Terrace
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Terrace clear sky chart from 2005-11-06 to 2024-01-15.

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	22% 16% 5% 9% 7% 4 5% 4 4 5% 18%	74852
	Night	22% 18% 5% 9% 7% 4 5% 4 4 5% 17%	74929
Transparency	Day	72% 8% 9% 8% 3	74524
	Night	72% 8% 8% 8% 3	74775
Seeing	Day	30% 15% 24% 21% 9% 2	15012
	Night	32% 12% 25% 23% 9%	22616

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	21% 23% 8% 11% 9% 4 5% 5% 4 2 11%	1239
	11th to 20th	Day	27% 22% 5% 10% 8% 4 4 3 2 3 11%	1288
	21st to end	Day	21% 22% 7% 8% 7% 3 6% 6% 4 4 13%	1436
February	1st to 10th	Day	22% 18% 6% 10% 5% 3 4 4 3 4 22%	1440
	11th to 20th	Day	12% 20% 7% 11% 6% 4 5% 5% 4 6% 21%	1502
	21st to end	Day	9% 19% 6% 8% 8% 5% 5% 5% 5% 4 26%	1368
March	1st to 10th	Day	13% 17% 5% 10% 7% 5% 6% 5% 4 5% 23%	1800
	11th to 20th	Day	13% 19% 7% 10% 9% 6% 7% 6% 5% 4 14%	1952
	21st to end	Day	14% 18% 6% 11% 8% 4 6% 5% 4 5% 18%	2218
April	1st to 10th	Day	16% 19% 7% 11% 10% 4 5% 4 4 4 16%	2258
	11th to 20th	Day	18% 18% 6% 10% 9% 4 4 4 5% 4 18%	2308
	21st to end	Day	19% 17% 6% 11% 7% 4 5% 4 4 4 18%	2348
May	1st to 10th	Day	22% 15% 5% 10% 8% 4 5% 4 4 5% 18%	2690
	11th to 20th	Day	19% 13% 5% 9% 8% 4 6% 5% 5% 6% 21%	2656
	21st to end	Day	19% 10% 5% 10% 8% 5% 6% 5% 5% 6% 21%	2917
June	1st to 10th	Day	22% 13% 5% 10% 8% 4 5% 5% 4 6% 18%	2749
	11th to 20th	Day	24% 14% 6% 10% 8% 4 5% 5% 4 6% 15%	2880
	21st to end	Day	24% 11% 6% 9% 8% 4 5% 5% 4 5% 20%	2880
July	1st to 10th	Day	21% 11% 3 8% 7% 3 5% 4 5% 7% 26%	2880
	11th to 20th	Day	24% 11% 5% 9% 7% 4 5% 4 5% 6% 21%	2724
	21st to end	Day	24% 13% 4 8% 6% 3 6% 4 4 4 23%	2938
August	1st to 10th	Day	16% 9% 4 7% 6% 3 5% 5% 6% 7% 32%	2612
	11th to 20th	Day	21% 9% 4 7% 8% 4 4 5% 5% 6% 28%	2401
	21st to end	Day	29% 13% 6% 9% 7% 4 5% 4 3 5% 15%	2574
September	1st to 10th	Day	30% 14% 4 8% 6% 3 4 3 4 5% 18%	2306
	11th to 20th	Day	25% 13% 4 7% 7% 4 6% 4 5% 4 22%	1980
	21st to end	Day	35% 18% 5% 10% 5% 3 3 4 3 3 10%	1980
October	1st to 10th	Day	27% 15% 4 8% 6% 4 5% 3 4 5% 18%	1868
	11th to 20th	Day	32% 19% 5% 10% 7% 4 5% 4 4 4 7%	1630
	21st to end	Day	28% 17% 5% 8% 7% 3 3 3 4 5% 17%	1754
November	1st to 10th	Day	28% 22% 5% 10% 8% 4 4 4 3 3 10%	1483
	11th to 20th	Day	26% 19% 7% 10% 8% 4 5% 4 3 3 10%	1330
	21st to end	Day	22% 24% 7% 10% 7% 3 5% 2 3 5% 12%	1330
December	1st to 10th	Day	19% 20% 7% 11% 6% 4 4 4 4 5% 18%	1330
	11th to 20th	Day	25% 21% 7% 9% 6% 4 5% 4 3 3 13%	1685
	21st to end	Day	23% 27% 7% 11% 8% 4 4 3 2 3 9%	2118
January	1st to 10th	Night	22% 22% 8% 12% 8% 4 4 4 4 3 9%	2920
	11th to 20th	Night	27% 22% 7% 9% 7% 3 4 3 2 4 11%	2766
	21st to end	Night	24% 21% 7% 8% 7% 4 5% 4 3 5% 12%	2918
February	1st to 10th	Night	22% 21% 6% 8% 6% 3 4 4 3 4 18%	2520
