How do you read the dewpoint and relative humidity charts on the Earth Science Reference Tables?

First find the wet-bulb depression: subtract the wet-bulb temperature from the dry-bulb temperature. Then on the Dewpoint Temperature chart, read across from the dry-bulb temperature (left column) and down from the depression (top row) to the dewpoint. Use the same dry-bulb row and depression column on the Relative Humidity chart to read the percentage. A large depression means dry air and low relative humidity; a small depression means moist air and high relative humidity; a zero depression means the air is saturated at 100 percent.

How do you decode the barometric pressure on a weather station model?

The pressure is given as a three-digit code at the top right of the station model. To decode it, put a 9 or 10 in front and place a decimal before the last digit, choosing whichever value is closest to 1000 mb. For example, the code 248 becomes 1024.8 mb, and the code 850 becomes 985.0 mb. Pressures above about 1013 mb are high (fair weather); pressures below are low (more unsettled weather).

What is the difference between dewpoint and relative humidity?

Dewpoint is the temperature to which air must be cooled to become saturated, so it measures the actual amount of water vapor in the air (a higher dewpoint means more moisture). Relative humidity is the ratio of the vapor present to the maximum the air could hold at its current temperature, written as a percentage. Cooling air toward its dewpoint raises the relative humidity to 100 percent; warming the same air lowers its relative humidity. Keep the amount (dewpoint) and the percentage of capacity (relative humidity) separate.

What weather do high-pressure and low-pressure systems bring?

High-pressure systems (anticyclones) have sinking air that warms and dries, so clouds cannot form, giving clear, fair, dry weather; surface winds diverge outward (clockwise in the Northern Hemisphere). Low-pressure systems (cyclones) have rising air that cools to its dewpoint, forming clouds and precipitation, giving cloudy, stormy weather; surface winds converge inward (counterclockwise in the Northern Hemisphere). The rule to remember is that high means fair and low means stormy.

New YorkEarth and Environmental Science

Weather and water: decoding the Reference Tables for the NY Regents hydrosphere and meteorology unit

Q: What factors control the climate of a region?

Latitude (lower latitudes are warmer because insolation is more direct), elevation (higher is colder), proximity to large bodies of water (coasts are milder, with smaller temperature ranges, because water has a high specific heat), ocean currents (warm currents warm and moisten coasts, cold currents cool and dry them), mountain barriers (the windward side is wet, the leeward side is a dry rain shadow), and prevailing winds (which carry heat and moisture). Climate is the long-term average weather, not the day-to-day weather.

A deep-dive guide to the hydrosphere and meteorology unit for the NY Regents: the water cycle and groundwater, energy transfer in the atmosphere, reading the dewpoint and relative humidity charts, decoding the station model and pressure code, air masses, fronts and pressure systems, and the factors that control climate, with worked chart readings.

Generated by Claude Opus 4.819 min readESCI-HYDROUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

The weather unit is a reading-the-tables unit
The water cycle and groundwater
Energy transfer in the atmosphere
Reading the moisture charts
Decoding the station model
Air masses, fronts and pressure systems
The factors that control climate
Check your knowledge

The weather unit is a reading-the-tables unit

The hydrosphere and meteorology unit rewards two skills above all: reading the moisture charts and decoding the station model. Both are pure Reference Tables tasks, and both appear every administration. This guide ties the dot-point pages together: the water cycle and groundwater, the oceans and surface currents, the atmosphere and energy transfer, moisture, dewpoint and humidity, weather systems, air masses and fronts and climate and the factors that control it.

The water cycle and groundwater

The water cycle (evaporation, transpiration, condensation, precipitation, infiltration, runoff, storage) is driven by solar energy and gravity. How much rain infiltrates rather than runs off depends on porosity (percentage of pore space), permeability (how easily water passes through, set by pore size and connection), slope, saturation and vegetation. The classic distinction: clay is porous but nearly impermeable (high runoff), while gravel is porous and highly permeable (fast infiltration). Below the water table lies the saturated zone (groundwater).

