Due West
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Due West 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% 6% 2 6% 4 4 4 4 6% 8% 43%	126505
	Night	13% 6% 2 6% 4 4 4 4 6% 7% 44%	123214
Transparency	Day	38% 6% 12% 20% 19% 6%	130216
	Night	37% 5% 10% 18% 21% 7%	127572
Seeing	Day	17% 9% 23% 25% 19% 8%	16408
	Night	16% 8% 22% 30% 20% 3	26429

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	15% 4 2 4 3 3 3 3 4 6% 53%	4056
	11th to 20th	Day	12% 6% 2 4 3 2 3 2 3 5% 59%	4060
	21st to end	Day	19% 7% 2 6% 4 3 3 3 3 5% 45%	4466
February	1st to 10th	Day	14% 7% 2 6% 5% 3 5% 4 6% 7% 42%	1915
	11th to 20th	Day	22% 5% 2 5% 4 3 3 3 5% 5% 43%	1948
	21st to end	Day	19% 6% 2 5% 4 4 3 3 5% 5% 44%	1650
March	1st to 10th	Day	21% 7% 3 6% 4 3 4 3 4 5% 40%	4881
	11th to 20th	Day	9% 5% 2 5% 4 4 3 3 5% 6% 54%	4980
	21st to end	Day	17% 6% 2 7% 3 4 4 3 4 4 45%	5465
April	1st to 10th	Day	11% 5% 2 5% 5% 4 4 4 6% 6% 48%	2200
	11th to 20th	Day	11% 5% 2 5% 4 4 4 3 5% 7% 50%	2332
	21st to end	Day	9% 5% 2 5% 4 3 5% 3 6% 8% 49%	2600
May	1st to 10th	Day	11% 7% 3 7% 4 4 5% 5% 7% 9% 38%	5862
	11th to 20th	Day	18% 7% 3 7% 5% 5% 5% 4 6% 9% 31%	5861
	21st to end	Day	12% 6% 3 6% 4 4 4 4 7% 8% 43%	6433
June	1st to 10th	Day	9% 6% 2 6% 6% 5% 6% 5% 9% 11% 35%	2600
	11th to 20th	Day	5% 5% 2 5% 5% 6% 5% 6% 8% 11% 43%	2600
	21st to end	Day	5% 4 2 6% 5% 7% 6% 5% 9% 12% 39%	2600
July	1st to 10th	Day	7% 5% 2 7% 6% 6% 6% 5% 8% 11% 36%	5900
	11th to 20th	Day	5% 5% 2 6% 5% 6% 6% 6% 9% 14% 38%	5900
	21st to end	Day	7% 6% 2 8% 6% 6% 7% 6% 10% 12% 28%	6424
August	1st to 10th	Day	16% 7% 3 8% 5% 6% 6% 5% 7% 11% 27%	4234
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 5% 8% 11% 37%	4174
	21st to end	Day	7% 4 2 5% 5% 4 6% 5% 7% 9% 47%	4234
September	1st to 10th	Day	8% 5% 3 6% 5% 5% 5% 5% 8% 10% 40%	2200
	11th to 20th	Day	13% 7% 2 6% 4 4 4 4 6% 8% 40%	2200
	21st to end	Day	15% 6% 2 6% 4 4 5% 4 5% 7% 43%	2200
October	1st to 10th	Day	13% 5% 2 5% 2 3 3 2 4 6% 54%	2169
	11th to 20th	Day	11% 5% 2 4 3 2 3 2 5% 6% 58%	1980
	21st to end	Day	11% 4 2 5% 3 3 3 2 3 4 58%	2090
November	1st to 10th	Day	14% 6% 2 5% 3 3 3 2 4 5% 52%	1922
	11th to 20th	Day	14% 5% 2 4 3 3 4 3 4 7% 52%	1800
	21st to end	Day	14% 6% 2 4 3 4 3 3 5% 6% 50%	1800
December	1st to 10th	Day	17% 6% 2 4 3 3 3 3 4 5% 51%	2930
	11th to 20th	Day	17% 6% 2 5% 3 4 4 4 6% 6% 43%	3373
	21st to end	Day	23% 6% 2 4 3 3 3 2 5% 6% 44%	4466
January	1st to 10th	Night	14% 5% 1 4 3 3 3 2 4 4 57%	5889
	11th to 20th	Night	12% 6% 2 5% 3 3 4 3 4 5% 53%	5900
	21st to end	Night	17% 8% 2 5% 3 3 3 3 4 5% 48%	6490
February	1st to 10th	Night	15% 7% 2 5% 4 4 4 3 5% 5% 46%	2480
	11th to 20th	Night	20% 7% 2 6% 4 3 3 3 4 5% 44%	2338
	21st to end	Night	16% 6% 2 5% 3 4 4 3 6% 7% 45%	1980
March	1st to 10th	Night	19% 6% 2 5% 3 3 4 3 5% 7% 41%	4970
