Scopehill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Scopehill Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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	13% 6% 3 5% 5% 3 4 4 5% 7% 45%	74573
	Night	12% 6% 2 5% 4 2 4 4 5% 7% 49%	75233
Transparency	Day	39% 3 12% 27% 16% 3	74296
	Night	36% 3 11% 27% 20% 4	75030
Seeing	Day	15% 7% 25% 28% 19% 6%	14436
	Night	15% 7% 29% 34% 14% 1	24414

By Month

Percentage Cloud Cover SE (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% 7% 3 5% 5% 2 2 4 4 7% 47%	1707
	11th to 20th	Day	14% 10% 4 5% 5% 2 4 3 4 6% 41%	1670
	21st to end	Day	11% 8% 4 7% 5% 2 5% 4 4 6% 43%	1782
February	1st to 10th	Day	16% 10% 4 8% 5% 2 4 3 3 5% 41%	1620
	11th to 20th	Day	13% 9% 4 6% 6% 3 4 3 3 5% 44%	1575
	21st to end	Day	14% 10% 3 5% 5% 3 4 4 5% 6% 40%	1564
March	1st to 10th	Day	14% 8% 4 6% 5% 3 3 3 4 6% 44%	1980
	11th to 20th	Day	22% 9% 3 6% 4 2 4 4 4 5% 37%	1980
	21st to end	Day	21% 10% 4 6% 5% 3 5% 4 5% 5% 34%	2178
April	1st to 10th	Day	12% 7% 3 4 6% 4 5% 4 5% 7% 44%	2070
	11th to 20th	Day	16% 7% 2 5% 5% 3 5% 5% 5% 7% 41%	2134
	21st to end	Day	21% 7% 3 6% 5% 3 4 4 4 6% 37%	2250
May	1st to 10th	Day	19% 8% 3 6% 5% 3 4 4 5% 6% 37%	2340
	11th to 20th	Day	16% 7% 2 5% 6% 3 5% 4 6% 7% 40%	2302
	21st to end	Day	13% 6% 3 7% 7% 4 5% 5% 6% 9% 34%	2531
June	1st to 10th	Day	6% 4 2 4 5% 3 5% 6% 7% 10% 48%	2416
	11th to 20th	Day	8% 4 3 4 4 3 4 5% 7% 9% 48%	2515
	21st to end	Day	6% 2 2 3 4 3 5% 5% 6% 9% 56%	2340
July	1st to 10th	Day	10% 4 2 5% 5% 4 5% 6% 7% 9% 41%	2340
	11th to 20th	Day	5% 2 2 4 4 3 4 5% 7% 9% 56%	2340
	21st to end	Day	8% 4 3 5% 6% 3 6% 6% 7% 10% 44%	2546
August	1st to 10th	Day	7% 3 2 5% 6% 3 5% 6% 7% 11% 46%	2328
	11th to 20th	Day	8% 4 2 4 6% 3 5% 6% 6% 9% 48%	2301
	21st to end	Day	7% 4 2 4 4 3 5% 4 6% 9% 52%	2408
September	1st to 10th	Day	12% 5% 2 4 3 4 4 3 4 7% 52%	2160
	11th to 20th	Day	11% 4 2 6% 4 3 4 5% 5% 9% 48%	1989
	21st to end	Day	9% 4 1 4 4 2 4 4 4 6% 57%	1972
October	1st to 10th	Day	12% 5% 2 5% 3 4 3 4 4 7% 52%	1800
	11th to 20th	Day	13% 5% 2 4 3 1 2 3 4 6% 56%	1800
	21st to end	Day	19% 6% 2 5% 3 2 3 3 4 4 48%	1980
November	1st to 10th	Day	11% 4 2 4 3 2 3 3 4 6% 57%	1858
	11th to 20th	Day	15% 6% 2 3 3 2 3 3 4 6% 52%	1754
	21st to end	Day	16% 6% 3 5% 4 2 2 2 3 6% 51%	1525
December	1st to 10th	Day	12% 7% 3 6% 5% 3 4 4 4 6% 47%	1520
	11th to 20th	Day	14% 9% 4 5% 5% 3 4 3 3 5% 45%	1918
	21st to end	Day	19% 15% 5% 6% 5% 3 4 4 5% 6% 29%	3080
January	1st to 10th	Night	12% 7% 2 4 5% 3 4 4 4 6% 50%	2458
	11th to 20th	Night	13% 6% 3 6% 4 2 4 4 4 6% 47%	2418
	21st to end	Night	11% 7% 3 6% 4 3 4 3 5% 6% 49%	2574
February	1st to 10th	Night	13% 11% 4 5% 5% 2 4 4 5% 5% 42%	2340
	11th to 20th	Night	14% 7% 3 6% 4 3 4 5% 5% 6% 44%	2273
