Beacon Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Beacon Hill Observatory clear sky chart from 2005-11-06 to 2024-06-30.

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% 10% 3 8% 5% 5% 5% 4 5% 6% 32%	89400
	Night	17% 10% 3 8% 5% 5% 4 4 5% 6% 32%	89759
Transparency	Day	51% 7% 11% 19% 10% 1	89119
	Night	51% 7% 12% 19% 10% 1	89594
Seeing	Day	25% 17% 28% 19% 9% 3	14320
	Night	22% 13% 27% 28% 9% 1	23304

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	13% 12% 4 11% 6% 5% 6% 5% 5% 6% 28%	2517
	11th to 20th	Day	14% 12% 7% 13% 7% 5% 5% 4 5% 5% 23%	2520
	21st to end	Day	14% 16% 6% 11% 6% 7% 4 4 5% 5% 21%	2772
February	1st to 10th	Day	13% 13% 4 9% 6% 7% 5% 4 6% 7% 28%	1675
	11th to 20th	Day	9% 13% 5% 12% 7% 6% 5% 5% 5% 6% 27%	1848
	21st to end	Day	16% 12% 3 9% 6% 4 4 4 5% 5% 30%	1570
March	1st to 10th	Day	21% 16% 4 9% 6% 5% 4 3 3 4 24%	3116
	11th to 20th	Day	14% 15% 6% 8% 5% 4 5% 4 5% 6% 28%	3251
	21st to end	Day	19% 12% 3 11% 5% 5% 4 3 4 5% 31%	4155
April	1st to 10th	Day	20% 13% 3 7% 5% 4 4 3 4 6% 32%	2280
	11th to 20th	Day	19% 10% 2 6% 5% 5% 4 4 5% 6% 35%	2254
	21st to end	Day	22% 10% 4 8% 5% 3 5% 4 4 6% 29%	2460
May	1st to 10th	Day	26% 10% 3 8% 5% 4 4 3 4 5% 28%	4106
	11th to 20th	Day	18% 8% 3 8% 6% 5% 5% 4 6% 7% 31%	4395
	21st to end	Day	13% 6% 2 6% 5% 4 5% 4 6% 9% 40%	4837
June	1st to 10th	Day	20% 7% 3 7% 6% 5% 5% 4 6% 7% 29%	2660
	11th to 20th	Day	18% 7% 2 6% 5% 5% 4 4 5% 9% 35%	2660
	21st to end	Day	20% 8% 2 7% 6% 6% 6% 4 6% 9% 27%	2646
July	1st to 10th	Day	18% 7% 2 5% 4 5% 4 4 6% 7% 38%	2520
	11th to 20th	Day	13% 6% 2 6% 6% 6% 6% 5% 7% 9% 34%	2520
	21st to end	Day	15% 7% 2 5% 6% 6% 5% 5% 7% 8% 35%	2744
August	1st to 10th	Day	14% 6% 2 8% 5% 6% 5% 5% 7% 8% 36%	2464
	11th to 20th	Day	14% 7% 3 7% 6% 5% 5% 4 6% 8% 35%	2248
	21st to end	Day	12% 6% 2 6% 5% 4 4 4 5% 8% 42%	2376
September	1st to 10th	Day	14% 5% 1 6% 4 5% 4 4 6% 8% 44%	2160
	11th to 20th	Day	15% 7% 2 5% 4 3 4 2 5% 6% 47%	2027
	21st to end	Day	17% 6% 2 5% 4 4 4 4 6% 8% 42%	1980
October	1st to 10th	Day	24% 8% 3 6% 5% 4 4 3 4 6% 34%	1954
	11th to 20th	Day	15% 9% 3 9% 5% 6% 4 3 5% 6% 37%	1728
	21st to end	Day	25% 12% 3 7% 4 5% 4 3 4 5% 27%	1782
November	1st to 10th	Day	13% 9% 3 6% 6% 4 4 4 6% 7% 38%	1671
	11th to 20th	Day	17% 11% 3 7% 5% 4 4 4 6% 6% 32%	1710
	21st to end	Day	18% 10% 4 8% 5% 5% 5% 3 5% 6% 30%	1596
December	1st to 10th	Day	19% 15% 4 10% 6% 6% 4 4 5% 5% 23%	1520
	11th to 20th	Day	21% 14% 2 8% 4 4 4 3 4 6% 29%	1906
	21st to end	Day	24% 13% 5% 12% 6% 5% 4 2 4 3 22%	2772
January	1st to 10th	Night	16% 11% 3 9% 6% 4 5% 4 6% 7% 30%	4429
	11th to 20th	Night	12% 15% 5% 14% 7% 6% 5% 4 5% 5% 22%	4440
	21st to end	Night	19% 15% 5% 11% 6% 5% 5% 3 5% 4 22%	4884
February	1st to 10th	Night	12% 12% 4 9% 6% 6% 5% 5% 7% 7% 26%	2503
	11th to 20th	Night	10% 11% 5% 10% 7% 6% 5% 5% 7% 5% 28%	2218
