Welland
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Welland clear sky chart from 2005-11-06 to 2026-05-02.

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	16% 10% 4 8% 5% 5% 4 4 5% 7% 32%	131942
	Night	16% 11% 4 9% 5% 5% 4 3 5% 6% 32%	130500
Transparency	Day	50% 7% 12% 19% 10% 1	135537
	Night	52% 7% 11% 18% 11% 1	134992
Seeing	Day	20% 12% 26% 23% 14% 5%	16324
	Night	21% 10% 27% 28% 12% 1	25882

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	17% 18% 7% 12% 5% 4 4 3 4 5% 22%	4661
	11th to 20th	Day	12% 22% 8% 20% 8% 6% 4 3 4 3 9%	5168
	21st to end	Day	15% 16% 7% 15% 10% 8% 5% 3 3 4 13%	5780
February	1st to 10th	Day	17% 18% 5% 12% 8% 7% 5% 3 4 4 17%	1890
	11th to 20th	Day	15% 17% 6% 15% 7% 5% 4 4 5% 5% 19%	1843
	21st to end	Day	20% 14% 5% 12% 7% 4 5% 3 6% 5% 20%	1640
March	1st to 10th	Day	18% 11% 5% 11% 5% 4 4 3 4 5% 29%	5042
	11th to 20th	Day	11% 16% 4 7% 5% 5% 5% 3 4 7% 31%	5090
	21st to end	Day	19% 13% 4 9% 5% 4 4 3 4 5% 31%	5586
April	1st to 10th	Day	19% 14% 3 7% 4 4 4 3 5% 5% 30%	2483
	11th to 20th	Day	18% 11% 2 7% 4 4 4 3 5% 6% 35%	2539
	21st to end	Day	20% 9% 3 7% 5% 5% 4 4 4 5% 34%	2730
May	1st to 10th	Day	21% 9% 3 7% 4 5% 4 3 5% 7% 31%	5901
	11th to 20th	Day	16% 10% 4 8% 5% 4 5% 4 5% 7% 31%	6226
	21st to end	Day	18% 7% 3 6% 4 4 4 3 5% 7% 38%	6949
June	1st to 10th	Day	15% 6% 2 6% 4 4 5% 4 6% 8% 39%	2800
	11th to 20th	Day	10% 5% 3 7% 6% 4 5% 5% 7% 10% 38%	2800
	21st to end	Day	14% 5% 2 5% 4 4 4 4 6% 9% 43%	2800
July	1st to 10th	Day	9% 5% 2 5% 4 4 5% 4 6% 9% 45%	6360
	11th to 20th	Day	11% 5% 2 6% 4 5% 5% 4 6% 9% 43%	6344
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 6% 8% 47%	6424
August	1st to 10th	Day	16% 6% 2 6% 4 4 5% 4 6% 8% 39%	4234
	11th to 20th	Day	13% 5% 2 6% 5% 4 6% 5% 7% 11% 35%	4174
	21st to end	Day	11% 5% 2 6% 4 4 4 4 6% 9% 46%	4454
September	1st to 10th	Day	14% 7% 2 5% 4 3 5% 4 7% 8% 42%	2400
	11th to 20th	Day	10% 5% 2 6% 4 5% 4 4 5% 8% 48%	2291
	21st to end	Day	15% 7% 3 8% 5% 6% 4 5% 7% 7% 34%	2150
October	1st to 10th	Day	19% 7% 2 6% 4 4 4 4 5% 6% 39%	2000
	11th to 20th	Day	19% 10% 3 8% 5% 5% 5% 3 5% 6% 31%	2000
	21st to end	Day	23% 12% 4 9% 6% 5% 4 3 5% 6% 24%	2200
November	1st to 10th	Day	16% 8% 4 8% 5% 5% 5% 4 5% 6% 34%	1829
	11th to 20th	Day	15% 12% 4 7% 6% 5% 5% 3 5% 7% 31%	1680
	21st to end	Day	21% 15% 3 7% 5% 5% 4 3 6% 7% 23%	1680
December	1st to 10th	Day	21% 16% 5% 10% 6% 6% 5% 5% 5% 6% 16%	2680
	11th to 20th	Day	21% 17% 5% 9% 5% 4 3 3 5% 4 23%	3066
	21st to end	Day	26% 18% 6% 11% 6% 5% 4 3 4 4 12%	4048
January	1st to 10th	Night	16% 17% 6% 11% 6% 4 4 3 4 5% 25%	8234
	11th to 20th	Night	13% 19% 7% 15% 6% 5% 4 3 5% 6% 17%	7699
	21st to end	Night	17% 15% 6% 14% 7% 6% 4 3 4 5% 19%	8401
February	1st to 10th	Night	17% 17% 5% 11% 7% 5% 4 4 4 5% 21%	2730
	11th to 20th	Night	16% 13% 5% 11% 6% 5% 5% 4 6% 6% 23%	2663
