Drumheller
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Drumheller clear sky chart from 2005-11-06 to 2026-10-29.

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	8% 8% 3 8% 5% 5% 5% 4 6% 8% 39%	154584
	Night	6% 9% 4 10% 6% 6% 6% 5% 7% 9% 35%	146218
Transparency	Day	40% 7% 16% 26% 10% 1	154281
	Night	44% 7% 16% 23% 9% 1	146104
Seeing	Day	12% 7% 25% 31% 19% 5%	18082
	Night	11% 6% 28% 38% 15% 2	27059

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	4 12% 8% 16% 9% 6% 5% 4 6% 6% 25%	4125
	11th to 20th	Day	3 8% 6% 11% 6% 6% 7% 6% 9% 8% 31%	4119
	21st to end	Day	3 10% 4 10% 6% 5% 5% 4 6% 9% 37%	5043
February	1st to 10th	Day	7% 14% 4 12% 9% 6% 5% 4 6% 8% 25%	1717
	11th to 20th	Day	5% 12% 6% 12% 7% 7% 7% 5% 7% 7% 26%	1845
	21st to end	Day	3 12% 5% 14% 8% 6% 7% 6% 5% 8% 26%	1615
March	1st to 10th	Day	3 10% 5% 12% 8% 6% 7% 4 6% 9% 31%	4659
	11th to 20th	Day	5% 10% 5% 10% 6% 5% 5% 4 6% 8% 35%	5109
	21st to end	Day	9% 18% 6% 13% 6% 5% 5% 4 6% 6% 23%	5607
April	1st to 10th	Day	8% 13% 3 9% 5% 6% 5% 5% 6% 9% 31%	2604
	11th to 20th	Day	10% 11% 4 8% 6% 5% 5% 5% 7% 8% 31%	2700
	21st to end	Day	12% 9% 3 9% 5% 5% 5% 5% 6% 7% 33%	2730
May	1st to 10th	Day	12% 8% 2 5% 4 4 5% 4 7% 8% 41%	8112
	11th to 20th	Day	13% 10% 3 9% 5% 5% 5% 5% 7% 9% 29%	8755
	21st to end	Day	12% 5% 3 7% 5% 5% 5% 5% 6% 7% 40%	9649
June	1st to 10th	Day	14% 7% 2 7% 5% 4 5% 4 8% 8% 35%	3150
	11th to 20th	Day	17% 9% 3 8% 6% 5% 6% 5% 7% 8% 29%	3150
	21st to end	Day	10% 4 2 5% 4 5% 6% 5% 8% 10% 41%	3150
July	1st to 10th	Day	6% 4 1 5% 4 4 5% 4 6% 9% 51%	8820
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 4 7% 8% 46%	8820
	21st to end	Day	6% 3 1 5% 3 3 4 4 5% 8% 57%	9569
August	1st to 10th	Day	6% 5% 2 5% 4 5% 5% 4 6% 9% 48%	6439
	11th to 20th	Day	8% 6% 2 7% 4 4 5% 4 6% 9% 43%	5925
	21st to end	Day	7% 4 1 4 3 3 4 3 6% 9% 55%	6570
September	1st to 10th	Day	11% 6% 2 5% 3 4 4 3 6% 8% 48%	2634
	11th to 20th	Day	14% 6% 2 6% 3 4 4 4 5% 7% 46%	2310
	21st to end	Day	7% 6% 2 6% 4 4 4 4 6% 9% 47%	2310
October	1st to 10th	Day	7% 6% 3 8% 5% 5% 5% 4 6% 8% 44%	3478
	11th to 20th	Day	6% 5% 2 6% 6% 5% 6% 4 7% 11% 43%	3189
	21st to end	Day	8% 8% 3 7% 4 5% 6% 4 6% 6% 42%	3272
November	1st to 10th	Day	10% 15% 5% 12% 6% 5% 4 4 5% 6% 27%	1649
	11th to 20th	Day	6% 13% 4 11% 7% 4 5% 4 5% 7% 33%	1643
