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 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	16% 11% 4 9% 5% 5% 4 4 5% 7% 30%	108637
	Night	16% 13% 4 10% 5% 5% 4 3 5% 6% 28%	109599
Transparency	Day	53% 6% 12% 19% 10% 1	112274
	Night	55% 6% 11% 18% 9% 1	114125
Seeing	Day	24% 17% 29% 19% 8% 3	14736
	Night	24% 15% 28% 23% 9% 1	24254

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% 22% 8% 20% 8% 5% 4 2 3 3 13%	3597
	11th to 20th	Day	12% 24% 9% 19% 10% 5% 4 3 3 4 8%	3870
	21st to end	Day	19% 17% 9% 15% 8% 7% 6% 3 4 3 10%	4466
February	1st to 10th	Day	19% 18% 5% 14% 8% 6% 5% 3 3 4 15%	1755
	11th to 20th	Day	13% 18% 8% 17% 9% 5% 4 3 3 4 16%	1663
	21st to end	Day	20% 15% 5% 13% 9% 4 5% 3 4 5% 17%	1477
March	1st to 10th	Day	19% 13% 5% 10% 6% 5% 4 3 3 4 28%	3432
	11th to 20th	Day	11% 22% 4 9% 4 5% 5% 3 4 5% 27%	3480
	21st to end	Day	17% 12% 3 9% 5% 4 4 3 6% 7% 31%	3815
April	1st to 10th	Day	18% 15% 4 7% 5% 4 3 3 5% 5% 30%	2270
	11th to 20th	Day	20% 10% 3 7% 4 4 3 3 4 6% 35%	2265
	21st to end	Day	21% 11% 3 8% 5% 4 4 3 4 5% 33%	2470
May	1st to 10th	Day	20% 10% 3 7% 5% 5% 4 3 5% 7% 33%	4125
	11th to 20th	Day	16% 10% 3 8% 5% 4 6% 4 6% 8% 30%	4398
	21st to end	Day	12% 6% 2 6% 3 4 4 4 6% 7% 46%	4837
June	1st to 10th	Day	17% 6% 2 6% 4 4 4 4 6% 8% 38%	2660
	11th to 20th	Day	13% 5% 2 6% 5% 5% 5% 5% 6% 9% 39%	2660
	21st to end	Day	15% 5% 2 6% 5% 4 4 4 7% 8% 40%	2660
July	1st to 10th	Day	10% 4 2 6% 4 5% 5% 4 6% 10% 44%	4440
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 5% 7% 9% 40%	4440
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 9% 41%	4538
August	1st to 10th	Day	18% 6% 2 6% 4 4 5% 4 5% 8% 38%	4104
	11th to 20th	Day	14% 5% 2 7% 5% 5% 6% 5% 7% 10% 34%	4044
	21st to end	Day	12% 5% 2 6% 4 4 4 4 6% 9% 44%	4285
September	1st to 10th	Day	14% 7% 2 6% 4 4 4 4 6% 8% 42%	2280
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 4 6% 8% 43%	2214
	21st to end	Day	15% 8% 3 7% 5% 5% 6% 4 6% 8% 33%	2024
October	1st to 10th	Day	21% 7% 3 7% 5% 4 4 3 5% 7% 33%	1900
	11th to 20th	Day	20% 11% 4 8% 5% 4 3 4 6% 6% 28%	1900
	21st to end	Day	26% 14% 4 9% 5% 4 4 3 4 5% 22%	2090
November	1st to 10th	Day	15% 12% 3 8% 6% 5% 4 4 5% 7% 30%	1732
	11th to 20th	Day	14% 17% 4 10% 5% 4 4 4 5% 7% 26%	1600
	21st to end	Day	20% 18% 4 10% 6% 5% 4 3 4 6% 20%	1600
December	1st to 10th	Day	19% 21% 7% 11% 6% 5% 5% 4 5% 5% 12%	2600
	11th to 20th	Day	22% 18% 6% 10% 6% 4 4 3 4 4 19%	2986
	21st to end	Day	23% 20% 7% 13% 7% 5% 4 3 3 4 11%	3960
January	1st to 10th	Night	15% 18% 6% 19% 7% 5% 4 3 3 4 14%	6349
	11th to 20th	Night	11% 19% 10% 19% 8% 6% 5% 3 4 4 11%	6000
	21st to end	Night	20% 18% 7% 17% 7% 5% 4 3 4 4 12%	6490
February	1st to 10th	Night	18% 19% 6% 13% 7% 5% 5% 2 4 4 18%	2536
	11th to 20th	Night	15% 15% 6% 12% 8% 5% 6% 3 4 5% 20%	2403
