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 2025-02-21.

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% 12% 4 10% 6% 5% 5% 4 5% 6% 26%	108517
	Night	15% 13% 5% 12% 7% 5% 5% 3 5% 5% 25%	110072
Transparency	Day	55% 7% 11% 17% 8% 1	112046
	Night	58% 7% 10% 15% 8% 1	114707
Seeing	Day	26% 18% 27% 18% 8% 2	15021
	Night	24% 15% 25% 26% 10% 1	24067

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	11% 13% 8% 23% 12% 7% 4 3 3 3 13%	3597
	11th to 20th	Day	9% 12% 6% 20% 14% 8% 6% 3 4 4 14%	3570
	21st to end	Day	9% 16% 8% 15% 6% 7% 6% 3 4 5% 20%	3960
February	1st to 10th	Day	13% 14% 7% 15% 10% 6% 6% 3 3 4 19%	1716
	11th to 20th	Day	12% 12% 8% 16% 9% 7% 5% 3 5% 4 18%	1948
	21st to end	Day	13% 16% 5% 14% 9% 5% 5% 3 5% 5% 21%	1570
March	1st to 10th	Day	19% 18% 5% 11% 7% 5% 4 3 3 4 20%	3116
	11th to 20th	Day	14% 18% 7% 12% 6% 4 5% 3 3 5% 23%	3276
	21st to end	Day	17% 13% 5% 13% 5% 3 4 2 4 4 30%	4124
April	1st to 10th	Day	20% 16% 4 9% 5% 4 4 2 3 5% 27%	2280
	11th to 20th	Day	19% 11% 4 8% 5% 5% 4 3 4 5% 32%	2273
	21st to end	Day	22% 12% 4 9% 5% 4 3 3 5% 6% 26%	2470
May	1st to 10th	Day	25% 13% 4 7% 5% 4 5% 4 4 5% 25%	4128
	11th to 20th	Day	17% 10% 3 9% 7% 7% 5% 4 6% 7% 25%	4398
	21st to end	Day	13% 8% 3 7% 5% 5% 6% 5% 7% 9% 33%	4837
June	1st to 10th	Day	25% 9% 2 7% 5% 4 6% 3 6% 6% 26%	2660
	11th to 20th	Day	16% 8% 3 6% 5% 5% 5% 4 6% 7% 34%	2660
	21st to end	Day	21% 10% 3 7% 6% 6% 6% 4 5% 7% 25%	2660
July	1st to 10th	Day	13% 7% 3 7% 5% 5% 6% 5% 8% 11% 30%	4440
	11th to 20th	Day	11% 7% 3 7% 6% 6% 6% 6% 9% 11% 29%	4440
	21st to end	Day	16% 8% 3 7% 5% 5% 5% 4 6% 8% 34%	4814
August	1st to 10th	Day	15% 8% 3 9% 7% 6% 6% 5% 7% 7% 27%	4361
	11th to 20th	Day	16% 9% 2 9% 6% 5% 6% 5% 8% 9% 25%	4018
	21st to end	Day	12% 8% 3 7% 5% 5% 6% 5% 6% 8% 36%	4180
September	1st to 10th	Day	14% 7% 3 6% 4 4 5% 4 6% 7% 41%	2280
	11th to 20th	Day	15% 8% 2 5% 4 3 4 3 5% 8% 43%	2130
	21st to end	Day	17% 7% 3 6% 5% 4 6% 4 6% 8% 34%	2090
October	1st to 10th	Day	23% 11% 3 8% 5% 4 4 3 6% 6% 28%	2071
	11th to 20th	Day	15% 13% 3 8% 5% 5% 5% 4 5% 7% 30%	1800
	21st to end	Day	22% 20% 5% 7% 5% 4 3 3 3 4 25%	1881
November	1st to 10th	Day	14% 16% 4 10% 5% 4 4 4 5% 6% 28%	1761
	11th to 20th	Day	15% 15% 7% 12% 5% 4 6% 3 4 5% 24%	1800
	21st to end	Day	18% 15% 6% 12% 6% 4 3 3 5% 6% 21%	1662
December	1st to 10th	Day	13% 21% 7% 22% 9% 5% 4 3 3 4 11%	2600
	11th to 20th	Day	17% 16% 9% 14% 5% 4 3 2 4 4 22%	2986
	21st to end	Day	18% 17% 7% 14% 5% 4 5% 3 3 3 20%	3960
January	1st to 10th	Night	8% 15% 6% 26% 13% 8% 4 3 3 3 11%	6349
	11th to 20th	Night	9% 15% 6% 20% 14% 7% 5% 3 4 4 11%	6315
	21st to end	Night	10% 14% 6% 15% 7% 8% 7% 4 5% 5% 18%	6996
February	1st to 10th	Night	11% 16% 6% 14% 10% 6% 6% 4 4 5% 18%	2680
	11th to 20th	Night	11% 12% 5% 14% 10% 8% 6% 5% 4 5% 19%	2338
