4 4 5% 15%	7635
	11th to 20th	Night	12% 17% 8% 16% 7% 7% 6% 3 4 4 16%	7680
	21st to end	Night	18% 14% 5% 9% 6% 5% 4 3 5% 6% 25%	8401
February	1st to 10th	Night	19% 13% 4 10% 6% 4 4 3 5% 4 28%	2730
	11th to 20th	Night	18% 12% 4 10% 6% 4 5% 4 5% 6% 26%	2474
	21st to end	Night	19% 14% 5% 11% 4 5% 3 3 4 5% 27%	2076
March	1st to 10th	Night	21% 12% 3 7% 4 4 4 3 4 5% 33%	5306
	11th to 20th	Night	12% 8% 3 6% 4 4 4 4 6% 8% 40%	5090
	21st to end	Night	18% 10% 3 7% 4 4 4 3 4 6% 36%	5588
April	1st to 10th	Night	17% 9% 3 7% 5% 4 5% 3 5% 8% 34%	2261
	11th to 20th	Night	14% 8% 3 7% 5% 4 5% 5% 5% 7% 36%	1920
	21st to end	Night	16% 8% 3 7% 4 4 4 4 5% 7% 38%	1890
May	1st to 10th	Night	23% 9% 3 8% 5% 4 5% 4 6% 6% 27%	4123
	11th to 20th	Night	17% 8% 3 7% 3 4 4 3 5% 9% 36%	4122
	21st to end	Night	13% 6% 3 6% 5% 5% 5% 4 6% 9% 38%	4520
June	1st to 10th	Night	9% 6% 2 5% 4 4 5% 4 6% 8% 46%	1800
	11th to 20th	Night	9% 5% 2 6% 4 5% 5% 4 8% 9% 43%	1706
	21st to end	Night	9% 4 2 4 4 4 5% 5% 7% 10% 45%	1600
July	1st to 10th	Night	9% 6% 2 7% 4 4 4 4 6% 8% 45%	3635
	11th to 20th	Night	8% 5% 2 7% 5% 5% 5% 5% 7% 9% 41%	4057
	21st to end	Night	8% 6% 3 7% 5% 5% 5% 4 6% 10% 41%	4427
August	1st to 10th	Night	9% 7% 3 7% 5% 5% 5% 5% 7% 9% 38%	2915
	11th to 20th	Night	7% 8% 3 9% 5% 5% 5% 3 5% 7% 44%	2875
	21st to end	Night	4 4 2 5% 4 5% 6% 5% 7% 9% 49%	3310
September	1st to 10th	Night	12% 5% 2 4 3 3 4 3 4 7% 53%	2161
	11th to 20th	Night	8% 4 2 4 3 4 3 3 6% 6% 56%	2200
	21st to end	Night	15% 6% 2 6% 4 4 4 3 4 6% 45%	2200
October	1st to 10th	Night	10% 5% 2 6% 4 3 4 4 4 6% 52%	2250
	11th to 20th	Night	16% 7% 3 5% 4 4 5% 3 5% 6% 43%	2400
	21st to end	Night	18% 8% 3 8% 4 4 4 3 4 5% 38%	2715
November	1st to 10th	Night	13% 7% 2 5% 4 4 5% 3 5% 6% 46%	2666
	11th to 20th	Night	17% 10% 3 7% 5% 5% 4 3 5% 7% 35%	2730
	21st to end	Night	20% 12% 4 10% 4 5% 4 4 4 6% 28%	2730
December	1st to 10th	Night	18% 12% 5% 10% 6% 6% 5% 3 5% 6% 25%	4380
	11th to 20th	Night	18% 18% 6% 12% 5% 4 4 3 4 5% 21%	4952
	21st to end	Night	24% 20% 6% 11% 5% 3 4 3 4 5% 15%	6633

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	74% 2 10% 11% 3	5259
	11th to 20th	Day	71% 2 8% 8% 11% 1	5280
	21st to end	Day	73% 2 9% 11% 4	5780
February	1st to 10th	Day	65% 3 9% 15% 8%	1890
	11th to 20th	Day	64% 2 8% 17% 8% 1	2037
	21st to end	Day	58% 2 12% 19% 7% 2	1730
March	1st to 10th	Day	62% 1 7% 22% 6%	4766
	11th to 20th	Day	47% 2 9% 31% 11% 1	5090
	21st to end	Day	57% 2 9% 22% 9% 2	5586
April	1st to 10th	Day	54% 2 11% 19% 13% 1	2377
	11th to 20th	Day	50% 3 12% 20% 13% 2	2649
	21st to end	Day	51% 3 9% 23% 12% 2	2730
May	1st to 10th	Day	58% 3 10% 17% 11% 1	6027
	11th to 20th	Day	50% 3 12% 22% 11% 1	5995
	21st to end	Day	46% 3 11% 25% 12% 3	6590
June	1st to 10th	Day	44% 3 10% 26% 15% 2	2600
	11th to 20th	Day	41% 4 14% 24% 16% 1	2686
	21st to end	Day	39% 5% 15% 22% 16% 3	2800
July	1st to 10th	Day	39% 5% 10% 28% 16% 2	6255
