Evergreen Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Evergreen 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	17% 11% 5% 7% 6% 3 4 4 5% 7% 31%	74709
	Night	18% 14% 5% 8% 6% 3 4 4 5% 6% 28%	75244
Transparency	Day	54% 6% 12% 18% 10% 1	74424
	Night	58% 6% 10% 16% 9% 1	75049
Seeing	Day	23% 14% 29% 22% 9% 3	13449
	Night	26% 13% 26% 23% 11% 2	23175

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	18% 20% 8% 16% 8% 4 5% 3 4 4 10%	1631
	11th to 20th	Day	17% 20% 11% 16% 8% 3 3 3 3 4 11%	1710
	21st to end	Day	16% 20% 9% 16% 10% 4 4 4 4 4 9%	1791
February	1st to 10th	Day	19% 18% 8% 14% 12% 4 4 3 2 3 13%	1620
	11th to 20th	Day	17% 17% 10% 13% 9% 4 5% 3 3 3 15%	1573
	21st to end	Day	20% 15% 6% 12% 4 3 4 5% 4 5% 21%	1447
March	1st to 10th	Day	17% 15% 6% 10% 5% 2 4 4 5% 7% 25%	1976
	11th to 20th	Day	16% 14% 7% 8% 5% 3 4 4 4 6% 28%	1980
	21st to end	Day	21% 14% 5% 6% 5% 3 5% 4 4 4 28%	2178
April	1st to 10th	Day	17% 12% 5% 6% 5% 3 4 4 4 7% 33%	2074
	11th to 20th	Day	20% 12% 3 5% 5% 2 4 4 5% 6% 34%	2220
	21st to end	Day	21% 9% 3 6% 6% 3 5% 4 4 6% 34%	2340
May	1st to 10th	Day	18% 9% 3 6% 5% 3 5% 4 6% 7% 34%	2340
	11th to 20th	Day	19% 7% 3 5% 6% 3 5% 5% 6% 6% 36%	2314
	21st to end	Day	12% 5% 3 5% 5% 3 5% 5% 6% 8% 42%	2725
June	1st to 10th	Day	16% 6% 2 4 5% 3 4 5% 6% 9% 39%	2520
	11th to 20th	Day	13% 5% 3 6% 5% 3 5% 4 6% 9% 40%	2520
	21st to end	Day	14% 4 2 4 5% 3 5% 4 6% 9% 43%	2520
July	1st to 10th	Day	10% 4 3 4 4 3 5% 5% 7% 9% 47%	2520
	11th to 20th	Day	12% 5% 3 5% 5% 3 4 4 7% 10% 42%	2356
	21st to end	Day	11% 5% 2 4 6% 4 4 6% 7% 9% 42%	2547
August	1st to 10th	Day	11% 5% 2 5% 5% 3 4 5% 7% 9% 43%	2327
	11th to 20th	Day	12% 6% 3 5% 6% 4 5% 6% 7% 9% 38%	2303
	21st to end	Day	13% 5% 2 5% 6% 3 5% 5% 6% 8% 41%	2493
September	1st to 10th	Day	17% 7% 3 5% 4 4 4 5% 6% 8% 38%	2159
	11th to 20th	Day	13% 5% 2 4 5% 3 5% 4 6% 9% 43%	1980
	21st to end	Day	18% 7% 3 6% 6% 3 5% 5% 4 7% 36%	1980
October	1st to 10th	Day	20% 6% 2 7% 5% 3 5% 5% 6% 7% 35%	1806
	11th to 20th	Day	25% 10% 3 7% 5% 4 5% 3 6% 6% 27%	1800
	21st to end	Day	30% 12% 4 8% 6% 3 4 4 4 6% 20%	1980
November	1st to 10th	Day	17% 14% 5% 6% 6% 3 4 4 4 6% 30%	1718
	11th to 20th	Day	19% 20% 4 8% 6% 3 3 4 5% 5% 23%	1539
	21st to end	Day	21% 26% 5% 8% 5% 3 4 3 3 6% 14%	1520
December	1st to 10th	Day	19% 29% 12% 8% 4 2 3 3 3 4 12%	1520
	11th to 20th	Day	19% 24% 12% 11% 5% 2 4 3 4 4 12%	1910
	21st to end	Day	24% 24% 8% 12% 7% 3 3 3 3 2 10%	2772
January	1st to 10th	Night	21% 22% 9% 14% 8% 3 3 3 2 3 12%	2531
	11th to 20th	Night	20% 22% 9% 14% 8% 3 3 2 3 3 13%	2470
	21st to end	Night	15% 22% 10% 14% 9% 4 4 4 3 3 12%	2599
February	1st to 10th	Night	21% 18% 6% 13% 10% 3 3 3 4 4 15%	2340
	11th to 20th	Night	18% 16% 8% 13% 8% 4 4 4 3 4 17%	2273
	21st to end	Night	21% 14% 7% 13% 7% 3 4 3 2 4 21%	1808