	21st to end	Night	12% 13% 4 12% 8% 7% 4 3 5% 5% 26%	1885
March	1st to 10th	Night	20% 17% 5% 7% 6% 6% 4 3 4 5% 24%	3724
	11th to 20th	Night	14% 16% 7% 11% 6% 5% 4 3 4 4 26%	3682
	21st to end	Night	16% 17% 6% 12% 7% 3 3 2 3 3 28%	3506
April	1st to 10th	Night	20% 12% 3 9% 6% 5% 4 4 5% 5% 27%	1900
	11th to 20th	Night	19% 11% 4 8% 6% 4 3 3 5% 5% 32%	1856
	21st to end	Night	19% 12% 3 7% 5% 4 3 4 6% 6% 31%	1710
May	1st to 10th	Night	24% 12% 3 8% 5% 4 4 3 5% 6% 26%	2834
	11th to 20th	Night	20% 9% 3 9% 5% 4 5% 4 6% 6% 31%	2501
	21st to end	Night	15% 6% 2 7% 5% 5% 5% 4 6% 8% 38%	2738
June	1st to 10th	Night	22% 10% 3 7% 5% 4 4 3 5% 8% 29%	1520
	11th to 20th	Night	17% 6% 2 6% 5% 4 4 5% 7% 7% 36%	1520
	21st to end	Night	21% 8% 3 6% 5% 5% 5% 4 6% 6% 30%	1520
July	1st to 10th	Night	11% 8% 3 7% 6% 6% 6% 6% 7% 9% 32%	2520
	11th to 20th	Night	16% 7% 2 7% 4 4 5% 4 6% 8% 37%	2520
	21st to end	Night	14% 7% 2 7% 5% 5% 5% 4 6% 7% 37%	2739
August	1st to 10th	Night	18% 9% 3 7% 5% 5% 4 4 6% 7% 31%	2538
	11th to 20th	Night	14% 8% 4 8% 5% 5% 6% 4 6% 8% 33%	2806
	21st to end	Night	13% 7% 3 7% 5% 5% 5% 4 6% 8% 37%	3476
September	1st to 10th	Night	14% 6% 2 6% 4 4 4 4 4 8% 43%	1900
	11th to 20th	Night	14% 7% 2 6% 3 3 4 3 5% 6% 46%	2049
	21st to end	Night	18% 8% 2 4 3 3 4 3 5% 7% 42%	2090
October	1st to 10th	Night	20% 10% 3 8% 5% 4 4 3 5% 6% 32%	2107
	11th to 20th	Night	16% 12% 4 8% 6% 5% 4 4 5% 7% 31%	2380
	21st to end	Night	21% 15% 4 9% 5% 4 4 3 4 6% 24%	2717
November	1st to 10th	Night	16% 12% 4 11% 5% 5% 5% 4 5% 5% 30%	2537
	11th to 20th	Night	16% 12% 6% 13% 5% 4 4 3 4 6% 27%	2600
	21st to end	Night	17% 14% 6% 14% 7% 5% 3 3 5% 5% 21%	2735
December	1st to 10th	Night	13% 18% 7% 18% 10% 7% 6% 4 4 4 9%	4580
	11th to 20th	Night	21% 16% 6% 13% 7% 5% 4 3 4 4 18%	5208
	21st to end	Night	19% 16% 5% 14% 6% 5% 5% 3 3 4 20%	6996

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	75% 5% 8% 11% 2	3598
	11th to 20th	Day	69% 8% 7% 8% 5% 2	3600
	21st to end	Day	63% 7% 8% 11% 10% 1	3960
February	1st to 10th	Day	68% 5% 7% 11% 6% 2	1716
	11th to 20th	Day	68% 5% 9% 11% 5% 1	1957
	21st to end	Day	65% 5% 8% 12% 8% 3	1570
March	1st to 10th	Day	68% 6% 7% 9% 9% 2	3119
	11th to 20th	Day	62% 7% 9% 11% 9% 3	3276
	21st to end	Day	58% 5% 6% 15% 15% 2	4124
April	1st to 10th	Day	61% 4 8% 16% 9% 2	2280
	11th to 20th	Day	53% 7% 9% 19% 12% 1	2273
	21st to end	Day	59% 6% 8% 16% 8% 3	2470
May	1st to 10th	Day	57% 9% 10% 17% 6% 1	4142
	11th to 20th	Day	52% 12% 14% 14% 7% 1	4402
	21st to end	Day	42% 11% 14% 23% 9% 1	4837
June	1st to 10th	Day	56% 8% 11% 17% 8%	2660
	11th to 20th	Day	47% 8% 11% 21% 12% 2	2660
	21st to end	Day	55% 10% 10% 18% 7% 1	2660
July	1st to 10th	Day	44% 7% 16% 23% 10%	4370
	11th to 20th	Day	44% 9% 16% 26% 5%	4365
	21st to end	Day	47% 8% 11% 25% 10%	4661
August	1st to 10th	Day	52% 8% 14% 19% 7%	4347
