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 2024-11-03.

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	9% 8% 3 8% 5% 5% 5% 4 6% 8% 38%	101893
	Night	7% 9% 4 9% 6% 6% 6% 5% 7% 8% 34%	97164
Transparency	Day	42% 8% 16% 23% 10% 1	101634
	Night	44% 8% 15% 22% 9% 2	97069
Seeing	Day	12% 7% 26% 31% 19% 5%	15453
	Night	11% 6% 28% 38% 15% 2	24496

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 11% 6% 13% 7% 6% 6% 5% 8% 8% 27%	2219
	11th to 20th	Day	5% 9% 5% 11% 6% 6% 8% 7% 8% 8% 27%	2199
	21st to end	Day	4 10% 4 11% 6% 5% 5% 4 8% 9% 32%	2707
February	1st to 10th	Day	7% 14% 4 12% 9% 6% 5% 4 6% 8% 25%	1553
	11th to 20th	Day	6% 13% 7% 12% 7% 6% 7% 6% 6% 6% 25%	1665
	21st to end	Day	3 12% 5% 14% 8% 7% 7% 6% 5% 7% 24%	1465
March	1st to 10th	Day	4 12% 5% 14% 9% 7% 7% 4 5% 5% 26%	3165
	11th to 20th	Day	7% 9% 3 7% 5% 4 4 4 7% 10% 40%	3489
	21st to end	Day	9% 19% 7% 14% 6% 4 4 3 5% 5% 23%	3825
April	1st to 10th	Day	8% 13% 3 9% 6% 6% 5% 5% 6% 8% 31%	2356
	11th to 20th	Day	10% 11% 4 9% 6% 5% 5% 5% 6% 7% 31%	2440
	21st to end	Day	13% 9% 3 9% 6% 5% 5% 5% 6% 8% 32%	2470
May	1st to 10th	Day	19% 12% 3 7% 5% 5% 4 4 6% 6% 29%	4379
	11th to 20th	Day	11% 8% 3 8% 5% 6% 5% 5% 7% 9% 33%	4694
	21st to end	Day	16% 7% 4 9% 6% 6% 6% 5% 6% 7% 29%	5161
June	1st to 10th	Day	14% 7% 2 7% 5% 4 5% 5% 8% 8% 35%	2850
	11th to 20th	Day	18% 9% 3 8% 5% 5% 5% 5% 7% 8% 28%	2850
	21st to end	Day	9% 4 2 5% 4 5% 5% 5% 8% 10% 42%	2850
July	1st to 10th	Day	7% 4 1 5% 4 4 5% 4 6% 9% 50%	4740
	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	11% 9% 3 8% 5% 4 3 3 5% 7% 41%	1963
	11th to 20th	Day	9% 7% 3 6% 6% 5% 6% 4 5% 9% 41%	1807
	21st to end	Day	7% 11% 4 8% 4 5% 6% 5% 6% 7% 36%	1881
November	1st to 10th	Day	10% 15% 5% 12% 7% 5% 4 4 5% 6% 27%	1505
	11th to 20th	Day	6% 13% 4 11% 7% 5% 5% 4 6% 6% 33%	1487
	21st to end	Day	6% 12% 3 11% 7% 6% 5% 4 6% 7% 34%	1330
December	1st to 10th	Day	7% 13% 5% 11% 7% 7% 8% 5% 5% 5% 27%	1330
	11th to 20th	Day	3 10% 6% 15% 9% 6% 6% 5% 6% 7% 26%	1682
	21st to end	Day	3 9% 4 9% 5% 4 4 3 6% 8% 44%	2442
January	1st to 10th	Night	4 13% 5% 13% 7% 6% 6% 6% 7% 9% 23%	4726
	11th to 20th	Night	4 8% 4 13% 7% 6% 7% 7% 6% 8% 29%	4699
	21st to end	Night	4 10% 5% 11% 6% 5% 6% 5% 7% 8% 32%	4978
February	1st to 10th	Night	5% 13% 5% 12% 7% 6% 5% 4 6% 8% 28%	2627
	11th to 20th	Night	5% 11% 5% 12% 8% 7% 6% 5% 7% 7% 27%	2402
	21st to end	Night	4 10% 5% 11% 7% 6% 6% 5% 7% 8% 32%	1985
March	1st to 10th	Night	3 8% 5% 16% 9% 8% 6% 5% 6% 7% 27%	3709
	11th to 20th	Night	8% 7% 3 7% 6% 7% 6% 6% 7% 8% 35%	3490
	21st to end	Night	7% 18% 7% 16% 7% 6% 6% 5% 6% 7% 16%	3828
April	1st to 10th	Night	10% 10% 4 10% 6% 6% 5% 4 7% 8% 30%	1819
	11th to 20th	Night	8% 11% 5% 10% 7% 6% 5% 5% 5% 8% 30%	1691
	21st to end	Night	10% 11% 4 7% 6% 5% 5% 4 7% 8% 33%	1710
May	1st to 10th	Night	22% 11% 3 9% 5% 5% 4 3 5% 6% 28%	2583
	11th to 20th	Night	10% 6% 2 8% 5% 6% 7% 5% 8% 9% 33%	2199
	21st to end	Night	16% 11% 4 10% 7% 7% 6% 4 6% 6% 23%	2414
June	1st to 10th	Night	15% 9% 3 8% 6% 5% 5% 4 6% 8% 31%	1330
	11th to 20th	Night	13% 7% 3 9% 8% 6% 5% 5% 8% 7% 28%	1330
	21st to end	Night	8% 7% 2 6% 5% 5% 5% 4 8% 8% 42%	1330
July	1st to 10th	Night	6% 4 2 7% 6% 5% 6% 5% 7% 8% 44%	2220
	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	10% 9% 2 6% 5% 4 5% 5% 6% 7% 41%	2216
	11th to 20th	Night	8% 8% 2 8% 5% 5% 6% 4 6% 8% 39%	2370
	21st to end	Night	6% 9% 3 10% 5% 5% 6% 4 7% 10% 35%	2717
November	1st to 10th	Night	10% 14% 4 8% 5% 5% 5% 4 6% 8% 32%	2598
	11th to 20th	Night	4 10% 4 10% 7% 7% 6% 5% 7% 9% 32%	2688
	21st to end	Night	5% 11% 4 10% 7% 5% 5% 5% 7% 9% 32%	2850
December	1st to 10th	Night	6% 14% 5% 11% 7% 6% 5% 4 5% 8% 29%	2850
	11th to 20th	Night	3 11% 4 13% 7% 7% 6% 5% 6% 9% 29%	3510
	21st to end	Night	3 8% 3 8% 6% 5% 5% 4 6% 9% 43%	5214

