Wheelock
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wheelock clear sky chart from 2005-11-06 to 2024-01-31.

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% 8% 7% 3 5% 5% 5% 6% 27%	74929
	Night	17% 11% 5% 9% 7% 4 5% 4 5% 6% 27%	75198
Transparency	Day	56% 7% 11% 17% 9% 1	74634
	Night	57% 7% 11% 16% 8% 1	75027
Seeing	Day	26% 18% 27% 18% 9% 2	13681
	Night	24% 15% 25% 26% 10% 1	22734

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	13% 15% 9% 16% 9% 4 5% 5% 4 4 15%	1517
	11th to 20th	Day	15% 15% 7% 12% 10% 4 5% 4 4 4 19%	1520
	21st to end	Day	11% 15% 8% 11% 9% 3 6% 6% 4 5% 22%	1664
February	1st to 10th	Day	14% 14% 8% 13% 12% 5% 5% 5% 3 4 18%	1544
	11th to 20th	Day	13% 11% 9% 15% 9% 5% 6% 4 5% 4 19%	1748
	21st to end	Day	13% 15% 7% 14% 10% 3 6% 4 4 6% 20%	1480
March	1st to 10th	Day	13% 13% 7% 11% 9% 4 5% 4 4 5% 24%	1797
	11th to 20th	Day	14% 15% 5% 9% 7% 3 5% 4 3 5% 29%	1860
	21st to end	Day	16% 14% 6% 11% 8% 3 4 3 4 4 28%	2348
April	1st to 10th	Day	20% 14% 6% 9% 7% 2 4 3 3 5% 28%	2160
	11th to 20th	Day	19% 11% 4 6% 6% 4 5% 4 4 5% 33%	2152
	21st to end	Day	23% 12% 4 8% 6% 3 4 4 5% 6% 25%	2340
May	1st to 10th	Day	23% 12% 4 6% 5% 3 4 4 4 5% 29%	2348
	11th to 20th	Day	19% 10% 4 5% 6% 3 4 5% 5% 6% 33%	2478
	21st to end	Day	16% 9% 4 6% 5% 3 5% 5% 6% 8% 32%	2725
June	1st to 10th	Day	26% 9% 3 7% 6% 3 5% 4 5% 7% 26%	2520
	11th to 20th	Day	17% 8% 3 6% 6% 3 5% 5% 5% 7% 34%	2520
	21st to end	Day	22% 9% 3 7% 6% 4 6% 5% 5% 8% 25%	2520
July	1st to 10th	Day	17% 7% 3 6% 5% 4 5% 5% 6% 9% 32%	2520
	11th to 20th	Day	13% 7% 3 6% 7% 5% 7% 6% 8% 9% 28%	2520
	21st to end	Day	17% 7% 3 6% 6% 4 6% 6% 7% 9% 29%	2744
August	1st to 10th	Day	15% 7% 3 6% 8% 4 5% 5% 6% 7% 32%	2471
	11th to 20th	Day	14% 9% 2 8% 6% 3 6% 5% 7% 9% 30%	2291
	21st to end	Day	14% 7% 3 5% 5% 4 6% 6% 7% 8% 35%	2376
September	1st to 10th	Day	15% 7% 3 5% 5% 3 5% 5% 6% 7% 40%	2160
	11th to 20th	Day	16% 8% 2 5% 5% 1 4 4 4 8% 43%	2018
	21st to end	Day	17% 7% 3 5% 6% 3 5% 5% 5% 8% 35%	1980
October	1st to 10th	Day	24% 11% 3 7% 6% 3 4 4 6% 6% 28%	1962
	11th to 20th	Day	15% 13% 3 7% 5% 3 5% 5% 5% 7% 30%	1706
	21st to end	Day	24% 19% 5% 6% 7% 3 3 4 3 4 23%	1782
November	1st to 10th	Day	14% 15% 4 9% 5% 4 4 4 6% 6% 28%	1671
	11th to 20th	Day	16% 14% 8% 11% 7% 3 6% 4 4 5% 22%	1710
	21st to end	Day	18% 14% 6% 11% 6% 3 4 4 5% 6% 22%	1579
December	1st to 10th	Day	19% 18% 8% 13% 8% 3 4 3 4 4 16%	1520
	11th to 20th	Day	20% 15% 10% 9% 6% 3 3 3 4 5% 23%	1906
	21st to end	Day	22% 17% 7% 14% 8% 3 4 4 3 3 15%	2772
January	1st to 10th	Night	13% 15% 7% 14% 12% 5% 7% 5% 5% 5% 13%	2649
	11th to 20th	Night	15% 16% 7% 12% 9% 5% 6% 5% 5% 5% 17%	2660
	21st to end	Night	10% 13% 7% 13% 9% 5% 6% 6% 6% 5% 20%	2919
February	1st to 10th	Night	12% 14% 7% 12% 11% 5% 7% 5% 4 5% 16%	2412
	11th to 20th	Night	11% 11% 6% 12% 10% 6% 7% 6% 4 6% 20%	2098
