New Astronomy Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the New Astronomy Observatory clear sky chart from 2005-11-06 to 2024-11-10.

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	23% 9% 3 7% 4 4 4 3 5% 6% 32%	105855
	Night	27% 10% 3 7% 4 4 4 3 4 5% 29%	101872
Transparency	Day	55% 6% 13% 17% 8% 1	105613
	Night	58% 6% 12% 15% 7% 1	101760
Seeing	Day	29% 9% 20% 25% 14% 3	15040
	Night	34% 6% 14% 26% 18% 2	23406

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	44% 16% 4 7% 4 3 3 2 2 3 11%	2348
	11th to 20th	Day	29% 12% 4 8% 4 4 3 3 5% 6% 23%	2516
	21st to end	Day	45% 12% 3 7% 3 3 4 2 3 4 12%	2835
February	1st to 10th	Day	34% 12% 4 8% 5% 4 4 3 5% 5% 16%	1710
	11th to 20th	Day	31% 14% 3 9% 5% 3 4 3 4 4 20%	1665
	21st to end	Day	30% 16% 3 7% 5% 4 3 3 5% 4 20%	1489
March	1st to 10th	Day	23% 14% 4 9% 6% 5% 5% 3 5% 5% 20%	3187
	11th to 20th	Day	22% 11% 3 6% 4 4 4 3 4 4 35%	3490
	21st to end	Day	28% 15% 4 10% 5% 5% 4 4 5% 6% 15%	3973
April	1st to 10th	Day	29% 13% 4 9% 6% 5% 4 4 5% 5% 16%	2280
	11th to 20th	Day	25% 10% 4 9% 6% 4 5% 3 4 6% 23%	2261
	21st to end	Day	25% 11% 3 9% 6% 5% 5% 5% 6% 5% 20%	2532
May	1st to 10th	Day	20% 9% 3 7% 4 4 4 3 4 5% 36%	4420
	11th to 20th	Day	21% 7% 3 6% 4 4 4 3 6% 9% 34%	4377
	21st to end	Day	32% 11% 3 8% 5% 4 4 3 5% 5% 19%	4813
June	1st to 10th	Day	26% 9% 3 7% 6% 4 5% 4 5% 6% 25%	2679
	11th to 20th	Day	28% 10% 3 8% 5% 5% 4 5% 6% 6% 20%	2850
	21st to end	Day	15% 8% 2 8% 5% 4 4 5% 6% 7% 37%	2850
July	1st to 10th	Day	9% 4 2 4 3 3 3 3 4 5% 61%	4740
	11th to 20th	Day	7% 3 1 3 2 3 3 2 4 5% 66%	4489
	21st to end	Day	13% 5% 1 4 2 3 3 2 5% 5% 56%	4790
August	1st to 10th	Day	7% 3 1 3 2 3 4 4 5% 8% 60%	4358
	11th to 20th	Day	11% 7% 3 7% 5% 5% 5% 5% 7% 9% 38%	4026
	21st to end	Day	18% 7% 3 7% 4 3 3 2 5% 7% 41%	4314
September	1st to 10th	Day	11% 7% 3 6% 4 3 4 2 4 6% 48%	2280
	11th to 20th	Day	18% 7% 4 7% 5% 4 4 3 5% 6% 37%	2280
	21st to end	Day	20% 8% 2 6% 3 4 4 3 5% 5% 38%	2015
October	1st to 10th	Day	18% 7% 2 6% 3 4 5% 3 6% 10% 36%	3180
	11th to 20th	Day	34% 7% 3 7% 4 4 4 3 4 6% 24%	3161
	21st to end	Day	32% 11% 4 9% 5% 5% 4 3 5% 5% 19%	3227
November	1st to 10th	Day	41% 10% 3 6% 4 4 4 3 4 5% 17%	1615
	11th to 20th	Day	38% 11% 3 6% 4 3 3 3 4 3 22%	1520
	21st to end	Day	37% 11% 3 7% 4 4 3 2 4 3 21%	1520
December	1st to 10th	Day	34% 11% 2 7% 5% 4 4 2 4 5% 23%	1520
	11th to 20th	Day	35% 13% 4 7% 5% 4 3 3 4 5% 17%	1925
	21st to end	Day	31% 13% 4 11% 6% 5% 4 3 4 4 15%	2620
January	1st to 10th	Night	43% 15% 3 7% 4 4 4 3 3 3 11%	4589
	11th to 20th	Night	31% 13% 4 6% 4 3 3 3 5% 7% 22%	4382
	21st to end	Night	42% 12% 3 8% 4 3 4 3 4 4 13%	4820
February	1st to 10th	Night	34% 13% 4 7% 5% 4 4 3 4 5% 18%	2470
	11th to 20th	Night	31% 14% 3 7% 5% 4 4 3 4 5% 22%	2402
	21st to end	Night	33% 15% 3 7% 3 3 3 3 4 5% 20%	1965
