Princeton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Princeton 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	5% 8% 3 7% 5% 5% 6% 5% 7% 8% 41%	135746
	Night	4 8% 3 8% 6% 6% 6% 5% 7% 9% 39%	136042
Transparency	Day	43% 10% 15% 20% 10% 2	135464
	Night	43% 11% 16% 19% 8% 1	135903
Seeing	Day	14% 19% 29% 23% 11% 3	17014
	Night	10% 13% 29% 31% 15% 2	25649

By Month

Percentage Cloud Cover NW (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	6% 13% 4 11% 7% 7% 8% 6% 9% 9% 21%	4165
	11th to 20th	Day	2 8% 3 8% 6% 7% 6% 5% 8% 9% 38%	4716
	21st to end	Day	3 8% 3 10% 7% 7% 6% 5% 7% 10% 33%	5186
February	1st to 10th	Day	5% 11% 5% 10% 9% 7% 8% 6% 8% 8% 24%	1782
	11th to 20th	Day	4 11% 5% 12% 8% 6% 7% 6% 7% 9% 27%	1845
	21st to end	Day	2 10% 6% 14% 10% 9% 7% 6% 8% 6% 23%	1677
March	1st to 10th	Day	2 6% 3 9% 6% 6% 8% 6% 9% 10% 36%	4618
	11th to 20th	Day	2 8% 3 8% 6% 6% 8% 6% 8% 10% 35%	4972
	21st to end	Day	7% 12% 5% 10% 6% 6% 7% 5% 8% 8% 25%	5961
April	1st to 10th	Day	5% 12% 4 9% 7% 7% 7% 5% 9% 8% 26%	2520
	11th to 20th	Day	6% 11% 4 10% 6% 5% 7% 5% 7% 9% 31%	2492
	21st to end	Day	7% 12% 4 9% 6% 5% 6% 4 7% 7% 33%	2885
May	1st to 10th	Day	6% 8% 2 6% 4 4 5% 5% 6% 7% 47%	6460
	11th to 20th	Day	7% 9% 3 7% 6% 6% 6% 5% 7% 10% 34%	6417
	21st to end	Day	11% 10% 3 7% 5% 5% 5% 4 6% 8% 35%	7057
June	1st to 10th	Day	13% 8% 4 7% 6% 5% 5% 5% 7% 6% 34%	2871
	11th to 20th	Day	12% 8% 4 8% 7% 5% 5% 5% 6% 7% 32%	3183
	21st to end	Day	8% 6% 3 7% 6% 5% 5% 5% 6% 8% 42%	3004
July	1st to 10th	Day	3 3 1 3 3 3 3 2 4 7% 68%	6780
	11th to 20th	Day	3 2 1 3 2 2 3 2 4 6% 72%	6504
	21st to end	Day	3 2 1 4 3 3 4 4 6% 8% 63%	6874
August	1st to 10th	Day	3 2 1 3 3 4 5% 4 6% 6% 64%	4541
	11th to 20th	Day	4 4 2 5% 5% 5% 5% 4 6% 8% 53%	4349
	21st to end	Day	7% 4 1 5% 5% 4 5% 4 5% 8% 50%	4409
September	1st to 10th	Day	7% 4 2 7% 4 4 4 4 4 7% 54%	2400
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 3 6% 7% 46%	2400
	21st to end	Day	11% 7% 2 7% 5% 4 5% 3 5% 8% 42%	2144
October	1st to 10th	Day	5% 8% 3 7% 4 5% 4 5% 6% 8% 45%	3280
	11th to 20th	Day	7% 9% 3 8% 6% 6% 6% 5% 6% 8% 36%	3234
	21st to end	Day	6% 9% 3 9% 6% 6% 7% 5% 6% 9% 34%	3200
November	1st to 10th	Day	6% 11% 5% 11% 8% 7% 7% 6% 7% 8% 24%	1723
	11th to 20th	Day	5% 9% 5% 10% 7% 7% 8% 6% 8% 8% 27%	1680
	21st to end	Day	6% 11% 4 10% 8% 6% 7% 6% 8% 8% 27%	1680
December	1st to 10th	Day	3 8% 3 8% 6% 7% 9% 7% 9% 9% 32%	2414
	11th to 20th	Day	4 9% 5% 10% 9% 8% 9% 7% 8% 9% 23%	2730
	21st to end	Day	3 11% 5% 13% 9% 7% 9% 6% 8% 9% 21%	3593
January	1st to 10th	Night	4 13% 6% 11% 7% 8% 9% 7% 8% 10% 18%	8712
	11th to 20th	Night	3 7% 3 7% 6% 6% 6% 5% 7% 8% 41%	8182
	21st to end	Night	3 7% 3 9% 7% 7% 6% 5% 7% 9% 36%	8994
February	1st to 10th	Night	5% 12% 6% 12% 8% 7% 7% 5% 7% 8% 24%	2835
	11th to 20th	Night	3 13% 4 11% 7% 5% 6% 5% 7% 8% 31%	2662
