Tomahawk
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tomahawk clear sky chart from 2005-11-06 to 2025-10-19.

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	13% 10% 4 9% 5% 5% 5% 4 5% 7% 34%	127034
	Night	12% 11% 4 10% 5% 5% 4 4 5% 6% 34%	122962
Transparency	Day	48% 6% 12% 23% 11% 1	130483
	Night	49% 6% 12% 21% 11% 1	127586
Seeing	Day	18% 13% 31% 23% 12% 3	17386
	Night	18% 10% 32% 29% 10% 1	27202

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	8% 12% 6% 19% 10% 6% 5% 3 5% 6% 21%	3617
	11th to 20th	Day	7% 14% 7% 14% 8% 7% 9% 6% 5% 6% 17%	3620
	21st to end	Day	9% 17% 9% 14% 4 4 6% 3 5% 6% 24%	4010
February	1st to 10th	Day	7% 14% 5% 10% 7% 5% 7% 4 5% 7% 28%	1800
	11th to 20th	Day	8% 11% 5% 12% 7% 5% 5% 4 7% 7% 29%	1811
	21st to end	Day	11% 14% 6% 10% 7% 6% 4 3 5% 8% 26%	1650
March	1st to 10th	Day	14% 10% 2 6% 4 4 4 3 5% 6% 42%	4480
	11th to 20th	Day	10% 9% 3 11% 6% 5% 6% 4 7% 8% 31%	4834
	21st to end	Day	22% 12% 5% 10% 5% 4 4 3 4 4 28%	5870
April	1st to 10th	Day	16% 13% 4 9% 5% 4 5% 3 5% 5% 29%	2400
	11th to 20th	Day	19% 13% 4 8% 5% 4 4 3 5% 5% 30%	2374
	21st to end	Day	19% 11% 3 8% 6% 5% 5% 3 5% 6% 29%	2512
May	1st to 10th	Day	16% 7% 2 6% 4 4 4 3 4 6% 46%	6319
	11th to 20th	Day	12% 8% 3 7% 4 5% 5% 4 6% 7% 40%	6319
	21st to end	Day	15% 9% 3 7% 5% 4 4 3 6% 7% 37%	6935
June	1st to 10th	Day	13% 6% 2 8% 4 5% 5% 4 6% 8% 40%	2800
	11th to 20th	Day	16% 7% 2 7% 5% 5% 5% 5% 5% 7% 36%	2800
	21st to end	Day	12% 7% 3 7% 5% 4 5% 5% 7% 9% 36%	2800
July	1st to 10th	Day	11% 4 2 6% 4 4 5% 5% 7% 9% 42%	6360
	11th to 20th	Day	8% 5% 2 5% 4 3 4 4 6% 9% 49%	6360
	21st to end	Day	9% 6% 3 7% 4 5% 4 4 6% 8% 43%	6924
August	1st to 10th	Day	13% 7% 3 8% 5% 4 5% 4 6% 7% 39%	4563
	11th to 20th	Day	14% 5% 2 5% 3 3 4 3 6% 8% 46%	4117
	21st to end	Day	12% 9% 3 7% 5% 5% 5% 4 6% 8% 38%	4312
September	1st to 10th	Day	13% 7% 2 6% 4 3 4 3 5% 8% 43%	2400
	11th to 20th	Day	12% 7% 3 7% 4 4 4 4 6% 8% 41%	2355
	21st to end	Day	15% 9% 2 7% 4 5% 5% 4 6% 7% 37%	2200
October	1st to 10th	Day	15% 9% 3 6% 4 5% 5% 4 5% 7% 38%	2000
	11th to 20th	Day	19% 13% 4 9% 5% 4 5% 4 4 6% 26%	1980
	21st to end	Day	21% 13% 4 9% 4 4 3 3 5% 6% 28%	1885
November	1st to 10th	Day	16% 11% 3 8% 5% 4 4 3 6% 7% 34%	1762
	11th to 20th	Day	14% 17% 4 13% 6% 5% 4 4 5% 5% 23%	1665
	21st to end	Day	14% 15% 4 12% 7% 4 5% 4 6% 6% 24%	1600
December	1st to 10th	Day	8% 13% 8% 25% 11% 8% 4 3 4 4 14%	2610
	11th to 20th	Day	9% 21% 7% 15% 8% 5% 5% 3 3 5% 19%	3008
	21st to end	Day	19% 24% 6% 11% 6% 5% 4 3 3 4 16%	3982
January	1st to 10th	Night	8% 13% 6% 16% 11% 6% 5% 4 4 5% 22%	6348
	11th to 20th	Night	6% 14% 6% 16% 7% 7% 7% 5% 6% 7% 18%	6360
	21st to end	Night	7% 17% 8% 17% 6% 4 4 3 5% 5% 24%	6954
February	1st to 10th	Night	8% 12% 5% 12% 7% 6% 5% 5% 5% 6% 29%	2600
	11th to 20th	Night	8% 12% 3 11% 6% 6% 5% 5% 5% 7% 32%	2475
