M & T Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the M & T Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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% 12% 5% 8% 7% 3 5% 5% 5% 7% 28%	74694
	Night	15% 13% 5% 9% 7% 3 5% 4 5% 6% 28%	75104
Transparency	Day	56% 6% 11% 17% 9% 1	74410
	Night	57% 6% 11% 16% 8% 1	74922
Seeing	Day	26% 18% 28% 17% 9% 3	13496
	Night	25% 16% 26% 22% 9% 1	23047

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% 18% 10% 13% 8% 4 5% 4 4 4 16%	1517
	11th to 20th	Day	14% 19% 8% 15% 10% 4 4 3 4 4 16%	1631
	21st to end	Day	10% 17% 10% 15% 13% 4 5% 5% 4 4 12%	1782
February	1st to 10th	Day	17% 18% 8% 11% 10% 5% 5% 4 3 4 15%	1620
	11th to 20th	Day	12% 15% 9% 14% 9% 4 5% 4 5% 5% 18%	1573
	21st to end	Day	19% 15% 5% 11% 9% 4 4 3 4 6% 21%	1417
March	1st to 10th	Day	16% 13% 7% 11% 7% 4 4 4 4 5% 25%	1977
	11th to 20th	Day	15% 15% 8% 8% 7% 3 4 4 4 6% 27%	1980
	21st to end	Day	19% 13% 6% 9% 7% 2 4 4 4 5% 28%	2178
April	1st to 10th	Day	15% 15% 6% 8% 5% 2 4 5% 5% 6% 29%	2157
	11th to 20th	Day	19% 13% 3 5% 5% 2 5% 5% 5% 6% 32%	2134
	21st to end	Day	21% 10% 3 7% 6% 3 4 5% 4 6% 31%	2302
May	1st to 10th	Day	20% 10% 4 8% 6% 4 5% 4 5% 6% 28%	2340
	11th to 20th	Day	20% 9% 3 8% 6% 3 4 4 4 7% 31%	2460
	21st to end	Day	13% 7% 3 5% 6% 4 6% 5% 5% 8% 38%	2725
June	1st to 10th	Day	16% 7% 3 7% 5% 3 5% 5% 5% 8% 35%	2520
	11th to 20th	Day	15% 7% 3 6% 6% 4 5% 6% 6% 9% 33%	2520
	21st to end	Day	15% 7% 3 5% 6% 3 5% 5% 7% 8% 36%	2520
July	1st to 10th	Day	11% 7% 2 6% 6% 4 7% 5% 7% 8% 37%	2520
	11th to 20th	Day	12% 7% 3 6% 6% 4 6% 6% 7% 10% 32%	2520
	21st to end	Day	12% 7% 3 6% 7% 3 5% 7% 6% 9% 33%	2558
August	1st to 10th	Day	11% 6% 3 7% 7% 3 5% 7% 8% 9% 35%	2327
	11th to 20th	Day	13% 8% 3 7% 7% 5% 7% 6% 8% 9% 29%	2303
	21st to end	Day	15% 6% 4 8% 6% 4 5% 5% 7% 8% 32%	2404
September	1st to 10th	Day	19% 7% 3 6% 5% 3 5% 5% 5% 7% 36%	2160
	11th to 20th	Day	15% 6% 3 6% 5% 3 5% 6% 4 7% 40%	2116
	21st to end	Day	18% 8% 3 7% 6% 3 5% 6% 6% 6% 32%	1904
October	1st to 10th	Day	18% 9% 4 7% 5% 4 5% 5% 4 8% 31%	1800
	11th to 20th	Day	22% 11% 4 7% 7% 3 4 3 5% 6% 29%	1800
	21st to end	Day	24% 17% 3 7% 6% 3 4 4 4 5% 23%	1980
November	1st to 10th	Day	13% 14% 4 7% 6% 3 5% 4 5% 6% 32%	1711
	11th to 20th	Day	16% 17% 6% 8% 5% 3 4 5% 4 7% 23%	1520
	21st to end	Day	18% 20% 6% 10% 7% 3 5% 4 4 5% 18%	1520
December	1st to 10th	Day	17% 21% 10% 15% 7% 3 3 4 5% 4 12%	1520
	11th to 20th	Day	15% 19% 8% 15% 7% 3 4 4 5% 4 16%	1906
	21st to end	Day	22% 24% 8% 11% 7% 3 3 3 3 4 12%	2772
January	1st to 10th	Night	14% 18% 10% 13% 9% 4 6% 4 5% 4 14%	2649
	11th to 20th	Night	15% 18% 9% 13% 9% 4 4 4 4 5% 16%	2446
	21st to end	Night	11% 17% 9% 13% 11% 5% 6% 4 5% 4 14%	2574
February	1st to 10th	Night	14% 17% 8% 12% 11% 4 4 4 4 5% 16%	2340
