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 2026-06-01.

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% 11% 4 9% 6% 5% 4 4 5% 6% 30%	132415
	Night	16% 12% 4 10% 6% 5% 4 3 5% 6% 28%	130724
Transparency	Day	53% 6% 11% 19% 10% 1	136007
	Night	56% 6% 11% 18% 9% 1	135218
Seeing	Day	25% 17% 29% 18% 8% 3	16453
	Night	24% 15% 28% 23% 8% 1	25912

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	15% 21% 7% 18% 8% 5% 4 2 3 3 13%	4661
	11th to 20th	Day	11% 25% 9% 20% 9% 5% 4 3 3 3 8%	5048
	21st to end	Day	15% 15% 8% 16% 11% 10% 6% 3 3 3 8%	5780
February	1st to 10th	Day	18% 18% 6% 14% 8% 6% 5% 3 3 4 14%	1890
	11th to 20th	Day	14% 18% 8% 17% 9% 5% 5% 3 3 4 15%	1843
	21st to end	Day	20% 16% 5% 12% 9% 4 5% 3 4 5% 17%	1627
March	1st to 10th	Day	17% 14% 6% 11% 6% 4 4 3 3 5% 28%	5042
	11th to 20th	Day	11% 18% 4 9% 5% 5% 5% 3 4 6% 30%	5090
	21st to end	Day	20% 13% 4 9% 5% 4 4 3 5% 5% 28%	5586
April	1st to 10th	Day	18% 15% 4 8% 5% 4 4 3 5% 5% 29%	2509
	11th to 20th	Day	20% 12% 3 7% 5% 4 4 4 4 6% 34%	2507
	21st to end	Day	21% 10% 2 8% 5% 4 4 3 4 5% 34%	2730
May	1st to 10th	Day	22% 10% 3 7% 5% 4 4 3 5% 6% 30%	6035
	11th to 20th	Day	17% 10% 3 9% 5% 4 5% 3 5% 7% 32%	6458
	21st to end	Day	19% 8% 2 6% 3 4 4 3 6% 6% 37%	7103
June	1st to 10th	Day	16% 7% 2 6% 4 4 4 4 7% 8% 38%	2800
	11th to 20th	Day	13% 6% 3 7% 5% 5% 5% 5% 6% 9% 38%	2800
	21st to end	Day	15% 5% 2 6% 5% 4 4 4 7% 8% 40%	2800
July	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 4 7% 10% 43%	6360
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 9% 40%	6360
	21st to end	Day	9% 5% 2 6% 5% 4 5% 4 7% 8% 46%	6524
August	1st to 10th	Day	18% 6% 2 6% 4 4 5% 4 6% 8% 38%	4234
	11th to 20th	Day	14% 6% 2 7% 5% 5% 6% 5% 7% 9% 34%	4174
	21st to end	Day	12% 5% 2 6% 4 4 4 4 6% 9% 43%	4420
September	1st to 10th	Day	14% 7% 2 6% 4 4 4 4 6% 8% 43%	2400
	11th to 20th	Day	12% 5% 2 5% 4 5% 5% 4 6% 8% 44%	2330
	21st to end	Day	15% 8% 3 7% 5% 5% 6% 4 6% 8% 33%	2130
October	1st to 10th	Day	20% 6% 2 7% 5% 4 4 3 5% 7% 36%	2000
	11th to 20th	Day	20% 11% 4 9% 5% 4 4 4 6% 7% 28%	2000
	21st to end	Day	26% 13% 4 9% 5% 5% 4 3 4 5% 23%	2200
November	1st to 10th	Day	15% 13% 3 8% 6% 5% 4 4 5% 7% 30%	1820
	11th to 20th	Day	14% 16% 4 10% 5% 4 4 4 5% 7% 27%	1680
	21st to end	Day	20% 19% 5% 10% 6% 5% 4 3 4 6% 19%	1680
December	1st to 10th	Day	19% 21% 7% 11% 6% 5% 5% 4 5% 5% 12%	2680
	11th to 20th	Day	22% 18% 6% 11% 6% 4 4 4 4 4 19%	3066
	21st to end	Day	23% 20% 7% 13% 7% 5% 4 3 3 4 11%	4048
January	1st to 10th	Night	15% 19% 7% 18% 7% 5% 4 2 3 4 14%	8234
	11th to 20th	Night	12% 20% 9% 19% 8% 6% 5% 3 4 4 11%	7822
	21st to end	Night	16% 16% 7% 16% 10% 8% 5% 3 4 4 11%	8401
February	1st to 10th	Night	17% 18% 5% 13% 7% 6% 5% 2 4 4 19%	2730
	11th to 20th	Night	15% 15% 6% 12% 7% 5% 6% 3 4 5% 19%	2663