	21st to end	Night	12% 12% 5% 10% 6% 6% 5% 4 5% 6% 30%	1980
March	1st to 10th	Night	10% 7% 3 8% 4 4 4 4 5% 8% 43%	5373
	11th to 20th	Night	9% 12% 4 11% 6% 6% 5% 4 6% 8% 31%	5169
	21st to end	Night	20% 13% 4 9% 5% 4 4 3 5% 6% 29%	5058
April	1st to 10th	Night	20% 11% 3 8% 4 3 4 4 6% 6% 31%	2000
	11th to 20th	Night	22% 10% 3 5% 4 3 3 4 4 6% 35%	1975
	21st to end	Night	18% 9% 4 7% 4 3 3 4 5% 7% 36%	1883
May	1st to 10th	Night	14% 9% 2 6% 4 4 4 3 4 6% 45%	3596
	11th to 20th	Night	9% 7% 2 7% 4 4 5% 3 5% 7% 47%	3596
	21st to end	Night	14% 8% 3 7% 5% 4 4 3 5% 6% 40%	3942
June	1st to 10th	Night	12% 6% 2 6% 3 5% 5% 4 7% 8% 43%	1600
	11th to 20th	Night	13% 7% 3 7% 5% 5% 5% 5% 6% 7% 37%	1600
	21st to end	Night	10% 7% 3 5% 5% 4 4 4 6% 10% 42%	1600
July	1st to 10th	Night	12% 5% 2 5% 4 3 5% 4 6% 9% 44%	3618
	11th to 20th	Night	9% 5% 2 5% 3 4 3 3 6% 9% 53%	3617
	21st to end	Night	9% 5% 2 6% 4 4 4 3 6% 8% 49%	3947
August	1st to 10th	Night	10% 6% 3 7% 5% 5% 5% 4 7% 8% 41%	2597
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 5% 7% 49%	2935
	21st to end	Night	13% 8% 3 6% 4 4 4 3 5% 6% 44%	3586
September	1st to 10th	Night	11% 7% 2 5% 3 4 4 3 5% 7% 49%	2000
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 5% 7% 48%	2024
	21st to end	Night	13% 8% 3 6% 5% 3 4 4 5% 8% 43%	2200
October	1st to 10th	Night	14% 7% 3 6% 4 4 4 3 5% 6% 43%	2395
	11th to 20th	Night	16% 12% 4 7% 5% 3 4 4 4 7% 34%	2377
	21st to end	Night	18% 13% 3 7% 5% 5% 5% 3 6% 6% 28%	2712
November	1st to 10th	Night	15% 11% 4 8% 4 3 3 3 5% 7% 36%	2536
	11th to 20th	Night	14% 15% 4 11% 6% 5% 4 3 5% 6% 28%	2734
	21st to end	Night	14% 14% 5% 12% 6% 5% 4 3 5% 6% 25%	2800
December	1st to 10th	Night	10% 12% 6% 15% 9% 6% 5% 4 6% 6% 19%	4580
	11th to 20th	Night	10% 20% 8% 14% 6% 5% 4 3 5% 5% 21%	5199
	21st to end	Night	21% 18% 5% 11% 6% 4 4 3 5% 5% 18%	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	63% 4 8% 17% 7%	3618
	11th to 20th	Day	65% 5% 6% 13% 10%	3620
	21st to end	Day	60% 4 11% 16% 8%	4010
February	1st to 10th	Day	54% 6% 11% 17% 12% 1	1800
	11th to 20th	Day	51% 7% 11% 21% 8% 1	1820
	21st to end	Day	56% 6% 11% 18% 8% 1	1650
March	1st to 10th	Day	42% 4 15% 19% 16% 4	4480
	11th to 20th	Day	47% 5% 17% 18% 11% 1	4834
	21st to end	Day	60% 5% 8% 15% 12%	5872
April	1st to 10th	Day	55% 5% 11% 17% 11% 1	2400
	11th to 20th	Day	56% 5% 10% 19% 9% 1	2374
	21st to end	Day	55% 5% 9% 17% 12% 1	2512
May	1st to 10th	Day	39% 5% 10% 28% 16% 1	6357
	11th to 20th	Day	41% 6% 13% 27% 12% 1	6323
	21st to end	Day	46% 5% 10% 22% 16% 1	6949
June	1st to 10th	Day	40% 8% 14% 26% 11% 1	2800
	11th to 20th	Day	45% 7% 14% 20% 13% 1	2800
	21st to end	Day	42% 8% 17% 22% 10% 1	2800
July	1st to 10th	Day	36% 7% 16% 33% 8%	6291
	11th to 20th	Day	31% 6% 17% 34% 12% 1	6285
	21st to end	Day	37% 6% 13% 28% 15%	6728
August	1st to 10th	Day	43% 6% 14% 23% 12% 1	4548
