Amateur Astronomers Association of Princeton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Amateur Astronomers Association of Princeton clear sky chart from 2005-11-06 to 2024-11-04.

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	16% 7% 2 6% 4 4 4 4 5% 7% 39%	101312
	Night	17% 7% 2 6% 4 4 4 3 5% 6% 42%	97718
Transparency	Day	42% 6% 12% 22% 16% 2	104857
	Night	41% 6% 11% 23% 16% 3	102262
Seeing	Day	21% 15% 29% 22% 10% 3	15006
	Night	19% 12% 31% 28% 9% 1	24199

By Month

Percentage Cloud Cover NE (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	15% 8% 3 5% 4 5% 4 3 4 4 47%	2517
	11th to 20th	Day	15% 10% 3 8% 5% 4 4 3 5% 7% 36%	2520
	21st to end	Day	22% 9% 3 7% 5% 4 4 3 4 5% 35%	3101
February	1st to 10th	Day	17% 7% 3 7% 4 3 4 4 5% 7% 39%	1710
	11th to 20th	Day	16% 8% 3 6% 5% 4 3 3 5% 5% 41%	1833
	21st to end	Day	20% 9% 3 5% 4 4 4 4 5% 5% 37%	1570
March	1st to 10th	Day	27% 8% 2 5% 4 3 3 2 4 5% 36%	3116
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 6% 7% 46%	3409
	21st to end	Day	21% 6% 2 5% 3 4 4 2 3 5% 43%	4092
April	1st to 10th	Day	17% 8% 3 6% 4 4 4 3 5% 6% 39%	2280
	11th to 20th	Day	15% 6% 2 5% 5% 4 4 4 6% 8% 40%	2254
	21st to end	Day	17% 5% 2 6% 4 5% 4 3 6% 6% 40%	2280
May	1st to 10th	Day	27% 8% 3 6% 4 4 4 3 4 8% 29%	4083
	11th to 20th	Day	25% 9% 2 6% 5% 4 4 4 5% 6% 29%	4398
	21st to end	Day	13% 6% 2 6% 4 4 4 3 6% 9% 42%	4837
June	1st to 10th	Day	13% 6% 2 5% 5% 4 6% 5% 7% 10% 37%	2660
	11th to 20th	Day	13% 7% 2 6% 5% 4 5% 5% 6% 9% 37%	2660
	21st to end	Day	11% 6% 2 5% 5% 5% 5% 5% 6% 8% 41%	2660
July	1st to 10th	Day	8% 6% 2 8% 6% 6% 7% 5% 8% 10% 35%	4440
	11th to 20th	Day	11% 7% 2 6% 5% 4 6% 5% 7% 9% 39%	4440
	21st to end	Day	13% 9% 3 8% 5% 5% 5% 5% 7% 8% 32%	4814
August	1st to 10th	Day	19% 6% 2 7% 5% 5% 5% 4 6% 8% 32%	4088
	11th to 20th	Day	13% 6% 2 6% 4 5% 5% 4 7% 9% 42%	3730
	21st to end	Day	12% 5% 2 5% 3 3 4 4 5% 8% 48%	4180
September	1st to 10th	Day	14% 6% 3 6% 5% 3 4 3 5% 7% 45%	2280
	11th to 20th	Day	13% 5% 1 6% 3 3 4 3 5% 8% 50%	2223
	21st to end	Day	19% 6% 2 7% 4 4 4 3 4 7% 39%	2090
October	1st to 10th	Day	19% 6% 2 5% 3 5% 4 3 5% 6% 42%	1945
	11th to 20th	Day	14% 7% 2 5% 3 3 4 3 5% 6% 49%	1900
	21st to end	Day	19% 6% 2 5% 3 2 4 3 4 7% 45%	1981
November	1st to 10th	Day	13% 6% 2 5% 3 4 3 2 5% 6% 52%	1698
