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 2024-01-13.

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	25% 9% 3 6% 5% 3 4 4 5% 6% 29%	75071
	Night	28% 9% 3 6% 5% 3 4 4 4 5% 29%	74656
Transparency	Day	58% 6% 13% 16% 7% 1	74766
	Night	59% 6% 12% 14% 7% 1	74479
Seeing	Day	29% 9% 20% 25% 14% 3	13884
	Night	34% 6% 14% 26% 18% 2	22428

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	43% 11% 4 7% 5% 3 3 3 2 3 15%	1406
	11th to 20th	Day	35% 9% 4 6% 5% 3 4 4 5% 5% 20%	1456
	21st to end	Day	35% 10% 3 5% 4 3 5% 4 5% 6% 19%	1611
February	1st to 10th	Day	35% 11% 5% 7% 6% 3 4 5% 5% 5% 17%	1620
	11th to 20th	Day	32% 14% 3 8% 6% 2 4 4 4 4 20%	1575
	21st to end	Day	30% 15% 4 7% 6% 3 4 4 5% 4 20%	1404
March	1st to 10th	Day	31% 14% 4 6% 7% 3 4 4 5% 5% 18%	1823
	11th to 20th	Day	33% 13% 4 5% 6% 3 5% 4 4 4 19%	1980
	21st to end	Day	31% 14% 5% 8% 6% 3 5% 5% 4 5% 14%	2262
April	1st to 10th	Day	29% 12% 5% 8% 6% 4 5% 5% 5% 6% 16%	2160
	11th to 20th	Day	26% 9% 4 9% 7% 4 5% 4 4 7% 21%	2140
	21st to end	Day	25% 10% 4 8% 7% 3 6% 5% 6% 5% 20%	2398
May	1st to 10th	Day	20% 10% 4 8% 6% 3 5% 4 4 6% 30%	2520
	11th to 20th	Day	21% 8% 4 7% 7% 3 5% 5% 6% 8% 27%	2477
	21st to end	Day	29% 10% 4 6% 6% 3 4 4 4 5% 24%	2723
June	1st to 10th	Day	26% 8% 3 7% 6% 3 5% 5% 5% 7% 25%	2538
	11th to 20th	Day	29% 9% 4 6% 6% 3 5% 5% 6% 6% 20%	2700
	21st to end	Day	15% 7% 3 7% 6% 3 4 6% 6% 7% 37%	2700
July	1st to 10th	Day	13% 6% 3 5% 4 3 4 4 5% 6% 47%	2700
	11th to 20th	Day	12% 5% 2 4 4 3 4 4 6% 7% 49%	2561
	21st to end	Day	11% 4 2 3 3 2 3 3 5% 5% 61%	2741
August	1st to 10th	Day	12% 5% 2 4 3 3 4 4 4 6% 53%	2490
	11th to 20th	Day	10% 4 2 3 3 2 3 4 5% 9% 55%	2298
	21st to end	Day	16% 6% 3 6% 4 3 4 3 5% 7% 44%	2458
September	1st to 10th	Day	12% 6% 3 5% 5% 2 4 3 4 7% 49%	2160
	11th to 20th	Day	18% 6% 4 6% 6% 3 4 4 4 6% 38%	2160
	21st to end	Day	21% 8% 2 5% 4 3 3 4 5% 5% 39%	1910
October	1st to 10th	Day	21% 8% 3 7% 5% 3 5% 5% 5% 7% 30%	1800
	11th to 20th	Day	31% 8% 3 5% 5% 2 4 4 5% 7% 27%	1800
	21st to end	Day	35% 7% 3 6% 5% 3 5% 4 5% 6% 23%	1854
November	1st to 10th	Day	41% 10% 3 5% 5% 3 4 4 4 5% 17%	1541
	11th to 20th	Day	38% 10% 3 6% 5% 3 3 3 4 3 22%	1520
	21st to end	Day	38% 10% 3 7% 4 3 3 3 4 3 21%	1520
December	1st to 10th	Day	34% 9% 3 5% 6% 3 4 3 4 5% 23%	1520
	11th to 20th	Day	36% 12% 4 6% 5% 3 4 4 4 5% 17%	1925
	21st to end	Day	31% 12% 5% 9% 7% 4 5% 4 4 5% 15%	2620
January	1st to 10th	Night	44% 10% 3 5% 5% 2 4 4 4 5% 15%	2751
	11th to 20th	Night	37% 10% 3 5% 5% 3 3 3 5% 5% 21%	2548
	21st to end	Night	34% 10% 3 6% 5% 2 4 4 5% 5% 22%	2743