	11th to 20th	Day	36% 6% 13% 33% 12%	4157
	21st to end	Day	43% 6% 14% 25% 10% 1	4311
September	1st to 10th	Day	39% 6% 14% 28% 12% 1	2400
	11th to 20th	Day	40% 7% 18% 28% 7%	2355
	21st to end	Day	45% 7% 12% 27% 10%	2200
October	1st to 10th	Day	33% 4 13% 30% 19%	3260
	11th to 20th	Day	50% 5% 10% 20% 14% 1	3220
	21st to end	Day	50% 5% 17% 22% 6%	3085
November	1st to 10th	Day	49% 6% 11% 22% 10% 1	1762
	11th to 20th	Day	61% 6% 10% 15% 7% 1	1665
	21st to end	Day	59% 7% 14% 16% 3 1	1600
December	1st to 10th	Day	73% 5% 7% 12% 4	2610
	11th to 20th	Day	67% 4 8% 12% 8%	3005
	21st to end	Day	72% 4 6% 11% 7%	3982
January	1st to 10th	Night	60% 6% 10% 16% 8% 1	6348
	11th to 20th	Night	62% 6% 9% 13% 10%	6360
	21st to end	Night	61% 5% 9% 18% 8%	6954
February	1st to 10th	Night	54% 5% 9% 20% 11% 2	2600
	11th to 20th	Night	50% 6% 12% 21% 10% 2	2476
	21st to end	Night	53% 6% 13% 18% 8% 2	1980
March	1st to 10th	Night	39% 4 16% 23% 16% 3	5373
	11th to 20th	Night	49% 5% 16% 20% 9% 1	5169
	21st to end	Night	57% 5% 9% 17% 11% 1	5058
April	1st to 10th	Night	52% 6% 8% 17% 16% 1	2000
	11th to 20th	Night	50% 5% 10% 20% 13% 2	1975
	21st to end	Night	47% 7% 10% 22% 14% 1	1883
May	1st to 10th	Night	39% 6% 13% 24% 15% 3	3620
	11th to 20th	Night	35% 8% 14% 24% 18% 2	3597
	21st to end	Night	44% 7% 12% 20% 16% 1	3950
June	1st to 10th	Night	37% 8% 13% 28% 13% 1	1600
	11th to 20th	Night	43% 7% 13% 21% 14% 2	1600
	21st to end	Night	36% 8% 16% 28% 11% 1	1600
July	1st to 10th	Night	35% 7% 19% 28% 9% 2	3580
	11th to 20th	Night	28% 6% 17% 36% 12% 1	3572
	21st to end	Night	33% 6% 15% 28% 17% 1	3832
August	1st to 10th	Night	38% 7% 16% 21% 16% 1	2588
	11th to 20th	Night	34% 6% 15% 31% 12% 2	2961
	21st to end	Night	40% 6% 13% 26% 13% 2	3586
September	1st to 10th	Night	34% 6% 13% 33% 15% 1	2000
	11th to 20th	Night	34% 7% 15% 33% 10%	2024
	21st to end	Night	40% 6% 13% 28% 13% 1	2200
October	1st to 10th	Night	31% 5% 8% 34% 20% 2	3915
	11th to 20th	Night	39% 5% 11% 30% 15%	3876
	21st to end	Night	54% 5% 9% 22% 10%	4462
November	1st to 10th	Night	48% 5% 12% 22% 12%	2536
	11th to 20th	Night	56% 6% 13% 15% 10% 1	2734
	21st to end	Night	58% 6% 13% 18% 5% 1	2800
December	1st to 10th	Night	61% 7% 14% 15% 2	4580
	11th to 20th	Night	66% 4 8% 13% 8%	5201
	21st to end	Night	68% 4 7% 14% 8%	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	14% 17% 34% 25% 8% 2	300
	11th to 20th	Day	20% 14% 36% 23% 6% 1	300
	21st to end	Day	17% 15% 39% 23% 6%	330
February	1st to 10th	Day	17% 16% 46% 14% 6% 2	300
	11th to 20th	Day	15% 23% 42% 14% 6%	300
	21st to end	Day	17% 22% 45% 15% 1	249
March	1st to 10th	Day	18% 20% 39% 17% 6%	300
	11th to 20th	Day	20% 18% 37% 14% 7% 3	300
	21st to end	Day	26% 14% 32% 20% 6% 2	505
April	1st to 10th	Day	26% 16% 31% 22% 5%	500
	11th to 20th	Day	30% 16% 24% 21% 7% 1	492
	21st to end	Day	24% 15% 30% 20% 9% 2	502