	11th to 20th	Day	52% 8% 15% 20% 5%	4057
	21st to end	Day	43% 8% 16% 19% 13% 2	4180
September	1st to 10th	Day	41% 8% 11% 28% 12% 1	2280
	11th to 20th	Day	39% 6% 12% 29% 14% 1	2130
	21st to end	Day	45% 8% 14% 21% 11% 1	2090
October	1st to 10th	Day	49% 8% 12% 21% 11%	3448
	11th to 20th	Day	46% 7% 14% 23% 9%	2964
	21st to end	Day	51% 5% 13% 22% 9% 1	3081
November	1st to 10th	Day	54% 7% 12% 17% 9% 1	1761
	11th to 20th	Day	61% 8% 8% 10% 10% 2	1800
	21st to end	Day	63% 7% 14% 13% 3	1662
December	1st to 10th	Day	78% 5% 9% 7% 2	2600
	11th to 20th	Day	65% 6% 5% 15% 9%	2986
	21st to end	Day	69% 5% 5% 10% 10% 2	3960
January	1st to 10th	Night	77% 6% 9% 7% 1	6349
	11th to 20th	Night	74% 9% 8% 6% 2 1	6360
	21st to end	Night	64% 7% 8% 10% 10% 1	6996
February	1st to 10th	Night	67% 7% 8% 11% 6% 1	2680
	11th to 20th	Night	65% 6% 9% 12% 7% 1	2340
	21st to end	Night	59% 7% 10% 15% 8% 2	1885
March	1st to 10th	Night	62% 5% 10% 14% 7% 2	3734
	11th to 20th	Night	60% 8% 8% 14% 10% 1	3683
	21st to end	Night	61% 5% 6% 13% 14% 2	3506
April	1st to 10th	Night	57% 6% 10% 16% 11% 1	1900
	11th to 20th	Night	53% 6% 9% 21% 11%	1856
	21st to end	Night	51% 9% 10% 16% 12% 2	1710
May	1st to 10th	Night	57% 9% 9% 16% 7% 1	2840
	11th to 20th	Night	50% 11% 14% 16% 8% 1	2502
	21st to end	Night	40% 12% 16% 19% 13% 1	2739
June	1st to 10th	Night	53% 9% 9% 20% 9% 1	1520
	11th to 20th	Night	44% 9% 16% 19% 10% 3	1520
	21st to end	Night	50% 11% 13% 18% 8% 1	1520
July	1st to 10th	Night	46% 9% 14% 20% 9%	2480
	11th to 20th	Night	42% 8% 14% 28% 8%	2472
	21st to end	Night	44% 10% 12% 25% 9%	2651
August	1st to 10th	Night	51% 8% 14% 22% 5%	2528
	11th to 20th	Night	48% 8% 18% 17% 9%	2833
	21st to end	Night	43% 7% 16% 22% 12%	3476
September	1st to 10th	Night	38% 9% 15% 27% 10%	1900
	11th to 20th	Night	37% 7% 12% 31% 12% 1	2049
	21st to end	Night	41% 8% 14% 24% 11% 2	2090
October	1st to 10th	Night	52% 7% 13% 20% 8%	3510
	11th to 20th	Night	45% 10% 10% 22% 12%	3955
	21st to end	Night	50% 6% 14% 20% 9% 1	4467
November	1st to 10th	Night	54% 8% 12% 18% 8% 1	2537
	11th to 20th	Night	56% 7% 9% 16% 9% 2	2600
	21st to end	Night	64% 6% 12% 13% 5%	2735
December	1st to 10th	Night	76% 8% 8% 7% 1	4580
	11th to 20th	Night	69% 6% 6% 10% 9%	5208
	21st to end	Night	68% 5% 7% 11% 7% 1	6996

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% 23% 33% 16% 5%	200
	11th to 20th	Day	30% 24% 32% 10% 4 1	200
	21st to end	Day	25% 25% 30% 15% 5%	220
February	1st to 10th	Day	34% 31% 25% 7% 2 1	268
	11th to 20th	Day	27% 27% 30% 13% 2	496
	21st to end	Day	33% 19% 28% 16% 4	382
March	1st to 10th	Day	32% 19% 28% 18% 3	460
	11th to 20th	Day	34% 20% 27% 14% 4 1	460
	21st to end	Day	33% 20% 25% 13% 7% 2	506
April	1st to 10th	Day	36% 19% 23% 15% 5% 2	460
	11th to 20th	Day	32% 25% 21% 14% 7% 2	452
	21st to end	Day	35% 19% 26% 12% 7% 1	460
