Weather 101

Air pressure is the force exerted by the weight of the air column above a given point. Higher altitudes have lower pressure because there is less air above them. Differences in pressure drive wind and are the foundation of every weather pattern — air always moves from high pressure toward low pressure.

Wind is air moving horizontally from areas of higher pressure to areas of lower pressure. The steeper the pressure gradient — the faster the pressure changes over distance — the stronger the wind. Earth's rotation adds a twist: it deflects wind to the right in the Northern Hemisphere (Coriolis effect), which is why storms spin counterclockwise here.

Relative humidity tells you how close the air is to holding all the water vapor it can at its current temperature — 100% means it's saturated. The dew point is the temperature at which that saturation is reached and condensation begins. A dew point above 60°F (15°C) starts to feel muggy; above 70°F (21°C) it becomes oppressively humid.

The jet stream is a band of strong winds 5–9 miles up, typically blowing from west to east at 100–200 mph. It forms along the boundary between cold polar air and warmer subtropical air. The jet stream steers storm systems across the country — where it dips south, cold air invades; where it bows north, warmth follows. Its position and waviness are the core of mid-range weather forecasting.

An air mass is a vast body of air — often spanning hundreds of miles — that takes on the temperature and moisture characteristics of the region it sits over. A continental polar air mass over Canada is cold and dry; a maritime tropical air mass over the Gulf of Mexico is warm and humid. U.S. weather is largely driven by the collision of these contrasting air masses.

The atmosphere is divided into layers. The troposphere (0–7 miles) is where all weather happens — temperature generally drops with altitude here. Above it, the stratosphere (7–31 miles) contains the ozone layer and actually warms with height, which is why it acts as a lid on convection. The polar vortex lives in the stratosphere, and its disruption can trigger major cold outbreaks weeks later.

A cold front marks the leading edge of an advancing mass of cold air. Because cold air is denser, it wedges under the warmer air ahead of it and forces it to rise sharply. That rapid lifting triggers tall cumulus clouds, heavy rain, and sometimes severe thunderstorms. After a cold front passes, temperatures drop noticeably, dew points fall, and winds shift to the northwest.

A warm front marks where advancing warm air rides up and over colder, denser air that retreats slowly ahead of it. Because the lifting is gradual, clouds thicken and lower over many hours before rain arrives — typically stratus and nimbostratus producing steady, prolonged drizzle or light rain. Temperatures and dew points rise steadily as the front passes.

Occlusion happens when a fast-moving cold front overtakes the warm front ahead of it, lifting the warm air mass off the surface entirely. The result is a complex boundary with characteristics of both front types — often bringing prolonged precipitation and a transition zone where temperatures and wind direction are ambiguous. Occluded fronts are common in mature, weakening storm systems.

A stationary front forms when the boundary between cold and warm air masses stops moving. Neither side has enough push to advance on the other. The result is often days of cloudy, rainy weather along the front line, sometimes with localized heavy rainfall as moisture repeatedly converges along the same narrow zone. Many flooding events are tied to a slow-moving or stationary front.

A low pressure system (or cyclone) forms where air converges at the surface, rises, and creates a region of lower atmospheric pressure. Air spirals counterclockwise into a low in the Northern Hemisphere. Rising air cools, moisture condenses, and clouds and precipitation form. Lows are responsible for most of the country's significant storm events — from nor'easters to Gulf Coast depressions.

A high pressure system forms where air sinks and spreads outward at the surface, clockwise in the Northern Hemisphere. Sinking air warms and dries out, suppressing cloud formation and precipitation. Highs typically bring fair, settled weather — but they can also amplify heat in summer and lock cold air in place in winter. A strong, persistent high is called a blocking high and can park weather patterns for weeks.

Rain forms when water droplets in a cloud collide and merge until they are too heavy to stay aloft. Drizzle has very small droplets (less than 0.5 mm), while heavy rain can produce drops several millimeters across. Rain intensity is categorized as light (under 0.1 in/hr), moderate, and heavy (over 0.3 in/hr). Duration matters as much as intensity for flooding — a moderate rain over 12 hours can saturate soil as effectively as a brief downpour.