	21st to end	Night	13% 10% 3 9% 7% 5% 5% 4 6% 7% 31%	1884
March	1st to 10th	Night	23% 14% 3 7% 5% 4 4 3 5% 6% 26%	3724
	11th to 20th	Night	14% 14% 4 9% 6% 6% 4 4 4 5% 31%	3708
	21st to end	Night	19% 13% 3 9% 4 4 3 3 4 5% 33%	3475
April	1st to 10th	Night	19% 10% 3 8% 6% 4 5% 4 4 6% 32%	1900
	11th to 20th	Night	19% 12% 3 5% 4 4 4 3 5% 5% 35%	1875
	21st to end	Night	22% 9% 3 6% 4 4 4 3 5% 7% 33%	1720
May	1st to 10th	Night	28% 10% 2 6% 5% 4 4 4 4 6% 26%	2834
	11th to 20th	Night	19% 10% 3 9% 5% 6% 5% 4 5% 5% 29%	2527
	21st to end	Night	15% 5% 2 5% 4 4 5% 4 6% 9% 40%	2738
June	1st to 10th	Night	19% 6% 3 6% 7% 5% 5% 4 6% 9% 32%	1520
	11th to 20th	Night	19% 7% 2 5% 4 4 4 4 6% 6% 39%	1520
	21st to end	Night	19% 5% 2 6% 6% 5% 5% 4 6% 7% 35%	1512
July	1st to 10th	Night	16% 5% 2 5% 4 4 4 5% 7% 6% 42%	1440
	11th to 20th	Night	15% 4 2 5% 4 2 4 5% 7% 8% 43%	1440
	21st to end	Night	11% 6% 2 6% 4 4 5% 4 6% 7% 44%	1574
August	1st to 10th	Night	13% 5% 2 5% 4 4 4 4 6% 7% 45%	1455
	11th to 20th	Night	14% 5% 2 6% 5% 4 4 4 6% 8% 41%	1649
	21st to end	Night	15% 6% 2 5% 4 4 4 3 6% 7% 45%	1980
September	1st to 10th	Night	13% 5% 2 5% 4 4 4 4 5% 7% 47%	1800
	11th to 20th	Night	15% 6% 2 4 3 4 3 2 5% 7% 49%	1932
	21st to end	Night	15% 6% 2 4 3 4 4 4 5% 6% 45%	1980
October	1st to 10th	Night	21% 7% 3 6% 4 4 3 3 6% 7% 34%	2004
	11th to 20th	Night	17% 9% 3 8% 5% 4 5% 4 5% 7% 34%	2231
	21st to end	Night	23% 10% 3 7% 5% 5% 5% 3 6% 6% 28%	2574
November	1st to 10th	Night	15% 10% 3 7% 5% 5% 4 4 5% 6% 36%	2407
	11th to 20th	Night	17% 8% 3 5% 6% 4 6% 4 6% 8% 34%	2470
	21st to end	Night	16% 12% 4 8% 5% 5% 4 3 5% 6% 32%	2580
December	1st to 10th	Night	19% 15% 5% 10% 6% 4 5% 4 5% 6% 21%	2660
	11th to 20th	Night	20% 12% 4 8% 5% 4 4 3 5% 7% 28%	3288
	21st to end	Night	23% 14% 6% 11% 5% 4 4 3 4 4 21%	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	54% 8% 10% 23% 5%	2518
	11th to 20th	Day	62% 8% 9% 16% 5% 1	2520
	21st to end	Day	63% 6% 9% 14% 6% 1	2772
February	1st to 10th	Day	54% 7% 12% 17% 8% 2	1675
	11th to 20th	Day	56% 7% 10% 17% 9% 2	1857
	21st to end	Day	55% 6% 10% 16% 12% 2	1570
March	1st to 10th	Day	64% 5% 8% 11% 10% 2	3119
	11th to 20th	Day	54% 8% 13% 13% 11% 2	3251
	21st to end	Day	56% 4 7% 13% 18% 3	4155
April	1st to 10th	Day	54% 4 9% 19% 11% 3	2280
	11th to 20th	Day	49% 6% 11% 18% 14% 2	2254
	21st to end	Day	56% 5% 9% 17% 12% 2	2460
May	1st to 10th	Day	57% 7% 10% 18% 7% 1	4120
	11th to 20th	Day	50% 10% 15% 15% 9% 2	4399
	21st to end	Day	38% 9% 13% 27% 12% 1	4837
June	1st to 10th	Day	52% 7% 14% 17% 10%	2660
	11th to 20th	Day	46% 7% 12% 20% 14% 2	2660
	21st to end	Day	53% 8% 11% 18% 9% 1	2646
July	1st to 10th	Day	43% 8% 11% 22% 15% 1	2450
	11th to 20th	Day	43% 9% 14% 24% 9% 1	2445
	21st to end	Day	43% 8% 15% 23% 11%	2591
August	1st to 10th	Day	43% 10% 14% 23% 10%	2450
