Fall River Pkwy Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fall River Pkwy Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	9% 6% 2 7% 5% 5% 6% 5% 7% 9% 40%	131157
	Night	9% 6% 2 6% 4 4 5% 4 6% 9% 43%	130008
Transparency	Day	36% 9% 16% 19% 14% 6%	134855
	Night	36% 8% 14% 19% 16% 7%	134380
Seeing	Day	10% 5% 19% 28% 24% 13%	17197
	Night	11% 5% 19% 32% 27% 6%	27326

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	7% 6% 2 6% 4 4 4 3 5% 7% 52%	5258
	11th to 20th	Day	16% 8% 3 7% 4 4 4 3 5% 7% 39%	5280
	21st to end	Day	10% 6% 3 6% 4 4 4 3 5% 9% 47%	5780
February	1st to 10th	Day	14% 7% 2 7% 4 4 4 3 4 6% 45%	2074
	11th to 20th	Day	13% 6% 2 6% 4 3 4 3 5% 6% 48%	2048
	21st to end	Day	12% 6% 2 5% 4 3 4 3 5% 6% 48%	1814
March	1st to 10th	Day	14% 8% 3 7% 4 4 4 3 6% 7% 40%	4957
	11th to 20th	Day	6% 3 2 4 3 3 4 4 5% 8% 58%	4980
	21st to end	Day	12% 6% 2 6% 4 4 4 4 5% 8% 47%	5465
April	1st to 10th	Day	7% 5% 2 5% 4 4 5% 4 6% 9% 49%	2200
	11th to 20th	Day	8% 6% 2 7% 4 5% 5% 4 6% 7% 47%	2258
	21st to end	Day	4 3 1 5% 4 4 4 4 7% 10% 53%	2437
May	1st to 10th	Day	7% 4 2 6% 5% 5% 5% 5% 7% 10% 45%	5862
	11th to 20th	Day	9% 6% 2 6% 4 4 5% 4 8% 10% 41%	5861
	21st to end	Day	4 3 1 5% 5% 5% 6% 6% 9% 12% 44%	6433
June	1st to 10th	Day	10% 7% 2 7% 5% 7% 7% 6% 9% 11% 30%	2600
	11th to 20th	Day	4 5% 2 7% 6% 7% 8% 6% 10% 12% 32%	2600
	21st to end	Day	6% 6% 3 8% 7% 8% 7% 7% 9% 11% 29%	2600
July	1st to 10th	Day	6% 6% 3 9% 8% 8% 10% 8% 10% 11% 20%	5900
	11th to 20th	Day	3 3 2 7% 6% 7% 9% 8% 11% 15% 30%	5900
	21st to end	Day	3 4 2 6% 6% 6% 8% 7% 9% 14% 36%	6424
August	1st to 10th	Day	11% 6% 3 9% 6% 7% 8% 6% 10% 11% 24%	4234
	11th to 20th	Day	5% 5% 2 6% 5% 6% 6% 5% 10% 13% 36%	4149
	21st to end	Day	8% 6% 2 8% 6% 7% 8% 6% 8% 10% 32%	3998
September	1st to 10th	Day	10% 9% 3 9% 7% 7% 7% 6% 8% 10% 25%	2200
	11th to 20th	Day	13% 7% 3 9% 7% 7% 7% 6% 9% 8% 26%	2200
	21st to end	Day	13% 7% 2 9% 5% 5% 6% 5% 7% 9% 32%	2200
October	1st to 10th	Day	12% 6% 3 7% 5% 5% 6% 5% 6% 10% 34%	2150
	11th to 20th	Day	6% 4 3 5% 4 4 4 3 6% 8% 52%	2000
	21st to end	Day	9% 5% 2 5% 4 4 5% 3 6% 8% 49%	2200
November	1st to 10th	Day	10% 6% 3 6% 4 4 5% 3 5% 8% 47%	2057
	11th to 20th	Day	9% 6% 3 6% 4 4 5% 3 5% 7% 47%	2100
	21st to end	Day	11% 6% 2 8% 4 4 4 4 5% 7% 45%	1890
December	1st to 10th	Day	6% 4 2 5% 5% 4 5% 4 5% 7% 54%	3020
	11th to 20th	Day	14% 8% 3 6% 5% 5% 4 4 5% 7% 39%	3463
	21st to end	Day	15% 8% 3 8% 5% 4 4 3 5% 5% 41%	4565
January	1st to 10th	Night	8% 6% 2 5% 3 3 3 3 4 8% 55%	7635
	11th to 20th	Night	14% 7% 3 6% 4 4 5% 4 6% 8% 39%	7680
	21st to end	Night	13% 8% 3 7% 4 4 4 4 5% 7% 42%	8401
February	1st to 10th	Night	13% 6% 3 7% 4 4 4 4 4 6% 45%	2541
	11th to 20th	Night	13% 7% 2 7% 5% 4 4 3 4 6% 46%	2458
	21st to end	Night	10% 6% 3 6% 5% 3 4 3 5% 7% 47%	1990
March	1st to 10th	Night	17% 9% 3 6% 4 4 3 3 4 5% 41%	4941