	11th to 20th	Night	10% 4 2 3 3 3 4 4 6% 8% 55%	4980
	21st to end	Night	16% 5% 2 5% 4 4 4 3 4 6% 48%	5467
April	1st to 10th	Night	11% 5% 2 5% 4 4 4 4 6% 7% 48%	2200
	11th to 20th	Night	10% 5% 2 5% 4 3 3 2 4 7% 54%	2031
	21st to end	Night	9% 5% 2 6% 5% 4 4 4 4 6% 52%	1800
May	1st to 10th	Night	11% 7% 3 7% 5% 5% 5% 4 6% 10% 37%	4033
	11th to 20th	Night	19% 7% 2 6% 5% 4 5% 4 6% 9% 32%	4032
	21st to end	Night	12% 5% 2 5% 5% 4 5% 4 7% 9% 42%	4421
June	1st to 10th	Night	10% 5% 2 6% 5% 5% 6% 4 10% 11% 35%	1800
	11th to 20th	Night	7% 6% 2 6% 5% 6% 4 5% 8% 13% 40%	1800
	21st to end	Night	8% 5% 2 7% 6% 4 6% 6% 9% 10% 37%	1800
July	1st to 10th	Night	10% 7% 3 9% 6% 6% 7% 5% 9% 11% 27%	4058
	11th to 20th	Night	6% 5% 2 7% 5% 6% 8% 7% 10% 14% 29%	4057
	21st to end	Night	10% 9% 3 10% 6% 6% 7% 6% 8% 11% 24%	4427
August	1st to 10th	Night	15% 7% 3 8% 6% 5% 7% 6% 9% 12% 23%	2915
	11th to 20th	Night	11% 7% 3 8% 6% 6% 6% 4 8% 9% 30%	2875
	21st to end	Night	6% 5% 2 6% 4 6% 5% 4 6% 8% 48%	3623
September	1st to 10th	Night	9% 6% 2 6% 5% 6% 5% 4 7% 9% 41%	2200
	11th to 20th	Night	13% 7% 3 7% 5% 5% 4 4 7% 8% 38%	2200
	21st to end	Night	14% 6% 2 6% 4 4 5% 3 6% 6% 45%	2200
October	1st to 10th	Night	11% 6% 1 6% 3 3 3 3 4 5% 54%	2230
	11th to 20th	Night	11% 5% 2 4 4 3 3 3 4 5% 56%	2377
	21st to end	Night	11% 4 2 4 3 3 4 2 4 5% 58%	2508
November	1st to 10th	Night	13% 6% 2 4 4 3 3 2 4 5% 54%	2371
	11th to 20th	Night	16% 5% 2 4 3 3 3 3 4 5% 52%	2600
	21st to end	Night	15% 4 2 5% 4 3 4 4 5% 6% 49%	2600
December	1st to 10th	Night	15% 5% 2 5% 3 3 4 3 4 4 52%	4250
	11th to 20th	Night	17% 7% 2 5% 3 3 3 3 5% 5% 46%	4822
	21st to end	Night	21% 5% 2 5% 3 4 4 3 4 5% 43%	6490

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	34% 2 5% 22% 29% 7%	4057
	11th to 20th	Day	31% 3 4 18% 36% 9%	4060
	21st to end	Day	44% 2 3 17% 23% 11%	4466
February	1st to 10th	Day	40% 4 9% 21% 20% 6%	1915
	11th to 20th	Day	43% 4 5% 19% 23% 6%	1958
	21st to end	Day	43% 3 8% 15% 22% 8%	1650
March	1st to 10th	Day	48% 3 6% 14% 19% 11%	4881
	11th to 20th	Day	31% 4 6% 14% 25% 20%	4980
	21st to end	Day	43% 3 6% 20% 22% 6%	5465
April	1st to 10th	Day	35% 4 11% 20% 25% 5%	2200
	11th to 20th	Day	34% 4 10% 21% 21% 10%	2332
	21st to end	Day	32% 5% 11% 28% 20% 4	2600
May	1st to 10th	Day	40% 8% 16% 22% 12% 2	5897
	11th to 20th	Day	49% 6% 14% 20% 10% 1	5865
	21st to end	Day	38% 7% 13% 21% 19% 2	6447
June	1st to 10th	Day	38% 9% 17% 25% 9% 2	2600
	11th to 20th	Day	32% 9% 20% 26% 13% 1	2600
	21st to end	Day	33% 10% 19% 21% 14% 3	2600
July	1st to 10th	Day	39% 9% 18% 25% 8% 1	5835
	11th to 20th	Day	32% 13% 31% 20% 3 1	5831
	21st to end	Day	43% 12% 19% 20% 6%	6240
August	1st to 10th	Day	49% 8% 16% 21% 5%	4221
	11th to 20th	Day	38% 8% 18% 19% 16% 2	4213
	21st to end	Day	30% 8% 15% 24% 18% 5%	4234
September	1st to 10th	Day	36% 8% 15% 19% 19% 3	2200
