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 2025-02-06.

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	6% 8% 3 8% 6% 5% 6% 5% 6% 8% 40%	111955
	Night	4 8% 3 8% 6% 6% 6% 5% 7% 9% 38%	114879
Transparency	Day	44% 10% 15% 18% 11% 3	111714
	Night	43% 12% 16% 18% 8% 2	114767
Seeing	Day	15% 20% 30% 22% 11% 3	15283
	Night	11% 13% 30% 31% 15% 2	24014

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% 14% 4 12% 8% 7% 8% 5% 8% 8% 21%	3205
	11th to 20th	Day	2 9% 4 9% 7% 7% 6% 5% 9% 10% 32%	3616
	21st to end	Day	3 8% 4 11% 7% 7% 6% 5% 6% 10% 33%	4004
February	1st to 10th	Day	6% 11% 5% 11% 9% 7% 8% 6% 8% 8% 23%	1653
	11th to 20th	Day	4 11% 5% 12% 9% 5% 6% 6% 7% 9% 26%	1665
	21st to end	Day	2 10% 6% 14% 11% 9% 7% 6% 7% 6% 23%	1521
March	1st to 10th	Day	2 7% 4 11% 6% 6% 8% 7% 9% 9% 31%	3138
	11th to 20th	Day	3 9% 2 7% 5% 5% 6% 5% 7% 10% 42%	3394
	21st to end	Day	7% 13% 4 12% 7% 7% 7% 6% 7% 7% 23%	4067
April	1st to 10th	Day	6% 13% 4 10% 8% 8% 7% 5% 8% 7% 24%	2280
	11th to 20th	Day	6% 12% 4 11% 6% 5% 7% 5% 7% 9% 30%	2252
	21st to end	Day	8% 12% 4 9% 6% 6% 6% 5% 7% 7% 30%	2609
May	1st to 10th	Day	6% 10% 2 7% 5% 4 5% 4 6% 7% 43%	4420
	11th to 20th	Day	6% 9% 2 6% 5% 5% 5% 4 7% 10% 41%	4377
	21st to end	Day	16% 14% 4 9% 6% 6% 6% 4 5% 5% 26%	4813
June	1st to 10th	Day	14% 8% 4 7% 6% 5% 5% 6% 6% 6% 33%	2726
	11th to 20th	Day	13% 9% 4 8% 7% 5% 5% 5% 6% 7% 31%	3024
	21st to end	Day	8% 6% 3 7% 6% 5% 5% 5% 6% 8% 42%	2854
July	1st to 10th	Day	3 3 1 3 3 3 3 3 4 5% 71%	4740
	11th to 20th	Day	3 2 1 3 3 2 3 2 4 5% 72%	4548
	21st to end	Day	3 2 1 4 3 3 3 3 5% 7% 66%	4790
August	1st to 10th	Day	3 2 1 3 3 4 4 3 6% 6% 65%	4401
	11th to 20th	Day	4 4 2 5% 5% 5% 5% 4 6% 8% 53%	4213
	21st to end	Day	7% 4 1 5% 5% 4 5% 4 5% 8% 49%	4274
September	1st to 10th	Day	7% 4 2 7% 4 4 4 4 4 7% 53%	2280
	11th to 20th	Day	9% 6% 2 6% 6% 5% 5% 3 6% 7% 44%	2280
	21st to end	Day	11% 7% 2 7% 5% 4 5% 4 5% 8% 42%	2038
October	1st to 10th	Day	5% 8% 3 7% 4 5% 4 5% 6% 8% 45%	3180
	11th to 20th	Day	7% 9% 3 8% 5% 6% 6% 5% 6% 8% 36%	3135
	21st to end	Day	6% 9% 3 9% 6% 6% 7% 5% 6% 9% 34%	3101
November	1st to 10th	Day	5% 11% 5% 11% 8% 7% 7% 6% 7% 8% 24%	1639
	11th to 20th	Day	5% 9% 5% 10% 7% 7% 8% 7% 7% 8% 28%	1600
	21st to end	Day	6% 10% 4 11% 8% 6% 7% 6% 8% 8% 27%	1600
December	1st to 10th	Day	3 8% 3 8% 6% 7% 9% 6% 9% 9% 32%	2342
	11th to 20th	Day	4 9% 5% 11% 9% 8% 9% 6% 8% 9% 23%	2660
	21st to end	Day	3 11% 6% 13% 9% 7% 9% 6% 8% 9% 21%	3516
January	1st to 10th	Night	4 12% 6% 11% 7% 8% 10% 7% 8% 9% 17%	6722
	11th to 20th	Night	2 8% 3 9% 7% 7% 6% 5% 7% 9% 38%	6282
	21st to end	Night	4 7% 3 10% 8% 7% 6% 5% 7% 9% 33%	6952
February	1st to 10th	Night	6% 13% 6% 12% 8% 6% 6% 5% 7% 8% 23%	2635
	11th to 20th	Night	2 13% 4 11% 6% 6% 6% 5% 7% 8% 31%	2402
	21st to end	Night	5% 10% 6% 14% 8% 6% 5% 5% 7% 9% 26%	1932