	11th to 20th	Night	5% 5% 1 4 3 4 4 3 6% 11% 53%	4980
	21st to end	Night	9% 7% 2 6% 4 3 4 3 5% 6% 49%	5467
April	1st to 10th	Night	7% 6% 2 5% 4 4 5% 4 6% 8% 49%	2200
	11th to 20th	Night	8% 6% 2 5% 3 4 4 4 5% 7% 53%	2090
	21st to end	Night	5% 4 1 4 3 4 4 5% 8% 9% 53%	1977
May	1st to 10th	Night	5% 4 2 6% 5% 4 5% 5% 8% 10% 47%	4033
	11th to 20th	Night	9% 6% 2 7% 4 4 5% 5% 7% 10% 41%	4032
	21st to end	Night	4 4 2 6% 5% 4 4 4 7% 12% 47%	4421
June	1st to 10th	Night	8% 5% 2 6% 4 6% 6% 6% 9% 12% 36%	1800
	11th to 20th	Night	6% 5% 2 6% 6% 6% 7% 6% 9% 13% 36%	1800
	21st to end	Night	6% 5% 2 8% 5% 6% 6% 5% 9% 13% 34%	1800
July	1st to 10th	Night	7% 5% 3 10% 6% 8% 8% 7% 10% 12% 24%	4058
	11th to 20th	Night	3 4 2 7% 6% 6% 7% 5% 11% 14% 35%	4057
	21st to end	Night	5% 5% 2 7% 5% 7% 7% 6% 10% 12% 35%	4427
August	1st to 10th	Night	10% 6% 3 8% 6% 7% 6% 6% 9% 11% 26%	2915
	11th to 20th	Night	4 4 2 6% 5% 5% 6% 5% 8% 12% 42%	2875
	21st to end	Night	7% 6% 3 8% 5% 5% 6% 4 7% 11% 38%	3871
September	1st to 10th	Night	9% 8% 3 9% 6% 7% 7% 6% 7% 10% 27%	2200
	11th to 20th	Night	12% 7% 3 7% 7% 7% 6% 6% 9% 10% 27%	2200
	21st to end	Night	9% 6% 3 8% 5% 6% 5% 4 7% 10% 38%	2200
October	1st to 10th	Night	12% 7% 3 7% 5% 5% 5% 4 6% 8% 39%	2248
	11th to 20th	Night	7% 4 2 5% 3 4 4 4 5% 8% 54%	2400
	21st to end	Night	8% 5% 2 6% 4 4 4 4 7% 6% 51%	2640
November	1st to 10th	Night	10% 6% 2 7% 4 4 4 3 6% 7% 47%	2461
	11th to 20th	Night	11% 6% 2 7% 4 4 4 3 5% 6% 48%	2520
	21st to end	Night	11% 7% 2 6% 4 4 4 3 5% 7% 46%	2725
December	1st to 10th	Night	10% 6% 2 5% 4 3 4 3 5% 6% 54%	4380
	11th to 20th	Night	12% 9% 3 8% 4 4 3 3 4 6% 45%	4952
	21st to end	Night	13% 8% 3 7% 4 4 4 3 5% 7% 41%	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	32% 5% 11% 15% 27% 10%	5259
	11th to 20th	Day	45% 4 5% 14% 17% 15%	5280
	21st to end	Day	35% 6% 9% 23% 15% 11%	5780
February	1st to 10th	Day	39% 6% 8% 15% 21% 11%	2074
	11th to 20th	Day	36% 5% 9% 19% 21% 11%	2058
	21st to end	Day	36% 6% 10% 19% 18% 12%	1814
March	1st to 10th	Day	42% 6% 11% 15% 19% 7%	4957
	11th to 20th	Day	23% 6% 13% 18% 21% 19%	4980
	21st to end	Day	35% 5% 10% 20% 18% 12%	5465
April	1st to 10th	Day	31% 6% 12% 22% 19% 10%	2200
	11th to 20th	Day	34% 6% 11% 20% 20% 9%	2258
	21st to end	Day	25% 7% 14% 24% 23% 6%	2437
May	1st to 10th	Day	31% 11% 16% 27% 13% 3	5897
	11th to 20th	Day	34% 9% 16% 28% 11% 1	5865
	21st to end	Day	27% 10% 18% 28% 15% 2	6447
June	1st to 10th	Day	42% 12% 18% 18% 9%	2600
	11th to 20th	Day	36% 14% 21% 23% 6%	2600
	21st to end	Day	41% 15% 22% 16% 5%	2600
July	1st to 10th	Day	45% 19% 26% 10% 1	5835
	11th to 20th	Day	31% 17% 38% 11% 2	5831
	21st to end	Day	30% 16% 29% 21% 4	6240
August	1st to 10th	Day	46% 14% 28% 11% 1	4221
	11th to 20th	Day	31% 13% 25% 21% 10%	4188
	21st to end	Day	40% 14% 20% 24% 1	3998
