Rideau Ferry Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rideau Ferry Observatory clear sky chart from 2005-11-06 to 2025-08-05.

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	17% 10% 3 8% 5% 4 4 4 5% 7% 32%	126147
	Night	17% 11% 4 9% 5% 4 4 3 5% 6% 31%	121894
Transparency	Day	51% 4 12% 21% 11% 1	129638
	Night	53% 4 11% 20% 10% 1	126490
Seeing	Day	23% 16% 31% 19% 8% 2	15414
	Night	23% 14% 30% 24% 9% 1	24506

By Month

Percentage Cloud Cover NE (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	14% 17% 6% 12% 7% 5% 4 3 4 4 23%	3597
	11th to 20th	Day	12% 17% 7% 14% 6% 5% 5% 3 4 5% 22%	3570
	21st to end	Day	14% 17% 6% 13% 7% 6% 5% 3 4 5% 20%	4218
February	1st to 10th	Day	16% 14% 4 12% 6% 4 5% 4 5% 5% 23%	1800
	11th to 20th	Day	13% 16% 5% 11% 7% 5% 5% 2 4 5% 27%	1753
	21st to end	Day	20% 14% 5% 9% 6% 6% 4 3 5% 5% 23%	1649
March	1st to 10th	Day	19% 14% 4 9% 6% 4 4 3 4 5% 27%	4537
	11th to 20th	Day	11% 13% 4 8% 6% 5% 6% 4 6% 7% 32%	4980
	21st to end	Day	18% 11% 3 7% 4 4 3 3 3 4 38%	5734
April	1st to 10th	Day	20% 13% 4 6% 5% 4 4 3 4 6% 32%	2400
	11th to 20th	Day	21% 10% 3 7% 4 4 3 2 4 5% 36%	2374
	21st to end	Day	23% 9% 3 8% 5% 3 4 3 5% 5% 32%	2510
May	1st to 10th	Day	25% 9% 3 8% 5% 4 4 4 5% 6% 26%	6346
	11th to 20th	Day	17% 9% 3 8% 5% 5% 5% 3 5% 7% 33%	6318
	21st to end	Day	20% 8% 3 7% 4 5% 4 3 5% 7% 34%	6949
June	1st to 10th	Day	21% 8% 3 7% 5% 5% 5% 4 5% 7% 31%	2800
	11th to 20th	Day	15% 5% 3 6% 5% 5% 5% 4 6% 9% 38%	2800
	21st to end	Day	17% 8% 3 7% 5% 6% 5% 4 5% 7% 33%	2800
July	1st to 10th	Day	13% 6% 3 7% 5% 5% 5% 4 7% 9% 36%	6360
	11th to 20th	Day	11% 4 2 6% 5% 4 5% 4 6% 9% 43%	6360
	21st to end	Day	10% 5% 2 5% 4 4 5% 4 7% 10% 44%	6926
August	1st to 10th	Day	17% 7% 3 7% 5% 4 5% 4 5% 7% 37%	4333
	11th to 20th	Day	18% 6% 3 5% 4 4 4 4 6% 9% 38%	3994
	21st to end	Day	15% 5% 2 6% 5% 4 4 4 6% 8% 40%	4180
September	1st to 10th	Day	14% 5% 2 7% 4 5% 5% 5% 5% 7% 42%	2280
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 6% 7% 47%	2280
	21st to end	Day	18% 6% 2 7% 5% 4 4 3 5% 6% 40%	2021
October	1st to 10th	Day	20% 8% 3 7% 4 4 3 3 5% 5% 37%	1900
	11th to 20th	Day	21% 10% 3 8% 5% 4 4 4 5% 7% 30%	1900
	21st to end	Day	25% 11% 3 7% 6% 4 3 4 5% 5% 27%	1964
November	1st to 10th	Day	15% 9% 2 7% 6% 5% 6% 4 5% 6% 33%	1761
	11th to 20th	Day	17% 13% 4 9% 6% 4 4 3 5% 6% 30%	1607
