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 2024-02-05.

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	7% 8% 4 8% 7% 4 6% 6% 6% 8% 36%	77895
	Night	5% 8% 4 8% 7% 4 6% 6% 7% 9% 35%	80852
Transparency	Day	48% 11% 14% 16% 9% 2	77612
	Night	46% 13% 15% 16% 8% 1	80716
Seeing	Day	15% 21% 30% 22% 10% 3	13990
	Night	11% 13% 30% 30% 15% 2	22752

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% 16% 5% 10% 9% 6% 7% 7% 7% 8% 20%	2231
	11th to 20th	Day	3 10% 5% 10% 10% 7% 6% 6% 7% 8% 28%	2516
	21st to end	Day	4 9% 5% 11% 11% 6% 7% 7% 7% 11% 22%	2794
February	1st to 10th	Day	6% 11% 5% 10% 9% 6% 7% 8% 8% 8% 24%	1559
	11th to 20th	Day	4 10% 5% 11% 10% 4 7% 7% 7% 9% 26%	1575
	21st to end	Day	2 9% 6% 12% 12% 6% 9% 8% 7% 6% 23%	1434
March	1st to 10th	Day	3 8% 5% 10% 8% 5% 8% 8% 8% 10% 28%	1800
	11th to 20th	Day	4 11% 4 8% 8% 5% 8% 7% 8% 9% 27%	1923
	21st to end	Day	6% 10% 5% 11% 8% 6% 8% 7% 9% 8% 21%	2317
April	1st to 10th	Day	6% 12% 5% 9% 8% 6% 7% 7% 8% 8% 25%	2160
	11th to 20th	Day	6% 11% 5% 9% 7% 3 7% 7% 7% 9% 28%	2132
	21st to end	Day	8% 11% 4 8% 8% 4 6% 6% 7% 8% 31%	2471
May	1st to 10th	Day	9% 9% 3 7% 7% 4 6% 5% 6% 8% 36%	2520
	11th to 20th	Day	7% 9% 3 7% 6% 4 6% 6% 7% 9% 34%	2477
	21st to end	Day	12% 8% 4 8% 7% 5% 7% 6% 6% 7% 30%	2723
June	1st to 10th	Day	14% 8% 4 6% 6% 4 5% 6% 6% 7% 33%	2582
	11th to 20th	Day	14% 8% 4 7% 8% 4 5% 6% 6% 7% 31%	2865
	21st to end	Day	8% 6% 3 6% 7% 4 5% 6% 6% 8% 42%	2704
July	1st to 10th	Day	5% 4 2 4 6% 4 5% 5% 6% 7% 52%	2700
	11th to 20th	Day	5% 3 1 4 4 3 4 4 5% 7% 59%	2592
	21st to end	Day	3 2 1 3 3 2 3 4 5% 6% 69%	2741
August	1st to 10th	Day	5% 4 1 4 4 3 4 4 5% 6% 60%	2507
	11th to 20th	Day	4 3 1 3 5% 3 4 4 5% 7% 61%	2410
	21st to end	Day	5% 5% 2 5% 5% 3 5% 5% 6% 9% 49%	2431
September	1st to 10th	Day	7% 4 2 6% 6% 3 4 4 4 7% 53%	2160
	11th to 20th	Day	9% 6% 3 6% 6% 4 5% 5% 6% 7% 44%	2160
	21st to end	Day	12% 7% 2 6% 6% 3 5% 5% 5% 9% 42%	1932
October	1st to 10th	Day	8% 9% 4 8% 7% 4 4 6% 5% 7% 37%	1800
	11th to 20th	Day	8% 7% 3 7% 7% 5% 6% 7% 7% 8% 34%	1787
	21st to end	Day	8% 9% 4 8% 8% 4 6% 6% 6% 9% 31%	1782
November	1st to 10th	Day	6% 11% 5% 9% 9% 4 7% 8% 7% 9% 24%	1556
	11th to 20th	Day	4 8% 5% 9% 8% 6% 8% 9% 7% 8% 29%	1520
	21st to end	Day	6% 9% 5% 9% 9% 4 7% 7% 9% 8% 28%	1520
December	1st to 10th	Day	5% 10% 5% 10% 7% 5% 6% 8% 8% 9% 26%	1368
	11th to 20th	Day	3 8% 6% 9% 12% 6% 9% 8% 8% 10% 21%	1686
	21st to end	Day	4 11% 5% 11% 10% 5% 8% 8% 8% 9% 21%	2460
January	1st to 10th	Night	5% 13% 6% 9% 8% 5% 8% 9% 8% 10% 18%	4696
	11th to 20th	Night	4 10% 5% 9% 9% 5% 7% 6% 8% 8% 29%	4382
	21st to end	Night	5% 9% 5% 10% 11% 7% 8% 8% 7% 10% 22%	4862
February	1st to 10th	Night	6% 12% 6% 10% 9% 4 7% 7% 7% 8% 25%	2492
	11th to 20th	Night	3 12% 6% 10% 8% 4 6% 6% 7% 8% 31%	2272
	21st to end	Night	5% 9% 7% 12% 9% 4 6% 6% 7% 10% 26%	1821
