Here in the Delaware Valley, we are heading into our “fog season.” But why? Why are there so many foggy mornings in the Fall?

Firstly, fog is a type of cloud. It is the only type that forms at ground level. Fog can be particularly crippling to motorists as a result of its high water particle density, making visibility levels extremely poor.  There are many classifications of fog that form in different areas of the world. In our area, we, for the most part, experience radiation fog.

Radiation fog can only occur when:

1. The temperature and dew point (temperature for water vapor in the air to condense into water) are less than 4 degrees.

2. The humidity is 100% (meaning the air is saturated with water vapor particles)

And most importantly,

3. After a clear and chilly night

Clear and chilly nights (common in the fall) are crucial in the formation of radiation fog. Radiational cooling of the air occurs, lowering the saturation point of water vapor. When the air becomes cool enough, the relative humidity will eventually reach 100%. Then, if the dew point and temperature are less than 4 degrees apart, fog will form. Radiation fog will form before sunrise and end shortly after as daytime heating begins and the saturation point goes back up.

I figured it would be a good time to talk about cold fronts and how they contribute to our weather pattern.

Cold Fronts: So what are cold fronts exactly? Usually portrayed by a blue line with triangles pointing in the direction of travel, a cold front is cooler air replacing warmer air ahead of it. Cold fronts originate from a low pressure system and are always complimented by a warm front. This is shown in the graph below along with the process of the cooler air replacing the warmer air:

Cold fronts are most common during the summer where they bring nasty thunderstorms. Here is an example of a classic summer day involving a cold front:

The morning starts out extremely humid as the warm front passes to the north. By noontime, temperatures are in the 80s and are approaching 90 fast. By the afternoon hours, the inevitable cold front is approaching fast with thunderstorms starting to build along it. By the 4 or 5 o’clock hour, the front passes through, bringing in nasty thunderstorms and gusty winds. By the nighttime hours, skies clear and the temperature is noticeably cooler. The next day should be a beauty!

The main reason why precipitation, especially thunderstorms, form along  a cold front is because of the upward motion of the warmer air as the front barrels from west to east. The greater the upward motion, the warmer the temperature and the more unstable the atmosphere ahead of the front. This is why we get the most severe thunderstorms during the summer.

Lesson Number 2: Low Pressure

I decided to focus on Low Pressure systems for my second lesson. For the most part, when people hear “low pressure”, they generally associate it with storminess, Nor Easters, snowstorms, hurricanes, and cold fronts. All of the previous listed occurrences come from a low pressure system.

Low pressure systems are generally more complicated than high-pressure systems. They are actually the polar opposite of high pressures. Low pressure systems are areas of counterclockwise circulation where the pressure at sea level is lower than the actual Earth pressure (29.92 inHg). Usually a low pressure system is connected to a cold front spreading to the south and a warm front spreading to the east (see map). As I said before, a low pressure system always means cloudy, rainy, and/or snowy weather.

In a low pressure area, warm air coming from the south rises and cools. This cooling causes cloud formation and eventually the clouds become dense enough to produce precipitation. The lower the pressure, the more precipitation that will be associated with the system, hence the reason why hurricanes are so expansive.

I want Nor Easters and Hurricanes to be their own separate lesson, but I’ll talk briefly about them here because of their relation to low pressure systems:

The reason why a Nor Easter gives us snow is that all the moisture from the warm front in the ocean is brought around the storm and ultimately comes from the northeast, mixing with the arctic air and giving us a major snowstorm.

Here is a simple map of a Nor Easter:

Hurricanes:

A strong hurricane forms and strengthens over warm water. This is because as the low pressure is moving west, it gathers moisture from the warm air and ocean. This moisture continues to circulate around the system as it keeps moving, causing something like the snowball effect. Eventually cyclogenesis occurs and the low pressure starts rotating around itself. I will talk more in detail about this phenomenon in the hurricane lesson.

Here is an example of the amount of moisture a hurricane can make during its trek across the Atlantic:

For the rest of the winter into spring and summer, I am introducing weather lessons for anyone who is curious about the weather and how it works. I am hoping that by next winter, most weather terms will be covered and described. I hope to educate my readers and myself about the weather and its inter-workings.

Lesson 1: High Pressure

High pressure is a term many of you have probably heard of on the news. It is generally associated with dry, cooler, and sunny weather, but there are certain exceptions. Without high pressures in the atmosphere, there wouldn’t be many dry and sunny days!

But what really is a high pressure? It is where the pressure on the surface of the earth is greater than the surrounding environment. The high pressure part, therefore, only describes what is going on at the surface compared to the atmosphere above. High pressure is usually an area of clockwise rotating winds surrounded by a hypothetical center marked  with a H.

Depending on where the high pressure is determines our weather. If we have a “Bermuda High” (high pressure over the Bermuda area), we get warmer temperatures because of the southerly winds coming from the clockwise spin of the high.

Here is an example of a weaker Bermuda high (high is east of Bermuda), notice the arrows from the south coming upward:

A high to the west of our area generally brings in the colder air, thus giving us the perfect setup for snowstorms. The perfect placement of a high pressure system for a snowstorm is in Quebec, where the winds come in from the north. The reason why the last storm (the ice storm) didn’t have enough cold air aloft was because the high was stuck up in southern Ontario, giving us a westerly flow instead of a northerly one.

Here is an example of the Quebec high setup:

I will have another Meteorological Lesson next week, this time talking about low pressures. 5 day forecast to come on Wednesday or Thursday.