	21st to end	Night	13% 12% 5% 11% 10% 5% 6% 4 4 5% 25%	1776
March	1st to 10th	Night	13% 13% 7% 10% 8% 5% 6% 5% 4 5% 24%	2150
	11th to 20th	Night	16% 12% 5% 9% 7% 4 5% 4 5% 5% 28%	2098
	21st to end	Night	16% 12% 5% 9% 7% 3 4 4 4 4 31%	2006
April	1st to 10th	Night	21% 11% 5% 8% 8% 3 4 5% 5% 5% 27%	1800
	11th to 20th	Night	18% 9% 5% 7% 7% 3 3 4 5% 5% 34%	1757
	21st to end	Night	19% 11% 4 6% 7% 2 4 5% 6% 6% 30%	1620
May	1st to 10th	Night	21% 11% 4 7% 5% 3 4 4 5% 6% 30%	1620
	11th to 20th	Night	19% 7% 4 6% 5% 3 4 4 6% 6% 36%	1421
	21st to end	Night	17% 7% 2 7% 6% 3 4 5% 5% 8% 36%	1550
June	1st to 10th	Night	23% 9% 3 6% 5% 3 5% 4 5% 8% 29%	1440
	11th to 20th	Night	18% 6% 3 6% 6% 3 4 5% 6% 7% 36%	1440
	21st to end	Night	20% 6% 3 6% 5% 3 7% 4 6% 8% 31%	1440
July	1st to 10th	Night	15% 7% 3 5% 6% 3 4 6% 6% 8% 36%	1440
	11th to 20th	Night	14% 6% 2 5% 4 3 5% 6% 6% 11% 38%	1440
	21st to end	Night	13% 6% 2 6% 6% 4 5% 5% 7% 8% 38%	1574
August	1st to 10th	Night	13% 6% 2 5% 5% 3 4 5% 7% 10% 41%	1450
	11th to 20th	Night	14% 7% 3 5% 6% 3 4 4 5% 9% 39%	1606
	21st to end	Night	16% 7% 3 5% 5% 3 5% 3 5% 8% 41%	1980
September	1st to 10th	Night	14% 6% 2 5% 5% 3 5% 5% 4 8% 43%	1800
	11th to 20th	Night	15% 7% 3 5% 4 2 4 3 5% 6% 46%	1941
	21st to end	Night	18% 7% 3 4 4 2 4 4 5% 7% 42%	1980
October	1st to 10th	Night	21% 9% 3 7% 5% 3 4 4 5% 7% 32%	1996
	11th to 20th	Night	17% 12% 3 7% 6% 4 4 5% 5% 7% 30%	2254
	21st to end	Night	23% 15% 4 8% 7% 3 4 4 4 6% 23%	2574
November	1st to 10th	Night	17% 12% 5% 9% 6% 3 5% 5% 5% 5% 30%	2407
	11th to 20th	Night	17% 10% 7% 12% 7% 3 4 4 4 6% 27%	2470
	21st to end	Night	17% 12% 7% 13% 8% 4 4 4 4 5% 21%	2598
December	1st to 10th	Night	19% 18% 8% 13% 9% 4 5% 5% 3 4 13%	2660
	11th to 20th	Night	20% 14% 8% 12% 9% 4 5% 3 4 4 16%	3288
	21st to end	Night	23% 15% 6% 15% 8% 4 4 3 4 3 15%	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	71% 5% 8% 12% 4	1518
	11th to 20th	Day	68% 5% 8% 10% 7% 1	1520
	21st to end	Day	63% 5% 10% 13% 8% 1	1664
February	1st to 10th	Day	70% 5% 7% 10% 6% 1	1544
	11th to 20th	Day	68% 5% 10% 12% 5% 1	1757
	21st to end	Day	66% 5% 8% 11% 8% 2	1480
March	1st to 10th	Day	63% 5% 9% 13% 7% 3	1800
	11th to 20th	Day	58% 4 11% 12% 12% 2	1860
	21st to end	Day	59% 5% 7% 16% 10% 3	2348
April	1st to 10th	Day	61% 4 8% 16% 9% 2	2160
	11th to 20th	Day	52% 7% 9% 19% 12% 1	2152
	21st to end	Day	59% 6% 8% 16% 7% 3	2340
May	1st to 10th	Day	55% 5% 9% 19% 10% 1	2348
	11th to 20th	Day	51% 6% 9% 19% 13% 2	2482
	21st to end	Day	48% 9% 13% 19% 9% 1	2725
June	1st to 10th	Day	57% 8% 11% 17% 7%	2520
	11th to 20th	Day	48% 8% 10% 22% 12% 2	2520
	21st to end	Day	55% 10% 10% 17% 7% 1	2520
July	1st to 10th	Day	46% 7% 15% 21% 11%	2450
	11th to 20th	Day	47% 11% 13% 22% 7%	2445
	21st to end	Day	48% 10% 14% 19% 7%	2591
