Ashcroft
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ashcroft clear sky chart from 2005-11-06 to 2025-06-10.

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	5% 8% 3 8% 6% 6% 6% 5% 7% 8% 36%	122789
	Night	5% 10% 4 9% 6% 6% 6% 5% 7% 8% 33%	124121
Transparency	Day	43% 10% 17% 20% 9% 2	122540
	Night	45% 11% 16% 19% 8% 1	124013
Seeing	Day	11% 14% 30% 27% 14% 3	16336
	Night	10% 8% 29% 33% 18% 2	24708

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	6% 14% 6% 16% 8% 8% 7% 6% 6% 7% 15%	3179
	11th to 20th	Day	7% 8% 5% 9% 6% 6% 6% 6% 11% 12% 25%	3177
	21st to end	Day	4 15% 6% 11% 7% 7% 6% 5% 8% 9% 23%	4004
February	1st to 10th	Day	6% 14% 5% 10% 6% 7% 7% 6% 7% 8% 24%	1690
	11th to 20th	Day	4 11% 4 10% 6% 6% 6% 5% 8% 9% 31%	1755
	21st to end	Day	2 12% 4 11% 9% 10% 7% 6% 8% 7% 23%	1549
March	1st to 10th	Day	1 8% 4 9% 6% 7% 8% 6% 9% 10% 32%	4518
	11th to 20th	Day	1 5% 3 8% 7% 7% 8% 7% 9% 10% 35%	4944
	21st to end	Day	3 10% 4 10% 8% 7% 7% 6% 8% 10% 26%	5784
April	1st to 10th	Day	4 10% 5% 10% 8% 7% 8% 6% 8% 9% 26%	2400
	11th to 20th	Day	5% 10% 5% 9% 8% 7% 6% 5% 8% 8% 28%	2419
	21st to end	Day	6% 11% 4 10% 7% 6% 5% 5% 7% 9% 29%	2770
May	1st to 10th	Day	6% 9% 3 8% 5% 5% 5% 4 6% 8% 42%	6320
	11th to 20th	Day	7% 9% 3 11% 7% 8% 7% 6% 8% 10% 25%	6277
	21st to end	Day	10% 10% 3 8% 6% 5% 6% 5% 7% 8% 31%	7031
June	1st to 10th	Day	11% 9% 3 9% 7% 6% 6% 4 7% 7% 31%	3118
	11th to 20th	Day	14% 9% 4 9% 9% 7% 6% 5% 7% 7% 23%	3040
	21st to end	Day	7% 6% 3 9% 7% 7% 6% 5% 7% 8% 37%	3010
July	1st to 10th	Day	4 3 1 4 4 4 4 4 5% 7% 59%	4740
	11th to 20th	Day	3 3 1 3 3 3 3 3 5% 8% 64%	4740
	21st to end	Day	5% 4 1 4 4 4 4 4 6% 8% 55%	4855
August	1st to 10th	Day	3 3 1 4 3 4 5% 4 7% 9% 57%	4401
	11th to 20th	Day	2 3 2 6% 5% 5% 7% 5% 9% 11% 47%	4214
	21st to end	Day	7% 5% 2 7% 4 5% 5% 5% 7% 9% 44%	4349
September	1st to 10th	Day	6% 5% 2 6% 4 5% 4 3 6% 8% 51%	2280
	11th to 20th	Day	9% 7% 3 8% 4 5% 5% 5% 6% 7% 42%	2280
	21st to end	Day	9% 7% 3 8% 5% 6% 5% 4 6% 8% 39%	2024
October	1st to 10th	Day	5% 7% 3 8% 5% 5% 5% 4 6% 7% 44%	3180
	11th to 20th	Day	9% 7% 3 7% 4 6% 6% 5% 8% 9% 37%	3143
	21st to end	Day	6% 9% 3 7% 6% 6% 6% 5% 9% 9% 35%	3088
November	1st to 10th	Day	7% 12% 5% 10% 7% 6% 6% 5% 6% 7% 28%	1568
	11th to 20th	Day	4 10% 4 9% 7% 6% 6% 5% 7% 9% 35%	1600