March	1st to 10th	Night	16% 16% 7% 9% 6% 2 5% 4 4 6% 24%	1983
	11th to 20th	Night	18% 14% 5% 8% 5% 3 4 3 4 5% 31%	1980
	21st to end	Night	19% 13% 6% 9% 7% 2 4 4 4 4 29%	2178
April	1st to 10th	Night	18% 14% 5% 6% 5% 3 4 4 5% 6% 30%	1886
	11th to 20th	Night	19% 10% 3 6% 5% 3 4 4 6% 7% 32%	1705
	21st to end	Night	16% 11% 3 5% 6% 3 5% 5% 5% 7% 33%	1620
May	1st to 10th	Night	20% 8% 3 5% 6% 3 5% 5% 5% 6% 34%	1620
	11th to 20th	Night	16% 8% 4 7% 6% 3 5% 4 6% 8% 35%	1579
	21st to end	Night	12% 5% 2 4 4 2 4 5% 7% 10% 45%	1550
June	1st to 10th	Night	11% 6% 3 6% 6% 4 5% 5% 6% 9% 40%	1440
	11th to 20th	Night	12% 6% 3 7% 6% 3 4 5% 5% 7% 43%	1440
	21st to end	Night	13% 5% 3 6% 5% 3 5% 5% 6% 9% 41%	1440
July	1st to 10th	Night	10% 5% 2 5% 5% 3 4 4 5% 8% 48%	1440
	11th to 20th	Night	11% 5% 2 4 5% 2 6% 4 6% 11% 43%	1600
	21st to end	Night	10% 5% 3 4 5% 4 6% 5% 7% 9% 43%	1771
August	1st to 10th	Night	9% 6% 2 6% 5% 3 4 5% 7% 9% 45%	1611
	11th to 20th	Night	11% 5% 3 5% 6% 3 6% 5% 7% 7% 43%	1598
	21st to end	Night	13% 6% 3 5% 5% 3 5% 4 6% 8% 43%	1844
September	1st to 10th	Night	15% 7% 2 5% 4 2 4 4 6% 7% 45%	1800
	11th to 20th	Night	14% 5% 3 5% 4 3 4 5% 5% 7% 46%	1979
	21st to end	Night	17% 5% 2 7% 7% 3 5% 5% 5% 7% 37%	1980
October	1st to 10th	Night	20% 8% 3 7% 4 2 4 4 5% 7% 38%	2153
	11th to 20th	Night	26% 9% 3 6% 6% 3 5% 5% 5% 6% 27%	2160
	21st to end	Night	31% 11% 4 6% 5% 3 4 4 4 5% 24%	2376
November	1st to 10th	Night	19% 15% 4 6% 5% 2 4 4 4 5% 31%	2360
	11th to 20th	Night	19% 20% 4 7% 6% 3 5% 4 5% 6% 21%	2641
	21st to end	Night	22% 23% 6% 7% 4 3 3 4 3 4 19%	2660
December	1st to 10th	Night	22% 28% 10% 8% 5% 2 3 3 3 3 12%	2660
	11th to 20th	Night	19% 26% 10% 10% 5% 3 3 3 3 5% 13%	3285
	21st to end	Night	24% 24% 7% 10% 7% 3 4 4 4 4 8%	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	78% 4 8% 8% 2	1632
	11th to 20th	Day	80% 3 9% 7% 1	1710
	21st to end	Day	80% 3 8% 6% 3	1791
February	1st to 10th	Day	78% 4 8% 6% 4	1620
	11th to 20th	Day	75% 3 7% 10% 5%	1582
	21st to end	Day	65% 5% 10% 13% 5% 2	1447
March	1st to 10th	Day	60% 5% 13% 17% 5%	1976
	11th to 20th	Day	58% 5% 10% 17% 9% 2	1980
	21st to end	Day	60% 4 7% 18% 9% 2	2178
April	1st to 10th	Day	53% 5% 13% 17% 12%	2074
	11th to 20th	Day	51% 5% 10% 19% 12% 2	2220
	21st to end	Day	53% 5% 10% 19% 11% 3	2340
May	1st to 10th	Day	50% 7% 10% 21% 10% 2	2340
	11th to 20th	Day	48% 7% 11% 18% 13% 3	2318
	21st to end	Day	39% 8% 14% 24% 13% 2	2725
June	1st to 10th	Day	41% 8% 13% 23% 13% 1	2520
	11th to 20th	Day	42% 7% 14% 22% 14% 1	2520
	21st to end	Day	38% 9% 13% 24% 15% 1	2520
July	1st to 10th	Day	34% 8% 13% 25% 18% 2	2450
	11th to 20th	Day	37% 7% 14% 25% 15% 1	2286
	21st to end	Day	38% 9% 14% 22% 15% 1	2405
August	1st to 10th	Day	37% 9% 16% 27% 12%	2314
	11th to 20th	Day	40% 11% 16% 23% 8% 2	2303