February	1st to 10th	Night	35% 12% 4 6% 6% 3 4 4 3 5% 19%	2340
	11th to 20th	Night	32% 13% 3 6% 5% 3 4 3 4 5% 22%	2272
	21st to end	Night	32% 14% 3 6% 4 2 3 4 4 6% 21%	1852
March	1st to 10th	Night	31% 11% 3 6% 4 3 4 4 4 6% 24%	2134
	11th to 20th	Night	30% 10% 4 7% 5% 3 4 4 4 5% 23%	1979
	21st to end	Night	31% 13% 4 8% 6% 3 4 4 5% 5% 17%	2093
April	1st to 10th	Night	29% 11% 4 7% 6% 4 5% 5% 6% 7% 18%	1800
	11th to 20th	Night	25% 10% 3 7% 6% 3 6% 4 5% 6% 25%	1770
	21st to end	Night	27% 10% 4 7% 6% 4 5% 4 5% 7% 22%	1562
May	1st to 10th	Night	21% 9% 3 6% 5% 3 4 3 6% 6% 34%	1440
	11th to 20th	Night	22% 7% 4 5% 5% 3 4 7% 5% 6% 32%	1416
	21st to end	Night	23% 8% 4 8% 7% 3 4 5% 5% 5% 28%	1552
June	1st to 10th	Night	24% 7% 4 6% 7% 4 5% 4 4 6% 29%	1418
	11th to 20th	Night	29% 10% 3 8% 5% 3 5% 3 4 5% 24%	1260
	21st to end	Night	14% 5% 3 5% 5% 4 4 4 5% 6% 43%	1260
July	1st to 10th	Night	13% 5% 3 5% 4 3 4 4 4 6% 49%	1260
	11th to 20th	Night	10% 5% 2 4 4 2 3 3 4 5% 56%	1394
	21st to end	Night	9% 4 1 3 3 2 3 3 4 5% 63%	1576
August	1st to 10th	Night	9% 4 1 3 3 2 4 4 5% 7% 57%	1447
	11th to 20th	Night	8% 4 2 3 3 2 2 2 4 7% 62%	1602
	21st to end	Night	15% 5% 2 4 3 2 3 4 4 6% 52%	1898
September	1st to 10th	Night	12% 5% 2 5% 4 2 3 4 5% 6% 53%	1800
	11th to 20th	Night	19% 6% 4 6% 5% 2 3 4 4 4 43%	1800
	21st to end	Night	20% 7% 3 6% 5% 3 3 3 5% 5% 40%	2046
October	1st to 10th	Night	22% 7% 3 5% 5% 3 5% 5% 4 6% 34%	2160
	11th to 20th	Night	29% 7% 3 5% 4 3 4 4 5% 5% 31%	2160
	21st to end	Night	33% 9% 2 5% 4 3 3 3 4 6% 28%	2501
November	1st to 10th	Night	38% 10% 2 6% 5% 3 3 4 5% 5% 20%	2536
	11th to 20th	Night	37% 9% 3 7% 5% 2 4 3 3 4 23%	2660
	21st to end	Night	36% 9% 3 6% 6% 3 4 3 4 4 22%	2660
December	1st to 10th	Night	32% 10% 4 6% 5% 3 3 4 5% 4 24%	2660
	11th to 20th	Night	34% 13% 5% 7% 5% 3 4 4 4 5% 16%	3280
	21st to end	Night	32% 13% 4 9% 7% 5% 5% 4 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	76% 4 6% 10% 4 1	1406
	11th to 20th	Day	66% 7% 10% 9% 6% 2	1456
	21st to end	Day	66% 6% 10% 12% 5%	1611
February	1st to 10th	Day	72% 7% 9% 8% 3 1	1620
	11th to 20th	Day	70% 5% 10% 8% 6% 1	1584
	21st to end	Day	69% 5% 10% 10% 5%	1404
March	1st to 10th	Day	69% 5% 12% 12% 3	1823
	11th to 20th	Day	72% 5% 9% 11% 3 1	1980
	21st to end	Day	75% 7% 10% 7% 1	2262
April	1st to 10th	Day	71% 7% 14% 8% 1	2160
	11th to 20th	Day	67% 6% 11% 12% 4 1	2140
	21st to end	Day	68% 7% 11% 9% 4 1	2398
May	1st to 10th	Day	58% 7% 16% 14% 5%	2520
	11th to 20th	Day	57% 8% 18% 13% 4	2480
	21st to end	Day	64% 7% 14% 14% 1	2724