March	1st to 10th	Night	23% 15% 3 9% 5% 6% 6% 5% 5% 6% 18%	3684
	11th to 20th	Night	23% 10% 2 6% 3 3 3 2 4 5% 38%	3489
	21st to end	Night	34% 14% 4 9% 5% 4 4 3 5% 4 16%	3679
April	1st to 10th	Night	29% 11% 3 9% 5% 5% 5% 5% 6% 5% 17%	1900
	11th to 20th	Night	24% 10% 3 7% 6% 4 5% 3 5% 5% 27%	1869
	21st to end	Night	28% 11% 3 7% 6% 5% 4 3 5% 6% 21%	1648
May	1st to 10th	Night	22% 9% 2 7% 4 4 3 2 4 4 39%	2540
	11th to 20th	Night	21% 7% 3 6% 3 3 4 4 4 6% 38%	2516
	21st to end	Night	33% 10% 4 9% 6% 4 3 3 4 4 20%	2762
June	1st to 10th	Night	24% 8% 3 7% 6% 5% 5% 3 5% 6% 29%	1496
	11th to 20th	Night	29% 12% 2 8% 5% 4 5% 3 4 5% 24%	1330
	21st to end	Night	14% 6% 3 6% 5% 5% 4 4 5% 6% 41%	1330
July	1st to 10th	Night	11% 5% 2 4 2 3 3 2 3 4 60%	2220
	11th to 20th	Night	6% 3 1 3 2 2 2 2 3 4 72%	2466
	21st to end	Night	13% 5% 2 4 3 3 3 3 4 5% 56%	2762
August	1st to 10th	Night	5% 3 1 3 2 2 3 3 5% 6% 67%	2558
	11th to 20th	Night	16% 8% 3 6% 3 3 3 2 4 6% 47%	2812
	21st to end	Night	21% 7% 2 5% 3 2 2 2 4 5% 47%	3360
September	1st to 10th	Night	12% 5% 2 6% 3 3 3 3 4 6% 53%	1900
	11th to 20th	Night	19% 7% 4 7% 4 3 3 4 4 4 41%	1900
	21st to end	Night	20% 8% 2 7% 4 4 2 3 5% 5% 40%	2161
October	1st to 10th	Night	18% 7% 2 6% 4 4 5% 4 6% 9% 36%	3800
	11th to 20th	Night	32% 8% 3 5% 3 4 4 3 4 5% 29%	3777
	21st to end	Night	33% 14% 3 8% 4 3 3 3 4 5% 20%	4334
November	1st to 10th	Night	37% 11% 2 6% 4 4 3 3 5% 5% 19%	2665
	11th to 20th	Night	37% 10% 2 7% 4 4 3 2 3 4 23%	2660
	21st to end	Night	36% 10% 3 7% 5% 3 4 3 4 4 22%	2660
December	1st to 10th	Night	32% 11% 3 6% 5% 4 3 3 5% 4 24%	2660
	11th to 20th	Night	34% 14% 4 8% 4 4 3 3 4 5% 16%	3280
	21st to end	Night	32% 14% 3 10% 6% 6% 5% 3 4 4 14%	5026

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	80% 4 6% 7% 2 1	2348
	11th to 20th	Day	64% 5% 10% 13% 6% 1	2540
	21st to end	Day	78% 5% 7% 7% 3	2835
February	1st to 10th	Day	72% 7% 10% 8% 3 1	1710
	11th to 20th	Day	70% 5% 9% 8% 6% 1	1674
	21st to end	Day	69% 5% 10% 10% 4	1489
March	1st to 10th	Day	64% 6% 14% 13% 3	3187
	11th to 20th	Day	54% 5% 14% 25% 2 1	3490
	21st to end	Day	72% 7% 12% 8% 1	3973
April	1st to 10th	Day	71% 7% 14% 8%	2280
	11th to 20th	Day	65% 6% 11% 12% 5% 1	2261
	21st to end	Day	68% 7% 11% 9% 4 1	2532
May	1st to 10th	Day	52% 5% 15% 19% 9%	4420
	11th to 20th	Day	48% 7% 25% 17% 3	4380
	21st to end	Day	69% 8% 14% 8%	4814
June	1st to 10th	Day	61% 7% 17% 10% 5%	2679
	11th to 20th	Day	66% 7% 15% 10% 3	2850
	21st to end	Day	47% 9% 18% 18% 7% 1	2850
July	1st to 10th	Day	28% 5% 9% 21% 23% 14%	4666
	11th to 20th	Day	23% 5% 8% 29% 33% 3	4411
	21st to end	Day	31% 5% 15% 33% 14% 2	4639
August	1st to 10th	Day	24% 5% 13% 40% 17% 1	4341
	11th to 20th	Day	44% 9% 18% 20% 7% 1	4065
	21st to end	Day	45% 4 12% 27% 11% 1	4313
