Mattatuck Astronomical Society Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mattatuck Astronomical Society 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	17% 7% 2 6% 4 4 4 4 6% 7% 37%	125642
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 40%	122399
Transparency	Day	45% 5% 11% 22% 15% 2	129158
	Night	44% 5% 12% 22% 16% 3	126970
Seeing	Day	24% 18% 31% 18% 7% 2	15906
	Night	22% 16% 32% 23% 7% 1	24908

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	10% 9% 3 9% 5% 5% 4 4 4 5% 42%	3597
	11th to 20th	Day	14% 9% 3 9% 5% 5% 4 3 5% 5% 38%	3570
	21st to end	Day	17% 10% 4 7% 4 4 4 3 4 5% 38%	4098
February	1st to 10th	Day	16% 8% 3 8% 5% 6% 5% 3 6% 6% 35%	1840
	11th to 20th	Day	14% 7% 3 7% 5% 5% 4 4 5% 6% 39%	1948
	21st to end	Day	19% 10% 3 6% 4 5% 4 3 5% 5% 37%	1650
March	1st to 10th	Day	21% 9% 2 7% 4 3 4 3 5% 5% 37%	4476
	11th to 20th	Day	14% 6% 2 7% 5% 5% 6% 4 6% 7% 39%	4658
	21st to end	Day	20% 8% 2 6% 4 5% 4 3 5% 7% 36%	5970
April	1st to 10th	Day	20% 9% 2 7% 5% 4 4 4 5% 6% 35%	2400
	11th to 20th	Day	18% 7% 2 6% 4 5% 4 4 5% 6% 39%	2374
	21st to end	Day	20% 6% 2 6% 4 4 4 3 5% 6% 39%	2500
May	1st to 10th	Day	32% 8% 3 6% 4 4 4 3 5% 8% 25%	5886
	11th to 20th	Day	20% 8% 3 7% 4 4 5% 4 6% 6% 33%	6220
	21st to end	Day	21% 7% 2 6% 4 4 5% 4 6% 8% 33%	6949
June	1st to 10th	Day	18% 6% 2 6% 5% 5% 6% 5% 7% 8% 33%	2800
	11th to 20th	Day	18% 6% 2 6% 5% 5% 4 4 7% 8% 35%	2800
	21st to end	Day	14% 7% 2 7% 5% 5% 5% 4 6% 9% 36%	2800
July	1st to 10th	Day	13% 7% 2 7% 5% 5% 5% 5% 7% 9% 35%	6360
	11th to 20th	Day	12% 5% 2 7% 5% 6% 6% 5% 7% 11% 34%	6360
	21st to end	Day	16% 6% 2 6% 4 5% 5% 4 5% 7% 39%	6926
August	1st to 10th	Day	20% 7% 2 7% 5% 5% 5% 4 6% 8% 31%	4362
	11th to 20th	Day	13% 6% 3 6% 4 5% 5% 4 7% 9% 38%	3732
	21st to end	Day	10% 5% 2 6% 4 5% 4 4 7% 9% 43%	4180
September	1st to 10th	Day	14% 7% 3 6% 4 5% 4 4 5% 6% 42%	2280
	11th to 20th	Day	13% 6% 3 5% 4 3 3 3 5% 6% 50%	2162
	21st to end	Day	18% 6% 2 6% 4 4 4 4 5% 7% 40%	2090
October	1st to 10th	Day	22% 7% 2 4 3 4 4 3 4 5% 40%	2036
	11th to 20th	Day	14% 7% 2 6% 3 3 5% 4 5% 7% 44%	1895
	21st to end	Day	21% 8% 2 5% 4 4 4 3 5% 6% 38%	1881
November	1st to 10th	Day	14% 6% 2 6% 4 4 2 2 5% 7% 48%	1761
	11th to 20th	Day	18% 7% 1 4 3 3 3 3 5% 7% 46%	1800
	21st to end	Day	18% 9% 2 5% 4 3 3 2 5% 6% 42%	1735
December	1st to 10th	Day	16% 8% 3 6% 4 4 3 3 4 6% 43%	2600
	11th to 20th	Day	21% 8% 2 5% 3 3 3 3 5% 5% 42%	2986
	21st to end	Day	18% 10% 3 8% 4 4 4 3 4 6% 36%	3960
January	1st to 10th	Night	13% 6% 2 6% 4 4 4 4 6% 7% 44%	6349
	11th to 20th	Night	13% 8% 3 8% 5% 4 4 3 5% 6% 39%	6315
	21st to end	Night	17% 10% 3 6% 4 5% 5% 3 4 5% 38%	6841
February	1st to 10th	Night	16% 9% 3 6% 4 4 5% 3 5% 6% 38%	2560
	11th to 20th	Night	14% 9% 3 6% 5% 4 5% 4 6% 6% 39%	2338
	21st to end	Night	18% 9% 2 7% 4 3 4 3 5% 5% 39%	1980