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	49% 10% 12% 20% 8% 1	2219
	11th to 20th	Day	46% 9% 13% 15% 15% 2	2220
	21st to end	Day	42% 9% 16% 26% 6%	2707
February	1st to 10th	Day	56% 7% 13% 16% 8% 1	1553
	11th to 20th	Day	55% 8% 13% 15% 8% 1	1674
	21st to end	Day	56% 6% 11% 17% 9%	1465
March	1st to 10th	Day	54% 10% 14% 13% 9%	3165
	11th to 20th	Day	37% 10% 19% 23% 11%	3490
	21st to end	Day	59% 9% 12% 15% 5%	3825
April	1st to 10th	Day	48% 9% 18% 18% 8% 1	2356
	11th to 20th	Day	48% 9% 13% 21% 8% 1	2440
	21st to end	Day	47% 9% 15% 21% 7% 1	2470
May	1st to 10th	Day	53% 9% 13% 21% 5%	4379
	11th to 20th	Day	41% 12% 14% 24% 9%	4698
	21st to end	Day	48% 11% 16% 19% 6% 1	5162
June	1st to 10th	Day	44% 10% 16% 21% 9% 1	2850
	11th to 20th	Day	51% 8% 15% 18% 6% 1	2850
	21st to end	Day	33% 11% 20% 26% 7% 2	2850
July	1st to 10th	Day	29% 8% 18% 28% 16% 2	4666
	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	43% 6% 16% 24% 10% 1	1963
	11th to 20th	Day	39% 8% 17% 25% 10% 1	1807
	21st to end	Day	45% 8% 13% 23% 11% 1	1881
November	1st to 10th	Day	57% 6% 13% 17% 6% 1	1505
	11th to 20th	Day	49% 8% 13% 21% 8% 1	1487
	21st to end	Day	47% 8% 17% 19% 8% 1	1330
December	1st to 10th	Day	55% 7% 13% 16% 7% 2	1330
	11th to 20th	Day	54% 8% 14% 20% 4 1	1684
	21st to end	Day	36% 5% 13% 25% 15% 5%	2442
January	1st to 10th	Night	50% 10% 13% 20% 5% 1	4726
	11th to 20th	Night	49% 6% 12% 17% 13% 3	4740
	21st to end	Night	44% 8% 14% 26% 8% 1	4978
February	1st to 10th	Night	52% 6% 12% 19% 10%	2627
	11th to 20th	Night	51% 8% 14% 15% 10% 1	2403
	21st to end	Night	46% 8% 14% 21% 10% 1	1985
March	1st to 10th	Night	49% 9% 15% 20% 6%	3709
	11th to 20th	Night	39% 10% 21% 22% 8% 1	3490
	21st to end	Night	63% 10% 11% 11% 4	3828
April	1st to 10th	Night	49% 8% 16% 16% 10% 1	1819
	11th to 20th	Night	50% 7% 13% 21% 8% 1	1691
	21st to end	Night	45% 9% 13% 24% 8% 2	1710
May	1st to 10th	Night	56% 6% 10% 21% 6% 1	2583
	11th to 20th	Night	39% 11% 16% 24% 10%	2199
	21st to end	Night	54% 13% 14% 15% 4 1	2414
June	1st to 10th	Night	48% 7% 15% 21% 8%	1330
	11th to 20th	Night	50% 12% 14% 16% 7% 1	1330
	21st to end	Night	36% 10% 19% 25% 9% 1	1330
July	1st to 10th	Night	33% 10% 20% 20% 15% 2	2185
	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	40% 6% 16% 25% 11% 1	2216
	11th to 20th	Night	41% 7% 17% 24% 10% 2	2370
	21st to end	Night	43% 9% 16% 24% 7% 1	2717
November	1st to 10th	Night	49% 7% 14% 21% 8% 1	2598
	11th to 20th	Night	47% 8% 16% 19% 10% 1	2688
	21st to end	Night	46% 10% 17% 19% 6% 2	2850
December	1st to 10th	Night	53% 7% 12% 20% 8% 1	2850
	11th to 20th	Night	47% 10% 20% 17% 6%	3507
	21st to end	Night	37% 6% 14% 23% 16% 4	5214

