Mason Bald Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mason Bald Observatory clear sky chart from 2005-11-06 to 2025-02-06.

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	15% 10% 3 8% 5% 5% 5% 4 6% 7% 35%	108625
	Night	16% 10% 3 8% 5% 4 5% 4 5% 7% 34%	109605
Transparency	Day	46% 7% 13% 22% 11% 1	112227
	Night	48% 7% 12% 20% 11% 1	114167
Seeing	Day	20% 15% 31% 21% 10% 3	14694
	Night	20% 12% 31% 26% 10% 1	24081

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% 13% 4 8% 5% 4 4 4 6% 7% 32%	3597
	11th to 20th	Day	10% 19% 7% 14% 7% 6% 4 3 5% 4 20%	3592
	21st to end	Day	15% 18% 7% 13% 6% 5% 5% 3 5% 5% 20%	4466
February	1st to 10th	Day	13% 16% 4 11% 7% 5% 5% 4 6% 5% 25%	1755
	11th to 20th	Day	12% 14% 5% 12% 7% 6% 4 3 5% 5% 28%	1663
	21st to end	Day	19% 13% 4 10% 6% 6% 4 4 4 6% 27%	1511
March	1st to 10th	Day	21% 12% 4 7% 5% 4 3 3 4 6% 31%	3306
	11th to 20th	Day	10% 15% 4 9% 6% 5% 5% 3 5% 6% 32%	3480
	21st to end	Day	14% 8% 2 6% 4 3 4 3 5% 6% 45%	3898
April	1st to 10th	Day	19% 12% 3 7% 6% 4 3 3 5% 5% 32%	2280
	11th to 20th	Day	21% 8% 3 6% 4 4 4 2 4 6% 38%	2254
	21st to end	Day	20% 9% 3 8% 6% 4 5% 3 5% 6% 33%	2437
May	1st to 10th	Day	18% 9% 3 7% 4 5% 4 4 5% 6% 34%	4299
	11th to 20th	Day	15% 9% 3 8% 4 5% 5% 4 5% 8% 33%	4398
	21st to end	Day	12% 5% 2 6% 5% 4 5% 4 6% 9% 43%	4837
June	1st to 10th	Day	17% 8% 2 7% 4 5% 5% 5% 7% 8% 33%	2660
	11th to 20th	Day	12% 6% 2 5% 5% 5% 5% 5% 7% 10% 39%	2660
	21st to end	Day	15% 8% 3 6% 4 4 4 5% 7% 8% 37%	2660
July	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 5% 6% 10% 41%	4440
	11th to 20th	Day	11% 4 2 5% 4 4 5% 4 7% 10% 45%	4440
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 8% 9% 42%	4770
August	1st to 10th	Day	15% 7% 3 6% 4 4 5% 4 5% 8% 38%	4104
	11th to 20th	Day	13% 7% 3 6% 4 4 5% 4 7% 9% 38%	4044
	21st to end	Day	10% 5% 2 6% 4 4 5% 5% 7% 9% 42%	4224
September	1st to 10th	Day	12% 6% 3 5% 4 4 4 4 6% 8% 44%	2280
	11th to 20th	Day	11% 4 2 5% 4 4 4 3 6% 8% 50%	2280
	21st to end	Day	15% 7% 2 7% 4 4 4 4 5% 7% 41%	1986
October	1st to 10th	Day	17% 8% 3 8% 4 4 6% 4 5% 7% 35%	1900
	11th to 20th	Day	19% 9% 3 7% 5% 5% 5% 4 6% 6% 32%	1900
	21st to end	Day	23% 10% 3 7% 5% 5% 4 4 5% 6% 27%	2052
November	1st to 10th	Day	14% 9% 3 8% 5% 5% 5% 3 6% 7% 35%	1706
	11th to 20th	Day	15% 11% 3 9% 5% 4 5% 5% 6% 6% 33%	1600
	21st to end	Day	20% 12% 4 8% 6% 4 6% 4 5% 6% 26%	1600
December	1st to 10th	Day	18% 16% 5% 11% 6% 5% 5% 4 4 7% 21%	2600
	11th to 20th	Day	17% 17% 6% 9% 4 5% 4 3 4 4 27%	2986
	21st to end	Day	25% 13% 3 9% 5% 5% 4 3 4 5% 24%	3960
January	1st to 10th	Night	15% 12% 7% 11% 6% 6% 5% 4 5% 5% 25%	6349
	11th to 20th	Night	12% 13% 6% 13% 8% 7% 6% 5% 6% 6% 18%	6291
	21st to end	Night	16% 14% 6% 11% 5% 5% 5% 4 5% 6% 24%	6490
February	1st to 10th	Night	14% 15% 4 10% 6% 5% 5% 4 5% 6% 26%	2536
	11th to 20th	Night	14% 13% 5% 9% 5% 5% 5% 4 6% 6% 28%	2403