March	1st to 10th	Night	2 8% 3 9% 6% 7% 6% 5% 8% 11% 34%	3736
	11th to 20th	Night	2 8% 2 6% 4 4 5% 5% 7% 12% 45%	3582
	21st to end	Night	7% 12% 4 9% 7% 6% 7% 6% 8% 8% 27%	3586
April	1st to 10th	Night	3 11% 3 8% 6% 6% 6% 5% 8% 10% 34%	1900
	11th to 20th	Night	4 10% 3 7% 6% 5% 6% 5% 8% 8% 36%	1879
	21st to end	Night	6% 11% 4 8% 7% 5% 6% 4 7% 8% 35%	1566
May	1st to 10th	Night	5% 8% 3 7% 5% 5% 5% 4 5% 7% 45%	2540
	11th to 20th	Night	5% 7% 2 5% 4 5% 5% 4 7% 10% 47%	2516
	21st to end	Night	11% 12% 3 8% 4 5% 5% 5% 7% 8% 32%	2762
June	1st to 10th	Night	8% 8% 2 7% 6% 6% 5% 5% 6% 9% 38%	1450
	11th to 20th	Night	10% 6% 2 5% 5% 6% 6% 5% 7% 10% 38%	1145
	21st to end	Night	5% 5% 2 5% 5% 5% 4 4 7% 9% 50%	1313
July	1st to 10th	Night	3 2 1 2 2 2 2 3 4 5% 73%	2220
	11th to 20th	Night	1 1 1 2 2 2 2 3 4 5% 77%	2409
	21st to end	Night	3 2 1 2 2 2 2 2 5% 6% 73%	2762
August	1st to 10th	Night	2 2 1 2 2 3 4 3 6% 9% 67%	2529
	11th to 20th	Night	1 3 1 5% 3 3 4 4 7% 9% 59%	2615
	21st to end	Night	8% 4 1 4 3 4 4 4 7% 9% 51%	3399
September	1st to 10th	Night	5% 2 1 4 3 3 4 4 7% 7% 61%	1900
	11th to 20th	Night	8% 4 2 5% 3 4 4 4 7% 7% 51%	1900
	21st to end	Night	8% 6% 2 6% 4 4 5% 4 6% 9% 46%	2140
October	1st to 10th	Night	4 7% 3 7% 5% 4 6% 4 6% 9% 46%	3800
	11th to 20th	Night	7% 11% 3 7% 5% 4 5% 4 6% 8% 40%	3802
	21st to end	Night	7% 9% 4 8% 5% 6% 6% 4 6% 9% 37%	4457
November	1st to 10th	Night	6% 11% 5% 10% 7% 6% 6% 5% 7% 9% 28%	2657
	11th to 20th	Night	5% 11% 4 8% 7% 7% 6% 6% 8% 10% 29%	2800
	21st to end	Night	4 11% 4 10% 8% 6% 7% 6% 9% 9% 26%	2800
December	1st to 10th	Night	2 8% 3 7% 6% 6% 7% 5% 7% 9% 41%	4786
	11th to 20th	Night	3 10% 5% 13% 9% 9% 9% 6% 7% 8% 23%	5550
	21st to end	Night	3 9% 4 12% 8% 8% 8% 6% 8% 10% 24%	7453

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	59% 15% 12% 10% 3	3205
	11th to 20th	Day	46% 10% 20% 15% 5% 4	3640
	21st to end	Day	48% 14% 15% 14% 9%	4004
February	1st to 10th	Day	60% 12% 13% 9% 6% 1	1653
	11th to 20th	Day	57% 13% 14% 9% 6% 1	1674
	21st to end	Day	62% 11% 11% 12% 3	1521
March	1st to 10th	Day	49% 15% 13% 12% 11% 1	3138
	11th to 20th	Day	39% 12% 19% 24% 6%	3394
	21st to end	Day	60% 13% 13% 7% 7%	4067
April	1st to 10th	Day	59% 15% 13% 12% 1	2280
	11th to 20th	Day	54% 11% 13% 13% 8% 1	2252
	21st to end	Day	55% 12% 15% 12% 6%	2609
May	1st to 10th	Day	42% 12% 14% 22% 9%	4420
	11th to 20th	Day	42% 12% 22% 21% 4	4380
	21st to end	Day	64% 9% 12% 12% 3	4814
June	1st to 10th	Day	53% 11% 13% 13% 7% 3	2726
	11th to 20th	Day	56% 10% 13% 14% 6%	3024
	21st to end	Day	41% 12% 15% 19% 11% 1	2854
July	1st to 10th	Day	20% 6% 8% 20% 30% 16%	4666
	11th to 20th	Day	17% 5% 9% 20% 34% 14%	4469
	21st to end	Day	22% 7% 14% 29% 23% 7%	4639
August	1st to 10th	Day	19% 6% 10% 28% 29% 8%	4384
	11th to 20th	Day	28% 9% 25% 25% 10% 3	4255
