Summit Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Summit Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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	14% 5% 2 5% 5% 3 5% 5% 6% 8% 42%	74729
	Night	15% 6% 3 5% 5% 3 4 4 5% 7% 43%	75027
Transparency	Day	39% 7% 13% 23% 16% 3	74434
	Night	40% 6% 12% 22% 17% 3	74842
Seeing	Day	17% 11% 26% 26% 15% 5%	13877
	Night	18% 9% 30% 31% 12% 1	23342

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	12% 6% 3 5% 6% 2 4 5% 5% 6% 45%	1517
	11th to 20th	Day	15% 6% 4 5% 5% 3 5% 5% 4 7% 42%	1558
	21st to end	Day	15% 6% 3 4 6% 3 6% 4 5% 6% 42%	1782
February	1st to 10th	Day	16% 7% 3 5% 5% 3 5% 5% 5% 6% 40%	1620
	11th to 20th	Day	17% 6% 2 4 4 3 4 6% 5% 7% 42%	1691
	21st to end	Day	18% 9% 3 5% 5% 3 4 4 6% 4 39%	1480
March	1st to 10th	Day	13% 5% 2 5% 5% 3 4 3 4 6% 50%	1836
	11th to 20th	Day	16% 5% 2 5% 5% 2 4 4 5% 7% 43%	1980
	21st to end	Day	19% 6% 3 5% 6% 2 5% 4 5% 6% 39%	2280
April	1st to 10th	Day	14% 6% 3 5% 5% 3 5% 5% 4 7% 44%	2160
	11th to 20th	Day	13% 5% 3 5% 4 3 5% 4 5% 8% 45%	2134
	21st to end	Day	17% 5% 2 5% 4 3 4 5% 6% 8% 40%	2160
May	1st to 10th	Day	17% 6% 3 6% 4 3 5% 4 5% 8% 38%	2269
	11th to 20th	Day	18% 5% 3 4 5% 4 5% 5% 6% 9% 36%	2478
	21st to end	Day	13% 6% 2 5% 4 3 5% 5% 6% 9% 42%	2725
June	1st to 10th	Day	11% 4 2 5% 5% 4 5% 6% 7% 11% 39%	2520
	11th to 20th	Day	12% 5% 2 6% 5% 4 5% 5% 6% 9% 40%	2520
	21st to end	Day	9% 4 2 5% 5% 2 5% 6% 6% 11% 44%	2520
July	1st to 10th	Day	9% 4 2 6% 6% 3 6% 6% 6% 10% 42%	2520
	11th to 20th	Day	6% 3 2 4 5% 4 5% 6% 8% 13% 45%	2520
	21st to end	Day	7% 5% 2 6% 7% 5% 6% 6% 9% 11% 37%	2635
August	1st to 10th	Day	9% 6% 2 5% 5% 4 5% 6% 8% 10% 39%	2327
	11th to 20th	Day	9% 5% 2 5% 5% 4 6% 5% 8% 8% 42%	2142
	21st to end	Day	11% 5% 2 5% 6% 3 5% 6% 7% 9% 42%	2376
September	1st to 10th	Day	14% 6% 3 5% 5% 3 4 5% 5% 8% 43%	2160
	11th to 20th	Day	12% 4 2 4 4 3 4 4 5% 7% 49%	2152
	21st to end	Day	18% 6% 3 5% 5% 3 5% 3 5% 7% 40%	1949
October	1st to 10th	Day	17% 6% 2 5% 4 2 4 5% 4 6% 46%	1800
	11th to 20th	Day	16% 5% 2 4 4 2 4 4 5% 6% 48%	1800
	21st to end	Day	17% 5% 2 5% 4 3 4 4 4 5% 47%	1958
November	1st to 10th	Day	12% 6% 2 4 4 3 3 3 5% 7% 52%	1671
	11th to 20th	Day	14% 5% 2 4 4 3 4 4 6% 6% 47%	1710
	21st to end	Day	15% 4 3 5% 4 3 5% 4 4 7% 48%	1577
December	1st to 10th	Day	18% 6% 2 5% 5% 3 4 4 7% 5% 42%	1520
	11th to 20th	Day	19% 7% 3 3 4 2 4 4 5% 6% 43%	1910
	21st to end	Day	15% 7% 3 6% 5% 4 5% 5% 6% 7% 38%	2772
January	1st to 10th	Night	16% 6% 2 5% 4 3 4 4 4 6% 46%	2649
	11th to 20th	Night	17% 7% 3 5% 4 3 4 5% 6% 6% 39%	2471
	21st to end	Night	15% 6% 2 5% 5% 3 3 4 5% 6% 46%	2574
February	1st to 10th	Night	16% 8% 3 5% 5% 3 4 4 5% 7% 39%	2340
	11th to 20th	Night	18% 8% 3 5% 5% 2 4 4 4 6% 42%	2153
	21st to end	Night	18% 7% 3 3 4 3 5% 4 4 6% 42%	1776