	11th to 20th	Night	16% 21% 6% 11% 8% 4 4 4 4 5% 16%	2345
	21st to end	Night	9% 17% 6% 10% 7% 5% 6% 5% 4 5% 26%	1887
March	1st to 10th	Night	15% 17% 5% 7% 8% 4 5% 5% 6% 6% 23%	2160
	11th to 20th	Night	11% 23% 7% 10% 8% 4 6% 4 5% 6% 15%	2007
	21st to end	Night	14% 19% 6% 10% 8% 5% 6% 6% 5% 6% 16%	2136
April	1st to 10th	Night	15% 17% 5% 8% 9% 5% 6% 6% 5% 5% 18%	1702
	11th to 20th	Night	16% 15% 6% 10% 7% 3 6% 6% 5% 7% 19%	1601
	21st to end	Night	19% 13% 5% 10% 8% 4 5% 4 5% 5% 23%	1610
May	1st to 10th	Night	19% 14% 6% 8% 9% 5% 5% 5% 5% 6% 20%	1270
	11th to 20th	Night	16% 11% 4 9% 7% 3 7% 7% 8% 8% 20%	1239
	21st to end	Night	16% 9% 3 8% 8% 4 5% 6% 6% 7% 27%	1358
June	1st to 10th	Night	19% 11% 4 9% 7% 4 6% 5% 5% 7% 25%	1206
	11th to 20th	Night	21% 13% 5% 10% 8% 4 7% 5% 4 7% 16%	1080
	21st to end	Night	20% 11% 4 8% 8% 4 5% 7% 6% 6% 22%	1080
July	1st to 10th	Night	21% 10% 3 9% 8% 4 5% 5% 3 4 28%	1080
	11th to 20th	Night	23% 13% 5% 8% 7% 3 5% 4 5% 5% 22%	1232
	21st to end	Night	19% 11% 5% 7% 6% 4 6% 5% 5% 7% 26%	1379
August	1st to 10th	Night	14% 9% 3 5% 7% 4 6% 5% 5% 6% 36%	1324
	11th to 20th	Night	18% 8% 3 7% 5% 3 5% 5% 6% 8% 31%	1498
	21st to end	Night	26% 12% 4 10% 7% 3 4 5% 5% 6% 18%	1782
September	1st to 10th	Night	25% 11% 4 9% 8% 4 4 4 4 5% 21%	1651
	11th to 20th	Night	22% 12% 3 7% 6% 4 4 4 4 6% 27%	1980
	21st to end	Night	37% 15% 4 9% 6% 3 4 3 4 4 11%	1980
October	1st to 10th	Night	26% 15% 3 8% 6% 3 4 5% 4 6% 20%	2092
	11th to 20th	Night	29% 20% 6% 9% 7% 4 4 4 4 4 10%	2327
	21st to end	Night	28% 18% 4 8% 7% 3 4 4 4 5% 15%	2601
November	1st to 10th	Night	31% 23% 6% 9% 7% 3 3 2 2 3 9%	2594
	11th to 20th	Night	24% 24% 7% 12% 8% 3 4 4 3 3 10%	2850
	21st to end	Night	23% 25% 6% 10% 7% 3 4 3 4 4 10%	2850
December	1st to 10th	Night	18% 21% 6% 10% 7% 4 4 4 4 5% 18%	2850
	11th to 20th	Night	25% 25% 6% 9% 6% 3 4 3 3 3 13%	3520
	21st to end	Night	25% 26% 7% 10% 8% 3 4 3 3 3 9%	5534

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	79% 7% 6% 6% 2	1239
	11th to 20th	Day	80% 7% 3 4 3 3	1288
	21st to end	Day	78% 5% 8% 6% 3	1436
February	1st to 10th	Day	70% 7% 7% 7% 9% 2	1440
	11th to 20th	Day	67% 9% 9% 11% 3 1	1510
	21st to end	Day	63% 7% 10% 13% 4 2	1368
March	1st to 10th	Day	68% 7% 9% 10% 6% 1	1800
	11th to 20th	Day	76% 9% 6% 7% 2	1952
	21st to end	Day	71% 11% 9% 9% 1	2218
April	1st to 10th	Day	76% 8% 8% 7% 2	2258
	11th to 20th	Day	74% 6% 7% 10% 3	2308
	21st to end	Day	75% 7% 7% 9% 3	2348
May	1st to 10th	Day	71% 9% 10% 9% 2	2690
	11th to 20th	Day	68% 8% 11% 10% 3 1	2659
	21st to end	Day	68% 10% 12% 9% 1	2918
June	1st to 10th	Day	72% 8% 10% 6% 3	2749
	11th to 20th	Day	75% 7% 9% 8% 1	2880
	21st to end	Day	71% 9% 12% 8% 1	2880
July	1st to 10th	Day	63% 9% 13% 13% 2	2801
	11th to 20th	Day	68% 10% 13% 8% 1	2641
	21st to end	Day	66% 10% 10% 12% 2	2775