	21st to end	Night	16% 17% 6% 13% 6% 4 4 4 4 6% 19%	1973
March	1st to 10th	Night	22% 12% 3 9% 6% 4 3 3 4 4 31%	3408
	11th to 20th	Night	18% 17% 3 8% 5% 3 4 4 4 6% 27%	3479
	21st to end	Night	16% 9% 3 8% 4 3 3 3 5% 6% 39%	3817
April	1st to 10th	Night	20% 14% 4 7% 6% 4 4 3 4 4 30%	1910
	11th to 20th	Night	18% 9% 3 7% 5% 4 5% 4 5% 6% 35%	1875
	21st to end	Night	17% 10% 4 7% 4 5% 5% 3 5% 7% 34%	1715
May	1st to 10th	Night	22% 9% 4 7% 4 3 4 3 5% 6% 32%	2815
	11th to 20th	Night	14% 7% 3 8% 6% 6% 4 4 7% 8% 33%	2495
	21st to end	Night	12% 6% 2 5% 4 3 5% 4 6% 9% 44%	2738
June	1st to 10th	Night	12% 6% 3 7% 5% 7% 6% 6% 7% 8% 34%	1520
	11th to 20th	Night	11% 6% 2 6% 5% 5% 6% 4 6% 8% 41%	1520
	21st to end	Night	13% 7% 2 6% 5% 5% 5% 3 6% 8% 40%	1520
July	1st to 10th	Night	9% 6% 3 7% 4 4 4 3 7% 9% 43%	2520
	11th to 20th	Night	11% 5% 2 6% 4 4 4 4 5% 9% 45%	2520
	21st to end	Night	10% 4 2 5% 3 5% 4 4 7% 7% 48%	3012
August	1st to 10th	Night	11% 6% 2 7% 4 4 4 4 6% 9% 42%	2825
	11th to 20th	Night	14% 7% 2 7% 4 4 5% 3 5% 8% 40%	2785
	21st to end	Night	9% 5% 2 5% 5% 4 5% 4 7% 10% 45%	3338
September	1st to 10th	Night	12% 7% 2 6% 4 3 5% 4 5% 6% 45%	1900
	11th to 20th	Night	13% 5% 2 5% 3 3 4 3 5% 7% 50%	1963
	21st to end	Night	17% 7% 3 6% 4 3 5% 4 6% 8% 36%	2156
October	1st to 10th	Night	17% 7% 3 7% 4 4 4 3 5% 7% 38%	2280
	11th to 20th	Night	22% 10% 3 8% 4 5% 4 3 5% 6% 30%	2280
	21st to end	Night	27% 10% 3 7% 5% 4 4 4 5% 5% 26%	2508
November	1st to 10th	Night	16% 10% 4 8% 4 4 4 3 4 6% 37%	2565
	11th to 20th	Night	16% 13% 4 9% 6% 6% 4 3 6% 7% 27%	2800
	21st to end	Night	18% 16% 5% 9% 5% 5% 5% 5% 6% 6% 21%	2800
December	1st to 10th	Night	17% 22% 7% 12% 6% 5% 5% 3 5% 5% 13%	4580
	11th to 20th	Night	20% 22% 5% 10% 5% 5% 4 3 4 4 19%	5208
	21st to end	Night	25% 17% 6% 11% 6% 6% 4 3 4 4 14%	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	78% 4 8% 7% 3	3598
	11th to 20th	Day	82% 5% 5% 6% 3 1	3904
	21st to end	Day	78% 6% 9% 5% 2	4466
February	1st to 10th	Day	74% 3 10% 11% 1	1755
	11th to 20th	Day	72% 6% 6% 12% 3	1672
	21st to end	Day	71% 4 9% 10% 5%	1477
March	1st to 10th	Day	59% 5% 9% 20% 7%	3435
	11th to 20th	Day	57% 7% 13% 14% 8% 1	3480
	21st to end	Day	53% 5% 10% 23% 8% 1	3815
April	1st to 10th	Day	57% 4 9% 19% 10% 1	2270
	11th to 20th	Day	52% 4 9% 21% 13% 2	2265
	21st to end	Day	53% 4 9% 19% 13% 2	2470
May	1st to 10th	Day	51% 5% 11% 20% 12% 2	4137
	11th to 20th	Day	51% 7% 12% 21% 8% 1	4402
	21st to end	Day	37% 5% 13% 26% 17% 3	4837
June	1st to 10th	Day	43% 5% 13% 21% 15% 2	2660
	11th to 20th	Day	41% 6% 14% 24% 15% 1	2660
	21st to end	Day	40% 6% 13% 25% 15% 1	2660
July	1st to 10th	Day	35% 7% 12% 27% 18% 2	4370
	11th to 20th	Day	36% 7% 15% 29% 13% 1	4365
	21st to end	Day	36% 9% 15% 25% 14% 1	4393
August	1st to 10th	Day	44% 7% 14% 22% 12% 1	4091