May	1st to 10th	Day	34% 21% 23% 14% 6% 2	463
	11th to 20th	Day	24% 17% 32% 19% 6% 3	487
	21st to end	Day	19% 19% 34% 18% 8% 2	529
June	1st to 10th	Day	30% 18% 27% 16% 8% 2	490
	11th to 20th	Day	21% 16% 31% 20% 9% 3	490
	21st to end	Day	26% 17% 31% 17% 7% 1	490
July	1st to 10th	Day	19% 12% 31% 26% 10% 2	485
	11th to 20th	Day	14% 14% 32% 25% 12% 2	484
	21st to end	Day	17% 12% 34% 25% 10% 2	532
August	1st to 10th	Day	17% 13% 26% 25% 16% 2	486
	11th to 20th	Day	21% 10% 28% 23% 14% 5%	479
	21st to end	Day	17% 16% 24% 24% 13% 6%	539
September	1st to 10th	Day	17% 14% 26% 22% 15% 6%	490
	11th to 20th	Day	18% 12% 22% 18% 20% 10%	490
	21st to end	Day	18% 12% 27% 22% 15% 6%	490
October	1st to 10th	Day	27% 14% 24% 18% 12% 5%	490
	11th to 20th	Day	25% 18% 25% 19% 10% 3	437
	21st to end	Day	36% 17% 24% 17% 7%	429
November	1st to 10th	Day	30% 19% 20% 24% 8%	395
	11th to 20th	Day	26% 22% 26% 20% 6% 1	400
	21st to end	Day	34% 17% 28% 18% 3	262
December	1st to 10th	Day	31% 22% 27% 13% 8%	200
	11th to 20th	Day	38% 20% 31% 11% 2	200
	21st to end	Day	33% 21% 27% 16% 3	220
January	1st to 10th	Night	23% 22% 32% 19% 4	800
	11th to 20th	Night	27% 19% 29% 21% 4	800
	21st to end	Night	19% 20% 30% 26% 4	880
February	1st to 10th	Night	21% 20% 34% 21% 4	800
	11th to 20th	Night	23% 21% 33% 20% 3	783
	21st to end	Night	22% 19% 32% 22% 4	629
March	1st to 10th	Night	25% 19% 28% 24% 3 1	760
	11th to 20th	Night	28% 18% 26% 20% 7%	760
	21st to end	Night	27% 18% 26% 21% 8% 1	836
April	1st to 10th	Night	28% 17% 26% 23% 6%	760
	11th to 20th	Night	25% 20% 26% 22% 8%	732
	21st to end	Night	27% 15% 30% 23% 5%	570
May	1st to 10th	Night	28% 14% 28% 22% 7% 1	570
	11th to 20th	Night	22% 12% 27% 30% 8% 1	380
	21st to end	Night	19% 12% 22% 35% 12%	411
June	1st to 10th	Night	26% 9% 22% 32% 9% 1	380
	11th to 20th	Night	19% 8% 21% 35% 16%	380
	21st to end	Night	23% 6% 21% 37% 12% 1	380
July	1st to 10th	Night	16% 6% 18% 44% 15% 1	370
	11th to 20th	Night	15% 4 21% 40% 18% 2	368
	21st to end	Night	14% 5% 17% 45% 19%	416
August	1st to 10th	Night	17% 4 14% 41% 24%	397
	11th to 20th	Night	16% 6% 22% 33% 20% 2	573
	21st to end	Night	17% 6% 19% 32% 23% 3	836
September	1st to 10th	Night	16% 9% 21% 34% 18% 2	760
	11th to 20th	Night	17% 10% 17% 28% 25% 3	760
	21st to end	Night	21% 10% 22% 25% 19% 2	760
October	1st to 10th	Night	27% 13% 18% 26% 15% 1	760
	11th to 20th	Night	24% 13% 23% 26% 13% 2	760
	21st to end	Night	31% 12% 27% 24% 6%	836
November	1st to 10th	Night	24% 17% 25% 26% 7% 1	780
	11th to 20th	Night	24% 17% 28% 21% 8% 2	800
	21st to end	Night	27% 19% 26% 20% 7% 1	800
December	1st to 10th	Night	29% 20% 27% 20% 5%	800
	11th to 20th	Night	29% 18% 27% 22% 5%	800
	21st to end	Night	31% 17% 26% 23% 3	880

Page generated at 2025-02-18 03:26:31. Version ac07

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