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 2026-06-16.

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% 11% 4 10% 6% 5% 5% 4 5% 6% 26%	137555
	Night	15% 12% 5% 12% 7% 6% 5% 3 5% 6% 25%	134505
Transparency	Day	56% 7% 11% 18% 8% 1	141038
	Night	58% 7% 10% 16% 8% 1	139111
Seeing	Day	26% 18% 28% 18% 8% 2	16750
	Night	24% 15% 26% 26% 10% 1	25680

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	9% 12% 8% 23% 13% 8% 4 3 3 3 12%	4661
	11th to 20th	Day	9% 14% 7% 19% 14% 9% 6% 3 4 4 12%	4650
	21st to end	Day	8% 14% 7% 14% 9% 12% 7% 3 5% 5% 18%	5124
February	1st to 10th	Day	12% 14% 7% 15% 10% 6% 6% 3 4 4 20%	1802
	11th to 20th	Day	12% 11% 8% 17% 9% 7% 5% 3 5% 4 19%	2048
	21st to end	Day	12% 15% 6% 15% 9% 5% 5% 3 5% 5% 20%	1730
March	1st to 10th	Day	18% 16% 6% 15% 7% 6% 4 2 3 3 20%	4576
	11th to 20th	Day	13% 15% 6% 10% 5% 4 5% 3 4 5% 29%	4795
	21st to end	Day	21% 13% 5% 11% 5% 3 4 2 4 5% 26%	6044
April	1st to 10th	Day	20% 16% 4 9% 5% 4 4 2 3 5% 27%	2520
	11th to 20th	Day	18% 12% 4 8% 6% 5% 4 3 4 5% 31%	2515
	21st to end	Day	22% 11% 4 9% 5% 4 4 3 5% 6% 27%	2730
May	1st to 10th	Day	26% 14% 4 8% 5% 4 5% 4 4 5% 19%	7688
	11th to 20th	Day	15% 9% 4 9% 7% 6% 6% 4 6% 7% 26%	8221
	21st to end	Day	18% 9% 3 7% 5% 5% 6% 5% 7% 7% 28%	9060
June	1st to 10th	Day	23% 9% 3 7% 5% 5% 6% 3 6% 7% 27%	2940
	11th to 20th	Day	16% 8% 3 7% 5% 5% 5% 4 6% 7% 34%	2870
	21st to end	Day	21% 10% 3 7% 6% 6% 6% 4 5% 7% 26%	2800
July	1st to 10th	Day	12% 7% 3 8% 5% 6% 6% 6% 9% 11% 27%	6360
	11th to 20th	Day	12% 7% 3 7% 5% 6% 6% 6% 8% 10% 31%	6360
	21st to end	Day	13% 7% 2 7% 4 5% 5% 4 6% 9% 37%	6926
August	1st to 10th	Day	15% 8% 3 9% 7% 6% 6% 4 7% 7% 28%	4499
	11th to 20th	Day	16% 9% 2 9% 6% 5% 6% 5% 7% 9% 26%	4147
	21st to end	Day	11% 8% 3 7% 5% 5% 6% 5% 6% 8% 35%	4312
September	1st to 10th	Day	14% 7% 3 6% 4 4 5% 4 6% 7% 42%	2400
	11th to 20th	Day	15% 7% 2 6% 4 3 4 3 5% 8% 44%	2242
	21st to end	Day	17% 7% 3 6% 5% 4 6% 4 6% 8% 35%	2200
October	1st to 10th	Day	22% 10% 3 7% 5% 4 4 3 6% 6% 30%	2180
	11th to 20th	Day	15% 13% 3 8% 4 4 5% 4 5% 7% 30%	1895
	21st to end	Day	21% 19% 5% 7% 6% 4 3 3 3 4 24%	1980
November	1st to 10th	Day	14% 17% 4 10% 5% 5% 4 4 5% 6% 27%	1851
	11th to 20th	Day	14% 16% 7% 13% 5% 4 6% 3 4 4 23%	1890
	21st to end	Day	18% 16% 6% 13% 6% 4 3 3 5% 6% 20%	1745
December	1st to 10th	Day	12% 20% 7% 22% 9% 5% 4 3 4 4 11%	2680
	11th to 20th	Day	17% 15% 9% 14% 6% 5% 3 2 4 4 21%	3066
	21st to end	Day	18% 17% 7% 15% 5% 4 5% 3 4 3 20%	4048
January	1st to 10th	Night	8% 14% 6% 25% 13% 8% 5% 3 4 3 12%	8234
	11th to 20th	Night	9% 15% 7% 20% 14% 8% 6% 3 4 4 10%	8235
	21st to end	Night	8% 13% 5% 14% 8% 11% 9% 4 5% 5% 17%	9057
February	1st to 10th	Night	11% 15% 6% 14% 10% 6% 6% 4 4 5% 18%	2814
	11th to 20th	Night	10% 12% 5% 15% 10% 8% 6% 5% 4 5% 19%	2458
