Elliott-Astronomy Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Elliott-Astronomy Observatory clear sky chart from 2005-11-06 to 2024-01-22.

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% 10% 4 7% 6% 3 5% 5% 5% 7% 31%	74746
	Night	17% 11% 4 8% 6% 3 5% 5% 5% 6% 30%	75185
Transparency	Day	51% 9% 13% 17% 9% 1	74461
	Night	53% 8% 12% 16% 9% 1	75004
Seeing	Day	21% 16% 28% 21% 10% 3	13412
	Night	22% 15% 29% 24% 10% 1	22946

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	16% 17% 8% 12% 10% 4 6% 4 4 5% 14%	1517
	11th to 20th	Day	18% 18% 6% 13% 10% 4 5% 5% 4 4 14%	1657
	21st to end	Day	14% 17% 7% 13% 12% 6% 6% 5% 5% 5% 11%	1791
February	1st to 10th	Day	19% 18% 5% 13% 9% 5% 5% 4 4 5% 14%	1620
	11th to 20th	Day	14% 17% 8% 14% 10% 4 5% 3 4 4 16%	1573
	21st to end	Day	21% 14% 6% 11% 10% 3 5% 4 4 5% 17%	1393
March	1st to 10th	Day	16% 11% 6% 12% 8% 5% 5% 4 4 4 23%	1977
	11th to 20th	Day	14% 16% 5% 7% 5% 4 5% 4 5% 7% 29%	1980
	21st to end	Day	19% 12% 4 8% 6% 4 5% 4 4 6% 29%	2178
April	1st to 10th	Day	17% 14% 5% 7% 5% 3 4 4 5% 6% 30%	2146
	11th to 20th	Day	19% 9% 3 6% 6% 3 4 4 4 6% 36%	2151
	21st to end	Day	21% 9% 4 7% 6% 3 4 4 5% 5% 34%	2340
May	1st to 10th	Day	16% 9% 3 7% 6% 4 5% 5% 5% 7% 34%	2340
	11th to 20th	Day	16% 7% 3 6% 5% 3 4 5% 5% 7% 38%	2471
	21st to end	Day	12% 6% 3 4 4 3 4 4 6% 9% 44%	2725
June	1st to 10th	Day	16% 5% 3 5% 4 3 5% 5% 6% 7% 40%	2520
	11th to 20th	Day	13% 5% 3 5% 6% 3 4 5% 7% 10% 40%	2520
	21st to end	Day	15% 5% 2 5% 5% 3 4 5% 7% 9% 42%	2520
July	1st to 10th	Day	8% 4 2 5% 4 3 5% 6% 6% 9% 49%	2520
	11th to 20th	Day	12% 5% 2 5% 6% 3 6% 6% 7% 10% 37%	2520
	21st to end	Day	11% 5% 3 5% 6% 4 6% 6% 7% 11% 37%	2578
August	1st to 10th	Day	9% 6% 2 5% 5% 3 5% 6% 7% 10% 42%	2327
	11th to 20th	Day	10% 5% 2 6% 5% 5% 6% 6% 7% 10% 37%	2303
	21st to end	Day	12% 5% 2 5% 5% 3 5% 5% 5% 8% 43%	2441
September	1st to 10th	Day	14% 7% 3 5% 5% 2 4 5% 7% 8% 41%	2160
	11th to 20th	Day	12% 5% 1 4 5% 3 5% 5% 7% 8% 43%	2089
	21st to end	Day	15% 7% 3 5% 6% 5% 5% 5% 6% 8% 35%	1922
October	1st to 10th	Day	21% 7% 3 6% 6% 3 5% 5% 4 7% 33%	1800
	11th to 20th	Day	20% 10% 4 9% 6% 3 5% 4 5% 6% 27%	1800
	21st to end	Day	27% 14% 4 8% 6% 3 4 3 4 6% 22%	1980
November	1st to 10th	Day	15% 10% 3 8% 7% 4 6% 4 5% 6% 32%	1649
	11th to 20th	Day	16% 13% 4 9% 6% 3 4 4 5% 8% 28%	1520
	21st to end	Day	21% 17% 5% 8% 7% 4 4 4 4 6% 21%	1520
December	1st to 10th	Day	20% 19% 7% 11% 7% 3 5% 4 4 5% 15%	1520
	11th to 20th	Day	20% 19% 7% 11% 7% 3 3 5% 5% 5% 15%	1906
	21st to end	Day	27% 22% 6% 11% 5% 3 4 4 3 4 11%	2772
January	1st to 10th	Night	18% 18% 6% 11% 7% 5% 5% 5% 3 4 17%	2649
	11th to 20th	Night	19% 19% 7% 12% 8% 4 4 4 4 4 14%	2520
	21st to end	Night	15% 16% 7% 12% 10% 5% 6% 4 4 5% 16%	2599
February	1st to 10th	Night	19% 17% 6% 11% 9% 3 5% 4 4 5% 18%	2340
