ActonStarParty
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ActonStarParty clear sky chart from 2005-11-06 to 2024-05-13.

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	17% 7% 2 6% 4 4 4 4 5% 6% 39%	83354
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 40%	86115
Transparency	Day	44% 6% 11% 21% 15% 3	83059
	Night	45% 5% 12% 21% 15% 2	85944
Seeing	Day	23% 16% 32% 19% 8% 2	14348
	Night	22% 13% 33% 25% 7% 1	23393

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	14% 7% 3 6% 4 5% 4 3 4 5% 44%	2517
	11th to 20th	Day	16% 7% 4 7% 4 4 5% 4 5% 5% 40%	2520
	21st to end	Day	21% 11% 3 5% 5% 4 4 3 4 4 35%	2772
February	1st to 10th	Day	19% 8% 3 6% 4 4 5% 4 5% 6% 35%	1764
	11th to 20th	Day	13% 8% 3 7% 5% 4 6% 3 5% 6% 40%	1848
	21st to end	Day	19% 10% 3 7% 4 5% 4 3 4 5% 37%	1570
March	1st to 10th	Day	25% 14% 3 7% 4 4 4 2 4 5% 30%	3116
	11th to 20th	Day	13% 7% 2 6% 4 4 4 4 7% 7% 41%	3320
	21st to end	Day	22% 9% 3 7% 4 3 4 3 5% 6% 34%	4038
April	1st to 10th	Day	19% 10% 2 6% 5% 4 4 3 4 7% 36%	2280
	11th to 20th	Day	18% 7% 2 5% 4 4 5% 4 6% 7% 38%	2263
	21st to end	Day	21% 6% 3 7% 5% 5% 4 3 4 6% 36%	2470
May	1st to 10th	Day	24% 9% 3 6% 4 4 4 4 5% 7% 31%	2470
	11th to 20th	Day	18% 6% 2 6% 5% 4 4 3 5% 7% 40%	2378
	21st to end	Day	14% 5% 2 6% 4 5% 4 3 5% 7% 44%	2725
June	1st to 10th	Day	19% 6% 2 7% 5% 5% 4 4 7% 7% 35%	2520
	11th to 20th	Day	17% 6% 2 6% 5% 4 6% 5% 7% 7% 37%	2520
	21st to end	Day	18% 6% 2 7% 5% 5% 5% 4 6% 8% 34%	2520
July	1st to 10th	Day	14% 6% 2 5% 5% 5% 4 4 6% 7% 41%	2520
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 4 6% 9% 40%	2520
	21st to end	Day	11% 6% 3 6% 5% 4 5% 4 7% 9% 40%	2744
August	1st to 10th	Day	9% 6% 2 7% 5% 6% 7% 4 7% 9% 39%	2366
	11th to 20th	Day	13% 6% 2 7% 4 5% 6% 4 6% 8% 40%	2271
	21st to end	Day	12% 4 2 5% 5% 5% 5% 4 6% 7% 45%	2376
September	1st to 10th	Day	14% 6% 2 5% 4 4 4 4 6% 7% 45%	2160
	11th to 20th	Day	15% 6% 2 5% 3 3 3 3 4 6% 51%	1980
	21st to end	Day	17% 5% 2 6% 4 3 4 2 5% 6% 46%	1980
October	1st to 10th	Day	23% 6% 2 6% 4 4 4 3 5% 5% 40%	1980
	11th to 20th	Day	14% 6% 2 5% 4 4 3 4 5% 7% 46%	1756
	21st to end	Day	22% 6% 2 6% 4 4 3 3 5% 6% 40%	1782
November	1st to 10th	Day	16% 7% 2 6% 4 3 2 2 4 4 49%	1671
	11th to 20th	Day	19% 7% 2 5% 3 3 3 4 4 5% 47%	1710
	21st to end	Day	18% 8% 2 5% 3 4 3 4 6% 5% 42%	1710
December	1st to 10th	Day	18% 8% 3 7% 4 4 5% 3 5% 5% 39%	1539
	11th to 20th	Day	23% 6% 3 4 4 2 3 3 4 4 45%	1906
	21st to end	Day	23% 10% 3 6% 4 4 3 3 4 6% 34%	2772
January	1st to 10th	Night	18% 8% 2 5% 3 3 3 3 4 5% 47%	4429
	11th to 20th	Night	14% 9% 4 9% 5% 4 3 3 4 5% 39%	4440
	21st to end	Night	22% 13% 4 7% 4 4 4 3 4 5% 30%	4884
February	1st to 10th	Night	16% 9% 2 7% 4 4 4 3 5% 6% 38%	2411
	11th to 20th	Night	15% 9% 3 6% 5% 4 4 3 5% 5% 41%	2218
	21st to end	Night	18% 10% 2 6% 4 3 4 3 4 6% 41%	1884
