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 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	18% 11% 4 10% 5% 5% 5% 4 5% 6% 28%	132037
	Night	17% 14% 5% 11% 6% 5% 4 3 4 5% 25%	131119
Transparency	Day	54% 7% 11% 18% 8% 1	135518
	Night	60% 7% 10% 15% 8% 1	135725
Seeing	Day	27% 18% 29% 17% 7% 2	16650
	Night	25% 16% 26% 24% 8% 1	25674

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	12% 16% 8% 19% 10% 5% 4 3 4 4 14%	4661
	11th to 20th	Day	10% 15% 6% 20% 10% 6% 4 3 4 4 17%	4650
	21st to end	Day	10% 16% 7% 12% 9% 8% 5% 4 5% 4 19%	5124
February	1st to 10th	Day	13% 14% 6% 15% 9% 5% 5% 3 5% 5% 20%	1843
	11th to 20th	Day	11% 17% 6% 15% 8% 6% 4 3 5% 5% 20%	2021
	21st to end	Day	14% 16% 6% 13% 8% 5% 4 3 3 4 23%	1730
March	1st to 10th	Day	21% 17% 6% 12% 5% 5% 4 3 4 4 21%	4576
	11th to 20th	Day	14% 16% 5% 11% 5% 5% 4 3 5% 5% 27%	4783
	21st to end	Day	20% 13% 5% 11% 4 4 4 3 4 4 27%	6102
April	1st to 10th	Day	21% 16% 4 8% 6% 3 3 3 4 5% 28%	2520
	11th to 20th	Day	20% 13% 3 8% 4 4 4 3 4 5% 32%	2494
	21st to end	Day	23% 12% 4 7% 4 4 4 3 4 5% 30%	2698
May	1st to 10th	Day	31% 12% 3 8% 5% 4 4 3 4 5% 20%	6174
	11th to 20th	Day	19% 9% 3 8% 5% 5% 6% 4 6% 7% 27%	6458
	21st to end	Day	21% 9% 3 7% 4 4 5% 4 6% 7% 29%	7103
June	1st to 10th	Day	25% 7% 2 6% 5% 5% 5% 5% 6% 8% 27%	2800
	11th to 20th	Day	18% 7% 2 6% 5% 5% 5% 4 6% 9% 33%	2800
	21st to end	Day	21% 7% 3 6% 6% 4 5% 4 7% 7% 29%	2800
July	1st to 10th	Day	14% 6% 2 7% 6% 5% 6% 5% 7% 10% 32%	6360
	11th to 20th	Day	13% 5% 2 6% 5% 5% 6% 5% 7% 10% 36%	6360
	21st to end	Day	15% 6% 2 6% 4 4 5% 4 6% 9% 39%	6926
August	1st to 10th	Day	18% 7% 3 7% 5% 4 6% 4 6% 8% 32%	4563
	11th to 20th	Day	18% 7% 3 8% 5% 4 5% 4 6% 8% 33%	4025
	21st to end	Day	16% 5% 2 5% 5% 3 4 4 6% 8% 41%	4312
September	1st to 10th	Day	17% 6% 2 5% 3 3 4 3 6% 7% 45%	2400
	11th to 20th	Day	16% 6% 2 4 3 3 4 3 5% 6% 48%	2280
	21st to end	Day	16% 8% 2 6% 4 4 5% 5% 6% 7% 38%	2200
October	1st to 10th	Day	23% 10% 2 7% 4 4 3 3 5% 5% 33%	2123
	11th to 20th	Day	19% 12% 3 7% 4 4 4 3 5% 6% 32%	1940
	21st to end	Day	25% 16% 3 8% 5% 4 4 3 4 4 24%	1980
November	1st to 10th	Day	17% 16% 4 8% 6% 4 4 3 4 5% 29%	1851
	11th to 20th	Day	18% 18% 5% 9% 5% 4 3 3 4 5% 25%	1890
	21st to end	Day	18% 17% 6% 12% 5% 4 5% 4 5% 5% 21%	1696
December	1st to 10th	Day	14% 18% 7% 22% 7% 5% 5% 3 4 5% 11%	2680
	11th to 20th	Day	17% 18% 8% 13% 5% 4 4 3 4 4 21%	3066
	21st to end	Day	17% 17% 6% 13% 6% 5% 7% 3 4 4 18%	4048
January	1st to 10th	Night	11% 18% 10% 20% 9% 5% 4 2 3 3 14%	8234
	11th to 20th	Night	10% 19% 9% 21% 9% 6% 4 3 4 3 11%	8235
	21st to end	Night	11% 13% 5% 15% 12% 10% 6% 4 4 5% 15%	9057
February	1st to 10th	Night	14% 16% 6% 16% 8% 5% 4 3 5% 4 19%	2773
	11th to 20th	Night	12% 16% 7% 16% 9% 5% 3 3 5% 4 20%	2484
	21st to end	Night	13% 17% 5% 12% 7% 5% 4 2 4 5% 25%	2076