June	1st to 10th	Day	61% 7% 17% 10% 4	2538
	11th to 20th	Day	67% 7% 15% 9% 2	2700
	21st to end	Day	47% 9% 17% 18% 7% 1	2700
July	1st to 10th	Day	40% 8% 14% 24% 12% 2	2626
	11th to 20th	Day	37% 7% 12% 29% 14%	2483
	21st to end	Day	27% 5% 15% 35% 16% 3	2589
August	1st to 10th	Day	34% 6% 16% 28% 14% 2	2473
	11th to 20th	Day	29% 7% 20% 28% 13% 3	2298
	21st to end	Day	42% 5% 15% 29% 7% 1	2458
September	1st to 10th	Day	37% 6% 16% 25% 12% 4	2160
	11th to 20th	Day	49% 5% 12% 17% 15% 2	2160
	21st to end	Day	49% 6% 15% 18% 9% 3	1910
October	1st to 10th	Day	52% 8% 12% 21% 6% 1	1800
	11th to 20th	Day	57% 6% 13% 18% 5% 1	1800
	21st to end	Day	63% 6% 10% 12% 9%	1854
November	1st to 10th	Day	71% 5% 11% 9% 4	1541
	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	74% 5% 8% 9% 4 1	2751
	11th to 20th	Night	67% 6% 10% 9% 6% 2	2548
	21st to end	Night	65% 5% 10% 13% 6% 2	2743
February	1st to 10th	Night	71% 6% 10% 9% 4 1	2340
	11th to 20th	Night	66% 7% 10% 9% 6% 2	2274
	21st to end	Night	68% 5% 10% 12% 4	1852
March	1st to 10th	Night	63% 6% 14% 13% 5%	2134
	11th to 20th	Night	65% 6% 11% 13% 4 1	1980
	21st to end	Night	71% 7% 13% 8% 1	2093
April	1st to 10th	Night	67% 9% 12% 11% 2	1800
	11th to 20th	Night	61% 6% 14% 11% 7% 1	1770
	21st to end	Night	65% 7% 11% 11% 5% 1	1562
May	1st to 10th	Night	53% 6% 20% 15% 6% 1	1440
	11th to 20th	Night	54% 7% 19% 15% 5% 1	1416
	21st to end	Night	59% 9% 17% 14% 1	1552
June	1st to 10th	Night	59% 6% 17% 14% 5%	1418
	11th to 20th	Night	65% 7% 15% 10% 2	1260
	21st to end	Night	43% 9% 21% 19% 6% 2	1260
July	1st to 10th	Night	39% 7% 16% 22% 13% 3	1225
	11th to 20th	Night	34% 5% 17% 28% 16% 1	1349
	21st to end	Night	23% 6% 14% 33% 19% 4	1488
August	1st to 10th	Night	28% 6% 14% 32% 17% 3	1439
	11th to 20th	Night	27% 6% 17% 33% 14% 3	1602
	21st to end	Night	37% 5% 15% 32% 10% 1	1898
September	1st to 10th	Night	34% 7% 15% 27% 14% 4	1800
	11th to 20th	Night	47% 6% 10% 18% 15% 3	1800
	21st to end	Night	47% 7% 14% 18% 11% 2	2046
October	1st to 10th	Night	50% 7% 14% 21% 7% 1	2160
	11th to 20th	Night	57% 7% 12% 18% 7% 1	2160
	21st to end	Night	60% 6% 10% 14% 10%	2501
November	1st to 10th	Night	68% 6% 13% 9% 3 1	2536
	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% 9% 15% 15% 12% 6%	180
	11th to 20th	Day	40% 15% 11% 16% 16% 2	164
	21st to end	Day	49% 8% 11% 17% 12% 3	203
February	1st to 10th	Day	47% 8% 20% 17% 8% 1	340
	11th to 20th	Day	45% 10% 21% 16% 7%	335
	21st to end	Day	39% 12% 19% 22% 7% 1	312
March	1st to 10th	Day	41% 13% 20% 18% 8%	420
	11th to 20th	Day	34% 13% 18% 22% 10% 2	420
