North Ashburnham Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the North Ashburnham Observatory 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% 8% 3 7% 5% 4 4 4 6% 7% 36%	101256
	Night	17% 8% 3 7% 4 4 4 4 5% 6% 38%	97766
Transparency	Day	45% 5% 11% 22% 14% 2	104781
	Night	44% 5% 12% 23% 15% 2	102317
Seeing	Day	26% 17% 32% 18% 6% 2	14960
	Night	24% 17% 32% 20% 6% 1	23884

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	12% 9% 3 7% 5% 4 4 3 5% 6% 41%	2517
	11th to 20th	Day	15% 8% 4 9% 5% 5% 5% 4 5% 6% 36%	2520
	21st to end	Day	18% 14% 3 8% 3 4 4 3 5% 5% 32%	2772
February	1st to 10th	Day	17% 8% 3 7% 5% 5% 5% 5% 5% 6% 33%	1747
	11th to 20th	Day	12% 8% 3 8% 6% 5% 5% 4 6% 7% 37%	1848
	21st to end	Day	17% 11% 3 7% 5% 5% 4 3 4 4 36%	1570
March	1st to 10th	Day	20% 15% 4 7% 6% 4 4 2 4 4 30%	3116
	11th to 20th	Day	12% 8% 2 6% 5% 5% 6% 4 6% 8% 38%	3283
	21st to end	Day	19% 9% 3 7% 4 3 5% 3 6% 6% 34%	4097
April	1st to 10th	Day	16% 11% 2 8% 6% 4 3 4 5% 5% 35%	2280
	11th to 20th	Day	16% 9% 2 6% 5% 4 5% 4 6% 7% 36%	2254
	21st to end	Day	19% 8% 3 8% 5% 5% 4 4 5% 5% 34%	2457
May	1st to 10th	Day	22% 11% 4 8% 5% 4 4 4 5% 7% 26%	4106
	11th to 20th	Day	21% 10% 3 8% 5% 4 3 3 5% 6% 33%	4257
	21st to end	Day	14% 5% 2 6% 4 4 4 4 7% 9% 42%	4837
June	1st to 10th	Day	18% 7% 2 5% 5% 5% 5% 4 7% 8% 33%	2660
	11th to 20th	Day	17% 7% 2 6% 5% 4 5% 4 7% 8% 35%	2660
	21st to end	Day	17% 7% 3 7% 5% 5% 5% 4 6% 8% 31%	2660
July	1st to 10th	Day	13% 6% 3 7% 5% 5% 6% 5% 7% 9% 35%	4440
	11th to 20th	Day	12% 6% 2 6% 4 5% 6% 5% 7% 11% 37%	4440
	21st to end	Day	18% 8% 3 7% 4 4 4 4 6% 8% 33%	4814
August	1st to 10th	Day	16% 6% 3 7% 5% 5% 5% 5% 6% 8% 34%	4238
	11th to 20th	Day	13% 7% 3 8% 5% 5% 5% 5% 7% 8% 34%	3927
	21st to end	Day	9% 4 2 6% 5% 5% 5% 4 8% 9% 43%	4180
September	1st to 10th	Day	13% 6% 2 6% 4 4 5% 5% 5% 7% 44%	2280
	11th to 20th	Day	15% 6% 2 5% 3 3 3 3 5% 6% 50%	2114
	21st to end	Day	17% 6% 3 5% 4 4 4 3 5% 8% 42%	2090
October	1st to 10th	Day	23% 7% 2 7% 5% 4 3 3 4 5% 37%	2090
	11th to 20th	Day	14% 7% 2 6% 4 4 4 3 5% 7% 44%	1833
	21st to end	Day	20% 9% 3 7% 4 3 3 3 5% 6% 38%	1881
November	1st to 10th	Day	13% 8% 3 7% 3 3 3 3 4 5% 48%	1698
	11th to 20th	Day	19% 7% 2 5% 3 3 4 3 5% 5% 45%	1710
	21st to end	Day	17% 9% 2 6% 4 4 4 3 5% 6% 40%	1682
December	1st to 10th	Day	17% 9% 3 8% 5% 5% 4 4 5% 5% 35%	1520
	11th to 20th	Day	22% 7% 2 5% 3 4 3 4 5% 5% 40%	1906
	21st to end	Day	20% 12% 3 7% 4 4 4 4 5% 7% 30%	2772
January	1st to 10th	Night	16% 8% 2 5% 3 3 4 3 5% 6% 43%	4429
	11th to 20th	Night	13% 9% 3 9% 6% 4 5% 3 4 7% 36%	4440
	21st to end	Night	21% 12% 4 8% 5% 4 5% 3 4 5% 29%	4884
February	1st to 10th	Night	15% 9% 3 7% 4 4 4 5% 6% 6% 37%	2428
	11th to 20th	Night	12% 9% 3 6% 4 5% 5% 4 6% 7% 39%	2218
	21st to end	Night	16% 9% 3 7% 4 4 4 3 5% 7% 39%	1884
