Andrew
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Andrew clear sky chart from 2005-11-06 to 2025-08-05.

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	10% 9% 3 9% 6% 5% 5% 4 6% 8% 35%	130180
	Night	8% 10% 4 10% 6% 6% 6% 4 6% 8% 33%	126588
Transparency	Day	46% 7% 17% 23% 7%	129877
	Night	47% 7% 16% 21% 8% 1	126471
Seeing	Day	12% 7% 26% 31% 18% 6%	16513
	Night	12% 6% 28% 36% 16% 2	25443

By Month

Percentage Cloud Cover NW (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	4 12% 5% 15% 11% 6% 7% 6% 7% 6% 21%	3179
	11th to 20th	Day	4 10% 3 11% 8% 6% 8% 7% 9% 10% 25%	3159
	21st to end	Day	8% 13% 4 11% 6% 5% 5% 4 6% 8% 31%	3613
February	1st to 10th	Day	7% 13% 5% 13% 9% 7% 4 4 8% 7% 21%	1600
	11th to 20th	Day	5% 12% 6% 14% 8% 6% 5% 5% 7% 8% 26%	1732
	21st to end	Day	4 13% 4 10% 7% 8% 6% 6% 6% 9% 26%	1485
March	1st to 10th	Day	6% 8% 4 11% 8% 6% 7% 5% 7% 10% 27%	4556
	11th to 20th	Day	5% 11% 4 15% 7% 5% 4 4 6% 9% 30%	4999
	21st to end	Day	10% 14% 6% 14% 6% 5% 4 3 5% 7% 25%	5486
April	1st to 10th	Day	8% 14% 4 8% 6% 6% 5% 4 5% 7% 33%	2528
	11th to 20th	Day	11% 11% 3 9% 5% 5% 5% 4 7% 8% 32%	2570
	21st to end	Day	12% 10% 3 8% 5% 5% 5% 4 7% 8% 32%	2615
May	1st to 10th	Day	12% 7% 3 8% 6% 5% 5% 4 6% 6% 38%	6638
	11th to 20th	Day	18% 11% 3 7% 5% 5% 6% 4 6% 7% 28%	6734
	21st to end	Day	10% 7% 3 8% 5% 5% 5% 4 6% 8% 39%	7405
June	1st to 10th	Day	13% 8% 3 8% 6% 5% 6% 5% 7% 7% 31%	3000
	11th to 20th	Day	19% 10% 4 8% 7% 5% 6% 4 7% 8% 23%	3000
	21st to end	Day	12% 6% 2 5% 5% 6% 6% 5% 9% 10% 34%	3000
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 5% 4 7% 9% 46%	6780
	11th to 20th	Day	10% 4 2 6% 5% 4 5% 5% 6% 9% 43%	6780
	21st to end	Day	11% 5% 2 6% 4 4 5% 4 6% 8% 44%	7376
August	1st to 10th	Day	12% 7% 3 6% 4 4 5% 4 5% 8% 44%	4694
	11th to 20th	Day	8% 5% 3 6% 4 5% 5% 4 7% 10% 43%	4097
	21st to end	Day	10% 4 2 5% 5% 4 5% 4 7% 9% 44%	4521
September	1st to 10th	Day	12% 6% 2 7% 4 5% 5% 4 6% 7% 43%	2427
	11th to 20th	Day	13% 6% 2 7% 5% 5% 6% 3 5% 7% 40%	2101
	21st to end	Day	9% 6% 3 7% 5% 5% 5% 4 6% 8% 42%	2090
October	1st to 10th	Day	10% 8% 3 8% 5% 5% 5% 5% 6% 8% 37%	3234
	11th to 20th	Day	10% 9% 3 7% 5% 5% 4 4 5% 9% 38%	2949
	21st to end	Day	7% 12% 4 9% 5% 6% 5% 5% 5% 7% 36%	2963
November	1st to 10th	Day	9% 16% 4 10% 7% 5% 5% 4 5% 7% 28%	1568
	11th to 20th	Day	7% 12% 4 12% 6% 6% 6% 5% 8% 8% 26%	1471