	21st to end	Night	16% 9% 3 5% 4 2 3 4 4 6% 44%	1705
March	1st to 10th	Night	13% 6% 2 5% 5% 2 4 4 4 7% 48%	1980
	11th to 20th	Night	17% 8% 3 4 5% 3 4 3 4 6% 43%	1980
	21st to end	Night	19% 8% 2 6% 6% 2 4 4 5% 7% 37%	2178
April	1st to 10th	Night	11% 6% 2 4 5% 2 4 4 4 7% 50%	1889
	11th to 20th	Night	14% 7% 2 5% 4 2 3 4 4 6% 48%	1775
	21st to end	Night	17% 6% 2 5% 5% 3 4 4 4 5% 44%	1706
May	1st to 10th	Night	14% 6% 3 6% 5% 3 5% 5% 5% 6% 41%	1620
	11th to 20th	Night	14% 5% 3 5% 4 2 4 4 5% 6% 49%	1593
	21st to end	Night	11% 6% 3 4 4 3 5% 4 6% 11% 42%	1745
June	1st to 10th	Night	5% 2 2 4 4 3 4 5% 6% 10% 53%	1530
	11th to 20th	Night	5% 3 1 4 4 3 5% 5% 8% 11% 51%	1440
	21st to end	Night	5% 3 1 3 3 3 3 4 6% 7% 62%	1611
July	1st to 10th	Night	6% 4 2 5% 4 3 4 6% 6% 10% 50%	1620
	11th to 20th	Night	3 1 1 3 4 3 4 5% 6% 9% 60%	1620
	21st to end	Night	7% 3 2 3 4 3 5% 5% 6% 11% 52%	1772
August	1st to 10th	Night	5% 3 2 4 5% 3 6% 5% 6% 11% 52%	1611
	11th to 20th	Night	6% 4 2 4 3 3 5% 4 5% 8% 57%	1600
	21st to end	Night	4 3 2 4 4 2 3 3 5% 8% 61%	1948
September	1st to 10th	Night	11% 5% 2 4 2 2 3 4 4 7% 56%	1800
	11th to 20th	Night	11% 4 1 3 4 3 4 5% 6% 7% 52%	1968
	21st to end	Night	6% 4 2 4 4 2 4 4 5% 7% 59%	1980
October	1st to 10th	Night	9% 4 2 3 3 2 3 4 5% 6% 59%	2150
	11th to 20th	Night	14% 4 2 4 3 2 3 3 4 5% 56%	2160
	21st to end	Night	16% 6% 2 4 4 2 3 3 3 5% 53%	2376
November	1st to 10th	Night	10% 5% 2 3 3 2 2 3 3 5% 63%	2220
	11th to 20th	Night	12% 6% 2 5% 4 2 3 4 4 6% 53%	2422
	21st to end	Night	17% 6% 2 3 4 1 3 3 3 6% 52%	2650
December	1st to 10th	Night	12% 8% 3 4 4 2 3 4 5% 7% 47%	2660
	11th to 20th	Night	13% 8% 3 6% 4 3 3 4 4 6% 46%	3279
	21st to end	Night	19% 14% 5% 6% 5% 3 3 4 3 5% 33%	4582

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	39% 2 11% 28% 19% 1	1708
	11th to 20th	Day	46% 2 11% 26% 14% 1	1670
	21st to end	Day	43% 3 14% 27% 13% 1	1782
February	1st to 10th	Day	49% 4 9% 26% 10% 2	1620
	11th to 20th	Day	46% 2 13% 22% 15% 3	1584
	21st to end	Day	45% 2 8% 26% 14% 5%	1564
March	1st to 10th	Day	44% 2 10% 22% 18% 4	1980
	11th to 20th	Day	51% 2 10% 22% 12% 3	1980
	21st to end	Day	53% 3 13% 20% 9% 1	2178
April	1st to 10th	Day	40% 3 13% 25% 17% 2	2070
	11th to 20th	Day	45% 3 11% 24% 14% 2	2134
	21st to end	Day	50% 2 11% 25% 11% 1	2250
May	1st to 10th	Day	49% 4 12% 21% 13% 1	2340
	11th to 20th	Day	44% 4 14% 26% 12% 1	2306
	21st to end	Day	46% 5% 15% 23% 9% 1	2531
June	1st to 10th	Day	31% 4 17% 29% 17% 2	2416
	11th to 20th	Day	32% 4 13% 32% 16% 2	2515
	21st to end	Day	24% 4 15% 35% 19% 3	2340
July	1st to 10th	Day	37% 4 14% 26% 16% 3	2276
	11th to 20th	Day	25% 3 16% 32% 20% 3	2271
	21st to end	Day	34% 4 16% 31% 15% 1	2405
August	1st to 10th	Day	31% 5% 20% 27% 15% 1	2314
	11th to 20th	Day	33% 4 12% 33% 17% 1	2301
