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 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	17% 11% 4 10% 5% 5% 5% 4 5% 6% 28%	108382
	Night	18% 14% 5% 11% 6% 5% 4 3 4 5% 26%	109872
Transparency	Day	54% 8% 11% 19% 8% 1	111911
	Night	59% 7% 10% 16% 8% 1	114507
Seeing	Day	26% 17% 29% 18% 7% 2	14926
	Night	25% 16% 26% 24% 8% 1	24031

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% 17% 8% 21% 10% 4 4 2 4 4 15%	3597
	11th to 20th	Day	11% 14% 6% 20% 9% 6% 4 3 4 5% 19%	3570
	21st to end	Day	11% 18% 9% 12% 6% 6% 5% 3 5% 4 21%	3960
February	1st to 10th	Day	14% 14% 6% 14% 9% 5% 4 3 6% 5% 21%	1710
	11th to 20th	Day	11% 18% 6% 15% 8% 6% 4 3 4 5% 20%	1823
	21st to end	Day	14% 16% 6% 12% 7% 5% 4 3 3 5% 24%	1570
March	1st to 10th	Day	23% 19% 4 10% 5% 4 4 3 4 4 19%	3116
	11th to 20th	Day	14% 21% 6% 13% 6% 5% 4 2 4 4 22%	3265
	21st to end	Day	17% 13% 6% 11% 4 4 4 3 3 5% 31%	4166
April	1st to 10th	Day	21% 16% 3 8% 6% 3 3 3 3 6% 28%	2280
	11th to 20th	Day	20% 12% 3 8% 4 4 4 3 4 5% 32%	2254
	21st to end	Day	24% 12% 4 7% 4 4 4 3 4 5% 29%	2441
May	1st to 10th	Day	25% 11% 3 9% 5% 5% 4 4 4 5% 26%	4219
	11th to 20th	Day	17% 9% 3 8% 6% 5% 6% 4 6% 7% 28%	4398
	21st to end	Day	14% 7% 2 6% 5% 5% 5% 4 7% 9% 35%	4837
June	1st to 10th	Day	25% 6% 2 6% 5% 5% 5% 5% 6% 8% 26%	2660
	11th to 20th	Day	18% 7% 2 6% 5% 5% 5% 4 6% 9% 33%	2660
	21st to end	Day	21% 8% 3 6% 6% 4 5% 4 7% 7% 28%	2660
July	1st to 10th	Day	14% 6% 2 6% 5% 5% 5% 4 7% 10% 35%	4440
	11th to 20th	Day	13% 6% 2 7% 4 5% 6% 5% 7% 10% 35%	4440
	21st to end	Day	14% 6% 2 7% 5% 5% 6% 5% 6% 8% 37%	4814
August	1st to 10th	Day	19% 8% 3 7% 5% 4 6% 4 6% 8% 31%	4423
	11th to 20th	Day	18% 7% 3 8% 5% 4 5% 4 6% 8% 33%	3900
	21st to end	Day	16% 5% 2 5% 5% 3 4 4 6% 8% 41%	4180
September	1st to 10th	Day	18% 6% 2 5% 3 3 4 3 6% 7% 44%	2280
	11th to 20th	Day	16% 7% 2 4 3 3 4 3 5% 6% 47%	2166
	21st to end	Day	16% 8% 2 6% 4 4 5% 5% 6% 6% 38%	2090
October	1st to 10th	Day	24% 11% 3 7% 4 4 4 3 5% 5% 31%	2017
	11th to 20th	Day	19% 12% 3 7% 4 4 4 3 5% 6% 32%	1843
	21st to end	Day	25% 15% 3 8% 5% 4 4 3 4 4 25%	1881
November	1st to 10th	Day	17% 15% 4 8% 6% 4 4 3 4 5% 30%	1761
	11th to 20th	Day	18% 17% 5% 9% 5% 4 3 3 5% 5% 25%	1800
	21st to end	Day	18% 17% 5% 12% 5% 3 5% 4 5% 5% 21%	1615
December	1st to 10th	Day	14% 18% 7% 22% 7% 5% 5% 3 4 5% 10%	2600
	11th to 20th	Day	18% 18% 8% 13% 5% 4 4 3 4 4 21%	2986
	21st to end	Day	17% 17% 6% 13% 5% 5% 7% 3 4 4 18%	3960
January	1st to 10th	Night	12% 17% 11% 22% 9% 5% 3 2 3 3 13%	6349
	11th to 20th	Night	10% 19% 8% 22% 9% 6% 4 3 4 4 12%	6315
	21st to end	Night	13% 15% 6% 15% 9% 7% 5% 4 4 5% 16%	6996
February	1st to 10th	Night	15% 17% 6% 15% 8% 5% 4 3 5% 4 19%	2577
	11th to 20th	Night	12% 16% 7% 15% 9% 5% 4 3 5% 4 20%	2242