	21st to end	Night	12% 13% 5% 13% 9% 7% 4 3 5% 4 26%	2076
March	1st to 10th	Night	18% 16% 5% 11% 7% 8% 5% 3 4 4 20%	5484
	11th to 20th	Night	14% 14% 6% 10% 6% 5% 4 3 5% 5% 27%	5383
	21st to end	Night	17% 16% 5% 13% 7% 4 4 3 4 4 24%	5127
April	1st to 10th	Night	20% 12% 3 10% 6% 5% 4 4 5% 5% 27%	2100
	11th to 20th	Night	19% 12% 4 9% 6% 4 3 3 5% 4 31%	2054
	21st to end	Night	18% 12% 3 8% 5% 3 4 4 6% 6% 31%	1890
May	1st to 10th	Night	25% 13% 4 9% 5% 4 4 3 4 6% 24%	5259
	11th to 20th	Night	16% 10% 3 8% 4 5% 5% 4 5% 7% 32%	4645
	21st to end	Night	17% 9% 3 8% 5% 4 4 3 5% 8% 34%	5114
June	1st to 10th	Night	22% 10% 3 7% 5% 4 4 3 5% 8% 30%	1680
	11th to 20th	Night	17% 6% 3 6% 5% 4 4 4 7% 7% 36%	1640
	21st to end	Night	20% 8% 3 6% 4 5% 5% 4 6% 7% 31%	1600
July	1st to 10th	Night	10% 7% 3 7% 6% 6% 6% 6% 7% 10% 32%	3600
	11th to 20th	Night	15% 9% 3 7% 4 4 5% 4 6% 8% 35%	3600
	21st to end	Night	10% 6% 2 7% 5% 5% 5% 4 7% 8% 42%	3926
August	1st to 10th	Night	17% 9% 3 7% 5% 5% 4 4 6% 7% 33%	2619
	11th to 20th	Night	14% 8% 4 8% 5% 5% 5% 4 5% 8% 34%	2897
	21st to end	Night	12% 7% 3 7% 5% 5% 5% 4 6% 8% 37%	3586
September	1st to 10th	Night	13% 7% 2 6% 4 4 4 3 4 8% 44%	2000
	11th to 20th	Night	13% 7% 2 6% 3 3 4 3 5% 7% 47%	2157
	21st to end	Night	18% 8% 2 5% 4 3 4 3 5% 7% 42%	2200
October	1st to 10th	Night	19% 9% 3 7% 4 4 4 3 5% 7% 34%	2218
	11th to 20th	Night	16% 11% 4 8% 6% 5% 4 3 5% 7% 31%	2505
	21st to end	Night	21% 14% 4 9% 6% 4 4 3 5% 6% 24%	2860
November	1st to 10th	Night	16% 13% 4 11% 5% 4 4 4 4 5% 29%	2667
	11th to 20th	Night	16% 11% 8% 13% 5% 4 4 3 4 6% 26%	2730
	21st to end	Night	16% 14% 6% 14% 7% 5% 3 3 5% 5% 21%	2872
December	1st to 10th	Night	13% 18% 7% 18% 10% 7% 6% 4 4 4 10%	4720
	11th to 20th	Night	21% 16% 6% 13% 7% 5% 5% 3 4 4 17%	5348
	21st to end	Night	19% 16% 5% 14% 6% 5% 5% 3 3 4 20%	7150

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% 6% 8% 9% 2	4662
	11th to 20th	Day	73% 7% 6% 7% 5% 1	4680
	21st to end	Day	65% 7% 8% 10% 10% 1	5124
February	1st to 10th	Day	67% 5% 7% 12% 7% 2	1802
	11th to 20th	Day	67% 6% 9% 12% 5% 1	2057
	21st to end	Day	65% 5% 8% 12% 8% 2	1730
March	1st to 10th	Day	68% 6% 6% 8% 8% 4	4579
	11th to 20th	Day	56% 5% 13% 14% 8% 3	4795
	21st to end	Day	60% 5% 5% 14% 14% 1	6044
April	1st to 10th	Day	61% 4 8% 16% 9% 2	2520
	11th to 20th	Day	54% 7% 8% 18% 12% 1	2515
	21st to end	Day	57% 6% 8% 17% 9% 3	2730
May	1st to 10th	Day	63% 7% 8% 15% 6% 1	7700
	11th to 20th	Day	52% 9% 13% 17% 7% 1	8225
	21st to end	Day	49% 8% 12% 23% 7%	9061
June	1st to 10th	Day	55% 8% 11% 18% 8%	2940
	11th to 20th	Day	47% 8% 11% 22% 12% 1	2870
	21st to end	Day	54% 9% 10% 18% 7% 1	2800
July	1st to 10th	Day	46% 7% 17% 21% 9%	6290
	11th to 20th	Day	44% 8% 15% 24% 9%	6285
	21st to end	Day	42% 7% 11% 30% 10%	6728
August	1st to 10th	Day	51% 8% 14% 20% 7%	4485