March	1st to 10th	Night	3 9% 4 8% 7% 4 8% 6% 7% 10% 32%	2160
	11th to 20th	Night	3 8% 4 7% 7% 5% 7% 8% 8% 11% 34%	2033
	21st to end	Night	5% 10% 4 9% 8% 4 7% 7% 7% 9% 30%	2039
April	1st to 10th	Night	2 10% 3 8% 7% 4 7% 6% 8% 10% 34%	1800
	11th to 20th	Night	5% 10% 3 7% 7% 4 6% 6% 8% 8% 35%	1779
	21st to end	Night	6% 10% 4 7% 7% 4 6% 5% 6% 8% 36%	1484
May	1st to 10th	Night	6% 6% 3 6% 7% 3 6% 6% 7% 8% 42%	1440
	11th to 20th	Night	7% 6% 2 5% 5% 3 6% 7% 9% 10% 41%	1416
	21st to end	Night	11% 8% 3 7% 5% 3 4 4 5% 9% 42%	1552
June	1st to 10th	Night	8% 7% 3 6% 7% 4 6% 5% 6% 10% 38%	1374
	11th to 20th	Night	10% 5% 3 5% 5% 4 6% 6% 8% 9% 39%	1085
	21st to end	Night	5% 4 3 4 5% 4 5% 4 7% 9% 50%	1243
July	1st to 10th	Night	5% 4 2 3 4 3 4 5% 6% 8% 58%	1260
	11th to 20th	Night	2 2 1 3 3 2 4 5% 5% 9% 65%	1365
	21st to end	Night	2 2 1 1 2 2 2 3 4 6% 76%	1576
August	1st to 10th	Night	3 2 1 3 3 2 3 4 5% 7% 66%	1432
	11th to 20th	Night	2 2 1 2 3 2 3 4 6% 7% 69%	1486
	21st to end	Night	5% 3 1 3 3 3 5% 5% 7% 9% 56%	1923
September	1st to 10th	Night	5% 2 1 4 3 2 4 5% 7% 7% 60%	1800
	11th to 20th	Night	9% 4 2 5% 4 3 4 5% 6% 8% 50%	1800
	21st to end	Night	8% 6% 2 5% 4 3 5% 5% 6% 9% 46%	2026
October	1st to 10th	Night	6% 7% 4 6% 6% 3 6% 5% 5% 8% 44%	2160
	11th to 20th	Night	8% 9% 3 7% 6% 3 6% 6% 6% 8% 40%	2173
	21st to end	Night	8% 10% 4 7% 6% 4 6% 5% 6% 10% 33%	2574
November	1st to 10th	Night	7% 11% 5% 9% 7% 4 7% 6% 6% 9% 28%	2520
	11th to 20th	Night	5% 10% 5% 7% 7% 5% 7% 7% 7% 10% 29%	2660
	21st to end	Night	5% 9% 5% 9% 9% 4 7% 8% 8% 9% 26%	2660
December	1st to 10th	Night	3 11% 5% 8% 9% 6% 8% 7% 7% 8% 28%	2788
	11th to 20th	Night	4 12% 6% 11% 10% 5% 8% 8% 8% 9% 21%	3510
	21st to end	Night	4 9% 4 10% 10% 5% 8% 7% 8% 11% 24%	5209

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	60% 17% 11% 8% 3 1	2231
	11th to 20th	Day	54% 10% 15% 11% 5% 5%	2540
	21st to end	Day	56% 16% 13% 9% 5%	2794
February	1st to 10th	Day	60% 12% 13% 9% 5% 1	1559
	11th to 20th	Day	57% 13% 14% 9% 5% 1	1584
	21st to end	Day	62% 11% 11% 12% 3	1434
March	1st to 10th	Day	51% 15% 16% 12% 5% 1	1800
	11th to 20th	Day	56% 14% 12% 13% 4	1923
	21st to end	Day	62% 13% 14% 9% 2	2317
April	1st to 10th	Day	59% 14% 13% 12% 1	2160
	11th to 20th	Day	56% 11% 13% 12% 8% 1	2132
	21st to end	Day	55% 12% 15% 12% 6%	2471
May	1st to 10th	Day	50% 11% 15% 15% 8% 1	2520
	11th to 20th	Day	49% 13% 17% 17% 5%	2480
	21st to end	Day	57% 9% 14% 15% 4	2724
June	1st to 10th	Day	54% 11% 13% 13% 6% 3	2582
	11th to 20th	Day	56% 11% 14% 14% 5%	2865
	21st to end	Day	41% 12% 15% 19% 11% 1	2704
July	1st to 10th	Day	35% 9% 13% 22% 16% 5%	2626
	11th to 20th	Day	28% 8% 15% 26% 20% 4	2513
	21st to end	Day	19% 7% 12% 28% 27% 6%	2589
August	1st to 10th	Day	26% 8% 14% 25% 21% 6%	2490
	11th to 20th	Day	24% 7% 18% 27% 18% 5%	2410
	21st to end	Day	33% 11% 16% 26% 12% 2	2431
