Sugarloaf Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sugarloaf Observatory clear sky chart from 2005-11-06 to 2025-08-05.

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	12% 6% 2 6% 4 4 5% 4 6% 8% 42%	125733
	Night	13% 6% 2 6% 4 4 4 4 6% 7% 44%	122338
Transparency	Day	39% 5% 12% 20% 19% 6%	129352
	Night	39% 4 10% 18% 21% 7%	126788
Seeing	Day	15% 7% 22% 25% 21% 11%	16443
	Night	15% 7% 23% 33% 20% 3	26514

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	13% 5% 2 5% 3 2 2 2 3 6% 56%	4056
	11th to 20th	Day	11% 5% 2 4 3 3 2 2 3 4 59%	4060
	21st to end	Day	14% 8% 3 7% 5% 4 3 3 4 5% 45%	4466
February	1st to 10th	Day	14% 8% 2 6% 4 4 4 4 5% 6% 43%	1800
	11th to 20th	Day	21% 7% 2 5% 3 2 3 2 5% 5% 45%	1866
	21st to end	Day	18% 7% 2 5% 4 4 4 3 5% 6% 43%	1770
March	1st to 10th	Day	21% 7% 2 7% 4 3 3 3 4 5% 39%	4935
	11th to 20th	Day	9% 5% 2 5% 4 4 4 3 5% 6% 55%	4980
	21st to end	Day	13% 6% 2 7% 4 4 3 3 4 5% 49%	5465
April	1st to 10th	Day	9% 5% 2 4 3 3 4 4 6% 8% 52%	2240
	11th to 20th	Day	10% 4 2 6% 3 4 5% 4 6% 6% 50%	2374
	21st to end	Day	10% 5% 2 5% 4 3 4 3 5% 7% 51%	2440
May	1st to 10th	Day	9% 6% 3 8% 5% 5% 5% 5% 7% 10% 37%	5862
	11th to 20th	Day	18% 7% 3 7% 5% 5% 5% 5% 7% 9% 31%	5861
	21st to end	Day	11% 5% 2 6% 5% 4 5% 4 6% 9% 43%	6433
June	1st to 10th	Day	8% 6% 3 6% 6% 5% 6% 5% 8% 11% 36%	2600
	11th to 20th	Day	8% 4 2 5% 4 5% 6% 6% 8% 11% 41%	2600
	21st to end	Day	7% 4 2 6% 5% 6% 6% 5% 8% 11% 40%	2600
July	1st to 10th	Day	10% 6% 3 9% 7% 7% 7% 6% 8% 10% 27%	5900
	11th to 20th	Day	6% 6% 2 7% 6% 6% 7% 7% 9% 14% 29%	5900
	21st to end	Day	8% 6% 3 8% 6% 7% 7% 6% 9% 11% 29%	6424
August	1st to 10th	Day	23% 7% 3 7% 5% 6% 6% 5% 8% 9% 22%	4156
	11th to 20th	Day	11% 6% 2 7% 5% 5% 6% 4 7% 11% 35%	3992
	21st to end	Day	6% 4 2 4 4 4 5% 4 8% 11% 48%	4180
September	1st to 10th	Day	9% 4 2 6% 6% 6% 6% 4 6% 8% 43%	2199
	11th to 20th	Day	13% 6% 2 7% 6% 5% 4 4 5% 7% 41%	2090
	21st to end	Day	17% 7% 3 6% 3 4 4 3 5% 7% 40%	2090
October	1st to 10th	Day	14% 6% 2 5% 3 3 3 3 5% 6% 49%	1928
	11th to 20th	Day	9% 4 1 5% 4 3 4 3 4 5% 57%	1900
	21st to end	Day	11% 4 2 4 3 3 3 2 4 6% 58%	2090
November	1st to 10th	Day	12% 5% 2 5% 4 3 3 2 4 5% 54%	1957
	11th to 20th	Day	14% 5% 2 4 4 2 3 3 4 6% 53%	1950
	21st to end	Day	12% 6% 3 5% 4 3 3 4 6% 5% 50%	1800
December	1st to 10th	Day	11% 6% 2 6% 4 4 4 4 4 5% 51%	2930
	11th to 20th	Day	16% 5% 2 6% 4 3 5% 3 5% 6% 47%	3373
	21st to end	Day	18% 7% 2 5% 3 3 3 3 4 6% 46%	4466
January	1st to 10th	Night	12% 4 1 5% 3 3 3 3 4 4 58%	5889
	11th to 20th	Night	12% 6% 2 5% 4 3 4 3 4 5% 52%	5900
	21st to end	Night	13% 9% 3 6% 4 4 3 3 4 6% 46%	6490
February	1st to 10th	Night	16% 6% 2 6% 4 3 4 3 5% 6% 45%	2600
	11th to 20th	Night	19% 7% 2 6% 4 3 4 3 4 5% 43%	2415
	21st to end	Night	14% 6% 2 6% 4 3 4 3 5% 7% 46%	1860