	11th to 20th	Day	39% 8% 11% 19% 21% 2	2200
	21st to end	Day	40% 5% 12% 20% 17% 6%	2200
October	1st to 10th	Day	28% 2 7% 18% 34% 11%	3533
	11th to 20th	Day	21% 1 5% 16% 29% 28%	3220
	21st to end	Day	26% 3 4 23% 30% 14%	3428
November	1st to 10th	Day	35% 3 7% 18% 28% 9%	1922
	11th to 20th	Day	34% 3 6% 22% 26% 10%	1800
	21st to end	Day	35% 4 8% 22% 25% 7%	1800
December	1st to 10th	Day	37% 2 5% 17% 28% 9%	2930
	11th to 20th	Day	41% 5% 8% 19% 19% 8%	3370
	21st to end	Day	42% 2 7% 24% 23% 2	4466
January	1st to 10th	Night	32% 3 5% 19% 34% 8%	5889
	11th to 20th	Night	34% 3 4 17% 34% 8%	5900
	21st to end	Night	40% 2 4 15% 29% 9%	6490
February	1st to 10th	Night	40% 4 7% 21% 21% 7%	2480
	11th to 20th	Night	44% 4 5% 18% 22% 7%	2339
	21st to end	Night	39% 4 7% 18% 21% 11%	1980
March	1st to 10th	Night	42% 4 8% 12% 26% 7%	4970
	11th to 20th	Night	28% 3 8% 12% 30% 19%	4980
	21st to end	Night	39% 3 5% 20% 22% 11%	5467
April	1st to 10th	Night	35% 5% 9% 20% 25% 8%	2200
	11th to 20th	Night	32% 4 6% 23% 24% 10%	2031
	21st to end	Night	32% 5% 11% 28% 21% 5%	1800
May	1st to 10th	Night	40% 8% 16% 22% 12% 2	4060
	11th to 20th	Night	47% 7% 17% 17% 12% 1	4033
	21st to end	Night	35% 7% 13% 24% 19% 2	4429
June	1st to 10th	Night	39% 9% 18% 20% 11% 2	1800
	11th to 20th	Night	34% 8% 19% 24% 13% 2	1800
	21st to end	Night	35% 13% 17% 18% 14% 4	1800
July	1st to 10th	Night	46% 11% 19% 17% 6% 1	4015
	11th to 20th	Night	38% 14% 27% 17% 3 1	4006
	21st to end	Night	51% 11% 18% 15% 5% 1	4297
August	1st to 10th	Night	49% 10% 19% 17% 4	2904
	11th to 20th	Night	46% 9% 19% 17% 9%	2902
	21st to end	Night	32% 8% 15% 24% 19% 3	3623
September	1st to 10th	Night	37% 9% 16% 16% 17% 5%	2200
	11th to 20th	Night	41% 7% 13% 17% 19% 3	2200
	21st to end	Night	38% 5% 13% 19% 19% 6%	2200
October	1st to 10th	Night	28% 3 9% 17% 34% 8%	3646
	11th to 20th	Night	20% 2 5% 17% 27% 28%	3876
	21st to end	Night	26% 3 4 21% 28% 18%	4120
November	1st to 10th	Night	33% 3 7% 19% 26% 11%	2371
	11th to 20th	Night	34% 3 5% 18% 28% 12%	2600
	21st to end	Night	36% 4 6% 20% 26% 8%	2600
December	1st to 10th	Night	36% 3 4 18% 31% 8%	4250
	11th to 20th	Night	40% 4 10% 16% 21% 9%	4824
	21st to end	Night	43% 3 7% 25% 20% 3	6490

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	20% 19% 39% 18% 3	300
	11th to 20th	Day	26% 20% 33% 18% 4	300
	21st to end	Day	22% 24% 38% 12% 3 1	330
February	1st to 10th	Day	18% 18% 43% 17% 3	300
	11th to 20th	Day	37% 16% 34% 13% 1	300
	21st to end	Day	29% 20% 37% 12% 1	249
March	1st to 10th	Day	25% 13% 29% 25% 7%	470
	11th to 20th	Day	18% 17% 41% 18% 6% 1	500
	21st to end	Day	22% 14% 36% 21% 7%	550
April	1st to 10th	Day	20% 17% 31% 23% 9% 1	500
	11th to 20th	Day	14% 13% 34% 26% 13% 2	492
	21st to end	Day	14% 10% 27% 29% 16% 5%	500
May	1st to 10th	Day	14% 11% 33% 23% 14% 6%	503
