Cascade
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cascade clear sky chart from 2005-11-06 to 2024-03-23.

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	8% 11% 4 7% 5% 3 4 4 5% 6% 43%	78652
	Night	9% 11% 4 7% 5% 3 4 4 5% 6% 42%	81251
Transparency	Day	42% 6% 11% 18% 15% 7%	78356
	Night	42% 7% 12% 20% 14% 6%	81068
Seeing	Day	14% 17% 31% 25% 11% 2	13502
	Night	14% 7% 20% 36% 20% 3	22961

By Month

Percentage Cloud Cover SW (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	6% 17% 10% 13% 6% 4 6% 4 5% 6% 23%	2539
	11th to 20th	Day	9% 19% 7% 13% 6% 3 4 3 3 4 28%	2540
	21st to end	Day	12% 21% 6% 7% 6% 3 4 3 4 7% 26%	2794
February	1st to 10th	Day	9% 19% 7% 11% 7% 2 4 3 5% 4 28%	1621
	11th to 20th	Day	9% 19% 7% 10% 6% 3 5% 4 5% 5% 27%	1850
	21st to end	Day	8% 19% 10% 15% 7% 2 3 4 4 6% 22%	1570
March	1st to 10th	Day	9% 19% 9% 10% 7% 3 6% 4 5% 5% 22%	1900
	11th to 20th	Day	11% 16% 4 6% 7% 4 5% 4 4 6% 32%	2009
	21st to end	Day	11% 20% 6% 10% 7% 4 5% 4 3 6% 23%	2295
April	1st to 10th	Day	11% 17% 4 8% 7% 4 5% 5% 6% 6% 27%	2160
	11th to 20th	Day	9% 14% 4 8% 7% 3 4 4 4 6% 36%	2189
	21st to end	Day	10% 14% 3 7% 5% 3 5% 5% 4 6% 37%	2340
May	1st to 10th	Day	7% 10% 4 7% 6% 4 5% 4 5% 6% 43%	2340
	11th to 20th	Day	11% 9% 3 7% 6% 4 6% 6% 5% 9% 35%	2400
	21st to end	Day	12% 11% 4 8% 7% 4 5% 6% 5% 8% 30%	2723
June	1st to 10th	Day	13% 7% 3 7% 7% 5% 6% 6% 6% 7% 33%	2520
	11th to 20th	Day	13% 7% 3 5% 6% 3 4 4 4 7% 44%	2520
	21st to end	Day	4 4 2 5% 4 2 4 3 4 5% 63%	2520
July	1st to 10th	Day	2 2 1 3 3 2 3 4 5% 6% 69%	2520
	11th to 20th	Day	2 2 1 2 2 1 3 3 4 5% 75%	2520
	21st to end	Day	3 2 1 3 3 2 4 4 4 5% 70%	2741
August	1st to 10th	Day	4 3 1 4 4 3 3 4 5% 7% 62%	2432
	11th to 20th	Day	2 1 1 3 3 2 3 3 4 5% 73%	2299
	21st to end	Day	4 2 1 3 3 2 3 4 4 6% 69%	2423
September	1st to 10th	Day	4 3 1 3 2 2 3 4 4 5% 70%	2160
	11th to 20th	Day	7% 3 2 4 4 3 4 4 5% 7% 58%	1994
	21st to end	Day	7% 4 2 4 3 3 4 4 4 6% 58%	1980
October	1st to 10th	Day	11% 8% 3 4 5% 3 4 3 4 5% 49%	1980
	11th to 20th	Day	9% 9% 2 6% 4 3 4 4 3 6% 50%	1720
	21st to end	Day	10% 11% 3 7% 5% 4 5% 4 5% 6% 40%	1782
November	1st to 10th	Day	15% 13% 5% 7% 6% 3 4 3 4 8% 34%	1674
	11th to 20th	Day	14% 14% 5% 6% 6% 3 4 3 6% 5% 34%	1710