	21st to end	Day	5% 12% 5% 11% 8% 6% 6% 5% 8% 9% 24%	1556
December	1st to 10th	Day	4 11% 5% 10% 7% 6% 10% 9% 10% 8% 21%	2241
	11th to 20th	Day	6% 16% 7% 13% 8% 7% 8% 6% 6% 5% 17%	2262
	21st to end	Day	3 11% 5% 13% 9% 9% 9% 6% 8% 7% 19%	3283
January	1st to 10th	Night	6% 16% 7% 15% 7% 7% 8% 6% 8% 7% 13%	6766
	11th to 20th	Night	7% 11% 4 12% 7% 5% 5% 4 9% 9% 28%	6720
	21st to end	Night	4 12% 6% 11% 7% 7% 8% 5% 8% 8% 24%	6952
February	1st to 10th	Night	8% 12% 5% 12% 9% 7% 6% 5% 6% 7% 23%	2706
	11th to 20th	Night	5% 12% 3 8% 7% 6% 7% 5% 7% 8% 33%	2532
	21st to end	Night	3 14% 5% 11% 7% 7% 7% 5% 7% 8% 26%	2080
March	1st to 10th	Night	1 10% 4 9% 5% 6% 7% 5% 8% 10% 33%	5376
	11th to 20th	Night	1 6% 2 6% 5% 5% 6% 6% 9% 10% 44%	5053
	21st to end	Night	6% 10% 3 10% 7% 6% 7% 5% 8% 9% 30%	5191
April	1st to 10th	Night	3 9% 3 10% 7% 6% 7% 5% 7% 8% 35%	2000
	11th to 20th	Night	2 8% 4 8% 7% 6% 6% 6% 8% 9% 36%	1930
	21st to end	Night	5% 9% 4 9% 6% 4 6% 5% 7% 10% 36%	1630
May	1st to 10th	Night	5% 9% 4 10% 6% 5% 5% 3 5% 6% 42%	3640
	11th to 20th	Night	7% 9% 4 8% 6% 6% 5% 5% 7% 9% 34%	3616
	21st to end	Night	7% 8% 3 8% 6% 6% 6% 4 7% 10% 35%	3839
June	1st to 10th	Night	12% 6% 3 8% 6% 5% 5% 5% 8% 8% 35%	1261
	11th to 20th	Night	9% 8% 3 8% 7% 6% 6% 6% 6% 9% 31%	1140
	21st to end	Night	5% 6% 3 7% 5% 6% 6% 5% 7% 7% 44%	1165
July	1st to 10th	Night	5% 5% 2 5% 4 4 4 4 6% 7% 55%	2220
	11th to 20th	Night	3 3 2 3 3 3 5% 3 6% 7% 63%	2220
	21st to end	Night	4 4 1 4 4 4 5% 4 7% 8% 53%	2695
August	1st to 10th	Night	3 2 1 6% 3 4 4 4 7% 8% 57%	2529
	11th to 20th	Night	4 4 1 5% 5% 5% 6% 5% 8% 11% 45%	2615
	21st to end	Night	7% 5% 2 7% 4 5% 5% 5% 7% 7% 45%	3323
September	1st to 10th	Night	4 4 2 6% 4 4 5% 3 5% 7% 55%	1900
	11th to 20th	Night	8% 6% 2 5% 5% 4 5% 4 6% 8% 47%	1900
	21st to end	Night	7% 7% 3 6% 4 4 5% 4 6% 8% 45%	2154
October	1st to 10th	Night	5% 7% 2 7% 5% 6% 6% 5% 7% 8% 43%	3800
	11th to 20th	Night	5% 7% 3 8% 5% 5% 6% 5% 8% 10% 37%	3793
	21st to end	Night	6% 8% 3 9% 7% 6% 6% 7% 9% 7% 31%	4457
November	1st to 10th	Night	9% 12% 4 10% 7% 6% 5% 5% 7% 8% 27%	2730
	11th to 20th	Night	4 10% 4 10% 7% 6% 6% 6% 8% 8% 30%	2800
	21st to end	Night	6% 15% 5% 11% 7% 7% 6% 5% 7% 9% 22%	2820
December	1st to 10th	Night	4 11% 4 9% 7% 6% 10% 7% 9% 7% 27%	4923