September	1st to 10th	Day	34% 8% 14% 25% 16% 3	2160
	11th to 20th	Day	42% 8% 10% 19% 17% 3	2160
	21st to end	Day	41% 12% 15% 17% 12% 3	1932
October	1st to 10th	Day	48% 8% 15% 22% 8%	1800
	11th to 20th	Day	49% 10% 14% 18% 7% 1	1787
	21st to end	Day	52% 11% 14% 17% 6%	1782
November	1st to 10th	Day	61% 13% 11% 10% 5%	1556
	11th to 20th	Day	56% 11% 13% 13% 5% 2	1520
	21st to end	Day	52% 14% 13% 13% 6% 2	1520
December	1st to 10th	Day	54% 15% 14% 13% 4 1	1368
	11th to 20th	Day	51% 17% 16% 12% 3	1688
	21st to end	Day	56% 15% 12% 13% 4	2460
January	1st to 10th	Night	61% 18% 11% 8% 2 1	4696
	11th to 20th	Night	53% 11% 13% 13% 6% 4	4420
	21st to end	Night	54% 19% 13% 9% 5% 1	4862
February	1st to 10th	Night	60% 11% 12% 11% 5%	2492
	11th to 20th	Night	54% 13% 13% 13% 5% 1	2274
	21st to end	Night	55% 11% 13% 16% 5%	1821
March	1st to 10th	Night	50% 12% 17% 13% 7% 1	2160
	11th to 20th	Night	45% 17% 17% 17% 5%	2034
	21st to end	Night	52% 15% 16% 14% 3	2039
April	1st to 10th	Night	47% 15% 19% 15% 4	1800
	11th to 20th	Night	45% 13% 18% 14% 9%	1779
	21st to end	Night	48% 13% 16% 17% 7%	1484
May	1st to 10th	Night	42% 15% 18% 17% 7% 1	1440
	11th to 20th	Night	37% 16% 20% 19% 8%	1416
	21st to end	Night	44% 11% 20% 22% 4	1552
June	1st to 10th	Night	44% 12% 16% 19% 6% 3	1374
	11th to 20th	Night	42% 14% 22% 17% 5%	1085
	21st to end	Night	33% 11% 23% 22% 10% 1	1243
July	1st to 10th	Night	26% 10% 17% 29% 15% 4	1225
	11th to 20th	Night	20% 8% 22% 30% 18% 2	1322
	21st to end	Night	12% 4 18% 33% 28% 6%	1488
August	1st to 10th	Night	20% 8% 18% 26% 22% 6%	1424
	11th to 20th	Night	16% 9% 20% 29% 22% 4	1486
	21st to end	Night	26% 11% 19% 29% 14% 2	1923
September	1st to 10th	Night	24% 10% 19% 28% 16% 3	1800
	11th to 20th	Night	35% 10% 13% 19% 19% 4	1800
	21st to end	Night	38% 10% 18% 22% 9% 3	2026
October	1st to 10th	Night	41% 8% 18% 24% 9% 1	2160
	11th to 20th	Night	46% 11% 12% 21% 9% 1	2173
	21st to end	Night	50% 12% 12% 19% 7%	2574
November	1st to 10th	Night	56% 14% 14% 11% 5% 1	2520
	11th to 20th	Night	52% 15% 14% 13% 5% 1	2660
	21st to end	Night	54% 16% 11% 10% 6% 2	2660
December	1st to 10th	Night	55% 14% 13% 13% 4 1	2788
	11th to 20th	Night	57% 15% 13% 11% 3	3507
	21st to end	Night	52% 16% 15% 13% 4	5209

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% 28% 27% 20% 13% 3	180
	11th to 20th	Day	11% 14% 28% 30% 12% 4	180
	21st to end	Day	8% 18% 39% 25% 9%	198
February	1st to 10th	Day	22% 28% 32% 15% 3	255
	11th to 20th	Day	16% 27% 35% 16% 5% 1	335
	21st to end	Day	13% 26% 35% 20% 6% 1	326
March	1st to 10th	Day	17% 29% 32% 14% 7% 1	420
	11th to 20th	Day	15% 27% 28% 20% 9% 1	420
	21st to end	Day	19% 30% 25% 15% 9% 1	462
April	1st to 10th	Day	20% 25% 33% 16% 6% 1	420
	11th to 20th	Day	19% 27% 25% 19% 7% 2	412
	21st to end	Day	20% 25% 28% 17% 7% 2	420
May	1st to 10th	Day	16% 21% 33% 21% 6% 2	423
