Markham
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Markham clear sky chart from 2005-11-06 to 2025-04-11.

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	14% 9% 3 9% 5% 5% 5% 4 6% 7% 32%	113614
	Night	14% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	114646
Transparency	Day	48% 6% 12% 20% 11% 1	117251
	Night	48% 6% 12% 21% 12% 1	119172
Seeing	Day	18% 9% 27% 27% 14% 5%	14883
	Night	19% 8% 29% 30% 13% 1	24431

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	13% 9% 3 11% 6% 5% 5% 4 5% 7% 29%	3597
	11th to 20th	Day	10% 16% 7% 18% 9% 7% 5% 4 4 5% 16%	3870
	21st to end	Day	16% 16% 6% 14% 7% 6% 6% 4 4 5% 18%	4466
February	1st to 10th	Day	15% 15% 5% 12% 6% 6% 5% 4 5% 5% 21%	1800
	11th to 20th	Day	12% 15% 7% 12% 7% 4 5% 3 5% 7% 25%	1753
	21st to end	Day	17% 14% 4 11% 7% 4 5% 3 4 5% 26%	1540
March	1st to 10th	Day	19% 10% 4 9% 5% 4 5% 3 5% 5% 32%	4932
	11th to 20th	Day	10% 11% 4 9% 5% 5% 6% 4 6% 7% 31%	4980
	21st to end	Day	18% 11% 3 8% 5% 4 4 3 5% 5% 33%	5465
April	1st to 10th	Day	17% 12% 4 9% 5% 4 4 4 6% 6% 29%	2369
	11th to 20th	Day	19% 10% 3 7% 5% 5% 3 3 5% 6% 35%	2319
	21st to end	Day	20% 8% 2 8% 5% 5% 4 3 5% 6% 33%	2470
May	1st to 10th	Day	18% 8% 3 8% 5% 4 4 4 6% 8% 32%	4108
	11th to 20th	Day	15% 9% 4 8% 5% 5% 4 4 6% 8% 32%	4393
	21st to end	Day	11% 5% 2 6% 4 4 4 4 6% 8% 46%	4837
June	1st to 10th	Day	14% 7% 2 6% 6% 5% 6% 4 7% 8% 35%	2660
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 5% 7% 10% 39%	2660
	21st to end	Day	14% 6% 2 6% 5% 5% 5% 4 6% 8% 38%	2660
July	1st to 10th	Day	10% 5% 2 6% 5% 5% 6% 5% 8% 10% 38%	4440
	11th to 20th	Day	10% 4 2 5% 4 5% 5% 4 7% 10% 44%	4440
	21st to end	Day	7% 5% 2 7% 5% 5% 6% 5% 8% 9% 40%	4534
August	1st to 10th	Day	16% 7% 3 6% 4 4 4 4 7% 9% 36%	4104
	11th to 20th	Day	10% 7% 3 7% 5% 5% 6% 5% 7% 10% 36%	4044
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 4 6% 10% 41%	4350
September	1st to 10th	Day	13% 5% 2 7% 5% 4 5% 5% 6% 7% 43%	2280
	11th to 20th	Day	11% 5% 3 6% 4 4 5% 4 6% 8% 45%	2177
	21st to end	Day	13% 6% 3 9% 5% 6% 5% 4 6% 7% 37%	2045
October	1st to 10th	Day	18% 7% 3 7% 5% 4 5% 3 5% 7% 36%	1900
	11th to 20th	Day	18% 9% 3 8% 5% 5% 5% 4 6% 6% 31%	1900
	21st to end	Day	23% 11% 4 8% 6% 5% 5% 5% 6% 6% 23%	2090
November	1st to 10th	Day	13% 10% 3 10% 5% 5% 5% 4 6% 8% 32%	1685
	11th to 20th	Day	15% 11% 4 9% 5% 5% 6% 6% 6% 8% 25%	1600
	21st to end	Day	17% 13% 5% 10% 6% 6% 5% 5% 5% 8% 21%	1600
December	1st to 10th	Day	18% 16% 5% 11% 7% 6% 5% 5% 5% 7% 15%	2600
	11th to 20th	Day	19% 16% 4 9% 6% 5% 4 3 6% 5% 22%	2986
	21st to end	Day	25% 15% 6% 14% 6% 5% 4 3 4 4 14%	3960
January	1st to 10th	Night	14% 11% 4 10% 6% 5% 6% 4 6% 8% 27%	6349
	11th to 20th	Night	12% 12% 5% 14% 9% 7% 7% 4 6% 6% 18%	5997
	21st to end	Night	15% 13% 5% 11% 6% 6% 5% 4 6% 7% 23%	6490
February	1st to 10th	Night	13% 15% 4 10% 7% 5% 5% 4 6% 6% 25%	2600