	11th to 20th	Night	7% 15% 6% 15% 8% 7% 7% 6% 6% 6% 16%	5930
	21st to end	Night	5% 13% 5% 12% 8% 7% 8% 7% 7% 7% 21%	7715

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% 16% 11% 11% 2	3179
	11th to 20th	Day	46% 13% 18% 16% 3 4	3198
	21st to end	Day	56% 11% 14% 14% 5%	4004
February	1st to 10th	Day	56% 12% 13% 10% 8% 1	1690
	11th to 20th	Day	47% 11% 16% 17% 8%	1764
	21st to end	Day	57% 10% 14% 14% 5% 1	1549
March	1st to 10th	Day	43% 10% 20% 18% 8%	4518
	11th to 20th	Day	41% 9% 20% 23% 6%	4944
	21st to end	Day	50% 11% 17% 14% 7%	5784
April	1st to 10th	Day	52% 12% 17% 14% 4	2400
	11th to 20th	Day	51% 10% 13% 16% 9% 1	2419
	21st to end	Day	51% 8% 16% 18% 8%	2770
May	1st to 10th	Day	37% 7% 13% 28% 14% 1	6320
	11th to 20th	Day	49% 10% 18% 18% 5%	6280
	21st to end	Day	46% 10% 18% 21% 3 2	7032
June	1st to 10th	Day	51% 8% 13% 15% 9% 3	3118
	11th to 20th	Day	58% 8% 15% 13% 6%	3040
	21st to end	Day	42% 7% 19% 18% 11% 2	3010
July	1st to 10th	Day	24% 5% 11% 23% 24% 13%	4666
	11th to 20th	Day	19% 5% 12% 29% 28% 7%	4659
	21st to end	Day	25% 5% 19% 26% 18% 7%	4701
August	1st to 10th	Day	22% 7% 13% 35% 21% 2	4384
	11th to 20th	Day	27% 12% 25% 24% 11% 1	4256
	21st to end	Day	34% 9% 24% 27% 5% 1	4348
September	1st to 10th	Day	31% 8% 17% 28% 13% 3	2280
	11th to 20th	Day	41% 8% 14% 23% 12% 2	2280
	21st to end	Day	44% 9% 20% 17% 8% 1	2024
October	1st to 10th	Day	39% 8% 19% 25% 7% 1	3180
	11th to 20th	Day	44% 10% 16% 25% 5%	3143
	21st to end	Day	42% 11% 22% 22% 3	3088
November	1st to 10th	Day	55% 10% 13% 17% 4 1	1568
	11th to 20th	Day	46% 15% 15% 17% 6%	1600
	21st to end	Day	51% 16% 17% 12% 4 1	1556
December	1st to 10th	Day	52% 13% 21% 12% 1	2241
	11th to 20th	Day	62% 13% 11% 9% 5%	2264
	21st to end	Day	56% 18% 13% 11% 2	3283
January	1st to 10th	Night	59% 15% 13% 10% 3	6766
	11th to 20th	Night	51% 10% 16% 15% 4 4	6761
	21st to end	Night	55% 13% 14% 12% 5%	6952
February	1st to 10th	Night	57% 12% 11% 13% 6% 1	2706
	11th to 20th	Night	48% 9% 17% 18% 8% 1	2534
	21st to end	Night	53% 10% 17% 14% 5%	2080
March	1st to 10th	Night	39% 11% 21% 21% 7%	5376
	11th to 20th	Night	33% 10% 21% 23% 12%	5054
	21st to end	Night	48% 11% 16% 16% 10%	5191
April	1st to 10th	Night	44% 11% 20% 19% 6%	2000
	11th to 20th	Night	42% 12% 16% 19% 11%	1930
	21st to end	Night	41% 11% 17% 20% 11%	1630