	21st to end	Day	29% 4 13% 28% 20% 6%	2408
September	1st to 10th	Day	34% 3 11% 29% 16% 7%	2160
	11th to 20th	Day	34% 3 9% 26% 26% 2	1989
	21st to end	Day	30% 3 9% 28% 26% 5%	1972
October	1st to 10th	Day	35% 2 10% 27% 23% 4	1800
	11th to 20th	Day	33% 2 6% 29% 24% 6%	1800
	21st to end	Day	42% 2 8% 23% 19% 6%	1980
November	1st to 10th	Day	30% 2 10% 31% 22% 5%	1858
	11th to 20th	Day	35% 2 8% 27% 25% 4	1754
	21st to end	Day	39% 2 9% 28% 21% 1	1525
December	1st to 10th	Day	40% 2 9% 34% 14% 1	1520
	11th to 20th	Day	44% 1 9% 26% 17% 2	1915
	21st to end	Day	59% 2 9% 21% 8% 2	3080
January	1st to 10th	Night	37% 3 13% 27% 18% 3	2458
	11th to 20th	Night	41% 2 10% 29% 16% 2	2418
	21st to end	Night	38% 2 12% 32% 13% 3	2574
February	1st to 10th	Night	45% 3 10% 25% 15% 2	2340
	11th to 20th	Night	42% 2 10% 26% 17% 3	2274
	21st to end	Night	42% 2 8% 24% 18% 6%	1705
March	1st to 10th	Night	38% 2 9% 24% 21% 6%	1980
	11th to 20th	Night	44% 3 9% 22% 18% 4	1980
	21st to end	Night	48% 4 11% 23% 12% 2	2178
April	1st to 10th	Night	36% 3 10% 24% 23% 4	1889
	11th to 20th	Night	40% 3 8% 25% 20% 4	1775
	21st to end	Night	45% 3 10% 25% 17% 1	1706
May	1st to 10th	Night	44% 5% 10% 24% 16% 2	1620
	11th to 20th	Night	37% 3 14% 28% 16% 2	1594
	21st to end	Night	37% 5% 17% 29% 11% 1	1746
June	1st to 10th	Night	25% 3 19% 32% 17% 3	1530
	11th to 20th	Night	26% 4 17% 30% 20% 2	1440
	21st to end	Night	22% 3 13% 35% 22% 4	1611
July	1st to 10th	Night	30% 4 14% 30% 17% 4	1575
	11th to 20th	Night	20% 4 13% 34% 25% 4	1566
	21st to end	Night	27% 3 15% 31% 22% 1	1673
August	1st to 10th	Night	26% 5% 17% 35% 15% 3	1601
	11th to 20th	Night	26% 3 10% 33% 25% 2	1600
	21st to end	Night	23% 3 13% 28% 25% 7%	1948
September	1st to 10th	Night	29% 3 12% 28% 21% 7%	1800
	11th to 20th	Night	30% 4 9% 25% 27% 6%	1968
	21st to end	Night	27% 3 7% 29% 30% 5%	1980
October	1st to 10th	Night	27% 3 10% 27% 26% 7%	2150
	11th to 20th	Night	32% 2 6% 27% 26% 7%	2160
	21st to end	Night	37% 2 7% 23% 25% 6%	2376
November	1st to 10th	Night	27% 2 10% 29% 25% 6%	2220
	11th to 20th	Night	35% 2 6% 27% 27% 4	2422
	21st to end	Night	37% 2 8% 26% 23% 3	2650
December	1st to 10th	Night	37% 3 12% 27% 19% 1	2660
	11th to 20th	Night	41% 2 8% 28% 19% 2	3281
	21st to end	Night	57% 2 8% 20% 11% 2	4582

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	21% 11% 33% 24% 10% 1	270
	11th to 20th	Day	26% 16% 34% 21% 2	257
	21st to end	Day	16% 12% 45% 21% 6%	264
February	1st to 10th	Day	23% 16% 35% 22% 4	240
	11th to 20th	Day	19% 18% 42% 17% 4 1	240
	21st to end	Day	23% 21% 38% 14% 3 1	198
March	1st to 10th	Day	21% 15% 37% 23% 4	240
	11th to 20th	Day	28% 11% 30% 23% 8%	240
	21st to end	Day	31% 11% 39% 15% 2 1	264
April	1st to 10th	Day	15% 17% 37% 22% 8% 1	330
	11th to 20th	Day	19% 13% 36% 25% 8%	412
