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 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	12% 6% 2 6% 4 4 4 4 6% 8% 42%	101159
	Night	13% 6% 2 6% 4 4 4 4 6% 7% 43%	97808
Transparency	Day	39% 4 12% 20% 19% 6%	104822
	Night	40% 4 10% 19% 20% 7%	102284
Seeing	Day	15% 6% 21% 25% 21% 11%	15444
	Night	15% 6% 22% 33% 20% 3	25518

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	15% 5% 2 5% 3 3 3 2 4 8% 50%	2836
	11th to 20th	Day	12% 6% 2 4 3 3 3 3 4 5% 54%	2840
	21st to end	Day	17% 9% 3 7% 4 3 3 2 4 5% 43%	3124
February	1st to 10th	Day	15% 8% 2 6% 4 4 4 4 5% 6% 42%	1710
	11th to 20th	Day	20% 7% 2 5% 3 2 3 3 5% 5% 46%	1770
	21st to end	Day	19% 7% 2 6% 4 4 4 3 5% 5% 41%	1684
March	1st to 10th	Day	23% 9% 3 7% 4 3 3 3 4 5% 37%	3435
	11th to 20th	Day	9% 5% 2 5% 3 4 4 3 5% 6% 54%	3480
	21st to end	Day	18% 7% 2 7% 4 4 3 3 4 6% 44%	3815
April	1st to 10th	Day	9% 5% 2 4 3 3 4 4 6% 7% 52%	2128
	11th to 20th	Day	11% 5% 2 6% 4 4 4 4 6% 6% 49%	2254
	21st to end	Day	10% 5% 2 5% 4 3 4 3 5% 6% 52%	2318
May	1st to 10th	Day	8% 7% 3 8% 5% 6% 4 5% 7% 10% 38%	4082
	11th to 20th	Day	16% 7% 3 7% 5% 4 4 4 6% 7% 37%	4081
	21st to end	Day	6% 4 2 5% 5% 4 5% 4 7% 10% 47%	4476
June	1st to 10th	Day	8% 6% 3 6% 6% 5% 6% 5% 8% 11% 35%	2470
	11th to 20th	Day	8% 4 2 6% 4 5% 6% 6% 8% 11% 41%	2470
	21st to end	Day	7% 5% 2 7% 6% 6% 6% 5% 8% 11% 39%	2470
July	1st to 10th	Day	9% 5% 3 9% 7% 7% 6% 6% 8% 10% 30%	4120
	11th to 20th	Day	8% 7% 2 7% 6% 6% 7% 6% 8% 13% 29%	4120
	21st to end	Day	9% 7% 3 9% 6% 7% 7% 6% 9% 11% 26%	4466
August	1st to 10th	Day	23% 7% 3 7% 5% 6% 6% 5% 8% 10% 22%	4104
	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 4 3 3 3 3 4 5% 56%	1887
	11th to 20th	Day	14% 5% 2 5% 4 2 3 3 4 6% 53%	1853
	21st to end	Day	13% 6% 3 5% 4 3 3 4 6% 5% 49%	1710
December	1st to 10th	Day	17% 7% 3 6% 5% 4 4 4 6% 6% 38%	1710
	11th to 20th	Day	15% 4 2 4 3 3 3 3 4 5% 54%	2153
	21st to end	Day	17% 7% 3 5% 4 3 3 3 5% 6% 45%	3124
January	1st to 10th	Night	16% 5% 2 5% 3 3 3 3 4 5% 50%	4109
	11th to 20th	Night	11% 7% 3 6% 4 4 5% 4 5% 6% 45%	4120
	21st to end	Night	16% 9% 3 6% 4 4 3 3 4 5% 44%	4532
February	1st to 10th	Night	16% 6% 2 6% 4 3 4 3 5% 6% 45%	2470
	11th to 20th	Night	18% 7% 2 6% 4 3 4 3 4 5% 43%	2291
	21st to end	Night	15% 6% 2 7% 4 3 4 3 5% 7% 45%	1770
March	1st to 10th	Night	22% 10% 3 7% 5% 3 4 3 4 5% 34%	3418
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 5% 8% 49%	3480
	21st to end	Night	17% 5% 2 5% 3 3 4 4 5% 7% 45%	3817
April	1st to 10th	Night	11% 6% 2 5% 4 3 4 3 5% 8% 51%	2050
	11th to 20th	Night	10% 5% 2 5% 3 5% 3 3 5% 7% 52%	1875
	21st to end	Night	10% 3 1 5% 4 5% 5% 4 6% 7% 50%	1876
May	1st to 10th	Night	10% 7% 2 6% 4 5% 5% 5% 6% 8% 42%	2813
	11th to 20th	Night	14% 7% 3 7% 5% 4 4 3 6% 8% 39%	2812
	21st to end	Night	7% 4 2 7% 5% 4 5% 5% 7% 10% 44%	3079
June	1st to 10th	Night	10% 7% 2 7% 6% 6% 6% 5% 7% 9% 35%	1710
	11th to 20th	Night	9% 6% 2 7% 5% 6% 6% 4 8% 9% 37%	1710