	21st to end	Night	14% 17% 5% 11% 7% 5% 4 2 4 5% 25%	1884
March	1st to 10th	Night	21% 16% 5% 8% 6% 4 3 2 4 4 25%	3724
	11th to 20th	Night	15% 21% 5% 10% 5% 4 3 2 3 4 26%	3691
	21st to end	Night	18% 18% 4 11% 4 3 3 2 3 4 30%	3466
April	1st to 10th	Night	20% 17% 4 8% 4 4 4 2 4 5% 27%	1900
	11th to 20th	Night	23% 14% 4 7% 5% 4 3 3 3 5% 29%	1875
	21st to end	Night	23% 12% 4 7% 4 4 3 3 4 5% 32%	1738
May	1st to 10th	Night	26% 12% 3 6% 4 5% 4 4 5% 6% 25%	2751
	11th to 20th	Night	21% 11% 4 8% 5% 4 4 3 5% 5% 31%	2495
	21st to end	Night	17% 6% 2 6% 4 5% 4 4 7% 9% 36%	2738
June	1st to 10th	Night	27% 7% 3 6% 5% 4 5% 3 5% 6% 30%	1520
	11th to 20th	Night	19% 6% 2 7% 5% 4 4 4 6% 8% 35%	1520
	21st to end	Night	22% 7% 3 7% 6% 4 5% 3 7% 6% 31%	1520
July	1st to 10th	Night	17% 5% 2 7% 5% 5% 6% 6% 5% 8% 34%	2520
	11th to 20th	Night	18% 7% 2 6% 4 4 5% 4 6% 7% 37%	2520
	21st to end	Night	16% 7% 3 7% 5% 5% 5% 4 6% 7% 37%	2739
August	1st to 10th	Night	21% 8% 3 6% 5% 4 4 3 5% 6% 34%	2506
	11th to 20th	Night	17% 7% 3 7% 6% 4 4 4 5% 6% 36%	2892
	21st to end	Night	16% 5% 2 5% 4 4 4 4 6% 8% 43%	3476
September	1st to 10th	Night	17% 6% 2 5% 5% 3 4 3 5% 6% 44%	1900
	11th to 20th	Night	15% 7% 1 5% 3 3 4 3 4 7% 48%	2014
	21st to end	Night	18% 6% 2 6% 4 4 4 3 5% 6% 42%	2090
October	1st to 10th	Night	24% 9% 2 6% 4 5% 4 4 5% 5% 31%	2160
	11th to 20th	Night	23% 12% 2 7% 5% 4 4 3 4 6% 30%	2336
	21st to end	Night	27% 15% 3 7% 4 4 4 3 4 5% 24%	2717
November	1st to 10th	Night	20% 15% 4 8% 4 3 4 4 4 5% 29%	2537
	11th to 20th	Night	19% 17% 5% 9% 5% 4 3 3 4 5% 27%	2600
	21st to end	Night	19% 19% 5% 12% 5% 4 4 3 5% 5% 20%	2780
December	1st to 10th	Night	16% 22% 11% 18% 6% 5% 4 2 3 3 9%	4580
	11th to 20th	Night	21% 19% 5% 14% 6% 5% 4 2 3 3 18%	5208
	21st to end	Night	17% 18% 6% 14% 6% 6% 5% 3 4 3 19%	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	72% 7% 8% 11% 2	3598
	11th to 20th	Day	67% 8% 7% 11% 6% 1	3600
	21st to end	Day	63% 7% 10% 11% 8% 1	3960
February	1st to 10th	Day	63% 8% 7% 15% 5% 2	1710
	11th to 20th	Day	66% 7% 8% 14% 5%	1832
	21st to end	Day	64% 6% 8% 12% 9% 2	1570
March	1st to 10th	Day	67% 6% 9% 10% 6% 1	3119
	11th to 20th	Day	63% 8% 9% 10% 8% 1	3265
	21st to end	Day	55% 6% 6% 15% 16% 2	4166
April	1st to 10th	Day	59% 5% 7% 19% 9% 2	2280
	11th to 20th	Day	53% 5% 7% 19% 14% 1	2254
	21st to end	Day	57% 5% 9% 17% 10% 2	2441
May	1st to 10th	Day	57% 9% 8% 19% 7% 1	4233
	11th to 20th	Day	50% 10% 15% 17% 8% 1	4402
	21st to end	Day	41% 10% 13% 25% 10% 1	4837
June	1st to 10th	Day	53% 8% 12% 17% 9%	2660
	11th to 20th	Day	46% 8% 11% 22% 12% 2	2660
	21st to end	Day	53% 8% 11% 19% 8% 1	2660
July	1st to 10th	Day	41% 8% 15% 25% 11%	4370
	11th to 20th	Day	41% 9% 16% 28% 5%	4365
	21st to end	Day	44% 9% 11% 29% 7%	4661
August	1st to 10th	Day	48% 8% 15% 21% 7%	4409