March	1st to 10th	Night	19% 16% 6% 12% 7% 6% 3 2 4 4 22%	5484
	11th to 20th	Night	14% 17% 5% 10% 6% 5% 4 3 4 6% 27%	5393
	21st to end	Night	20% 17% 5% 12% 5% 4 3 3 3 4 25%	5072
April	1st to 10th	Night	21% 16% 4 8% 4 4 4 2 4 5% 27%	2100
	11th to 20th	Night	24% 14% 4 8% 4 4 3 3 3 5% 29%	2075
	21st to end	Night	22% 12% 4 7% 4 4 3 3 4 5% 33%	1920
May	1st to 10th	Night	30% 13% 3 7% 4 4 4 4 5% 6% 20%	4017
	11th to 20th	Night	21% 11% 4 7% 5% 4 5% 3 5% 7% 29%	3655
	21st to end	Night	22% 10% 3 7% 4 4 4 4 6% 7% 30%	4014
June	1st to 10th	Night	27% 7% 3 6% 5% 4 5% 3 4 6% 30%	1600
	11th to 20th	Night	19% 6% 2 7% 5% 4 4 4 6% 8% 35%	1600
	21st to end	Night	22% 6% 3 6% 6% 4 5% 3 7% 6% 32%	1600
July	1st to 10th	Night	15% 6% 2 6% 5% 5% 6% 6% 6% 9% 35%	3600
	11th to 20th	Night	18% 7% 2 7% 4 4 5% 4 6% 7% 37%	3600
	21st to end	Night	14% 7% 3 7% 4 5% 5% 3 6% 7% 39%	3926
August	1st to 10th	Night	21% 8% 3 6% 4 4 4 3 5% 6% 35%	2586
	11th to 20th	Night	17% 7% 3 7% 5% 4 4 4 5% 6% 37%	2985
	21st to end	Night	16% 5% 2 5% 4 4 4 4 6% 8% 42%	3586
September	1st to 10th	Night	17% 6% 2 5% 5% 3 4 3 5% 6% 45%	2000
	11th to 20th	Night	14% 7% 1 5% 3 3 4 3 4 7% 48%	2120
	21st to end	Night	19% 6% 2 6% 4 4 4 3 4 6% 42%	2200
October	1st to 10th	Night	23% 9% 2 6% 4 5% 4 4 5% 5% 33%	2274
	11th to 20th	Night	22% 12% 3 7% 5% 4 4 4 4 6% 30%	2459
	21st to end	Night	27% 15% 3 7% 4 4 4 3 4 5% 24%	2860
November	1st to 10th	Night	21% 15% 4 8% 4 3 4 4 4 5% 29%	2667
	11th to 20th	Night	18% 18% 5% 8% 5% 4 3 3 4 5% 27%	2730
	21st to end	Night	19% 19% 6% 12% 5% 4 4 3 4 5% 20%	2919
December	1st to 10th	Night	15% 22% 11% 18% 6% 5% 4 2 3 3 10%	4720
	11th to 20th	Night	21% 18% 5% 14% 6% 5% 4 2 3 3 18%	5348
	21st to end	Night	17% 18% 6% 14% 6% 6% 5% 3 4 3 19%	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	71% 8% 9% 11% 2	4662
	11th to 20th	Day	69% 7% 7% 10% 5% 1	4680
	21st to end	Day	65% 7% 9% 11% 8%	5124
February	1st to 10th	Day	64% 7% 7% 15% 5% 2	1843
	11th to 20th	Day	65% 7% 8% 15% 5%	2030
	21st to end	Day	65% 6% 8% 11% 9% 2	1730
March	1st to 10th	Day	67% 6% 8% 9% 7% 3	4579
	11th to 20th	Day	56% 8% 12% 14% 9% 2	4783
	21st to end	Day	58% 7% 6% 12% 15% 1	6102
April	1st to 10th	Day	59% 5% 7% 18% 9% 2	2520
	11th to 20th	Day	55% 5% 7% 18% 14% 1	2494
	21st to end	Day	56% 5% 9% 18% 10% 2	2698
May	1st to 10th	Day	65% 8% 7% 15% 6% 1	6185
	11th to 20th	Day	53% 9% 14% 16% 8% 1	6462
	21st to end	Day	50% 10% 11% 21% 7% 1	7103
June	1st to 10th	Day	53% 8% 12% 17% 9%	2800
	11th to 20th	Day	46% 8% 12% 21% 12% 1	2800
	21st to end	Day	52% 8% 11% 19% 9% 1	2800
July	1st to 10th	Day	44% 9% 16% 23% 9%	6290
	11th to 20th	Day	40% 10% 15% 26% 9%	6285
	21st to end	Day	40% 8% 12% 33% 6%	6728
August	1st to 10th	Day	47% 8% 15% 22% 7%	4549
	11th to 20th	Day	48% 7% 16% 23% 6%	4064