	21st to end	Night	7% 5% 2 7% 6% 5% 6% 6% 9% 11% 35%	1710
July	1st to 10th	Night	10% 7% 3 9% 6% 5% 7% 5% 8% 11% 30%	2840
	11th to 20th	Night	11% 7% 2 8% 6% 5% 6% 5% 9% 12% 28%	2840
	21st to end	Night	13% 9% 3 10% 6% 6% 6% 6% 7% 10% 23%	3087
August	1st to 10th	Night	24% 7% 3 7% 5% 4 5% 5% 7% 9% 24%	2825
	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 2 3 4 5% 55%	2264
	11th to 20th	Night	12% 7% 2 5% 4 3 3 3 4 6% 52%	2322
	21st to end	Night	14% 5% 2 6% 3 3 3 4 6% 6% 49%	2470
December	1st to 10th	Night	17% 8% 2 6% 4 4 4 3 4 6% 42%	2470
	11th to 20th	Night	17% 6% 2 5% 3 3 3 2 4 4 52%	3042
	21st to end	Night	18% 6% 2 6% 4 3 3 3 5% 6% 44%	4532

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	37% 2 5% 25% 25% 7%	2837
	11th to 20th	Day	34% 2 6% 18% 30% 10%	2840
	21st to end	Day	47% 1 3 17% 24% 9%	3124
February	1st to 10th	Day	43% 3 8% 21% 20% 5%	1710
	11th to 20th	Day	42% 3 5% 19% 24% 7%	1779
	21st to end	Day	45% 3 7% 16% 21% 7%	1684
March	1st to 10th	Day	52% 1 5% 18% 15% 9%	3435
	11th to 20th	Day	33% 3 8% 16% 29% 12%	3480
	21st to end	Day	44% 2 7% 21% 22% 5%	3815
April	1st to 10th	Day	30% 3 11% 21% 28% 6%	2128
	11th to 20th	Day	35% 3 10% 22% 21% 10%	2254
	21st to end	Day	33% 3 11% 26% 22% 4	2318
May	1st to 10th	Day	42% 3 18% 24% 13% 2	4120
	11th to 20th	Day	47% 2 11% 22% 17% 1	4085
	21st to end	Day	32% 4 17% 21% 23% 4	4489
June	1st to 10th	Day	40% 5% 18% 25% 11% 2	2470
	11th to 20th	Day	33% 7% 19% 24% 15% 2	2470
	21st to end	Day	36% 7% 17% 23% 14% 2	2470
July	1st to 10th	Day	44% 11% 18% 17% 9% 1	4055
	11th to 20th	Day	41% 12% 27% 15% 4 1	4051
	21st to end	Day	48% 11% 17% 17% 7%	4324
August	1st to 10th	Day	55% 8% 20% 12% 5%	4091
	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	32% 2 7% 18% 29% 11%	1887
	11th to 20th	Day	35% 2 7% 19% 27% 9%	1853
	21st to end	Day	36% 2 8% 20% 24% 9%	1710
December	1st to 10th	Day	47% 3 7% 20% 19% 4	1710
	11th to 20th	Day	34% 2 6% 23% 26% 9%	2150
	21st to end	Day	43% 1 7% 21% 25% 3	3124
January	1st to 10th	Night	38% 2 5% 20% 28% 8%	4109
	11th to 20th	Night	41% 2 7% 17% 25% 7%	4120
	21st to end	Night	45% 2 4 15% 27% 7%	4532
February	1st to 10th	Night	41% 3 7% 19% 22% 8%	2470
	11th to 20th	Night	45% 2 6% 16% 23% 8%	2293
	21st to end	Night	41% 2 8% 19% 21% 9%	1770
March	1st to 10th	Night	55% 2 7% 14% 16% 6%	3418
	11th to 20th	Night	35% 3 7% 14% 24% 17%	3480
	21st to end	Night	40% 2 8% 21% 21% 7%	3817
April	1st to 10th	Night	33% 3 9% 21% 24% 9%	2050
	11th to 20th	Night	33% 2 9% 23% 22% 10%	1875
	21st to end	Night	32% 4 12% 25% 22% 5%	1876
May	1st to 10th	Night	40% 4 16% 25% 13% 3	2840
	11th to 20th	Night	45% 4 11% 23% 17% 1	2813
	21st to end	Night	35% 4 12% 28% 19% 2	3087
June	1st to 10th	Night	45% 6% 14% 23% 11% 2	1710
	11th to 20th	Night	40% 6% 18% 20% 13% 2	1710
	21st to end	Night	38% 11% 18% 20% 12% 3	1710
July	1st to 10th	Night	44% 11% 21% 16% 7% 1	2795
	11th to 20th	Night	45% 12% 25% 12% 5% 1	2786
	21st to end	Night	55% 9% 15% 15% 6% 1	2988
August	1st to 10th	Night	55% 7% 18% 16% 3 1	2814