	21st to end	Day	6% 12% 4 12% 7% 6% 5% 3 6% 7% 31%	1470
December	1st to 10th	Day	9% 13% 4 10% 6% 6% 7% 5% 6% 8% 27%	2360
	11th to 20th	Day	6% 9% 5% 15% 9% 6% 6% 5% 6% 7% 25%	2712
	21st to end	Day	2 7% 4 10% 5% 5% 5% 4 7% 9% 41%	3575
January	1st to 10th	Night	4 13% 5% 15% 8% 6% 6% 5% 7% 8% 22%	8768
	11th to 20th	Night	3 9% 4 10% 6% 5% 5% 6% 6% 10% 36%	8779
	21st to end	Night	3 9% 5% 11% 6% 6% 6% 5% 7% 8% 35%	9194
February	1st to 10th	Night	5% 12% 5% 13% 8% 6% 5% 4 7% 9% 27%	2903
	11th to 20th	Night	5% 11% 5% 12% 8% 7% 6% 5% 8% 7% 27%	2662
	21st to end	Night	4 10% 5% 11% 7% 6% 6% 5% 7% 8% 32%	2187
March	1st to 10th	Night	3 7% 4 13% 8% 7% 6% 5% 8% 10% 29%	5455
	11th to 20th	Night	6% 8% 3 10% 7% 7% 7% 6% 8% 9% 31%	5110
	21st to end	Night	7% 18% 7% 15% 7% 6% 6% 5% 6% 6% 19%	5610
April	1st to 10th	Night	11% 10% 4 9% 6% 6% 5% 4 6% 8% 31%	2010
	11th to 20th	Night	7% 12% 5% 10% 6% 6% 6% 5% 5% 8% 30%	1871
	21st to end	Night	9% 11% 4 7% 6% 5% 5% 4 6% 8% 34%	1890
May	1st to 10th	Night	13% 9% 3 8% 5% 5% 5% 3 6% 7% 36%	4811
	11th to 20th	Night	12% 8% 3 9% 6% 6% 6% 5% 8% 9% 28%	4105
	21st to end	Night	11% 8% 3 9% 6% 6% 6% 4 6% 8% 33%	4522
June	1st to 10th	Night	15% 9% 3 8% 6% 5% 5% 4 7% 8% 31%	1470
	11th to 20th	Night	12% 8% 3 9% 8% 6% 6% 5% 8% 7% 28%	1470
	21st to end	Night	9% 7% 2 6% 5% 5% 5% 5% 8% 9% 40%	1470
July	1st to 10th	Night	4 4 1 6% 5% 5% 5% 5% 6% 9% 50%	4140
	11th to 20th	Night	7% 4 2 6% 4 5% 5% 5% 6% 10% 47%	4140
	21st to end	Night	7% 4 2 6% 3 4 5% 3 6% 9% 51%	4499
August	1st to 10th	Night	9% 6% 2 7% 5% 4 4 3 6% 8% 45%	3684
	11th to 20th	Night	9% 6% 3 8% 6% 6% 6% 5% 8% 10% 35%	4141
	21st to end	Night	7% 4 2 5% 4 4 5% 4 7% 9% 50%	4578
September	1st to 10th	Night	10% 6% 2 6% 3 4 4 4 5% 7% 48%	1982
	11th to 20th	Night	12% 7% 2 5% 4 3 4 3 5% 8% 47%	2310
	21st to end	Night	7% 6% 2 5% 4 4 4 4 7% 8% 49%	2310
October	1st to 10th	Night	7% 7% 2 6% 5% 4 5% 4 6% 8% 45%	3934
	11th to 20th	Night	6% 7% 2 7% 4 5% 5% 4 7% 8% 43%	4176
	21st to end	Night	6% 8% 2 8% 5% 5% 6% 5% 7% 9% 39%	4704
November	1st to 10th	Night	10% 14% 4 9% 5% 5% 5% 4 6% 7% 32%	2848
	11th to 20th	Night	4 9% 4 10% 7% 7% 6% 5% 6% 9% 32%	2972
	21st to end	Night	5% 12% 4 10% 7% 6% 6% 5% 7% 8% 30%	3150
December	1st to 10th	Night	6% 13% 5% 12% 7% 7% 6% 5% 6% 10% 24%	5040