March	1st to 10th	Night	23% 12% 3 5% 4 3 4 3 4 6% 32%	3724
	11th to 20th	Night	11% 8% 3 6% 5% 5% 6% 5% 6% 7% 38%	3674
	21st to end	Night	21% 10% 3 5% 3 4 4 4 6% 5% 34%	3530
April	1st to 10th	Night	17% 9% 2 8% 5% 4 4 4 6% 6% 35%	1900
	11th to 20th	Night	19% 8% 2 5% 4 4 4 4 5% 7% 39%	1875
	21st to end	Night	20% 7% 3 7% 4 4 3 3 4 6% 39%	1721
May	1st to 10th	Night	22% 11% 4 8% 4 4 5% 4 5% 7% 26%	2834
	11th to 20th	Night	17% 11% 4 9% 6% 6% 5% 5% 6% 6% 26%	2665
	21st to end	Night	16% 7% 2 6% 4 4 3 3 6% 8% 41%	2738
June	1st to 10th	Night	19% 8% 3 6% 5% 3 5% 4 4 6% 38%	1520
	11th to 20th	Night	18% 7% 3 6% 4 4 4 4 7% 6% 37%	1520
	21st to end	Night	16% 7% 2 5% 4 3 4 4 7% 8% 40%	1520
July	1st to 10th	Night	12% 7% 2 7% 5% 5% 5% 5% 7% 9% 37%	2520
	11th to 20th	Night	15% 7% 2 6% 4 4 4 4 6% 8% 40%	2520
	21st to end	Night	16% 8% 3 7% 5% 4 4 3 5% 7% 39%	2739
August	1st to 10th	Night	22% 7% 2 6% 4 4 4 4 5% 6% 34%	2661
	11th to 20th	Night	13% 8% 3 9% 6% 4 4 4 5% 6% 36%	2896
	21st to end	Night	9% 4 1 4 3 3 3 3 6% 8% 57%	3476
September	1st to 10th	Night	13% 7% 2 5% 3 4 3 3 5% 7% 48%	1900
	11th to 20th	Night	13% 6% 2 6% 4 3 3 3 4 6% 52%	2065
	21st to end	Night	15% 6% 3 6% 5% 4 4 2 5% 6% 45%	2090
October	1st to 10th	Night	20% 7% 2 6% 4 4 4 3 4 6% 40%	2090
	11th to 20th	Night	15% 6% 2 5% 4 3 4 3 6% 6% 45%	2340
	21st to end	Night	20% 8% 3 6% 3 4 4 3 4 6% 38%	2717
November	1st to 10th	Night	14% 6% 2 6% 4 4 4 3 4 5% 47%	2453
	11th to 20th	Night	17% 7% 2 6% 4 4 4 3 4 5% 44%	2470
	21st to end	Night	14% 8% 2 6% 4 4 4 3 5% 7% 42%	2493
December	1st to 10th	Night	18% 11% 3 6% 4 3 4 4 5% 7% 36%	2660
	11th to 20th	Night	18% 8% 2 6% 4 4 4 4 5% 6% 37%	3288
	21st to end	Night	19% 11% 4 8% 4 4 4 3 5% 6% 33%	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	43% 3 8% 35% 11% 2	2518
	11th to 20th	Day	48% 5% 10% 25% 11% 2	2520
	21st to end	Day	53% 5% 9% 22% 10% 2	2772
February	1st to 10th	Day	50% 4 13% 19% 12% 3	1747
	11th to 20th	Day	45% 6% 10% 21% 16% 3	1857
	21st to end	Day	51% 3 9% 20% 15% 3	1570
March	1st to 10th	Day	58% 3 10% 14% 12% 2	3119
	11th to 20th	Day	39% 5% 16% 23% 14% 3	3283
	21st to end	Day	50% 3 5% 17% 20% 5%	4097
April	1st to 10th	Day	50% 4 8% 20% 15% 3	2280
	11th to 20th	Day	45% 4 10% 22% 16% 3	2254
	21st to end	Day	50% 4 7% 19% 16% 3	2457
May	1st to 10th	Day	56% 7% 10% 16% 10% 1	4120
	11th to 20th	Day	52% 6% 14% 17% 10% 1	4261
	21st to end	Day	36% 7% 12% 26% 17% 1	4837
June	1st to 10th	Day	46% 7% 12% 23% 12% 1	2660
	11th to 20th	Day	43% 6% 13% 21% 15% 2	2660
	21st to end	Day	49% 6% 11% 21% 12% 1	2660
July	1st to 10th	Day	42% 7% 13% 21% 16% 1	4370
	11th to 20th	Day	38% 7% 16% 26% 12% 1	4365
	21st to end	Day	47% 5% 12% 20% 14% 1	4661
August	1st to 10th	Day	46% 7% 15% 22% 10% 1	4224
	11th to 20th	Day	44% 7% 16% 25% 8% 1	3966