	11th to 20th	Day	44% 9% 16% 22% 10%	2248
	21st to end	Day	39% 7% 15% 25% 13% 1	2376
September	1st to 10th	Day	37% 8% 13% 28% 13% 1	2160
	11th to 20th	Day	38% 5% 9% 29% 17% 2	2027
	21st to end	Day	39% 7% 14% 26% 13% 2	1980
October	1st to 10th	Day	52% 6% 10% 19% 12% 2	1954
	11th to 20th	Day	48% 6% 13% 25% 7% 1	1728
	21st to end	Day	60% 5% 10% 15% 8% 2	1782
November	1st to 10th	Day	44% 7% 14% 21% 13% 1	1671
	11th to 20th	Day	50% 7% 13% 15% 13% 2	1710
	21st to end	Day	55% 6% 17% 17% 5% 1	1596
December	1st to 10th	Day	62% 7% 13% 13% 5% 1	1520
	11th to 20th	Day	57% 6% 10% 17% 9% 1	1906
	21st to end	Day	66% 6% 8% 12% 8%	2772
January	1st to 10th	Night	51% 8% 16% 18% 7%	4429
	11th to 20th	Night	60% 7% 13% 12% 7% 1	4440
	21st to end	Night	64% 7% 9% 13% 6% 1	4884
February	1st to 10th	Night	53% 9% 11% 17% 9% 1	2503
	11th to 20th	Night	53% 6% 11% 19% 10% 1	2220
	21st to end	Night	50% 6% 14% 18% 9% 3	1884
March	1st to 10th	Night	58% 5% 11% 16% 9% 1	3734
	11th to 20th	Night	53% 8% 10% 16% 12% 1	3709
	21st to end	Night	54% 4 8% 13% 18% 4	3475
April	1st to 10th	Night	52% 5% 9% 19% 13% 2	1900
	11th to 20th	Night	50% 6% 9% 21% 13% 1	1875
	21st to end	Night	51% 5% 10% 17% 14% 3	1720
May	1st to 10th	Night	57% 7% 10% 16% 9% 1	2840
	11th to 20th	Night	54% 9% 12% 16% 8% 1	2528
	21st to end	Night	39% 10% 16% 20% 13% 2	2739
June	1st to 10th	Night	47% 10% 13% 19% 11% 1	1520
	11th to 20th	Night	43% 8% 15% 21% 11% 2	1520
	21st to end	Night	46% 8% 13% 22% 10%	1512
July	1st to 10th	Night	38% 9% 17% 21% 14% 1	1400
	11th to 20th	Night	35% 9% 18% 26% 12%	1392
	21st to end	Night	36% 8% 15% 28% 12% 1	1486
August	1st to 10th	Night	36% 9% 15% 29% 11%	1445
	11th to 20th	Night	40% 7% 17% 24% 11%	1649
	21st to end	Night	37% 7% 15% 27% 13%	1980
September	1st to 10th	Night	34% 8% 16% 29% 13%	1800
	11th to 20th	Night	35% 6% 11% 30% 16% 1	1932
	21st to end	Night	37% 8% 12% 26% 14% 2	1980
October	1st to 10th	Night	49% 7% 11% 21% 11% 2	2004
	11th to 20th	Night	48% 8% 13% 23% 7%	2231
	21st to end	Night	54% 7% 11% 18% 9% 1	2574
November	1st to 10th	Night	47% 8% 13% 20% 11% 2	2407
	11th to 20th	Night	46% 8% 12% 20% 12% 2	2470
	21st to end	Night	53% 6% 14% 20% 7%	2580
December	1st to 10th	Night	62% 8% 12% 15% 4	2660
	11th to 20th	Night	56% 6% 12% 14% 11% 1	3288
	21st to end	Night	66% 6% 9% 16% 4	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	24% 22% 29% 18% 6% 1	180
	11th to 20th	Day	33% 22% 27% 13% 4 1	180
	21st to end	Day	23% 21% 32% 16% 8% 1	198
February	1st to 10th	Day	31% 28% 28% 11% 2	265
	11th to 20th	Day	29% 26% 28% 14% 2	456
	21st to end	Day	31% 21% 29% 15% 4	382
March	1st to 10th	Day	31% 15% 33% 17% 3	460
	11th to 20th	Day	32% 23% 29% 11% 5% 1	460
	21st to end	Day	33% 20% 25% 13% 6% 3	506
April	1st to 10th	Day	38% 16% 26% 13% 6% 1	460
	11th to 20th	Day	34% 22% 22% 13% 8% 1	452
	21st to end	Day	30% 23% 27% 12% 7% 1	460