	21st to end	Day	39% 7% 17% 22% 13% 1	4312
September	1st to 10th	Day	38% 7% 12% 30% 12% 1	2400
	11th to 20th	Day	37% 5% 11% 31% 15% 1	2280
	21st to end	Day	44% 8% 15% 22% 10% 2	2200
October	1st to 10th	Day	45% 6% 11% 26% 11%	3461
	11th to 20th	Day	49% 6% 13% 21% 11%	3143
	21st to end	Day	50% 6% 13% 25% 7% 1	3180
November	1st to 10th	Day	57% 7% 11% 17% 8%	1851
	11th to 20th	Day	61% 6% 10% 13% 8% 2	1890
	21st to end	Day	64% 8% 12% 13% 3	1696
December	1st to 10th	Day	76% 6% 8% 7% 2	2680
	11th to 20th	Day	66% 6% 8% 15% 4	3066
	21st to end	Day	66% 9% 7% 11% 7% 1	4048
January	1st to 10th	Night	75% 6% 9% 7% 2	8234
	11th to 20th	Night	75% 7% 7% 7% 4 1	8280
	21st to end	Night	69% 8% 9% 8% 7% 1	9057
February	1st to 10th	Night	68% 6% 8% 12% 6% 1	2773
	11th to 20th	Night	66% 6% 7% 14% 7%	2486
	21st to end	Night	61% 7% 8% 16% 7% 1	2076
March	1st to 10th	Night	67% 4 8% 12% 8% 1	5494
	11th to 20th	Night	57% 6% 13% 14% 8% 1	5394
	21st to end	Night	62% 6% 5% 13% 13% 1	5072
April	1st to 10th	Night	60% 5% 7% 16% 11% 1	2100
	11th to 20th	Night	59% 6% 7% 16% 12%	2075
	21st to end	Night	53% 5% 9% 17% 13% 3	1920
May	1st to 10th	Night	63% 8% 8% 13% 6% 1	4023
	11th to 20th	Night	54% 9% 10% 18% 9% 1	3656
	21st to end	Night	53% 7% 11% 18% 11% 1	4015
June	1st to 10th	Night	54% 8% 10% 19% 10% 1	1600
	11th to 20th	Night	45% 8% 14% 20% 11% 2	1600
	21st to end	Night	49% 8% 13% 20% 9%	1600
July	1st to 10th	Night	43% 9% 18% 21% 9%	3560
	11th to 20th	Night	44% 9% 12% 25% 10%	3552
	21st to end	Night	41% 8% 14% 28% 8%	3809
August	1st to 10th	Night	49% 7% 13% 22% 8%	2576
	11th to 20th	Night	46% 8% 15% 21% 9% 1	3012
	21st to end	Night	38% 8% 15% 25% 12% 1	3586
September	1st to 10th	Night	40% 7% 14% 28% 12%	2000
	11th to 20th	Night	37% 6% 14% 30% 12%	2120
	21st to end	Night	43% 6% 12% 24% 14% 2	2200
October	1st to 10th	Night	53% 7% 10% 21% 9%	3716
	11th to 20th	Night	52% 5% 10% 21% 12%	3995
	21st to end	Night	54% 5% 12% 22% 6% 1	4610
November	1st to 10th	Night	57% 6% 11% 17% 8% 1	2667
	11th to 20th	Night	60% 5% 10% 15% 8% 2	2730
	21st to end	Night	66% 7% 10% 13% 4	2919
December	1st to 10th	Night	79% 7% 7% 7% 1	4720
	11th to 20th	Night	71% 6% 6% 11% 6%	5348
	21st to end	Night	69% 6% 6% 10% 6% 2	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	27% 27% 28% 13% 5%	220
	11th to 20th	Day	35% 25% 29% 8% 3 1	220
	21st to end	Day	27% 28% 31% 10% 4	242
February	1st to 10th	Day	33% 30% 28% 8% 2	305
	11th to 20th	Day	32% 23% 34% 9% 2	509
	21st to end	Day	31% 24% 33% 10% 2	446
March	1st to 10th	Day	35% 20% 28% 15% 2	540
	11th to 20th	Day	33% 25% 26% 12% 3	540
	21st to end	Day	36% 21% 24% 12% 5% 2	594
April	1st to 10th	Day	39% 19% 25% 12% 4 1	540
	11th to 20th	Day	37% 23% 21% 12% 5% 2	532
	21st to end	Day	32% 24% 25% 11% 6% 2	540
May	1st to 10th	Day	36% 19% 26% 12% 6% 2	543
