MT Scopehead Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the MT Scopehead Observatory clear sky chart from 2005-11-06 to 2024-01-27.

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	18% 11% 4 8% 6% 3 5% 5% 5% 7% 29%	74882
	Night	19% 12% 5% 9% 6% 3 4 4 4 6% 27%	75159
Transparency	Day	54% 7% 11% 18% 9% 1	74587
	Night	58% 7% 10% 17% 8% 1	74988
Seeing	Day	26% 17% 29% 18% 8% 2	13632
	Night	25% 15% 26% 24% 9% 1	22742

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	14% 18% 8% 11% 9% 4 6% 4 4 5% 16%	1517
	11th to 20th	Day	17% 14% 5% 13% 8% 4 5% 4 4 5% 21%	1520
	21st to end	Day	12% 15% 7% 12% 8% 4 7% 5% 5% 6% 20%	1632
February	1st to 10th	Day	14% 14% 6% 13% 9% 4 5% 4 6% 5% 19%	1580
	11th to 20th	Day	12% 17% 7% 12% 9% 5% 5% 3 4 6% 21%	1725
	21st to end	Day	14% 14% 7% 12% 8% 5% 5% 4 3 5% 23%	1480
March	1st to 10th	Day	15% 17% 6% 10% 7% 4 5% 4 4 4 24%	1797
	11th to 20th	Day	18% 15% 6% 9% 6% 3 5% 3 4 5% 26%	1849
	21st to end	Day	19% 14% 7% 8% 5% 3 4 3 3 5% 30%	2376
April	1st to 10th	Day	21% 14% 5% 7% 6% 2 3 4 3 6% 28%	2160
	11th to 20th	Day	20% 11% 3 6% 5% 3 4 4 4 6% 33%	2134
	21st to end	Day	25% 12% 4 6% 4 3 4 4 4 6% 29%	2312
May	1st to 10th	Day	22% 9% 4 7% 6% 3 4 5% 5% 5% 31%	2397
	11th to 20th	Day	18% 9% 3 6% 4 3 5% 4 5% 7% 36%	2478
	21st to end	Day	18% 8% 3 5% 5% 4 5% 5% 7% 8% 34%	2725
June	1st to 10th	Day	26% 6% 3 5% 6% 4 5% 6% 6% 8% 26%	2520
	11th to 20th	Day	19% 6% 2 5% 5% 3 5% 5% 5% 9% 34%	2520
	21st to end	Day	21% 7% 3 5% 6% 3 5% 5% 7% 8% 28%	2520
July	1st to 10th	Day	15% 6% 2 6% 5% 3 5% 5% 6% 8% 38%	2520
	11th to 20th	Day	13% 6% 3 6% 5% 4 6% 6% 7% 9% 35%	2520
	21st to end	Day	16% 5% 3 5% 6% 4 6% 6% 7% 10% 34%	2744
August	1st to 10th	Day	14% 6% 3 6% 6% 3 6% 6% 6% 9% 36%	2506
	11th to 20th	Day	15% 5% 2 7% 6% 3 5% 5% 7% 9% 35%	2229
	21st to end	Day	13% 6% 3 6% 6% 3 4 5% 6% 9% 38%	2376
September	1st to 10th	Day	18% 6% 2 4 4 2 4 4 6% 7% 43%	2160
	11th to 20th	Day	17% 6% 2 3 4 2 4 4 4 6% 47%	2052
	21st to end	Day	16% 7% 2 5% 5% 3 5% 6% 6% 7% 38%	1980
October	1st to 10th	Day	25% 11% 3 7% 5% 3 4 4 5% 5% 30%	1911
	11th to 20th	Day	19% 12% 3 7% 5% 3 5% 4 5% 6% 30%	1746
	21st to end	Day	27% 15% 3 7% 7% 3 3 5% 4 4 23%	1782
November	1st to 10th	Day	17% 13% 5% 7% 6% 3 5% 4 4 6% 30%	1671
	11th to 20th	Day	19% 15% 5% 9% 6% 3 4 4 5% 5% 24%	1710
	21st to end	Day	18% 15% 6% 11% 5% 2 5% 5% 5% 5% 22%	1535
December	1st to 10th	Day	21% 18% 7% 14% 6% 3 4 3 3 5% 16%	1520
	11th to 20th	Day	19% 17% 6% 9% 6% 3 4 4 4 5% 21%	1906
	21st to end	Day	22% 17% 6% 13% 7% 4 6% 4 3 4 14%	2772
January	1st to 10th	Night	16% 18% 7% 15% 9% 4 5% 4 3 4 14%	2649
	11th to 20th	Night	16% 17% 7% 12% 9% 4 5% 4 4 4 17%	2660
	21st to end	Night	12% 16% 7% 12% 9% 4 6% 4 4 5% 21%	2868
February	1st to 10th	Night	15% 16% 6% 15% 10% 4 4 4 5% 4 18%	2377