	21st to end	Night	18% 15% 4 10% 7% 6% 5% 4 5% 6% 21%	2164
March	1st to 10th	Night	18% 10% 5% 8% 6% 5% 4 3 4 5% 32%	5018
	11th to 20th	Night	14% 10% 3 7% 4 4 4 3 5% 8% 37%	5089
	21st to end	Night	19% 9% 3 7% 4 3 4 3 4 5% 37%	5588
April	1st to 10th	Night	20% 11% 3 9% 5% 4 4 3 5% 5% 31%	2136
	11th to 20th	Night	19% 8% 3 7% 5% 4 4 3 5% 7% 35%	2048
	21st to end	Night	16% 8% 4 7% 5% 4 6% 5% 5% 6% 36%	1890
May	1st to 10th	Night	23% 11% 4 8% 4 4 4 3 4 5% 30%	4062
	11th to 20th	Night	12% 7% 3 7% 5% 6% 6% 4 6% 7% 37%	3650
	21st to end	Night	18% 6% 2 6% 4 4 4 3 5% 7% 40%	3926
June	1st to 10th	Night	12% 7% 3 6% 5% 5% 6% 5% 7% 8% 37%	1600
	11th to 20th	Night	9% 6% 2 6% 6% 5% 5% 4 6% 8% 43%	1600
	21st to end	Night	13% 6% 2 6% 5% 5% 5% 4 5% 7% 43%	1600
July	1st to 10th	Night	9% 5% 2 7% 5% 5% 4 4 6% 9% 45%	3600
	11th to 20th	Night	10% 5% 2 6% 4 5% 4 4 6% 8% 47%	3600
	21st to end	Night	9% 4 2 5% 4 4 4 4 6% 7% 50%	4427
August	1st to 10th	Night	10% 6% 2 7% 4 5% 4 4 6% 8% 44%	2915
	11th to 20th	Night	14% 7% 3 6% 4 5% 4 3 6% 8% 40%	2875
	21st to end	Night	8% 4 1 6% 4 5% 5% 5% 7% 8% 47%	3411
September	1st to 10th	Night	13% 6% 2 6% 3 4 4 3 5% 7% 47%	2000
	11th to 20th	Night	11% 4 2 5% 3 3 3 3 5% 7% 55%	2105
	21st to end	Night	15% 7% 3 8% 5% 5% 4 4 6% 6% 38%	2248
October	1st to 10th	Night	17% 6% 2 7% 4 5% 4 3 5% 6% 42%	2400
	11th to 20th	Night	21% 9% 3 7% 4 5% 4 4 5% 5% 33%	2400
	21st to end	Night	22% 9% 3 6% 5% 4 5% 4 4 6% 31%	2640
November	1st to 10th	Night	16% 9% 4 6% 4 4 4 3 4 7% 39%	2683
	11th to 20th	Night	15% 11% 4 7% 5% 5% 5% 4 5% 7% 31%	2940
	21st to end	Night	19% 13% 4 8% 5% 4 4 5% 6% 7% 25%	2940
December	1st to 10th	Night	18% 18% 5% 11% 5% 5% 4 4 6% 7% 18%	4720
	11th to 20th	Night	17% 21% 5% 9% 5% 5% 3 3 3 5% 24%	5348
	21st to end	Night	26% 17% 5% 10% 5% 5% 4 3 5% 5% 15%	7150

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	65% 8% 11% 9% 7% 1	4662
	11th to 20th	Day	79% 5% 6% 5% 4 1	5202
	21st to end	Day	74% 6% 10% 7% 4	5780
February	1st to 10th	Day	70% 6% 10% 11% 3	1890
	11th to 20th	Day	66% 7% 10% 13% 4	1852
	21st to end	Day	65% 6% 10% 13% 5%	1640
March	1st to 10th	Day	56% 6% 13% 17% 8%	5045
	11th to 20th	Day	51% 7% 16% 16% 7% 2	5090
	21st to end	Day	55% 6% 7% 22% 10% 1	5586
April	1st to 10th	Day	55% 5% 11% 18% 11% 1	2483
	11th to 20th	Day	50% 6% 11% 20% 13% 2	2539
	21st to end	Day	51% 6% 11% 18% 12% 2	2730
May	1st to 10th	Day	52% 7% 11% 16% 10% 5%	5910
	11th to 20th	Day	49% 9% 10% 19% 12% 1	6230
	21st to end	Day	44% 7% 11% 23% 12% 2	6949
June	1st to 10th	Day	41% 8% 13% 21% 15% 2	2800
	11th to 20th	Day	39% 8% 14% 25% 13% 1	2800
	21st to end	Day	37% 8% 13% 26% 15% 1	2800
July	1st to 10th	Day	33% 8% 13% 27% 17% 1	6290
	11th to 20th	Day	36% 8% 13% 28% 15% 1	6269
	21st to end	Day	32% 8% 15% 26% 18% 1	6240
August	1st to 10th	Day	40% 9% 14% 23% 13% 1	4221