	21st to end	Night	16% 17% 6% 14% 6% 5% 5% 4 4 6% 18%	2174
March	1st to 10th	Night	19% 12% 5% 10% 7% 5% 4 3 4 4 28%	5018
	11th to 20th	Night	14% 14% 3 8% 4 3 4 4 5% 7% 33%	5089
	21st to end	Night	19% 10% 4 9% 5% 4 4 3 5% 6% 32%	5588
April	1st to 10th	Night	20% 13% 4 9% 6% 4 4 3 4 4 29%	2111
	11th to 20th	Night	18% 9% 3 7% 5% 4 5% 4 5% 6% 35%	2075
	21st to end	Night	17% 9% 4 7% 4 5% 5% 3 5% 7% 35%	1895
May	1st to 10th	Night	25% 11% 4 8% 4 4 4 3 4 5% 29%	4125
	11th to 20th	Night	13% 7% 3 8% 5% 6% 4 4 7% 8% 35%	3655
	21st to end	Night	19% 7% 3 6% 4 3 4 4 5% 8% 37%	4014
June	1st to 10th	Night	12% 6% 3 7% 5% 7% 6% 6% 7% 8% 34%	1600
	11th to 20th	Night	11% 6% 2 6% 5% 5% 6% 4 6% 8% 41%	1600
	21st to end	Night	13% 7% 2 6% 5% 5% 5% 4 6% 8% 41%	1600
July	1st to 10th	Night	10% 6% 3 7% 4 4 4 3 6% 9% 43%	3600
	11th to 20th	Night	10% 5% 3 7% 5% 4 4 4 5% 9% 45%	3600
	21st to end	Night	10% 5% 2 5% 3 4 4 4 6% 7% 50%	4325
August	1st to 10th	Night	10% 6% 2 7% 4 4 4 4 7% 9% 42%	2915
	11th to 20th	Night	14% 7% 2 7% 4 4 5% 3 5% 8% 40%	2875
	21st to end	Night	9% 5% 2 5% 5% 4 5% 4 7% 10% 45%	3444
September	1st to 10th	Night	12% 7% 2 6% 4 3 5% 4 6% 6% 46%	2000
	11th to 20th	Night	12% 5% 2 5% 3 3 4 3 5% 7% 51%	2067
	21st to end	Night	17% 7% 3 6% 4 3 5% 4 6% 7% 37%	2270
October	1st to 10th	Night	16% 7% 3 7% 4 4 4 3 5% 7% 40%	2400
	11th to 20th	Night	21% 10% 3 8% 4 5% 4 3 5% 6% 31%	2400
	21st to end	Night	26% 10% 3 7% 5% 4 4 3 5% 5% 27%	2640
November	1st to 10th	Night	16% 11% 4 8% 4 4 4 3 5% 6% 36%	2696
	11th to 20th	Night	16% 13% 4 9% 6% 6% 4 3 6% 7% 27%	2940
	21st to end	Night	18% 16% 5% 9% 5% 5% 5% 5% 5% 6% 20%	2940
December	1st to 10th	Night	17% 22% 7% 12% 6% 5% 5% 3 5% 5% 13%	4720
	11th to 20th	Night	19% 21% 5% 10% 5% 5% 4 3 4 4 19%	5348
	21st to end	Night	25% 18% 6% 11% 6% 6% 4 3 4 4 13%	7150

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	76% 5% 9% 7% 3	4662
	11th to 20th	Day	82% 4 4 5% 4 1	5082
	21st to end	Day	81% 6% 8% 4 2	5780
February	1st to 10th	Day	74% 4 10% 11% 1	1890
	11th to 20th	Day	73% 6% 7% 11% 2	1852
	21st to end	Day	71% 5% 9% 10% 5%	1627
March	1st to 10th	Day	60% 4 12% 16% 8%	5045
	11th to 20th	Day	54% 6% 16% 15% 8% 1	5090
	21st to end	Day	57% 5% 8% 21% 9% 1	5586
April	1st to 10th	Day	58% 5% 9% 18% 10% 1	2509
	11th to 20th	Day	53% 4 9% 21% 13% 2	2507
	21st to end	Day	53% 4 9% 19% 12% 2	2730
May	1st to 10th	Day	54% 5% 10% 17% 10% 4	6044
	11th to 20th	Day	51% 6% 10% 19% 13% 1	6462
	21st to end	Day	47% 5% 11% 22% 13% 2	7103
June	1st to 10th	Day	43% 5% 13% 21% 16% 2	2800
	11th to 20th	Day	42% 6% 14% 24% 14% 1	2800
	21st to end	Day	40% 6% 13% 25% 15% 1	2800
July	1st to 10th	Day	35% 7% 12% 27% 17% 1	6290
	11th to 20th	Day	37% 7% 14% 28% 14% 1	6285
	21st to end	Day	33% 7% 13% 30% 16%	6337
August	1st to 10th	Day	43% 7% 14% 22% 13% 1	4221