	11th to 20th	Night	14% 12% 5% 10% 7% 5% 5% 4 6% 6% 26%	2533
	21st to end	Night	13% 12% 4 10% 6% 5% 5% 4 6% 6% 29%	2088
March	1st to 10th	Night	15% 9% 4 9% 6% 5% 5% 4 5% 6% 32%	4908
	11th to 20th	Night	10% 8% 3 7% 5% 5% 6% 5% 7% 8% 37%	4979
	21st to end	Night	20% 8% 2 6% 4 4 4 3 4 6% 38%	5467
April	1st to 10th	Night	18% 10% 3 7% 5% 5% 5% 3 6% 7% 31%	2030
	11th to 20th	Night	16% 9% 3 6% 5% 5% 5% 4 4 6% 38%	1832
	21st to end	Night	15% 8% 2 7% 5% 4 4 4 6% 9% 36%	1710
May	1st to 10th	Night	17% 7% 3 6% 4 4 4 4 5% 6% 39%	2833
	11th to 20th	Night	14% 8% 3 6% 4 5% 4 4 5% 8% 37%	2499
	21st to end	Night	9% 4 2 5% 4 4 3 3 6% 8% 51%	2738
June	1st to 10th	Night	13% 6% 3 6% 4 4 4 3 6% 9% 41%	1520
	11th to 20th	Night	9% 5% 2 7% 5% 4 5% 4 6% 10% 43%	1520
	21st to end	Night	13% 6% 2 6% 4 4 4 4 6% 7% 44%	1520
July	1st to 10th	Night	6% 5% 2 5% 4 4 5% 4 6% 10% 50%	2520
	11th to 20th	Night	11% 4 2 4 3 4 4 3 7% 8% 51%	2520
	21st to end	Night	7% 5% 2 5% 4 4 4 4 7% 8% 51%	3015
August	1st to 10th	Night	9% 5% 2 6% 5% 5% 4 4 5% 7% 47%	2825
	11th to 20th	Night	11% 5% 2 5% 3 4 3 3 5% 8% 50%	2785
	21st to end	Night	9% 6% 2 6% 4 5% 4 4 6% 8% 47%	3274
September	1st to 10th	Night	9% 5% 2 6% 4 5% 5% 5% 6% 7% 47%	1900
	11th to 20th	Night	11% 4 2 4 4 3 3 3 5% 7% 55%	2000
	21st to end	Night	15% 7% 2 6% 4 4 4 3 6% 6% 42%	2133
October	1st to 10th	Night	15% 6% 3 8% 5% 4 4 4 6% 7% 39%	2280
	11th to 20th	Night	18% 8% 3 7% 5% 5% 5% 4 6% 7% 34%	2280
	21st to end	Night	20% 9% 3 7% 5% 4 4 4 6% 7% 30%	2508
November	1st to 10th	Night	17% 8% 3 6% 4 4 4 4 6% 6% 38%	2609
	11th to 20th	Night	17% 10% 3 8% 5% 5% 5% 5% 6% 7% 30%	2800
	21st to end	Night	19% 11% 3 8% 6% 6% 5% 4 7% 8% 25%	2800
December	1st to 10th	Night	16% 15% 5% 10% 6% 6% 6% 4 7% 7% 19%	4580
	11th to 20th	Night	15% 17% 5% 10% 6% 5% 5% 4 5% 6% 23%	5208
	21st to end	Night	25% 13% 6% 12% 6% 5% 5% 3 5% 5% 15%	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	52% 7% 14% 16% 11%	3598
	11th to 20th	Day	70% 4 10% 10% 5% 1	3904
	21st to end	Day	66% 5% 10% 11% 8%	4466
February	1st to 10th	Day	62% 6% 11% 17% 4 1	1800
	11th to 20th	Day	58% 6% 10% 17% 7% 1	1762
	21st to end	Day	57% 8% 12% 16% 6% 1	1540
March	1st to 10th	Day	53% 5% 14% 13% 14% 1	4935
	11th to 20th	Day	49% 6% 16% 18% 8% 2	4980
	21st to end	Day	50% 6% 8% 20% 14% 1	5465
April	1st to 10th	Day	54% 5% 11% 17% 11% 1	2369
	11th to 20th	Day	48% 7% 10% 21% 14% 1	2319
	21st to end	Day	50% 7% 10% 18% 12% 3	2470
May	1st to 10th	Day	47% 8% 12% 18% 14% 2	4120
	11th to 20th	Day	47% 12% 13% 18% 8% 1	4397
	21st to end	Day	33% 9% 14% 24% 17% 4	4837
June	1st to 10th	Day	42% 9% 12% 21% 15% 1	2660
	11th to 20th	Day	38% 7% 14% 24% 17% 2	2660
	21st to end	Day	39% 8% 13% 24% 14% 1	2660
July	1st to 10th	Day	38% 5% 14% 26% 15% 2	4370
	11th to 20th	Day	34% 5% 12% 36% 12% 1	4365
	21st to end	Day	38% 8% 13% 22% 18% 1	4389
