High humidity

The current weather pattern is feeding subtropical air onto the North Island.

Pohutukawa blosom , 3 Dec 2009

Pohutukawa blosom , 3 Dec 2009

Pohutukawa trees are revelling in the warmth and high humidity.  This one, near Auckland Harbour Bridge, is already near full bloom and it will not be long before its colleagues follow, all around the coastal North Island.

Weather pattern , 1pm Wed 2 Dec 2009

Weather pattern , 1pm Wed 2 Dec 2009

On the weather map shown here, for 1pm on Wednesday in the middle of the first week of December, some typical signs of summer can be seen:  The incoming high pressure system is, for a while, at near 45°S. The low pressure system in the Tasman Sea has been transporting some warm air from the subtropics on to northern New Zealand, raising our humidity.  (Next week, the  high is expected to return to its typical El Nino summer latitude of 35°S, so let’s enjoy some subtropical warmth when we can.)

The relative humidity has been “flat-lining” at 100% in many places all around the country this week.  The bottom part of the graph shown here gives the relative humidity at Whangarei, Auckland, Wellington, Christchurch and Dunedin for comparative purposes.  Out of all these locations, Christchurch was the place hugging 100% the most, so did it feel the most humid?

Dew point (degrees C) above and Relative Humidity % below for WRA Whangarei, AAA Auckland, WNA Wellington, CHA Christchurch and DNA Dunedin  . Timesteps are in UTC so 010000=1pm Tues 1 Dec 2009, 020000=1pm Wed 2 Dec, etc

Dew point (degrees C) above and Relative Humidity % below for WRA Whangarei, AAA Auckland, WNA Wellington, CHA Christchurch and DNA Dunedin . Timesteps are in UTC so 010000=1pm Tues 1 Dec 2009, 020000=1pm Wed 2 Dec, etc

The answer is NO.

Relative humidity measures how close the air is to saturation (see here for more) but not necessarily how muggy it feels to people.

In cool air, saturation is reached at relatively low amounts of water vapour – when the air temperature is below around 8°C your breath

can often contain sufficient moisture  to produce saturated air or misty cloud – at standard pressure that’s 6 g of water vapour per kg of air,

and this has been the amount of water vapour in Christchurch at times this week.    Places such as Auckland or Whangarei have this week been having around 12 g of water vapour per kg of air – twice the ‘humidity’ of Christchurch,  even when the relative humidity is 100%!

A better index for measuring mugginess is the dew point … the temperature at which, if cooled, water vapour from the air will condense on to a surface such as grass.  This condensation requires 100% relative humidity and occurs when the air temperature equals the dew point.

The air feels muggy when we get hot and sticky – and that’s when 1) the air is warm enough to make us perspire, and 2) the air contains sufficient water vapour to interfere with the evaporation of our perspiration and not allow us to cool down much from that.  So mugginess depends on both heat and humidity.

In the graph shown above the dew point is plotted on the upper table.  Notice how Whangarei and Auckland have both been on top this week.  They have often been having nearly the same dew point and so have been nearly equally muggy. However, only Auckland has, occasionally at night, been flat-lining at 100% relative humidity.  Even though both places are equally muggy, Whangarei has slightly warmer air than Auckland and thus slightly lower relative humidity.

The dew point temperature is a great mugginess parameter, for it combines heat and humidity into one number. Wikipedia has more information about the human perception of mugginess depending on the dew point, ranging from extreme discomfort above around 24°C, to noticeably humid from 16°C-18°C, to noticeably dry below 10°C

I hope this has posting has helped to clear up any misconceptions between relative humidity and perception of mugginess.  However, leave me a comment if you have any queries.

Jon Tunster of Wellington comments:

Here’s a photo of the same tree at Oriental Bay, Wellington taken on 6 Dec 2008, and a year later on 7 Dec 2009.

Oriental Parade Pohutukawa 6 Dec 2008

Oriental Parade Pohutukawa 6 Dec 2008

Oriental Parade Pohutukawa 7 Dec 2009

Oriental Parade Pohutukawa 7 Dec 2009

Flowering is considerably delayed this year, maybe due to the cold October?

Bob McDavitt answers:

The name for the study of plant (and animal) life cycle events in relation to the changing seasons is Phenology.