	11th to 20th	Day	42% 8% 15% 25% 8%	4213
	21st to end	Day	36% 5% 12% 34% 11% 1	4420
September	1st to 10th	Day	39% 6% 13% 29% 12% 1	2400
	11th to 20th	Day	37% 6% 13% 29% 14% 1	2330
	21st to end	Day	49% 5% 13% 23% 9% 1	2130
October	1st to 10th	Day	39% 6% 15% 25% 13% 1	3260
	11th to 20th	Day	48% 3 15% 20% 13%	3240
	21st to end	Day	51% 6% 14% 21% 7% 1	3538
November	1st to 10th	Day	55% 6% 11% 19% 9% 1	1820
	11th to 20th	Day	58% 6% 13% 14% 8% 1	1680
	21st to end	Day	67% 5% 12% 12% 4	1680
December	1st to 10th	Day	72% 7% 13% 7% 1	2680
	11th to 20th	Day	68% 7% 8% 9% 7% 1	3066
	21st to end	Day	78% 5% 9% 7% 2	4048
January	1st to 10th	Night	74% 6% 9% 9% 2	8234
	11th to 20th	Night	77% 6% 7% 7% 4	7863
	21st to end	Night	75% 6% 7% 8% 4	8401
February	1st to 10th	Night	70% 4 9% 13% 4	2730
	11th to 20th	Night	66% 6% 10% 13% 4	2665
	21st to end	Night	66% 4 12% 13% 6%	2174
March	1st to 10th	Night	59% 4 11% 17% 8%	5028
	11th to 20th	Night	50% 5% 15% 19% 8% 2	5090
	21st to end	Night	52% 6% 8% 20% 13% 1	5588
April	1st to 10th	Night	59% 4 8% 18% 10% 1	2111
	11th to 20th	Night	51% 5% 9% 21% 12% 2	2075
	21st to end	Night	49% 5% 10% 20% 14% 2	1895
May	1st to 10th	Night	57% 5% 8% 14% 11% 4	4132
	11th to 20th	Night	45% 7% 14% 21% 12% 1	3656
	21st to end	Night	46% 6% 10% 21% 16% 1	4015
June	1st to 10th	Night	44% 6% 10% 22% 16% 2	1600
	11th to 20th	Night	40% 6% 13% 22% 18% 2	1600
	21st to end	Night	41% 6% 14% 21% 17% 2	1600
July	1st to 10th	Night	37% 6% 13% 25% 18% 2	3560
	11th to 20th	Night	36% 6% 13% 28% 16% 1	3552
	21st to end	Night	32% 6% 14% 25% 22% 1	4197
August	1st to 10th	Night	37% 6% 15% 27% 13% 1	2904
	11th to 20th	Night	41% 6% 14% 28% 11%	2902
	21st to end	Night	32% 7% 14% 32% 13% 1	3444
September	1st to 10th	Night	37% 6% 14% 28% 14% 1	2000
	11th to 20th	Night	33% 5% 11% 33% 18% 1	2067
	21st to end	Night	44% 6% 13% 23% 12% 1	2270
October	1st to 10th	Night	41% 6% 9% 26% 18% 1	3920
	11th to 20th	Night	49% 4 13% 21% 12% 1	3899
	21st to end	Night	45% 6% 15% 25% 9% 1	4252
November	1st to 10th	Night	50% 4 10% 24% 11% 2	2696
	11th to 20th	Night	57% 5% 12% 16% 8% 1	2940
	21st to end	Night	63% 7% 13% 14% 3	2940
December	1st to 10th	Night	71% 8% 11% 8% 2	4720
	11th to 20th	Night	69% 6% 7% 11% 7% 1	5348
	21st to end	Night	74% 5% 9% 9% 3	7150

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	32% 32% 23% 9% 3 1	220
	11th to 20th	Day	33% 30% 26% 9% 2 1	294
	21st to end	Day	31% 26% 29% 12% 2	363
February	1st to 10th	Day	35% 28% 29% 7% 1	330
	11th to 20th	Day	33% 33% 28% 5% 1	330
	21st to end	Day	42% 23% 26% 7% 1	338
March	1st to 10th	Day	28% 25% 30% 13% 4	540
	11th to 20th	Day	34% 21% 31% 11% 3 1	540
	21st to end	Day	33% 22% 26% 14% 3 1	594
April	1st to 10th	Day	31% 21% 30% 13% 4 1	540
	11th to 20th	Day	36% 18% 24% 15% 5% 2	532
	21st to end	Day	29% 20% 31% 13% 6% 1	540