	21st to end	Day	21% 16% 4 9% 6% 5% 5% 3 5% 5% 21%	1600
December	1st to 10th	Day	22% 18% 5% 10% 6% 4 5% 3 5% 5% 16%	2600
	11th to 20th	Day	18% 18% 5% 11% 6% 4 4 3 3 4 24%	2986
	21st to end	Day	22% 17% 5% 11% 5% 4 3 3 3 4 22%	3960
January	1st to 10th	Night	14% 17% 8% 12% 7% 5% 4 3 4 5% 22%	6349
	11th to 20th	Night	12% 17% 7% 16% 7% 6% 5% 3 4 5% 20%	6315
	21st to end	Night	16% 17% 6% 13% 7% 6% 4 3 4 4 20%	6720
February	1st to 10th	Night	15% 16% 5% 10% 6% 5% 4 4 6% 6% 23%	2600
	11th to 20th	Night	14% 15% 5% 10% 6% 5% 4 3 4 5% 28%	2533
	21st to end	Night	15% 15% 5% 8% 6% 4 4 3 5% 6% 28%	1980
March	1st to 10th	Night	22% 14% 4 9% 4 5% 4 3 3 3 30%	5300
	11th to 20th	Night	14% 13% 3 7% 4 5% 5% 4 6% 8% 31%	4979
	21st to end	Night	18% 11% 3 8% 4 4 4 3 5% 6% 34%	5197
April	1st to 10th	Night	21% 10% 4 8% 4 4 4 3 5% 5% 32%	2000
	11th to 20th	Night	21% 9% 3 7% 5% 4 3 3 4 5% 37%	1975
	21st to end	Night	20% 11% 3 7% 5% 4 4 3 5% 6% 33%	1904
May	1st to 10th	Night	25% 9% 3 8% 5% 4 5% 4 5% 6% 25%	3594
	11th to 20th	Night	15% 8% 3 7% 5% 4 4 4 5% 7% 40%	3575
	21st to end	Night	18% 9% 3 6% 4 4 4 4 6% 7% 34%	3926
June	1st to 10th	Night	17% 6% 2 7% 5% 5% 6% 4 6% 7% 34%	1600
	11th to 20th	Night	15% 6% 2 6% 4 4 4 4 6% 7% 41%	1600
	21st to end	Night	17% 6% 3 7% 5% 4 5% 5% 6% 8% 35%	1600
July	1st to 10th	Night	13% 6% 2 8% 6% 4 5% 5% 7% 9% 36%	3600
	11th to 20th	Night	13% 5% 2 5% 4 4 5% 4 6% 7% 45%	3600
	21st to end	Night	10% 5% 2 6% 4 4 4 4 6% 7% 48%	3926
August	1st to 10th	Night	11% 6% 2 7% 4 5% 4 3 6% 7% 44%	2682
	11th to 20th	Night	12% 7% 3 6% 4 5% 4 3 6% 7% 43%	2802
	21st to end	Night	11% 4 2 6% 4 5% 5% 4 6% 9% 43%	3476
September	1st to 10th	Night	12% 6% 2 7% 5% 4 3 3 4 7% 47%	1900
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 5% 7% 52%	1900
	21st to end	Night	17% 6% 2 6% 4 4 3 3 5% 7% 43%	2157
October	1st to 10th	Night	17% 8% 3 7% 4 4 4 3 5% 7% 37%	2280
	11th to 20th	Night	22% 8% 2 7% 5% 4 4 4 5% 7% 33%	2280
	21st to end	Night	23% 10% 3 7% 5% 4 4 3 5% 6% 28%	2633
November	1st to 10th	Night	18% 11% 3 6% 4 4 4 4 4 5% 36%	2537
	11th to 20th	Night	17% 12% 4 7% 5% 5% 4 4 5% 5% 33%	2790
	21st to end	Night	21% 14% 4 8% 6% 4 5% 3 5% 7% 24%	2800
December	1st to 10th	Night	19% 19% 5% 11% 7% 5% 4 3 4 5% 17%	4580
	11th to 20th	Night	23% 14% 4 9% 5% 4 4 3 4 4 26%	5208
	21st to end	Night	26% 17% 4 12% 5% 3 3 3 4 4 19%	6996