	21st to end	Day	34% 7% 12% 30% 15% 1	4180
September	1st to 10th	Day	38% 5% 12% 26% 18% 2	2280
	11th to 20th	Day	36% 4 7% 27% 24% 2	2114
	21st to end	Day	41% 5% 8% 23% 20% 3	2090
October	1st to 10th	Day	44% 6% 8% 23% 17% 2	3480
	11th to 20th	Day	33% 4 9% 28% 23% 4	3010
	21st to end	Day	35% 4 10% 31% 17% 4	3081
November	1st to 10th	Day	39% 4 12% 25% 17% 3	1698
	11th to 20th	Day	41% 4 10% 23% 19% 3	1710
	21st to end	Day	44% 4 15% 23% 12% 2	1682
December	1st to 10th	Day	49% 4 14% 16% 15% 1	1520
	11th to 20th	Day	44% 3 12% 20% 19% 2	1906
	21st to end	Day	53% 5% 11% 19% 11% 1	2772
January	1st to 10th	Night	40% 5% 12% 28% 12% 3	4429
	11th to 20th	Night	47% 5% 11% 22% 12% 3	4440
	21st to end	Night	58% 4 9% 17% 11% 2	4884
February	1st to 10th	Night	46% 5% 13% 21% 13% 2	2428
	11th to 20th	Night	43% 5% 11% 21% 16% 3	2220
	21st to end	Night	45% 3 11% 24% 13% 4	1884
March	1st to 10th	Night	53% 5% 8% 21% 10% 3	3734
	11th to 20th	Night	43% 7% 13% 21% 15% 2	3675
	21st to end	Night	48% 4 7% 14% 21% 7%	3530
April	1st to 10th	Night	47% 4 10% 20% 16% 2	1900
	11th to 20th	Night	44% 4 10% 21% 18% 2	1875
	21st to end	Night	47% 4 10% 19% 16% 4	1721
May	1st to 10th	Night	55% 8% 10% 18% 7% 2	2840
	11th to 20th	Night	54% 8% 13% 15% 9% 2	2666
	21st to end	Night	41% 6% 13% 23% 15% 1	2739
June	1st to 10th	Night	47% 6% 10% 25% 12% 1	1520
	11th to 20th	Night	44% 7% 13% 21% 13% 2	1520
	21st to end	Night	40% 8% 14% 22% 15% 1	1520
July	1st to 10th	Night	41% 7% 14% 23% 15% 1	2480
	11th to 20th	Night	40% 6% 15% 23% 15%	2472
	21st to end	Night	44% 5% 12% 22% 16% 1	2651
August	1st to 10th	Night	50% 6% 13% 20% 11% 1	2651
	11th to 20th	Night	46% 6% 14% 24% 10%	2923
	21st to end	Night	25% 5% 16% 34% 19% 1	3476
September	1st to 10th	Night	36% 5% 12% 28% 19% 1	1900
	11th to 20th	Night	34% 4 10% 26% 24% 2	2065
	21st to end	Night	40% 4 9% 27% 17% 3	2090
October	1st to 10th	Night	40% 4 15% 24% 15% 2	3480
	11th to 20th	Night	28% 5% 12% 26% 26% 4	3889
	21st to end	Night	35% 3 12% 33% 14% 2	4467
November	1st to 10th	Night	39% 5% 10% 23% 20% 3	2453
	11th to 20th	Night	42% 3 11% 23% 19% 3	2470
	21st to end	Night	41% 3 16% 25% 14% 1	2493
December	1st to 10th	Night	47% 4 13% 23% 13% 1	2660
	11th to 20th	Night	45% 5% 13% 20% 16% 2	3288
	21st to end	Night	50% 5% 10% 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	31% 25% 28% 14% 1	180
	11th to 20th	Day	37% 22% 30% 8% 2 1	180
	21st to end	Day	27% 27% 35% 9% 2	198
February	1st to 10th	Day	34% 31% 25% 9% 1	332
	11th to 20th	Day	37% 26% 29% 8%	456
	21st to end	Day	36% 21% 31% 10% 2	382
March	1st to 10th	Day	32% 21% 35% 9% 2	460
	11th to 20th	Day	31% 27% 29% 8% 3 1	460
	21st to end	Day	32% 23% 30% 9% 4 1	506
April	1st to 10th	Day	35% 20% 29% 10% 5%	460
	11th to 20th	Day	31% 21% 34% 11% 4 1	452
	21st to end	Day	32% 18% 31% 13% 5% 1	460