March	1st to 10th	Night	14% 5% 3 4 4 3 4 4 5% 7% 48%	2120
	11th to 20th	Night	17% 6% 3 5% 4 2 4 5% 5% 6% 43%	1980
	21st to end	Night	17% 6% 3 6% 5% 3 4 4 4 6% 42%	2075
April	1st to 10th	Night	16% 7% 3 5% 5% 3 4 4 5% 7% 42%	1800
	11th to 20th	Night	14% 6% 2 5% 5% 2 4 4 5% 8% 44%	1775
	21st to end	Night	20% 5% 2 4 4 2 3 4 5% 7% 44%	1800
May	1st to 10th	Night	17% 6% 3 7% 5% 3 5% 5% 4 7% 37%	1704
	11th to 20th	Night	18% 7% 3 5% 4 2 5% 5% 6% 6% 38%	1415
	21st to end	Night	15% 7% 3 5% 5% 4 4 5% 5% 9% 40%	1550
June	1st to 10th	Night	13% 5% 2 5% 5% 3 5% 5% 5% 8% 44%	1440
	11th to 20th	Night	10% 7% 4 5% 5% 4 5% 6% 5% 10% 40%	1440
	21st to end	Night	10% 5% 2 5% 4 3 4 6% 8% 10% 43%	1440
July	1st to 10th	Night	8% 5% 3 6% 7% 4 5% 6% 6% 11% 38%	1440
	11th to 20th	Night	5% 4 3 5% 5% 4 6% 6% 8% 11% 42%	1440
	21st to end	Night	8% 4 3 6% 6% 4 5% 5% 6% 10% 41%	1679
August	1st to 10th	Night	8% 5% 2 5% 6% 3 6% 7% 7% 10% 41%	1611
	11th to 20th	Night	10% 6% 3 6% 6% 4 6% 6% 6% 8% 40%	1740
	21st to end	Night	12% 6% 2 4 5% 3 5% 5% 6% 9% 42%	1980
September	1st to 10th	Night	14% 6% 3 5% 5% 3 4 3 5% 7% 45%	1800
	11th to 20th	Night	11% 5% 2 4 3 2 3 4 4 7% 54%	1808
	21st to end	Night	17% 7% 2 6% 5% 3 4 4 5% 7% 42%	2006
October	1st to 10th	Night	19% 6% 3 5% 5% 2 3 4 5% 6% 42%	2160
	11th to 20th	Night	16% 5% 2 4 4 3 3 3 5% 7% 49%	2160
	21st to end	Night	16% 5% 2 5% 4 2 5% 4 4 6% 48%	2397
November	1st to 10th	Night	12% 4 2 5% 4 2 3 4 5% 6% 53%	2407
	11th to 20th	Night	16% 6% 2 5% 4 2 3 4 4 6% 48%	2470
	21st to end	Night	13% 4 2 4 5% 3 5% 4 6% 7% 47%	2598
December	1st to 10th	Night	19% 7% 2 5% 5% 3 4 4 4 7% 40%	2660
	11th to 20th	Night	17% 5% 3 5% 5% 3 4 4 6% 7% 43%	3285
	21st to end	Night	16% 7% 3 5% 5% 3 5% 6% 6% 8% 37%	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	40% 5% 13% 22% 16% 4	1518
	11th to 20th	Day	42% 6% 13% 22% 14% 3	1558
	21st to end	Day	43% 5% 11% 24% 14% 4	1782
February	1st to 10th	Day	43% 6% 14% 24% 11% 2	1620
	11th to 20th	Day	42% 5% 9% 27% 14% 3	1700
	21st to end	Day	47% 4 10% 21% 14% 4	1480
March	1st to 10th	Day	37% 4 12% 28% 18% 2	1836
	11th to 20th	Day	40% 5% 11% 21% 17% 5%	1980
	21st to end	Day	45% 5% 8% 20% 17% 4	2280
April	1st to 10th	Day	39% 6% 12% 20% 17% 6%	2160
	11th to 20th	Day	37% 5% 11% 24% 18% 6%	2134
	21st to end	Day	40% 7% 12% 18% 19% 4	2160
May	1st to 10th	Day	43% 7% 11% 20% 17% 3	2269
	11th to 20th	Day	43% 7% 14% 20% 14% 2	2482
	21st to end	Day	37% 9% 14% 24% 13% 2	2725
June	1st to 10th	Day	36% 9% 14% 24% 15% 2	2520
	11th to 20th	Day	39% 8% 14% 24% 14% 2	2520
	21st to end	Day	32% 8% 16% 25% 18% 1	2520
July	1st to 10th	Day	36% 9% 14% 22% 16% 3	2450
	11th to 20th	Day	28% 9% 16% 28% 15% 3	2445
	21st to end	Day	36% 10% 14% 22% 16% 2	2488
August	1st to 10th	Day	34% 10% 16% 25% 13% 1	2314
	11th to 20th	Day	36% 9% 16% 21% 15% 2	2142
	21st to end	Day	37% 8% 13% 22% 16% 4	2376