	11th to 20th	Night	13% 15% 8% 13% 10% 5% 4 3 4 4 20%	2120
	21st to end	Night	14% 17% 6% 10% 8% 4 5% 3 4 5% 24%	1776
March	1st to 10th	Night	16% 18% 8% 8% 8% 3 4 3 3 4 26%	2150
	11th to 20th	Night	17% 13% 6% 11% 6% 3 4 3 3 5% 28%	2107
	21st to end	Night	20% 14% 4 8% 5% 3 2 4 3 5% 32%	1980
April	1st to 10th	Night	22% 15% 5% 7% 5% 3 4 3 4 6% 27%	1800
	11th to 20th	Night	24% 13% 4 6% 5% 3 4 4 3 5% 30%	1775
	21st to end	Night	23% 12% 4 6% 5% 3 4 4 4 5% 31%	1647
May	1st to 10th	Night	25% 11% 3 5% 3 3 4 4 5% 7% 30%	1579
	11th to 20th	Night	21% 9% 3 5% 5% 3 4 4 5% 6% 37%	1415
	21st to end	Night	18% 6% 3 4 5% 4 5% 5% 6% 8% 36%	1550
June	1st to 10th	Night	27% 6% 3 5% 5% 3 4 5% 4 6% 30%	1440
	11th to 20th	Night	20% 5% 2 6% 6% 3 4 4 6% 9% 35%	1440
	21st to end	Night	21% 6% 3 6% 6% 3 5% 5% 6% 8% 32%	1440
July	1st to 10th	Night	17% 4 2 5% 5% 3 6% 6% 5% 8% 39%	1440
	11th to 20th	Night	13% 5% 2 5% 6% 3 6% 6% 7% 9% 38%	1440
	21st to end	Night	15% 6% 3 5% 5% 3 6% 5% 6% 10% 37%	1574
August	1st to 10th	Night	13% 5% 2 6% 5% 3 5% 5% 6% 9% 41%	1432
	11th to 20th	Night	14% 6% 3 5% 6% 3 4 5% 5% 8% 40%	1649
	21st to end	Night	15% 6% 2 6% 5% 3 4 5% 6% 8% 40%	1980
September	1st to 10th	Night	17% 5% 2 4 5% 2 4 4 5% 6% 44%	1800
	11th to 20th	Night	16% 7% 2 5% 4 2 3 4 4 7% 47%	1908
	21st to end	Night	19% 5% 2 5% 4 3 4 4 5% 6% 42%	1980
October	1st to 10th	Night	24% 9% 2 5% 5% 4 4 5% 5% 5% 31%	2046
	11th to 20th	Night	23% 11% 3 6% 6% 3 4 4 5% 6% 29%	2213
	21st to end	Night	28% 15% 3 6% 5% 3 4 4 4 5% 23%	2574
November	1st to 10th	Night	20% 14% 3 7% 5% 3 4 5% 4 5% 29%	2407
	11th to 20th	Night	19% 15% 6% 8% 6% 3 3 3 4 5% 27%	2470
	21st to end	Night	19% 17% 6% 11% 6% 3 3 4 5% 5% 20%	2641
December	1st to 10th	Night	20% 21% 9% 11% 7% 4 5% 3 3 4 13%	2660
	11th to 20th	Night	22% 16% 6% 11% 7% 4 5% 4 4 4 18%	3288
	21st to end	Night	22% 18% 7% 13% 7% 3 4 4 3 3 15%	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	69% 8% 7% 13% 4	1518
	11th to 20th	Day	65% 7% 9% 12% 7% 1	1520
	21st to end	Day	64% 6% 12% 10% 7% 1	1632
February	1st to 10th	Day	65% 8% 7% 14% 5% 2	1580
	11th to 20th	Day	66% 6% 8% 14% 5%	1734
	21st to end	Day	65% 6% 8% 10% 9% 1	1480
March	1st to 10th	Day	63% 5% 10% 13% 7% 2	1800
	11th to 20th	Day	61% 5% 9% 12% 11% 2	1849
	21st to end	Day	59% 4 7% 16% 11% 3	2376
April	1st to 10th	Day	58% 5% 7% 19% 9% 2	2160
	11th to 20th	Day	52% 5% 8% 19% 14% 1	2134
	21st to end	Day	58% 5% 10% 16% 10% 2	2312
May	1st to 10th	Day	54% 5% 8% 19% 12% 2	2397
	11th to 20th	Day	47% 6% 10% 21% 14% 2	2482
	21st to end	Day	47% 9% 13% 19% 10% 1	2725
June	1st to 10th	Day	54% 8% 12% 17% 8%	2520
	11th to 20th	Day	46% 8% 11% 22% 12% 2	2520
	21st to end	Day	53% 8% 12% 18% 9% 1	2520
July	1st to 10th	Day	41% 8% 14% 24% 13%	2450
	11th to 20th	Day	43% 8% 15% 25% 8%	2445
	21st to end	Day	45% 9% 14% 23% 9%	2591