These photos dramatically illustrate the difference between the start of  a La Nina summer (Dec 2008) and an El Nino summer (Dec 2009). The chilly air and soil in October is indeed a likely source of the delayed flowering of the pohutukawa.

Continue reading

The mid-July northern low

On the night of 17th July and early on the 18th, New Zealand was affected by a fast-moving and rapidly deepening depression originating in the north Tasman Sea. Sustained southwesterly winds of more than 60 knots were recorded in Colville Channel as the low passed by. Severe Weather Warnings were issued for wind in Coromandel/Great Barrier Island and rain in the eastern North Island.

This post will have a look at some of the reasons that the low deepened so rapidly, and whether the computer models did a good job of predicting its path. We’ll also see that weather is a lot more complicated than simply following what happens at the surface only.

MODIS imagery of the low as it moved away from the country on Saturday 18th July
MODIS imagery of the low as it moved away from the country on Saturday 18th July

On Friday 17th July at 6PM, a low was analysed just west of Northland with a central pressure of about 997 hPa.

24 hours later the low lay a few hundred kilometres east of the North Island and had deepened to around 975 hPa. It continued deepening as it headed southeastwards – New Zealand was only affected by the first stages of the rapid development.

Midday 17th July
Midday 17th July – a “slack low”
Midnight 17th July
Midnight 17th July – 12 hours later the low has quickly evolved

The rapid development of this low cannot be adequately explained by considering surface conditions only. Chris Webster here states “Per­haps we inad­ver­tently rein­force a per­cep­tion that our weather is 2-D by pub­lish­ing lots of weather maps that are valid only at the Earth’s sur­face (e.g., see Weather Maps). Rarely do we show what’s hap­pen­ing higher up through the atmos­phere or, more pre­cisely, the tro­pos­phere —?the part of the atmos­phere that con­tains our weather.”

Our low was in fact strongly influenced by what was happening at the highest reaches of the troposphere – the regions where jet streams tend to be present.

Upper levels

ECMWF forecast for 3PM on the 17th. Green contours are surface isobars. Shaded regions are those of high upper level winds (black being the strongest).

ECMWF forecast for 3PM on the 17th. Green contours are surface isobars. Shaded regions are those of high upper level winds (black being the strongest).

This forecast chart is entirely computer generated, but shows why the low began organising itself and deepening from the Friday evening. The surface low pressure centre lies near the equator-ward entrance region of an upper level jet stream. This is a particularly favourable position for cyclogenesis (meaning development of a low). This is because it is an area of upper level divergence, which favours convergence at the surface and upward motion  – both of which are conducive to developing depressions.
The other favourable position is the pole-ward exit region. Where these coincide (the exit of one jet, and the entrance of another), and where a low lies downstream from a sharpening upper trough then a recipe exists for “explosive cyclogenesis”:

uppertrough

Here are the comments made by senior forecasters regarding the development of this low:

“A sharpening upper trough over the Tasman Sea with double jet structure should provide strong upper level divergence for the low developing there.”

“Deepening caused by very strong divergence and vorticity associated with a broad very strong subtropical jet with winds reported 150-200 kt by aircraft”

…………………………………………………………………………………………………………………

The actual behaviour of the low

06z18z

21z00z

Analysis charts above, drawn every 1 hPa, show the actual track of the low as analysed by a senior meteorologist. Chart times are 6PM 17th (top left); 6AM 18th (top right); 9AM 18th (bottom left) and Noon 18th (bottom right).

Contrast the 6AM analysis with these forecast model prognoses for that same time:

UKMO Unified Model and ECMWF IFS model output - runtime midnight 15th July

Surface pressure contours. UKMO Unified Model (red) and ECMWF IFS model (blue) output for 6AM 18th July. Model runtime was midnight 15th July

Analysis for 6AM on the 18th July

Surface analysis for 6AM on the 18th July

There was considerable disagreement between models as to the track and depth of this low,  and, as suggested by the analysis, no single model got it quite right.
Even though computer models are becoming ever more sophisticated, it’s important not to slavishly follow their predictions. As we have seen with this fairly brief event, it’s crucial – especially for high-impact weather events – to have professional meteorologists monitoring the situation and applying their judgement and conceptual knowledge of meteorology to the output of the models. This is something that is not likely to change in the future.