Snow forms when water vapor freezes directly into ice crystals in clouds where temperatures are below freezing. The shape of each snowflake depends on temperature and humidity during its descent. Snow accumulation depends not just on how much falls but on its water content — light, fluffy snow in very cold air might be 30:1 (30 inches of snow for 1 inch of liquid), while wet, heavy snow near 32°F (0°C) might be closer to 8:1.

Both freezing rain and sleet involve a warm layer aloft above a shallow layer of cold surface air. With freezing rain, drops stay liquid through the warm layer and freeze on contact with cold surfaces — coating everything in a glaze of ice. With sleet, the surface cold layer is deep enough that drops refreeze before hitting the ground, producing ice pellets instead. Freezing rain is far more disruptive: a quarter inch can make roads impassable and bring down power lines.

Hail forms inside powerful thunderstorm updrafts. A hailstone begins as a small ice pellet and grows each time the updraft carries it back up into the supercooled liquid water zone, adding a new layer of ice. The stronger the updraft, the longer the hailstone stays aloft and the larger it grows. Quarter-sized hail (1 inch) is common in supercells; baseball-sized or larger (2.75 inches) is severe and can shatter windshields and strip vegetation.

Fog is simply a cloud that forms at or near the surface when air cools to its dew point. Radiation fog forms on clear, calm nights as the ground radiates heat away — common in valleys and low-lying areas. Advection fog forms when warm, moist air moves over a cold surface, like the warm Gulf Stream meeting colder coastal waters. Fog dissipates when the sun warms the surface or wind mixes in drier air from above.

A thunderstorm requires three ingredients: moisture (fuel), lift (a trigger to start air rising), and instability (an atmospheric profile that keeps air rising once started). As towering cumulonimbus clouds build to 50,000 feet or higher, ice crystals and water droplets collide and separate electrical charge. Lightning is the discharge that equalizes this separation — thunder is the shockwave from the rapid heating of air along the lightning channel.

Tornadoes typically form within supercell thunderstorms when the rotating updraft (mesocyclone) tightens and stretches toward the ground. Wind speeds range from 65 mph in a weak EF0 to over 200 mph in a violent EF5. The Enhanced Fujita scale rates tornadoes based on damage to structures. The U.S. averages about 1,200 tornadoes per year — more than any other country — concentrated in the Great Plains' Tornado Alley but possible anywhere.

Hurricanes are organized tropical systems that develop over warm ocean water (at least 79°F / 26°C). They are powered by the latent heat released when warm, moist air rises and condenses. The Saffir-Simpson scale ranks hurricanes from Category 1 (74–95 mph) to Category 5 (157+ mph) based on sustained wind speed. Storm surge — the dome of water pushed ashore by the storm's winds — is responsible for the majority of hurricane fatalities.

The official NWS definition of a blizzard requires sustained winds or frequent gusts of 35 mph or more, combined with falling or blowing snow that reduces visibility to a quarter mile or less for at least three consecutive hours. Temperature does not factor into the definition — a blizzard can technically occur near freezing if strong winds drive heavy wet snow. The danger is primarily whiteout conditions and wind chill, not necessarily heavy accumulation.

A derecho is a widespread, long-lived wind storm associated with a band of rapidly moving showers and thunderstorms. To qualify, it must produce wind damage reports spanning at least 240 miles with wind gusts reaching 58 mph or more along most of the path. Derechos are driven by a cold, fast-moving outflow boundary and can cross multiple states in a matter of hours, causing damage that rivals a weak tornado — but in long, straight swaths rather than narrow paths.

Flash floods develop within six hours (often less) of a heavy rain event. They occur when rainfall intensity exceeds the ground's ability to absorb water — especially over urban pavement, saturated soil, or in narrow mountain canyons. Just six inches of fast-moving water can knock a person off their feet; two feet can carry away most vehicles. Flash floods are the number one weather-related cause of death in the U.S., killing roughly 90 people per year.