August	1st to 10th	Day	48% 10% 16% 20% 7%	2457
	11th to 20th	Day	47% 9% 16% 19% 9%	2291
	21st to end	Day	43% 9% 13% 22% 12% 1	2376
September	1st to 10th	Day	42% 8% 12% 26% 11% 1	2160
	11th to 20th	Day	40% 6% 11% 29% 14% 1	2018
	21st to end	Day	44% 8% 14% 21% 11% 1	1980
October	1st to 10th	Day	56% 6% 11% 17% 9% 1	1962
	11th to 20th	Day	50% 8% 17% 19% 6%	1706
	21st to end	Day	66% 5% 9% 12% 7% 2	1782
November	1st to 10th	Day	54% 7% 12% 17% 9% 1	1671
	11th to 20th	Day	62% 8% 9% 10% 9% 2	1710
	21st to end	Day	62% 7% 14% 13% 3	1579
December	1st to 10th	Day	73% 5% 9% 10% 3	1520
	11th to 20th	Day	64% 5% 8% 15% 8% 1	1906
	21st to end	Day	74% 6% 5% 9% 5%	2772
January	1st to 10th	Night	71% 7% 9% 9% 3	2649
	11th to 20th	Night	69% 6% 10% 10% 4 1	2660
	21st to end	Night	63% 7% 11% 11% 7% 1	2919
February	1st to 10th	Night	68% 7% 8% 10% 6% 1	2412
	11th to 20th	Night	64% 6% 9% 13% 7% 1	2100
	21st to end	Night	60% 7% 9% 15% 8% 1	1776
March	1st to 10th	Night	63% 5% 8% 15% 8% 2	2160
	11th to 20th	Night	57% 7% 9% 16% 10% 1	2099
	21st to end	Night	55% 6% 8% 17% 11% 2	2006
April	1st to 10th	Night	58% 5% 9% 16% 10% 1	1800
	11th to 20th	Night	51% 6% 9% 22% 12%	1757
	21st to end	Night	52% 9% 10% 16% 11% 2	1620
May	1st to 10th	Night	56% 6% 7% 17% 12% 2	1620
	11th to 20th	Night	46% 8% 11% 19% 14% 2	1422
	21st to end	Night	46% 9% 14% 20% 10% 1	1551
June	1st to 10th	Night	53% 8% 10% 20% 9% 1	1440
	11th to 20th	Night	45% 9% 14% 19% 10% 3	1440
	21st to end	Night	49% 11% 13% 18% 8%	1440
July	1st to 10th	Night	44% 12% 13% 19% 12% 1	1400
	11th to 20th	Night	37% 12% 17% 26% 8%	1392
	21st to end	Night	40% 12% 14% 24% 9% 1	1486
August	1st to 10th	Night	38% 10% 18% 28% 7%	1440
	11th to 20th	Night	42% 8% 18% 21% 11%	1606
	21st to end	Night	42% 7% 15% 25% 11%	1980
September	1st to 10th	Night	38% 10% 16% 26% 10% 1	1800
	11th to 20th	Night	38% 8% 11% 30% 12% 1	1941
	21st to end	Night	40% 8% 14% 25% 11% 2	1980
October	1st to 10th	Night	52% 7% 12% 18% 10% 1	1996
	11th to 20th	Night	52% 9% 13% 20% 6%	2254
	21st to end	Night	61% 7% 11% 12% 7% 1	2574
November	1st to 10th	Night	54% 7% 12% 17% 8% 1	2407
	11th to 20th	Night	57% 7% 10% 16% 8% 2	2470
	21st to end	Night	64% 6% 13% 13% 5%	2598
December	1st to 10th	Night	74% 6% 8% 9% 2	2660
	11th to 20th	Night	71% 5% 8% 9% 6%	3288
	21st to end	Night	73% 6% 7% 11% 3	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	24% 19% 33% 18% 5%	180
	11th to 20th	Day	32% 24% 29% 11% 4 1	180
	21st to end	Day	24% 24% 30% 16% 6%	196
February	1st to 10th	Day	36% 30% 26% 6% 3	216
	11th to 20th	Day	27% 27% 29% 13% 3	416
	21st to end	Day	32% 20% 27% 17% 4	346
March	1st to 10th	Day	31% 20% 29% 17% 3	420
	11th to 20th	Day	33% 20% 27% 14% 5% 1	420
	21st to end	Day	31% 19% 26% 14% 8% 2	462
April	1st to 10th	Day	35% 19% 24% 16% 5% 1	420
	11th to 20th	Day	32% 24% 21% 15% 8% 2	412