March	1st to 10th	Night	28% 11% 2 5% 3 4 3 3 4 5% 32%	3724
	11th to 20th	Night	12% 9% 3 7% 5% 4 5% 4 5% 6% 40%	3625
	21st to end	Night	24% 9% 3 5% 3 4 4 4 6% 5% 34%	3578
April	1st to 10th	Night	18% 8% 3 7% 6% 4 3 3 5% 7% 36%	1900
	11th to 20th	Night	20% 7% 2 6% 4 4 4 3 4 6% 39%	1866
	21st to end	Night	20% 7% 3 5% 5% 4 4 3 5% 5% 39%	1710
May	1st to 10th	Night	24% 8% 3 7% 5% 4 4 4 5% 6% 32%	1710
	11th to 20th	Night	18% 6% 3 6% 4 4 5% 4 6% 7% 37%	1585
	21st to end	Night	17% 6% 3 6% 5% 4 4 5% 4 7% 38%	1550
June	1st to 10th	Night	19% 9% 2 7% 4 4 3 3 7% 7% 36%	1440
	11th to 20th	Night	19% 8% 3 6% 5% 4 4 4 5% 8% 37%	1440
	21st to end	Night	14% 6% 2 6% 5% 3 4 4 7% 10% 40%	1440
July	1st to 10th	Night	14% 5% 2 6% 6% 4 4 4 7% 7% 41%	1440
	11th to 20th	Night	12% 8% 3 6% 5% 4 5% 4 7% 9% 38%	1440
	21st to end	Night	12% 6% 2 5% 5% 4 5% 4 6% 8% 44%	1574
August	1st to 10th	Night	9% 5% 2 5% 4 5% 5% 5% 6% 9% 45%	1553
	11th to 20th	Night	11% 5% 2 6% 6% 6% 6% 5% 8% 8% 38%	1626
	21st to end	Night	15% 5% 2 5% 3 4 3 4 6% 8% 48%	1980
September	1st to 10th	Night	13% 6% 3 6% 4 4 3 3 5% 7% 48%	1800
	11th to 20th	Night	15% 5% 2 4 3 3 3 3 4 6% 52%	1978
	21st to end	Night	17% 6% 2 5% 3 3 4 4 5% 6% 46%	1980
October	1st to 10th	Night	22% 6% 2 6% 4 3 3 2 5% 5% 42%	1980
	11th to 20th	Night	16% 5% 1 5% 4 3 4 3 6% 7% 45%	2201
	21st to end	Night	23% 8% 3 5% 4 3 4 3 5% 5% 38%	2574
November	1st to 10th	Night	16% 5% 2 5% 4 3 4 3 5% 5% 48%	2407
	11th to 20th	Night	19% 7% 1 5% 4 3 3 3 4 5% 46%	2470
	21st to end	Night	16% 8% 3 4 4 3 3 3 5% 7% 45%	2470
December	1st to 10th	Night	19% 9% 2 6% 5% 4 3 3 5% 6% 38%	2636
	11th to 20th	Night	20% 7% 2 5% 4 4 3 3 4 6% 41%	3288
	21st to end	Night	21% 10% 3 6% 4 4 4 3 4 6% 35%	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	41% 4 10% 32% 11% 2	2518
	11th to 20th	Day	46% 6% 10% 24% 13% 2	2520
	21st to end	Day	51% 7% 9% 22% 10% 2	2772
February	1st to 10th	Day	48% 6% 12% 20% 12% 3	1764
	11th to 20th	Day	44% 6% 11% 20% 15% 3	1857
	21st to end	Day	49% 5% 9% 19% 14% 3	1570
March	1st to 10th	Day	58% 4 11% 15% 10% 3	3119
	11th to 20th	Day	38% 7% 15% 23% 15% 3	3320
	21st to end	Day	50% 4 7% 16% 16% 7%	4038
April	1st to 10th	Day	49% 4 9% 18% 16% 4	2280
	11th to 20th	Day	43% 6% 11% 19% 17% 3	2263
	21st to end	Day	49% 6% 7% 17% 17% 4	2470
May	1st to 10th	Day	53% 6% 8% 18% 13% 2	2470
	11th to 20th	Day	44% 6% 11% 18% 18% 3	2382
	21st to end	Day	40% 7% 13% 23% 14% 2	2725
June	1st to 10th	Day	46% 8% 13% 22% 11% 1	2520
	11th to 20th	Day	43% 8% 12% 19% 16% 2	2520
	21st to end	Day	45% 9% 12% 21% 12% 1	2520
July	1st to 10th	Day	40% 8% 10% 23% 16% 2	2450
	11th to 20th	Day	39% 8% 13% 26% 13% 1	2445
	21st to end	Day	37% 8% 15% 22% 14% 3	2591
August	1st to 10th	Day	39% 9% 15% 23% 14% 1	2352
	11th to 20th	Day	41% 9% 13% 24% 13% 1	2271