	11th to 20th	Night	6% 12% 5% 14% 8% 7% 7% 5% 7% 8% 21%	5700
	21st to end	Night	3 7% 3 10% 6% 6% 5% 5% 7% 10% 38%	7623

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	57% 6% 9% 18% 9%	4125
	11th to 20th	Day	45% 6% 15% 16% 16% 2	4140
	21st to end	Day	42% 6% 18% 28% 6%	5044
February	1st to 10th	Day	55% 6% 12% 16% 9% 1	1717
	11th to 20th	Day	54% 8% 13% 17% 9% 1	1854
	21st to end	Day	54% 6% 11% 19% 9%	1615
March	1st to 10th	Day	48% 7% 17% 19% 9%	4659
	11th to 20th	Day	44% 8% 15% 22% 11%	5110
	21st to end	Day	59% 7% 11% 17% 6%	5607
April	1st to 10th	Day	47% 8% 18% 19% 8% 1	2604
	11th to 20th	Day	48% 8% 14% 21% 8% 1	2700
	21st to end	Day	47% 9% 14% 22% 7% 1	2730
May	1st to 10th	Day	40% 6% 10% 29% 13% 3	8112
	11th to 20th	Day	47% 8% 12% 24% 8%	8759
	21st to end	Day	40% 8% 15% 27% 9%	9650
June	1st to 10th	Day	43% 9% 15% 22% 10% 1	3150
	11th to 20th	Day	51% 8% 15% 19% 6% 1	3150
	21st to end	Day	34% 11% 20% 27% 7% 1	3150
July	1st to 10th	Day	28% 7% 18% 32% 14% 1	8746
	11th to 20th	Day	34% 6% 17% 28% 12% 3	8739
	21st to end	Day	25% 6% 16% 32% 18% 4	9362
August	1st to 10th	Day	31% 8% 17% 31% 11% 2	6421
	11th to 20th	Day	36% 7% 24% 27% 6%	5964
	21st to end	Day	26% 5% 19% 37% 12% 1	6569
September	1st to 10th	Day	34% 7% 16% 27% 12% 4	2634
	11th to 20th	Day	38% 6% 15% 30% 10% 1	2310
	21st to end	Day	32% 8% 17% 28% 11% 3	2310
October	1st to 10th	Day	38% 4 20% 28% 9% 1	3478
	11th to 20th	Day	35% 6% 19% 28% 11% 2	3189
	21st to end	Day	41% 6% 10% 32% 10% 1	3272
November	1st to 10th	Day	57% 6% 13% 17% 6% 2	1649
	11th to 20th	Day	49% 7% 14% 21% 8% 1	1643
	21st to end	Day	50% 7% 16% 18% 8% 1	1470
December	1st to 10th	Day	54% 6% 19% 15% 4 1	2360
	11th to 20th	Day	58% 6% 15% 17% 3 1	2714
	21st to end	Day	39% 5% 12% 28% 13% 3	3575
January	1st to 10th	Night	56% 7% 12% 15% 8% 1	8769
	11th to 20th	Night	42% 5% 18% 20% 14% 2	8820
	21st to end	Night	44% 6% 19% 24% 7%	9194
February	1st to 10th	Night	52% 6% 13% 19% 11%	2903
	11th to 20th	Night	50% 8% 13% 17% 11% 1	2663
	21st to end	Night	46% 7% 14% 21% 10% 1	2187
March	1st to 10th	Night	45% 8% 21% 18% 7%	5455
	11th to 20th	Night	44% 8% 18% 23% 7% 1	5110
	21st to end	Night	62% 8% 10% 14% 5% 1	5610
April	1st to 10th	Night	49% 7% 16% 17% 10% 1	2010
	11th to 20th	Night	50% 6% 13% 21% 8% 1	1871