	11th to 20th	Day	48% 8% 16% 23% 5%	3939
	21st to end	Day	39% 7% 17% 22% 13% 1	4180
September	1st to 10th	Day	39% 7% 12% 29% 12% 1	2280
	11th to 20th	Day	38% 5% 11% 31% 14% 1	2166
	21st to end	Day	44% 9% 15% 22% 10% 2	2090
October	1st to 10th	Day	47% 6% 11% 26% 10%	3355
	11th to 20th	Day	49% 6% 13% 20% 11%	3046
	21st to end	Day	49% 5% 13% 25% 6% 1	3081
November	1st to 10th	Day	55% 7% 12% 17% 8%	1761
	11th to 20th	Day	60% 7% 10% 13% 9% 2	1800
	21st to end	Day	63% 8% 12% 13% 3	1615
December	1st to 10th	Day	77% 6% 8% 7% 2	2600
	11th to 20th	Day	66% 6% 8% 15% 4	2986
	21st to end	Day	66% 9% 7% 11% 7% 1	3960
January	1st to 10th	Night	76% 6% 9% 7% 2	6349
	11th to 20th	Night	74% 8% 8% 7% 3 1	6360
	21st to end	Night	67% 8% 9% 8% 8% 1	6996
February	1st to 10th	Night	68% 6% 9% 11% 6% 1	2577
	11th to 20th	Night	65% 6% 7% 14% 7%	2244
	21st to end	Night	61% 7% 8% 16% 7% 1	1884
March	1st to 10th	Night	63% 4 10% 14% 8% 1	3734
	11th to 20th	Night	60% 6% 8% 15% 9% 1	3692
	21st to end	Night	58% 5% 6% 14% 15% 2	3466
April	1st to 10th	Night	60% 5% 7% 16% 11% 1	1900
	11th to 20th	Night	57% 7% 7% 17% 12%	1875
	21st to end	Night	55% 5% 9% 16% 12% 3	1738
May	1st to 10th	Night	56% 9% 9% 16% 8% 1	2757
	11th to 20th	Night	53% 8% 10% 19% 9% 1	2496
	21st to end	Night	42% 8% 14% 20% 15% 1	2739
June	1st to 10th	Night	53% 8% 10% 19% 8% 1	1520
	11th to 20th	Night	46% 7% 14% 20% 11% 2	1520
	21st to end	Night	50% 9% 12% 20% 9%	1520
July	1st to 10th	Night	45% 9% 14% 19% 12% 1	2480
	11th to 20th	Night	43% 10% 13% 26% 8%	2472
	21st to end	Night	45% 9% 12% 25% 9%	2651
August	1st to 10th	Night	50% 7% 12% 22% 8%	2496
	11th to 20th	Night	47% 8% 15% 21% 8% 1	2919
	21st to end	Night	38% 8% 15% 25% 12% 1	3476
September	1st to 10th	Night	40% 7% 14% 27% 12%	1900
	11th to 20th	Night	38% 5% 15% 30% 12%	2014
	21st to end	Night	42% 6% 13% 24% 13% 2	2090
October	1st to 10th	Night	54% 7% 10% 21% 8%	3602
	11th to 20th	Night	52% 5% 10% 21% 12%	3872
	21st to end	Night	54% 5% 12% 22% 6% 1	4467
November	1st to 10th	Night	56% 6% 12% 17% 8% 1	2537
	11th to 20th	Night	59% 5% 10% 16% 8% 2	2600
	21st to end	Night	65% 7% 11% 13% 4	2780
December	1st to 10th	Night	79% 7% 7% 6% 1	4580
	11th to 20th	Night	71% 6% 6% 11% 6%	5208
	21st to end	Night	69% 7% 6% 10% 6% 2	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	28% 27% 27% 14% 5%	200
	11th to 20th	Day	33% 24% 31% 9% 3 1	200
	21st to end	Day	25% 29% 32% 9% 5%	220
February	1st to 10th	Day	34% 32% 26% 6% 2	262
	11th to 20th	Day	31% 24% 34% 9% 2	431
	21st to end	Day	31% 22% 33% 11% 2	382
March	1st to 10th	Day	35% 17% 29% 16% 2	460
	11th to 20th	Day	31% 25% 27% 13% 4	460
	21st to end	Day	34% 21% 25% 13% 6% 2	506
April	1st to 10th	Day	37% 18% 26% 13% 5% 1	460
	11th to 20th	Day	34% 23% 21% 13% 6% 2	452
	21st to end	Day	33% 22% 27% 11% 6% 2	460