September	1st to 10th	Day	49% 13% 17% 13% 8% 1	2200
	11th to 20th	Day	49% 13% 15% 14% 7% 1	2200
	21st to end	Day	44% 9% 15% 17% 13% 2	2200
October	1st to 10th	Day	49% 7% 10% 11% 15% 8%	3501
	11th to 20th	Day	28% 5% 5% 17% 33% 11%	3240
	21st to end	Day	24% 6% 6% 21% 32% 9%	3538
November	1st to 10th	Day	35% 6% 10% 20% 18% 10%	2057
	11th to 20th	Day	37% 6% 8% 17% 21% 11%	2100
	21st to end	Day	38% 5% 10% 20% 20% 7%	1890
December	1st to 10th	Day	29% 6% 11% 24% 20% 9%	3020
	11th to 20th	Day	43% 7% 10% 15% 19% 7%	3460
	21st to end	Day	44% 5% 8% 20% 17% 6%	4565
January	1st to 10th	Night	29% 5% 10% 16% 30% 10%	7636
	11th to 20th	Night	43% 4 6% 15% 19% 13%	7680
	21st to end	Night	41% 5% 9% 19% 15% 11%	8401
February	1st to 10th	Night	38% 7% 8% 14% 21% 12%	2541
	11th to 20th	Night	39% 5% 9% 19% 17% 11%	2459
	21st to end	Night	35% 5% 11% 20% 17% 12%	1990
March	1st to 10th	Night	44% 5% 9% 15% 15% 11%	4941
	11th to 20th	Night	25% 5% 11% 21% 18% 19%	4980
	21st to end	Night	35% 5% 10% 18% 19% 14%	5467
April	1st to 10th	Night	32% 6% 13% 21% 17% 11%	2200
	11th to 20th	Night	31% 7% 9% 24% 20% 10%	2090
	21st to end	Night	24% 8% 15% 27% 21% 5%	1977
May	1st to 10th	Night	28% 11% 19% 27% 12% 2	4060
	11th to 20th	Night	34% 9% 19% 26% 10% 1	4033
	21st to end	Night	27% 8% 19% 28% 17% 1	4429
June	1st to 10th	Night	36% 11% 24% 20% 9%	1800
	11th to 20th	Night	37% 12% 23% 22% 7%	1800
	21st to end	Night	36% 15% 24% 18% 5% 1	1800
July	1st to 10th	Night	46% 17% 25% 11% 1	4015
	11th to 20th	Night	33% 18% 35% 11% 2	4006
	21st to end	Night	36% 14% 27% 21% 2	4297
August	1st to 10th	Night	45% 15% 29% 10% 2	2904
	11th to 20th	Night	30% 14% 26% 20% 10%	2902
	21st to end	Night	38% 12% 24% 23% 3	3871
September	1st to 10th	Night	47% 14% 19% 14% 6% 1	2200
	11th to 20th	Night	48% 12% 17% 17% 7%	2200
	21st to end	Night	40% 11% 14% 20% 14% 2	2200
October	1st to 10th	Night	50% 8% 9% 13% 12% 8%	3677
	11th to 20th	Night	27% 4 6% 17% 36% 10%	3899
	21st to end	Night	23% 6% 7% 20% 33% 11%	4252
November	1st to 10th	Night	35% 7% 10% 19% 18% 12%	2461
	11th to 20th	Night	36% 6% 8% 18% 21% 10%	2520
	21st to end	Night	36% 5% 12% 19% 18% 9%	2725
December	1st to 10th	Night	32% 4 11% 21% 19% 12%	4380
	11th to 20th	Night	41% 5% 9% 21% 16% 9%	4954
	21st to end	Night	42% 6% 10% 19% 18% 4	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	10% 9% 43% 29% 9% 1	330
	11th to 20th	Day	10% 10% 34% 35% 9% 1	330
	21st to end	Day	14% 16% 37% 28% 5%	363
February	1st to 10th	Day	12% 12% 33% 31% 11% 1	330
	11th to 20th	Day	14% 12% 41% 26% 7%	330
	21st to end	Day	11% 8% 40% 32% 9% 1	357
March	1st to 10th	Day	11% 13% 32% 32% 12% 1	508
	11th to 20th	Day	10% 10% 32% 31% 14% 3	500
	21st to end	Day	12% 8% 33% 31% 15% 1	550
April	1st to 10th	Day	9% 10% 32% 32% 15% 3	500
	11th to 20th	Day	9% 9% 25% 35% 16% 7%	492
	21st to end	Day	4 5% 26% 34% 22% 9%	500