	11th to 20th	Day	17% 6% 2 5% 3 3 3 3 6% 5% 47%	1710
	21st to end	Day	15% 7% 2 4 4 3 3 3 5% 7% 46%	1615
December	1st to 10th	Day	18% 8% 3 6% 3 4 4 3 5% 8% 38%	1520
	11th to 20th	Day	20% 6% 2 4 4 3 5% 3 5% 6% 42%	1906
	21st to end	Day	17% 8% 2 7% 5% 5% 5% 4 5% 8% 34%	2772
January	1st to 10th	Night	16% 7% 2 5% 3 4 3 3 5% 6% 46%	4429
	11th to 20th	Night	17% 9% 3 6% 4 4 4 3 4 5% 43%	4440
	21st to end	Night	24% 9% 2 5% 4 3 3 3 4 6% 36%	4550
February	1st to 10th	Night	17% 9% 3 6% 3 3 4 4 5% 6% 41%	2470
	11th to 20th	Night	17% 9% 3 6% 4 4 3 4 4 4 42%	2233
	21st to end	Night	19% 7% 2 6% 3 4 4 3 4 6% 43%	1884
March	1st to 10th	Night	24% 9% 2 5% 4 4 4 2 4 5% 37%	3724
	11th to 20th	Night	12% 6% 2 6% 4 4 4 3 5% 6% 48%	3548
	21st to end	Night	19% 8% 3 6% 4 3 3 2 3 5% 44%	3540
April	1st to 10th	Night	17% 7% 2 7% 3 4 4 4 6% 7% 39%	1900
	11th to 20th	Night	16% 8% 3 6% 5% 4 3 3 5% 6% 40%	1875
	21st to end	Night	19% 6% 2 4 3 3 4 3 6% 7% 43%	1900
May	1st to 10th	Night	23% 11% 3 7% 4 4 3 4 5% 7% 30%	2872
	11th to 20th	Night	24% 10% 3 7% 4 3 3 3 5% 7% 31%	2506
	21st to end	Night	14% 7% 2 5% 4 3 4 3 6% 10% 41%	2738
June	1st to 10th	Night	15% 7% 1 6% 4 5% 4 5% 5% 7% 43%	1520
	11th to 20th	Night	12% 6% 2 6% 5% 5% 4 4 6% 8% 42%	1520
	21st to end	Night	11% 4 2 6% 5% 4 4 4 7% 10% 42%	1520
July	1st to 10th	Night	8% 5% 3 8% 5% 6% 6% 5% 7% 9% 38%	2520
	11th to 20th	Night	13% 6% 2 6% 5% 5% 5% 5% 7% 10% 37%	2520
	21st to end	Night	15% 7% 2 7% 5% 5% 5% 5% 7% 7% 35%	2739
August	1st to 10th	Night	23% 6% 2 6% 4 5% 5% 4 5% 8% 31%	2805
	11th to 20th	Night	13% 7% 2 7% 4 4 5% 4 6% 7% 40%	3097
	21st to end	Night	10% 5% 2 6% 4 4 4 4 5% 7% 50%	3476
September	1st to 10th	Night	15% 6% 2 6% 4 4 4 4 4 6% 45%	1900
	11th to 20th	Night	13% 5% 2 4 3 3 3 4 4 7% 52%	1957
	21st to end	Night	18% 8% 2 6% 4 3 3 3 4 6% 42%	2090
October	1st to 10th	Night	18% 6% 3 5% 3 3 4 3 4 6% 44%	2232
	11th to 20th	Night	14% 5% 2 4 4 3 4 3 5% 6% 50%	2280
	21st to end	Night	18% 7% 2 5% 3 3 3 3 5% 5% 46%	2615
November	1st to 10th	Night	11% 5% 2 5% 4 4 3 2 5% 5% 54%	2453
	11th to 20th	Night	18% 6% 2 5% 3 3 3 3 5% 5% 47%	2470
	21st to end	Night	14% 7% 3 5% 4 4 4 3 6% 7% 45%	2563
December	1st to 10th	Night	20% 7% 2 6% 4 3 4 3 5% 6% 39%	2660
	11th to 20th	Night	18% 6% 2 5% 4 3 4 4 6% 6% 42%	3288
	21st to end	Night	18% 9% 3 6% 4 4 4 4 5% 5% 38%	4884