	21st to end	Night	44% 8% 13% 25% 8% 2	1890
May	1st to 10th	Night	46% 5% 8% 26% 13% 2	4808
	11th to 20th	Night	45% 8% 14% 24% 8%	4105
	21st to end	Night	45% 9% 15% 24% 6% 1	4526
June	1st to 10th	Night	49% 7% 14% 21% 9%	1470
	11th to 20th	Night	50% 11% 14% 17% 7% 1	1470
	21st to end	Night	37% 10% 19% 25% 8% 1	1470
July	1st to 10th	Night	28% 8% 23% 26% 14% 1	4105
	11th to 20th	Night	32% 7% 18% 32% 10% 3	4098
	21st to end	Night	29% 6% 16% 29% 17% 3	4401
August	1st to 10th	Night	37% 6% 15% 30% 10% 1	3677
	11th to 20th	Night	40% 10% 20% 26% 5% 1	4168
	21st to end	Night	29% 5% 18% 37% 10% 1	4578
September	1st to 10th	Night	34% 8% 15% 25% 13% 6%	1982
	11th to 20th	Night	36% 6% 14% 29% 12% 3	2310
	21st to end	Night	31% 7% 17% 28% 15% 2	2310
October	1st to 10th	Night	37% 5% 20% 27% 11% 1	3934
	11th to 20th	Night	36% 6% 16% 31% 10% 2	4176
	21st to end	Night	38% 7% 15% 32% 7% 1	4704
November	1st to 10th	Night	50% 6% 13% 21% 8% 1	2848
	11th to 20th	Night	47% 8% 16% 19% 10% 1	2972
	21st to end	Night	48% 10% 16% 19% 6% 1	3150
December	1st to 10th	Night	55% 7% 18% 15% 5% 1	5040
	11th to 20th	Night	57% 8% 17% 13% 4	5697
	21st to end	Night	40% 6% 12% 24% 15% 3	7623

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	8% 5% 25% 33% 23% 6%	220
	11th to 20th	Day	9% 6% 30% 32% 20% 3	220
	21st to end	Day	9% 9% 25% 33% 20% 4	242
February	1st to 10th	Day	12% 7% 28% 33% 16% 5%	239
	11th to 20th	Day	10% 6% 28% 33% 18% 5%	330
	21st to end	Day	6% 6% 23% 40% 20% 5%	331
March	1st to 10th	Day	10% 6% 31% 32% 16% 4	540
	11th to 20th	Day	9% 5% 34% 31% 16% 4	540
	21st to end	Day	12% 9% 30% 29% 16% 4	594
April	1st to 10th	Day	12% 12% 32% 29% 10% 4	540
	11th to 20th	Day	13% 11% 30% 27% 16% 3	532
	21st to end	Day	16% 10% 26% 29% 15% 3	540
May	1st to 10th	Day	14% 10% 23% 29% 19% 5%	667
	11th to 20th	Day	13% 8% 22% 34% 19% 4	775
	21st to end	Day	15% 5% 22% 30% 21% 7%	844
June	1st to 10th	Day	16% 8% 25% 32% 15% 3	780
	11th to 20th	Day	17% 9% 26% 30% 14% 3	780
	21st to end	Day	13% 6% 24% 28% 23% 7%	780
July	1st to 10th	Day	9% 5% 18% 36% 25% 8%	771
	11th to 20th	Day	10% 6% 21% 32% 23% 7%	768
	21st to end	Day	6% 4 21% 32% 28% 9%	849
August	1st to 10th	Day	9% 5% 23% 31% 23% 10%	587
	11th to 20th	Day	9% 4 22% 32% 26% 7%	557
	21st to end	Day	9% 9% 20% 31% 23% 9%	627
September	1st to 10th	Day	13% 6% 25% 29% 21% 5%	570