	21st to end	Day	34% 10% 18% 23% 13% 2	4273
September	1st to 10th	Day	33% 8% 14% 25% 17% 3	2280
	11th to 20th	Day	42% 9% 10% 20% 16% 3	2280
	21st to end	Day	42% 11% 17% 17% 11% 3	2038
October	1st to 10th	Day	43% 7% 18% 27% 5%	3180
	11th to 20th	Day	50% 8% 10% 25% 6% 1	3135
	21st to end	Day	49% 9% 18% 18% 6%	3101
November	1st to 10th	Day	61% 13% 11% 10% 5%	1639
	11th to 20th	Day	57% 11% 13% 12% 5% 2	1600
	21st to end	Day	53% 14% 13% 13% 5% 2	1600
December	1st to 10th	Day	46% 12% 22% 13% 6%	2342
	11th to 20th	Day	55% 14% 16% 12% 4	2662
	21st to end	Day	59% 13% 13% 12% 3	3516
January	1st to 10th	Night	60% 16% 14% 8% 2	6722
	11th to 20th	Night	45% 11% 20% 15% 6% 3	6320
	21st to end	Night	46% 15% 16% 15% 7%	6952
February	1st to 10th	Night	61% 11% 12% 11% 5%	2635
	11th to 20th	Night	53% 13% 13% 13% 6% 2	2404
	21st to end	Night	55% 12% 12% 16% 5%	1932
March	1st to 10th	Night	40% 16% 16% 14% 12% 1	3736
	11th to 20th	Night	33% 14% 21% 26% 6%	3583
	21st to end	Night	50% 17% 16% 13% 4	3586
April	1st to 10th	Night	48% 15% 19% 14% 4	1900
	11th to 20th	Night	44% 13% 18% 15% 9% 1	1879
	21st to end	Night	48% 13% 16% 16% 8%	1566
May	1st to 10th	Night	40% 14% 16% 22% 7%	2540
	11th to 20th	Night	33% 15% 22% 24% 5%	2516
	21st to end	Night	50% 15% 18% 15% 2	2762
June	1st to 10th	Night	44% 12% 16% 18% 7% 2	1450
	11th to 20th	Night	42% 13% 21% 17% 6%	1145
	21st to end	Night	34% 10% 22% 22% 10% 1	1313
July	1st to 10th	Night	16% 6% 10% 25% 29% 13%	2185
	11th to 20th	Night	12% 5% 12% 24% 31% 15%	2366
	21st to end	Night	15% 6% 16% 32% 21% 10%	2674
August	1st to 10th	Night	16% 6% 13% 32% 30% 4	2521
	11th to 20th	Night	19% 11% 30% 26% 12% 2	2639
	21st to end	Night	33% 10% 19% 25% 13% 1	3399
September	1st to 10th	Night	23% 10% 19% 28% 17% 3	1900
	11th to 20th	Night	35% 10% 13% 20% 18% 4	1900
	21st to end	Night	38% 10% 19% 22% 8% 3	2140
October	1st to 10th	Night	39% 8% 19% 26% 7%	3800
	11th to 20th	Night	45% 9% 14% 23% 9%	3802
	21st to end	Night	49% 9% 15% 21% 6%	4457
November	1st to 10th	Night	56% 13% 14% 11% 5% 1	2657
	11th to 20th	Night	53% 14% 14% 13% 4 1	2800
	21st to end	Night	54% 16% 12% 11% 6% 2	2800
December	1st to 10th	Night	42% 12% 18% 21% 5% 1	4786
	11th to 20th	Night	58% 12% 14% 13% 3	5547
	21st to end	Night	55% 14% 14% 13% 3	7453

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	10% 26% 27% 22% 13% 4	200
	11th to 20th	Day	11% 13% 27% 35% 11% 4	200
	21st to end	Day	7% 16% 38% 28% 11%	220
February	1st to 10th	Day	21% 25% 33% 16% 4 1	283
	11th to 20th	Day	14% 25% 36% 17% 6% 2	365
	21st to end	Day	16% 25% 34% 19% 6% 1	359
March	1st to 10th	Day	16% 28% 31% 16% 8% 1	460
	11th to 20th	Day	14% 27% 27% 20% 10% 2	460
	21st to end	Day	19% 30% 25% 16% 9% 1	506
April	1st to 10th	Day	20% 25% 33% 15% 6% 1	460
	11th to 20th	Day	19% 26% 26% 19% 8% 2	452
	21st to end	Day	20% 26% 28% 17% 8% 2	460