	11th to 20th	Day	24% 17% 33% 18% 6% 2	567
	21st to end	Day	24% 19% 31% 19% 6% 1	617
June	1st to 10th	Day	29% 16% 30% 17% 6% 1	530
	11th to 20th	Day	23% 16% 32% 21% 7% 2	530
	21st to end	Day	24% 18% 31% 19% 6% 1	530
July	1st to 10th	Day	17% 13% 34% 23% 10% 2	525
	11th to 20th	Day	17% 10% 35% 26% 10% 2	524
	21st to end	Day	16% 10% 37% 28% 7% 1	576
August	1st to 10th	Day	16% 10% 30% 27% 14% 3	526
	11th to 20th	Day	19% 10% 27% 25% 13% 5%	519
	21st to end	Day	15% 15% 31% 25% 11% 4	583
September	1st to 10th	Day	18% 14% 31% 19% 12% 6%	530
	11th to 20th	Day	16% 13% 22% 22% 20% 7%	530
	21st to end	Day	19% 14% 31% 21% 12% 4	530
October	1st to 10th	Day	27% 13% 25% 18% 14% 3	530
	11th to 20th	Day	28% 18% 26% 21% 6% 1	497
	21st to end	Day	38% 17% 24% 17% 3	462
November	1st to 10th	Day	32% 21% 22% 18% 6%	425
	11th to 20th	Day	29% 26% 24% 15% 5% 1	430
	21st to end	Day	37% 22% 28% 12% 1	236
December	1st to 10th	Day	30% 22% 31% 14% 4	220
	11th to 20th	Day	35% 27% 29% 8% 1	220
	21st to end	Day	30% 24% 33% 10% 3	242
January	1st to 10th	Night	26% 26% 30% 16% 2	840
	11th to 20th	Night	27% 24% 29% 17% 3	840
	21st to end	Night	21% 21% 31% 22% 5%	924
February	1st to 10th	Night	23% 19% 35% 21% 3	840
	11th to 20th	Night	24% 22% 33% 18% 3	823
	21st to end	Night	26% 21% 30% 19% 4	692
March	1st to 10th	Night	29% 19% 28% 23% 2	840
	11th to 20th	Night	28% 23% 26% 18% 5%	840
	21st to end	Night	31% 18% 25% 19% 6% 1	924
April	1st to 10th	Night	29% 18% 27% 22% 4	840
	11th to 20th	Night	33% 18% 24% 18% 7%	831
	21st to end	Night	30% 15% 27% 22% 6%	660
May	1st to 10th	Night	30% 14% 25% 23% 7% 1	581
	11th to 20th	Night	22% 11% 35% 25% 7% 1	414
	21st to end	Night	21% 10% 28% 30% 10% 1	455
June	1st to 10th	Night	27% 8% 28% 29% 9% 1	400
	11th to 20th	Night	22% 8% 25% 35% 10% 1	400
	21st to end	Night	23% 7% 25% 38% 8% 1	400
July	1st to 10th	Night	14% 5% 24% 42% 14% 1	390
	11th to 20th	Night	14% 5% 23% 40% 18% 1	388
	21st to end	Night	15% 8% 21% 44% 13%	438
August	1st to 10th	Night	17% 5% 16% 41% 20% 1	398
	11th to 20th	Night	16% 6% 20% 36% 20% 2	652
	21st to end	Night	17% 7% 23% 33% 19% 1	880
September	1st to 10th	Night	19% 9% 24% 32% 16% 2	800
	11th to 20th	Night	18% 7% 18% 31% 23% 3	800
	21st to end	Night	20% 12% 24% 27% 16% 2	800
October	1st to 10th	Night	28% 10% 20% 26% 15% 1	800
	11th to 20th	Night	29% 13% 23% 25% 9% 1	800
	21st to end	Night	34% 14% 26% 20% 5%	880
November	1st to 10th	Night	30% 19% 25% 18% 7% 1	820
	11th to 20th	Night	29% 18% 27% 17% 8% 1	840
	21st to end	Night	30% 23% 26% 16% 5% 1	840
December	1st to 10th	Night	30% 23% 28% 17% 3	840
	11th to 20th	Night	29% 22% 25% 20% 3	840
	21st to end	Night	31% 19% 29% 17% 3	924

Page generated at 2026-05-29 01:53:32. Version ac07

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