August	1st to 10th	Day	44% 6% 13% 22% 14% 1	4091
	11th to 20th	Day	41% 6% 15% 27% 11%	4083
	21st to end	Day	37% 5% 11% 36% 10% 1	4350
September	1st to 10th	Day	38% 8% 13% 28% 11% 1	2280
	11th to 20th	Day	34% 8% 14% 29% 15% 1	2177
	21st to end	Day	43% 9% 13% 26% 9%	2045
October	1st to 10th	Day	39% 4 14% 27% 15% 1	3160
	11th to 20th	Day	40% 4 16% 25% 15%	3140
	21st to end	Day	53% 5% 14% 22% 6%	3428
November	1st to 10th	Day	47% 8% 14% 21% 9% 1	1685
	11th to 20th	Day	54% 9% 13% 15% 8% 1	1600
	21st to end	Day	59% 7% 15% 16% 3	1600
December	1st to 10th	Day	67% 4 15% 10% 4	2600
	11th to 20th	Day	61% 7% 12% 10% 9%	2986
	21st to end	Day	72% 6% 6% 9% 6%	3960
January	1st to 10th	Night	54% 6% 11% 22% 6% 1	6349
	11th to 20th	Night	65% 8% 10% 12% 5% 1	6038
	21st to end	Night	60% 5% 11% 12% 11% 1	6490
February	1st to 10th	Night	57% 6% 12% 18% 5% 1	2600
	11th to 20th	Night	56% 7% 10% 18% 8%	2535
	21st to end	Night	53% 5% 13% 20% 8%	2088
March	1st to 10th	Night	53% 4 10% 18% 13% 2	4918
	11th to 20th	Night	43% 5% 17% 23% 10% 2	4980
	21st to end	Night	47% 4 8% 21% 17% 2	5467
April	1st to 10th	Night	51% 6% 10% 19% 13% 1	2030
	11th to 20th	Night	47% 6% 8% 21% 16% 2	1832
	21st to end	Night	45% 6% 11% 20% 15% 2	1710
May	1st to 10th	Night	44% 6% 13% 19% 15% 3	2840
	11th to 20th	Night	43% 10% 15% 18% 11% 2	2500
	21st to end	Night	29% 8% 13% 25% 21% 3	2739
June	1st to 10th	Night	38% 6% 11% 25% 17% 2	1520
	11th to 20th	Night	36% 6% 13% 22% 22% 2	1520
	21st to end	Night	37% 7% 12% 25% 18% 1	1520
July	1st to 10th	Night	31% 4 15% 27% 21% 3	2480
	11th to 20th	Night	32% 4 13% 34% 16% 1	2472
	21st to end	Night	30% 6% 15% 31% 16% 2	2919
August	1st to 10th	Night	36% 4 14% 29% 15% 2	2814
	11th to 20th	Night	33% 5% 15% 34% 12% 1	2812
	21st to end	Night	36% 4 11% 33% 14% 1	3274
September	1st to 10th	Night	33% 8% 15% 28% 15% 1	1900
	11th to 20th	Night	29% 6% 12% 34% 19% 1	2000
	21st to end	Night	41% 7% 13% 27% 12%	2133
October	1st to 10th	Night	35% 4 8% 27% 25% 1	3800
	11th to 20th	Night	43% 4 14% 27% 11%	3779
	21st to end	Night	41% 5% 15% 28% 11% 1	4120
November	1st to 10th	Night	44% 7% 12% 24% 12% 1	2609
	11th to 20th	Night	51% 7% 14% 18% 10% 1	2800
	21st to end	Night	55% 8% 18% 16% 4	2800
December	1st to 10th	Night	62% 5% 14% 15% 4	4580
	11th to 20th	Night	60% 6% 10% 13% 11% 1	5208
	21st to end	Night	70% 6% 10% 10% 5%	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	20% 17% 34% 21% 8% 1	200
	11th to 20th	Day	21% 19% 36% 15% 8% 1	267
	21st to end	Day	23% 14% 36% 22% 4 1	330
February	1st to 10th	Day	21% 18% 40% 17% 3	300
	11th to 20th	Day	22% 20% 38% 17% 3	300
	21st to end	Day	27% 22% 27% 22% 3	274
March	1st to 10th	Day	21% 12% 38% 23% 6%	500
	11th to 20th	Day	17% 16% 35% 24% 6% 1	500
	21st to end	Day	26% 11% 34% 22% 7% 1	550
April	1st to 10th	Day	21% 13% 34% 24% 7%	500
	11th to 20th	Day	24% 12% 30% 21% 10% 3	452