Energy transfer in the atmosphere

Energy moves by radiation (electromagnetic waves, no medium, the Sun to Earth), conduction (direct contact) and convection (a heated fluid rising while cool sinks, the main mover in the troposphere and oceans). Dark, rough, dull surfaces absorb insolation best (and radiate well); light, smooth, shiny surfaces reflect it (high albedo). The atmosphere is layered by its temperature profile (troposphere, stratosphere, mesosphere, thermosphere), and pressure always decreases with altitude.

Reading the moisture charts

Dewpoint and relative humidity

A psychrometer reads a dry-bulb of 18 degrees Celsius and a wet-bulb of 13 degrees Celsius. Find the dewpoint and the relative humidity.

step 1 Find the depression

$\text{depression} = 18\,^\circ\text{C} - 13\,^\circ\text{C} = 5\,^\circ\text{C}$ .

step 2 Read the dewpoint

On the Dewpoint Temperature chart, the 18 degree dry-bulb row and the 5 degree depression column give a dewpoint of about 10 degrees Celsius.

step 3 Read the relative humidity

On the Relative Humidity chart, the same row and column give about 55 percent (accept the NYSED value).

step 4 Interpret

Cooling this air from 18 degrees down to its 10 degree dewpoint would raise the relative humidity to 100 percent and start condensation, forming cloud or dew.

Decoding the station model

The circle's shading is sky cover; temperature sits top left and dewpoint bottom left (degrees Fahrenheit). Combine the readings to describe the air, then use the rule that New York weather moves west to east to forecast.

Air masses, fronts and pressure systems

Air masses: mT (warm, moist), cP (cold, dry), mP (cool, moist), cT (hot, dry). Cold fronts bring a narrow band of heavy storms then cooler, clearing weather; warm fronts bring a wide band of lighter precipitation then warming. High pressure (sinking, diverging) is fair; low pressure (rising, converging) is stormy.

The factors that control climate

Climate (the long-term average) is set by latitude (lower is warmer), elevation (higher is colder), proximity to water (coasts are milder), ocean currents (warm currents warm and moisten coasts), mountain barriers (wet windward side, dry leeward rain shadow) and prevailing winds.

Check your knowledge

Attempt these with the Reference Tables open, then check the solutions.

A dry-bulb of 22 degrees Celsius and a wet-bulb of 16 degrees Celsius: find the depression, dewpoint and relative humidity. (3 marks)
Decode the station-model pressure code 174. (1 mark)
State the weather (sky and winds) of a high-pressure system. (2 marks)
Distinguish porosity from permeability with an example. (2 marks)
Name the three modes of energy transfer and which moves heat through the troposphere. (2 marks)
Explain why a coastal city has a smaller temperature range than an inland city at the same latitude. (2 marks)
Explain how a rain shadow forms. (2 marks)

Solutions

Q1: Depression = 22 - 16 = 6 degrees Celsius; dewpoint about 13 degrees Celsius; relative humidity about 56 percent (accept the NYSED chart values).
Q2: 174 becomes 1017.4 mb (add 10, decimal before the last digit, nearest 1000 mb).
Q3: Clear, fair, dry skies with surface winds diverging outward (clockwise in the Northern Hemisphere), because the air is sinking.
Q4: Porosity is the percentage of open pore space; permeability is how easily water passes through. Clay is porous but nearly impermeable; gravel is porous and highly permeable.
Q5: Radiation, conduction and convection; convection moves heat through the troposphere (warm air rises, cool air sinks).
Q6: The nearby water has a high specific heat, so it warms and cools slowly, moderating the coastal air; land heats and cools quickly, so inland temperatures swing more.
Q7: Moist air rises up the windward side of a range, cools and rains; descending the leeward side it warms and dries, leaving a dry rain shadow.

Sources & how we know this

Reference Tables for Physical Setting/Earth Science (2011 edition) — New York State Education Department (2011)
Regents Examination in Physical Setting/Earth Science — New York State Education Department (2026)