The Geek Shed
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Geek Shed 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	15% 7% 3 7% 6% 4 5% 5% 6% 7% 35%	74692
	Night	15% 7% 3 6% 6% 3 5% 5% 5% 7% 38%	75050
Transparency	Day	47% 6% 12% 21% 12% 2	74406
	Night	45% 6% 13% 22% 13% 2	74870
Seeing	Day	21% 19% 27% 19% 10% 3	13657
	Night	20% 16% 28% 26% 10% 1	23172

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% 9% 4 9% 7% 3 6% 6% 5% 6% 33%	1517
	11th to 20th	Day	13% 12% 5% 10% 10% 5% 5% 5% 5% 6% 23%	1593
	21st to end	Day	12% 10% 6% 11% 9% 5% 5% 5% 4 6% 26%	1782
February	1st to 10th	Day	16% 13% 5% 9% 6% 4 5% 4 5% 7% 26%	1620
	11th to 20th	Day	13% 9% 4 9% 8% 4 5% 6% 5% 7% 30%	1587
	21st to end	Day	17% 11% 4 9% 7% 4 4 5% 4 5% 30%	1480
March	1st to 10th	Day	12% 9% 4 7% 6% 2 4 5% 5% 6% 39%	1917
	11th to 20th	Day	16% 8% 3 7% 8% 4 6% 4 6% 6% 32%	1980
	21st to end	Day	19% 9% 3 7% 6% 3 4 3 4 6% 35%	2220
April	1st to 10th	Day	14% 8% 4 7% 6% 4 5% 4 6% 7% 35%	2160
	11th to 20th	Day	16% 8% 3 6% 6% 3 5% 5% 5% 6% 37%	2134
	21st to end	Day	18% 7% 3 6% 6% 4 4 5% 6% 7% 35%	2185
May	1st to 10th	Day	20% 6% 4 7% 6% 3 5% 5% 5% 7% 33%	2353
	11th to 20th	Day	20% 7% 3 6% 6% 3 5% 5% 5% 6% 32%	2478
	21st to end	Day	15% 6% 3 5% 5% 3 5% 5% 6% 8% 39%	2725
June	1st to 10th	Day	13% 6% 3 6% 6% 4 6% 7% 6% 8% 36%	2520
	11th to 20th	Day	15% 6% 3 5% 6% 4 5% 4 5% 9% 36%	2520
	21st to end	Day	11% 5% 3 5% 5% 3 6% 6% 7% 11% 39%	2520
July	1st to 10th	Day	10% 5% 3 5% 7% 5% 7% 6% 6% 10% 37%	2520
	11th to 20th	Day	9% 4 2 5% 5% 4 6% 6% 7% 11% 39%	2520
	21st to end	Day	11% 6% 3 7% 6% 4 6% 6% 7% 9% 34%	2587
August	1st to 10th	Day	11% 5% 2 6% 7% 5% 6% 7% 8% 9% 35%	2327
	11th to 20th	Day	14% 7% 2 6% 6% 4 5% 6% 7% 9% 34%	2250
	21st to end	Day	15% 6% 3 6% 6% 4 5% 6% 7% 8% 35%	2376
September	1st to 10th	Day	18% 5% 2 5% 6% 3 5% 4 5% 6% 42%	2160
	11th to 20th	Day	15% 4 2 4 4 3 3 4 5% 7% 49%	2160
	21st to end	Day	19% 6% 2 6% 4 3 3 4 5% 6% 41%	1891
October	1st to 10th	Day	17% 7% 3 6% 5% 3 4 4 4 7% 40%	1800
	11th to 20th	Day	19% 7% 2 5% 5% 4 4 5% 5% 7% 38%	1800
	21st to end	Day	18% 8% 3 6% 6% 4 5% 5% 5% 6% 35%	1980
November	1st to 10th	Day	12% 7% 2 5% 5% 2 4 4 6% 7% 45%	1701
	11th to 20th	Day	15% 8% 3 7% 5% 3 6% 4 6% 6% 36%	1611
	21st to end	Day	18% 6% 4 5% 6% 4 6% 6% 6% 6% 33%	1520
December	1st to 10th	Day	17% 12% 5% 8% 8% 5% 5% 4 4 7% 26%	1520
	11th to 20th	Day	19% 9% 4 8% 6% 3 5% 4 6% 7% 30%	1906
	21st to end	Day	19% 11% 4 11% 9% 4 5% 5% 5% 6% 21%	2772
January	1st to 10th	Night	13% 8% 3 7% 7% 3 5% 5% 6% 9% 34%	2649
	11th to 20th	Night	15% 10% 3 7% 7% 5% 5% 5% 6% 7% 30%	2439
	21st to end	Night	12% 7% 4 8% 6% 4 6% 6% 5% 8% 34%	2574
February	1st to 10th	Night	15% 9% 4 6% 7% 3 5% 5% 6% 7% 32%	2340
	11th to 20th	Night	15% 8% 4 7% 7% 4 5% 5% 6% 7% 32%	2257
	21st to end	Night	16% 9% 4 8% 6% 4 5% 5% 4 6% 32%	1776