	11th to 20th	Day	52% 8% 15% 20% 5%	4186
	21st to end	Day	43% 8% 16% 19% 13% 2	4312
September	1st to 10th	Day	40% 7% 11% 29% 12% 1	2400
	11th to 20th	Day	38% 6% 11% 29% 15% 1	2242
	21st to end	Day	44% 7% 14% 21% 12% 1	2200
October	1st to 10th	Day	48% 8% 12% 22% 11%	3557
	11th to 20th	Day	46% 7% 14% 23% 9%	3059
	21st to end	Day	51% 5% 13% 22% 9% 1	3180
November	1st to 10th	Day	55% 7% 11% 17% 9% 1	1851
	11th to 20th	Day	62% 8% 9% 10% 10% 2	1890
	21st to end	Day	64% 7% 13% 12% 3	1745
December	1st to 10th	Day	77% 5% 9% 7% 3	2680
	11th to 20th	Day	65% 6% 5% 15% 8%	3066
	21st to end	Day	69% 5% 5% 10% 10% 2	4048
January	1st to 10th	Night	75% 7% 8% 7% 3	8234
	11th to 20th	Night	75% 8% 7% 6% 4 1	8280
	21st to end	Night	66% 7% 7% 10% 9% 1	9057
February	1st to 10th	Night	66% 7% 8% 12% 6% 1	2814
	11th to 20th	Night	64% 6% 9% 13% 7% 1	2460
	21st to end	Night	59% 6% 9% 15% 8% 1	2076
March	1st to 10th	Night	66% 6% 7% 11% 8% 1	5494
	11th to 20th	Night	57% 7% 14% 13% 8% 1	5384
	21st to end	Night	63% 6% 4 13% 13% 1	5127
April	1st to 10th	Night	57% 6% 10% 16% 11% 1	2100
	11th to 20th	Night	54% 6% 8% 20% 11%	2054
	21st to end	Night	50% 9% 9% 17% 13% 2	1890
May	1st to 10th	Night	60% 7% 10% 15% 8%	5265
	11th to 20th	Night	50% 8% 12% 22% 7% 1	4646
	21st to end	Night	46% 8% 12% 23% 10% 1	5115
June	1st to 10th	Night	51% 8% 9% 21% 10% 1	1680
	11th to 20th	Night	44% 9% 15% 20% 10% 2	1640
	21st to end	Night	49% 10% 12% 19% 8% 1	1600
July	1st to 10th	Night	44% 8% 19% 21% 7%	3560
	11th to 20th	Night	46% 7% 12% 26% 9%	3552
	21st to end	Night	37% 9% 15% 30% 9%	3809
August	1st to 10th	Night	50% 8% 14% 22% 5%	2609
	11th to 20th	Night	48% 8% 17% 17% 9% 1	2924
	21st to end	Night	43% 7% 15% 23% 12%	3586
September	1st to 10th	Night	38% 9% 15% 27% 11%	2000
	11th to 20th	Night	36% 7% 12% 32% 13% 1	2157
	21st to end	Night	41% 8% 13% 24% 12% 2	2200
October	1st to 10th	Night	50% 7% 13% 21% 9% 1	3621
	11th to 20th	Night	45% 10% 10% 23% 12%	4080
	21st to end	Night	50% 6% 14% 20% 9% 1	4610
November	1st to 10th	Night	55% 7% 12% 17% 8% 1	2667
	11th to 20th	Night	58% 7% 10% 15% 9% 2	2730
	21st to end	Night	64% 6% 12% 13% 5%	2872
December	1st to 10th	Night	75% 8% 8% 8% 1	4720
	11th to 20th	Night	69% 6% 6% 10% 9%	5348
	21st to end	Night	68% 5% 7% 11% 8% 1	7150

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	23% 22% 33% 17% 5%	220
	11th to 20th	Day	29% 25% 33% 9% 4 1	220
	21st to end	Day	23% 25% 31% 15% 5%	242
February	1st to 10th	Day	31% 31% 26% 9% 3 1	294
	11th to 20th	Day	25% 27% 30% 15% 3	536
	21st to end	Day	31% 20% 30% 16% 3	446
March	1st to 10th	Day	31% 21% 29% 17% 2	540
	11th to 20th	Day	36% 20% 28% 13% 4 1	540
	21st to end	Day	34% 21% 25% 12% 6% 2	594
April	1st to 10th	Day	37% 19% 24% 14% 4 1	540
	11th to 20th	Day	35% 23% 22% 12% 6% 1	532
	21st to end	Day	34% 20% 25% 12% 6% 2	540