May	1st to 10th	Day	16% 21% 32% 22% 6% 2	463
	11th to 20th	Day	17% 19% 29% 21% 11% 3	488
	21st to end	Day	19% 22% 27% 22% 9% 2	529
June	1st to 10th	Day	17% 25% 34% 17% 6% 1	556
	11th to 20th	Day	19% 20% 34% 17% 9% 2	854
	21st to end	Day	12% 20% 31% 22% 12% 2	684
July	1st to 10th	Day	9% 14% 37% 28% 11% 1	671
	11th to 20th	Day	8% 17% 34% 26% 12% 3	556
	21st to end	Day	6% 13% 31% 31% 15% 5%	531
August	1st to 10th	Day	8% 11% 27% 30% 17% 7%	487
	11th to 20th	Day	6% 12% 28% 30% 18% 6%	477
	21st to end	Day	9% 18% 28% 25% 14% 5%	539
September	1st to 10th	Day	11% 20% 26% 23% 14% 6%	490
	11th to 20th	Day	12% 17% 25% 29% 14% 3	490
	21st to end	Day	19% 20% 26% 21% 11% 3	490
October	1st to 10th	Day	17% 20% 27% 20% 13% 4	490
	11th to 20th	Day	19% 22% 23% 20% 13% 3	476
	21st to end	Day	21% 18% 25% 27% 7% 2	330
November	1st to 10th	Day	22% 11% 35% 23% 8% 1	237
	11th to 20th	Day	20% 24% 25% 17% 14% 2	200
	21st to end	Day	15% 14% 33% 22% 11% 6%	200
December	1st to 10th	Day	16% 15% 19% 24% 21% 6%	200
	11th to 20th	Day	11% 19% 34% 27% 9% 2	200
	21st to end	Day	13% 11% 35% 28% 11% 1	220
January	1st to 10th	Night	14% 17% 33% 26% 8% 1	890
	11th to 20th	Night	9% 12% 39% 28% 11% 1	800
	21st to end	Night	9% 14% 36% 30% 9% 2	880
February	1st to 10th	Night	14% 17% 32% 27% 8% 2	780
	11th to 20th	Night	9% 14% 34% 31% 10% 1	744
	21st to end	Night	13% 16% 28% 29% 13% 2	628
March	1st to 10th	Night	9% 16% 31% 27% 15% 1	760
	11th to 20th	Night	8% 16% 35% 28% 11% 1	760
	21st to end	Night	10% 17% 34% 28% 10% 1	836
April	1st to 10th	Night	11% 15% 29% 31% 13% 2	760
	11th to 20th	Night	9% 17% 31% 27% 14% 1	751
	21st to end	Night	13% 15% 28% 32% 10% 1	426
May	1st to 10th	Night	6% 14% 27% 36% 16% 2	380
	11th to 20th	Night	10% 8% 19% 40% 22% 1	374
	21st to end	Night	13% 7% 23% 35% 21% 1	411
June	1st to 10th	Night	13% 8% 29% 35% 14% 1	380
	11th to 20th	Night	13% 11% 28% 30% 18%	380
	21st to end	Night	7% 9% 26% 34% 22% 2	380
July	1st to 10th	Night	6% 7% 19% 41% 26% 2	370
	11th to 20th	Night	4 7% 24% 40% 24% 1	368
	21st to end	Night	3 6% 19% 44% 28%	416
August	1st to 10th	Night	4 3 13% 42% 35% 3	378
	11th to 20th	Night	3 2 16% 42% 33% 4	442
	21st to end	Night	8% 7% 23% 38% 22% 2	776
September	1st to 10th	Night	6% 8% 25% 33% 24% 4	760
	11th to 20th	Night	9% 7% 21% 28% 28% 8%	760
	21st to end	Night	13% 11% 26% 30% 18% 3	760
October	1st to 10th	Night	12% 10% 24% 33% 19% 2	760
	11th to 20th	Night	14% 12% 27% 29% 16% 2	760
	21st to end	Night	15% 18% 28% 26% 10% 2	836
November	1st to 10th	Night	14% 14% 38% 27% 6%	780
	11th to 20th	Night	15% 20% 30% 24% 10% 2	800
	21st to end	Night	12% 18% 33% 25% 10% 3	800
December	1st to 10th	Night	9% 14% 33% 30% 12% 2	935
	11th to 20th	Night	12% 15% 38% 28% 7% 1	1000
	21st to end	Night	11% 14% 38% 31% 6%	993

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