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 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	8% 8% 3 8% 5% 5% 5% 4 6% 8% 38%	123789
	Night	6% 9% 4 10% 6% 6% 6% 5% 7% 9% 34%	123700
Transparency	Day	42% 8% 15% 24% 10% 1	123530
	Night	44% 7% 15% 22% 9% 1	123602
Seeing	Day	12% 7% 25% 31% 19% 5%	16299
	Night	11% 6% 28% 38% 15% 2	25406

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	3 11% 8% 16% 9% 6% 5% 4 6% 6% 26%	3179
	11th to 20th	Day	4 10% 5% 11% 7% 6% 7% 7% 9% 8% 26%	3159
	21st to end	Day	4 10% 4 10% 6% 5% 5% 4 7% 9% 35%	3889
February	1st to 10th	Day	7% 14% 4 13% 10% 7% 5% 4 5% 7% 24%	1635
	11th to 20th	Day	5% 12% 6% 12% 7% 7% 7% 5% 7% 6% 24%	1755
	21st to end	Day	3 12% 5% 14% 8% 7% 7% 6% 5% 8% 25%	1540
March	1st to 10th	Day	3 10% 5% 12% 8% 7% 7% 4 6% 9% 31%	4558
	11th to 20th	Day	5% 10% 5% 10% 6% 5% 4 4 6% 8% 36%	4999
	21st to end	Day	9% 18% 6% 13% 6% 5% 5% 4 6% 6% 23%	5486
April	1st to 10th	Day	8% 13% 3 9% 6% 6% 5% 5% 6% 8% 32%	2480
	11th to 20th	Day	11% 11% 4 8% 5% 5% 5% 5% 7% 7% 31%	2570
	21st to end	Day	13% 9% 3 9% 6% 5% 5% 5% 6% 7% 33%	2600
May	1st to 10th	Day	15% 9% 3 6% 4 4 5% 4 6% 6% 39%	6259
	11th to 20th	Day	13% 10% 3 9% 5% 5% 5% 5% 6% 9% 29%	6734
	21st to end	Day	12% 6% 3 8% 5% 5% 5% 4 6% 7% 41%	7405
June	1st to 10th	Day	13% 7% 2 7% 5% 4 5% 4 8% 8% 36%	3000
	11th to 20th	Day	17% 9% 2 8% 5% 5% 5% 5% 7% 8% 29%	3000
	21st to end	Day	9% 4 2 5% 4 5% 6% 5% 8% 10% 42%	3000
July	1st to 10th	Day	7% 4 1 5% 4 4 5% 4 6% 9% 50%	4816
	11th to 20th	Day	7% 4 2 5% 3 4 5% 4 7% 9% 51%	4740
	21st to end	Day	4 3 1 4 3 4 4 4 6% 9% 58%	5138
August	1st to 10th	Day	7% 6% 2 6% 5% 5% 5% 4 7% 9% 44%	4398
	11th to 20th	Day	6% 4 2 6% 4 5% 5% 4 7% 11% 46%	4028
	21st to end	Day	10% 5% 1 5% 3 3 4 3 6% 8% 52%	4521
September	1st to 10th	Day	10% 6% 2 6% 3 4 4 3 6% 8% 49%	2384
	11th to 20th	Day	16% 6% 2 6% 3 4 4 4 5% 7% 44%	2090
	21st to end	Day	7% 7% 2 6% 4 4 4 4 6% 9% 46%	2090
October	1st to 10th	Day	7% 6% 3 9% 5% 5% 5% 4 5% 7% 43%	3272
	11th to 20th	Day	6% 5% 2 6% 6% 5% 6% 4 7% 12% 42%	2999
	21st to end	Day	8% 9% 3 7% 4 5% 6% 4 6% 6% 42%	3101
November	1st to 10th	Day	10% 15% 5% 12% 7% 5% 4 4 5% 6% 27%	1569
	11th to 20th	Day	6% 13% 4 11% 7% 4 5% 4 5% 6% 33%	1565
	21st to end	Day	6% 12% 4 12% 7% 6% 5% 4 5% 7% 32%	1400