August	1st to 10th	Day	55% 11% 15% 15% 4	2594
	11th to 20th	Day	59% 11% 13% 13% 4	2401
	21st to end	Day	75% 9% 9% 4 2	2574
September	1st to 10th	Day	72% 8% 11% 9% 1	2306
	11th to 20th	Day	66% 9% 12% 11% 3	1980
	21st to end	Day	83% 6% 5% 5% 2	1980
October	1st to 10th	Day	68% 10% 11% 8% 3	1868
	11th to 20th	Day	83% 8% 4 3 1	1630
	21st to end	Day	74% 7% 7% 10% 2	1754
November	1st to 10th	Day	83% 5% 5% 4 1	1483
	11th to 20th	Day	82% 6% 4 3 4	1330
	21st to end	Day	79% 5% 5% 7% 4 1	1330
December	1st to 10th	Day	69% 9% 10% 9% 2 1	1330
	11th to 20th	Day	79% 6% 5% 6% 4 1	1688
	21st to end	Day	80% 10% 3 3 3	2118
January	1st to 10th	Night	82% 6% 5% 5% 3	2920
	11th to 20th	Night	80% 5% 4 5% 3 2	2766
	21st to end	Night	76% 8% 6% 6% 4	2918
February	1st to 10th	Night	72% 7% 4 7% 9% 1	2520
	11th to 20th	Night	73% 6% 8% 8% 4 1	2348
	21st to end	Night	63% 8% 10% 12% 5% 2	1887
March	1st to 10th	Night	64% 8% 10% 10% 7% 1	2160
	11th to 20th	Night	74% 10% 7% 8% 1	2008
	21st to end	Night	71% 9% 11% 8% 1	2136
April	1st to 10th	Night	72% 9% 8% 7% 4	1702
	11th to 20th	Night	66% 9% 12% 11% 2	1601
	21st to end	Night	67% 7% 12% 11% 2	1610
May	1st to 10th	Night	69% 11% 10% 8% 2	1270
	11th to 20th	Night	63% 14% 10% 10% 3 1	1239
	21st to end	Night	63% 11% 14% 11% 1	1358
June	1st to 10th	Night	64% 11% 13% 10% 3	1206
	11th to 20th	Night	71% 11% 11% 7% 1	1080
	21st to end	Night	64% 13% 16% 5% 1	1080
July	1st to 10th	Night	61% 11% 14% 14% 1	1050
	11th to 20th	Night	68% 10% 14% 7% 1	1192
	21st to end	Night	59% 11% 17% 9% 4	1302
August	1st to 10th	Night	51% 13% 21% 12% 3	1317
	11th to 20th	Night	53% 16% 17% 12% 2	1498
	21st to end	Night	71% 10% 11% 6% 2	1782
September	1st to 10th	Night	68% 9% 12% 9% 2	1651
	11th to 20th	Night	60% 9% 13% 15% 3	1980
	21st to end	Night	82% 6% 5% 5% 2	1980
October	1st to 10th	Night	66% 9% 13% 10% 3	2092
	11th to 20th	Night	81% 7% 6% 4 2	2327
	21st to end	Night	74% 9% 7% 9% 2	2601
November	1st to 10th	Night	83% 6% 4 4 2	2594
	11th to 20th	Night	82% 6% 5% 4 4	2850
	21st to end	Night	79% 6% 5% 5% 3	2850
December	1st to 10th	Night	70% 7% 10% 10% 2	2850
	11th to 20th	Night	78% 7% 5% 5% 4 1	3516
	21st to end	Night	78% 11% 4 4 3	5534

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	27% 19% 29% 17% 7% 1	180
	11th to 20th	Day	35% 24% 25% 11% 5% 1	168
	21st to end	Day	35% 11% 28% 19% 7%	226
February	1st to 10th	Day	34% 17% 27% 17% 3	340
	11th to 20th	Day	27% 13% 33% 19% 7%	335
	21st to end	Day	26% 23% 27% 17% 6% 1	280
March	1st to 10th	Day	22% 23% 24% 19% 11% 2	340
	11th to 20th	Day	28% 20% 25% 19% 7% 1	408
	21st to end	Day	26% 21% 23% 21% 6% 2	462
April	1st to 10th	Day	30% 19% 28% 15% 7% 1	420
	11th to 20th	Day	27% 15% 24% 21% 11% 3	412
	21st to end	Day	30% 17% 25% 17% 9% 2	428