September	1st to 10th	Day	39% 7% 11% 20% 19% 3	2160
	11th to 20th	Day	32% 6% 10% 24% 24% 4	2152
	21st to end	Day	45% 5% 8% 19% 21% 3	1949
October	1st to 10th	Day	39% 5% 13% 23% 18% 2	1800
	11th to 20th	Day	36% 5% 10% 23% 23% 3	1800
	21st to end	Day	40% 4 8% 27% 18% 2	1958
November	1st to 10th	Day	34% 3 12% 28% 19% 4	1671
	11th to 20th	Day	36% 5% 10% 23% 23% 3	1710
	21st to end	Day	38% 4 10% 31% 14% 3	1577
December	1st to 10th	Day	42% 6% 15% 24% 12% 2	1520
	11th to 20th	Day	42% 4 12% 29% 11% 2	1906
	21st to end	Day	42% 7% 16% 21% 13%	2772
January	1st to 10th	Night	41% 4 10% 22% 18% 4	2649
	11th to 20th	Night	44% 6% 12% 22% 14% 1	2471
	21st to end	Night	40% 4 9% 26% 18% 3	2574
February	1st to 10th	Night	44% 5% 13% 23% 14% 1	2340
	11th to 20th	Night	44% 4 9% 21% 17% 5%	2155
	21st to end	Night	44% 4 10% 19% 17% 5%	1776
March	1st to 10th	Night	36% 5% 12% 25% 19% 3	2120
	11th to 20th	Night	41% 5% 12% 20% 18% 4	1980
	21st to end	Night	44% 5% 9% 19% 18% 6%	2075
April	1st to 10th	Night	41% 5% 10% 19% 20% 5%	1800
	11th to 20th	Night	39% 5% 12% 20% 19% 6%	1775
	21st to end	Night	41% 6% 10% 19% 19% 6%	1800
May	1st to 10th	Night	45% 6% 11% 18% 17% 3	1704
	11th to 20th	Night	44% 9% 11% 15% 18% 3	1416
	21st to end	Night	41% 9% 15% 19% 13% 3	1551
June	1st to 10th	Night	38% 8% 11% 23% 19% 2	1440
	11th to 20th	Night	39% 8% 14% 23% 14% 2	1440
	21st to end	Night	33% 8% 15% 24% 17% 3	1440
July	1st to 10th	Night	38% 11% 17% 18% 13% 3	1400
	11th to 20th	Night	32% 11% 17% 24% 13% 3	1392
	21st to end	Night	35% 9% 14% 25% 16% 1	1586
August	1st to 10th	Night	35% 11% 17% 22% 15% 1	1600
	11th to 20th	Night	40% 9% 15% 18% 16% 2	1740
	21st to end	Night	37% 9% 14% 21% 17% 3	1980
September	1st to 10th	Night	39% 7% 10% 21% 19% 4	1800
	11th to 20th	Night	31% 5% 9% 25% 26% 4	1808
	21st to end	Night	42% 6% 9% 20% 19% 4	2006
October	1st to 10th	Night	43% 5% 11% 21% 18% 2	2160
	11th to 20th	Night	35% 5% 12% 27% 18% 3	2160
	21st to end	Night	38% 5% 10% 26% 19% 3	2397
November	1st to 10th	Night	33% 5% 12% 26% 19% 6%	2407
	11th to 20th	Night	39% 4 9% 23% 20% 5%	2470
	21st to end	Night	36% 5% 12% 27% 18% 2	2598
December	1st to 10th	Night	44% 5% 12% 24% 14% 1	2660
	11th to 20th	Night	40% 5% 12% 21% 18% 4	3288
	21st to end	Night	43% 8% 14% 23% 11% 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	18% 22% 36% 18% 5% 1	180
	11th to 20th	Day	19% 34% 26% 17% 3	208
	21st to end	Day	20% 19% 41% 13% 6% 1	264
February	1st to 10th	Day	22% 20% 39% 15% 4	240
	11th to 20th	Day	23% 20% 41% 15% 1	359
	21st to end	Day	24% 21% 33% 19% 3	346
March	1st to 10th	Day	15% 24% 33% 23% 5%	420
	11th to 20th	Day	21% 19% 32% 20% 5% 2	420
	21st to end	Day	23% 15% 32% 23% 6% 1	462
April	1st to 10th	Day	18% 18% 37% 19% 6% 2	420
	11th to 20th	Day	16% 15% 30% 25% 9% 4	412
	21st to end	Day	22% 15% 33% 21% 7% 2	420