September	1st to 10th	Day	38% 6% 15% 24% 12% 5%	2280
	11th to 20th	Day	49% 6% 12% 17% 14% 2	2280
	21st to end	Day	49% 6% 16% 18% 9% 3	2015
October	1st to 10th	Day	44% 6% 14% 29% 6%	3180
	11th to 20th	Day	62% 5% 10% 15% 8% 1	3161
	21st to end	Day	67% 5% 11% 9% 7% 1	3227
November	1st to 10th	Day	71% 5% 11% 9% 4	1615
	11th to 20th	Day	69% 5% 9% 8% 7% 2	1520
	21st to end	Day	69% 5% 6% 11% 7% 2	1520
December	1st to 10th	Day	65% 6% 8% 12% 8% 1	1520
	11th to 20th	Day	71% 5% 10% 12% 2 1	1928
	21st to end	Day	71% 7% 10% 9% 3	2620
January	1st to 10th	Night	79% 5% 6% 6% 2 1	4589
	11th to 20th	Night	64% 5% 9% 16% 6% 1	4420
	21st to end	Night	75% 5% 7% 8% 4 1	4820
February	1st to 10th	Night	71% 6% 10% 9% 4 1	2470
	11th to 20th	Night	65% 7% 11% 9% 6% 2	2404
	21st to end	Night	69% 5% 10% 11% 4	1965
March	1st to 10th	Night	63% 7% 14% 11% 4	3684
	11th to 20th	Night	50% 5% 19% 22% 2 1	3490
	21st to end	Night	72% 8% 12% 7% 1	3679
April	1st to 10th	Night	67% 9% 12% 10% 2	1900
	11th to 20th	Night	60% 6% 14% 10% 8% 1	1869
	21st to end	Night	66% 7% 11% 11% 5% 1	1648
May	1st to 10th	Night	51% 4 14% 23% 7%	2540
	11th to 20th	Night	49% 6% 20% 20% 3 1	2516
	21st to end	Night	70% 8% 14% 8% 1	2762
June	1st to 10th	Night	58% 6% 17% 14% 5%	1496
	11th to 20th	Night	64% 7% 15% 11% 2	1330
	21st to end	Night	44% 8% 21% 18% 6% 2	1330
July	1st to 10th	Night	31% 5% 9% 20% 17% 20%	2185
	11th to 20th	Night	20% 3 11% 26% 40% 1	2421
	21st to end	Night	31% 6% 13% 28% 19% 2	2674
August	1st to 10th	Night	19% 4 14% 40% 21% 2	2550
	11th to 20th	Night	40% 7% 20% 22% 8% 2	2839
	21st to end	Night	43% 4 14% 30% 9%	3360
September	1st to 10th	Night	34% 6% 15% 27% 14% 4	1900
	11th to 20th	Night	48% 6% 11% 18% 14% 3	1900
	21st to end	Night	47% 7% 15% 18% 11% 2	2161
October	1st to 10th	Night	45% 6% 14% 29% 5%	3800
	11th to 20th	Night	58% 5% 10% 17% 9%	3777
	21st to end	Night	68% 4 10% 11% 7%	4334
November	1st to 10th	Night	68% 6% 13% 10% 3 1	2665
	11th to 20th	Night	69% 5% 10% 10% 5% 2	2660
	21st to end	Night	68% 5% 6% 10% 9% 1	2660
December	1st to 10th	Night	66% 6% 9% 11% 8% 1	2660
	11th to 20th	Night	71% 5% 9% 10% 3 1	3276
	21st to end	Night	73% 6% 11% 7% 3	5026

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	43% 10% 16% 14% 11% 6%	180
	11th to 20th	Day	40% 14% 11% 18% 16% 2	180
	21st to end	Day	50% 9% 13% 15% 11% 3	226
February	1st to 10th	Day	44% 9% 20% 18% 8% 1	370
	11th to 20th	Day	44% 13% 21% 16% 7%	365
	21st to end	Day	38% 14% 18% 22% 6% 1	343
March	1st to 10th	Day	38% 14% 20% 18% 9%	460
	11th to 20th	Day	32% 13% 18% 22% 11% 3	460
	21st to end	Day	41% 12% 25% 15% 7% 1	506
April	1st to 10th	Day	37% 12% 23% 17% 9% 2	460
	11th to 20th	Day	30% 16% 27% 15% 9% 3	452
	21st to end	Day	32% 13% 26% 19% 10% 1	460
May	1st to 10th	Day	23% 10% 28% 27% 10% 2	463