	21st to end	Night	5% 10% 6% 13% 7% 6% 5% 5% 7% 9% 26%	2128
March	1st to 10th	Night	1 7% 3 8% 5% 6% 5% 5% 8% 12% 39%	5496
	11th to 20th	Night	2 9% 3 7% 5% 5% 6% 6% 8% 11% 40%	5244
	21st to end	Night	7% 13% 4 10% 7% 7% 7% 5% 7% 9% 26%	5256
April	1st to 10th	Night	3 11% 3 8% 6% 6% 6% 5% 8% 10% 35%	2100
	11th to 20th	Night	4 10% 3 7% 6% 6% 6% 6% 8% 8% 36%	2079
	21st to end	Night	6% 10% 3 8% 7% 5% 5% 4 7% 8% 37%	1730
May	1st to 10th	Night	4 6% 2 6% 5% 5% 5% 5% 6% 7% 49%	3720
	11th to 20th	Night	7% 10% 3 7% 4 5% 5% 5% 7% 9% 38%	3696
	21st to end	Night	8% 9% 2 6% 4 5% 4 4 7% 10% 41%	4060
June	1st to 10th	Night	8% 7% 2 7% 6% 6% 5% 5% 6% 9% 39%	1526
	11th to 20th	Night	9% 6% 2 6% 4 5% 6% 5% 7% 10% 39%	1205
	21st to end	Night	6% 5% 2 5% 5% 5% 4 4 7% 9% 50%	1382
July	1st to 10th	Night	2 2 1 3 2 2 3 3 4 6% 71%	3180
	11th to 20th	Night	1 1 1 2 2 2 2 3 4 6% 76%	3453
	21st to end	Night	4 3 1 3 2 2 2 2 5% 6% 69%	3969
August	1st to 10th	Night	2 2 1 3 2 3 4 3 6% 9% 66%	2609
	11th to 20th	Night	2 3 1 5% 3 3 4 4 7% 9% 58%	2699
	21st to end	Night	8% 5% 1 4 3 4 4 4 7% 9% 51%	3506
September	1st to 10th	Night	5% 2 1 4 3 3 4 4 7% 7% 61%	2000
	11th to 20th	Night	8% 4 2 5% 3 4 4 3 7% 7% 51%	2000
	21st to end	Night	8% 7% 2 6% 4 4 5% 4 6% 9% 47%	2253
October	1st to 10th	Night	4 6% 3 7% 5% 4 6% 4 6% 9% 46%	3920
	11th to 20th	Night	7% 11% 3 7% 5% 4 5% 4 6% 8% 40%	3923
	21st to end	Night	7% 9% 4 8% 5% 6% 6% 4 6% 9% 37%	4600
November	1st to 10th	Night	6% 11% 5% 10% 7% 6% 6% 5% 7% 9% 28%	2793
	11th to 20th	Night	5% 11% 4 8% 6% 7% 6% 6% 8% 10% 28%	2940
	21st to end	Night	5% 11% 4 10% 8% 6% 7% 6% 8% 9% 25%	2940
December	1st to 10th	Night	2 8% 3 7% 6% 6% 7% 5% 7% 9% 40%	4933
	11th to 20th	Night	3 10% 5% 12% 9% 9% 8% 6% 7% 8% 23%	5700
	21st to end	Night	3 9% 4 12% 8% 8% 8% 6% 8% 10% 24%	7617

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	62% 13% 11% 10% 3	4165
	11th to 20th	Day	42% 9% 19% 19% 7% 4	4740
	21st to end	Day	48% 12% 14% 17% 8%	5187
February	1st to 10th	Day	59% 11% 13% 9% 7% 1	1782
	11th to 20th	Day	56% 12% 13% 11% 6% 1	1854
	21st to end	Day	62% 11% 11% 12% 4	1677
March	1st to 10th	Day	42% 14% 16% 12% 14% 1	4618
	11th to 20th	Day	45% 11% 16% 21% 7%	4972
	21st to end	Day	56% 12% 14% 11% 7%	5961
April	1st to 10th	Day	56% 14% 14% 14% 1	2520
	11th to 20th	Day	52% 11% 14% 14% 8% 1	2492
	21st to end	Day	52% 11% 15% 14% 7% 1	2885
May	1st to 10th	Day	38% 9% 12% 29% 11% 1	6460
	11th to 20th	Day	49% 12% 19% 17% 3	6420
	21st to end	Day	50% 8% 16% 22% 3	7058
June	1st to 10th	Day	51% 11% 13% 14% 8% 3	2871
	11th to 20th	Day	55% 10% 13% 16% 6%	3183
	21st to end	Day	41% 12% 16% 19% 11% 1	3004
July	1st to 10th	Day	20% 5% 14% 25% 25% 12%	6706
	11th to 20th	Day	17% 5% 14% 27% 27% 10%	6425
	21st to end	Day	22% 7% 17% 31% 18% 5%	6681
August	1st to 10th	Day	20% 6% 10% 28% 29% 8%	4524
	11th to 20th	Day	28% 9% 25% 25% 10% 3	4391