	21st to end	Day	36% 7% 11% 24% 19% 3	2376
September	1st to 10th	Day	37% 7% 12% 24% 18% 2	2160
	11th to 20th	Day	36% 4 9% 25% 23% 3	1980
	21st to end	Day	39% 5% 10% 21% 23% 2	1980
October	1st to 10th	Day	47% 5% 9% 21% 14% 4	1980
	11th to 20th	Day	36% 6% 12% 30% 15% 2	1756
	21st to end	Day	46% 6% 12% 20% 14% 3	1782
November	1st to 10th	Day	39% 4 12% 25% 17% 2	1671
	11th to 20th	Day	41% 4 10% 22% 21% 3	1710
	21st to end	Day	43% 5% 15% 23% 13% 2	1710
December	1st to 10th	Day	47% 5% 14% 16% 17% 1	1539
	11th to 20th	Day	43% 4 14% 18% 18% 4	1906
	21st to end	Day	52% 6% 11% 20% 11% 1	2772
January	1st to 10th	Night	40% 4 12% 29% 12% 3	4429
	11th to 20th	Night	46% 5% 11% 21% 12% 4	4440
	21st to end	Night	56% 5% 9% 17% 11% 2	4884
February	1st to 10th	Night	46% 5% 13% 20% 13% 2	2411
	11th to 20th	Night	44% 5% 10% 20% 17% 4	2220
	21st to end	Night	45% 4 8% 24% 15% 4	1884
March	1st to 10th	Night	54% 4 9% 19% 10% 3	3734
	11th to 20th	Night	43% 7% 13% 18% 16% 3	3626
	21st to end	Night	51% 3 8% 12% 18% 7%	3578
April	1st to 10th	Night	48% 4 11% 18% 16% 3	1900
	11th to 20th	Night	46% 4 9% 20% 19% 3	1866
	21st to end	Night	46% 5% 10% 18% 16% 4	1710
May	1st to 10th	Night	52% 6% 10% 17% 12% 3	1710
	11th to 20th	Night	44% 7% 12% 17% 16% 3	1586
	21st to end	Night	45% 6% 14% 21% 13% 1	1551
June	1st to 10th	Night	47% 6% 12% 23% 11% 1	1440
	11th to 20th	Night	46% 7% 12% 19% 15% 2	1440
	21st to end	Night	37% 10% 15% 22% 14% 1	1440
July	1st to 10th	Night	39% 9% 13% 23% 16% 1	1400
	11th to 20th	Night	40% 8% 16% 26% 10% 1	1392
	21st to end	Night	36% 8% 13% 29% 12% 2	1486
August	1st to 10th	Night	33% 10% 14% 24% 16% 2	1543
	11th to 20th	Night	41% 8% 17% 21% 13%	1626
	21st to end	Night	35% 6% 11% 26% 20% 1	1980
September	1st to 10th	Night	37% 5% 13% 25% 19% 1	1800
	11th to 20th	Night	34% 5% 10% 22% 27% 2	1978
	21st to end	Night	38% 6% 9% 24% 21% 3	1980
October	1st to 10th	Night	45% 5% 12% 22% 12% 4	1980
	11th to 20th	Night	37% 6% 14% 27% 15% 1	2201
	21st to end	Night	48% 5% 12% 20% 14% 1	2574
November	1st to 10th	Night	38% 6% 11% 21% 20% 3	2407
	11th to 20th	Night	41% 4 11% 22% 19% 3	2470
	21st to end	Night	40% 6% 13% 25% 14% 2	2470
December	1st to 10th	Night	48% 4 13% 22% 13% 1	2636
	11th to 20th	Night	43% 5% 15% 17% 18% 2	3288
	21st to end	Night	51% 4 9% 25% 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	28% 23% 29% 15% 4	180
	11th to 20th	Day	32% 21% 34% 9% 4	180
	21st to end	Day	26% 23% 35% 12% 4	198
February	1st to 10th	Day	31% 19% 35% 14% 1	362
	11th to 20th	Day	27% 28% 32% 11% 2	456
	21st to end	Day	30% 19% 33% 14% 3 1	382
March	1st to 10th	Day	30% 18% 35% 13% 3	460
	11th to 20th	Day	30% 21% 33% 12% 3 1	460
	21st to end	Day	31% 20% 32% 11% 5% 2	506
April	1st to 10th	Day	30% 23% 27% 14% 5% 1	460
	11th to 20th	Day	30% 23% 30% 10% 6% 1	452
	21st to end	Day	28% 19% 32% 16% 3 2	460