	11th to 20th	Day	40% 6% 14% 24% 16% 1	5831
	21st to end	Day	39% 6% 13% 22% 18% 3	6240
August	1st to 10th	Day	38% 5% 16% 26% 14% 1	4221
	11th to 20th	Day	35% 6% 11% 32% 15% 1	4213
	21st to end	Day	33% 5% 13% 32% 14% 3	4584
September	1st to 10th	Day	35% 3 10% 26% 22% 4	2217
	11th to 20th	Day	35% 3 10% 28% 22% 3	2200
	21st to end	Day	45% 2 8% 22% 22% 1	2200
October	1st to 10th	Day	27% 3 10% 35% 24% 1	3500
	11th to 20th	Day	34% 3 10% 28% 23% 2	3240
	21st to end	Day	42% 3 10% 26% 19% 1	3409
November	1st to 10th	Day	41% 3 10% 26% 18% 2	1852
	11th to 20th	Day	52% 2 9% 22% 12% 2	1890
	21st to end	Day	59% 2 9% 20% 9% 1	1890
December	1st to 10th	Day	64% 2 14% 17% 4	3020
	11th to 20th	Day	67% 2 10% 16% 5%	3460
	21st to end	Day	77% 2 7% 9% 4	4565
January	1st to 10th	Night	71% 3 9% 14% 4	7636
	11th to 20th	Night	72% 2 7% 11% 7% 1	7680
	21st to end	Night	61% 2 11% 17% 7% 1	8401
February	1st to 10th	Night	60% 2 10% 18% 8% 1	2730
	11th to 20th	Night	58% 3 10% 17% 11% 1	2475
	21st to end	Night	62% 2 9% 16% 9% 2	2076
March	1st to 10th	Night	55% 2 7% 24% 11% 1	5306
	11th to 20th	Night	42% 2 10% 33% 11% 1	5090
	21st to end	Night	51% 2 7% 24% 14% 1	5588
April	1st to 10th	Night	49% 3 11% 22% 14% 2	2261
	11th to 20th	Night	46% 4 11% 18% 18% 3	1920
	21st to end	Night	47% 3 11% 23% 15% 2	1890
May	1st to 10th	Night	57% 3 9% 17% 10% 3	4150
	11th to 20th	Night	46% 4 10% 24% 14% 1	4123
	21st to end	Night	43% 3 12% 28% 12% 2	4528
June	1st to 10th	Night	36% 4 12% 24% 21% 2	1800
	11th to 20th	Night	36% 5% 13% 27% 17% 3	1706
	21st to end	Night	33% 5% 15% 26% 18% 4	1600
July	1st to 10th	Night	36% 5% 11% 26% 18% 4	3597
	11th to 20th	Night	36% 7% 13% 28% 14% 1	4006
	21st to end	Night	38% 6% 13% 21% 20% 3	4297
August	1st to 10th	Night	40% 5% 14% 25% 14% 1	2904
	11th to 20th	Night	39% 5% 10% 27% 19%	2902
	21st to end	Night	30% 6% 14% 30% 18% 2	3310
September	1st to 10th	Night	33% 2 11% 27% 21% 6%	2161
	11th to 20th	Night	29% 2 8% 30% 27% 4	2200
	21st to end	Night	41% 3 8% 21% 25% 2	2200
October	1st to 10th	Night	26% 3 7% 30% 33% 2	3679
	11th to 20th	Night	31% 3 9% 29% 24% 3	3899
	21st to end	Night	36% 3 7% 27% 23% 4	4376
November	1st to 10th	Night	39% 2 11% 25% 20% 2	2666
	11th to 20th	Night	49% 3 8% 24% 14% 3	2730
	21st to end	Night	58% 2 10% 21% 8% 1	2730
December	1st to 10th	Night	59% 3 12% 20% 6%	4380
	11th to 20th	Night	66% 2 12% 15% 6%	4954
	21st to end	Night	73% 2 9% 12% 3	6633

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% 13% 37% 15% 6% 2	330
	11th to 20th	Day	30% 17% 36% 13% 3 1	330
	21st to end	Day	30% 16% 33% 17% 5%	363
February	1st to 10th	Day	28% 15% 34% 18% 4	330
	11th to 20th	Day	26% 17% 39% 15% 3	330
	21st to end	Day	26% 17% 42% 14% 1	273
March	1st to 10th	Day	23% 17% 34% 21% 5%	412
	11th to 20th	Day	26% 14% 34% 18% 6% 1	540
	21st to end	Day	29% 12% 32% 21% 5% 1	594
April	1st to 10th	Day	24% 12% 33% 20% 9% 2	540