May	1st to 10th	Day	28% 16% 31% 17% 5% 3	543
	11th to 20th	Day	22% 15% 29% 21% 10% 3	567
	21st to end	Day	19% 16% 32% 20% 10% 3	617
June	1st to 10th	Day	20% 18% 34% 20% 6% 1	530
	11th to 20th	Day	16% 16% 33% 24% 10% 2	530
	21st to end	Day	19% 13% 34% 23% 8% 2	530
July	1st to 10th	Day	11% 8% 30% 30% 17% 4	525
	11th to 20th	Day	16% 9% 30% 27% 15% 4	524
	21st to end	Day	13% 8% 35% 29% 12% 3	576
August	1st to 10th	Day	14% 7% 27% 29% 16% 7%	526
	11th to 20th	Day	13% 7% 29% 29% 16% 7%	519
	21st to end	Day	14% 9% 30% 25% 14% 7%	583
September	1st to 10th	Day	15% 11% 27% 24% 15% 8%	530
	11th to 20th	Day	12% 8% 23% 27% 20% 11%	530
	21st to end	Day	19% 13% 30% 21% 11% 5%	530
October	1st to 10th	Day	24% 13% 25% 21% 12% 4	530
	11th to 20th	Day	28% 20% 30% 16% 5% 1	530
	21st to end	Day	37% 17% 27% 14% 4	583
November	1st to 10th	Day	31% 20% 31% 12% 5% 1	367
	11th to 20th	Day	32% 25% 29% 8% 4 2	220
	21st to end	Day	38% 24% 29% 9%	220
December	1st to 10th	Day	33% 24% 30% 10% 3	220
	11th to 20th	Day	35% 27% 28% 8% 2	220
	21st to end	Day	38% 30% 22% 7% 2 1	242
January	1st to 10th	Night	32% 24% 29% 11% 3	840
	11th to 20th	Night	34% 25% 26% 13% 2	840
	21st to end	Night	26% 22% 31% 17% 4	924
February	1st to 10th	Night	29% 28% 26% 15% 2	840
	11th to 20th	Night	27% 27% 29% 16% 2	823
	21st to end	Night	29% 24% 30% 15% 1	692
March	1st to 10th	Night	27% 19% 33% 17% 4	840
	11th to 20th	Night	29% 19% 29% 18% 4 1	840
	21st to end	Night	26% 15% 32% 20% 6% 1	924
April	1st to 10th	Night	28% 18% 32% 20% 3	840
	11th to 20th	Night	25% 17% 31% 20% 6% 1	831
	21st to end	Night	23% 11% 34% 25% 7%	635
May	1st to 10th	Night	23% 10% 27% 26% 12% 2	625
	11th to 20th	Night	16% 8% 35% 34% 6%	414
	21st to end	Night	14% 11% 28% 34% 12% 1	455
June	1st to 10th	Night	15% 8% 36% 33% 9%	400
	11th to 20th	Night	14% 8% 31% 36% 12%	400
	21st to end	Night	14% 8% 35% 36% 7%	400
July	1st to 10th	Night	11% 4 27% 40% 16% 1	390
	11th to 20th	Night	11% 5% 29% 36% 17% 2	388
	21st to end	Night	12% 2 29% 39% 17% 2	606
August	1st to 10th	Night	10% 3 21% 44% 20% 2	597
	11th to 20th	Night	11% 4 21% 36% 26% 2	594
	21st to end	Night	13% 5% 26% 34% 21% 2	790
September	1st to 10th	Night	15% 8% 23% 35% 17% 2	800
	11th to 20th	Night	15% 4 20% 32% 25% 5%	800
	21st to end	Night	21% 9% 25% 28% 14% 3	800
October	1st to 10th	Night	21% 8% 29% 25% 16% 2	800
	11th to 20th	Night	29% 15% 27% 22% 6% 1	800
	21st to end	Night	33% 14% 28% 21% 4	880
November	1st to 10th	Night	25% 16% 29% 22% 8%	820
	11th to 20th	Night	27% 18% 29% 18% 7% 1	840
	21st to end	Night	29% 23% 28% 16% 5%	840
December	1st to 10th	Night	35% 23% 23% 14% 4	840
	11th to 20th	Night	29% 27% 27% 14% 3	840
	21st to end	Night	36% 20% 28% 14% 2	924

Page generated at 2026-05-29 01:53:27. Version ac07

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