	11th to 20th	Day	18% 7% 19% 26% 21% 8%	527
	21st to end	Day	18% 5% 18% 24% 22% 13%	573
June	1st to 10th	Day	13% 5% 15% 31% 26% 10%	530
	11th to 20th	Day	7% 2 17% 28% 28% 16%	530
	21st to end	Day	9% 3 14% 32% 26% 16%	530
July	1st to 10th	Day	10% 2 10% 26% 31% 22%	525
	11th to 20th	Day	8% 2 6% 25% 40% 19%	524
	21st to end	Day	5% 2 9% 30% 36% 18%	576
August	1st to 10th	Day	18% 5% 10% 25% 25% 17%	526
	11th to 20th	Day	13% 3 9% 27% 31% 17%	519
	21st to end	Day	13% 3 12% 24% 29% 19%	583
September	1st to 10th	Day	9% 2 12% 32% 32% 13%	530
	11th to 20th	Day	18% 3 11% 23% 31% 15%	530
	21st to end	Day	19% 5% 18% 26% 24% 8%	530
October	1st to 10th	Day	16% 5% 23% 29% 16% 10%	530
	11th to 20th	Day	11% 8% 22% 32% 22% 5%	522
	21st to end	Day	14% 6% 27% 29% 21% 3	539
November	1st to 10th	Day	21% 9% 25% 27% 14% 4	460
	11th to 20th	Day	19% 9% 40% 22% 8% 1	300
	21st to end	Day	14% 13% 36% 25% 10% 2	300
December	1st to 10th	Day	28% 15% 32% 20% 4	300
	11th to 20th	Day	24% 11% 34% 23% 6% 1	300
	21st to end	Day	29% 16% 29% 17% 6% 2	330
January	1st to 10th	Night	22% 18% 27% 27% 7%	800
	11th to 20th	Night	20% 16% 31% 27% 6%	800
	21st to end	Night	21% 16% 33% 27% 3	880
February	1st to 10th	Night	19% 12% 32% 32% 5%	800
	11th to 20th	Night	25% 14% 35% 24% 2	783
	21st to end	Night	21% 15% 32% 28% 3	660
March	1st to 10th	Night	19% 12% 31% 32% 6%	800
	11th to 20th	Night	16% 13% 35% 26% 9% 1	800
	21st to end	Night	17% 12% 36% 26% 9%	880
April	1st to 10th	Night	14% 15% 32% 30% 9%	800
	11th to 20th	Night	14% 13% 28% 31% 14% 1	740
	21st to end	Night	12% 6% 27% 35% 18% 1	600
May	1st to 10th	Night	11% 8% 25% 36% 19% 1	600
	11th to 20th	Night	15% 4 22% 34% 22% 3	591
	21st to end	Night	14% 3 11% 34% 32% 6%	650
June	1st to 10th	Night	13% 3 14% 34% 30% 6%	600
	11th to 20th	Night	9% 2 16% 29% 34% 10%	600
	21st to end	Night	10% 3 11% 33% 36% 8%	600
July	1st to 10th	Night	10% 3 8% 29% 39% 11%	585
	11th to 20th	Night	9% 2 6% 28% 44% 11%	582
	21st to end	Night	9% 2 8% 28% 45% 7%	657
August	1st to 10th	Night	15% 4 6% 28% 38% 9%	597
	11th to 20th	Night	14% 3 7% 29% 39% 8%	594
	21st to end	Night	11% 4 9% 26% 40% 10%	741
September	1st to 10th	Night	10% 3 10% 31% 38% 9%	800
	11th to 20th	Night	17% 3 10% 26% 39% 6%	800
	21st to end	Night	17% 3 16% 31% 30% 4	800
October	1st to 10th	Night	13% 4 18% 31% 28% 6%	800
	11th to 20th	Night	13% 7% 22% 35% 19% 4	793
	21st to end	Night	14% 8% 20% 33% 22% 3	836
November	1st to 10th	Night	16% 5% 23% 32% 21% 3	780
	11th to 20th	Night	18% 9% 27% 30% 14% 1	800
	21st to end	Night	16% 10% 25% 34% 14% 1	800
December	1st to 10th	Night	21% 13% 26% 32% 8% 1	800
	11th to 20th	Night	20% 11% 25% 33% 11% 1	800
	21st to end	Night	28% 12% 23% 30% 7%	880

Page generated at 2025-10-16 01:50:23. Version ac07

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