March	1st to 10th	Night	18% 8% 3 6% 5% 3 4 3 5% 4 41%	4918
	11th to 20th	Night	10% 4 2 4 3 3 4 3 5% 8% 54%	4980
	21st to end	Night	13% 4 2 5% 3 4 4 3 5% 7% 49%	5467
April	1st to 10th	Night	11% 6% 2 5% 4 3 4 4 4 8% 50%	2158
	11th to 20th	Night	10% 5% 2 5% 3 4 3 3 5% 7% 53%	1975
	21st to end	Night	10% 4 1 5% 4 5% 5% 4 6% 7% 49%	1975
May	1st to 10th	Night	10% 7% 2 6% 4 5% 5% 5% 6% 8% 41%	4033
	11th to 20th	Night	17% 7% 3 7% 4 4 4 4 6% 9% 35%	4032
	21st to end	Night	12% 5% 2 6% 4 4 5% 4 7% 9% 41%	4421
June	1st to 10th	Night	10% 6% 2 7% 6% 6% 6% 5% 7% 9% 35%	1800
	11th to 20th	Night	9% 6% 2 7% 5% 6% 6% 4 8% 9% 37%	1800
	21st to end	Night	6% 4 2 7% 6% 5% 6% 6% 9% 11% 37%	1800
July	1st to 10th	Night	11% 7% 3 10% 6% 6% 7% 6% 8% 11% 25%	4058
	11th to 20th	Night	10% 6% 2 9% 6% 6% 7% 6% 10% 14% 25%	4057
	21st to end	Night	12% 9% 3 10% 7% 6% 7% 6% 7% 11% 22%	4427
August	1st to 10th	Night	24% 7% 3 7% 5% 4 5% 5% 7% 9% 24%	2861
	11th to 20th	Night	9% 6% 2 8% 6% 5% 6% 6% 9% 9% 33%	2803
	21st to end	Night	7% 4 1 5% 4 3 5% 4 7% 11% 48%	3476
September	1st to 10th	Night	9% 5% 2 7% 6% 5% 4 4 7% 10% 41%	1976
	11th to 20th	Night	15% 7% 3 7% 4 5% 5% 3 5% 6% 40%	2090
	21st to end	Night	16% 7% 3 7% 5% 4 5% 4 4 6% 40%	2090
October	1st to 10th	Night	14% 6% 2 5% 3 2 3 2 4 5% 52%	2251
	11th to 20th	Night	12% 5% 2 4 3 3 3 3 5% 6% 56%	2280
	21st to end	Night	9% 6% 2 4 3 3 3 2 5% 5% 58%	2508
November	1st to 10th	Night	13% 6% 1 5% 3 3 3 3 4 5% 54%	2341
	11th to 20th	Night	12% 6% 3 5% 4 3 3 2 4 6% 52%	2445
	21st to end	Night	13% 5% 2 5% 3 3 3 4 6% 6% 50%	2600
December	1st to 10th	Night	13% 6% 2 5% 3 3 3 2 4 5% 54%	4250
	11th to 20th	Night	16% 8% 3 6% 4 4 3 3 4 5% 44%	4822
	21st to end	Night	18% 6% 2 6% 4 3 3 3 5% 5% 45%	6490

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	33% 2 4 20% 34% 8%	4057
	11th to 20th	Day	31% 2 4 16% 36% 11%	4060
	21st to end	Day	43% 2 3 17% 23% 12%	4466
February	1st to 10th	Day	42% 3 9% 21% 19% 6%	1800
	11th to 20th	Day	43% 3 5% 19% 24% 6%	1875
	21st to end	Day	44% 3 7% 17% 22% 8%	1770
March	1st to 10th	Day	49% 2 5% 14% 20% 10%	4935
	11th to 20th	Day	32% 3 7% 15% 27% 17%	4980
	21st to end	Day	39% 3 7% 21% 22% 8%	5465
April	1st to 10th	Day	30% 3 12% 22% 27% 6%	2240
	11th to 20th	Day	34% 3 10% 23% 21% 10%	2374
	21st to end	Day	33% 4 11% 26% 22% 4	2440
May	1st to 10th	Day	41% 5% 18% 24% 11% 1	5897
	11th to 20th	Day	49% 5% 14% 19% 12%	5865
	21st to end	Day	38% 4 14% 20% 20% 4	6447
June	1st to 10th	Day	40% 5% 18% 24% 12% 2	2600
	11th to 20th	Day	33% 7% 21% 23% 14% 2	2600
	21st to end	Day	35% 7% 18% 24% 14% 2	2600
July	1st to 10th	Day	46% 11% 18% 17% 7% 1	5835
	11th to 20th	Day	39% 15% 32% 12% 3 1	5831
	21st to end	Day	43% 12% 18% 19% 8%	6240
August	1st to 10th	Day	56% 8% 19% 12% 5%	4143
	11th to 20th	Day	40% 9% 17% 22% 12%	4031
	21st to end	Day	28% 5% 17% 34% 14% 2	4180