	21st to end	Day	10% 13% 6% 7% 6% 3 5% 5% 5% 6% 35%	1645
December	1st to 10th	Day	11% 18% 5% 10% 4 2 3 3 5% 7% 32%	1520
	11th to 20th	Day	11% 19% 6% 10% 8% 3 4 3 5% 6% 25%	1928
	21st to end	Day	9% 13% 5% 11% 5% 3 4 4 4 7% 34%	2794
January	1st to 10th	Night	8% 17% 8% 11% 7% 3 4 4 5% 5% 27%	4406
	11th to 20th	Night	10% 18% 8% 13% 7% 2 3 3 4 5% 27%	4420
	21st to end	Night	10% 24% 5% 7% 6% 3 4 3 4 5% 28%	4862
February	1st to 10th	Night	10% 17% 7% 8% 7% 3 5% 4 5% 6% 30%	2554
	11th to 20th	Night	9% 16% 6% 9% 6% 3 4 4 5% 5% 33%	2218
	21st to end	Night	9% 17% 9% 12% 6% 3 4 4 5% 5% 27%	1884
March	1st to 10th	Night	9% 18% 6% 8% 6% 4 5% 4 4 6% 32%	2280
	11th to 20th	Night	9% 13% 5% 6% 5% 3 5% 5% 5% 7% 35%	2170
	21st to end	Night	10% 14% 5% 7% 6% 3 5% 5% 6% 7% 31%	2104
April	1st to 10th	Night	10% 11% 4 8% 5% 3 5% 4 5% 6% 39%	1800
	11th to 20th	Night	11% 10% 4 6% 5% 3 4 4 5% 6% 41%	1717
	21st to end	Night	8% 7% 3 6% 5% 3 4 5% 5% 6% 48%	1620
May	1st to 10th	Night	8% 6% 2 4 3 2 5% 6% 5% 8% 50%	1620
	11th to 20th	Night	6% 6% 4 6% 6% 4 5% 5% 7% 9% 42%	1488
	21st to end	Night	11% 8% 3 5% 5% 3 6% 4 6% 10% 38%	1552
June	1st to 10th	Night	13% 5% 2 4 5% 3 5% 6% 5% 9% 43%	1440
	11th to 20th	Night	11% 4 1 4 4 2 3 4 5% 7% 54%	1440
	21st to end	Night	5% 2 2 3 4 2 4 4 5% 5% 64%	1440
July	1st to 10th	Night	2 1 1 2 2 3 3 5% 5% 7% 68%	1440
	11th to 20th	Night	1 1 2 2 1 3 4 4 6% 75%	1440
	21st to end	Night	2 2 1 2 2 1 3 4 4 5% 73%	1576
August	1st to 10th	Night	4 3 2 3 4 2 3 4 4 7% 64%	1507
	11th to 20th	Night	2 1 1 2 3 2 3 3 4 5% 75%	1602
	21st to end	Night	3 2 1 2 3 2 3 4 4 5% 69%	1930
September	1st to 10th	Night	3 3 1 2 3 2 3 3 3 5% 72%	1800
	11th to 20th	Night	6% 4 2 3 3 2 4 4 5% 6% 62%	1966
	21st to end	Night	8% 3 1 3 3 2 3 3 4 5% 64%	1980
October	1st to 10th	Night	9% 5% 2 3 4 2 4 4 6% 8% 53%	1980
	11th to 20th	Night	9% 6% 3 5% 4 2 4 4 5% 7% 51%	2237
	21st to end	Night	10% 9% 2 5% 5% 3 4 5% 6% 7% 44%	2574
November	1st to 10th	Night	14% 11% 3 6% 6% 3 4 4 5% 7% 37%	2405
	11th to 20th	Night	14% 13% 3 5% 5% 3 5% 4 5% 7% 36%	2470
	21st to end	Night	9% 12% 4 7% 7% 4 4 4 4 6% 40%	2531
December	1st to 10th	Night	11% 14% 5% 8% 5% 3 5% 4 5% 6% 33%	2660
	11th to 20th	Night	11% 18% 5% 9% 7% 5% 5% 4 5% 5% 27%	3276
	21st to end	Night	8% 15% 5% 9% 6% 3 5% 4 5% 6% 34%	4862