	21st to end	Day	4 12% 4 15% 7% 5% 7% 4 5% 7% 29%	1400
December	1st to 10th	Day	8% 12% 4 11% 10% 8% 7% 5% 7% 8% 20%	2290
	11th to 20th	Day	11% 14% 4 11% 6% 6% 6% 4 7% 7% 22%	2642
	21st to end	Day	3 8% 3 11% 8% 6% 6% 4 7% 8% 36%	3498
January	1st to 10th	Night	4 9% 5% 16% 10% 8% 8% 6% 8% 7% 20%	6766
	11th to 20th	Night	3 10% 4 9% 8% 6% 6% 5% 5% 8% 36%	6739
	21st to end	Night	7% 12% 4 10% 6% 5% 5% 4 6% 7% 32%	7386
February	1st to 10th	Night	6% 12% 4 11% 8% 8% 7% 4 6% 8% 25%	2800
	11th to 20th	Night	6% 10% 4 11% 8% 7% 6% 6% 7% 6% 30%	2554
	21st to end	Night	3 9% 5% 10% 7% 8% 7% 5% 7% 9% 31%	2144
March	1st to 10th	Night	4 9% 4 11% 7% 6% 6% 6% 8% 10% 30%	5337
	11th to 20th	Night	4 9% 4 12% 8% 7% 6% 5% 7% 9% 29%	5000
	21st to end	Night	10% 15% 6% 13% 7% 5% 4 3 6% 7% 23%	5489
April	1st to 10th	Night	8% 9% 4 8% 6% 6% 6% 5% 7% 8% 34%	1868
	11th to 20th	Night	10% 9% 5% 9% 6% 6% 7% 4 7% 6% 31%	1781
	21st to end	Night	12% 10% 4 7% 5% 6% 5% 4 6% 7% 36%	1784
May	1st to 10th	Night	15% 7% 2 6% 4 4 5% 3 5% 6% 42%	3325
	11th to 20th	Night	15% 10% 4 9% 6% 6% 6% 4 6% 7% 28%	3159
	21st to end	Night	12% 9% 3 8% 4 5% 6% 4 7% 9% 34%	3470
June	1st to 10th	Night	15% 7% 4 9% 5% 5% 7% 5% 5% 7% 32%	1400
	11th to 20th	Night	18% 8% 3 8% 7% 5% 6% 5% 7% 8% 25%	1400
	21st to end	Night	8% 7% 3 7% 5% 6% 5% 4 7% 9% 40%	1400
July	1st to 10th	Night	7% 5% 2 6% 5% 5% 6% 5% 8% 9% 43%	3180
	11th to 20th	Night	9% 6% 2 6% 5% 4 5% 4 7% 8% 42%	3180
	21st to end	Night	12% 5% 2 5% 4 4 5% 4 6% 8% 43%	3467
August	1st to 10th	Night	12% 6% 2 7% 5% 5% 4 4 6% 7% 42%	2323
	11th to 20th	Night	10% 6% 2 7% 5% 6% 5% 5% 7% 9% 39%	2738
	21st to end	Night	10% 5% 2 6% 4 4 4 4 6% 8% 46%	3146
September	1st to 10th	Night	12% 5% 3 7% 4 5% 5% 3 5% 7% 45%	1753
	11th to 20th	Night	12% 6% 2 5% 4 4 5% 4 4 8% 46%	2076
	21st to end	Night	9% 6% 2 6% 5% 4 5% 4 6% 8% 45%	2090
October	1st to 10th	Night	7% 8% 2 8% 5% 5% 5% 4 7% 9% 40%	3738
	11th to 20th	Night	10% 8% 2 6% 5% 4 5% 4 7% 8% 40%	3978
	21st to end	Night	8% 10% 4 9% 5% 4 5% 4 7% 9% 34%	4564
November	1st to 10th	Night	12% 15% 5% 10% 6% 5% 4 4 6% 6% 27%	2730
	11th to 20th	Night	8% 13% 5% 13% 8% 6% 5% 4 6% 7% 24%	2925
	21st to end	Night	5% 13% 5% 13% 8% 6% 5% 4 7% 8% 27%	3000
December	1st to 10th	Night	10% 15% 5% 13% 8% 6% 5% 4 6% 7% 22%	4890
	11th to 20th	Night	7% 15% 6% 15% 8% 7% 6% 5% 6% 6% 19%	5550
	21st to end	Night	3 7% 3 11% 6% 6% 6% 5% 7% 7% 38%	7458