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	39% 5% 11% 27% 13% 4	2518
	11th to 20th	Day	46% 6% 11% 21% 13% 2	2520
	21st to end	Day	52% 5% 10% 21% 10% 2	3101
February	1st to 10th	Day	45% 5% 13% 24% 11% 2	1710
	11th to 20th	Day	45% 5% 8% 23% 16% 3	1842
	21st to end	Day	47% 6% 10% 20% 13% 4	1570
March	1st to 10th	Day	50% 5% 9% 25% 9% 2	3119
	11th to 20th	Day	33% 7% 13% 29% 15% 3	3409
	21st to end	Day	44% 4 8% 17% 22% 5%	4092
April	1st to 10th	Day	44% 6% 10% 19% 16% 5%	2280
	11th to 20th	Day	39% 6% 13% 21% 17% 3	2254
	21st to end	Day	42% 7% 10% 18% 19% 4	2280
May	1st to 10th	Day	53% 8% 11% 15% 11% 2	4097
	11th to 20th	Day	54% 7% 13% 17% 9% 1	4402
	21st to end	Day	35% 8% 12% 25% 17% 2	4837
June	1st to 10th	Day	39% 9% 13% 22% 15% 1	2660
	11th to 20th	Day	42% 9% 13% 22% 12% 2	2660
	21st to end	Day	39% 9% 13% 22% 16% 1	2660
July	1st to 10th	Day	43% 8% 14% 19% 13% 2	4370
	11th to 20th	Day	40% 6% 16% 22% 15% 1	4365
	21st to end	Day	48% 5% 11% 18% 16% 2	4661
August	1st to 10th	Day	48% 7% 13% 22% 9%	4074
	11th to 20th	Day	40% 5% 13% 27% 14% 1	3766
	21st to end	Day	34% 5% 10% 27% 22% 2	4180
September	1st to 10th	Day	39% 7% 11% 20% 20% 3	2280
	11th to 20th	Day	34% 5% 11% 23% 24% 3	2223
	21st to end	Day	46% 6% 8% 17% 19% 3	2090
October	1st to 10th	Day	39% 4 10% 17% 25% 5%	3231
	11th to 20th	Day	25% 3 10% 25% 31% 6%	3140
	21st to end	Day	27% 3 13% 25% 27% 5%	3245
November	1st to 10th	Day	34% 4 13% 25% 21% 2	1698
	11th to 20th	Day	38% 5% 9% 23% 22% 3	1710
	21st to end	Day	37% 5% 13% 29% 12% 3	1615
December	1st to 10th	Day	44% 6% 15% 21% 13% 1	1520
	11th to 20th	Day	43% 5% 13% 25% 12% 2	1906
	21st to end	Day	45% 9% 13% 22% 10% 1	2772
January	1st to 10th	Night	38% 5% 10% 30% 12% 4	4429
	11th to 20th	Night	44% 4 13% 24% 12% 3	4440
	21st to end	Night	49% 5% 9% 23% 12% 2	4550
February	1st to 10th	Night	43% 6% 13% 24% 13% 2	2470
	11th to 20th	Night	44% 5% 9% 19% 19% 4	2235
	21st to end	Night	42% 5% 8% 23% 18% 4	1884
March	1st to 10th	Night	49% 5% 9% 22% 12% 3	3734
	11th to 20th	Night	35% 7% 13% 24% 18% 2	3549
	21st to end	Night	44% 4 7% 16% 22% 6%	3540
April	1st to 10th	Night	42% 7% 10% 19% 17% 4	1900
	11th to 20th	Night	43% 6% 10% 18% 20% 3	1875
	21st to end	Night	39% 6% 10% 21% 20% 4	1900
May	1st to 10th	Night	53% 7% 10% 19% 10% 1	2878
	11th to 20th	Night	52% 7% 14% 14% 10% 2	2507
	21st to end	Night	37% 9% 15% 22% 14% 2	2739
June	1st to 10th	Night	39% 7% 12% 21% 20% 2	1520
	11th to 20th	Night	39% 8% 14% 26% 13% 1	1520
	21st to end	Night	36% 9% 16% 24% 14% 1	1520
July	1st to 10th	Night	42% 8% 18% 18% 13% 2	2480
	11th to 20th	Night	42% 6% 16% 24% 12% 1	2472
	21st to end	Night	45% 6% 14% 16% 16% 2	2651
August	1st to 10th	Night	50% 7% 19% 16% 8% 1	2795
	11th to 20th	Night	41% 6% 12% 29% 12% 1	3127
	21st to end	Night	33% 5% 9% 31% 21% 1	3476
September	1st to 10th	Night	40% 7% 10% 20% 20% 2	1900
	11th to 20th	Night	32% 6% 10% 22% 25% 4	1957
	21st to end	Night	42% 6% 8% 21% 21% 2	2090
October	1st to 10th	Night	40% 3 10% 23% 22% 2	3726
	11th to 20th	Night	25% 4 9% 29% 29% 5%	3779
	21st to end	Night	29% 3 11% 31% 24% 3	4301
November	1st to 10th	Night	33% 4 12% 24% 22% 4	2453
	11th to 20th	Night	41% 4 9% 23% 19% 5%	2470
	21st to end	Night	38% 5% 13% 27% 16% 2	2563
December	1st to 10th	Night	45% 5% 12% 23% 14% 2	2660
	11th to 20th	Night	41% 6% 12% 21% 17% 4	3288
	21st to end	Night	46% 6% 12% 24% 11% 1	4884