May	1st to 10th	Day	22% 15% 31% 22% 9% 2	703
	11th to 20th	Day	17% 16% 31% 24% 9% 3	725
	21st to end	Day	19% 14% 28% 24% 13% 3	789
June	1st to 10th	Day	13% 13% 31% 27% 12% 4	730
	11th to 20th	Day	18% 10% 34% 25% 10% 3	730
	21st to end	Day	15% 10% 33% 25% 14% 4	730
July	1st to 10th	Day	10% 5% 25% 31% 21% 8%	721
	11th to 20th	Day	12% 8% 28% 26% 17% 8%	718
	21st to end	Day	7% 6% 27% 34% 18% 8%	794
August	1st to 10th	Day	12% 6% 27% 26% 23% 6%	726
	11th to 20th	Day	16% 5% 22% 26% 24% 7%	655
	21st to end	Day	13% 11% 24% 27% 19% 6%	583
September	1st to 10th	Day	15% 12% 25% 24% 18% 5%	530
	11th to 20th	Day	12% 9% 27% 31% 17% 4	530
	21st to end	Day	18% 14% 22% 26% 16% 3	530
October	1st to 10th	Day	16% 16% 27% 28% 8% 4	330
	11th to 20th	Day	21% 24% 33% 17% 6%	324
	21st to end	Day	30% 17% 37% 14% 2	330
November	1st to 10th	Day	23% 13% 37% 21% 5% 2	300
	11th to 20th	Day	24% 22% 38% 11% 4	300
	21st to end	Day	21% 20% 41% 13% 4	300
December	1st to 10th	Day	17% 16% 41% 20% 6%	300
	11th to 20th	Day	22% 20% 40% 14% 2 1	300
	21st to end	Day	28% 21% 32% 16% 3 1	330
January	1st to 10th	Night	15% 12% 40% 27% 6%	1000
	11th to 20th	Night	18% 11% 39% 25% 6% 1	1000
	21st to end	Night	17% 9% 36% 30% 6%	1100
February	1st to 10th	Night	15% 13% 38% 28% 6%	1000
	11th to 20th	Night	13% 15% 41% 25% 6%	921
	21st to end	Night	18% 14% 39% 24% 5%	660
March	1st to 10th	Night	15% 13% 35% 28% 8% 1	800
	11th to 20th	Night	16% 13% 38% 26% 6%	685
	21st to end	Night	21% 10% 31% 27% 10% 1	660
April	1st to 10th	Night	24% 10% 36% 25% 6% 1	600
	11th to 20th	Night	27% 10% 27% 29% 7%	593
	21st to end	Night	23% 9% 32% 28% 7% 1	600
May	1st to 10th	Night	22% 11% 32% 28% 7% 1	600
	11th to 20th	Night	19% 9% 34% 30% 8% 1	591
	21st to end	Night	16% 8% 26% 36% 14% 1	650
June	1st to 10th	Night	13% 8% 28% 40% 12%	600
	11th to 20th	Night	15% 8% 30% 38% 9%	600
	21st to end	Night	13% 7% 24% 41% 15% 1	600
July	1st to 10th	Night	9% 3 23% 40% 23% 2	585
	11th to 20th	Night	9% 6% 26% 36% 22% 1	582
	21st to end	Night	8% 2 22% 44% 21% 3	657
August	1st to 10th	Night	9% 3 21% 41% 24% 1	597
	11th to 20th	Night	14% 3 19% 36% 26% 2	594
	21st to end	Night	13% 7% 21% 35% 22% 2	660
September	1st to 10th	Night	15% 6% 22% 35% 18% 4	600
	11th to 20th	Night	13% 5% 27% 35% 18% 2	624
	21st to end	Night	18% 9% 26% 31% 14% 2	800
October	1st to 10th	Night	18% 9% 31% 27% 14% 1	800
	11th to 20th	Night	21% 11% 32% 27% 8%	793
	21st to end	Night	25% 8% 33% 25% 8%	836
November	1st to 10th	Night	20% 11% 37% 27% 5% 1	780
	11th to 20th	Night	20% 14% 38% 22% 6%	934
	21st to end	Night	20% 15% 35% 24% 5%	1000
December	1st to 10th	Night	18% 14% 37% 25% 6% 1	1000
	11th to 20th	Night	19% 13% 37% 23% 7% 1	1000
	21st to end	Night	27% 12% 33% 24% 4	1100

Page generated at 2025-10-16 00:50:29. Version ac07

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