	11th to 20th	Day	21% 14% 30% 19% 10% 5%	532
	21st to end	Day	21% 10% 33% 21% 12% 3	540
May	1st to 10th	Day	26% 10% 29% 22% 10% 3	543
	11th to 20th	Day	23% 5% 27% 23% 15% 6%	567
	21st to end	Day	16% 7% 20% 25% 21% 10%	617
June	1st to 10th	Day	13% 5% 21% 33% 19% 8%	530
	11th to 20th	Day	13% 5% 21% 30% 23% 9%	616
	21st to end	Day	12% 5% 24% 28% 22% 8%	730
July	1st to 10th	Day	13% 3 12% 28% 26% 18%	711
	11th to 20th	Day	13% 3 18% 23% 26% 18%	524
	21st to end	Day	11% 2 14% 27% 29% 18%	576
August	1st to 10th	Day	12% 3 11% 25% 29% 20%	526
	11th to 20th	Day	12% 3 15% 28% 25% 18%	519
	21st to end	Day	9% 3 15% 25% 30% 18%	583
September	1st to 10th	Day	13% 4 13% 26% 26% 18%	530
	11th to 20th	Day	10% 1 10% 24% 32% 23%	530
	21st to end	Day	21% 8% 17% 23% 21% 10%	530
October	1st to 10th	Day	12% 5% 20% 30% 21% 12%	530
	11th to 20th	Day	16% 9% 29% 28% 14% 5%	530
	21st to end	Day	26% 10% 33% 22% 8% 1	503
November	1st to 10th	Day	17% 12% 34% 24% 10% 4	330
	11th to 20th	Day	25% 14% 31% 19% 9% 2	330
	21st to end	Day	28% 14% 36% 20% 3	330
December	1st to 10th	Day	24% 14% 38% 18% 5% 2	330
	11th to 20th	Day	26% 13% 40% 16% 5%	330
	21st to end	Day	37% 11% 27% 21% 4	363
January	1st to 10th	Night	25% 12% 35% 23% 4 1	840
	11th to 20th	Night	29% 13% 33% 22% 4	840
	21st to end	Night	27% 10% 33% 27% 3	924
February	1st to 10th	Night	27% 9% 31% 29% 4	840
	11th to 20th	Night	25% 10% 36% 27% 2	823
	21st to end	Night	27% 11% 37% 23% 2	692
March	1st to 10th	Night	24% 11% 32% 27% 6%	840
	11th to 20th	Night	22% 11% 33% 26% 7%	840
	21st to end	Night	24% 10% 32% 27% 8%	924
April	1st to 10th	Night	22% 11% 31% 30% 6% 1	791
	11th to 20th	Night	18% 10% 32% 28% 11% 1	623
	21st to end	Night	19% 5% 34% 31% 9% 1	630
May	1st to 10th	Night	24% 5% 26% 34% 10% 1	630
	11th to 20th	Night	20% 5% 25% 35% 15% 1	621
	21st to end	Night	11% 4 20% 39% 24% 2	683
June	1st to 10th	Night	10% 2 23% 46% 18% 2	600
	11th to 20th	Night	11% 2 18% 45% 23% 2	600
	21st to end	Night	11% 2 17% 42% 26% 2	600
July	1st to 10th	Night	11% 2 9% 39% 30% 8%	585
	11th to 20th	Night	9% 3 11% 39% 34% 4	582
	21st to end	Night	10% 2 8% 40% 37% 2	657
August	1st to 10th	Night	8% 2 10% 43% 34% 4	597
	11th to 20th	Night	10% 2 8% 41% 34% 5%	594
	21st to end	Night	6% 1 10% 40% 36% 7%	660
September	1st to 10th	Night	13% 4 12% 35% 31% 5%	761
	11th to 20th	Night	11% 3 10% 35% 38% 5%	800
	21st to end	Night	17% 5% 17% 33% 26% 2	800
October	1st to 10th	Night	13% 4 22% 35% 23% 4	800
	11th to 20th	Night	20% 4 28% 33% 13% 1	800
	21st to end	Night	23% 7% 29% 31% 9% 1	880
November	1st to 10th	Night	17% 6% 28% 33% 15% 1	820
	11th to 20th	Night	22% 12% 30% 27% 8% 1	840
	21st to end	Night	25% 8% 29% 30% 8%	840
December	1st to 10th	Night	28% 8% 31% 27% 5%	840
	11th to 20th	Night	26% 10% 31% 28% 5% 1	840
	21st to end	Night	33% 9% 28% 26% 4	924

Page generated at 2026-05-29 01:53:27. Version ac07

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