	21st to end	Day	24% 12% 32% 21% 10%	510
May	1st to 10th	Day	20% 9% 33% 26% 11% 1	603
	11th to 20th	Day	20% 7% 29% 29% 12% 4	625
	21st to end	Day	16% 5% 26% 26% 19% 7%	679
June	1st to 10th	Day	6% 3 15% 35% 30% 11%	630
	11th to 20th	Day	8% 2 21% 38% 23% 8%	630
	21st to end	Day	7% 4 27% 34% 24% 4	630
July	1st to 10th	Day	11% 4 11% 29% 32% 14%	621
	11th to 20th	Day	5% 1 11% 36% 38% 9%	618
	21st to end	Day	9% 1 8% 32% 35% 13%	684
August	1st to 10th	Day	9% 1 12% 36% 34% 8%	626
	11th to 20th	Day	7% 2 13% 32% 38% 8%	615
	21st to end	Day	9% 2 9% 27% 37% 17%	527
September	1st to 10th	Day	14% 3 11% 30% 33% 9%	450
	11th to 20th	Day	5% 1 15% 37% 31% 11%	279
	21st to end	Day	12% 2 19% 36% 25% 7%	270
October	1st to 10th	Day	14% 6% 27% 27% 16% 10%	270
	11th to 20th	Day	13% 9% 40% 29% 8% 1	270
	21st to end	Day	28% 10% 31% 23% 8% 1	297
November	1st to 10th	Day	16% 8% 33% 26% 13% 4	270
	11th to 20th	Day	16% 10% 35% 25% 13% 1	270
	21st to end	Day	23% 10% 33% 25% 7% 2	270
December	1st to 10th	Day	13% 12% 41% 21% 9% 3	270
	11th to 20th	Day	17% 11% 35% 29% 7% 1	270
	21st to end	Day	20% 17% 41% 21% 1	297
January	1st to 10th	Night	15% 11% 38% 29% 6%	950
	11th to 20th	Night	17% 12% 39% 27% 5%	930
	21st to end	Night	14% 11% 39% 31% 4	990
February	1st to 10th	Night	19% 11% 39% 27% 4	900
	11th to 20th	Night	18% 11% 40% 29% 3	879
	21st to end	Night	21% 13% 36% 25% 3	521
March	1st to 10th	Night	17% 9% 38% 32% 4	540
	11th to 20th	Night	22% 8% 32% 32% 6% 1	540
	21st to end	Night	23% 10% 37% 26% 4 1	594
April	1st to 10th	Night	13% 10% 38% 31% 7%	540
	11th to 20th	Night	19% 10% 33% 32% 6%	533
	21st to end	Night	21% 6% 38% 27% 8%	540
May	1st to 10th	Night	17% 6% 31% 37% 10%	540
	11th to 20th	Night	18% 3 26% 37% 15% 1	531
	21st to end	Night	13% 5% 23% 35% 20% 3	584
June	1st to 10th	Night	6% 4 14% 43% 33% 1	540
	11th to 20th	Night	5% 1 22% 49% 21% 1	540
	21st to end	Night	6% 1 24% 49% 19% 1	540
July	1st to 10th	Night	8% 2 9% 42% 35% 4	525
	11th to 20th	Night	3 1 11% 46% 36% 2	522
	21st to end	Night	8% 1 9% 40% 40% 2	591
August	1st to 10th	Night	6% 2 10% 44% 36% 3	537
	11th to 20th	Night	9% 1 9% 46% 33% 3	534
	21st to end	Night	6% 1 9% 37% 43% 4	594
September	1st to 10th	Night	13% 2 11% 43% 28% 4	540
	11th to 20th	Night	11% 2 14% 38% 29% 6%	540
	21st to end	Night	9% 1 20% 43% 24% 4	540
October	1st to 10th	Night	12% 2 25% 36% 22% 2	710
	11th to 20th	Night	15% 6% 36% 33% 9%	720
	21st to end	Night	18% 8% 33% 31% 9% 1	792
November	1st to 10th	Night	13% 6% 37% 32% 12% 1	740
	11th to 20th	Night	16% 9% 35% 30% 10%	902
	21st to end	Night	20% 9% 35% 29% 7%	950
December	1st to 10th	Night	18% 7% 38% 30% 6%	950
	11th to 20th	Night	22% 7% 34% 32% 5%	950
	21st to end	Night	20% 12% 36% 28% 4	1045

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