	11th to 20th	Night	13% 14% 8% 14% 10% 4 7% 5% 5% 5% 16%	2273
	21st to end	Night	17% 14% 7% 14% 8% 3 4 4 4 4 21%	1837
March	1st to 10th	Night	12% 13% 7% 12% 8% 5% 4 4 5% 5% 24%	1970
	11th to 20th	Night	16% 14% 7% 10% 6% 4 4 3 4 5% 28%	1979
	21st to end	Night	14% 15% 6% 8% 7% 3 4 4 5% 6% 28%	2178
April	1st to 10th	Night	18% 12% 6% 8% 5% 3 4 5% 4 7% 28%	1801
	11th to 20th	Night	18% 14% 3 6% 4 3 3 4 4 6% 36%	1775
	21st to end	Night	16% 12% 3 7% 5% 3 5% 4 5% 7% 33%	1674
May	1st to 10th	Night	17% 10% 2 7% 7% 4 4 5% 5% 7% 33%	1620
	11th to 20th	Night	13% 9% 3 6% 7% 3 6% 5% 5% 6% 37%	1433
	21st to end	Night	12% 6% 3 5% 6% 3 3 4 6% 8% 44%	1550
June	1st to 10th	Night	14% 6% 3 6% 6% 3 4 5% 7% 9% 37%	1440
	11th to 20th	Night	13% 6% 2 7% 5% 3 5% 5% 5% 10% 40%	1440
	21st to end	Night	12% 6% 3 5% 5% 3 5% 4 6% 8% 43%	1440
July	1st to 10th	Night	7% 7% 2 5% 6% 3 4 4 6% 9% 46%	1440
	11th to 20th	Night	9% 6% 3 5% 4 3 4 4 5% 7% 48%	1440
	21st to end	Night	11% 5% 3 5% 5% 4 5% 5% 6% 9% 41%	1757
August	1st to 10th	Night	7% 5% 3 6% 5% 4 5% 6% 6% 8% 44%	1611
	11th to 20th	Night	13% 6% 3 7% 5% 3 4 6% 7% 9% 37%	1598
	21st to end	Night	12% 6% 3 6% 6% 3 4 5% 6% 8% 42%	1932
September	1st to 10th	Night	16% 6% 2 5% 5% 3 4 5% 5% 7% 42%	1800
	11th to 20th	Night	16% 5% 3 5% 4 2 3 4 4 7% 47%	1842
	21st to end	Night	17% 8% 3 5% 6% 4 5% 4 5% 7% 35%	2055
October	1st to 10th	Night	17% 9% 2 6% 6% 3 4 4 5% 8% 37%	2160
	11th to 20th	Night	20% 10% 3 7% 6% 3 4 4 5% 7% 31%	2160
	21st to end	Night	26% 13% 3 6% 5% 3 4 4 5% 6% 25%	2376
November	1st to 10th	Night	14% 13% 4 6% 6% 2 4 3 5% 7% 36%	2362
	11th to 20th	Night	16% 16% 5% 9% 6% 3 5% 4 6% 6% 24%	2660
	21st to end	Night	16% 17% 8% 10% 6% 3 5% 5% 4 5% 20%	2660
December	1st to 10th	Night	19% 19% 10% 13% 7% 3 5% 4 4 4 12%	2660
	11th to 20th	Night	17% 17% 8% 15% 8% 3 5% 4 4 4 15%	3288
	21st to end	Night	22% 20% 9% 11% 7% 3 4 4 5% 4 10%	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	72% 6% 8% 11% 4	1518
	11th to 20th	Day	75% 4 8% 8% 5%	1631
	21st to end	Day	75% 5% 7% 7% 5% 1	1782
February	1st to 10th	Day	76% 4 9% 8% 3	1620
	11th to 20th	Day	70% 4 6% 14% 5%	1582
	21st to end	Day	66% 5% 10% 14% 4 2	1417
March	1st to 10th	Day	62% 5% 10% 17% 6% 1	1980
	11th to 20th	Day	59% 5% 9% 16% 8% 2	1980
	21st to end	Day	60% 5% 6% 17% 10% 2	2178
April	1st to 10th	Day	56% 5% 11% 16% 11% 1	2157
	11th to 20th	Day	52% 6% 11% 18% 12% 1	2134
	21st to end	Day	55% 5% 10% 17% 12% 1	2302
May	1st to 10th	Day	55% 7% 9% 17% 9% 2	2340
	11th to 20th	Day	54% 6% 10% 18% 11% 1	2464
	21st to end	Day	43% 9% 15% 21% 11% 1	2725
June	1st to 10th	Day	47% 7% 12% 20% 13% 1	2520
	11th to 20th	Day	47% 8% 12% 22% 11%	2520
	21st to end	Day	45% 7% 13% 23% 12%	2520
July	1st to 10th	Day	43% 9% 10% 24% 13% 1	2450
	11th to 20th	Day	44% 9% 15% 21% 11%	2445
	21st to end	Day	46% 8% 14% 20% 12% 1	2415