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	64% 2 12% 15% 5% 1	3598
	11th to 20th	Day	66% 3 10% 12% 8% 1	3600
	21st to end	Day	67% 4 9% 10% 9% 1	4218
February	1st to 10th	Day	62% 4 8% 19% 6% 1	1800
	11th to 20th	Day	59% 3 10% 17% 10% 1	1762
	21st to end	Day	65% 3 10% 12% 9% 1	1649
March	1st to 10th	Day	61% 3 8% 14% 11% 3	4540
	11th to 20th	Day	52% 4 17% 15% 11% 2	4980
	21st to end	Day	51% 3 5% 20% 18% 3	5734
April	1st to 10th	Day	54% 4 6% 19% 13% 3	2400
	11th to 20th	Day	52% 2 7% 20% 17% 2	2374
	21st to end	Day	55% 3 8% 17% 14% 4	2510
May	1st to 10th	Day	59% 4 8% 18% 9% 2	6357
	11th to 20th	Day	52% 4 12% 20% 12% 1	6322
	21st to end	Day	49% 4 12% 22% 11% 1	6949
June	1st to 10th	Day	53% 4 9% 20% 13% 1	2800
	11th to 20th	Day	43% 5% 11% 26% 13% 2	2800
	21st to end	Day	49% 5% 13% 20% 11% 1	2800
July	1st to 10th	Day	42% 6% 14% 27% 11%	6290
	11th to 20th	Day	36% 6% 14% 29% 15%	6285
	21st to end	Day	34% 6% 13% 37% 10%	6728
August	1st to 10th	Day	47% 4 15% 23% 11% 1	4319
	11th to 20th	Day	42% 5% 15% 28% 10% 1	4033
	21st to end	Day	40% 5% 15% 28% 11%	4180
September	1st to 10th	Day	41% 5% 13% 28% 13% 1	2280
	11th to 20th	Day	36% 5% 12% 31% 16%	2280
	21st to end	Day	45% 5% 13% 26% 11%	2021
October	1st to 10th	Day	44% 4 13% 31% 8%	3160
	11th to 20th	Day	46% 4 15% 26% 9%	3140
	21st to end	Day	49% 5% 16% 23% 7%	3215
November	1st to 10th	Day	50% 4 15% 22% 8% 1	1761
	11th to 20th	Day	56% 3 12% 17% 10% 1	1607
	21st to end	Day	65% 5% 12% 11% 7%	1600
December	1st to 10th	Day	70% 4 14% 9% 2	2600
	11th to 20th	Day	66% 4 10% 14% 6%	2986
	21st to end	Day	67% 3 8% 16% 6%	3960
January	1st to 10th	Night	67% 2 12% 14% 5%	6349
	11th to 20th	Night	69% 3 11% 11% 6%	6360
	21st to end	Night	68% 4 8% 9% 11% 1	6720
February	1st to 10th	Night	62% 4 9% 17% 6% 1	2600
	11th to 20th	Night	59% 3 9% 20% 8% 1	2535
	21st to end	Night	58% 4 8% 19% 10% 1	1980
March	1st to 10th	Night	60% 2 7% 17% 11% 3	5310
	11th to 20th	Night	50% 4 18% 16% 8% 3	4980
	21st to end	Night	52% 4 6% 23% 14% 2	5197
April	1st to 10th	Night	54% 3 7% 21% 13% 2	2000
	11th to 20th	Night	51% 3 7% 20% 17% 1	1975
	21st to end	Night	53% 4 7% 17% 16% 3	1904
May	1st to 10th	Night	60% 3 9% 17% 8% 3	3600
	11th to 20th	Night	45% 4 14% 21% 14% 1	3576
	21st to end	Night	49% 4 13% 19% 14% 1	3927
June	1st to 10th	Night	48% 5% 12% 22% 12% 1	1600
	11th to 20th	Night	41% 5% 12% 23% 18% 2	1600
	21st to end	Night	46% 7% 13% 22% 12% 1	1600
July	1st to 10th	Night	41% 6% 17% 19% 15% 1	3560
	11th to 20th	Night	36% 6% 14% 32% 12%	3552
	21st to end	Night	34% 6% 13% 31% 15% 1	3809
August	1st to 10th	Night	39% 6% 13% 27% 13% 1	2672
	11th to 20th	Night	40% 4 15% 31% 9%	2829
	21st to end	Night	36% 7% 16% 28% 11% 1	3476
September	1st to 10th	Night	37% 6% 11% 31% 14% 1	1900
	11th to 20th	Night	33% 4 14% 35% 12% 1	1900
	21st to end	Night	41% 4 11% 31% 11% 1	2157
October	1st to 10th	Night	41% 5% 11% 30% 12%	3800
	11th to 20th	Night	44% 4 15% 27% 9%	3779
	21st to end	Night	48% 5% 15% 24% 8%	4332
November	1st to 10th	Night	49% 5% 13% 22% 10% 1	2537
	11th to 20th	Night	53% 4 12% 20% 11% 1	2790
	21st to end	Night	61% 5% 11% 17% 5%	2800
December	1st to 10th	Night	70% 4 13% 11% 3	4580
	11th to 20th	Night	62% 5% 9% 14% 10% 1	5208
	21st to end	Night	69% 3 7% 12% 9%	6996