	11th to 20th	Night	16% 15% 5% 10% 8% 4 6% 5% 5% 5% 21%	2273
	21st to end	Night	19% 15% 6% 12% 7% 3 4 4 4 6% 19%	1858
March	1st to 10th	Night	14% 12% 5% 9% 8% 3 5% 5% 4 6% 27%	1970
	11th to 20th	Night	18% 11% 4 8% 5% 3 5% 4 4 6% 31%	1979
	21st to end	Night	17% 11% 4 9% 6% 3 4 4 4 5% 34%	2178
April	1st to 10th	Night	19% 11% 4 8% 7% 3 4 4 4 5% 31%	1814
	11th to 20th	Night	17% 9% 3 7% 5% 3 5% 5% 5% 6% 35%	1775
	21st to end	Night	18% 9% 4 6% 4 3 6% 6% 5% 6% 35%	1620
May	1st to 10th	Night	18% 8% 3 6% 5% 3 5% 5% 6% 7% 34%	1620
	11th to 20th	Night	13% 6% 3 5% 5% 5% 5% 5% 6% 8% 40%	1420
	21st to end	Night	12% 6% 2 5% 5% 2 5% 5% 6% 7% 45%	1550
June	1st to 10th	Night	12% 6% 3 6% 6% 4 7% 6% 7% 8% 35%	1440
	11th to 20th	Night	11% 4 3 6% 6% 3 6% 5% 5% 7% 43%	1440
	21st to end	Night	13% 5% 3 7% 4 4 5% 5% 5% 9% 42%	1440
July	1st to 10th	Night	6% 5% 2 5% 5% 3 4 4 6% 8% 52%	1440
	11th to 20th	Night	11% 5% 2 5% 5% 3 5% 5% 5% 9% 45%	1440
	21st to end	Night	11% 4 2 5% 5% 3 5% 6% 6% 8% 45%	1736
August	1st to 10th	Night	8% 6% 2 4 5% 3 5% 6% 7% 9% 45%	1611
	11th to 20th	Night	10% 5% 3 6% 5% 3 4 6% 5% 9% 45%	1598
	21st to end	Night	10% 6% 2 5% 6% 3 4 5% 6% 8% 46%	1895
September	1st to 10th	Night	13% 7% 2 5% 5% 3 4 4 6% 6% 44%	1800
	11th to 20th	Night	14% 4 2 4 4 2 4 4 5% 7% 51%	1867
	21st to end	Night	15% 8% 3 5% 6% 3 5% 5% 5% 8% 38%	2038
October	1st to 10th	Night	16% 8% 3 6% 5% 3 5% 5% 5% 7% 38%	2160
	11th to 20th	Night	22% 10% 3 6% 6% 3 5% 5% 5% 6% 29%	2160
	21st to end	Night	26% 11% 3 7% 5% 3 4 5% 4 6% 25%	2376
November	1st to 10th	Night	17% 9% 4 6% 4 3 4 4 4 7% 38%	2427
	11th to 20th	Night	15% 12% 5% 7% 7% 4 5% 5% 5% 7% 29%	2660
	21st to end	Night	19% 13% 6% 8% 6% 3 5% 5% 5% 6% 23%	2660
December	1st to 10th	Night	22% 18% 8% 9% 7% 3 4 4 4 4 17%	2660
	11th to 20th	Night	17% 21% 6% 10% 6% 3 5% 4 4 5% 19%	3288
	21st to end	Night	30% 15% 6% 8% 6% 4 5% 4 5% 5% 11%	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	72% 9% 8% 9% 2	1518
	11th to 20th	Day	74% 9% 8% 7% 1	1657
	21st to end	Day	76% 7% 9% 5% 2	1791
February	1st to 10th	Day	73% 7% 11% 8% 1	1620
	11th to 20th	Day	71% 8% 9% 10% 2	1582
	21st to end	Day	69% 8% 10% 9% 4	1393
March	1st to 10th	Day	61% 9% 12% 13% 5% 1	1980
	11th to 20th	Day	55% 7% 12% 16% 9% 1	1980
	21st to end	Day	55% 7% 9% 19% 9% 2	2178
April	1st to 10th	Day	55% 6% 12% 17% 9% 1	2146
	11th to 20th	Day	48% 7% 12% 19% 14% 2	2151
	21st to end	Day	51% 7% 11% 17% 12% 2	2340
May	1st to 10th	Day	48% 7% 12% 20% 11% 2	2340
	11th to 20th	Day	44% 10% 11% 22% 12% 2	2475
	21st to end	Day	36% 9% 17% 22% 14% 2	2725
June	1st to 10th	Day	41% 9% 14% 20% 15% 2	2520
	11th to 20th	Day	38% 8% 16% 23% 14% 1	2520
	21st to end	Day	37% 9% 15% 23% 15% 1	2520
July	1st to 10th	Day	30% 10% 15% 25% 19% 2	2450
	11th to 20th	Day	37% 10% 18% 24% 11%	2445
	21st to end	Day	38% 11% 18% 21% 12% 1	2434