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	58% 8% 10% 16% 8%	3179
	11th to 20th	Day	51% 7% 14% 13% 15% 1	3180
	21st to end	Day	50% 7% 19% 18% 5%	3613
February	1st to 10th	Day	59% 6% 14% 12% 9%	1600
	11th to 20th	Day	53% 8% 15% 16% 8%	1741
	21st to end	Day	51% 8% 15% 18% 7%	1485
March	1st to 10th	Day	50% 7% 19% 19% 6%	4556
	11th to 20th	Day	51% 6% 16% 21% 7%	5000
	21st to end	Day	56% 7% 11% 21% 5%	5486
April	1st to 10th	Day	49% 8% 15% 19% 8% 1	2528
	11th to 20th	Day	47% 8% 15% 21% 9% 1	2570
	21st to end	Day	46% 8% 14% 23% 7% 1	2615
May	1st to 10th	Day	45% 6% 11% 22% 15% 1	6638
	11th to 20th	Day	52% 6% 12% 22% 7% 1	6738
	21st to end	Day	40% 8% 19% 20% 11% 1	7406
June	1st to 10th	Day	47% 9% 15% 19% 9% 1	3000
	11th to 20th	Day	57% 9% 14% 16% 4	3000
	21st to end	Day	40% 12% 21% 20% 6% 1	3000
July	1st to 10th	Day	35% 4 24% 29% 7%	6706
	11th to 20th	Day	38% 5% 15% 31% 11%	6699
	21st to end	Day	38% 5% 23% 28% 6%	7169
August	1st to 10th	Day	39% 7% 19% 32% 3	4676
	11th to 20th	Day	36% 6% 25% 25% 8%	4137
	21st to end	Day	36% 6% 28% 27% 3	4520
September	1st to 10th	Day	39% 9% 18% 26% 7% 2	2427
	11th to 20th	Day	41% 9% 18% 27% 6%	2101
	21st to end	Day	37% 10% 17% 25% 9% 2	2090
October	1st to 10th	Day	45% 4 22% 24% 4	3234
	11th to 20th	Day	43% 5% 16% 28% 8%	2949
	21st to end	Day	47% 6% 17% 26% 3	2963
November	1st to 10th	Day	54% 9% 17% 15% 5% 1	1568
	11th to 20th	Day	53% 9% 19% 16% 4	1471
	21st to end	Day	52% 10% 17% 17% 4 1	1400
December	1st to 10th	Day	60% 6% 18% 11% 4	2290
	11th to 20th	Day	58% 6% 17% 14% 4	2644
	21st to end	Day	45% 5% 13% 29% 8% 1	3498
January	1st to 10th	Night	58% 8% 12% 13% 9% 1	6766
	11th to 20th	Night	47% 5% 14% 18% 16% 1	6780
	21st to end	Night	49% 6% 22% 18% 5%	7386
February	1st to 10th	Night	54% 8% 13% 15% 9% 1	2800
	11th to 20th	Night	50% 9% 13% 17% 10%	2555
	21st to end	Night	47% 9% 13% 21% 9% 1	2144
March	1st to 10th	Night	45% 6% 20% 20% 8%	5337
	11th to 20th	Night	48% 7% 15% 22% 8%	5000
	21st to end	Night	56% 9% 11% 15% 9%	5489
April	1st to 10th	Night	43% 8% 15% 20% 13% 1	1868
	11th to 20th	Night	48% 7% 13% 17% 11% 3	1781
	21st to end	Night	45% 8% 13% 21% 11% 2	1784
May	1st to 10th	Night	41% 5% 10% 24% 16% 3	3325
	11th to 20th	Night	52% 7% 14% 20% 7% 1	3159
	21st to end	Night	43% 9% 17% 22% 8%	3470
June	1st to 10th	Night	48% 8% 12% 21% 9% 2	1400
	11th to 20th	Night	53% 10% 15% 16% 5%	1400
	21st to end	Night	39% 10% 21% 23% 6% 1	1400
July	1st to 10th	Night	36% 6% 23% 21% 13% 1	3145
	11th to 20th	Night	39% 4 15% 32% 9% 1	3138
	21st to end	Night	38% 5% 23% 28% 6%	3369
August	1st to 10th	Night	40% 7% 19% 32% 2	2316
	11th to 20th	Night	40% 7% 21% 24% 8%	2764
	21st to end	Night	35% 5% 25% 29% 6%	3146
September	1st to 10th	Night	37% 8% 19% 25% 10% 2	1753
	11th to 20th	Night	36% 8% 17% 28% 10% 1	2076
	21st to end	Night	37% 8% 15% 25% 13% 2	2090
October	1st to 10th	Night	40% 4 22% 27% 7%	3738
	11th to 20th	Night	40% 4 15% 34% 7%	3978
	21st to end	Night	45% 5% 22% 22% 4	4564
November	1st to 10th	Night	56% 6% 12% 20% 6%	2730
	11th to 20th	Night	56% 9% 14% 16% 5%	2925
	21st to end	Night	53% 9% 17% 16% 4	3000
December	1st to 10th	Night	62% 6% 15% 12% 5% 1	4890
	11th to 20th	Night	64% 7% 16% 10% 4	5547
	21st to end	Night	43% 6% 14% 22% 13% 2	7458