May	1st to 10th	Night	40% 9% 11% 26% 13% 2	3637
	11th to 20th	Night	41% 13% 15% 25% 6%	3616
	21st to end	Night	40% 13% 18% 23% 6%	3839
June	1st to 10th	Night	45% 8% 14% 19% 11% 3	1261
	11th to 20th	Night	47% 9% 16% 17% 10%	1140
	21st to end	Night	35% 8% 21% 21% 11% 4	1165
July	1st to 10th	Night	28% 7% 11% 16% 27% 11%	2185
	11th to 20th	Night	18% 8% 12% 25% 33% 4	2178
	21st to end	Night	27% 7% 14% 25% 15% 12%	2610
August	1st to 10th	Night	22% 8% 12% 30% 26% 2	2521
	11th to 20th	Night	30% 10% 20% 25% 13% 2	2639
	21st to end	Night	34% 8% 24% 24% 8% 1	3323
September	1st to 10th	Night	28% 8% 16% 30% 15% 4	1900
	11th to 20th	Night	36% 7% 17% 24% 14% 3	1900
	21st to end	Night	36% 10% 22% 19% 11% 2	2154
October	1st to 10th	Night	39% 8% 19% 26% 8%	3800
	11th to 20th	Night	44% 9% 16% 24% 7%	3793
	21st to end	Night	45% 9% 18% 25% 4	4457
November	1st to 10th	Night	54% 11% 16% 14% 5% 1	2730
	11th to 20th	Night	49% 16% 15% 14% 6% 1	2800
	21st to end	Night	55% 15% 15% 10% 4	2820
December	1st to 10th	Night	48% 12% 19% 18% 4	4923
	11th to 20th	Night	61% 13% 13% 9% 3	5927
	21st to end	Night	53% 14% 14% 15% 3	7715

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	7% 14% 36% 29% 10% 6%	200
	11th to 20th	Day	7% 13% 31% 34% 15% 3	200
	21st to end	Day	15% 9% 36% 27% 13% 1	220
February	1st to 10th	Day	9% 15% 44% 21% 8% 3	290
	11th to 20th	Day	10% 11% 37% 27% 13% 2	395
	21st to end	Day	9% 16% 39% 24% 10% 2	364
March	1st to 10th	Day	10% 19% 34% 24% 13% 1	500
	11th to 20th	Day	8% 20% 35% 20% 14% 3	500
	21st to end	Day	13% 23% 31% 22% 9% 2	550
April	1st to 10th	Day	14% 22% 30% 25% 8% 1	500
	11th to 20th	Day	16% 21% 29% 20% 11% 3	492
	21st to end	Day	12% 21% 29% 24% 11% 3	500
May	1st to 10th	Day	12% 20% 31% 25% 10% 2	503
	11th to 20th	Day	13% 11% 29% 28% 14% 4	528
	21st to end	Day	14% 15% 27% 29% 12% 3	623
June	1st to 10th	Day	14% 18% 30% 27% 10% 1	856
	11th to 20th	Day	17% 13% 27% 27% 14% 2	870
	21st to end	Day	10% 10% 28% 30% 19% 2	840
July	1st to 10th	Day	8% 11% 31% 31% 18% 2	671
	11th to 20th	Day	6% 13% 27% 30% 19% 5%	668
	21st to end	Day	5% 7% 24% 33% 26% 4	570
August	1st to 10th	Day	6% 7% 22% 30% 28% 7%	487
	11th to 20th	Day	5% 10% 21% 34% 22% 8%	477
	21st to end	Day	8% 12% 27% 32% 16% 6%	539
September	1st to 10th	Day	8% 14% 28% 29% 16% 4	490
