Rannie Road
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rannie Road clear sky chart from 2005-11-06 to 2025-07-06.

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	29% 12% 3 6% 4 3 3 3 4 5% 28%	123677
	Night	32% 13% 3 5% 3 3 3 2 4 4 28%	123805
Transparency	Day	58% 7% 11% 16% 7% 1	123427
	Night	60% 6% 11% 15% 7% 1	123699
Seeing	Day	32% 11% 21% 23% 10% 2	16136
	Night	37% 7% 19% 23% 13% 1	24359

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	41% 18% 4 7% 4 4 3 2 3 3 11%	3179
	11th to 20th	Day	23% 13% 3 7% 3 3 4 2 3 5% 34%	3174
	21st to end	Day	47% 9% 2 4 2 2 2 2 2 2 26%	4020
February	1st to 10th	Day	37% 15% 4 7% 4 3 3 3 4 4 16%	1800
	11th to 20th	Day	34% 16% 3 6% 3 3 3 3 3 4 21%	1755
	21st to end	Day	28% 22% 4 7% 4 5% 3 2 3 3 18%	1515
March	1st to 10th	Day	30% 24% 5% 8% 4 3 3 2 3 3 13%	4637
	11th to 20th	Day	27% 23% 4 8% 4 3 2 1 2 3 23%	5000
	21st to end	Day	36% 19% 3 7% 5% 3 3 3 3 4 13%	5656
April	1st to 10th	Day	36% 16% 3 7% 6% 4 3 3 5% 4 13%	2400
	11th to 20th	Day	28% 14% 3 7% 6% 4 4 3 4 5% 21%	2454
	21st to end	Day	33% 14% 3 6% 5% 3 4 3 4 4 20%	2677
May	1st to 10th	Day	20% 13% 3 6% 3 3 3 3 4 6% 36%	6320
	11th to 20th	Day	28% 12% 3 7% 4 5% 4 4 5% 6% 21%	6277
	21st to end	Day	31% 11% 3 7% 4 4 5% 4 5% 6% 21%	6995
June	1st to 10th	Day	33% 9% 2 6% 4 4 4 3 4 6% 25%	3000
	11th to 20th	Day	35% 11% 3 9% 5% 5% 4 3 4 5% 18%	3000
	21st to end	Day	26% 9% 3 6% 5% 4 4 3 5% 6% 31%	3000
July	1st to 10th	Day	12% 5% 2 5% 3 3 3 3 3 5% 57%	4816
	11th to 20th	Day	10% 4 1 3 2 2 2 2 3 4 66%	4740
	21st to end	Day	19% 5% 2 4 3 3 3 3 5% 6% 47%	4853
August	1st to 10th	Day	10% 4 1 4 2 2 2 2 4 6% 63%	4401
	11th to 20th	Day	22% 8% 3 6% 4 3 3 3 5% 6% 36%	4028
	21st to end	Day	25% 7% 2 5% 4 4 4 3 4 5% 35%	4402
September	1st to 10th	Day	21% 8% 3 5% 4 4 2 2 4 6% 42%	2280
	11th to 20th	Day	26% 9% 2 6% 4 3 3 3 4 5% 36%	2280
	21st to end	Day	33% 7% 2 5% 4 3 3 2 4 4 33%	2002
October	1st to 10th	Day	30% 11% 3 7% 4 4 4 4 5% 4 25%	3180
	11th to 20th	Day	43% 7% 2 7% 3 4 4 3 3 4 21%	3161
	21st to end	Day	39% 17% 3 8% 4 3 3 2 3 4 14%	3162
November	1st to 10th	Day	46% 12% 3 6% 4 3 3 3 2 4 15%	1568
	11th to 20th	Day	47% 13% 2 5% 2 2 2 2 2 4 19%	1600
	21st to end	Day	44% 13% 2 4 3 3 3 3 4 4 17%	1600
December	1st to 10th	Day	28% 12% 3 6% 5% 4 5% 3 4 5% 26%	2602
	11th to 20th	Day	49% 17% 4 6% 3 2 2 2 2 2 11%	2645
	21st to end	Day	47% 20% 3 7% 4 3 3 2 2 2 7%	3498
January	1st to 10th	Night	40% 18% 3 7% 3 3 3 2 2 3 15%	6766
	11th to 20th	Night	24% 10% 2 6% 4 3 4 3 5% 7% 32%	6722
	21st to end	Night	43% 11% 2 3 2 2 2 2 2 3 28%	6935
February	1st to 10th	Night	36% 14% 4 6% 4 4 3 2 4 5% 18%	2600
	11th to 20th	Night	30% 19% 3 7% 3 3 2 2 4 3 23%	2532
	21st to end	Night	32% 15% 3 7% 4 3 3 3 4 3 23%	2115