	21st to end	Night	16% 10% 4 8% 5% 4 5% 4 6% 7% 31%	1941
March	1st to 10th	Night	19% 10% 3 8% 5% 5% 4 3 5% 5% 32%	3532
	11th to 20th	Night	15% 13% 3 9% 5% 5% 4 4 6% 6% 30%	3479
	21st to end	Night	16% 8% 2 5% 3 3 3 3 5% 6% 45%	3733
April	1st to 10th	Night	19% 10% 3 7% 4 4 4 3 5% 6% 35%	1900
	11th to 20th	Night	18% 8% 2 8% 4 4 4 3 5% 6% 37%	1875
	21st to end	Night	19% 7% 2 7% 4 4 4 4 6% 6% 35%	1758
May	1st to 10th	Night	20% 9% 3 6% 4 5% 5% 4 5% 6% 35%	2641
	11th to 20th	Night	15% 8% 3 7% 4 4 4 4 6% 8% 37%	2495
	21st to end	Night	10% 6% 2 5% 4 4 4 4 6% 7% 47%	2738
June	1st to 10th	Night	13% 6% 2 7% 5% 5% 5% 3 6% 8% 38%	1520
	11th to 20th	Night	10% 5% 2 7% 5% 4 4 5% 6% 8% 43%	1520
	21st to end	Night	12% 8% 3 7% 5% 4 4 4 6% 6% 42%	1520
July	1st to 10th	Night	8% 5% 2 7% 5% 5% 6% 4 7% 8% 44%	2520
	11th to 20th	Night	11% 6% 3 4 4 4 4 4 5% 7% 49%	2520
	21st to end	Night	9% 5% 1 5% 4 4 4 4 6% 8% 50%	2768
August	1st to 10th	Night	10% 6% 2 6% 4 4 4 3 6% 8% 46%	2825
	11th to 20th	Night	12% 6% 2 5% 3 3 4 4 5% 8% 46%	2785
	21st to end	Night	8% 4 2 5% 4 4 5% 5% 7% 10% 47%	3399
September	1st to 10th	Night	11% 6% 2 6% 4 4 4 4 6% 8% 46%	1900
	11th to 20th	Night	11% 4 1 5% 3 3 4 3 5% 7% 53%	1900
	21st to end	Night	16% 6% 2 6% 4 4 4 3 4 6% 43%	2189
October	1st to 10th	Night	16% 6% 3 7% 4 4 4 4 6% 7% 38%	2280
	11th to 20th	Night	20% 8% 3 8% 5% 5% 4 3 6% 6% 33%	2280
	21st to end	Night	21% 9% 4 8% 5% 4 5% 3 4 7% 30%	2546
November	1st to 10th	Night	18% 7% 3 5% 5% 4 4 4 5% 7% 39%	2588
	11th to 20th	Night	17% 10% 3 6% 5% 5% 5% 4 6% 7% 32%	2800
	21st to end	Night	21% 10% 3 7% 5% 5% 5% 4 6% 7% 27%	2800
December	1st to 10th	Night	19% 14% 4 8% 5% 4 5% 4 6% 7% 23%	4580
	11th to 20th	Night	18% 14% 4 10% 4 4 4 4 5% 6% 26%	5208
	21st to end	Night	26% 15% 4 9% 4 4 4 3 4 5% 21%	6996

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	50% 6% 13% 21% 10% 1	3598
	11th to 20th	Day	66% 6% 10% 10% 7% 1	3622
	21st to end	Day	66% 5% 9% 10% 10% 1	4466
February	1st to 10th	Day	60% 7% 12% 16% 5% 1	1755
	11th to 20th	Day	57% 6% 11% 15% 10% 1	1672
	21st to end	Day	59% 6% 11% 14% 9% 1	1511
March	1st to 10th	Day	53% 6% 13% 18% 9% 1	3309
	11th to 20th	Day	51% 6% 17% 15% 10% 2	3480
	21st to end	Day	40% 6% 9% 26% 17% 3	3898
April	1st to 10th	Day	52% 5% 10% 19% 10% 3	2280
	11th to 20th	Day	48% 4 10% 22% 16% 1	2254
	21st to end	Day	52% 6% 10% 18% 13% 2	2437
May	1st to 10th	Day	47% 7% 11% 21% 12% 2	4311
	11th to 20th	Day	46% 9% 14% 20% 10% 1	4402
	21st to end	Day	35% 8% 14% 25% 15% 3	4837
June	1st to 10th	Day	45% 8% 14% 18% 13% 1	2660
	11th to 20th	Day	38% 8% 14% 24% 14% 2	2660
	21st to end	Day	41% 8% 14% 22% 13% 1	2660
July	1st to 10th	Day	36% 6% 15% 29% 14% 1	4370
	11th to 20th	Day	32% 7% 16% 35% 10%	4365
	21st to end	Day	34% 9% 16% 29% 12%	4618
August	1st to 10th	Day	43% 7% 15% 23% 12% 1	4091