August	1st to 10th	Day	34% 11% 18% 25% 11% 1	2314
	11th to 20th	Day	39% 10% 17% 23% 10% 1	2303
	21st to end	Day	37% 8% 16% 26% 12% 2	2441
September	1st to 10th	Day	39% 9% 16% 24% 11% 1	2160
	11th to 20th	Day	36% 9% 15% 26% 14% 1	2089
	21st to end	Day	44% 9% 15% 21% 9% 1	1922
October	1st to 10th	Day	50% 8% 14% 16% 11% 1	1800
	11th to 20th	Day	54% 9% 15% 14% 6% 1	1800
	21st to end	Day	66% 6% 11% 11% 5% 1	1980
November	1st to 10th	Day	52% 9% 13% 17% 8% 1	1649
	11th to 20th	Day	54% 9% 17% 13% 6% 1	1520
	21st to end	Day	64% 9% 13% 11% 3	1520
December	1st to 10th	Day	70% 9% 13% 7% 1	1520
	11th to 20th	Day	70% 11% 11% 7% 2	1906
	21st to end	Day	77% 9% 9% 5% 1	2772
January	1st to 10th	Night	70% 9% 9% 9% 3	2649
	11th to 20th	Night	74% 9% 9% 6% 1	2520
	21st to end	Night	70% 9% 10% 7% 3 1	2599
February	1st to 10th	Night	69% 9% 9% 10% 3	2340
	11th to 20th	Night	65% 8% 12% 11% 4	2275
	21st to end	Night	65% 8% 11% 11% 5%	1858
March	1st to 10th	Night	56% 8% 13% 16% 6% 1	1980
	11th to 20th	Night	55% 6% 11% 17% 9% 1	1980
	21st to end	Night	53% 6% 9% 18% 12% 2	2178
April	1st to 10th	Night	55% 6% 10% 18% 10% 1	1814
	11th to 20th	Night	48% 6% 10% 20% 13% 2	1775
	21st to end	Night	49% 8% 11% 17% 13% 2	1620
May	1st to 10th	Night	46% 8% 11% 19% 14% 2	1620
	11th to 20th	Night	41% 8% 13% 22% 14% 2	1421
	21st to end	Night	35% 10% 13% 22% 17% 3	1551
June	1st to 10th	Night	43% 8% 11% 20% 16% 1	1440
	11th to 20th	Night	38% 9% 12% 20% 19% 2	1440
	21st to end	Night	39% 8% 15% 20% 16% 2	1440
July	1st to 10th	Night	29% 8% 14% 25% 20% 4	1400
	11th to 20th	Night	34% 8% 16% 25% 15% 2	1392
	21st to end	Night	34% 9% 17% 22% 16% 2	1639
August	1st to 10th	Night	31% 10% 18% 27% 13% 1	1600
	11th to 20th	Night	35% 9% 16% 27% 12%	1598
	21st to end	Night	34% 9% 14% 28% 13% 2	1895
September	1st to 10th	Night	38% 8% 16% 23% 14% 1	1800
	11th to 20th	Night	33% 6% 14% 29% 17% 1	1867
	21st to end	Night	43% 9% 14% 22% 13%	2038
October	1st to 10th	Night	45% 8% 13% 19% 14% 1	2160
	11th to 20th	Night	53% 9% 14% 16% 8%	2160
	21st to end	Night	61% 7% 12% 12% 8% 1	2376
November	1st to 10th	Night	47% 8% 12% 20% 11% 2	2427
	11th to 20th	Night	53% 9% 14% 15% 7% 1	2660
	21st to end	Night	60% 11% 15% 12% 2	2660
December	1st to 10th	Night	69% 10% 13% 7% 2	2660
	11th to 20th	Night	67% 11% 10% 9% 2	3288
	21st to end	Night	74% 9% 11% 5% 1	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	27% 30% 23% 13% 6% 1	180
	11th to 20th	Day	28% 29% 27% 13% 3 1	245
	21st to end	Day	22% 25% 30% 18% 4	267
February	1st to 10th	Day	33% 31% 30% 7%	240
	11th to 20th	Day	27% 32% 34% 8%	240
	21st to end	Day	38% 18% 32% 10% 1	250
March	1st to 10th	Day	25% 22% 34% 14% 4	420
	11th to 20th	Day	26% 18% 35% 17% 3 1	420
	21st to end	Day	29% 17% 30% 18% 5% 2	462
April	1st to 10th	Day	25% 21% 31% 17% 6% 1	420
	11th to 20th	Day	29% 20% 22% 18% 8% 3	412