March	1st to 10th	Night	14% 7% 3 5% 5% 4 5% 5% 5% 7% 41%	2027
	11th to 20th	Night	15% 8% 3 6% 5% 2 4 4 5% 5% 40%	1979
	21st to end	Night	16% 7% 3 7% 5% 4 4 5% 5% 7% 37%	2135
April	1st to 10th	Night	16% 8% 5% 7% 6% 3 5% 5% 4 8% 34%	1800
	11th to 20th	Night	15% 6% 3 6% 5% 3 4 6% 6% 8% 38%	1775
	21st to end	Night	19% 5% 2 5% 5% 2 5% 5% 5% 7% 40%	1791
May	1st to 10th	Night	19% 9% 3 6% 5% 2 4 5% 4 6% 36%	1604
	11th to 20th	Night	17% 7% 2 5% 5% 3 4 5% 5% 6% 40%	1415
	21st to end	Night	14% 7% 2 5% 5% 4 4 4 5% 8% 41%	1550
June	1st to 10th	Night	14% 5% 3 5% 5% 3 4 4 5% 9% 42%	1440
	11th to 20th	Night	13% 5% 3 6% 5% 2 5% 6% 6% 8% 41%	1440
	21st to end	Night	11% 4 2 5% 5% 3 5% 5% 7% 9% 44%	1440
July	1st to 10th	Night	11% 5% 3 5% 5% 4 5% 5% 5% 7% 46%	1440
	11th to 20th	Night	6% 5% 2 6% 6% 4 4 5% 7% 10% 45%	1440
	21st to end	Night	10% 6% 3 6% 4 3 5% 5% 6% 10% 42%	1727
August	1st to 10th	Night	9% 5% 2 6% 5% 3 4 5% 7% 8% 45%	1611
	11th to 20th	Night	11% 7% 3 6% 7% 3 4 5% 5% 7% 41%	1631
	21st to end	Night	13% 5% 2 4 4 3 5% 6% 6% 10% 42%	1980
September	1st to 10th	Night	17% 5% 2 5% 6% 3 4 4 5% 7% 45%	1800
	11th to 20th	Night	13% 5% 2 4 3 2 3 3 5% 6% 52%	1800
	21st to end	Night	17% 5% 3 4 4 3 4 5% 5% 7% 43%	2062
October	1st to 10th	Night	19% 6% 2 5% 3 2 3 4 5% 7% 43%	2160
	11th to 20th	Night	16% 7% 2 5% 5% 2 4 4 5% 8% 41%	2160
	21st to end	Night	20% 8% 3 5% 5% 3 4 4 6% 6% 38%	2376
November	1st to 10th	Night	10% 7% 3 5% 5% 3 4 3 4 5% 50%	2375
	11th to 20th	Night	17% 7% 3 4 6% 3 5% 5% 5% 7% 39%	2565
	21st to end	Night	15% 7% 3 7% 5% 3 4 4 6% 8% 37%	2660
December	1st to 10th	Night	16% 10% 4 7% 5% 4 5% 5% 5% 8% 29%	2660
	11th to 20th	Night	17% 9% 3 6% 6% 3 5% 5% 5% 7% 33%	3288
	21st to end	Night	18% 9% 4 8% 7% 4 6% 5% 6% 7% 26%	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	48% 6% 15% 21% 9% 1	1518
	11th to 20th	Day	59% 5% 12% 15% 7% 1	1593
	21st to end	Day	58% 5% 10% 18% 8% 1	1782
February	1st to 10th	Day	58% 5% 12% 17% 8% 1	1620
	11th to 20th	Day	52% 6% 10% 23% 8% 1	1596
	21st to end	Day	55% 4 11% 19% 8% 3	1480
March	1st to 10th	Day	45% 6% 12% 24% 11% 2	1920
	11th to 20th	Day	52% 5% 10% 19% 11% 4	1980
	21st to end	Day	51% 5% 8% 19% 15% 2	2220
April	1st to 10th	Day	49% 5% 10% 20% 13% 4	2160
	11th to 20th	Day	46% 6% 9% 21% 15% 3	2134
	21st to end	Day	47% 6% 10% 20% 13% 4	2185
May	1st to 10th	Day	50% 5% 10% 19% 13% 3	2353
	11th to 20th	Day	51% 6% 11% 16% 13% 2	2482
	21st to end	Day	42% 7% 15% 22% 12% 2	2725
June	1st to 10th	Day	43% 7% 13% 22% 14% 2	2520
	11th to 20th	Day	43% 8% 13% 21% 13% 2	2520
	21st to end	Day	37% 8% 13% 28% 13% 1	2520
July	1st to 10th	Day	41% 9% 13% 22% 13% 2	2450
	11th to 20th	Day	35% 7% 16% 25% 15% 2	2445
	21st to end	Day	43% 8% 12% 20% 16% 1	2442
August	1st to 10th	Day	41% 9% 13% 24% 13%	2314