	11th to 20th	Day	42% 8% 16% 25% 8% 1	4083
	21st to end	Day	36% 5% 12% 34% 12% 1	4285
September	1st to 10th	Day	40% 6% 14% 27% 12% 1	2280
	11th to 20th	Day	37% 6% 13% 28% 15% 1	2214
	21st to end	Day	49% 5% 13% 23% 9% 1	2024
October	1st to 10th	Day	40% 6% 16% 25% 13% 1	3160
	11th to 20th	Day	48% 3 16% 20% 12%	3140
	21st to end	Day	51% 6% 14% 21% 7%	3428
November	1st to 10th	Day	54% 6% 11% 19% 9% 1	1732
	11th to 20th	Day	57% 6% 14% 15% 8% 1	1600
	21st to end	Day	67% 5% 12% 12% 4	1600
December	1st to 10th	Day	72% 7% 13% 7% 1	2600
	11th to 20th	Day	68% 7% 8% 9% 7% 1	2986
	21st to end	Day	78% 4 9% 7% 2	3960
January	1st to 10th	Night	73% 6% 10% 9% 2	6349
	11th to 20th	Night	77% 6% 8% 7% 3	6041
	21st to end	Night	74% 6% 8% 7% 4	6490
February	1st to 10th	Night	71% 4 9% 13% 3	2536
	11th to 20th	Night	66% 6% 10% 13% 4	2405
	21st to end	Night	66% 4 12% 13% 6%	1973
March	1st to 10th	Night	57% 4 10% 22% 6%	3418
	11th to 20th	Night	58% 5% 13% 16% 7% 1	3480
	21st to end	Night	45% 6% 9% 25% 15% 1	3817
April	1st to 10th	Night	58% 4 8% 19% 11% 1	1910
	11th to 20th	Night	50% 5% 9% 21% 13% 2	1875
	21st to end	Night	50% 5% 10% 19% 14% 2	1715
May	1st to 10th	Night	52% 5% 11% 19% 12% 2	2822
	11th to 20th	Night	46% 7% 15% 21% 8% 1	2496
	21st to end	Night	36% 6% 11% 25% 20% 2	2739
June	1st to 10th	Night	44% 6% 10% 21% 17% 1	1520
	11th to 20th	Night	40% 6% 12% 22% 19% 2	1520
	21st to end	Night	42% 6% 13% 21% 16% 1	1520
July	1st to 10th	Night	37% 6% 12% 24% 19% 2	2480
	11th to 20th	Night	35% 6% 14% 30% 13% 1	2472
	21st to end	Night	33% 7% 17% 24% 18% 1	2916
August	1st to 10th	Night	38% 6% 14% 28% 13% 1	2814
	11th to 20th	Night	42% 6% 14% 28% 10%	2812
	21st to end	Night	33% 7% 14% 32% 13% 1	3338
September	1st to 10th	Night	38% 6% 14% 27% 14% 1	1900
	11th to 20th	Night	34% 5% 11% 31% 18% 1	1963
	21st to end	Night	44% 6% 13% 23% 12% 1	2156
October	1st to 10th	Night	42% 6% 10% 25% 17% 1	3800
	11th to 20th	Night	49% 4 13% 21% 12% 1	3779
	21st to end	Night	45% 5% 15% 25% 8% 1	4120
November	1st to 10th	Night	50% 4 10% 24% 11% 2	2565
	11th to 20th	Night	57% 5% 12% 16% 9% 1	2800
	21st to end	Night	62% 7% 13% 14% 3	2800
December	1st to 10th	Night	72% 8% 11% 7% 2	4580
	11th to 20th	Night	69% 6% 7% 11% 7% 1	5208
	21st to end	Night	74% 5% 9% 9% 3	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	32% 30% 24% 10% 4 1	200
	11th to 20th	Day	30% 30% 27% 10% 2 1	267
	21st to end	Day	30% 25% 31% 12% 2	330
February	1st to 10th	Day	33% 31% 28% 6% 1	285
	11th to 20th	Day	30% 36% 28% 5% 1	270
	21st to end	Day	40% 24% 26% 8% 2	284
March	1st to 10th	Day	27% 24% 32% 13% 4	460
	11th to 20th	Day	30% 22% 33% 12% 2 1	460
	21st to end	Day	32% 21% 27% 15% 4 1	506
April	1st to 10th	Day	29% 20% 32% 14% 4 1	460
	11th to 20th	Day	34% 19% 23% 17% 5% 2	452
	21st to end	Day	29% 19% 32% 13% 6% 1	460