May	1st to 10th	Day	33% 17% 27% 14% 7% 2	463
	11th to 20th	Day	22% 16% 34% 20% 7% 2	487
	21st to end	Day	22% 17% 32% 21% 7% 1	529
June	1st to 10th	Day	29% 16% 30% 17% 7% 1	490
	11th to 20th	Day	23% 15% 32% 22% 7% 2	490
	21st to end	Day	25% 18% 31% 19% 7% 1	490
July	1st to 10th	Day	18% 12% 33% 25% 11% 2	485
	11th to 20th	Day	17% 10% 35% 26% 10% 2	484
	21st to end	Day	16% 10% 37% 29% 8% 1	532
August	1st to 10th	Day	17% 10% 30% 26% 14% 3	486
	11th to 20th	Day	19% 10% 28% 26% 13% 5%	479
	21st to end	Day	14% 14% 30% 25% 11% 5%	539
September	1st to 10th	Day	19% 14% 31% 18% 12% 6%	490
	11th to 20th	Day	17% 13% 22% 21% 20% 7%	490
	21st to end	Day	19% 13% 31% 21% 12% 5%	490
October	1st to 10th	Day	28% 13% 26% 17% 14% 3	490
	11th to 20th	Day	29% 17% 25% 21% 7% 1	460
	21st to end	Day	38% 17% 24% 17% 4	429
November	1st to 10th	Day	31% 21% 23% 19% 6% 1	395
	11th to 20th	Day	31% 25% 24% 15% 5% 1	400
	21st to end	Day	35% 21% 30% 13% 1	215
December	1st to 10th	Day	28% 23% 32% 14% 4	200
	11th to 20th	Day	35% 27% 28% 9% 2	200
	21st to end	Day	29% 24% 33% 10% 3	220
January	1st to 10th	Night	26% 26% 30% 16% 2	800
	11th to 20th	Night	26% 24% 30% 18% 3	800
	21st to end	Night	20% 21% 32% 22% 4	880
February	1st to 10th	Night	24% 19% 35% 20% 2	781
	11th to 20th	Night	24% 23% 32% 19% 3	743
	21st to end	Night	26% 21% 29% 19% 5%	628
March	1st to 10th	Night	28% 18% 28% 24% 2	760
	11th to 20th	Night	26% 22% 26% 20% 5%	760
	21st to end	Night	30% 17% 25% 20% 7% 1	836
April	1st to 10th	Night	28% 17% 27% 24% 4	760
	11th to 20th	Night	31% 18% 24% 18% 8% 1	751
	21st to end	Night	30% 15% 27% 22% 7%	598
May	1st to 10th	Night	29% 13% 25% 24% 8% 1	526
	11th to 20th	Night	22% 9% 35% 25% 7% 1	374
	21st to end	Night	19% 9% 28% 31% 11% 1	411
June	1st to 10th	Night	27% 7% 27% 29% 9% 1	380
	11th to 20th	Night	21% 7% 24% 36% 11% 1	380
	21st to end	Night	23% 6% 25% 37% 8% 1	380
July	1st to 10th	Night	15% 5% 23% 42% 14% 1	370
	11th to 20th	Night	14% 4 23% 40% 18% 1	368
	21st to end	Night	15% 7% 20% 44% 13%	416
August	1st to 10th	Night	17% 5% 16% 41% 20% 1	378
	11th to 20th	Night	16% 6% 21% 35% 20% 2	619
	21st to end	Night	17% 7% 22% 33% 20% 2	836
September	1st to 10th	Night	19% 9% 23% 31% 16% 2	760
	11th to 20th	Night	19% 7% 19% 31% 22% 3	760
	21st to end	Night	20% 11% 25% 26% 16% 2	760
October	1st to 10th	Night	29% 10% 20% 25% 15% 1	760
	11th to 20th	Night	29% 13% 22% 26% 9% 1	760
	21st to end	Night	34% 14% 26% 20% 5%	836
November	1st to 10th	Night	29% 19% 25% 19% 8% 1	780
	11th to 20th	Night	28% 18% 27% 18% 8% 1	800
	21st to end	Night	30% 23% 26% 16% 6% 1	800
December	1st to 10th	Night	30% 22% 28% 17% 3	800
	11th to 20th	Night	29% 22% 26% 20% 3	800
	21st to end	Night	30% 19% 30% 17% 3	880

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