	11th to 20th	Day	39% 7% 16% 28% 9%	4083
	21st to end	Day	36% 7% 13% 33% 10% 1	4224
September	1st to 10th	Day	36% 8% 15% 29% 12% 1	2280
	11th to 20th	Day	31% 6% 15% 32% 15% 1	2280
	21st to end	Day	42% 8% 13% 28% 9%	1986
October	1st to 10th	Day	39% 6% 13% 29% 12% 1	3160
	11th to 20th	Day	40% 7% 17% 24% 11% 1	3140
	21st to end	Day	49% 6% 14% 21% 9%	3366
November	1st to 10th	Day	46% 6% 17% 20% 10% 1	1706
	11th to 20th	Day	49% 8% 15% 17% 10% 1	1600
	21st to end	Day	57% 9% 13% 13% 7%	1600
December	1st to 10th	Day	64% 5% 16% 10% 4	2600
	11th to 20th	Day	61% 7% 9% 14% 9%	2986
	21st to end	Day	63% 6% 9% 15% 8%	3960
January	1st to 10th	Night	59% 6% 11% 15% 8%	6349
	11th to 20th	Night	63% 8% 12% 11% 5% 1	6336
	21st to end	Night	60% 6% 11% 11% 11% 1	6490
February	1st to 10th	Night	59% 6% 12% 17% 5% 1	2536
	11th to 20th	Night	54% 6% 12% 19% 9%	2405
	21st to end	Night	50% 7% 13% 21% 9% 1	1941
March	1st to 10th	Night	53% 6% 10% 21% 10% 1	3542
	11th to 20th	Night	51% 8% 14% 17% 10% 1	3480
	21st to end	Night	39% 4 10% 25% 19% 2	3733
April	1st to 10th	Night	49% 5% 10% 21% 13% 2	1900
	11th to 20th	Night	47% 6% 10% 18% 17% 1	1875
	21st to end	Night	46% 7% 12% 17% 16% 3	1758
May	1st to 10th	Night	48% 8% 11% 16% 15% 2	2648
	11th to 20th	Night	44% 7% 14% 23% 10% 2	2496
	21st to end	Night	34% 8% 14% 20% 21% 2	2739
June	1st to 10th	Night	42% 7% 12% 23% 14% 2	1520
	11th to 20th	Night	35% 7% 14% 21% 20% 3	1520
	21st to end	Night	41% 7% 13% 23% 14% 2	1520
July	1st to 10th	Night	35% 7% 14% 24% 20% 1	2480
	11th to 20th	Night	34% 6% 12% 33% 14%	2472
	21st to end	Night	31% 7% 16% 33% 11% 2	2680
August	1st to 10th	Night	36% 5% 13% 30% 15% 2	2814
	11th to 20th	Night	35% 7% 17% 28% 12% 1	2812
	21st to end	Night	30% 8% 13% 37% 12% 1	3399
September	1st to 10th	Night	34% 7% 16% 27% 14% 1	1900
	11th to 20th	Night	30% 8% 13% 32% 17% 1	1900
	21st to end	Night	42% 6% 11% 27% 13% 1	2189
October	1st to 10th	Night	35% 6% 10% 28% 21% 1	3800
	11th to 20th	Night	42% 5% 15% 29% 9%	3779
	21st to end	Night	45% 5% 14% 26% 9%	4182
November	1st to 10th	Night	44% 7% 13% 23% 12% 1	2588
	11th to 20th	Night	49% 7% 13% 19% 10% 2	2800
	21st to end	Night	53% 10% 16% 16% 5% 1	2800
December	1st to 10th	Night	58% 7% 14% 17% 4	4580
	11th to 20th	Night	58% 7% 12% 13% 10% 1	5208
	21st to end	Night	65% 6% 10% 11% 9%	6996

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% 24% 33% 13% 7% 1	200
	11th to 20th	Day	27% 22% 32% 15% 3 1	205
	21st to end	Day	27% 24% 30% 15% 4 1	330
February	1st to 10th	Day	28% 28% 31% 12% 2	285
	11th to 20th	Day	25% 26% 39% 9% 1	270
	21st to end	Day	33% 22% 34% 10% 1	323
March	1st to 10th	Day	23% 19% 37% 16% 4	460
	11th to 20th	Day	24% 22% 32% 18% 3	460
	21st to end	Day	30% 18% 32% 13% 6% 1	506
April	1st to 10th	Day	26% 22% 32% 12% 7% 1	460
	11th to 20th	Day	28% 22% 23% 15% 8% 3	452
	21st to end	Day	26% 20% 32% 15% 6% 2	460