March	1st to 10th	Night	28% 23% 4 6% 4 3 3 2 3 4 22%	5254
	11th to 20th	Night	30% 20% 4 5% 3 2 3 2 3 4 24%	4999
	21st to end	Night	44% 16% 3 5% 3 3 2 2 3 3 16%	5319
April	1st to 10th	Night	32% 14% 3 8% 4 4 3 3 5% 5% 19%	2000
	11th to 20th	Night	26% 13% 3 7% 5% 4 4 3 4 5% 27%	1892
	21st to end	Night	30% 13% 3 6% 4 4 4 2 5% 6% 23%	1722
May	1st to 10th	Night	19% 12% 3 5% 3 2 2 2 2 4 45%	3640
	11th to 20th	Night	23% 10% 3 6% 3 4 4 4 5% 7% 31%	3616
	21st to end	Night	26% 10% 3 7% 4 4 4 4 5% 6% 26%	3863
June	1st to 10th	Night	27% 8% 2 8% 5% 4 3 3 5% 6% 30%	1400
	11th to 20th	Night	25% 13% 3 7% 5% 4 4 4 5% 6% 25%	1400
	21st to end	Night	22% 8% 2 5% 3 4 4 4 5% 6% 37%	1400
July	1st to 10th	Night	8% 4 1 3 3 2 2 2 5% 5% 63%	2255
	11th to 20th	Night	7% 3 1 3 2 2 2 2 3 4 74%	2220
	21st to end	Night	13% 4 1 4 3 3 4 3 5% 6% 54%	2696
August	1st to 10th	Night	6% 2 1 2 2 2 2 3 5% 6% 69%	2529
	11th to 20th	Night	14% 7% 2 5% 3 2 4 3 5% 7% 47%	2807
	21st to end	Night	20% 8% 2 5% 4 3 4 3 5% 6% 39%	3269
September	1st to 10th	Night	14% 5% 2 5% 3 4 4 3 3 6% 51%	1900
	11th to 20th	Night	25% 7% 2 6% 3 3 3 3 3 4 40%	1900
	21st to end	Night	29% 8% 2 5% 3 4 3 2 4 5% 35%	2175
October	1st to 10th	Night	28% 14% 3 7% 4 4 4 3 4 4 26%	3800
	11th to 20th	Night	40% 10% 2 5% 3 3 3 3 4 4 24%	3777
	21st to end	Night	39% 16% 4 6% 3 2 2 2 2 4 20%	4394
November	1st to 10th	Night	46% 12% 2 5% 4 3 3 2 3 4 16%	2730
	11th to 20th	Night	44% 13% 2 5% 3 3 2 2 3 3 20%	2800
	21st to end	Night	41% 13% 3 5% 3 3 3 2 3 4 20%	2800
December	1st to 10th	Night	30% 12% 3 5% 3 3 3 3 5% 5% 28%	4560
	11th to 20th	Night	47% 17% 3 6% 3 3 2 2 2 2 11%	5560
	21st to end	Night	51% 19% 3 5% 3 2 2 1 2 2 9%	7458

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	77% 6% 6% 8% 2 1	3179
	11th to 20th	Day	54% 6% 10% 17% 9% 4	3195
	21st to end	Day	68% 2 5% 16% 8%	4020
February	1st to 10th	Day	72% 5% 7% 8% 5% 3	1800
	11th to 20th	Day	68% 6% 7% 11% 7% 2	1764
	21st to end	Day	72% 5% 7% 10% 4 2	1515
March	1st to 10th	Day	73% 8% 5% 10% 3	4637
	11th to 20th	Day	68% 6% 5% 17% 4	5000
	21st to end	Day	74% 6% 9% 9% 2	5656
April	1st to 10th	Day	73% 7% 10% 8% 2	2400
	11th to 20th	Day	65% 7% 7% 12% 8% 2	2454
	21st to end	Day	66% 7% 9% 11% 6%	2677
May	1st to 10th	Day	48% 9% 14% 22% 8%	6320
	11th to 20th	Day	58% 13% 14% 13% 2	6280
	21st to end	Day	59% 11% 15% 14% 2	6996
June	1st to 10th	Day	60% 7% 15% 12% 5% 2	3000
	11th to 20th	Day	69% 8% 11% 9% 3	3000
	21st to end	Day	54% 7% 15% 15% 7% 1	3000
July	1st to 10th	Day	29% 6% 10% 26% 21% 8%	4742
	11th to 20th	Day	23% 4 12% 41% 19% 2	4659
	21st to end	Day	36% 6% 21% 25% 10% 2	4700
August	1st to 10th	Day	24% 5% 11% 42% 17% 1	4384
	11th to 20th	Day	46% 7% 19% 18% 8% 2	4067
	21st to end	Day	49% 8% 15% 19% 9% 1	4401
September	1st to 10th	Day	44% 6% 11% 23% 11% 4	2280