	11th to 20th	Day	31% 9% 19% 22% 15% 4	488
	21st to end	Day	33% 9% 20% 24% 11% 3	529
June	1st to 10th	Day	28% 9% 21% 26% 12% 3	509
	11th to 20th	Day	29% 11% 24% 24% 10% 2	680
	21st to end	Day	17% 6% 22% 36% 16% 3	680
July	1st to 10th	Day	14% 5% 22% 38% 19% 2	671
	11th to 20th	Day	12% 6% 19% 38% 21% 4	524
	21st to end	Day	11% 4 17% 40% 23% 5%	531
August	1st to 10th	Day	14% 4 16% 37% 21% 8%	487
	11th to 20th	Day	10% 4 17% 37% 26% 7%	477
	21st to end	Day	19% 6% 22% 32% 15% 6%	539
September	1st to 10th	Day	16% 9% 23% 33% 15% 4	490
	11th to 20th	Day	20% 10% 17% 33% 18% 2	490
	21st to end	Day	24% 7% 21% 27% 17% 3	490
October	1st to 10th	Day	27% 7% 21% 23% 18% 4	490
	11th to 20th	Day	36% 6% 17% 20% 16% 4	490
	21st to end	Day	39% 11% 17% 19% 9% 4	406
November	1st to 10th	Day	43% 12% 14% 20% 10% 1	226
	11th to 20th	Day	47% 10% 11% 11% 17% 5%	180
	21st to end	Day	41% 10% 16% 13% 11% 9%	180
December	1st to 10th	Day	47% 8% 12% 13% 12% 8%	180
	11th to 20th	Day	42% 12% 18% 19% 9%	180
	21st to end	Day	43% 6% 18% 21% 11% 2	198
January	1st to 10th	Night	50% 9% 13% 16% 11% 1	809
	11th to 20th	Night	42% 9% 13% 23% 12% 2	760
	21st to end	Night	42% 7% 13% 26% 12%	836
February	1st to 10th	Night	42% 6% 17% 25% 8% 1	760
	11th to 20th	Night	40% 8% 13% 28% 10% 1	744
	21st to end	Night	41% 9% 16% 24% 9% 1	628
March	1st to 10th	Night	39% 7% 17% 23% 13% 1	760
	11th to 20th	Night	36% 8% 18% 25% 12% 1	760
	21st to end	Night	40% 9% 19% 23% 8% 1	836
April	1st to 10th	Night	36% 9% 17% 25% 12% 1	760
	11th to 20th	Night	33% 7% 18% 28% 13% 1	741
	21st to end	Night	34% 8% 18% 27% 12% 1	508
May	1st to 10th	Night	25% 9% 13% 34% 18% 1	380
	11th to 20th	Night	25% 6% 13% 28% 24% 3	374
	21st to end	Night	28% 5% 16% 25% 23% 3	411
June	1st to 10th	Night	28% 5% 14% 31% 20% 2	380
	11th to 20th	Night	35% 6% 14% 25% 19% 1	380
	21st to end	Night	18% 3 14% 34% 28% 3	380
July	1st to 10th	Night	18% 3 11% 29% 36% 4	370
	11th to 20th	Night	13% 3 9% 35% 36% 4	368
	21st to end	Night	12% 2 8% 28% 44% 6%	416
August	1st to 10th	Night	11% 2 7% 24% 48% 7%	394
	11th to 20th	Night	13% 2 6% 26% 43% 10%	564
	21st to end	Night	19% 5% 7% 29% 35% 4	750
September	1st to 10th	Night	16% 3 9% 35% 31% 6%	760
	11th to 20th	Night	23% 5% 8% 26% 32% 6%	760
	21st to end	Night	25% 5% 12% 28% 27% 3	760
October	1st to 10th	Night	26% 5% 12% 29% 25% 3	760
	11th to 20th	Night	34% 6% 13% 25% 19% 3	760
	21st to end	Night	41% 6% 16% 24% 11% 2	836
November	1st to 10th	Night	44% 7% 19% 25% 6%	778
	11th to 20th	Night	44% 7% 17% 20% 10% 2	760
	21st to end	Night	42% 7% 17% 21% 12% 1	760
December	1st to 10th	Night	38% 7% 16% 26% 12% 2	760
	11th to 20th	Night	47% 9% 15% 23% 5% 1	760
	21st to end	Night	51% 8% 16% 20% 5%	883

Page generated at 2024-11-07 03:55:45. Version ac07

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