	11th to 20th	Day	44% 7% 13% 22% 12% 1	2250
	21st to end	Day	43% 7% 11% 22% 15% 3	2376
September	1st to 10th	Day	41% 7% 11% 22% 16% 3	2160
	11th to 20th	Day	36% 5% 12% 24% 21% 3	2160
	21st to end	Day	45% 5% 11% 20% 18% 2	1891
October	1st to 10th	Day	45% 7% 11% 22% 14% 1	1800
	11th to 20th	Day	45% 6% 12% 24% 13%	1800
	21st to end	Day	48% 5% 12% 22% 11% 1	1980
November	1st to 10th	Day	37% 5% 15% 24% 16% 3	1701
	11th to 20th	Day	47% 4 12% 22% 12% 2	1611
	21st to end	Day	48% 7% 13% 23% 8% 1	1520
December	1st to 10th	Day	58% 6% 15% 15% 7%	1520
	11th to 20th	Day	53% 5% 15% 20% 6% 1	1906
	21st to end	Day	62% 6% 14% 14% 4	2772
January	1st to 10th	Night	47% 6% 13% 22% 10% 2	2649
	11th to 20th	Night	52% 6% 16% 16% 7% 1	2439
	21st to end	Night	48% 5% 13% 21% 11% 2	2574
February	1st to 10th	Night	51% 6% 14% 21% 8% 1	2340
	11th to 20th	Night	49% 6% 12% 19% 12% 1	2259
	21st to end	Night	53% 4 9% 19% 13% 2	1776
March	1st to 10th	Night	42% 5% 11% 25% 14% 2	2037
	11th to 20th	Night	45% 6% 10% 22% 14% 2	1980
	21st to end	Night	48% 5% 11% 18% 16% 3	2135
April	1st to 10th	Night	50% 5% 10% 20% 14% 2	1800
	11th to 20th	Night	42% 7% 10% 23% 16% 3	1775
	21st to end	Night	44% 6% 11% 21% 14% 4	1791
May	1st to 10th	Night	48% 6% 9% 22% 11% 4	1604
	11th to 20th	Night	44% 8% 11% 17% 18% 2	1416
	21st to end	Night	41% 9% 14% 20% 14% 3	1551
June	1st to 10th	Night	39% 7% 13% 23% 17% 1	1440
	11th to 20th	Night	40% 8% 13% 21% 16% 2	1440
	21st to end	Night	34% 8% 14% 28% 15% 1	1440
July	1st to 10th	Night	39% 9% 12% 22% 14% 4	1400
	11th to 20th	Night	34% 9% 13% 26% 17% 2	1392
	21st to end	Night	34% 10% 14% 23% 17% 2	1631
August	1st to 10th	Night	35% 10% 14% 25% 15% 1	1600
	11th to 20th	Night	42% 6% 14% 20% 17% 1	1631
	21st to end	Night	37% 9% 12% 23% 17% 3	1980
September	1st to 10th	Night	40% 6% 11% 24% 17% 3	1800
	11th to 20th	Night	32% 4 11% 30% 20% 2	1800
	21st to end	Night	40% 5% 11% 23% 20% 2	2062
October	1st to 10th	Night	42% 5% 13% 24% 14% 1	2160
	11th to 20th	Night	41% 6% 13% 25% 14% 2	2160
	21st to end	Night	45% 5% 12% 24% 13% 1	2376
November	1st to 10th	Night	36% 4 13% 26% 18% 3	2375
	11th to 20th	Night	44% 5% 11% 23% 14% 3	2565
	21st to end	Night	45% 5% 15% 24% 10% 1	2660
December	1st to 10th	Night	51% 6% 15% 19% 9%	2660
	11th to 20th	Night	52% 5% 12% 21% 10% 1	3288
	21st to end	Night	56% 6% 15% 19% 4	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	29% 32% 26% 11% 1 2	180
	11th to 20th	Day	28% 29% 32% 9% 1	224
	21st to end	Day	23% 37% 29% 8% 4	264
February	1st to 10th	Day	29% 37% 26% 7% 1	240
	11th to 20th	Day	31% 31% 35% 3	254
	21st to end	Day	34% 25% 25% 14% 1	346
March	1st to 10th	Day	30% 25% 30% 13% 3	420
	11th to 20th	Day	28% 28% 28% 11% 3 1	420
	21st to end	Day	29% 24% 27% 14% 4 2	462
April	1st to 10th	Day	23% 30% 26% 14% 6% 1	420
	11th to 20th	Day	21% 28% 23% 17% 7% 3	412