May	1st to 10th	Day	35% 19% 27% 13% 5% 1	463
	11th to 20th	Day	22% 18% 37% 16% 5% 1	487
	21st to end	Day	21% 13% 34% 24% 7% 1	529
June	1st to 10th	Day	22% 14% 35% 19% 7% 2	490
	11th to 20th	Day	20% 12% 38% 22% 8% 1	490
	21st to end	Day	21% 13% 37% 19% 7% 3	490
July	1st to 10th	Day	18% 9% 31% 29% 11% 2	485
	11th to 20th	Day	16% 6% 34% 32% 10% 2	484
	21st to end	Day	15% 10% 36% 28% 8% 2	532
August	1st to 10th	Day	14% 10% 33% 28% 13% 2	486
	11th to 20th	Day	18% 10% 30% 27% 12% 3	479
	21st to end	Day	14% 11% 33% 28% 10% 4	539
September	1st to 10th	Day	19% 12% 29% 23% 14% 4	490
	11th to 20th	Day	20% 11% 26% 22% 13% 8%	490
	21st to end	Day	20% 12% 33% 21% 8% 5%	490
October	1st to 10th	Day	30% 12% 26% 18% 10% 3	490
	11th to 20th	Day	24% 15% 32% 21% 5% 2	447
	21st to end	Day	31% 15% 32% 16% 4 1	429
November	1st to 10th	Day	25% 21% 29% 16% 8% 1	374
	11th to 20th	Day	34% 21% 27% 15% 2 1	370
	21st to end	Day	31% 20% 30% 14% 4	342
December	1st to 10th	Day	30% 29% 26% 12% 3	180
	11th to 20th	Day	39% 22% 33% 5% 1	180
	21st to end	Day	34% 23% 31% 8% 3 1	198
January	1st to 10th	Night	25% 27% 35% 11% 2	760
	11th to 20th	Night	31% 24% 29% 13% 2	760
	21st to end	Night	24% 23% 35% 16% 2	836
February	1st to 10th	Night	25% 24% 35% 14% 1	760
	11th to 20th	Night	24% 28% 36% 11% 1	743
	21st to end	Night	29% 26% 29% 13% 2	628
March	1st to 10th	Night	27% 22% 34% 14% 2	760
	11th to 20th	Night	26% 24% 34% 14% 2	760
	21st to end	Night	28% 22% 32% 15% 4	836
April	1st to 10th	Night	28% 19% 32% 18% 3	760
	11th to 20th	Night	26% 21% 32% 17% 4	751
	21st to end	Night	28% 18% 34% 18% 2	581
May	1st to 10th	Night	29% 13% 34% 20% 3	570
	11th to 20th	Night	22% 16% 36% 22% 4	479
	21st to end	Night	20% 9% 38% 27% 6%	411
June	1st to 10th	Night	22% 12% 34% 24% 9%	380
	11th to 20th	Night	20% 10% 34% 30% 5% 1	380
	21st to end	Night	17% 8% 38% 31% 7%	380
July	1st to 10th	Night	16% 6% 35% 32% 10%	370
	11th to 20th	Night	17% 4 30% 38% 10% 2	368
	21st to end	Night	13% 5% 35% 37% 10%	416
August	1st to 10th	Night	13% 4 34% 35% 13%	468
	11th to 20th	Night	16% 6% 30% 36% 11% 1	624
	21st to end	Night	17% 5% 27% 35% 15% 1	836
September	1st to 10th	Night	16% 8% 34% 31% 11% 1	760
	11th to 20th	Night	17% 8% 30% 27% 16% 3	760
	21st to end	Night	19% 11% 30% 26% 12% 2	760
October	1st to 10th	Night	25% 13% 28% 22% 10% 1	760
	11th to 20th	Night	21% 15% 32% 23% 8% 1	760
	21st to end	Night	28% 18% 30% 19% 5%	836
November	1st to 10th	Night	21% 21% 34% 17% 7% 1	755
	11th to 20th	Night	24% 25% 29% 15% 6% 1	760
	21st to end	Night	26% 23% 33% 14% 4	760
December	1st to 10th	Night	30% 22% 32% 12% 3	760
	11th to 20th	Night	28% 24% 31% 14% 3	760
	21st to end	Night	32% 24% 28% 15% 1	836

Page generated at 2024-11-01 22:47:38. Version ac07

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