May	1st to 10th	Day	24% 14% 32% 19% 8% 4	463
	11th to 20th	Day	17% 12% 34% 24% 10% 3	487
	21st to end	Day	14% 13% 31% 26% 12% 4	529
June	1st to 10th	Day	18% 14% 33% 25% 9% 2	490
	11th to 20th	Day	12% 13% 33% 26% 13% 3	490
	21st to end	Day	19% 11% 31% 25% 11% 3	490
July	1st to 10th	Day	9% 9% 28% 32% 18% 5%	485
	11th to 20th	Day	14% 8% 29% 27% 17% 5%	484
	21st to end	Day	9% 8% 33% 33% 12% 4	532
August	1st to 10th	Day	13% 6% 30% 29% 16% 7%	486
	11th to 20th	Day	12% 6% 26% 30% 20% 6%	479
	21st to end	Day	12% 8% 31% 27% 14% 8%	539
September	1st to 10th	Day	12% 9% 30% 28% 13% 8%	490
	11th to 20th	Day	9% 8% 26% 25% 18% 14%	490
	21st to end	Day	15% 13% 28% 24% 15% 5%	490
October	1st to 10th	Day	20% 17% 26% 18% 14% 6%	490
	11th to 20th	Day	22% 18% 32% 18% 8% 2	490
	21st to end	Day	29% 18% 28% 21% 4	519
November	1st to 10th	Day	25% 17% 26% 22% 9% 1	340
	11th to 20th	Day	23% 22% 36% 16% 4 1	200
	21st to end	Day	24% 24% 33% 15% 4 1	200
December	1st to 10th	Day	29% 16% 35% 15% 6%	200
	11th to 20th	Day	31% 25% 31% 12% 2 1	200
	21st to end	Day	35% 17% 31% 13% 3	220
January	1st to 10th	Night	26% 19% 32% 19% 4	800
	11th to 20th	Night	26% 17% 32% 20% 5%	800
	21st to end	Night	23% 15% 35% 23% 5%	880
February	1st to 10th	Night	22% 18% 38% 19% 2	781
	11th to 20th	Night	21% 19% 36% 18% 4	743
	21st to end	Night	24% 19% 34% 20% 3	628
March	1st to 10th	Night	21% 16% 36% 24% 3	760
	11th to 20th	Night	23% 18% 32% 22% 4 1	760
	21st to end	Night	21% 14% 33% 24% 7% 1	836
April	1st to 10th	Night	23% 13% 36% 24% 3	760
	11th to 20th	Night	22% 16% 32% 22% 9% 1	751
	21st to end	Night	22% 12% 31% 25% 10% 1	618
May	1st to 10th	Night	22% 7% 29% 27% 13% 1	456
	11th to 20th	Night	14% 7% 35% 34% 10%	374
	21st to end	Night	11% 6% 34% 34% 15% 1	411
June	1st to 10th	Night	15% 4 36% 32% 13% 1	380
	11th to 20th	Night	11% 7% 29% 36% 17% 1	380
	21st to end	Night	14% 6% 28% 37% 14% 1	380
July	1st to 10th	Night	6% 6% 27% 42% 18% 1	370
	11th to 20th	Night	10% 2 26% 38% 22% 1	368
	21st to end	Night	10% 2 27% 44% 17% 1	433
August	1st to 10th	Night	11% 4 18% 40% 25% 3	567
	11th to 20th	Night	9% 5% 20% 37% 25% 3	564
	21st to end	Night	9% 4 27% 36% 22% 2	785
September	1st to 10th	Night	13% 5% 28% 32% 19% 3	760
	11th to 20th	Night	14% 4 20% 29% 27% 6%	760
	21st to end	Night	19% 8% 21% 30% 19% 4	760
October	1st to 10th	Night	19% 9% 29% 24% 16% 3	760
	11th to 20th	Night	23% 13% 29% 25% 9% 1	760
	21st to end	Night	25% 14% 32% 23% 6%	836
November	1st to 10th	Night	22% 15% 25% 28% 9% 1	780
	11th to 20th	Night	23% 16% 33% 22% 5% 1	800
	21st to end	Night	28% 16% 31% 19% 7%	800
December	1st to 10th	Night	29% 17% 32% 19% 4	800
	11th to 20th	Night	25% 19% 34% 19% 4	800
	21st to end	Night	34% 13% 33% 17% 3	880

Page generated at 2025-02-03 03:05:50. Version ac07

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