	21st to end	Day	40% 8% 14% 22% 15% 2	2493
September	1st to 10th	Day	44% 7% 14% 20% 13% 2	2159
	11th to 20th	Day	39% 8% 14% 24% 15%	1980
	21st to end	Day	48% 7% 12% 22% 11% 1	1980
October	1st to 10th	Day	49% 6% 14% 18% 12% 1	1806
	11th to 20th	Day	57% 6% 13% 16% 7% 1	1800
	21st to end	Day	66% 5% 10% 12% 6% 1	1980
November	1st to 10th	Day	54% 5% 11% 18% 10% 2	1718
	11th to 20th	Day	63% 4 11% 13% 6% 2	1539
	21st to end	Day	73% 4 9% 9% 4	1520
December	1st to 10th	Day	78% 5% 8% 8% 1	1520
	11th to 20th	Day	76% 5% 9% 8% 1	1906
	21st to end	Day	82% 4 7% 7% 1	2772
January	1st to 10th	Night	79% 4 7% 7% 3	2531
	11th to 20th	Night	78% 4 8% 8% 2	2470
	21st to end	Night	78% 4 8% 8% 2 1	2599
February	1st to 10th	Night	74% 5% 9% 9% 3	2340
	11th to 20th	Night	71% 4 9% 11% 4	2275
	21st to end	Night	68% 5% 9% 11% 7%	1808
March	1st to 10th	Night	61% 5% 9% 17% 7%	1983
	11th to 20th	Night	57% 5% 10% 18% 10% 1	1980
	21st to end	Night	60% 4 5% 15% 13% 2	2178
April	1st to 10th	Night	55% 5% 11% 17% 11% 2	1886
	11th to 20th	Night	51% 6% 11% 16% 14% 2	1705
	21st to end	Night	50% 6% 11% 20% 11% 4	1620
May	1st to 10th	Night	51% 6% 9% 20% 11% 4	1620
	11th to 20th	Night	47% 7% 10% 19% 14% 3	1580
	21st to end	Night	34% 9% 14% 27% 15% 2	1551
June	1st to 10th	Night	40% 8% 11% 23% 16% 2	1440
	11th to 20th	Night	40% 7% 12% 22% 17% 3	1440
	21st to end	Night	39% 9% 12% 24% 15% 1	1440
July	1st to 10th	Night	34% 8% 10% 24% 20% 4	1400
	11th to 20th	Night	35% 8% 11% 25% 19% 3	1548
	21st to end	Night	35% 9% 14% 23% 17% 2	1672
August	1st to 10th	Night	35% 7% 12% 31% 14% 2	1600
	11th to 20th	Night	38% 9% 14% 27% 12% 1	1598
	21st to end	Night	38% 8% 14% 22% 15% 3	1844
September	1st to 10th	Night	38% 7% 13% 25% 13% 3	1800
	11th to 20th	Night	36% 6% 13% 25% 19% 1	1979
	21st to end	Night	46% 6% 12% 21% 14% 1	1980
October	1st to 10th	Night	45% 6% 12% 20% 16% 1	2153
	11th to 20th	Night	56% 5% 12% 16% 10% 1	2160
	21st to end	Night	63% 5% 10% 14% 8% 1	2376
November	1st to 10th	Night	54% 7% 11% 18% 9% 2	2360
	11th to 20th	Night	63% 7% 9% 13% 6% 2	2641
	21st to end	Night	69% 6% 9% 12% 4	2660
December	1st to 10th	Night	78% 4 8% 7% 2	2660
	11th to 20th	Night	76% 4 11% 8% 1	3288
	21st to end	Night	81% 6% 8% 5%	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	30% 24% 31% 10% 5%	234
	11th to 20th	Day	33% 26% 30% 9% 3	270
	21st to end	Day	27% 30% 33% 8% 2	267
February	1st to 10th	Day	33% 23% 36% 9%	240
	11th to 20th	Day	34% 26% 31% 8%	240
	21st to end	Day	31% 32% 28% 9%	198
March	1st to 10th	Day	29% 22% 35% 9% 4	416
	11th to 20th	Day	27% 21% 35% 13% 3 1	420
	21st to end	Day	34% 18% 29% 16% 4	462
April	1st to 10th	Day	21% 25% 34% 14% 5% 1	420
	11th to 20th	Day	29% 16% 29% 19% 5% 2	412
	21st to end	Day	26% 15% 34% 19% 5% 1	420
May	1st to 10th	Day	29% 14% 32% 17% 5% 3	423