	11th to 20th	Day	15% 8% 20% 30% 21% 5%	570
	21st to end	Day	10% 11% 29% 31% 15% 5%	570
October	1st to 10th	Day	14% 6% 25% 31% 18% 7%	429
	11th to 20th	Day	11% 8% 31% 33% 14% 4	360
	21st to end	Day	12% 12% 32% 30% 12% 2	387
November	1st to 10th	Day	18% 8% 26% 32% 14% 2	330
	11th to 20th	Day	13% 10% 30% 34% 12% 2	311
	21st to end	Day	9% 7% 28% 36% 11% 9%	220
December	1st to 10th	Day	9% 5% 28% 36% 16% 6%	220
	11th to 20th	Day	10% 8% 32% 36% 12% 2	220
	21st to end	Day	8% 5% 22% 35% 25% 6%	242
January	1st to 10th	Night	9% 7% 32% 36% 14% 2	1050
	11th to 20th	Night	10% 7% 31% 39% 13% 1	1050
	21st to end	Night	10% 8% 29% 40% 12% 1	994
February	1st to 10th	Night	10% 7% 25% 39% 18% 3	840
	11th to 20th	Night	10% 5% 29% 40% 15% 1	824
	21st to end	Night	8% 4 21% 43% 19% 4	692
March	1st to 10th	Night	8% 4 30% 40% 16% 3	840
	11th to 20th	Night	8% 6% 30% 41% 14% 2	840
	21st to end	Night	12% 6% 30% 36% 14% 2	924
April	1st to 10th	Night	13% 6% 30% 33% 16% 1	729
	11th to 20th	Night	12% 10% 33% 34% 10% 1	623
	21st to end	Night	13% 7% 28% 38% 13% 1	630
May	1st to 10th	Night	15% 8% 24% 36% 16% 1	579
	11th to 20th	Night	11% 6% 30% 35% 17% 1	414
	21st to end	Night	17% 6% 25% 32% 18% 1	455
June	1st to 10th	Night	19% 9% 25% 35% 10% 2	420
	11th to 20th	Night	15% 7% 31% 29% 17%	420
	21st to end	Night	12% 3 27% 36% 20% 2	420
July	1st to 10th	Night	8% 2 18% 50% 20%	410
	11th to 20th	Night	10% 3 24% 43% 19% 1	408
	21st to end	Night	6% 3 22% 43% 24% 2	460
August	1st to 10th	Night	9% 4 22% 39% 24% 3	428
	11th to 20th	Night	10% 3 19% 45% 21% 2	624
	21st to end	Night	9% 5% 21% 44% 21%	693
September	1st to 10th	Night	13% 4 21% 44% 16% 2	662
	11th to 20th	Night	15% 6% 18% 42% 19% 1	840
	21st to end	Night	10% 5% 30% 40% 13% 2	840
October	1st to 10th	Night	13% 4 28% 39% 15% 2	840
	11th to 20th	Night	12% 5% 30% 37% 13% 2	840
	21st to end	Night	10% 9% 31% 37% 12%	912
November	1st to 10th	Night	16% 7% 32% 35% 8% 1	1003
	11th to 20th	Night	8% 8% 36% 35% 10% 3	1050
	21st to end	Night	10% 5% 33% 39% 12% 2	1050
December	1st to 10th	Night	12% 8% 28% 35% 15% 2	1050
	11th to 20th	Night	10% 6% 34% 37% 12% 2	1050
	21st to end	Night	10% 4 31% 38% 15% 2	1155

Page generated at 2026-10-26 01:37:46. Version ac07

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