Invest
A weather system designated for investigation by the National Hurricane Center. An Invest designation triggers enhanced data collection and model runs but does not indicate expected development. Many Invests never become tropical cyclones.
More Tropical Weather Terms
Hurricane
A tropical cyclone with maximum one-minute sustained surface winds of 74 mph or greater in the Atlantic or eastern and central North Pacific. Hurricanes are categorized 1-5 on the Saffir-Simpson Hurricane Wind Scale, with Category 5 beginning at 157 mph.
Typhoon
A tropical cyclone with sustained winds of 74 mph or greater in the Western Pacific. Typhoons are meteorologically identical to hurricanes; the different name reflects only their geographic location.
Cyclone
A rotating low-pressure weather system. Tropical cyclone terminology and intensity categories vary by region: in Australia, a tropical cyclone has maximum sustained winds of at least 34 knots (63 km/h, about 39 mph), averaged over ten minutes. The term does not universally require hurricane-force winds of 74 mph.
Eye Wall
The ring of intense thunderstorms surrounding a hurricane's eye, containing the storm's strongest winds and heaviest rainfall. Eye wall replacement cycles can temporarily weaken a hurricane before it potentially re-intensifies.
Storm Surge
Storm surge is an abnormal rise in sea level during a hurricane or other intense storm, measured as the height of the water above the normal predicted tide. It is produced mainly by the storm's winds pushing water toward the shore. Storm surge is the leading cause of deaths from hurricanes.
Tropical Depression
An organized system of clouds and thunderstorms with defined circulation and maximum sustained winds of 38 mph or less. Tropical depressions can strengthen into tropical storms and eventually hurricanes.
Tropical Storm
A tropical cyclone with maximum sustained winds between 39-73 mph. At this stage, the storm receives a name. Tropical storms can produce dangerous flooding, winds, and tornadoes even without reaching hurricane strength.
Saffir-Simpson Scale
The Saffir-Simpson Hurricane Wind Scale rates hurricanes from Category 1 to Category 5 by maximum sustained wind speed. Category 1 starts at 74 mph and Category 5 at 157 mph, and Category 3 and higher are called major hurricanes. The scale measures wind only: it does not account for storm surge, rainfall flooding, or tornadoes.
Post-Tropical Cyclone
A post-tropical cyclone is a former tropical cyclone that no longer has enough tropical characteristics to be classified as one. It can still bring heavy rain and strong winds. The two kinds are extratropical cyclones, which draw their energy from temperature contrasts between air masses, and remnant lows, which have winds below 34 knots (39 mph).
Invest: Guides and Explainers
These explainers connect this definition to real-world forecasting and decision-making.
Weather Education
Spaghetti Models Explained: What Those Tangled Hurricane Lines Actually Mean
Spaghetti models are a tangle of individual forecast tracks, not a forecast. Here is what each line is, what the labels like AVNO and EMX stand for, and how to read the spread instead of the scariest strand.
Weather Education
Typhoon vs Hurricane vs Cyclone: Same Storm, Three Names
A typhoon and a hurricane are the same kind of storm with the same 74 mph threshold. What changes is the ocean basin, the agency, and how the wind is measured. Here is the full naming map.
Weather Education
What Is an Invest in Weather? What 90L, 91L, and 92L Actually Mean
An Invest is a data-collection label, not a forecast. Here is what the National Hurricane Center means by Invest 90L, how the numbering and basin codes work, and how an Invest becomes a named storm.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Live Tracker
Live Hurricane Tracker
Follow every active Atlantic and Pacific storm with the official NHC cone of uncertainty and forecast track, refreshed continuously.
Guide
Hurricane Categories Explained
Understand what each Saffir-Simpson category says and what it does not say about a storm's risk.
Seasonal Hub
Hurricane Season Guide
Track tropical risk, categories, and preparedness advice in one place.
Feature
Hurricane Tracker
Track cones, watches, warnings, and tropical systems from formation through landfall.
Explore Other Terms
CAPE
CAPE (Convective Available Potential Energy) is the amount of energy a rising parcel of air can draw from the atmosphere, measured in joules per kilogram (J/kg). The higher the CAPE, the faster a storm's updraft can rise, and the stronger thunderstorms can become once something sets them off.
VFR
Visual Flight Rules allow flight using visual references when the applicable visibility and cloud-clearance minimums are met. Those minimums vary by airspace class, altitude, and day or night under 14 CFR 91.155. Separately, the VFR weather category means a ceiling above 3,000 feet and visibility greater than 5 statute miles.
Frost
Frost is a thin layer of ice crystals that forms on surfaces when they cool to 32°F (0°C) or below and water vapor in the air freezes onto them. It is the surface that has to reach freezing, not the air: frost can form while the official air temperature is still in the mid 30s.
Small Craft Advisory
A Small Craft Advisory is a National Weather Service alert for winds or seas that are hazardous to small boats in coastal and Great Lakes waters. The wind and sea thresholds are set separately for each geographic area. There is no precise definition of a small craft: any vessel that the conditions could affect counts.
Red Flag Warning
A Red Flag Warning is a National Weather Service alert that weather conditions could cause extreme fire behavior. It is issued when those conditions are occurring or expected within 24 hours. Typical criteria combine dry fuels with sustained winds of 15 mph or more and relative humidity of 25% or less, and exact thresholds vary by state and local office.
El Niño and La Niña
El Niño and La Niña are the warm and cool phases of a recurring climate pattern in the tropical Pacific called the El Niño-Southern Oscillation (ENSO). During El Niño, the trade winds weaken and the central and eastern tropical Pacific runs warmer than average. During La Niña, the trade winds strengthen and the same waters run cooler. Both shift weather patterns worldwide.
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