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	60% 11% 13% 14% 2 1	2539
	11th to 20th	Day	60% 7% 9% 15% 8% 1	2540
	21st to end	Day	59% 8% 16% 13% 3 1	2794
February	1st to 10th	Day	60% 7% 12% 16% 4 1	1621
	11th to 20th	Day	60% 8% 11% 15% 6%	1860
	21st to end	Day	63% 7% 10% 13% 6% 1	1570
March	1st to 10th	Day	61% 8% 10% 13% 7%	1900
	11th to 20th	Day	54% 7% 12% 20% 8%	2009
	21st to end	Day	63% 7% 11% 14% 4	2295
April	1st to 10th	Day	56% 8% 13% 15% 6% 1	2160
	11th to 20th	Day	49% 7% 11% 20% 11% 2	2192
	21st to end	Day	48% 8% 13% 22% 10%	2340
May	1st to 10th	Day	40% 7% 14% 25% 13% 1	2340
	11th to 20th	Day	44% 7% 14% 23% 10% 1	2403
	21st to end	Day	50% 9% 13% 17% 9% 1	2724
June	1st to 10th	Day	46% 10% 12% 21% 10%	2520
	11th to 20th	Day	38% 6% 10% 22% 18% 6%	2520
	21st to end	Day	23% 4 8% 21% 33% 11%	2520
July	1st to 10th	Day	16% 4 7% 18% 28% 26%	2451
	11th to 20th	Day	12% 3 5% 11% 32% 36%	2445
	21st to end	Day	15% 3 6% 11% 27% 37%	2589
August	1st to 10th	Day	21% 5% 9% 18% 26% 21%	2415
	11th to 20th	Day	13% 5% 6% 21% 35% 21%	2299
	21st to end	Day	17% 3 8% 20% 34% 17%	2423
September	1st to 10th	Day	17% 4 7% 19% 30% 23%	2160
	11th to 20th	Day	26% 5% 9% 21% 27% 12%	1994
	21st to end	Day	27% 5% 9% 28% 25% 7%	1980
October	1st to 10th	Day	38% 6% 10% 24% 19% 4	1980
	11th to 20th	Day	36% 5% 11% 28% 18% 2	1720
	21st to end	Day	44% 9% 11% 20% 14% 1	1782
November	1st to 10th	Day	51% 6% 13% 20% 9% 1	1674
	11th to 20th	Day	53% 6% 15% 18% 8% 1	1710
	21st to end	Day	50% 7% 13% 16% 12% 2	1645
December	1st to 10th	Day	53% 8% 16% 14% 7% 2	1520
	11th to 20th	Day	60% 10% 12% 12% 6%	1928
	21st to end	Day	50% 6% 15% 19% 9% 1	2794
January	1st to 10th	Night	57% 9% 13% 17% 3 1	4406
	11th to 20th	Night	59% 7% 9% 15% 8% 1	4420
	21st to end	Night	58% 8% 12% 15% 5% 1	4862
February	1st to 10th	Night	56% 8% 13% 16% 5% 1	2554
	11th to 20th	Night	53% 8% 13% 16% 9% 1	2219
	21st to end	Night	60% 6% 10% 15% 7% 2	1884
March	1st to 10th	Night	53% 7% 14% 17% 7% 2	2280
	11th to 20th	Night	48% 9% 12% 22% 9% 1	2170
	21st to end	Night	51% 9% 17% 17% 6%	2104
April	1st to 10th	Night	45% 8% 14% 22% 11% 1	1800
	11th to 20th	Night	43% 6% 14% 22% 13% 3	1717
	21st to end	Night	36% 8% 16% 26% 14%	1620
May	1st to 10th	Night	29% 9% 13% 31% 18% 1	1620
	11th to 20th	Night	36% 8% 16% 28% 12%	1488
	21st to end	Night	40% 8% 17% 24% 9% 1	1552
June	1st to 10th	Night	36% 9% 18% 28% 9%	1440
	11th to 20th	Night	29% 5% 13% 29% 19% 5%	1440
	21st to end	Night	21% 4 9% 24% 30% 11%	1440
July	1st to 10th	Night	13% 5% 7% 18% 35% 22%	1400
	11th to 20th	Night	10% 4 6% 18% 26% 36%	1392
	21st to end	Night	13% 4 6% 11% 30% 37%	1488
August	1st to 10th	Night	19% 5% 8% 18% 28% 22%	1499
	11th to 20th	Night	12% 3 7% 19% 34% 24%	1602
	21st to end	Night	16% 4 7% 19% 34% 20%	1930
September	1st to 10th	Night	16% 3 7% 21% 30% 23%	1800
	11th to 20th	Night	23% 5% 8% 22% 27% 16%	1966
	21st to end	Night	23% 4 8% 28% 26% 11%	1980
October	1st to 10th	Night	28% 7% 11% 26% 22% 5%	1980
	11th to 20th	Night	33% 5% 13% 27% 20% 3	2237
	21st to end	Night	38% 7% 14% 21% 17% 2	2574
November	1st to 10th	Night	47% 6% 12% 22% 11% 2	2405
	11th to 20th	Night	48% 8% 13% 22% 9%	2470
	21st to end	Night	47% 6% 11% 20% 13% 3	2531
December	1st to 10th	Night	51% 8% 14% 15% 9% 2	2660
	11th to 20th	Night	59% 8% 11% 16% 5% 1	3276
	21st to end	Night	49% 8% 15% 17% 9% 3	4862