May	1st to 10th	Day	32% 19% 30% 13% 4 2	463
	11th to 20th	Day	23% 14% 41% 14% 5% 1	456
	21st to end	Day	19% 13% 36% 21% 9% 2	485
June	1st to 10th	Day	17% 15% 39% 18% 9% 2	450
	11th to 20th	Day	17% 15% 40% 21% 7% 1	450
	21st to end	Day	17% 13% 37% 21% 9% 2	450
July	1st to 10th	Day	18% 9% 29% 27% 14% 2	445
	11th to 20th	Day	14% 7% 34% 31% 12% 3	444
	21st to end	Day	13% 8% 34% 33% 10% 1	488
August	1st to 10th	Day	12% 9% 30% 33% 13% 3	446
	11th to 20th	Day	16% 8% 32% 28% 13% 3	439
	21st to end	Day	13% 8% 34% 26% 14% 5%	495
September	1st to 10th	Day	20% 9% 26% 24% 16% 5%	450
	11th to 20th	Day	20% 11% 31% 18% 12% 8%	450
	21st to end	Day	19% 13% 31% 22% 10% 5%	450
October	1st to 10th	Day	28% 12% 26% 20% 10% 4	450
	11th to 20th	Day	19% 17% 29% 24% 10% 2	403
	21st to end	Day	28% 14% 30% 19% 9% 1	396
November	1st to 10th	Day	22% 14% 33% 19% 9% 2	365
	11th to 20th	Day	30% 19% 27% 19% 5% 1	370
	21st to end	Day	28% 17% 31% 17% 6% 1	370
December	1st to 10th	Day	20% 29% 31% 17% 4	199
	11th to 20th	Day	31% 27% 31% 9% 2	180
	21st to end	Day	24% 26% 30% 15% 4 2	198
January	1st to 10th	Night	22% 17% 39% 19% 3	760
	11th to 20th	Night	28% 17% 34% 19% 2	760
	21st to end	Night	20% 19% 37% 20% 3	836
February	1st to 10th	Night	23% 16% 38% 21% 2	760
	11th to 20th	Night	23% 17% 40% 18% 1	743
	21st to end	Night	26% 18% 32% 20% 3	628
March	1st to 10th	Night	26% 17% 35% 19% 3	760
	11th to 20th	Night	21% 20% 39% 16% 4	760
	21st to end	Night	26% 17% 34% 18% 5%	836
April	1st to 10th	Night	24% 19% 33% 20% 4	760
	11th to 20th	Night	24% 18% 33% 19% 6%	742
	21st to end	Night	23% 18% 36% 20% 3	570
May	1st to 10th	Night	29% 13% 32% 22% 4	570
	11th to 20th	Night	21% 15% 38% 21% 5%	506
	21st to end	Night	19% 9% 33% 32% 6% 1	389
June	1st to 10th	Night	23% 5% 37% 27% 9% 1	360
	11th to 20th	Night	19% 9% 34% 29% 8% 1	360
	21st to end	Night	16% 4 40% 33% 8%	360
July	1st to 10th	Night	16% 4 28% 41% 11% 1	350
	11th to 20th	Night	14% 5% 26% 44% 11% 1	348
	21st to end	Night	12% 5% 30% 44% 9% 1	394
August	1st to 10th	Night	11% 5% 29% 42% 13% 1	479
	11th to 20th	Night	12% 5% 30% 39% 13% 1	562
	21st to end	Night	15% 5% 24% 39% 15% 2	792
September	1st to 10th	Night	16% 5% 31% 33% 14% 1	720
	11th to 20th	Night	17% 7% 27% 29% 17% 2	720
	21st to end	Night	20% 7% 28% 30% 13% 2	720
October	1st to 10th	Night	24% 10% 26% 26% 12% 2	720
	11th to 20th	Night	20% 12% 32% 24% 11% 1	720
	21st to end	Night	27% 13% 32% 22% 5% 1	792
November	1st to 10th	Night	19% 14% 30% 28% 9% 1	740
	11th to 20th	Night	24% 16% 34% 20% 7% 1	760
	21st to end	Night	23% 18% 33% 20% 5% 1	760
December	1st to 10th	Night	27% 17% 34% 18% 4	760
	11th to 20th	Night	25% 16% 33% 23% 3	760
	21st to end	Night	26% 16% 36% 20% 3	836

Page generated at 2024-05-10 12:39:01. Version ac07

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