	21st to end	Day	26% 16% 35% 14% 7% 2	420
May	1st to 10th	Day	25% 14% 31% 17% 9% 3	423
	11th to 20th	Day	20% 12% 29% 25% 11% 3	447
	21st to end	Day	15% 11% 28% 27% 13% 5%	485
June	1st to 10th	Day	18% 15% 33% 25% 8% 2	450
	11th to 20th	Day	16% 14% 26% 30% 12% 2	450
	21st to end	Day	17% 10% 32% 30% 10% 2	450
July	1st to 10th	Day	9% 6% 25% 33% 22% 4	445
	11th to 20th	Day	14% 8% 27% 28% 17% 6%	444
	21st to end	Day	12% 8% 32% 31% 13% 4	488
August	1st to 10th	Day	11% 8% 22% 33% 18% 9%	446
	11th to 20th	Day	11% 5% 26% 30% 20% 8%	439
	21st to end	Day	12% 8% 29% 27% 15% 9%	495
September	1st to 10th	Day	14% 10% 25% 29% 14% 8%	450
	11th to 20th	Day	12% 8% 22% 25% 22% 10%	450
	21st to end	Day	18% 14% 29% 21% 12% 7%	450
October	1st to 10th	Day	22% 15% 25% 18% 15% 5%	450
	11th to 20th	Day	24% 22% 30% 18% 5% 1	450
	21st to end	Day	37% 18% 24% 15% 5%	495
November	1st to 10th	Day	24% 21% 31% 15% 7% 2	311
	11th to 20th	Day	29% 28% 23% 14% 3 2	180
	21st to end	Day	30% 27% 29% 13% 1	180
December	1st to 10th	Day	31% 24% 31% 8% 5% 1	180
	11th to 20th	Day	31% 28% 27% 12% 2	180
	21st to end	Day	29% 37% 24% 9% 1 1	198
January	1st to 10th	Night	29% 23% 31% 13% 4	760
	11th to 20th	Night	31% 23% 29% 15% 2	760
	21st to end	Night	24% 19% 33% 19% 4	800
February	1st to 10th	Night	27% 27% 31% 13% 2	720
	11th to 20th	Night	24% 23% 33% 17% 3	703
	21st to end	Night	29% 20% 31% 17% 2	592
March	1st to 10th	Night	23% 17% 34% 21% 5%	720
	11th to 20th	Night	25% 17% 31% 20% 5% 1	720
	21st to end	Night	22% 13% 33% 22% 9% 1	792
April	1st to 10th	Night	24% 17% 33% 23% 4	720
	11th to 20th	Night	22% 16% 31% 22% 8% 1	711
	21st to end	Night	22% 9% 32% 27% 10%	540
May	1st to 10th	Night	20% 9% 27% 29% 13% 2	540
	11th to 20th	Night	15% 7% 30% 40% 7% 1	359
	21st to end	Night	12% 9% 28% 35% 15% 2	389
June	1st to 10th	Night	13% 7% 34% 34% 12%	360
	11th to 20th	Night	14% 7% 32% 33% 14% 1	360
	21st to end	Night	13% 6% 31% 40% 9%	360
July	1st to 10th	Night	8% 5% 25% 41% 20% 1	350
	11th to 20th	Night	10% 6% 29% 36% 17% 1	348
	21st to end	Night	11% 3 26% 40% 18% 2	556
August	1st to 10th	Night	9% 6% 17% 42% 23% 3	537
	11th to 20th	Night	9% 4 22% 34% 28% 2	534
	21st to end	Night	12% 5% 23% 35% 21% 3	707
September	1st to 10th	Night	15% 7% 22% 34% 20% 3	720
	11th to 20th	Night	14% 5% 21% 30% 25% 5%	720
	21st to end	Night	17% 10% 24% 29% 16% 4	720
October	1st to 10th	Night	20% 10% 29% 23% 16% 2	720
	11th to 20th	Night	27% 18% 26% 22% 7% 1	720
	21st to end	Night	33% 15% 26% 22% 4	792
November	1st to 10th	Night	24% 15% 29% 22% 10% 1	740
	11th to 20th	Night	22% 21% 32% 16% 8% 1	760
	21st to end	Night	27% 21% 31% 16% 6%	760
December	1st to 10th	Night	34% 22% 24% 16% 3	760
	11th to 20th	Night	26% 24% 32% 15% 3	760
	21st to end	Night	33% 19% 32% 13% 2	836

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