May	1st to 10th	Day	27% 16% 24% 24% 7% 1	603
	11th to 20th	Day	24% 12% 24% 24% 11% 4	625
	21st to end	Day	23% 13% 28% 25% 10% 2	680
June	1st to 10th	Day	31% 18% 20% 24% 6%	630
	11th to 20th	Day	30% 13% 26% 22% 7% 2	630
	21st to end	Day	26% 12% 26% 25% 9% 2	630
July	1st to 10th	Day	23% 10% 26% 27% 12% 1	621
	11th to 20th	Day	27% 13% 26% 22% 10% 2	618
	21st to end	Day	29% 14% 21% 20% 12% 3	684
August	1st to 10th	Day	20% 12% 20% 28% 15% 4	626
	11th to 20th	Day	26% 10% 26% 24% 12% 1	540
	21st to end	Day	34% 16% 24% 17% 8% 1	495
September	1st to 10th	Day	42% 12% 17% 19% 10% 1	450
	11th to 20th	Day	31% 12% 19% 21% 13% 4	450
	21st to end	Day	50% 14% 18% 11% 6% 1	450
October	1st to 10th	Day	36% 12% 20% 18% 12% 2	450
	11th to 20th	Day	43% 14% 20% 13% 9% 1	365
	21st to end	Day	37% 15% 20% 19% 8% 1	368
November	1st to 10th	Day	46% 16% 22% 11% 4 1	180
	11th to 20th	Day	37% 15% 22% 17% 7% 1	180
	21st to end	Day	44% 12% 18% 16% 9% 1	180
December	1st to 10th	Day	23% 16% 22% 25% 14%	180
	11th to 20th	Day	27% 9% 33% 22% 8% 1	180
	21st to end	Day	32% 15% 22% 19% 13%	198
January	1st to 10th	Night	34% 16% 28% 18% 4	850
	11th to 20th	Night	41% 15% 27% 15% 2	822
	21st to end	Night	36% 14% 27% 19% 5%	880
February	1st to 10th	Night	35% 13% 29% 20% 4	800
	11th to 20th	Night	28% 13% 26% 27% 5%	738
	21st to end	Night	19% 16% 34% 24% 7%	592
March	1st to 10th	Night	26% 13% 30% 21% 10% 1	720
	11th to 20th	Night	26% 17% 28% 23% 6%	720
	21st to end	Night	26% 12% 27% 22% 13% 1	750
April	1st to 10th	Night	26% 14% 26% 23% 10% 1	540
	11th to 20th	Night	25% 14% 22% 28% 11%	533
	21st to end	Night	27% 13% 26% 24% 9%	540
May	1st to 10th	Night	23% 10% 25% 29% 13%	370
	11th to 20th	Night	21% 12% 25% 31% 11% 1	354
	21st to end	Night	19% 8% 29% 33% 12%	389
June	1st to 10th	Night	24% 11% 20% 31% 15%	360
	11th to 20th	Night	24% 9% 25% 31% 11%	360
	21st to end	Night	24% 6% 27% 32% 10%	360
July	1st to 10th	Night	21% 8% 21% 39% 11%	350
	11th to 20th	Night	29% 7% 18% 32% 13%	348
	21st to end	Night	23% 9% 21% 31% 15% 1	394
August	1st to 10th	Night	18% 7% 17% 29% 28% 1	429
	11th to 20th	Night	24% 7% 18% 31% 19% 1	534
	21st to end	Night	35% 8% 19% 28% 10% 1	594
September	1st to 10th	Night	37% 8% 19% 26% 11%	540
	11th to 20th	Night	31% 8% 17% 24% 19% 2	720
	21st to end	Night	46% 13% 18% 15% 8% 1	720
October	1st to 10th	Night	38% 8% 24% 20% 10%	720
	11th to 20th	Night	44% 12% 20% 16% 8%	720
	21st to end	Night	41% 13% 18% 22% 6%	792
November	1st to 10th	Night	45% 15% 23% 13% 4 1	742
	11th to 20th	Night	36% 16% 26% 18% 4	850
	21st to end	Night	39% 14% 26% 16% 4	850
December	1st to 10th	Night	30% 13% 26% 23% 8%	850
	11th to 20th	Night	30% 14% 30% 20% 5%	850
	21st to end	Night	32% 14% 29% 20% 5%	935

Page generated at 2024-01-12 04:31:48. Version ac07

Copyright 2024 Attilla Danko