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% 24% 32% 23% 8%	180
	11th to 20th	Day	9% 3 33% 28% 22% 4	180
	21st to end	Day	10% 9% 25% 29% 22% 5%	198
February	1st to 10th	Day	13% 7% 27% 35% 13% 5%	195
	11th to 20th	Day	11% 6% 30% 31% 18% 4	270
	21st to end	Day	6% 6% 22% 40% 20% 5%	277
March	1st to 10th	Day	11% 7% 32% 33% 15% 3	460
	11th to 20th	Day	8% 5% 33% 32% 17% 5%	460
	21st to end	Day	11% 8% 30% 30% 17% 4	506
April	1st to 10th	Day	11% 13% 35% 29% 8% 3	460
	11th to 20th	Day	12% 9% 30% 29% 16% 3	452
	21st to end	Day	17% 10% 28% 29% 14% 2	460
May	1st to 10th	Day	15% 11% 22% 28% 20% 5%	575
	11th to 20th	Day	11% 7% 23% 34% 20% 5%	675
	21st to end	Day	16% 6% 22% 28% 21% 7%	734
June	1st to 10th	Day	15% 8% 26% 32% 16% 3	680
	11th to 20th	Day	18% 9% 27% 29% 15% 3	680
	21st to end	Day	11% 6% 24% 28% 23% 7%	680
July	1st to 10th	Day	9% 5% 18% 35% 24% 9%	671
	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	19% 8% 27% 30% 13% 2	276
	11th to 20th	Day	13% 11% 29% 35% 11% 2	255
	21st to end	Day	7% 8% 31% 32% 11% 11%	180
December	1st to 10th	Day	7% 5% 30% 34% 17% 7%	180
	11th to 20th	Day	7% 7% 32% 39% 13% 2	180
	21st to end	Day	9% 4 20% 37% 24% 6%	198
January	1st to 10th	Night	9% 7% 32% 35% 15% 2	950
	11th to 20th	Night	10% 6% 30% 39% 14% 1	950
	21st to end	Night	10% 8% 28% 39% 13% 2	900
February	1st to 10th	Night	10% 7% 25% 36% 18% 3	760
	11th to 20th	Night	10% 5% 31% 38% 16% 1	744
	21st to end	Night	8% 4 20% 43% 20% 5%	628
March	1st to 10th	Night	8% 4 31% 39% 16% 3	760
	11th to 20th	Night	8% 5% 29% 41% 15% 2	760
	21st to end	Night	12% 6% 30% 36% 15% 2	836
April	1st to 10th	Night	12% 6% 31% 33% 16% 1	660
	11th to 20th	Night	12% 9% 33% 34% 10% 1	563
	21st to end	Night	14% 6% 29% 37% 13% 1	570
May	1st to 10th	Night	15% 8% 24% 35% 16% 1	524
	11th to 20th	Night	11% 6% 31% 34% 17% 1	374
	21st to end	Night	18% 7% 25% 31% 18% 1	411
June	1st to 10th	Night	18% 8% 25% 36% 11% 2	380
	11th to 20th	Night	16% 6% 30% 29% 18% 1	380
	21st to end	Night	11% 3 27% 36% 20% 2	380
July	1st to 10th	Night	9% 2 18% 48% 21% 1	370
	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	915
	11th to 20th	Night	8% 8% 37% 34% 10% 3	950
	21st to end	Night	9% 5% 35% 37% 12% 2	950
December	1st to 10th	Night	12% 7% 27% 35% 16% 3	950
	11th to 20th	Night	9% 6% 33% 38% 13% 2	950
	21st to end	Night	10% 4 30% 37% 16% 2	1045

Page generated at 2024-10-31 01:39:23. Version ac07

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