	11th to 20th	Day	41% 9% 16% 24% 9% 1	4213
	21st to end	Day	34% 6% 13% 34% 12% 1	4454
September	1st to 10th	Day	39% 6% 14% 27% 12% 1	2400
	11th to 20th	Day	34% 7% 12% 31% 15% 1	2291
	21st to end	Day	46% 8% 15% 21% 9% 1	2150
October	1st to 10th	Day	35% 7% 15% 26% 16% 1	3260
	11th to 20th	Day	44% 6% 15% 21% 14% 1	3240
	21st to end	Day	51% 6% 15% 20% 8%	3538
November	1st to 10th	Day	51% 7% 12% 19% 9% 2	1829
	11th to 20th	Day	53% 7% 15% 16% 8% 1	1680
	21st to end	Day	61% 8% 14% 13% 4	1680
December	1st to 10th	Day	68% 8% 14% 9% 1	2680
	11th to 20th	Day	64% 7% 10% 10% 8% 1	3066
	21st to end	Day	75% 6% 10% 7% 3	4048
January	1st to 10th	Night	62% 8% 9% 16% 5%	8234
	11th to 20th	Night	67% 8% 10% 8% 6%	7740
	21st to end	Night	68% 6% 7% 10% 8% 1	8401
February	1st to 10th	Night	66% 6% 10% 13% 5%	2730
	11th to 20th	Night	62% 6% 12% 15% 5%	2665
	21st to end	Night	63% 7% 10% 13% 7%	2164
March	1st to 10th	Night	55% 5% 10% 19% 10% 1	5028
	11th to 20th	Night	46% 6% 17% 20% 10% 2	5090
	21st to end	Night	48% 5% 10% 20% 16% 2	5588
April	1st to 10th	Night	56% 5% 8% 18% 11% 2	2136
	11th to 20th	Night	49% 6% 10% 19% 14% 2	2048
	21st to end	Night	48% 6% 10% 19% 14% 2	1890
May	1st to 10th	Night	57% 5% 8% 13% 12% 5%	4069
	11th to 20th	Night	43% 9% 14% 21% 11% 1	3651
	21st to end	Night	43% 6% 10% 24% 16% 1	3927
June	1st to 10th	Night	41% 10% 10% 21% 16% 2	1600
	11th to 20th	Night	39% 6% 13% 23% 17% 2	1600
	21st to end	Night	39% 7% 13% 22% 17% 2	1600
July	1st to 10th	Night	35% 6% 12% 23% 22% 2	3560
	11th to 20th	Night	34% 7% 13% 30% 16% 1	3552
	21st to end	Night	32% 7% 14% 24% 23% 2	4297
August	1st to 10th	Night	36% 7% 14% 26% 14% 2	2904
	11th to 20th	Night	41% 7% 14% 27% 11%	2902
	21st to end	Night	31% 8% 14% 32% 14% 1	3411
September	1st to 10th	Night	36% 6% 14% 26% 16% 1	2000
	11th to 20th	Night	29% 7% 11% 34% 19% 1	2105
	21st to end	Night	46% 7% 11% 22% 13% 1	2248
October	1st to 10th	Night	37% 6% 9% 26% 21% 1	3920
	11th to 20th	Night	48% 5% 12% 22% 13%	3899
	21st to end	Night	41% 6% 16% 26% 10% 1	4252
November	1st to 10th	Night	46% 6% 11% 24% 12% 1	2683
	11th to 20th	Night	51% 7% 14% 17% 9% 1	2940
	21st to end	Night	59% 9% 13% 15% 4	2940
December	1st to 10th	Night	66% 9% 14% 9% 2	4720
	11th to 20th	Night	64% 6% 8% 11% 10% 1	5348
	21st to end	Night	69% 8% 11% 9% 3	7150

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	22% 20% 35% 15% 6% 1	220
	11th to 20th	Day	28% 22% 32% 12% 5% 1	316
	21st to end	Day	27% 18% 31% 17% 6% 1	363
February	1st to 10th	Day	25% 19% 36% 17% 3	330
	11th to 20th	Day	25% 25% 33% 16% 2	330
	21st to end	Day	29% 23% 31% 15% 1	315
March	1st to 10th	Day	24% 15% 34% 19% 6% 1	540
	11th to 20th	Day	24% 18% 31% 20% 5% 1	540
	21st to end	Day	28% 13% 30% 22% 6% 1	594
April	1st to 10th	Day	25% 15% 30% 21% 8% 1	540
	11th to 20th	Day	27% 16% 27% 17% 10% 3	532
