Titan U glossary

Relative HumidityRH

Relative humidity (RH) is the ratio of the water vapor actually in the air to the amount the air could hold at its current temperature, expressed as a percent.

Storm Forecasting Moisture

Relative humidity (RH) is the ratio of the water vapor actually in the air to the amount the air could hold at its current temperature, expressed as a percent.

The physics of the ratio

The air's capacity to hold water vapor rises sharply with temperature. Because RH divides the actual vapor by that temperature-dependent capacity, warming the air lowers RH even when no vapor has left, and cooling it raises RH even when no vapor has arrived. Saturation occurs at 100 percent RH, which is also where the temperature equals the dewpoint.

This is why RH is best thought of as a distance-to-saturation measure rather than a moisture measure. Two locations at 60 percent RH can hold very different amounts of water vapor if one is at 45 degrees Fahrenheit and the other is at 90.

How forecasters use it

Forecasters read RH by layer. Near the surface, high RH favors fog, drizzle, and low ceilings. In the middle troposphere, roughly 700 to 500 mb, lower RH lets falling precipitation evaporate and cool the air, which strengthens downdrafts and can support strong outflow winds. In the upper troposphere, RH shapes cloud tops and anvil coverage.

For severe weather setups, forecasters often prefer to look at the whole vertical profile of RH on a sounding rather than a single-level value. A moist boundary layer under a drier middle troposphere is a common signature associated with strong downdrafts and hail-producing storms.

Where it misleads

A high RH does not mean a lot of moisture. Cool morning air at 95 percent RH can hold less total vapor than a warm afternoon at 40 percent RH. To compare moisture between times or places, use dewpoint or mixing ratio, which do not depend on temperature.

RH is also sensitive to small errors in the temperature reading, especially in dry air near saturation. Forecasters cross-check with dewpoint and, when available, direct moisture soundings before drawing conclusions about how close a layer is to producing clouds or precipitation.

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How Relative Humidity connects

Leads to 12

  • Fine Fuels

    Fine fuel moisture can change very quickly with relative humidity, which is why fine fuels drive most of the daily swing in fire danger.

  • Fire Weather Watch

    A Fire Weather Watch points to the local Red Flag criteria, which often pair low relative humidity with strong wind.

  • Fuel Moisture

    Dead fuel moisture follows relative humidity closely, so fuels dry out and catch fire more easily when humidity is low.

  • Red Flag Warning

    Very low relative humidity is one of the weather ingredients in Red Flag criteria, though each office sets its own numbers.

  • Virga

    Virga depends on how dry the air below the cloud is, because falling rain or snow only vanishes before landing where that layer is far from saturation.

  • Elevated Fire Weather Area

    An Elevated area is drawn from fuels plus forecast wind, relative humidity and temperature that come close to, but stay short of, critical.

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  • Evaporative Cooling

    Evaporative cooling depends on how far the air is from saturation, which relative humidity describes, since drier air allows more evaporation.

  • Fire Behavior

    Low relative humidity dries fuels so they catch more easily, which is one way the weather side of fire behavior works.

  • Fuel Timelag Classes

    The fastest timelag class leans on humidity, since 1-hour fuel moisture is governed mainly by the temperature and humidity of the air touching the fuel.

  • National Fire Danger Rating System

    NFDRS depends on relative humidity, one of the weather inputs to the model that now supplies its dead fuel moisture values.

  • Precipitation Efficiency

    Moist air through the depth of a storm raises precipitation efficiency, because less dry air gets mixed into the storm.

  • Wet-Bulb Temperature

    Wet-bulb temperature can be calculated from temperature, relative humidity and pressure, and the classic wet-bulb reading is also used to find humidity.

Contrasts with 4

  • Dewpoint

    Dewpoint tracks how much moisture is actually in the air, while relative humidity swings with temperature through the day even when moisture stays the same.

  • Mixing Ratio

    Mixing ratio stays put when air simply warms or cools, so it shows whether real moisture arrived, while relative humidity changes with heating alone.

  • Dewpoint Depression

    Relative humidity and dewpoint depression both show how close air is to saturation, and a wide spread on a warm afternoon means low relative humidity.

  • Vapor Pressure Deficit

    Relative humidity is a ratio that hides temperature, while vapor pressure deficit adds the temperature effect to show how hard the air is drying fuels.

Used to measure 2

  • Midlevel Dry Air

    Midlevel dry air is defined by low relative humidity in the middle troposphere, usually between about 700 and 500 mb.

  • Humidity Recovery

    Humidity recovery is measured as the overnight rise in relative humidity as the air cools.

Seen in 1

  • Atmospheric Sounding

    Forecasters often read relative humidity through the whole vertical profile on a sounding rather than from a single level.