December	1st to 10th	Day	9% 13% 4 10% 6% 6% 7% 5% 6% 7% 27%	2290
	11th to 20th	Day	6% 9% 5% 15% 9% 7% 6% 5% 6% 7% 25%	2642
	21st to end	Day	2 7% 4 10% 5% 5% 5% 4 6% 9% 41%	3498
January	1st to 10th	Night	3 14% 5% 14% 8% 7% 6% 5% 7% 8% 22%	6766
	11th to 20th	Night	3 10% 5% 12% 7% 6% 6% 6% 6% 10% 30%	6739
	21st to end	Night	3 10% 5% 11% 6% 5% 6% 5% 7% 8% 36%	7110
February	1st to 10th	Night	5% 13% 5% 13% 8% 6% 5% 4 7% 8% 27%	2765
	11th to 20th	Night	5% 11% 5% 12% 8% 7% 6% 5% 8% 7% 27%	2532
	21st to end	Night	4 9% 5% 11% 7% 6% 6% 5% 7% 8% 32%	2086
March	1st to 10th	Night	3 7% 4 13% 8% 7% 6% 5% 8% 10% 28%	5336
	11th to 20th	Night	6% 8% 3 10% 7% 7% 7% 6% 8% 9% 31%	5000
	21st to end	Night	7% 17% 7% 15% 7% 6% 5% 5% 6% 6% 19%	5489
April	1st to 10th	Night	10% 10% 4 9% 6% 6% 5% 4 6% 8% 31%	1914
	11th to 20th	Night	8% 12% 5% 11% 7% 6% 6% 5% 5% 8% 30%	1781
	21st to end	Night	9% 10% 4 7% 7% 5% 6% 4 6% 9% 33%	1800
May	1st to 10th	Night	16% 9% 3 8% 5% 5% 4 3 6% 7% 34%	3697
	11th to 20th	Night	12% 8% 3 9% 6% 6% 6% 5% 8% 9% 29%	3159
	21st to end	Night	12% 9% 3 9% 6% 6% 6% 4 6% 6% 33%	3470
June	1st to 10th	Night	14% 10% 3 8% 6% 5% 5% 4 6% 8% 31%	1400
	11th to 20th	Night	13% 8% 3 9% 8% 6% 6% 5% 8% 7% 28%	1400
	21st to end	Night	8% 7% 2 6% 5% 5% 5% 5% 8% 9% 41%	1400
July	1st to 10th	Night	6% 4 2 7% 6% 5% 6% 5% 7% 8% 44%	2255
	11th to 20th	Night	5% 3 1 5% 4 5% 5% 6% 8% 12% 45%	2220
	21st to end	Night	4 3 2 6% 4 4 5% 4 7% 10% 52%	2414
August	1st to 10th	Night	9% 5% 2 7% 4 4 5% 4 7% 9% 44%	2518
	11th to 20th	Night	6% 5% 2 6% 5% 5% 6% 5% 8% 11% 42%	2811
	21st to end	Night	8% 4 1 5% 4 4 4 4 7% 9% 50%	3146
September	1st to 10th	Night	10% 6% 1 6% 4 4 4 4 5% 7% 48%	1792
	11th to 20th	Night	13% 8% 3 5% 4 4 4 3 5% 7% 45%	2090
	21st to end	Night	8% 7% 2 5% 4 4 4 4 7% 8% 48%	2090
October	1st to 10th	Night	8% 7% 2 6% 5% 4 5% 4 6% 8% 45%	3700
	11th to 20th	Night	6% 7% 2 7% 4 5% 5% 4 7% 9% 43%	3926
	21st to end	Night	6% 7% 2 8% 5% 5% 6% 5% 7% 9% 39%	4457
November	1st to 10th	Night	10% 14% 4 8% 5% 5% 5% 4 6% 7% 32%	2709
	11th to 20th	Night	4 10% 4 10% 7% 7% 6% 5% 6% 8% 31%	2830
	21st to end	Night	5% 12% 4 10% 7% 6% 5% 5% 7% 8% 30%	3000
December	1st to 10th	Night	6% 13% 5% 12% 7% 7% 6% 5% 6% 10% 24%	4890
	11th to 20th	Night	6% 12% 5% 14% 8% 7% 7% 5% 7% 8% 21%	5550
