Recipe for Record Lows

August 20, 2012

Low dewpoints, clear skies and light winds were the perfect recipe for record low temperatures this weekend. Let’s take a look at how each of these factors contributed to some cool mornings…

START WITH – Low Dewpoints: The dewpoint is a measure of the amount of moisture in the atmosphere. When dewpoints climb above 60 degrees in the summer, the air feels warm and muggy. Warm dewpoints on a summer night also limit cooling. This is because as the air temperature gets close to the dewpoint, relative humidity increases and condensation occurs. The process of condensation releases heat into the air causing the temperature to level-off near the dewpoint. So when the dewpoint is lower, that “level-off” happens at a lower temperature. That was the case this weekend as dewpoints dropped into the 40s.

ADD IN – Clear Skies: Starry skies are needed for temperatures to fall close to the dewpoint. During the day pavement, buildings, vegetation and soil absorb heat from the sun. During the night, that heat gets released back toward space. When the nighttime sky is cloudy, those clouds act like a blanket at night, keeping temperatures warm. However, when the nighttime sky is clear, heat released from earth escapes into space causing temperatures to drop. Locations that saw record lows this weekend all had clear skies for a majority of the night.

In winter, we welcome the cloudy blanket for warmth.

MIX WITH – Light Winds: Wind speed is the final factor in how low temperatures can fall. Since air normally cools from the ground up, the air closest to the ground is the coolest overnight. As you increase in elevation off the ground, the temperature normally gets warmer. If the wind is strong overnight, the cool air near the surface mixes with the warmer air above, which limits the potential for cooling. However, on a night with light winds, the air near the ground continues to cool with no influence from warm air above. Wind speeds were generally less than 5 mph this weekend.

Coolest Air Since June Coming!

August 11, 2012

A couple weeks ago we were begging for highs below 100; would it be crazy to hope for the 70s? The last time the Missouri River Valley saw high temperatures below 80 was in early June. After the hottest July on the planet, a few days in August might not get out of the 70s.

Models are coming into agreement that a fairly strong trough of low pressure will dive southward this Thursday into Friday opening the door to cool Canadian air. Highs could potentially drop 10-15 degrees below normal in some locations. While it’s still early to pinpoint exactly how cool we will get, triple digits are history for now.

When will the heat return? The first signs of any heat returning are during the following week, but models are unclear on how hot it will get, and how long the heat could last. For now, bring on the 70s!

Is the Reprieve Here to Stay?

August 6, 2012

Expected Pattern 8-5

While rejoicing about this morning (lows 56 to 68), many are asking, will the persistent heat return? Thankfully, the answer is not yet. Earlier indications were that the scorching heat would return for the 10th through the 15th, but some welcome changes have occurred. Models now agree that the hot dome of high pressure stays west of the area, at least through the middle of next week. This means multiple cold fronts for the Missouri River Valley. Highs will climb above 90 ahead of the fronts, and drop below 90 behind the fronts, with no persistent 100+ stretches in sight. We look forward to occasional rain chances; not drought busting rains, but certainly welcome nonetheless. The jury is still out on the second half of the month. With the drought continuing, another heat wave could certainly be in store, but for now, watch for more pleasant days ahead!