May	1st to 10th	Day	27% 13% 31% 19% 6% 4	463
	11th to 20th	Day	21% 13% 30% 22% 10% 3	487
	21st to end	Day	16% 15% 31% 23% 11% 4	529
June	1st to 10th	Day	20% 17% 34% 21% 6% 1	490
	11th to 20th	Day	16% 15% 31% 26% 11% 2	490
	21st to end	Day	20% 12% 34% 23% 8% 2	490
July	1st to 10th	Day	11% 7% 30% 29% 18% 4	485
	11th to 20th	Day	16% 8% 29% 27% 15% 4	484
	21st to end	Day	13% 8% 34% 30% 12% 3	532
August	1st to 10th	Day	15% 7% 27% 28% 16% 7%	486
	11th to 20th	Day	13% 6% 30% 28% 16% 7%	479
	21st to end	Day	15% 9% 29% 26% 14% 8%	539
September	1st to 10th	Day	15% 11% 28% 24% 14% 8%	490
	11th to 20th	Day	12% 8% 23% 25% 20% 11%	490
	21st to end	Day	20% 14% 29% 20% 12% 6%	490
October	1st to 10th	Day	26% 14% 25% 21% 11% 4	490
	11th to 20th	Day	27% 20% 29% 17% 6% 1	490
	21st to end	Day	38% 17% 27% 14% 5%	539
November	1st to 10th	Day	29% 19% 32% 13% 6% 1	339
	11th to 20th	Day	31% 27% 30% 8% 4 2	200
	21st to end	Day	36% 25% 30% 10% 1	200
December	1st to 10th	Day	33% 24% 31% 9% 3 1	200
	11th to 20th	Day	35% 27% 28% 9% 2	200
	21st to end	Day	35% 30% 24% 7% 2 1	220
January	1st to 10th	Night	32% 24% 29% 12% 4	800
	11th to 20th	Night	33% 25% 27% 14% 2	800
	21st to end	Night	26% 22% 31% 17% 4	880
February	1st to 10th	Night	30% 29% 25% 14% 2	781
	11th to 20th	Night	26% 27% 30% 15% 3	743
	21st to end	Night	30% 24% 29% 16% 2	628
March	1st to 10th	Night	26% 19% 34% 18% 4	760
	11th to 20th	Night	28% 19% 29% 19% 4 1	760
	21st to end	Night	25% 14% 33% 21% 7% 1	836
April	1st to 10th	Night	28% 17% 32% 21% 3	760
	11th to 20th	Night	25% 17% 31% 19% 7% 1	751
	21st to end	Night	24% 10% 34% 25% 8%	575
May	1st to 10th	Night	22% 9% 27% 27% 12% 2	565
	11th to 20th	Night	16% 7% 33% 37% 6% 1	374
	21st to end	Night	12% 10% 27% 36% 13% 1	411
June	1st to 10th	Night	15% 7% 36% 32% 10%	380
	11th to 20th	Night	14% 7% 30% 36% 12%	380
	21st to end	Night	15% 8% 34% 36% 7%	380
July	1st to 10th	Night	12% 5% 27% 39% 16% 1	370
	11th to 20th	Night	11% 5% 30% 35% 16% 2	368
	21st to end	Night	12% 2 29% 39% 16% 2	575
August	1st to 10th	Night	11% 3 22% 42% 20% 2	567
	11th to 20th	Night	12% 4 22% 34% 26% 2	564
	21st to end	Night	13% 4 25% 34% 21% 2	750
September	1st to 10th	Night	16% 8% 23% 34% 16% 3	760
	11th to 20th	Night	15% 4 21% 30% 25% 5%	760
	21st to end	Night	21% 9% 25% 28% 15% 4	760
October	1st to 10th	Night	21% 8% 29% 24% 16% 2	760
	11th to 20th	Night	30% 15% 26% 22% 7% 1	760
	21st to end	Night	33% 15% 28% 20% 4	836
November	1st to 10th	Night	24% 15% 29% 22% 8% 1	780
	11th to 20th	Night	27% 18% 29% 17% 7% 1	800
	21st to end	Night	28% 23% 28% 16% 5%	800
December	1st to 10th	Night	36% 23% 24% 14% 4 1	800
	11th to 20th	Night	29% 26% 27% 14% 3	800
	21st to end	Night	35% 20% 28% 14% 3	880

Page generated at 2025-02-03 03:05:51. Version ac07

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