March	1st to 10th	Night	19% 9% 2 6% 3 3 4 3 4 6% 39%	5364
	11th to 20th	Night	15% 8% 3 7% 4 5% 4 4 5% 6% 40%	5269
	21st to end	Night	20% 10% 3 6% 3 4 3 3 5% 6% 38%	4962
April	1st to 10th	Night	19% 9% 3 7% 5% 4 4 4 6% 7% 34%	2000
	11th to 20th	Night	19% 7% 2 6% 5% 5% 4 4 4 6% 38%	1975
	21st to end	Night	19% 7% 2 6% 4 4 3 3 5% 5% 42%	1900
May	1st to 10th	Night	32% 10% 2 6% 4 3 4 3 4 7% 26%	4054
	11th to 20th	Night	21% 10% 3 8% 5% 4 4 3 5% 6% 32%	3715
	21st to end	Night	22% 8% 3 6% 4 3 4 4 5% 7% 34%	3926
June	1st to 10th	Night	19% 7% 2 6% 4 4 3 4 4 6% 42%	1600
	11th to 20th	Night	17% 6% 2 6% 4 4 4 3 8% 9% 36%	1600
	21st to end	Night	14% 6% 3 5% 4 3 5% 4 6% 9% 42%	1600
July	1st to 10th	Night	14% 7% 3 6% 4 4 4 4 6% 8% 40%	3600
	11th to 20th	Night	17% 9% 3 7% 4 4 4 4 7% 9% 33%	3600
	21st to end	Night	13% 7% 2 6% 3 3 4 3 4 6% 47%	3926
August	1st to 10th	Night	23% 7% 3 6% 4 5% 5% 4 5% 7% 31%	2624
	11th to 20th	Night	16% 6% 3 7% 4 5% 4 4 5% 7% 41%	3091
	21st to end	Night	11% 5% 2 4 3 3 3 3 5% 7% 53%	3476
September	1st to 10th	Night	16% 7% 2 6% 4 4 3 2 4 7% 45%	1900
	11th to 20th	Night	13% 5% 2 5% 2 3 3 3 5% 6% 53%	2017
	21st to end	Night	18% 6% 3 6% 3 3 4 3 4 6% 44%	2090
October	1st to 10th	Night	20% 7% 2 6% 4 4 2 3 4 7% 41%	2141
	11th to 20th	Night	14% 7% 2 6% 4 4 3 3 4 6% 47%	2285
	21st to end	Night	23% 6% 2 5% 3 3 4 3 5% 6% 39%	2717
November	1st to 10th	Night	13% 7% 2 5% 3 3 3 4 5% 5% 48%	2537
	11th to 20th	Night	18% 7% 2 6% 3 3 4 3 4 6% 44%	2600
	21st to end	Night	15% 7% 2 6% 4 3 4 3 5% 7% 44%	2663
December	1st to 10th	Night	16% 8% 2 6% 3 4 3 3 4 6% 45%	4580
	11th to 20th	Night	24% 8% 2 5% 3 3 4 3 4 4 39%	5208
	21st to end	Night	18% 10% 4 7% 4 3 3 3 4 5% 38%	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	43% 4 7% 29% 14% 3	3598
	11th to 20th	Day	46% 4 10% 21% 12% 7%	3600
	21st to end	Day	48% 4 8% 23% 16% 1	4098
February	1st to 10th	Day	48% 5% 13% 19% 14% 1	1840
	11th to 20th	Day	44% 6% 10% 23% 14% 3	1957
	21st to end	Day	49% 4 10% 21% 13% 3	1650
March	1st to 10th	Day	49% 3 10% 16% 13% 9%	4479
	11th to 20th	Day	40% 5% 15% 29% 9% 2	4658
	21st to end	Day	48% 3 8% 14% 21% 6%	5970
April	1st to 10th	Day	48% 6% 10% 19% 14% 3	2400
	11th to 20th	Day	43% 6% 10% 21% 17% 2	2374
	21st to end	Day	45% 5% 9% 21% 16% 4	2500
May	1st to 10th	Day	58% 5% 13% 14% 9% 1	5897
	11th to 20th	Day	51% 5% 12% 19% 12% 1	6224
	21st to end	Day	47% 6% 11% 22% 12% 1	6949
June	1st to 10th	Day	45% 7% 12% 22% 12% 1	2800
	11th to 20th	Day	44% 7% 14% 22% 12% 1	2800
	21st to end	Day	43% 8% 13% 22% 13% 1	2800
July	1st to 10th	Day	46% 6% 11% 20% 15% 1	6290
	11th to 20th	Day	44% 4 16% 24% 11% 1	6285
	21st to end	Day	44% 4 11% 23% 17% 1	6728
August	1st to 10th	Day	51% 5% 14% 20% 9%	4348
	11th to 20th	Day	41% 5% 15% 29% 10% 1	3768