May	1st to 10th	Day	27% 14% 26% 20% 11% 2	423
	11th to 20th	Day	21% 9% 27% 30% 11% 2	447
	21st to end	Day	18% 4 19% 28% 23% 8%	485
June	1st to 10th	Day	12% 6% 24% 30% 20% 8%	450
	11th to 20th	Day	15% 8% 22% 30% 19% 6%	450
	21st to end	Day	10% 5% 22% 34% 20% 8%	450
July	1st to 10th	Day	10% 3 17% 30% 28% 11%	445
	11th to 20th	Day	6% 2 14% 34% 33% 12%	444
	21st to end	Day	9% 2 19% 35% 27% 8%	488
August	1st to 10th	Day	10% 2 17% 31% 28% 11%	446
	11th to 20th	Day	12% 3 12% 29% 32% 13%	439
	21st to end	Day	10% 3 19% 31% 23% 14%	495
September	1st to 10th	Day	17% 6% 18% 23% 23% 14%	450
	11th to 20th	Day	13% 3 20% 28% 22% 14%	450
	21st to end	Day	21% 8% 23% 27% 16% 4	450
October	1st to 10th	Day	23% 5% 20% 27% 15% 9%	450
	11th to 20th	Day	16% 12% 24% 27% 18% 3	450
	21st to end	Day	20% 9% 32% 27% 10% 3	484
November	1st to 10th	Day	15% 9% 30% 27% 15% 4	365
	11th to 20th	Day	16% 17% 33% 22% 12%	370
	21st to end	Day	16% 16% 30% 27% 11%	237
December	1st to 10th	Day	20% 19% 32% 21% 7% 1	180
	11th to 20th	Day	26% 21% 25% 22% 6%	180
	21st to end	Day	23% 16% 29% 24% 8% 1	198
January	1st to 10th	Night	20% 15% 40% 21% 4	760
	11th to 20th	Night	21% 15% 33% 27% 5%	735
	21st to end	Night	18% 13% 41% 25% 2	792
February	1st to 10th	Night	21% 12% 40% 25% 2	720
	11th to 20th	Night	22% 17% 41% 18% 1	703
	21st to end	Night	22% 17% 38% 20% 3	592
March	1st to 10th	Night	17% 15% 38% 25% 4	720
	11th to 20th	Night	21% 13% 37% 22% 6% 1	720
	21st to end	Night	20% 8% 41% 24% 7%	792
April	1st to 10th	Night	20% 13% 38% 22% 6%	720
	11th to 20th	Night	16% 11% 33% 32% 8%	711
	21st to end	Night	23% 9% 29% 30% 8%	720
May	1st to 10th	Night	21% 7% 33% 32% 7%	624
	11th to 20th	Night	20% 7% 31% 33% 10%	354
	21st to end	Night	17% 4 17% 39% 20% 2	389
June	1st to 10th	Night	14% 3 22% 41% 19% 1	360
	11th to 20th	Night	12% 1 26% 43% 18%	360
	21st to end	Night	10% 2 20% 50% 18%	360
July	1st to 10th	Night	11% 2 18% 42% 25% 2	350
	11th to 20th	Night	7% 1 12% 47% 32% 1	348
	21st to end	Night	10% 2 14% 46% 26% 2	499
August	1st to 10th	Night	10% 1 13% 44% 30% 2	537
	11th to 20th	Night	13% 2 14% 38% 31% 3	676
	21st to end	Night	13% 3 16% 38% 27% 4	792
September	1st to 10th	Night	16% 2 17% 37% 24% 3	720
	11th to 20th	Night	14% 3 16% 36% 27% 5%	720
	21st to end	Night	20% 4 21% 34% 21% 1	720
October	1st to 10th	Night	22% 5% 20% 30% 18% 4	720
	11th to 20th	Night	18% 6% 28% 34% 13% 1	720
	21st to end	Night	19% 8% 34% 27% 10% 1	792
November	1st to 10th	Night	14% 7% 35% 31% 12% 1	740
	11th to 20th	Night	19% 12% 32% 29% 8% 1	760
	21st to end	Night	15% 13% 33% 30% 8% 1	760
December	1st to 10th	Night	22% 13% 34% 23% 8% 1	760
	11th to 20th	Night	18% 12% 39% 26% 6%	760
	21st to end	Night	21% 10% 37% 30% 2	836

Page generated at 2024-01-11 04:22:37. Version ac07

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