	21st to end	Day	24% 11% 30% 23% 9% 2	540
May	1st to 10th	Day	24% 12% 25% 25% 10% 5%	507
	11th to 20th	Day	16% 10% 24% 27% 18% 6%	527
	21st to end	Day	14% 8% 24% 27% 21% 7%	573
June	1st to 10th	Day	16% 9% 30% 29% 14% 2	530
	11th to 20th	Day	12% 9% 26% 30% 19% 4	530
	21st to end	Day	17% 6% 28% 31% 15% 4	530
July	1st to 10th	Day	8% 5% 19% 31% 28% 8%	525
	11th to 20th	Day	15% 6% 20% 28% 22% 10%	524
	21st to end	Day	10% 5% 24% 33% 21% 7%	576
August	1st to 10th	Day	10% 5% 19% 30% 24% 12%	526
	11th to 20th	Day	14% 5% 17% 29% 24% 12%	519
	21st to end	Day	12% 6% 21% 29% 21% 11%	583
September	1st to 10th	Day	14% 8% 18% 26% 23% 10%	530
	11th to 20th	Day	10% 5% 14% 25% 28% 19%	530
	21st to end	Day	16% 11% 23% 24% 18% 8%	530
October	1st to 10th	Day	20% 9% 25% 20% 18% 8%	530
	11th to 20th	Day	22% 15% 29% 23% 8% 2	530
	21st to end	Day	31% 15% 26% 21% 8%	583
November	1st to 10th	Day	23% 16% 29% 21% 8% 3	359
	11th to 20th	Day	25% 21% 25% 20% 5% 3	220
	21st to end	Day	31% 20% 35% 11% 3	220
December	1st to 10th	Day	27% 19% 32% 16% 5% 1	220
	11th to 20th	Day	28% 22% 30% 15% 5%	220
	21st to end	Day	28% 21% 33% 13% 4 1	242
January	1st to 10th	Night	27% 18% 33% 16% 6%	840
	11th to 20th	Night	29% 18% 30% 19% 4	840
	21st to end	Night	24% 12% 35% 23% 6%	924
February	1st to 10th	Night	26% 17% 32% 22% 3	840
	11th to 20th	Night	24% 15% 36% 22% 4	823
	21st to end	Night	27% 12% 35% 23% 3	692
March	1st to 10th	Night	21% 13% 34% 26% 5%	840
	11th to 20th	Night	25% 13% 32% 23% 7%	840
	21st to end	Night	22% 10% 32% 25% 10% 1	924
April	1st to 10th	Night	23% 11% 33% 27% 5%	840
	11th to 20th	Night	23% 9% 30% 28% 8% 1	804
	21st to end	Night	21% 7% 27% 34% 11%	630
May	1st to 10th	Night	21% 5% 24% 34% 13% 3	603
	11th to 20th	Night	15% 4 26% 41% 13% 1	430
	21st to end	Night	15% 4 24% 35% 21% 2	433
June	1st to 10th	Night	13% 5% 29% 37% 16% 1	400
	11th to 20th	Night	12% 5% 24% 41% 18% 1	400
	21st to end	Night	13% 6% 22% 42% 17%	400
July	1st to 10th	Night	7% 3 17% 45% 26% 2	390
	11th to 20th	Night	11% 3 20% 38% 26% 3	388
	21st to end	Night	11% 3 17% 43% 24% 2	656
August	1st to 10th	Night	9% 3 13% 41% 31% 4	597
	11th to 20th	Night	9% 3 15% 32% 37% 3	594
	21st to end	Night	11% 2 16% 41% 27% 4	770
September	1st to 10th	Night	14% 4 18% 35% 24% 5%	800
	11th to 20th	Night	13% 4 15% 30% 32% 7%	800
	21st to end	Night	18% 6% 20% 30% 21% 5%	800
October	1st to 10th	Night	18% 6% 23% 30% 21% 3	800
	11th to 20th	Night	26% 11% 26% 26% 9% 2	800
	21st to end	Night	28% 10% 28% 28% 5%	880
November	1st to 10th	Night	21% 11% 28% 28% 10% 1	820
	11th to 20th	Night	22% 15% 34% 19% 9% 2	840
	21st to end	Night	26% 13% 35% 19% 6%	840
December	1st to 10th	Night	32% 16% 28% 20% 5%	840
	11th to 20th	Night	26% 19% 32% 18% 5%	840
	21st to end	Night	31% 14% 31% 21% 3	924

Page generated at 2026-04-29 02:21:40. Version ac07

Copyright © 2026 Attilla Danko.