	21st to end	Day	38% 5% 11% 29% 15% 1	4180
September	1st to 10th	Day	41% 6% 10% 23% 18% 1	2280
	11th to 20th	Day	34% 6% 10% 24% 24% 2	2162
	21st to end	Day	43% 5% 11% 21% 18% 3	2090
October	1st to 10th	Day	42% 3 7% 19% 25% 4	3387
	11th to 20th	Day	30% 3 9% 25% 29% 4	3123
	21st to end	Day	34% 3 14% 24% 21% 4	3081
November	1st to 10th	Day	37% 5% 13% 24% 19% 2	1761
	11th to 20th	Day	39% 4 11% 24% 19% 4	1800
	21st to end	Day	44% 4 13% 26% 11% 2	1735
December	1st to 10th	Day	42% 3 16% 28% 11% 1	2600
	11th to 20th	Day	43% 4 14% 20% 17% 2	2986
	21st to end	Day	49% 4 7% 20% 14% 5%	3960
January	1st to 10th	Night	40% 3 9% 27% 18% 2	6349
	11th to 20th	Night	44% 4 13% 19% 14% 5%	6360
	21st to end	Night	48% 3 11% 17% 17% 4	6841
February	1st to 10th	Night	46% 5% 13% 23% 11% 2	2560
	11th to 20th	Night	44% 6% 11% 19% 17% 4	2340
	21st to end	Night	47% 4 10% 22% 14% 5%	1980
March	1st to 10th	Night	47% 4 11% 18% 16% 6%	5374
	11th to 20th	Night	43% 6% 15% 21% 11% 4	5270
	21st to end	Night	48% 3 6% 14% 24% 4	4962
April	1st to 10th	Night	47% 6% 10% 21% 13% 3	2000
	11th to 20th	Night	45% 6% 9% 19% 19% 1	1975
	21st to end	Night	44% 5% 9% 22% 16% 3	1900
May	1st to 10th	Night	58% 6% 14% 11% 10% 1	4060
	11th to 20th	Night	52% 6% 12% 16% 11% 2	3716
	21st to end	Night	49% 6% 13% 21% 10% 1	3927
June	1st to 10th	Night	45% 6% 10% 23% 15% 1	1600
	11th to 20th	Night	43% 7% 15% 24% 10% 1	1600
	21st to end	Night	38% 8% 13% 25% 15% 1	1600
July	1st to 10th	Night	42% 6% 14% 21% 15% 1	3560
	11th to 20th	Night	47% 5% 16% 21% 10% 1	3552
	21st to end	Night	40% 3 12% 25% 18% 1	3809
August	1st to 10th	Night	52% 7% 14% 18% 9% 1	2614
	11th to 20th	Night	43% 6% 13% 29% 9%	3121
	21st to end	Night	31% 5% 12% 35% 16% 1	3476
September	1st to 10th	Night	40% 6% 10% 24% 19% 1	1900
	11th to 20th	Night	33% 6% 10% 25% 25% 2	2017
	21st to end	Night	41% 6% 9% 23% 20% 2	2090
October	1st to 10th	Night	41% 4 13% 20% 21% 2	3570
	11th to 20th	Night	29% 4 8% 27% 28% 5%	3796
	21st to end	Night	34% 3 14% 29% 18% 2	4467
November	1st to 10th	Night	37% 6% 11% 21% 21% 4	2537
	11th to 20th	Night	42% 4 10% 22% 19% 3	2600
	21st to end	Night	41% 4 14% 29% 11% 1	2663
December	1st to 10th	Night	42% 3 12% 32% 11% 1	4580
	11th to 20th	Night	48% 4 13% 17% 17% 2	5208
	21st to end	Night	49% 3 8% 19% 16% 5%	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	33% 26% 28% 13% 2	200
	11th to 20th	Day	30% 32% 28% 9% 2	200
	21st to end	Day	24% 32% 31% 8% 4 1	250
February	1st to 10th	Day	32% 29% 30% 9% 1	340
	11th to 20th	Day	31% 27% 31% 10% 1	496
	21st to end	Day	33% 22% 31% 11% 2	414
March	1st to 10th	Day	28% 25% 33% 12% 2	500
	11th to 20th	Day	28% 23% 36% 10% 3 1	500
	21st to end	Day	34% 21% 33% 8% 4 1	550
April	1st to 10th	Day	33% 25% 26% 11% 5% 1	500
	11th to 20th	Day	32% 23% 29% 11% 4 1	492
	21st to end	Day	29% 21% 33% 12% 4 1	500
