Holiday Weather Havoc: In the week before Christmas, the US East Coast from Florida to Maine was raked by strong winds and intense rainfall. Severe floods impacted all coastal states and many lost power. Streets were flooded in Florida from heavy rains and storm surges driven by high winds. Parts of the Carolinas received over a foot of rain in 24 hrs. High winds toppled trees as they lost grip in the rain-soaked soils in New York, leading to power outages there and throughout New England. Six people died from South Carolina to Maine and power went out for more than 600,000 and remained out for several days after the storm. Water rescues from homes and vehicles had to be done. Then just before New Years, California was hit with an intense storm that generated 25 ft waves that caused extensive coastal flooding in Ventura County. People and vehicles were swept away and beach front properties were damaged. As we get into the second week of January storms Ember and Finn are again raking the East coast with snow, rain, flooding and high winds.

We have previously noted that intensifying rainstorms are a direct consequence of our warming atmosphere. The moisture-holding capacity of the atmosphere rises exponentially with increasing temperature. But what of the strong winds that hit hurricane strength, and developed those huge waves that hit California? Some insight comes from observations during the pre-Christmas New England storm. Massachusetts’ Blue Hill Observatory recorded a gust of 90 mph while at New Hampshire’s Mt. Washington winds gusted to 132 mph. One factor is the change in wind strength with height. Blue Hill, the nation’s oldest meteorological station, has a height of 636 ft, while Mt Washington reaches 6,288 ft. Winds aloft are always stronger.The fast east-flowing winds of the jet stream at 30,000 ft make west-to-east flights significantly shorter than east-to-west flights. But how do those strong winds aloft make it down to ground level during a storm?

The answer lies in a simple aspect of the water vapor: it is a very light molecule! Consider the major constituents of the atmosphere from high school chemistry. Table 1 shows the atomic numbers and atomic weights of the main isotopes of the elements in the atmosphere.

Aside from the inert gas Argon, which comprises less than 1% of the atmosphere, individual atoms are bound up in molecules such as O₂, N₂ and H₂O. So the molecular weights of the atmosphere’s constituents are:

The water molecule is much lighter than the other gasses in the atmosphere, and thus is very significantly buoyant. When water evaporates from the surface of a leaf, the soil, a lake or the ocean, we know it is going to cause the parcels of air that it enters to be more buoyant than drier air. The consequences for the atmosphere are significant. Lighter gasses rise, so evaporation of water leads to a less dense and buoyant atmosphere. This is termed “moist convection”, with moisture laden rising air, as opposed to dry convection in which low humidity air rises because of a higher temperature. Moist convection is characterized by cloud formation, as the rising air eventually reaches a level where a lower temperature induces condensation of the water vapor into liquid droplets. When they reach sufficient size they will rain out, with that condensation representing a transfer of the latent heat energy initially put into the water vapor by the Sun at the surface, that is transferred to the air at the height of the clouds. The added heat energy can drive further convection. Such upward vertical motions must be compensated by downward flows that can bring very strong high-altitude winds to ground level. These strong vertical motions also contribute to lightning and thunderstorms, much the way we can generate static electricity by rubbing a non-conductor against hair or fur. Many afternoon thunderstorms are generated by the solar heating of moist ground. This is a local recycling process. Depending on the available moisture, the intensity of solar heating and the vertical structure of the atmosphere, such storms can be severe. If there are strong winds aloft then straight line “derechos” can cause significant damage to trees and houses.  If there is rotational shear aloft then the formation of tornadoes may be possible. And the large cyclonic ocean storms known as hurricanes and typhoons are powered by the buoyancy and latent heat cycle made possible by the structure of the water molecule. They are tapping into the thermal energy stored in the upper ocean by the Sun. Higher ocean temperatures and a deeper extent of warm water will lead to intensification of hurricanes. The unique properties of the very light water molecule give it an ability to store and release large amounts of energy that are the dominant drivers of atmospheric motions. Thus, the presence of water on our Blue Planet dictates much about our climate and weather and is key to the most severe storms that we encounter. We are coming off the warmest year on record and our weather can only become more volatile as the planet continues to warm.

Takeaways

• Winds aloft are stronger than surface winds
• Water vapor is much lighter than all other atmospheric gasses
• Buoyant moist convection brings strong winds down to the surface
• The warming atmosphere holds more water vapor, driving more severe wind storms and more intense flooding.

Salient has developed the world’s most skillful weather forecasts on the timescales of weeks to months ahead and can advise on future extreme weather risks in time for you to do something about it.