May	1st to 10th	Day	5% 4 24% 38% 20% 9%	503
	11th to 20th	Day	7% 3 19% 34% 26% 11%	527
	21st to end	Day	8% 2 12% 27% 33% 18%	573
June	1st to 10th	Day	13% 2 9% 31% 29% 16%	530
	11th to 20th	Day	5% 1 10% 24% 32% 28%	530
	21st to end	Day	6% 1 5% 24% 35% 29%	530
July	1st to 10th	Day	5% 2 4 17% 33% 39%	525
	11th to 20th	Day	3 1 2 14% 40% 40%	524
	21st to end	Day	7% 1 4 16% 40% 31%	576
August	1st to 10th	Day	12% 1 4 17% 35% 31%	526
	11th to 20th	Day	7% 1 2 15% 39% 36%	519
	21st to end	Day	12% 3 4 21% 32% 28%	583
September	1st to 10th	Day	14% 3 7% 20% 33% 24%	530
	11th to 20th	Day	14% 3 9% 30% 31% 14%	530
	21st to end	Day	13% 2 10% 26% 32% 16%	530
October	1st to 10th	Day	17% 4 19% 34% 20% 6%	530
	11th to 20th	Day	8% 5% 19% 35% 28% 5%	530
	21st to end	Day	10% 5% 19% 37% 26% 4	583
November	1st to 10th	Day	14% 6% 22% 32% 21% 5%	535
	11th to 20th	Day	14% 7% 26% 34% 16% 3	540
	21st to end	Day	12% 10% 31% 36% 11% 1	330
December	1st to 10th	Day	11% 8% 35% 29% 15% 2	330
	11th to 20th	Day	16% 11% 29% 32% 9% 2	330
	21st to end	Day	15% 8% 27% 35% 13% 2	363
January	1st to 10th	Night	13% 9% 34% 32% 10% 1	840
	11th to 20th	Night	13% 8% 29% 37% 11% 1	840
	21st to end	Night	16% 10% 33% 35% 7%	924
February	1st to 10th	Night	15% 7% 30% 35% 12% 1	840
	11th to 20th	Night	16% 7% 31% 37% 9%	823
	21st to end	Night	14% 7% 31% 42% 6%	692
March	1st to 10th	Night	10% 9% 30% 41% 11%	809
	11th to 20th	Night	9% 7% 29% 40% 14% 1	800
	21st to end	Night	11% 6% 29% 39% 14% 1	880
April	1st to 10th	Night	9% 6% 30% 40% 15% 1	800
	11th to 20th	Night	10% 7% 26% 37% 20% 1	791
	21st to end	Night	7% 3 21% 44% 23% 2	777
May	1st to 10th	Night	7% 2 17% 44% 27% 3	600
	11th to 20th	Night	6% 2 16% 38% 31% 6%	591
	21st to end	Night	7% 1 9% 31% 44% 8%	650
June	1st to 10th	Night	11% 2 8% 31% 40% 8%	600
	11th to 20th	Night	7% 2 7% 25% 44% 16%	600
	21st to end	Night	9% 2 5% 22% 46% 16%	600
July	1st to 10th	Night	7% 1 4 20% 45% 23%	585
	11th to 20th	Night	7% 1 2 13% 50% 27%	582
	21st to end	Night	5% 1 4 18% 55% 16%	657
August	1st to 10th	Night	8% 1 3 17% 50% 21%	597
	11th to 20th	Night	6% 1 3 13% 50% 27%	594
	21st to end	Night	11% 2 6% 20% 42% 18%	870
September	1st to 10th	Night	12% 2 7% 21% 44% 14%	800
	11th to 20th	Night	14% 3 7% 24% 43% 9%	800
	21st to end	Night	13% 2 9% 25% 41% 10%	800
October	1st to 10th	Night	15% 2 16% 30% 32% 5%	800
	11th to 20th	Night	7% 5% 15% 35% 32% 5%	800
	21st to end	Night	10% 4 17% 34% 31% 4	880
November	1st to 10th	Night	12% 4 21% 34% 25% 4	820
	11th to 20th	Night	15% 7% 22% 34% 20% 2	840
	21st to end	Night	12% 6% 27% 34% 19% 1	840
December	1st to 10th	Night	13% 8% 25% 32% 20% 2	840
	11th to 20th	Night	13% 8% 25% 33% 20% 2	840
	21st to end	Night	15% 7% 22% 37% 17% 1	924

Page generated at 2026-02-28 01:40:25. Version ac07

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