August	1st to 10th	Day	43% 9% 16% 24% 7%	2492
	11th to 20th	Day	45% 7% 16% 23% 9%	2229
	21st to end	Day	43% 8% 12% 25% 12% 1	2376
September	1st to 10th	Day	40% 7% 12% 28% 12% 1	2160
	11th to 20th	Day	39% 5% 10% 30% 15% 1	2052
	21st to end	Day	43% 9% 15% 22% 9% 2	1980
October	1st to 10th	Day	56% 6% 9% 19% 10%	1911
	11th to 20th	Day	52% 8% 14% 19% 7%	1746
	21st to end	Day	65% 5% 8% 13% 7% 1	1782
November	1st to 10th	Day	56% 7% 12% 17% 8%	1671
	11th to 20th	Day	61% 7% 10% 12% 8% 2	1710
	21st to end	Day	63% 8% 13% 13% 3	1535
December	1st to 10th	Day	71% 6% 10% 10% 3	1520
	11th to 20th	Day	65% 6% 11% 13% 5%	1906
	21st to end	Day	70% 10% 8% 9% 3	2772
January	1st to 10th	Night	74% 7% 8% 8% 3	2649
	11th to 20th	Night	70% 6% 9% 10% 4	2660
	21st to end	Night	64% 7% 8% 12% 8% 1	2868
February	1st to 10th	Night	69% 6% 9% 9% 6% 1	2377
	11th to 20th	Night	66% 5% 7% 13% 8%	2122
	21st to end	Night	62% 7% 7% 15% 8% 1	1776
March	1st to 10th	Night	64% 4 9% 15% 7% 1	2160
	11th to 20th	Night	60% 5% 8% 17% 9% 2	2108
	21st to end	Night	56% 5% 8% 17% 12% 3	1980
April	1st to 10th	Night	61% 5% 7% 16% 10% 1	1800
	11th to 20th	Night	57% 6% 7% 17% 12%	1775
	21st to end	Night	55% 5% 9% 16% 11% 3	1647
May	1st to 10th	Night	53% 6% 8% 17% 14% 2	1579
	11th to 20th	Night	47% 7% 9% 20% 15% 2	1416
	21st to end	Night	45% 7% 14% 20% 13% 1	1551
June	1st to 10th	Night	54% 7% 11% 19% 8% 1	1440
	11th to 20th	Night	46% 7% 14% 20% 11% 2	1440
	21st to end	Night	49% 9% 13% 21% 9%	1440
July	1st to 10th	Night	40% 9% 15% 22% 13% 1	1400
	11th to 20th	Night	39% 10% 14% 27% 10%	1392
	21st to end	Night	40% 10% 15% 25% 10% 1	1486
August	1st to 10th	Night	38% 9% 16% 27% 10%	1422
	11th to 20th	Night	41% 9% 14% 24% 10% 1	1649
	21st to end	Night	41% 8% 13% 26% 12%	1980
September	1st to 10th	Night	41% 7% 14% 27% 11% 1	1800
	11th to 20th	Night	39% 5% 14% 30% 12%	1908
	21st to end	Night	43% 6% 13% 25% 12% 2	1980
October	1st to 10th	Night	54% 6% 11% 19% 10% 1	2046
	11th to 20th	Night	55% 7% 11% 20% 6%	2213
	21st to end	Night	64% 6% 9% 14% 7% 1	2574
November	1st to 10th	Night	56% 6% 12% 17% 8% 1	2407
	11th to 20th	Night	60% 5% 10% 16% 7% 2	2470
	21st to end	Night	66% 7% 10% 13% 4	2641
December	1st to 10th	Night	76% 5% 8% 9% 2	2660
	11th to 20th	Night	70% 8% 7% 10% 5%	3288
	21st to end	Night	73% 7% 7% 10% 2	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	29% 24% 26% 15% 6%	180
	11th to 20th	Day	33% 24% 29% 9% 3 1	180
	21st to end	Day	22% 31% 33% 10% 4	188
February	1st to 10th	Day	35% 34% 25% 6% 1	222
	11th to 20th	Day	32% 23% 34% 9% 2 1	393
	21st to end	Day	31% 23% 33% 11% 2	346
March	1st to 10th	Day	35% 17% 30% 16% 2	420
	11th to 20th	Day	31% 24% 27% 14% 4	420
	21st to end	Day	33% 20% 25% 14% 6% 2	462
April	1st to 10th	Day	36% 17% 27% 14% 5% 1	420
	11th to 20th	Day	33% 22% 21% 15% 7% 2	412