May	1st to 10th	Day	35% 18% 27% 12% 6% 1	503
	11th to 20th	Day	23% 21% 35% 15% 6%	527
	21st to end	Day	23% 14% 32% 21% 8% 2	573
June	1st to 10th	Day	20% 14% 33% 22% 9% 2	530
	11th to 20th	Day	20% 15% 34% 23% 8% 1	530
	21st to end	Day	18% 15% 32% 26% 7% 2	530
July	1st to 10th	Day	16% 10% 34% 25% 13% 2	525
	11th to 20th	Day	13% 8% 32% 30% 14% 2	524
	21st to end	Day	14% 11% 37% 28% 8% 2	576
August	1st to 10th	Day	14% 9% 31% 31% 10% 4	502
	11th to 20th	Day	15% 10% 31% 28% 12% 5%	479
	21st to end	Day	12% 12% 32% 24% 15% 5%	539
September	1st to 10th	Day	20% 12% 26% 22% 15% 5%	490
	11th to 20th	Day	16% 12% 27% 24% 12% 9%	490
	21st to end	Day	23% 12% 33% 20% 8% 5%	490
October	1st to 10th	Day	28% 12% 28% 17% 11% 4	490
	11th to 20th	Day	18% 20% 30% 23% 6% 2	487
	21st to end	Day	28% 19% 26% 19% 7% 1	429
November	1st to 10th	Day	21% 19% 30% 20% 9% 1	395
	11th to 20th	Day	30% 23% 27% 16% 4	400
	21st to end	Day	29% 20% 30% 17% 4	335
December	1st to 10th	Day	26% 25% 31% 14% 5%	200
	11th to 20th	Day	36% 25% 28% 10% 2	200
	21st to end	Day	27% 26% 27% 15% 5%	220
January	1st to 10th	Night	24% 22% 38% 14% 3	800
	11th to 20th	Night	27% 22% 33% 18% 2	800
	21st to end	Night	21% 22% 38% 17% 2	880
February	1st to 10th	Night	24% 21% 35% 19% 2	800
	11th to 20th	Night	25% 24% 36% 13% 1	783
	21st to end	Night	28% 22% 33% 16% 2	660
March	1st to 10th	Night	24% 22% 35% 16% 3	800
	11th to 20th	Night	22% 23% 34% 18% 2	800
	21st to end	Night	26% 22% 32% 16% 4	880
April	1st to 10th	Night	27% 20% 33% 17% 2	800
	11th to 20th	Night	25% 20% 34% 17% 4	791
	21st to end	Night	23% 21% 35% 17% 4	700
May	1st to 10th	Night	29% 16% 30% 21% 4	600
	11th to 20th	Night	24% 14% 38% 20% 4	456
	21st to end	Night	22% 9% 30% 32% 6% 1	433
June	1st to 10th	Night	20% 8% 33% 31% 8%	400
	11th to 20th	Night	20% 8% 31% 32% 9%	400
	21st to end	Night	17% 5% 31% 37% 10%	400
July	1st to 10th	Night	16% 5% 26% 36% 17%	390
	11th to 20th	Night	16% 6% 23% 41% 13% 1	388
	21st to end	Night	13% 6% 30% 39% 12%	438
August	1st to 10th	Night	12% 6% 29% 40% 12% 1	433
	11th to 20th	Night	15% 4 27% 36% 17% 1	744
	21st to end	Night	15% 5% 25% 35% 19% 1	836
September	1st to 10th	Night	19% 6% 30% 31% 12% 2	760
	11th to 20th	Night	16% 7% 27% 30% 18% 2	760
	21st to end	Night	22% 8% 28% 26% 14% 2	760
October	1st to 10th	Night	24% 10% 28% 26% 11% 2	760
	11th to 20th	Night	19% 14% 32% 25% 8% 1	760
	21st to end	Night	28% 14% 32% 20% 6%	836
November	1st to 10th	Night	17% 17% 34% 23% 8% 1	780
	11th to 20th	Night	24% 18% 34% 18% 6% 1	800
	21st to end	Night	24% 19% 31% 21% 5% 1	800
December	1st to 10th	Night	26% 19% 32% 19% 5% 1	800
	11th to 20th	Night	25% 21% 31% 19% 3	800
	21st to end	Night	27% 20% 35% 17% 2	880

Page generated at 2025-08-02 01:32:26. Version ac07

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