	11th to 20th	Day	16% 19% 29% 21% 12% 3	448
	21st to end	Day	19% 20% 27% 22% 9% 3	485
June	1st to 10th	Day	17% 25% 34% 16% 6% 1	512
	11th to 20th	Day	19% 20% 34% 17% 9% 1	795
	21st to end	Day	12% 21% 31% 22% 11% 3	634
July	1st to 10th	Day	10% 15% 37% 27% 10% 1	621
	11th to 20th	Day	8% 19% 35% 26% 10% 2	512
	21st to end	Day	6% 13% 31% 31% 14% 5%	487
August	1st to 10th	Day	9% 12% 28% 30% 15% 6%	447
	11th to 20th	Day	7% 12% 29% 30% 17% 4	437
	21st to end	Day	9% 19% 29% 25% 13% 5%	495
September	1st to 10th	Day	12% 22% 28% 23% 12% 4	450
	11th to 20th	Day	13% 17% 26% 28% 13% 3	450
	21st to end	Day	20% 20% 26% 20% 11% 4	450
October	1st to 10th	Day	17% 20% 28% 19% 12% 4	450
	11th to 20th	Day	19% 22% 24% 20% 12% 3	437
	21st to end	Day	21% 20% 26% 23% 8% 2	297
November	1st to 10th	Day	24% 12% 35% 21% 7% 1	214
	11th to 20th	Day	19% 26% 24% 16% 14% 2	180
	21st to end	Day	14% 16% 33% 21% 10% 6%	180
December	1st to 10th	Day	17% 17% 19% 23% 18% 5%	180
	11th to 20th	Day	11% 20% 34% 25% 8% 2	180
	21st to end	Day	14% 12% 37% 27% 9% 1	198
January	1st to 10th	Night	15% 18% 34% 24% 8% 1	841
	11th to 20th	Night	10% 13% 39% 26% 11% 1	760
	21st to end	Night	10% 14% 36% 29% 9% 2	836
February	1st to 10th	Night	15% 17% 32% 27% 8% 1	738
	11th to 20th	Night	10% 14% 34% 30% 11% 1	704
	21st to end	Night	11% 15% 29% 29% 13% 2	592
March	1st to 10th	Night	10% 16% 31% 27% 14% 2	720
	11th to 20th	Night	9% 16% 36% 27% 10% 2	720
	21st to end	Night	11% 17% 34% 27% 10% 1	792
April	1st to 10th	Night	9% 15% 29% 32% 13% 2	720
	11th to 20th	Night	10% 17% 31% 27% 14% 1	711
	21st to end	Night	13% 15% 28% 32% 11% 1	404
May	1st to 10th	Night	6% 15% 27% 36% 16% 2	360
	11th to 20th	Night	10% 7% 18% 40% 24% 1	354
	21st to end	Night	13% 5% 23% 35% 23% 1	389
June	1st to 10th	Night	13% 8% 29% 34% 15% 1	360
	11th to 20th	Night	13% 11% 28% 31% 17%	360
	21st to end	Night	7% 9% 26% 34% 21% 2	360
July	1st to 10th	Night	6% 7% 20% 39% 26% 2	350
	11th to 20th	Night	4 8% 25% 41% 22% 1	348
	21st to end	Night	3 6% 19% 45% 28% 1	394
August	1st to 10th	Night	5% 3 14% 40% 36% 2	358
	11th to 20th	Night	3 2 16% 43% 32% 3	418
	21st to end	Night	7% 8% 23% 37% 23% 2	735
September	1st to 10th	Night	6% 8% 26% 33% 23% 4	720
	11th to 20th	Night	10% 7% 21% 27% 28% 8%	720
	21st to end	Night	13% 11% 26% 29% 18% 3	720
October	1st to 10th	Night	13% 10% 23% 33% 20% 2	720
	11th to 20th	Night	14% 12% 28% 28% 16% 2	720
	21st to end	Night	16% 19% 28% 25% 10% 3	792
November	1st to 10th	Night	14% 15% 39% 26% 6%	740
	11th to 20th	Night	15% 21% 31% 23% 9% 2	760
	21st to end	Night	12% 18% 33% 24% 10% 3	760
December	1st to 10th	Night	10% 15% 33% 29% 11% 2	888
	11th to 20th	Night	12% 15% 38% 28% 7% 1	950
	21st to end	Night	11% 14% 38% 30% 7%	938

Page generated at 2024-02-02 02:20:27. Version ac07

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