	11th to 20th	Day	14% 12% 26% 26% 17% 5%	490
	21st to end	Day	15% 18% 32% 23% 10% 2	490
October	1st to 10th	Day	14% 14% 31% 24% 13% 4	490
	11th to 20th	Day	14% 20% 28% 23% 14% 1	484
	21st to end	Day	16% 12% 33% 26% 12% 1	329
November	1st to 10th	Day	15% 18% 36% 18% 13% 1	200
	11th to 20th	Day	12% 10% 39% 24% 13% 4	200
	21st to end	Day	10% 11% 35% 29% 9% 7%	200
December	1st to 10th	Day	9% 7% 30% 28% 22% 6%	200
	11th to 20th	Day	10% 10% 32% 29% 14% 6%	200
	21st to end	Day	10% 9% 34% 34% 10% 3	220
January	1st to 10th	Night	15% 9% 35% 29% 10% 2	900
	11th to 20th	Night	13% 7% 34% 34% 11% 1	896
	21st to end	Night	10% 6% 36% 32% 14% 2	880
February	1st to 10th	Night	14% 7% 33% 32% 13% 2	800
	11th to 20th	Night	10% 6% 31% 37% 15% 1	784
	21st to end	Night	7% 9% 36% 29% 17% 2	660
March	1st to 10th	Night	10% 9% 32% 31% 17% 1	800
	11th to 20th	Night	5% 12% 35% 31% 16% 2	800
	21st to end	Night	8% 9% 31% 32% 17% 2	880
April	1st to 10th	Night	9% 10% 26% 33% 20% 2	800
	11th to 20th	Night	5% 9% 35% 34% 16% 1	742
	21st to end	Night	7% 9% 33% 32% 18% 1	430
May	1st to 10th	Night	8% 8% 30% 34% 19% 2	400
	11th to 20th	Night	9% 5% 23% 39% 22% 2	394
	21st to end	Night	15% 4 23% 34% 22% 2	433
June	1st to 10th	Night	13% 8% 25% 36% 18% 1	399
	11th to 20th	Night	13% 6% 23% 38% 18% 2	380
	21st to end	Night	6% 6% 25% 36% 26% 2	380
July	1st to 10th	Night	7% 4 22% 41% 24% 2	370
	11th to 20th	Night	7% 5% 18% 38% 29% 3	368
	21st to end	Night	6% 3 13% 42% 34% 3	416
August	1st to 10th	Night	5% 2 13% 34% 40% 6%	378
	11th to 20th	Night	5% 4 15% 35% 36% 6%	442
	21st to end	Night	8% 5% 19% 37% 30% 2	726
September	1st to 10th	Night	7% 5% 19% 40% 26% 4	760
	11th to 20th	Night	11% 5% 16% 29% 31% 7%	760
	21st to end	Night	13% 9% 24% 31% 19% 4	760
October	1st to 10th	Night	10% 9% 23% 34% 22% 2	760
	11th to 20th	Night	13% 9% 27% 31% 17% 3	760
	21st to end	Night	14% 9% 32% 32% 12% 2	836
November	1st to 10th	Night	16% 9% 33% 32% 10% 1	780
	11th to 20th	Night	12% 9% 33% 31% 13% 2	800
	21st to end	Night	14% 11% 30% 32% 10% 3	820
December	1st to 10th	Night	10% 6% 30% 37% 15% 2	1010
	11th to 20th	Night	14% 8% 35% 32% 10% 1	1100
	21st to end	Night	13% 9% 36% 32% 10% 1	1104

Page generated at 2025-06-07 07:28:15. Version ac07

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