	21st to end	Night	3 7% 3 10% 6% 6% 5% 5% 7% 10% 38%	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	55% 7% 10% 19% 8% 1	3179
	11th to 20th	Day	47% 8% 13% 15% 16% 2	3180
	21st to end	Day	41% 7% 16% 28% 7%	3889
February	1st to 10th	Day	57% 6% 12% 15% 8% 1	1635
	11th to 20th	Day	55% 8% 13% 16% 8% 1	1764
	21st to end	Day	55% 6% 11% 19% 9%	1540
March	1st to 10th	Day	48% 7% 17% 19% 9%	4558
	11th to 20th	Day	44% 8% 15% 22% 12%	5000
	21st to end	Day	59% 7% 11% 17% 6%	5486
April	1st to 10th	Day	47% 8% 17% 19% 8% 1	2480
	11th to 20th	Day	48% 9% 14% 21% 8% 1	2570
	21st to end	Day	47% 9% 14% 21% 7% 1	2600
May	1st to 10th	Day	44% 7% 10% 22% 14% 4	6259
	11th to 20th	Day	48% 9% 13% 22% 7%	6738
	21st to end	Day	39% 9% 16% 25% 10% 1	7406
June	1st to 10th	Day	43% 9% 15% 22% 10% 1	3000
	11th to 20th	Day	50% 8% 15% 19% 6% 1	3000
	21st to end	Day	33% 11% 21% 27% 7% 1	3000
July	1st to 10th	Day	28% 8% 18% 28% 16% 2	4742
	11th to 20th	Day	28% 8% 17% 34% 13%	4659
	21st to end	Day	23% 7% 14% 30% 21% 5%	4976
August	1st to 10th	Day	35% 9% 15% 31% 9% 1	4380
	11th to 20th	Day	30% 7% 24% 30% 7% 1	4067
	21st to end	Day	29% 6% 22% 31% 10% 1	4520
September	1st to 10th	Day	33% 8% 17% 26% 12% 5%	2384
	11th to 20th	Day	39% 7% 15% 28% 9% 1	2090
	21st to end	Day	33% 8% 17% 28% 10% 4	2090
October	1st to 10th	Day	40% 4 20% 27% 9% 1	3272
	11th to 20th	Day	35% 6% 19% 28% 10% 1	2999
	21st to end	Day	41% 6% 10% 33% 9% 1	3101
November	1st to 10th	Day	57% 6% 13% 17% 6% 1	1569
	11th to 20th	Day	49% 8% 13% 21% 8% 1	1565
	21st to end	Day	50% 7% 16% 18% 8% 1	1400
December	1st to 10th	Day	54% 6% 20% 15% 4 1	2290
	11th to 20th	Day	58% 6% 16% 17% 3 1	2644
	21st to end	Day	38% 5% 12% 28% 13% 3	3498
January	1st to 10th	Night	55% 8% 14% 17% 5% 1	6766
	11th to 20th	Night	47% 6% 13% 17% 15% 2	6780
	21st to end	Night	42% 6% 17% 27% 7% 1	7110
February	1st to 10th	Night	53% 6% 12% 18% 11%	2765
	11th to 20th	Night	51% 8% 14% 16% 11% 1	2533
	21st to end	Night	45% 8% 15% 21% 11% 1	2086
March	1st to 10th	Night	45% 8% 21% 18% 7%	5336
	11th to 20th	Night	43% 8% 18% 24% 6% 1	5000
	21st to end	Night	62% 9% 10% 14% 5% 1	5489
April	1st to 10th	Night	48% 7% 15% 17% 11% 1	1914
	11th to 20th	Night	50% 6% 13% 21% 8% 1	1781
	21st to end	Night	44% 9% 13% 24% 8% 2	1800