May	1st to 10th	Day	37% 21% 23% 13% 5% 1	543
	11th to 20th	Day	24% 18% 32% 18% 5% 2	567
	21st to end	Day	21% 19% 33% 18% 7% 1	617
June	1st to 10th	Day	28% 17% 27% 18% 8% 2	570
	11th to 20th	Day	22% 17% 31% 20% 8% 3	550
	21st to end	Day	25% 16% 32% 18% 7% 2	530
July	1st to 10th	Day	19% 12% 32% 26% 9% 2	525
	11th to 20th	Day	15% 14% 32% 26% 11% 2	524
	21st to end	Day	17% 12% 35% 25% 10% 2	576
August	1st to 10th	Day	16% 12% 26% 26% 17% 3	526
	11th to 20th	Day	20% 10% 27% 23% 14% 6%	519
	21st to end	Day	17% 15% 25% 24% 13% 5%	583
September	1st to 10th	Day	16% 14% 26% 22% 15% 6%	530
	11th to 20th	Day	17% 12% 22% 18% 21% 10%	530
	21st to end	Day	18% 13% 26% 22% 15% 6%	530
October	1st to 10th	Day	25% 14% 24% 19% 13% 5%	530
	11th to 20th	Day	26% 18% 25% 18% 10% 3	472
	21st to end	Day	35% 17% 25% 17% 7%	462
November	1st to 10th	Day	31% 19% 20% 24% 7%	425
	11th to 20th	Day	26% 21% 27% 19% 6% 1	430
	21st to end	Day	34% 18% 28% 17% 3	285
December	1st to 10th	Day	30% 21% 28% 13% 8%	220
	11th to 20th	Day	38% 20% 31% 10% 1	220
	21st to end	Day	33% 22% 27% 15% 3	242
January	1st to 10th	Night	22% 22% 33% 20% 4	840
	11th to 20th	Night	27% 19% 29% 21% 4	840
	21st to end	Night	19% 20% 30% 25% 5%	924
February	1st to 10th	Night	20% 19% 35% 21% 5%	840
	11th to 20th	Night	22% 20% 33% 21% 3	823
	21st to end	Night	22% 19% 32% 23% 4	692
March	1st to 10th	Night	26% 19% 29% 23% 3	840
	11th to 20th	Night	29% 18% 25% 20% 6%	840
	21st to end	Night	27% 17% 27% 21% 7% 1	924
April	1st to 10th	Night	29% 16% 27% 23% 5%	840
	11th to 20th	Night	26% 20% 26% 21% 7%	810
	21st to end	Night	26% 15% 31% 23% 5%	630
May	1st to 10th	Night	29% 13% 28% 23% 7%	630
	11th to 20th	Night	20% 12% 29% 30% 8% 1	420
	21st to end	Night	20% 11% 24% 34% 11%	455
June	1st to 10th	Night	25% 10% 21% 33% 10%	420
	11th to 20th	Night	20% 8% 22% 35% 15%	410
	21st to end	Night	22% 6% 21% 38% 12% 1	400
July	1st to 10th	Night	15% 6% 18% 45% 15% 1	390
	11th to 20th	Night	16% 4 20% 40% 18% 2	388
	21st to end	Night	14% 5% 18% 45% 18%	438
August	1st to 10th	Night	16% 4 14% 41% 24%	418
	11th to 20th	Night	16% 6% 22% 33% 21% 2	604
	21st to end	Night	17% 6% 19% 32% 23% 3	880
September	1st to 10th	Night	15% 9% 21% 35% 19% 2	800
	11th to 20th	Night	17% 9% 17% 29% 26% 3	800
	21st to end	Night	21% 10% 22% 26% 19% 2	800
October	1st to 10th	Night	26% 12% 18% 27% 16% 1	800
	11th to 20th	Night	24% 13% 23% 27% 12% 2	800
	21st to end	Night	31% 12% 27% 24% 6%	880
November	1st to 10th	Night	25% 17% 25% 26% 7% 1	820
	11th to 20th	Night	24% 18% 28% 21% 8% 2	840
	21st to end	Night	27% 19% 27% 20% 6% 1	840
December	1st to 10th	Night	29% 20% 27% 20% 5%	840
	11th to 20th	Night	29% 18% 27% 21% 4	840
	21st to end	Night	31% 17% 26% 23% 3	924

Page generated at 2026-06-13 02:31:39. Version ac07

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