	21st to end	Day	33% 10% 18% 24% 14% 2	4408
September	1st to 10th	Day	32% 8% 14% 25% 18% 3	2400
	11th to 20th	Day	40% 8% 11% 21% 17% 3	2400
	21st to end	Day	41% 11% 17% 17% 11% 2	2144
October	1st to 10th	Day	42% 6% 19% 27% 5%	3280
	11th to 20th	Day	50% 8% 10% 25% 6% 1	3234
	21st to end	Day	49% 10% 18% 18% 6%	3200
November	1st to 10th	Day	60% 12% 12% 11% 5%	1723
	11th to 20th	Day	57% 11% 13% 12% 5% 2	1680
	21st to end	Day	53% 13% 13% 13% 5% 2	1680
December	1st to 10th	Day	47% 12% 21% 13% 6%	2414
	11th to 20th	Day	55% 13% 16% 12% 4	2732
	21st to end	Day	59% 13% 13% 12% 3	3593
January	1st to 10th	Night	61% 14% 14% 8% 3	8712
	11th to 20th	Night	42% 10% 18% 21% 7% 2	8220
	21st to end	Night	46% 13% 16% 17% 7%	8994
February	1st to 10th	Night	60% 11% 12% 11% 6% 1	2835
	11th to 20th	Night	53% 12% 13% 14% 6% 2	2664
	21st to end	Night	56% 11% 13% 15% 5%	2128
March	1st to 10th	Night	37% 14% 17% 19% 12% 1	5496
	11th to 20th	Night	39% 12% 19% 22% 8%	5245
	21st to end	Night	54% 13% 14% 14% 4	5256
April	1st to 10th	Night	47% 14% 20% 15% 4	2100
	11th to 20th	Night	44% 13% 18% 15% 9% 1	2079
	21st to end	Night	46% 12% 16% 17% 8%	1730
May	1st to 10th	Night	36% 11% 16% 26% 11%	3717
	11th to 20th	Night	45% 13% 19% 20% 4	3696
	21st to end	Night	40% 13% 22% 22% 3	4060
June	1st to 10th	Night	43% 12% 16% 19% 8% 2	1526
	11th to 20th	Night	41% 13% 20% 18% 7%	1205
	21st to end	Night	34% 10% 23% 22% 10% 1	1382
July	1st to 10th	Night	18% 6% 16% 25% 27% 10%	3145
	11th to 20th	Night	12% 6% 19% 27% 26% 11%	3410
	21st to end	Night	19% 5% 16% 36% 17% 7%	3857
August	1st to 10th	Night	16% 6% 13% 32% 30% 3	2601
	11th to 20th	Night	20% 11% 30% 26% 12% 2	2723
	21st to end	Night	33% 9% 19% 25% 13% 2	3506
September	1st to 10th	Night	23% 10% 19% 28% 18% 3	2000
	11th to 20th	Night	34% 10% 13% 21% 18% 4	2000
	21st to end	Night	38% 10% 19% 23% 8% 3	2253
October	1st to 10th	Night	39% 8% 20% 26% 7%	3920
	11th to 20th	Night	45% 9% 13% 23% 9%	3923
	21st to end	Night	49% 9% 15% 21% 6%	4600
November	1st to 10th	Night	56% 13% 14% 12% 5% 1	2793
	11th to 20th	Night	54% 14% 14% 13% 4 1	2940
	21st to end	Night	55% 16% 12% 10% 5% 2	2940
December	1st to 10th	Night	43% 12% 18% 20% 5% 1	4933
	11th to 20th	Night	58% 12% 14% 13% 3	5697
	21st to end	Night	54% 14% 14% 13% 4	7617

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	9% 25% 29% 22% 12% 3	220
	11th to 20th	Day	10% 13% 25% 35% 12% 4	220
	21st to end	Day	7% 15% 36% 29% 13%	242
February	1st to 10th	Day	19% 23% 30% 17% 8% 2	322
	11th to 20th	Day	13% 22% 33% 21% 8% 2	425
	21st to end	Day	16% 24% 34% 20% 6%	419
March	1st to 10th	Day	16% 25% 30% 19% 9% 1	540
	11th to 20th	Day	14% 27% 27% 20% 9% 1	540
	21st to end	Day	19% 28% 27% 17% 9% 1	594
April	1st to 10th	Day	18% 24% 32% 17% 8% 1	540
	11th to 20th	Day	18% 24% 26% 20% 8% 3	532
	21st to end	Day	18% 24% 27% 17% 11% 3	540
May	1st to 10th	Day	14% 20% 31% 24% 9% 2	543