May	1st to 10th	Day	34% 20% 26% 12% 6% 2	463
	11th to 20th	Day	23% 15% 33% 20% 6% 2	487
	21st to end	Day	19% 18% 32% 21% 9% 1	529
June	1st to 10th	Day	22% 16% 37% 15% 9% 1	490
	11th to 20th	Day	19% 17% 31% 21% 11% 2	490
	21st to end	Day	22% 16% 32% 19% 9% 2	486
July	1st to 10th	Day	17% 12% 31% 25% 12% 4	445
	11th to 20th	Day	16% 9% 33% 28% 11% 4	444
	21st to end	Day	14% 7% 40% 25% 11% 2	488
August	1st to 10th	Day	13% 12% 29% 23% 19% 4	446
	11th to 20th	Day	17% 10% 26% 24% 15% 8%	439
	21st to end	Day	13% 14% 26% 25% 14% 7%	495
September	1st to 10th	Day	16% 12% 24% 25% 15% 8%	450
	11th to 20th	Day	19% 12% 25% 19% 14% 10%	450
	21st to end	Day	19% 12% 27% 23% 12% 7%	450
October	1st to 10th	Day	28% 13% 22% 18% 14% 6%	450
	11th to 20th	Day	20% 17% 26% 23% 11% 2	414
	21st to end	Day	37% 16% 21% 19% 8%	396
November	1st to 10th	Day	24% 15% 22% 23% 13% 2	365
	11th to 20th	Day	27% 21% 25% 19% 8% 1	370
	21st to end	Day	31% 16% 28% 18% 7%	256
December	1st to 10th	Day	31% 17% 31% 13% 7% 1	180
	11th to 20th	Day	36% 19% 29% 12% 3	180
	21st to end	Day	34% 21% 23% 17% 5%	198
January	1st to 10th	Night	22% 17% 34% 23% 3	760
	11th to 20th	Night	28% 16% 28% 24% 4	760
	21st to end	Night	18% 16% 34% 27% 4	836
February	1st to 10th	Night	21% 18% 34% 24% 3	760
	11th to 20th	Night	20% 19% 35% 23% 3	743
	21st to end	Night	24% 17% 30% 27% 3	628
March	1st to 10th	Night	25% 14% 32% 26% 3	760
	11th to 20th	Night	25% 18% 29% 22% 6%	760
	21st to end	Night	28% 15% 26% 23% 8%	836
April	1st to 10th	Night	27% 17% 28% 24% 5%	760
	11th to 20th	Night	25% 20% 25% 22% 7%	751
	21st to end	Night	29% 16% 26% 23% 7%	580
May	1st to 10th	Night	28% 14% 28% 22% 8%	570
	11th to 20th	Night	19% 13% 29% 33% 6% 1	393
	21st to end	Night	18% 8% 27% 34% 13%	411
June	1st to 10th	Night	20% 8% 28% 34% 11%	380
	11th to 20th	Night	19% 9% 25% 33% 13%	380
	21st to end	Night	17% 6% 25% 39% 12% 1	378
July	1st to 10th	Night	17% 4 19% 43% 16% 1	350
	11th to 20th	Night	14% 3 22% 40% 19% 1	348
	21st to end	Night	12% 5% 21% 44% 17% 1	394
August	1st to 10th	Night	15% 3 15% 43% 23%	381
	11th to 20th	Night	17% 4 22% 36% 18% 3	585
	21st to end	Night	15% 7% 18% 35% 23% 2	792
September	1st to 10th	Night	16% 5% 24% 34% 20% 1	720
	11th to 20th	Night	18% 8% 20% 29% 22% 4	720
	21st to end	Night	18% 7% 26% 30% 17% 2	720
October	1st to 10th	Night	26% 12% 17% 28% 16% 2	720
	11th to 20th	Night	22% 10% 23% 31% 12% 1	720
	21st to end	Night	27% 14% 27% 25% 8%	792
November	1st to 10th	Night	21% 16% 25% 30% 8%	740
	11th to 20th	Night	22% 15% 28% 25% 8% 1	760
	21st to end	Night	24% 16% 26% 27% 7%	760
December	1st to 10th	Night	27% 15% 33% 21% 4	760
	11th to 20th	Night	26% 16% 28% 25% 4	760
	21st to end	Night	29% 16% 31% 21% 3	836

Page generated at 2024-06-26 22:18:40. Version ac07

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