	21st to end	Day	35% 19% 26% 12% 7% 1	420
May	1st to 10th	Day	35% 21% 24% 12% 7% 2	423
	11th to 20th	Day	25% 18% 32% 17% 5% 3	447
	21st to end	Day	20% 18% 34% 18% 9% 2	485
June	1st to 10th	Day	31% 18% 28% 15% 7% 2	450
	11th to 20th	Day	22% 17% 30% 19% 9% 3	450
	21st to end	Day	25% 16% 32% 18% 8% 2	450
July	1st to 10th	Day	20% 12% 29% 27% 10% 2	445
	11th to 20th	Day	15% 14% 32% 25% 12% 2	444
	21st to end	Day	17% 11% 35% 25% 10% 2	488
August	1st to 10th	Day	17% 12% 26% 26% 16% 2	446
	11th to 20th	Day	20% 10% 28% 22% 15% 5%	439
	21st to end	Day	18% 16% 25% 23% 13% 6%	495
September	1st to 10th	Day	17% 13% 26% 22% 15% 6%	450
	11th to 20th	Day	19% 13% 24% 18% 18% 8%	450
	21st to end	Day	18% 13% 27% 22% 14% 6%	450
October	1st to 10th	Day	28% 14% 23% 19% 12% 5%	450
	11th to 20th	Day	24% 18% 26% 19% 9% 2	403
	21st to end	Day	37% 17% 23% 17% 7%	396
November	1st to 10th	Day	29% 18% 19% 25% 8%	365
	11th to 20th	Day	27% 22% 25% 19% 5% 1	370
	21st to end	Day	33% 16% 28% 19% 4	239
December	1st to 10th	Day	32% 21% 26% 13% 8%	180
	11th to 20th	Day	35% 22% 30% 12% 2	180
	21st to end	Day	35% 21% 26% 16% 3	198
January	1st to 10th	Night	23% 22% 31% 20% 4	760
	11th to 20th	Night	28% 19% 29% 20% 4	760
	21st to end	Night	20% 21% 29% 26% 5%	835
February	1st to 10th	Night	21% 20% 35% 20% 4	720
	11th to 20th	Night	22% 21% 33% 20% 4	703
	21st to end	Night	22% 20% 32% 23% 4	592
March	1st to 10th	Night	24% 19% 28% 24% 3 1	720
	11th to 20th	Night	28% 18% 26% 20% 7%	720
	21st to end	Night	27% 18% 26% 21% 8% 1	792
April	1st to 10th	Night	28% 17% 26% 23% 6%	720
	11th to 20th	Night	24% 20% 26% 23% 8%	693
	21st to end	Night	26% 15% 30% 23% 6%	540
May	1st to 10th	Night	28% 14% 28% 22% 8% 1	540
	11th to 20th	Night	22% 11% 28% 30% 8% 1	360
	21st to end	Night	19% 11% 23% 35% 12%	389
June	1st to 10th	Night	27% 9% 23% 31% 10% 1	360
	11th to 20th	Night	20% 8% 21% 36% 16%	360
	21st to end	Night	21% 6% 21% 39% 13% 1	360
July	1st to 10th	Night	16% 5% 19% 43% 16% 1	350
	11th to 20th	Night	15% 3 21% 40% 19% 2	348
	21st to end	Night	13% 5% 18% 46% 18%	394
August	1st to 10th	Night	16% 4 14% 40% 25%	376
	11th to 20th	Night	16% 7% 22% 34% 20% 2	542
	21st to end	Night	18% 6% 19% 31% 23% 3	792
September	1st to 10th	Night	15% 9% 22% 34% 18% 2	720
	11th to 20th	Night	18% 10% 18% 28% 23% 3	720
	21st to end	Night	21% 10% 23% 25% 19% 2	720
October	1st to 10th	Night	27% 13% 18% 25% 15% 1	720
	11th to 20th	Night	24% 13% 23% 26% 13% 2	720
	21st to end	Night	32% 13% 26% 23% 6%	792
November	1st to 10th	Night	24% 17% 24% 26% 8% 1	740
	11th to 20th	Night	24% 17% 28% 21% 8% 2	760
	21st to end	Night	27% 20% 25% 20% 7% 1	760
December	1st to 10th	Night	30% 20% 27% 19% 4	760
	11th to 20th	Night	29% 18% 27% 22% 4	760
	21st to end	Night	31% 18% 26% 23% 3	836

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