August	1st to 10th	Day	41% 10% 15% 23% 10%	2314
	11th to 20th	Day	48% 11% 14% 19% 8% 1	2303
	21st to end	Day	47% 8% 11% 21% 11% 1	2404
September	1st to 10th	Day	47% 7% 10% 22% 13% 1	2160
	11th to 20th	Day	43% 6% 11% 24% 15% 1	2116
	21st to end	Day	50% 7% 10% 20% 12% 1	1904
October	1st to 10th	Day	52% 7% 12% 16% 12% 1	1800
	11th to 20th	Day	57% 6% 13% 17% 7% 1	1800
	21st to end	Day	65% 5% 8% 14% 7% 2	1980
November	1st to 10th	Day	53% 6% 11% 17% 11% 2	1711
	11th to 20th	Day	62% 5% 11% 15% 6% 2	1520
	21st to end	Day	69% 5% 8% 13% 4 1	1520
December	1st to 10th	Day	76% 5% 8% 8% 3	1520
	11th to 20th	Day	71% 5% 11% 9% 2	1906
	21st to end	Day	78% 4 8% 9% 1	2772
January	1st to 10th	Night	74% 4 8% 8% 5%	2649
	11th to 20th	Night	72% 5% 10% 8% 3 1	2446
	21st to end	Night	74% 5% 8% 9% 5% 1	2574
February	1st to 10th	Night	72% 5% 9% 10% 4	2340
	11th to 20th	Night	70% 5% 8% 11% 5%	2275
	21st to end	Night	67% 5% 7% 12% 7% 1	1837
March	1st to 10th	Night	63% 6% 8% 18% 5% 1	1980
	11th to 20th	Night	60% 5% 10% 16% 9% 1	1980
	21st to end	Night	58% 5% 8% 14% 12% 2	2178
April	1st to 10th	Night	58% 5% 10% 17% 10%	1801
	11th to 20th	Night	51% 5% 12% 17% 13% 2	1775
	21st to end	Night	51% 7% 10% 19% 12% 2	1674
May	1st to 10th	Night	51% 7% 10% 19% 10% 2	1620
	11th to 20th	Night	47% 8% 10% 22% 12% 1	1434
	21st to end	Night	38% 9% 14% 24% 13% 2	1551
June	1st to 10th	Night	43% 7% 13% 23% 14% 1	1440
	11th to 20th	Night	40% 8% 15% 22% 13% 2	1440
	21st to end	Night	39% 9% 13% 26% 13% 1	1440
July	1st to 10th	Night	36% 9% 13% 23% 18% 1	1400
	11th to 20th	Night	35% 8% 14% 26% 16% 1	1392
	21st to end	Night	38% 9% 16% 22% 15% 1	1659
August	1st to 10th	Night	37% 8% 16% 29% 9% 1	1600
	11th to 20th	Night	40% 11% 15% 25% 9%	1598
	21st to end	Night	39% 8% 14% 25% 12% 2	1932
September	1st to 10th	Night	40% 8% 12% 25% 12% 3	1800
	11th to 20th	Night	38% 6% 13% 25% 16% 1	1842
	21st to end	Night	48% 7% 13% 19% 12% 1	2055
October	1st to 10th	Night	45% 8% 12% 19% 14% 1	2160
	11th to 20th	Night	54% 6% 13% 17% 10% 1	2160
	21st to end	Night	62% 5% 10% 15% 7% 1	2376
November	1st to 10th	Night	49% 6% 12% 20% 10% 3	2362
	11th to 20th	Night	60% 6% 10% 15% 7% 2	2660
	21st to end	Night	66% 5% 11% 13% 3 1	2660
December	1st to 10th	Night	77% 5% 7% 8% 2	2660
	11th to 20th	Night	74% 5% 9% 10% 2	3288
	21st to end	Night	78% 5% 8% 8% 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% 29% 28% 8% 4	180
	11th to 20th	Day	34% 32% 25% 5% 3	239
	21st to end	Day	30% 30% 29% 8% 2	264
February	1st to 10th	Day	40% 31% 26% 3	240
	11th to 20th	Day	36% 34% 28% 2	240
	21st to end	Day	42% 30% 21% 7% 1	283
March	1st to 10th	Day	37% 23% 26% 10% 4	420
	11th to 20th	Day	30% 24% 30% 11% 3 2	420
	21st to end	Day	35% 21% 26% 13% 4 1	462
April	1st to 10th	Day	32% 24% 24% 15% 5% 1	420
	11th to 20th	Day	31% 21% 26% 11% 8% 3	412