	21st to end	Day	43% 10% 24% 14% 8% 1	462
April	1st to 10th	Day	38% 11% 23% 16% 10% 2	420
	11th to 20th	Day	32% 15% 27% 14% 9% 3	412
	21st to end	Day	32% 13% 26% 20% 9% 1	420
May	1st to 10th	Day	23% 10% 28% 28% 10% 2	423
	11th to 20th	Day	33% 8% 19% 22% 15% 3	448
	21st to end	Day	32% 8% 20% 25% 12% 3	485
June	1st to 10th	Day	28% 9% 22% 25% 13% 3	468
	11th to 20th	Day	29% 11% 25% 24% 10% 2	630
	21st to end	Day	16% 5% 23% 36% 17% 3	630
July	1st to 10th	Day	15% 5% 21% 38% 19% 2	621
	11th to 20th	Day	13% 7% 20% 37% 20% 3	482
	21st to end	Day	11% 3 17% 41% 23% 5%	487
August	1st to 10th	Day	15% 5% 15% 35% 22% 8%	447
	11th to 20th	Day	9% 3 17% 37% 26% 7%	437
	21st to end	Day	18% 6% 22% 34% 15% 6%	495
September	1st to 10th	Day	16% 9% 24% 33% 14% 3	450
	11th to 20th	Day	21% 10% 16% 33% 18% 2	450
	21st to end	Day	25% 7% 20% 28% 17% 3	450
October	1st to 10th	Day	27% 7% 21% 23% 18% 4	450
	11th to 20th	Day	36% 6% 17% 21% 15% 5%	450
	21st to end	Day	39% 12% 16% 20% 9% 5%	369
November	1st to 10th	Day	42% 12% 15% 20% 10% 1	206
	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% 12% 23% 13% 2	728
	21st to end	Night	42% 6% 12% 27% 13%	792
February	1st to 10th	Night	44% 6% 17% 25% 7% 1	720
	11th to 20th	Night	41% 8% 12% 28% 10% 1	704
	21st to end	Night	41% 8% 16% 25% 9% 1	592
March	1st to 10th	Night	40% 7% 16% 23% 13% 1	720
	11th to 20th	Night	37% 8% 18% 25% 11% 1	720
	21st to end	Night	41% 9% 18% 23% 8% 1	792
April	1st to 10th	Night	36% 9% 17% 25% 12% 2	720
	11th to 20th	Night	33% 7% 17% 28% 13% 2	702
	21st to end	Night	34% 7% 18% 27% 13% 1	482
May	1st to 10th	Night	25% 9% 12% 35% 19% 1	360
	11th to 20th	Night	26% 6% 12% 29% 24% 3	354
	21st to end	Night	28% 5% 15% 24% 24% 4	389
June	1st to 10th	Night	29% 4 14% 31% 20% 2	360
	11th to 20th	Night	36% 6% 14% 25% 19% 1	360
	21st to end	Night	17% 3 15% 34% 29% 3	360
July	1st to 10th	Night	18% 3 10% 28% 37% 4	350
	11th to 20th	Night	14% 3 9% 35% 36% 4	348
	21st to end	Night	11% 2 8% 28% 45% 6%	394
August	1st to 10th	Night	12% 2 8% 24% 47% 7%	373
	11th to 20th	Night	12% 1 6% 27% 44% 11%	534
	21st to end	Night	18% 5% 7% 30% 36% 4	710
September	1st to 10th	Night	16% 3 10% 36% 30% 5%	720
	11th to 20th	Night	23% 5% 8% 26% 33% 6%	720
	21st to end	Night	25% 5% 13% 28% 27% 3	720
October	1st to 10th	Night	26% 5% 12% 29% 25% 3	720
	11th to 20th	Night	33% 7% 13% 25% 19% 3	720
	21st to end	Night	40% 6% 16% 23% 12% 2	792
November	1st to 10th	Night	43% 7% 19% 25% 6%	740
	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

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