	11th to 20th	Day	22% 11% 31% 24% 9% 2	447
	21st to end	Day	14% 7% 29% 30% 15% 5%	485
June	1st to 10th	Day	17% 6% 37% 29% 9% 3	450
	11th to 20th	Day	16% 10% 28% 30% 14% 2	450
	21st to end	Day	17% 7% 32% 30% 13% 2	450
July	1st to 10th	Day	10% 4 25% 34% 20% 7%	445
	11th to 20th	Day	16% 4 27% 32% 17% 5%	444
	21st to end	Day	10% 6% 31% 32% 16% 5%	488
August	1st to 10th	Day	12% 5% 23% 34% 19% 7%	446
	11th to 20th	Day	15% 5% 21% 35% 18% 7%	439
	21st to end	Day	15% 7% 22% 32% 16% 6%	495
September	1st to 10th	Day	18% 7% 23% 30% 15% 6%	450
	11th to 20th	Day	12% 5% 24% 30% 21% 8%	450
	21st to end	Day	20% 9% 29% 24% 13% 6%	450
October	1st to 10th	Day	20% 10% 28% 23% 15% 5%	450
	11th to 20th	Day	31% 16% 28% 20% 5% 1	450
	21st to end	Day	40% 15% 27% 16% 2	495
November	1st to 10th	Day	29% 21% 25% 17% 7% 1	316
	11th to 20th	Day	38% 21% 25% 11% 5% 1	189
	21st to end	Day	36% 20% 34% 10%	180
December	1st to 10th	Day	36% 24% 27% 11% 2 1	180
	11th to 20th	Day	31% 31% 27% 11% 1	180
	21st to end	Day	38% 20% 30% 10% 1 1	198
January	1st to 10th	Night	33% 24% 28% 11% 4	760
	11th to 20th	Night	35% 21% 28% 14% 2	760
	21st to end	Night	27% 22% 33% 15% 3	800
February	1st to 10th	Night	35% 18% 29% 16% 2	720
	11th to 20th	Night	28% 23% 30% 17% 2	703
	21st to end	Night	31% 22% 28% 17% 1	592
March	1st to 10th	Night	27% 18% 28% 23% 3	720
	11th to 20th	Night	26% 17% 31% 21% 6% 1	720
	21st to end	Night	26% 14% 28% 24% 7% 1	792
April	1st to 10th	Night	25% 14% 30% 24% 6%	720
	11th to 20th	Night	25% 11% 32% 23% 8% 2	641
	21st to end	Night	20% 12% 29% 31% 8% 1	540
May	1st to 10th	Night	25% 7% 22% 34% 12% 1	540
	11th to 20th	Night	19% 8% 25% 36% 11% 1	518
	21st to end	Night	13% 5% 25% 30% 24% 3	389
June	1st to 10th	Night	14% 4 31% 32% 17% 2	360
	11th to 20th	Night	14% 3 26% 37% 19% 1	360
	21st to end	Night	13% 8% 26% 32% 21% 1	360
July	1st to 10th	Night	12% 3 19% 38% 22% 6%	350
	11th to 20th	Night	13% 4 21% 32% 26% 4	504
	21st to end	Night	12% 3 19% 39% 25% 2	591
August	1st to 10th	Night	10% 2 19% 39% 25% 5%	537
	11th to 20th	Night	13% 3 13% 36% 30% 4	534
	21st to end	Night	15% 4 21% 29% 26% 5%	656
September	1st to 10th	Night	19% 7% 16% 30% 24% 6%	720
	11th to 20th	Night	15% 5% 17% 32% 26% 5%	720
	21st to end	Night	19% 9% 22% 27% 17% 5%	720
October	1st to 10th	Night	24% 9% 23% 23% 18% 3	720
	11th to 20th	Night	32% 13% 27% 21% 7% 1	720
	21st to end	Night	39% 14% 27% 17% 3	792
November	1st to 10th	Night	29% 12% 28% 23% 8% 1	740
	11th to 20th	Night	35% 20% 24% 14% 5% 1	760
	21st to end	Night	38% 19% 28% 11% 4	760
December	1st to 10th	Night	41% 22% 23% 10% 4	760
	11th to 20th	Night	36% 25% 24% 12% 3	760
	21st to end	Night	33% 22% 30% 14% 1	836

Page generated at 2024-01-19 09:28:36. Version ac07

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