September	1st to 10th	Day	37% 6% 15% 19% 18% 4	2199
	11th to 20th	Day	42% 4 12% 16% 21% 4	2090
	21st to end	Day	43% 4 9% 21% 17% 6%	2090
October	1st to 10th	Day	33% 4 7% 20% 27% 9%	3201
	11th to 20th	Day	20% 2 4 18% 27% 29%	3140
	21st to end	Day	26% 3 5% 23% 27% 17%	3428
November	1st to 10th	Day	33% 3 7% 19% 28% 11%	1957
	11th to 20th	Day	35% 2 7% 20% 27% 9%	1950
	21st to end	Day	36% 2 8% 20% 25% 9%	1800
December	1st to 10th	Day	37% 3 5% 17% 28% 10%	2930
	11th to 20th	Day	39% 3 11% 21% 20% 6%	3370
	21st to end	Day	42% 2 8% 22% 24% 2	4466
January	1st to 10th	Night	31% 2 4 17% 39% 8%	5889
	11th to 20th	Night	36% 2 5% 15% 34% 8%	5900
	21st to end	Night	42% 2 4 14% 29% 9%	6490
February	1st to 10th	Night	41% 3 7% 21% 21% 8%	2600
	11th to 20th	Night	45% 2 6% 16% 23% 7%	2417
	21st to end	Night	39% 2 8% 18% 22% 10%	1860
March	1st to 10th	Night	47% 2 6% 12% 26% 7%	4918
	11th to 20th	Night	30% 3 7% 12% 27% 20%	4980
	21st to end	Night	36% 2 8% 22% 22% 10%	5467
April	1st to 10th	Night	34% 3 9% 21% 23% 9%	2158
	11th to 20th	Night	32% 2 9% 24% 22% 11%	1975
	21st to end	Night	32% 4 12% 26% 22% 5%	1975
May	1st to 10th	Night	39% 5% 16% 24% 14% 2	4060
	11th to 20th	Night	47% 5% 15% 20% 12% 1	4033
	21st to end	Night	39% 5% 12% 25% 17% 2	4429
June	1st to 10th	Night	45% 6% 14% 23% 11% 2	1800
	11th to 20th	Night	40% 7% 19% 20% 12% 2	1800
	21st to end	Night	36% 10% 18% 20% 12% 3	1800
July	1st to 10th	Night	48% 12% 20% 14% 5% 1	4015
	11th to 20th	Night	45% 15% 27% 9% 3 1	4006
	21st to end	Night	53% 11% 16% 15% 5%	4297
August	1st to 10th	Night	56% 7% 18% 16% 3 1	2850
	11th to 20th	Night	42% 10% 22% 19% 6% 1	2830
	21st to end	Night	28% 6% 15% 32% 17% 3	3476
September	1st to 10th	Night	38% 6% 16% 20% 14% 6%	1976
	11th to 20th	Night	44% 5% 13% 16% 19% 3	2090
	21st to end	Night	45% 5% 11% 18% 16% 5%	2090
October	1st to 10th	Night	35% 4 9% 20% 24% 7%	3758
	11th to 20th	Night	22% 2 4 17% 29% 27%	3779
	21st to end	Night	26% 3 6% 20% 27% 18%	4120
November	1st to 10th	Night	35% 2 6% 16% 26% 14%	2341
	11th to 20th	Night	36% 2 5% 17% 28% 12%	2445
	21st to end	Night	34% 3 7% 22% 26% 9%	2600
December	1st to 10th	Night	34% 3 7% 17% 30% 10%	4250
	11th to 20th	Night	43% 4 9% 17% 19% 9%	4824
	21st to end	Night	42% 2 8% 23% 23% 3	6490

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	21% 12% 46% 17% 5%	300
	11th to 20th	Day	15% 14% 40% 22% 8%	300
	21st to end	Day	22% 15% 38% 21% 4 1	330
February	1st to 10th	Day	17% 16% 33% 26% 8%	300
	11th to 20th	Day	34% 13% 35% 15% 3	300
	21st to end	Day	21% 12% 37% 23% 6% 1	369
March	1st to 10th	Day	21% 13% 30% 25% 11%	500
	11th to 20th	Day	18% 11% 39% 23% 7% 1	500
	21st to end	Day	17% 12% 35% 25% 9% 2	550
April	1st to 10th	Day	14% 14% 33% 23% 14% 1	500
	11th to 20th	Day	10% 8% 32% 27% 16% 6%	492
	21st to end	Day	14% 6% 27% 26% 22% 5%	500