	21st to end	Day	29% 21% 31% 11% 7% 2	420
May	1st to 10th	Day	29% 21% 28% 12% 7% 3	423
	11th to 20th	Day	22% 24% 26% 17% 8% 2	447
	21st to end	Day	19% 12% 29% 19% 17% 4	485
June	1st to 10th	Day	14% 18% 29% 25% 11% 2	450
	11th to 20th	Day	19% 17% 26% 25% 12% 2	450
	21st to end	Day	13% 14% 34% 26% 11% 2	450
July	1st to 10th	Day	12% 8% 26% 30% 18% 6%	445
	11th to 20th	Day	8% 8% 27% 34% 18% 5%	444
	21st to end	Day	13% 10% 30% 28% 14% 5%	488
August	1st to 10th	Day	13% 6% 27% 28% 21% 6%	446
	11th to 20th	Day	15% 7% 25% 25% 20% 7%	439
	21st to end	Day	14% 9% 26% 25% 16% 9%	495
September	1st to 10th	Day	21% 10% 20% 23% 18% 8%	450
	11th to 20th	Day	16% 7% 25% 24% 21% 7%	450
	21st to end	Day	23% 13% 26% 18% 16% 4	450
October	1st to 10th	Day	23% 13% 23% 20% 17% 4	450
	11th to 20th	Day	22% 18% 30% 22% 9%	450
	21st to end	Day	24% 20% 30% 19% 7%	495
November	1st to 10th	Day	20% 18% 27% 22% 12% 1	379
	11th to 20th	Day	24% 27% 25% 18% 5%	271
	21st to end	Day	23% 31% 31% 13% 2 1	180
December	1st to 10th	Day	28% 32% 26% 11% 4 1	180
	11th to 20th	Day	31% 27% 28% 13% 2	180
	21st to end	Day	26% 29% 30% 12% 2	198
January	1st to 10th	Night	19% 28% 34% 16% 3	760
	11th to 20th	Night	24% 21% 31% 20% 4	735
	21st to end	Night	16% 26% 36% 21% 2	792
February	1st to 10th	Night	24% 22% 33% 20% 1	720
	11th to 20th	Night	23% 28% 34% 15% 1	703
	21st to end	Night	25% 26% 31% 16% 3	592
March	1st to 10th	Night	23% 23% 31% 20% 3	720
	11th to 20th	Night	24% 22% 29% 19% 5%	720
	21st to end	Night	21% 19% 35% 19% 5% 1	792
April	1st to 10th	Night	23% 23% 30% 18% 6%	720
	11th to 20th	Night	20% 20% 31% 21% 8% 1	711
	21st to end	Night	22% 17% 29% 26% 6%	711
May	1st to 10th	Night	24% 12% 26% 25% 12%	524
	11th to 20th	Night	19% 10% 31% 30% 9%	354
	21st to end	Night	17% 9% 22% 34% 18% 1	389
June	1st to 10th	Night	14% 6% 26% 37% 17%	360
	11th to 20th	Night	14% 9% 24% 38% 15%	360
	21st to end	Night	11% 3 30% 43% 13% 1	360
July	1st to 10th	Night	11% 5% 24% 38% 20% 1	350
	11th to 20th	Night	7% 4 19% 43% 25% 1	348
	21st to end	Night	12% 3 21% 42% 20% 1	547
August	1st to 10th	Night	12% 2 19% 41% 24% 2	537
	11th to 20th	Night	14% 5% 16% 33% 29% 3	567
	21st to end	Night	14% 3 18% 35% 27% 2	792
September	1st to 10th	Night	19% 4 22% 30% 22% 3	720
	11th to 20th	Night	16% 6% 20% 29% 25% 5%	720
	21st to end	Night	20% 8% 21% 28% 20% 3	720
October	1st to 10th	Night	22% 10% 22% 25% 18% 3	720
	11th to 20th	Night	19% 15% 27% 29% 9% 1	720
	21st to end	Night	26% 14% 28% 25% 6%	792
November	1st to 10th	Night	16% 16% 29% 28% 11% 1	740
	11th to 20th	Night	23% 21% 24% 24% 7% 1	760
	21st to end	Night	20% 22% 30% 23% 5% 1	760
December	1st to 10th	Night	25% 22% 31% 16% 5%	760
	11th to 20th	Night	23% 22% 31% 20% 4	760
	21st to end	Night	23% 22% 30% 23% 2	836

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