May	1st to 10th	Night	48% 5% 8% 23% 13% 2	3694
	11th to 20th	Night	46% 10% 15% 23% 7%	3159
	21st to end	Night	46% 10% 15% 22% 6% 1	3470
June	1st to 10th	Night	48% 7% 15% 21% 9%	1400
	11th to 20th	Night	50% 12% 14% 17% 7% 1	1400
	21st to end	Night	37% 10% 19% 25% 8% 1	1400
July	1st to 10th	Night	33% 10% 20% 21% 15% 2	2220
	11th to 20th	Night	28% 9% 18% 35% 8% 2	2178
	21st to end	Night	25% 7% 15% 28% 21% 4	2337
August	1st to 10th	Night	34% 8% 17% 30% 11% 1	2511
	11th to 20th	Night	32% 10% 20% 30% 7% 1	2838
	21st to end	Night	29% 6% 21% 33% 10% 2	3146
September	1st to 10th	Night	33% 9% 15% 23% 13% 7%	1792
	11th to 20th	Night	38% 6% 14% 27% 12% 4	2090
	21st to end	Night	31% 7% 17% 28% 15% 2	2090
October	1st to 10th	Night	36% 5% 20% 26% 11% 1	3700
	11th to 20th	Night	36% 6% 17% 30% 9% 2	3926
	21st to end	Night	39% 7% 15% 32% 6% 1	4457
November	1st to 10th	Night	50% 6% 13% 21% 8% 1	2709
	11th to 20th	Night	48% 8% 15% 19% 10% 1	2830
	21st to end	Night	48% 10% 16% 19% 6% 2	3000
December	1st to 10th	Night	55% 7% 18% 15% 5% 1	4890
	11th to 20th	Night	57% 9% 17% 13% 4	5547
	21st to end	Night	39% 5% 12% 24% 15% 3	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	8% 5% 26% 31% 24% 7%	200
	11th to 20th	Day	9% 5% 32% 31% 22% 4	200
	21st to end	Day	10% 9% 24% 32% 20% 5%	220
February	1st to 10th	Day	12% 7% 28% 33% 14% 6%	217
	11th to 20th	Day	10% 6% 30% 32% 19% 4	300
	21st to end	Day	6% 5% 22% 41% 21% 5%	304
March	1st to 10th	Day	10% 7% 31% 32% 17% 4	500
	11th to 20th	Day	9% 5% 32% 32% 17% 5%	500
	21st to end	Day	12% 8% 29% 30% 18% 4	550
April	1st to 10th	Day	11% 12% 34% 29% 10% 5%	500
	11th to 20th	Day	13% 11% 31% 27% 15% 3	492
	21st to end	Day	16% 10% 27% 29% 15% 3	500
May	1st to 10th	Day	14% 11% 23% 28% 19% 5%	621
	11th to 20th	Day	12% 8% 22% 34% 20% 5%	725
	21st to end	Day	15% 6% 22% 29% 21% 7%	789
June	1st to 10th	Day	15% 8% 25% 32% 16% 3	730
	11th to 20th	Day	18% 9% 26% 29% 15% 3	730
	21st to end	Day	12% 6% 24% 28% 24% 7%	730
July	1st to 10th	Day	9% 5% 18% 36% 24% 9%	697
	11th to 20th	Day	10% 5% 20% 32% 25% 8%	668
	21st to end	Day	5% 4 21% 31% 29% 9%	739
August	1st to 10th	Day	9% 5% 23% 32% 23% 9%	505
	11th to 20th	Day	9% 3 21% 33% 27% 7%	477
	21st to end	Day	10% 9% 20% 30% 22% 9%	539
September	1st to 10th	Day	12% 7% 27% 29% 20% 6%	490