May	1st to 10th	Day	8% 10% 33% 26% 17% 6%	503
	11th to 20th	Day	18% 4 16% 27% 24% 11%	527
	21st to end	Day	14% 5% 14% 24% 25% 17%	573
June	1st to 10th	Day	11% 2 14% 33% 27% 14%	530
	11th to 20th	Day	9% 2 12% 30% 26% 20%	530
	21st to end	Day	8% 2 9% 27% 34% 19%	530
July	1st to 10th	Day	10% 1 6% 22% 33% 29%	525
	11th to 20th	Day	10% 3 3 21% 36% 26%	524
	21st to end	Day	7% 3 6% 25% 37% 22%	576
August	1st to 10th	Day	17% 3 8% 23% 28% 21%	502
	11th to 20th	Day	12% 2 4 26% 33% 25%	479
	21st to end	Day	12% 2 7% 21% 33% 24%	539
September	1st to 10th	Day	11% 3 7% 27% 31% 21%	490
	11th to 20th	Day	16% 4 11% 20% 34% 16%	490
	21st to end	Day	21% 3 12% 28% 27% 10%	490
October	1st to 10th	Day	16% 3 18% 27% 23% 13%	490
	11th to 20th	Day	10% 5% 23% 29% 24% 9%	490
	21st to end	Day	14% 4 23% 26% 26% 7%	539
November	1st to 10th	Day	18% 4 23% 31% 19% 5%	495
	11th to 20th	Day	19% 8% 32% 28% 11% 2	450
	21st to end	Day	11% 9% 37% 26% 14% 3	300
December	1st to 10th	Day	20% 10% 38% 23% 9%	300
	11th to 20th	Day	21% 7% 36% 24% 10% 3	300
	21st to end	Day	24% 9% 35% 20% 9% 3	330
January	1st to 10th	Night	19% 11% 36% 26% 7%	800
	11th to 20th	Night	17% 11% 34% 31% 7%	800
	21st to end	Night	18% 9% 36% 33% 3	880
February	1st to 10th	Night	19% 9% 35% 31% 6%	800
	11th to 20th	Night	23% 11% 39% 25% 2	783
	21st to end	Night	18% 12% 36% 30% 4	660
March	1st to 10th	Night	18% 8% 34% 32% 7%	800
	11th to 20th	Night	17% 10% 35% 29% 9% 1	800
	21st to end	Night	15% 9% 36% 33% 7%	880
April	1st to 10th	Night	13% 13% 34% 31% 9% 1	800
	11th to 20th	Night	10% 12% 29% 35% 13% 1	791
	21st to end	Night	11% 6% 27% 41% 14% 1	775
May	1st to 10th	Night	11% 6% 29% 42% 13%	600
	11th to 20th	Night	13% 6% 19% 40% 19% 2	591
	21st to end	Night	12% 4 11% 37% 35% 2	650
June	1st to 10th	Night	14% 2 12% 41% 28% 4	600
	11th to 20th	Night	11% 3 12% 35% 30% 9%	600
	21st to end	Night	8% 2 10% 39% 36% 6%	600
July	1st to 10th	Night	9% 3 5% 33% 44% 6%	585
	11th to 20th	Night	10% 1 4 32% 46% 7%	582
	21st to end	Night	9% 3 5% 33% 44% 6%	657
August	1st to 10th	Night	14% 3 6% 27% 43% 7%	579
	11th to 20th	Night	14% 2 5% 32% 39% 7%	569
	21st to end	Night	12% 2 5% 30% 43% 6%	836
September	1st to 10th	Night	10% 2 8% 32% 43% 7%	760
	11th to 20th	Night	18% 3 9% 29% 36% 5%	760
	21st to end	Night	19% 3 13% 34% 28% 3	760
October	1st to 10th	Night	18% 3 16% 29% 29% 5%	760
	11th to 20th	Night	14% 4 23% 33% 22% 3	760
	21st to end	Night	13% 6% 22% 34% 22% 3	836
November	1st to 10th	Night	16% 6% 22% 35% 18% 4	780
	11th to 20th	Night	15% 9% 26% 35% 13% 1	800
	21st to end	Night	15% 7% 27% 32% 17% 1	800
December	1st to 10th	Night	19% 8% 28% 37% 7% 1	800
	11th to 20th	Night	19% 7% 26% 37% 10% 1	800
	21st to end	Night	23% 9% 24% 33% 9% 1	880

Page generated at 2025-08-02 01:32:25. Version ac07

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