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	8% 5% 30% 35% 19% 4	200
	11th to 20th	Day	10% 11% 26% 36% 14% 5%	200
	21st to end	Day	10% 6% 42% 26% 11% 4	220
February	1st to 10th	Day	9% 11% 24% 38% 15% 4	200
	11th to 20th	Day	8% 5% 31% 36% 17% 4	289
	21st to end	Day	8% 4 34% 30% 16% 7%	249
March	1st to 10th	Day	10% 7% 30% 35% 16% 2	500
	11th to 20th	Day	10% 5% 31% 32% 15% 6%	500
	21st to end	Day	16% 4 29% 32% 17% 3	550
April	1st to 10th	Day	8% 10% 33% 35% 12% 3	500
	11th to 20th	Day	13% 11% 29% 30% 14% 4	492
	21st to end	Day	17% 8% 28% 30% 14% 4	515
May	1st to 10th	Day	12% 11% 25% 31% 15% 5%	703
	11th to 20th	Day	14% 8% 24% 30% 17% 7%	725
	21st to end	Day	16% 6% 22% 30% 19% 7%	789
June	1st to 10th	Day	16% 10% 27% 32% 12% 3	730
	11th to 20th	Day	20% 9% 23% 24% 17% 7%	730
	21st to end	Day	13% 5% 18% 31% 23% 10%	730
July	1st to 10th	Day	11% 4 19% 33% 25% 8%	721
	11th to 20th	Day	11% 6% 21% 32% 24% 8%	718
	21st to end	Day	10% 4 21% 31% 25% 10%	794
August	1st to 10th	Day	13% 4 18% 30% 24% 11%	649
	11th to 20th	Day	7% 6% 26% 31% 22% 7%	477
	21st to end	Day	10% 8% 23% 34% 17% 9%	539
September	1st to 10th	Day	13% 8% 29% 24% 19% 6%	490
	11th to 20th	Day	16% 9% 25% 31% 17% 2	490
	21st to end	Day	12% 12% 33% 25% 16% 3	490
October	1st to 10th	Day	14% 9% 27% 27% 19% 3	338
	11th to 20th	Day	17% 5% 33% 32% 11% 1	300
	21st to end	Day	17% 13% 35% 21% 11% 4	330
November	1st to 10th	Day	14% 7% 30% 33% 14% 2	300
	11th to 20th	Day	10% 8% 34% 36% 11% 1	235
	21st to end	Day	4 7% 38% 30% 14% 8%	200
December	1st to 10th	Day	9% 7% 38% 30% 14% 4	200
	11th to 20th	Day	12% 7% 32% 32% 16% 2	200
	21st to end	Day	9% 4 26% 37% 17% 8%	220
January	1st to 10th	Night	11% 7% 32% 36% 13% 2	1000
	11th to 20th	Night	9% 8% 31% 38% 12% 2	1000
	21st to end	Night	12% 10% 33% 32% 11% 2	1067
February	1st to 10th	Night	10% 7% 27% 37% 17% 3	800
	11th to 20th	Night	9% 5% 31% 37% 16% 2	784
	21st to end	Night	7% 4 28% 40% 17% 4	660
March	1st to 10th	Night	10% 5% 26% 44% 14% 2	800
	11th to 20th	Night	11% 6% 28% 36% 15% 3	800
	21st to end	Night	13% 6% 25% 36% 17% 2	880
April	1st to 10th	Night	10% 6% 26% 39% 17% 3	656
	11th to 20th	Night	13% 5% 31% 34% 15% 2	593
	21st to end	Night	16% 6% 25% 38% 15% 1	600
May	1st to 10th	Night	15% 7% 24% 33% 19% 3	467
	11th to 20th	Night	14% 7% 21% 39% 17% 3	394
	21st to end	Night	18% 5% 20% 35% 20% 2	433
June	1st to 10th	Night	16% 9% 22% 39% 14% 1	400
	11th to 20th	Night	22% 6% 20% 30% 21% 2	400
	21st to end	Night	12% 3 19% 36% 27% 4	400
July	1st to 10th	Night	11% 2 19% 41% 26%	390
	11th to 20th	Night	10% 4 19% 44% 22% 2	388
	21st to end	Night	9% 4 16% 41% 26% 5%	438
August	1st to 10th	Night	10% 4 14% 38% 31% 4	386
	11th to 20th	Night	11% 4 19% 39% 25% 2	515
	21st to end	Night	11% 3 24% 38% 23% 1	627
September	1st to 10th	Night	13% 5% 22% 37% 22% 2	570
	11th to 20th	Night	13% 5% 21% 41% 19% 1	746
	21st to end	Night	12% 5% 31% 36% 15% 1	760
October	1st to 10th	Night	15% 4 29% 36% 16% 1	760
	11th to 20th	Night	15% 5% 29% 37% 12% 2	760
	21st to end	Night	14% 8% 32% 33% 13% 1	894
November	1st to 10th	Night	18% 6% 30% 33% 10% 2	975
	11th to 20th	Night	13% 7% 33% 33% 12% 3	1000
	21st to end	Night	8% 7% 38% 34% 11% 2	1000
December	1st to 10th	Night	13% 6% 34% 30% 13% 3	1000
	11th to 20th	Night	11% 7% 34% 35% 12% 2	1000
	21st to end	Night	11% 3 29% 37% 17% 3	1100

Page generated at 2025-08-01 23:32:22. Version ac07

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