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% 21% 28% 18% 3 1	180
	11th to 20th	Day	34% 27% 27% 10% 2	180
	21st to end	Day	23% 25% 34% 13% 5%	292
February	1st to 10th	Day	29% 25% 33% 10% 3	270
	11th to 20th	Day	29% 22% 36% 12% 2	441
	21st to end	Day	31% 20% 30% 16% 3	382
March	1st to 10th	Day	28% 21% 29% 18% 3	460
	11th to 20th	Day	26% 27% 30% 13% 3 1	460
	21st to end	Day	27% 23% 30% 14% 5% 1	506
April	1st to 10th	Day	28% 21% 33% 10% 7% 1	460
	11th to 20th	Day	23% 22% 31% 13% 9% 2	452
	21st to end	Day	28% 20% 31% 15% 4 2	460
May	1st to 10th	Day	30% 19% 28% 14% 7% 2	463
	11th to 20th	Day	23% 16% 31% 20% 9% 1	487
	21st to end	Day	19% 10% 25% 28% 15% 3	529
June	1st to 10th	Day	17% 11% 31% 26% 13% 2	490
	11th to 20th	Day	17% 13% 28% 29% 11% 3	490
	21st to end	Day	16% 11% 32% 28% 11% 2	490
July	1st to 10th	Day	13% 6% 23% 31% 21% 6%	485
	11th to 20th	Day	9% 5% 26% 34% 20% 6%	484
	21st to end	Day	13% 7% 31% 30% 15% 4	532
August	1st to 10th	Day	12% 6% 25% 35% 15% 7%	486
	11th to 20th	Day	14% 5% 21% 34% 21% 5%	479
	21st to end	Day	14% 6% 26% 29% 15% 9%	539
September	1st to 10th	Day	17% 9% 26% 24% 16% 9%	490
	11th to 20th	Day	14% 7% 27% 28% 16% 9%	490
	21st to end	Day	24% 12% 25% 23% 10% 6%	490
October	1st to 10th	Day	26% 8% 25% 23% 13% 6%	490
	11th to 20th	Day	18% 16% 27% 26% 10% 3	490
	21st to end	Day	21% 17% 28% 21% 11% 2	482
November	1st to 10th	Day	20% 11% 30% 23% 13% 3	374
	11th to 20th	Day	25% 17% 32% 18% 8% 1	370
	21st to end	Day	20% 17% 36% 19% 7% 1	275
December	1st to 10th	Day	27% 25% 30% 10% 8%	180
	11th to 20th	Day	38% 19% 26% 12% 6%	180
	21st to end	Day	29% 17% 32% 15% 6% 2	198
January	1st to 10th	Night	19% 19% 39% 19% 3	760
	11th to 20th	Night	24% 18% 33% 22% 3	760
	21st to end	Night	20% 20% 37% 21% 1	836
February	1st to 10th	Night	21% 18% 35% 23% 2	760
	11th to 20th	Night	24% 22% 36% 17% 1	743
	21st to end	Night	25% 18% 37% 17% 2	628
March	1st to 10th	Night	24% 19% 34% 20% 3	760
	11th to 20th	Night	24% 18% 35% 18% 4	760
	21st to end	Night	21% 14% 40% 19% 5%	836
April	1st to 10th	Night	22% 19% 37% 18% 4	760
	11th to 20th	Night	21% 14% 37% 23% 5%	751
	21st to end	Night	21% 13% 36% 25% 5%	760
May	1st to 10th	Night	24% 16% 31% 23% 6%	595
	11th to 20th	Night	22% 10% 36% 27% 6%	385
	21st to end	Night	16% 7% 25% 38% 12% 1	411
June	1st to 10th	Night	17% 6% 28% 37% 12% 1	380
	11th to 20th	Night	12% 6% 30% 41% 11%	380
	21st to end	Night	10% 3 28% 45% 13%	380
July	1st to 10th	Night	11% 3 24% 42% 18% 1	370
	11th to 20th	Night	11% 4 22% 42% 20% 1	368
	21st to end	Night	12% 2 25% 44% 17%	416
August	1st to 10th	Night	14% 2 18% 44% 20% 2	547
	11th to 20th	Night	13% 3 20% 37% 24% 3	750
	21st to end	Night	15% 4 18% 41% 21% 2	836
September	1st to 10th	Night	15% 5% 26% 34% 17% 2	760
	11th to 20th	Night	16% 5% 21% 33% 22% 4	760
	21st to end	Night	22% 6% 20% 34% 16% 1	760
October	1st to 10th	Night	22% 8% 25% 27% 16% 3	760
	11th to 20th	Night	16% 11% 30% 32% 11%	760
	21st to end	Night	22% 11% 31% 27% 8%	836
November	1st to 10th	Night	15% 10% 33% 28% 12% 1	755
	11th to 20th	Night	21% 16% 29% 26% 7%	760
	21st to end	Night	17% 16% 34% 24% 7%	760
December	1st to 10th	Night	25% 18% 30% 20% 6% 1	760
	11th to 20th	Night	21% 16% 35% 24% 4	760
	21st to end	Night	22% 17% 33% 26% 3	836

Page generated at 2024-11-01 22:47:45. Version ac07

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