Rochester Weather Science

Bundle up! How dew point affects temperature as Rochester chills for fall.

As seasons change and days become shorter, Rochesterians begin to brace themselves for another long winter. Opening the front door on an early fall morning, you may expect familiar summer air only to be rudely welcomed by a blanket of frost.

Long before winter makes her presence felt on the northeast, the transition into fall not only brings the beauty of radiant deciduous trees but also the cool, crisp air of autumn.  Did you ever wonder why during this period, daytime temperatures may still climb into the 80s but come nightfall, temperatures dip well into the 40s? Or in a matter of minutes following sunset, temperatures will drop ten or almost twenty degrees?

This is a direct result of a decrease of daily dew point temperatures. The dew point temperature is the temperature at which the air needs to cool to reach saturation. What does this mean? Essentially, dew point temperature is the measure of water vapor or moisture in the air. To grasp the idea of dew point temperature, think back two months to the unbearably muggy days of July. At some point everyone has said, “It’s way too hot outside”. Hot temperatures are exacerbated by exceptionally high dew point temperatures (approximately 65° and greater) not allowing perspiration or sweat to evaporate efficiently, which creates an oppressively hot, sticky feeling.

This week, Rochesterians have begun to experience “fall” like weather, especially in the mornings. Sure colder temperatures are associated with this feeling.  However, fall brings a certain crisp, dry air, allowing more evaporation of perspiration, and thus comfortable conditions.

Since the dew point temperature is the temperature at which air needs to cool to reach saturation, it is important to note that temperature cannot be lower than dew point temperature. Thus, one can use this fact to help predict the low temperature for a given day.  When the sun sets and earth cools due to lack of incoming solar radiation (heat from sun), the temperature will usually cool to around the dew point temperature. This is why lows in the summer often stay in the mid 60s or even 70s while lows in early autumn will drop into the 40s or below.

Dew point temperature and air temperature can be equal to one another though, which results in 100% relative humidity causing the air to condense and form early morning dew. As fall progresses, temperatures continue to drop, along with the dew point temperature, eventually falling below the freezing mark (32°F).  When the relative humidity is 100% and the temperature is below freezing, water in the air deposits on the ground as frost.

When frost coats the ground for the first time in autumn, it’s a sure thing that beach season has come to an unfortunate end.