	21st to end	Day	23% 9% 31% 25% 10% 2	460
May	1st to 10th	Day	20% 8% 26% 29% 12% 5%	463
	11th to 20th	Day	15% 6% 22% 34% 18% 5%	487
	21st to end	Day	12% 5% 22% 32% 22% 7%	529
June	1st to 10th	Day	14% 6% 24% 38% 15% 3	490
	11th to 20th	Day	14% 7% 23% 32% 20% 3	490
	21st to end	Day	17% 5% 20% 34% 19% 5%	490
July	1st to 10th	Day	9% 4 16% 35% 27% 8%	485
	11th to 20th	Day	13% 3 19% 32% 24% 9%	484
	21st to end	Day	8% 4 19% 39% 21% 8%	532
August	1st to 10th	Day	13% 3 16% 33% 23% 12%	486
	11th to 20th	Day	10% 3 17% 29% 29% 12%	479
	21st to end	Day	12% 5% 18% 31% 25% 9%	539
September	1st to 10th	Day	13% 4 22% 29% 21% 12%	490
	11th to 20th	Day	10% 3 16% 31% 23% 17%	490
	21st to end	Day	14% 6% 27% 29% 15% 9%	490
October	1st to 10th	Day	21% 8% 25% 25% 15% 6%	490
	11th to 20th	Day	19% 9% 37% 23% 10% 2	490
	21st to end	Day	26% 10% 33% 23% 7% 1	539
November	1st to 10th	Day	19% 16% 27% 26% 11% 1	287
	11th to 20th	Day	19% 21% 32% 23% 5% 2	200
	21st to end	Day	25% 16% 37% 19% 4	200
December	1st to 10th	Day	25% 13% 39% 18% 6% 1	200
	11th to 20th	Day	26% 19% 33% 17% 6% 1	200
	21st to end	Day	27% 15% 39% 16% 3	220
January	1st to 10th	Night	23% 12% 37% 21% 6% 1	800
	11th to 20th	Night	25% 13% 32% 25% 5%	800
	21st to end	Night	18% 10% 38% 27% 8%	880
February	1st to 10th	Night	20% 13% 38% 25% 3	800
	11th to 20th	Night	21% 12% 36% 25% 5%	783
	21st to end	Night	19% 13% 39% 24% 5%	660
March	1st to 10th	Night	19% 12% 35% 27% 7%	800
	11th to 20th	Night	20% 13% 34% 25% 6% 1	800
	21st to end	Night	20% 9% 31% 29% 9% 1	880
April	1st to 10th	Night	24% 10% 31% 29% 6%	800
	11th to 20th	Night	18% 10% 33% 28% 10% 1	704
	21st to end	Night	19% 6% 30% 33% 12%	570
May	1st to 10th	Night	18% 4 27% 33% 16% 2	570
	11th to 20th	Night	15% 3 25% 38% 19% 1	378
	21st to end	Night	10% 3 22% 41% 22% 1	411
June	1st to 10th	Night	15% 2 22% 42% 18% 1	380
	11th to 20th	Night	12% 3 25% 39% 20% 1	380
	21st to end	Night	14% 4 19% 43% 19% 1	380
July	1st to 10th	Night	6% 2 18% 44% 27% 2	370
	11th to 20th	Night	12% 3 17% 40% 24% 3	368
	21st to end	Night	7% 1 16% 47% 25% 3	578
August	1st to 10th	Night	10% 1 14% 40% 31% 4	567
	11th to 20th	Night	7% 2 16% 38% 34% 4	564
	21st to end	Night	12% 3 19% 36% 27% 3	712
September	1st to 10th	Night	11% 5% 20% 36% 23% 6%	760
	11th to 20th	Night	12% 4 18% 30% 30% 7%	760
	21st to end	Night	18% 4 20% 32% 22% 4	760
October	1st to 10th	Night	19% 6% 25% 28% 21% 2	760
	11th to 20th	Night	21% 7% 29% 31% 11% 1	760
	21st to end	Night	26% 7% 29% 32% 6%	836
November	1st to 10th	Night	21% 10% 26% 31% 11% 1	780
	11th to 20th	Night	21% 11% 33% 26% 8% 2	800
	21st to end	Night	24% 10% 33% 26% 7% 1	800
December	1st to 10th	Night	27% 10% 35% 22% 6% 1	800
	11th to 20th	Night	22% 12% 39% 23% 4	800
	21st to end	Night	31% 9% 34% 22% 3	880

Page generated at 2025-04-08 09:17:28. Version ac07

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