	11th to 20th	Day	18% 8% 21% 30% 19% 3	490
	21st to end	Day	11% 11% 28% 31% 15% 4	490
October	1st to 10th	Day	16% 6% 25% 30% 17% 7%	363
	11th to 20th	Day	11% 7% 32% 33% 14% 4	300
	21st to end	Day	12% 12% 32% 30% 12% 2	330
November	1st to 10th	Day	18% 8% 26% 31% 15% 2	300
	11th to 20th	Day	12% 10% 30% 34% 12% 1	283
	21st to end	Day	10% 8% 29% 33% 12% 10%	200
December	1st to 10th	Day	9% 5% 29% 35% 17% 7%	200
	11th to 20th	Day	9% 7% 30% 40% 14% 2	200
	21st to end	Day	8% 4 19% 36% 26% 6%	220
January	1st to 10th	Night	9% 7% 32% 35% 15% 2	1000
	11th to 20th	Night	10% 6% 31% 39% 13% 1	1000
	21st to end	Night	10% 9% 28% 40% 12% 1	947
February	1st to 10th	Night	10% 7% 25% 37% 18% 3	800
	11th to 20th	Night	10% 5% 30% 39% 16% 1	784
	21st to end	Night	7% 4 20% 43% 20% 5%	660
March	1st to 10th	Night	8% 4 30% 39% 16% 3	800
	11th to 20th	Night	8% 5% 28% 42% 15% 2	800
	21st to end	Night	12% 6% 29% 36% 15% 2	880
April	1st to 10th	Night	12% 6% 31% 33% 16% 1	694
	11th to 20th	Night	12% 10% 33% 34% 10% 1	593
	21st to end	Night	13% 6% 29% 38% 13% 1	600
May	1st to 10th	Night	15% 8% 24% 36% 16% 1	551
	11th to 20th	Night	11% 6% 31% 34% 18% 1	394
	21st to end	Night	17% 7% 25% 31% 18% 1	433
June	1st to 10th	Night	18% 9% 26% 36% 11% 2	400
	11th to 20th	Night	16% 7% 31% 29% 18% 1	400
	21st to end	Night	11% 3 27% 37% 20% 2	400
July	1st to 10th	Night	9% 2 18% 48% 21% 1	380
	11th to 20th	Night	10% 2 23% 43% 20% 1	368
	21st to end	Night	5% 3 22% 44% 24% 2	416
August	1st to 10th	Night	9% 3 22% 39% 24% 3	387
	11th to 20th	Night	10% 3 19% 45% 22% 2	564
	21st to end	Night	9% 5% 21% 43% 22%	627
September	1st to 10th	Night	13% 4 21% 43% 17% 2	598
	11th to 20th	Night	16% 6% 19% 41% 17% 1	760
	21st to end	Night	11% 5% 30% 38% 14% 2	760
October	1st to 10th	Night	13% 3 29% 37% 16% 2	760
	11th to 20th	Night	11% 5% 30% 37% 14% 2	760
	21st to end	Night	10% 8% 32% 36% 13%	836
November	1st to 10th	Night	16% 7% 31% 35% 9% 1	954
	11th to 20th	Night	8% 8% 37% 34% 10% 3	1000
	21st to end	Night	10% 5% 34% 37% 13% 2	1000
December	1st to 10th	Night	12% 7% 27% 35% 16% 3	1000
	11th to 20th	Night	9% 6% 33% 37% 13% 2	1000
	21st to end	Night	10% 4 30% 38% 16% 3	1100

Page generated at 2025-07-02 23:30:13. Version ac07

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