	11th to 20th	Day	17% 19% 30% 21% 11% 3	568
	21st to end	Day	17% 21% 28% 22% 9% 3	617
June	1st to 10th	Day	16% 24% 33% 18% 8% 1	601
	11th to 20th	Day	18% 19% 33% 18% 10% 2	913
	21st to end	Day	12% 19% 31% 22% 13% 2	734
July	1st to 10th	Day	9% 14% 36% 27% 12% 1	721
	11th to 20th	Day	8% 17% 33% 27% 13% 2	600
	21st to end	Day	6% 13% 29% 31% 16% 6%	575
August	1st to 10th	Day	7% 10% 27% 30% 18% 8%	527
	11th to 20th	Day	6% 12% 29% 30% 18% 5%	517
	21st to end	Day	9% 17% 26% 24% 17% 7%	583
September	1st to 10th	Day	10% 18% 25% 22% 16% 9%	530
	11th to 20th	Day	12% 16% 23% 30% 15% 4	530
	21st to end	Day	19% 20% 26% 21% 12% 3	530
October	1st to 10th	Day	15% 18% 27% 22% 13% 4	530
	11th to 20th	Day	19% 21% 24% 20% 12% 3	515
	21st to end	Day	20% 17% 28% 26% 7% 2	363
November	1st to 10th	Day	22% 11% 34% 24% 7% 1	261
	11th to 20th	Day	19% 22% 25% 18% 14% 2	220
	21st to end	Day	16% 13% 31% 25% 10% 5%	220
December	1st to 10th	Day	15% 15% 23% 22% 19% 5%	220
	11th to 20th	Day	12% 19% 34% 25% 8% 1	220
	21st to end	Day	12% 11% 36% 29% 11% 1	242
January	1st to 10th	Night	14% 16% 33% 28% 8% 1	939
	11th to 20th	Night	9% 11% 38% 29% 11% 1	840
	21st to end	Night	9% 13% 36% 31% 10% 2	924
February	1st to 10th	Night	13% 16% 31% 30% 9% 1	840
	11th to 20th	Night	9% 14% 33% 33% 10% 1	824
	21st to end	Night	13% 15% 30% 29% 12% 1	692
March	1st to 10th	Night	9% 15% 31% 29% 15% 1	840
	11th to 20th	Night	8% 17% 35% 29% 10% 1	840
	21st to end	Night	10% 16% 34% 29% 10% 1	924
April	1st to 10th	Night	10% 14% 29% 32% 13% 2	840
	11th to 20th	Night	10% 16% 31% 27% 15% 1	831
	21st to end	Night	12% 15% 26% 33% 12% 2	470
May	1st to 10th	Night	5% 13% 25% 38% 17% 1	420
	11th to 20th	Night	11% 7% 19% 41% 22% 1	414
	21st to end	Night	13% 6% 22% 38% 21% 1	455
June	1st to 10th	Night	13% 8% 29% 36% 15% 1	400
	11th to 20th	Night	12% 11% 28% 32% 18%	400
	21st to end	Night	8% 9% 26% 34% 23% 2	400
July	1st to 10th	Night	5% 7% 19% 42% 26% 2	390
	11th to 20th	Night	3 7% 23% 41% 24% 1	388
	21st to end	Night	3 5% 19% 43% 29% 1	438
August	1st to 10th	Night	5% 3 13% 42% 35% 3	398
	11th to 20th	Night	4 2 15% 43% 32% 4	466
	21st to end	Night	8% 7% 22% 38% 23% 2	817
September	1st to 10th	Night	6% 7% 24% 33% 26% 5%	800
	11th to 20th	Night	9% 6% 20% 29% 29% 7%	800
	21st to end	Night	13% 10% 26% 31% 17% 3	800
October	1st to 10th	Night	12% 9% 24% 34% 19% 2	800
	11th to 20th	Night	14% 12% 27% 30% 15% 2	800
	21st to end	Night	15% 18% 29% 26% 10% 2	880
November	1st to 10th	Night	14% 14% 38% 28% 6%	820
	11th to 20th	Night	15% 19% 30% 25% 9% 2	840
	21st to end	Night	12% 17% 32% 26% 10% 2	840
December	1st to 10th	Night	10% 15% 33% 30% 12% 2	982
	11th to 20th	Night	12% 15% 39% 27% 6%	1050
	21st to end	Night	10% 14% 38% 31% 6%	1047

Page generated at 2026-05-28 22:53:29. Version ac07

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