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% 21% 38% 12% 4 1	200
	11th to 20th	Day	32% 19% 32% 15% 3 1	200
	21st to end	Day	25% 22% 32% 17% 4	277
February	1st to 10th	Day	33% 22% 33% 11% 1	300
	11th to 20th	Day	31% 27% 33% 7% 1	300
	21st to end	Day	33% 19% 36% 10% 2	413
March	1st to 10th	Day	28% 20% 33% 15% 4	500
	11th to 20th	Day	30% 19% 33% 14% 4	500
	21st to end	Day	32% 21% 28% 12% 7%	550
April	1st to 10th	Day	30% 23% 29% 12% 6%	500
	11th to 20th	Day	32% 20% 23% 14% 8% 2	492
	21st to end	Day	29% 21% 32% 12% 5% 2	500
May	1st to 10th	Day	25% 17% 31% 19% 6% 2	503
	11th to 20th	Day	18% 17% 35% 20% 8% 2	527
	21st to end	Day	20% 15% 30% 23% 9% 2	573
June	1st to 10th	Day	23% 14% 34% 21% 7% 2	530
	11th to 20th	Day	14% 18% 33% 22% 11% 2	530
	21st to end	Day	20% 14% 32% 21% 10% 3	530
July	1st to 10th	Day	13% 8% 33% 30% 13% 3	525
	11th to 20th	Day	13% 11% 31% 27% 14% 3	524
	21st to end	Day	13% 9% 32% 30% 11% 4	576
August	1st to 10th	Day	16% 9% 29% 29% 14% 4	502
	11th to 20th	Day	15% 6% 27% 29% 15% 6%	479
	21st to end	Day	16% 10% 33% 24% 11% 7%	539
September	1st to 10th	Day	14% 11% 32% 24% 12% 7%	490
	11th to 20th	Day	12% 9% 24% 25% 19% 11%	490
	21st to end	Day	20% 11% 26% 25% 14% 3	490
October	1st to 10th	Day	26% 16% 25% 18% 11% 4	490
	11th to 20th	Day	28% 20% 28% 16% 7% 2	490
	21st to end	Day	29% 21% 26% 19% 4	472
November	1st to 10th	Day	27% 17% 30% 18% 7%	395
	11th to 20th	Day	26% 18% 35% 12% 6% 2	207
	21st to end	Day	31% 23% 33% 11% 3	200
December	1st to 10th	Day	31% 23% 30% 14% 3	200
	11th to 20th	Day	36% 24% 29% 8% 4 1	200
	21st to end	Day	33% 17% 32% 13% 5%	220
January	1st to 10th	Night	28% 20% 32% 17% 3	800
	11th to 20th	Night	29% 17% 29% 20% 4 1	800
	21st to end	Night	20% 17% 34% 24% 5%	880
February	1st to 10th	Night	25% 21% 33% 19% 2	800
	11th to 20th	Night	23% 20% 37% 17% 3	783
	21st to end	Night	25% 19% 33% 19% 4	660
March	1st to 10th	Night	27% 16% 32% 22% 4	800
	11th to 20th	Night	26% 20% 29% 20% 4 1	800
	21st to end	Night	24% 17% 31% 21% 7%	880
April	1st to 10th	Night	28% 16% 31% 23% 3	800
	11th to 20th	Night	26% 20% 29% 18% 7% 1	791
	21st to end	Night	26% 14% 27% 24% 8% 1	704
May	1st to 10th	Night	25% 11% 26% 27% 10% 2	400
	11th to 20th	Night	16% 11% 34% 32% 7%	394
	21st to end	Night	13% 11% 37% 27% 11%	433
June	1st to 10th	Night	16% 9% 29% 36% 10% 1	400
	11th to 20th	Night	17% 10% 31% 30% 12% 1	400
	21st to end	Night	19% 6% 30% 33% 13% 1	400
July	1st to 10th	Night	9% 7% 32% 36% 15% 1	390
	11th to 20th	Night	13% 5% 28% 38% 16% 1	388
	21st to end	Night	10% 5% 29% 41% 13% 1	438
August	1st to 10th	Night	12% 6% 26% 38% 18% 1	465
	11th to 20th	Night	11% 5% 22% 35% 24% 2	568
	21st to end	Night	11% 8% 30% 32% 16% 3	836
September	1st to 10th	Night	14% 7% 32% 30% 16% 1	760
	11th to 20th	Night	16% 7% 21% 28% 23% 5%	760
	21st to end	Night	19% 10% 25% 26% 18% 2	760
October	1st to 10th	Night	22% 13% 26% 22% 15% 2	760
	11th to 20th	Night	26% 12% 30% 23% 8% 1	760
	21st to end	Night	30% 14% 27% 23% 6%	836
November	1st to 10th	Night	28% 14% 29% 21% 8%	780
	11th to 20th	Night	25% 17% 32% 17% 9%	800
	21st to end	Night	30% 18% 28% 19% 6%	800
December	1st to 10th	Night	30% 18% 31% 17% 4	800
	11th to 20th	Night	28% 19% 31% 19% 3 1	800
	21st to end	Night	33% 16% 29% 19% 3	880

Page generated at 2025-08-02 01:32:26. Version ac07

Copyright © 2025 Attilla Danko.