	11th to 20th	Day	51% 7% 9% 18% 13% 2	2280
	21st to end	Day	54% 8% 12% 15% 9% 2	2002
October	1st to 10th	Day	60% 5% 13% 16% 6%	3180
	11th to 20th	Day	66% 6% 9% 12% 6% 1	3161
	21st to end	Day	73% 6% 6% 10% 5% 1	3162
November	1st to 10th	Day	75% 5% 7% 8% 4 1	1568
	11th to 20th	Day	72% 4 7% 8% 6% 2	1600
	21st to end	Day	71% 5% 5% 10% 8% 1	1600
December	1st to 10th	Day	59% 7% 7% 16% 10% 1	2602
	11th to 20th	Day	82% 4 5% 7% 1 1	2648
	21st to end	Day	84% 6% 4 5% 1	3498
January	1st to 10th	Night	75% 5% 5% 10% 4	6766
	11th to 20th	Night	50% 7% 12% 19% 10% 3	6763
	21st to end	Night	64% 3 7% 18% 7% 1	6935
February	1st to 10th	Night	70% 5% 9% 8% 5% 3	2600
	11th to 20th	Night	67% 6% 6% 11% 8% 2	2534
	21st to end	Night	65% 6% 8% 13% 6% 2	2115
March	1st to 10th	Night	68% 5% 8% 11% 8%	5254
	11th to 20th	Night	65% 7% 8% 15% 5%	5000
	21st to end	Night	74% 5% 12% 7% 2	5319
April	1st to 10th	Night	68% 7% 13% 11% 2	2000
	11th to 20th	Night	60% 7% 10% 15% 7% 2	1892
	21st to end	Night	62% 7% 11% 13% 6% 1	1722
May	1st to 10th	Night	44% 5% 15% 24% 11% 1	3637
	11th to 20th	Night	50% 13% 18% 16% 4	3616
	21st to end	Night	57% 9% 16% 16% 2	3863
June	1st to 10th	Night	56% 7% 17% 13% 6% 3	1400
	11th to 20th	Night	59% 10% 13% 15% 3	1400
	21st to end	Night	46% 9% 19% 16% 7% 3	1400
July	1st to 10th	Night	23% 6% 12% 32% 20% 8%	2220
	11th to 20th	Night	18% 4 12% 43% 23% 1	2178
	21st to end	Night	29% 5% 17% 29% 17% 3	2611
August	1st to 10th	Night	17% 5% 16% 38% 23% 2	2521
	11th to 20th	Night	36% 7% 27% 18% 8% 3	2834
	21st to end	Night	45% 7% 18% 21% 9% 1	3269
September	1st to 10th	Night	35% 8% 15% 23% 14% 5%	1900
	11th to 20th	Night	48% 5% 11% 16% 15% 4	1900
	21st to end	Night	53% 6% 13% 18% 8% 2	2175
October	1st to 10th	Night	61% 5% 14% 13% 6%	3800
	11th to 20th	Night	63% 5% 8% 15% 8%	3777
	21st to end	Night	70% 4 6% 13% 6%	4394
November	1st to 10th	Night	74% 5% 8% 8% 4 1	2730
	11th to 20th	Night	71% 5% 7% 9% 6% 2	2800
	21st to end	Night	70% 5% 7% 8% 9% 1	2800
December	1st to 10th	Night	59% 5% 11% 18% 6% 1	4560
	11th to 20th	Night	81% 4 5% 7% 3 1	5556
	21st to end	Night	83% 4 3 6% 3 1	7458

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	48% 10% 20% 13% 8% 3	200
	11th to 20th	Day	49% 9% 15% 19% 8%	200
	21st to end	Day	49% 7% 19% 17% 8%	223
February	1st to 10th	Day	46% 12% 19% 17% 5% 1	400
	11th to 20th	Day	43% 13% 18% 22% 3 1	395
	21st to end	Day	41% 15% 23% 15% 6%	347
March	1st to 10th	Day	41% 14% 20% 18% 6% 1	500
	11th to 20th	Day	42% 13% 17% 19% 9% 1	500
	21st to end	Day	47% 14% 18% 15% 6% 1	550
April	1st to 10th	Day	41% 15% 18% 19% 6% 1	500
	11th to 20th	Day	32% 14% 26% 17% 9% 1	492
	21st to end	Day	34% 15% 23% 17% 11% 1	500
May	1st to 10th	Day	27% 14% 24% 25% 10% 1	503