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	17% 15% 30% 24% 11% 3	180
	11th to 20th	Day	21% 16% 25% 21% 11% 7%	180
	21st to end	Day	17% 15% 33% 21% 10% 5%	198
February	1st to 10th	Day	21% 21% 33% 15% 8% 2	239
	11th to 20th	Day	20% 20% 35% 16% 8%	455
	21st to end	Day	22% 23% 30% 18% 7%	382
March	1st to 10th	Day	22% 24% 34% 17% 2	460
	11th to 20th	Day	19% 18% 31% 21% 9% 1	460
	21st to end	Day	20% 24% 32% 17% 6%	471
April	1st to 10th	Day	21% 23% 27% 19% 8% 2	420
	11th to 20th	Day	20% 29% 26% 17% 6% 1	412
	21st to end	Day	18% 25% 28% 19% 9% 2	420
May	1st to 10th	Day	10% 22% 32% 26% 9% 1	423
	11th to 20th	Day	20% 17% 28% 24% 9% 2	448
	21st to end	Day	14% 16% 24% 29% 14% 4	485
June	1st to 10th	Day	14% 19% 26% 24% 14% 3	450
	11th to 20th	Day	17% 22% 33% 18% 10% 1	450
	21st to end	Day	5% 12% 34% 33% 14% 2	450
July	1st to 10th	Day	3 11% 41% 33% 11% 2	446
	11th to 20th	Day	3 14% 39% 32% 11% 2	444
	21st to end	Day	3 8% 38% 34% 15% 1	487
August	1st to 10th	Day	6% 11% 36% 32% 13% 2	447
	11th to 20th	Day	2 9% 30% 38% 18% 3	437
	21st to end	Day	5% 11% 32% 33% 16% 2	495
September	1st to 10th	Day	6% 10% 31% 34% 16% 3	450
	11th to 20th	Day	9% 12% 32% 31% 16% 1	450
	21st to end	Day	8% 14% 32% 30% 14% 1	450
October	1st to 10th	Day	15% 15% 27% 29% 14% 1	450
	11th to 20th	Day	16% 16% 26% 22% 16% 5%	329
	21st to end	Day	19% 16% 27% 26% 9% 3	297
November	1st to 10th	Day	24% 12% 25% 25% 13% 1	270
	11th to 20th	Day	14% 19% 25% 26% 12% 3	270
	21st to end	Day	15% 12% 28% 29% 13% 3	239
December	1st to 10th	Day	23% 13% 22% 22% 14% 6%	180
	11th to 20th	Day	26% 17% 28% 19% 9% 2	180
	21st to end	Day	20% 17% 28% 23% 9% 3	198
January	1st to 10th	Night	22% 9% 22% 29% 17% 2	760
	11th to 20th	Night	23% 8% 21% 27% 18% 3	760
	21st to end	Night	17% 9% 24% 28% 20% 3	836
February	1st to 10th	Night	20% 9% 28% 30% 10% 2	760
	11th to 20th	Night	19% 10% 26% 30% 14% 2	744
	21st to end	Night	18% 11% 25% 33% 11% 2	628
March	1st to 10th	Night	20% 12% 25% 29% 12% 2	760
	11th to 20th	Night	17% 9% 23% 33% 17% 1	760
	21st to end	Night	17% 11% 26% 34% 11% 1	800
April	1st to 10th	Night	16% 11% 22% 32% 18% 3	720
	11th to 20th	Night	14% 13% 28% 28% 14% 2	649
	21st to end	Night	11% 9% 24% 39% 15% 2	540
May	1st to 10th	Night	9% 7% 24% 38% 21% 1	540
	11th to 20th	Night	9% 6% 23% 35% 24% 4	426
	21st to end	Night	13% 3 20% 36% 26% 2	389
June	1st to 10th	Night	14% 5% 14% 39% 26% 2	360
	11th to 20th	Night	14% 1 19% 41% 22% 2	360
	21st to end	Night	4 2 13% 48% 32% 1	360
July	1st to 10th	Night	1 2 10% 53% 32% 2	350
	11th to 20th	Night	2 1 15% 58% 23% 1	348
	21st to end	Night	2 10% 54% 31% 2	394
August	1st to 10th	Night	6% 2 9% 48% 33% 3	433
	11th to 20th	Night	1 1 10% 47% 36% 4	534
	21st to end	Night	4 2 12% 44% 33% 5%	742
September	1st to 10th	Night	4 2 13% 47% 32% 3	720
	11th to 20th	Night	8% 3 12% 38% 33% 6%	720
	21st to end	Night	10% 3 15% 37% 27% 7%	720
October	1st to 10th	Night	11% 4 17% 41% 24% 2	720
	11th to 20th	Night	12% 6% 16% 38% 23% 5%	720
	21st to end	Night	15% 6% 22% 36% 17% 4	792
November	1st to 10th	Night	20% 7% 20% 35% 16% 2	740
	11th to 20th	Night	22% 8% 23% 31% 14% 2	760
	21st to end	Night	16% 8% 26% 29% 18% 4	760
December	1st to 10th	Night	18% 8% 24% 32% 16% 2	760
	11th to 20th	Night	22% 11% 22% 29% 14% 2	760
	21st to end	Night	23% 12% 25% 27% 12% 1	836

Page generated at 2024-03-19 21:31:04. Version ac07

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