	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	34% 2 6% 16% 27% 14%	2264
	11th to 20th	Night	36% 2 6% 17% 27% 12%	2322
	21st to end	Night	35% 3 7% 21% 25% 9%	2470
December	1st to 10th	Night	44% 3 9% 17% 20% 6%	2470
	11th to 20th	Night	38% 2 6% 20% 25% 10%	3044
	21st to end	Night	43% 1 6% 21% 25% 4	4532

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% 10% 46% 18% 5%	270
	11th to 20th	Day	16% 14% 39% 23% 9%	270
	21st to end	Day	22% 14% 38% 20% 5% 1	297
February	1st to 10th	Day	19% 16% 33% 24% 8%	270
	11th to 20th	Day	31% 13% 36% 17% 3	270
	21st to end	Day	22% 14% 37% 20% 6% 1	339
March	1st to 10th	Day	20% 12% 30% 26% 12%	460
	11th to 20th	Day	19% 10% 40% 24% 7% 1	460
	21st to end	Day	18% 11% 35% 24% 9% 2	506
April	1st to 10th	Day	15% 15% 32% 23% 15% 1	460
	11th to 20th	Day	11% 8% 32% 26% 17% 6%	452
	21st to end	Day	15% 6% 28% 26% 20% 6%	460
May	1st to 10th	Day	8% 9% 33% 25% 18% 6%	463
	11th to 20th	Day	17% 4 15% 26% 25% 12%	487
	21st to end	Day	13% 4 12% 25% 27% 19%	529
June	1st to 10th	Day	11% 2 12% 33% 27% 14%	490
	11th to 20th	Day	10% 2 12% 29% 25% 21%	490
	21st to end	Day	9% 2 10% 28% 32% 18%	490
July	1st to 10th	Day	9% 1 5% 21% 32% 31%	485
	11th to 20th	Day	11% 3 4 23% 36% 24%	484
	21st to end	Day	8% 2 6% 26% 38% 20%	532
August	1st to 10th	Day	16% 2 8% 23% 29% 22%	486
	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% 30% 19% 5%	467
	11th to 20th	Day	19% 7% 31% 30% 10% 2	413
	21st to end	Day	11% 8% 36% 27% 15% 3	270
December	1st to 10th	Day	22% 10% 33% 25% 9%	270
	11th to 20th	Day	21% 7% 36% 23% 10% 3	270
	21st to end	Day	24% 9% 35% 20% 10% 3	297
January	1st to 10th	Night	20% 10% 35% 27% 7%	760
	11th to 20th	Night	17% 11% 33% 32% 7%	760
	21st to end	Night	18% 9% 36% 33% 3	836
February	1st to 10th	Night	19% 9% 34% 31% 6%	760
	11th to 20th	Night	22% 11% 39% 25% 2	743
	21st to end	Night	18% 13% 35% 30% 4	628
March	1st to 10th	Night	19% 8% 33% 33% 7%	760
	11th to 20th	Night	17% 10% 35% 29% 9% 1	760
	21st to end	Night	16% 8% 35% 34% 7%	836
April	1st to 10th	Night	13% 12% 34% 31% 9% 1	760
	11th to 20th	Night	11% 12% 28% 34% 14% 1	751
	21st to end	Night	12% 7% 27% 41% 13% 1	736
May	1st to 10th	Night	11% 5% 28% 42% 13%	570
	11th to 20th	Night	13% 6% 19% 40% 20% 2	561
	21st to end	Night	11% 3 10% 37% 37% 3	617
June	1st to 10th	Night	13% 2 11% 41% 28% 4	570
	11th to 20th	Night	11% 3 12% 34% 31% 9%	570
	21st to end	Night	8% 2 10% 38% 35% 6%	570
July	1st to 10th	Night	9% 3 5% 33% 44% 6%	555
	11th to 20th	Night	10% 1 4 32% 45% 7%	552
	21st to end	Night	9% 2 5% 35% 43% 6%	624
August	1st to 10th	Night	13% 2 6% 27% 44% 7%	567
	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% 5% 21% 36% 18% 4	755
	11th to 20th	Night	15% 9% 26% 35% 13% 1	760
	21st to end	Night	16% 7% 26% 33% 18% 1	760
December	1st to 10th	Night	19% 8% 28% 37% 8% 1	760
	11th to 20th	Night	19% 7% 26% 37% 10% 1	760
	21st to end	Night	24% 9% 25% 33% 9% 1	836

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