	21st to end	Day	30% 21% 28% 15% 5% 1	420
May	1st to 10th	Day	31% 19% 26% 14% 7% 3	423
	11th to 20th	Day	27% 17% 29% 16% 8% 3	447
	21st to end	Day	17% 16% 26% 23% 12% 5%	485
June	1st to 10th	Day	22% 17% 30% 20% 9% 2	450
	11th to 20th	Day	20% 15% 27% 25% 12% 2	450
	21st to end	Day	18% 13% 36% 22% 8% 2	450
July	1st to 10th	Day	11% 9% 29% 27% 18% 5%	445
	11th to 20th	Day	13% 10% 28% 28% 16% 5%	444
	21st to end	Day	15% 9% 35% 24% 13% 4	488
August	1st to 10th	Day	12% 8% 26% 28% 16% 9%	446
	11th to 20th	Day	17% 9% 28% 23% 15% 8%	439
	21st to end	Day	15% 12% 29% 23% 13% 9%	495
September	1st to 10th	Day	20% 12% 25% 21% 15% 8%	450
	11th to 20th	Day	15% 9% 24% 23% 19% 11%	450
	21st to end	Day	22% 14% 27% 19% 13% 4	450
October	1st to 10th	Day	22% 15% 26% 19% 14% 5%	450
	11th to 20th	Day	31% 20% 27% 14% 7% 1	450
	21st to end	Day	39% 16% 26% 16% 2	495
November	1st to 10th	Day	25% 19% 29% 16% 9% 2	371
	11th to 20th	Day	41% 19% 24% 11% 3 1	180
	21st to end	Day	35% 26% 29% 8% 1 1	180
December	1st to 10th	Day	38% 29% 23% 4 3 2	180
	11th to 20th	Day	38% 27% 27% 7% 2	180
	21st to end	Day	40% 24% 30% 5% 1	198
January	1st to 10th	Night	32% 25% 29% 12% 3	760
	11th to 20th	Night	31% 24% 28% 15% 2	744
	21st to end	Night	25% 28% 30% 15% 2	792
February	1st to 10th	Night	31% 26% 29% 14% 1	720
	11th to 20th	Night	29% 29% 30% 10% 1	703
	21st to end	Night	33% 25% 26% 14% 2	592
March	1st to 10th	Night	26% 24% 32% 15% 2	720
	11th to 20th	Night	32% 22% 23% 18% 4	720
	21st to end	Night	25% 20% 31% 17% 6% 1	792
April	1st to 10th	Night	28% 19% 29% 20% 3	720
	11th to 20th	Night	29% 15% 28% 20% 7% 1	711
	21st to end	Night	24% 11% 32% 26% 7%	594
May	1st to 10th	Night	24% 10% 21% 31% 13%	540
	11th to 20th	Night	17% 12% 29% 34% 9%	372
	21st to end	Night	16% 7% 26% 30% 20% 1	389
June	1st to 10th	Night	16% 7% 27% 36% 13% 1	360
	11th to 20th	Night	16% 11% 23% 37% 13%	360
	21st to end	Night	10% 7% 32% 36% 14%	360
July	1st to 10th	Night	10% 4 21% 43% 19% 3	350
	11th to 20th	Night	10% 3 24% 39% 23% 1	348
	21st to end	Night	14% 4 20% 39% 21% 1	577
August	1st to 10th	Night	12% 4 21% 36% 26% 2	537
	11th to 20th	Night	16% 4 16% 32% 30% 2	534
	21st to end	Night	14% 5% 21% 35% 22% 3	744
September	1st to 10th	Night	19% 8% 21% 30% 20% 3	720
	11th to 20th	Night	18% 8% 18% 27% 26% 4	720
	21st to end	Night	21% 11% 19% 26% 19% 4	720
October	1st to 10th	Night	22% 11% 24% 24% 16% 3	720
	11th to 20th	Night	28% 15% 28% 23% 6%	720
	21st to end	Night	35% 15% 28% 18% 4	792
November	1st to 10th	Night	22% 16% 30% 24% 7%	740
	11th to 20th	Night	30% 19% 28% 15% 6% 2	760
	21st to end	Night	30% 22% 28% 15% 5%	760
December	1st to 10th	Night	35% 25% 25% 11% 3	760
	11th to 20th	Night	31% 24% 28% 13% 3	760
	21st to end	Night	33% 25% 28% 13% 1	836

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