Catalina eddy? June gloom? Santa Ana winds? We’ve all heard terms like these in L.A.-area weather forecasts, but do we actually know what they mean?
UCLA staff meteorologist James Murakami, profiled in the UCLA Newsroom story, He's been UCLA's weatherman — rain or shine for three decades — helps us out with this glossary of common weather terms:
A conveyor belt, in effect, for water vapor flowing from the tropics. Excessively high precipitation occurs when large-scale storms tap into these rivers. An atmospheric river originating near the Hawaiian Islands is known as the "Pineapple Express" or "Pineapple Connection."
Catalina eddy/coastal eddy
Low-level wind that circulates over coastal waters is called a coastal eddy. The phenomenon gets dubbed a "Catalina eddy" when its center occurs near Santa Catalina Island. A Catalina eddy can influence the character of marine layer clouds throughout the area.
Dew point temperature
The temperature at which some of the water vapor contained in air condenses into precipitation. The higher the dew point temperature — which is different from air temperature — the more water vapor the air can hold.
El Niño/ La Niña
El Niño is an ocean phenomenon where tropical waters west of South America are warmer than normal for several months — typically at their warmest when winter is starting in the northern hemisphere — and influencing the wind-flow pattern around the world. In Southern California, a west-to-east-oriented jet stream, which guides winds in large-scale storms, ends up passing through more often than normal in winter. This can result in a wetter-than-usual winter — but not always.
La Niña is a similar phenomenon, but it occurs when tropical waters west of South America are colder than usual, typically at their coldest at the start of Southern California winters. La Niña can lead to a drier winter, but not always.
The boundary between two air masses of different temperatures and densities, both of them in motion and affecting each other. All fronts are capable of producing inclement weather. A cold front is one where a mass of cold air is displacing a mass of warmer air. In a warm front, warm air is displacing colder air. An occluded front occurs when cold air displaces warmer air above it. A stationary front shows little motion.
High-pressure system/ low-pressure system
A high-pressure system is an area where the atmospheric pressure is high compared to its surroundings. High-pressure systems are most often associated with dry, mostly sunny weather. A low-pressure system has atmospheric pressure that is low relative to its surroundings. Low-pressure systems are most often associated with inclement weather. Storms, in fact, are low-pressure systems of varying sizes.
Humidity (relative humidity)
A percentage measure of how saturated with water vapor the air is for a given temperature. A low percentage of humidity is associated with dry air, while a high percentage makes for moist air.
A layer of air that is warmer — and usually drier — than the air mass that lies below it. Southern California’s cool marine layers are always capped by an inversion layer.
A narrow band of strong winds — jet streams passing over California can exceed speeds of 150 miles per hour — occurring at altitudes several miles above the Earth and capable of having a large impact on weather. The polar jet stream guides the forward motion of large-scale storms and influences storm strength. The sub-tropical jet stream can be a source of upper-level moisture for storms.
A southern California term used to describe persistent, low clouds along the coast, commonly occurring in late spring — called “May Gray” during the month of May — and early summer. The phenomenon occurs when the marine layer is dense enough for low clouds to produce early-morning mist or drizzle.
A layer of cool, moist air that that originates from a large body of water — the Pacific Ocean, for instance — and makes contact with the ground. It is capped by a layer of warm, dry air. The more moist a marine layer is, the more likely clouds and fog accompany it.
Off-shore flow/on-shore flow
An off-shore flow — also also known as a “land breeze” — is a wind pattern where the air is coming from land and moving toward a large body of water. Depending on the depth of the off-shore flow, it can dilute or altogether purge a marine layer. In an on-shore flow — essentially, a sea breeze — the air comes from a large body of water and moves inland. The strength of an on-shore flow influences the character of the marine layer.
Santa Ana winds
A type of off-shore flow, Santa Ana winds occur when surface high pressure over the Great Basin states — most of Nevada and parts of Utah, Oregon, Idaho and California — induces persistent and sometimes strong winds from the northeast to flow over parts of Southern California. Santa Ana winds are dry and usually produce warmer than normal weather in our region, usually during the autumn and winter months.The winds also pose the danger of spreading wildfires.
A summer wind-flow pattern where air with high water vapor content — coming from the Gulf of Mexico, the Gulf of California or the tropical Pacific south of mainland Mexico — reaches the desert Southwest and may even briefly extend to the California coast. A southwest monsoon results in scattered showers and thunderstorms.
A type of off-shore flow pattern that occurs in coastal Santa Barbara County when strong winds from the northwest or north occur, producing a brief period of significantly warm, dry weather. These winds typically occur during late-afternoon or early-evening hours — hence the name “sundowner.”
Read this UCLA Newsroom story about Murakami: He's been UCLA's weatherman — rain or shine — for three decades