	11th to 20th	Day	31% 10% 21% 24% 11% 4	528
	21st to end	Day	27% 15% 24% 22% 9% 4	613
June	1st to 10th	Day	28% 13% 25% 24% 8% 2	730
	11th to 20th	Day	32% 14% 23% 22% 8% 2	730
	21st to end	Day	25% 11% 22% 28% 12% 2	730
July	1st to 10th	Day	20% 8% 27% 31% 11% 3	697
	11th to 20th	Day	15% 9% 26% 34% 12% 4	668
	21st to end	Day	13% 7% 25% 36% 12% 6%	568
August	1st to 10th	Day	17% 9% 19% 34% 14% 7%	487
	11th to 20th	Day	14% 5% 23% 35% 17% 5%	477
	21st to end	Day	20% 12% 24% 28% 12% 3	539
September	1st to 10th	Day	24% 10% 23% 29% 13% 2	490
	11th to 20th	Day	30% 9% 19% 26% 13% 2	490
	21st to end	Day	30% 13% 19% 26% 11% 3	490
October	1st to 10th	Day	33% 9% 21% 22% 13% 2	490
	11th to 20th	Day	43% 8% 14% 20% 12% 3	490
	21st to end	Day	43% 12% 19% 17% 8% 2	389
November	1st to 10th	Day	47% 10% 17% 21% 6%	200
	11th to 20th	Day	56% 8% 10% 12% 13% 3	200
	21st to end	Day	51% 9% 18% 10% 8% 6%	200
December	1st to 10th	Day	48% 11% 12% 8% 18% 5%	200
	11th to 20th	Day	57% 9% 13% 16% 4 3	200
	21st to end	Day	52% 10% 17% 16% 4	220
January	1st to 10th	Night	54% 10% 19% 13% 4	900
	11th to 20th	Night	46% 6% 22% 23% 3	897
	21st to end	Night	46% 10% 19% 21% 4	880
February	1st to 10th	Night	44% 9% 25% 18% 4	800
	11th to 20th	Night	41% 9% 22% 21% 5%	784
	21st to end	Night	40% 11% 24% 18% 7%	660
March	1st to 10th	Night	40% 9% 21% 21% 8% 1	800
	11th to 20th	Night	45% 9% 18% 21% 5%	800
	21st to end	Night	46% 9% 17% 20% 8% 1	880
April	1st to 10th	Night	39% 8% 21% 24% 9% 1	800
	11th to 20th	Night	31% 10% 26% 23% 9% 1	704
	21st to end	Night	37% 7% 21% 25% 9%	522
May	1st to 10th	Night	23% 7% 22% 31% 17% 1	400
	11th to 20th	Night	24% 4 13% 34% 22% 2	394
	21st to end	Night	25% 5% 17% 32% 20%	433
June	1st to 10th	Night	29% 5% 16% 33% 16% 2	400
	11th to 20th	Night	25% 10% 17% 27% 19% 2	400
	21st to end	Night	23% 5% 16% 31% 23% 3	400
July	1st to 10th	Night	13% 6% 10% 36% 33% 3	380
	11th to 20th	Night	12% 4 10% 33% 37% 5%	368
	21st to end	Night	8% 3 7% 33% 45% 3	416
August	1st to 10th	Night	11% 2 6% 29% 46% 6%	378
	11th to 20th	Night	10% 2 8% 36% 40% 5%	559
	21st to end	Night	19% 3 12% 35% 29% 2	698
September	1st to 10th	Night	15% 3 16% 37% 27% 1	760
	11th to 20th	Night	26% 4 14% 26% 26% 3	760
	21st to end	Night	33% 6% 13% 32% 16% 1	760
October	1st to 10th	Night	32% 4 19% 28% 16% 1	760
	11th to 20th	Night	43% 5% 18% 22% 11% 1	760
	21st to end	Night	43% 8% 22% 20% 7% 1	836
November	1st to 10th	Night	51% 9% 21% 17% 2	780
	11th to 20th	Night	52% 7% 20% 15% 6% 1	800
	21st to end	Night	47% 10% 20% 17% 7% 1	800
December	1st to 10th	Night	39% 9% 23% 23% 6%	800
	11th to 20th	Night	55% 11% 19% 13% 2	900
	21st to end	Night	52% 11% 24% 12% 1	990

Page generated at 2025-07-02 22:30:13. Version ac07

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