	21st to end	Day	33% 21% 26% 11% 6% 2	420
May	1st to 10th	Day	33% 18% 26% 13% 7% 2	423
	11th to 20th	Day	22% 17% 34% 19% 6% 2	447
	21st to end	Day	23% 16% 31% 22% 8% 1	485
June	1st to 10th	Day	31% 15% 31% 17% 6% 1	450
	11th to 20th	Day	23% 14% 32% 22% 7% 2	450
	21st to end	Day	24% 16% 31% 20% 7% 1	450
July	1st to 10th	Day	18% 12% 31% 26% 11% 2	445
	11th to 20th	Day	17% 9% 35% 26% 11% 2	444
	21st to end	Day	16% 9% 37% 28% 8% 1	488
August	1st to 10th	Day	16% 10% 31% 26% 15% 3	446
	11th to 20th	Day	18% 9% 28% 26% 14% 5%	439
	21st to end	Day	14% 15% 31% 25% 11% 5%	495
September	1st to 10th	Day	19% 14% 31% 18% 12% 6%	450
	11th to 20th	Day	18% 14% 22% 20% 19% 7%	450
	21st to end	Day	18% 14% 31% 20% 12% 5%	450
October	1st to 10th	Day	29% 13% 25% 16% 14% 4	450
	11th to 20th	Day	28% 17% 27% 22% 6% 1	423
	21st to end	Day	39% 17% 23% 17% 4	396
November	1st to 10th	Day	30% 19% 23% 21% 7% 1	365
	11th to 20th	Day	31% 26% 24% 14% 5% 1	370
	21st to end	Day	33% 18% 33% 15% 1	195
December	1st to 10th	Day	28% 22% 32% 13% 4	180
	11th to 20th	Day	33% 27% 28% 10% 2	180
	21st to end	Day	31% 25% 32% 9% 3 1	198
January	1st to 10th	Night	26% 24% 30% 17% 3	760
	11th to 20th	Night	26% 23% 30% 18% 3	760
	21st to end	Night	20% 21% 31% 23% 4	820
February	1st to 10th	Night	24% 19% 35% 20% 2	720
	11th to 20th	Night	23% 22% 33% 19% 3	703
	21st to end	Night	26% 21% 29% 19% 5%	592
March	1st to 10th	Night	27% 18% 28% 25% 2	720
	11th to 20th	Night	27% 22% 26% 20% 5%	720
	21st to end	Night	30% 17% 25% 20% 7% 1	792
April	1st to 10th	Night	28% 17% 26% 24% 4	720
	11th to 20th	Night	31% 17% 25% 19% 8% 1	711
	21st to end	Night	29% 15% 28% 22% 7%	567
May	1st to 10th	Night	29% 13% 25% 24% 8% 1	499
	11th to 20th	Night	23% 9% 34% 26% 7% 1	354
	21st to end	Night	20% 9% 28% 32% 11% 1	389
June	1st to 10th	Night	28% 6% 28% 28% 9% 1	360
	11th to 20th	Night	21% 6% 24% 36% 11% 1	360
	21st to end	Night	22% 5% 26% 39% 8% 1	360
July	1st to 10th	Night	14% 5% 24% 41% 15% 1	350
	11th to 20th	Night	12% 4 24% 41% 18% 1	348
	21st to end	Night	15% 7% 20% 44% 14%	394
August	1st to 10th	Night	16% 4 16% 42% 20% 1	358
	11th to 20th	Night	16% 6% 19% 36% 20% 2	585
	21st to end	Night	17% 7% 22% 33% 20% 2	792
September	1st to 10th	Night	18% 9% 23% 32% 17% 2	720
	11th to 20th	Night	20% 8% 19% 30% 21% 3	720
	21st to end	Night	20% 11% 25% 25% 17% 2	720
October	1st to 10th	Night	29% 10% 20% 25% 15% 1	720
	11th to 20th	Night	28% 14% 23% 26% 9% 1	720
	21st to end	Night	34% 14% 26% 21% 5%	792
November	1st to 10th	Night	28% 18% 25% 19% 8% 1	740
	11th to 20th	Night	28% 19% 27% 17% 8% 1	760
	21st to end	Night	29% 22% 26% 16% 6% 1	760
December	1st to 10th	Night	30% 23% 28% 17% 3	760
	11th to 20th	Night	28% 22% 26% 20% 3	760
	21st to end	Night	31% 19% 30% 17% 3	836

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